U.S. patent application number 10/589399 was filed with the patent office on 2007-08-09 for nanoscale metal paste for interconnect and method of use.
Invention is credited to Guofeng Bai, Jesus Calata, Guo-Quan Lu, Zhiye Zhang.
Application Number | 20070183920 10/589399 |
Document ID | / |
Family ID | 38334253 |
Filed Date | 2007-08-09 |
United States Patent
Application |
20070183920 |
Kind Code |
A1 |
Lu; Guo-Quan ; et
al. |
August 9, 2007 |
Nanoscale metal paste for interconnect and method of use
Abstract
A paste including metal or metal alloy particles (which are
preferably silver or silver alloy), a dispersant material, and a
binder is used to form an electrical, mechanical or thermal
interconnect between a device and a substrate. By using nanoscale
particles (i.e., those which are less than 500 nm in size and most
preferably less than 100 nm in size), the metal or metal alloy
particles can be sintered at a low temperature to form a metal or
metal alloy layer which is desired to allow good electrical,
thermal and mechanical bonding, yet the metal or metal alloy layer
can enable usage at a high temperature such as would be desired for
SiC, GaN, or diamond (e.g., wide bandgap devices). Furthermore,
significant application of pressure to form the densified layers is
not required, as would be the case with micrometer sized particles.
In addition, the binder can be varied so as to insulate the metal
particles until a desired sintering temperature is reached; thereby
permitting fast and complete sintering to be achieved.
Inventors: |
Lu; Guo-Quan; (Blacksburg,
VA) ; Bai; Guofeng; (Redmond, WA) ; Calata;
Jesus; (Blacksburg, VA) ; Zhang; Zhiye; (San
Jose, CA) |
Correspondence
Address: |
BLANK ROME LLP
600 NEW HAMPSHIRE AVENUE, N.W.
WASHINGTON
DC
20037
US
|
Family ID: |
38334253 |
Appl. No.: |
10/589399 |
Filed: |
February 14, 2005 |
PCT Filed: |
February 14, 2005 |
PCT NO: |
PCT/US05/04567 |
371 Date: |
August 14, 2006 |
Current U.S.
Class: |
419/9 ;
75/252 |
Current CPC
Class: |
H01L 2924/01074
20130101; H01L 2924/3512 20130101; B22F 7/064 20130101; H01L
2224/32225 20130101; H01L 2924/0665 20130101; H01L 2224/27332
20130101; H01L 2924/01322 20130101; B22F 1/0059 20130101; H01L
2924/01047 20130101; H01L 2924/0132 20130101; H01L 2924/0132
20130101; H01L 2924/014 20130101; H01L 2924/3025 20130101; H01L
2224/29111 20130101; H01L 2224/83801 20130101; H01L 2924/0132
20130101; H01L 2924/01006 20130101; B22F 2998/00 20130101; H01L
2924/01046 20130101; H01L 2924/01011 20130101; H01L 2924/19041
20130101; H01L 24/83 20130101; H01L 2224/2919 20130101; H01L
2924/0665 20130101; H01L 24/29 20130101; H01L 2924/01049 20130101;
H01L 2224/29111 20130101; H01L 2224/29139 20130101; H01L 2224/29139
20130101; H01L 2924/01033 20130101; H01L 2224/29339 20130101; H01L
2924/01019 20130101; H01L 2924/01029 20130101; H01L 2924/0132
20130101; H01L 2224/29101 20130101; H01L 2924/203 20130101; H01L
2924/01023 20130101; H01L 2924/12041 20130101; H01L 2924/01079
20130101; H01L 2924/01082 20130101; H01L 2924/14 20130101; H01L
2924/01078 20130101; H01L 2924/203 20130101; H01L 2224/2919
20130101; H01L 2924/01005 20130101; H01L 2224/8384 20130101; B22F
2001/0066 20130101; H01L 2224/29101 20130101; H01L 2224/29111
20130101; H01L 2924/01013 20130101; B22F 2998/00 20130101; H01L
2924/00015 20130101; H01L 2924/00 20130101; H01L 2924/01079
20130101; H01L 2924/01082 20130101; H01L 2924/0665 20130101; H01L
2924/00 20130101; B22F 1/0018 20130101; H01L 2924/01079 20130101;
H01L 2924/0002 20130101; H01L 2924/0105 20130101; H01L 2924/00
20130101; H01L 2924/00012 20130101; H01L 2924/0105 20130101; H01L
2924/01047 20130101; H01L 2924/01079 20130101; H01L 2924/0105
20130101; H01L 2924/01082 20130101; H01L 2924/014 20130101; H01L
2924/00015 20130101; H01L 2924/00 20130101 |
Class at
Publication: |
419/009 ;
075/252 |
International
Class: |
B22F 7/04 20060101
B22F007/04 |
Claims
1. A composition for forming electrical interconnect, comprising: a
metal or metal alloy powder composed of a plurality of particles of
a particle size of 500 nm or less; a dispersant associated with
particles of the metal or metal alloy powder, said dispersant being
present in sufficient quantity to reduce or prevent agglomeration
of said particles of the metal or metal alloy powder; and a binder
having a temperature of volatilization below the sintering
temperature of said metal or metal alloy powder.
