Benzocyclobutene-based die attach adhesive compositions

The invention is a die attach adhesive composition comprising a homogeneous blend of an organopolysiloxane-bridged bisbenzocyclobutene monomer, an organosilane adhesion aid, and a finely divided electrically conductive metal. The composition is easy to apply, exhibits good adhesive strength and thermal stability at high temperatures, and does not require solvents, initiators, or curing agents. Weight loss on cure is minimal.

BACKGROUND OF THE INVENTION 
This invention relates to die attach adhesive compositions. More 
specifically, it relates to die attach adhesive compositions containing at 
least one bisbenzocyclobutene monomer. 
A die attach adhesive bonds the inactive side of a semiconductor chip to a 
semiconductor package. During assembly of the semiconductor package, the 
die attach adhesive holds the chip firmly in place during wire bonding and 
encapsulation. After assembly, the adhesive provides a conductive path to 
remote heat from the chip and to provide an electrical ground. Examples of 
commonly used die attach adhesives are eutectic solders, conductive 
epoxies, and conductive polyimides. 
Eutectic solders are metal alloys typically made with gold. A "preform", 
which is a metal foil cut to the shape and size of the semiconductor chip, 
is deposited on the desired substrate of the package and is heated to a 
temperature near the melting point of the preform. The chip can then be 
placed onto the preform with a scrubbing motion. Although eutectic solders 
are very reliable and exhibit outstanding thermal stability, they are 
expensive and difficult to process. 
Conductive epoxies are typically low viscosity pastes containing at least 
70 percent of an electrically conductive metal, typically silver. The 
epoxy is applied to the substrate of the package by conventional means and 
the chip is then placed in contact with the coated substrate. The epoxy 
can then be cured in one step. Epoxies are less expensive than eutectic 
solders, are easy to process, and exhibit excellent adhesive strength. 
However, they possess poor thermal stability at high temperatures and an 
unacceptable coefficient of thermal expansion at high temperatures, 
properties which are necessary for advanced applications in electronics. 
Conductive polyimides are similar to conductive epoxies. They possess 
acceptable adhesive strength, excellent thermal stability at high 
temperatures, and an acceptable coefficient of thermal expansion. 
Unfortunately, polyimides must be dissolved in a high boiling solvent, 
such as N-methylpyrrolidone, for use as a die attach adhesive. The solvent 
must be evaporated during a two-step cure. This results in a substantial 
weight loss during cure and can lead to void formation under the 
semiconductor chip. Voids can reduce adhesive strength and thermal 
conductivity, problems which become more serious as the chip size 
increases. 
Copending U.S. application Ser. No. 005,189 (filed Jan. 20, 1987) discloses 
die attach adhesive compositions comprising an arylcyclobutene monomer 
(more commonly referred to as a "cyclobutarene" monomer) and an 
electrically conductive metal. The compositions are easy to process, 
exhibit outstanding thermal stability at high temperatures, and exhibit an 
acceptable coefficient of thermal expansion at high temperatures. 
Furthermore, unlike conductive polyimides, solvents are unnecessary and 
weight loss on cure is minimal. However, the adhesive strength of these 
conductive arylcylobutanes is less than the adhesive strength of the 
polyimides. 
In view of the deficiencies of the prior art a die attach adhesive 
composition, that is easy to process, exhibits excellent adhesive 
strength, has thermal stability at high temperatures, and an acceptable 
coefficient of thermal expansion, is needed. Furthermore, a composition, 
that does not require any solvent, initiator, or curing agent for use as a 
die attach, does not require a two-step cure, and does not exhibit a 
substantial weight loss on cure, is needed. 
