Radiation sensitive adhesive composition and method of photoimagingsame

A method of photodelineating an adhesive on a substrate. The first step (10) is to coat the substrate with a layer of photopolymeric adhesive, typically spin coated. The adhesive is then briefly `soft baked` at a moderate temperature to set it (20). Portions of the adhesive are then selectively exposed to actinic radiation to partially cure them, while other portions are not exposed (30). A photomask is used to selectively expose the photopolymeric adhesive to ultraviolet light at an intensity and for a time sufficient to partially cure the photopolymeric adhesive. The adhesive is developed (40) to selectively remove those portions that were not exposed to radiation, usually in an appropriate solvent, creating a pattern in the adhesive. The developed adhesive pattern is then heated for a time and temperature sufficient to completely cure it (50).

TECHNICAL FIELD 
This invention relates in general to a radiation sensitive composition and 
a method of producing an image in the composition, and more particularly 
to a photopolymeric adhesive and a method of photoimaging the adhesive. 
BACKGROUND 
Adhesives find use in an unending variety of applications, and are an 
integral part of our daily lives. A good adhesive must have many 
properties, some of which are mutually exclusive, and therefore require 
compromise in selection. For example, it must adhere tenaciously to the 
substrate upon which it is to be bonded, it must be able to be converted 
from a `non-adhesive` state to an adhesive state, it must be able to be 
properly applied or located on the substrate of choice, and must meet 
numerous other criteria such as cost, environmental resistance, toxicity, 
reliability, shear strength, peel strength, color, viscosity, etc. Of 
these, we are concerned here with the ability to be properly applied or 
located on the substrate of choice and the ability to be converted from a 
`non-adhesive` state to an adhesive state. 
Conventional adhesives are applied in myriad ways, for example, dispensing, 
pouring, roller coating, spraying, dipping, painting, stenciling, and 
electrostatic coating. Each of these are performed manually or by machine. 
Those skilled in the art will appreciate that each technique poses 
problems in uniform and accurate application, and can be messy. Creation 
of complex patterns of adhesive is difficult, and is typically achieved by 
stenciling or dispensing. Both of these methods have a limit to their 
usefulness when trying to achieve fine resolution and accurate placement 
of the adhesive. 
Conventional adhesives, such as ultraviolet (UV) light-curable adhesives, 
cannot be patterned with existing photodelineation methods. To do so would 
completely cure the adhesive, thereby rendering it useless as an adhesive. 
The UV adhesives must also be fully exposed to be cured, which requires 
that either the part to be bonded is very small, or it is transparent to 
UV light, so that the adhesive can be cured under the part. This severely 
limits their use. 
Photoresists have been used as masks to create very fine patterns on 
printed circuit boards and semiconductor wafers. These polymers are 
typically applied as a dry film or a liquid, and are selectively exposed, 
and developed. While these materials are widely used in the electronics 
industry, they suffer from poor adhesion to the substrate and cannot be 
used as true adhesives to bond articles together due to the need to 
irradiate the adhesive to cure it. For example, Gurtler, in U.S. Pat. No. 
3,909,930 describes the use of RISTON photoresist to create a cavity in a 
liquid crystal display. Those skilled in the art will appreciate that 
while RISTON is an excellent photoresist for masking and etching circuit 
patterns, it can not be used as an adhesive due to the marginal adhesion 
to the substrate. Sullivan, in U.S. Pat. No. 4,966,827 describes a 
cumbersome method to overcome the stated poor adhesion of dry film 
resists. Further, the resist must be fully and uniformly exposed by the 
light for proper results, thereby precluding its use as an adhesive to 
bond two opaque materials together. For example, RISTON cannot be used to 
adhesively bond a metal heat sink to a printed circuit board, because it 
is a very poor adhesive, and because the light cannot get through the 
opaque metal to initiate the curing photoreaction. Attempts to thermally 
cure these types of resists produce minimal, if any, adhesive bonding. 
Therefore, a need has existed for a material and a method to produce fine 
line, high resolution patterns of adhesives to bond opaque materials 
together. 
