U.S. patent application number 11/642724 was filed with the patent office on 2007-05-10 for low viscosity precursor compositions and methods for the deposition of conductive electronics features.
This patent application is currently assigned to Cabot Corporation. Invention is credited to Paolina Atanassova, Hugh Denham, Mark J. Hampden-Smith, Toivo T. Kodas, Klaus Kunze, Allen B. Schult, Aaron D. Stump, Karel Vanheusden.
Application Number | 20070104882 11/642724 |
Document ID | / |
Family ID | 27668496 |
Filed Date | 2007-05-10 |
United States Patent
Application |
20070104882 |
Kind Code |
A1 |
Kodas; Toivo T. ; et
al. |
May 10, 2007 |
Low viscosity precursor compositions and methods for the deposition
of conductive electronics features
Abstract
A precursor composition for the deposition and formation of an
electrical feature such as a conductive feature. The precursor
composition advantageously has a low viscosity enabling deposition
using direct-write tools. The precursor composition also has a low
conversion temperature, enabling the deposition and conversion to
an electrical feature on low temperature substrates. A particularly
preferred precursor composition includes silver metal for the
formation of highly conductive silver features.
Inventors: |
Kodas; Toivo T.;
(Albuquerque, NM) ; Hampden-Smith; Mark J.;
(Albuquerque, NM) ; Vanheusden; Karel; (Placitas,
NM) ; Denham; Hugh; (Albuquerque, NM) ; Stump;
Aaron D.; (Albuquerque, NM) ; Schult; Allen B.;
(Albuquerque, NM) ; Atanassova; Paolina;
(Albuquerque, NM) ; Kunze; Klaus; (Albuquerque,
NM) |
Correspondence
Address: |
Jaimes Sher, Esq.;Cabot Corporation
5401 Venice Avenue NE
Albuquerque
NM
87113
US
|
Assignee: |
Cabot Corporation
Boston
MA
|
Family ID: |
27668496 |
Appl. No.: |
11/642724 |
Filed: |
December 21, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10265351 |
Oct 4, 2002 |
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11642724 |
Dec 21, 2006 |
|
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60327620 |
Oct 5, 2001 |
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Current U.S.
Class: |
427/375 ;
252/500 |
Current CPC
Class: |
H01L 2224/053 20130101;
H01L 24/05 20130101; H05K 2203/1131 20130101; H01L 2224/05294
20130101; H05K 3/105 20130101; H01L 24/03 20130101; H05K 2201/0154
20130101; H01L 2924/12042 20130101; C23C 18/08 20130101; H01L
2924/12044 20130101; C23C 18/06 20130101; H01B 1/026 20130101; H01L
2924/14 20130101; H05K 1/097 20130101; H01L 2924/12042 20130101;
H01L 2924/00 20130101; H01L 2924/12044 20130101; H01L 2924/00
20130101; H01L 2924/14 20130101; H01L 2924/00 20130101 |
Class at
Publication: |
427/375 ;
252/500 |
International
Class: |
B05D 3/02 20060101
B05D003/02 |
Claims
1. A process for forming a flat panel display conductive feature,
comprising: (a) direct printing a precursor composition onto a
surface modified substrate, wherein the precursor composition
comprises metallic particles; and (b) heating the precursor
composition to form the flat panel display conductive feature on
the substrate, the flat panel display conductive feature having a
minimum feature size of not greater than 100 .mu.m.
2. The process of claim 1, wherein the direct printing comprises
syringe printing.
3. The process of claim 1, wherein the direct printing comprises
aerosol jet deposition.
4. The process of claim 1, wherein the direct printing comprises,
ink jet printing.
5. The process of claim 4, wherein the process further comprises:
(c) surface modifying selected regions of a surface of an initial
substrate to form the surface modified substrate.
6. The process of claim 5, wherein the surface modifying comprises
surface energy patterning by increasing or decreasing the surface
energy of the surface in the selected regions corresponding to
where it is desired to confine the precursor composition.
7. The process of claim 6, wherein the surface modifying is
performed with a laser.
8. The process of claim 7, wherein the laser removes hydroxyl
groups from the surface.
9. The process of claim 7, wherein the laser increases or decreases
hydrophilicity of the surface.
10. The process of claim 7, wherein the laser forms pores on the
substrate.
11. The process of claim 5, wherein the surface modifying comprises
surface energy patterning by increasing or decreasing surface
energy of the surface in selected regions corresponding to where it
is desired to eliminate the precursor composition.
12. The process of claim 11, wherein the surface modifying is
performed with a laser.
13. The process of claim 12, wherein the laser removes hydroxyl
groups from the surface.
14. The process of claim 12, wherein the laser increases or
decreases hydrophilicity of the surface.
15. The process of claim 12, wherein the laser forms pores on the
substrate.
16. The process of claim 5, wherein the surface modifying increases
adhesion of the precursor composition to the substrate.
17. The process of claim 5, wherein the surface modifying is
performed with a thermal print head.
18. The process of claim 5, wherein the surface modifying comprises
chemically modifying the surface.
19. The process of claim 5, wherein the surface modifying comprises
electrostatic printing.
20. The process of claim 5, wherein the surface modifying comprises
micro-contact printing.
21. The process of claim 5, wherein the substrate comprises a
polymer.
22. The process of claim 5, wherein the substrate comprises
glass.
23. The process of claim 22, wherein the surface modifying
comprises increasing the surface energy of the glass in selected
regions corresponding to where it is desired to confine or
eliminate the precursor composition.
24. The process of claim 5, wherein the precursor composition has a
surface tension of 20 to 50 dynes/cm.
25. The process of claim 5, wherein the direct printing comprises
directing droplets of the precursor composition toward a surface of
the substrate, the droplets having an average droplet size not
greater than about 10 .mu.m.
26. The process of claim 25, wherein the average droplet size is
not greater than about 5 .mu.m.
27. The process of claim 5, wherein the direct printing comprises
depositing droplets onto the substrate at a rate of 1000 drops per
second or higher.
28. The process of claim 27, wherein each droplet comprise from
about 25 to 100 picoliters of the precursor composition.
29. The process of claim 5, wherein the minimum feature size is not
greater than 75 .mu.m.
30. The process of claim 5, wherein the minimum feature size is not
greater than 50 .mu.m.
31. The process of claim 5, wherein the minimum feature size is not
greater than 25 .mu.m.
32. The process of claim 5, wherein the flat panel display
conductive feature has a width not greater than 200 .mu.m.
33. The process of claim 5, wherein the flat panel display
conductive feature has a width not greater than 100 .mu.m.
34. The process of claim 5, wherein the flat panel display
conductive feature has a width not greater than 75 .mu.m.
35. The process of claim 5, wherein the flat panel display
conductive feature has a width not greater than 50 .mu.m.
36. The process of claim 5, wherein the metallic particles comprise
a metal selected from the group consisting of silver, palladium,
copper, gold, platinum and nickel.
37. The process of claim 5, wherein the precursor composition
further comprises metal oxide particles.
38. The process of claim 5, wherein the precursor composition
further comprises glass particles.
39. The process of claim 5, wherein the metallic particles have a
volume median particle size of not greater than 100 nanometers.
40. The process of claim 5, wherein the metallic particles have a
volume median particle size of not greater than 0.3 .mu.m.
41. The process of claim 40, wherein the metallic particles
comprise a cap or coating thereon.
42. The process of claim 41, wherein the cap or coating comprises
an inorganic cap or coating.
43. The process of claim 41, wherein the cap or coating comprises
silica.
44. The process of claim 41, wherein the cap or coating comprises
glass.
45. The process of claim 41, wherein the cap or coating comprises
an organic cap or coating.
46. The process of claim 41, wherein the cap or coating comprises a
polymer.
47. The process of claim 41, wherein the cap or coating comprises
an intrinsically conductive polymer, a sulfonated
perfluorohydrocarbon polymer, polystyrene,
polystyrene/methacrylate, sodium bis(2-ethylhexyl) sulfosuccinate,
tetra-n-octyl-ammonium bromide or an alkane thiolate.
48. The process of claim 41, wherein the cap or coating comprises
PVP.
49. The process of claim 40, wherein at least 80 volume percent of
the metallic particles are not. larger than twice the average
particle size.
50. The process of claim 5, wherein the heating comprises heating
in air at from about 450.degree. C. to 600.degree. C.
51. The process of claim 5, wherein the flat panel display
conductive feature comprises an electrode.
52. The process of claim 5, wherein the flat panel display
conductive feature comprises a bus line.
53. The process of claim 5, wherein the flat panel display
conductive feature comprises a transparent conductive feature.
54. The process of claim 5, wherein the flat panel display
conductive feature comprises indium-tin oxide or antimony-tin
oxide.
55. The process of claim 5, wherein the metallic particle comprise
a metal and wherein the conductivity of the flat panel display
conductive feature is no less than 10 percent the conductivity of
the equivalent pure metal.
56. The process of claim 5, wherein the metallic particle comprise
a metal and wherein the flat panel display conductive feature has a
resistivity that is not greater than 4 times the resistivity of the
equivalent pure metal.
57. The process of claim 5, wherein the metallic particle comprise
a metal and wherein the flat panel display conductive feature has a
resistivity that is not greater than 2 times the resistivity of the
equivalent pure metal.
58. The process of claim 5, wherein the flat panel display
conductive feature has a thickness greater than 1 .mu.m.
59. The process of claim 5, wherein the flat panel display
conductive feature has a thickness greater than 5 .mu.m.
60. The process of claim 5, wherein the flat panel display
conductive feature comprises a metal-glass composition.
61. The process of claim 5, wherein the process further comprises
high shear mixing the precursor composition.
62. A process for forming a flat panel display conductive feature,
the process comprising heating an ink jet printed precursor
composition to form the flat panel display conductive feature on a
surface modified substrate, wherein the precursor composition
comprises metallic particles, and wherein the flat panel display
conductive feature has a width of not greater than 100 .mu.m.
63. The process of claim 62, wherein the process further comprises
surface modifying selected regions of a surface of an initial
substrate to form the surface modified substrate.
64. The process of claim 63, wherein the surface modifying
comprises surface energy patterning by increasing or decreasing the
surface energy of the surface in the selected regions corresponding
to where it is desired to confine the precursor composition.
65. The process of claim 64, wherein the surface modifying is
performed with a laser.
66. The process of claim 65, wherein the laser removes hydroxyl
groups from the surface.
67. The process of claim 65, wherein the laser increases or
decreases hydrophilicity of the surface.
68. The process of claim 65, wherein the laser forms pores on the
substrate.
69. The process of claim 63, wherein the surface modifying
comprises surface energy patterning by increasing or decreasing
surface energy of the surface in selected regions corresponding to
where it is desired to eliminate the precursor composition.
70. The process of claim 69, wherein the surface modifying is
performed with a laser.
71. The process of claim 70, wherein the laser removes hydroxyl
groups from the surface.
72. The process of claim 70, wherein the laser increases or
decreases hydrophilicity of the surface.
73. The process of claim 70, wherein the laser forms pores on the
substrate.
74. The process of claim 63, wherein the surface modifying
increases adhesion of the precursor composition to the
substrate.
75. The process of claim 63, wherein the surface modifying is
performed with a thermal print head.
76. The process of claim 63, wherein the surface modifying
comprises chemically modifying the surface.
77. The process of claim 63, wherein the surface modifying
comprises electrostatic printing.
78. The process of claim 63, wherein the surface modifying
comprises micro-contact printing.
79. The process of claim 63, wherein the substrate comprises a
polymer.
80. The process of claim 63, wherein the substrate comprises
glass.
81. The process of claim 80, wherein the surface modifying
comprises increasing the surface energy of the glass in selected
regions corresponding to where it is desired to confine or
eliminate the precursor composition.
82. The process of claim 63, wherein the precursor composition has
a surface tension of 20 to 50 dynes/cm.
83. The process of claim 63, wherein the direct printing comprises
directing droplets of the precursor composition toward a surface of
the substrate, the droplets having an average droplet size not
greater than about 10 .mu.m.
84. The process of claim 83, wherein the average droplet size is
not greater than about 5 .mu.m.
85. The process of claim 63, wherein the direct printing comprises
depositing droplets onto the substrate at a rate of 1000 drops per
second or higher.
86. The process of claim 85, wherein each droplet comprise from
about 25 to 100 picoliters of the precursor composition.
87. The process of claim 63, wherein the minimum feature size is
not greater than 75 .mu.m.
88. The process of claim 63, wherein the minimum feature size is
not greater than 50 .mu.m.
89. The process of claim 63, wherein the minimum feature size is
not greater than 25 .mu.m.
90. The process of claim 63, wherein the flat panel display
conductive feature has a width not greater than 200 .mu.m.
91. The process of claim 63, wherein the flat panel display
conductive feature has a width not greater than 100 .mu.m.
92. The process of claim 63, wherein the flat panel display
conductive feature has a width not greater than 75 .mu.m.
93. The process of claim 63, wherein the flat panel display
conductive feature has a width not greater than 50 .mu.m.
94. The process of claim 63, wherein the metallic particles
comprise a metal selected from the group consisting of silver,
palladium, copper, gold, platinum and nickel.
95. The process of claim 63, wherein the precursor composition
further comprises metal oxide particles.
96. The process of claim 63, wherein the precursor composition
further comprises glass particles.
97. The process of claim 63, wherein the metallic particles have a
volume median particle size of not greater than 100 nanometers.
98. The process of claim 63, wherein the metallic particles have a
volume median particle size of not greater than 0.3 .mu.m.
99. The process of claim 98, wherein the metallic particles
comprise a cap or coating thereon.
100. The process of claim 99, wherein the cap or coating comprises
an inorganic cap or coating.
101. The process of claim 99, wherein the cap or coating comprises
silica.
102. The process of claim 99, wherein the cap or coating comprises
glass.
103. The process of claim 99, wherein the cap or coating comprises
an organic cap or coating.
104. The process of claim 99, wherein the cap or coating comprises
a polymer.
105. The process of claim 99, wherein the cap or coating comprises
an intrinsically conductive polymer, a sulfonated
perfluorohydrocarbon polymer, polystyrene,
polystyrene/methacrylate, sodium bis(2-ethylhexyl) sulfosuccinate,
tetra-n-octyl-ammonium bromide or an alkane thiolate.
106. The process of claim 99, wherein the cap or coating comprises
PVP.
107. The process of claim 98, wherein at least 80 volume percent of
the metallic particles are not larger than twice the average
particle size.
108. The process of claim 63, wherein the heating comprises heating
in air at from about 450.degree. C. to 600.degree. C.
109. The process of claim 63, wherein the flat panel display
conductive feature comprises an electrode.
110. The process of claim 63, wherein the flat panel display
conductive feature comprises a bus line.
111. The process of claim 63, wherein the flat panel display
conductive feature comprises a transparent conductive feature.
112. The process of claim 63, wherein the flat panel display
conductive feature comprises indium-tin oxide or antimony-tin
oxide.
113. The process of claim 63, wherein the metallic particle
comprise a metal and wherein the conductivity of the flat panel
display conductive feature is no less than 10 percent the
conductivity of the equivalent pure metal.
114. The process of claim 63, wherein the metallic particle
comprise a metal and wherein the flat panel display conductive
feature has a resistivity that is not greater than 4 times the
resistivity of the equivalent pure metal.
115. The process of claim 63, wherein the metallic particle
comprise a metal and wherein the flat panel display conductive
feature has a resistivity that is not greater than 2 times the
resistivity of the equivalent pure metal.
116. The process of claim 63, wherein the flat panel display
conductive feature has a thickness greater than 1 .mu.m.
117. The process of claim 63, wherein the flat panel display
conductive feature has a thickness greater than 5 .mu.m.
118. The process of claim 63, wherein the flat panel display
conductive feature comprises a metal-glass composition.
119. The process of claim 63, wherein the process further comprises
high shear mixing the precursor composition.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation application of co-pending
U.S. patent application Ser. No. 10/265,351, filed Oct. 4, 2002,
which claims the benefit of U.S. Provisional Application No.
60/327,620 filed Oct. 5, 2001. Each of the foregoing referenced
patent applications is incorporated by reference herein as if set
forth below in its entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to precursor compositions that
are useful for the deposition of conductive electronic features.
The precursor compositions can advantageously have a low conversion
temperature to enable low-temperature treatment of the precursors
to form conductive electronic features on a variety of substrates.
The precursor compositions can also have a low viscosity to enable
the deposition of the compositions using direct-write tools, such
as ink-jet devices.
[0004] 2. Description of Related Art
[0005] The electronics, display and energy industries rely on the
formation of coatings and patterns of conductive materials to form
circuits on organic and inorganic substrates. The primary methods
for generating these patterns are screen printing for features
larger than about 100 .mu.m and thin film and etching methods for
features smaller than about 100 .mu.m. Other subtractive methods to
attain fine feature sizes include the use of photo-patternable
pastes and laser trimming.
[0006] One consideration with respect to patterning of conductors
is cost. Non-vacuum, additive methods generally entail lower costs
than vacuum and subtractive approaches. Some of these printing
approaches utilize high viscosity flowable liquids.
Screen-printing, for example, uses flowable mediums with
viscosities of thousands of centipoise. At the other extreme, low
viscosity compositions can be deposited by methods such as ink-jet
printing. However, this latter family of low viscosity compositions
is not as well developed as the high viscosity compositions.
[0007] Ink-jet printing of conductors has been explored, but the
approaches to date have been inadequate for producing well-defined
features with good electrical properties. For example, ink-jet
printable conductor compositions have been described by R. W. Vest
(Metallo-Organic Materials for Improved Thick Film Reliability,
Nov. 1, 1980, Final Report, Contract #N00163-79-C-0352, National
Avionic Center). The compositions disclosed by Vest included a
precursor and a solvent for the precursor. These compositions were
not designed for processing at low temperatures, and as a result
the processing temperatures were relatively high, such as greater
than 250.degree. C.
[0008] U.S. Pat. Nos. 5,882,722 and 6,036,889 by Kydd disclose
conductor precursor compositions that contain metallic particles, a
precursor and a vehicle and are capable of forming conductors at
low temperatures on organic substrates. However, the formulations
have a relatively high viscosity and are not useful for alternative
deposition methods such as ink-jet printing.
[0009] Attempts have also been made to produce metal-containing
compositions at low temperatures by using a composition containing
a polymer and a precursor to a metal. See, for example, U.S. Pat.
No. 6,019,926 by Southward et al. However, the deposits were chosen
for optical properties and were either not conductive or were
poorly conductive.
[0010] U.S. Pat. Nos. 5,846,615 and 5,894,038, both by Sharma et
al., disclose precursors to Au and Pd that have low reaction
temperatures thereby conceptually enabling processing at low
temperatures to form metals. It is disclosed that a variety of
methods can be used to apply the precursors, including ink-jet
printing and screen printing. However, the printing of these
compositions is not disclosed in detail.
[0011] U.S. Pat. No. 5,332,646 by Wright et al. discloses a method
of making colloidal palladium and/or platinum metal dispersions by
reducing a palladium and/or platinum metal of a metallo-organic
palladium and/or platinum metal salt which lacks halide
functionality. However, formulations for depositing electronic
features are not disclosed.
[0012] U.S. Pat. No. 5,176,744 by Muller discloses the use of
Cu-formate precursor compositions for the direct laser writing of
copper metal. The compositions include a crystallization inhibitor
to prevent crystallization of copper formate during drying.
[0013] U.S. Pat. No. 5,997,044 by Behm et al. discloses a document,
such as a lottery ticket, having simple circuitry deposited
thereon. The circuitry can be formed from inks containing
conductive carbon and other additives as well as metallic
particles. It is disclosed that the inks can be deposited by
methods such as gravure printing.
[0014] U.S. Pat. No. 6,238,734 by Senzaki et al. is directed to
compositions for the chemical vapor deposition of mixed metal or
metal compound layers. The method uses a solventless common ligand
mixture of metals in a liquid state for deposition by direct liquid
injection.
[0015] There exists a need for low viscosity precursor compositions
for the fabrication of conductive features for use in electronics,
displays, and other applications. Further, there is a need for
precursor compositions that have low processing temperatures to
allow deposition onto organic substrates and subsequent heat
treatment. It would also be advantageous if the compositions could
be deposited with a fine feature size, such as not greater than 100
.mu.m, while still providing electronic features with adequate
electrical and mechanical properties.
[0016] The ideal low viscosity precursor composition and its
associated deposition technique for the fabrication of electronic
features such as a conductor would combine a number of attributes.
The conductive feature would have high conductivity, preferably
close to that of a dense, pure metal. The processing temperature
would be low enough to allow formation of conductors on a variety
of organic substrates. The deposition technique would allow
deposition onto surfaces that are non-planar (e.g., not flat). The
conductive feature would have high resistance to electromigration,
solder leaching and oxidation. The conductor would also have good
adhesion to the substrate.
[0017] Further, there is a need for electronic circuit elements and
complete electronic circuits fabricated on inexpensive, thin and/or
flexible substrates, such as paper, using high volume printing
techniques such as reel-to-reel printing. Recent developments in
organic thin film transistor (TFT) technology and organic light
emitting device (OLED) technology have accelerated the need for
complimentary circuit elements that can be written directly onto
low cost substrates. Such elements include conductive
interconnects, electrodes, conductive contacts and via fills.
DESCRIPTION OF THE INVENTION
[0018] The present invention is directed to low viscosity precursor
compositions that can be deposited onto a substrate using, for
example, direct-write methods such as ink-jet deposition. The
precursor compositions preferably have a low decomposition
temperature, thereby enabling the formation of electronic features
on a variety of substrates, including organic substrates. The
precursor compositions can include various combinations of
molecular metal precursors, solvents, micron-sized particles,
nanoparticles, vehicles, reducing agents and other additives. The
precursor compositions can advantageously include one or more
conversion reaction inducing agents adapted to reduce the
conversion temperature of the precursor composition. The precursor
compositions can be deposited onto a substrate and reacted to form
highly conductive electronic features having good electrical and
mechanical properties.
[0019] The precursor compositions according to the present
invention can be formulated to have a wide range of properties and
a wide range of relative cost. For example, in high volume
applications that do not require well-controlled properties,
inexpensive precursor compositions can be deposited on
cellulose-based materials, such as paper, to form simple disposable
circuits.
[0020] On the other hand, the precursor compositions of the present
invention can be utilized to form complex, high precision circuitry
having good electrical properties. For example, the compositions
and methods of the present invention can be utilized to form
conductive features on a substrate, wherein the features have a
feature size (i.e., average width of the smallest dimension) of not
greater than about 200 micrometers (.mu.m), more preferably not
greater than about 100 .mu.m, even more preferably not greater than
about 75 .mu.m, even more preferably not greater than about 50
.mu.m and most preferably not greater than about 25 .mu.m.
[0021] The conductive electronic features formed according to the
present invention can have good electrical properties. For example,
the conductive features according to the present invention can have
a resistivity that is not greater than 20 times the resistivity of
the bulk conductor, such as not greater than 10 times the
resistivity of the bulk conductor, preferably not greater than 6
times the resistivity of the bulk conductor, more preferably not
greater than 4 times the resistivity of the bulk conductor and even
more preferably not greater than 2 times the resistivity of the
bulk conductor.
[0022] The method for forming the electronic features according to
the present invention can also use relatively low processing
temperatures. In one embodiment, the conversion temperature is not
greater than about 250.degree. C., such as not greater than about
225.degree. C., more preferably is not greater than about
200.degree. C. and even more preferably is not greater than about
185.degree. C. In certain embodiments, the conversion temperature
can be not greater than about 150.degree. C., such as not greater
than about 125.degree. C. and even not greater than about
100.degree. C.
Definitions
[0023] As used herein, the term low viscosity precursor composition
refers to a flowable composition that has a viscosity of not
greater than about 1000 centipoise. According to one embodiment,
the low viscosity precursor composition has a viscosity of not
greater than about 500 centipoise, more preferably not greater than
about 100 centipoise and even more preferably not greater than
about 50 centipoise. As used herein, the viscosity is measured at a
shear rate of about 132 Hz and under the relevant deposition
conditions, particularly temperature. For example, some precursor
compositions may be heated prior to deposition to reduce the
viscosity.
[0024] As used herein, the term molecular metal precursor refers to
a molecular compound that includes a metal atom. Examples include
organometallics (molecules with carbon-metal bonds), metal organics
(molecules containing organic ligands with metal bonds to other
types of elements such as oxygen, nitrogen or sulfur) and inorganic
compounds such as metal nitrates, metal halides and other metal
salts.
[0025] The low viscosity precursor compositions in accordance with
the present invention can also include particulates of a metal
other material phases. The particulates can fall in two size ranges
referred to herein as nanoparticles and micron-size particles.
Nanoparticles have an average size of not greater than about 100
nanometers. Micron-size particles have an average particle size of
greater than about 0.1 .mu.m. Nanoparticles and micron-size
particles are collectively referred to herein as particles or
powders.
[0026] In addition, the low viscosity precursor compositions can
include a solvent for the molecular metal precursor. A solvent is a
chemical that is capable of dissolving at least a portion of the
molecular metal precursor. The low viscosity precursor composition
can also include a vehicle. As used herein, a vehicle is a flowable
medium that facilitates deposition of the precursor composition,
such as by imparting sufficient flow properties or supporting
dispersed particles. As will be appreciated from the following
discussion, the same chemical compound can have multiple functions
in the precursor composition, such as one that is both a solvent
and a vehicle.
[0027] Other chemicals, referred to herein simply as additives, can
also be included in the low viscosity precursor compositions of the
present invention. As is discussed below, such additives can
include, but are not limited to, crystallization inhibitors,
polymers, polymer precursors (oligomers or monomers), reducing
agents, binders, dispersants, surfactants, humectants, defoamers
and the like.
Precursor Compositions
[0028] As is discussed above, the low viscosity precursor
compositions according to the present invention can optionally
include particulates in the form of nanoparticles and/or
micron-size particles.
[0029] Nanoparticles have an average size of not greater than about
100 nanometers, such as from about 10 to 80 nanometers.
Particularly preferred for low viscosity precursor compositions are
nanoparticles having an average size in the range of from about 25
to 75 nanometers.
[0030] Nanoparticles that are particularly preferred for use in the
present invention are not substantially agglomerated. Preferred
nanoparticle compositions include Al.sub.2O.sub.3, CuO.sub.x,
SiO.sub.2 and TiO.sub.2, conductive metal oxides such as
In.sub.2O.sub.3, indium-tin oxide (ITO) and antimony-tin oxide
(ATO), silver, palladium, copper, gold, platinum and nickel. Other
useful nanoparticles of metal oxides include pyrogenous silica such
as HS-5 or M5 or others (Cabot Corp., Boston, Mass.) and AEROSIL
200 or others (Degussa AG, Dusseldorf, Germany) or surface modified
silica such as TS530 or TS720 (Cabot Corp., Boston, Mass.) and
AEROSIL 380 (Degussa AG, Dusseldorf, Germany). In one embodiment of
the present invention, the nanoparticles are composed of the same
metal that is contained in the metal precursor compound, discussed
below. Nanoparticles can be fabricated using a number of methods
and one preferred method, referred to as the Polyol process, is
disclosed in U.S. Pat. No. 4,539,041 by Figlarz et al., which is
incorporated herein by reference in its entirety.
[0031] The precursor compositions according to the present
invention can also include micron-size particles, having an average
size of at least about 0.1 .mu.m. Preferred compositions of
micron-size particles are similar to the compositions described
above with respect to nanoparticles. The particles are preferably
spherical, such as those produced by spray pyrolysis. Particles in
the form of flakes increase the viscosity of the precursor
composition and are not amenable to deposition using tools having a
restricted orifice size, such as an ink-jet device. When
substantially spherical particles are described herein, the
particle size refers to the particle diameter. In one preferred
embodiment, the low viscosity precursor compositions according to
the present invention do not include any particles in the form of
flakes.
[0032] Generally, the volume median particle size of the
micron-size particles utilized in the low viscosity precursor
compositions according to the present invention is at least about
0.1 .mu.m, such as at least about 0.3 .mu.m. Further, the volume
median particle size is preferably not greater than about 20 .mu.m.
