PHOTORESIST STRIPPING PROCESS USING N,N-DIMETHYL-BIS(2-HYDROXYETHYL) QUATERNARY AMMONIUM HYDROXIDE

Abstract: A process for stripping photo-resists and an aqueous bath for use in the process are disclosed. The process is carried out at 40 to 100 C for about 0.1 to 10 minutes. The aqueous bath contains 1 to 10 weight percent N,N-dimethyl-bis(2-hydroxyethyl) quaternary ammonium hydroxide and preferably 0.5 to 10 weight percent of a metal complexing agent such as monoethanol amine, ethylene diamine, ethylene diamine tetraacetic, melamine, nitrillotriacetic acid, morpholine, acetonylacetone, and preferably from 0.1 to 5 weight percent ammonia.

Description: TITLE PHOTORESIST STRIPPING PROCESS USING N,N-DIMETHYL-BIS(2-HYDROXYETHYL) QUATERNARY AMMONIUM HYDROXIDE Field of the Invention: 1. The present invention relates to a process for stripping photo¬ resists used in etching and plating operations for forming printed circuits wherein N,N-dimethyl-bis(2-hydroxyethyl) quaternary ammonium hydroxide is used as the stripping agent. Background of the Invention: In the past choline [(CH^N+C^CH j OH] [OH]- has been used as a stripping agent for photo-resists. In this application choline has the advantages of being an organic base which leaves no non-volatile residues, low toxicity, relatively low cost, easily made, being a strong base and being biodegradable. On the other hand, choline suffers the disadvantages of having a serious odor problem in use and of decomposing to give colored solutions and precipitates. Summary of the Invention The present invention involves the use of N,N-dimethyl-bis(2- hydroxyethyl) quaternary ammonium hydroxide (choline II) as a stripping agent for removal of unexposed areas of photo-resists prior to etching or plating. The choline II is used as a 0.1 to 15 wt % aqueous solution. Brief Description of the Drawing Figure I is a graphic representation of the stripping agents described in Example 2 below. Detailed Description The choline II used in the present invention is prepared by reacting two moles of ethylene oxide with one mole of dimethyl amine. The preferred technique is to mix dimethyl amine with about twice the molar amount of ethylene oxide at 35° to 60° C under a pressure of 15 to 150 psi (3.5 to 35 KPa) at a rate of from 0.5 to 5 moles per hour after which time the reaction is substantially complete. The stripping action of choline II on photo-resists has been found to be substantially identical with that of choline I. [(CH 3 ) 3 N + -CH 2 - CH 2 OH][OH] " which in spite of its disadvantages is widely used as a stripping agent for photo-resists. Further, the base strength of choline II is substantially identical with that of choline I. Generally, a 1 to 10% aqueous solution of choline π is used to perform the stripping process. Generally the stripping process is carried out at 40° to 100°C for 0.1 to 10 minutes. In a particularly preferred aspect of the invention the stripping solution contains from 0.5 to 10 wt % of a metal complexing agent such as monoethanol amine, ethylene diamime, or ethylene diamine tetraacetic acid. The metal complexing agent serves to break the bond between the metal (copper) on the metal clad substrate and the photoresist, thereby facilitating removal of the photoresist from the metal clad substrate. It has also been found that ammonia (NH 3 ) exerts very significant favorable effects when added to the compositions of the invention. Even amounts as little as 0.1% by weight noticeably improve stripping speed, the sensitivity of stripping speed to changes in pH and the sensitivity of stripping speed to changes in temperature (i.e. the activation energy of stripping). There is little advantage in using more than 5 weight percent ammonia in the solution. Such improvements are found both in the presence and the absence of metal complexing agents, however, the improvements in the above-mentioned properties function synergistically with the improvements observed on mixing metal-complexing agents such as ethanol amine, ethylene diamine, ethylenediamine tetraacetic acid with the choline π base compositions of the invention. Thus the most highly preferred compositions of the invention will consist of ternary mixtures of choline π bases or basic salts; ammonia; and a metal complexing agent. Metal complexing agents suitable for the purposes of this invention comprise those whose complexes with copper π ions have complexity constants of less than 10 -2 . That is, in the reaction, Cu + 2 + L Cu(L)<2 + ι> the equilibrium constant K [Cu +2 ] [L] = K, < 10- 2 [Cu L < +2+ ι>] is less than 10" 2 . In the above expression, the symbol "L" signifies the ligand or metal complexing agent concentration in moles per liter, [Cu +E ] stands for the concentration of cupric ions in moles per liter, and (Cu L +2 - l ) the concentration of cupric complexed ion. "1" is the valence (usually negative) of the ionized complexing agent in those cases where the complexing agent is a charged species. O For example, glycine H 2 N-C 2 H 4 -C-OH would have a charge of (-1) due to the ionization of the proton of its