WO1993011465A1

WO1993011465A1 - Photopolymerizable composition containing interlinked allylic and epoxy polymer networks

Info

Publication number: WO1993011465A1
Application number: PCT/EP1992/002332
Authority: WO
Inventors: Ricardo Henry Breeveld; Jan André Jozef SCHUTYSER
Original assignee: Akzo Nobel N.V.
Priority date: 1991-12-06
Filing date: 1992-10-09
Publication date: 1993-06-10

Abstract

The invention pertains to a photopolymerizable composition comprising a mixture of interpenetrating network-forming monomers and a photo-initiator, the interpenetrating network-forming monomers comprising ethylenically unsaturated compounds capable of forming a polymer network, and a mixture capable of forming an epoxy resin network. The polymer network is formed, at least partially, from allylic compounds. Essentially the polymer network and the epoxy resin network are interlinked by means of a compound having both an ethylenically unsaturated functional group and a functional group reactive towards at least one of the ingredients in the epoxy resin network. Preferably, said compound is an ethylenically unsaturated epoxy crosslinker, such as maleic anhydride. The photopolymerizable composition, which optionally further comprises photopolymerizable vinylic monomers, a film-forming binder, solvent, pigments, and other additives, proves particularly useful as an additive plating resist, or - if additive catalysts are added - as an electroless platable resist.

Description

PHOTOPOLYMERIZABLE COMPOSITION CONTAINING INTERLINKED ALLYLIC AND EPOXY POLYMER NETWORKS

The invention pertains to a photopolymerizable composition comprising a mixture of interpenetrating network-forming monomers and a photo- initiator, the interpenetrating network-forming monomers comprising

(a) ethylenically unsaturated compounds capable of forming a polymer network, the ethylenically unsaturated compounds comprising a compound having at least two allylic functional groups,

(b) a mixture capable of forming an epoxy resin network, the mixture comprising at least one difunctional epoxide and a crosslinker reactive towards epoxy.

Photopolymerizable, or photosensitive compositions, also referred to as "photoresists", are compositions which upon irradiation undergo a change in physical properties, notably involving solubility or extractability in a developer liquid. In the case of "negative" or "permanent" resists, the initial composition is solvent-extractable and becomes solvent-resistant upon irradiation. In the case of a "positive" resist, there is a change from solvent-resistant to solvent-extractable.

Photoresists can be used to define patterns made up of covered and uncovered portions of a substrate. They are widely used, e.g. in manufacturing printed wiring boards (PWBs), either as an imaging resist at the stage where a conductive pattern is defined, or as a solder mask to protect selected areas of the conductive pattern from the action of molten solder.

The present invention particularly pertains to photopolymerizable compositions that can be used as imaging resists in additive plating processes involving hot alkaline plating baths. In such a process a permanent resist is applied to a substrate and irradiated through a master image, in order to define the pattern where metallization should take place. Of course, the developed photosensitive composition should be resistant to the hot alkaline medium. Such photoresists may be referred to as permanent additive resists.

The present invention also pertains to photopolymerizable compositions that can be adapted for use as a platable surface. Platable resists are used in processes for the manufacture of multilayer printed circuit boards in which, usually, additive plating techniques are employed. Such processes ("sequential multilayer processes") have been described in WO 91/6423 and US 4,737,446, both of which are incorporated by reference for all purposes. A basic requirement for electroless pl tability is that conventional additive catalysts can be added to the bulk and/or onto the pre-treated surface of the resist. The main requirement is that, once such catalysts are present, the 'photoresist can be plated so as to achieve favourable metallization with sufficient bond strength.

A photopolymerizable composition of the type mentioned in the opening paragraph is known from Japanese Patent Application Laid-Open No. 62/265-321. In the disclosed resin compositions the compound having at least two allylic functional groups is a prepolymer of diallyl phthalate. The cross!inker reactive towards epoxy is a mixture of a diamino triazine-modified imidazole compound and dicyanodiamide.

The specific ingredients necessary for providing the disclosed composition with acceptable thermal resistance, solvent resistance, and resistance towards hot alkali are attended, in this known composition, with several drawbacks. For example, dicyanodiamide has a tendency to crystallize in the photopolymerizable resin composition, which leads to ineffective curing. The presence of crystallized dicyanodiamide will adversely affect the general appearance and performance of the photoresist. Furthermore, dicyanodiamide is objectionable because of its toxicity. On the other hand, according to the Japanese disclosure, the specified mixture is suitable for preventing spelling of the resist. One object of the present invention is to provide a photocurable composition without a dicyanodiamide epoxy crosslinker in which the resist's properties are not adversely affected, e.g. no spelling.

Further, free amino groups are present in the disclosed compositions, which is known to have a negative impact on the storage-stability of epoxy-containing compositions. In particular this is a drawback in the case of one-pot systems, which in commercial practice are preferred. In the photopolymerizable compositions of the instant invention it is very well possible, and indeed preferred, to avoid the use of amino and/or cyano groups-containing epoxy curing agents. In this respect the invention has for one of its objects to provide a photopolymerizable composition which allows of a relatively wide choice of ingredients but is sufficiently resistant still towards increased temperatures, organic solvents, and hot alkali. Another object of the invention is to further improve these properties as compared with the known photoresist.

It should be noted that photopolymerizable compositions having some resistance towards hot alkali are known. Such permanent additive resists have been described, e.g., in EP-A-270 945. The disclosed photopolymerizable composition comprises a monomeric component comprising a half-acryloyl ester of bisphenol-A epoxy monomer, an initiating system activated by actinic radiation, and an elastomeric polymeric binder.

Although the EP-A-270 945 compositions are said to withstand contact for 24 hours with a liquid at pH 12 maintained at a temperature of 70°C, the alkali resistance should be improved for the compositions to function favourably in a commercial additive plating process. A further drawback is the presence of an elastomeric binder. These rubber-type components tend to lead to a high oxygen sensitivity, which has a negative effect on the product's performance and properties.

A permanent additive resist which in practice displays more acceptable results is the PAR1 Riston^® dry film photoresist ex DuPont de Nemours and Company. Though the alkali resistance is satisfactory, there is still room for further improvement. Besides, the above-mentioned drawback of oxygen sensitivity applies.