2. The composition of claim 1 wherein said particle size is 100 nm
or less.
3. The composition of claim 1 wherein said metal or metal alloy is
silver or a silver alloy.
4. The composition of claim 1 wherein the dispersant is a fatty
acid or a fish oil.
5. The composition of claim 1 wherein said binder is a polymeric
material.
6. The composition of claim 1 further comprising a viscosity
adjusting component.
7. A method for forming an interconnect which performs at least one
of mechanically, thermally or electrically connecting a device to a
substrate, comprising the step of: sintering metal or metal alloy
particles that have a size of 500 nm or less which are positioned
on contacts on the device and the substrate and sandwiched
therebetween, said sintering step forming a metal or metal alloy
layer from said metal or metal alloy particles which performs one
or more of mechanically, thermally, or electrically interconnecting
the device and the substrate.
8. The method of claim 7 further comprising the step of depositing
on at least one electrical contact of at least one of the device
and the substrate said metal or metal alloy particles.
9. The method of claim 8 wherein said step of depositing is
performed by screening, printing or stenciling.
10. The method of claim 7 wherein said metal or metal alloy
particles are of a size of 100 nm or less.
11. The method of claim 7 further comprising the step of holding
the device and the substrate together during the step of
sintering.
12. The method of claim 7 wherein said metal or metal alloy is
silver or silver alloy.
13. The method of claim 7 wherein said metal or metal alloy, prior
to said step of sintering, is present in the form of a paste which
comprises a dispersant associated with the metal or metal alloy
particles, said dispersant being present in sufficient quantity to
reduce or prevent agglomeration of said metal or metal alloy
particles, and a binder having a temperature of volatilization
below the sintering temperature of said metal or metal alloy
particles.
14. A method for connecting a substrate and a device, comprising:
positioning a paste between contacts of said substrate and said
device which comprises a metal or metal alloy powder composed of a
plurality of particles of a particle size of 500 nm or less, a
dispersant associated with particles of the metal or metal alloy
powder, said dispersant being present in sufficient quantity to
reduce or prevent agglomeration of said particles of the metal or
metal alloy powder, and a binder having a temperature of
volatilization below the sintering temperature of said metal or
metal alloy powder; and heating said paste to a temperature and for
a time sufficient to remove said binder and said dispersant, and to
sinter metal particles of said metal or metal alloy powder together
to form a metal or metal alloy layer from said metal or metal alloy
particles which performs at least one of mechanically, thermally,
or electrically interconnecting the device and the substrate.
15. The method of claim 14 wherein said metal or metal alloy is
silver or silver alloy.
16. The method of claim 14 wherein said particles are 100 nm or
less in size.
17. The method of claim 14 wherein said positioning step is
performed by stenciling, printing, or screening.
18. The method of claim 14 further comprising the step of selecting
said binder in said paste based on a desired temperature of
volatilization.
19. The method of claim 14 further comprising the step of isolating
said metal or metal alloy particles with said binder until a preset
temperature during said heating step, wherein said preset
temperature is determined based on said binder and a sintering
temperature for said metal or metal alloy particles.
20. The method of claim 19 wherein said preset temperature is the
same as or slightly below a sintering temperature for said metal or
metal alloy particles.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention generally relates to materials used
for interconnecting electronic devices and, particularly, devices
which either generate high temperatures during use or devices which
are used in high temperature applications. Furthermore, the
invention is generally related to a fabrication method which
reduces or eliminates the need for high pressure application during
fabrication of an interconnection, such as during die attach.
[0003] 2. Background Description
[0004] All semiconductor chips have to be fastened or attached to a
substrate to function in an electronic product. The
state-of-the-art technology for interconnecting these chips
typically uses a lead or lead-free solder alloy, or a conductive
polymeric glue, such as an epoxy. However, these materials have
poor thermal properties and do not dissipate the heat generated by
the chips. They also have poor electrical properties and fail to
effectively reduce loss of electrical power, and poor robustness
for mechanical strength and reliability. Furthermore, because of
the low melting temperatures of solder alloys and low decomposition
temperatures of epoxies, these materials may not be generally
suitable for allowing some chips, such as SiC or GaN chips, to
function at high temperatures.
[0005] Sintering of microscale metal powder paste is commonly used
in hybrid electronic packages for producing electrical circuit
patterns. However, the high processing temperatures
(>600.degree. C.) prevents its use in joining electronic
components to substrates. The current practice is to use solder
that is reflowed at temperatures low enough for the devices to
withstand. The advantage of low melting temperatures becomes a
liability for solder alloys because they cannot meet the
requirements of high temperature operation or use in high
temperature applications. Furthermore, solder materials have
relatively poor electrical and thermal properties, and poor fatigue
resistance, compared to other metals such as copper and silver,
which detrimentally affect the performance of the whole electronic
system.
[0006] Pressure-assisted sintering using commercial silver metal
paste to attach electronic components was discussed in Zhang et
al., "Pressure-Assisted Low-Temperature Sintering of Silver Paste
as an Alternative Die-Attach Solution to Solder Reflow", IEEE
Transactions on Electronics Packaging Manufacturing, vol. 25, no.