carboxylic acid group. Similarly ethylene diamine tetraacetic acid would have a charge of (-4), nitilo-triacetic acid a charge of (-3), etc. The mechanism of action of each component in the preferred ternary mixtures of the compositions of the invention are not totally understood, but the following explanations seem consistent with known experimental facts. The choline II base component seems to function primarily via hydroysis of hydrolyzable species in the photo-resist. The photo-resists usually contain a multiplicity of linkages such as ester linkages, amide linkages, urethane linkages, ether linkages, products of aldol condensations, etc. Strong bases such as choline π furnish a high concentration of hydroxyl ions which catalyze the hydrolysis of such linkages. Also, in the case of linkages in the photo-resist such as esters which form acidic groups upon hydrolysis, strong bases, such as choline II , can neutralize the acid group to form a salt. In accordance with this view of the role of strong bases, the logarithm of the rate of stripping ( i.e. str i p i ng t i me ) is a linear function of the logarithm of the hydroxyl ion concentration with a slope of one. The activation energy for stripping is of the order of 20 kilocalories per mole, as expected for ester hydrolysis, or for amide hydrolysis. The role of the metal complexing agent is almost certainly to form complex ions with the copper surface atoms of the copper-coated, fiberglass laminate, thus weakening the attachment of such atoms to groups such as carboxylic acid groups in the photo-resist polymer. This in turn weakens the bond between the copper surface and the photo-resist polymer coating. Although the logarithm of the stripping rates of choline "II" mixtures with metal complexing agents are still linear functions of the logarithm of the hydroxl ion concentration, the slope is no longer equal to one; and the activation energy for stripping in such mixtures is only about half that found when stripping with systems containing no complexing agents. This is very useful in a practical sense, since decreases in pH or in temperatures decrease stripping rates less rapidly than is observed in the absence of the complexing agents. In addition, stripping speed is increased to a striking degree. The effect of ammonia is very dramatic but little understood. Although ammonia is a good metal complexing agent for cupric ions in its own right, it improves mixtures which already have metal complexing agents present just as much as those which have none. It also further decreases activation energies for stripping and the sensitivity of stripping rates to changes in pH. These both are very important in practical applications. Normally with the stripping compositions of the prior art, stripping speeds would decrease rapidly as the pH of the bath decreased. The lower Curve (1) in Figure I shows this effect in the case of choline π hydroxide. The stripping rate increases ten-fold for every single unit increase in pH, i.e. from 1200 seconds at pH= 12 to 90 seconds at pH=13. All basic stripping baths decrease their pH during use, due to pick up of CO 2 from the air, reaction with acid groups in the photo-resist, and loss of base via chemical decomposition, volitilization, mechanical carry-over and adsorption into the discarded photo-resist flakes. Thus, if stripping rates decrease rapidly as the pH drops, the baths soon must be discarded or production rates will be seriously affected. Loss of expensive chemicals of the bath and the necessity of disposing of them after stripping only a limited number of plates causes an undesired environmental impact and increases costs. Figure I, Curve (2) shows the variation in stripping rates with pH when equi-molar mixtures of choline II hydroxide and monoethanol amine, one of the metal complexing agents of the invention, are used together. It is evident that stripping rates are much faster (by factors ranging from 2-5) in the presence of the complexing agent. It can also be seen that the loss of stripping speed as the pH decreases is much lower than with choline π alone. A stripping speed of 70 seconds is found at pH=13 vs. 220 seconds at pH= 12. This is a drop of only three fold vs. the more than ten fold drop over the same pH range in the absence of the metal complexing agent. Curve (3) of Figure I shows the effect of addition of equal amounts of NH 3 to choline II and ethanol amine to form one of the highly preferred ternary compositions of the invention. It can be seen that stripping speeds are increased further by factors ranging from 3 to 5 fold, and that the system is very insensitive to pH. There is virtually no loss of stripping speed from pH=13.5 down to pH=ll. This corresponds to going from 5% compositions in the three ingredients to 0.5%. Thus even when 90% of the active ingredients are exhausted, stripping speeds are scarcely affected. Activation energies are also very low for such mixtures (in the range of 3 to 5 kilocalories per mole). This means that stripping speeds are relatively insensitive to stripping temperatures. This