PHOTORESIST STRIPPER COMPOSITION AND PROCESS OF USE

Abstract: A composition, adapted to strip photoresists from substrates after the protective function of the photoresist has been completed, comprises a mixture of (a) morpholine or an N-alkyl or N-hydroxyalkylmorpholine and (b) pyrrolidone or an N-alkyl or N-hydroxyalkylpyrrolidone in a ratio by weight of (a) to (b) of about 4:1 to about 0.25:1. The composition is effective in stripping, rapidly and completely, a wide variety of photoresists at room temperature or at elevated temperatures below the flash point of the composition.

Description: "PHOTORESIST STRIPPER COMPOSITION AND PROCESS OF USE" 1 BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to improved resist stripping 5 compositions and to processes for stripping resists from substrates using said stripping compositions and is more particularly concerned with stripping compositions comprising a mixture of morpholine or an N-substituted » derivative thereof and pyrrolidone or an N-substituted 10 derivative thereof * and with processes for stripping resists from substrates using said compositions. 2. Description of the Prior Art Various types of resist materials are employed to protect selected areas of a substrate, such as a wafer 15 for microelectronics fabrication, whilst the substrate is subjected to processing such as pattern generation by techniques including etching, ion implantation, metal deposition and the like. When this processing has been completed it is necessary to remove the resist material 20 from the substrate. This step is generally accomplished by the use of stripper solvent compositions. It is essen¬ tial that the stripper remove the resist completely i.e., without leaving any residue, and that this be done with¬ out the necessity for scrubbing or like abrasive tech- 25 niques which could cause damage to the substrate. It is also necessary that the stripping composition be free of any material which could cause corrosion of the substrate or affect the latter deleteriously in any manner. A wide variety of stripper compositions has been 30 disclosed in the prior art. Thus, stripper compositions have been described containing one or more components such :- as halogenated hydrocarbons (methylene chloride, tetrachloroethylene) , phenols and phenolic compounds, glycol ethers, ketones (acetone, methylethyl ketone) , dioxane, sulfonic acids and the like. These various agents suffer one or more disadvantages such as potential toxicity to workers exposed thereto, environmental and pollution problems in disposal after use, volatility, corrosion of equipment, and the safety hazard of operat¬ ing at temperatures above the flash point of the stripper. Illustratively, U.S. Patent 3,075,923 discloses a methylene chloride-based stripper composition containing benzenesulfoniσ acid or an alkylbenzenesulfonic acid as activator. U.S. Patent 4,011,351 describes the removal of polymethacrylate positive resists using solvents such as chloroform, nitromethane, dioxane, formamides and the like. U.S. Patent 4,165,294 teaches the use of stripper compositions comprising a mixture of an alkarylsulfonic acid, a hydrotropic aromatic sulfonic acid and, optional¬ ly, a halogen-free aromatic hydrocarbon having a boiling point about 150°C. U.S. Patent 4,165,295 discloses stripper composi¬ tions comprising an organic sulfonic acid, in association with a source of fluoride ions, optionally phenol, and a solvent. The solvents listed include N-methylpyrroli- done. Related U.S. Patent 4,215,005 describes stripper compositions comprising an organic sulfonic acid in association with a complex of hydrogen fluoride and a complexing agent of which morpholine is an example. U.S. Patent 4,242,218 teaches the use of an alkarylsulfonic acid in a hydrocarbon solvent which can include chlori¬ nated aromatic hydrocarbons. All of the above patents disclose stripper composi¬ tions: which are clearly undesirable for use commercially either by reason of exposure of workers to potentially toxic materials or by reason of the difficulty or expense of disposing of the composition in an environmentally acceptable manner after use. A number of compositions containing N-methylpyrroli- done as active ingredient have been described. Illustra¬ tively, U.S. Patent 3,673,099 describes a stripper com- position for removing cured resins such as silicones and polyvinylcinnamate from substrates. The composition com¬ prises a mixture of N-methyl-2-pyrrolidone, a strong base and optionally ethylene glycol mono-ethyl ether. U.S. Patent 4,276,186 discloses a composition comprising N-methyl-2-pyrrolidone and an alkanolamine for cleaning substrates such as integrated circuit carriers to remove contaminants such as solder flux. U.S. Patent 4,428,871 describes a stripper com¬ position which is a mixture of about 55 to 80 percent by weight of pyrrolidone or certain N-substituted pyrroli- dones and about 20 to 45 percent by weight of