A serious further drawback to the aforementioned permanent additive resists is the occurrence of extraneous growth of the conductive metal when the photo-imaged substrate is immersed in the additive plating bath, and the leaching of organic constituents of the composition into the plating bath. These drawbacks may lead to metallization beyond the desired pattern and contamination of the plating bath, resulting in a less favourable yield in PWB production. Said problems of extraneous metal-growth and bath contamination associated with PAR1 are not incurred with the composition according to the present invention.

Further, it should be noted that a photopolymerizable composition which upon curing forms an interpenetrating polymer network (IPN) is known from EP-A-359 216. Disclosed are IPNs comprising an epoxy resin network produced from an acidified epoxy resin and a network of a polymer produced by the polymerization under the influence of radicals of a monomer which at room temperature is solid or has a viscosity greater than 1 kPa.s. Specifically disclosed are di- and triacrylates. The networks which make up the IPN are not interlinked. The acidified epoxy oligomer prescribed in EP-A-359216 preferably is avoided in the composition of the present invention. A drawback to said oligomer is its low stability upon storage. Also, the presence of ions negatively affects the resist's electrical properties. Further, the reactivity of the EP-A-359216 composition is enhanced by means of corrosive, hence disadvantageous Lewis acids. The concept of a polymeric material comprising chemically linked allylic and epoxy polymer networks is known from EP-A-417837. The disclosure does not pertain to photopolymerizable compositions.

In EP 366333 a photosensitive resin is disclosed which comprises a half ester of a hydroxyalkyl (meth)acrylate and a styrene-maleic anhydride copolymer, a multifunctional (meth)acrylate monomer, a multifunctional epoxide, and a photo-initiator. According to the disclosure, which does not pertain to allylic networks, the epoxide serves to provide more stable crosslinking of the photopoly er.

In JP 57/16446 the use of triallyl trimellitate as a photosensitive compound is disclosed. The disclosure does not pertain to IPNs.

In addition to the above, still another object of the present invention is to provide a generally favourable photosensitive composition which is especially viable as a permanent additive resist or a platable resist, and has a particularly good resistance towards hot alkali .

To this end, the invention consists of a photopolymerizable composition of the above known type which comprises at least one compound having both an ethylenically unsaturated functional group and a functional group reactive towards at least one of the ingredients in the mixture capable of forming an epoxy resin network. Essentially, the invention thus pertains to a photocurable mixture in which at least one ingredient serves as a means for interlinking the unsaturated double bond based polymer network and the epoxy resin network.

This can be further explained as follows. A curable mixture generally comprises monome.ric, oligomeric, or prepolymeric compounds which, upon polymerization, form a three-dimensional polymer network. In the case of the curable mixture employed in the present invention, two such networks, viz. the unsaturated double bond based polymer network and the epoxy resin network, are produced by different chemical mechanisms. This results in the formation of an interpenetrating network (IPN). In accordance with the instant invention, the networks producing the IPN are linked to each other. The unsaturated double bond based polymer network will be referred to hereinafter as allylic polymer network.

Various means may be provided for this interlinking. One possibility resides in employing a multifunctional compound containing both allylic and epoxy moieties instead of, or in addition to, the allylic compound contained in monomer mixture (a). Such a compound may also be used instead of, or in addition to, the epoxide contained in monomer mixture (b) . Such a multifunctional compound is capable of reacting according to both network-forming mechanisms, resulting in the two networks being linked to each other. Al ernatively, the networks are interlinked by means of the epoxy crosslinker. To this end a cross!inker is to be employed which has one or more functional groups reactive towards epoxy, and which further comprises an unsaturated chemical bond copolymerizable with the monomers contained in network- forming mixture (a). Also, an epoxy compound containing a polymerizable unsaturated chemical bond may be employed. At any rate, it is essential that, upon curing, the photopolymerizable composition of the present invention forms a polymeric material comprising chemically linked allylic and epoxy polymer networks.

It should be noted that the above-mentioned disclosure of interl nked IPNs, EP 417837, does not teach the man skilled in the art to employ a chemically linked interpenetrating polymer network of the disclosed type in a photopolymerizable composition, nor does it lead him to expect any of the advantages obtained with the photopolymerizable composition of the instant invention. Further, it should be noted that photo-initiation leads to different reaction kinetics than thermal initiation. It is believed, though such theory should not be considered to be binding, that in the thermally cured interlinked IPN of EP 417 837 the two networks are formed simultaneously. In the instant photopolymerizable composition the polymer network formed by photo-initiated polymerization of the ethylenically unsaturated compounds (a) is believed to be formed first. In the presence of a photo-initiated polymer network the epoxy resin network is formed by means of the (thermally initiated) crosslinking reaction of mixture (b). Hence, not only does the photopolymerizable composition according to the invention differ from the compositions disclosed in EP 417837 in the uncured state, also the resulting photo-cured products are different from the thermally cured products known from EP-A-417837.

The photopolymerizable composition according to the present invention displays further advantages, both of general value to photoresists and of specific value to permanent plating resists.

General advantages include superior chemical resistance, excellent Persoz hardness, high scratch resistance, and the possibility to avoid the use of halogen-containing solvents and developing agents. A further advantage of the instant photoresists with regard to additive plating processes besides favourable hot-alkali resistance is the high resolution obtained, i.e. the possibility of defining patterns of metallic conductors having narrow widths. In this respect the favourable side-wall geometry which may be obtained with the photopolymerizable composition according to the present invention is of paramount importance. In other words: the instant photoresist is capable of precisely covering conductive tracks, without the photopolymerizable composition flowing beyond the tracks. Another advantage of the photopolymerizable composition of the present invention is the good adhesion to copper obtained. A discussion of the various aspects of the current invention and the preferred embodiments follows below.

As has been stated above, several means for interlinking the allyl and epoxy networks can be provided. Examples of multifunctional molecules containing allylic and epoxy moieties include those described in J. Appl. Polym. Sci. 36(7), DE-OS-3037 094, and Japanese Patent Applications Laid-Open Nos. 61/120746, 60/081225, 55/021455, 55/072304.