is a distinct advantage since relatively inexpensive handling equipment such as polyvinyl chloride plastics can be used in the stripping line instead of the more expensive stainless steel. Lower temperatures of operation also minimize corrosive attack on the copper surfaces, solder connections and other metallic components of the circuit boards. How ammonia achieves these effects is unclear. Speculatively, the NH 3 molecule may be small enough to penetrate rapidly through the photo-resist polymer and start a process of swelling the polymer while simultaneously forming copper amine complexes with surface copper atoms on the copper-coated fiberglass laminate. Both of these actions may facilitate the attack by strong base and metal complexing agents. The photo-resists being stripped by the process of the present invention generally are polymer films which change their solubility upon exposure to light. Positive photo-resists become less soluble due to photo- induced cross-linking. Negative photo-resists become more soluble due to photo-induced depolymerization. Generally photo-resists comprise a binder phase, a solubilizing or cross-linking polymer, a photo-sensitive group which catalyzes cross- linking reactions or depolymerization reactions when exposed to light and a dye to enhance light absorption. Generally the binder phase is a phenol- formaldehyde resin, an epoxy resin, a polyamide, polyimide, polyurethane, etc. Generally the solubilizing or cross-linking polymers are polyacrylic acid, polymethacrylic acid, maleic anhydride- vinyl copolymers or other polyacids. The photo-resist polymer films are bonded by heat and pressure to copper-plated fiber glass-resin laminate sheets. Alternatively the photo-resist can be applied as a coating to a copper clad substrate. (Silver or gold plated alumina substrates are used for high quality electronics applications.) The photo-resist is exposed to light through a pattern containing the shape of the desired electronic circuit. Exposed (negative resist) or unexposed (positive resist) areas are removed by developing with a mild developer solution. Then various other operations are conducted on mild developer solution. Then various other operations are conducted on exposed copper areas such as etching, plating, soldering of connections, or deposition of other patterns. The final step involves removing the "insoluble" protecting part of the photoresist film which is removed by a stripping agent. The stripping agent functions to remove the protective photo¬ resist by a combination of actions including one or more of hydrolysis, metal complexing, dissolving of photo-resist components, salt formation, swelling, solvation, and mechanical shearing forces. The features sought after in stripping agents generally are low temperature operation, tolerable working environment, i.e., odor-free and non-toxic, control of flake size, speed of stripping, predictable constant speed of stripping, clean stable bath, i.e., no color, no precipitates and no decomposition, no residues on the work piece, a low corrosion of copper plate and connectors, ease of disposal, i.e., low toxicity and biodegradability, and competitive price. Choline I undergoes the following Hoffman degradation reaction in use: [(CH 3 ) 3 N-CH 2 -CH 2 OH] + [OH]- - (H 3 C) 3 N + H 3 C-CHO (Trimethyl Amine) (Acetaldehyde) These degradation products are undesirable because trimethyl amine smells at parts per billion levels and acetaldehyde polymerizes to give colored products and precipitates. On the other hand when N,N-dimethyl- bis(2-hydroxyethyl) quaternary ammonium hydroxide (choline II) undergoes a Hoffman degradation CH 2 -CH 2 -OH (CH 3 ) 2 N - (CH 3 )N-CH 2 -CH 2 -OH +H 3 CCHO CH 2 -CH 2 -OH N,N-dimethyl-2-hydroxyethyl/amine + acetaldehyde The N,N-dimethyl-2-hydroxyethyl amine is a non-volatile and non-smelly amine. The acetaldehyde is the same color forming precipitate formed by the degradation of choline I and the things which slow this problem when using choline I also work with choline II. These Hoffman degradation products of choline π do not undergo further degradation and the choline II remains odor free even after a month at 50°C. Examples Example 1 To a 500 ml. 4-neck round bottom flask equipped with a mechanical stirrer, immersion thermometer, submerged feed mbe for feeding ethylene oxide or nitrogen, and a dry-ice condenser is added 200 g of a 26% aqueous solution of dimethyl amine (1.18 moles). Under a slow nitrogen purge, the flask and contents are heated to 40°C with slow agitation and ethylene oxide feed is started to maintain a feed rate of approximately 25 grams/hour. While ethylene oxide is being fed, the temperature is held between 45° and 56°C. After 4.25 hours, the ethylene oxide feed is stopped and the flask cooled and purged with nitrogen to remove residual ethylene oxide. 