SUMMARY OF THE INVENTION 
Briefly, according to the invention, there is provided a method of 
photodelineating an adhesive on a substrate. The substrate is first coated 
with a layer of photopolymeric adhesive, and heated at a moderate 
temperature for a brief time. Portions of the adhesive are selectively 
exposed to actinic radiation to partially cure them, while other portions 
are not exposed. The adhesive is developed to selectively remove those 
portions that were not exposed to radiation, thereby creating a pattern in 
the adhesive. The developed adhesive pattern is then heated to completely 
cure it. 
In an alternate embodiment of the invention, the photopolymeric adhesive is 
spin coated onto a substrate. A `soft bake` at a moderate temperature for 
a brief time is performed to `set` the adhesive. A photomask is used to 
selectively expose the photopolymeric adhesive to ultraviolet light at an 
intensity and for a time sufficient to partially cure the photopolymeric 
adhesive. The adhesive is developed in a suitable solvent to remove the 
adhesive that was not selectively exposed, thereby creating a pattern. The 
pattern is further cured by heating it for a time and temperature 
sufficient to fully cure the adhesive.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
While the specification concludes with claims defining the features of the 
invention that are regarded as novel, it is believed that the invention 
will be better understood from a consideration of the following 
description in conjunction with the sole drawing figure, in which like 
reference numerals are carried forward. 
The main thrust of this invention is to obtain adhesive compositions 
suitable for use as a photoimageable adhesive, and methods for imaging 
these adhesives. A particularly advantageous embodiment provides a 
composition and a method of patterning an adhesive that can be used for 
tightly bonding two opaque materials together. The preferred adhesive 
composition is a modified commercial formulation which, when processed in 
a particular way, produces the desired result of UV imaging and thermal 
curing, and has the requisite adhesive bonding and thixotropic properties. 
The formulation consists of an UV thermally active resin that is UV 
photoactive. Typically, the resin contains prepolymers and monomers, a UV 
active catalyst to polymerize the resin, and heat active catalysts for the 
final cure. The catalysts can also be dual function catalysts. These types 
of formulations harden and cure upon exposure to UV light and then become 
fully cured upon exposure to heat. Alternatively, a single photoactive 
catalyst, or a UV active resin, susceptible to varying amounts of 
radiation can be employed. 
Referring now to FIG. 1, a process flow for the adhesive is disclosed. In 
step 10, a UV curable adhesive, such as the family of acrylic adhesives, 
typified by LOCTITE UV 352 from Loctite Corporation, is coated onto a 
substrate. Other types of adhesives might also be suitable, for example 
adhesives curable by electron beams, visible or infrared light, x-rays, 
etc. The substrate can be any number of materials, and is used herein to 
designate the member upon which the adhesive is desired to be patterned. 
The adhesive is preferably spin coated onto the substrate in order to 
create a thin, uniform layer of adhesive of the desired and requisite 
thickness. At this point, the adhesive is a viscous liquid, and must be 
handled carefully. 
Step 20 is a partial curing of the adhesive coating. For the particular 
adhesive described herein, this step has been found to be critical in the 
successful implementation of the process. One way to partially cure the 
adhesive is to `soft bake` it. If the soft bake step is omitted, the 
adhesive does not cure in the desired manner, and does not photoimage 
properly. The exact mechanisms occurring during soft bake are not fully 
understood, but it is postulated that the heat causes removal of various 
chemicals that affect the curing mechanism, or acts as a catalyst to 
initiate the curing reaction, and "partially" cures the adhesive. The 
range of soft-bake conditions found to be workable with the selected UV 
adhesive are between 40.degree. C. and 85.degree. C. for between 1 and 15 
minutes, with 5 minutes at 55.degree. C. the preferred conditions. Other 
methods of partially curing might be substituted for the soft bake, such 
as exposure to low levels of UV radiation. 
The adhesive is then masked with a phototool and imaged in step 30. The 
phototool can assume many forms, from a crude cardboard mask to a 
sophisticated glass phototool. The goal is to selectively expose portions 
of the thin adhesive coating to UV light in order to partially cure the 
adhesive. The phototool must have areas that can pass the light with 
minimal attenuation, such as holes in a mask or transparent areas in a 
glass tool. Other areas must obstruct the light from reaching the 
adhesive, and are typically opaque. In the mask, these would be solid 
portions, and would be black portions on a glass phototool. Any manner of 
achieving this selective exposure will deliver equivalent results. For 
example, the phototool could be placed directly on the adhesive, slightly 
above the adhesive, or at a distance from the adhesive, or it may be 
eliminated entirely by using direct imaging. Direct imaging uses a moving 
right beam, such as a laser, to directly expose portions of the adhesive. 
