U.S. patent application number 11/117443 was filed with the patent office on 2005-12-15 for copper nanocrystals and methods of producing same.
Invention is credited to Dezelah, Charles L. IV, Winter, Charles H., Yu, Zhengkun.
Application Number | 20050277297 11/117443 |
Document ID | / |
Family ID | 32229012 |
Filed Date | 2005-12-15 |
United States Patent
Application |
20050277297 |
Kind Code |
A1 |
Winter, Charles H. ; et
al. |
December 15, 2005 |
Copper nanocrystals and methods of producing same
Abstract
The invention relates to methods of making monodisperse
nanocrystals comprising the steps of reducing a copper salt with a
reducing agent, providing a passivating agent comprising a nitrogen
and/or an oxygen donating moitey and isolating the copper
nanocrystals. Moreover, the invention relates to methods for making
a copper film comprising the steps of applying a solvent comprising
copper nanocrystals onto a substrate and heating the substrate to
form a film of continuous bulk copper from said nanocrystals.
Finally, the invention also relates to methods for filling a
feature on a substrate with copper comprising the steps of applying
a solvent comprising copper nanocrystals onto the featured
substrate and heating the substrate to fill the feature by forming
continuous bulk copper in the feature.
Inventors: |
Winter, Charles H.;
(Bloomfield Hills, MI) ; Yu, Zhengkun; (Detroit,
MI) ; Dezelah, Charles L. IV; (Detroit, MI) |
Correspondence
Address: |
KENYON & KENYON
1500 K STREET NW
SUITE 700
WASHINGTON
DC
20005
US
|
Family ID: |
32229012 |
Appl. No.: |
11/117443 |
Filed: |
April 29, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11117443 |
Apr 29, 2005 |
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10290303 |
Nov 8, 2002 |
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6887297 |
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Current U.S.
Class: |
438/687 |
Current CPC
Class: |
B22F 2998/00 20130101;
B22F 2999/00 20130101; B82Y 30/00 20130101; B22F 2998/00 20130101;
B22F 9/24 20130101; B22F 1/0018 20130101; B22F 1/0059 20130101;
B22F 1/0062 20130101; B22F 2999/00 20130101; B22F 1/0088 20130101;
B22F 9/24 20130101; B22F 1/0088 20130101; B22F 1/0062 20130101;
B22F 1/0059 20130101 |
Class at
Publication: |
438/687 |
International
Class: |
H01L 021/00; H01L
021/16; H01L 021/44 |
Claims
What is claimed is:
1. A method of making monodisperse nanocrystals comprising the
steps: (a) reducing a copper salt with a reducing agent to obtain
reduced copper; (b) providing a passivating agent to contact said
reduced copper, wherein said passivating agent comprises at least
one moiety selected from the group consisting of a nitrogen donor
and an oxygen donor; (c) isolating said nanocrystals comprising
said reduced copper.
2. The method of claim 1, wherein the passivating agent comprises
an imidazole moiety.
3. The method of claim 1, wherein the nitrogen donor is selected
from the group consisting of imidazole, pyridine, bipyridine,
phenanthroline, ammine, amine, nitrile, and nitride.
4. The method of claim 1, wherein the passivating agent is
1-decyl-2-methylimidazole.
5. The method of claim 1, wherein the passivating agent comprises a
ammonium or alkylammonium alkylcarboxylate moiety.
6. The method of claim 1, wherein the oxygen donor is selected from
the group consisting of tetraoctylammonium, alkoxides, oxides,
ethers, carboxylates, acetoacenoates, sulphoxide, hydroxide,
acetate, lactate, propionate, oxalate, and maltolate.
7. The method of claim 1, wherein the oxygen donor is selected from
the group consisting of sodium decanoate, tetraoctylammonium
octylxanthate, potassium octylxanthate and tetraoctylammonium
octylxanthate.
8. The method of claim 1, wherein the passivating agent is
tetra-n-octylammonium decanoate.
9. The method of claim 1, wherein said nanocrystals are about 1 to
about 20 nanometers in size.
10. The method of claim 1, where the nanocrystals melt at less than
about 350.degree. C.
11. The method of claim 1, wherein the nanocrystals are soluble in
a solvent.
12. The method of claim 11, wherein the solvent is selected from
the group consisting of water, acetonitrile, methanol, isopropanol,
hexanes, dodecane, toluene, cyclohexanone, diethyl ether,
tetrahydrofuran, ethyl lactate, dichloromethane, acetone and
ethanol.
13. The method of claim 1, wherein the passivating agent
volatilizes between about 125.degree. C. to about 250.degree.
C.
14. The method of claim 1, wherein weight loss upon heating said
nanocrystals, is in the range of about 15 to about 40%.
15. The method of claim 1, wherein the isolation of said
nanocrystals takes place in the presence of a surfactant.
16. The method of claim 15, wherein said surfactant
1-heptyl-4-(4-pyridyl)pyridium bromide.
Description
[0001] This is a continuation of application Ser. No. 10/290,303
filed 8 Nov. 2002, U.S. Pat. No. 6,887,297, the content of which is
incorporated by reference.
FIELD OF THE INVENTION
[0002] The invention relates to methods of making macroscopic
quantities of monodisperse, soluble copper nanocrystals. The
present invention also relates to microelectronic trenched feature
formation and more particularly to the formation of an interconnect
from a copper nanocrystal solution.
BACKGROUND OF THE INVENTION
[0003] An integrated circuit requires conductive interconnects
between semiconducting domains in order to communicate signals
therebetween. In order to create ever faster microprocessors,
smaller dimension interconnects of higher conductivity materials is
an ongoing goal.
[0004] As microelectronic efficiencies have increased,
interconnects have decreased in dimensional size and efforts have
been made to increase the electrical conductivity of interconnect
features. It is becoming increasingly difficult to design and
fabricate ultralarge scale integrated circuits (ULSI) chips with
the desired high packing density and high-speed operation. This is
not due to the difficulty of transistor scaling but to the
interconnect delay in distributing high frequency signals across
the ULSI chip. Therefore, the significance of interconnect
technologies has become much greater in view of the ongoing need
for ever smaller interconnects. Accordingly, there is a need for
novel interconnect-forming technologies.
[0005] The rapid miniaturization of interconnects is occurring
simultaneously with the transition from aluminum (Al) to copper
(Cu) metallization for sub-0.25 .mu.m interconnects (IC). It is now
becoming apparent that a major component of improved interconnect
performance will consist in replacing aluminum, the previous metal
of choice, with copper. Murakami et al., J. Vac. Sci. Technol. B.
1999, 17, 2321-2324, have indicated that due to its electromagnetic
resistance, low resistivity, and high reliability against
electromigration Cu is thought to be the most attractive substitute
for Al alloys in integrated circuit manufacturing. The transition
from Al to Cu has led to a change in the way interconnects are
formed. While Al has been deposited as a blanket layer which is
then patterned by reactive ion etching, Cu interconnects are formed
by evaporative deposition into preformed (damascene) trenches and
vias followed by chemical mechanical polishing (CMP).
[0006] As the interconnect width decreases and the aspect ratio
increases, conventional vacuum deposition techniques approach the
theoretical resolution threshold. Deep, narrow trenches and vias
preferentially collect material at the damascene feature edges,
leading to void formation. Blanket and selective chemical vapor
deposition (CVD) are well-established Cu deposition techniques that
have a demonstrated ability to fill current interconnect trenches.
(A. E. Kaloyeros and M. A. Fury, MRS Bull. (June 1993), pp. 22-29).
This process of involves heating a metal until it vaporizes and
then condensing the vaporized metal condenses onto a cold surface.
The process is cumbersome in terms of time, money and energy.
Additionally, heating of the IC substrate during CVD to assure
crystalline growth degrades fine architecture structures on the
substrate.
[0007] The cost of Physical vapor deposition (PVD) and chemical
vapor deposition (CVD) equipment capable of performing either of
these processes is about $1 million. This cost does not include the
cost, time, and materials associated with intermediate polishing
prior to fill. Additionally, PVD is a competing technique to CVD.
