Photosensitive layer

Abstract: A photosensitive resin composition comprising (A) a binder polymer having carboxyl groups, (B) a photopolymerizable compound having at least one polymerizable ethylenically unsaturated group in the molecule, and (C) a photoinitiator can provide a film of high mechanical strength, chemical resistance, flexibility, and suitable for producing a photosensitive element.

Description: The present invention relates to a photosensitive layer formed by using a photosensitive resin composition and a photosensitive element using the same. In the art of manufacture of printed circuit boards, photosensitive elements made of photosensitive resin compositions with a support and a protective film have been popularly used as resist in such treatments as etching and plating. A printed circuit board is manufactured by a process comprising the steps of laminating a photosensitive film on a copper substrate, subjecting it to pattern exposure, removing the unexposed portion with a developer, carrying out etching or plating to form a pattern, and then stripping the cured portion from the substrate. An alkaline developer comprising a sodium bicarbonate solution or the like is popularly used as developer for removing the unexposed portion. Usually developer is used as long as it maintains the ability to dissolve the photosensitive layer to a desired degree, and in use thereof, the photosensitive resin composition is dissolved or dispersed in the developing solution. With the recent trend toward higher density of printed circuit boards, which entails reduction of contact area between the copper substrate and the pattern-formed photosensitive layer, it is required of the composition used for forming the photosensitive layer to show excellent adhesiveness, mechanical strength, chemical resistance and flexibility in the development, etching or plating step. For the improvement of chemical resistance, methods using a photopolymerizable compound having isocyanurate rings are disclosed in JP-A-61-77844, JP-A-62-290705, JP-A-61-14212, JP-A-59-222480, JP-A-1-14190, JP-A-57-55914, JP-A-5-216224, JP-A-5-273754, etc., but these methods have the problem that the cured film of said compound is hard and frangible. On the other hand, from the viewpoint of improvement of processability, a photosensitive resin composition having high sensitivity and little possibility of causing contamination of the plating bath is desired, but these properties are dependent on the type and amount of the photoinitiator used in the composition. High-sensitivity photoinitiators are disclosed in DE-AS 2027467, EP-A 11786, EP-A 220, EP-A 589, JP-A 6-69631, etc., but these photoinitiators have the disadvantage that they may cause contamination of the plating bath. Photoinitiators with low possibility of causing plating bath contamination and high sensitivity are proposed in JP-A-60-239744, JP-A-2-226149, etc., but they are at a disadvantage in that their sensitivity is low. USP 3479185 discloses a photosensitive resin composition comprising a 2,4,5-triphenylimidazole dimer, which is a photoinitiator with low possibility of causing plating bath contamination, combined with a hydrogen donative compound to elevate sensitivity. This composition, however, has the drawbacks that in case sensitivity of the composition has been adjusted to a desired level, when the amount of said 2,4,5-triphenylimidazole dimer used is increased, the line width of the resist is enlarged, while when the amount of the hydrogen donative compound is increased, adhesiveness to copper and storage stability are deteriorated. Combination of a 2,4,5-triphenylimidazole dimer and a compound having a highly reactive ethylenically unsaturated group (such as polyethylene . glycol dimethacrylate) has also been studied, but this combination is very poor in chemical resistance. The present invention proposes to solve the problems of the prior art by means of a photosensitive layer formed by using a photosensitive resin composition which can satisfy various property requirements such as mentioned above. This can be done by using as an ethylenically unsaturated compound a compound in which an isocyanurate ring and an acrylate (or methacrylate) are bonded by a flexible alkylene oxide group (such as polyethylene oxide group or polypropylene oxide group) and an alkylene group (such as hexamethylene group), and if necessary, to improve plating resistance by adding a dicyanediamide thereto. This