SUMMARY OF THE INVENTION 
The present invention is a die attach adhesive composition comprising a 
homogeneous blend of: 
(a) a bisbenzocyclobutene monomer of the formula: 
##STR1## 
wherein: each R is independently alkyl of 1 to 6 carbon atoms, cycloalkyl, 
aralkyl, or phenyl; 
each R.sup.1 is independently vinyl, allyl, methallyl, or styryl; 
each R.sup.2 is independently substituted or unsubstituted alkyl of 1 to 6 
carbon atoms, methoxy, methyl, carboxy, trifluoromethyl carboxy, nitro, 
chloro, bromo, or iodo; 
each R.sup.3 is independently chloro, bromo, iodo, nitro, substituted or 
unsubstituted alkyl of 1 to 6 carbon atoms, and cyano; 
n is an integer of 1 or more; and 
each q and r are independently integers of zero or 1; 
(b) an organosilane adhesion aid; and 
(c) a finely divided electrically conductive metal. 
The die attach adhesive composition of this invention is a low viscosity 
paste that can be applied and processed by conventional methods. It does 
not require solvents, initiators, or curing agents and can be cured in one 
step. Unlike conductive polyimides, the weight loss on cure is minimal. 
The composition exhibits outstanding thermal stability at high 
temperatures and an acceptable coefficient of thermal expansion at high 
temperatures. 
Surprisingly, the combination of the organopolysiloxane-bridged 
bisbenzocyclobutene monomer and the organosilane adhesion aid dramatically 
increases the adhesive strength of the die attach adhesive composition 
relative to the adhesive strength achieved by conductive arylcyclobutene 
compositions. 
DETAILED DESCRIPTION OF THE INVENTION 
The bisbenzocyclobutene monomer of the die attach adhesive composition is 
bridged by an organopolysiloxane group. The monomer has the following 
formula: 
##STR2## 
wherein: each R is independently alkyl of 1 to 6 carbon atoms, cycloalkyl, 
aralkyl, or phenyl; 
each R.sup.1 is independently vinyl, allyl, methallyl, or styryl; 
each R.sup.2 is independently substituted or unsubstituted alkyl of 1 to 6 
carbon atoms, methoxy, methyl carboxy, trifluoromethyl carboxy, nitro, 
chloro, bromo, or iodo; 
each R.sup.3 is independently chloro, bromo, iodo, nitro, substituted or 
unsubstituted alkyl of 1 to 6 carbon atoms, and cyano; 
n is an integer of 1 or more; and 
each q and r are independently integers of zero or 1. 
Preferably, n is 1 so that the organopolysiloxane bridging member is an 
organodisiloxane. The most preferred organodisiloxane bridging member is 
commonly referred to as 1,1,3,3-tetramethyl-1,3-divinyldisiloxane. The 
most preferred bisbenzocyclobutene monomer has the following formula: 
##STR3## 
The bisbenzocyclobutene monomers of this invention can be prepared by 
reacting a bromobenzocyclobutene with the desired organopolysiloxane 
bridging member. The reaction is possible because the organopolysiloxane 
bridging member is a bisvinyl or bisallyl bridging member. The 
substitution reaction of an olefinic compound possessing at least one 
hydrogen on a vinylic position with an organic halide is known and 
disclosed in U.S. Pat. No. 3,922,299 (Heck), which is incorporated by 
reference herein. 
Heck discloses the substitution reaction of aryl halides with olefinic 
compounds in the presence of a Group VIII metal, a trivalent arsenic or 
phosphorous compound, and a soluble trialkylamine. The reaction displaces 
a hydrogen on a vinylic or allylic position with the organic compound. For 
example, the most preferred bisbenzocyclobutene monomer can be prepared by 
reacting about 2 moles of bromobenzocyclobutene with about one mole of 
1,1,3,3-tetramethyl-1,3-divinyldisiloxane in the presence of a catalytic 
amount of palladium acetate and tri(ortho-tolyl)phosphine, in addition to 
triethylamine, which acts as an acid scavenger. 
Organopolysiloxanes and processes for preparing them are known and 
disclosed in U.S. Pat. Nos. 3,584,027; 3,701,195; 3,770,768; and 
3,803,196. A process for preparing bromobenzocyclobutene is disclosed by 
Lloyd et al., Tetrahedron, Vol. 21, pp. 245-254 (1965) at page 253. 
