Multilayer dry-film positive-acting laminable photoresist with two photoresist layers wherein one layer includes thermal adhesive

Dry-film, positive-acting photoresist layers are used in the formation of many articles such as circuit boards, printing plates and the like. Laminable monolayers of photoresist suffer from slow speeds, brittleness, and narrow latitude during development and exposure. The use of a laminable positive-acting photoresist integral adhesive layer on the dry-film, positive-acting photoresist layer improves the properties and performance of the photoresist.

FIELD OF THE INVENTION 
The present invention relates to laminable dry-film, positive-acting 
photoresists. Such photoresists are useful for processes in which 
substrates are to be etched or are to have material deposited on the 
surface of the substrate in an imagewise fashion. 
BACKGROUND OF THE INVENTION 
Background of the Art 
Initially, resists were applied to substrates in liquid form. The resist 
was imagewise subjected to radiation to form relatively soluble and 
relatively insoluble areas. Upon treatment with an appropriate developer 
solution, the relatively soluble areas were removed and a physical mask in 
the desired image pattern was left on the surface of the substrate. The 
exposed area of the substrate would then be treated by either deposition 
of material onto the exposed areas (e.g., metal deposition by vapor 
coating or electrolytic processes) or etching of the surface of the 
substrate in the area exposed through the mask. 
One of the first advances in the use of dry-film photoresist materials was 
discussed in U.S. Pat. No. 3,469,982 in which a laminable negative-acting 
single layer film was adhered to a substrate and used as a photoresist. 
U.S. Pat. No. 4,193,797 discloses the use of single layer laminable films 
of positive-acting dry-film photoresist compositions for use in 
photoresist processes. 
Improved dry-film photoresist compositions are disclosed in U.S. Pat. No. 
4,247,616. This composition provides good light sensitivity, stability, 
and excellent resist characteristics. 
U.S. Pat. No. 4,349,620 discloses a multi-layer photosensitive film resist 
(positive- or negative-acting) having a plurality of layers with differing 
adhesive properties. In particular, a carrier layer having a first 
photosensitive resist composition thereon and then a second photoresist 
with different adhesive properties on the first photosensitive composition 
is disclosed. 
Other developments in the art have lead to the use of two layer or 
multilayer laminable or coatings of photoresist materials in order to 
improve the sensitometric or physical properties of the dry-film resist. 
U.K. Pat. No. 1,493,833 shows the use of a photoresist comprising a 
carrier layer, a photosensitive layer (including positive-acting 
photosensitive layers) and a non-photosensitive layer which is soluble in 
aqueous or organic solvents. A host of natural and synthetic polymers are 
disclosed for these non-photosensitive layers. U.S. Pat. No. 4,204,009 
discloses the use of a resist having two photosensitive layers of 
different reactivities adjacent each other on top of the substrate to be 
treated. U.S. Pat. No. 4,217,407 discloses the use of multiple layer 
resist materials which comprise an o-quinone diazide containing layer in 
combination with at least one other layer which is permeable, swellable, 
or soluble by alkaline envelopers. A brief list of non-photosensitive 
compositions is provided. 
U.S. Pat. No. 4,191,573 shows a photosensitive image element comprising the 
support to be plated or etched with two independently applied liquid 
resists of photosolubilizable layers. The resist layers comprise an azide 
compound and a polyamide. 
U.S. Pat. No. 4,180,604 discloses the use of two separately coated liquid 
photosensitive resist layers. The compositions appear to be the same, with 
larger amounts of solvent diluent used in coating the second layer. The 
lower layer is more readily soluble, promoting undercutting in the 
formation of the resist image. 
U.S. patent application Ser. No. 428,475, filed on Sept. 29, 1982, in the 
name of P. M. Koelsch and J. P. Vikesland discloses an improved two layer 
photoresist construction in which one layer is a photosensitive, 
positive-acting resist composition and the other layer bonding the 
composite to the surface of the article to be etched or plated is a 
crosslinked, light-insensitive thermally laminable adhesive layer. The 
crosslinking provides for reduced undercutting of the resist image. 
BRIEF DESCRIPTION OF THE INVENTION 
The present invention relates to positive-acting, multilayer, laminable, 
dry-film photoresist articles which are useful in etching and plating 
processes. The laminable article comprises at least one positive-acting 
photoresist layer adhered to a thermoplastic positive-acting adhesive 
second photoresist layer which contains a lower effective amount of 
photosolubilizer than contained in the first positive-acting photoresist 
layer. The second layer is preferably crosslinked or crosslinkable to 
provide improved physical properties in the composite and in the bonding 
properties of that second layer. 
