Photoimageable composition having improved flexibility, adhesion and stripping characteristics

Abstract: A negative-acting photoimageable composition useful as a photoresist comprises an acid-functional binder, a photopolymerizable component, a photoinitiator chemical system comprising 9-phenyl acridine or a homologue thereof, and a dibenzoate plasticizer.

Description: The present invention is directed to negative-acting photoimageable compositions such as those used as photoresists. The photoimageable composition uses 9-phenyl acridine or a homologue thereof as the photoinitiator, and the photoimageable composition is plasticized with a dibenzoate plasticizer. Background of the Invention This invention is directed to negative-acting photoimageable compositions which are developable in alkaline aqueous solutions. The invention is particularly applicable to primary photoimaging resists, but is applicable, as well, to components of compositions that are hardenable so as to form solder masks and the like. A variety of such photoimageable compositions are described. Essential compositions of the type to which the present invention is directed are A) a binder polymer; B) photopolymerizable α,β-ethylenically unsaturated compound(s), and C) a photoinitiator chemical system. The binder polymer A) has sufficient acid functionality, generally carboxylic acid functionality, that the binder polymer is soluble in alkaline aqueous solution and thereby renders the photoimageable composition developable in alkaline aqueous solutions. The photoimageable compound(s) B) are monomers and/or short chain oligomers, a substantial portion of which have multiple α,β-ethylenic unsaturated functionality. Herein, the photoinitiator C), or at least a significant portion of the photoinitiator C) is 9-phenyl acridine or a homologue thereof. Summary of the Invention In accordance with the present invention a negative-acting photoimageable composition comprises A) an organic polymeric binder having sufficient acid functionality to render the photoimageable composition developable in alkaline aqueous solution, B) an addition-polymerizable, non-gaseous ethylenically unsaturated compound(s) capable of forming a polymer by free-radical initiated chain-propagating addition polymerization, and C) an organic, radiation-sensitive free-radical generating system activatable by actinic radiation to initiate chain-propagating addition polymerization of the addition-polymerizable material, the free-radical generating system herein comprising 9-phenyl acridine or a homologue thereof, and D) a dibenzoate plasticizer Detailed Description of Certain Preferred Embodiments Herein, unless otherwise noted, all percentages are weight percentages. Component A) (the binder polymer), Component B) (the photoimageable compounds(s)) and Component C) (the photoinitiator chemical system) are herein considered to equal 100 wt%, and other components, including the dibenzoate plasticizer, are calculated as parts relative to 100 parts of A) plus B) plus C). The invention is directed to photoimageable compositions which are developable in alkaline aqueous solution and which therefore have substantial acid functionality. Such photoimageable compositions typically have a binder A) having acid functionality, typically an acid number of at least 80, preferably at least 100 and more preferably 150 or more, up to 250. The acid functionality is typically carboxylic acid functionality, but may also include, for example, sulfonic acid functionality or phosphoric acid functionality. Binder polymers for photoimageable compositions typically have weight average molecular weights between 20,000 and 200,000, preferably at least 80,000. The polymers are typically derived from a mixture of acid functional monomers and non-acid functional monomers. Some specific examples of suitable acid functional monomers are acrylic acid, methacrylic acid, maleic acid, fumaric acid, citraconic acid, 2-acrylamido-2-methylpropanesulfonic acid, 2-hydroxyethyl acrylolyl phosphate, 2-hydroxypropyl acrylol phosphate, 2-hydroxy-alpha-acryloyl phosphate, etc. One or more of such acid functional monomers may be used to form the binder polymer. The acid functional monomers may be copolymerized with non-acid functional monomers, such as esters of acrylic acids, for