The formation of a seed layer is essential prior to trench filling
by electrochemical deposition (ECD). ECD may be used to achieve
conformal fill of ICs, trenches, and vias into which a seed layer
has been grown by CVD or PVD. Thus, existing methods require
additional steps of (a) depositing seed layers prior to fill, (b)
intermediate cleaning between seed layer deposition, and (c) a
final chemical mechanical polishing step to remove the conformal
metal coating after fill. In addition to the high purchase cost of
a separate chamber to perform each of these additional steps, each
step adds about $1 per metallization layer in consumable materials
cost.
[0008] In any case, CVD does not inherently fill trenches
preferentially over any other portion of substrate having
nucleation sites. Therefore a method that preferentially deposits
Cu into trenches based on differential solvent evaporation
associated with trenches is needed.
[0009] Thus, the semiconductor industry is in need of a copper
interconnect formation process capable of achieving higher
resolution at lower temperature and ideally, at a lower cost. The
successful synthesis of Cu nanocrystals will offer the
semiconductor industry a thermodynamically metastable source for
copper metallization. Based on the literature and reagent costs,
copper nanocrystal synthesis could be expected to yield a suitable
precursor at a lower cost than CVD.
[0010] The mesoscopic size regime between atoms and bulk materials
is characterized by unusual properties. Mesoscopic systems exhibit
collective atomic behavior, but not to a sufficient extent so as to
preclude quantized effects. Many of the unusual thermodynamic and
spectroscopic anomalies associated with mesoscopic systems are
attributable to surface effects. Studies have shown surface
energies that are 10 to 400% greater for nanocrystals than for bulk
gold (Au) and platinum (Pt) (C. Solliard and M. Flueli, Surf. Sci.
156 (1985), pp. 487-494), and Al (J. Wolterdorf, A. S. Nepijko and
E. Pippel, Surf. Sci. 106 (1981), pp. 64-72). In the bulk, surface
atoms represent such a small percentage of the total that surface
effects are largely inconsequential. Surfaces generally possess
modified atomic coordination numbers, geometries and diminished
lattice energies relative to the bulk. The result of these
modifications is that physical, spectroscopic, and thermodynamic
properties, which are constant in the bulk, become size dependent
variables in nanocrystals. The ability to modify the thermodynamic
properties of nanocrystals, particularly the melting temperature,
is exploited in the present invention to produce thin film copper
IC structures at low temperature.
[0011] Metallic nanocrystals have been shown to reduce melting
temperatures compared with the bulk. (Ph. Buffat and J-P. Borel,
Phys. Rev. A, 13 (1976), pp. 2287-2298; C. J. Coombes, J. Phys. 2
(1972), pp. 441-449; J. Eckert, J. C. Holzer, C. C. Ahn, Z. Fu and
W. L. Johnson, Nanostruct. Matls. 2 (1993), pp. 407-413; C. R. M.
Wronski, Brit. J. Appl. Phys. 18 (1967), pp. 1731-1737 and M.
Wautelet, J. Phys. D, 24 (1991), pp. 343-346). The depression in
melting and annealing temperature is evident throughout the
nanocrystal size regime, with the most dramatic effects observed in
nanocrystals having a diameter from 2 to 6 nm. Melting studies on a
range of nanocrystals have established that the melting temperature
is size dependent in the nanometer size regime and is approximately
proportional to the inverse particle radius regardless of the
material identity. The size dependent melting temperature of
metallic nanocrystals has included studies of Au, Pb and In, Al and
Sn. (Au: Ph. Buffat and J-P. Borel, Phys. Rev. A, 13 (1976), pp.
2287-2298; Pb and In: C. J. Coombes, J. Phys. 2 (1972), pp.
441-449; Al: J. Eckert, J. C. Holzer, C. C. Ahn, Z. Fu and W. L.
Johnson, Nanostruct. Matis 2 (1993), pp. 407-413; and Sn: C. R. M.
Wronski, Brit. J. Appl. Phys. 18 (1967), pp. 1731-1737). The
reduction in melting temperature as a function of nanocrystal size
can be enormous. For example, 2 nm Au nanocrystals are predicted to
melt at about 300 degrees Celsius, as compared to 1065 degrees
Celsius for bulk gold. (M. Wautelet, J. Phys. D, 24 (1991), pp.
343-346).
[0012] Numerous techniques exist for the preparation of metal
colloids. Surprisingly, little attention has been paid to
monodispersity and solubility of Cu nanocrystals. Monodispersity
refers to a unimodality or uniformity of nanocrystal size in
solution as opposed to polydispersity which connotes a solution
having a range of nanocrystal sizes. Monodisperse Au nanocrystals
having an alkane-thiol capped surface are readily synthesized in a
two-phase reaction, with resulting particles being highly soluble
in toluene and/or hexane. Many liquid phase syntheses also exist
for Ag nanocrystals, but with less control over particle size
distribution. Silver nanoparticle syntheses are only to a very
limited extent adapable to making Cu nanocrystals. Inherent
inadequacies in conventional nanoparticle synthesis technology with
respect to polydispersity and solubility, preclude the formation of
high conductivity interconnects necessary for internal
communication within an IC. For example, the polydispersity of Cu
nanocrystals made by conventional methodology, results in a broad
range of melting temperatures and leads to comparatively high
resitivity in interfacial regions between particles melting at
variable temperatures.
[0013] Unsuccessful attempts have been made to obtain Cu
nanocrystals involving a number of methodologies. INn one method
hydrazine carboxylate was used as a ligand for Cu.sup.2+. The
intent was to produce copper nanocrystals from the decomposition of
the hydrazine carboxylate complex of Cu.sup.2+ in the presence of a
passivating agent. Hydazine carboxylate would act as a reducing
agent for copper, itself being oxidized to gaseous products. This
experiment failed to produce any copper nanocrystals. Problems
included but are not limited to: poor solubility of the copper
hydrazine carboxalate complex and recovery of only bulk copper from
the reaction mixtures. Additionally, hydrazine carboxylate and
complexes thereof are potentially explosive making them diffuclt to
work with. As such, this method is not capable of forming suitable
nanocrystal colloids.
[0014] A proposed alternative to make Cu nanocrystals involved
reducing Cu salts in water and pyridine, followed by the addition
of sulfur-containing ligands, such as alkylthiols, alkylthiophenes,
and alkylxanthates initially showed great promise. They afforded
nanocrystals that generally possessed good solubility in a wide
range of organic solvents. Air-stability of nanocrystals passivated
by the above ligands was found to be excellent. Copper nanocrystals
passivated by the above mentioned sulfur-containing ligands were
characterized by Transmission Electron Microscopy(TEM), Powder
X-ray Diffraction(XRD), Thermogravimetric Analysis(TGA),
Differential Thermal Analysis(DTA), and Atomic Force Microscopy
(AFM). Data generally agreed with that expected for copper
nanocrystals having a size between 2 nm and 10 nm. However, the
sulfur-containing set of passivating agents/ligands was ultimately
abandoned when it became apparent that nanocrystals passivated by
such ligands led to Cu.sub.2S or Cu.sub.2S/Cu mixtures under
thermolysis conditions. Generally thermolysis under an inert
atmosphere (nitrogen or argon) gave Cu.sub.2S, whereas thermolysis
under a reducing atmosphere (5% Hydrogen/95% nitrogen). Thermolysis
products were determined by XRD.
[0015] Finally, syntheses using alkylamine-ligands and reduction in
the presence of pyridine, also yielded copper nanocrystals.
However, such syntheses generally yielded a black, insoluble powder
(>95%) and a very faintly colored amber solution. The solution
was found to contain copper nanocrystals in the range of 2 nm to 10
nm in size by AFM. The amount of soluble material present was
certainly so miniscule (probably microgram scale) that any
application of such nanocrystals would be impossible.
SUMMARY OF THE INVENTION
[0016] One aspect of the current invention relates to a method of
making monodisperse nanocrystals comprising the steps of reducing a
copper salt with a reducing agent to obtain reduced copper;
providing a passivating agent to contact the reduced copper,
wherein the passivating agent comprises a nitrogen and/or an oxygen
donating moitey and isolating the nanocrystals comprising the
reduced copper.
[0017] In one embodiment of this aspect of the invention, the
passivating agent comprises an imidazole moiety. In another
embodiment of this aspect of the invention the passivating agent is
1-decyl-2-methylimidazole.
[0018] In another embodiment of this aspect of the invention, the
passivating agent comprises an ammonium or alkylammonium
alkylcarboxylate moiety. In another embodiment of this aspect of
the invention the passivating agent is tetra-n-octylammonium
decanoate.
