WO1997002511A1

WO1997002511A1 - Electrodepositable photoimageable compositions with improved edge coverage

Info

Publication number: WO1997002511A1
Application number: PCT/US1996/009367
Authority: WO
Inventors: Michael G. Sandala
Original assignee: Ppg Industries, Inc.
Priority date: 1995-07-03
Filing date: 1996-06-07
Publication date: 1997-01-23
Also published as: AU6097796A

Abstract

The present invention provides an improved electrodepositable photoresist composition which includes inorganic extender pigments. The use of these particular extender pigments at a pigment to binder ratio of at least 0.05:1 promotes edge coverage, while not adversely affecting the photosensitivity and/or developability properties of the photoresist into which they are incorporated. Embodiments of this invention encompass negative-acting and positive-acting photoresists, either of which may be cationically or anionically electrodeposited, and which include a sufficient quantity of an inorganic extender pigment such that edge coverage is promoted, while adequate photosensitivity and developability are maintained.

Description

ELECTRODEPOSITABLE PHOTOIMAGEABLE COMPOSITIONS WITH IMPROVED EDGE COVERAGE

Field of the Invention

The present invention relates to electrodepositable photoimageable compositions ("photoresists"). Particularly, the present invention relates to such compositions which are especially suitable for use in the manufacture of circuit patterns with conductive through-holes passing therethrough.

grief Description ofthe Prior Art

Processes for forming resist patterns on the surfaces of substrates are known in the art. Such processes typically comprise forming a photosensitive layer onto the surface of a substrate, irradiating portions ofthe photosensitive layer with actinic light, and developing the irradiated layer. Ifthe solubilization ofthe photoresist increases when exposed to the actinic light, it is referred to as a "positive-acting" photoresist. On the other hand, ifthe solubilization ofthe photoresist decreases (i.e., ifthe photoresist crosslinks) when exposed to the actinic light, it is referred to as a "negative-acting" photoresist. In most instances, the purpose ofthe photoresist is to protect the underlying substrate from the effects of a subsequent etching process. Therefore, defects in the resist pattern, such as inadequate coverage over certain parts ofthe substrate or inadequate development ofthe irradiated layer, can result in an incomplete or shorted circuit. Accordingly, it is important to employ photoresist whose irradiated layer can adequately be developed, and which can form a uniform layer over all surfaces ofthe substrate which are to be protected. The importance of employing such a photoresist becomes even more critical when it is used to form high density circuit boards. These high density boards typically have circuit patterns on their surfaces which are interconnected at predetermined locations. Such interconnection is often accomplished by the passage of a circuit through precisely positioned conductive holes ("through-holes") which pass through the substrate.

Since the electrodeposition process is known as a means of forming smooth, uniform films on substrates, particularly those having three dimensional features, it is often the preferred method of applying photoresists for substrates having conductive through- holes. There are, however, problems associated with the electrodeposition of photoresists onto substrates having a through-hole design. For example, conventional electrodeposited photoresists tend to flow away from the edges of conductive through-holes. This phenomenon can result in the formation of a very thin film, if any, ofthe photoresist around these edges. As such, the edges ofthe through-holes can be prone to attack by etchants during subsequent processing procedures. This, in turn, may result in the formation of an incomplete circuit.

One method of trying to resolve this problem is set out in U.S. Patent 4,755,551 which discloses a six-step process for preparing a printed circuit board having a plated through-hole. In this process, a resist layer, having a reverse pattern for a desired circuit, is formed on the surface of a copper-plated substrate having a conductive through¬ hole passing therethrough. This resist layer is not electrodeposited and does not enter the substrate's through-hole.

Thereafter, a resinous paint layer, having an inorganic filler to promote edge coverage, is electrodeposited onto the exposed area ofthe plated layer (i.e., on that portion of the plated layer which is not covered by the resist). Thereafter, it is heat cured. The resinous paint layer passes into the substrate's through-hole. It should be noted, however, that this resinous paint is not photoimageable (i.e., it is not a photoresist).

As can be seen from the above, the process disclosed in U.S. Patent 4,755,551 uses a non-photoimageable resinous paint to protect the edges ofthe conductive through-hole. Although U.S. Patent 4,755,551 discloses that the process can be used to form printed circuit boards with conductive through-holes, it requires the use of: (a) a separate resist and a separate electrodepositable resinous paint, and (b) a separate resist layer removing solution and a separate electrodepositable resinous paint layer removing solution. As the need for higher dertsity and higher precision circuit boards continues to grow, so does the need for improved methods of producing such boards. This includes a need for the development of improved electrodepositable photoresists. Summary of tbe Invention

The present invention provides an improved electrodepositable photoresist. Specifically, it has now been discovered that inorganic extender pigments can be effectively used as flow control agents for electrodepositable photoresists. The use of these particular extender pigments promotes edge coverage, but do not adversely affect the photosensitivity and/or developability properties ofthe photoresist into which they are incoφorated.

In accordance with the foregoing, embodiments ofthe present invention encompass negative-acting and positive-acting photoresists, either of which may be cationically or anionically electrodeposited, and which include a sufficient quantity of an inorganic extender pigment such that edge coverage is promoted, while adequate photosensitivity and developability are maintained.

These and other objects, aspects and advantages of this invention will become apparent to those skilled in the art upon reading the following Detailed Description in light ofthe appended claims.

Detailed Description of the Invention The term "photosensitivity" as used herein refers to the light exposure dose which is required to insolubilize or solubilize a film, depending upon whether the film is a positive-acting or negative-acting photoresist, thereby permitting subsequent development of a pattem by contact with a developing solution. Generally, the higher the sensitivity, the lower is the required light exposure dose to accomplish the desired result. In this specification, light exposure dose is measured in terms of millijoules per square centimeter.

The term "developability" as used herein refers to the susceptibility of a photoresist film, which was exposed or not exposed to actinic radiation, depending upon whether the film is positive-acting or negative-acting, to be removed or washed away by contact with a developing solution.