diethylene glycol monoalkyl ether. It is specifically stated that N-methylpyrrolidone alone has proved not effective as a stripper for certain positive resists and that the mix- tures disclosed in the patent are more effective than either of the components when used alone. The disclosure and experimental data in the patent are confined to the stripping of positive photoresists. Although the speci¬ fication states that the stripper compositions are effective at about 75 β C or less the bulk of the data was derived using stripping temperatures of 75°C with only 3 exceptions each carried out at 25°C. Of the latter 3 experiments only one resulted in 100 percent removal of photoresist. Further, all the data was derived using -H - photoresists which had been baked at 150°C for 45 minutes to 1 hour. No data is given for photoresists baked at higher temperatures. In actual practice, the photo¬ resists are frequently exposed to temperatures signifi- cantly in excess of 150°C and even as high as 220°C, during the processing of substrate to achieve pattern generation using the techniques discussed previously. It has now been found that stripping compositions which contain as the principal active components a mix- ture of (a) morpholine or certain derivatives thereof and (b) pyrrolidone or certain derivatives thereof possess markedly improved properties in terms of their ability to strip a wide variety of resists including resists which have been exposed to processing temperatures of the order of 180°C or higher. These stripper compositions can be employed effectively at temperatures approximating room temperature in most cases and certainly at temperatures which are safely below the flash point of the composi¬ tions. Other advantages in these compositions will become apparent form the description and specific exemplification which follows. SUMMARY OF THE INVENTION It is an object of the invention to provide a resist stripper composition which is effective in rapidly removing resist material from a substrate at temperatures which are safely below the flash point of the composition and preferably are of the order of 25°C. It is another object of the invention to provide a resist stripper composition which is effective in remov- ing from a substrate a resist material which has been exposed to processing temperatures as high as about 220°C. It is yet another object of the invention to pro¬ vide : a stripper composition which is effective in remov¬ ing from a substrate a resist material which has been subjected to a resist hardening process such as by expo- sure to deep UV radiation with or without post-bake treat¬ ment, or like known techniques for thermal stabilization of the resist prior to exposure to pattern generation. It is a further object of the invention to provide a resist stripper composition which does not pose any significant toxicity hazard to operators handling the same or create any undue problems of an ecological nature in the ultimate disposal of the compositions after use. These objects, and other objects which will become apparent from the description which follows, are achieved by the stripper compositions of the invention which com¬ prise a mixture of (a) a compound of the formula: R —N p \ and (b) a compound of the formula: ^< II R 1 wherein R and R 1 are independently selected from the group consisting of hydrogen, alkyl from 1 to 3 carbon atoms, inclusive, and hydroxyalkyl from 1 to 3 carbon atoms, inclusive. The term "alkyl from 1 to 3 carbon atoms, inclu¬ sive", means methyl, ethyl, propyl and isopropyl. -έ>- The term "hydroxyalkyl from 1 to 3 carbon atoms, inclusive", means hydroxymethyl, 1- and 2- hydroxyethyl, 1-, 2- and 3-hydroxypropy1, and 1-hydroxymeth lethyl. The invention also comprises a process for stripp- ing a resist from a substrate after the protective func¬ tion of the resist with respect to the substrate has been completed wherein the process comprises employing a stripper composition of the invention to effect the stripping. While the compositions and process of the invention can be employed to strip a wide variety of resist mate¬ rials from a wide variety of substrates known in the art they are particularly adapted for use in stripping posi¬ tive photoresists from wafers employed in fabrication of microelectronic circuitry. Illustrative of the latter substrates are silicon wafers whose surface has been treated to form a layer of silicon oxide, aluminum, alloys of aluminum with copper and like metals, chromium, chromium alloys, silicon nitride and the like. DETAILED DESCRIPTION OF THE INVENTION As set forth above the stripper compositions of the invention comprise, as the principal active component, a mixture of a compound of the formula (I) above and a com¬ pound of the formula (II) above. Advantageously the ratio by weight of the compound of formula (I) to the com¬ pound of formula (II) is within the range of about 4:1 to about 0.25:1 and preferably is