Aqueous-developable dry film photoresist

Abstract: In an aqueous-developable dry-film photoresist containing a carboxyl-group containing film-forming polymeric binder, the improvement wherein the binder is obtained by polymerizing a mixture including: (a) at least one monomer of the formula H₂C=CRCOO[(C n H 2n )X(C p H 2p )] m R', wherein R is hydrogen or methyl and R' is a saturated C₅-C₁₂ bridged alkyl optionally substituted by at least one C₁-C₄ alkyl or halogen, X is oxygen or sulfur, n is 2-4, p is 0-4, m is 0-2; and (b) at least one C₃-C₁₅ α,β-unsaturated carboxyl-containing or anhydride-containing monomer having 3-15 carbon atoms.

Description: The present invention relates to aqueous-developable dry film photoresists. In particular, it relates to aqueous-developable dry film photoresists that contain a film-forming polymeric binder. Aqueous-developable dry-film photoresists are photopolymerizable compositions used in the manufacture of printed circuit boards. Such photoresists are generally made by applying the solvated resist material to a carrier, such as a transparent polyester film, and then evaporating the solvent to produce the dry film. In typical use, a dry-film photoresist is applied to a copper-clad substrate along with the carrier, exposed in certain areas through the carrier to actinic radiation that will cure the film, and then washed with an alkaline aqueous solution to remove the unexposed film from the copper. The exposed copper surface can then be removed in etching solutions leaving the protected area under the cured photoresist to form the electrical circuit. Manufacturing operations such as alkaline etching and gold plating typically involve alkaline conditions that cause particular problems, such as loss of adhesion of the cured photoresist. Accordingly, it is important that the cured photoresist be tough enough to withstand such procedures. It is also advantageous to have a photoresist that in unexposed areas is readily washed from the substrate in order to achieve as rapid a development time as possible. To facilitate easy removal of uncured photoresist, one of the ingredients in photoresists is a carboxyl-group containing film-forming polymeric binder. However, a delicate balance of ingredients must be maintained in photoresist formulations. Materials that advantageously affect toughness of the cured resist might adversely affect exposure time or development time or might cause the cured photoresist to become brittle. U.S. Patent No. 4,777,115 is directed to photopolymerizable compositions containing an ethylene terpolymer binder which may contain, among other things, dicyclopentadienyl acrylate. These binders must be prepared at high temperatures and high pressures. The process is difficult and expensive. European Patent Application No. 217,137 is directed to photopolymerizable composition of acrylic copolymers containing dicyclopentenyl acrylate or methacrylate. These compounds have been found to be useful binders, but photoresists made with these binders have poor stripping performance and improved physical properties (e.g., processing characteristics, etc.) are desired. EP A 0 237 309 discloses an active energy ray-curable resin composition which comprises (A) a graft copolymerized polymer comprising graft chains composed mainly of structural units derived from at least one monomer selected among others from carboxyl containing (meth)acrylic or vinyl monomers and (B) a monomer having an ethylenically unsaturated bond. The present invention is an aqueous-developable dry-film photoresist comprising a carboxyl-group containing film-forming polymeric binder, characterized in that the binder is obtained by polymerizing a mixture comprising: (a) at least one monomer of the formula H₂C=CRCOO[(C n H 2n )X(C p H 2p )] m R′, wherein R is hydrogen or methyl and R′ is a saturated C₅-C₁₂ bridged alkyl optionally substituted by at least one C₁-C₄ alkyl or halogen, X is oxygen or sulfur, n is 2-4, p is 0-4, m is 0-2; and (b) at least one C₃-C₁₅ α,β-unsaturated carboxyl-containing or anhydride-containing monomer. The present invention is also characterized by an article comprising the dry-film photoresist disposed on a carrier, a method of using the dry-film photoresist. The carboxyl-group containing film-forming polymeric binder useful in accordance with this invention is prepared using a minimum of two monomers. The first monomer a) has the formula H₂C=CRCOO[(C n H 2n )X(C p H 2p )] m R′, wherein R is hydrogen or methyl and R′ is a saturated C₅-C₁₂ bridged alkyl optionally substituted by at least one C₁-C₄ alkyl or halogen, X is oxygen or sulfur, n is 2-4, p is 0-4, m is 0-2. The saturated C₅-C₁₂ bridged alkyl group