The improved electrodepositable photoresist ofthe present invention can be either positive-acting or negative-acting. Moreover, with regards to each of these, the electrodepositable photoresist ofthe present invention can also be either anionic or cationic. It is preferred to use a positive-acting electrodepositable photoresist for preparing circuit boards with conductive through-holes in accordance with the present invention. This preference is due, in part, to the difficulty encountered when attempting to insolubilize a negative-acting photoresist located within the through-hole with actinic radiation.

However, regardless of whether the photoresist being employed when practicing this invention is either positive-acting or negative-acting, it is preferred that it be cationic. This preference is due, in part, to the fact that anionic solutions tend to react with the copper over which they are electrodeposited more so than their cationic counterparts.

Positive-Acting Photoresists The positive-acting electrodepositable photoresists which can be employed when practicing this invention must be rendered more soluble in a developing solution after being exposed to actinic radiation. Moreover, in order that they may be electrodepositable, they should have salt-forming groups in the molecule.

A number of materials, or mixtures of materials, are known to have these properties (e.g., the polyoxymethylene polymers described in U.S. Patent 3,991,003; the o- nitrocarbinol esters described in U.S. Patent 3,849,137; the o-nitrophenyl acetals, their polyesters, and end-capped derivatives described in U.S. Patent 4,086,210; and benzo- and naphthquinonediazide sulfonic esters such as those described in U.S. Patent 4,306,010, and in British Patent Specification Nos. 1,227,602, 1,329,888 and 1,330,932).

In one preferred embodiment, the positive-acting electrodepositable photoresist into which a sufficient quantity of an inorganic extender pigment is added is the salt of polyoxymethylene polymers bearing salt-forming groups, o-nitrocarbinol esters bearing salt forming groups, o-nitrophenyl acetals, their polyesters, and end-capped derivatives bearing salt-forming groups, and benzo- and naphthquinonediazide sulfonic esters bearing salt forming groups. The preferred salt-forming groups in this embodiment of the invention are amine groups, and carboxylic, sulphonic or phosphoric acid groups.

Processes of preparing salts ofthe amine group-containing materials and of the acid group-containing materials are known to those skilled in the art. Examples of such processes include those which are set out in U.S. Patent 4,632,900, column 4, lines 24-33, incoφorated herein by reference. Moreover, examples of processes of preparing a photosensitive o-nitrophenyl acetal, a photosensitive o-nitrocarbinol ester of an unsaturated acid, a photosensitive quinone diazide group-containing salt, as well as other preferred electrodepositable positive-acting photoresists which can be used when practicing this invention, include those which are set out in U.S. Patent 4,632,900, column 4, line 34 through column 9, line 42, also incoφorated herein by reference. In another, preferred embodiment of this invention, wherein a sufficient quantity of an inorganic extender pigment is added to a positive-acting electrodepositable photoresist, the positive-acting photoreactive compounds are synthesized from monomers including:

or

where X and Y may be the same or different member selected from the group consisting of: halogen, -OH, -OR, -O-SO2R, -SR, and -NRR'. R and R' may be hydrogen or any of a wide variety of organic substituents, including substituted or unsubstituted alkyl, aryl, or aralkyl substituents. To form adducts or polymers, the R or R' groups include a reactive group such as a hydroxyl group. After exposure to actinic radiation such as ultraviolet light, the bond is broken between the carbon and the X in the CH2X group, thus providing photoactivity. The 2,6-dinitro- 1 ,4-bis(dichloromethyl)benzene species of structure (1) has been found to be particularly useful, and the corresponding diol species can be derived from the dichloro monomer. Both the chloride and hydroxyl groups are reactive with a wide variety of substances whereby intermediates and polymers can be synthesized from the dichloro or diol monomers of structure (1) or from the corresponding 2,5-dinitro monomers. The oligomers or polymers thus formed are highly photoreactive, and include the photoreactive groups as defined above and at least one ether, ester, urethane, carbonate, thio, or amino group or combinations thereof. Each of these substituents may include a reactive group (e.g., OH) to enable further reaction or copolymerization, if desired.

The polymers described above can be prepared from monomers and intermediates having the defined bis(chloromethyl)dinitrobenzene or dinitrobenzene dimethanol structures which are hydrolytically and thermally stable to the processing conditions required for photoimaging, such as in the manufacture of circuit boards. Polymers such as polyurethanes, polysulfides, and polyethers can be produced and are known to be stable in electrocoating baths. Polyesters, polyamines, and polyquatemized amine polymers have also been prepared witii the desirable dinitro groups set out above. Optionally, the photoreactive polymers may include salt forming groups or may be blended with another polymer that has salt forming groups to permit aqueous dispersion and electrodeposition of the photoresist composition onto conductive substrates. A detailed description ofthe aforementioned positive-acting electrodepositable photoresists into which inorganic extender pigments can be added in accordance with the present invention is set out in co-pending U.S. Application Serial No. 08/274,614, entitled "Positive Photoactive Compounds Based on 2,6- Dinitro Benzyl Groups" and filed July 13, 1994.

Negative-Acting Photoresists Negative-acting electrodepositable photoresists into which inorganic extender pigments can be added in accordance with the present invention, one example is a photoresist composition which comprise an unsaturated ionic polymeric material. Other embodiments additionally comprise an nonionic unsaturated material. Negative-acting electrodepositable photoresists typically include a photoinitiator. This photoinitiator can be either incoφorated into the photoresist's backbone, or blended in the photoresist as a separate component. A particularly preferred embodiment of a negative-acting electrodepositable photoresist into which inorganic extender pigments can be added in accordance with the present invention includes photoresist compositions which comprise: (a) an unsaturated ionic polymeric material, and (b) a nonionic unsaturated material. The process of making a negative-acting electrodepositable photoresist is known to those skilled in the art. One particular example of such a process is set out in co¬ pending U.S. Application Serial No. 08/268,778, entitled "Photoimageable Electro¬ depositable Photoresist Composition" and filed June 30, 1994. The unsaturated ionic polymeric component is preferably a cationic, acid- neutralized, unsaturated amine-functional polymeric material. One example of such a polymeric material is an unsaturated epoxy-amine adduct. The epoxy materials useful in making up this component can be monomeric or polymeric compounds or a mixture of compounds having an average of at least one epoxy group per molecule. Although monoepoxides can be utilized, it is presently preferred that the epoxy materials contain more than one epoxy group per molecule.