DETAILED DESCRIPTION OF THE INVENTION 
The present positive-acting, laminable dry-film photoresist article 
comprises at least two functional layers and optionally a strippable 
carrier layer. One functional layer is a positive-acting dry-film 
photoresist composition and the second functional layer is a thermally 
laminable positive-acting photoresist adhesive layer. 
Dry-film photoresist compositions are well described in the art. A 
polymeric binder (either thermoplastic or crosslinked) usually carries 
within it or pendant from the polymer itself a light-sensitive material or 
moiety, the solubility of which in selected solvents increases when 
exposed to radiation. This photosoluble moiety may also be a part of the 
polymer backbone. The most preferred class of positive-acting 
photosensitizers are those with o-quinone diazide groups. These materials 
are generally referred to in the art as o-quinone diazides or diazo 
oxides. These compounds are well disclosed in the literature as, for 
example, in U.S. Pat. Nos. 4,349,620; 4,345,020; 4,346,163; 4,193,797; 
4,217,407; 4,247,616; 4,211,834; 4,007,047; 3,666,473; 3,201,239 and 
4,180,604. These positive-acting photosensitizers are generally used in 
amounts of from 5 to 80% by weight of the resist layer, and preferably 
from 10 to 50% of the layer. These patents also describe the various 
different binders that can be used with the positive-acting o-quinone 
diazides. The preferred binder material tends to be phenol formaldehyde 
resins (either novolaks or resoles alone or in combination with 
crosslinked epoxies as disclosed in U.S. Pat. No. 4,247,616), but a wide 
variety of various binders are used in the art. These binders include 
acrylic resins (e.g., alkyl methacrylate, ethylacrylate, copolymers and 
the like) polymeric polyols (e.g., polyvinyl alcohol, and hydroxy 
substituted addition polyesters such as polyacrylate and 
polymethylacrylate polyesters), hydroxy substituted copolymers (such as 
poly[methyl methacrylate/hydroxy methacrylate]), natural colloids (for 
example, gelatin and shellac), polyvinyl hydrogenphthalate, ethylene oxide 
polymers and copolymers, polyacrylamides, polyethylincally unsaturated 
materials (such as polystyrene and its copolymers), polyamides, 
polyesters, and the other various polymeric materials both natural and 
synthetic, thermoplastic and crosslinked as known in the art. The only 
requirement is that these materials have at least some reasonable 
solubility in aqueous alkaline or organic developer solutions. The 
thickness of the first (also the primary) photosensitive layer may 
generally be between 0.05 and 2.0 mils, preferably between 0.10 and 0.50 
mils, most preferably at least 0.1 and less than 0.3 mils. 
The thermoplastic adhesive layer is essential to successful commercial 
practice of dry-film, positive-acting photoresist technology. Monolayer 
constructions have to date been unable to meet even existing commercial 
needs for (1) resistance to crazing of the film during storage (because of 
the brittle nature of the film), (2) photosensitivity, the speed of the 
film being too slow for commercial processes, and (3) thermal dimensional 
stability of the film during plating and etching processes. The use of the 
distinct, non-photosensitive adhesive layer to make the photoresist 
article laminable according to the teachings of U.S. Ser. No. 428,475, 
cited above, overcomes all three of these problems. It is the presence of 
the integral adhesive layer between the photoresist layer and the receptor 
layer which improves these areas of positive-acting, dry-film photoresist 
technology, while monlayer constructions have not satisfied the needs of 
the technology. 
The thermoplastic adhesive layer of the present invention is also a 
photosensitive thermally laminable adhesive layer. This layer must contain 
a lower effective amount of photosolubilizer (positive-acting 
photosensitizer) than does the primary photosensitive layer. This enables 
the more sensitive primary layer to control the imaging while the 
laminated adhesive layer controls the degree of undercutting. The term 
"lower effective amount" of photosolubilizer means that the actual amount 
of the particular photosolubilizer present in the laminable adhesive layer 
acts at a reduced or less efficient rate in photosolubilizing that layer 
than does the amount of photosolubilizer in the primary photosensitive 
layer at a given activating exposure. Thus the increase in solubility of 
the adhesive layer would be less than the increase in solubility for the 
top resist layer. For example, if the primary and adhesive layers both 
comprised a phenol-formaldehyde resin, and the positive-acting 
photosensitizer in both layers were the same, a lower concentration or 
percentage of photosolubilizer would be used in the adhesive layer. If the 
polymeric composition of the primary photosensitive layer and the adhesive 
layer were the same, but positive-acting photosensitizers of differing 
speeds were to be used, the slower-acting or less efficient 
photosensitizer would probably be used in the adhesive up to an amount 
where it approached but was still less than the effective ability of the 
photosolubilizer in the primary photosensitive layer to solubilize that 
layer. A greater amount of less efficient photosolubilizer could be used 
in the primary layer and a lower effective amount of a more sensitive 
photosolubilzer could also be used in the adhesive layer. This last 
variation provides no benefit over the previously described construction. 