Suitable multifunctional molecules can generally be prepared by epoxidation of part of the ally! groups in polyallyl compounds. Such epoxidation has been described in CA. May's Epoxy Resins (to be referred to later).

Examples of crosslinkers which are reactive towards epoxy and copolymerizable with allyl include cyclic carboxylic anhydrides containing a polymerizable unsaturated carbon-carbon bond, e.g., maleic anhydride, itaconic anhydride, citraconic anhydride, half-esters of the corresponding dicarboxylic acids and of isomeric d carboxylic acids, such as fumaric acid. The esterifying alcohol or alcohols are not particularly critical, but lower alcohols, such as methanol, ethanol, propanol, iso-propanol, butanol, pentanol, hexanol, and the like, are preferred. Also advantageous are half-esters of dicarboxylic acids in which the esterifying alcohol contains a polymerizable unsaturated carbon-carbon bond, such as allylic alcohol, hydroxy methyl acrylate, hydroxy methyl methacrylate, hydroxy ethyl acrylate, hydroxy ethyl methacrylate, as well as other hydroxy(alkyl) (meth)aerylates, and the like. Other suitable crosslinkers include carboxyl terminated oligo(ester) reaction products of dicarboxylic acids having a polymerizable double bond, such as maleic acid, and aliphatic or aromatic dio s. Such a crosslinker having a polymerizable unsaturated carbon-carbon bond is the preferred means of interlinking the allylic and the epoxy polymer networks. By further preference this crosslinker is an anhydride or a half-ester of an α,β-unsaturated dicarboxylic acid, or a mixture thereof. The best results are obtained with maleic anhydride. An additional advantage of maleic anhydride is that it enhances the polymerization rate of the allylic monomers.

In those embodiments where the means of interlinking the allyl polymer network and the epoxy resin is provided by allylic and/or epoxy functional compounds rather than by the crosslinker, the crosslinker does not need to contain a polymerizable unsaturated carbon-carbon bond. Suitable crosslinkers that may be used in this respect include saturated carboxylic anhydrides, such as hexahydrophthalic anhydride, methyl hexahydrophthalic anhydride, methyl tetrahydrophthalic anhydride, Nadic® methyl anhydride, phthalic anhydride, tetrahydrophthalic anhydride, succinic anhydride, trimellitic anhydride.

The presence of accelerators for the thermal reaction of the epoxide and the crosslinker is not excluded. Such accelerators are, e.g., imidazoles, notably alkyl substituted imidazoles such as 2-methyl imidazole and 2-ethyl 4-methyl imidazole.

Examples of compounds having at least two allylic functional groups include cyanuric compounds, such as triallyl cyanurate (TAC) monomer or trially! isocyanurate (TAIC) monomer, TAC or TAIC oligomers or prepolymers; aromatic polyallyl esters, such as diallyl phthalate, diallyl isophthalate, triallyl trimellitate, tetraallyl pyromellitate, diallyl tetrabromophthalate; aliphatic polyallyl esters such as diallyl adipate, diallyl maleate, diallyl fumarate; polyallyl carbonates such as diethylene glycol diallyl carbonate, and the like. It is very well possible to use mixtures of allyl compounds. It is preferred to use TAC or TAIC monomers, oligo ers or prepolymers, or multifunctional allyl compounds in which the allylic moieties are TAC or TAIC moieties.

In this respect it is remarked that another disadvantage of the composition disclosed in JP-A-62/265-321 is the prescribed presence of diallyl phthalate prepolymer, which is attended with the necessity of using a relatively large amount of solvent to provide a photopolymerizable composition with the desired processability. The present invention provides a photopolymerizable composition which allows the use of monomers which may simultaneously serve as a solvent. An example of such a monomer is TAC.

The term "difunctional epoxide" is used to indicate the compounds present in a curable composition of oxirane ring-containing compounds. Such a curable composition may also be referred to simply as "epoxy resin". Said compounds have been described in CA. May's Epoxy Resins, 2nd Edition, Marcel Dekker Inc., New York & Basle, 1988. Further reference can be made to the aforementioned EP 417 837, which is hereby incorporated by reference for all purposes.

Preferred epoxy resins are phenol types, such as those based on the diglycidyl ether of Bisphenol-A, on polyglycidyl ethers of phenol- formaldehyde Novolac or cresol-formaldehyde Novolac, on the triglycidyl ether of tris(p-hydroxyphenol)methane, or on the tetraglycidyl ether of tetraphenyl ethane; amine types, such as those -based on tetraglycidyl methylene diam^"line or the triglycidyl ether of p-aminoglycol; cycloaliphatic types, such as those based on 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate.

As has been indicated above, the epoxide may also be a multifunctional compound containing both allylic and epoxy moieties. Multifunctional compounds having epoxy and vinylic moieties copolymerizable with allylic compounds are suitable as well. In one embodiment the ethylenically unsaturated compounds (a) may be exclusively allylic compounds (al). In another embodiment, the photochemical curing speed may be improved by incorporating into the photopolymerizable composition according to the present invention monomers present in conventional photoresists, i.e. photopolymerizable vinylic monomers (a2). Due to the presence of these monomers faster curing speeds and hence the economically most feasible manufacturing processes are realized.

Examples of suitable photopolymerizable vinyl ic monomers include monomers polymerizable under the influence of radicals-generating photo-initiators and monomers polymerizable under the influence of cationic photo-initiators. Examples of vinyl ic monomers polymerizable under the influence of photo-generated radicals are monomers, such as styrene, diisopropenyl benzene, triisopropenyl benzene. Other examples include acrylic and methacrylic monomers, such as t-butyl

(meth) acrylate, pentanediol di (meth)acrylate, N,N-diethyl aminoethyl

(meth) acrylate, ethylene glycol di (meth)acrylate, butanediol di (meth) acrylate, diethylene glycol di (meth) acrylate, hexa ethylene glycol di (meth) acrylate, prooanediol di ( eth)acrylate, decamethylene glycol di (meth) acrylate, cyclohexanediol di (meth) acrylate, dimethyl ol propane di (meth) acrylate, glycerol di (meth)acrylate, tripropylene glycol di (meth) acrylate, glycerol tri (meth) acrylate, trimethylol propane tri (meth) acrylate, pentaerythritol tri (meth) acrylate, pentaerythritol tetraacrylate, tri s (2-hydroxyethyl ) i socyanurate tetraacryl ate .