The epoxy materials can be essentially any ofthe epoxides known to those skilled in the art. However, a particular class of polyepoxides which are useful include polyglycidyl ethers of polyphenols such as bisphenol A. These can be produced, for example, by etherification of a polyphenol with epichlorohydrin in the presence of an alkali. The phenolic compound may be, for example, bis(4-hydroxyphenyl)2,2- propane, 4,4'- dihydroxy benzophenone, bis(4- hydroxyphenyl) 1,1 -ethane, nonyl phenol, resorcinol, catechol, bis(4-hydroxyphenyl)l,l-isobutane, bis(4-hydroxy tertiarybutyl phenyl)2,2- propane, bis(2-hydroxynaphthyl) methane, 1,5-dihydroxy-naphthylene, or the like.

In many instances, it is desirable to employ such polyepoxides having somewhat higher molecular weight and, preferably, containing aromatic groups. These polyepoxides can be made by reacting the diglycidyl ether set forth above with a polyphenol such as bisphenol A. Preferably, the polyglycidyl ether of a polyphenol contains free hydroxyl groups in addition to epoxide groups. While the polyglycidyl ethers of polyphenols may be employed, per se, it is frequently desirable to react a portion ofthe reactive sites (hydroxyl or in some instances epoxy) with a modifying material to vary the film characteristics ofthe resin.

Another quite useful class of polyepoxides are produced similarly from novolac resins or similar polyphenol resins. Also suitable are the similar polyglycidyl ethers of polyhydric alcohols which may be derived from such polyhydric alcohols as ethylene glycol, diethylene glycol, triethylene glycol, 1,2-propylene glycol, 1,4-propylene glycol, 1,5- pentanediol, 1,2,6-hexanetriol, glycerol, bis(4-hydroxycyclohexyl)2,2-propane and the like. Polyglycidyl esters of polycarboxylic acids can also be used. These are produced by the reaction of epichlorohydrin or similar epoxy compounds with an aliphatic or aromatic polycarboxylic acid such as oxalic acid, succinic acid, glutaric acid, terephthalic acid, 2,6-naphthylene dicarboxylic acid, dimerized linolenic acid and the like. Examples include: glycidyl adipate and glycidyl phthalate. Also useful are polyepoxides derived from the epoxidation of an olefinically unsaturated alicyclic compound. Included are diepoxides comprising in part one or more monoepoxides. These polyepoxides are non-phenolic and are obtained by the epoxidation of alicyclic olefins, for example, by oxygen and selected metal catalysts, by perbenzoic acids, by acetaldehyde monoperacetate, or by peracetic acid. Among such polyepoxides are the epoxy alicyclic ethers and esters which are well-known in the art.

Other epoxy-containing compounds and resins include nitrogeneous diepoxides such as those disclosed in U.S. Patent 3,365,471; epoxy resins from 1,1- methylene bis(5-substituted hydantoin) such as those disclosed in U.S. Patent 3,391,097; bisimide-containing diepoxides such as those disclosed in U.S. Patent 3,450,711 ; epoxylated aminoethyldiphenyl oxides such as those disclosed in U.S. Patent 3,312,644; heterocyclic N,N'-diglycidyl compounds such as those disclosed in U.S. Patent 3,503,979; amine epoxy phosphonates such as those disclosed in British Patent 1,172,916; and 1,3,5-triglycidyl isocyanurates, as well as other epoxy-containing materials known in the art. The epoxy equivalent weight of the epoxy material (grams of solid resin per equivalent of epoxy) will typically range from between 100 to 5,000. Preferably, the equivalent weight ofthe epoxy material ranges from between 200 to 1,000.

The amines useful in preparing the polyepoxide amine adduct can be ammonia, primary, secondary and tertiary amines and or a mixture thereof. Examples of useful amines include: dibutylamine, methyl ethanolamine, dimethylamine, diethanolamine, and the diketimine of diethylene triamine, polyoxyalkylene polyamine (e.g., JEFF AMINES® from Texaco Co.), diethylamine, moφholine, dimethylamine, propylamine, diisopropanol¬ amine, butylamine, ethylamine, triethylamine, triethanolamine, dimethylethanolamine and the like and/or a mixture thereof (e.g., dibutylamine and methylethanolamine). The epoxy-amine adduct can be at least partially neutralized with an acid to form a polymeric product containing amine salt and or quatemary ammonium salt groups. For example, U.S. Patent 4,260,720, column 5, line 20, to column 7, line 4, the portions of which are hereby incoφorated by reference, discloses polyepoxide amine adducts and methods of preparing the same. With regard to the amount of organic amine and polyepoxide which are reacted with one another, the relative amounts depend upon the extent of cationic base, such as cationic salt group formation, desired. This, in turn, will depend upon the molecular weight ofthe polymer. The extent of cationic salt group formation and the molecular weight of the reaction product should be selected such that, when the resultant cationic polymer is mixed with aqueous medium, a stable dispersion will form. A stable dispersion is one which does not settle or is one which is easily dispersible if some sedimentation occurs. The dispersion should additionally be of sufficient cationic character that the dispersed polymer particles will migrate towards the cathode when an electrical potential is impressed between an anode and a cathode immersed in the aqueous dispersion.

The molecular weight, structure and extent of cationic salt group formation should be controlled such that the dispersed polymer will have the required flow to form a continuous film on the electrode and, in the case of cationic electrodeposition, to form a continuous film on the cathode. The film should be insensitive to moisture to the extent that it will not appreciably redissolve in the electrodeposition bath or be easily rinsed away by an aqueous solution from the coated surface after removal from the bath. On the other hand, the molecular weight, structure and extent of cationic salt group formation should also be controlled such that the deposited and dehydrated film will be dissolvable in aqueous acids during development ofthe photoresist film.