[0019] Another aspect of the invention relates to a method for
producing a copper film comprising the steps of applying a solvent
comprising copper nanocrystals dissolved therein onto a substrate,
wherein the copper nanocrystals are made using the methodology
described herein; and heating the substrate to form a film of
continuous bulk copper from the nanocrystals on the substrate.
[0020] In one embodiment of this aspect of the invention, the
substrate comprises silicon. In another embodiment of this aspect
of the invention, the substrate comprises SiO.sub.2.
[0021] Yet a further aspect of the invention provides for methods
to fill a feature on a substrate with copper comprising the steps
of applying a solvent comprising copper nanocrystals onto the
featured substrate, wherein
[0022] the copper nanocrystals are made using the methodology
described herein; and heating the substrate to fill the feature by
forming continuous bulk copper from the nanocrystals in the
feature. In one embodiment of this aspect of the invention, the
substrate comprises silicon.
[0023] In one embodiment of this aspect of the invention, the
feature is a via. In another embodiment of this aspect of the
invention, the feature is a trench with an aspect ratio in the
range of about 1.6 to about 10. In yet another embodiment of this
aspect of the invention, the feature is in the form of an
interconnect (IC). In yet a further embodiment of this aspect of
the invention, the nanocrystals selectively collect within recesses
of said features as opposed to plateaus of the substrate.
DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 shows Thermogravimetric analysis of passivated copper
nanocrystals.
[0025] FIG. 2 shows X-Ray diffraction spectrum of copper
nanocrystals after heating to 350.degree. C. with reference copper
metal spectrum overlaid.
[0026] FIG. 3 shows conformal, bottom-up fill of trenches with
copper nanocrystals prior to heating.
[0027] FIG. 4 shows a reverse contrast cross-sectional micrograph
of copper-filled 0.1.times.1.0 .mu.m trenches on TaN-coated silicon
substrate.
DETAILED DESCRIPTION OF THE INVENTION
[0028] The present invention relates to methods of making
macroscopic scale syntheses of monodisperse, copper nanocrystals
suitable for use as IC seed layers or interconnects, where none
existed before. The nanocrystals made according to the subject
invention, will possess a passivating agent to confer solubility
and air stability and stability against aggregation. The
passivating agent is used to arrest growth of the particles at a
pre-determined and uniform size, yet upon heating will volatilize.
This is interchangeably referred to as thermolysis. The
effectiveness of various passivating agents and reaction conditions
have been assessed with regard to particle polydispersity,
solubility, passivating agent volatilization and compatibility with
other process steps associated with IC manufacture. The inventors
have discovered that passivating agents comprising nitrogen and/or
oxygen donor moieties efficiently facilitate the macroscopic
production of monodisperse and soluble Cu nanocrystals which are of
adjustable domain size, typically from about 1 to about 20 nm, and
are amenable to bulk formation upon heating. Copper nanocrystals
made using the methodology described herein may be used to produce
copper films or fill features on a substrate. Conventional methods
of rendering copper nanocrystals do not provide sufficient
quantities or workable amounts of monodisperse copper nanocrystals
to carry out such methods. This is the challenge that has been
overcome by the present invention.
[0029] Monodispersity refers to a relative unimodality or
uniformity of nanocrystal size in solution as opposed to
polydispersity which connotes a solution having a range of
nanocrystal sizes. Monodisperse Cu nanocrystals have a relatively
uniform melting temperature resulting in a narrow melting
temperature range for a population of monodisperse Cu
nanocrystals.
[0030] The nanocrystal is defined as a particle having a linear
dimension in any direction on the order of nanometers, namely about
1 to about 100 nm in diameter. Preferably, the nanocrystal of the
present invention is between about 1 and about 20 nm. More
preferably, the nanocrystal of the present invention is between
about 1.5 and about 10 nm. Most preferably the nanocrystal of the
present invention is between about 2 and about 6 nm. The size
distribution of the nanocrystals in solution is of some importance,
since the sintering temperature of particles in this size regime is
size dependent. Likewise, the melting temperature is size
dependent. Preferably, a size distribution of less than 30% of the
average particle diameter is used for nanocrystals of an average
diameter of less than about 6 nm with increasingly greater
tolerances being preferred as the average particle increases beyond
6 nm. Alternatively, smaller nanocrystals having a lower melting
temperature form a flux that facilitates uniform melting of larger
nanocrystals within the distribution. The nanocrystals are
optionally either dispersed in a solvent by conventional means
illustratively including sonication, agitation, solution shearing
and the like. The nanocrystal surface is coupled to a passivating
agent by adsorption or chemically bonding thereto. The passivating
agent preferably being soluble in the solvent and thus imparting
solubility to the particles. Preferably, the nanocrystals are
soluble to promote segregation into low volatilization rate, trench
and via regions as compared to substrate plateaus.
[0031] The nanocrystal size is selected to take advantage of size
dependent sintering and melting temperatures. The stability of the
underlying substrate architecture is typically the controlling
factor in determining optimal melting temperature and thus
nanocrystal size. Preferably, nanocrystals are selected having a
minimal heating requirement to obtain desired electrical
conductivities from the resulting bulk structure of less than about
450 degrees Celsius. More preferably, the nanocrystals are heated
to less than about 350 degrees Celsius. Most preferably, the
nanocrystals are heated to less than about 300 degrees Celsius. The
melting temperature of various sizes of nanocrystals is calculable
(Wautelet, J. Phys. D, 24 (1991), p. 343).
[0032] Sintering is defined herein as the interfacial coalescence
of contiguous particles while the particle cores retain prior
crystalline properties. Sintering temperature is calculable or
approximated as two-thirds of the substance melting temperature in
degrees Kelvin. Further, heating beyond the sintering temperature
brings the nanocrystals to a size dependent melting temperature.
Melting of a contiguous matrix of nanocrystals results in a
densified polycrystalline bulk structure limiting the interfacial
resistivity between sintered domains. Once the passivating agent is
volatilized contiguous nanocrystals are able to sinter. The
relevant volatilization sintering and melting temperatures for a
given nanocrystal solution are determinable through thermal
analysis, techniques such as differential scanning calorimetry
(DSC), thermal gravimetric analysis (TGA), temperature dependent
spectroscopies, differential thermal analysis (DTA) and
conductivity.
[0033] The passivating agent, as defined herein, is a compound
introduced to prevent nanocrystal growth beyond a pre-selected size
and preferably to also impart solubility on the nanocrystal in a
solvent. The passivating agent is used to arrest growth of the
particles at a pre-determined and uniform size, yet upon heating
will volatilize such that nanocrystals decompose to pure copper
metal with complete evaporation of the passivating agent. The
passivating agent illustratively includes a variety of thermally
volatile organics including those of the formula XRY where X is a
moiety capable of chemically bonding to a surface Cu atom of the Cu
nanocrystal and illustratively includes imidazole, pyridine,
bipyridine, phenanthroline, ammine, amine, nitrile, nitride or
other nitrogen donors; alkoxides, oxides, ethers, carboxylates,
acetoacenoates, sulphoxide, hydroxide, acetate, lactate,
propionate, oxalate, maltolate or other oxygen donors. R is
C.sub.1-C.sub.30 aliphatic, aryl or heteroatom substituted
derivative thereof and may also include substituent moieties
extending from the alkyl chain or heteroatoms within the chain. Y
is X, hydrogen, or may also include moieties other than hydrogen
that are not capable of forming a bond to copper. Such terminal
moieties could find potential use in tailoring the passivating
agent for maximum solubility in solvent systems optimal for spin
coating. Examples of such terminal "Y" groups can include the
hydroxyl moiety, alkoxy groups, a ketone moiety, or even alkyl or
aryl ester moieties. The RY portion of the passivating agent is
optionally chosen to interact with the solvent to impart
solubility. In another embodiment, Y bonds to the surface of a
second nanocrystal to tether nanocrystals together. Further,
dendritic or polymeric variants of XRY are operative to form an
extended matrix of nanocrystals. The nanocrystal surface binding
passivating agent moiety being dictated by established
organometallic chemistry. Thermolysis, i.e. volatilization upon
heating, of the passivating agent is provided by induction heating,
photolysis, sonication, laser ablation and introduction of a heated
gas stream. The invention also contemplates a passivating agent
comprising an electrically conductive polymer that electrically
couples contiguous nanocrystals to one another obviating the need
for passivating agent volatilization. It is understood that the
"XRY" model is useful in describing the characteristics of a
typical passivating agent, but it should not limit the breadth of
possible passivating molecules. For example, a number of the
moieties described as being a possible choice as "X" themselves
exist as molecules capable of acting in a passivating fashion in
other circumstances, exclusive of the above described "R" or "Y"
groups.
