U.S. patent application number 10/399541 was filed with the patent office on 2005-04-07 for composite adhesive.
Invention is credited to Andersson, Clarence A., Roach, Philip J.
Application Number | 20050075438 10/399541 |
Document ID | / |
Family ID | 22910720 |
Filed Date | 2005-04-07 |
United States Patent
Application |
20050075438 |
Kind Code |
A1 |
Andersson, Clarence A. ; et
al. |
April 7, 2005 |
Composite adhesive
Abstract
A composite adhesive featuring a matrix phase that includes a
cyanate ester and a filler or reinforcement phase that includes a
plurality of bodies of at least one material comprising a high
shear strength and/or high modulus material. Preferably, the filler
also possesses at least one of high thermal conductivity and low
coefficient of thermal expansion. Unlike certain commercially
available cyanate esters, those of the instant invention
substantially maintain or even increase in strength upon addition
of the filler to the system. The instant composite adhesives may
also display reduced coefficients of moisture expansion relative to
the unfilled or "neat" resin. Such a composite adhesive is
extremely useful for joining articles where high strength and
minimal swelling in moist environments are required, such as in the
precision equipment industry. In particular, the instant adhesives
find great utility in jointing components for semiconductor
fabrication equipment, such as those that support the optics in a
lithography machine.
Inventors: |
Andersson, Clarence A.;
(Wallingford, PA) ; Roach, Philip J; (Townsend,
DE) |
Correspondence
Address: |
Jeffrey R Ramberg
M Cubed Technologies Inc
1 Tralee Industrial Park
Newark
DE
19711
US
|
Family ID: |
22910720 |
Appl. No.: |
10/399541 |
Filed: |
October 2, 2003 |
PCT Filed: |
October 17, 2001 |
PCT NO: |
PCT/US01/32389 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60241443 |
Oct 18, 2000 |
|
|
|
Current U.S.
Class: |
524/404 ;
524/424 |
Current CPC
Class: |
C09J 179/04 20130101;
C09J 11/04 20130101; C08L 79/00 20130101; C08K 3/38 20130101; C08K
3/20 20130101; C09J 7/10 20180101; C09J 2301/408 20200801; C09J
2479/08 20130101; C08K 3/14 20130101; C08K 3/28 20130101; C08K
3/013 20180101 |
Class at
Publication: |
524/404 ;
524/424 |
International
Class: |
C08K 003/38 |
Claims
1. A composite adhesive, comprising: (a) a matrix phase comprising
a cyanate ester; and (b) a filler or reinforcement phase comprising
a plurality of bodies of at least one material, said bodies making
up no more than 40 percent by volume of said composite adhesive;
and wherein said composite adhesive is free of weak, soft or
friable metals and ceramics; and wherein (c) in the cured
condition, said composite adhesive possesses at least about 85
percent of a strength possessed by said matrix phase alone.
2. A composite adhesive, comprising: (a) a matrix phase comprising
a cyanate ester; and (b) a filler or reinforcement phase comprising
a plurality of bodies of at least one material, said bodies making
up no more than 40 percent by volume of said composite adhesive;
and wherein said composite adhesive is free of weak, soft or
friable metals and ceramics, wherein upon curing, the composite
adhesive possesses a flexural strength that is at least about 85
percent of the flexural strength possessed by a similarly cured
neat cyanate ester having substantially the same composition as
said cyanate ester of said matrix phase.
3. The composite adhesive of claim 2, wherein said filler comprises
at least one material possessing at least one property selected
from the group consisting of high shear strength and high elastic
modulus.
4. The composite adhesive of claim 1, wherein said filler comprises
silicon carbide.
5. (canceled)
6. The composite adhesive of claim 2, wherein said filler possesses
a high shear modulus and does not consist of boron nitride.
7. The composite adhesive of claim 2, wherein said filler possesses
a high shear modulus and does not consist of a metal selected from
the group consisting of silver, gold and copper.
8. The composite adhesive of claim 2, wherein said filler comprises
a material selected from the group consisting of silicon carbide
and aluminum nitride.
9. The composite adhesive of claim 2, wherein said filler comprises
a material selected from the group consisting of boron carbide,
silicon nitride and aluminum oxide.
10. The composite adhesive of claim 2, wherein said filler
comprises a material selected from the group consisting of oxides,
carbides, borides and nitrides.