The amines, including aromatic amines in the presence ofcertain photoinitiators, are realized to be "co-initiators" of photopolymerization. Hence, the amine employed herein, as to type and/or amount, can have an appreciable effect on the photosensitivity and/or developability of these negative-acting photoresist compositions. Without being bound by any particular theory, it is believed that the amines as aforestated increase the number of potential crosslinkable sites in this particular class of negative-acting photoresist compositions which are capable of participating in the insolubilization reaction. Differently put, the concentration ofthe reactive sites in these negative-acting photoresist compositions is thereby increased. Thus, these negative-acting photoresist compositions can be more readily insolubilized with a relatively small but effective content of unsaturated groups. Epoxy-amine adducts, and preferably epoxy polymers containing an aromatic group (e.g., aromatic epoxy-amine adducts), are believed to be particularly effective in improving photosensitivity of these negative-acting photoresist compositions. In general, most ofthe cationic polymers (e.g., the epoxy-amine adducts useful in the practice ofthe invention) will typically have weight average molecular weights within the range ofbetween 1,000 to 500,000, and will typically contain from between 0.1 to 2, and preferably from between 0.2 to 1.0 milliequivalents of a basic group (e.g., cationic group) per gram of resin solids. Given the teachings herein, one can couple the molecular weight with the cationic group content to arrive at a satisfactory polymer. The epoxy equivalent weight can range from between 100 to 5,000, and preferably from between 200 to 1,000. The cationic groups can alternately be introduced into the polyepoxide by reacting into the epoxy groups with a sulfide or phosphine and an acid, thus forming cationic sulfonium or phosphonium groups. Cationic resins that contain both sulfonium and amine salt groups can also be made as described in U.S. Patent 3,935,087 which is hereby incoφorated by reference.

An unsaturated moiety can be introduced into the cationic polymeric material or a precursor thereof. The resultant unsaturated cationic polymeric material contains on the average at least one ethylenically unsaturated moiety per molecule. Preferably, the unsaturated moiety is derived from the partial reaction of isophorone diisocyanate with an active hydrogen containing ethylenically unsaturated compound such as 2-hydroxyethyl acrylate. The resultant product can be described as a partially capped or partially blocked isocyanate. The partially capped isocyanate is then reacted with the aforedescribed polyepoxide adduct. Other than isophorone diisocyanate and the hydroxyethyl acrylate, other polyisocyanates and other compounds, which are preferably diisocyanates, can be substituted therefor in order to produce the half-capped isocyanates containing an unsaturated moiety. Illustrative examples ofthe other polyisocyanates include: aliphatic isocyanates such as alkylene isocyanates, e.g., trimethylene, tetramethylene, pentamethylene, hexamethylene, 1,2-propylene, 1,2-butylene, 2,3-butylene, 1,3-butylene, ethylidene and butylidene diisocyanates, cycloalkylene isocyanates, e.g., 1,3-cyclopentane, 1,4-cyclohexane, 1,2-cyclohexane diisocyanates, aromatic isocyanates such as arylene isocyanates, e.g., m-phenylene, p-phenylene, 4,4'-diphenyl, 1,5-naphthalene and 1,4- naphthalene diisocyanates and alkarylene isocyanates, e.g., 4,4'-diphenyl methane, 2,4- or 2,6-tolylene, or mixtures thereof, 4,4'-toluidine, and 1,4-xylylene diisocyanates; nuclear- substituted aromatic compounds, e.g., dianisidine diisocyanate, 4,4'- diphenylether diisocyanate and chlorodiphenylene diisocyanate. Triisocyanates, such as triphenyl methane- 4,4',4"- triisocyanate, 1,3,5-triisocyanato benzene and 2,4,6-triisocyanato toluene; the tetraisocyanates, such as 4,4'-diphenyldimethyl methane-2,2',5,5'-tetraisocyanate; polymerized polyisocyanates, such as tolylene diisocyanate dimers and trimers and the like can also be used. -I l -

In addition, the polyisocyanates may be prepolymers derived from polyols such as polyether polyols or polyester polyols, including polyols which are reacted with excess polyisocyanates, such as mentioned above, to form isocyanate-terminated prepolymers. Examples ofthe suitable isocyanate prepolymers include those which are described in U.S. Patent 3,799,854, column 2, lines 22 to 53. These are incoφorated herein by reference.

Illustrative examples of other compounds from which the unsaturated moiety can be derived include: hydroxypropyl acrylate, hydroxyethyl methacrylate, t-butylamino ethyl methacrylate, N-methylolacrylamide, caprolactone, acrylic acid adducts, meta isopropenyl a,a-dimethylisocyanate and mixtures thereof. The resultant unsaturated polymeric material has pendant unsaturation. By "pendant" is meant unsaturation which is attached to the polymer by means of a covalent or ionic bond.

The useful unsaturated cationic polymeric materials, which are typically amine-functional polymeric materials with pendant unsaturation, generally have weight average molecular weights ranging from between 1,500 to 1,000,000, and preferably from between 2,000 to 500,000. Moreover, unsaturation equivalent (gram solid resin/equivalent of unsaturation) ofthe material generally range from between 750 to 1,000,000, and preferably from between 1,000 to 50,000. Typically, the glass transition temperature (T_g) ofthe unsaturated cationic polymeric material ranges from between -20°C to 130°C, and preferably from between 10°C to 80°C. Milliequivalents ofthe basic group (e.g., the cationic group) typically range from between 0.1 to 5.0, and preferably from between 0.2 to 1.0 per gram of resin solids.