Examples of monomers polymerizable under the influence of cationic photo-initiators include vinyl ethers, e.g. alkyl vinyl ethers such as ethyl vinyl ether, isopropenyl vinyl ether, isobutyl vinyl ether, tertiary butyl vinyl ether; substituted alkyl vinyl ethers, such as chloroethyl vinyl ether, aryl vinyl ethers such as phenyl vinyl ether, methoxyphenyl vinyl ether, tolyl vinyl ether; substituted aryl vinyl ethers, such as chlorophenyl vinyl ether.

Preferred photopolymerizable vinylic monomers are trimethylol propane triacrylate and trimethylol propane tri ethacrylate.

A wide range of photo-initiators may be used. Photo-initiators are known in the art, and can be generally described as compounds producing highly reactive species, such as free radicals, upon actinic radiation, e.g. UV radiation.

Examples of radicals-generating photo-initi tors include well-known vicinal ketaldonyl alcohols, such as benzoin, pivaloin, acyloin ethers, e.g. benzoin methyl and ethyl ethers; -hydrocarbon substituted aromatic acyloins such as α-methyl benzoin, σ-allyl benzoin, α-phenyl benzoin; phenazine, oxazines, quinones; Michler's ketone (tetramethyl diamino benzophenone), benzophenone; substituted or unsubstituted polynuclear quinones, e.g. anthraquinone, chloroanthraquinone, methyl nthraquinone, ethylanthraquinone, t-butyl anthraquinone, octamethyl anthraquinone, naphthoquinone, phenanthrenequ none, benzanthraquinone, methyl naphthoquinone, dichloronaphthoquinone, dimethylanthraquinone, phenylanthraquinone, diphenylanthraquinone, and the like. Combinations of two or more photo-initiators are frequently applied. In this respect a mixture of benzophenone and a minor amount of Michler's ketone is found to be advantageous.

Other suitable photo-initiators include those initiating cationic polymerization reactions, such as the polymerization of vinyl ethers, and those liberating free acids and thus initiating acid-hardening reactions, such as epoxy polymerization. Such cationic photo- initiators are, e.g., arylsulphoπiurn and aryliodonium salts. These and other unli itative examples of suitable cationic photo-initiators are known from, e.g., EP 166682, EP 84515, EP 89922. It should be noted that, as the artisan knows, photo-initiators frequently are present in the form of a photo-initiating system. In such a system there may be present a sensitizer, i.e. a compound which is capable of absorbing radiation energy and transferring said energy to the actual photo-initiator. Photoreducible dyes may also be present. Examples of photo-initiating systems include those described in EP-A-270945, which is hereby incorporated by reference for all purposes.

Which specific combination of initiator and sensitizer is viable will depend on such circumstances as the source of irradiation used, the monomers to be polymerized, and so on. In a preferred embodiment photo-initiating systems sensitive to 488 nm may be used. These systems are advantageous in the case of so-called "direct imaging", where an Ar⁺ ion laser is employed to create the desired image without a mask being used.

A further possible constituent of the photopolymerizable compositions of the present invention is a film-forming polymeric binder. Though such a binder may be an elastomer, these are not to be preferred, as indicated above. It is an advantage of the current invention that even without a rubber component the photoresists obtained still display the required elasticity. Examples of non-rubber film-forming binders include polyacrylate and α-alkyl polyacrylate esters, e.g. polymethyl methacrylate and polyethyl methacrylate, polyvinyl esters such as polyvinyl acetate, and copolymers thereof with polyvinyl (meth)acrylate, hydrolyzed polyvinyl acetate, ethylene/vinyl acetate copolymers, polystyrene, and copolymers of polystyrene with, e.g., maleic anhydride and esters, copolymers of vinylidene chloride and, e.g., acrylonitrile, methacrylate, vinyl acetate, polyvinyl chloride, and copolymers, e.g. with polyvinyl acetate. Unsaturated polyesters are also suitable binders, i.e., the condensation products obtained by polycondensation of unsaturated dicarboxylic acids or anhydrides, such as maleic anhydride, citraconic anhydride, fumaric acid, with polyhydric alcohols such as ethylene glycol or 1,2-propylene glycol.

The aforementioned ingredients may be present in a wide range of ratios. Preferably the allylic compounds (al) and the epoxy resin network forming mixture (b) are present in a weight ratio of (al):(b) in the range of from 70:30 to 30:70, more preferably of from 60:40 to 40:60. The crosslinker will generally be present in a molar ratio of about 0.5 to 1.5 functional groups reactive towards epoxy per epoxy functional group. A preferred range is of from about 0.8 to about 1.2. According to a further preferred embodiment, the molar ratio of allylic functional groups to epoxy functional groups preferably is in the range of from about 6 to about 1. The crosslinker most preferably is present in an amount about equimolar to the epoxy functional groups.

The photo-initiator generally is present in an amount of 0.5 to 20 wt%, preferably 2-10 wt%, of the total composition. If present, the photopolymerizable monomers may make up any amount up to about 80% of the entire composition. The same holds for the film-forming binder.

In a further preferred embodiment the instant invention pertains to a photopolymerizable composition comprising about 29-74 wt% of the curable mixture of ingredients (al) and (b) described hereinbefore, about 3-6 wt% of the photo-initiator, about 15-46 wt% of the photopolymerizable monomers (a2), and up to 40 wt% of the film-forming binder.

The photosensitive compositions of the present invention may further contain additives such as those commonly present in photoresists. These include dyes, pigments, inhibitors, wetting agents, release agents, fillers. Examples include talc, aluminium silicate pigments, alumina tetrahydrate, alumina spheres, barium sulphate pigment, fumed silica and amorphous silica. These and other additives are known in the art, and need not be discussed here to explain the instant invention.