4, October, 2002 (pp 279-283); and Zhang et al., "Pressure-Assisted
Low Temperature Sintering of Silver Paste as an Alternative
Die-Attach Solution to Reflow", The Fifth International IEEE
Symposium on High Density Packaging and Component Failure Analysis
in Electronics Manufacturing (HDP 2002) The metal powder in
commercial silver metal paste typically has a particle size in the
micrometer range. Because of the large particle size, a high
sintering temperature is required (600.degree. C. and up) under
normal firing conditions. At reduced firing temperature, a large
pressure is applied on the assembly to assist the sintering
process. However, the application of pressure can be undesirable
because of increased difficulty in manufacturing with a
corresponding increase in the production cost. Applying pressure
also increases the likelihood of damage to the device during
processing.
SUMMARY OF THE INVENTION
[0007] It has been discovered that by using very fine conductive
metal and metal alloy particles on the order of 500 nm or less, and
most preferably on the order of 100 nm or less (e.g., 1-100 nm),
densified metallic interconnections can be established by
relatively low temperature sintering with reduced or no pressure
application being required. The materials of this invention can be
applied and processed like a solder paste or epoxy (e.g.,
dispensing, stencil/screen printing, etc.). However, the thermal,
electrical and mechanical properties of the joint formed with the
fine powders and compositions thereof are far superior to those of
traditional lead or lead free solders, epoxy materials, and even
micron sized powders (sintered at low temperature).
[0008] By using metal particles in the nanoscale range, it is
possible to both reduce the bonding temperature (i.e., the
sintering temperature in the context of the present invention), and
to eliminate or reduce the need for high applied pressure. Without
the need for high applied pressure, it is thus possible to make use
of existing hybrid microelectronics processing techniques and
fabrication equipment and, therefore, enable mass manufacturing of
such components. The nanopowder of the present invention can be
prepared using known techniques, or purchased directly at a price
comparable to that that of micron-size powder. A dispersant is
preferably used for reducing agglomeration of the particles which
could lead to undesirable/low silver particle loading during mixing
of the paste. The nanopowder of the present invention, preferably
together with the dispersant, can be combined with a polymer binder
that preferably has a volatilization temperature below the desired
sintering temperature. Using a binder that preferably does not
volatilize until close to the sintering temperature for the metal
or metal alloy powder, assists in achieving denser interconnections
since sintering occurs more uniformly throughout the composition
(i.e., the binder is preferably chosen and formulated into the
composition such that the metal or metal alloy powder on the edges
closer to the source of heat does not start to fuse with
neighboring particles until the bulk of the particles begins to
fuse). Dispersion of the metal or metal alloy powder in the binder
can be facilitated by ultrasonic or mechanical or methods, or
combinations of the same.
[0009] The compositions of the present invention have a wide range
of applications. For example, they can be used to bond silicon
integrated circuit chips in computers, or silicon power chips in
power supplies, or optoelectronic chips in telecommunications
modules. Also, in the case of silver powder, and silver alloys,
where the metal melts at temperatures over 700.degree. C. or
800.degree. C., the invention is suitable for attaching
semiconductor chips that can be operated at high temperatures,
e.g., SiC or GaN power chips. That is, by sintering silver or
silver alloy that is in the form of a nanopowder (one that is less
than 500 nm in size, and most preferably below 100 nm in size) at a
relatively low temperatures (e.g., on the order of 300.degree. C.,
a dense, conductive metal interconnection is achieved that can be
operated at high temperatures without risking melting of the
interconnect, as would be the case with commercial lead and lead
free solders as well as conductive epoxies. The ability to allow
these chips to operate at a high temperatures cuts down their
cooling requirement, leading to savings in materials and energy in
the manufacture and operations of the product.
[0010] The nanosilver paste of this invention, due to its high
melting temperature and low processing temperature, is also useful
for applications other than the attachment of silicon devices and
heatsinks. It may be used to attach/interconnect wide bandgap
devices that need to operate at elevated temperature such as SiC,
GaN and diamond. It is also useful for attaching devices that
generate substantial amounts of heat such as light-emitting diodes
(LED) and semiconductor lasers.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The foregoing and other objects, aspects and advantages will
be better understood from the following detailed description of a
preferred embodiment of the invention with reference to the
drawings, in which:
[0012] FIG. 1 is a schematic view of metal particles (e.g.,
nanosilver particles formed as a paste utilizing dispersants to
prevent agglomeration and binder to prevent paste cracking during
handling and dry processing;
[0013] FIG. 2 is a schematic view showing an exemplary two step
procedure for formulating a nano scale metal particle paste for use
in the present invention;
[0014] FIG. 3 is a schematic view showing the use of metal paste
according to the invention for attachment of devices to a
substrate;
[0015] FIGS. 4a-c are graphs showing a comparison of the relative
electrical conductivity, relative thermal conductivity, and elastic
modulus, respectively, of a various prior art interconnect
materials and the interconnect material of the present
invention;
[0016] FIGS. 5a-b are SEM images of the nanoscale silver paste of
the present invention and the commercial silver paste (Heraeus
C1075) that has micrometer-sized silver, respectively after the
pastes are sintered at 300.degree. C. for ten minutes;
[0017] FIG. 6 is a graph showing the expansion/shrinkage curves for
different particle sizes of silver that are to be sintered; and
[0018] FIGS. 7a and 7b show SEM images of 100 nm silver particles
formulated respectively with higher burn out temperature PVB and
fatty acid dispersants, and lower burnout temperature PRV914.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE
INVENTION
[0019] It has been discovered that sintering a nanoscale metal
paste is a viable solution to forming electrical interconnect
because it enables circumventing the requirements of a high
processing temperature and high processing pressure which are
required with micrometer sized metal powder. Preferably, the metal
powder in the nanoscale metal paste has a particle size less than
500 nm, and most preferably, the particle size is less than 100 nm
(e.g., 1-100 nm or 1-60 nm, etc.).