11. The composite adhesive of claim 2, wherein said filler
comprises a morphology selected from the group consisting of
particulate, platelets and discontinuous fibers.
12. The composite adhesive of claim 2, wherein said composite
adhesive is provided in the form of a sheet or film.
13. The composite adhesive of claim 2, wherein said composite
adhesive is provided in bulk form.
14. An adhesive system comprising the composite adhesive of claim 2
disposed as a layer on at least one face of a release film.
15. The composite adhesive of claim 1, wherein the flexural
strength of said composite adhesive is higher than that of said
matrix phase alone.
16. The composite adhesive of claim 2, wherein the flexural
strength of said composite adhesive is higher than that of said
similarly cured neat cyanate ester.
17. A composite adhesive, comprising: (a) a matrix phase comprising
a cyanate ester; and (b) a filler or reinforcement phase comprising
a plurality of bodies of at least one material selected from the
group consisting of intrinsically strong or hard metals and
intrinsically strong or hard ceramics, the latter group comprising
at least one ceramic material selected from the group consisting of
aluminum oxide, zirconium oxide, titanium diboride, silicon
tetraboride, aluminum nitride and silicon nitride, said bodies
making up no more than about 50 percent by volume of said composite
adhesive.
18. The composite adhesive of claim 17, wherein said composite
adhesive possesses at least about 85 percent of a strength
possessed by said matrix phase alone.
19. The composite adhesive of claim 17, wherein said intrinsically
strong or hard metals comprises at least one metal selected from
the group consisting of molybdenum and hardened steel.
20. The composite adhesive of claim 1, wherein in a cured condition
said composite adhesive has a lower coefficient of moisture
expansion than does a similarly cured neat cyanate ester having
substantially the same chemical composition as said cyanate ester
of said matrix.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Technical Field
[0002] The instant invention relates to high strength adhesives,
particularly those having low coefficients of moisture expansion,
such as those based on monomers or oligomers of cyanate esters. The
invention furthermore relates to composite adhesives, wherein the
matrix of the composite comprises the adhesive, and the filler or
additive comprises one or more strong or elastically rigid
materials.
[0003] 2. Background Art
[0004] For certain applications, the preferred means of fastening
or joining one structure to another is by gluing or adhesive
bonding. Adhesives used to bond components used in applications
requiring precise motion control should be stable with respect to
mechanical loads, temperature changes and changes in humidity. For
mechanical stability, an adhesive should be stiff and strong. For
thermal stability, an adhesive should have a low coefficient of
thermal expansion (CTE) and a high thermal conductivity. For
moisture stability, an adhesive should absorb as little water as
possible and have a low coefficient of moisture expansion
(CME).
[0005] To elaborate further on this moisture stability issue,
consider, for example, a lithography machine for semiconductor
fabrication featuring an optical projection mirror mounted on a
cantilever beam, the end of which is adhesively bonded to a
supporting structure. If one side of the adhesive joint is kept in
a moisture-free environment but the other is exposed to the ambient
atmosphere, it is conceivable that the side of the joint exposed to
the atmosphere will absorb some water vapor and expand, whereas the
side of the joint kept moisture-free will remain in its original
size. This differential expansion will tend to bend the cantilever
beam, which in turn will change the path of light reflected from
the mirror mounted on the cantilever beam. An optics system whose
light path is affected by the humidity of the ambient atmosphere is
not a robust design.
[0006] The cyanate esters are a class of resins that in general
have favorable moisture stability characteristics., They also
possess high tensile and lap shear strengths. While epoxies
generally possess adequate strength for these applications, they
have higher CME's than do the cyanate esters, and also absorb
greater quantities of moisture than the equivalent amount of
cyanate ester.
[0007] Other properties of the adhesive resin that may be important
are its coefficient of thermal expansion (CTE) and its thermal
conductivity, among others. The CTE of the adhesive is important
for much the same reason as described above for CME. Even if the
entire adhesive joint were to heat up uniformly, the joint would
expand. Typically, the CTE of adhesives is greater, often much
greater, than the materials to which they are bonded. Thus, the
adhesive would be expanding much more than the bonded materials for
the same rise in temperature. Such differential expansions are
rarely beneficial, as they often create internal stresses that can
potentially lead to strains or distortions.