Preferably, the adhesive layer contains an amount of photosolubilizer 
which is capable of generating greater than 10% and fewer than 90% of the 
number amount volume of soluble species as does the primary photoresist 
layer at a given exposure to radiation to which the primary photosensitive 
layer is sensitive. Preferably, the adhesive layer is capable of 
generating between 30 and 90% of the number amount/volume of soluble 
species as does the primary photoresist layer and most preferably it is 
capable of generating between 40 and 85% of those species. It is preferred 
to have the same or approximately equivalent photosolubilizers in both 
layers with a smaller concentration in the adhesive layer. 
The laminable adhesive layer as previously noted may comprise either a 
two-dimensional, three-dimensional (crosslinked) or crosslinkable polymer. 
A crosslinked polymer is preferred in the present invention. 
The crosslinked or crosslinkable laminable layer may be made of any 
compositions which are soluble in aqueous alkaline or organic solvent 
solutions and are not sufficiently crosslinked so as to prevent bonding of 
that layer to a substrate when pressed and heated. Typically, the 
crosslinked or crosslinkable layer should be capable of adhering to at 
least one substrate of mildly abraded copper, aluminum, tin, or polyester 
when pressed against such a surface with a force of no more than ten 
pounds per square inch at a temperature no greater than 150.degree. C. for 
no longer than twenty seconds. The layer, to be laminable, must have a 
major surface exposed or exposable (as by removing a physically strippable 
cover sheet without melting or chemical treatment) so that it can be 
laminated. For example, a layer fused to two other layers and being 
sandwiched by them cannot be laminable. These characteristics define the 
term laminable in the practice of the present invention. The intrinsically 
crosslinkable resin (that is, without the external application of chemical 
crosslinking materials) may already be crosslinked to any degree 
consistent with its also being laminable and yet be capable of further 
crosslinkability. This can be achieved by various means known to the 
ordinarily skilled artisan such as inclusion of photoactivatable 
crosslinking agents, partial crosslinking of the original composition 
which may be further crosslinked by heating, etc. 
The desirability for crosslinking or crosslinkability in the laminable 
layer derives from the fact that the many various applications of the 
dry-film photoresist article will require different properties in the 
various applications. This is why the crosslinkable ability of the layer, 
which is controlable by the degree of crosslinking stimulation given that 
layer, is the preferred embodiment of the present invention. Crosslinking 
is an improvement over the general use of thermoplastic materials as the 
laminable layer in the prior art because of the improved control of 
solubility which can be given those layers. The crosslinkable compositions 
must be crosslinkable to a degree that satisfies the definition of 
crosslinked according to the present invention. 
The control of solubility in thermoplastic laminable layers quickly reaches 
a point of diminishing returns. Particularly when organic solvents are 
being used as a developer, increasing the molecular weight of 
thermoplastic polymers used in the laminable layer becomes more difficult 
and less productive in reducing the natural solubility of that layer in 
developer solvents. Doubling the molecular weight of a thermoplastic 
polymer from 500,000 to a million, for example, does not provide for 
facile control of solubility properties and does not easily produce a 
polymer with consistent properties because of the significant distribution 
of molecular weights within the polymerized material. Controlled 
crosslinking, on the other hand, tends to provide a more consistent 
composition with more accurately controlled solubility properties. This is 
particularly true with respect to thermal dimensional stability in the 
laminated resist which is extremely important in many fine detail 
processes where elevated temperatures are used, such as in plating 
processes. 