[0020] The preferred metal or metal alloy within the practice of
this invention is a silver or silver alloy. This is because of a
combination of low cost, as compared to gold, and the amenability
of being fired in ordinary atmosphere. While it is processed at
temperatures comparable to that of solder reflow, it can withstand
subsequent exposure to higher temperature, which solder cannot.
[0021] Suitable nanosilver powder (e.g., less than 500 or 100 nm in
particle size) is commercially available from various suppliers in
various sizes at a cost of roughly $1/gram. Exemplary commercial
suppliers include Nanostructured & Amorphous Materials, Inc.,
Inframat Advanced Materials, Inc., Sumitomo electric U.S.A., Inc.,
and Kemco International Associates. Nanosilver powder has been used
in a variety of applications. For example, silver can be used as an
antibacterial additive to fabric products such as carpets, napkins
and surgical masks. Silver has been medically proven to kill a wide
range of disease causing organisms in the body and is also
relatively safe. For this reason, numerous vendors tout the use of
silver colloid for attacking bacterial organisms in the body.
Samsung also uses silver nanoparticles to enhance food preservation
in its refrigerator product lines. Silver nanopowder is also being
used as an additive in consumer products such as toothpaste,
toothbrush, and soap, as well as socks because of the antibacterial
property. In electronic applications, silver nanopowder is being
sold for use in conductive traces, resistors, electrodes, optical
filters and EMI-shielding (its application to the creation of an
interconnect by sintering at low temperatures for use in high
temperature applications being heretofore unrecognized until the
invention described herein). Nanosilver particles are also used as
a coloring additive in paints, glass, ink, and cosmetics.
[0022] Suitable nanosilver pastes having application in the present
invention may also be produced using a modified Carey Lea method.
The Carey Lea method was first applied to making photographic
emulsions. However, a modified process can be used to synthesize
nanosilver particles (see, for example, S. M. Heard, F. Grieser, C.
G. Barraclough and J. V. Sanders, J. Colloid Interface Sci. 93 (2):
545-555 1983; and F. C. Meldrum, N. A. Kotov, and J. H. Fendler,
"Utilization of surfactant-stabilized colloidal silver
nanocraystallites in the constructruction of mono- and
multiparticulate Langmuir-Boldgett films", Langmuir 10(7):
2035-2040, 1994). In this technique, a reducing agent is prepared
by mixing solutions of sodium citrate and ferrous sulfate. This
mixture is added to a solution of silver nitrate under vigorous
stirring to form a blue-black precipitate which is recovered by
centrifuging the solution. An example for this procedure would be
as follows: Preparation of reducing agent by mixing 3.5 ml of 40%
Na.sub.3citrate.2H.sub.2O and 2.5 ml of freshly prepared 30%
FeSO.sub.4.7H.sub.2O. This is added to 2.5 ml of 10% AgNO.sub.3
solution to precipitate the nanosilver particles.
[0023] It will be apparent to those of skill in the art that the
invention can be practiced with a wide variety of metal and metal
alloy powders. And, in particular, in the case of the preferred
silver powder, the composition and technique are not limited to
pure silver. In fact, it is common practice to modify the alloy
composition and paste components to make them suitable for a wide
variety of applications. Often these metals are also of the
precious metals type such as Au, Pt and Pd. They do have the effect
of raising the firing temperature of the paste and melting point of
the alloy, which could be necessary in some cases. A small amount
of the palladium (Pd) could be added to silver to prevent the
silver migration. Au can also be added to form a gold-silver alloy
with still considerably high melting temperature. Adhesion/bonding
to the die and substrate can be enhanced with the addition of small
amounts of a lower-melting temperature metal such as indium. If
present in small amounts, the operating temperature will still be
higher than high-temperature solder such as eutectic AuSn, yet can
be processed at comparable temperature. Techniques that make use of
the presence of indium in the bonding layer have been developed to
form high-temperature joints but typically require long processing
times (see, for example, R. W. Chuang and C. C. Lee, "Silver-Indium
Joints Produced at Low Temperature for High Temperature Devices,"
IEEE Transactions on Components and Packaging Technologies, 25 (3)
(2002) pp 453-458).