The amine-functional polymeric material is at least partially neutralized with an acid, preferably in the presence of water. Illustrative examples ofthe acid include: lactic and acetic acid, formic, sulfuric, phosphoric, propionic, hydroxy acetic, acrylic, methacrylic, glycolic, butyric, dimethylol propionic, 1,2 hydroxy stearic, and the like, and/or combinations thereof. The neutralized material can be dispersed in water during or after the neutralization. The resultant dispersion is useful in a cationic electrodeposition negative- acting photoresist composition. Besides the cationic polymeric material, anionic polymeric materials can also be employed in such electrodepositable photoresist compositions. As aforestated, a nonionic unsaturated material is employed in combination with the aforedescribed unsaturated polymeric cationic material. Typically the nonionic unsaturated materials are blended with the unsaturated cationic materials prior to dispersion in water. The nonionic unsaturated material can be a mono or multifunctional unsaturated material, e.g., a diacrylate of a polyepoxide, or an acrylate ester of an ethoxylated or propoxylated polyphenol, an acrylate ester of trimethylolpropane or pentaerythritol or a mixture thereof. Amounts typically ranging from between 1 to 70 percent, and preferably from between 5 to 40 percent, by weight ofthe nonionic unsaturated material, based on total resin solids weight ofthe bath, can be employed. Unsaturation equivalent weight of these nonionic materials are typically greater than 90. In many instances, the unsaturation equivalent weight of these nonionic materials range from between 90 to 1000. Illustrative examples ofthe unsaturated nonionic materials include: epoxy acrylates, urethane acrylates, ether acrylates, ester acrylates and ethylenically unsaturated resin such as acrylated melamine resins and acrylated acrylic resins.

Preferably, the nonionic unsaturated materials contain at least two unsaturated groups per molecule. Examples ofthe preferred nonionic materials include: a diacrylate of a diglycidyl ether of bisphenol A, trimethylol-propane triacrylate, pentaerythritol triacrylate, a diacrylate ester of ethoxylated diphenol and/or mixtures thereof. The percent of equivalents of unsaturation derived from the cationic material is typically from between 5 to 80 percent, and preferably from between 10 to 50 percent of the total unsaturation equivalents ofthe photoresist composition. The percent of equivalents of unsaturation derived from the nonionic material is typically from between 95 to 20 percent, and preferably from between 90 to 50 percent. As photoinitiators for such negative-acting photoresists, there are typically employed compounds which will initiate polymerization ofthe unsaturated groups ofthe electrodepositable photoresist composition when a film ofthe photoresist composition is exposed to actinic radiation of an appropriate energy. Illustrative examples of photoinitiators which can be used for this class of negative-acting electrodepositable photoresists include: thioxanthones, phosphine oxides, quinones, benzophenones, benzoin, acetophenones, benzoin ethers, and benzil ketals. Examples of photoinitiators which are presently preferred include: isopropyl thioxanthone, 1 -chloro thioxanthone (or other thioxanthane derivatives), Benzoin ethers (e.g., IRGACURE 907 from Ciba Geigy Co.), and combinations thereof. These photoinitiators can be made an integral part ofthe resinous materials used herein. However, in one preferred embodiment, the photoinitiator is extemal to the resinous material, namely the unsaturated cationic polymeric material.

The photoinitiator is typically employed in an amount ranging from between 0.05 to 25 percent, and preferably, from between 0.5 to 20 percent based on total resin solids. Moreover, the photoinitiator can be added before, during or after the dispersion is prepared.

Inorganic Extender Pigments

The extender pigment component used any embodiment of this invention is selected so as to promote edge coverage on the edges of conductive through-holes in order to protect these areas from etchants encountered in subsequent etching processes. It has been discovered that, in order to promote adequate edge coverage in these areas, the pigment to binder ratio within the resulting electrodepositable photoresist composition should be at least 0.05 : 1. The term "pigment to binder ratio" as used herein refers to the weight ratio ofthe weight of inorganic extender pigment solids to the total weight of all organic resinous solids in the photoresist.

In addition to the above, the extender pigment should also not significantly adversely affect the photosensitivity and/or developability properties ofthe photoresist composition into which it is incoφorated when present in a pigment to binder ratio of at least 0.05 : 1. It has been discovered that inorganic extender pigments possess these properties.

When practicing any ofthe embodiments ofthe invention, the inorganic extender pigment component is typically present in an amount such that the pigment to binder ratio is at least 0.05.1, and preferably at least 0.1 : 1, and even more preferably at least

0.2: 1. It was suφrising to discover that, when the inorganic extender pigment was present in such high concentrations, it does not adversely affect the photosensitivity and/or developability ofthe photoresist into which it is incoφorated.

Examples of inorganic extender pigments which can be used when practicing this invention include: calcites (e.g., calcium carbonate), talcs, silicates, clays (e.g., hydrated aluminum silicate), barytes, silicas (e.g., diatomaceous silica, crystalline silica, amoφhous silica, and synthetic silica), mica (i.e., hydrous aluminum potassium silicate), asbestos (i.e., fibrous magnesium silicate), wollastonite (i.e., calcium metasilicate), aluminum oxide, titanium dioxide, silicon oxide, antimony oxide, zinc oxide, barium sulfate and basic lead sulfate. The preferred extender pigment depends, at least in part, upon the developing solution being employed. For example, for negative-acting cationic photoresist compositions, one ofthe preferred inorganic extender pigments is hydrated aluminum silicate. When preparing the electrodepositable photoresist compositions in accordance with the present invention, the aforedescribed components can be mixed together to produce a dispersion. The mixture can then be dispersed into an aqueous solution. Additional water can be added to the dispersion to reduce the solids content. Also, the dispersion can be stripped in order to remove any volatile organic solvents that may be present.

The resultant dispersion typically has a resin solids content ranging from between 5 to 50 percent, and preferably from between 10 to 40 percent. Moreover, the photoresist compositions can have number average molecular weight (Mn) which typically range from between 500 to 50,000, unsaturation equivalent (gram solid resin per equivalent of unsaturation) which typically range from between 300 to 25,000, and T_g ofthe resultant dehydrated film which typically range from between 0°C to 100°C.

Due to the nature ofthe inorganic extender pigment component, it is presently preferred to have it in the form of a paste prior to mixing it with the other components. One preferred method of preparing such a paste includes dispersing the pigment using a water soluble organic grinding vehicle. The paste may, in addition, contain optional ingredients such as plasticizers, wetting agents, surfactants and/or defoamers.

Grinding is usually accomplished by the use of ball mills, sand mills, Cowles dissolvers, continuous attritors and the like until the extender pigment has been reduced to the desired size, preferably has been wet by and dispersed by the grinding vehicle. After grinding, the particle should typically be in the range of 20 microns or less, preferably as small as practical. Generally, a Hegman grind gauge reading of about 2 to about 15 is employed.