In one embodiment the invention provides a platable resist. Platable resists can be employed to define a pattern that should be metallized, the metallization in this case occurring on the surface of the resist. Platable resists thus can be used to build a multilayer printed wire board with conductive vias rather than plated through holes. In order to be platable a photoresist should be rendered catalytic for additive (electroless) plating. However, the mere addition of catalyst to just any photoresist does not generally render such a photoresist suitable for additive plating. Of course, one requirement is that the resist withstand the plating conditions mentioned earlier. The resist's mechanical, thermal, and electrical properties must also be sufficient to serve as a substrate for conductive circuitry. The photopolymerizable compositions of the invention satisfy these requirements, and can be rendered suitable for additive plating by the mere addition of a catalyst. Suitable catalysts include noble metals and noble metal compounds, palladium being preferred. More specifically: as a catalyst to be added to the bulk of the photoresist palladium and/or palladium chloride are preferred, dispersed or not in an inorganic or organic matrix (e.g. Pd in clay). As a surface treatment so-called 9F seeding is preferred, as well as the methods described in EP 430333 and US 4,701,351.

Solvents may also be present in the photopolymerizable compositions of the present invention. Suitable solvents include dimethyl formamide; glycol ethers, such as ethylene glycol onoethyl ether or propylene glycol monoethyl ether, and esters thereof, such as ethylene glycol monoethyl ether . acetate; ketones, such as methyl isobutyl ketone, methyl ethyl ketone (MEK), acetone, methyl isopropyl ketone; lactones such as gamma-butyrolactone; aromatic hydrocarbons, such as toluene and xylene, N-methyl pyrrolidone (NMP). Also, use may be made of mixtures of solvents. The preferred solvents are ketones, notably MEK, and NMP, by further preference in admixture with isopropyl alcohol (IPA); and Dowanol®PMA (propyleen glycol methylether acetate). A further advantage of the instant photopolymerizable compositions is that the use of solvents may be omitted, notably if TAC is used as the allylic monomer. Thus another disadvantage of the composition disclosed in JP-A-62/265-321, viz. the necessity to use a relatively large amount of solvent in order to provide the diallyl phthalate prepolymer-based photopolymerizable composition with the desired processability, can be overcome.

The compositions of the present invention may be used either in the form of a liquid resist, which is applied, e.g., by spraying onto a substrate, or in the form of a so-called dry film resist, which is usually applied to a substrate by roll lamination.

As a liquid resist, the compositions of the present invention are particularly useful in curtain coating processes. In such processes, a substrate is passed through, and is perpendicular to, a freely falling liquid resist curtain and so coated with the photopolymerizable composition. In order to be well-suited for use in such a process, the liquid resist generally has a viscosity of about 150 to 20000 Pa. With the compositions of the present invention viscosities in the desired range can easily be obtained, notably by varying the solid content.

In the case of a dry film resist, use may be made of a sandwich in which the photopolymerizable composition is present between a support layer, e.g. a polymeric layer such as polyethylene, and a.protective layer (e.g., polyethylene terephthalate foil). Upon lamination the support layer is removed, the photoresist thus being adhered -to the substrate with the protective layer on top. Then the photoresist- covered substrate may be subjected to photo-imaging, after which said protective layer is removed.

The instant invention also pertains to a dry film resist comprising a photopolymerizable composition as described hereinbefore.

The photo-imaging process itself is known in the art and involves irradiating the photoresist-covered substrate through a master image, resulting in certain portions being polymerized and other portions remaining uncured. The resist of the instant invention is also suitable for screen-printing. In such a process, known in itself, the photocurable composition is applied to a substrate by being passed through a screen which defines the desired image.

The photocuring treatment is carried out until the polymerized parts are sufficiently resistant towards the developer liquid used for the subsequent removal of the portions not subjected to irradiation. The irradiation generally is UV irradiation. A preferred light source used is a high pressure mercury arc.

A typical irradiation process as used in commercially operating PWB plants will involve an initial exposure of several seconds and a holdtime of a few minutes, followed by development. After the developing process the remaining portions of the photoresist are subjected to heat treatment in order to obtain complete curing. Suitable developers include the solvents in which the photopolymerizable composition of the present invention may be dissolved. Of course it is of importance that the irradiated, though not completely cured portions of the photosensitive composition be resistant towards the developer. Examples of such suitable developers include acetone, 1,1,1-trichloroethane, heptane, Dowanol PMA, IPA/heptane mixture, y-butyro lactone, methyl isobutyl ketone, xylene. For health and environmental reasons halogenated hydrocarbons such as trichloroethane are not to be preferred. A further advantage of the present invention to be mentioned is that the instant photopolymerizable compositions display a significantly higher initial •₅ solvent resistance towards less hazardous developers, such as heptane, IPA, Dowanol PMA, than state of the art additive plating resists do.

The invention a so pertains to a method of using a photopolymerizable composition according to any one of the embodiments described hereinbefore to provide a substrate with a photodefined layer, the 0 method comprising the steps of (i) applying the photopolymerizable composition to a substrate, resulting in the substrate being coated with a photosensitive layer, (ii) exposing selected portions of the photosensitive layer to

UV-light of sufficient energy to result in the exposed portions being initially cured, while the non-exposed portions remain uncured, (iii) contacting the partially irradiated layer with a developer liquid to remove the uncured portions of the photosensitive layer, thus leaving a photodefined coating on the substrate, (iv) subjecting the photodefined coating to thermal treatment in order to increase the degree of curing, (v) exposing the photodefined coating to UV-light, optionally in conjunction with heat, in order to effect further curing, to give a photodefined laminate comprised of the substrate coated with a permanent photodefined layer.

In such a method the substrate may be a base material for the additive manufacture of printed wiring boards. The permanent photodefined layer in such a case is used as a permanent plating resist mask. The mask is resistant towards hot alkali and does not lead to extraneous copper growth, nor does it cause contamination of the plating bath with leachables, thus leading to a higher yield of printed wiring boards and causing the manufacture thereof to be a more economically advantageous operation. The manufacture of printed wiring boards by additive metallization of a substrate susceptible to additive plating is known in the art, and need not be explained in detail here. Instead, reference is made to Clyde F. Coombs Jr., Printed Circuits Handbook, third edition, 1988, McGraw-Hill Book Company, New York.