R′ is a polycyclic hydrocarbon having 5-12 carbon atoms that contains a single or fused ring in which a valence bond, atom, or atom chain connects different parts of the ring. The C₁-C₄ alkyl group can be methyl, ethyl, propyl, isopropyl, n-butyl, or isobutyl, and halogen can be chloro, bromo, iodo, or fluoro. Preferred examples of the optionally substituted bridged alkyl include single ring bridged systems such as isobornyl, bornyl, norbornyl, bicyclo[4.3.2]undecyl, tricyclo[5.3.1.1 2,6 ]dodecyl, tricyclo[3.2.1.0 2,7 ]octyl, 2-chloronorbornyl, 2,3-dichloronorbornyl, as well as fused ring bridged systems. Methods for making the acrylates and methacrylates of the bridged groups named above are well known, such as by esterification of the alcohol of the bridged group with acrylic or methacrylic acid. Exemplary first monomers include isobornyl acrylate and methacrylate having the formula wherein R is as defined hereinabove. Other exemplary monomers include isobornyloxyethyl acrylate and methacrylate. The second monomer (b) that is used to make the binder that is used in accordance with the present invention is at least one C₃-C₁₅ α,β-unsaturated carboxyl-containing or anhydride-containing monomer. Examples of the useful carboxyl-containing monomers are cinnamic acid, crotonic acid, sorbic acid, acrylic acid, methacrylic acid, itaconic acid, propionic acid, maleic acid, fumaric acid, and half esters and anhydrides of these acids. Acrylic acid and methacrylic acid are preferred. Preferably, the binder used in accordance with the present is made using a third monomer (c). This third monomer can be a compound (i) C₁-C₈ alkyl acrylate or methacrylate, a compound (ii) C₂-C₈ hydroxyalkyl acrylate or methacrylate, or a compound (iii) styrene, α-C₁-C₆ alkyl styrene, α-halostyrene, or a ring-substituted derivative thereof. The compound (i) is any alkyl acrylate or methacrylate wherein the alkyl group is a straight or branched chain group having 1-8 carbon atoms, such as methyl, ethyl, propyl, sec-butyl, t-pentyl, n-hexyl, n-heptyl, or 2-ethylhexyl acrylate or methacrylate. The compound (ii) is any hydroxyalkyl acrylate or methacrylate wherein the hydroxyalkyl group is a straight or branched chain group having 2-8 carbon atoms, such as 2-hydroxyethyl, 2-hydroxypropyl, or 2-hydroxy-n-butyl acrylate or methacrylate. The compound (iii) is, for example, styrene, α-methyl styrene, para-methyl styrene, or para-t-butyl styrene. The benzene ring of the compound (iii) can be substituted with, for example, 1-5 functional substituents selected from nitro, alkyl having 1-6 carbon atoms such as methyl, ethyl, or t-butyl, alkoxy having 1-6 carbon atoms, acyl having 1-6 carbon atoms, carboxyl having 1-6 carbon atoms, sulpho, hydroxyl, or halo such as fluoro, chloro, iodo, or bromo. The binder used in accordance with the present invention can be made by known procedures, such as disclosed in U.S. Pat. No. 4,985,343, the disclosure of which is incorporated herein by reference. Polymerization is normally carried out in solution at standard pressure and slightly elevated temperatures. That is, the binder synthesis is typically carried out at ambient pressure in a closed reactor at reflux. Sealed reactors in which higher pressures or temperatures are used are suitable; however, sealed reactor polymerization may sometimes be difficult because of the reaction exotherm. Solution polymerization is carried out at temperatures of about 50-about 120°C at about 1-about 4 atmospheres, more preferably at about 60-about 100°C at about 1 atmosphere, and most preferably at temperatures of about 70-about 80°C at about 1 atmosphere. Preferably, the binder has a weight-average molecular weight of 40,000-200,000, more preferably 70,000-100,000, as determined by size-exclusion chromotography using polystyrene as the calibrating standard. In making the binder, the amount of the monomer (a) preferably varies from 15-50 weight%, more preferably 15-30 weight%, most preferably 20-29 weight%, based on the weight of the binder. The amount of the monomer (b) useful in making the binder varies from 15-50 weight%, more preferably 15-30 weight%, most preferably 18-32 weight%, based on the weight of the binder. The amount of the optional monomer (c) useful in making the binder varies from 0-40 weight%, more preferably 10-40 weight%, most preferably 15-35 weight%, based on the weight of the binder. Polymeric binder-containing aqueous-developable dry-film photoresists are well known. Accordingly, in general, ingredients other than the specific polymeric binder disclosed herein that are useful in the aqueous-developable dry-film photoresist of the present invention will be readily apparent to the skilled