This technology finds wide use in exposing dry film photoresists, and can 
be employed here. In any case, the time and intensity of exposure is 
dependent on the particular material and thickness of the coating, and 
should be of such level as to partially, but not completely, cure the 
adhesive. If it is too high, the adhesive will cure too much and cannot be 
properly further cured. Too little exposure will not allow the image to be 
properly developed. Exposure intensities between 500 and 2000 millijoules 
were found effective, with 1500 millijoules being preferred. Collimated 
ultraviolet light at a wavelength of about 365 nm is best for the 
particular adhesive described here. At this point, a latent image is 
present in the adhesive. That is, the adhesive has been selectively 
exposed (polymerized), but the image cannot be seen with the naked eye. 
The next step (40) is to develop the latent image. Typically, one or more 
suitable solvents are used to dissolve away the unexposed (unpolymerized) 
portions of the adhesive. Solvents such as water, alcohol, xylene, 
chlorinated hydrocarbons, ethers, esters, and mixtures thereof are 
typically used. The solvent is chosen such that it dissolves the monomeric 
resin, but does not attack or degrade the exposed resin. The image can 
also be developed in other ways, such as plasma developing, where the 
uncured adhesive is removed by plasma ashing. After developing, the 
desired image or pattern can be seen in the adhesive, because unwanted 
portions have been removed. The adhesive is now partially cured, and is 
dry to the touch, but not fully cured. This is commonly referred to as a 
`B-Stage` of material. The photoinitiators have reacted with the adhesive 
and converted it from a liquid state to a solid, but further cure is still 
available. 
The final step (50) is the attachment of the desired second material, such 
as an electronic component, another substrate, or other desired part, onto 
the B-Stage adhesive. Since the adhesive is still somewhat tacky, this is 
easily performed. The adhesive is completely cured by heating at a time 
and temperature sufficient to cure and securely bond the second material 
to the first material. Temperatures between 125.degree. and 150.degree. C. 
were found effective, with 145.degree. C. for 1-2 hours being preferred. 
This treatment converts the adhesive from a B-Stage material to a C-Stage 
material. Other methods of fully curing the adhesive such as flood 
exposure at high levels of UV energy might also be useful. 
EXAMPLE 1 
LOCTITE 352 adhesive was spin coated onto a polyester substrate by spinning 
the substrate at 5000 revolutions per minute for 30 seconds using a spin 
coater made for semiconductor wafers. The viscosity of the adhesive prior 
to spin coating was about 20,000 centipoise. The substrate with the coated 
adhesive was soft baked at 55.degree. C. for 5 minutes. After soft baking, 
the adhesive was masked with a glass phototool and exposed to collimated 
ultraviolet light at 365 nm. The light intensity was 1500 millijoules. 
After exposure, the latent image in the adhesive was developed for 
approximately 1 minute in n-butyl acetate, and then rinsed in xylene, 
followed by deionized water. The resulting pattern was between 10 and 12 
microns thick, and features as small as 0.025 mm could be discerned. A 
second polyester substrate was placed onto the adhesive, and the assembly 
was cured with heat and pressure in a 145.degree. C. oven for 1 hour. The 
composite was found to be tightly bonded after the process, and exhibited 
20 lbs of adhesion when tested in the shear mode. All adhesion tests were 
performed on a 6.times.25 mm section of the substrates. 
EXAMPLE 2 
LOCTITE 352 adhesive was spin coated onto a polyester substrate by spinning 
the substrate at 5000 revolutions per minute for 30 seconds using a spin 
coater made for semiconductor wafers. The viscosity of the adhesive prior 
to spin coating was about 20,000 centipoise. The adhesive was masked with 
a glass phototool and exposed to collimated ultraviolet light at 365 nm. 
The light intensity was 1500 millijoules. After exposure, the latent image 
in the adhesive was developed for approximately 1 minute in n-butyl 
acetate, and then rinsed in xylene, followed by deionized water. The 
resulting pattern was poorly developed, and had poorly discernible 
features. A second polyester substrate wets placed onto the adhesive, and 
the assembly was cured with heat arid pressure in a 145.degree. C. oven 
for 1 hour. The composite was found to be tightly bonded after the 
process, and exhibited 20 lbs of adhesion when tested in a combination 
shear/tension mode. 