Any polymeric material which can be crosslinked and in its incompletely 
crosslinked stage (i.e., at least partially crosslinked), is soluble in 
aqueous alkaline solutions or organic solvents is useful in the practice 
of the present invention. By crosslinked, it is understood that the 
polymeric composition has at least some three dimensional structure to it 
and is at least ten percent (preferably at least 15 or 20 percent and more 
preferably 25 or 50 percent) less soluble (either in absolute amounts or 
in its rate of solubilization) in the selected developer solution than the 
polymeric composition without the crosslinking therein. Amongst the 
various classes of materials that can be used for the laminable layer are 
phenol formaldehyde resins (including novolaks), epoxy resins, acrylic 
resins (and copolymers), polyesters, polyamides, and the like. Each of 
these materials is well known in the art to be crosslinkable or can be 
mixed with a crosslinked resin, and the skilled artisan is well aware of 
the various crosslinking agents which can be used for the various 
polymeric materials. These, for example, include diisocyanates and 
epoxies, diacid chlorides, dianhydrides, diacids, polyisocyanates, 
polyepoxides, polyacids, aziridines, azlactones, dihalides, polyhalides, 
and the like. When already crosslinked, the laminable layer must remain 
laminable. No crosslinked layer should be in combination with the 
photosensitive resist layer prior to lamination if it is so crosslinked 
that it is neither laminable nor soluble in both aqueous alkaline 
solutions and organic solvent solutions. This control is well understood 
by those of ordinary skill in the art and can be determined for any 
crosslinkable material and any particular crosslinking agent by routine 
experimentation. Mixtures of these crosslinkable materials together or in 
combination with thermoplastic materials may be readily practiced by one 
of ordinary skill in the art. For example, the composition of U.S. Pat. 
No. 4,247,616 may be used as the laminable layer as well as the binding 
composition for the positive-acting photosensitizers. The preferred 
compositions of the present invention for the laminable layer comprise 
phenol formaldehyde resins blended with acrylic resins and particularly 
resole resins and acrylic terpolymers. A typical developer solution for 
determining relative solubility for crosslinked materials, as defined 
above, would be aqueous sodium hydroxide solutions of pH 13.0 to 13.5. 
The photosensitive and laminable layers may vary in thickness depending on 
their particular needs. Generally the various layers will range from 2 to 
25 microns, preferably from 4 to 15 microns, and most preferably from 6 to 
12 microns for the primary photosensitive resist layer and the adhesive 
layer. The laminable photosensitive layer may be of the same, smaller, or 
larger thickness, but should remain within the broad limits given for the 
photosensitive resist layer. The composite should be at least 4 and 
preferably at least 6 microns to enable physical transfer of the composite 
without structural damage thereto such as wrinkling and folding.

Various other ingredients may be included in these layers as is well 
understood in the art. Surfactants, spectral sensitizers, dyes, fillers, 
lubricants, coating aids, spectral absorbers (such as ultraviolet 
radiation absorbers) and the like may be used as is understood in the art. 
These and other aspects of the present invention will be disclosed in the 
following examples. 
EXAMPLE 1 
A photosensitive coating solution was prepared by mixing the following: 
______________________________________ 
Acetone 40 g 
"Resinox", a novolak phenolic 
60 g 
(phenol-formaldehyde) resin 
Acryloid AT-70, a styrene, ethyl- 
4 g 
acrylate acrylic acid terpolymer 
(50% solids resin in a xylene and 
Glycol monomethyl ether acetate 
50/50 solvent) 
DMP-30, 2,4,6-tris(dimethylamino- 
0.2 g 
methyl) phenol amine catalyst 
DDI-1410, a 36 carbon atom aliphatic 
1.3 g 
diisocyanate 
Phenyl isocyanate 2.5 g 
______________________________________ 
After a reaction time of two hours, this solution was mixed with the 
following additional materials: 
______________________________________ 
DER 732, propylene glycol 
3.2 g 
di(2,3-glycidylpropyl)ether 
Diaminodiphenylsulfone 1.3 g 
Phthalic anhydride 0.4 g 
2,4-dihydroxybenzophenone 
7.4 g 
bis-[naphthoquinone-(1,2)-diazide- 
(2)-5-sulfonate] the photosensitizer 
______________________________________ 
This solution was then coated onto a polyester web bearing a release layer 
of "Gantrez" S-95, an acidified copolymer of maleic anhydride and methyl 
vinyl ether. The drying temperature was 100.degree. C. for 3 minutes. The 
dry thickness of the photosensitive layer was 0.2-0.3 mil. Other 
photosensitive coatings as described in U.S. Pat. No. 4,247,616 may be 
used. 
The solution providing the laminable photosensitive layer was coated onto 
the dried photosensitive layer to a thickness of 0.25-0.35 mil after 
drying at 100.degree. C. for 3 minutes. The preferred coating solution is: 
______________________________________ 
30 g Acetone 
5.4 g BKR 2620, a resole type phenol formaldehyde 
resin 
0.05 g Triethylenediamine 
0.2 g DDI-1410, a 36 carbon atom aliphatic diiso- 
cyanate 
2.0 g "Carboset" 525, ethyl acrylate, methyl 
acrylate, acrylic acid terpolymer 
2.5 g 2,4-dihydroxybenzophenone bis-[naphtho- 
quinone-(1,2)-diazide-(2)-5-sulfonate] 
photosolubilizer. 