For most applications, the volume median particle size is more
preferably not greater than about 10 .mu.m and even more preferably
is not greater than about 5 .mu.m. A particularly preferred median
particle size for the micron-size particles is from about 0.3 .mu.m
to about 3 .mu.m. According to one embodiment of the present
invention, it is preferred that the volume median particle size of
the micron-size particles is at least 10 times smaller than the
orifice diameter in the tool using the composition, such as not
greater than about 5 .mu.m for an ink-jet head having a 50 .mu.m
orifice.
[0033] There are many difficulties typically associated with
depositing low viscosity compositions containing particulates. If
many of the particulates are too small in size (nanoparticles), the
viscosity of the composition can be too high. At the other extreme,
larger micron-size particles tend to settle quickly out of the
liquid leading to a short shelf life for the suspensions. Larger
particles and particle agglomerates also tend to clog the orifices
of many direct-write tools such as syringes and ink-jets. Flakes do
not flow as easily through narrow channels and therefore spherical
particulates are preferred. However, spherical particles of many
materials are not readily available. For these and other reasons,
many electronic materials have not been readily deposited using
such direct-write tools.
[0034] The micron-size particles that are useful in the precursor
compositions of the present invention advantageously have settling
velocities that correspond to relatively small particle sizes when
measured by a sedimentation technique, corresponding to porous or
hollow particles. There are numerous ways to measure and quantify
particle size including by mass, volume and number. For the low
viscosity compositions of the present invention, one of the most
important aspects is that the particles do not settle rapidly and
the means by which the particle size is measured and reported must
be carefully interpreted in this context. The geometric sizes of
the particles, as might be observed using a microscope, do not
reflect that particles with the same size can have different
densities and therefore significantly different settling
velocities. Hollow or porous micron-size particles settle more
slowly than dense particles. Similarly, optically determined
particle sizes as from light scattering in liquids or gases also
provide data that only reflect the geometric size of the particles
and these measurements do not reflect that particles of the same
size can have different densities. Any measurement that provides
the actual physical size of the particles, such as optical
techniques, can provide numbers that must be interpreted with
caution. For example, the volume median diameter of particles
determined from light scattering cannot easily be related to mass
median diameter in the absence of information about the apparent
density of the particles, such as whether they are hollow or porous
and the extent of the porosity. Likewise, the particle size data
determined from settling velocities can provide information about
the settling behavior of the particles, but does not provide
information about the true geometric particle size if the particles
are hollow or porous.
[0035] However, the combination of size data measured from
optically-based approaches and data measured from sedimentation
velocity provides a measure of particle performance in low
viscosity compositions, where control of the particle settling rate
is crucial. A small size calculated from settling velocity of a
dense particle along with a large geometric size is an indication
of hollow or porous particles. It is preferred that the average
size of the micron-size particles utilized in the precursor
compositions of the present invention, as measured by sedimentation
techniques corresponds to dense particles having a settling
velocity corresponding to an average particle size of not greater
than about 4 .mu.m, more preferably not greater than about 1 .mu.m,
even more preferably not greater than about 0.5 .mu.m and even more
preferably not greater than about 0.1 .mu.m.
[0036] Thus, the micron-size particles according to the present
invention preferably include particles having a low settling rate
as measured by settling techniques while having a larger size when
measured by geometric techniques. One such geometric technique is
to measure the particle size by light scattering using a MICROTRAC
particle size analyzer (Honeywell Industrial Automation and
Control, Fort Washington, Pa.), which yields a geometric (volume)
average particle size.
[0037] It is desirable to maintain a substantially neutral buoyancy
of the micron-size particles in the suspension while maintaining a
relatively large physical size. The buoyancy is required for
stability while the larger size maintains liquid properties, such
as viscosity or light scattering ability, within useful ranges.
Stated another way, it is often desirable to provide micron-size
particles having a low settling velocity but at high loadings. The
settling velocity of the particles is proportional to the apparent
density of the particle (pp) minus the density of the liquid
(.rho..sub.L). Ideally, the particles will have an apparent density
that is approximately equal to the density of the liquid, which is
typically about 1 g/cm.sup.3 (e.g., the density of water). Since a
typical metal has a theoretical density in the range of from about
6 to about 20 g/cm.sup.3, it is preferable that the apparent
density of such micron-size particles be a fraction of the
theoretical density. According to one embodiment, the micron-size
particles have an apparent density that is not greater than about
75 percent of the theoretical density for the particle, more
preferably not greater than about 50 percent of the theoretical
density.
[0038] One preferred method for obtaining a reduced apparent
density of the micron-size particles according to the present
invention is to produce particles having a hollow microstructure.
That is, one preferred particle morphology is a particle comprised
of a dense shell having an inner radius and an outer radius.
Preferably, the shell has a high density and is substantially
impermeable. For such a hollow particle, the equation representing
the conditions for neutral buoyancy can be written: r 2 = [ .rho. p
.rho. p - 1 3 ] .times. r 1 ##EQU1##
[0039] where: [0040] r.sub.1=inner radius [0041] .rho..sub.L=1
(water) [0042] r.sub.2=outer radius [0043] .rho..sub.p=theoretical
density of the particle
[0044] For example, if a hollow particle has an outer radius of 2
.mu.m (4 .mu.m diameter) and a density of 5 g/cm.sup.3, then the
optimum average wall thickness would be about 0.15 .mu.m for the
particle to be neutrally buoyant in a liquid having a density of 1
g/cm.sup.3.
[0045] Although hollow micron-size particles can be preferred
according to the present invention, it will be appreciated that
other particle morphologies can be utilized while maintaining an
apparent density within the desired range. For example, the
particles could have a sufficient amount of closed porosity to
yield a particle having an apparent density that is lower than the
theoretical density. Open (surface) porosity can also decrease the
apparent density if the surface tension of the liquid medium (the
liquid precursor components) does not enable the liquid to
substantially penetrate the surface pores. For example, supported
electrocatalyst particles, such as those disclosed in U.S. Pat. No.
6,103,393 by Kodas et al., can have a high level of porosity and
surface area.
[0046] Thus, the particles that are particularly useful in low
viscosity precursor compositions according to the present invention
have a low settling velocity in the liquid medium. The settling
velocity according to Stokes Law can be defined as: V = D st 2
.function. ( .rho. s - .rho. l ) .times. g 18 .times. .times. .eta.
##EQU2##
[0047] where: [0048] D.sub.st=Stokes diameter [0049] .eta.=fluid
viscosity [0050] .rho..sub.s=apparent density of the particle
[0051] .rho..sub.l=density of the liquid [0052] V=settling velocity
[0053] g=acceleration due to gravity
[0054] Preferably, the average settling velocity of the particles
is sufficiently low such that the precursor compositions have a
useful shelf life without the necessity of mechanical mixing
techniques. Thus, it is preferred that a large mass fraction of the
particles, such as at least about 50 weight percent remains
suspended in the liquid for at least 1 hour. Stated another way,
the micron-size particles preferably have a settling velocity that
is not greater than 50 percent, more preferably not greater than 20
percent, of a theoretically dense particle of the same composition.
Further, the particles can be completely redispersed after
settling, such as by mixing, to provide the same particle size
distribution in suspension as measured before settling.
[0055] According to a preferred embodiment of the present
invention, the particles (nanoparticles and micron-size particles)
also have a narrow particle size distribution, such that the
majority of particles are about the same size and so that there are
a minimal number of large particles that can clog an orifice, such
as an orifice in an ink-jet head. A narrow particle size
distribution is particularly advantageous for direct-write
applications due to reduced clogging of the orifice by large
particles and due to the ability to form surface features having a:
fine line width, high resolution and high packing density.
Preferably, at least about 70 volume percent and more preferably at
least about 80 volume percent of the particles within the same size
classification (nanoparticles or micron-size particles) are not
larger than twice the average particle size. For example, when the
average particle size of micron-size particles is about 2 .mu.m, it
is preferred that at least about 70 volume percent of the
micron-size particles are not larger than 4 .mu.m and it is more
preferred that at least about 80 volume percent of the micron-size
particles are not larger than 4 .mu.m. Further, it is preferred
that at least about 70 volume percent and more preferably at least
about 80 volume percent of the particles are not larger than about
1.5 times the average particle size. Thus, when the average
particle size of the micron-size particles is about 2 .mu.m, it is
preferred that at least about 70 volume percent of the micron-size
particles are not larger than 3 .mu.m and it is more preferred that
at least about 80 volume percent of the micron-size particles are
not larger than 3 .mu.m.
[0056] It is known that micron-size particles and nanoparticles
often form soft agglomerates as a result of their relatively high
surface energies, as compared to larger particles. It is also known
that such soft agglomerates may be dispersed easily by treatments
such as exposure to ultrasound in a liquid medium, sieving, high
shear mixing and 3-roll milling. The average particle size and
particle size distributions described herein are measured by mixing
samples of the powders in a liquid medium, such as water and a
surfactant, and exposing the suspension to ultrasound through
either an ultrasonic bath or horn. The ultrasonic treatment
supplies sufficient energy to disperse the soft agglomerates into
primary particles. The primary particle size and size distribution
are then measured by light scattering in a MICROTRAC instrument.
This provides a good measure of the useful dispersion
characteristics of the powder because this simulates the dispersion
of the particles in a liquid vehicle, such as an ink-jet
suspension. Thus, the references to particle size herein refer to
the primary particle size, such as after lightly dispersing soft
agglomerates of the particles.
[0057] It is also possible to provide micron-size particles or
nanoparticles having a bimodal particle size distribution. That is,
the particles can have two distinct and different average particle
sizes. Preferably, each of the distinct particle size distributions
will meet the foregoing size distribution limitations. A bimodal or
trimodal particle size distribution can advantageously enhance the
packing efficiency of the particles when deposited according to the
present invention. In one embodiment, the larger mode includes
hollow or porous particles while the smaller mode includes dense
particles. The two modes can include particles of different
composition. In one embodiment, the two modes have average particle
sizes at about 1 .mu.m and 5 .mu.m, and in another embodiment the
average particle size of the 2 modes is about 0.5 .mu.m and 2.5
.mu.m. The bimodal particle size distribution can also be achieved
using nanoparticles and in one embodiment the larger mode has an
average particle size of from about 1 .mu.m to 10 .mu.m and the
smaller mode has an average particle size of from about 10 to 100
nanometers.
[0058] The particles that are useful in precursor compositions
according to the present invention also preferably have a high
degree of purity and it is preferred that the particles include not
greater than about 1.0 atomic percent impurities and more
preferably not greater than about 0.1 atomic percent impurities and
even more preferably not greater than about 0.01 atomic percent
impurities. Impurities are those materials that are not intended in
the final product (i.e., the conductive feature) and that
negatively affect the properties of the final product. For many
electronic applications, the most critical impurities to avoid are
Na, K, Cl, S and F. As is discussed below, it will be appreciated
that the particles can include composite particles having one or
more second phases. Such second phases are not considered
impurities.
[0059] The particles for use in the precursor compositions
according to the present invention can also be coated particles
wherein the particle includes a surface coating surrounding the
particle core. Coatings can be generated on the particle surface by
a number of different mechanisms. One preferred mechanism is spray
pyrolysis. One or more coating precursors can vaporize and fuse to
the hot particle surface and thermally react resulting in the
formation of a thin film coating by chemical vapor deposition
(CVD). Preferred coatings deposited by CVD include metal oxides and
elemental metals. Further, the coating can be formed by physical
vapor deposition (PVD) wherein a coating material physically
deposits on the surface of the particles. Preferred coatings
deposited by PVD include organic materials and elemental metals.
Alternatively, a gaseous precursor can react in the gas phase
forming small particles, for example, less than about 5 nanometers
in size, which then diffuse to the larger particle surface and
sinter onto the surface, thus forming a coating. This method is
referred to as gas-to-particle conversion (GPC). Whether such
coating reactions occur by CVD, PVD or GPC is dependent on the
reactor conditions, such as temperature, precursor partial
pressure, water partial pressure and the concentration of particles
in the gas stream. Another possible surface coating method is
surface conversion of the particles by reaction with a vapor phase
reactant to convert the surface of the particles to a different
material than that originally contained in the particles.
[0060] In addition, a volatile coating material such as lead oxide,
molybdenum oxide or vanadium oxide can be introduced into the
reactor such that the coating deposits on the particles by
condensation. Further, the particles can be coated using other
techniques. For example, soluble precursors to both the particle
and the coating can be used in the precursor solution. In another
embodiment, a colloidal precursor and a soluble precursor can be
used to form a particulate colloidal coating on the composite
particle. It will be appreciated that multiple coatings can be
deposited on the surface of the particles if such multiple coatings
are desirable.
[0061] The coatings are preferably as thin as possible while
maintaining conformity about the particles such that the core of
the particle is not substantially exposed. For example, the
coatings on a micron-size particle can have an average thickness of
not greater than about 200 nanometers, preferably not greater than
about 100 nanometers and more preferably not more than about 50
nanometers. For most applications, the coating has an average
thickness of at least about 5 nanometers. A specific example of
useful coated particles is silica coated silver particles.
[0062] Nanoparticles can also be coated by utilizing the coating
strategies as described above. In addition, it may be advantageous
to coat nanoparticles with materials such as a polymer, to prevent
agglomeration of the nanoparticles due to high surface energy. This
is described by P. Y. Silvert et al. (Preparation of colloidal
silver dispersions by the polyol process, Journal of Material
Chemistry, 1997, volume 7(2), pp. 293-299). In another embodiment,
the particles can be coated with an intrinsically conductive
polymer, preventing agglomeration in the composition and providing
a conductive patch after solidification of the composition. In yet
another embodiment, the polymer can decompose during heating
enabling the nanoparticles to sinter together. In one embodiment,
the nanoparticles are generated in-situ and are coated with a
polymer. Preferred coatings for nanoparticles according to the
present invention include sulfonated perfluorohydrocarbon polymer
(e.g., NAFION, available from E.I. duPont deNemours, Wilmington,
Del.), polystyrene, polystyrene/methacrylate, polyvinyl pyrolidone,
sodium bis(2-ethylhexyl)sulfosuccinate, tetra-n-octyl-ammonium
bromide and alkane thiolates.
[0063] The particles that are useful with the present invention can
also be "capped" with other compounds. The term capped refers to
having compounds bonded to the outer surface of the particles
without necessarily creating a coating over the outer surface. The
particles used with the present invention can be capped with any
functional group including organic compounds such as polymers,
organometallic compounds, and metal organic compounds. These
capping agents can serve a variety of functions including the
prevention of agglomeration of the particles, prevention of
oxidation, enhancement of bonding of the particles to a surface,
and enhancement of the flowability of the particles in a precursor
composition. Preferred capping agents that are useful with the
particles of the present invention include amine compounds,
organometallic compounds, and metal organic compounds.
[0064] The particulates in accordance with the present invention
can also be composite particles wherein the particles include a
first phase and a second phase associated with the first phase.
Preferred composite particulates include carbon-metal,
carbon-polymer, carbon-ceramic, carbon1-carbon2, ceramic-ceramic,
ceramic-metal, metal1-metal2, metal-polymer, ceramic-polymer, and
polymer1-polymer2. Also preferred are certain 3-phase combinations
such as metal-carbon-polymer. In one embodiment, the second phase
is uniformly dispersed throughout the first phase. The second phase
can be an electronic compound or it can be a non-electronic
compound. For example, sintering inhibitors such as metal oxides
can be included as a second phase in a first phase of a metallic
material, such as silver metal to inhibit sintering of the metal
without substantially affecting the conductivity.
[0065] As a further example, the particles can be electrocatalyst
particles, such as those composed of a metal or a metal oxide
dispersed on a support such as carbon. Such particles are disclosed
in U.S. Pat. No. 6,103,393 by Kodas et al., which is incorporated
herein by reference in its entirety. Such particles can be used in
fuel cells such as direct methanol fuel cells (DMFC) and proton
exchange membrane fuel cells (PEMFC), as well as metal-air
batteries and similar devices.
[0066] Further, the micron-size particles can be hollow particles,
as is discussed above, wherein the shell includes a first phase and
a second phase dispersed throughout the first phase.
[0067] The particulates according to a preferred embodiment of the
present invention are also substantially spherical in shape. That
is, the particulates are not jagged or irregular in shape.
Spherical particles are particularly advantageous because they are
able to disperse more readily in a liquid suspension and impart
advantageous flow characteristics to the precursor composition,
particularly for deposition using an ink-jet device or similar
tool. For a given level of solids loading, a low viscosity
precursor composition having spherical particles will have a lower
viscosity than a composition having non-spherical particles, such
as flakes. Spherical particles are also less abrasive than jagged
or plate-like particles, reducing the amount of abrasion and wear
on the deposition tool.
[0068] Thus, micron-size particles with low settling densities
derived from their porosity or hollowness can be used to provide
the low viscosity precursor compositions. Such micron-size
particles can be produced, for example, by spray pyrolysis. Spray
pyrolysis for production of micron-size particles is described in
U.S. Pat. No. 6,103,393 by Kodas, et al., which is incorporated
herein by reference in its entirety.
[0069] Hollow particles with well-controlled apparent density can
advantageously be formed by the spray-pyrolysis process disclosed
above. In the case of metals such as silver, it is often necessary
to add small amounts of metal oxide precursors, or salts to the
starting solution that can be removed after particle formation. The
metal oxides or the salts inhibit the densification of the metal
particles during the residence time in the reactor. As an example,
porous and/or hollow particles of conductors with reduced density
can be formed by adding metal oxide precursors such as alumina,
silica, copper oxides, glasses (e.g., barium aluminum borosilicate,
calcium silicate or lead borosilicate) and virtually any other
metal oxide that has a melting point significantly greater than the
metal. These additives can also serve the dual purpose of providing
adhesion to substrates, inhibiting sintering (as in the case of
silver that has a low sintering temperature), modifying temperature
coefficients of resistivity and other functions.
[0070] Another method for providing hollow particles in a spray
pyrolysis process is the use of particle precursors having low
solubility. Precursors having a low solubility precipitate at the
surface of the droplet thereby forming a shell. As the remainder of
the solvent leaves, more metal precursor precipitates onto the
shell, forming a hollow particle. Examples include organometallics,
metal organics and inorganic precursors. Preferred particle
precursors according to this embodiment have solubilities of not
greater than about 20 wt. %, more preferably not greater than about
10 wt. % and even more preferably not greater than about 5 wt. %.
Inorganic precursors can be selected, for example, from metal
nitrates, metal halides, metal sulfates, metal hydroxides and metal
carbonates.
[0071] Another method for forming hollow particles is to use
particle precursors that have low effective yields during
conversion from the precursor to the particulate product. Once the
droplets are dried and the particles consist only of precursor,
reaction to form the product results in a porous or hollow particle
because of the large volume change from reactant to product with a
relatively constant particle diameter. One example is the formation
of alumina from aluminum nitrate. Preferred particle precursors
according to this embodiment have a volumetric yield of not greater
than about 50%, more preferably not greater than about 25% and even
more preferably not greater than about 10%.
[0072] Another method is to use particle precursors that liberate
gases and inflate the particles during reaction of the precursor.
Preferred particle precursors according to this embodiment release,
for example, NO.sub.x or CO.sub.2 gas.
[0073] Metal salts such as nitrates, chlorides, sulfates,
hydroxides or oxalates can be used as particle precursors in a
spray pyrolysis process. Preferred metal salts include the metal
nitrates. For example, a preferred precursor to platinum metal
according to the present invention is nitrated diamine
dinitroplatinum (II). Another preferred metal is silver and a
preferred precursor to silver metal particles is silver nitrate,
AgNO.sub.3, or a silver carboxylate compound.
[0074] The precursor compositions according to the present
invention can also include molecular metal precursors, either alone
or in combination with particulates. Preferred examples include
molecular metal precursors to silver (Ag), nickel (Ni), platinum
(Pt), gold (Au), palladium (Pd), copper (Cu), indium (In) and tin
(Sn). Other molecular metal precursors can include precursors to
aluminum (Al), zinc (Zn), iron (Fe), tungsten (W), molybdenum (Mo),
ruthenium (Ru), lead (Pb), bismuth (Bi) and similar metals. The
molecular metal precursors can be either soluble or insoluble in
the precursor composition.
[0075] In general, molecular metal precursor compounds that
eliminate ligands by a radical mechanism upon conversion to metal
are preferred, especially if the species formed are stable radicals
and therefore lower the decomposition temperature of that precursor
compound.
[0076] Furthermore, molecular metal precursors containing ligands
that eliminate cleanly upon precursor conversion and escape
completely from the substrate (or the formed functional structure)
are preferred because they are not susceptible to carbon
contamination or contamination by anionic species such as nitrates.
Therefore, preferred precursors for metals used for conductors are
carboxylates, alkoxides or combinations thereof that would convert
to metals, metal oxides or mixed metal oxides by eliminating small
molecules such as carboxylic acid anhydrides, ethers or esters.
Metal carboxylates, particularly halogenocarboxylates such as
fluorocarboxylates, are particularly preferred metal precursors due
to their high solubility.
[0077] Particularly preferred metal precursor compounds are metal
precursor compounds containing silver, nickel, platinum, gold,
palladium, copper and ruthenium.
[0078] Examples of silver metal precursors that can be used in the
low viscosity precursor compositions according to the present
invention are illustrated in Table 1. TABLE-US-00001 TABLE 1 Silver
Precursor Molecular Compounds and Salts General Class Examples
Chemical Formula Nitrates Silver nitrate AgNO.sub.3 Nitrites Silver
nitrite AgNO.sub.2 Oxides Silver oxide Ag.sub.2O, AgO Carbonates
Silver carbonate Ag.sub.2CO.sub.3 Oxalates Silver oxalate
Ag.sub.2C.sub.2O.sub.4 (Pyrazolyl)borates Silver
trispyrazolylborate Ag[(N.sub.2C.sub.3H.sub.3).sub.3]BH Silver
tris(dimethylpyrazolyl)borate
Ag[((CH.sub.3).sub.2N.sub.2C.sub.3H.sub.3).sub.3]BH Azides Silver
azide AgN.sub.3 Fluoroborates Silver tetrafluoroborate AgBF.sub.4
Carboxylates Silver acetate AgO.sub.2CCH.sub.3 Silver propionate
AgO.sub.2CC.sub.2H.sub.5 Silver butanoate AgO.sub.2CC.sub.3H.sub.7
Silver ethylbutyrate AgO.sub.2CCH(C.sub.2H.sub.5)C.sub.2H.sub.5
Silver pivalate AgO.sub.2CC(CH.sub.3).sub.3 Silver
cyclohexanebutyrate AgO.sub.2C(CH.sub.2).sub.3C.sub.6H.sub.11
Silver ethylhexanoate AgO.sub.2CCH(C.sub.2H.sub.5)C.sub.4H.sub.9
Silver neodecanoate AgO.sub.2CC.sub.9H.sub.19 Halogenocarboxylates
Silver trifluoroacetate AgO.sub.2CCF.sub.3 Silver
pentafluoropropionate AgO.sub.2CC.sub.2F.sub.5 Silver
heptafluorobutyrate AgO.sub.2CC.sub.3F.sub.7 Silver
trichloroacetate AgO.sub.2CCCl.sub.3 Silver
6,6,7,7,8,8,8-heptafluoro-2,2- AgFOD dimethyl-3,5-octanedionate
Hydroxycarboxylates Silver lactate AgO.sub.2CH(OH)CH.sub.3 Silver
citrate Ag.sub.3C.sub.6H.sub.5O.sub.7 Silver glycolate
AgOOCCH(OH)CH.sub.3 Aminocarboxylates Silver glyconate Aromatic and
nitro and/or Silver benzoate AgO.sub.2CCH.sub.2C.sub.6H.sub.5
fluoro substituted aromatic Silver phenylacetate
AgOOCCH.sub.2C.sub.6H.sub.5 Carboxylates Silver nitrophenylacetates
AgOOCCH.sub.2C.sub.6H.sub.4NO.sub.2 Silver dinitrophenylacetate
AgOOCCH.sub.2C.sub.6H.sub.3(NO.sub.2).sub.2 Silver
difluorophenylacetate AgOOCCH.sub.2C.sub.6H.sub.3F.sub.2 Silver
2-fluoro-5-nitrobenzoate AgOOCC.sub.6H.sub.3(NO.sub.2)F Beta
diketonates Silver acetylacetonate
Ag[CH.sub.3COCH.dbd.C(O--)CH.sub.3] Silver
hexafluoroacetylacetonate Ag[CF.sub.3COCH.dbd.C(O--)CF.sub.3]
Silver trifluoroacetylacetonate Ag[CH.sub.3COCH.dbd.C(O--)CF.sub.3]
Silver sulfonates Silver tosylate AgO.sub.3SC.sub.6H.sub.4CH.sub.3
Silver triflate AgO.sub.3SCF.sub.3
[0079] In addition to the foregoing, complex silver salts
containing neutral inorganic or organic ligands can also be used as
precursors. These salts are usually in the form of nitrates,
halides, perchlorates, hydroxides or tetrafluoroborates. Examples
are listed in Table 2. TABLE-US-00002 TABLE 2 Complex Silver Salt
Precursors Class Examples (Cation) Amines
[Ag(RNH.sub.2).sub.2].sup.+, Ag(R.sub.2NH).sub.2].sup.+,
[Ag(R.sub.3N).sub.2].sup.+, R = aliphatic or aromatic
N-Heterocycles [Ag(L).sub.x].sup.+, (L = aziridine, pyrrol, indol,
piperidine, pyridine, aliphatic substituted and amino substituted
pyridines, imidazole, pyrimidine, piperazine, triazoles, etc.)
Amino alcohols [Ag(L).sub.x].sup.+, L = Ethanolamine Amino acids
[Ag(L).sub.x].sup.+, L = Glycine Acid amides [Ag(L).sub.x].sup.+, L
= Formamides, acetamides Nitriles [Ag(L).sub.x].sup.+, L =
Acetonitriles
[0080] The molecular metal precursors can be utilized in an
aqueous-based solvent or an organic solvent. Organic solvents are
typically used for ink-jet deposition. Preferred molecular metal
precursors for silver in an organic solvent include Ag-nitrate,
Ag-neodecanoate, Ag-trifluoroacetate, Ag-acetate, Ag-lactate,
Ag-cyclohexanebutyrate, Ag-carbonate, Ag-oxide, Ag-ethylhexanoate,
Ag-acetylacetonate, Ag-ethylbutyrate, Ag-pentafluoropropionate,
Ag-benzoate, Ag-citrate, Ag-heptafluorobutyrate, Ag-salicylate,
Ag-decanoate and Ag-glycolate. Among the foregoing, particularly
preferred molecular metal precursors for silver include Ag-acetate,
Ag-nitrate, Ag-trifluoroacetate and Ag-neodecanoate. Most preferred
among the foregoing silver precursors are Ag-trifluoroacetate and
Ag-acetate. The preferred precursors generally have a high
solubility and high metal yield. For example, Ag-trifluoroacetate
has a solubility in dimethylacetamide (DMAc) of about 78 wt. % and
Ag-trifluoroacetate is a particularly preferred silver precursor
according to the present invention.
[0081] Preferred molecular silver precursors for aqueous-based
solvents include Ag-nitrates, Ag-fluorides such as silver fluoride
or silver hydrogen fluoride (AgHF.sub.2), Ag-thiosulfate,
Ag-trifluoroacetate and soluble diamine complexes of silver
salts.
[0082] Silver precursors in solid form that decompose at a low
temperature, such as not greater than about 200.degree. C., can
also be used. Examples include Ag-oxide, Ag-nitrite, Ag-carbonate,
Ag-lactate, Ag-sulfite and Ag-citrate.
[0083] When a more volatile molecular silver precursor is desired,
such as for spray deposition of the precursor composition, the
precursor can be selected from alkene silver betadiketonates,
R.sub.2(CH).sub.2Ag([R'COCH.dbd.C(O--)CR''] where R=methyl or ethyl
and R', R''=CF.sub.3, C.sub.2F.sub.5, C.sub.3F.sub.7, CH.sub.3,
C.sub.mH.sub.2m+1 (m=2 to 4), or trialkylphosphine and
triarylphosphine derivatives of silver carboxylates, silver beta
diketonates or silver cyclopentadienides.