Preferably, grinding is conducted in an aqueous dispersion ofthe vehicle. The amount of water present in the aqueous grind should be sufficient to produce a continuous aqueous phase. The aqueous grind usually contains about 20 to 80 percent total solids. The use of more water merely reduces the effective capacity of the mill and, while less water can be employed, higher resultant viscosity may create problems in certain instances. In addition to the aforementioned components of a photoresist composition made in accordance with the present invention , other various formulating additives can optionally be employed. For instance, since film-smoothness is often of great importance, additives which decrease surface defects, (e.g., crater control agents) can be employed. Examples of such additives include: chain-extended epoxy materials such as polyepoxide- polyoxyalkylene polyamine adducts, polypropylene oxides, and other high molecular weight cationic polymers. There can also be employed as crater control agents silicone materials such as organo silicone fluids that can also increase slip and improve release ofthe photoresist from the photomask. The resin solids content ofthe electrodepositable photoresist compositions prepared in accordance with the present invention will typically range from between 1 to 50 percent, preferably from between 3 to 40 percent, and more preferably from between 5 to 30 percent; pH will typically range from between 2 to 10, and preferably from between 3 to 9; and conductivity will typically range from between 100 to 4,000 micromhos per centimeter, and preferably from between 200 to 2,000 micromhos per centimeter.

One method of electrodepositing a photoresist prepared in accordance with the present invention includes placing the aqueous dispersion ofthe photoresist composition in contact with an electrically-conductive anode and an electrically-conductive cathode. The surface to be coated can be made the cathode or the anode. In the case of cationic electrodeposition, the surface to be coated is the cathode; and in the case of anionic electrodeposition, the surface to be contacted is the anode.

Following contact with the aqueous dispersion, an adherent film ofthe coating composition is deposited onto the electrode being coated when a sufficient voltage is impressed between the electrodes. Conditions under which electrodeposition is carried out can be as follows. The applied voltage may be varied and can be, for example, as low as one volt or as high as several thousand volts. Typically however, the applied voltage will range from between 10 to 400 volts. On the other hand, current density will typically range from between 1 to 10 amperes per square foot and will tend to decrease during electrodeposition indicating the formation of an insulating film. Electrodeposition time typically varies from between 5 to 200 seconds. The resulting electrodeposited film typically has a thickness ranging from between 0.2 to 130 microns. In most instances, however, film thicknesses will range from between 2 to 80 microns. Generally, the electrodeposited film is dehydrated by an infrared bake, a short oven bake, or an air blow-off. In the process for preparing the pattemed photoresist in accordance with the present invention, a photomask is typically placed on the dehydrated photoresist. The masked photoresist is then exposed to actinic radiation.

In accordance with this invention, the dehydrated photoresist can be directly covered with a photomask to make an intimate contact therewith. The photomask (or tool) is a film on which is printed a patte which is the negative (or positive) image ofthe desired circuit. Actinic radiation is shown through the regions ofthe photoresist film which are transparent to said radiation in order to transfer the image ofthe photomask onto the photoresist film.

When practicing this invention, less than 800 millijoules per square centimeter, preferably less than 500 millijoules per square centimeter and even more preferably less than 300 millijoules per square centimeter, of light exposure dose (irradiation) is typically required to solubilize or insolubilize an electrodeposited and dehydrated film having a thickness which is less than or equal to 3 microns (on a flat surface) when it is exposed to dilute aqueous acidic solution, depending upon whether the photoresist is positive-acting or negative-acting. Irradiation can be conducted with ultra-violet (UV) light produced by various lamps having an average emission wavelength ranging from 200 to 700 nanometers (nm). Generally, the conditions for irradiation will depend on the nature ofthe source of irradiation, film thickness and the like.

After irradiation and removal ofthe photomask, the photoresist film is developed. Development ofthe photoresist film entails subjecting it to a developing solution by spraying, dipping, or the like. The developing solution for positive-acting and negative- acting photoresists is usually an acidic aqueous solution for cationic photoresists and a basic aqueous solution for anionic photoresists.

Usually, the photoresist film may be developed at a temperature which ranges from between 0°C to 85°C over a period which ranges from between 10 seconds to 10 minutes. With negative-acting photoresists, the exposed areas become insolubilized. However, the opposite is true for positive-acting photoresists. Hence, there is a solubility differential between the exposed and unexposed areas ofthe photoresist film, and the developing solution removes one or the other.

The term solubility differential denotes the rate of removal, by a developing solution, of a photoresist film which has been exposed to actinic radiation. For example, with regard to negatives-acting photoresists, the removal ofthe film by the developing solution is slower in the exposed areas relative to the unexposed areas. Accordingly, for a negative-acting photoresist, the unexposed areas ofthe film are the first to be removed. On the other hand, with regard to positive-acting photoresists, the removal ofthe film by the developing solution is slower in the unexposed areas relative to the exposed areas. Accordingly, for a positive-acting photoresist, the exposed areas ofthe film are the first to be removed.

The etching process involves the removal ofthe conductive substrate that is not covered with the photoresist film after the developing process. Etching is conducted by subjecting the uncovered substrate to an etchant which is designed to attack the exposed conductive material (e.g., copper) but not significantly attack the photoresist . Typically, etchants comprise ferric chloride solutions, cupric chloride, alkaline ammoniacal etchants or peroxide.

The etchant is usually sprayed onto the developed surfaces. Moreover, the etchant is usually maintained at a temperature which ranges from between 0°C to 75°C and is permitted to remain on the surface ofthe substrate for a sufficient time (i.e., from about 1 to about 20 minutes) to dissolve the exposed conductive material. Amine functional epoxy resins are not expected to swell in alkaline solutions (etchants). Thus, resolution is not compromised. Films derived from epoxy resins, in general, will resist attack by the various etchants used in the industry, (e.g. CuCl₂, FeCl₃, and peroxide: sulfuric acid). Usually, stripping is conducted by placing the etched substrate into the stripping solution at a temperature ranging from about 0°C to 100°C over a period from about 10 seconds to about 10 minutes.