artisan. In a preferred embodiment of the photoresist of the present invention there is included the polymeric binder, a free-radical photoinitiator, an addition-polymerizable monomer, and a thermal polymerization inhibitor. In a preferred embodiment, the photoresist also contains a plasticizer. Optional additives such as dyes and adhesive promoting agents are also contemplated. "Dry" films are those in which the solvent has been evaporated. In the preferred embodiment, based on the total weight of the dry-film photoresist, the amount of polymeric binder varies from 40-70%, more preferably from 50-60%, the amount of photoinitiator varies from 0.5-10%, more preferably from 3-7%, the amount of addition-polymerizable monomer varies from 5-40%, more preferably 15-35%, the amount of plasticizer varies from 2-30%, more preferably from 9-15%, and the amount of thermal polymerization inhibitor varies from 0.003%-0.04%, more preferably from 0.01%-0.02%. The plasticizer useful in accordance with the present invention can be any well known plasticizer in the photoresist art. Examples include polymeric and non-polymeric plasticizers. Non-polymeric plasticizers include, for example, tri-n-butyl citrate, N-ethyl-toluene sulfonamide, and glycerol triacetate. Polymeric plasticizers include the reaction product of (1) an isocyanate-terminated block copolymer of polyesterdiol blocks and polyalkylene glycol/ethylene oxide blocks and (2) at least one hydroxy-substituted acrylate or methacrylate, such as disclosed in U.S. Pat. No. 4,686,171. Carboxyl-modified polyurethanes are also useful plasticizers. Other useful plasticizers will be readily apparent to the skilled artisan. The free-radical photoinitiator useful in accordance with this invention is a conventional photoinitiator activatable by actinic radiation that is thermally inactive below about 185°C. Examples of useful photoinitiators are found in U.S. Pat. No. 4,268,610. Exemplary photoinitiators are aromatic ketones, such as benzophenone and dimethoxyphenyl acetophenone, Michler's ketone, 4,4′-bis(diethylamino)benzophenone, tert-butylanthroquinone, 2-ethylanthraquinone, thioxanthones, benzoin alkyl ethers, and benzyl ketals. Other useful photoinitiators will be apparent to those skilled in the art. The addition-polymerizable monomer useful in accordance with the present invention is a compound containing at least 1, preferably at least 2, more preferably 2 to 4, most preferably 2 to 3 ethylenic double bonds, or a mixture of said compounds, i.e., the preferred addition-polymerizable monomers use polyfunctional (multifunctional). Suitable addition-polymerizable monomers include alkylene or polyalkylene glycol diacrylates. Monomers containing vinylidene groups conjugated with ester linkages are particularly suitable. Illustrative examples include but are not limited to ethylene diacrylate; diethylene glycol diacrylate; glycerol diacrylate; glycerol triacrylate; 1,3-propanediol dimethacrylate; 1,2,4-butanetriol trimethacrylate; 1,4-benzenediol dimethacrylate; 1,4-cyclohexanediol diacrylate; pentaerythritol tri- and tetramethacrylate; pentaerythritol tri- and tetraacrylate; tetraethylene glycol dimethacrylate; trimethylolpropane trimethacrylate; triethylene glycol diacrylate; tetraethylene glycol diacrylate; pentaerythritol triacrylate; trimethylol propane triacrylate; pentaerythritol tetraacrylate; 1,3-propanediol diacrylate; 1,5-pentanediol dimethacrylate; and the bis-acrylates and bis-methacrylates of polyethylene glycols, polypropylene glycols, and copolymers thereof of molecular weight from about 100 to about 500 (number average). Other useful addition-polymerizable monomers will be apparent to those skilled in the art. The thermal polymerization inhibitor useful in accordance with the instant invention prevents thermal polymerization during drying and storage. Examples of useful thermal polymerization inhibitors are p-methoxyphenol, hydroquinone, alkyl and aryl-substituted hydroquinones and quinones, tertbutyl catechol, pyrogallol, copper resinate, β-naphthol, 2,6-di-tert-butyl-p-cresol, 2,2'-methylene-bis(4-ethyl-6-t-butylphenol), p-tolylquinone, chloranil, aryl phosphites, and aryl alkyl phosphites. Other useful thermal polymerization inhibitors will be apparent to those skilled in the art. The photoresist of the present invention optionally includes additives well known in the art of photopolymerizable compositions, such as leuco (i.e., printout) dyes, background dyes, adhesion promoters, and antioxidants as disclosed in U.S. Pat. No. 4,297,435 and the heretofore mentioned U.S. Pat. No. 4,268,610. Other optional additives will be apparent to those skilled in the art. While desirable, the optional