[0084] Molecular metal precursors for nickel that are preferred
according to the present invention are illustrated in Table 3. A
particularly preferred nickel precursor for use with an
aqueous-based solvent is Ni-acetylacetonate. TABLE-US-00003 TABLE 3
Molecular Metal Precursors for Nickel General Class Example
Chemical Formula Inorganic Salts Ni-nitrate Ni(NO.sub.3).sub.2
Ni-sulfate NiSO.sub.4 Nickel ammine [Ni(NH.sub.3).sub.6].sup.n+ (n
= 2, 3) complexes Ni-tetrafluoroborate Ni(BF.sub.4).sub.2 Metal
Organics Ni-oxalate NiC.sub.2O.sub.4 (Alkoxides, Beta-
Ni-isopropoxide Ni(OC.sub.3H.sub.7).sub.2 diketonates,
Ni-methoxyethoxide Ni(OCH.sub.2CH.sub.2OCH.sub.3).sub.2
Carboxylates, Ni-acetylacetonate [Ni(acac).sub.2].sub.3 or
Ni(acac).sub.2(H.sub.2O).sub.2 Fluorocarboxylates Ni-hexafluoro-
Ni[CF.sub.3COCH.dbd.C(O--)CF.sub.3].sub.2 acetylacetonate
Ni-formate Ni(O.sub.2CH).sub.2 Ni-acetate
Ni(O.sub.2CCH.sub.3).sub.2 Ni-octanoate
Ni(O.sub.2CC.sub.7H.sub.15).sub.2 Ni-ethylhexanoate
Ni(O.sub.2CCH(C.sub.2H.sub.5)C.sub.4H.sub.9).sub.2
Ni-trifluoroacetate Ni(OOCCF.sub.3).sub.2
[0085] Various molecular precursors can be used for platinum metal.
Preferred molecular precursors include ammonium salts of platinates
such as ammonium hexachloro platinate (NH.sub.4).sub.2PtCl.sub.6,
and ammonium tetrachloro platinate (NH.sub.4).sub.2PtCl.sub.4;
sodium and potassium salts of halogeno, pseudohalogeno or nitrito
platinates such as potassium hexachloro platinate
K.sub.2PtCl.sub.6, sodium tetrachloro platinate Na.sub.2PtCl.sub.4,
potassium hexabromo platinate K.sub.2PtBr.sub.6, potassium
tetranitrito platinate K.sub.2Pt(NO.sub.2).sub.4; dihydrogen salts
of hydroxo or halogeno platinates such as hexachloro platinic acid
H.sub.2PtCl.sub.6, hexabromo platinic acid H.sub.2PtBr.sub.6,
dihydrogen hexahydroxo platinate H.sub.2Pt(OH).sub.6; diamine and
tetraammine platinum compounds such as diamine platinum chloride
Pt(NH.sub.3).sub.2Cl.sub.2, tetraammine platinum chloride
[Pt(NH.sub.3).sub.4]Cl.sub.2, tetraammine platinum hydroxide
[Pt(NH.sub.3).sub.4](OH).sub.2, tetraammine platinum nitrite
[Pt(NH.sub.3).sub.4](NO.sub.2).sub.2, tetraammine platinum nitrate
Pt(NH.sub.3).sub.4(NO.sub.3).sub.2, tetraammine platinum
bicarbonate [Pt(NH.sub.3).sub.4](HCO.sub.3).sub.2, tetraammine
platinum tetrachloroplatinate [Pt(NH.sub.3).sub.4]PtCl.sub.4;
platinum diketonates such as platinum (II) 2,4-pentanedionate
Pt(C.sub.5H.sub.7O.sub.2).sub.2; platinum nitrates such as
dihydrogen hexahydroxo platinate H.sub.2Pt(OH).sub.6 acidified with
nitric acid; other platinum salts such as Pt-sulfite and
Pt-oxalate; and platinum salts comprising other N-donor ligands
such as [Pt(CN).sub.6].sup.4+.
[0086] Platinum precursors useful in organic-based precursor
compositions include Pt-carboxylates or mixed carboxylates.
Examples of carboxylates include Pt-formate, Pt-acetate,
Pt-propionate, Pt-benzoate, Pt-stearate, Pt-neodecanoate. Other
precursors useful in organic vehicles include aminoorgano platinum
compounds including Pt(diaminopropane)(ethylhexanoate).
[0087] Preferred combinations of platinum precursors and solvents
include: PtCl.sub.4 in H.sub.2O; Pt-nitrate solution from
H.sub.2Pt(OH).sub.6; H.sub.2Pt(OH).sub.6 in H.sub.2O;
H.sub.2PtCl.sub.6 in H.sub.2O; and
[Pt(NH.sub.3).sub.4](NO.sub.3).sub.2 in H.sub.2O.
[0088] Gold precursors that are particularly useful for aqueous
based precursor compositions include Au-chloride (AuCl.sub.3) and
tetrachloric auric acid (HAuCl.sub.4).
[0089] Gold precursors useful for organic based formulations
include: Au-thiolates, Au-carboxylates such as Au-acetate
Au(O.sub.2CCH.sub.3).sub.3; aminoorgano gold carboxylates such as
imidazole gold ethylhexanoate; mixed gold carboxylates such as gold
hydroxide acetate isobutyrate; Au-thiocarboxylates and
Au-dithiocarboxylates.
[0090] In general, preferred gold molecular metal precursors for
low temperature conversion are compounds comprising a set of
different ligands such as mixed carboxylates or mixed alkoxo metal
carboxylates. As one example, gold acetate isobutyrate hydroxide
decomposes at 155.degree. C., a lower temperature than gold
acetate. As another example, gold acetate neodecanoate hydroxide
decomposes to gold metal at even lower temperature, 125.degree. C.
Still other examples can be selected from gold acetate
trifluoroacetate hydroxide, gold bis(trifluoroacetate) hydroxide
and gold acetate pivalate hydroxide.
[0091] Other useful gold precursors include Au-azide and
Au-isocyanide. When a more volatile molecular gold precursor is
desired, such as for spray deposition, the precursor can be
selected from: [0092] dialkyl and monoalkyl gold carboxylates,
R.sub.3-nAu(O.sub.2CR').sub.n (n=1,2) R=methyl, ethyl; R'=CF.sub.3,
C.sub.2F.sub.5, C.sub.3F.sub.7, CH.sub.3, C.sub.mH.sub.2m+1 (m=2-9)
[0093] dialkyl and monoalkyl gold beta diketonates, R.sub.3-nAu
[R'COCH.dbd.C(O--)CR''].sub.n (n=1,2), R=methyl, ethyl; R',
R''=CF.sub.3, C.sub.2F.sub.5, C.sub.3F.sub.7, CH.sub.3,
C.sub.mH.sub.2m+1 (m=2-4) [0094] dialkyl and monoalkyl gold
alkoxides, R.sub.3-nAu(OR').sub.n (n=1,2) R=methyl, ethyl;
R'=CF.sub.3, C.sub.2F.sub.5, C.sub.3F.sub.7, CH.sub.3,
C.sub.mH.sub.2m+1 (m=2-4), SiR.sub.3'' (R''=methyl, ethyl, propyl,
isopropyl, n-butyl, isobutyl, tert. Butyl) [0095] phosphine gold
complexes: [0096] RAu(PR'.sub.3) R, R'=methyl, ethyl, propyl,
isopropyl, n-butyl, isobutyl, tert. butyl. [0097]
R.sub.3Au(PR'.sub.3) R, R'=methyl, ethyl, propyl, isopropyl,
n-butyl, isobutyl, tert. butyl.
[0098] Particularly useful molecular precursors to palladium for
organic based precursor compositions according to the present
invention include Pd-carboxylates, including Pd-fluorocarboxylates
such as Pd-acetate, Pd-propionate, Pd-ethylhexanoate,
Pd-neodecanoate and Pd-trifluoroacetate as well as mixed
carboxylates such as Pd(OOCH)(OAc), Pd(OAc)(ethylhexanoate),
Pd(ethylhexanoate).sub.2, Pd(OOCH).sub.1.5
(ethylhexanoate).sub.0.5, Pd(OOCH)(ethylhexanoate),
Pd(OOCCH(OH)CH(OH)COOH).sub.m (ethylhexanoate), Pd(OPr).sub.2,
Pd(QAc)(OPr), Pd-oxalate,
Pd(OOCCHO).sub.m(OOCCH.sub.2OH).sub.n=(Glyoxilic palladium
glycolate) and Pd-alkoxides. A particularly preferred palladium
precursor is Pd-trifluoroacetate.
[0099] Palladium precursors useful for aqueous based precursor
compositions include: tetraammine palladium hydroxide
[Pd(NH.sub.3).sub.4](OH).sub.2; Pd-nitrate Pd(NO.sub.3).sub.2;
Pd-oxalate Pd(O.sub.2CCO.sub.2).sub.2; Pd-chloride PdCl.sub.2; Di-
and tetraammine palladium chlorides, hydroxides or nitrates such as
tetraammine palladium chloride [Pd(NH.sub.3).sub.4]Cl.sub.2,
tetraammine palladium hydroxide [Pd(NH.sub.3).sub.4](OH).sub.2,
tetraammine palladium nitrate [Pd(NH.sub.3).sub.4](NO.sub.3).sub.2,
diamine palladium nitrate [Pd(NH.sub.3).sub.2](NO.sub.3).sub.2 and
tetraammine palladium tetrachloropalladate [Pd(NH.sub.3).sub.4]
[PdCl.sub.4].
[0100] When selecting a molecular copper precursor compound, it is
desired that the compound react during processing to metallic
copper without the formation of copper oxide or other species that
are detrimental to the conductivity of the conductive copper
feature. Some copper molecular precursors form copper by thermal
decomposition at elevated temperatures. Other molecular copper
precursors require a reducing agent to convert to copper metal.
Reducing agents are materials that are oxidized, thereby causing
the reduction of another substance. The reducing agent loses one or
more electrons and is referred to as having been oxidized. The
introduction of the reducing agent can occur in the form of a
chemical agent (e.g., formic acid) that is soluble in the precursor
composition to afford a reduction to copper either during transport
to the substrate or on the substrate. In some cases, the ligand of
the molecular copper precursor has reducing characteristics, such
as in Cu-formate or Cu-hypophosphite, leading to reduction to
copper metal. However, formation of metallic copper or other
undesired side reactions that occur prematurely in the ink or
precursor composition should be avoided.
[0101] Accordingly, the ligand can be an important factor in the
selection of suitable copper molecular precursors. During thermal
decomposition or reduction of the precursor, the ligand needs to
leave the system cleanly, preferably without the formation of
carbon or other residues it could be incorporated into the copper
feature. Copper precursors containing inorganic ligands are
preferred in cases where carbon contamination is detrimental. Other
desired characteristics for molecular copper precursors are low
decomposition temperature or processing temperature for reduction
to copper metal, high solubility in the selected solvent/vehicle to
increase metallic yield and achieve dense features and the compound
should be environmentally benign.
[0102] Preferred copper metal precursors according to the present
invention include Cu-formate and Cu-neodecanoate. Molecular copper
precursors that are useful for aqueous-based precursor compositions
include: Cu-nitrate and amine complexes thereof; Cu-carboxylates
including Cu-formate and Cu-acetate; and Cu beta-diketonates such
as Cu-hexafluoroacetylacetonate and copper salts such as
Cu-chloride.
[0103] Molecular copper precursors generally useful for organic
based formulations include: Cu-carboxylates and
Cu-fluorocarboxylates, such as Cu-formate; Cu-ethylhexanoate;
Cu-neodecanoate; Cu-methacrylate; Cu-trifluoroacetate;
Cu-hexanoate; and copper beta-diketonates such as cyclooctadiene Cu
hexafluoroacetylacetonate.
[0104] Among the foregoing, Cu-formate is particularly preferred as
it is highly soluble in water and results in the in-situ formation
of formic acid, which is an effective reducing agent.
[0105] Copper precursors useful in this invention can also be
categorized as copper I and copper II compounds. They can be
categorized as inorganic, metal organic, and organometallic. They
can also be categorized as copper hydrides, copper amides, copper
alkenes, copper allyls, copper carbonyls, copper metallocenes,
copper cyclopentadienyls, copper arenes, copper carbonates, copper
hydroxides, copper carboxylates, copper oxides, organo copper,
copper beta-diketonates, copper alkoxides, copper
beta-ketoiminates, copper halides, copper alkyls. The copper
compounds can have neutral donor ligands or not have neutral
ligands. Copper I compounds are particularly useful for
disproportionation reactions. The disproportionation products are
copper metal and a copper II compound. In some cases a neutral
ligand is also a product.
[0106] In a novel approach, the copper II product can be rapidly
converted back to a copper I compound using a reducing agent.
Appropriate reducing agents for reducing copper II to copper I are
known in the art. Useful reducing agents for copper precursors
include ethylene diamine, tetramethylethylenediamine, 3
aminopropanol, mono, di and triethanolamine. Useful reducing agents
are described in U.S. Pat. No. 5,378,508, which is incorporated
herein by reference in its entirety. The resulting copper I
compound reacts further via disproportionation to form more copper
and copper II compound. Through this approach with excess reducing
agent, copper I compounds can be used to form pure copper metal
without any copper II compounds.
[0107] The copper compounds can also be used as capping agents to
cap copper particles. The copper particles can be nanoparticles.
U.S. Pat. No. 6,294,401 by Jacobsen describes capping procedures
and is incorporated herein in its entirety by reference.
[0108] As is discussed above, two or more molecular metal
precursors can be combined in the precursor composition to form
metal alloys and/or metal compounds. For example, preferred
combinations of metal precursors to form alloys based on silver
include: Ag-nitrate and Pd-nitrate; Ag-acetate and
[Pd(NH.sub.3).sub.4](OH).sub.2; Ag-trifluoroacetate and
[Pd(NH.sub.3).sub.4](OH).sub.2; and Ag-neodecanoate and
Pd-neodecanoate. One particularly preferred combination of
molecular metal precursors is Ag-trifluoroacetate and
Pd-trifluoroacetate. Another preferred alloy is Ag/Cu.
[0109] To form alloys, the two (or more) molecular metal precursors
should have similar decomposition temperatures to avoid the
formation of one of the metal species before the other species.
Preferably, the decomposition temperatures of the different
molecular metal precursors are within 50.degree. C., more
preferably within 25.degree. C.
[0110] Some applications require the utilization of a transparent
or semi-transparent conductive feature. For example, indium tin
oxide (ITO) is useful for the formation of transparent conductive
features, such as for use in display applications. Antimony tin
oxide (ATO) is useful as a color tunable oxide layer that finds use
in electrochromic applications.
[0111] Such transparent conductive features can also be fabricated
according to the present invention. For ITO, useful molecular
precursors for indium include: In-nitrate; In-chloride;
In-carboxylates such as In-acetate; In-propionates including
fluoro, chloro or bromo derivatives thereof; beta diketonates such
as In-acetylacetonate, In-hexafluoroacetylacetonate and
In-trifluoroacetylacetonate; pyrazolyl borohydrides such as
In(Pz).sub.3BH; In-alkoxides and In-fluoroalkoxides; and In-amides.
Mixed alkoxo In-carboxylates such as indium isopropoxide
ethylhexanoate are also useful.
[0112] Useful molecular precursors for tin in ITO or ATO include:
Sn-halides such as Sn-tetrachloride; Sn-dichloride; Sn-carboxylates
such as Sn-acetate or Sn-ethylhexanoate; Sn-alkoxides such as
Sn(O.sup.tBu).sub.4; Sn-hydroxycarboxylates such as Sn-glycolate;
and beta diketonates such as Sn-hexafluoroacetylacetonate.
[0113] Useful molecular precursors for antimony include:
Sb-trichloride; antimony carboxylates such as Sb-acetate or
Sb-neodecanoate; antimony alkoxides such as Sb-methoxide,
Sb-ethoxide, Sb-butoxide.
[0114] The low viscosity precursor compositions according to the
present invention preferably also include a solvent capable of
solubilizing the molecular metal precursor discussed above. The
solvent can be water thereby forming an aqueous-based precursor
composition. Water is more environmentally acceptable than organic
solvents. However, water cannot typically be used for deposition of
the precursor composition onto hydrophobic substrates, such as
tetrafluoroethylene fluorocarbon substrates (e.g., TEFLON, E.I.
duPont deNemours, Wilmington, Del.) without modification of the
substrate or the aqueous composition.
[0115] The solvent can also include an organic solvent, by itself
or in addition to water. The selected solvent should be capable of
solubilizing the selected molecular metal precursor to a high
level. A low solubility of the molecular metal precursor in the
solvent leads to low yields of the conductor, thin deposits and low
conductivity. The precursor compositions of the present invention
exploit combinations of solvents and precursors that advantageously
provide high solubility of the molecular precursor while still
allowing low temperature conversion of the precursor to the
conductor.
[0116] According to one embodiment of the present invention, the
solubility of the molecular metal precursor in the solvent is
preferably at least about 20 weight percent metal precursor, more
preferably at least 40 weight percent metal precursor, even more
preferably at least about 50 weight percent metal precursor and
most preferably at least about 60 weight percent metal precursor.
Such high levels of metal precursor lead to increased metal yield
and the ability to deposit features having adequate thickness.
[0117] The solvents can be polar or non-polar. Solvents that are
useful according to the present invention include amines, amides,
alcohols, water, ketones, unsaturated hydrocarbons, saturated
hydrocarbons, mineral acids organic acids and bases, Preferred
solvents include alcohols, amines, amides, water, ketone, ether,
aldehydes and alkenes. Particularly preferred organic solvents
according to the present invention include N,N,-dimethylacetamide
(DMAc), diethyleneglycol butylether (DEGBE), ethanolamine and
N-methylpyrrolidone.
[0118] In some cases, the solvent can be a high melting point
solvent, such as one having a melting point of at least about
30.degree. C. and not greater than about 100.degree. C. In this
embodiment, a heated ink-jet head can be used to deposit the
precursor composition while in a flowable state whereby the solvent
solidifies upon contacting the substrate. Subsequent processing can
then remove the solvent by other means and then convert the
material to the final product, thereby retaining resolution.
Preferred solvents according to this embodiment are waxes, high
molecular weight fatty acids, alcohols, acetone,
N-methyl-2-pyrrolidone, toluene, tetrahydrofuran and the like.
Alternatively, the precursor composition may be a liquid at room
temperature, wherein the substrate is kept at a lower temperature
below the freezing point of the composition.
[0119] The solvent can also be a low melting point solvent. A low
melting point is required when the precursor composition must
remain as a liquid on the substrate until dried. A preferred low
melting point solvent according to this embodiment is DMAc, which
has a melting point of about -20.degree. C.
[0120] In addition, the solvent can be a low vapor pressure
solvent. A lower vapor pressure advantageously prolongs the work
life of the composition in cases where evaporation in the ink-jet
head, syringe or other tool leads to problems such as clogging. A
preferred solvent according to this embodiment is terpineol. Other
low vapor pressure solvents include diethylene glycol, ethylene
glycol, hexylene glycol, N-methyl-2-pyrrolidone, and tri(ethylene
glycol) dimethyl ether.
[0121] The solvent can also be a high vapor pressure solvent, such
as one having a vapor pressure of at least about 1 kPa. A high
vapor pressure allows rapid removal of the solvent by drying. High
vapor pressure solvents include acetone, tetrahydrofuran, toluene,
xylene, ethanol, methanol, 2-butanone and water.
[0122] As is discussed above, a vehicle is a flowable medium that
facilitates the deposition of the precursor composition. In cases
where the liquid serves only to carry particles and not to dissolve
molecular species, the terminology of vehicle is often used for the
liquid. However, in many precursor compositions, the solvent can
also be considered the vehicle. The metal, such as silver, can be
bound to the vehicle, for example, by synthesizing a silver
derivative of terpineol that simultaneously acts as both a
precursor to silver and as a vehicle. This improves the metallic
yield and reduces the porosity of the conductive feature.
[0123] Examples of preferred vehicles are listed in Table 4.
Particularly preferred vehicles according to the present invention
include alpha terpineol, toluene and ethylene glycol.
TABLE-US-00004 TABLE 4 Organic Vehicles Useful in Precursor
Compositions Formula/Class Name Alcohols 2-Octanol Benzyl alcohol
4-hydroxy-3methoxy benzaldehyde Isodeconol Butylcarbitol Terpene
alcohol Alpha-terpineol Beta-terpineol Cineol Esters 2,2,4
trimethylpentanediol-1,3 monoisobutyrate Butyl carbitol acetate
Butyl oxalate Dibutyl phthalate Dibutyl benzoate Butyl cellosolve
acetate Ethylene glycol diacetate Ethylene glycol diacetate
N-methyl-2-pyrrolidone Amides N,N-dimethyl formamide N,N-dimethyl
acetamide Aromatics Xylenes Aromasol Substituted Nitrobenzene
aromatics o-nitrotoluene Terpenes Alpha-pinene, beta-pinene,
dipentene, dipentene oxide Essential Oils Rosemary, lavender,
fennel, sassafras, wintergreen, anise oils, camphor, turpentine
[0124] The low viscosity precursor compositions in accordance with
the present invention can also include one or more polymers. The
polymers can be thermoplastic polymers or thermoset polymers.
Thermoplastic polymers are characterized by being fully
polymerized. They do not take part in any reactions to further
polymerize or cross-link to form a final product. Typically, such
thermoplastic polymers are melt-cast, injection molded or dissolved
in a solvent. Examples include polyimide films, ABS plastics,
vinyl, acrylic, styrene polymers of medium or high molecular weight
and the like.
[0125] The polymers can also be thermoset polymers, which are
characterized by not being fully polymerized or cured. The
components that make up thermoset polymers must undergo further
reactions to form fully polymerized, cross-linked or dense final
products. Thermoset polymers tend to be resistant to solvents,
heat, moisture and light.
[0126] A typical thermoset polymer mixture initially includes a
monomer, resin or low molecular weight polymer. These components
require heat, hardeners, light or a combination of the three to
fully polymerize. Hardeners are used to speed the polymerization
reactions. Some thermoset polymer systems are two part epoxies that
are mixed at consumption or are mixed, stored and used as
needed.
[0127] Specific examples of thermoset polymers include amine or
amide-based epoxies such as diethylenetriamine, polyglycoldianine
and triethylenetetramine. Other examples include imidazole,
aromatic epoxies, brominated epoxies, thermoset PET, phenolic
resins such as bisphenol-A, polymide, acrylics, urethanes and
silicones. Hardeners can include isophoronediamine and
meta-phenylenediamene.
[0128] The polymer can also be an ultraviolet or other
light-curable polymer. The polymers in this category are typically
UV and light-curable materials that require photoinitiators to
initiate the cure. Light energy is absorbed by the photoinitiators
in the formulation causing them to fragment into reactive species,
which can polymerize or cross-link with other components in the
formulation. In acrylate-based adhesives, the reactive species
formed in the initiation step are known as free radicals. Another
type of photoinitiator, a cationic salt, is used to polymerize
epoxy functional resins generating an acid, which reacts to create
the cure. Examples of these polymers include cyanoacrylates such as
z-cyanoacrylic acid methyl ester with an initiator as well as
typical epoxy resin with a cationic salt.
[0129] The polymers can also be conductive polymers such as
intrinsically conductive polymers. Conductive polymers are
disclosed, for example, in U.S. Pat. No. 4,959,430 by Jonas et al.,
which is incorporated herein by reference in its entirety. Other
examples of intrinsically conductive polymers are listed in Table 5
below. TABLE-US-00005 TABLE 5 Intrinsically Conductive Polymers
Examples Class/Monomers Catalyst/Dopant Polyacetylene
Poly[bis(benzylthio) acetylene] Phenyl vinyl sulfoxide Ti
alkylidene Poly[bis(ethylthio)acetylene] 1,3,5,7-Cyclooctatetraene
Poly[bis(methylthio)acetylene] Polyaniline Fully reduced organic
sulfonic acids such as: Half oxidized Dinonylnaphthalenedisulfonc
acid Dinonylnaphthaleneusulfonic acid Dodecylbenzenesulfonic acid
Poly(anilinesulfonic acid) Self-doped state Polypyrrole Organic
sulfonic acid Polythiophene Poly(thiophine-2.5-diyl)
2,5-Dibromo-3-alkyl/arylthiophene Poly(3-alkylthiophene-2.5-diyl)
alkyl = butyl, hexyl, octyl, alkyl = butyl, hexyl, octyl, decyl,
decyl, dodecyl dodecyl aryl = phenyl Poly(styrenesulfonate)/poly-
Dibromodithiophene (2,3-dihydrothieno-[3,4-b]-1,4- Terthiophene
dioxin) Other substituted thiophenes Poly(1,4-phenylenevinylene)
(PPV) p-Xylylenebis (tetrahydrothiopheniumchloride))
Poly(1,4-phenylene sulfide) Poly(fluroenyleneethynylene)
[0130] Other additives can be included in the low viscosity
precursor compositions in accordance with the present invention.
Among these are reducing agents to prevent the undesirable
oxidation of metal species. Reducing agents are materials that are
oxidized, thereby causing the reduction of another substance. The
reducing agent loses one or more electrons and is referred to as
having been oxidized. For example, copper and nickel metal have a
strong tendency to oxidize. The compositions including nickel or
copper precursors according to the present invention should
preferably include reducing agents as additives to provide reaction
conditions for the formation of the metal at the desired
temperature, rather than the metal oxide. Reducing agents are
particularly applicable when using molecular metal precursor
compounds where the ligand is not reducing by itself. Examples of
reducing agents include amino alcohols. Alternatively, the
precursor conversion process can take place under reducing
atmosphere, such as hydrogen or forming gas.
[0131] In some cases, the addition of a reducing agent results in
the formation of the metal even under ambient conditions. The
reducing agent can be part of the precursor itself, for example in
the case of certain ligands. An example is Cu-formate where the
precursor forms copper metal even in ambient air at low
temperatures. In addition, the Cu-formate precursor is highly
soluble in water, results in a relatively high metallic yield and
forms only gaseous byproducts, which are reducing in nature and
protect the in-situ formed copper from oxidation. Copper formate is
therefore a preferred copper precursor for aqueous based precursor
compositions. Other examples of molecular metal precursors
containing a ligand that is a reducing agent are Ni-acetylacetonate
and Ni-formate.
[0132] The precursor compositions can also include crystallization
inhibitors and a preferred crystallization inhibitor is lactic
acid. Such inhibitors reduce the formation of large crystallites
directly from the molecular metal precursor, which can be
detrimental to conductivity. Other crystallization inhibitors
include ethylcellulose and polymers such as styrene allyl alcohol
(SAA) and polyvinyl pirolydone (PVP). For example, in some silver
precursor compositions small additions of lactic acid completely
prevent crystallization. In other cases, such as in aqueous
Cu-formate compositions, small amounts of glycerol can act as a
crystallization inhibitor. Other compounds useful for reducing
crystallization are other polyalcohols such as malto dextrin,
sodium carboxymethylcellulose and TRITON X100. In general, solvents
with a higher melting point and lower vapor pressure inhibit
crystallization of any given compound more than a lower melting
point solvent with a higher vapor pressure. In one embodiment, not
greater than about 10 weight percent crystallization inhibitor as a
percentage of total composition is added, preferably not greater
than 5 weight percent and more preferably not greater than 2 weight
percent.
[0133] The low viscosity precursor compositions can also include an
adhesion promoter adapted to improve the adhesion of the conductive
feature to the underlying substrate. For example, polyamic acid can
improve the adhesion of the composition to a polymer substrate. In
addition, the precursor compositions can include rheology
modifiers. As an example, styrene allyl alcohol (SAA) can be added
to the precursor composition to reduce spreading on the
substrate.
[0134] The low viscosity precursor compositions can also include
complexing agents. Complexing agents are a molecule or species that
binds to a metal atom and isolates the metal atom from solution.