layer has excellent mechanical strength, chemical resistance and flexibility. Improved properties such as adhesiveness, photosensitivity, plating bath anti-contamination property can be achieved by use of a specific combination of a photopolymerizable compound containing an ethylenically unsaturated group and a photoinitiator. Thus, the present invention provides a photosensitive layer formed by using a photosensitive resin composition comprising: (A) a binder polymer having carboxyl groups; (B') a photopolymerizable compound having at least one polymerizable ethylenically unsaturated group in the molecule and having an isocyanurate ring and an acrylate or methacrylate group bonded by (i) a flexible alkylene oxide group and an alkylene group; and (C) a 2,4,5-triphenylimidazole dimer as photoinitiator. Each nitrogen atom in the isocyanurate ring may be bound to a group of formula (VI): The present invention also provides a photosensitive element comprising a base material and said layer formed thereon. The binder polymers having carboxyl groups usable as component (A) in this invention include homopolymers of acrylic and methacrylic acid [when the term "(meth)acrylic acid" is used in the present specification, it means both acrylic acid and methacrylic acid] and copolymers thereof with vinyl monomers copolymerizable therewith. Examples of said vinyl monomers, which are not specified in this invention, include methyl (meth)-acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, tetrahydrofurfuryl (meth)-acrylate, dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, glycidyl methacrylate, 2,2,2-trifluoroethyl (meth)acrylate, 2,2,3,3-tetrafluoropropyl (meth)acrylate (and corresponding acrylates), acrylamide, diacetoacrylamide, styrene, vinyltoluene and the like. The binder polymer (A) used in the present invention is preferably soluble or swellable in alkaline solutions (such as 1-3 wt% sodium carbonate or potassium carbonate solutions) for facilitation of development or for environmental protection. The acid value of the binder polymer (A) preferably falls in a range of 100 to 500. When the acid value is less than 100, the time required for the development tends to be elongated, and when the acid value exceeds 500, the photocured resist tends to exhibit reduced resistance to developer. The weight-average molecular weight (measured by gel permeation chromatography (GPC) and calculated as polystyrene) of said binder polymer is preferably in a range of 20,000 to 300,000. When the weight-average molecular weight is less than 20,000, developer resistance of said polymer tends to lower, and when it exceeds 300,000, the development time tends to be elongated. The photopolymerizable compound can be obtained by reacting, for instance, a compound of formula (VII): wherein q is an integer of 1 to 9, with a compound of the formula: wherein R 3 is a hydrogen atom or a methyl group, and n is an integer of 1 to 14. In the formula (VII), q is preferably 3 to 6. When q is greater than 9, mechanical strength of the composition is low. The photopolymerizable compounds usable in the present invention include those commercially available, such as NK Oligo UA-21 (available from Shin-Nakamura Chemical Industries Co., Ltd. in which a group of formula (VI) is bound to each nitrogen atom in the isocyanurate ring: It is essential that the photoinitiator used as component (C) in the composition of the layer of the present invention contain a 2,4,5-triphenylimidazole dimer such as 2,2'-bis(o-chlorophenyl)-4,4',5,5'-tetraphenylbiimidazole, 2,2'-bis(o-chlorophenyl)-4,4',5,5'-tetrakis(m-methoxyphenyl)biimidazole, 2,2'-bis(p-carboxyphenyl)-4,4',5,5'-tetraphenylbiimidazole, 2,2'-bis(p-chlorophenyl)-4,4',5,5'-tetrakis(p-methoxyphenyl)-biimidazole, 2,2'-di-o-tolyl-4,4',5,5'-tetraphenylbiimidazole, 2,2'-di-p-tolyl-4,4'-di-o-tolyl-5,5'-diphenylbiimidazole and the like. These dimers may be used singly or in combination of two or more of them. The specific combination of said component (B') and said component (C) is instrumental in enhancing photosensitivity and improving adhesiveness, chemical resistance, flexibility and plating bath anti-contamination properties. The ratio of the component (A) is preferably 40 to 80 parts by weight per 100 parts by weight of the total of the components (A) and (B'). When the ratio of the component (A) is less than 40 parts by weight, the photocured product of the composition tends to become frangible, and the photosensitive