Though the photopolymerizable compositions according to the present invention are particularly advantageous as permanent additive resists used in the step of generating a desired metallization pattern, the use of the instant compositions is not confined to this particular aspect of PWB manufacture. The instant photoresists display generally favourable properties and so are widely utilizable. Still with regard to PWB manufacture, for instance, the photopolymerizable compositions as described hereinbefore can advantageously be used to protect selected areas of the conductive pattern of a printed wiring board from the action of molten solder. This essentially is another embodiment of the above method, with the substrate used being a printed wiring board and the permanent photodefined layer forming a solder mask.

The composition of the instant invention may also be used in the subtractive manufacture of printed wiring boards. Subtractive manufacture, as described in the above-mentioned handbook, involves the process step of etching away copper from a copper-laminated substrate in order to define a conductive pattern. By virtue of the good copper adhesion obtained, the composition of the instant invention is particularly suitable in defining said pattern (which is protected from the etchant by the photoresist coating.

The invention also pertains to a printed wiring board comprising a substrate covered partially with a conductive metal pattern and partially with a polymeric permanent plating resist, the permanent plating resist being a cured photopolymerizable composition of the type described hereinbefore, in any embodiment.

The invention will be further described hereinafter with reference to the following Examples. These Examples should be construed to be explanatory rather than limitative.

EXAMPLES

General

In the compositions used in the Examples the following ingredients according to the invention have been used:

(a) ethylenically unsaturated monomers

a.l allylic monomers:

- tr ally! cyanurate (TAC) - tr allyl isocyanurate (TAIC)

- partially epoxidated TAIC mixture (EpTAIC) comprising 31% epoxide and 69% allylic functionality, present in the form of 6.6 t% triglycidyl isocyanurate (TGIC), 17.2 wt% DiGIC, 37.6 wt% Mo^'noGIC, and 26.7 wt% TAIC.

- TAC prepolymer, having a viscosity of 2700 cP, in a solvent-free mixture of 39 wt% of prepolymer having Mw 58550 and Mn 5453, and 61 wt% TAC monomer.

a.2 vinyl ic monomers:

- ethoxylated bispheπol -A di methacryl te (Diacryl 101)

- propoxylated bisphenol-A di methacrylate (Diacryl 201)

- trimethylol propane tri acrylate (TMPTA) (b) curable epoxy mixture

b.l epoxy resin:

(A) diglycidyl ether of bisphenol A having an epoxy equivalent weight (EEW) of 184 (Epikote 828)

(B) polyglycidyl ether of phenol-formaldehyde novolak (EEW 178)

(C) polyglycidyl ether of phenol-formaldehyde novolak having an epoxy functionality of 2.5 and an EEW of 175

b.2 crosslinker:

- mal eic anhydride (MA)

- mal ei c aci d monobutyl ester (MAMBE)

- maleic acid monoethyl ester (MAMEE)

- methyl hexahydrophthal i c anhydride (MHHPA)

(c) photo-initiator

(see the i ndi vidual Exampl es)

(d) polymeric binder

- poly(methyl methacrylate) with M_w 90000 - 400000 (Elvacite®2041)

- poly(methyl methacrylate - methacrylic acid) with M_w 20000 - 200000

(e) sol vent

(see the individual Examples)

(f) pigment - Sunfast blue (pigment blue 15:1)

In the Examples I-X the following general procedure was followed:

(1) Ingredients (al), optionally (a2), (bl), (b2), and optionally (f) were mixed to form a curable mixture.

(2) Polymeric binder (d) was dissolved in solvent (e) .

(3) The mixture formed under (1) was added to the solution formed under (2), and the whole was stirred until homogeneous.

(4) Photo-initiator (c) was added to the homogeneous mixture formed under (3) to form a photopolymerizable composition.

(5) The photopolymerizable composition formed under (4) was coated onto a support material of either glass, FR-4 (epoxy resin) board, or PET-film, dried for 5 minutes at room-temperature in a so-called "forced-dry" oven and for 30 minutes at 50°C, and the resulting tacky photopolymerizable coating was covered with a protective PET layer.

(6) The photoresist coated substrates were subjected to photopolymerization conditions as follows:

- initial exposure (of the order of seconds)

- hold time (a few minutes)

- development

- thermal treatment (150°C, 1 hour) - hold time (a few minutes)

- UV bump

For the initial exposure the light source used is a Philips HOK-6 80 W/cm (moving belt) mercury lamp. The UV bump is carried through using a Philips HPK 125 W (static) mercury lamp.

For comparison, using essentially the same photopolymerization procedure, PAR1 ex Dupont de Nemours was employed. The resulting photo-cured products were subjected to the following test methods:

Viscosity

The viscosity of the photoresist formulation was measured with a

Brookfield viscometer at 21°C (in mPa.s).

Cure speed

The cure speed was examined by passing the photoresist-coated substrate under a 80 W/cm lamp at four different belt speeds (hence four different irradiation times). The photoresist was irradiated through the protective PET layer, which was removed 20 minutes thereafter. Reported is the scratch- and/or tackfree state of the photoresist after irradiation and post-exposure hold time. A photoresist which upon being touched did not display any stickiness was considered tackfree. A photoresist which could not be scratched with a fingernail edge was considered scratchfree, and cured.

Persoz hardness

The Persoz hardness was measured on samples having glass as the support layer, using standard Persoz hardness equipment. The hardness of the samples was measured twice, viz. after initial exposure and hold time, and after the complete curing procedure.

Chemical resistance

Solvent resistance after the initial as well as the final cure was determined by placing samples having glass as the support layer in a solvent during 1 hour at 21°C A sample was considered solvent- resistant if the coating layer did not show any visible change, such as deterioration, curling, swelling or lift-up from the substrate. Exampl e I

In Example I a photopolymerizable composition was formed in accordance with the procedure given above, in the following manner:

^'5

(1) 27.5 g of TAC (a.l), 19.8 g of Epikote 828 (b.l), 8.7 g of maleic anhydride (b.2), and 70.4 g of (a. ) trimethylolpropane triacrylate (TMPTA) were mixed to form a curable mixture.

(2) 26.6 g of PMMA (d) were dissolved in 38.3 g of (e) a 1/1 mixture ₀ of methyl ethyl ketone (MEK) and isopropyl alcohol (IPA).