additives are not essential to the instant invention. The photoresist of the present invention is prepared by mixing the various components in a solvent. Suitable solvents are alcohols, ketones, halogenated hydrocarbons, and ethers. Other solvents will be apparent to those skilled in the art. After mixing, the composition is then coated onto a support or carrier, and the solvent is evaporated. The photoresist has a preferable thickness of 10-100 µm, more preferably 12.2-50.8 µm. Carriers are preferably about 0.0254-0.0508 mm thick. U.S. Pat. Nos. 3,469,982 and 4,293,635 disclose useful methods for preparing the photopolymerizable composition of this invention, i.e., as rolls of dry film sandwiched between a flexible support member and a flexible cover member. It will be apparent to those skilled in the art that dry films of the instant invention can be made on inflexible supports as well as flexible supports and may be supplied as stacks of laminated sheets as disclosed in the heretofore mentioned U.S. Pat. No. 4,268,610, as well as rolls. The photoresist of this invention is useful in the manufacture of printed circuit boards. Generally, the resist is coated onto the copper surface of a copper clad substrate, exposed to actinic radiation through a negative to create a latent image of photopolymerized material, and developed in a known aqueous developing solution to remove the unpolymerized composition from the copper surface. The portions of the surface not covered by the photopolymerized material are then modifiable by known processes, e.g., by plating or etching procedures, while the photoresist protects the covered surface. The photopolymerized material can be ultimately removed from the substrate by washing with known stripping solutions. The photoresist of the present invention is applied to the copper clad substrate by known procedures, such as hot shoe or hot roll lamination of the dry film attached to a transparent, peelable support, which support is removed after polymerization, as disclosed in the heretofore mentioned U.S. Pat. No. 4,293,635. Generally, the amount of actinic radiation used to polymerize the composition varies from about 35 to about 150 mJ/cm², with precise amounts determinable by those skilled in the art based on the specific composition used. The copper clad substrate is any known copper/dielectric laminate used in circuit board manufacture, such as a copper clad board of fiberglass reinforced epoxy resin. Other useful dielectrics will be apparent to those skilled in the art. The aqueous developing solutions used in accordance with this invention have, by weight, about 0.5-10% alkaline agents, preferably about 0.5-1%, and the latent imaged board is washed in the solution for a time sufficient to remove the unpolymerized composition. Useful alkaline agents are alkali metal hydroxides, e.g., lithium, sodium and potassium hydroxide, the base reacting alkali metal salts of weak acids, e.g., sodium carbonate and bicarbonate, and alkali metal phosphates and pyrophosphates. Sodium carbonate is preferred. The circuit board can be submerged in the developing solution or, preferably, the solution is high pressure sprayed on the board. In general, the stripping solutions useful in removing the photopolymerized material in accordance with the instant invention are heated aqueous alkaline solutions, using the same alkaline agents as the developing solutions, but having a higher alkaline concentration, i.e., generally, by weight, from about 1%-10%, preferably from about 1%-3%. Generally, the stripping solution is heated to a temperature of about 45-65°C, preferably about 50-55°C. Washing the substrate to remove the photopolymerized material is by methods well known to those skilled in the art, such as spraying the substrate with the heated stripping solution or, preferably, agitating the substrate in a heated bath of the stripping solution. Examples of the photoimaging techniques and equipment, including radiation sources, exposure intensity and duration, developing and stripping solutions and techniques, and laminated board composition useful in the preferred embodiment are disclosed in the heretofore described U.S. Pat. Nos. 3,469,982, 4,293,635, and 4,268,610. The photoresist of the present invention is particularly useful in manufacturing operations on printing circuit boards involving alkaline etching and gold plating processes as well as chemical milling processes. The binder useful in accordance with the present invention imparts the toughness and resistance to ammoniacal chemistries needed during these operations. Alkaline etching and gold plating procedures are well known, such as disclosed in U.S. Pat. Nos. 3,705,061, 