[0024] With reference to FIG. 1, the nanosilver particles 10 are
preferably used in a paste which includes a dispersant 12 to
disperse the silver particles 10 and prevent agglomeration, a
binder 14 to prevent paste cracking during the handling and dry
processing, and, in some instances, a thinner 16 to adjust the
paste viscosity to allow for screen or stencil printing (the
current practice of applying paste to substrates). A wide variety
of dispersants 12 can be used in the practice of the invention
including fatty acids, fish oils, poly(diallyldimethyl ammonium
chloride)(PDDA), polyacrylic acid (PAA), polystyrene sulfonate
(PSS), etc.
[0025] As shown in FIG. 1, in the case of a fatty acid or fish oil,
the dispersant 12 can associate a polar head group with the surface
of a nanosilver particle 10 by hydrogen bonding or other means, and
the hydrophobic tail serves to space adjacent particles apart from
one another and prevent agglomeration. Agglomeration leads to low
solid loading and ultimately interconnections of poor electrical,
thermal or mechanical properties.
[0026] The preferred binder 14 may be a low boiling organic, such
as terpineol (bp of 220.degree. C.) that enables unhindered
densification of the powder at up to 300.degree. C. Examples of
other suitable binders 14 include, for example, polyvinyl alcohol
(PVA), polyvinyl butyral (PVB), and wax. The properties of the
binder 14 (e.g., volatilization temperature) need to match the
sintering kinetics of the nanopowder (i.e., the binder must boil,
vaporize, or otherwise decompose below the sintering temperature)
and the temperature limitations imposed by the device being
attached. As will be discussed in more detail below (see for
example, Comparative Example 1 and Example 1), judicious selection
of or formulation of binder 14 can be used to assure more uniform
sintering of the particles. To reduce viscosity of the paste, which
may be required for enabling stenciling and other operations to be
performed, a thinner 16, such as RV 912 from Heraus, Inc. may be
added. Depending on the choice of binder 14, terpineol may be used
as the thinner 16. The choice of thinner is wide ranging, and will
depend on the needs of the fabricator, the choice of materials, and
other factors. Suitable thinners may include Haraeus HVS 100,
texanol, terpineol, Heraeus RV-372, Heraeus RV-507, etc. As with
the binder 14, the volatilization temperature of the thinner 16
should match the sintering kinetics of the metal particles 10. The
total binder 14 and thinner 16 addition will vary depending on the
application and may constitute, for example, up to 20% by weight or
more (in certain embodiments, the preferred weight percentage is
between 5-20%).
[0027] FIG. 2 shows a two step procedure for formulating a
nano-scale metal paste which may be used in the present invention.
Commercially obtained metal particles 20, of a size less than 500
nm and most preferably less than 100 nm in diameter, are combined
with a fish oil or other suitable dispersant 22 that has been
dissolved in acetone 24. This yields a free flowing powder
(non-agglomerated) 26 of particles that have dispersant associated
on their surfaces. The powder 26 is combined with a solution 27
which includes binder 28 dispersed or dissolved in a carrier such
as a thinner, which ultimately yields a paste 30 that includes the
metal particles dispersed in the binder material. Dispersion of the
metal particles can be aided by immersion in an ultrasonic bath
using a room temperature or cold water bath to prevent heating and
sintering of the metal powder. Additionally, mechanical mechanisms
for stirring, vibrating, etc., can be used to assist in dispersing
the metal particles in the binder. In the process shown in FIG. 2,
excess acetone can significantly aid in a fatty acid dispersant
dispersing silver particles during ultrasonic treatment. Further,
the nonpolar acetone is easily separated from a mixture of silver
plus fatty acid without a centrifuge. The process of FIG. 2 has the
advantage of making control of paste quality easier since the
particle dispersing step is separate from the paste quality
adjustment.
[0028] While FIG. 2 illustrates the dissolved dispersant being
combined with the metal particles, it should be understood that the
particles made by the Carey Lea method described above may have
citrate moieties hydrogen bonded to the surface, and the citrate
may serve as a dispersant. Alternatively, the citrate moieties may
be displaced by a longer chain fatty acid or fish oil dispersant in
a manner similar to that shown in FIG. 2.
[0029] FIG. 3 illustrates an exemplary process for attaching
electronic components to substrates in the practice of the
invention. Initially, a nanoscale silver powder 32 is combined with
polymers 34 to form a nanoscale silver paste 36. As discussed in
conjunction with FIGS. 1 and 2, dispersing the silver powder in the
binder can be enhanced or augmented by ultrasonic methods. The
nanoscale silver powder 32 can be converted to a paste 36 form by
the addition of an organic solvent with a low boiling point (e.g.,
terpineol) and thinner (e.g., RV 912 from Heraeus). Electronic
devices 38, such as silicon or wide bandgap devices, can be joined
to substrates 40 by sintering the nanopowder paste 36 to form a
solid bond layer between the devices 38 and mounting substrate 40.
The process shown in FIG. 3 may be employed with silver particles,
silver alloys, as well as other metals and metal alloys.
[0030] Gold and silver plating can be used to improve the
interconnection in the practice of this invention. For example, to
prevent copper oxidation, since copper oxide cannot form a good
bond with silver by interdiffusion, a thin coating of gold or
silver can be applied to the bonding site and/or contacts on the
device (not shown) prior to screening, stenciling or printing the
nanoscale silver past 36. Utilizing a coating of silver or gold
will not pose a significant deviation from current practices since
the copper substrates in the current commercially available
high-performance electronic packages usually are gold coated
already.