After the etching process is completed, a stripping means is employed to remove the remaining photoresist film from the substrate. The particular stripping solutions employed will depend upon the chemical composition ofthe photoresist. Stripping solutions for photoresist made from cationic polymers comprise aqueous acids. Stripping solutions for photoresists made from anionic polymers comprise aqueous bases. EXAMPLES

The examples which follow are intended to assist in a further understanding of this invention. Particular materials employed, species and conditions are intended to be illustrative ofthe invention and are not limitive ofthe reasonable scope thereof.

Preparation of Electrodepositable Photoresist Bath Isophorone diisocyanate "IPDI" (687.4 grams (g) having 6.21 NCO equivalents) were charged into a 3-liter four-neck round bottom flask which was equipped with a mechanical stirrer, a thermometer, a dropping funnel, and a condenser. A calcium sulfate drying tube was attached to the condenser to protect it from moisture. The flask was heated to 50°C under stirring. Dibutyltin dilaurate (0.4 g) was added to the IPDI and a mixture of 2-hydroxyethyl acrylate (367.8 g, 3.17 OH equivalents) and IONOL®, which is di-t-butyl-p-cresol (6.3 g), was added to the IPDI at 50°C to 55°C through the dropping funnel over the period of 1.5 hours. After the addition was completed, the mixture was stirred at 50°C for an additional 4 hours to give urethane acrylate (NCO equivalent weight of 350.8) in which free IPDI was estimated to be 0.2 percent (wt wt) present by HPLC (High Pressure Liquid Chromatography). The resin made from this process is hereinafter referred to as resin A.

A mixture of EPON® 880 (10.6 parts by weight) epoxy resin from Shell Chemical Co. and bis-phenol A (4.3 parts by weight) were heated in 4-methyl-2-pentanone (5% on resin solids) to form an epoxy having epoxy equivalent weight of 800 to 850. The resin was then cooled to 80°C. A mixture of methylethanolamine (1.01 parts by weight) and dibutylamine (0.57 parts by weight) was then added to react with the epoxies.

After 3 hours, 5.94 parts by weight of resin A was added at a batch temperature of 68°C. The resin was held at this temperature until isocyanate was no longer detected by infra-red spectroscopy.

SARTOMER® 349, an ethoxylated bis-phenol diacrylate (6.92 parts by weight) and isopropylthioxanthone (2.44 parts by weight) were then added to the resin. Sufficient 4-methyl-2-pentanone was added to adjust the solids to about 72%. The resin was then dispersed into a mixture of deionized water and lactic acid (0.98 parts by weight) to form a dispersion with a final solids of 30% after azeotroping off the 4-methyl-2-pentanone. The electrocoat paint bath was then prepared by blending together 33.3 parts by weight ofthe resin with 66.7 parts by weight of deionized water. Prior to the coat-out process, the electrocoat paint bath had a pH of about 3.89 and a conductivity of about 900 μmhos. Preparation of Grind Vehicles used to Make the Cationic Inorganic Extender Pigment Paste

A quaternized epoxy amine urethane grind vehicle was prepared in accordance with the process set out in Examples I and II of U.S. Patent 4,081,343 (see, column 7, lines 6 - 60 of that patent). This grind vehicle is hereinafter referred to as "grind vehicle A."

A second grind vehicle was prepared by combining epoxy resin EPON® 880 with ARMEEN® DM- 18, an aliphatic amine, in a 1:2 molar ratio, in a solution of acetic acid, water and butyl ether of ethylene glycol. The solution was heated to produce a bis- quatamine-acid salt. The final solids were 62%. This grind vehicle is hereinafter referred to as "grind vehicle B."

Preparation of Cationic Inorganic Extender Pigment Paste

A cationic pigment paste was made by blending together 428.4 parts deionized water, 183.9 parts of grind vehicle A, 16 parts of grind vehicle B, 8.4 parts of SILWET™ L-7602, a surface active copolymer available from Union Carbide, and 363.3 parts of ASP- 170, a hydrous aluminum silicate available from Engelhard. This mixture was dispersed on an airmixer at high speed for approximately two hours using a ceramic media. The paste was then blended with the electrocoat paint bath prepared in accordance with the process set out above to give theoretical pigment to binder ratios of 0: 1 ; 0.1 : 1 ; 0.2: 1 ; and 0.25: 1.

Photosensitivity and Developability Experiments

The tests which follow are intended to demonstrate that, contrary to what was expected, the addition of relatively large amounts of inorganic extender pigments into an electrodepositable photoresist composition does not adversely affect the photosensitivity or developability ofthe resulting composition. This is especially suφrising since the inorganic extender pigments which are employed have no photosensitive functional groups. In these tests, copper foil clad laminate samples (obtained from Nelco Co.) were electrocoated at each pigment to binder level. During the coat-out process, the electrocoat bath temperature was maintained at 35°C. Voltage was varied from between about 170 volts to 230 volts in order to obtain a dry film thickness on the substrate of 0.5 mil for each pigment to binder level. After the electrocoating process, each panel was baked at a temperature of 125°C for about 3 minutes.

Following the baking process, the electrocoated panels were exposed on a vacuum frame U.V. light source. A phototool, with a test pattem having various line and space pairs thereon (i.e., 254 micron line/254 micron space; 203 micron line/203 micron space; 152 micron line/152 micron space; 127 micron line/127 micron space; 102 micron line/102 micron space; 51 micron line/51 micron space), was used to image the test panels. A Stouffer 21 -Step Sensitivity Guide was placed between the coated panels and the phototool to evaluate film cure. All panels were given an exposure dosage of 225 mj/c at the coating surface. Following the exposure process, the panels were dipped into a developing solution consisting of 0.23% glycolic acid, 3.5% butyl carbitol and 96.27% deionized water. The developer temperature was maintained at 32°C. Each test panel was moderately hand agitated in the developer for approximately 40 seconds. Following development, the panels were rinsed with deionized water. It was suφrising to discover that, when compared to the test panel which had no inorganic extender pigment added, there was no noticeable difference in the photosensitivity of any ofthe electrodepositable photoresists samples made in accordance with this invention, regardless ofthe high concentrations ofthe inorganic extender pigment incoφorated therein. Specifically, all pigment to binder levels showed development of unexposed areas clean to copper in 40 seconds.