104.5 g (2.37 moles) of ethylene oxide is absorbed. The product is a water-white, clear, odorless solution. Proton NMR analysis shows the (CH 3 ) 2 /NCH 2 /OCH 2 proton ratio to be ca. 6.00/4.1/3.8. These data are consistent with a 2 ethylene oxide adduct to dimethyl amine. Example 2 A plurality of stripping runs are made using an aqueous solution of N,N-dimethyl-bis(2-hydroxyethyl) quaternary ammonium hydroxide alone and in conjunction with equal weight amounts of monoethanolamine and/or ammonia. The pH of the resulting mixture was measured with a pH meter to the value reported in Figure I. The photo- resist being stripped is approximately 1.5 mils in thickness laminated to a copper clad, phenolic substrate reinforced with a woven glass fabric. The composition of the photo-resists are proprietory but generally contain the following: 1) About 60% by weight of a polymer binder such as an acrylic resin. 2) About 30% by weight of acrylic polyfunctional monomers such as pentaerythritol triacrylate. 3) Photo initiator systems such as Michlers ketone, benzophenone, thioanthones, etc. ( 5%). 4) Dyes such as malachite green or Victoria blue ( 0.1 %). 5) Other additives such as adhesion promoters, stabilizers, plasticizers, etc. This particular resist was DuPont Riston inner layer resist number 3813. In Figure I the points indicated with a circle are obtained using an aqueous solution of choline II alone, the points indicated with a triangle are obtained using equal weight of an aqueous solution of choline II and ethanol amine, the points indicated with a square are obtained using equal weight aqueous solution of choline II, monoethanol amine and ammonia. The pH is varied by adjusting the concentrations from 0.1 % to 5% in each instance. In each case the stripping is done using an agitated bath at 50°C into which the laminate bearing the resist coating is immersed. The time required for stripping is reported in Figure 1. Example 3 A series of runs are made using the aqueous solutions reported in Table I_ below. The solutions contain the amounts of choline II and the metal complexing agent reported in Table I. The time reported is the time required to remove the resist from the copper clad circuit board. Table I Experiment C D Choline II (wt %) 5 5 5 5 5 Ethylene diamine (wt %) 1 Tetraacetic acid Melamine (wt %) 1 Nitrillotriacetic acid (wt %) 1 Morpholine (wt %) 1 Triethylenetetra ine (wt %) Acetonylacetone (wt %) Monoethanol amine (wt X) Ethylene Diamine (wt ) pH Time (minutes) Temperature (°C) Example 4 A series of runs are made using the aqueous solutions reported in the Table II below. The solutions contain the amounts of choline II and ammonia reported in Table II. The time reported is the time required to remove the resist from the copper clad circuit board. The pronounced effect of NH 3 on stripping speed is evident even in the absence of other metal complexing agents. Table II Composition 1) 5% Choline π Hydroxide + 9% urea 2) As above + 0.25% NH 3 3) As in (l) + 0.5% NH 3 4) As in (l) + 1.0% NH 3 The above table shows that as little as 0.25% ammonia improves the stripping speed of choline II hydroxide by a factor of 3.5 fold and 0.5% by 5.3 fold. Doubling this to 1.0% causes no further improvement. These tests were performed at 50°C in an agitated bath on fiberglass laminate coated with copper which was bonded to a 1.5 mil thick photo-resist of Du Pont Riston grade 4220 plating resist (positive) photo¬ resist polymer by heat and pressure under vacuum. The photo-resist was fully exposed with u.v. light prior to stripping. The resist is similar in composition to that described in Example 2.

Claims: CLAIMS: 1. A process for stripping photo-resists from a conductive metal clad substrate comprising applying an aqueous solution comprising 1 to 10 weight percent N,N-dimethyl-bis(2-hydroxyethyl) quaternary ammomum hydroxide and from 0.5 to 10 weight percent of a metal complexing agent to said photoresist at 40° to 100°C, for about 0.1 to about 10 minutes. 2. The process of claim 1 wherein the aqueous solution contains from 0.1 to 5 weight percent ammonia. 3. The process of claim 2 wherein the metal complexing agent is selected from the group consisting of monoethanol amine, ethylene diamine, ethylene diamine tetraacetic acid, melamine, nitrillotriacetic acid, morpholine and acetonylocetene. 4. The process of claim 3 wherein the metal complexing agent is monoethanol amine. 5. The process of claim 4 wherein the metal complexing agent is ethylene diamine. 6. An aqueous bath containing 1 to 10 weight percent N,N- dimethyl-bis(2-hydroxyethyl) quarternary ammomum hydroxide and from 0.5 to 10 weight percent of a metal complexing agent. 7. The aqueous bath of claim 6 containing from 0.1 to 5 weight percent ammonia. 8. The aqueous bath of claim 7 wherein the metal complexing agent is selected from the group consisting of monoethanol amine, ethylene diamine, ethylene diamine tetraacetic acid, melamine, nitrillotriacetic acid, morpholine and acetonylactone. 9. The aqueous bath of claim 8 wherein the metal complexing agent is monoethanol amine. 10. The aqueous bath of claim 8 wherein the metal complexing agent is ethylene diamine.

Inventors: YATES PAUL C, YATES, PAUL, C.
Applicants: DUCOA L P, DUCOA L.P.
Application year: 1993
Main classification: G03F7
IPC codes: G03F7, H01L21