In one embodiment of the invention, the organosilane adhesion aid of the 
die attach adhesive composition has the following formula: 
EQU R.sub.n SiX.sub.(4-n) 
wherein: 
R is a nonhydrolyzable organic radical that prossesses a 
benzocyclobutene-reactive functionality; 
X is alkoxy, acyloxy, or amino; and 
n is an integer of 1, 2, or 3. 
A benzocyclobutene-reactive functionality is a dienophilic functionality 
capable of reacting with the bisbenzocyclobutene monomer component of the 
invention. When the adhesive composition is cured, the cyclobutane rings 
of the benzocyclobutene groups open. The opened rings form conjugated 
dienes (orthoquinodimethanes) that react with dienophilic ("diene loving") 
functionalities. Typically, opened rings react with other opened rings. 
However, opened rings can also react with olefinic or acetylenic 
functionalities via Diels-Alder reactions as disclosed in Feiser and 
Feiser, Organic Chemistry, 3rd ed., (1980). 
Organosilane adhesion aids of this invention are known and sold 
commercially by Petrarch Systems Inc. (see, for example, Petrarch's 
catalog of silicon compounds, published in 1984, at pages 71-76). 
The preferred organosilane adhesion aid is that depicted when the subscript 
n of the formula disclosed hereinbefore is 1 and R contains a 
benzocyclobutene group. A preferred adhesion aid is 
2-vinyl(4-benzocyclobutenyl)triethoxy silane and has the following 
formula: 
##STR4## 
This adhesion aid can be prepared by reacting equimolar amounts of 
4-bromobenzocyclobutene with vinyltriethoxysilane (sold commercially by 
Petrarch Systems, Inc.) using the process conditions disclosed in Heck. 
Preferably, the reaction is carried out in the presence of a suitable 
nonreacting diluent, such as acetonitrile, to control the reaction 
temperature during the exothermic reaction. 
Another preferred adhesion promoter is 3-amido(4-benzocyclobutenyl)propyl 
triethoxy silane and has the formula: 
##STR5## 
Yet still another preferred adhesion aid is 
amido(4-benzocyclobutenyl)phenyl trimethoxy silane and has the formula: 
##STR6## 
These monomers can be prepared by reacting equimolar amounts of 
benzocyclobutene 4-acid chloride with either 3-aminopropyltriethoxysilane 
or aminophenyltrimethoxysilane (both of which are sold commercially by 
Petrarch Systems Inc.). The reaction is carried out in the presence of a 
suitable nonreacting diluent, such as toluene or methylene chloride, and a 
tertiary amine which acts as an acid scavenger. Preferably, the reaction 
temperature is maintained below 20.degree. C. The process for preparing 
the acid chloride is disclosed in U.S. Pat. No. 4,540,763. 
Although the mechanism by which the organosilane adhesion aid enhances the 
adhesive strength of the die attach composition is not fully understood, 
it is believed that the adhesion aid promotes bonding between the 
substrate of the semi-conductor package and the bisbenzocyclobutene 
monomer. If desired, a mixture of two or more organosilane adhesion aids 
can be employed in the die attach composition to enhance the adhesive 
strength. 
The electrically conductive metal of the die attach adhesive composition is 
any metal which provides the desired electrical conductivity for the 
composition. The metal must be in a finely divided form suitable for 
intimate blending with the monomer and adhesion promoter, such as, for 
example, metal flake or powder. Examples of the electrically conductive 
metals of this invention include oxidation resistant metals such as 
silver, gold, copper, nickel and mixtures of these metals. The preferred 
electrically conductive metal is silver flake containing low ionic 
impurities. This metal as well as other electrically conductive metals are 
available commercially. 