Complexing agents are adapted to increase the solubility of the
molecular precursors in the solvent, resulting in a higher yield of
metal. One preferred complexing agent, particularly for use with
Cu-formate and Ni-formate, is 3-amino-1-proponal. For example, a
preferred precursor composition for the formation of copper
includes Cu-formate dissolved in water and 3-amino-1-propanol.
[0135] The low viscosity precursor compositions above can also
include rheology modifiers. Rheology modifiers can include SOLTHIX
250 (Avecia Limited), SOLSPERSE 21000 (Avecia Limited), styrene
allyl alcohol (SAA), ethyl cellulose, carboxy methylcellulose,
nitrocellulose, polyalkylene carbonates, ethyl nitrocellulose, and
the like. These additives can reduce spreading of the precursor
composition after deposition, as is discussed in more detail
below.
[0136] The precursor compositions above can also include other
components such as wetting angle modifiers, humectants and the
like.
[0137] In accordance with the foregoing, the low viscosity
precursor compositions according to the present invention can
include combinations of particles (nanoparticles and/or micron-size
particles), molecular metal precursor compounds, solvents,
vehicles, reducing agents, crystallization inhibitors, adhesion
promoters, and other minor additives to control properties such as
surface tension.
[0138] For low viscosity precursor compositions, it is preferred
that the total loading of particulates (nanoparticles and
micron-size particles) is not greater than about 75 weight percent,
such as from about 5 wt. % to about 50 wt. %. Loading in excess of
the preferred amount can lead to higher viscosities and undesirable
flow properties. It is particularly preferred that the total
loading of micron-size particles not exceed about 50 weight percent
and that the total loading of nanoparticles not exceed about 75
weight percent. In one preferred embodiment, the low viscosity
precursor composition includes from about 5 to about 50 weight
percent nanoparticles and substantially no micron-size
particles.
[0139] A preferred low viscosity precursor composition comprises at
least one molecular metal precursor where the precursor is highly
soluble in the selected aqueous or organic solvent. Preferably, the
precursor composition includes at least about 20 weight percent of
molecular metal precursor, such as from about 30 weight percent to
about 60 weight percent. It is particularly preferred that the
molecular metal precursor be added to the precursor composition up
to the solubility limit of the precursor compound in the
solvent.
[0140] According to the present invention, the precursor
composition is carefully selected to reduce the conversion
temperature required to convert the metal precursor compound to the
conductive metal. The precursor converts at a low temperature by
itself or in combination with other precursors and provides for a
high metal yield. As used herein, the conversion temperature is the
temperature at which the metal species contained in the molecular
metal precursor compound, is at least 95 percent converted to the
pure metal. As used herein, the conversion temperature is measured
using a thermogravimetric analysis (TGA) technique wherein a
50-milligram sample of the precursor composition is heated at a
rate of 10.degree. C./minute in air and the weight loss is
measured.
[0141] A preferred approach for reducing the conversion temperature
according to the present invention is to bring the molecular metal
precursor compound into contact with a conversion reaction inducing
agent. As used herein, a conversion reaction inducing agent is a
chemical compound that effectively reduces the temperature at which
the molecular metal precursor compound decomposes to the metal. The
conversion reaction inducing agent can either be added into the
original precursor composition or added in a separate step during
conversion on the substrate. The former method is preferred.
Preferably, the conversion temperature of the metal precursors can
be preferably lowered by at least about 25.degree. C., more
preferably by at least about 50.degree. C. even more preferably by
at least about 100.degree. C., as compared to the dry metal
precursor compound.
[0142] The reaction inducing agent can be the solvent or vehicle
that is used for the precursor composition. For example, the
addition of certain alcohols can reduce the conversion temperature
of the precursor composition. Preferred alcohols for use as
conversion reaction inducing agents according to the present
invention include terpineol and diethyleneglycol butylether
(DEGBE). It will be appreciated that the alcohol can also be the
vehicle, such as in the case of terpineol.
[0143] More generally, organic alcohols such as primary and
secondary alcohols that can be oxidized to aldehydes or ketones,
respectively, can advantageously be used as the conversion reaction
inducing agent. Examples are 1-butanol, diethyleneglycol, DEGBE,
octanol, and the like. The choice of the alcohol is determined by
its reducing capability as well as its boiling point, viscosity and
precursor solubilizing capability. It has been discovered that some
tertiary alcohols can also lower the conversion temperature of some
molecular precursors. For example, alpha-terpineol, which also
serves as a vehicle, significantly lowers the conversion
temperature of some molecular metal precursors. The boiling point
of the conversion reaction inducing agents is preferably high
enough to provide for the preferred ratio of metal ions to inducing
agent during conversion to metal. It should also be low enough for
the inducing agent to escape the deposit cleanly without unwanted
side reactions that could lead to carbon residues in the final
film. The preferred ratio of metal precursor to inducing agent is
stoichiometric for complete reduction. However, in some cases
catalytic amounts of the inducing agent are sufficient.
[0144] Some solvents, such as DMAc, can serve as a solvent, vehicle
and a conversion reaction inducing agent. It can also be regarded
as a complexing agent for silver. This means that precursors such
as Ag-nitrate that are otherwise not very soluble in organic
solvents can be brought into solution by complexing the metal ion
with a complexing agent such as DMAc. In this specific case,
Ag-nitrate can form a 1:1 adduct with DMAc which is soluble in
organic solvents such as N-methylpyrrolidinone (NMP) or DMAc.
[0145] Another preferred approach to reducing the conversion
temperature of the molecular precursor is utilizing a palladium
compound as a conversion reaction inducing agent. According to this
embodiment, a palladium precursor compound is added to the
precursor composition, which includes another precursor such as a
silver precursor. With addition of various Pd compounds, the
conversion temperature of the silver precursor can be
advantageously reduced by at least 25.degree. C. and more
preferably by at least 50.degree. C. Preferred palladium precursors
according to this embodiment of the present invention include
Pd-acetate, Pd-trifluoroacetate, Pd-neodecanoate and tetraammine
palladium hydroxide. Pd-acetate and Pd-trifluoroacetate are
particularly preferred as conversion reaction inducing agents to
reduce the conversion temperature of a silver metal carboxylate
compound. Small additions of Pd-acetate to a precursor composition
that includes Ag-trifluoroacetate in DMAc can lower the
decomposition temperature by up to 80.degree. C. Preferred are
additions of Pd-acetate or Pd-trifluoracetate in an amount of at
least about 1 weight percent, more preferably at least about 2
weight percent. The upper range for this Pd conversion reaction
inducing agent is limited by its solubility in the solvent and in
one embodiment does not exceed about 10 weight percent. Most
preferred is the use of Pd-trifluoroacetate because of its high
solubility in organic solvents. For example, a preferred precursor
composition for a silver/palladium alloy according to the present
invention is Ag-trifluoroacetate and Pd-trifluoracetate dissolved
in DMAc and lactic acid.
[0146] A complete range of homogenous Ag/Pd alloys can be formed
with a Ag-trifluoroacetate/Pd-trifluoroacetate combination in a
solvent such as DMAc. The molecular mixing of the metal precursors
provides preferred conditions for the formation of virtually any
Ag/Pd alloy at low temperature. The conversion temperature of the
silver precursor when dissolved in DMAc is preferably reduced by at
least 80.degree. C. when combined with Pd-trifluoroacetate. Pure
Pd-trifluoroacetate dissolved in DMAc can be converted to pure
palladium at a similar temperature.
[0147] Other conversion reaction inducing agents that can also
lower the conversion temperature for base metals such as nickel and
copper can be used such as amines (ammonia, ethylamine,
propylamine), amides (DMAc, dimethylformamide, methylformamide,
imidazole, pyridine), alcohols such as aminoalcohols (ethanol
amine, diethanolamine and triethanolamine), aldehydes
(formaldehyde, benzaldehyde, acetaldehyde); formic acid; thiols
such as ethyl thioalcohol, phosphines such as trimethylphosphine or
triethylphosphine and phosphides. Still other conversion reaction
inducing agents can be selected from boranes and borohydrides such
as borane-dimethylamine or borane-trimethylamine. Preferred
conversion reaction inducing agents are alcohols and amides.
[0148] Another factor that has been found to influence the
conversion temperature is the ratio of molecular metal precursor to
conversion reaction inducing agent. It has been found that the
addition of various amounts of DEGBE to a molecular silver
precursor such as Ag-trifluoroacetate in DMAc further reduces the
precursor conversion temperature, for example by up to about
70.degree. C. Most preferred is the addition of stoichiometric
amounts of the inducing agent such as DEGBE. Excess amounts of
conversion temperature inducing agent are not preferred because it
does not lower the temperature any further. In addition, higher
amounts of solvent or inducing agents lower the overall
concentration of molecular precursor in the precursor composition
and can negate other characteristics such as the composition being
in the preferred viscosity and surface tension range. The ratio of
inducing agent to metal ion that is reduced to metal during
conversion can be expressed as a molar ratio of functional group
(inducing part in the reducing agent) to metal ion. The ratio is
preferably about 1, such as in the range from about 1.5 to about
0.5 and more preferably in the range of about 1.25 to about 0.75
for univalent metal ions such as Ag. For divalent metal ions the
ratio is preferably about 2, such as in the range from about 3 to
1, and for trivalent metals the ratio is preferably about 3, such
as in the range from about 4.5 to 1.5.
[0149] The molecular precursor preferably provides as high a yield
of metal as possible. A preferred ratio of molecular precursor to
solvent is that corresponding to greater than 10% mass fraction of
metal relative to the total weight of the liquid (i.e., all
precursor components excluding particles). As an example, at least
10 grams of conductor is preferably contained in 100 grams of the
precursor composition. More preferably, greater than 20 wt. % of
the precursor composition is metal, even more preferably greater
than 30 wt. %, even more preferably greater than 40 wt. % and most
preferably greater than 50 wt. %.
[0150] Yet another preferred approach for reducing the conversion
temperature is to catalyze the reactions using particles,
particularly nanoparticles. Preferred powders that catalyze the
reaction include pure Pd, Ag/Pd alloy particles and other alloys of
Pd as well as Pt and alloys of Pt. Another approach for reducing
the conversion temperature is to use gaseous reducing agents such
as hydrogen or forming gas.
[0151] Yet another preferred approach for reducing the conversion
temperature is ester elimination, either solvent assisted or
without solvent assist. Solvent assist refers to a process wherein
the metal alkoxide is converted to an oxide by eliminating an
ester. In one embodiment, a metal carboxylate and metal alkoxide
are mixed in the precursor composition. At the processing
temperature the two precursors react and eliminate an organic ester
to form a metal oxide, which decomposes to the corresponding metal
at a lower temperature than the precursors themselves. This is also
useful for Ag and Au, where for Au the metal oxide formation is
skipped.
[0152] Another preferred approach for reducing the conversion
temperature is by photochemical reduction. For example,
photochemical reduction of Ag can be achieved by using precursors
containing silver bonds that can be broken photochemically. Another
method is to induce photochemical reduction of a silver precursor
on prepared surfaces where the surface catalyzes the photochemical
reaction.
[0153] Another preferred approach for reducing the conversion
temperature is in-situ precursor generation by reaction of ligands
with particles. For example, silver oxide particles can be a
starting material and can be incorporated into low viscosity
precursor compositions in the form of nanoparticles. The silver
oxide can react with deprotonateable organic compounds to form the
corresponding silver salts. For example, silver oxide can be mixed
with a carboxylic acid to form silver carboxylate. Preferred
carboxylic acids include acetic acid, neodecanoic acid and
trifluoroacetic acid. Other carboxylic acids work as well. For
example, DARVAN C (Vanderbilt Chemical) can react its carboxylic
function with the metal oxide. Small silver particles that are
coated with a thin silver oxide layer can also be reacted with
these compounds. Another potential benefit is simultaneously gained
with regard to rheology in that such a surface modification can
lead to improved particle loadings in low viscosity formulations.
Another example is the reaction of CuO coated silver powder with
carboxylic acids. This procedure can be applied more generally to
other oxides such as copper oxide, palladium oxide and nickel oxide
particles as well. Other deprotonateable compounds are halogeno-,
hydroxy- and other alkyl and aryl derivatives of carboxylic acids,
beta diketones, more acidic alcohols such as phenol, and
hydrogentetrafluoroborates.
[0154] Thus, as is discussed above and illustrated by the examples
below, the precursor compositions of the present invention can have
a precursor conversion temperature that is significantly lower than
the decomposition temperature of the dry metal precursor compound.
In one embodiment, the conversion temperature is not greater than
about 250.degree. C., such as not greater than about 225.degree.
C., more preferably is not greater than about 200.degree. C. and
even more preferably is not greater than about 185.degree. C. In
certain embodiments, the conversion temperature can be not greater
than about 150.degree. C., such as not greater than about
125.degree. C. and even not greater than about 100.degree. C.
Substrates
[0155] The precursor compositions according to the present
invention can be deposited and converted to the conductive feature
at low temperatures, thereby enabling the use of a variety of
substrates having a relatively low melting or decomposition
temperature. During conversion of low viscosity precursor
compositions to the conductive feature, the substrate surface that
the composition is printed onto significantly influences how the
overall conversion to a final structure occurs.
[0156] The types of substrates that are particularly useful
according to the present invention include polyfluorinated
compounds, polyimides, epoxies (including glass-filled epoxy),
polycarbonates and many other polymers. Particularly useful
substrates include cellulose-based materials such as wood or paper,
acetate, polyester, polyethylene, polypropylene, polyvinyl
chloride, acrylonitrile, butadiene (ABS), flexible fiber board,
non-woven polymeric fabric, cloth, metallic foil and thin glass.
The substrate can be coated, for example a dielectric on a metallic
foil. Although the present invention can be used for such
low-temperature substrates, it will be appreciated that traditional
substrates such as ceramic substrates can also be used in
accordance with the present invention.
[0157] According to a particularly preferred embodiment of the
present invention, the substrate onto which the precursor
composition is deposited and converted to a conductive feature has
a softening point of not greater than about 225.degree. C.,
preferably not greater than about 200.degree. C., even more
preferably not greater than about 185.degree. C. even more
preferably not greater than about 150.degree. C. and even more
preferably not greater than about 100.degree. C.
Deposition of Fine Features
[0158] One difficulty in printing and processing low viscosity
precursor compositions is that the composition can wet the surface
and rapidly spread to increase the width of the deposit, thereby
negating the advantages of fine line printing. This is particularly
true when ink-jet printing is employed to deposit fine features
such as interconnects, because ink-jet technology puts strict upper
boundaries on the viscosity of the composition that can be
employed. Nonetheless, ink-jet printing is a preferred low-cost,
large-area deposition technology that can be used to deposit the
precursor compositions of the present invention.
[0159] According to a preferred embodiment of the present
invention, the low viscosity precursor compositions can be confined
on the substrate, thereby enabling the formation of features having
a small minimum feature size, the minimum feature size being the
smallest dimension in the x-y axis, such as the width of a
conductive line. The precursor composition can be confined to
regions having a width of not greater than 100 .mu.m, preferably
not greater than 75 .mu.m, more preferably not greater than 50
.mu.m, even more preferably not greater than 25 .mu.m, and even
more preferably not greater than 10 .mu.m, such as not greater than
about 5 .mu.m. The present invention provides compositions and
methods of processing that advantageously reduce the spreading of
the low viscosity composition. For example, small amounts of
rheology modifiers such as styrene allyl alcohol (SAA) and other
polymers can be added to the precursor composition to reduce
spreading. The spreading can also be controlled through
combinations of nanoparticles and precursors. Spreading can also be
controlled by rapidly drying the compositions during printing by
irradiating the composition during deposition.
[0160] Spreading can also be controlled by the addition of a low
decomposition temperature polymer in monomer form. The monomer can
be cured during deposition by thermal or ultraviolet means,
providing a network structure to maintain line shape. The polymer
can then be either retained or removed during subsequent processing
of the conductor.
[0161] A preferred method is to pattern an otherwise non-wetting
substrate with wetting enhancement agents that control the
spreading and also yield increased adhesion. For example, this can
be achieved by functionalizing the substrate surface with hydroxide
or carboxylate groups.
[0162] Fabrication of conductor features with feature widths of not
greater than 100 .mu.m or features with minimum feature size of not
greater than 100 .mu.m from a low viscosity composition requires
the confinement of the low viscosity precursor compositions so that
the composition does not spread over certain defined boundaries.
Various methods can be used to confine the composition on a
surface, including surface energy patterning by increasing or
decreasing the hydrophobicity (surface energy) of the surface in
selected regions corresponding to where it is desired to confine
the precursor or eliminate the precursor. These can be classified
as physical barriers, electrostatic and magnetic barriers, surface
energy differences, and process related methods such as increasing
the precursor viscosity to reduce spreading, for example by
freezing or drying the composition very rapidly once it strikes the
surface.
[0163] In physical barrier approaches, a confining structure is
formed that keeps the precursor composition from spreading. These
confining structures may be trenches and cavities of various shapes
and depths below a flat or curved surface which confine the flow of
the precursor composition. Such trenches can be formed by chemical
etching or by photochemical means. The physical structure confining
the precursor can also be formed by mechanical means including
embossing a pattern into a softened surface or means of mechanical
milling, grinding or scratching features. Trenches can also be
formed thermally, for example by locally melting a low melting
point coating such as a wax coating. Alternatively, retaining
barriers and patches can be deposited to confine the flow of
composition within a certain region. For example, a photoresist
layer can be spin coated on a polymer substrate. Photolithography
can be used to form trenches and other patterns in this photoresist
layer. These patterns can be used to retain precursor that is
deposited onto these preformed patterns. After drying, the
photolithographic mask may or may not be removed with the
appropriate solvents without removing the deposited metal.
Retaining barriers can also be deposited with direct write
deposition approaches such as ink-jet printing or any other direct
writing approach, as disclosed herein.
[0164] For example, a polymer trench can be ink-jet printed onto a
flat substrate by depositing two parallel lines with narrow
parallel spacing. A precursor composition can be printed between
the two polymer lines to confine the composition. Another group of
physical barriers include printed lines or features with a certain
level of porosity that can retain a low viscosity composition by
capillary forces. The confinement layer may comprise particles
applied by any of the techniques disclosed herein. The particles
confine the precursor composition that is deposited onto the
particles to the spaces between the particles because of wetting of
the particles by the precursor composition. In one embodiment, the
particles are surface modified to make them hydrophilic and the
composition is hydrophilic with the substrate being
hydrophobic.
[0165] In one particular example, carbon particles are deposited
onto a substrate with a 75 .mu.m resolution using electrostatic
laser printing. A silver precursor composition can be subsequently
applied to this printed pattern and the resolution is retained
while the printed line has a bulk conductivity of 10 milli-ohm-cm
after heating at 200.degree. C.
[0166] Surface energy patterning can be classified by how the
patterning is formed, namely by mechanical, thermal, chemical or
photochemical means. In mechanical methods, the physical structure
confining the precursor composition is formed by mechanical means
including embossing a pattern into a softened surface, milling
features, or building up features to confine the composition. In
thermal methods, heating of the substrate is used to change the
surface energy of the surface, either across the entire surface or
in selected locations, such as by using a laser or thermal print
head. In chemical methods, the entire surface or portions of the
surface are chemically modified by reaction with some other
species. In one embodiment, the chemical reaction is driven by
local laser heating with either a continuous wave or pulsed laser.
In photochemical methods, light from either a conventional source
or from a laser is used to drive photochemical reactions that
result in changes in surface energy.
[0167] The methods of confining precursor compositions disclosed
herein can involve two steps in series--first the formation of a
confining pattern, that may be a physical or chemical confinement
method, and second, the application of a precursor composition to
the desired confinement areas.
[0168] Electrostatic printing can be used to print high resolution
patterns that correspond to at least two levels of surface
energies. In one embodiment, the electrostatic printing is carried
out on a hydrophobic surface and a hydrophilic material is printed.
The regions where no printing occurs correspond to hydrophobic
material. A hydrophobic precursor composition can then be printed
onto the hydrophobic regions thereby confining the composition.
Alternatively, a hydrophilic composition can be printed onto the
hydrophilic electrostatically printed regions. The width of the
hydrophobic and hydrophilic regions can be not greater than 100
.mu.m, more preferably are not greater than 75 .mu.m, more
preferably not greater than about 50 .mu.m and even more preferably
not greater than about 25 .mu.m.
[0169] The precursor composition confinement may be accomplished by
applying a photoresist and then laser patterning the photoresist
and removing portions of the photoresist. The confinement may be
accomplished by a polymeric resist that has been applied by another
jetting technique or by any other technique resulting in a
patterned polymer. In one embodiment, the polymeric resist is
hydrophobic and the substrate surface is hydrophilic. In that case,
the precursor composition utilized is hydrophilic resulting in
confinement of the composition in the portions of the substrate not
covered by the polymeric resist.
[0170] A laser can be used in various ways to modify the surface
energy of a substrate in a patterned manner. The laser can be used
to remove hydroxyl groups through local heating. These regions are
converted to more hydrophobic regions that can be used to confine a
hydrophobic or hydrophillic precursor composition. The laser can be
used to remove selectively a previously applied surface layer
formed by chemical reaction of the surface with a silanating
agent.
[0171] In one embodiment, a surface is laser processed to increase
the hydrophilicity in regions where the laser strikes the surface.
A polyimide substrate coated with a thin layer of hydrophobic
material, such as a fluorinated polymer. A laser, such as a pulsed
YAG, excimer or other UV or shorter wavelength pulsed laser, can be
used to remove the hydrophobic surface layer exposing the
hydrophilic layer underneath. Translating (e.g., on an x-y axis)
the laser allows patterns of hydrophilic material to be formed.
Subsequent application of a hydrophilic precursor composition to
the hydrophilic regions allows confinement of the composition.
Alternatively, a hydrophobic precursor composition can be used and
applied to the hydrophobic regions resulting in composition
confinement.
[0172] In another embodiment of the present invention, a surface is
laser processed to increase the hydrophobicity in regions where the
laser strikes the surface. A hydrophobic substrate such as a
fluorinated polymer can be chemically modified to form a
hydrophilic layer on its surface. Suitable modifying chemicals
include solutions of sodium naphthalenide. Suitable substrates
include polytetrafluoroethylene and other fluorinated polymers. The
dark hydrophilic material formed by exposing the polymer to the
solution can be removed in selected regions by using a laser.
Continuous wave and pulsed lasers can be used. Hydrophilic
precursor compositions, for example aqueous based compositions, can
be applied to the remaining dark material. Alternatively,
hydrophobic precursor compositions, such as those based on
solutions in non-polar solvents, can be applied to the regions
where the dark material was removed leaving the hydrophobic
material underneath. Ceramic surfaces can be hydroxylated by
heating in moist air or otherwise exposing the surface to moisture.
The hydroxylated surfaces can be silanated to create a monolayer of
hydrophobic molecules. The laser can be used to selectively remove
the hydrophobic surface layer exposing the hydrophilic material
underneath. A hydrophobic patterned layer can be formed directly by
micro-contact printing using a stamp to apply a material that
reacts with the surface to leave exposed a hydrophobic material
such as alkyl chain. The precursor composition can be applied
directly to the hydrophilic regions or hydrophobic regions using a
hydrophilic or hydrophobic precursor composition, respectively.
[0173] A surface with patterned regions of hydrophobic and
hydrophilic regions can be formed by micro-contact printing. In
this approach, a stamp is used to apply a reagent to selected
regions of a surface. This reagent can form a self-assembled
monolayer that provides a hydrophobic surface. The regions between
the hydrophobic surface regions can be used to confine a
hydrophilic precursor composition.
[0174] Precursor composition modification can also be employed to
confine the composition on the substrate. Such methods restrict
spreading of the compositions by methods other than substrate
modification. A precursor composition including a binder can be
used for surface confinement. The binder can be chosen such that it
is a solid at room temperature, but is a liquid suitable for
ink-jet deposition at higher temperatures. These compositions are
suitable for deposition through, for example, a heated ink-jet
head. The precursor composition can also include metal particles.
The precursor composition that is frozen on the surface can provide
linear features having widths of not greater than 100 .mu.m, more
preferably not greater than 75 .mu.m and more preferably not
greater than 50 .mu.m. The precursor composition can also include a
molecular precursor that is capable of forming a metal when heated
or irradiated by light. The precursor composition can combine
conductive particles and a molecular precursor.
[0175] Binders can also be used in the precursor compositions of
the present invention to provide mechanical cohesion and limit
spreading of the composition after deposition. In one preferred
embodiment, the binder is a solid at room temperature. During
ink-jet printing, the binder is heated and becomes flowable. The
binder can be a polymer or in some cases can be a precursor. In one
embodiment, the binder is a solid at room temperature, when heated
to greater than 50.degree. C. the binder melts and flows allowing
for ease of transfer and good wetting of the substrate, then upon
cooling to room temperature the binder becomes solid again
maintaining the shape of the deposited pattern. The binder can also
react in some instances. Preferred binders include waxes, styrene
allyl alcohols, poly alkylene carbonates, polyvinyl acetals,
cellulose based materials, tetradecanol, trimethylolpropane and
tetramethylbenzene. The preferred binders have good solubility in
the solvent used in the precursor composition and should be
processable in the melt form. For example, styrene allyl alcohol is
soluble in dimethylacetimide, solid at room temperature and becomes
fluid-like upon heating to 80.degree. C.
[0176] The binder in many cases should depart out of the ink-jet
printed feature or decompose cleanly during thermal processing,
leaving little or no residuals after processing the precursor
composition. The departure or decomposition can include
vaporization, sublimation, unzipping, partial polymer chain
breaking, combustion, or other chemical reactions induced by a
reactant present on the substrate material, or deposited on top of
the material.
[0177] An example of a precursor as a binder is the use of
Ag-trifluoroacetate with DMAc. The DMAc will form adducts with the
Ag-trifluoroacetate which then acts as a binder as well as the
silver precursor.
[0178] Other methods for controlling the spreading during printing
of a low viscosity precursor composition according to the present
invention include the steps of depositing a composition onto a
cooled substrate, freezing the composition as the droplets contact
the substrate, removing at least the solvent without melting the
composition, and converting the remaining components of the
composition to an electronic material. The melting point of the
composition is preferably less than 25.degree. C. Preferred
solvents include higher molecular weight alcohols. It is preferred
to cool the substrate to less than 10.degree. C.
[0179] The surface of a substrate can be pretreated with a
reactant, in one embodiment a reactant that does not contain a
metal. This reactant can be a reducing agent for a metal-containing
precursor. The surface can be completely coated or regions can be
coated with any approach including screen printing, ink-jet
printing, spin coating, dip coating, spraying, or any other
approach. In a second step, a metal containing reactant is ink-jet
printed onto the surface. The metal containing precursor reacts
with the co-reactant on the surface to form metal. The reaction is
rapid enough to confine the spreading of the metal on the surface.
In one embodiment, the metal containing precursor comprise silver
or copper. The co-reactant on the surface can comprise a reducing
agent for the metal. Alternatively, a reducing agent or reaction
inducing agent can be printed locally prior to or following the
deposition of the metal precursor composition. By performing both
printing steps within a short time frame and by ensuring that the
co-reactant triggered decomposition reaction occurs fast enough to
avoid spreading of the precursor composition, fine features can be
deposited. All the metal-containing compounds and reducing agents
discussed herein can be used in this approach.
[0180] Yet another method for controlling the spreading of a low
viscosity precursor composition during printing is provided. The
method comprises depositing a precursor composition onto a
substrate, simultaneously irradiating the deposited composition
with light to limit the spreading of the composition on the surface
and converting the composition to a conductive feature. UV light
can be used to photochemically decompose a metal precursor to a
metal before spreading of the precursor composition occurs.
[0181] Yet another method for controlling the spreading during
printing according to the present invention comprises the steps of
depositing a precursor composition onto a porous substrate, thereby
limiting the spreading of the composition, and converting the
composition to a conductive feature. In one embodiment, the
porosity in the substrate is created by laser patterning. The
porosity can be limited to the very surface of the substrate.