[0034] One aspect of the present invention provides a method for
the production of Cu nanocrystals to overcome the fact that the
prior art does not teach any methods for producing the sufficient
volumes of monodisperse Cu nanocrystals suitable for producing
interconnect formations. This aspect of the invention, utilizes a
passivating agent to prevent Cu nanocrystal growth beyond a
pre-selected size and preferably to also impart solubility on the
nanocrystal in a solvent. As opposed to previously used passivating
agents ineffectual for producing monodisperse Cu nanocrystals, the
inventors have found that passivating agents comprising nitrogen
and/or oxygen donor moieties efficiently facilitate the macroscopic
production of monodisperse and soluble Cu nanocrystals which are of
adjustable domain size, typically from about 1 to about 20 nm, and
are amenable to bulk formation upon heating. Copper nanocrystals
that are soluble in a chosen solvent through coordination of an
organic passivating agent to the surface of the nanocrystal, should
decompose to pure copper metal with complete thermolysis of the
passivating agent. The nanocrystals will be sized to melt to bulk
copper upon heating to temperatures of less than about 350.degree.
C., preferably less than about 300.degree. C. Heating of copper
nanocrystal-coated substrates proceeds optimally under a nitrogen
atmosphere containing a small amount of hydrogen. The hydrogen
reduces any copper oxides. Semiconductor tool manufacturers
indicate that spin coating equipment already in place in a
fabrication line can be used without modification with the smelting
of copper nanocrystals. Example 1 exemplifies a method of making Cu
nanocrystals.
[0035] One embodiment appreciates that the isolation of
nanocrystals as a solid precipitate is facilitated by the use of
such surfactants. For example, surfactants such as but not limited
to 1-heptyl-4-(4-pyridyl)pyr- idium bromide, added to reaction
mixtures subsequent to reduction, yields substantially improved
preparations. Preferably, such surfactants are used with for those
preparations using imidazole-based passivating agents.
[0036] Cu inorganic salts are commercially available at high purity
and low cost. Copper ions are readily reduced to copper metal in
the presence of a reducing agent. A reducing agent as defined
herein refers to a substance that reduces another substance and is
oxidized. Examples of reducing agents include but are not limited
to sodium borohydride or hydrazine. The addition of the reducing
agent prevents a copper ion bonded to the passivating agent from
bonding to other copper atoms at the bonding site occupied by the
passivating agent, ultimately making the passivating agent-bound
copper atom a surface atom of a growing nanocrystal. By Adjusting
the ratio of passivating agent to copper, the ratio of copper
surface atoms to copper interior atoms is controlled. For example,
a copper to passivating agent ratio of about 3:1 yields
nanocrystals of about 2.9 nm. These ratios are hypothetical
examples based on a proposed model of how nanocrystals are believed
to form. A spherical nanocrystal of a given size should have a
highly predictable number of surface copper atoms. If each surface
atom is chemically bonded to one molecule of passivating agent,
then in theory one can control the size by careful control of the
quantity of passivating agent, effectively dictating the number of
surface atoms during the stage of nanocrystal formation Copper
nanocrystal synthesis according to the present invention, may
proceed in conventional glassware.
[0037] Synthesis of copper nanocrystals of about 1 to about 20 nm
are passivated with either passivating agents comprising nitrogen
and/or oxygen donor moieties. These copper nanocrystals melt at
less than about 350.degree. C., exhibit X-ray diffraction spectra
consistent with copper metal, and are highly soluble (up to about
10 wt. %) in organic solvents ranging from organic solvents ranging
from fluorocarbons, n-alkanes to alcohols. Examples include but are
not limited to water, acetonitrile, methanol, isopropanol, hexanes,
dodecane, toluene, cyclohexanone, diethyl ether, tetrahydrofuran,
dichloromethane, acetone, and ethanol. With appropriate passivating
agents, water may even be used as a solvent for the nanocrystals.
Thermolysis of the nanocrystals at less than about 350.degree. C.
under a nitrogen/hydrogen atmosphere affords substantially pure
copper metal, with complete evaporation of the passivating agents.
Substantially pure copper refers to copper metal with low content
of carbon, oxygen, or other elements. Additionally, pure copper
refers to copper that possesses electrical resistivities equal to
or close to that of a standard pure copper metal reference. Weight
loss upon heating, as measured by thermogravimetric analysis, is
generally in the range of about 15 to about 40%, preferably about
20 to about 35 percent and most preferably about 27 to about 32%.
Thermogravimetric analysis indicates that evaporation of
passivating agents occurs between about 125-250.degree. C. Spin
coating of nanocrystal solutions onto TaN-coated silicon wafers,
followed heating at 350.degree. C. under a nitrogen/hydrogen
atmosphere, affords copper films that exhibit bulk resistivity
values. XPS analysis of these films indicated pure copper metal,
with no oxides present. The films of the present invention are
highly adherent, and pass the Scotch.RTM. tape test.
[0038] One embodiment of this aspect of the invention, utilizes a
passivating agent wherein X (of passivating agent structure XRY,
described supra) comprises tetraoctylammonium, alkoxides, oxides,
ethers, carboxylates, acetoacenoates, sulphoxide, hydroxide,
acetate, lactate, propionate, oxalate, maltolate or other oxygen
donating moieties to prevent Cu nanocrystal growth beyond a
pre-selected size and preferably to also impart solubility on the
nanocrystal in a solvent. The inventors have found that ammonium
moieties, including tetraoctylammonium ion are do not themselves
act as a ligand, due to the lack of ability to donate electrons to
a metal atom. Ammonium and alkylammonium groups are positively
charged ions and are able to act as counter-ion to a negatively
charged ligand species. Despite the lack of donor abilities of
ammonium ions, they help provide stability of the nanocrystal
toward air and toward agglomeration when coupled to an appropriate
negatively charged passivating ligand.
[0039] Preferably, the oxygen-donating passivating agent is a
linear C.sub.2-C.sub.24 having an oxygen donating moiety.
Preferable oxygen-donating passivating agents are sodium decanoate,
tetraoctylammonium octylxanthate, potassium octylxanthate and
tetraoctylammonium octylxanthate. Even more preferable for this
embodiment is utilization of ammonium or alkylammonium
alkylcarboxylates. Most preferably, tetra-n-octylammonium decanoate
is used as an oxygen donating passivating agent.
[0040] Another embodiment of this aspect of the invention utilizes
a passivating agent wherein X (of passivating agent structure XRY,
described supra) comprises an imidazole, pyridine, bipyridine,
phenanthroline, ammine, amine, nitrile, nitride or other nitrogen
dononating moieties to prevent Cu nanocrystal growth beyond a
pre-selected size and preferably to also impart solubility on the
nanocrystal in a solvent. In this embodiment, the preferable
passivating agent is a linear C.sub.2-C.sub.24 having a nitrogen
donating moiety. Even more preferable for this embodiment is
utilization of long chain alkyl imidazoles. Most preferably,
1-decyl-2-methylimidazole is used as a nitrogen donating
passivating agent.
[0041] In another aspect of the present invention, a method is
detailed herein to create copper films from copper nanocrystals,
wherein the copper nanocrystals are made using the methodology
described supra. First, a solvent comprising the copper
nanocrystals delivered to a substrate. The substrate is then heated
to form a continuous bulk copper from the nanocrystals onto the
substrate. The nanocrystal solutions or suspensions are applied by
spray or spin coating onto a substrate.
[0042] "Substrates" as defined herein may be any rigid or flexible
surface that could be coated with copper films. The terms substrate
therefore may also by used to refer to particles such as powders
that my be coated with the dislcosed methods. Exemplary substrates
are silicon, SiO.sub.2, TiN-coated (1000 .ANG. by PVD) flat silicon
wafers, and TaN-coated (1000 .ANG. by PVD) flat silicon wafers.