element made thereof tends to deteriorate in coating characteristics. When the ratio exceeds 80 parts by weight, the composition sensitivity tends to lower. The ratio of the component (C) preferably falls in a range of 0.1 to 20 parts by weight per 100 parts by weight of the total of the components (A) and (B'). When the ratio of the component (C) is less than 0.1 part by weight, the composition sensitivity tends to lower, and when the ratio is over 20 parts by weight, light absorption at the surface of the composition increases when exposed to light, causing imperfect photocuring of the inside portion of the composition. The ratio of the 2,4,5-triphenylimidazole dimer(s) used as an essential constituent of the component (C) is preferably 1 part by weight or more per 100 parts by weight of the total of the components (A) and (B'). When the ratio of said dimer(s) is less than 1 part by weight, the composition tends to lack sensitivity. The ratio of the component (B') is preferably 20 to 60 parts by weight per 100 parts by weight of the total of the components (A) and (B'). When the ratio of the component (B') is less than 20 parts by weight, the composition tends to lack sensitivity, and when the ratio exceeds 60 parts by weight, the photocured product of the composition tends to become frangible. The ratio of the compound of (B') is usually not less than 3 parts by weight, preferably 5-40 parts by weight per 100 parts by weight of the total of the components (A) and (B'). When the ratio of said compound is less than 3 parts by weight, the composition tends to lower in sensitivity, adhesiveness and plating resistance. In the photosensitive resin composition of the layer of the present invention, a hydrogen donative compound (N-phenylglycin, 2-mercaptobenzothiazole, 2-mercaptobenzoxazole, 2-mercaptobenzimidazole, etc.) may be contained within limits not affecting adhesiveness to copper and storage stability of the composition. Various additives such as dye, colorant, plasticizer, pigment, flame retardant, stabilizer, adhesiveness imparting agent, etc., may be contained as desired in the photosensitive resin compositions of the layer of the present invention. The photosensitive resin compositions of the layer of the present invention may be made into a homogeneous solution by mixing and dissolving the respective components in a solvent capable of dissolving them, such as toluene, acetone, methylethyl ketone (MEK), methyl isobutyl ketone, methyl Cellosolve, ethyl Cellosolve, chloroform, methylene chloride, methyl alcohol, ethyl alcohol, etc. The photosensitive element of the present invention can be produced by coating the photosensitive resin composition described above on a support and drying the coating. As the support, polymer films such as polyethylene terephthalate film, polypropylene film and polyethylene film can be used. Polyethylene terephthalate film is preferred. The polymer film used as support should not have such a surface treatment or should not be made of such a material that will make the film unseparable, since the film needs to be separated later from the photosensitive layer. The thickness of the polymer film is preferably in a range of 5 to 100 µm, more preferably 10 to 30 µm. The polymer films may be laminated on both sides of the photosensitive layer, with one of the polymer films used as a support film of the photosensitive layer and the other serving as a protective film of the photosensitive layer. For forming a photoresist image using the photosensitive element of the present invention, in case a protective film is present, a method may be employed in which after removal of the protective film the photosensitive layer is heated and pressed against the substrate to thereby laminate the layer on the substrate. The substrate surface on which the photosensitive layer is laminated is usually a metallic surface but not limited thereto. The photosensitive layer heating temperature may be properly decided but is preferably selected from a range of 90°C to 130°C. The pressure applied for bonding the photosensitive layer to the substrate is also not specified but preferably 3 kg/cm 2 . If the photosensitive layer is heated as described above, it is not necessary to preheat the substrate, but the substrate may be preheated for facilitating lamination. The thus laminated photosensitive layer is then exposed image wise to active light through a negative film or a positive film. In case the polymer film