[0182] Yet another method for controlling the spreading of a low
viscosity precursor composition according to the present invention
includes the steps of patterning the substrate to form regions with
two distinct levels of porosity where the porous regions form the
pattern of a desired feature. The precursor composition can then be
deposited, such as by ink-jet printing, onto the regions defining
the pattern thereby confining the precursor composition to these
regions, and converting the deposited precursor composition to a
conductive feature. Preferred substrates are polyimide, and epoxy
laminates. In one embodiment the patterning is carried out with a
laser. In another embodiment the patterning is carried out using
photolithography. In another embodiment, capillary forces pull at
least some portion of the composition into the porous
substrate.
[0183] Spreading of the precursor composition is influenced by a
number of factors. A drop of liquid placed onto a surface will
either spread or not depending on the surface tensions of the
liquid, the surface tension of the solid and the interfacial
tension between the solid and the liquid. If the contact angle is
greater than 90 degrees, the liquid is considered non-wetting and
the liquid tends to bead or shrink away from the surface. For
contact angles less than 90 degrees, the liquid can spread on the
surface. For the liquid to completely wet, the contact angle must
be zero. For spreading to occur, the surface tension of the liquid
must be lower than the surface tension of the solid on which it
resides.
[0184] In one embodiment, a precursor composition is applied, as by
ink-jet deposition, to an unpatterned substrate. Unpatterned refers
to the fact that the surface energy (tension) of the substrate has
not been intentionally patterned for the sole purpose of confining
the composition. It is to be understood that variations in surface
energy (used synonymously herein with surface tension) of the
substrate associated with devices, interconnects, vias, resists and
any other functional features may already be present. For
substrates with surface tensions of less than 30, a hydrophilic
precursor composition can be based on water, glycerol, glycol, and
other solvents or liquids having surface tensions of greater than
30 dynes/cm, more preferably greater than 40 dynes/cm and
preferably greater than 50 dynes/cm and even greater than 60
dynes/cm. For substrates with surface tensions of less than 40, the
solvents should have surface tensions of greater than 40 dynes/cm,
preferably greater than 50 dynes/cm and even more preferably
greater than 60 dynes/cm. For substrates with surface tensions less
than 50, the surface tension of the precursor composition should be
greater than 50 dynes/cm, preferably greater than 60 dynes/cm.
Alternatively, the surface tension of the composition can be chosen
to be 5, 10, 15, 20, or 25 dynes/cm greater than that of the
substrate. Continuous ink jet heads often require surface tensions
of 40 to 50 dynes/cm. Bubble-jet ink jet heads often require
surface tensions of 35 to 45 dynes/cm. The previously described
methods are particularly preferred for these types of deposition
approaches.
[0185] In another embodiment, a precursor composition is applied,
as by ink-jet deposition, to an unpatterned low surface energy
(hydrophobic) surface that has been surface modified to provide a
high surface energy (hydrophilic). The surface energy can be
increased by hydroxylating the surface by various means known to
those skilled in the art including exposing to oxidizing agents and
water, heating in moist air and the like. The surface tension of
the precursor composition can then be chosen to be 5, 10, 15, 20,
or 25 dynes/cm less than that of the substrate. Piezo-jet ink jet
heads operating with hot wax often require surface tensions of 25
to 30 dynes/cm. Piezo-jet ink jet heads operating with UV curable
inks often require surface tensions of 25 to 30 dynes/cm.
Bubble-jet ink jet heads operating with UV curable inks often
require surface tensions of 20 to 30 dynes/cm. Surface tensions of
roughly 20 to 30 dynes/cm are required for piezo-based ink jet
heads using solvents. The previously described methods are
particularly preferred for these types of applications.
[0186] Most electronic substrates with practical applications have
low values of surface tension, in the range of 18
(polytetrafluroethylene) to 45, often between 20 and 40 dynes/cm.
In one approach to confining a precursor composition to a narrow
line or other shape, a hydrophilic pattern corresponding to the
pattern of the desired conductor feature is formed on the surface
of a substrate through the methods discussed herein. A particularly
preferred method uses a laser. For example, a laser can be used to
remove a hydrophobic surface layer exposing a hydrophilic layer
underneath. In one embodiment, the hydrophilic material pattern on
the surface has a surface energy that is 5, 10, 15, 20, 25 or 30
dynes/cm greater than the surrounding substrate. In one embodiment,
the surface tension of the composition is chosen to be less than
the surface tension of the hydrophilic region but greater than the
surface tension of the hydrophobic region. The surface tension of
the composition can be chosen to be 5, 10, 15, 20 or 25 dynes/cm
less than that of the hydrophilic regions. The surface tension of
the composition can be chosen to be 5, 10, 15, 20 or 25 dynes/cm
greater than that of the hydrophobic regions. In another approach,
the surface energy of the composition is higher than the surface
energy of both the hydrophobic and hydrophilic regions. The surface
tension of the composition can be chosen to be 5, 10, 15, 20 or 25
dynes/cm greater than that of the hydrophilic regions. The surface
tension of the ink is chosen to be 5, 10, 15, 20 or 25 dynes/cm
less than that of the hydrophilic regions. This approach is
preferred for aqueous-based precursor compositions and compositions
with high surface tensions in general. Continuous ink jet heads
often require surface tensions of 40 to 50 dynes/cm. Bubble-jet ink
jet heads often require surface tensions of 35 to 45 dynes/cm. The
previously described methods are particularly preferred for these
types of applications that can handle compositions with high
surface tensions.
[0187] In one embodiment of this latter approach, a hydrophilic
composition is applied, as by inkjet deposition, to the hydrophilic
regions. For substrates with unpatterned hydrophobic regions with
surface tensions of less than 30, the hydrophilic composition can
be based on water, glycerol, glycol, and other solvents or liquids
to provide compositions having surface tensions of greater than 30
dynes/cm, more preferably greater than 40, greater than 45, greater
than 50, and even greater than 60 dynes/cm. For substrates with
surface tensions of less than 40 in the hydrophobic regions, the
compositions should have surface tensions of greater than 40,
greater than 45, greater than 50 and greater than 60 dynes/cm. For
substrates with surface tensions less than 50, the surface tension
of the composition should be greater than 50 greater than 55, or
greater than 60 dynes/cm. For hydrophilic regions with surface
tensions of less than 30, the hydrophilic precursor compositions
can be based on water, glycerol, glycol, and other solvents or
liquids having surface tensions of greater than 30 dynes/cm,
greater than 35 dynes/cm, greater than 40 and greater than 50, and
even greater than 60 dynes/cm. For hydrophilic regions with surface
tensions of less than 40, the compositions should have surface
tensions of greater than 40, greater than 50 and greater than 60
dynes/cm. For hydrophilic regions with surface tensions less than
50, the surface tension of the composition should be greater than
50 or greater than 60 dynes/cm. For continuous ink-jet heads that
require surface tensions of 40 to 50 dynes/cm. For bubble-jet
ink-jet heads that require surface tensions of 35 to 45 dynes/cm.
The previously described methods are particularly preferred for
these types of applications.
[0188] In another approach to confining a composition to a narrow
feature, a hydrophilic surface, or a hydrophobic surface that is
rendered hydrophilic by surface modification, is patterned with a
hydrophobic pattern. In one embodiment, the hydrophobic pattern has
a surface energy that is 5, 10, 15, 20, 25 or 30 dynes/cm less than
the surrounding substrate. This can be done by removing a
hydrophilic surface layer using a laser to expose a hydrophobic
region underneath. A hydrophobic precursor composition is applied
to the hydrophobic surface regions to confine the composition. In
one embodiment, the hydrophobic composition has a surface energy
that is 5, 10, 15, 20, 25 or 30 dynes/cm less than the surrounding
substrate. In one embodiment, the hydrophobic composition has a
surface energy that is 5, 10, 15, 20, 25 or 30 dynes/cm greater
than the surrounding substrate. In one embodiment, the hydrophobic
precursor composition has a surface energy that is 5, 10, 15, 20,
25 or 30 dynes/cm less than the hydrophobic surface pattern. In one
embodiment, the hydrophobic ink has a surface energy that is 5, 10,
15, 20, 25 or 30 dynes/cm greater than the hydrophobic surface
pattern. In another embodiment, the surface tension of the
composition is less than the hydrophilic regions and greater than
the hydrophobic regions. The hydrophilic surface can have a surface
tension of greater than 40, greater than 50 or greater than 60
dynes/cm. When the hydrophobic surface has a surface energy of
greater than 40 dynes/cm, it is preferred to use an ink with
surface tension of less than 40, even less than 30 dynes/cm, and
less than 25 dynes/cm. When the hydrophobic surface has a surface
energy of greater than 50 dynes/cm, it is preferred to use a
composition with a surface tension of less than 50, preferably less
than 40, even less than 30 dynes/cm, and more preferably less than
25 dynes/cm. When the hydrophobic surface has a surface tension of
greater than 40 dynes/cm, it is preferred to use a precursor
composition with a surface tension of less than 40, less than 35,
less than 30 and even less than 25 dynes/cm.
[0189] Piezo-jet ink jet heads operating with hot wax often require
surface tensions of 25 to 30 dynes/cm. Piezo-jet ink jet heads
operating with UV curable inks often require surface tensions of 25
to 30 dynes/cm. Bubble-jet ink jet heads operating with UV curable
inks often require surface tensions of 20-30 dynes/cm. Surface
tensions of roughly 20 to 30 dynes/cm are required for piezo-based
ink jet heads using solvents. The previously described methods are
particularly preferred for these types of applications.
[0190] For ink-jet heads and other deposition techniques that
require surface tensions greater than 30 dynes/cm, a particularly
preferred method for confining a precursor composition to a surface
involves increasing the hydrophilicity of the surface to provide a
surface tension greater than 40, greater than 45 or greater than 50
dynes/cm and then providing a hydrophobic surface pattern with
surface tension lower than the surrounding surface. The surface
tension of the pattern can be 5, 10, 15, 20 or 25 dynes/cm greater
than the surface tension of the surrounding substrate.
[0191] Surfactants, molecules with hydrophobic tails corresponding
to lower surface tension and hydrophilic ends corresponding to
higher surface tension can be use to modify the compositions and
substrates to achieve the values of surface tensions and
interfacial energies required.
[0192] For the purposes of this application, hydrophobic means a
material that has the opposite response to interaction with water.
Hydrophobic materials have low surface tensions. They also do not
have functional groups for making hydrogen bonds With water.
[0193] Hydrophilic means a material that has an affinity for water.
Hydrophilic surfaces are wetted by water. Hydrophilic materials
also have high values of surface tension. They can also form
hydrogen bonds with water. The surface tensions for different
liquids are listed in Table 6 and the surface energies for
different solids are listed in Table 7. TABLE-US-00006 TABLE 6
Surface Tensions of Various Liquids Surface Temp Tension Liquid
(.degree. C.) (dynes/cm) Water 20 72.75 Acetamide 85 C. 39.3
Acetone 20 C. 23.7 Acetonitrile 20 29.3 n-butyl 20 C. 24.6 alcohol
ethyl alcohol 20 24 Hexane 20 18.4 Isopropyl 20 22 alcohol Glycerol
20 63.4 Glycol 20 47.7 Tolulene 20 29
[0194] TABLE-US-00007 TABLE 7 Surface Energies of Various Solids
Surface Energy Material (dynes/cm) Glass 30 PTFE 18 Polyethylene 31
Polyvinychlorides 41 Polyvinylidene 25 fluoride Polypropylene 29
Polystyrene 33 Polyvinylchloride 39 Polysulfone 41 Polycarbonate 42
Polyethylene 43 terephthalate Polyacryonitrile 44 Cellulose 44
[0195] Another difficulty during printing and processing is that
during drying, precursors in the composition can crystallize and
form discontinuous lines that provide poor conductivity upon
conversion to the conductor. This can be substantially prevented by
adding small amounts of a crystallization inhibitor, as is
discussed above.
[0196] The present invention also provides compositions and methods
to increase adhesion of the conductive feature, referred to herein
as adhesion promoters. Various substrates have different surface
characteristics that result in varying degrees of adhesion.
According to the present invention, the surface can be modified by
hydroxylating or otherwise functionalizing the surface to provide
reaction sites from the precursor compositions. In one embodiment,
the surface of a polyfluorinated material is modified by a sodium
naphthalenide solution that provides reactive sites for bonding
during reaction with the precursor. In another embodiment, a thin
layer of metal is sputtered onto the surface to provide for better
adhesion of precursor composition or conductive feature to the
substrate. In another embodiment, polyamic acid or similar
materials are added to the composition that then bond with both the
conductor and surface to provide adhesion. Preferred amounts of
polyamic acid and related compounds are from about 1 to 10 weight
percent of the low viscosity precursor composition.
[0197] In accordance with the foregoing, the precursor compositions
according to the present invention can include a molecular
precursor and a vehicle, without nanoparticles or micron-size
particles. In one preferred embodiment, the precursor compositions
include a conversion reaction inducing agent, which can be either
or both of a powder, a molecular precursor or another inorganic or
organic compound. In another embodiment, the low viscosity
precursor composition includes additives to reduce spreading of the
composition by controlling the wetting angle of the composition on
the surface. In another embodiment, the combination of molecular
precursor and solvent is chosen to provide a high solubility of the
precursor in the solvent.
[0198] In another embodiment, the precursor composition includes
hollow or porous micron-size particles, a molecular precursor and a
vehicle. The molecular precursor is preferably a metal organic
compound. In another embodiment, the precursor composition includes
hollow or porous micron-size particles, nanoparticles and a
vehicle. In another embodiment, the precursor composition includes
hollow or porous micron-size particles, a molecular precursor,
nanoparticles and a vehicle. The precursor is preferably a metal
organic compound.
[0199] The precursor composition can also include a molecular
precursor, a vehicle and nanoparticles. The nanoparticles can be
selected from silver, copper and other metals, or can be
non-conductive nanoparticles such as silica, copper oxide and
aluminum oxide.
[0200] The precursor compositions can also include a molecular
precursor, a vehicle, and a polymer or polymer precursor, such as
in cases where adhesion to a polymeric substrate is desired. The
precursor to a polymer can be poly(amic) acid. The polymer can be
an epoxy, polyimide, phenolic resin, thermo set polyester,
polyacrylate and the like. The low viscosity precursor compositions
can include a low curing polymer, such as one that cures at not
greater than 200.degree. C., more preferably not greater than
150.degree. C.
[0201] The precursor compositions can also include carbon, a
molecular precursor and a vehicle. The compositions can include
particulate carbon, such as conductive carbon, e.g., graphitic
carbon. One preferred combination is conductive carbon with a
molecular precursor to silver metal.
[0202] The precursor compositions can also include conductive
transparent particles (e.g., ITO particles), a molecular precursor
and a vehicle. The molecular precursors can include ITO precursors
and metal precursors such as silver precursors.
[0203] The precursors compositions can also include a conductive
polymer, molecular precursor and a vehicle. The polymer can be
conductive for both electrons and protons. Electrically conductive
polymers can be selected from polyacetylene, polyaniline,
polyphenylene, polypyrrole, polythiophene,
polyethylenedioxythiophene and poly (paraphenylene vinylene).
Protonic conductive polymers include those with sulfonates or
phosphates, for example sulfonated polyaniline.
[0204] The precursor compositions can also include glass or metal
oxide nanoparticles, micron-size particles and a molecular
precursors. The compositions can include nanoparticles of metal
oxides such as silica, copper oxide, and aluminum oxide. Preferred
molecular precursors according to this embodiment are metal
organics.
[0205] The precursor compositions can also include conductive
nanoparticles and vehicle. The flowable composition can further
include a polymer precursor.
[0206] The low viscosity precursor compositions can also include an
electrocatalyst or catalyst and a precursor. The precursor can be
converted to a catalytically active material or can serve to fuse
the layer together.
[0207] In the low viscosity precursor compositions that include a
molecular precursor and powders (nanoparticles and/or micron-size
particles), the ratio of precursors to powders is close to that
corresponding to the amount needed to fill the spaces between
particulates with material derived from the precursors. However, a
significant improvement in conductivity can also be obtained for
lower levels of molecular precursor. When particles are included in
the precursor compositions of the present invention, it is
preferred that at least about 10 vol. %, more preferably at least
about 25 vol. % and even more preferably at least about 50 vol. %
of the final conductor be derived from precursor.
[0208] Other specific low viscosity precursor compositions
according to the present invention are preferred for different
applications. Typically, the formulation for the low viscosity
composition will take into account the deposition mechanism, the
desired performance of the features and the relative cost of the
features. For example, simple circuitry on a paper substrate
designed for a disposable, high-volume application will require a
low cost precursor composition but will not require electronic
features having superior properties. On the other hand, higher end
applications such as for repair of electronic circuitry will
require electronic features having very good electrical properties
and the relative cost of the low viscosity precursor composition
will typically not be a significant factor.
[0209] According to one embodiment, the precursor composition can
include particulates, including insulative particles such as
SiO.sub.2, particulates that are a precursor to a conductive phase
such as silver oxide or silver nitrate particles, Ag
trifluoroacetate crystallites, conductive micron-size particles and
nanoparticles of the conductive phase, and a liquid phase made up
of a vehicle and molecular metal precursors dissolved therein. For
low viscosity compositions, the particulate fraction of the
composition preferably is not greater than 25 volume percent of the
total composition volume. The precursor fraction of the
composition, both present in the form of precursor particles and
molecular precursor dissolved in the vehicle, is typically
expressed as a weight percent of the total composition weight and
can be up to about 80 weight percent of the total composition
weight.
[0210] In one embodiment, the low viscosity precursor composition
includes up to about 20 volume percent carbon and from about 10 to
about 15 weight percent of a molecular precursor, with the balance
being vehicle and other additives. In another embodiment, the low
viscosity precursor composition includes up to about 15 volume
percent carbon and up to about 5 volume percent metal
nanoparticles, with the balance being vehicle and other
additives.
[0211] According to another embodiment, the low viscosity precursor
composition includes up to about 75 weight percent metal
nanoparticles, such as from 5 to 50 weight percent metal
nanoparticles, and from about 10 to about 50 weight percent of a
molecular precursor, wherein the balance is vehicle and other
additives.
[0212] According to another embodiment, the low viscosity precursor
composition includes up to about 20 volume percent micron-size
metal particles and from about 10 to about 15 weight percent of a
molecular precursor with the balance being vehicle and other
additives. After heating at not greater than 250.degree. C., the
conductivity of the conductive feature is in the range from 1 to 5
times the bulk metal conductivity.
[0213] According to yet another embodiment, the low viscosity
precursor composition includes up to about 20 volume percent
micron-size metal particles, with the balance being a vehicle
containing a precursor to a conductive polymer. After heating at
not greater than 200.degree. C. the bulk conductivity is in the
range from 5 to 50 times the bulk conductivity of the metal
phase.
[0214] In one embodiment of a transparent conductor precursor
composition, the composition contains about 15 vol. % micron-size
particles selected from the group of ITO, ATO, ZnO, SnO.sub.2, and
5 vol. % Ag nanoparticles, and between 0 and 20 weight percent
molecular precursor to Ag with the balance being solvents, vehicle
and other additives.
[0215] In another embodiment of a transparent conductor precursor
formulation, the composition contains up to about 30 vol. %
micron-size particles selected from the group of ITO, ATO, ZnO,
SnO.sub.2, and between 5 and 40 weight percent precursor to Ag,
with the balance being solvents, vehicle and other additives.
[0216] In yet another embodiment, a transparent conductor precursor
composition contains up to about 15 vol. % micron size particles
selected from the group of ITO, ATO, ZnO, SnO.sub.2, and up to 10
vol. % conductive glass particles such as silver phosphate glass,
and between 0 and 20 weight percent precursor to Ag with the
balance being solvents, vehicle and other additives.
[0217] In addition to the foregoing, the low viscosity precursor
compositions according to the present invention can also include
carbon particles, such as graphitic particles. Depending upon the
other components in the low viscosity precursor composition, carbon
particle loading up to about 20 volume percent can be obtained in
the compositions. The average particle size of the carbon particles
is preferably not greater than about 1 .mu.m and the carbon
particles can advantageously have a bimodal or trimodal particle
size distribution. Graphitic carbon has a bulk resistivity of about
1375 .mu..OMEGA.-cm and is particularly useful in low viscosity
precursor compositions for conductive features that require a
relatively low cost.
Deposition of Precursor Compositions
[0218] The low viscosity precursor compositions of the present
invention can be deposited onto surfaces using a variety of
tools.
[0219] As used herein, a low viscosity deposition tool is a device
that deposits a liquid or liquid suspension onto a surface by
ejecting the composition through an orifice toward the surface
without the tool being in direct contact with the surface. The low
viscosity deposition tool is preferably controllable over an x-y
grid, referred to herein as a direct-write deposition tool. A
preferred direct-write deposition tool according to the present
invention is an ink-jet device. Other examples of direct-write
deposition tools include aerosol jets and automated syringes, such
as the MICROPEN tool, available from Ohmcraft, Inc., of Honeoye
Falls, N.Y.
[0220] For use in an ink-jet, the viscosity of the precursor
composition is preferably not greater than 50 centipoise, such as
in the range of from about 10 to about 40 centipoise. For use in an
aerosol jet atomization, the viscosity is preferably not greater
than about 20 centipoise. Automated syringes can use compositions
having a higher viscosity, such as up to about 5000 centipoise.
[0221] A preferred direct-write deposition tool according to the
present invention is an ink-jet device. Ink-jet devices operate by
generating droplets of the composition and directing the droplets
toward a surface. The position of the ink-jet head is carefully
controlled and can be highly automated so that discrete patterns of
the composition can be applied to the surface. Ink-jet printers are
capable of printing at a rate of 1000 drops per jet per second or
higher and can print linear features with good resolution at a rate
of 10 cm/sec or more, up to about 1000 cm/sec. Each drop generated
by the ink-jet head includes approximately 25 to 100 picoliters of
the composition, which is delivered to the surface. For these and
other reasons, ink-jet devices are a highly desirable means for
depositing materials onto a surface.
[0222] Typically, an ink-jet device includes an ink-jet head with
one or more orifices having a diameter of not greater than about
100 .mu.m, such as from about 50 .mu.m to 75 .mu.m. Droplets are
generated and are directed through the orifice toward the surface
being printed. Ink-jet printers typically utilize a piezoelectric
driven system to generate the droplets, although other variations
are also used. Ink-jet devices are described in more detail in, for
example, U.S. Pat. No. 4,627,875 by Kobayashi et al. and U.S. Pat.
No. 5,329,293 by Liker, each of which is incorporated herein by
reference in their entirety. However, such devices have primarily
been used to deposit inks of soluble dyes.
[0223] It is also important to simultaneously control the surface
tension and the viscosity of the precursor composition to enable
the use of industrial ink-jet devices. Preferably the surface
tension is from about 10 to 50 dynes/cm, such as from about 20 to
40 dynes/cm, while the viscosity is maintained at not greater than
about 50 centipoise.
[0224] According to one embodiment, the solids loading of particles
in the low viscosity precursor composition is preferably as high as
possible without adversely affecting the viscosity or other
necessary properties of the composition. For example, the low
viscosity precursor composition can have a particle loading of up
to about 75 weight percent, and in one embodiment the particle
loading is from about 5 to about 50 weight percent.
[0225] The precursor compositions for use in an ink-jet device can
also include water and an alcohol. Surfactants can also be used to
maintain the particles in suspension. Co-solvents, also known as
humectants, can be used to prevent the precursor composition from
crusting and clogging the orifice of the ink-jet head. Biocides can
also be added to prevent bacterial growth over time. Examples of
such ink-jet liquid vehicle compositions are disclosed in U.S. Pat.
No. 5,853,470 by Martin et al.; U.S. Pat. No. 5,679,724 by
Sacripante et al.; U.S. Pat. No. 5,725,647 by Carlson et al.; U.S.
Pat. No. 4,877,451 by Winnik et al.; U.S. Pat. No. 5,837,045 by
Johnson et al.; and U.S. Pat. No. 5,837,041 by Bean et al. Each of
the foregoing U.S. patents is incorporated by reference herein in
their entirety. The selection of such additives is based upon the
desired properties of the composition, as is known to those skilled
in the art. Particles can be mixed with the liquid vehicle using a
mill or, for example, an ultrasonic processor.
[0226] The low viscosity precursor compositions according to the
present invention can also be deposited by aerosol jet deposition.
Aerosol jet deposition can enable the formation of conductive
features having a feature width of not greater than about 200
.mu.m, such as not greater than 100 .mu.m, not greater than 75
.mu.m and even not greater than 50 .mu.m. In aersol jet deposition,
the precursor composition is aerosolized into droplets and the
droplets are transported to the substrate in a flow gas through a
flow channel. Typically, the flow channel is straight and
relatively short.
[0227] The aerosol can be created using a number of atomization
techniques. Examples include ultrasonic atomization, two-fluid
spray head, pressure atomizing nozzles and the like. Ultrasonic
atomization is preferred for compositions with low viscosities and
low surface tension. Two-fluid and pressure atomizers are preferred
for higher viscosity fluids. Solvent or other precursor components
can be added to the precursor composition during atomization, if
necessary, to keep the concentration of precursor components
substantially constant during atomization.
[0228] The size of the aerosol droplets can vary depending on the
atomization technique. In one embodiment, the average droplet size
is not greater than about 10 .mu.m and more preferably is not
greater than about 5 .mu.m. Large droplets can be optionally
removed from the aerosol, such as by the use of an impactor.
[0229] Low aerosol concentrations require large volumes of flow gas
and can be detrimental to the deposition of fine features. The
concentration of the aerosol can optionally be increased, such as
by using a virtual impactor. The concentration of the aerosol can
be greater than about 10.sup.6 droplets/cm and more preferably is
greater than 10.sup.7 droplets/cm.sup.3. The concentration of the
aerosol can be monitored and the information can be used to
maintain the mist concentration within, for example, 10% of the
desired mist concentration over a period of time.
[0230] The droplets are deposited onto the surface of the substrate
by inertial impaction of larger droplets, electrostatic deposition
of charged droplets, diffusional deposition of sub-micron droplets,
interception onto non-planar surfaces and settling of droplets,
such as those having a size in excess of about 10 .mu.m.
[0231] Examples of tools and methods for the deposition of fluids
using aerosol jet deposition include U.S. Pat. No. 6,251,488 by
Miller et al., U.S. Pat. No. 5,725,672 by Schmitt et al. and U.S.
Pat. No. 4,019,188 by Hochberg et al. Each of these U.S. patents is
incorporated herein by reference in their entirety.
[0232] The precursor compositions of the present invention can also
be deposited by a variety of other techniques including intaglio,
roll printer, spraying, dip coating, spin coating, and other
techniques that direct discrete units of fluid or continuous jets,
or continuous sheets of fluid to a surface. Other printing methods
include lithographic and gravure printing.
[0233] For example, gravure printing can be used with precursor
compositions having a viscosity of up to about 5000 centipoise. The
gravure method can deposit features having an average thickness of
from about 1 .mu.m to about 25 .mu.m micrometers and can deposit
such features at a high rate of speed, such as up to about 700
meters per minute. The gravure process also enables the direct
formation of patterns onto the surface.
[0234] Lithographic printing methods can also be utilized. In the
lithographic process, the inked printing plate contacts and
transfers a pattern to a rubber blanket and the rubber blanket
contacts and transfers the pattern to the surface being printed. A
plate cylinder first comes into contact with dampening rollers that
transfer an aqueous solution to the hydrophilic non-image areas of
the plate. A dampened plate then contacts an inking roller and
accepts the ink only in the oleophillic image areas.
[0235] Using one or more of the foregoing deposition techniques, it
is possible to deposit the precursor composition on one side or
both sides of a substrate. Further, the processes can be repeated
to deposit multiple layers of the same or different precursor
compositions on a substrate.
[0236] An optional first step, prior to deposition of the precursor
composition, is surface modification of the substrate as is
discussed above. The surface modification can be applied to the
entire substrate or can be applied in the form of a pattern, such
as by using photolithography. The surface modification can include
increasing or decreasing the hydrophilicity of the substrate
surface by chemical treatment. For example, a silanating agent can
be used on the surface of a glass substrate to increase the
adhesion and/or to control spreading of the precursor composition
through modification of the surface tension and/or wetting angle.