3,466,208, Printed Circuits Handbook , third edition (McGraw-Hill 1988). The binder of the instant invention imparts improved physical properties (e.g., processing characteristics, etc.) due to its narrow molecular weight distribution (less branching), as compared to photoresists containing binders comprising unsaturated monomers such as dicyclopentenyl acrylate or methacrylate. In order to more clearly describe the present invention, the following non-limiting examples are provided. All parts and percentages in the examples are by weight unless indicated otherwise. EXAMPLES 1-4 Photoresists are prepared and tested using polymeric binders, the compositions of which are recorded in the following Table 1. Each binder is synthesized as follows. A reaction vessel is charged with 422 g methyl ethyl ketone and 555 g of the monomer mixture as in Table 1. While under a nitrogen atmosphere, the contents are stirred for about one hour while heating at about 80°C. The vessel is then charged with 7 ml of a solution having a concentration of 0.05 g azobisisobutyro-nitrile per 1 ml of methyl ethyl ketone. After 5 hours, 19 ml of a solution having a concentration of 0.05 g of azobisiso-butyronitrile per 1 ml methyl ethyl ketone are added, and the polymerization allowed to continue for an additional nine hours. After polymerization is complete, the binder is placed in a glass container and methyl ethyl ketone added to bring the polymer solids content to 31%. Photoresists are prepared from the formulations in the following Table 2 by adding to the 31% binder/solvent mixture the other ingredients recorded. Accordingly, the amount of binder recorded in the table includes the actual binder plus the solvent. The photoresists are made as follows. Materials as in Table 2 are stirred in a propeller mixer at about 600 rpm. The mixture is then centrifuged for 3 minutes to remove the air bubbles. Drawdowns are made of the mixtures at a thickness of about 38.1 µm on a 23.4 µm (92 gauge) polyester substrate and then dried in an oven at 100 °C for 3-6 minutes. Drawdowns are laminated to a degreased and scrubbed double-sided copper pannel (56.7 g copper on an dielectric base) having a thickness of about 0.79 mm on a hot-roll laminator at a temperature of about 121 °C, a pressure of about 2.1 kg/cm², and a speed of about 121.9 cm/min. The panels are exposed imagewise (through a negative) at an exposure corresponding to Stouffer step 6-7 using a 21 Stouffer Step Guide. The panels are held for 15 minutes and then passed through a Chemcut™ 547 developer at a speed adjusted to give a 50% breakpoint in a developing solution of 0.90% w/w aqueous sodium carbonate monohydrate at a temperature of about 35 °C, a top spray pressure of about 1.41 kg/cm², and a rinse pressure of about 1.41 kg/cm². Panels are then placed in a conveyorized spray etcher (Chemcut™ 547) having an etch chamber about 91.4 cm long, a pollution control section about 61 cm long, and a rinse section about 30.5 cm long having a temperature of about 49°C. Etcher speed for the panels exposed at a Stouffer step of 6-7 is 45.72 cm/min for 1 pass and 91.44 cm/min for a second pass. The etcher speed is adjusted such that 1 pass gives about 90% complete etching, and the second pass completes the etch, leaving no residual copper. The etchant used is MacDermid ULTRAETCH™ 50 Etchant, which gives an optimum etch rate at a concentration of 427.95-570.6 g of copper per 3.785 l of etchant, 4.5-5.0 ppm chloride and a pH of 8-8.5. The replenishment chamber (pollution control chamber) is filled with MacDermid ULTRAETCH™ 50 Replenisher Solution (ammonium hydroxide and ammonium chloride mixture) giving the pollution control chamber a pH of 11. In operation, the replenisher solution is sprayed on the panels after coming out of the etch chamber. The solution then cascades into the etch chamber. Etched panels are evaluated for resistance to the alkaline etchant by measuring resist adhesion. Adhesion is visually checked by the adherence of a series of lines having a width of 40, 50, 60, 70, 80, 90, 100, 125, 150, 200, and 250 µm, with a 250 µm spacing between each line. The performance of the resist is based on the smallest line held at each etch speed. The smaller the line held indicates the greater resistance to the alkaline etchant. The smallest line held for each Example is recorded in the following Table 3. EXAMPLES 5-9 Photoresists are prepared using binder formulations as in the following Table 4. Binders and photoresists are prepared as in Examples 1-4 using the formulations in the following Table 5. Photoresists are exposed, developed, etched, and tested as in Examples 1-4. The results are recorded in the following Table 6.