SiO.sub.2 wafers flat silicon wafers are preferable. TaN-coated are
more preferable. TiN coated flat silicon wafers are most
preferable. One of skill in the art could recognize a range of
additional substrates given that the methods disclosed herein could
be readily extended to other systems by analogy. For example,
deposition in a microelectronics device usually entails involves a
substrate coated with a barrier layer, but future advances may
eliminate the need for a barrier layer.
[0043] This embodiment of this aspect of the invention further
envisages heating nanocrystal solution-coated substrates to about
300.degree. C. to afford copper films that exhibit bulk resistivity
values and a specular bright copper-colored film. Specular is a
term that refers to thin film reflectivity. A specular film is one
that is shiny and reflective, as a high quality copper film should
be. XPS analysis of the present films indicates pure copper metal,
with no oxides present. The films of this embodiment are highly
adherent, and pass the Scotch.RTM. tape test. Moreover, the copper
films of this aspect of the invention show no evidence of copper
oxides in the film, even before sputtering. Thus, this aspect of
the invention provides for pure copper films from copper
nanocrystals that also exhibit bulk electrical conductivity. In one
embodiment of this aspect of the invention, copper films have
domain sizes of about 1 nm to about 10 nm, preferably about 2 nm to
about 8 nm and most preferably about 4 nm to about 6 nm. The
Example 2 illustrates making copper films using the nanocrystals
provided in the present invention.
[0044] Another aspect of the current invention relates to a method
for producing a microscale structure. First a solvent comprising
copper nanocrystals is applied onto a substrate having a feature
cut therein, wherein the copper nanocrystals are made using the
methodology described supra. The substrate is then heated to form a
microscale structure of continuous bulk copper from said
nanocrystals within the feature.
[0045] "Features" as defined herein are contours machined or
fabricated onto the surfaces of substrates, i.e. a featured
substrate, using techniques such as lithography. Preferably
features are in the form of sub-micron vias and trenches.
Microelectronics substrates contain trenches and vias and both need
to be filled with copper metal. Usually trenches provide the
horizontal wiring, and vias provide the vertical wiring within the
chip. Most preferably the features are in the shape of
interconnects (IC).
[0046] In this embodiment of this aspect of the invention further
envisages substrates of silicon, SiO.sub.2, or TiN-coated (1000
.ANG. by PVD) flat silicon wafers, and TaN-coated (1000 .ANG. by
PVD) flat silicon wafers. SiO.sub.2 wafers flat silicon wafers are
preferable. TaN-coated are more preferable. TiN coated flat silicon
wafers are most preferable. These substrates preferably have an
array features such as 0.1.times.1.0 .mu.m blind vias.
[0047] This aspect of the invention further envisages that upon
heating to about 125.degree. C. to about 250.degree. C. (where loss
of the passivating agent occurs), and small crystallites of copper
on the surface after heating to about 280.degree. C. (near where
melting of the nanocrystals occurs). Upon heating to about
150.degree. C., wicking of nanoparticles from the plateau region to
the via is observed. At about 280.degree. C., a perfectly conformal
copper film has been obtained in the via (the bright region
corresponds to fluorescence from copper metal). In addition, the
invention provides for a small amount of bottom-up fill at about
280.degree. C. The observation of a perfectly conformal copper film
in a 0.1.times.1.0 .mu.m blind via (aspect ratio of 10) with the
nanocrystal method described herein, is extremely important. Most
notably, the methods of this invention can deposit copper seed
layers selectively in the features, without creating a blanket
copper coating on plateau regions. This embodiment of this aspect
of the invention allows for selective growth of copper metal only
in the features by electrochemical deposition, thus greatly
reducing the need for CMP after metallization.
[0048] One embodiment of this aspect of the invention relates to a
method for filling a micro-scale trench. The term trench as defined
herein refers to features on a substrate with a high aspect ratio.
The narrower the trench width relative to the trench depth, or the
deeper the trench relative to the width of the trench, the higher
the aspect ratio. The preferable aspect ratio is about 1.6 to about
5000, more preferably about 4 to about 1000 and most preferably
about 6 to about 1000. Even more preferably is an apect ration of
about 10 to about 100. First a solvent comprising copper
nanocrystals is applied to a substrate having a trench cut therein,
wherein the copper nanocrystals are made using the methodology
described supra. The substrate is then heated to form continuous
bulk copper from the nanocrystals within the trench. One embodiment
of this aspect of the invention relates to fabrication of
completely conformal copper seed layers in high aspect ratio etched
features. Conformal seed layers are required for feature filling by
electrochemical deposition, and the present invention provides an
inherently conformal process that can coat trenches, i.e. very
aggressive, i.e. high aspect ratio, features. Most notably, the
methods of this invention can deposit copper seed layers
selectively in the features, without creating a blanket copper
coating on plateau regions. This embodiment of this aspect of the
invention allows for selective growth of copper metal only in the
trenches by electrochemical deposition, thus greatly reducing the
need for CMP after metallization. Moreover, this embodiment may be
used to repair defects in existing seed layers that are deposited
by physical vapor deposition. Example 4 demonstrates a high degree
of trench filling, with little or no copper deposition on the
substrate plateau when using TaN-coated silicon wafers with
0.1.times.1.0 .mu.m trenches. Filling in a feature with an aspect
ratio of 10 and a pitch of 0.1 .mu.m assures that lower aspect
ratio features, such as those required for interconnect structures
in the International Technology Roadmap for Semiconductors,
(International Technology Roadmap for Semiconductiors:
http://public.itrs.net) should be easily filled with copper using
the nanocrystals provided herein. TGA and XRD experiments
demonstrate that the passivating agent used to create solublize
nanocrystals is quantitatively volatilized upon heating of the
coated substrates. Pure copper deposits are obtained, as
exemplified by the XPS data and observation of bulk copper
resistivity. Electron micrographs of filled trenches before and
after heating suggest that about 10% to about 30% shrinkage occurs
upon heating to about 300.degree. C. Due to the observed "wicking"
of nanocrystals into the features upon heating. As such, it is not
necessary to apply additional nanocrystals to compensate for
densification shrinkage. Instead, a slight excess of nanocrystals
can be applied to the substrates to allow movement of nanocrystals
on the plateau region to the copper deposits in the trenches. In
this way, any shrinkage due to densification can be eliminated
during a single heating cycle. As noted supra, a temperature of
only about 250.degree. C. to about 350.degree. C. is needed to
transform the passivated nanocrystals to high purity copper that
exhibits bulk resistivity. This temperature range fits well within
the thermal budget for semiconductor processing. See Example 4.
[0049] In another aspect of the invention, a substrate that has
been patterned by lithography to form a series of features is an
exemplary substrate. The preferred embodiment is silicon wafer that
has been patterned by lithography and etched to form a series of
trenches that have an exposed surface that contains SiO.sub.2.
Therefore, this aspect of the invention further appreciates that a
wetting layer may optionally be applied to the substrate to promote
interconnect wetting thereof and to prevent interdiffusion during
subsequent IC processing.
[0050] In a another aspect of the invention, a solution of
dissolved or suspended nanocrystals is applied to the featured
surface of a substrate. Preferably, the nanocrystal solution or
suspension is concentrated to opaqueness to facilitate rapid
deposition of features or preferably trenches and most preferably
interconnect structures. Following the evaporation of the solvent,
adjacent particles are heated to form bulk domains within the
trenches.
[0051] Another aspect of the invention contemplates the deposition,
delivery or application of nanocrystals to a substrate by range of
methods such as spray or spin coating. One of ordinary skill in the
art could contemplate further mechanisms for depositing, delivering
or coating a substrate with the nanocrystals made using the
methodology described herein. The present invention supplants
expensive vacuum evaporation equipment with a paint booth or spin
coating technology for the formation of integrated circuitry
interconnect structures. Please see Examples 2, 3 and 4. Unlike
chemical and physical vapor deposition techniques, the present
invention selectively deposits nanocrystal particulate in the
features or preferably IC trenches, by taking advantage of the
slower volatilization of a solvent carrier from trenches, causing
the nanocrystals to congregate in high aspect ratio features. As a
result, the quantity of extraneous deposition material, which must
be removed by CMP, is diminished. Lastly, an environmental benefit
results from pre-selecting aqueous and or benign organic solvents
the nanocrystal solution and foregoing the environmentally harmful
solvents currently used in chip manufacturing processes.