The surface modification can also include the use of a laser to
clean the substrate. The surface can also be subjected to
mechanical modification by contacting with another type of surface.
The substrate can also be modified by corona treatment. For the
deposition of organic-based precursor compositions, the activation
energy of the substrate surface can be modified.
[0237] For example, a line of polyimide can be printed prior to
deposition of a precursor composition, such as a silver metal
precursor composition, to prevent infiltration of the composition
into a porous substrate, such as paper. In another example, a
primer material may be printed onto a substrate to locally etch or
chemically modify the substrate, thereby inhibiting the spreading
of the precursor composition being deposited in the following
printing step. In yet another example, a via can be etched by
printing a dot of a chemical that is known to etch the substrate.
The via can then be filled in a subsequent printing process to
connect circuits being printed on the front and back of the
substrate.
[0238] As is discussed above, the deposition of the low viscosity
precursor composition can be carried out by pen/syringe, continuous
or drop on demand ink-jet, droplet deposition, spraying,
flexographic printing, lithographic printing, gravure printing,
other intaglio printing, and others. The precursor composition can
also be deposited by dip-coating or spin-coating, or by pen
dispensing onto a rod or fiber type substrates with the same
composition. Immediately after deposition, the composition may
spread, draw in upon itself, or form patterns depending on the
surface modification discussed above. In another embodiment, a
method is provided for processing the deposited composition using 2
or more jets or other ink sources. In one embodiment, the first
deposition step provides the precursor composition including a
molecular metal precursor compound while the second deposition step
provides a reducing agent or other co-reactant that converts the
precursor and/or reduces the conversion temperature. Another
example of a method for processing the deposited composition is
using infiltration into a porous bed formed by a previous
fabrication method. Another method for depositing the composition
is using multi-pass deposition to build the thickness of the
deposit. Another example of a method for depositing the composition
is using a heated head to decrease the viscosity of the
composition.
[0239] The properties of the deposited precursor composition can
also be subsequently modified. This can include freezing, melting
and otherwise modifying the properties such as viscosity with or
without chemical reactions or removal of material from the
precursor composition. For example, a low viscosity precursor
composition including a UV-curable polymer can be deposited and
immediately exposed to an ultraviolet lamp to polymerize and
thicken and reduce spreading of the composition. Similarly, a
thermoset polymer can be deposited and exposed to a heat lamp or
other infrared light source. After deposition, the precursor
composition is treated in one or more steps to convert the metal
precursor and obtain the desired final properties of the deposited
feature.
[0240] After deposition, the precursor composition is treated to
convert the precursor composition to the conductive feature. The
treatment can include multiple steps, or can occur in a single
step, such as when the precursor composition is rapidly heated and
held at the conversion temperature for a sufficient amount of time
to form the conductive feature.
[0241] An optional, initial step can include drying or subliming of
the composition by heating or irradiating. In this step, material
is removed from the composition and/or chemical reactions occur in
the composition. An example of a method for processing the
deposited composition in this manner is using a UV, IR, laser or a
conventional light source. Heating rates for drying the precursor
composition are preferably greater than about 10.degree. C./min,
more preferably greater than 100.degree. C./min and even more
preferably greater than 1000.degree. C./min. The temperature of the
deposited precursor composition can be raised using hot gas or by
contact with a heated substrate. This temperature increase may
result in further evaporation of solvents and other species. A
laser, such as an IR laser, can also be used for heating. IR lamps
or a belt furnace can also be utilized. It may also be desirable to
control the cooling rate of the deposited feature.
[0242] After drying, the next step is to react the molecular metal
precursors. In one embodiment, the precursor composition is reacted
using various gases to assist in the conversion of the precursor
composition to a conductive feature. For example, hydrogen,
nitrogen, and reducing-gases can be used to assist the reaction.
Copper, nickel, and other metals that oxidize when exposed to
oxygen may require the presence of reducing atmospheres. It has
been found that the precursor compositions of the present invention
can advantageously provide very short reaction times when processed
with light (e.g., a laser) that heats the materials. This is a
result of the high chemical reaction rates when sufficiently high
temperatures are provided for a specific precursor and the ability
of light to rapidly heat the materials over time scales of
milliseconds or even less. In the case of precursor compositions
including particles, phases having a low melting or softening point
allow short processing times.
[0243] The precursor compositions of the present invention can be
processed for very short times and still provide useful materials.
Short heating times can advantageously prevent damage to the
underlying substrate. Preferred thermal processing times for
deposits having a thickness on the order of about 10 .mu.m are not
greater than about 100 milliseconds, more preferably not greater
than about 10 milliseconds, more preferably not greater than about
1 millisecond. The short heating times can be provided using laser
(pulsed or continuous wave), lamps, or other radiation.
Particularly preferred are scanning lasers with controlled dwell
times. When processing with belt and box furnaces or lamps, the
hold time is preferably not greater than 60 seconds, and more
preferably not greater than 30 seconds, and even more preferably
not greater than 10 seconds. The heating time can even be not
greater than 1 second when processed with these heat sources, and
even not greater than 0.1 second while still providing conductive
materials that are useful in a variety of applications. The
preferred heating time and temperature will also depend on the
nature of the electronic feature. It will be appreciated that short
heating times may not be beneficial if the solvent or other
constituents boil rapidly and form porosity or other defects in the
feature.
[0244] When used to form conductors the deposited precursor
compositions can be substantially fully converted at temperatures
of not greater than 300.degree. C., more preferably not greater
than 250.degree. C., more preferably not greater than 225.degree.
C., even more preferably not greater than 200.degree. C., and even
more preferably not greater than 185.degree. C.
[0245] The particles in the precursor composition (if any) or the
material derived from the precursor can optionally be sintered
subsequent to decomposition of the metal precursor. The sintering
can be carried out using furnaces, light sources such as heat lamps
and/or lasers. In one embodiment, the use of a laser advantageously
provides very short sintering times and in one embodiment the
sintering time is not greater than 1 second, more preferably not
greater than 0.1 seconds and even more preferably not greater than
0.01 seconds. Laser types include pulsed and continuous wave. In
one embodiment, the laser pulse length is tailored to provide a
depth of heating that is equal to the thickness of the material to
be sintered. The components in the precursor composition can be
fully or partially reacted before contact with laser light. The
components can be reacted by exposure to the laser light and then
sintered.
[0246] The conductive feature can be post-treated after deposition
and conversion of the metal precursor. For example, the
crystallinity of the phases present can be increased, such as by
laser processing. The post-treatment can also include cleaning
and/or encapsulation of the electronic features, or other
modifications.
[0247] It will be appreciated from the foregoing discussion that
two or more of the latter process steps (drying, heating, reacting
and sintering) can be combined into a single process step.
[0248] One preferred process flow includes the steps of: forming a
structure by conventional methods such as lithographic, gravure,
flexo, screen printing, photo patterning, thin film or wet
subtractive approaches; identifying locations requiring addition of
material; adding material by a direct deposition of a low viscosity
composition; and processing to form the final product. In a
specific embodiment, a circuit is prepared by screen-printing and
is then repaired by localized printing of a low viscosity precursor
composition.
[0249] More specifically, the present invention provides a method
for the repair of a feature by ink-jet printing or syringe
dispensing. In one embodiment, the method includes the steps of
ink-jet printing a precursor composition onto a repair region and
heating to temperatures sufficient to convert the precursor
composition to a substantially pure conductor. According to one
embodiment of the present invention, the repair feature is a ball
grid array (BGA). According to another embodiment, the feature is a
circuit pattern in a low temperature cofire ceramic (LTCC) layer.
In one embodiment, the pattern is not yet sintered while in another
embodiment the pattern is already sintered. In one embodiment, a
laser can be used to heat the repair section. The repair can be
carried out prior to processing of the part. The repair can be made
to a metallic conductor or other electronic feature. The repaired
portion can have been formed by screen-printing or photopatterning
of a particle-containing composition. In one embodiment, laser
trimming is used to further define the repair region after ink-jet
deposition.
[0250] According to one embodiment of the present invention, the
repaired feature preferably has a minimum feature size that is not
greater than about 250 .mu.m and more preferably is not greater
than about 100 .mu.m. According to one embodiment, the repaired
feature has a minimum feature size not greater than about 10 .mu.m.
The repair can be made to features derived from various processes
such as chemical vapor deposition, evaporation, sputtering or other
thin film techniques.
[0251] In another embodiment, features larger than approximately
100 .mu.m are first prepared by screen-printing. Features not
greater than about 100 .mu.m are then deposited by a direct
deposition method using a low viscosity precursor composition.
[0252] In yet another embodiment, a polyimide surface is first
modified to promote adhesion of the low viscosity precursor
composition. The precursor composition is deposited, and then is
dried and converted at a temperature of not greater than
300.degree. C. After deposition and conversion, the feature can
then optionally be laser sintered.
[0253] Preferably, the conductive feature has a resistivity that is
not greater than 10 times the bulk resistivity of the metal,
preferably not greater than 6 times the bulk resistivity, more
preferably not greater than 4 times the bulk resistivity and most
preferably not greater than 2 times the bulk resistivity of the
metal.
[0254] According to the present invention, the low viscosity
precursor composition can be deposited, dried, and reacted with a
total reaction time of not greater than about 100 seconds, more
preferably not greater than about 10 seconds and even more
preferably not greater than about 1 second.
[0255] In yet another embodiment, the low viscosity precursor
composition can be deposited, dried, and reacted, wherein the total
time for deposition, drying and reaction is not greater than about
1 minute, more preferably not greater than about 10 seconds and
even more preferably not greater than about 1 second.
[0256] The product compositions derived from the printed low
viscosity precursor compositions of the present invention can
include a variety of material combinations.
[0257] In one embodiment, a conductive feature comprises silver and
copper. In a preferred embodiment, the feature includes discrete
regions of copper metal that are derived from particles, preferably
particles having an average size of not greater than 1 .mu.m.
According to this embodiment, the copper metal is dispersed in a
matrix of silver that is derived from a molecular metal precursor.
The silver and copper are not substantially interdiffused as when
derived from high fire compositions. In one embodiment, the feature
includes about 85 vol. % copper and 15 vol. % silver. In another
embodiment, the silver derived from the precursor also includes an
amount of copper, palladium, platinum or some other metal that
provides resistance to electromigration or powder
solderability.
[0258] In another embodiment, the conductive feature includes
silver and palladium. In a preferred embodiment, the feature
includes regions of substantially pure dispersed silver in a matrix
of silver-palladium that provides resistance to solder leaching. In
a particularly preferred embodiment, the silver-palladium is
derived from precursors and the overall feature includes not
greater than about 2 vol. % palladium, more preferably not greater
than about 1 vol. % palladium. In another embodiment, the palladium
is replaced with another metal derived from a precursor to provide
a silver matrix that includes an amount of copper, platinum or some
other metal that provides resistance to electromigration or solder
leaching.
[0259] In yet another embodiment, the feature comprises silver or
copper derived from a precursor and an insulating phase. The
insulating phase is preferably a glass or metal oxide. Preferred
glasses are aluminum borosilicates, lead borosilicates and the
like. Preferred metal oxides are silica, titania, alumina, and
other simple and complex metal oxides. The insulating phase can be
derived from particles or precursors. This embodiment is
particularly useful for the production of low ohm resistors.
[0260] In an embodiment preferred for transparent and conducting
materials, zinc oxide, antimony tin oxide (ATO), indium tin oxide
(ITO) and mixtures of these are contained in a feature. In a
preferred embodiment, the feature comprises a small amount of metal
to improve the conductivity while only slightly degrading
conductivity by choosing processing conditions to provide metal
regions not greater than about 100 nanometers in size.
[0261] The conductor composition can also be a composite of
dissimilar materials. The composite can include metal-metal oxide,
metal-polymer, metal-glass, carbon-metal, and other combinations.
The conductor composition can also include solder-like
compositions. The composition can include silver, lead, tin,
indium, copper, and other elements.
[0262] In accordance with the foregoing direct-write processes, the
present invention enables the formation of features for devices and
components having a small minimum feature size. For example, the
method of the present invention can be used to fabricate features
having a minimum feature size (the smallest feature dimension in
the x-y axis) of not greater than about 100 .mu.m, more preferably
not greater than about 75 .mu.m, even more preferably not greater
than 50 .mu.m and even more preferably not greater than 25 .mu.m.
The minimum feature size can even be not greater than about 10
.mu.m and even not greater than about 5 .mu.m. These feature sizes
can be provided using ink-jet printing and other printing
approaches that provide droplets or discrete units of low viscosity
composition to a surface. The small feature sizes can
advantageously be applied to various components and devices, as is
discussed below.
Conductor Properties and Structure
[0263] The conductors formed by the present invention have
combinations of various features that have not been attained using
other low viscosity precursors. The conductive features preferably
have a high purity, a high electrical conductivity and high
electromigration resistance. High conductivity can be provided
through the low viscosity compositions comprising precursors to
silver, platinum, palladium, gold, nickel or copper.
[0264] The present invention is particularly useful for fabrication
of conductors with resistivities that are not greater than 20 times
the resistivity of the substantially pure bulk conductor, more
preferably not greater than 10 times the substantially pure bulk
conductor, even more preferably not greater than 6 times and most
preferably not greater than 2 times that of the substantially pure
bulk conductor.
[0265] However, it will be appreciated that the properties of the
conductive feature can vary depending upon the particular
application. For example, it may be desirable for some applications
to process the feature at a very low temperature where low
resistivity is not a major factor. According to one embodiment, a
precursor composition can be deposited and converted at a
temperature of not greater than 125.degree. C., where the
resistivity of the feature is not greater than about 200 times the
resistivity of the pure bulk conductor, more preferably not greater
than about 100 times the resistivity of the bulk conductor and even
more preferably not greater than about 80 times the resistivity of
the bulk conductor.
[0266] After heating, the compositions of the present invention
will yield solids with specific bulk resistivity values. As a
background, bulk resistivity values of a number of solids are
provided in Table 8. TABLE-US-00008 TABLE 8 Bulk Resistivity of
Various Materials Bulk Resistvity Material (micro-.OMEGA. cm)
silver (Ag - thick film material fired at 850.degree. C.) 1.59
copper (Cu) 1.68 gold (Au) 2.24 aluminum (Al) 2.64 Ferro CN33-246
(Ag + low melting glass, 2.7-3.2 fired at 150.degree. C.) SMP Ag
flake + precursor formulation, 250.degree. C. 4.5 molybdenum (Mo)
5.2 Tungsten (W) 5.65 zinc (Zn) 5.92 nickel (Ni) 6.84 iron (Fe)
9.71 palladium (Pd) 10.54 tin (Sn) 11 solder (Pb--Sn; 50:50) 15
Lead 20.64 Titanium nitrate (TiN transparent conductor) 20 duPont
Polymer Thick Film 5029 (state of the art Ag filled polymer,
150.degree. C.) 18-50 duPont Polymer Thick Film (Cu filled polymer)
75-300 ITO indium tin oxide (In.sub.2O.sub.3:Sn) 100 zinc oxide
(ZnO doped-undoped) 120-450 carbon (C-graphite) 1375 KIA SCC-10
(doped silver phosphate glass, 3000 330.degree. C. soft point)
ruthenium oxide RuO.sub.2 type conductive oxides 5000-10,000 Bayer
conductive polymer Baytron-P 1,000,000
[0267] According to one embodiment of the present invention, a low
viscosity precursor composition includes up to about 20 volume
percent micron-size metal particles and from about 10 to about 15
weight percent of a molecular metal precursor with the balance
being vehicle and other additives. After heating at between
200.degree. C. and 300.degree. C., the feature can have a bulk
conductivity in the range from 1 to 5 times the bulk metal
conductivity.
[0268] According to another embodiment of the present invention, a
low viscosity precursor composition includes up to about 20 volume
percent micron-size metal particles, with the balance being a
vehicle containing a precursor to a conductive polymer. After
heating at between 100.degree. C. and 200.degree. C., the feature
can have a bulk conductivity in the range from 5 to 50 times the
bulk conductivity of the metal phase.
[0269] According to another embodiment of the present invention, a
transparent conductor ink formulation includes about 15 vol. %
micron-size particles selected from the group of ITO, ATO, ZnO,
SnO.sub.2, and 5 vol. % Ag nanoparticles, and between 0 and 20
weight percent precursor to Ag with the balance being solvents,
vehicle and other additives. After firing at between 250.degree. C.
and 400.degree. C. the feature can have a bulk conductivity in the
range from 500 to 1000 micro-ohm-centimeter.
[0270] A transparent conductor ink formulation includes up to about
30 vol. % micron-size particles selected from the group of ITO,
ATO, ZnO, SnO.sub.2, and between 5 and 40 weight percent precursor
to Ag, with the balance being solvents, vehicle and other
additives. After firing at between 150.degree. C. and 300.degree.
C., the feature can have a conductivity in the range from 500 to
1000 micro-ohm-centimeter.
[0271] According to another embodiment of the present invention, a
transparent conductor ink formulation includes up to about 15 vol.
% micron-size particles selected from the group of ITO, ATO, ZnO,
SnO.sub.2, and up to 10 vol. % conductive glass particles such as
silver phosphate glass, and between 0 and 20 weight percent
precursor to Ag, with the balance being solvents, vehicle and other
additives. After firing at between 300.degree. C. and 500.degree.
C., the feature can have a bulk conductivity in the range from 300
to 800 micro-ohm-centimeter.
[0272] According to another embodiment of the present invention, a
low cost conductor precursor composition includes between 5 and 20
vol. % micron-size particles selected from the group of amorphous
carbon, carbon graphite, iron, nickel, tungsten, molybdenum, and
between 0 and 5 vol. % nanoparticles selected from the group of Ag,
carbon, intrinsically conductive polymer, Fe, Cu, Mo, W, and
between 0 and 20 weight percent precursor to a metal such as Ag,
with the balance being solvents, vehicle and other additives. After
heating at between 250.degree. C. and 400.degree. C., the feature
can have a bulk conductivity in the range from 100 to 4000
micro-ohm-centimeter.
[0273] According to another embodiment of the present invention, a
low cost conductor precursor composition includes between 5 and 20
vol % micron-size particles selected from the group of amorphous
carbon, graphite, iron, nickel, tungsten, molybdenum, and between
20 and 50 weight percent precursor to an intrinsically conductive
polymer, with the balance being solvents, vehicle and other
additives. After heating at between 100.degree. C. and 200.degree.
C., the feature can have a bulk conductivity in the range from
5,000 to 15,000 micro-ohm-centimeter.
[0274] The silver-palladium compositions of the present invention
can also provide resistance to solder leaching. In one embodiment,
the compositions provide resistance to 3 dips in standard 60/40
lead-tin solder at its melting point.
[0275] The compositions and methods of the present invention
advantageously allow the fabrication of various unique
structures.
[0276] In one embodiment, the average thickness of the deposited
feature is greater than about 0.01 .mu.m, more preferably is
greater than about 0.05 .mu.m, even more preferably is greater than
about 0.1 .mu.m and even more preferably is greater than about 0.5
.mu.m. The thickness can even be greater than about 1 .mu.m, such
as greater than about 5 .mu.m. These thicknesses can be obtained by
ink-jet deposition or deposition of discrete units of material by
depositing more than a single layer. A single layer can be
deposited and dried, followed by repetitions of this cycle.
[0277] Vias can also be filled with the low viscosity precursor
compositions of the present invention. The via can be filled, dried
to remove the volume of the solvent, filled further and two or more
cycles of this type can be used to fill the via. The via can then
be processed to convert the material to its final composition.
After conversion, it is also possible to add more precursor
composition, dry and then convert the material to product to
replace the volume of material lost upon conversion to the final
product.
[0278] The compositions and methods of the present invention can
also be used to form dots, squares and other isolated regions of
material. The regions can have a minimum feature size of not
greater than 250 .mu.m, such as not greater than 100 .mu.m, and
even not greater than 50 .mu.m, such as not greater than 25 .mu.m
and even not greater than 10 .mu.m. These features can be deposited
by ink-jet printing of a single droplet or multiple droplets at the
same location with or without drying in between deposition of
droplets or periods of multiple droplet deposition. In one
embodiment, the surface tension of the precursor composition on the
substrate material is chosen to provide poor wetting of the surface
so that the composition contracts onto itself after printing. This
provides a method for producing deposits with sizes equal to or
smaller than the droplet diameter.
[0279] The compositions and methods of the present invention can
also be used to form lines. In one embodiment, the lines can
advantageously have an average width of not greater than 250 .mu.m,
such as not greater than 100 .mu.m, and even not greater than 50
.mu.m.
[0280] The compositions and methods of the present invention
produce features that have good adhesion to the substrates on which
they are formed. For example, the conductive features will adhere
to the substrate with a peel strength of at least 10 newtons/cm.
Adhesion can be measured using the scotch-tape test, wherein
scotch-tape is applied to the feature and is pulled perpendicular
to the plane of the trace and the substrate. This applies a force
of about 10 N/cm. A passing measure is when little or no residue
from the feature remains on the tape.
Applications
[0281] The low viscosity precursor compositions and methods of the
present invention can advantageously be used in a variety of
applications. The following is a non-limiting description of the
types of devices and components to which the methods and
compositions of the present invention are applicable.
[0282] The compositions and methods of the present invention can be
used to fabricate transparent antennas for RF (radio frequency)
tags and smart cards. This is enabled by compositions comprising a
transparent conductive metal oxide such as ITO. In another
embodiment, the compositions can include some metal to enhance
conductivity. In one embodiment, the antenna comprises a material
with a sheet resistivity of from about 10 to 100,000 ohms/square.
In another embodiment, the antenna comprises a conductor with a
resistivity that is not greater than three times the resistivity of
substantially pure silver. High conductivity traces are required
for inductively coupled antennas whereas conductive metal oxides
can be used for electrostatic (capacitively coupled) antennas.
[0283] The compositions can also serve as solder replacements. Such
compositions can include silver, lead or tin.
[0284] The compositions and methods can be utilized to provide
connection between chips and other components in smart cards and RF
tags.
[0285] In one embodiment, the surface to be printed onto is not
planar and a non-contact printing approach is used. The non-contact
printing approach can be ink-jet printing or another technique
providing deposition of discrete units of fluid onto the surface.
Examples of surfaces that are non-planar include in windshields,
electronic components, electronic packaging and visors.
[0286] The compositions and methods provide the ability to print
disposable electronics such as for games included in magazines. The
compositions can advantageously be deposited and reacted on
cellulose-based materials such as paper or cardboard. The
cellulose-based material can be coated if necessary to prevent
bleeding of the precursor composition into the substrate. For
example, the cellulose-based material could be coated with a UV
curable polymer.
[0287] The compositions and methods can be used to form under-bump
metallization, redistribution patterns and basic circuit
components.
[0288] The compositions and processes of the present invention can
also be used to fabricate microelectronic components such as
multichip modules, particularly for prototype designs or low-volume
production
[0289] Another technology where the direct-write deposition of
electronic features according to the present invention provides
significant advantages is for flat panel displays, such as plasma
display panels. Ink-jet deposition of electronic powders is a
particularly useful method for forming the electrodes for a plasma
display panel. The electronic powders and deposition method
according to the present invention can advantageously be used to
form the electrodes, as well as the bus lines and barrier ribs, for
the plasma display panel. Typically, a metal paste is printed onto
a glass substrate and is fired in air at from about 450.degree. C.
to 600.degree. C. Direct-write deposition of low viscosity
precursor compositions offers many advantages over paste techniques
including faster production time and the flexibility to
produce-prototypes and low-volume production applications. The
deposited features will have high resolution and dimensional
stability, and will have a high density.
[0290] Another type of flat panel display is a field emission
display (FED). The deposition method of the present invention can
advantageously be used to deposit the microtip emitters of such a
display. More specifically, a direct-write deposition process such
as an ink-jet deposition process can be used to accurately and
uniformly create the microtip emitters on the backside of the
display panel.
[0291] Another type of electronic powder to which the present
invention is applicable is transparent electrode powder,
particularly indium-tin oxide, referred to as ITO. Such materials
are used as electrodes in display applications, particularly for
thin-film electroluminescent (TFEL) displays. The electrode
patterns of ITO can advantageously be deposited using the
direct-write method of the present invention including an ink-jet,
particularly to form discrete patterns of indicia, or the like.
[0292] The present invention is also applicable to inductor-based
devices including transformers, power converters and phase
shifters. Examples of such devices are illustrated in U.S. Pat. No.
5,312,674 by Haertling et al.; U.S. Pat. No. 5,604,673 by Washburn
et al.; and U.S. Pat. No. 5,828,271 by Stitzer. Each of the
foregoing U.S. Patents is incorporated herein by reference in their
entirety. In such devices, the inductor is commonly formed as a
spiral coil of an electrically conductive trace, typically using a
thick-film paste method. To provide the most advantageous
properties, the metalized layer, which is typically silver, must
have a fine pitch (line spacing). The output current can be greatly
increased by decreasing the line width and decreasing the distance
between lines. The direct-write process of the present invention is
particularly advantageous for forming such devices, particularly
when used in a low-temperature cofired ceramic package (LTCC).
[0293] The present invention can also be used to fabricate antennas
such as antennas used for cellular telephones. The design of
antennas typically involves many trial and error iterations to
arrive at the optimum design. The direct-write process of the
present invention advantageously permits the formation of antenna
prototypes in a rapid and efficient manner, thereby reducing a
product development time. Examples of microstrip antennas are
illustrated in U.S. Pat. No. 5,121,127 by Toriyama; U.S. Pat. No.
5,444,453 by Lalezari; U.S. Pat. No. 5,767,810 by Hagiwara et al.;
and U.S. Pat. No. 5,781,158 by Ko et al. Each of these U.S. patents
is incorporated herein by reference in their entirety. The
methodology of the present invention can be used to form the
conductors of an antenna assembly.
[0294] The precursor compositions and methods of the present
invention can also be used to apply underfill materials that are
used below electronic chips to attach the chips to surfaces and
other components. Hollow particles are particularly advantageous
because they are substantially neutrally buoyant. This allows the
particles to be used in underfill applications without settling of
the particles in the liquid between the chip and surface below.
Further, the spherical morphology of the particles allows them to
flow better through the small gap. This significantly reduces the
stratification that is often observed with dense particles.
Further, very high thermal conductivity is not required and
therefore silica is often used in this application. In other
applications, the material must be thermally conductive but not
electrically conductive. Materials such as boron nitride (BN) can
then be used.
[0295] Additional applications enabled by the low viscosity
precursor compositions and deposition methods of the present
invention include low cost or disposable electronic devices such as
electronic displays, electrochromic, electrophoretic and
light-emitting polymer-based displays. Other applications include
circuits imbedded in a wide variety of devices such as low cost or
disposable light-emitting diodes, solar cells, portable computers,
pagers, cell phones and a wide variety of internet compatible
devices such as personal organizers and web-enabled cellular
phones. The present invention also enables a wide variety of
security and authentication applications. For example, with the
advent and growth of desktop publishing and color-photocopiers, the
opportunities for document and coupon fraud have increased
dramatically. The present invention has utility in a variety of
areas including coupon redemption, inventory security, currency
security, compact disk security and driver's license and passport
security. The present invention can also be utilized as an
effective alternative to magnetic strips. Presently, magnetic
strips include identification numbers such as credit card numbers
that are programmed at the manufacturer. These strips are prone to
failure and are subject to fraud because they are easily copied or
modified. To overcome these shortcomings, circuits can be printed
on the substrate and encoded with specific consumer information.
Thus, the present invention can be used to improve the security of
credit cards, ATM cards and any other tracking card, which uses
magnetic strips as a security measure.