[0008] Similarly, the thermal conductivity of polymer adhesives
typically is low, at least compared to that of the materials that
it bonds, such as metals, ceramics and composites. Where thermal
insulating properties are required, the neat resin (e.g., with no
added filler) may perform entirely satisfactorily. Semiconductor
fabrication operations such as lithography, however, generate heat
that needs to be dissipated, thus requiring high thermal
conductivity materials. Here the low thermal conductivity of the
adhesive is a disadvantage, and it would be desirable to have an
adhesive whose thermal conductivity is closer to that of the
substrates bonded by the adhesive.
[0009] Japanese Laid-open Patent Application No. JP 2-175,148 to
Kouichi et al. discloses an adhesive for bonding a skin layer of
thermoplastic resin to a core material. The adhesive may be a
curing type system such as epoxy, urethane or acrylic, or a hot
melt system such as ethylene-vinyl acetate, polyester or polyamide.
A filler is added to the adhesive to reduce the thermal expansion
coefficient. Candidate filler materials include inorganic salts
such as calcium carbonate or calcium sulfate, pulverized metals
such as aluminum or iron, ceramic such as silicon carbide or
silicon nitride, short fibers such as glass or carbon, or woodmeal
or resin powder.
[0010] U.S. Pat. No. 5,844,309 to Yukio et al. discloses an
adhesive composition particularly useful in bonding a semiconductor
device to a substrate. A filler material having a specific particle
size distribution is incorporated into the resin component of the
adhesive, thereby rendering the adhesive capable of completely
filling hollows and gaps during thermal pressing of the
semiconductor device to the substrate. The thermal conductivity of
the adhesive may be enhanced by using fillers having excellent
thermal conductivity, such as aluminum oxide, aluminum nitride,
silicon nitride, silicon carbide, crystalline silica, fused silica
and so forth. Where electrical conductivity is required, silver
powder is used as the filler. Neither Yukio nor Kouichi discloses
the low CME cyanate esters of the instant invention. Further,
"excellent" is a relative term in that while the fillers may be
much more thermally conductive than the adhesive, some on the list,
fused silica for example, are poor thermal conductors compared to
some of their ceramic peers, such as aluminum nitride.
[0011] Japanese Laid-open Patent Application No. JP 11-106,481 to
Masahiro et al. and entitled "Underfill Material for Liquid
Injection Sealing" also discloses an adhesive composition useful
for semiconductor bonding. The composition includes a spherical
inorganic filler having an average particle size of about 0.5-12
micrometers, with all particles smaller than 70 micrometers. The
resin matrix includes an epoxy resin, a cyanate ester and a
bisphenol compound.
[0012] Japanese Laid-open Patent Application No. JP 7-258,542 to
Akio discloses a resin composition based upon a cyanate ester that
is more resistant against separation of admixed microballoons
during curing of the resin. Curing yields a homogeneous,
lightweight material. The data sheet for what appears to be a
related product refers to its composition as a syntactic foam.
(BryteCor.RTM. EX-1541 Syntactic Foam, Bryte Technologies, Inc.,
Morgan Hill, Calif.). Typical applications of this syntactic foam
include foam cores for space structures and net molded foam parts,
e.g., for making tooling.
[0013] U.S. Pat. No. 4,931,496 to Qureshi et al. discloses a tough,
damage tolerant fiber-reinforced composite based upon a cyanate
ester resin formulation. The reinforcing fiber is a structural
fiber such as glass.
[0014] U.S. Pat. No. 5,955,543 to Sachdev discloses an adhesive for
bonding a die of an integrated circuit, the adhesive comprising an
aryl cyanate ester resin and an additive that is a functionalized
oligomeric/polymeric phenolic resin. The adhesive optionally may
contain an electrically or thermally conductive filler. The
electrically conductive filler is preferably a highly conductive
metal such as silver, gold, copper or nickel, and preferably is in
the form of flakes. The thermally conductive fillers can be AlN,
SiO.sub.2, SiC, BN, the like, and mixtures thereof. The weight
ratio of the resin/additive mixture to filler is preferably in the
range of about 15:85 to about 50:50.