The amount of the bisbenzocyclobutene monomer in the die attach adhesive 
composition can range from about 5 to about 30, more preferably from about 
10 to about 25, percent of the weight of the composition. The amount of 
organosilane adhesion aid can range from about 1 to about 10, more 
preferably from about 2 to about 6, percent of the weight of the 
composition. The electrically conductive metal must be present in an 
amount sufficient to reach the percolation threshold value for the 
composition, which is the value at which electrical conductivity can be 
measured. A further discussion of percolation thresholds can be found in 
Kirkpatrick, Reviews of Modern Physics, Vol. 45, No. 4, pp. 574-88, 
October 1973. 
In a preferred embodiment of this invention, a second bisbenzocyclobutene 
monomer is added to the die attach adhesive composition. This monomer is 
added to control the viscosity of the final blend and to maintain adhesion 
after the blend is cured. Examples of suitable bisbenzocyclobutene 
monomers and processes for preparing them are disclosed in U.S. Pat. No. 
4,540,763 and U.S. copending application Ser. No. 005,189 (filed Jan. 20, 
1987), both of which are incorporated by reference herein. 
Examples of the monomers disclosed include those having any of the 
formulae: 
##STR7## 
wherein R is separately in each occurrence hydrogen, an 
electron-withdrawing substituent, or an electron-donating substituent; 
X is --CH.sub.2 --.sub.p, phenylene, 
##STR8## 
p is an integer of between 2 and 10. The preferred monomers are diamide 
and diester-bridged bisbenzocyclobutenes having the following formulae: 
##STR9## 
wherein n is an integer from 2 to 12, inclusive; and 
##STR10## 
Another preferred monomer is a divinyl-bridged bisbenzocyclobutene monomer 
having the following formula: 
##STR11## 
The most preferred monomer is the monomer of Formula V wherein the 
subscript n is 6 or 7. 
The amount of the second bisbenzocyclobutene monomer in the die attach 
adhesive composition, if employed, can range from about 1 to about 20, 
more preferably from about 5 to about 15, percent of the weight of the 
composition. 
A homogeneous blend of the die attach adhesive composition is formed by 
initially mixing the polyorganosilane-bridged bisbenzocyclobutene monomer, 
the organosilane adhesion aid, and the second bisbenzocyclobutene monomer, 
if employed. The finely divided electrically conductive metal is then 
blended with the mixture to a degree sufficient to wet the surface area of 
the metal particles. Intimate mixing can then be achieved by further 
blending of the components on a high shear mixing device, such as a 
three-roll mill. 
After the homogeneous blend is formed, the composition is applied to the 
substrate of the semiconductor package. The composition can be applied by 
screen printing, syringe dispensing, or dot transferring using automated 
or manual die bonding equipment. The inactive side of the semiconductor 
chip is placed on the applied area and the composition is then cured. The 
composition can be cured thermally, without the use of catalysts, 
initiators, or solvents. Generally, as the cure temperature increases, the 
cure time decreases. For example, the composition can be sufficiently 
cured in about 1 hour at 220.degree. C. and in less than 5 minutes at 
270.degree. C. 
The following examples illustrate but do not limit the scope of this 
invention.

EXAMPLE 1 
0.7 Grams (g) of the monomer disclosed as Formula I, 0.375 g of the monomer 
disclosed as Formula V wherein the subscript n is 6, and 0.175 g of the 
organosilane adhesion aid disclosed as Formula III are mixed by hand with 
a flat edge spatula. 3.75 Grams of silver flake sold commercially by Metz 
Metallurgical Corporation as Fine Silver Flake #15 is added to the blend 
and mixed with the blend by hand until all of the silver flake is wetted. 
The blend is intimately mixed on a 3-roll mill under high shear to give a 
smooth paste that can be molded or applied for chip attachment by the 
usual accepted methods. The blended formulation can be cured in a 
convection oven at 240.degree. C. for 20 minutes. 
Tests for thermal properties, bond strength, coefficient of linear thermal 
expansion, and volume resistivity are performed according to the 
procedures described in Military Specification A-87172 and Military 
Standard 883C. Similar tests are performed for two commercially available 
epoxy-based formulations and two commercially available polyimide-based 
formulations. The results appear in Table I. 