(3) The curable mixture of (1) was added to the solution of (2), and the whole stirred until homogeneous.

(4) The total amount of (a.l)+(a.2)+(b.l)+(b.2)+(d)+(e) being defined as 100 wt%, 3.3 wt% of Irgacure 651 (c) was added to the homogeneous mixture formed under (3). The resulting 80 wt% solids 5 content photopolymerizable composition had a Brookfield viscosity of 3500 mPa.s.

Example II

As Example I, except that 5 wt% Irgacure 651 (c) was used and further 1.0 wt% of Sunfast Blue pigment (f) added.

The results obtained with the photocurable compositions of Examples I and II are outlined below together those of PAR1 for comparison.

TABLE A - PHOTOCURING SPEED ("TF" stands for "tackfree" and "SF" stands for "scratchfree")

From the table it can be learned that with the photopolymerizable compositions according to the present invention it is possible to obtain photocuring speeds at least equal to the photoresist currently preferred in practice.

TABLE B - PERSOZ HARDNESS

(In seconds, measured on a film of 25 μm thickness)

TABLE C - INITIAL SOLVENT RESISTANCE

"good": no visible change occurred "moderate": initially cured film curled

It is apparent from Table C that relatively safe solvents can be used as a developer.

TABLE D - FINAL SOLVENT RESISTANCE

Solvent Example I Example II Comparison

Dichloromethane Trichloroethane Acetone DowanolTM PMA Hot Alkali*)

*) Hot alkali resistance is tested at a pH of 12, a temperature of 82°C, and for a period of 24 hours.

Examples III - X

Following essentially the same procedure as used in the previous Examples photopolymerizable compositions III - X were prepared and tested.

The actual compositions are made up as fol lows, with the amounts of ingredients expressed in weight percentages:

27

TABLE E - COMPOSITIONS III - VII

TABLE F - TEST RESULTS III - VII

Test III IV VI VII

Final Persoz Hardness (sec.) 318 Trichloroethane resistance good Acetone resistance good MEK/IPA resistance good Dowanol resistance good Hot alkali resistance good

Samples III-VII and PARl were subjected to thermogravimetric analysis (TGA) in order to determine the content of volatile substances. A low content of volatile substances is of importance with regard to soldering. The "solder shock" (280°C) to which printed wiring boards are usually subjected may lead to blistering in the case of a high volatiles content. TGA (under nitrogen) III IV V VI VII PARl

Results (loss at 300°C) , % 4.0 2.0 8.0 7.0 5.0 9.0

TABLE G - COMPOSITIONS VIII - X

TABLE H - TEST RESULTS VIII - X

Test VIII IX X

Final Persoz Hardness (sec.) 333 306 312

TGA (as above) 7.0 6.0 4.6

Trichloroethane resistance good good good

Acetone resistance good good good

MEK/IPA resistance good good good

Dowanol resistance good good good

Hot alkali resistance good mod good

Samples I-X as well as a PARl sample were subjected to a test described in US 4,601,970, according to which cured films are immersed in a methylene blue solution for 5 minutes, after having been subjected to hot alkali for 24 hours. The samples in accordance with the invention did not or hardly display any blue coloration, which demonstrates that the samples where well resistent towards hot alkali. PARl subjected to hot alkali displayed a deep blue colour, which PARl not subjected to hot alkali did not.

Example XI permanent plating resist

A conventional adhesive-coated FR4 epoxy-glass laminate rendered suitable for additive plating was provided with photopolymerizable composition prepared (essentially according to the procedure used in the previous Examples) from the following ingredients. The amounts expressed are weight percentages. a. l TAC 17.0 a.2 TMPTA 14.4 a.2 Diacryl 101 29.4 b.l epikote 828 LV/EL 12.8 b.2 maleic anhydride 5.3 c. irgacure 651 4.7 d. PMMA 16.3 e. Dowanol PMA/o-xylene 50.0 f. Holcolest pigment green EP 91719) 0.1

The photopolymerizable composition was applied to the adhesive laminate by meter-bar coating. The wet coated layer was dried by heating to 60°C for 30 minutes. The dry film thickness was 25-30 μm. A polyethylene terephthalate cover sheet of 23 μm thickness was applied over the dried layer.

Using a photoprinter the coated laminate was subjected to exposure to UV irradiation through a phototool. After a holdtime of 30 minutes the latent image was developed in a 1/1 mixture of Dowanol PMA/o-xylene. The image exhibited excellent resolution and side-wall geometry. Subsequently the image was thermally cured at 150°C for 1 hour. After cooling to room temperature the image was subjected to UV irradiation for 3 minutes using a Philips HPK 125 W lamp.

The imaged surface was degreased by washing with a warm soapy solution having a pH of about 10, followed by a water rinse.

The exposed adhesive areas were adhesion promoted in a hot solution of chrome sulphuric acid and sodium fluoride, followed by neutralisation in a sodium bisulphite solution and rinsing in water.

The laminate was then immersed in an electroless copper plating solution (Kollmorgen PCK 570, pH 12, temperature 70°C). In about one day's time this resulted in deposition of a copper circuit pattern. The photoimaged resist layer was unaffected (no leachables, no extraneous copper, no nodules).

Example XII platable resist

The photopolymerizable composition of Example XI was applied to a conventional adhesive-coated FR4 epoxy-glass laminate. The conventional laminate was not rendered suitable for additive plating, i.e. the adhesive coating did not contain catalyst for electroless plating.

The application and hardening of the photopolymerizable composition were done according to the procedure outlined in Example XI.

The surface of the cured coating was roughened and subsequently degreased by washing with a warm soapy solution having a pH of about 10, followed by a water rinse.

After 30 seconds of immersion in an aqueous 1% hydrochloric acid solution the laminate was placed in a colloidal solution of palladium chloride. After soaking for 10 minutes the laminate was washed with demineralized water and then immediately placed in an electroless copper-plating bath (Shipley Cuposit^®328 Q) .

Within a few minutes a copper layer smoothly deposited onto the coated surface of the laminate. After this initial electroless plating the thickness of the copper layer was customarily increased to 26 μm by electroplating.