[0015] The prior art, as represented by the above-mentioned patent
disclosures, suffers from a variety of shortcomings. As can be
seen, many of the disclosures pertain to bonding integrated
circuits ("dies") to substrates, and many of these applications are
for hermetic environments. Thus, expansion due to moisture
absorption would not be as problematic compared to the product
applications facing the instant inventors, where the adhesive joint
will be exposed to moist environments, or even more challenging,
where one side of the joint will be exposed to the moist
environment. Thus, while some of these patents disclose low CTE
fillers for a resin matrix, the matrix is not the low CME cyanate
ester resin of the instant invention. Others disclose electrically
or thermally conductive fillers for a cyanate ester resin system,
but are silent on the issue of the resulting strength of the filled
cyanate ester adhesive system. Perhaps in these patents, the
strength of the joint is not critical because the component being
bonded is relatively small and lightweight. In contrast, the
instant inventors need a resin system for bonding large, heavy
structures. Thus, the strength of the bond is important. At the
same time, many of the above-mentioned properties are also
desirable, such as high thermal conductivity. There are
commercially available cyanate ester prepreg systems containing
highly conductive structural reinforcement such as graphite fibers.
There is also at least one commercially available cyanate ester
based adhesive system containing thermally conductive boron
nitride. Unfortunately, this composite adhesive system has a lap
shear strength of only about 1000 psi (6.9 MPa) versus about 6000
psi (41 MPa) for the cyanate ester adhesive without such boron
nitride filler.
[0016] In short, the prior art addresses some, but not all, of the
issues confronting the instant inventors.
[0017] Thus, it is an object of the instant invention to produce a
cyanate ester based adhesive featuring an additive filler that,
upon curing, maintains at least a significant fraction of the
tensile and shear strength of the neat cyanate ester.
[0018] It is an object of the instant invention to produce a high
strength cyanate ester based composite adhesive that, upon curing,
absorbs even less moisture than the neat cyanate ester.
[0019] It is an object of the instant invention to produce a high
strength cyanate ester based composite adhesive that, upon curing,
has a higher elastic modulus than the neat cyanate ester.
[0020] It is an object of the instant invention to produce a high
strength cyanate ester based composite adhesive that, upon curing,
has a higher thermal conductivity than the neat cyanate ester.
[0021] It is an object of the instant invention to produce a high
strength cyanate ester based composite adhesive that, upon curing,
has a lower CTE than the neat cyanate ester.
DISCLOSURE OF THE INVENTION
[0022] Accordingly, these and other desirable features of the
instant invention are realized by incorporating a physically strong
and/or stiff filler material into the neat resin, thereby producing
a "composite" cyanate ester based adhesive. Preferably, the filler
also possesses a low CTE and/or high thermal conductivity, at least
in comparison to the corresponding properties of the cured neat
resin. Such candidate filler materials include silicon carbide and
aluminum nitride. These filler materials typically are provided as
finely divided, discontinuous bodies, and are easily admixed with
the cyanate ester based resin system to produce a composite
adhesive. Unlike the commercially available BN filled cyanate ester
resin, it was surprisingly discovered that the strength of the
composite adhesive of the instant invention usually was greater
than that of the neat resin. Moreover, composite cyanate ester
adhesives containing the preferred fillers exhibit even lower
coefficient of moisture expansion (CME) values than the already
relatively low CME's for the neat resin.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a graph showing the strength of the composite
adhesive in flexure mode as a function of SiC filler loading.
[0024] FIG. 2 is a graph of weight increase as a function of time
for various composite adhesive compositions, indicating moisture
absorption.
BEST MODES FOR CARRYING OUT THE INVENTION
[0025] The instant invention comprises a cyanate ester resin system
having incorporated in it as a property-modifying agent one or more
filler materials of high strength and/or high elastic modulus,
thereby producing a "composite" cyanate ester based adhesive. Such
composite adhesives exhibit dramatically improved tensile strength
compared to their unreinforced counterparts. Preferably, the filler
also possesses a low CTE and/or high thermal conductivity. By the
terms "high" and "low", what is meant is that the CTE and thermal
conductivity exhibited by the filler is higher or lower than the
corresponding property exhibited by the neat cyanate ester based
adhesive in the cured condition.
[0026] As mentioned above, a boron nitride (BN) reinforced cyanate
ester system is commercially available because many customers want
the enhanced thermal conductivity provided by the BN, but also want
a low electrical conductivity not provided by metallic fillers.
However, the presence of the BN drastically reduces the bond
strength of the adhesive in the cured condition. The instant
inventors were pleasantly surprised to discover that this strength
reduction effect is not universal, but instead is filler-specific.
