A silver metallizing paste for attachment of silicon semi-conductive devices in lead-frame packages, specifically ceramic packages, which is less expensive than a gold preform but useable in hermetic packages, and provides better electrical and thermal conductivity, and higher bond strength, than silver polyimides. From 25 to 95% of silver is blended with a low-melting glass, preferably one having 95-96% PbO, and a paste or ink is formed with a suitable vehicle at 75-85% solids. Use of the paste follows conventional practice. Selection of Ag:glass ratio depends on the type of die bonding to be used. The paste is particularly useful in MOS technology, where low contact resistance is required, and also finds applications as a solder substitute and bonding chip capacitors. It is most advantageous in attachment of larger-area integrated circuits in that stress cracking associated with the gold-silicon eutectic is avoided.

BACKGROUND OF THE INVENTION 
The present invention relates generally to silver metallizations and, more 
particularly, the invention relates to a silver-filled glass composition 
specifically adapted to bond silicon semiconductive devices to substrates. 
Silver metallization compositions had their origins in decorative 
enamelling, but were adapted early on for use in thick film hybrid 
circuitry. The attention of early workers was, however, concentrated on 
designing compositions that would adhere strongly to the ceramic 
substrate. The so-called "Scotch tape test" became an early standard of 
adhesion. Knox, U.S. Pat. No. 2,385,580 disclosed high proportions of 
bismuth oxide in lead borosilicate glasses which was widely used with 
silver, but was not satisfactory with other noble metals. Hoffman, U.S. 
Pat. No. 3,440,182 disclosed additions of vanadium and copper oxides as 
improving adhesion, solderability and conductivity of noble metal 
metallizing compositions generally. These compositions were used as 
conductors, rather than as a medium for attachment of devices such as 
silicon integrated circuits to the substrates. 
In the latter category, gold-based inks or performs have been the most 
common, taking advantage of the low-temperature gold-silicon eutectic to 
achieve a good bond. Even though very substantial efforts have been made 
to reduce the amount of gold used to make such bonds, its expense 
mitigates against its use wherever possible. 
There has been a great deal of effort over the years to eliminate gold from 
hermetic packages in the electronics industry. One of the more difficult 
areas to eliminate gold has been in MOS technology, due to the necessity 
of having backside low resistance contact; as of now, gold is still the 
material of choice in this application. 
Plastic packaging has nearly eliminated the necessity for gold, with the 
exceptions of gold bonding wire and gold evaporated on the backside of the 
wafer. The gold on the frame and gold preform have been eliminated by the 
use of epoxy and polyimides filled with silver flake in such packages. 
Silver-filled polyimides have been used for die attachment in hermetic 
packages. Because of the problem of final cross linking of polyimides, and 
the generation of CO.sub.2 and H.sub.2 O during sealing, this has not 
achieved significant volume. 
There are no low-temperature phases in the silver-gold system, which is a 
continuous series of solid solutions, and the silver-silicon system has a 
eutectic, but a high temperature one (over 800.degree. C.), so systems 
based on silver must employ a fundamentally different bonding mechanism, 
indeed one where the silver per se plays little or no part. 
Thus, where a gold preform is used to attach a silicon die to a silver 
metallized surface, the mechanism on one side is the gold-silicon eutectic 
and on the other it is a solid-liquid diffusion, with the glass playing 
the major role in terms of bond strength. Being less than a metallurgical 
bond, the thermal and electrical conductivity are not as good as desired. 
Pure glass bonds have also been used in this service, but without a 
conductive element both conductivities suffer, as would be expected. 
Regarding silver polyimide compositions, the quantity of silver that can be 
incorporated is limited, and special processing is necessary (for 
high-volume manufacturing, uniformity of processing is an important cost 
consideration). The biggest drawback of polyimides, or any organic bonding 
system, is that they can be used in hermetic packages such as Cerdips, 
because they are moisture getters, can not be outgassed, and generally can 
not withstand high temperature used in assembling these packages. 
The present invention provides a silver-filled glass that produces strong 
bonds between the silicon die and the substrate, whether or not the latter 
is metallized, with controllable thermal and electrical conductivity, and 
which may be used in hermetic packages. 
OBJECTS OF THE INVENTION 
A general object of the present invention is to provide an improved medium 
for bonding silicon dies to substrates. 
A further object of the present invention is to provide a silver-filled 
glass adapted to make strong bonds between silicon dies and metallized or 
bare substrates under normal processing conditions. 