[0052] In one embodiment of this aspect of the present invention,
nanocrystals are applied to the substrate by applying the solution
thereto, such that it is applied to promote uniform deposition of
the nanocrystals across whole substrates. Such methods
illustratively include atomizing the solution and applying as a
fine nanocrystal paint spray; and spin coating the solution onto a
rotating substrate.
[0053] In one particular embodiment of this aspect of the
invention, a binder is dispersed in the solution to minimize
particle movement as the solvent pool evaporates. A binder useful
in the present invention includes low molecular weight and
polymeric organic substances. The binder being chosen such that
upon heating to a temperature of less than about 250 to about 300
degrees Celsius, the binder is volatilized. The binder is chosen to
minimize binder residues, the residues likely to decrease
conductivity of the bulk domain resulting from heating the
nanocrystals. Binders illustratively include waxes; aliphatic
compounds containing at least eight carbon atoms including
carboxylic acids alcohols, aldehydes, amines, thiols and salt
thereof wherein the cation is a conductive metal ion illustratively
including copper aluminum, magnesium, gold, silver, manganese,
molybdenum and the like; polymeric materials which volatilize to
greater than about 95% by weight below about 250 degrees Celsius
including poly(acrylic acid), polyglycols, polycarbonates,
polyalkyls, polyalcohols, polyesters and the like; proteinaceous
substrates such as albumin, gelatin and collagen; carbohydrates;
and organosilanes.
[0054] Another embodiment of the invention relates to passivating
agents and binders that decompose to volatile products upon heating
to moderate temperatures, and thus yield high purity copper metal
deposits. Such materials could include ammonium salts of the
formula RnH4-nN+X-, where R is a carbon chain and X- is anionic
group that can accept a hydrogen atom to provide a gaseous
product.
[0055] An example would be trioctylammonium decanoate, which would
decompose to the volatile species trioctylamine and decanoic acid
upon heating to about 100 to about 250 deg. C. Many other
combinations are possible, and will be apparent to a skilled
practitioner.
[0056] In a further embodiment of this aspect of the invention, it
is appreciated that a binder is operative herein to increase the
nanocrystal solution viscosity such that after solution application
to the substrate, a surface coating results which does not
generally penetrate trenches and vias associated with the
substrate. A surface coating upon heating volatilizes the binder
and nanocrystal passivating agent thereby causing the nanocrystals
to be wicked into the substrate trenches and vias so as to coat
trench and via wall surfaces and initiate top to bottom substrate
feature fill. This inventive feature fill technique is particularly
well suited for the formation of barrier layers and seed
layers.
[0057] This aspect of the present invention further identifies cost
savings associated with production defects. Nanocrystal deposition
of substrate features is both reversible and repairable. Poor
deposition technique is corrected by resolubilizing or
re-dissolving nanocrystals stranded on a feature-containing
substrate by solvent washing prior to sintering the nanocrystals.
Further, feature or trench regions showing incomplete filling or
separation from the trench walls can be remedied after sintering or
melting to form bulk material interconnect features through the
reapplication of a nanocrystal solution. Because the solvent
containing the nanocrystals can freely penetrate fissures and voids
associated with an IC patterned for an interconnect feature,
additional nanocrystals are deposited into poorly filled
features.
[0058] In another aspect of the invention, a nanocrystal solution
is applied to flood the substrate surface yet nanocrystals
selectively settle in the feature or trench recesses within the
substrate surface, including those shadowed or otherwise obscured
from evaporative methods. In the process nanocrystals dissolved or
suspended in the solution are carried into these regions. This
process is selective for nanocrystal deposition in substrate
features because the solvent volatilizes more quickly from the
substrate plateaus as compared to the feature recesses causing the
nanocrystals to become concentrated in the shrinking solvent pools
within the feature or trench recesses. Therefore, it is beneficial
to allow the solvent to evaporate at a temperature below sinter
temperature to allow for nanocrystal concentration in substrate
features. Evaporation time may range from a few minutes to tens of
hours. This is highly dependent on the solvent being used, the
temperature, and the pressure. Generally the evaporation time is
nearly inversely proportional to the vapor pressure of the solvent.
For the solutions studied typical evaporation times were in the
range of about 5 min to about 20 min. Eventually, the nanocrystals
become stranded in the dry trenches. Heating the substrate to a
temperature sufficient to sinter or melt the nanocrystals results
in the formation of bulk domains. Preferably, substantially pure
bulk Cu domains.
[0059] Another aspect of the invention contemplates a substrate
comprising an barrier layer. Preferably, the barrier layer is a
layer of TiN or TaN having a sufficient thickness to assure
continuity. There are many barrier materials that may be considered
such as TiN, TaN, WxN, Ta--Si--N, Ti--Si--N, Ta, Ta2N, and others.
Such barrier layers are formed by evaporation of a base metal layer
onto the featured substrate and thereafter exposing the base metal
layer to a nitrogen plasma to induce a surface nitriding as is
conventional to the art. Preferably, the nanocrystal solutions
disclosed herein are coated onto barrier layers.
[0060] Another aspect of the invention contemplates instances where
copper nanocrystals are deposited onto a substrate that is coated
with an intermediate conventional adhesion-promoting layer. In one
embodiment of this aspect of the invention, a hydrophobic solvent
is used to coat a hydrophilic oxide surface. The solvent can be
induced to wet the oxide surface by washing the substrate surface
with a silanol or similar surfactant prior to deposition of the
particle containing solvent. The silanol wash creates a thin layer
that projects organic functionalities away from the surface which
attractively interact with the particle surfaces of hydrophobic
particles and simultaneously bonds to the oxide surface through the
hydroxyl functionality. It is appreciated that other adhesion
promoters known to the art of laminating non-wettable oxide and
metallic layers are operative herein under the disclosed thermal
conditions.
[0061] In yet another aspect of the present invention, the solvent
utilized to form a solution of nanocrystals made using the
methodology described herein, is optionally aqueous or organic. The
solvent is chosen based on factors including, but not limited to
hydrophilicity of the substrate surface, solubility of the
nanocrystals therein, which passivating agent was used, vapor
pressure, toxicity, purity and the like. Solvents may include but
are not limited to water, acetonitrile, methanol, isopropanol,
hexanes, dodecane, toluene, cyclohexanone, diethyl ether,
tetrahydrofuran, dichloromethane, acetone, and ethanol. The
invention contemplates the use of solvents ranging from
fluorocarbons to water, and everything in between, through
manipulation of the chemical character of the passivating
agent.
[0062] Yet another aspect of the invention, recognizes that there
are three temperature ranges important to the process of converting
a feature of a substrate such as a trench on a substrate, filled
with nanocrystals into an operative IC structure. The nanocrystal
application temperature occurs at any temperature at which the
solvent is liquid. It is appreciated that higher application
temperature decreases the number of substrate surface sites a
nanocrystal contacts prior to being stranded on a dry substrate. In
a preferred embodiment, the nanocrystal surface is coated with a
passivating agent that imparts solvent solubility to the
nanocrystal. Upon heating a nanocrystal filled substrate above the
passivating agent thermalization temperature, the passivating agent
is volatilized. This provides for clean nanocrystal surfaces
because of contiguous nanocrystals contacts. While the passivating
agent volatilization temperature is largely size independent and
related to the chemisorption or physisorption energy between the
passivating agent and the nanocrystal surface atoms, nanocrystal
sintering and melting temperature are controllable over hundreds of
degrees Celsius through nanocrystal size domain selection. Once the
passivating agent is volatilized contiguous nanocrystals are able
to sinter.
[0063] A still a further aspect of the present invention, is
capable of overcoming prior art limitations regarding dimensional
shrinkage associated with densification, thin film instability
through repetitive nanocrystal solution application. (K. T. Miller
et al., J. Mater Res. 5 (1990), pp. 151-160; B. A. Korgel and D.
Fitzmaurice, Phys. Rev. Let. 80 (1998), pp. 3531-3534; A. G. Evans
et al., J. Mater. Res. 3 (1988), pp. 1043-1049). Significant
shrinkage is associated with passivating agent volatilization, thus
after applying a nanocrystal solution to a substrate, the substrate
is heated to the passivating agent volatilization temperature and
additional nanocrystal solution applied to fill feature voids
associated with volatilization. This process is optionally repeated
prior to raising the substrate to a sintering of melting
temperature for the nanocrystal mass filling a substrate feature
such as a trench or via. It is appreciated that nanocrystal
deposition is optionally carried out at a temperature above the
passivating agent volatilization temperature such that active
nanocrystal surfaces are formed upon contact with a trenched
substrate.