[0031] Except for the low temperature preferably used in the
practice of this invention, the methodology of joining electrical
devices 38 to substrates 40 is similar to conventional metal paste
firing techniques such as those performed for hybrid electronic
packages. The firing temperature, due to the size of the metal
particles (nano scale (preferably less than 100 nm in diameter) as
opposed to micrometer sized), is preferably comparable to solder
reflow, and, if required, only a moderate applied force may be
necessary to maintain intimate contact with the sintering metal
powder layer. As shown in FIG. 3, the nanoscale metal paste is
typically screen or stencil printed on the substrate in the form of
a thick film (e.g., 20 to 100 micrometers thick) pattern onto which
the device is mounted. After device placement, the die may be
pushed down with a moderate force and held in place while sintering
takes place. Depending on the thickness of the film, the size of
the particles, and the material of the particles (e.g., silver or
silver alloy) the sintering time and temperature will vary. In many
applications, the sintering temperature will be at least
250.degree. C. and the duration will generally be 2 minutes or
longer. Sintering can be carried out in a conventional belt oven in
a semi-continuous operation or in a box oven/furnace in a batch
type operation. FIG. 3 shows the electrical devices 38 mechanically
affixed to the substrate 40 in electrical contact with traces or
other contacts after the low temperature sintering operation. As
will be discussed in more detail below, the interconnect formed by
the process is a dense, conductive metal which can operate at
temperatures that are much higher than those used for sintering
(e.g., on the order of 600.degree. C., 700.degree. C. or
900.degree. C. or more).
COMPARATIVE EXAMPLE 1
Nanoscale Silver Paste Versus Micrometer-Size Silver Paste
[0032] Silver compares favorably with other known interconnect
materials such as solder and silver-filled conductive epoxy.
Eutectic Pb--Sn solder is used in the vast majority of
interconnections although lead-free alternatives are gaining
ground. For higher-temperature applications such as bonding of
light emitting diodes (LED) and semiconductor lasers, the eutectic
AuSn is often recommended because they can go to higher
temperatures than Pb-based or Sn-based solders. However, it is a
far more expensive solution. Silver-filled conductive epoxies are
currently used for silicon device interconnect applications. For
example, conductive epoxy is used in International Rectifier's
DirectFET.TM. to secure the silicon dice to a copper cavity. The
properties of these materials are listed in Table 1 and some are
also shown in FIGS. 4a-c. TABLE-US-00001 TABLE 1 Property
comparison of some common interconnect materials with the sintered
nanosilver paste of the present invention. Reflowed Reflowed Silver
filled Sintered eutectic PbSn eutectic AuSn conductive nanosilver
solder solder epoxy paste Composition (wt %) 37Pb63Sn 80Au20Sn
Silver filler and Pure Ag resin and hardener Density 8.47
g/cm.sup.3 14.7 g/cm.sup.3 .about.3 g/cm.sup.3 8.58 g/cm.sup.3
(.about.80% of bulk Ag due to porosity) Bonding mechanism Liquidus
reflow Liquidus reflow Epoxy curing Sintering/solidus
interdiffusion Peak processing 210.degree. C. 310.degree. C.
.about.150.degree. C. 280.degree. C. temperatures Application
<180.degree. C. <280.degree. C. <150.degree. C.
<900.degree. C. temperature range Electrical conductivity 0.69
.times. 10.sup.5 (.OMEGA.-cm).sup.-1 0.62 .times. 10.sup.5
(.OMEGA.-cm).sup.-1 .about.0.1 .times. 10.sup.5 (.OMEGA.-cm).sup.-1
2.6 .times. 10.sup.5 (.OMEGA.-cm).sup.-1 Thermal conductivity 51
(W/K-m) 58 (W/K-m) 0.1 (W/K-m) 240 (W/K-m) Young's modulus 16 GPa
(hard) 68 GPa (hard) <1 GPa (soft) 7 GPa Joint Strength N/A 50
Mpa 5-40 MPa >20 MPa CTE 25 (PPM/.degree. C.) 16 (PPM/.degree.
C.) .about.25 (PPM/.degree. C.) 19 (PPM/.degree. C.) Tensile/shear
>27 Mpa 275/275 Mpa .about.10/10 MPa 43 MPa strength Price
estimate $0.1/gram $40/gram $4/gram $4/gram
[0033] Nanoscale silver as used in the practice of the present
invention instead of micron-size silver is primarily to lower the
sintering temperature to the processing range of most solders. This
allows it to be used as a drop-in replacement for these
interconnect materials. The sintering temperature is sensitive to
the size and morphology of the particles. Silver, which has a very
high diffusion rate, is particularly attractive because it can be
sintered at well below its melting temperature (962.degree. C.) if
the particle size is made small enough. Current silver paste
materials must be fired to above 600.degree. C. to obtain
reasonable strength and density. The prescribed firing schedule is
usually to take the paste to around 900.degree. C. to densify it.
However, in the case of the nanoscale silver paste of the present
invention where the silver particle is less than 100 nm in size, it
can undergo densification starting at as low as 100.degree. C.