[0296] The compositions and methods of the present invention can
also produce conductive patterns that can be used in flat panel
displays. The conductive materials used for electrodes in display
devices have traditionally been manufactured by commercial
deposition processes such as etching, evaporation, and sputtering
onto a substrate. In electronic displays it is often necessary to
utilize a transparent electrode to ensure that the display images
can be viewed. Indium tin oxide (ITO), deposited by means of
vacuum-deposition or a sputtering process, has found widespread
acceptance for this application. U.S. Pat. No. 5,421,926 by
Yukinobu et al. discloses a process for printing ITO inks. For rear
electrodes (i.e., the electrodes other than those through which the
display is viewed) it is often not necessary to utilize transparent
conductors. Rear electrodes can therefore be formed from
conventional materials and by conventional processes. Again, the
rear electrodes have traditionally been formed using costly
sputtering or vacuum deposition methods. The compositions according
to the present invention allow the direct deposition of metal
electrodes onto low temperature substrates such as plastics. For
example, a silver precursor composition can be ink-jet printed and
heated at 150.degree. C. to form 150 .mu.m by 150 .mu.m square
electrodes with excellent adhesion and sheet resistivity values of
less than 1 ohms per square.
[0297] In one embodiment, the precursor compositions are used to
interconnect electrical elements on a substrate, such as non-linear
elements. Non-linear elements are defined herein as electronic
devices that exhibit nonlinear responses in relationship to a
stimulus. For example a diode is known to exhibit a nonlinear
output-current/input-voltage response. An electroluminescent pixel
is known to exhibit a non-linear light-output/applied-voltage
response. Nonlinear devices also include but are not limited to
transistors such as TFTs and OFETs, emissive pixels such as
electroluminescent pixels, plasma display pixels, field emission
display (FED) pixels and organic light emitting device (OLED)
pixels, non emissive pixels such as reflective pixels including
electrochromic material, rotatable microencapsulated microspheres,
liquid crystals, photovoltaic elements, and a wide range of sensors
such as humidity sensors.
[0298] Nonlinear elements, which facilitate matrix addressing, are
an essential part of many display systems. For a display of
M.times.N pixels, it is desirable to use a multiplexed, addressing
scheme whereby M column electrodes and N row electrodes are
patterned orthogonally with respect to each other. Such a scheme
requires only M+N address lines (as opposed to M.times.N lines for
a direct-address system requiring a separate address line for each
pixel). The use of matrix addressing results in significant savings
in terms of power consumption and cost of manufacture. As a
practical matter, the feasibility of using matrix addressing
usually hinges upon the presence of a nonlinearity in an associated
device. The nonlinearity eliminates crosstalk between electrodes
and provides a thresholding function. A traditional way of
introducing nonlinearity into displays has been to use a backplane
having devices that exhibit a nonlinear current/voltage
relationship. Examples of such devices include thin-film
transistors (TFT) and metal-insulator-metal (MIM) diodes. While
these devices achieve the desired result, they involve thin-film
processes, which suffer from high production costs as well as
relatively poor manufacturing yields.
[0299] The present invention allows the direct printing of the
conductive components of nonlinear devices including the source the
drain and the gate. These nonlinear devices may include directly
printed organic materials such as organic field effect transistors
(OFET) or organic thin film transistors (OTFT), directly printed
inorganic materials and hybrid organic/inorganic devices such as a
polymer based field effect transistor with an inorganic gate
dielectric. Direct printing of these conductive materials will
enable low cost manufacturing of large area flat displays.
[0300] The compositions and methods of the present invention
produce conductive patterns that can be used in flat panel displays
to form the address lines or data lines. The lines may be made from
transparent conducting polymers, transparent conductors such as
ITO, metals or other suitable conductors. The present invention
provides ways to form address and data lines using deposition tools
such as an ink-jet device. The precursor compositions of the
present invention allow printing on large area flexible substrates
such as plastic substrates and paper substrates, which are
particularly useful for large area flexible displays. Address lines
may additionally be insulated with an appropriate insulator such as
a non-conducting polymer or other suitable insulator.
Alternatively, an appropriate insulator may be formed so that there
is electrical isolation between row conducting lines, between row
and column address lines, between column address lines or for other
purposes. These, lines can be printed with a thickness of about one
.mu.m and a line width of 100 .mu.m by ink-jet printing the
precursor composition. These data lines can be printed continuously
on large substrates with an uninterrupted length of several meters.
Surface modification can be employed, as is discussed above, td
confine the composition and to enable printing of lines as narrow
as 10 .mu.m. The deposited lines can be heated to 200.degree. C. to
form metal lines with a bulk conductivity that is not less than 10
percent of the conductivity of the equivalent pure metal.
[0301] Flat panel displays may incorporate emissive or reflective
pixels. Some examples of emissive pixels include electroluminescent
pixels, photoluminescent pixels such as plasma display pixels,
field emission display (FED) pixels and organic light emitting
device (OLED) pixels. Reflective pixels include contrast media that
can be altered using an electric field. Contrast media may be
electrochromic material, rotatable microencapsulated microspheres,
polymer dispersed liquid crystals (PDLCs), polymer stabilized
liquid crystals, surface stabilized liquid crystals, smectic liquid
crystals, ferroelectric material, or other contrast media well
known in art. Many of these contrast media utilize particle-based
non-emissive systems. Examples of particle-based non-emissive
systems include encapsulated electrophoretic displays (in which
particles migrate within a dielectric fluid under the influence of
an electric field); electrically or magnetically driven
rotating-ball displays as disclosed in U.S. Pat. Nos. 5,604,027 and
4,419,383, which are incorporated herein by reference in their
entirety; and encapsulated displays based on micromagnetic or
electrostatic particles as disclosed in U.S. Pat. Nos. 4,211,668,
5,057,363 and 3,683,382, which are incorporated herein by reference
in their entirety. A preferred particle non-emissive system is
based on discrete, microencapsulated electrophoretic elements,
examples of which are disclosed in U.S. Pat. No. 5,930,026 by
Jacobson et al. which is incorporated herein by reference in its
entirety.
[0302] In one embodiment, the present invention relates to directly
printing conductive features, such as electrical interconnects and
electrodes for addressable, reusable, paper-like visual displays.
Examples of paper-like visual displays include "gyricon" (or
twisting particle) displays and forms of electronic paper such as
particulate electrophoretic displays (available from E-ink
Corporation, Cambridge, Mass.). A gyricon display is an addressable
display made up of optically anisotropic particles, with each
particle being selectively rotatable to present a desired face to
an observer. For example, a gyricon display can incorporate "balls"
where each ball has two distinct hemispheres, one black and the
other white. Each hemisphere has a distinct electrical
characteristic (e.g., zeta potential with respect to a dielectric
fluid) so that the ball is electrically as well as optically
anisotropic. The balls are electrically dipolar in the presence of
a dielectric fluid and are subject to rotation. A ball can be
selectively rotated within its respective fluid-filled cavity by
application of an electric field, so as to present either its black
or white hemisphere to an observer viewing the surface of the
sheet.
[0303] In another embodiment, the present invention relates to
electrical interconnects and electrodes for organic light emitting
displays (OLEDs). Organic light emitting displays are emissive
displays consisting of a transparent substrate coated with a
transparent conducting material (e.g., ITO), one or more organic
layers and a cathode made by evaporating or sputtering a metal of
low work function characteristics (e.g., calcium or magnesium). The
organic layer materials are chosen so as to provide charge
injection and transport from both electrodes into the
electroluminescent organic layer (EL), where the charges recombine
to emit light. There may be one or more organic hole transport
layers (HTL) between the transparent conducting material and the
EL, as well as one or more electron injection and transporting
layers between the cathode and the EL. The precursor compositions
according to the present invention allow the direct deposition of
metal electrodes onto low temperature substrates such as flexible
large area plastic substrates that are particularly preferred for
OLEDs. For example, a metal precursor composition can be ink-jet
printed and heated at 150.degree. C. to form a 150 .mu.m by 150
.mu.m square electrode with excellent adhesion and a sheet
resistivity value of less than 1 ohm per square. The compositions
and printing methods of the present invention also enable printing
of row and column address lines for OLEDs. These lines can be
printed with a thickness of about one .mu.m and a line width of 100
.mu.m using ink-jet printing. These data lines can be printed
continuously on large substrates with an uninterrupted length of
several meters. Surface modification can be employed, as is
discussed above, to confine the precursor composition and to enable
printing of such lines as narrow as 10 .mu.m. The printed ink lines
can be heated to 150.degree. C. and form metal lines with a bulk
conductivity that is no less than 5 percent of the conductivity of
the equivalent pure metal.
[0304] In one embodiment, the present invention relates to
electrical interconnects and electrodes for liquid crystal displays
(LCDs), including passive-matrix and active-matrix. Particular
examples of LCDs include twisted nematic (TN), supertwisted nematic
(STN), double supertwisted nematic (DSTN), retardation film
supertwisted nematic (RFSTN), ferroelectric (FLCD), guest-host
(GHLCD), polymer-dispersed (PD), polymer network (PN).
[0305] Thin film transistors (TFTS) are well known in the art, and
are of considerable commercial importance. Amorphous silicon-based
thin film transistors are used in active matrix liquid crystal
displays. One advantage of thin film transistors is that they are
inexpensive to make, both in terms of the materials and the
techniques used to make them. In addition to making the individual
TFTs as inexpensively as possible, it is also desirable to
inexpensively make the integrated circuit devices that utilize
TFTs. Accordingly, inexpensive methods for fabricating integrated
circuits with TFTs, such as those of the present invention, are an
enabling technology for printed logic.
[0306] For many applications, inorganic interconnects are not
adequately conductive to achieve the desired switching speeds of an
integrated circuit due to high RC time constants. Printed pure
metals, as enabled by the precursor compositions of the present
invention, achieve the required performance. A metal interconnect
printed by using a silver precursor composition as disclosed in the
present invention will result in a reduction of the resistance (R)
and an associated reduction in the time constant (RC) by a factor
of 100,000, more preferably by 1,000,000, as compared to current
conductive polymer interconnect material used to connect polymer
transistors.
[0307] Field-effect transistors (FETs), with organic semiconductors
as active materials, are the key switching components in
contemplated organic control, memory, or logic circuits, also
referred to as plastic-based circuits. An expected advantage of
such plastic electronics is the ability to fabricate them more
easily than traditional silicon-based devices. Plastic electronics
thus provide a cost advantage in cases where it is not necessary to
attain the performance level and device density provided by
silicon-based devices. For example, organic semiconductors are
expected to be much more readily printable than vapor-deposited
inorganics, and are also expected to be less sensitive to air than
recently proposed solution-deposited inorganic semiconductor
materials. For these reasons, there have been significant efforts
expended in the area of organic semiconductor materials and
devices.
[0308] Organic thin film transistors (TFTs) are expected to become
key components in the plastic circuitry used in display drivers of
portable computers and pagers, and memory elements of transaction
cards and identification tags. A typical organic TFT circuit
contains a source electrode, a drain electrode, a gate electrode, a
gate dielectric, an interlayer dielectric, electrical
interconnects, a substrate, and semiconductor material. The
precursor compositions of the present invention can be used to
deposit all the components of this circuit, with the exception of
the semiconductor material.
[0309] One of the most significant factors in bringing organic TFT
circuits into commercial use is the ability to deposit all the
components on a substrate quickly, easily and inexpensively as
compared with silicon technology (i.e., by reel-to-reel printing).
The precursor compositions of the present invention enable the use
of low cost deposition techniques, such as ink-jet printing, for
depositing these components.
[0310] The precursor compositions of the present invention are
particularly useful for the direct printing of electrical
connectors as well as antennae of smart tags, smart labels, and a
wide range of identification devices such as radio frequency
identification (RFID) tags. In a broad sense, the conductive
precursor compositions can be utilized for electrical connection of
semiconductor radio frequency transceiver devices to antenna
structures and particularly to radio frequency identification
device assemblies. A radio frequency identification device ("RFID")
by definition is an automatic identification and data capture
system comprising readers and tags. Data is transferred using
electric fields or modulated inductive or radiating electromagnetic
carriers. RFID devices are becoming more prevalent in such
configurations as, for example, smart cards, smart labels, security
badges, and livestock tags.
[0311] The precursor compositions of the present invention also
enable the low cost, high volume, highly customizable production of
electronic labels. Such labels can be formed in various sizes and
shapes for collecting, processing, displaying and/or transmitting
information related to an item in human or machine readable form.
The precursor compositions of the present invention can be used to
print the conductive features required to form the logic circuits,
electronic interconnections, antennae, and display features in
electronic labels. The electronic labels can be an integral part of
a larger printed item such as a lottery ticket structure with
circuit elements disclosed in a pattern as disclosed in U.S. Pat.
No. 5,599,046.
[0312] In another embodiment of the present invention, the
conductive patterns made in accordance with the present invention
can be used as electronic circuits for making photovoltaic panels.
Currently, conventional screen-printing is used in mass scale
production of solar cells. Typically, the top contact pattern of a
solar cell consists of a set of parallel narrow finger lines and
wide collector lines deposited essentially at a right angle to the
finger lines on a semiconductor substrate or wafer. Such front
contact formation of crystalline solar cells is performed with
standard screen-printing techniques. Direct printing of these
contacts with the precursor compositions of the present invention
provides the advantages of production simplicity, automation, and
low production cost.
[0313] Low series resistance and low metal coverage (low front
surface shadowing) are basic requirements for the front surface
metallization in solar cells. Minimum metallization widths of 100
to 150 .mu.m are obtained using conventional screen-printing. This
causes a relatively high shading of the front solar cell surface.
In order to decrease the shading, a large distance between the
contact lines, i.e., 2 to 3 mm is required. On the other hand, this
implies the use of a highly doped, conductive emitter layer.
However, the heavy emitter doping induces a poor response to short
wavelength light. Narrower conductive lines can be printed using
the precursor composition and printing methods of the present
invention. The conductive precursor compositions of the present
invention enable direct printing of finer features down to 20
.mu.m. The precursor compositions of the present invention further
enable the printing of pure metals with resistivity values of the
printed features as low as 2 times bulk resistivity after
processing at temperatures as low as 200.degree. C.
[0314] The low processing and direct-write deposition capabilities
according to the present invention are particularly enabling for
large area solar cell manufacturing on organic and flexible
substrates. This is particularly useful in manufacturing novel
solar cell technologies based on organic photovoltaic materials
such as organic semiconductors and dye sensitized solar cell
technology as disclosed in U.S. Pat. No. 5,463,057 by Graetzel et
al. The precursor compositions according to the present invention
can be directly printed and heated to yield a bulk conductivity
that is no less than 10 percent of the conductivity of the
equivalent pure metal, and achieved by heating the printed features
at temperatures below 200.degree. C. on polymer substrates such as
plexiglass (PMMA).
[0315] Another embodiment of the present invention enables the
production of an electronic circuit for making printed wiring board
(PWBs) and printed circuit boards (PCBs). In conventional
subtractive processes used to make printed-wiring boards, wiring
patterns are formed by preparing pattern films. The pattern films
are prepared by means of a laser plotter in accordance with wiring
pattern data outputted from a CAD (computer-aided design system),
and are etched on copper foil by using a resist ink or a dry film
resist.
[0316] In such conventional processes, it is necessary to first
form a pattern film, and to prepare a printing plate in the case
when a photo-resist ink is used, or to take the steps of
lamination, exposure and development in the case when a dry film
resist is used.
[0317] Such methods can be said to be methods in which the
digitized wiring data are returned to an analog image-forming step.
Screen-printing has a limited work size because of the printing
precision of the printing plate. The dry film process is a
photographic process and, although it provides high precision, it
requires many steps, resulting in a high cost especially for the
manufacture of small lots.
[0318] The precursor composition and printing methods of the
present invention offer solutions to overcome the limitations of
the current PWB formation process. For example, they do not
generate any waste. The printing methods of the present invention
are a single step direct printing process and are compatible with
small-batch and rapid turn around production runs. For example, a
copper precursor composition can be directly printed onto FR4 (a
polymer impregnated fiberglass) to form interconnection circuitry.
These features are formed by heating the printed copper precursor
in an N.sub.2 ambient at 150.degree. C. to form copper lines with a
line width of not greater than 100 .mu.m, a line thickness of not
greater than 5 .mu.m, and a bulk conductivity that is not less than
10 percent of the conductivity of the pure copper metal.
[0319] Patterned electrodes obtained by one embodiment of the
present invention can also be used for screening electromagnetic
radiation or earthing electric charges, in making touch screens,
radio frequency identification tags, electrochromic windows and in
imaging systems, e.g., silver halide photography or
electrophotography. A device such as the electronic book described
in U.S. Pat. No. 6,124,851 can be formed using the compositions of
the present invention.
EXAMPLES
[0320] The following examples illustrate the many advantages of the
low viscosity precursor compositions according to the present
invention. For reference purposes, pure Ag-trifluoroacetate has a
normal decomposition temperature of about 325.degree. C. as
indicated by thermogravimetric analysis. Pure Ag-acetate decomposes
at about 255.degree. C. As used in these examples,
thermogravimetric analysis consisted of heating samples (typically
50 milligrams) in air at a heating rate of 10.degree. C./minute and
observing the weight loss of the sample.
Example 1
Comparative Example
[0321] A silver metal precursor composition containing 50 grams
Ag-trifluoroacetate and 50 grams H.sub.2O was formulated. The
calculated silver content of the precursor composition was 24.4 wt.
% and thermogravimetric analysis showed the mass loss reached 78
wt. % at 340.degree. C. This data corresponds to the
above-described decomposition temperature for pure
Ag-trifluoroacetate, within a reasonable margin for error.
Example 2
Preferred Additive
[0322] A silver precursor composition was formulated containing 44
grams Ag-trifluoroacetate, 22 grams H.sub.2O, 33 grams DEGBE and 1
gram lactic acid. The calculated silver content was 21.5 wt. % and
thermogravimetric analysis showed the mass loss reached 79 wt. % at
215 EC. The addition of DEGBE as a conversion reaction inducing
agent advantageously reduced the conversion temperature by 125 EC
compared to the formulation described in Example 1, a decrease of
about 34 percent compared to pure Ag-trifluoracetate. The lactic
acid functions as a crystallization inhibitor.
Example 3
Comparative Example
[0323] A silver precursor composition was formulated containing 58
grams Ag-trifluoroacetate and 42 grams dimethylformamide. The
calculated silver content was 21.5 wt. % and thermogravimetric
analysis showed a mass loss of 78.5 wt. % at 335 EC, a conversion
temperature similar to the formulation in Example 1. This example
illustrates that a common solvent (dimethylformamide) had no affect
on the conversion temperature of the composition.
Example 4
Preferred Solvent
[0324] A silver precursor composition was formulated containing
64.8 grams Ag-trifluoroacetate, 34 grams DMAc and 1.1 grams of a
styrene allyl alcohol (SAA) copolymer binder. Thermogravimetric
analysis showed that precursor conversion to silver was complete at
275.degree. C. The use of DMAc reduced the conversion temperature
by about 65.degree. C. as compared to Example 1.
Example 5
[0325] A silver precursor composition was formulated containing 51
grams Ag-trifluoroacetate, 16 grams DMAc and 32 grams
alpha-terpineol. The calculated silver content was 25 wt. %.
Thermogravimetric analysis showed a mass loss of 77 wt. % at 205
EC. Compared to the composition described in Example 4, which does
not employ alpha-terpineol as an additive, the conversion
temperature was further reduced by 70.degree. C.
Example 6
[0326] A silver precursor composition was formulated containing
33.5 grams Ag-trifluoroacetate, 11 grams DMAc, 2 grams lactic acid
and 53.5 grams DEGBE. The calculated silver content was 16.3 wt. %.
Thermogravimetric analysis showed a mass loss of 83 wt. % at
205.degree. C. to 215.degree. C. The decomposition temperature is
60.degree. C. to 70.degree. C. lower as compared to the composition
described in Example 4, which does not employ DEGBE as an additive
in addition to DMAc.
Example 7
[0327] A silver precursor composition was formulated containing 49
grams Ag-trifluoroacetate, 16 grams DMAc, 32 grams alpha-terpineol
and 1.2 grams Pd-acetate. Thermogravimetric analysis indicated
complete conversion of the metal organic precursors at 170.degree.
C. This conversion temperature is 35 EC lower as compared to the
composition described in Example 5, which does not employ
Pd-acetate as a further additive.
Example 8
[0328] A silver precursor composition was formulated containing 46
grams Ag-trifluoroacetate, 49 grams DMAc and 2.3 grams Pd-acetate.
Thermogravimetric analysis indicated complete conversion of the
metal organic precursors at 195.degree. C. This conversion
temperature is 80.degree. C. lower compared to the composition
described in Example 4, which does not employ Pd-acetate as a
further additive.
Example 9
[0329] A silver precursor composition was formulated containing 6.8
grams Ag-acetate and 93.1 grams ethanolamine. Thermogravimetric
analysis showed that precursor conversion to silver was complete at
190.degree. C. This conversion temperature is 65.degree. C. lower
than the conversion temperature of pure Ag-acetate.
Example 10
[0330] A silver/palladium precursor composition was formulated
containing 8.2 grams Ag-trifluoroacetate, 18.7 grams
Pd-trifluoroacetate, 70.2 grams DMAc and 2.8 grams lactic acid. The
targeted ratio of Ag/Pd was 40/60 by mass. The calculated Ag/Pd
content of the precursor composition was 10 wt. %.
Thermogravimetric analysis showed a mass loss of 87 wt. % at
190.degree. C. The presence of Pd-trifluoroacetate reduced the
conversion temperature by 80.degree. C. compared to the composition
described in Example 4.
Example 11
[0331] A silver/palladium precursor composition was formulated
containing 5.2 grams Ag-trifluoroacetate, 23.4 grams
Pd-trifluoroacetate, 67.9 grams DMAc and 3.5 grams lactic acid. The
targeted ratio of Ag/Pd was 25/75 by mass and the calculated Ag/Pd
content was 10 wt. %. Thermogravimetric analysis showed a mass loss
of 88 wt. % at 190.degree. C. The presence of Pd-trifluoroacetate
reduced the conversion temperature by 80 EC compared to the
composition described in Example 4.
Example 12
[0332] Silver precursor compositions containing various amounts and
ratios of Ag-neodecanoate, solvents and additives were formulated.
Specific examples are outlined in Table 9. In general, the
differences in decomposition temperature are not as pronounced as
in formulations containing Ag-trifluoroacetate. Typically,
compositions containing Ag-neodecanoate in either DMAc or NMP,
together with DEGBE or ethyleneglycolbutylether and/or
alpha-terpineol as additives resulted in decomposition temperatures
that are between 40.degree. C. and 55.degree. C. lower than pure
Ag-neodecanoate. Xylene had no affect on the decomposition
temperature of the Ag-neodecanoate. TABLE-US-00009 TABLE 9 Examples
Utilizing Ag-Neodecanoate DECOMPOSITION TEMPERATURE RUN SOLVENT
ADDITIVE 1 ADDITIVE 2 (.degree. C.) Ag neodecanoate 265 Ag
neodecanoate Xylene 265 Ag neodecanoate DMAc Methoxyethanol 250 7.6
.times. 10.sup.-2 mole 7.9 .times. 10.sup.-2 mole 1.45 .times.
10.sup.-2 mole Ag neodecanoate Xylene 1-Butanol 250 1.52 .times.
10.sup.-2 mole 3.58 .times. 10.sup.-2 mole 2.67 .times. 10.sup.-2
mole Ag neodecanoate DMAc DEGBE 220-230 7.6 .times. 10.sup.-2 mole
6.43 .times. 10.sup.-2 mole 1.48 .times. 10.sup.-2 mole Ag DMAc
DEGBE Alpha-Terpineol 210-230 neodecanoate 2.66 .times. 10.sup.-2
6.43 .times. 10.sup.-2 mole 2.95 .times. 10.sup.-2 mole 5.19
.times. 10.sup.-2 mole mole Ag neodecanoate DMAc DEGBE EGBE 225
1.14 .times. 10.sup.-2 mole 2.64 .times. 10.sup.-2 mole 3.1 .times.
10.sup.-2 mole 1.69 .times. 10.sup.-2 mole Ag neodecanoate DMAc
EGBE 240 1.14 .times. 10.sup.-2 mole 5.05 .times. 10.sup.-2 mole
2.2 .times. 10.sup.-2 mole Ag neodecanoate THF EGBE 240 1.14
.times. 10.sup.-2 mole 6.1 .times. 10.sup.-2 mole 2.2 .times.
10.sup.-2 mole Ag neodecanoate Xylene DEGBE 230 1.55 .times.
10.sup.-5 mole 2.92 .times. 10.sup.-2 mole 1.79 .times. 10.sup.-2
mole Ag neodecanoate NMP Ethanolamine 220 9.5 .times. 10.sup.-2
mole 6.45 .times. 10.sup.-2 mole 1.8 .times. 10.sup.-2 mole Ag
neodecanoate THF Methoxyethanol 250 1.5 .times. 10.sup.-2 mole 5.27
.times. 10.sup.-2 mole 2.89 .times. 10.sup.-2 mole
Example 13
[0333] A gold containing precursor composition was prepared
containing 5 grams hydrated gold hydroxide, 15 ml acetic acid and 3
ml trifluoroacetic acid. The mixture was heated to 53.degree. C.
for 24 hours until all gold hydroxide had dissolved. The solution
was filtered through a microfilter and a clear golden solution of
the gold precursor was obtained. Thermogravimetric analysis showed
that precursor conversion to gold was complete at 125.degree.
C.
Example 14
[0334] A gold containing precursor composition was prepared
containing 5 grams hydrated gold hydroxide and 18 ml
trifluoroacetic acid. The mixture was heated to 53.degree. C. for 3
hours and subsequently stirred at room temperature for 21 hours
until all gold hydroxide has dissolved. The solution was filtered
through a microfilter and a clear purple solution of the gold
precursor was obtained. Thermogravimetric analysis showed that
precursor conversion to gold started at room temperature and was
complete at a conversion temperature of 90.degree. C.
Example 15
[0335] A silver precursor composition was formulated containing
48.1 grams Ag-trifluoroacetate, 48.1 grams DMAc and 3.8 grams
DEGBE. The precursor composition was deposited on a glass substrate
and fired on a hotplate at 200.degree. C. The resulting film showed
large crystal growth and was not conductive. This was believed to
be due to crystallization of the Ag-trifluoracetate.
Example 16
[0336] A silver precursor composition was formulated containing
48.1 grams Ag-trifluoroacetate, 48.1 grams DMAc and 3.8 grams
lactic acid as a crystallization inhibitor. The composition was
deposited on a glass substrate and fired on a hotplate at
200.degree. C. The resulting film showed reduced crystal growth,
demonstrating the effectiveness of lactic acid as a crystallization
inhibitor.
Example 17
[0337] A silver precursor composition was formulated containing
33.5 grams Ag-trifluoroacetate, 11.2 grams DMAc, 53.6 grams DEGBE
and 1.8 grams lactic acid. The composition was deposited on a
polyimide substrate (KAPTON HN, E.I. duPont deNemours Corp.,
Wilmington, Del.) using an ink-jet device. The resulting film
showed severe spreading and formed areas that no longer resembled
the original pattern. This example illustrates that additional
additives may be necessary to control spreading of the precursor
composition.
Example 18
[0338] A silver precursor composition was formulated containing 372
grams Ag-trifluoroacetate, 26.7 grams DMAc, 0.9 grams lactic acid,
34.5 grams DEGBE and 0.9 grams SAA to control spreading. The
composition was deposited on a polyimide substrate (KAPTON HN)
using an ink-jet device. No spreading was observed. After heating
in an oven at 250.degree. C. the resulting film showed some
shrinkage and had a bulk resistivity of 5.2 times the resistivity
of bulk silver.
Example 19
[0339] A silver precursor composition was formulated including 21.6
grams Ag-trifluoroacetate, 35.1 grams silver nanoparticles, 21.6
grams ethylene glycol and 21.6 grams water. All weight percentages
are relative to the weight of the final composition. The
composition was deposited using an ink-jet device and the deposited
precursor was heated at 220.degree. C. for 10 minutes to form a
conductive trace.