In fact, some fillers such as SiC particulate actually increased
the tensile strength of the composite adhesive, and dramatically so
under the right conditions. As will be discussed in more detail
below, at least when the filler consisted of this SiC particulate,
however, there was a limit to the strength-enhancing effect with
increasing concentration, or volunietric loading, of the SiC
filler. At a loading of about 40 volume percent SiC, for instance,
the flexural strength of the bond of the cured composite adhesive
was more than double that of the neat resin in the same cured
condition. The strength dropped quickly, however, with further SiC
additions. At a loading of about 50 volume percent, for instance,
the strength had dropped to below that of the neat (i.e.,
unreinforced) cyanate ester adhesive.
[0027] Without wishing to be bound to any particular theory or
explanation, it may be that certain fillers such as silicon carbide
have elastic moduli (e.g., Young's modulus and shear modulus)
sufficiently large as to act to constrain the matrix during
mechanical loading of the cured composite adhesive. Accordingly,
such fillers may be able to effectively transfer some of the
applied load from the resin matrix to the filler. In contrast,
other fillers, particularly those having the graphite crystal
structure, such as boron nitride, are weakly bonded in one or more
crystallographic directions, and therefore are easily distorted in
these directions under stress. In other words, in the elastic
region, such materials would have a low elastic constant in one or
more crystallographic directions. Similarly, in the plastic region,
they would yield at a low stress level. In a composite adhesive
body, such compliant or low-strength filler materials might act as
a void, thereby forcing the matrix to carry the entire applied
load. Under these conditions, then, the yield and fracture strength
of the composite adhesive might be expected to vary in some
proportion to the volume fraction of resin matrix, in other words,
in inverse proportion to the filler loading. Certain soft metals
often used to make electrically conductive resin compositions, such
as silver, gold, aluminum and copper, might also be expected to
exhibit similar deleterious mechanical behavior in composite
adhesive systems.
[0028] The candidate fillers of the instant invention generally are
the metal- or ceramic-containing materials. Usually, such materials
will inherently possess lower CTE and higher thermal conductivity
than does the cured matrix. Appropriate fillers then, can and
should first be selected or screened based on their effect on the
tensile or shear strength of the composite adhesive. Again, the use
of weak, soft or friable ceramics such as graphite, hexagonal boron
nitride, talc, etc. typically is counterproductive to the
objectives of the instant invention. Similarly, soft metals,
including unalloyed silver, aluminum, gold or copper are not
desirable. Rather, molybdenum or hardened steel, for instance,
should be suitable. Generally speaking, elastically stiff, strong,
intrinsically hard or hardenable metals, such as for example,
through alloying and/or heat treatment, should be useful as
fillers. Among ceramic materials, there are many strong or hard
ceramics from which to choose, including oxides such as aluminum
oxide or zirconium oxide, borides such as titanium diboride or
silicon tetraboride, carbides such as silicon carbide or boron
carbide, and nitrides such as aluminum nitride or silicon
nitride.
[0029] These filler materials typically are provided as finely
divided, discontinuous bodies. The morphology or shape of the
bodies is not particularly critical, although for reasons of
economy and commercial availability, particulate is a popular
choice. Discrete, discontinuous bodies of filler are preferred
because it is anticipated that the most common method for preparing
the instant composite adhesives will be to stir or blend the filler
into the adhesive matrix. Accordingly, while the inventors
anticipate that users may want to mix in any required catalysts
into the resin prior to adding filler, the resulting adhesive still
should have a viscosity that is not so high as to thwart the mixing
in of the filler. The instant inventors have found that a cyanate
ester adhesive having a paste consistency works entirely
satisfactorily. Conversely, exceptionally low viscosities could be
problematical because the added fillers may tend to settle out too
quickly, but such concerns may be largely academic. Specifically,
one can usually tailor viscosity (e.g., raise or lower) as needed
through controlled crosslinking of the resin and/or through
temperature adjustment.
[0030] Further, because the filler contemplated by the instant
invention mostly comprise the metal- or ceramic-containing
materials, and in general, nonporous ones at that, the filler phase
usually does not absorb moisture upon exposure to damp
environments. Accordingly, the composite adhesive will absorb less
water than the same volume of unfilled adhesive. Still further, due
to the mechanical constraining effect of the filler, the composite
adhesive typically will exhibit a lower CME than the unfilled
adhesive.
