Aqueous developing solutions for reduced developer residue

Abstract: An alkaline aqueous developing solution for developing photoresists or the like contains, as an anti-scum agent, an ethoxylated surfactant having the general formula: R-[O-(AO) n ] m -X where AO are alkylene oxide units selected from ethylene oxide units (CH 2 -CH 2 -O) and propylene oxide units (CH(CH 3 )-CH 2 -O) or (CH 2 -CH(CH 3 )-O), where R is a hydrophobic group, X is H or an anionic group, m is from 1 to 3, and n is at least about 8, and the molar ratio of total ethylene oxide units to total propylene oxide units is between 1:4 and 4:1, preferably between 2:3 and 3:2.

Description: The present invention is directed to alkaline aqueous development solutions such as are used to develop photoresists. In the process of manufacturing printed circuit boards, UV curable photoresist are used. The exposed portion of the photoresist becomes insoluble in developer solution (dilute alkaline solution) and forms a protective barrier to other processing chemicals (for example, etching and plating solutions). The unexposed portion of the photoresist needs to rinse freely from the circuit board with a mild alkaline solution (for example, 1% sodium carbonate, monohydrate in water). The development occurs because the polymer in the photoresist contains acid functionality. These acid groups within the organic polymer matrix are neutralized in alkaline solution forming a water soluble organic salt. As the dissolved photoresist builds up in solution (called developer loading), insoluble organic materials begin to form in the developing tank, eventually forming a water insoluble scum or residue. The presence of anti-foam additives (conventionally added to developing solutions to minimize foaming) greatly increases the tendency for scum to form. As the level of scum builds, chances increase for an inadvertent redeposit of these water insoluble residues onto the developed circuit board. These redeposited residues cause a retardation of the etching solution (etching chemistries have difficulty penetrating any organic residues). Where the etch is retarded, circuit shorts form causing a defective circuit board. In addition to increasing the potential for defective circuit boards, this residue also makes it difficult to clean equipment increasing maintenance time Accordingly, it is a primary object of the present invention to provide developing solutions in which the build-up of scum and residue is reduced. Moreover, the developer described herein will not only reduce the scum, but will minimize or even eliminate the need for a separate anti-foam agent. In accordance with the invention, alkaline aqueous developing solutions, containing between 0.1 and 3.0 wt% of a base, are improved by the addition of between 0.05 and 1.0 wt% of a surfactant or surfactant mixture, the surfactant(s) having the general formula: [R-O-(AO) n ] m -X where AO are alkylene oxide units selected from ethylene oxide units (CH 2 -CH 2 -O) and propylene oxide units (CH(CH 3 )-CH 2 -O) or (CH 2 -CH(CH 3 )-O), where R is a hydrophobic group, typically a hydrocarbon group, X is . phosphate, m is from 1 to 2, and n is at least about 8, preferably at least 10, provided that the molar ratio of total ethylene oxide units to total propylene oxide units in the surfactant or surfactant mixture is between 1:4 and 4:1, preferably between 3:2 and 2:3. As a practical matter, n may be up to about 150. The developing solution may also contain an anti-foam agent, e.g., at between 0.01 and 1.0 wt%. Aqueous developing solutions to which the present invention generally applies are well known for developing photoresist compositions, both primary photoresists and photoresists which are intended to form hard permanent layers, such as are suitable for solder masks. Examples of such photoresists and the alkaline aqueous development solutions are found, for example, in U.S. Patents nos. 5,576,145, 5,609,991, 5,393,643, and 5,364,736. The photoresist compositions vary in composition, but typically such photoresist compositions comprise A) between 20 and 80 wt% of a binder polymer having an acid number between 40 and 250, B) between 15 and 50 wt% of α,β-ethylenically unsaturated compounds, typically monomers and short-chain oligomers, and C) between 0.1 and 25 wt% of a photoinitiator or photoinitiator chemical system, these weight percentages being based on total weight of A) plus B) plus C) Typically, the alkaline aqueous solution is a solution of sodium carbonate, e.g., 1% sodium carbonate monohydrate. However, other bases, such as NaOH, KOH, triethanolamine, potassium carbonate etc. may be used to provide the alkalinity necessary for developing photoresists of this type. The alkalinity of the developing solution forms salts with the acid functionality of the binder polymer, typically carboxylic acid functionality, rendering the binder polymer soluble in the alkaline aqueous solution. Thus, photoresists of this type are applied as a layer to a substrate, exposed to patterned actinic radiation, and developed in alkaline aqueous solution which washes away non-exposed, unpolymerized portions of the photoresist layer. The ethoxylate/propoxylate surfactants used in accordance with the present invention significantly reduce photoresist scum without any reduction in developing performance. Ethoxylate surfactants by themselves are effective in reducing scum in alkaline aqueous developing solutions, particularly when the ethyoxylate chain is at least 10 units long, preferably at least 30 units long. Propoxylate surfactants are significantly less effective. Surprisingly and unexpectedly, however, is the discovery that when a surfactant or surfactant mixture contains a mixture of ethylene oxide and propylene oxide units, such units act synergistically in significantly reducing scum in the developing solution. Accordingly, surfactants having shorter alkylene oxide chains may be used or less surfactant may be used than, for example, using surfactants comprised entirely of ethylene oxide units. The hydrophobic end, i.e. R in the equation (I) above, may almost any hydrocarbon alcohol having a molecular weight of 43 or above, including alkyl, alkylaryl, arylalkyl, cycloalkyl alcohols, etc. Alcohols used to form the hydrophobic end include, for example, nonylphenol, octylphenol and tristyrylphenol. The most effective ethoxylate surfactants for reducing scum and residue are anionic ethoxylate surfactants where X is a phosphate (m=1-2). The ethylene oxide units and propylene oxide units may be incorporated in a chain within the same surfactant molecule. However, from a convenience standpoint in terms of what is commercially available, a mixture of a surfactant containing an ethylene oxide unit chain and a propylene oxide unit chain are used to provide the desired molar ratio of ethylene oxide units. Ethoxylate surfactants useful in accordance with the invention are sold, for example, under the trademarks Rhodafac and Igepal by Rhône-Polenc. Propoxylate surfactants useful in accordance with the invention are solf, for example under the trademark Macol by BASF. The alkaline aqueous solution may also contain an anti-foam agent, such as that sold as Antifoam 80 by Morton International. The invention will now be described in greater detail by way of specific examples: Examples 1-8 (1 - 4 comparative) To 300 ml. of solutions containing 1 wt% sodium carbonate monohydrate, 0.5 wt% anti-foam agent were added 5.4 grams of Morton International 38G297 photoresist and a total of 3 g (1 wt%) surfactants or surfactant mixture. The solutions were thoroughly mixed and the development of foam was noted. The solutions were filtered and the filtrate dried and weighed. Results are shown in the Table below. EO Component Moles of EO % of Mixture PO Component Moles of PO % of Mixture % Scum Reduction Foam Formation 1. RD209 9 100 % - - - 51 % Increase Moderate 2. - - - RD216 16 100 % 18 % Increase Very Slight 3. RE 960 50 100 % - - - 68 % Decrease Slight to Moderate 4. RD230 30 100 % - - - 21 % Decrease Moderate 5. RD230 30 50 % RD216 16 50 % 71 % Decrease Very Slight 6. RD209 9 50 % RD216 16 50% 62 % Decrease Very Slight 7. RE960 50 50 % RD216 16 50 % 73 % Decrease Very Slight 8. RE960 50 60 % RD216 16 40 % 88 % Decrease Very Slight