example, methyl acrylate, 2-ethyl hexyl acrylate, n-butyl acrylate, n-hexyl acrylate, methyl methacrylate, hydroxy ethyl acrylate, butyl methacrylate, octyl acrylate, 2-ethoxy ethyl methacrylate, t-butyl acrylate, 1,5-pentanediol diacrylate, N,N-diethylaminoethyl acrylate, ethylene glycol diacrylate, 1,3-propanediol diacrylate, decamethylene glycol diacrylate, decamethylene glycol dimethacrylate, 1,4-cyclohexanediol diacrylate, 2,2-dimethylol propane diacrylate, glycerol diacrylate, tripropylene glycol diacrylate, glycerol triacrylate, 2,2-di (p-hydroxphenyl)-propane dimethacrylate, triethylene glycol diacrylate, polyoxyethyl-2-2-di (p-hydroxyphenyl)-propane dimethacrylate, triethylene glycol dimethacrylate, polyoxypropyltrimethylol propane triacrylate, ethylene glycol dimethacrylate, butylene glycol dimethacrylate, 1,3-propanediol dimethacrylate, butylene glycol dimethacrylate, 1,3-propanediol dimethacrylate, 1,2,4-butanetriol trimethacrylate, 2,2,4-trimethyl-1,3-pentanediol dimethacrylate, pentaerythritol trimethacrylate, 1-phenyl ethylene-1,2-dimethacrylate, pentaerythritol tetramethacrylate, trimethylol propane trimethacrylate, 1,5-pentanediol dimethacrylate, and 1,4-benzenediol dimethacrylate; styrene and substituted styrene, such as 2-methyl styrene and vinyl toluene and vinyl esters, such as vinyl acrylate and vinyl methacrylate to provide the desired acid number. The amount of the binder polymer A) may vary over a wide range, typically comprising between 30 to 80 wt% of the composition based on total weight of A) plus B) plus C). Examples of such polymers and photoimageable compositions using such polymers are found, in the following U.S. Patent Nos: 3,953,309, 4,003,877, 4,610,951, and 4,695,527. The photopolymerizable component B) is typically a monomer, dimer or short chain oligomer having ethylenic unsaturation, particularly, α,β-ethylenic unsaturation, including monofunctional compounds and compounds having α,β-ethylenic unsaturation functionality 2 or greater. Typically, a mixture of mono-functional and multi-functional monomers will be used. Suitable photopolymerizable compounds include, but are not limited to, the monomers recited above as suitable for forming binder polymers, particularly the non-acid functional compounds. The amount of photopolymerizable component B) is typically between 19 and 69 wt% of the photoimageable composition based on total weight of A) plus B) plus C). To initiate polymerization of the monomers upon exposure to actinic radiation, the photoimageable composition contains a photoinitiator chemical system. In accordance with the invention, the photoinitiator chemical system is, or contains a significant proportion of, a 9-phenylacridine compound preferably having the formula: where R 1 and R 2 are the same or different and are selected from hydrogen, alkyl, substituted alkyl, acyl, and substituted acyl; R 3 and R 4 are the same or different and are selected from hydrogen, halogens, alkyl, substituted alkyl, acyl, and substituted acyl; and R 5 , R 6 and R 7 are the same or different and are selected from hydrogen, halogens, alkyl, substituted alkyl, acyl, substituted acyl and groups of formula II: where R 1 and R 2 are as described above. Such compounds are described, for example, in U.S. Patent No. 5,217,845. From an availability standpoint, unsubstituted 9-phenyl acridine is the currently preferred photoinitiator. Of these, 9-phenyl acridine is the currently preferred initiator from a practical standpoint. Between 0.05 wt% and 2 wt%, preferably between 0.1 and 0.5 wt% of the photoimageable composition (based on total weight of A) plus B) plus C) is 9-phenyl acridine or a homologue of the general formula set forth above. Such photoinitiators are used in the present invention because they tend to produce high cross-link densities. However, because of the high cross-link densities, the photoimageable compositions upon development become brittle. Photoresists are becoming required to tent ever larger through-holes in printed circuit boards, to which end brittleness is a significant detriment. Therefore, photoimageable compositions in accordance with the invention are plasticized to improve flexibility, and herein, it is found that dibenzoate plasticizers in conjunction with the photoinitiators described above provide