(although this is not the desirable temperature range).
[0034] With the addition of the proper types of dispersant, binder,
and solvent, the onset of sintering can be delayed until such time
that the preferred firing temperature is reached (.about.280 to
300.degree. C.) to enable very fast densification rates and attain
not only high density, but also good adhesion onto the device and
substrate. Therefore, in addition to the reduction in particle
size, an important ingredient to the usability of the paste is the
selection of the dispersant and binder system that can be
volatilized and burned off just below the sintering temperature. If
the binder system leaves the paste too early, the silver
nanoparticles will start sintering at a lower temperature, and
consequently with reduced kinetics, the activation of a
non-densifying mechanism, e.g. surface diffusion, occurs resulting
in a microstructure that is difficult to densify even at the higher
targeted sintering temperature. If the binder system components
burn off at a temperature higher than the desired firing
temperature, the silver particles will not sinter properly because
the polymer components will prevent the widespread contact between
particles. A top size of 500 nm (a size that is not traditionally
in the size range considered as "nanoscale") is a practical limit
for this technique because the sintering temperature will rise
accordingly, which could go beyond the desired range and, of
course, will no longer be suitable as a solder drop-in replacement.
Most of the experimental work performed to date and reported
herein, other than commercial Ag paste, has been performed on
powder 100 nm or smaller.
[0035] FIGS. 4a-b show that the nano-silver pastes of the present
invention provide superior relative electrical conductivity and
thermal conductivity compared to eutectic PbSn, eutectic AuSn, and
conductive epoxy. FIG. 4c shows that the elastic modulus of the
sinter nano-silver paste is satisfactory for interconnect
applications.
[0036] FIGS. 5a and 5b are SEM images of silver pastes sintered at
300.degree. C. for 10 minutes. FIG. 5a shows an SEM image of a
sintered nanoscale silver paste according to the present invention,
while FIG. 5b shows an SEM image of a sintered commercially
available silver paste which includes micrometer sized silver
(Heraeus C1075). FIG. 5a shows that a relatively high density
(approximately 80%) results from sintering a nanoscale silver paste
at 300.degree. C. for ten minutes, which is about two times the
green density (silver powder loading only before sintering;
organics are not included). FIG. 5b shows that the commercial paste
with the micrometer sized silver fired under the same condition.
However, the microstructure is very porous, and there is minimal
densification. Specifically, the only change in the structure in
FIG. 5b is the elimination of the sharp contacts. The annealing out
of these features makes the paste difficult to densify.
[0037] FIG. 6 presents a graph obtained from the web site of a
commercial supplier of silver powder (see Ferro's web site) which
presents the shrinkage of silver powder of various sizes with
increasing temperature. The data in this graph, along with the
experiments presented herein, demonstrate that the nanoscale silver
paste of the present invention can be sintered at lower
temperatures with decreasing size.
COMPARATIVE EXAMPLE 2
Comparison of Process for Different Types of Interconnect
Materials
[0038] Some high-temperature-melting solders are currently used for
high-temperature semiconductor device interconnect applications.
For example, eutectic Au80Sn20 solder can be reflowed at
310-330.degree. C. and used at a temperature below its melting
point 280.degree. C. The major differences between the solder
reflow and the nanoscale silver paste sintering of the present
invention include:
[0039] 1) Solder is processed by heating the alloy above its
melting temperature to form the bond. The alloy undergoes melting
and solidification after the completion of the procedure known as
solder reflow. The requirement to melt the alloy means that only
those with low melting points are suitable. This restriction also
limits the maximum operating temperature of the joint to below the
melting point.
[0040] 2) Conductive epoxy is hardened by curing above room
temperature to induce the epoxy to undergo a setting reaction.
While the process temperature is low and no melting is involved,
the maximum working temperature is limited by the decomposition
temperature of the epoxy component, which is in the range of the
curing temperature.
[0041] 3) Attachment/interconnection by nanosilver paste, according
to the present invention, is achieved through a sintering process
wherein the silver nanoparticles undergo consolidation through
diffusion processes rather than by melting. By doing so, high
processing temperature is avoided. On the other hand, because the
melting point of bulk silver is much higher than the sintering
temperature of nanosilver particles, the interconnections can be
operated at temperatures higher than the processing temperature. In
sum, the nanopowder sintering technique of this invention is a
low-temperature bonding solution for high-temperature applications.
The sintering temperature can be reduced significantly by making
the particle size of the powder smaller. As shown above and as is
discussed in comparative example 3, the sintering temperature of
silver can be drastically reduced if micrometer-size particles are
replaced by nanoscale particles. It is then possible to lower the
sintering temperature to that of the reflow temperature of many
solder alloys.
COMPARATIVE EXAMPLE 3
Prior Art on the use of Silver Paste Containing Micrometer Sized
Silver
[0042] Currently available commercial silver/silver alloy pastes
contain micrometer-size silver (silver particles larger than 500 nm
in size and typically on the order of 10-100 .mu.m in size).
Typically, these pastes have to be fired to a high temperature
approaching the melting point of the alloy to achieve high density.