Yet another object of the present invention is to provide a silver-filled 
glass for bonding silicon dies to substrates that is lower in cost than 
gold-based systems, higher in conductivity and bond strength than other 
silver or non-metallic systems, and which is adapted for use in hermetic 
packages. 
Another object of the invention is to provide a silver-filled glass useful 
as a solder-substitute and for bonding capacitor chips to substrates. 
A still further object of the invention is to provide a silver-filled glass 
for bonding silicon dies to alumina substrates that is as good as 
gold-silicon eutectic bonds in terms of adhesion but which is lower in 
thermally induced stresses than eutectic bonds. 
Various other objects and advantages of the invention will become clear 
from the following description of embodiments, and the novel features will 
be particularly pointed out in connection with the appended claims.

DESCRIPTION OF EMBODIMENTS 
In the selection of a silver powder for use with the invention, it has been 
determined that both spherical and flake powders function well, though the 
latter produces a shinier, more metallic-looking finish. It is of interest 
that some prior workers specified flake for silver conductives, but that 
was for a current-carrying "wire" rather than a bonding medium, where 
conductivity is through the thickness, rather than along the length. 
Satisfactory silvers for the invention are those having a surface area in 
the range of 0.2 to 1 m.sup.2 /gm, and a tap density of 2.2 to 2.8 g/cc. 
The glass is the second key component, and it is essential that it be 
low-melting, so as to be molten at the die-attach temperature, 
425.degree.-450.degree. C. The preferred glass selected meets this 
requirement, has a softening temperature of 325.degree. C., and the 
following composition: 
PbO: 95-96% 
SiO.sub.2 : 0.25-2.5% 
B.sub.2 O.sub.3 : remainder 
It has been found that small quantities of ZnO, under 0.5%, are not 
deleterious, but any sodium should be rigorously avoided, as it attacks 
silicon. While bismuth oxide can also be incorporated in low-melting 
glasses, it is harder to mill than lead oxide, and will attack platinum 
used in formulation procedures. Thus, substitution of bismuth for lead is 
not advised. 
The glass is fritted and ground in a high-purity alumina jar mill to meet 
the following specifications: 
surface area: 0.3-0.6 m.sup.2 /gm 
tap density: 2.8-3.6 g/cc 
Generally, glasses having a softening point in the range of 325.degree. to 
425.degree. C., and a coefficient of thermal expansion no higher than 
about 13 ppm/.degree.C., preferably in the range of 8-13 ppm/.degree.C., 
may be used. 
The softening point should be at least 325.degree. C. to insure that all 
organics are burned off. If the softening point is higher than 425.degree. 
C., the glass will not be sufficiently fluid at the die attach 
temperature. The glass is then mixed with the vehicle described 
hereinbelow (80% solids) and milled on a 3-roll mill to a particle size 
(F.O.G.) of 7-8 microns. 
Those skilled in the art appreciate that the selection of vehicle is not 
critical, and a variety of appropriate vehicles are readily available. Of 
course, burn-out must be complete at the indicated temperatures. In this 
case the vehicle selected comprised: 
Ethyl methacrylate: 12% 
Terpineol: 88% 
The silver is then added to the glass paste in a desired silver: glass 
ratio as discussed below, but falling within the limits 25:75 to 95:5. The 
percent (total) solids in then adjusted to within the range of 75-85% by 
addition of more of the vehicle. Outside of this range, rheological 
problems are likely to be encountered; generally a solid content in the 
range of 80-83% is preferred. At this level, typically, the paste will 
have a viscosity of 20-22 Kcps, as measured on a Brookfield RVT 
Viscometer, with a TF spindle, at 20 RPM and 25.degree. C. 
Use of the paste is essentially conventional. Depending on use, a dot, 
square or screened area of the paste is applied on a metallized or bare 
film (ceramic) substrate, machine dispensing, screen printing or stamping 
techniques all being useable. If it is dotted, the size of the dot is 
about 25% larger than the die. The die is attached by placing the die in 
the center of the wet paste and "setting" it by applying pressure, so that 
the paste flows about half way up the side of the die and leaves a thin 
film under the die. Drying in an oven is carried out at 
50.degree.-75.degree. C. for 20-40 minutes. Organic burn-out is done on a 
cycle time of 15-20 minutes, with 2-3 minutes at a peak temperature in the 
range of 325.degree.-450.degree. C. In the accompanying drawing, a 
substrate 10 is shown with a die 12 attached thereto with a layer of 
silver-filled glass 14, which has flowed up around the edges during 
"setting." For test purposes, the package is subjected to a simulated 
(package) sealing cycle is the range of 430.degree. to 525.degree. C., 
with 15 minutes at 430.degree. C. 