EXAMPLE 3 
LOCTITE 352 adhesive was coated onto a polyester substrate by forming a 
thin layer on the substrate with a blade. The viscosity of the adhesive 
was about 20,000 centipoise. The adhesive was masked with a glass 
phototool and exposed to collimated ultraviolet light at 365 nm and 1500 
millijoules. After exposure, the latent image was developed in n-butyl 
acetate, and then rinsed in xylene, followed by deionized water. The 
resulting pattern was poorly developed, and had poorly discernible 
features. A second polyester substrate was placed onto the adhesive, and 
the assembly was cured with heat and pressure in a 145.degree. C. oven for 
1 hour. The composite was found to be poorly bonded after the process, and 
exhibited 2 lbs of adhesion when tested in a shear mode. 
EXAMPLE 4 
LOCTITE 352 adhesive was coated onto a polyester substrate by forming a 
thin layer on the substrate with a blade. The viscosity of the adhesive 
was about 20,000 centipoise. The substrate with the coated adhesive was 
soft baked at 55.degree. C. for 5 minutes. After soft baking, the adhesive 
was masked with a glass phototool and exposed to collimated ultraviolet 
light at 365 nm and 1500 millijoules. After exposure, the latent image was 
developed. The resulting pattern was poorly developed, and had poorly 
discernible features. A second polyester substrate was; placed onto the 
adhesive, and the assembly was cured with heat and pressure in a 
145.degree. C. oven for 1-2 hours. The composite was found to be poorly 
bonded after the process, and exhibited 20 lbs of adhesion when tested in 
a shear mode. 
EXAMPLE 5 
RISTON 318 R photoresist was coated onto a polyester substrate by 
laminating under heat and pressure. The photoresist was masked with a 
glass phototool and exposed to collimated ultraviolet light at 365 nm. 
After exposure, the latent image was developed. The resulting pattern 
exhibited discernible features as small as 0.05 mm. A second polyester 
substrate was; placed onto the adhesive, and the assembly was cured with 
heat and pressure in a 145.degree. C. oven for 1-2 hours. The composite 
was found to be poorly bonded after the process, and exhibited 7 lbs of 
adhesion when tested in a shear mode. 
______________________________________ 
Soft Bake/ Adhesion 
Example 
Application 
Exp. Energy 
(Shear) 
Contrast 
______________________________________ 
1 Spin Coat 55.degree. C. for 5 
20 lbs Very Good - 
(5K RPM) minutes at least 1 mil 
1500 MJ Resolution 
2 Spin Coat None/ 20 lbs Poor 
(5K RPM) 1500 mJ 
3 Butter Coat 
None/ 2 lbs Good 
15000 mJ 
4 Butter Coat 
55.degree. C. for 5 
20 lbs Poor 
minutes 
1500 MJ 
5 Riston 318R 
None 7 lbs Fair 
______________________________________ 
In an alternate embodiment of the invention, a colloidally dispersed 
electrically conductive dispersion is added to the above disclosed 
adhesive composition to impart electrical conductivity to the patterned 
adhesive to obtain a conductive adhesive. Seventy to ninety weight percent 
of micron or submicron sized particles of metal or metal coated spheres 
such as polymer, glass or silica, are dispersed n the adhesive. Conductive 
particles dispersed in a volatile solvent containing a volatile surfactant 
consisting of a volatile solvent which is removed during spin coating to 
obtain a uniformly dispersed colloidal suspension with the desired 
thickness. Graphite or metals such as copper, nickel, gold, silver, 
platinum, and rhodium can be used. The conductive adhesive formulation is 
imaged at extended exposure times to overcome the effects of scattered 
radiation from the presence of the metal particles. 
The composition and method, according to the invention, provide the long 
desired ability to accurately create a high resolution pattern of adhesive 
material and bond two opaque substrates together, something heretofore 
unavailable. While the preferred embodiments of the invention have been 
illustrated and described, it will be clear that the invention is not so 
limited. Numerous modifications, changes, variations, substitutions and 
equivalents will occur to those skilled in the art without departing from 
the spirit and scope of the present invention as defined by the appended 
claims.