The photoresist surface was completely covered with copper, hence proven to be platable. Comparative Exampl e JP 62/265-321

In accordance with prior art reference JP 62/265-321 a photopolymerizabl e composition was prepared having the fol l owi ng ingredients (weight percentages, ratio according to the disclosure indicated in brackets)

DAP prepolymer 78.19 [100]

- TMPTA 3.74 [ 4]

- Epoxy resin (EPN 1139) 13.40 [ 15] Irgacure 907 0.77 [ 1] Dicyanodi amide 1.56 [ 2]

- Curezol^®2MZ azine 2.34 [ 3]

The composition was prepared as a 40% solution in Dowanol PMA.

Unfavourable solubility was observed for dicyanodiamide and Curezol®2MZ, resulting in a turbid mixture.

The composition was coated onto glass, thickness 26 μm. After drying the coating was tackfree.

Subjected to photocuring, the composition displayed a poor curing speed. The ^" composition was ten times less sensitive than the photoresist of the present invention.

Persoz hardness was measured. Initial: 228 sec, final: 275 sec.

Solvent resistance was measured, rendering the following data Solvent Initial Final

The final composition displayed a poor hot alkali resistance.

Claims

Cl aims :

1. A photopolymerizable composition comprising a mixture of interpenetrating network-forming monomers and a photo-initiator, .5 the interpenetrating network-forming monomers comprising

10 (b) a mixture capable of forming an epoxy resin network, the mixture comprising at least one difunctional epoxide and a crosslinker reactive towards epoxy, characterized in that the composition comprises at least one compound having both an ethylenically unsaturated functional group _τc and a functional group reactive towards at least one of the lb ingredients in the mixture capable of forming an epoxy resin network.

A photopolymerizable composition according to claim 1, 0 characterized in that the compound having both an ethylenically unsaturated functional group and a functional group reactive towards at least one of the ingredients in the mixture capable of forming an epoxy resin network is a multifunctional molecule having an allylic moiety and an epoxy moiety. 5

A photopolymerizable composition according to claim 1, characterized in that the crosslinker reactive towards epoxy comprises a functional group copolymerizable with the allylic functional groups. 0

A photopolymerizable composition according to claim 3, characterized in that the crosslinker is selected from the group consisting of anhydrides, half-esters, and diesters of c.,/3-unsaturated dicarboxylic acids, and mixtures thereof.

5. A photopolymerizable composition according to claim 4, characterized in that the crosslinker is maleic anhydride.

6. A photopolymerizable composition according to any one of the preceding claims, characterized in that the allylic functional groups are contained in allylic compounds (al) having allylic moieties in the form of triallyl cyanurate, triallyl isocyanurate, or mixtures thereof.

7. A photopolymerizable composition according to any one of the preceding claims, characterized in that the at least difunctional epoxide is a phenol type epoxy resin.

8. A photopolymerizable composition according to any one of the preceding claims, characterized in that it comprises (a2) photopolymerizable vinylic monomers.

9. A photopolymerizable composition according to claim 8, characterized in that the vinylic monomers (a2) are selected from the group consisting of acrylates, methacrylates, vinyl ethers, and mixtures thereof.

10. A photopolymerizable composition according to any one of the preceding claims, characterized in that the photo-initiator is selected from the group consisting of benzoin, benzoin ethers, Michler's ketone, benzophenone, and mixtures thereof.

11. A photopolymerizable composition according to any one of the claims 1-9, characterized in that the photo-initiator is a cationic photo-initiator selected from the group consisting of arylsulphoniurn salts, aryliodoniu salts, and mixtures thereof.

12. A photopolymerizable composition according to any one of the preceding claims, characterized in that it further comprises a film-forming polymeric binder.

13. A photopolymerizable composition according to any one of the preceding claims, characterized in that it comprises about 29-74 wt% of a curable mixture of ingredients (al) and (b) , about 3-6 wt% of the photo-initiator, about 15-46 wt% of the photopolymerizable monomers (a2) , and from 0 to about 40 wt% of the film-forming polymeric binder.

14. A photopolymerizable composition according to any one of the preceding claims, characterized in that in the curable mixture of ingredients (a) and (b) the molar ratio of allylic functional groups (al) to epoxy functional groups (bl) is in the range of from about 1 to about 6, and the crosslinker (b2) is present in an amount about equimolar to the epoxy functional groups.

15. A photopolymerizable composition according to any one of the preceding claims, characterized in that it has been rendered suitable for electroless metal plating.

16. A dry film photoresist comprising a photosensitive layer having on one side a polymeric support layer and on the other side a removable protective layer, characterized in that the photosensitive layer is a photopolymerizable composition according to any one of the claims 1-15.

17. A method of using a photopolymerizable composition according to any one of the claims 1-15, comprising the steps of i. applying the photopolymerizable composition to a substrate, resulting in the substrate being coated with a photosensitive layer, ii. exposing selected portions of the photosensitive layer to UV-light of sufficient energy to result in the exposed portions being initially cured, while the non-exposed portions remain uncured, ,r iii. contacting the partially irradiated layer with a developer liquid to remove the uncured portions of the photosensitive layer, thus leaving a photodefined coating on the substrate, iv. subjecting the photodefined coating to thermal treatment in order to increase the degree of curing, 0 v. exposing the photodefined coating to UV-light, optionally in conjunction with heat, in order to effect further curing, to give a photodefined laminate comprised of the substrate coated with a permanent photodefined layer.

18. A method according to claim 17, characterized in that the 5 substrate is a base material for the additive manufacture of printed wiring boards, the photopolymerizable composition is applied in the form of a dry film resist according to claim 15, and the permanent photodefined layer forms a permanent plating 0 resist mask.

19. A method according to claim 17, characterized in that the substrate is a printed wiring board and the permanent photodefined layer forms a solder mask. 5

20. A method according to claim 18, characterized in that in step (i) the photopolymerizable composition is applied by curtain coating.

21. A printed wiring board comprising a substrate covered partially 0 with a conductive metal pattern and partially with a polymeric permanent plating resist, the permanent plating resist being a cured photopolymerizable composition according to any one of the claims 1-14.

22. A multilayer printed wiring board comprising conductive layers and dielectric layers characterized in that at least one of the dielectric layers comprises a platable photoresist according to claim 15.