Alternatively, the die may be attached by known scrubbing techniques, or 
hot-stage vibratory bonding may be employed. 
A surprising aspect of the invention is that the mechanical strength of the 
bond is proportional to the silver content. Using a standard push test 
(Mil. Spec. 883B, method 2019.1), a range of 5 to 17 lbs. was recorded 
through the silver range of 30 to 95%. As would be expected, electrical 
conductivity also improves with silver content. At the low end, 
resistivity is comparable to the commercial epoxies (25-35 .mu.ohm. cm) 
for example EPO-TEK P-10, and this drops to 5-10 .mu.ohm. cm at high 
silver levels. 
When the substrate has been metallized, acceptable bonds are achieved at 
Ag:glass ratios of 25:75 to 95:5. On bare alumina, it is preferred to keep 
the ratio between 50:50 and 90:10. Note that "acceptable" is here defined 
as well above the mil spec of 4.2 lbs. 
With both bond strength and conductivity rising with silver content a 
question could be raised as to utility of the low-silver, high-glass 
compositions. The answer, generally, depends on intended use. More 
particularly, when the die is to be attached by mechanical scrubbing 
means, very good bonds are achieved with silver in the 25-40% range. In 
situations where it is desired to have the die sink into the ink to a 
degree, the higher silver ratios are preferred. At the very high silver 
end (e.g. 75-95%), tests indicate that the ink can be applied to a bare 
substrate and the chip can be ultrasonically down-bonded with good 
results. One would not want to go much above 90% silver as adhesion will 
start to drop off. There are thus a variety of possibilities, including 
elimination of certain processing steps, by, for example, attaching lead 
frames and dies at the same time. 
It is possible to substitute certain base metals for a portion of the 
silver but, generally, adhesion will drop and resistivity will rise with 
such substitution. Specifically, up to 10% Ni, up to 60% Sn and up to 
about 20% Cu were substituted and resulted in acceptable bond strengths, 
providing firing was carried out in air, not nitrogen, and at a combined 
metal:glass ratio of 80:20 (nitrogen firing reduces the lead oxide and 
destroys the glass). 
An important aspect of the invention is its applicability to the larger 
integrated circuits now coming into use. More particularly, it is known 
that the gold-silicon eutectic is a brittle intermetallic, and that any 
bonding material must accommodate the different thermal expansion rates of 
the die and the substrate. This is not a notable problem with small chips, 
but in the VLSIC range the sealing cycle temperature can cause both bond 
failure and chip cracking due to thermal stress. Because the composition 
of the present invention softens rather than melts, such thermal stresses 
are avoided, as has been shown by thermal shock test (mil spec standard 
883B, condition A). 
Lastly, the question arises as to whether there might be applications of 
the invention where it would be desireable to substitute a noble metal, 
particularly gold, for a portion of the silver. It was found that no 
particular advantages accrued by this expedient. More particularly, a 
standard gold paste was mixed with an 80:20 Ag:glass paste of the 
invention in proportions that ranged from a Au:Ag ratio of 10/90 to 80/20. 
While the conductivity of bonds to chips showed some tendency to rise with 
higher gold, results were inconclusive, and there was clearly no cost 
justification for such substitution. Moreover, the shear strength of the 
bonds tended to drop at higher gold, though it was acceptable at any 
level. No gold-silicon eutectic was observed, presumably due to features 
of the Au-Ag-Si ternary phase diagram. There is thus no apparent reason to 
sacrifice the considerable economies of the invention by trading-off 
silver for gold. 
A further important application of the invention is in bonding chip 
capacitors to substrates. For example, a 120.times.90.times.35 mil 
capacitor is "set" in a 5-7 mil pad of the silver-filled glass of the 
invention, dried and fired as noted above. Shear strength was 13.8 lb., 
and a good electrical contact was made around the sides. In terms of 
hybrid circuit manufacture, this has important ramifications, to wit, 
circuit chips and capacitors can be attached, dried and fired in a single 
cycle, with good bonds. Moreover, subsequent processing or operation may 
be carried out at temperatures that would melt conventional solder pastes. 
A further application for the invention is as a substitute for solder. More 
particularly, at the preferred 80:20 Ag:glass ratio, and the 80-85% solids 
content, the composition of the invention will "hold" the device to be 
soldered through the firing cycle, whereas solders will allow movement. 
Various changes in the details, steps, materials and arrangements of parts, 
which have been herein described and illustrated to explain the nature of 
the invention, may be made by those skilled in the art within the 
principle and scope of the invention as defined in the appended claims.