[0064] Yet a further aspect of the invention envisages filling a
trench of a substrate with enough particles such that the particles
as deposited are at or above the percolation threshold. The
percolation threshold is defined herein as a spatial density of
particles sufficient to traverse the region through at least one
continuous pathway of contiguous particles. To assure percolation
threshold attainment and therefore conductivity upon sintering or
fusing the particles, a trench feature is preferably at least 7
particle widths in the directions parallel to the substrate
surface. Thus, the instant invention is best utilized with features
larger than about 7 nm.
[0065] Having described the inventions, the following illustrative
examples are provided which are not intended to limit the scope of
the invention.
EXAMPLES
Example 1
[0066] Synthesis and Characterization of Copper Nanocrystals
Procedure 1A using 1-decyl-2-methylimidazole as passivating agent:
Under argon and at room temperature 20 mL of pyridine was added to
a vigorously stirred 0.25 M pale blue solution of copper nitrate
(25 mL, 6.25 mmol) and the color immediately became deep blue.
Hexanes (50 mL) was then added. To the mixture a freshly prepared
solution of NaBH.sub.4 (0.60 g) in water (40 mL) was added dropwise
in a period of 15 minutes and further stirring was continued for
another 15 minutes to afford a (red) brown mixture. If stirring was
stopped a brown aqueous phase and a colorless hexanes phase could
be observed. To the stirred red-brown mixture
1-heptyl-4-(4-pyridyl)pyridium bromide (0.697 g, 2.08 mmol, from
Aldrich) was added and stirred for 30 minutes followed by addition
of 1-decyl-2-methylimidazole (0.700 g, 3.125 mmol, from Aldrich)
and stirring for half an hour. Black insoluble material was formed
in the aqueous phase and partially precipitated. The mixture was
filtered on M-frit glass in air and the residue was washed with
water and hexanes. After dried in vacuo overnight at room
temperature black powder (0.430 g) was obtained. The black powder
was considerably soluble in EtOH, and soluble in acetone and
cyclohexanone. After heated under 5% H.sub.2/N.sub.2 at 320.degree.
C. for 2.5 hours, red-brown powder/thin film was obtained, XRD of
which revealed pure crystalline copper.
[0067] Procedure 1B using 1-decyl-2-methylimidazole as passivating
agent: Under argon and at room temperature 20 mL of pyridine was
added to a vigorously stirred 0.25 M pale blue solution of copper
nitrate (25 mL, 6.25 mmol) and the color immediately became deep
blue. Hexanes (50 mL) was then added. To the mixture a freshly
prepared solution of NaBH.sub.4 (0.60 g) in water (40 mL) was added
dropwise in a period of 15 minutes and further stirring was
continued for another 15 minutes to afford a (red) brown mixture.
If stirring was stopped a brown aqueous phase and a colorless
hexanes phase could be observed. To the stirred red-brown mixture
1-decyl-2-methylimidazole (1.390 g, 6.25 mmol, from Aldrich) and
1-dodecylpyridium chloride hydrate (1.000 g, 3.52 mmol, from
Aldrich) were successively added and stirred for half an hour.
Black insoluble material was formed and partially precipitated. The
hexanes phase was colorless and the aqueous phase is pale brown.
The mixture was filtered on M-frit glass in air and the collected
residue was washed with water and hexanes. During washing certain
amount of pale blue material was formed on the inside wall of the
funnel, indicating partial oxidization of the product. After the
residue was first washed with water, hexanes (15 mL) was added to
the residue and the black material on frit-glass was transferred to
a clean M-frit glass funnel and washed with heaxanes. After dried
in vacuo overnight at room temperature black powder (0.455 g) was
obtained. The black powder was considerably soluble in EtOH, and
soluble in acetone and cyclohexanone. After heated under 5%
H.sub.2/N.sub.2 at 320.degree. C. for 2.5 hours, red-brown
powder/thin film was obtained, XRD of which revealed pure
crystalline copper.
[0068] Procedure 2 using tetraoctylammonium decanoate as
passivating agent: Pyridine (25 mL) was added to an aqueous 0.10 M
solution of copper(II) nitrate (25 mL) with rapid stirring. The
resulting dark blue solution was treated dropwise with 0.40 M
sodium borohydride (20 mL) with continued stirring, to afford a
red-brown solution. When effervescence had subsided, the solution
was treated with tetraoctylammonium decanoate in 50 mL of toluene.
After approximately 10 min of rapid stirring, the dark crimson
colored mixture was centrifuged (.about.10 min, <5000 rpm) to
yield a biphasic mixture and a brown precipitate. The supernatant
blue toluene phase was discarded. The precipitate was collected by
vacuum filtration of the aqueous phase, washing any residual
precipitate from the centrifuge tube with water. The collected
solid was washed copiously with water. Copper nanoparticles were
extracted from the brown solid via multiple washings with hexanes,
with the soluble material passing through the filter frit. The
red-brown hexane solution of nanoparticles was passed through a 0.2
.mu.m syringe filter to remove any finely suspended material. A
brown solid could then be obtained by removal of solvent under
reduced pressure and stored under an atmosphere of argon using
standard Schlenk apparatus. The solid can easily be redissolved in
hydrocarbon solvents as desired.
[0069] Preparation of Tetraoctylammonium decanoate: Decanoic acid
(1.70 g) was dissolved in aqueous 0.30 M potassium hydroxide (100
mL). The solution was treated with 0.01 M tetraoctylammonium
bromide (50 mL) in toluene. The milky white mixture was stirred
rapidly and transferred to a separatory funnel. Upon standing, two
phases appeared. The toluene phase was collected and subsequently
used in the preparation of copper nanoparticles given above.
[0070] Procedure 3 using sodium decanoate as passivating agent:
Aqueous copper(II) nitrate (25.0 mL, 0.10 M) was treated with
pyridine (0.80 mL) to afford a deep blue solution. With rapid
stirring, sodium borohydride (20 mL, 0.40 M) was added dropwise.
Upon addition of the first few drops, the color of the reaction
mixture changed rapidly to green, then finally to dark brown.
Continued addition led to vigorous gas evolution and much foaming.
When the foaming had largely subsided, the muddy brown mixture was
treated with aqueous sodium decanoate (30 mL, 90 mM) under rapid
stirring. After approximately 15 min of continued stirring, the
reaction mixture was centrifuged until a dark brown precipitate and
a clear, colorless supernatant solution was observed. The brown
solid was washed with several portions of water, followed by
ethanol. The solid was taken to dryness under reduced atmosphere
using schlenk apparatus. It could be dissolved in organic solvents
such as diethyl ether or toluene.
[0071] Procedure 4 using tetraoctylammonium octylxanthate as
passivating agent: Aqueous copper(II) nitrate (25.0 mL, 0.10 M) was
treated with varying amounts of pyridine (0.8 mL, 1 mL, 10 mL, 25
mL, and 50 mL) to afford deep blue solutions. With rapid stirring,
each solution was treated dropwise with aqueous sodium borohydride
(20 mL, 0.40 M). Each reaction mixture exhibited a color change to
a dark brown or reddish-brown color. The reaction mixtures
containing 0.8 mL and 1 mL pyridine became muddy in appearance with
considerable foaming. The reaction mixtures containing greater
amounts of pyridine became clear, reddish-brown solutions. Upon
reduction, the reaction mixtures were each treated with
tetraoctylammonium octylxanthate in 100 mL toluene (see below).
With rapid stirring, the color was observed to migrate from the
aqueous phase to the organic phase. Some copper-colored opalescence
was noted during the transfer. After about 15 min, the aqueous
phase was observed to be colorless and clear. The organic and
aqueous phases were separated and the aqueous phase discarded. The
nanoparticle containing toluene phase was centrifuged to remove
suspended solids and was transferred to a schlenk flask for storage
under argon. The solution could be taken to dryness under reduced
pressure and redissolved in numerous organic solvents as
desired.