present on the photosensitive layer is transparent, exposure may be carried out with the polymer film allowed to stay, but when the polymer film is opaque, it should be removed before conducting exposure. It is desirable for protection of the photosensitive layer that the polymer film is transparent, and that exposure is conducted without removing the polymer film but through this film. As active light for exposure, there can be used the light generated from the known active light sources such as carbon arc lamp, mercury vapor lamp, xenon arc lamp, etc. Sensitivity of the photoinitiator contained in the photosensitive layer is usually maximized in the ultraviolet region, so that in this case it is desirable to use an active light source which can effectively emit the ultraviolet rays. In case the photoinitiator includes a substance (such as 9,10-phenanethrequinone) which is sensitive to visible light rays, visible light is used as active light, and as light source, photoflood lamp, solar lamp, etc., can be used beside those mentioned above. After exposure, the polymer film if present on the photosensitive layer is removed and then the non-exposed portion is removed by a known method such as spraying, dipping under shaking, brushing, scrubbing, etc., using an aqueous alkaline solution for development. As the base of the aqueous alkaline solution, there can be used alkali metal carbonates, alkali metal hydroxides (e.g. alkali metal being lithium, sodium or potassium), alkali metal phosphates (potassium phosphate, sodium phosphate, etc.), alkali metal pyrophosphates and the like. An aqueous solution of sodium carbonate is preferably used. The pH of the aqueous alkaline solution is preferably in a range of 9 to 11, and its temperature is adjusted inconformity to the developing performance of the photosensitive layer. A surfactant, deforming agent, small quantity of an organic solvent for promoting development, and other additives may also be contained in the aqueous alkaline solution. A known method may be employed for fabricating a printed wiring board. For instance, the surface of the substrate exposed with the developed photoresist image as mask is subjected to etching, plating and other necessary treatments in the known ways. The photoresist image can be eliminated with an aqueous alkaline solution having a stronger alkalinity than the alkaline solution used for the development. A 1-5 wt% aqueous solution of sodium hydroxide can be used as the strongly alkaline solution.

Claims: A photosensitive layer formed by using a photosensitive resin compoistion comprising: (A) a binder polymer having carboxyl groups; (B') a photopolymerizable compound having at least one polymerizable ethylenically unsaturated group in the molecule and having an isocyanurate ring and an acrylate of methacrylate group bonded by (i) a flexible alkylene oxide group and an alkylene group; and (C) 2,4,5-triphenylimidazole dimer as photoinitiator. A photosensitive layer according to claim 1, wherein the photopolymerizable compound has an isocyanurate ring of the formula: wherein q is an integer of 1 to 9. A photosensitive layer according to claim 1, wherein the photopolymerizable compound is obtained by reacting a compound of the formula: wherein q is an integer of 1 to 9, with a compound of the formula: wherein R 3 is a hydrogen atom or a methyl group; and n is an integer of 1 to 14. A photosensitive layer according to claim 1, wherein each nitrogen atom in the isocyanurate ring is bound to a group of formula (VI): A photosensitive element comprising a base material and a photosensitive layer as claimed in claims 1 to 4. A method of manufacturing a printed circuit board which method comprises imagewise exposing a photosensitive layer according to any one of claims 1 to 4 laminated to a copper substrate, removing the unexposed portions with a developer and etching or plating the exposed substrate. A photosensitive layer of any one of claims 1 to 3 which is formed by using a solution of the photosensitive resin composition defined in claim 1. Use of a photosensitive layer of any one of claims 1 to 3 for producing a photosensitive element. Use of a photosensitive layer of any of claims 1 to 3 for manufacturing a printed circuit board.

Inventors: CHIBA TATSUO, ICHIKAWA TATSUYA, CHIBA, TATSUO, ICHIKAWA, TATSUYA
Applicants: HITACHI CHEMICAL CO LTD, HITACHI CHEMICAL COMPANY, LTD.
Application year: 1996
Main classification: G03F7
IPC codes: G03F7, C08F265