[0031] The following example further illustrates the instant
invention.
EXAMPLE
[0032] Various volumetric loadings of SiC in a cyanate ester resin
were prepared. Specifically, the resin and catalyst components of
EX-1502-1 cyanate ester paste adhesive (Bryte Technologies, Inc.,
Morgan Hill, Calif.) were placed into a beaker in accordance with
the manufacturer's recommended proportions and heated to a
temperature of about 120.degree. F.-150.degree. F. (49.degree.
C.-71.degree. C.) to reduce their viscosity for improved mixing. At
this temperature, the two components were thoroughly stirred
together. Then, premeasured amounts of Grade 500 RG SIKA green
silicon carbide particulate (Norton-St. Gobain, Worchester, Mass.)
having an average particle size of about 12 micrometers and
preheated to a temperature of about 120.degree. F.-150.degree. F.
(49.degree. C.-71.degree. C.) were hand-stirred into the warmed,
homogeneous adhesive to produce a composite adhesive.
[0033] Pairs of rectangular prisms of Grade HSC-701
aluminum-toughened silicon carbide ceramic (M Cubed Technologies,
Inc., Monroe, Conn.) were bonded at their ends using this composite
adhesive, and then strength tested in flexure. More specifically,
the ends to be joined were prepared for resin bonding by first
"sandblasting" these surfaces with abrasive grit. Both the adhesive
and the prisms of ceramic material were then preheated to a
temperature of about 120.degree. F.-150.degree. F. (49.degree.
C.-71.degree. C.). At this temperature, the composite adhesive was
applied to the joint, and the joined pair was fixtured to prevent
movement during curing. Further heating to a temperature of about
250.degree. F. (121.degree. C.) for about 5 to 16 hours was
sufficient to cure the adhesive. The flexural strength testing was
then performed according to ASTM Procedure No. D790. The results
are presented both in the table below as well as in FIG. 1. As can
be seen, the strength of the composite adhesive 40 percent loaded
in SiC was about twice that of the adhesive containing no SiC.
[0034] The ambient temperature Young's Modulus was measured for the
40 volume percent SiC loaded cyanate ester adhesive using a Sintech
mechanical tester (Systems Integration Technology Inc., Stoughton,
Mass.); it was found to be about 3.9 GPa.
[0035] Further, gravimetric analysis was performed to indicate the
amount of water absorbed by the composite adhesive sample.
Specifically, bodies of cured composite resin corresponding to the
various SiC loadings were prepared by casting the composite
adhesive into discs measuring about 80 millimeters in diameter by
about 3 millimeters thick. The discs of cured composite adhesive
were then placed into a chamber flushed with 98 percent humid
nitrogen gas. This particular atmosphere was prepared by bubbling
commercially pure nitrogen gas through a water bath maintained at a
temperature of about 80.degree. C. The relative humidity was
measured with a hydrometer. The samples were removed, typically on
a daily basis, weighed, and returned to the approximately
20.degree. C. chamber. The results are reported in the table below,
specifically at the 21-day mark. The complete water absorption data
are also displayed graphically in FIG. 2, along with the data
obtained on a commercially available aluminum filled epoxy system
(Hysol EA 9394, Dexter Adhesive and Coating Systems, Bay Point,
Calif.) typically used for bonding aerospace structures.
1 Flexural Strength Vol % SiC Filler (MPa) 21 day Water Absorption
(Wt %) 0 65 0.32 30 110 0.14 40 140 -- 50 55 0.09
[0036] Still further, the coefficient of moisture expansion (CME)
was measured in the "as-cured" condition at ambient temperature
(e.g., about 20.degree. C.) for the samples of cured adhesive
containing zero and 30 volume percent SiC. The CME is a measure of
the fractional length change of the body per unit change in
moisture content. Specifically, the above-described humidity
chamber was placed into the length-measuring stage of a Mettler
TMA-40 thermo-mechanical analyzer (Mettler-Toledo GmbH, Greifensee,
Switzerland). A specimen measuring about 4 millimeters square by
about 16 millimeters long was inserted into the thermo-mechanical
analyzer lengthwise. Approximately daily, the length and mass of
the specimen was measured and recorded. The mass increase was used
as a measure of moisture absorption. At the 21 -day mark, a CME was
computed based on the total length and mass increases. The CME of
the 30 volume percent SiC filled resin was about 400 ppm linear
expansion per weight percent moisture absorbed, which value is
about one-third that of the neat resin.