Moreover, it was also suφrising to discover that, when compared to the test panel which had no inorganic extender pigment added, all electrodepositable photoresists made in accordance with this invention held a strong 2, (partial 3, 4) on the Stouffer Sensitivity Guide, regardless ofthe high concentrations ofthe inorganic extender pigment incoφorated therein. Furthermore, microscopic examination of line space pairs showed that all circuitry features were intact at all pigment to binder ratios following development. The aforementioned tests demonstrate that inorganic extender pigments do not adversely affect the photosensitivity or developability ofthe photoresist into which they are incoφorated.

Edge Coverage Experiments

The tests which follow are intended to demonstrate that, in addition to not adversely affecting the photosensitivity or developability ofthe photoresist into which they are incoφorated, the use of inorganic extender pigments significantly promote edge coverage. In these tests, two-sided copper clad boards, with a consistent plated hole pattem, were electrocoated at each pigment to binder level set out above. During the coat- out process, the electrocoat bath temperature was maintained at 35°C. Voltage was varied from between about 170 volts to about 230 volts in order to obtain a dry film thickness on the substrate of 0.6 mil. Edge coverage of each coat-out board was determined by a WACO Enamelrater.

Specifically, each test panel was attached to an electrode and immersed into a 1% NaCl solution attached to another electrode. A 6.3 voltage source was applied to the circuit for approximately 4 seconds; and the amperage draw through the coating was recorded. Lower amperage readings signify better edge coverage properties. Ideally, the amperage reading should be as close to zero as possible. Accordingly, amperage readings were used as a quantitative measurement of edge coverage.

For each ofthe test panels, their edges were taped to avoid panel edge effects due to shearing. Also, a small spot of coating was removed from the top of each panel so as to allow for good contact with the electrode. This area ofthe test panels was not immersed into the salt solution. The results observed after performing the edge coverage tests are listed below.

Pigment/Binder Ratio Conductivity

0: 1 274 milliamps 0.1 : 1 196 milliamps

0.2: 1 18 milliamps

0.25: 1 13 milliamps As can be seen, the Enamelrater readings demonstrate that edge coverage improves with the implementation of higher concentrations of inorganic extender pigments. Moreover, it was essentially suφrising to discover that, although employing a photoresist with a pigment to binder ratio of 0.1 : 1 reduced the amperage readings by 28%, the implementation of a photoresist with a pigment to binder ratio of greater than 0.1 : 1 reduced the amperage readings by 93%.

It is evident from the foregoing that various modifications, which are apparent to those skilled in the art, can be made to the embodiments of this invention without departing from the spirit or scope thereof. Having thus described the invention, it is claimed as follows.

Claims

THAT WHICH IS CLAIMED IS:

1. A photoimageable resist composition comprising: (a) a positive-acting, electrodepositable photoresist, and (b) an inorganic extender pigment, wherein the inorganic extender pigment is present in a pigment to binder ratio of at least 0.05: 1.

2. A photoimageable resist composition as recited in claim 1 wherein the positive-acting photoresist is cationic.

3. A photoimageable resist composition as recited in claim 1 wherein the positive-acting photoresist is anionic.

4. A photoimageable resist composition as recited in claim 1 wherein the inorganic extender pigment is present in a pigment to binder ratio of at least 0.1 :1.

5. A photoimageable resist composition as recited in claim 1 wherein the inorganic extender pigment is present in a pigment to binder ratio of at least 0.2: 1.

6. A photoimageable resist composition as recited in claim 1 wherein the inorganic extender pigment is selected from the group consisting of calcites, talcs, silicates, clays, barytes, silicas, mica, asbestos, wollastonite, aluminum oxide, titanium dioxide, silicon oxide, antimony oxide, zinc oxide, barium sulfate and basic lead sulfate.

7. A photoimageable resist composition as recited in claim 1 wherein the inorganic extender pigment comprises clay.

8. A photoimageable resist composition comprising: (a) a negative-acting, electrodepositable photoresist, and (b) an inorganic extender pigment, wherein the inorganic extender pigment is present in a pigment to binder ratio of at least 0.05: 1.

9. A photoimageable resist composition as recited in claim 8 wherein the negative-acting photoresist is cationic.

10. A photoimageable resist composition as recited in claim 8 wherein the negative-acting photoresist is anionic.

11. A photoimageable resist composition as recited in claim 8 wherein the inorganic extender pigment is present in a pigment to binder ratio of at least 0.1 : 1.

12. A photoimageable resist composition as recited in claim 8 wherein the inorganic extender pigment is present in a pigment to binder ratio of at least 0.2: 1.

13. A photoimageable resist composition as recited in claim 8 wherein the inorganic extender pigment is selected from the group consisting of calcites, talcs, silicates, clays, barytes, silicas, mica, asbestos, wollastonite, aluminum oxide, titanium dioxide, silicon oxide, antimony oxide, zinc oxide, barium sulfate and basic lead sulfate.

14. A photoimageable resist composition as recited in claim 8 wherein the inorganic extender pigment comprises clay.

15. A photoimageable resist composition as recited in claim 8 further comprising a photoinitiator.

16. A photoimageable resist composition as recited in claim 8 wherein the negative-acting photoresist comprises a water dispersible unsaturated cationic polymeric material.

17. A photoimageable resist composition as recited in claim 16 wherein the water-dispersible unsaturated cationic polymeric material is an amine-functional unsaturated polymeric material.

18. A photoimageable resist composition as recited in claim 16 further comprising a photoinitiator.

19. A photoimageable resist composition as recited in claim 16 wherein the negative-acting photoresist further comprises a nonionic unsaturated material.

20. A photoimageable resist composition as recited in claim 19 wherein the nonionic unsaturated material is selected from the groups consisting of a diacrylate of a diglycidyl ether of bisphenol A, trimethylolpropane triacrylate, pentaerythritol triacrylate or a diacrylate ester of ethoxylated diphenols and a mixture thereof.