Claims: An alkaline aqueous developing solution for photoresists comprising between 0.1 and 3.0 wt.% of a base, characterised in that said developing solution comprises between 0.05 and 1.0 wt.% of an alkylene oxide surfactant or surfactant mixture, the surfactant(s) having the general formula: [R-O-(AO) n ] m -X where AO are alkylene oxide units selected from ethylene oxide units (CH 2 -CH 2 -O) and propylene oxide units (CH(CH 3 )-CH 2 -O) or (CH 2 -CH(CH 3 )-O), where R is a hydrophobic group, X is a phosphate group, m is from 1 to 2, and n is at least 8, and the molar ratio of total ethylene oxide units to total propylene oxide units is from 1:4 to 4:1. A solution according to claim 1 wherein n is at least 10. A solution according to claim 1 or claim 2 wherein the molar ratio of total ethylene oxide units to total propylene oxide units is between 2:3 and 3:2. A solution according to any preceding claim further comprising from 0.01 to 1.0 wt.% of an anti-foam agent. A method of preparing a patterned photoresist pattern on a substrate comprising providing a layer of a photoresist to a substrate, said photoresist comprising an acid functional binder polymer and photopolymerizable compounds, exposing said layer of photoresist to patterned actinic radiation, and developing said photoresist layer in an alkaline aqueous developing solution according to any preceding claim.

Inventors: BARR ROBERT, LUNDY DANIEL E, BARR, ROBERT, LUNDY, DANIEL E.
Applicants: NICHIGO MORTON CO LTD, NICHIGO-MORTON CO LTD
Application year: 1999
Main classification: G03F7
IPC codes: G03F7