photoimageable compositions with good flexibility, fine line adhesion and stripping. Dibenzoate plasticizers in accordance with the invention have the general formula: C 6 H 5 -COO-[R] n -R'-C 6 H 5 , where R = -CH 2 -CH 2 -O- and/or -CH(CH 3 )-CH 2 -O-; n = 1 to 10, and R' is -CHX-CHX-OOC-,where both Xs are H or one X is H and the other is CH 3 ,or -OC-. Specific examples of suitable dibenzoates include, but are not limited to dipropyleneglycol dibenzoate, diethylene glycol dibenzoate, polypropyleneglycol dibenzoate, and polyethylene glycol dibenzoate. The dibenzoate plasticizer D) is used at levels of between 1 and 8 wt% relative to total weight of A) plus B) plus C), typically at between 2 and 6 wt%. Additionally, the photoimageable compositions may contain a wide variety of additional components as are known in the art, including additional polymers, such as those which might be used to effect a final hardened cure of a solder mask, dyes, stabilizers, flexibilizing agents, fillers etc. Processing of the photoimageable composition is in a conventional manner. In a typical procedure, a photoimageable composition layer, either formed from a liquid composition or transferred as a layer from a dry film, is applied to a copper surface of a copper-clad board. The photoimageable composition layer is exposed to actinic radiation through appropriate artwork. Exposure to actinic radiation polymerizes the monomer in the light-exposed areas, resulting in a cross-linked structure that is resistant to developer. Next, the composition is developed in dilute alkaline aqueous solution, such as a 1% sodium carbonate solution. The alkali solution causes salt formation with the carboxylic groups of the binder polymers, rendering them soluble and removable. After development, an etchant may be used to remove copper from those areas where the resist was removed, thereby forming a printed circuit. The remaining resist is then removed using an appropriate stripper. The invention provides a fine line (less than 75 µm) resist sidewall which adheres well to copper surfaces. It is believed that the dibenzoate plasticizer contributes a low overall glass transition temperature (T g ) to the photoimageable composition. The lower T g allows for better flow at lamination and better conformation to the copper surface. This property is particularly important on copper surfaces with nicks or scratches. Because a compositions with a dibenzoate plasticizer conforms better to a surface, more photoresist surface area is in contact with the surface. This results in more potential for chemical bonding to the copper surface and, therefore, improved adhesion properties. Because the dibenzoate plasticizer cannot incorporate into the backbone of the exposed acrylic monomer system, the inclusion of the dibenzoate plasticizer results in less overall shrinkage of the composition. This reduced shrinkage produces less stress on the copper/photoresist interface, contributing to the improved adhesion. Minimizing shrinkage is particularly important when using 9-phenyl acridine as the initiator because of the resulting high level of cross-link density. The most surprising discovery was the improvement in stripping. Because the dibenzoate plasticizer produced better adhesion, stripping of the resist from the copper surface was expected to be more difficult. For example, the sidewall of the photoresists containing the dibenzoate plasticizer showed less attack in an alkaline-based developing solution. However, in the alkaline-based stripping solution, the dibenzoate-based systems stripped up to 20% faster. This is likely related to the ester functionality in the backbone and the way it orients the polymerized matrix of the photoresist composition. The invention will now be described in greater detail by way of specific examples. Examples 1-8 (1 and 6-8 comparative) A base formulation is prepared having the following components: To the base formula in Table A were added the additional ingredients set forth in Column 1 of Table B below. All mixtures were prepared in 7:1 2-Butanone:2-Propanol at approximately 50% solids. The solutions were coated onto biaxially oriented 80 gauge polyester film and dried to approximately 1% or less retained solvent. The coated mixtures were then laminated onto mechanically scrubbed 1 