Example 20
[0340] A silver precursor composition was formulated including 27.5
grams Ag-trifluoroacetate, 17.7 grams silver nanoparticles, 9.4
grams DMAc, 43.9 grams DEGBE and 1.5 grams lactic acid. The
composition was deposited using an ink-jet device and the deposited
precursor was heated to 220.degree. C. for 10 minutes to form a
conductive trace having a resistivity of not more than about 10
times the resistivity of pure bulk silver.
[0341] Precursor Solubility
[0342] Solubility of precursor material in a variety of different
solvents was tested. Test solutions were prepared by dissolving the
precursor in the respective solvent. Small amounts of solid
precursor were added incrementally and the solution shaken for 10
to 30 minutes. When the solubility limit was reached by this method
the solution was shaken for 12 hours and re-evaluated. This
procedure was repeated until additional precursor did not dissolve
or precipitation occurred. Solvents tested included water, toluene,
xylene, N-methylpyrrolidinone (NMP), alpha-terpineol,
N,N-dimethylacetamide (DMAc), N-methylacetamide, nitromethane,
diethyleneglycolbutylether (DEGBE), triethyleneglycoldiethylether,
methyl alcohol, ethyl alcohol, isopropyl alcohol, 1-butyl alcohol,
methyl ethyl ketone, acetone, diethylether, tetrahydrofurane,
ethanolamine, 3-amino-1-propanol, pyridine, diethylentriamine,
tetraethylenediamine, 2-amino-butanol, isopropylaminoethanol.
[0343] In general, high solubilities were observed in particular
for fluorinated metal carboxylates, mixed carboxylates as well as
long chain carboxylates. Preferred solvents for these compounds
with regard to solubility are toluene, xylene, N-Methyl
pyrrolidinone, tetrahydrofurane and DMAc. Also, some precursors can
be successfully dissolved in high amounts in water. A particularly
preferred combination consists of silver trifluoroacetate in DMAc
where solutions with up to 78 wt. % precursor loading can be
achieved.
Example 21
[0344] Ratio of Precursor to Reducing Agent
[0345] DMAc based silver precursor compositions were formulated
containing different ratios of Ag-trifluoroacetate and DEGBE. As
illustrated in Table 10 thermogravimetric analysis showed that the
higher the ratio of DEGBE to Ag-trifluoroacetate, the lower the
conversion temperature to silver. A molar ratio of 1.2--slightly
above the stoichiometric ratio of DEGBE to Ag precursor--produces a
conversion temperature to silver of 210.degree. C. The use of DEGBE
reduced the decomposition temperature by about 65.degree. C. as
compared to Example 4, whose data are incorporated in Table 10 for
reference, where no DEGBE was used. Smaller ratios of DEGBE to
Ag-trifluoroacetate had a decreased effect on lowering the
conversion temperature. TABLE-US-00010 TABLE 10 Effect of Ratio of
DEGBE to Ag-trifluoroacetate DEGBE Ag-trifluoroacetate Conversion
temperature (pbw) (pbw) (.degree. C.) 0 40 275 30 100 285 38 59 270
26.5 50 245 38 41 250 45 46 240 53.5 33.5 205-215
Example 22
[0346] Conductivity as Function of Time and Temperature
[0347] A precursor composition was formulated containing 37 grams
Ag-trifluoroacetate, 34.5 grams DEGBE, 26.7 grams DMAc, 0.9 grams
SAA copolymer and 0.9 grams lactic acid and the composition was
applied to glass slides to form thin films. These slides were then
placed into a preheated oven and heated for controlled lengths of
time varying from 1 minute to 60 minutes. The oven was heated to
temperatures ranging from 130 EC to 250 EC. As is illustrated in
Table 11, the precursor composition formed conductive features at
200.degree. C. after 10 minutes in a convection oven. The numbers
listed in Table 11 are the resistivity expressed as a multiple of
the resistivity of bulk silver ("no" indicates a complete lack of
conductivity). The most conductive features were formed and the
most complete conversion occurred at 250.degree. C. TABLE-US-00011
TABLE 11 Resistivity as a Function of Time and Temperature Time
Temperature (mins) (.degree. C.) 1 2 5 7 10 30 60 130 No no No no
no no no 150 No no No no no no no 175 No no No no no no no 200 No
no no no yes yes yes 220 No no no no no yes/no yes 250 No no 850.5
161.8 119.3 5.2 5.8
[0348] The precursor composition does not form highly conductive
features unless exposed to temperatures above 200.degree. C. for a
period of time. The highest conductivities are achieved when the
composition is exposed to 250.degree. C. for 10 minutes or greater.
Thermogravimetric analysis shows that the composition shows
complete conversion at about 220.degree. C. Samples that are fired
below 200.degree. C. tended to form crystalline deposits caused by
evaporation of the solvent before the desired reactions occurred.
The formation of crystals is more of a problem in convection ovens
due to the mass transfer from the films to the air. This transfer
does not occur in a box furnace. The samples heated in the box
furnace tend to stay moist longer and not form crystals. The
solutions formed elemental silver as shown by x-ray diffraction
(XRD) analysis.
[0349] The above indicates that one can achieve similar
conductivities through heating in an oven, or by any other
conventional method by varying either the time or the temperature.
If one wishes to achieve a given conductivity all one has to do is
fire for a short time at an elevated temperature. If it is desired
to fire at lower temperatures one can fire at extended periods of
time at a lower temperature. The resulting materials should not
differ substantially.
[0350] Ink-Jet Deposition of Features
Example 23
[0351] A silver precursor composition was formulated comprising 1.2
grams styrene allyl alcohol, 49.2 grams DMAc, 46.2 grams
Ag-trifluoroacetate, 1.4 grams lactic acid and 2.1 grams
Pd-trifluoroacetate. The composition had a viscosity of 14
centipoise at a shear rate of 66 Hz. The surface tension was 37.4
dynes/cm. The composition was deposited using an ink-jet device and
the deposited precursor was heated to 250.degree. C. to form
substantially pure metal traces that were conductive.
Thermogravimetric analysis indicated that decomposition was
substantially complete at 230.degree. C.
[0352] This example illustrates an ink-jettable composition and the
use of Pd-trifluoroacetate as a reducing agent for the
Ag-trifluoroacetate. This composition has excellent adhesion to
KAPTON-HN, silicon and glass substrates.
Example 24
[0353] A silver precursor composition was formulated comprising 1.8
grams styrene allyl alcohol, 54.2 grams N-Methyl pyrolidone, 40.5
grams Ag-trifluoroacetate, 2.6 grams lactic acid and 0.9 grams
Pd-trifluoroacetate. The composition had a viscosity of 16
centipoise at a shear rate of 66 Hz. The surface tension was 40
dynes/cm. The composition was deposited using an ink-jet device and
the deposited precursor was heated to 250.degree. C. to form
substantially pure metal traces that were conductive.
Thermogravimetric analysis indicates that decomposition is
substantially complete at 225.degree. C.
[0354] This is an example of an ink-jettable composition and of the
use of Pd-trifluoroacetate as a reducing agent for
Ag-trifluoroacetate. This example also indicates the wide range of
solvents usable for this type of application. This composition has
excellent adhesion to KAPTON-HN, silicon and glass.
Example 25
[0355] A silver precursor composition was formulated comprising 1.3
grams styrene allyl alcohol, 46.7 grams DMAc, 42.5 grams
Ag-trifluoroacetate, 2.6 grams lactic acid and 6.9 grams
Pd-trifluoroacetate. This composition had a viscosity of 16.9
centipoise at a shear rate of 66 Hz. The surface tension was 37.8
dynes/cm. The composition was deposited using and ink-jet device
and the deposited precursor was heated to 250.degree. C. to form
substantially pure metal traces that were conductive.
Thermagravimetric analysis indicated that decomposition was
substantially complete at 205.degree. C.
[0356] This is an example of an ink-jettable composition and of the
use of Pd-trifluoroacetate as a reducing agent for
Ag-trifluoroacetate. This composition has excellent adhesion to
KAPTON-HN, silicon, and glass.
Example 26
[0357] A silver precursor and silver nanoparticle precursor
composition was formulated comprising 31.6 grams
Ag-trifluoroacetate, 31.6 grams water, 30.9 grams ethylene glycol
and 5.9 grams silver nanoparticles. The composition had a viscosity
of 10 centipoise at a shear rate of 66 Hz and the surface tension
was 51 dynes/cm. The composition was deposited using an ink-jet
device and heated to 100.degree. C. to produce traces that were
conductive and phase pure silver as measured by XRD. The
composition was also deposited using an ink-jet device and heated
to 200.degree. C. to produce traces that were conductive and were
phase pure silver by XRD. Thermogravimetric analysis indicates that
conversion was complete by about 185 C. This is an example of a
precursor and nanoparticle formulation that can be deposited by an
ink-jet and heated at low temperatures to produce phase pure silver
on low temperature substrates.
Example 27
[0358] A silver precursor composition was formulated comprising 33
grams Ag-trifluoroacetate, 33 grams DMAc, 33 grams diethylene
glycol butyl ether (DEGBE), and 3 grams lactic acid. The
composition had a viscosity of 10 centipoise at a shear rate of 66
Hz and the surface tension was 63 dynes/cm. This composition was
deposited using an ink-jet and heated to 200.degree. C. for 30
minutes. The resulting features were phase pure silver by XRD.
Thermogravimetric analysis indicated complete conversion at
210.degree. C. when heated at 10.degree. C./min. This same
composition, when deposited and heated at 250.degree. C. for 10
minutes, produced traces that were phase pure silver by XRD, and
had a bulk resistivity of not greater than 4 times that of bulk
silver. This composition has excellent adhesion to glass,
KAPTON-HN, silicon nitrite and silicon.
Example 28
[0359] A silver precursor composition was formulated comprising
38.3 grams Ag-Trifluoroacetate, 29.2 grams DMAc, 29.2 grams DEGBE,
and 3.4 grams lactic acid. This composition, when deposited and
heated to 250.degree. C., produced phase pure silver that was
highly conductive. This composition has excellent adhesion to
glass, KAPTON-HN, silicon nitrite and silicon.
Example 29
[0360] A silver nanoparticle composition was formulated comprising
16.6 grams silver nanoparticles, 41.7 grams water and 41.7 grams
ethylene glycol. This composition was deposited using an inkjet
and, when heated to 100.degree. C. on paper and KAPTON-HN, formed
conductive traces that were phase pure silver by XRD. This is an
example of a purely particle based composition that can be
deposited onto low temperature substrates such as mylar, paper and
others.
Example 30
[0361] A silver nanoparticle composition was formulated comprising
46.7 silver nanoparticles, 17.8 grams water, 17.8 grams
Ag-trifluoroacetate and 17.8 grams ethylene glycol. This
composition, when deposited and heated, formed phase pure silver by
XRD that was highly conductive.
Example 31
[0362] A silver nanoparticle composition was formulated comprising
35 grams ethyl alcohol and 65 grams silver nanoparticles. This
composition, when heated on a glass slide at 70.degree. C. for 4
hours, produced traces that were conductive, phase pure silver by
XRD, and had a bulk resistivity of 100 times that of bulk silver.
This composition, when deposited with an ink-jet device, produced
traces that were phase pure silver by XRD. This illustrates an
example of an ultra low temperature silver composition.
[0363] Copper Precursor Compositions
Example 32
[0364] Copper formate xH.sub.2O (x.about.2) was analyzed by
thermogravimetric analysis and was shown to undergo full
decomposition to copper by about 225.degree. C. in each of forming
gas, air and nitrogen. The example in air showed that full
decomposition to copper takes place before copper oxides begin to
form.
Example 33
[0365] Copper formate x6H.sub.2O found to have a solubility in
water of about 6% by weight forming a light blue solution. Droplets
were deposited onto glass slide. Deposits form copper when
processed on hotplate at 200.degree. C. under nitrogen flow gas.
Boiling of solvent causes some splattering and drying of solvent
causes recrystallization of salt and leads to slow decomposition
and formation of disconnected copper deposit. This shows that
additives are necessary to increase solubility of copper and
decrease volatility of solution to allow for good film
formation.
Example 34
[0366] Complexing agents for copper formate were added to aqueous
solutions. Examples of complexing agents used are ammonium
hydroxide, ethanolamine, ethylene diamene, 3-amino-1-propanol,
2-amino-1-butanol, 2-(isopropylamino)ethanol, and triethanolamine.
The addition of these complexing agents increased the solubility of
copper formate and produced a visible color change. The resulting
solutions were made as concentrated as possible and were decomposed
from 160.degree. C. to 200.degree. C. on a hotplate under nitrogen
flow. The higher vapor pressure complexing agents such as ammonium
hydroxide tended to boil and splatter, producing disconnected films
with some "halo" of vapor deposition. The lower vapor pressure
complexing agents such as 3-amino-1-propanol and 2-amino-1-butanol
did not splatter as much and did not vapor deposit copper. All the
complexing agents produced copper films. The best complexing agent
appeared to be 3-amino-1-propanol as it formed the most continuous
copper films that also showed high conductivity.
Example 35
[0367] Example 1 from U.S. Pat. No. 5,378,508 by Castro was
repeated using copper formate and Duomeen OL (Akzo Nobel,
Amersfoort, Netherlands) in a water, acetic acid, and methanol
solution to produce a tacky light blue deposit. Instead of
decomposing this deposit with a laser as is disclosed by Castro,
the material was decomposed on a hotplate at 200.degree. C. under
nitrogen flow. The resulting deposit showed some signs of
decomposition to copper but was very discontinuous and contained
residue from the surfactant.
Example 36
[0368] A composition was formulated with 3 wt. % Cu-formate
xH.sub.2O (x.about.2); 3 wt % nickel formate 2H.sub.2O; 9 wt. %
ammonium hydroxide; and 85 wt. % DI water. When decomposed under
Nitrogen in TGA gave a Cu--Ni alloy by XRD. This shows that
complexing agents and combinations of similar metal precursors work
to produce alloys.
Example 37
[0369] A 50:50 Cu:Ni precursor composition was prepared using
Cu-formate.xH.sub.2O (x.about.2) and Ni-formate.2H.sub.2O complexed
with 3-amino-1-propanol in DI H.sub.2O. The precursor composition
was decomposed under nitrogen cover/flow gas at 350.degree. C. to
produce a metallic looking and conductive deposit. The deposit had
a nickel colored sheen and was very porous. The deposit showed to
be a Cu--Ni alloy by XRD. This shows that an alloy deposit can be
produced by conventional processing.
Example 38
[0370] A copper precursor composition was formulated containing
equal parts by weight of Cu-formate.xH.sub.2O (x.about.2),
3-amino-1-propanol and water. The composition was deposited on a
glass substrate and rapidly heated to 350.degree. C. The
temperature was held at 350.degree. C. for less than 10 seconds,
and then rapidly cooled to room temperature. A Scanning Electron
Microscope (SEM) photomicrograph showed the film to be dense and
x-ray diffraction (XRD) showed that the film contained copper with
small amounts of copper oxide. The film had a resistivity of 40
times the bulk resistivity of pure copper metal. This shows that
with fast ramp rates, copper deposits can be produced in air.
Example 39
[0371] The same precursor composition as in above example was
processed at 300.degree. C. with rapid heating and cooling. The
conductivity of the resulting film was measured at approximately
3.times.10.sup.8 times the bulk copper resistivity while XRD
results were identical to Example 33. This shows that kinetics of
decomposition are critical to achieving a conductive deposit in
air.
Example 40
[0372] A precursor composition was formulated including 13 wt. %
Cu-formate; 16 wt. % 3-amino-1-propanol; 58 wt. % deionized water;
and 20 wt. % ethanol (95%). The precursor composition had a surface
tension of 31 dynes/cm and a viscosity of 5 centipoise at a shear
rate of 132 Hz. The composition was deposited on ink-jet paper
using an ink-jet printer. The precursor composition was rapidly
heated in air and cooled and resulted in conductive traces. This
shows that a modified precursor composition can be deposited using
an ink-jet to produce conductive traces, in air.
Example 41
[0373] A precursor composition including 30 weight percent
Cu-formate-xH.sub.2O (x.about.2), 40 weight percent
3-amino-1-propanol and 30 weight percent water was deposited and
processed on glass, FR-4, and Kapton at 200.degree. C. under a
nitrogen atmosphere. The resulting films when scraped with a razor
blade rolled up, behaving much like a foil. The films were dense
and had an average resistivity of 10 times the resistivity of bulk
copper. This example demonstrates that conductive copper features
can be formed by processing the precursor compositions under an
inert gas, such as nitrogen.
Example 42
[0374] The above precursor composition was deposited with a quill
pen onto FR-4 and glass. Conductive traces could be made when lines
were processed under nitrogen at 200.degree. C. In certain thin
areas solvent evaporation took place before decomposition occurred.
In theses cases recrystallization of the copper formate took place
and the film was discontinuous.
[0375] Large drops of precursor composition were also deposited and
dried. In one instance a deposit was dried at 90.degree. C. for 40
minutes leading to partial drying and some crystallization. Upon
decomposition at 200.degree. C., however, crystals redissolved and
decomposition to copper was complete. When a drop was dried further
until no remaining solvent was apparent and crystals changed from
dark blue to light blue-green before processing, the resulting
deposit took longer (few minutes) to fully convert to copper and
resulted in a porous, discontinuous and mechanically weak
deposit.
[0376] This shows that presence of the complexing agent and solvent
is necessary for proper film formation and conversion to a dense
film. Therefore, precursor composition should not be dried before
processing and decomposition kinetics need to be fairly rapid in
order that decomposition takes place faster than drying.
Example 43
[0377] The precursor composition of Example 42 was deposited on
glass, FR-4, and KAPTON and processed at 180.degree. C. and at
150.degree. C. under nitrogen. The resulting films averaged 47
times the bulk copper resistivity and 390 times the bulk copper
resistivity, respectively.
Example 44
[0378] A composition was formulated with 27 wt. % Cu formate
xH.sub.2O (x .about.2); 27 wt. % DI water; 32 wt. %
3-amino-1-propanol; and 14 wt. % dimethyacetamide. Drops were
decomposed on glass under nitrogen flow at 200.degree. C. on a hot
plate. Bulk resistivity measurements yielded 10.times. bulk copper
on average. The solution was modified with an addition of
tetraamine palladium hydroxide. Samples of both the solution with
and without the palladium precursor addition were decomposed on a
hotplate in air at 200.degree. C. The sample without the addition
did form copper by XRD with indication of copper oxides as well,
but the deposit was dark with residue and did not appear to be
completely decomposed. The sample with the tetraamine palladium
hydroxide addition decomposed in a few seconds to a coppery looking
conductive deposit which showed to have no oxide present in a short
XRD scar and no separate Pd peaks. The ratio of Cu to Pd was about
2% Pd by weight. This shows that an additional metal precursor may
act as a catalyst for decomposition while also forming an
alloy.
[0379] Electrocatalysts
[0380] The following examples illustrate low viscosity compositions
for the deposition of PEM MEA electrodes.
Example 45
[0381] 1 gram of a 20 wt. % platinum on carbon (Pt/C)
electrocatalyst, where the carbon support is an acetylene carbon
(SHAWINIGAN BLACK, available from Chevron Chemical Company,
Houston, Tex.), was dispersed in 2 ml of de-ionized water and 10 ml
of a 5% solution of a sulfonated perfluorohydrocarbon polymer
(NAFION, available from E.I. duPont deNemours, Wilmington, Del.) to
yield final composition after drying of the solvent of 67 wt. %
catalyst and 33 wt. % NAFION. The composition was sonicated in a
water bath for at least 10 min. The particle size distribution for
this composition is a d.sub.10 of 1.9 .mu.m, a d.sub.50 Of 4.7
.mu.m and a d.sub.95 of 16.0 .mu.m. The viscosity was measured to
be 10 centipoise in the range of 5 to 50 rpm.
Example 46
[0382] 1 gram of 60 wt. % Pt/C electrocatalyst, where the carbon
support is a high surface area carbon (KETJENBLACK, available from
Akzo Nobel, Amersfoort, Netherlands), was dispersed in 2 ml of
de-ionized water and 10 ml of 5 wt. % NAFION solution to yield a
final composition after drying the solvent of 60 wt. % catalyst and
40 wt. % NAFION. The composition was sonicated in a water bath for
at least 10 min. The particle size distribution for this
composition is a d.sub.10 of 3 .mu.m, a d.sub.50 of 6 .mu.m and a
d.sub.95 of 14 .mu.m.
[0383] The following examples illustrate ink-jettable compositions
that are useful for the fabrication of DMFC (direct methanol fuel
cell) electrodes.
Example 47
[0384] 1 gram of 60 wt. % precious metals (Pt, PtRu) on carbon
electrocatalysts, where the carbon support is KETJENBLACK, was
dispersed in 6 grams de-ionized water and NAFION solution (5% by
weight) to have a final weight ratio of 85:15 of dry catalyst to
NAFION in the final electrode structure. The composition was then
mildly sonicated using a bath. The particle size distribution for
this composition was a d.sub.10 of 3.4 .mu.m, a d.sub.50 of 6.5
.mu.m and a d.sub.95 of 16.8 .mu.m. Viscosity was measured to be 23
centipoise at 5 rpm and 92 centipoise at 50 rpm. The composition
had a surface tension of 30 mN/m.
Example 48
[0385] 1 gram of porous, micron-sized pure Pt particles prepared by
a spray conversion method was dispersed in 10 gram de-ionized water
by sonication using an ultrasonic horn. A NAFION solution (5% by
weight) was then added to have a final weight ratio of 90:10 of dry
catalyst to NAFION in the final electrode structure. The viscosity
of this ink was about 7 to 10 centipoise with a surface tension of
30 mN/m. The particle size distribution for this ink was a d.sub.10
of 1 .mu.m, a d.sub.50 of 3.2 .mu.m and a d.sub.95 of 10.6
.mu.m.
Example 49
[0386] gram of Pt blacks was dispersed in 10 grams de-ionized water
by sonication using an ultrasonic horn. A NAFION solution (5% by
weight) was then added to have a final weight ratio of 90:10 of dry
catalyst to NAFION in the final electrode structure. The particle
size distribution for this ink was a d.sub.10 of 1 .mu.m, a
d.sub.50 of 5 .mu.m and a d.sub.95 of 20 .mu.m.
Example 50
[0387] As is discussed above, preferred precursors for platinum
metal according to the present invention include chloroplatinic
acid (H.sub.2PtCl.sub.6.xH.sub.2O), tetraamineplatinum (II) nitrate
(Pt(NH.sub.3).sub.4(NO.sub.3).sub.2), tetraamineplatinum (II)
hydroxide (Pt(NH.sub.3).sub.4(OH).sub.2), tetraamineplatinum (II)
bis(bicarbonate) (Pt(NH.sub.3).sub.4(HCO.sub.3).sub.2), platinum
nitrate (Pt(NO.sub.3).sub.2), hexa-hydroxyplatinic acid
(H.sub.2Pt(OH).sub.6), platinum (II) 2,4-pentanedionate
(Pt(acac).sub.2), and platinum (II) 1,1,1,5,5,5-hexafluoro
2,4-pentanedionate (Pt(hfac).sub.2). Other platinum precursors
include Pt-nitrates, Pt-amine nitrates, Pt-hydroxides,
Pt-carboxylates, Na.sub.2PtCl.sub.4, and the like.
[0388] The Pt precursor is dissolved in either water or organic
based solvent up to 30 wt. % concentration. A portion of
appropriate solvent (water or organic based) is slowly added to a
carbon dispersion similar to GRAFO 1300 (Fuchs Lubricant, Harvey,
Ill.), while being shear mixed to achieve up to 30 wt. % solids
loading dispersion. A solution of Pt precursor is then slowly added
to the shearing carbon dispersion. The resulting composition is
then shear mixed for an additional 10 minutes. The viscosity for a
5 wt. % solids loading dispersion was measured to be 3 to 4
centipoise with surface tension of 77 mN/m.
Example 51
[0389] A TEFLON coated KAPTON substrate was selectively coated with
a removable protective coating exposing a 100 .mu.m wide trench of
the underlying TEFLON coating. This substrate was dipped into an
etchant (TETRA-ETCH, available from W.L. Gore and Associates) that
forms a hydrophilic surface, followed by a rinse in water and
removal of the adhesive protective coating. This resulted in a
hydrophobic surface (natural surface of TEFLON) with a 100 .mu.m
wide hydrophilic strip from the etchant. The substrate was
subsequently drop coated with a silver precursor composition
containing 33 grams Ag-trifluoroacetate, 33 grams H.sub.2O, 33
grams DEGBE and 1 gram lactic acid. The composition was observed to
be confined to the hydrophilic surface strip. After heating of the
confined composition to 200.degree. C. for 5 minutes, a 100 .mu.m
wide silver line was obtained with a bulk resistivity of about 3
times the bulk resistivity of pure solid silver.
[0390] This example demonstrates the ability to confine a low
viscosity precursor composition through surface modification of the
substrate.
Example 52
[0391] A precursor composition was formulated by combining 0.24
grams palladium trifluoroacetate, 7.3 grams silver
trifluoroacetate, 37.5 grams silver flake, 5.13 grams terpineol,
1.55 grams N-methyl-pyrolidone. This mixture was fired at
185.degree. C. for 60 minutes to yield a resistivity of 2.3 times
the bulk resistivity of pure silver.
Example 53
[0392] A precursor composition was formulated by combining 35 grams
silver flake, 7.55 grams silver (I) oxide and 5.35 grams terpineol.
This mixture was fired at 185.degree. C. for 60 minutes to yield a
resistivity of 2.4 times the bulk resistivity of pure silver.
Example 54
[0393] A precursor composition was formulated by combining 35.03
grams silver flake, 6.26 grams silver nitrite, 6.51 grams
terpineol. This mixture was fired at 185.degree. C. for 60 minutes
to yield a resistivity of 2.1 times the bulk resistivity of pure
silver.
Example 55
[0394] A precursor composition was formulated including 16.5 grams
metallic silver powder, 3.5 grams alpha-terpineol and 5 grams
silver carbonate. This composition was deposited and heated to
350.degree. C. The resulting conductive trace had a resistivity of
29 times the bulk resistivity of pure silver.
Example 56
[0395] A precursor composition was formulated including 10 grams
silver oxide, 0.9 grams silver nitrate, 20 grams metallic silver
powder, 2.1 grams DMAc and 5.0 grams terpineol. The composition was
deposited and heated to 350.degree. C. The resulting conductive
trace had a resistivity of about 11 times the bulk resistivity of
silver.
Examples of In-Situ Precursor Generation
Example 57
Comparative Example
[0396] Silver oxide (AgO) powder was tested using TGA at a constant
heating rate of 10.degree. C./min. The TGA showed the conversion to
pure silver was complete by about 460.degree. C.
Example 58
[0397] A mixture of 3.2 grams silver oxide and 3.0 grams
neodecanoic acid was analyzed in a TGA. The analysis demonstrated
that the conversion to pure silver was substantially complete by
about 250.degree. C.
Example 59
[0398] A mixture of 5.2 grams alpha terpineol, 4.9 grams silver
oxide and 1.1 grams neodecanoic acid was analyzed in a TGA. The TGA
demonstrated that the conversion to pure silver was substantially
complete by about 220.degree. C.
Example 60
[0399] The silver oxide/carboxylic acid chemistry was modified by
the addition of metallic silver powder. The reaction products from
the silver oxide and carboxylic acid weld the silver particles
together providing highly conductive silver traces and
features.
Example 61
[0400] A precursor composition was formulated that included 102.9
grams silver metal powder, 7.8 grams, silver oxide, 15.2 grams
silver nitrate, 10.1 grams terpineol and 1.5 grams SOLSPERSE 21000.
The precursor composition was deposited and was heated to
250.degree. C. The resulting conductive features had a resistivity
that was less than 6 times the bulk resistivity of pure silver. The
material was very dense and had low porosity. This mixture was
analyzed in a TGA and showed a conversion to silver at about
270.degree. C.
[0401] While various embodiments of the present invention have been
described in detail, it is apparent that modifications and
adaptations of those embodiments will occur to those skilled in the
art. However, it is to be expressly understood that such
modifications and adaptations are within the spirit and scope of
the present invention.
* * * * *