[0037] Thus, the above example shows that incorporation of some SiC
particulate into a cyanate ester adhesive increases strength,
reduces moisture absorption and reduces CME. Although the flexural
strength of the composite adhesive that was 50 percent loaded in
SiC was only about 85 percent that of the neat resin, such a
formulation still may have utility due to enhancement of other
properties, e.g., reduced water absorption.
[0038] The composite adhesive containing 30 volume percent SiC
particulate was further characterized with respect to it sound
propagating properties in the "as cured" condition. Specifically,
the velocity of sound waves propagating through the filled cyanate
ester composite body was measured using the pulse echo technique of
ASTM Standard D 2485 to determine the Poisson's ration and elastic
modulus ("sonic modulus"). Using the water immersion technique of
ASTM Standard B 311, the bulk density of the body was found to be
about 1631 kg/m.sup.3. From this density and the sound wave
velocities, the Poisson's ratio was calculated to be about 0.31 and
the elastic modulus was computed as being about 8 GPa.
[0039] The EX 1502-1 cyanate ester (Bryte Technologies, Inc.) of
the above example was selected because it exhibited viscosity
characteristics at room temperature that were sought by the instant
inventors, e.g., it had a paste consistency. Other cyanate esters
having somewhat different viscosities should also be useable in
accordance with the instant invention.
[0040] Further on this point, not only may composite cyanate ester
adhesives be prepared in bulk form, it should be possible also to
prepare the composite adhesive in sheet form. Specifically, one or
more mechanically strong and/or stiff fillers can be admixed into a
cyanate ester adhesive. As in the above example, it may be
necessary to heat at least the adhesive somewhat to place the
adhesive in a workable condition, e.g., to reduce viscosity. Before
the composite adhesive crosslinks excessively, it is spread into
sheet form onto a film featuring a release coating. Another
approach is to dissolve the cyanate ester adhesive in a solvent
such as tetrahydrofuran, add the filler(s) to the solution, spread
this mixture onto the release film such as by a doctor blade
technique, and remove the solvent by drying. A thickness of
adhesive of only a few thousandths of an inch, e.g., about 5
thousandths (about 130 microns) has been found to be entirely
satisfactory. The composite sheet adhesive is then stored until it
is ready to be used, preferably in contact with a covering release
film and preferably at a temperature at which little or no
crosslinking occurs. If the release film has release properties on
both sides, the composite sheet adhesive can be stored simply by
rolling the sheet on itself like a roll of wrapping paper.
[0041] Thus, the instant inventors have discovered that it is
possible to tailor or engineer, among other parameters, the
thermal, elastic and expansion properties of the cyanate ester
resin system through addition of the right filler. Not only may the
above-mentioned properties be achieved without sacrificing the
strength of the cured resin, but the strength can actually be
increased, and to a substantial degree. At least when the filler is
SiC, though, the composite adhesive should not be made excessively
loaded in the SiC filler. Thus, it may be necessary to choose the
filler and the volumetric loading of that filler judiciously when
preparing composite adhesives where strength is an important
parameter.
[0042] Further, because the composite adhesive system possesses a
tailorable viscosity, it should be possible to form or mold bulk
structures of the composite adhesive by known techniques to make
self-supporting structures of desired size and shape, e.g., using
the adhesive compositions to fabricate shaped articles as opposed
to joining other structures to one another.
[0043] The preceding example is by no means exhaustive.
Accordingly, the instant invention should in no way be construed as
being limited to the example. In fact, those skilled in the art
will readily appreciate that numerous modifications can be made to
the invention as described without departing from the scope of what
is sought to be protected, which is what is recited in the appended
claims.
INDUSTRIAL APPLICABILITY
[0044] The methods and compositions of the instant invention find
utility in applications in which an adhesive bond is required to
possess high strength and low coefficient of moisture expansion.
Accordingly, the composite adhesives of the instant invention
should be of interest to the precision equipment, robotics,
tooling, electronic packaging, and semiconductor fabrication
industries, among others. Specific applications contemplated by the
instant invention include bonding semiconductor chips to
substrates, and bonding structural components of semiconductor
lithography machines, such as in the bench, bridge and housing
structures for supporting the optics.
* * * * *