Claims: An aqueous-developable dry-film photoresist comprising a carboxyl-group containing film-forming polymeric binder, characterized in that the binder is obtained by polymerizing a mixture comprising: (a) at least one monomer of the formula H₂C=CRCOO[(C n H 2n )X(C p H 2p )] m R′, wherein R is hydrogen or methyl and R′ is a saturated C₅-C₁₂ bridged alkyl optionally substituted by at least one C₁-C₄ alkyl or halogen, X is oxygen or sulfur, n is 2-4, p is 0-4, m is 0-2; and (b) at least one C₃-C₁₅ α,β-unsaturated carboxyl-containing or anhydride-containing monomer. The photoresist of claim 1 further characterized in that the mixture comprises (c) at least one (i) C₁-C₈ alkyl acrylate or methacrylate, (ii) C₂-C₈ hydroxyalkyl acrylate or methacrylate, or (iii) styrene, α-C₁-C₆ alkyl styrene, α-halostyrene, or a ring-substituted derivative thereof. The photoresist of any of the preceding claims further characterized in that R' is selected from the group consisting of isobornyl, bornyl, norbornyl, bicyclo[4.3.2]undecyl, tricyclo[5.3.1.1 2,6 ]dodecyl, tricyclo[3.2.1.0 2,7 ]octyl, 2-chloronorbornyl, and 2,3-dichloronorbornyl. The photoresist of any of the preceding claims the improvement further comprising that the binder is obtained using (b) the at least one C₃-C₁₅ α,β-unsaturated carboxyl-containing monomer. The photoresist of any of claims 1, 2, 3 or 4, the improvement further comprising that the binder is obtained using (b) the at least one C₃-C₁₅ α,β-unsaturated anhydride-containing monomer. The photoresist of any of the preceding claims wherein R′ is isobornyl. The photoresist of any of the preceding claims further comprising a free-radical photoinitiator, an addition-polymerizable monomer, a plasticizer, and a thermal polymerization inhibitor. The photoresist as claimed in claim 7 wherein the addition-polymerizable monomer is polyfunctional. A composition comprising the photoresist of any of the preceding claims and a solvent. An article comprising the dry-film photoresist of any of claims 1-8 disposed on a carrier. An article comprising the dry-film photoresist of any claims 1-8 sandwiched between a flexible carrier film and a flexible cover film. In a method of using a dry-film photoresist containing a film-forming polymeric binder comprising the steps of applying to a substrate the article of claim 10, exposing the photoresist imagewise through the carrier to actinic radiation sufficient to cure the photoresist in exposed areas, removing the carrier from the photoresist, and washing the photoresist with an alkaline aqueous solution to remove uncured photoresist from the substrate. The product made by the method of claim 12.

Inventors: MAYES RICHARD THOMAS DR, POHL RUDOLPH LE ROY DR, MAYES, RICHARD THOMAS, DR., POHL, RUDOLPH LE ROY, DR.
Applicants: HERCULES INC, HERCULES INCORPORATED
Application year: 1992
Main classification: G03F7
IPC codes: H01L21, G03F7, H05K3