[0072] Preparation of tetraoctylammonim octylxantate: Potassium
octylxanthate was prepared using literature methods. A toluene
solution of tetraoctylammonium bromide (0.670 g in 100 mL toluene)
was treated with an aqueous solution of potassium octylxanthate
(0.300 g in 50 mL water) The biphasic mixture was stirred rapidly
for a period of approximately 2 h, after which time the yellowish
color of the xanthate had transferred to the toluene phase. The
toluene and aqueous phases were separated. The toluene phase,
containing tetraoctylammonium octylxanthate, was used in the above
nanoparticle preparation.
[0073] Analysis: Transmission electron microscopy (TEM)
demonstrated reproducible nanocrystal sizes of 5.5.+-.1.0 nm for
samples obtained using the methods described herein. Nanocrystal
size was not dependent upon the nature of the final passivating
agent, provided that the initial copper(II) salt and pyridine
concentrations were held constant. X-ray diffraction (XRD) spectra
revealed very broad reflections consistent with copper metal, with
crystalline domain sizes of <10 nm. The onset of nanocrystal
melting is generally in the range of 250.degree. C. to 450.degree.
C., with the melting point of a 5.5.+-.1.0 nm nanocrystal sample
typically occurring at or near 350.degree. C., as determined by
differential thermal analysis. The infrared and .sup.1H nuclear
magnetic resonance (NMR) spectra of the nanocrystals showed only
the final passivating agents to be present; no pyridine was
observed. The nanocrystals were highly soluble in many organic
solvents, including hexane, toluene, acetone, and ethyl lactate.
Solutions of up to 10 wt. % copper nanocrystals were obtained in
hexane, demonstrating the high solubility of the nanocrystals.
There was no evidence for light scattering by the solutions, ruling
out colloids. A 10 wt. % copper nanocrystal solution in hexane was
unchanged after 1 month at 23.degree. C. under an argon atmosphere,
demonstrating that the particles do not agglomerate or settle over
extended periods of time. The copper nanocrystals are stable for
days under ambient atmosphere at 23.degree. C. in the solid state.
In solution, exposure to ambient atmosphere results in oxidation to
green, insoluble copper oxide materials over 12-24 h. Accordingly,
solutions of the copper nanocrystals were handled under protective
nitrogen atmospheres. Thermogravimmetric analysis (TGA) of the
carboxylate- and imidazole-passivated nanocrystals revealed weight
loss between 150-300.degree. C. (sublimation of the passivating
agents) to afford ceramic yields of 68-73% (FIG. 1). With selection
of a suitable imidazole or ammonium or alkylammonium
alkylcarboxylate ligand, no weight loss is observed above
300.degree. C. XRD spectra of the residue from the TGA experiment
shown in FIG. 1 demonstrated that pure copper metal was the only
crystalline product (FIG. 2). Separate bulk thermolysis experiments
with the carboxylate- and imidazole-passivated nanocrystals at
350.degree. C. under a 5% hydrogen/95% nitrogen atmosphere led to
70-75% yields of copper metal (as determined by XRD). Since the
particles melt at about 350.degree. C., beautiful fractal-shaped,
shiny copper metal deposits were obtained in the bulk
thermolyses.
Example 2
[0074] Copper Films from the Passivated Nanocrystals. Synthesis of
5.5.+-.1.0 nm copper nanocrystals are passivated with either
tetra-n-octylammonium decanoate or 1-decyl-2-methylimidazole. These
copper nanocrystals melt at about 350.degree. C., exhibit X-ray
diffraction spectra consistent with copper metal, and are highly
soluble (up to 10 wt. %) in organic solvents ranging from n-alkanes
to ethyl lactate. Thermolysis of the nanocrystals at 350.degree. C.
under a nitrogen/hydrogen atmosphere afforded pure copper metal,
with complete evaporation of the passivating agents. Weight loss
upon heating, as measured by thermogravimetric analysis, was
27-32%. Thermogravimetric analysis also indicated that evaporation
of the passivating agents occurs between 150-250.degree. C. A 6 wt.
% dodecane solution of 5.5.+-.1.0 nm tetra-n-octylammonium
decanoate-passivated copper nanocrystals was spin coated onto a
TaN-coated (1000 .ANG. by PVD) flat silicon wafer. Spin coating of
nanocrystal solutions onto a TaN-coated silicon wafer, followed
heating at 350.degree. C. under a 5% hydrogen/95% nitrogen
atmosphere, afforded copper films that exhibited bulk resistivity
values and led to a specular, bright copper-colored film. XPS
analysis of these films indicated pure copper metal, with no oxides
present. The films were highly adherent, and passed the Scotch.RTM.
tape test. The XRD spectrum revealed reflections due only to copper
metal. X-ray photoelectron spectroscopy showed ionizations due to
copper metal (2p 3/2=932.5.+-.0.1 eV), oxygen, and carbon. The
oxygen and carbon signals were nearly eliminated after sputtering
for 150 s, suggesting that these elements are surface contaminants.
There was no evidence for copper oxides in the film, even before
sputtering. Thus, pure copper films were obtained from the
nanocrystals. The electrical resistivity of a 20-nm thick copper
film was 2.7 microhm cm, which is identical to the resistivity
obtained for a 20-nm thick pure copper film that has been reported
in a recent study. Kuan, et al., MRS Symp.Proc. 612, D7.1.1-8
(2000). Accordingly, these films exhibit bulk electrical
conductivity. A scanning electron micrograph of the copper film
showed a smooth surface, with domain sizes of 4-6 nm.
Example 3
[0075] Spin Coating and Heating of the Passivated Nanocrystals. A
dilute (<1 wt. %) pyridine solution of copper nanocrystals is
spin coated onto a silicon wafer containing an array of
0.1.times.1.0 .mu.m blind vias. Notably, substrates that are heated
to about 300.degree. C. do not have a continuous copper film on the
substrate plateau. The substrate following nanocrystal deposition
prior to heating shows completely conformal coverage of the vias
and plateau region. Upon heating to 150.degree. C., wicking of
nanoparticles from the plateau region to the via is observed. At
about 300.degree. C., a perfectly conformal copper film has been
obtained in the via.
Example 4
[0076] Complete trench filling. Conformal copper seed layers are
required for copper fill using electrochemical deposition, and the
fabrication of such layers in high aspect ratio features is
currently a severe problem. In addition, the selective deposition
of copper metal in the vias (and lack of a continuous copper film
on the plateau region) suggests that CMP could be greatly reduced
using the nanocrystals of the present invention. The other EXAMPLES
disclosed herein, sought to assess bottom-up filling of trenches
with copper nanocrystals prior to heating. FIG. 3 shows a sectioned
micrograph of an as-deposited trenched silicon wafer (SiO.sub.2
surface) prior to heating (the wafer was only cleaved and was not
polished on the edge to avoid any heating of the copper
nanocrystals, hence the rough edge). The trenches show conformal,
bottom-up fill. Due to potential adhesion problems between copper
and silicon dioxide, we sought to obtain a TaN-coated trenched
silicon wafer to assess feature filling on a metallized substrate.
The substrates were generously provided by Ron Powell of Novellus
Systems. FIG. 4 shows a cross sectional SEM image shown in negative
of an array of 0.1.times.1.0 .mu.m trenches onto which a 6%
dodecane solution of 5.5.+-.1.0 nm tetra-n-octylammonium
decanoate-passivated copper nanocrystals was spin coated. After
spin coating and solvent evaporation, the substrate was heated to
350.degree. C. under a 5% hydrogen/95% nitrogen atmosphere. This
test pattern is extremely aggressive (aspect ratio of about 10). In
spite of the aggressive pitch, the trenches are nearly all
completely filled with copper metal. There is not a continuous
copper film on top of the substrate plateau, consistent with
experiments described above. Notably, trenches #1,2,5,7 (counting
from left) are perfectly filled, while trenches #3, 4, 6 each show
a small void near the surface of the substrate. The fact that the
trenches have been nearly perfectly filled indicates that the
subject invention provides spin-coating conditions for trench fill
with very low void density. The subject invention's spin on copper
metallization can fill high aspect ratio features (aspect ratio of
about 10) that are well beyond those used in commercial
applications. Copper interconnect aspect ratios typical of the
current device generation is 1.6.
[0077] It is appreciated that various modification of the present
invention in addition to those shown and described herein will be
apparent to those skilled in the art from the above description.
Such modifications are also intended to fall within the scope of
the appended claims.
[0078] All references cited herein are intended to be incorporated
by reference to the same extent as if each was individually and
explicitly incorporated by reference.
* * * * *
References