For example, the recommended firing profile of silver paste is to
heat it to around 900.degree. C. (although it is possible to obtain
reasonably high density for mechanical strength at lower
temperatures, e.g. 700.degree. C.). They are most often used to
form conductive traces/patterns (package substrates) and electrodes
(capacitors) for various electronics applications. They are not
typically used for forming interconnects between devices and
substrates, as is proposed in the present application. There are
numerous vendors for these products such as DuPont, Heraeus, and
Ferro. Silver paste has also been considered as a die-attach and
interconnect material. To make it work, an external pressure is
applied to the assembly (about 40 MPa) to lower the sintering
temperature to 300.degree. C. or lower (see, for example, H.
Schwarzbauer, "Method of securing electronic components to a
substrate," U.S. Pat. No. 4,810,672; H. Schwarzbauer and R.
Kuhnert, "Novel large Area jointing technique for improved power
device performance", IEEE Trans. Ind. Appl. 27 (1): 93-95, 1991;
and Z. Zhang and G. Q. Lu, "Pressure-assisted low-temperature
sintering of silver paste as an alternative die-attach solution to
solder reflow", IEEE Trans. Electron. Pack. Manu., 25 (4): 279-283,
2002), which is basically the maximum temperature a semiconductor
device can be exposed to without destroying it. However, high
applied pressure is not the norm in the packaging industry and
could pose serious complications to the attachment/interconnection
process, which in turn could lead to more failures (e.g., cracked
die) and higher manufacturing costs. It may require major
modifications to existing production lines, thus it may not be
considered as a drop-in replacement for solder. The higher cost
alone may discourage industry from adopting it.
[0043] Results for pressure-assisted sintering of commercially
available silver paste (micrometer sized) are summarized in Table
2. A reasonably high density (80%) can be attained only if the
external pressure on the joint is dramatically increased. This is
also accompanied by substantial increases on some key parameter
values of the sintered Ag joint such as electrical conductivity,
thermal conductivity and shear strength. In contrast, with the
nanoscale silver paste of this invention (less than 500 nm and more
preferably less than 100 nm in size), it is not necessary to apply
such a high pressure to induce sintering and bonding, thus making
it a potential drop-in replacement for solder and/or epoxy for die
attachment and electrical interconnection. In practice, the
pressure used before the silver sintering with the silver paste of
the present invention may be used only for better initial interface
contacts, and it is recommended that this pressure not exceed 0.1
Mpa so that the silver paste does not get squeezed out (this
procedure is very common on solder reflow die-attaching).
TABLE-US-00002 TABLE 2 Effect of pressure on the properties of
sintered silver paste with micrometer-size silver. The pastes were
fired at 240.degree. C. for 5 min. Applied pressure Properties 10
MPa 40 MPa Relative Density 57% 80% Electrical conductivity 2.6
.times. 10.sup.5 4.2 .times. 10.sup.5 (.OMEGA. cm).sup.-1 Thermal
conductivity 30 78 W/K-m Shear Strength 15 50 MPa
EXAMPLE 1
Method for Regulating the Densification of Silver by Binder System
Composition
[0044] Regulation of the densification temperature/rate of the
metal particles in the paste of this invention can be achieved by
adjusting the type of components that go into the binder system. In
particular, it is possible to increase or reduce the firing
temperature for any given particle size of the silver (or other
metal or metal alloy). For example, if it is desired to increase
the effective onset of densification of, for example, a nanosilver
paste as discussed above, this can be achieved by substituting the
binder system components with alternatives that burn out at higher
temperatures to closely match the desired or target peak processing
temperature (e.g., the binder system might be chosen to vaporize or
otherwise decompose at a temperature that is the same as or
slightly below (e.g., within 50.degree. C. or 30.degree. C. or
10.degree. C.) the sintering temperature for the metal or metal
alloy particles. This has the added benefit of retaining the
nanoscale silver that will quickly densify when the temperature is
reached, thus keeping the processing time short.
[0045] There are applications where the peak processing temperature
need not be limited to 300.degree. C. or below. For example,
silicon carbide may be attached at temperatures as high as
600.degree. C. using gold or its alloy(s) but with contact pad
problems when fired in air. The present technique can be used to
make a paste that can be fired at a higher temperature to attain
higher density and stronger bonding (but still lower than
600.degree. C. since it is desired to retain the nanosilver
particles until the sintering temperature). An illustrative example
of this technique is shown in FIGS. 7a-7b where a nanosilver paste
containing 100 nm particles and fatty acids of different carbon
chain lengths (hence different burnout temperatures) was sintered
at 450.degree. C. The fired paste in FIG. 7a had a significantly
denser microstructure than that of the paste in FIG. 7b. The higher
burn-out temperature of the PVB and longer-chain fatty acid (C-24)
in the paste in FIG. 7a prevented the aggregation and sintering
reaction of 100 nm particles while still at low temperatures during
heating, thus enhancing the densification rate at the sintering
temperature. Being able to retain the original particle size (or
most of it) while heating to the sintering temperature (thus
preventing the annealing out of the energy needed by the sintering
process) helped achieve the result.
[0046] While the invention has been described in terms of its
preferred embodiments, those skilled in the art will recognize that
the invention can be practiced with modification within the spirit
and scope of the appended claims.
* * * * *