TABLE I 
__________________________________________________________________________ 
Benzocyclobutene- 
Based 
Property Measured 
Formulation 
EPOXY A.sup.5 
EPOXY B.sup.6 
POLYIMIDE A.sup.7 
POLYIMIDE B.sup.8 
__________________________________________________________________________ 
Percent Weight Loss 
1.08 2.9 2.7 17.5 20.0 
on Cure.sup.1 
Percent Weight Loss 
0.03 0.33 0.13 1.13 6.01 
at 300.degree. C..sup.1 
Volume Resistivity, 
9.0 .times. 10.sup.-5 
1.7 .times. 10.sup.-4 
5.3 .times. 10.sup.-4 
4.5 .times. 10.sup.-5 
9.3 .times. 10.sup.-5 
ohm .multidot. cm (25.degree. C.) 
Die Shear.sup.2, psi 
3437 4260 6875 3341 3121 
(40 .times. 40 mil die, 25.degree. C.) 
Glass Transition 
370 75 110 Not Not 
Temperature (T.sub.g).sup.3, .degree.C. 
Measured Measured 
Coefficient of Linear 
Thermal Expansion.sup.4, 
inch/inch/.degree.C.: 
-70.degree. C. to T.sub.g 
45 .times. 10.sup.-6 
57 .times. 10.sup.-6 
72 .times. 10.sup.-6 
Not Not 
Measured Measured 
T.sub.g to 150.degree. C. 
-- 189 .times. 10.sup.-6 
160 .times. 10.sup.-6 
__________________________________________________________________________ 
.sup.1 Measured by thermogravometric analysis (TGA) in air. 
.sup.2 Measured on a Model 520D Die Adhesion Tester from Anza Technology, 
Inc. 
.sup.3 Measured by thermomechanical analysis (TMA). 
.sup.4 Measured on a Mettler TMA40 connected to a Mettler TC10A Thermal 
Analysis Processor. 
.sup.5 Sold commercially by Ablestik Labs as 841LMI; recommended cure 
schedule = 150.degree. C./60 min. 
.sup.6 Sold commercially by Epoxy Tech, Inc. as H35175M; recommended cure 
schedule: 150.degree. C./90 min. 
.sup.7 Sold commercially by Epoxy Tech, Inc. as P10; recommended cure 
schedule: 150.degree. C./60 min. 
.sup.8 Sold commercially by Epoxy Tech, Inc. as P1011; recommended cure 
schedule: 70.degree. C./30 min, 150.degree. C./60 min. 
The data in Table I indicates that the die attach adhesive composition of 
this invention exhibits comparable adhesive strength relative to the 
adhesive strength exhibited by the polyimide compositions, without the 
attendant weight loss on cure caused by evaporation of solvents or other 
volatiles. Table I also indicates that, unlike the epoxy compositions, the 
compositions of this invention exhibit excellent thermal stability at high 
temperatures as well as a relatively low coefficient of thermal expansion. 
All of the compositions exhibit acceptable electrical conductivity as 
measured by the volume resistivity. 
EXAMPLE 2 
The procedure of Example 1 is followed to prepare 8 additional blends, 
except that the organosilane adhesion aid is not added to 3 of the blends 
and the amount of each component added to each of the 8 blends varied. The 
die shear for each blend is measured. The results appear in Table II. 
TABLE II 
__________________________________________________________________________ 
Weight Weight Percent 
Weight Weight Die Shear.sup.5, psi 
Blend 
Percent of 
of Percent of 
Percent of 
(40 .times. 40 mil die, 
Number 
Component 1.sup.1 
Component 2.sup.2 
Component 3.sup.3 
Silver Flake 
25.degree. C.) 
__________________________________________________________________________ 
1* 18.75 6.25.sup.4 
None 75 2283 
2* 17 13 None 70 1664 
3* 20 10 None 70 1911 
4 15.6 7.8 3.9 72.7 3493 
5 12 13.3 4.6 70 3740 
6 13 13 4 70 3534 
7 17 9 4 70 3355 
8 22 4 4 70 3231 
__________________________________________________________________________ 
*Not an example of this invention. 