______________________________________ 
The coated dry-film was laminated at a rate of 3 ft/minute to a copper foil 
using a heated-roll laminator set at 110.degree. C. The laminated resist 
was then exposed through a photomask and developed with an approximately 
1.0% solution of sodium hydroxide. The resulting image quality was 
excellent down to a linewidth resolution of 0.2 mil. The identical 
construction without any photosensitizer in the solution provided 
linewidth resolution of only 0.5 mil. This composition is thermally 
stable, giving negligible movement of the edge profile of the resist at 
bath temperature up to 170.degree. F. (which is higher than the maximum 
temperature of commonly employed electroplating solutions). Chemical 
resistance to FeCl.sub.3 etchant at 150.degree. F. was outstanding. 
A non-crosslinked composition identical to that used as the 
non-photosensitive composition of Example 1 was prepared, except that the 
triethylenediamine (crosslinking catalyst) and the diisocyanate (DDI-1410 
crosslinking agent) were excluded. That composition was coated onto the 
photosensitive layer and dried in the same manner as in Example 1. The 
bilayer was then laminated, exposed and developed as in that example. 
Microscopic examination of the developed photoresist showed significant 
image undercutting and loss of resolution in the non-crosslinked, bilayer 
image. 
EXAMPLES 2-3 
In addition to the above example, the following compositions have also been 
successfully employed as the adhesive layer with the same lower effective 
amounts of photosensitizer used in Example 1. 
______________________________________ 
A. 38.1 g Acetone 
5.1 g BKR 2620 
3.1 g Carboset 525 
0.13 g Gantrez AN119, (maleic anhydride, 
methyl vinyl ether copolymer) 
B. 40 g Acetone 
0.05 g Triethylene diamine 
5.4 g BKR 2620 
0.1 g Phenylisocyanate 
2.0 g Carboset 525 
______________________________________ 
Composition B is a crosslinkable composition. When initially laminated, the 
composition was capable of being undercut by development. By heating the 
laminated composition, the crosslinking reaction was initiated and various 
degrees of crosslinking could be generated. In this manner the precise 
degree of undercutting of the mask or image could be controlled offering a 
highly desirable degree of latitude in processing. Heating of composition 
B at 250.degree. F. for fifteen minutes after lamination produced an 
adhesive layer that was barely undercut by the developer (aqueous alkaline 
solution at pH 13+). 
EXAMPLE 4 
A photosensitive coating solution was prepared by mixing the following: 
______________________________________ 
Acetone 40 g 
Resinox, a novolak phenolic resin 
20 g 
Acryloid AT-70, a styrene, ethyl- 
4 g 
acrylate acrylic acid terpolymer 
(50% solids) 
DMP 30, amine catalyst 0.2 g 
DDI 1140, an aliphatic diisocyanate 
1.3 g 
Phenyl isocyanate 2.5 g 
______________________________________ 
After a reaction time of two hours, this solution was mixed with the 
following additional materials: 
______________________________________ 
DER 732, a bis epoxy 3.2 g 
Diaminodiphenylsulfone 
1.3 g 
Phthalic anhydride 0.4 g 
1,2 naphthaloquinone diazide 
7.4 g 
5-p-tert-butyl phenylsulfonate 
______________________________________ 
This solution was then coated onto a polyester web bearing a release layer 
of Gantrez S-95, an acidified copolymer of maleic anhydride and methyl 
vinyl ether. The drying temperature was 100.degree. C. for 3 minutes. The 
dry thickness of the photosensitive layer was 0.25 mil. 
The secondary photosensitive solution was coated to a thickness of 
0.25-0.35 mil after drying at 100.degree. C. for 3 minutes. The preferred 
non-photosensitive coating solution is: 
______________________________________ 
30 g Methyl ethyl keytone 
5.4 g BKR 2620 
0.05 g Triethylenediamine 
0.2 g DDI 1410 
2.0 g Carboset 525, ethyl acrylate, methyl 
acrylate, acrylic acid terpolymer 
1.0 g 1.2 naphthaloquinone diazide 5-p-tert- 
butyl phenylsulfonate 
______________________________________ 
The coated dry film was laminated at a rate of 3 ft/minute to a copper foil 
using heated roll laminator set at 110.degree. C. The laminated resist was 
then exposed through a photomask and developed with an approximately 1.0% 
solution of sodium hydroxide. The resulting image quality was excellent 
down to a linewidth resolution of 4 micrometers by immersion development.