oz./FR-4/1 oz. clad copper composite using a hot roll laminator at 110°C at 2 meters/minute and 3 bar pressure (3.10 5 Pa). The laminated material was then imaged on a UV printer through an appropriate phototool with an adjusted exposure to obtain a copper step of 7 as measured with a Stouffer® 21 step wedge (approximately 20 mJ/cm 2 ). The exposed panels were then developed in a 1% sodium carbonate monohydrate solution at 29°C using a conveyorized spray developer at about 26 psi with residence time adjusted so that the break point occurred at 40% to 50% of the chamber length (unless otherwise noted in the specific example), followed by several spray rinses using tap water and the deionized water. Etching was accomplished using a 2N cupric chloride/hydrochloric acid solution at 48°C in a conveyorized etcher equipped with multiple spray nozzles. The etched boards were then stripped of the imaged, developed and etched photo resist in a 3% sodium hydroxide solution at 54°C in a conveyorized stripping unit equipped with multiple spray nozzles followed by a spray rinse of tap water. Process responses for the examples are cited at various points throughout the above procedure. Processing results are shown in Columns 2-6 of Table B. Example 11 and comparative examples 9, 10 and 12 To the Master Batch of Table C were added ingredients listed in the first two columns of Table D below. Processing was as per Examples 1-8. Results are shown in Columns 3-7 of Table D below.

Claims: A negative-acting photoimageable composition comprising A) between 30 and 80 wt% of a binder polymer having acid functionality sufficient to render said photoimageable composition developable in alkaline aqueous solution, B) between 19 and 69 wt% of an addition-polymerizable, non-gaseous ethylenically unsaturated compound(s) capable of forming a high polymer by free-radical initiated chain-propagating addition polymerization, C) a photoinitiator chemical system comprising between 0.05 and 2 wt% of 9-phenyl acridine or a homologue thereof, and D) between 1 and 8 wt% of a dibenzoate plasticizer, said percentages being based on the total of (A) plus (B) plus (C). A photoimageable composition according to Claim 1 wherein said 9-phenyl acridine or homologue is present at between 0.1 and 0.5 wt%, based on the total of (A) plus (B) plus (C). A photoimageable composition according to claim 1 or claim 2 wherein said dibenzoate plasticizer is present at between 2 and 6 wt%, based on the total of (A) plus (B) plus (C) . A photoimageable composition according to any preceding claim wherein the binder polymer (A) has an acid number of at least 80. A photoimageable composition according to claim 4 wherein the binder polymer (A) has an acid number in the range from 100 to 250. A photoimageable composition according to any preceding claim wherein the binder polymer (A) has a weight average molecular weight from 20,000 to 200,000. A photoimageable composition according to any preceding claim wherein the binder polymer (A) is derived from a mixture of acid-functional and non-acid functional monomers. A photoimageable composition according to any preceding claim wherein the addition-polymerizable compound (B) is a monomer, dimer or short-chain oligomer having α,β-ethylenic unsaturation. A photoimageable composition according to any preceding claim wherein the photoinitiator chemical system (C) comprises 9-phenylacridine or a homologue thereof having the general formula: where R 1 and R 2 are the same or different and are selected from hydrogen, alkyl, substituted alkyl, acyl, and substituted acyl; R 3 and R 4 are the same or different and are selected from hydrogen, halogens, alkyl, substituted alkyl, acyl and substituted acyl; and R 5 , R 6 and R 7 are the same or different and are selected from hydrogen, halogens, alkyl, substituted alkyl, acyl, substituted acyl and groups of formula II: where R 1 and R 2 are as described above. A photoimageable composition according to any preceding claim which comprises from 0.1 to 0.5 wt% of 9-phenylacridine or a homologue thereof, based on the total of (A) plus (B) plus (C).

Inventors: BARR ROBERT, LUNDY DANIEL E, BARR, ROBERT, LUNDY, DANIEL E.
Applicants: NICHIGO MORTON CO LTD, NICHIGO-MORTON CO LTD
Application year: 1998
Main classification: G03F7
IPC codes: G03F7, C08L101