.sup.1 Component 1 is the monomer disclosed as Formula I. 
.sup.2 Component 2 is the monomer disclosed as Formula V wherein the 
subscript n is 6. 
.sup.3 Component 3 is the organosilane adhesion promoter disclosed as 
Formula III. 
.sup.4 Component 2 of blend 1 is the monomer disclosed as Formula V 
wherein the subscript n is 7. 
.sup.5 Measured on a Model 520D Die Adhesion Tester from Anza Technology, 
Inc. 
Table II indicates a dramatic improvement in adhesive strength for the die 
attach adhesive compositions of this invention relative to the adhesive 
strength exhibited by benzocyclobutene-based compositions without the 
organosilane adhesion promoter. 
EXAMPLE 3 
The procedure of Example 1 is followed to prepare 7 additional blends, 
except that different resin components are blended for each of the 7 
blends and the amount of some of the components added to each blend is 
varied. The volume resistivity and die shear for each blend is measured. 
The results appear in Table III. 
TABLE III 
__________________________________________________________________________ 
Die Shear.sup.11 
Volume PSI 
Blend 
Weight Percent of Component 
Silver 
Resistivity,.sup.10 
(40 .times. 40 mil 
Number 
1.sup.1 
2.sup.2 
3.sup.3 
4.sup.4 
5.sup.5 
6.sup.6 
7.sup.7 
8.sup.8 
9.sup.9 
Flake 
ohm .multidot. cm (25.degree. C.) 
die, 25.degree.) 
__________________________________________________________________________ 
1 17 
4 9 70 1.6 .times. 10.sup.-2 
3438 
2 17 
4 9 70 1.1 .times. 10.sup.-4 
3726 
3 17 4 9 70 1.3 .times. 10.sup.-3 
4153 
4 17 9 4 70 1.1 .times. 10.sup.-4 
3011 
5 17 9 4 70 2.2 .times. 10.sup.-4 
3369 
6 14 
3.5 7.5 9 75 9.1 .times. 10.sup.-5 
3520 
7 17 
4 70 1.1 .times. 10.sup.-2 
3548 
__________________________________________________________________________ 
.sup.1 Component 1 is the monomer disclosed as Formula I. 
.sup.2 Component 2 is the organosilane adhesion promoter disclosed as 
Formula III. 
.sup.3 Component 3 is the monomer disclosed as Formula VI. 
.sup.4 Component 4 is the monomer disclosed as Formula V wherein the 
subscript n is 12. 
.sup.5 Component 5 is the organosilane adhesion promoter disclosed as 
Formula II. 
.sup.6 Component 6 is the monomer disclosed as Formula V wherein the 
subscript n is 6. 
.sup.7 Component 7 is the monomer disclosed as Formula V wherein the 
subscript n is 7. 
.sup.8 Component 8 is vinyltriethoxysilane (sold commercially by Petrarch 
Systems, Inc.). 
.sup.9 Component 9 is the monomer disclosed as Formula VII. 
.sup.10 Sold commercially by Metz Metallurgical Corporation as Fine Silve 
Flake #15. 
.sup.11 Measured on a Model 520D Die Adhesion Tester from Anza Technology 
Inc. 
Table III indicates that acceptable electrical and adhesive properties are 
still obtained when different resin components at varying concentrations 
are blended to prepare the die attach adhesive compositions of this 
invention. 
EXAMPLE 4 
The procedure of Example 1 is followed to prepare an additional blend, 
except that the silver flake is replaced with nickel flake sold 
commercially by Metz Metallurgical Corporation as Nickel Flake Batch #109. 
The volume resistivity of the cured blend is 2.3.times.10.sup.-3 
ohm.multidot.cm and its die shear (40.times.40 mil die, 25.degree. C.) as 
measured on a Model 520D Die Adhesion Tester from Anza Technology, Inc. is 
3300 psi. This data indicate that excellent results can be obtained with 
conductive metals other than silver.