within the range of about 2:1 to about 0.5:1. Illustrative of compounds of the formula (I) above are morpholine, N-methylmorpholine, N-ethylmorpholine and N-(2-hydroxyethyl)morpholine and the like. Illustrative of compounds of the formula (II) above are pyrrolidone, N-methylpyrrolidone, N-ethylpyrrolidone, N-(2-hydroxy- ethyl) pyrrolidone and the like. A preferred stripper composition in accordance with the invention is one which comprises a mixture of morpho¬ line and N-methylpyrrolidone in a volume ratio within the ranges set forth above. A particularly preferred range of proportions by weight is about 1:1. In addition to the above principal components in the compositions of the invention there may also be pre¬ sent one or more diluents provided that said diluents are free from corrosive properties, do not attack the sub¬ strate, do not present any significant toxicity or environmental hazards, or interfere in any way with the efficacy of the compositions of the invention as resist strippers. Such diluents can be present in the composi¬ tions of the invention in amounts up to about 70 percent by weight based on total weight of the compositions of the invention. The actual amount of such diluents employed in any given instance will vary depending on the relative ease with which the particular resist can be stripped. Illustrative of such diluents are polar organic solvents such as glycols of which ethylene glycol, propy- lene glycol, dipropylene glycol, tripropylene glycol, tetrapropylene glycol and the like are typical, glycol mono-alkyl ethers such as the monomethyl, monoethyl, mono- propyl and mono-hexyl ethers of diethylene glycol and dipropylene glycol, glycol esters such as ethyleneglycol mono-acetate, ethyleneglycol diacetate, and the like, and glycol ether esters such as 2-ethoxyethyl acetate (cello- solve acetate), 2-butoxyethyl acetate (butylcellosolve acetate) and the like. These diluents can be employed alone or in mixtures of two or more provided that the total 1 amount of diluent employed is within the above stated range. Particularly advantageous diluents for use in the compositions of the invention are the ono-lower-alkyl ethers of dipropylene glycol. The term "lower-alkyl" means alkyl from 1 to 6 carbon atoms, inclusive, such as methyl, ethyl, propyl, butyl, pentyl, hexyl and isomeric forms thereof. The mono-methyl ether of dipropylene glycol is a particularly preferred diluent. ' The compositions of the invention can also contain water in amounts up to about 70 percent by weight, based on total weight of the composition. However, the composi¬ tions of the invention are preferably substantially free of any water other than that which may be inherently pre¬ sent and introduced in association with one or more of the other components. This is particularly the case where the substrate to be stripped is likely to be corroded by exposure to water. The compositions of the invention can be employed to strip any of a wide variety of resists, including both positive and negative photoresists, from a substrate after the resist has performed its protective function in respect of the substrate. Thus the stripping of the resist is the final step in a process which includes the following steps. The substrate is first coated with a layer of the resist and an image is developed in the photoresist layer by exposure of the latter to actinic radiation via an appropriate mask followed by removal of the unexposed material (in the case of a negative resist) or the exposed material (in the case of a positive resist) using the appropriate developer. The substrate with photoresist image formed thereon is generally subjected to a post-bake treatment at temperatures up to about- 220°C and or other thermal hardening techniques known in the art as discussed above, before being exposed to pattern generation by any of the procedures known in the art such as etching, ion implantation doping, metal deposition and the like. The step of pattern generation may also give rise to exposure of the resist material to elevated temperatures which can reach as high as about 180°C or even higher. Finally, the resist material is stripped from the substrate using a stripping composition of the invention. It is found that this stripping can be accomplished in many cases at temperatures approximating ambient tem¬ perature (circa 20-25°C) in a relatively short time mea- sured in minutes. However, where the resist has been subjected to relatively high temperatures, up to about 180°C or higher, it is found that the stripper composi¬ tion should be employed at elevated temperature up to about 100°C in order that the stripping of the resist can be accomplished completely in a short period of time i.e., of the order of about 10 minutes. The exact temperature and time at which to employ the stripper composition in any given instance is principally a function of the temperature to which the resist has been exposed during post-bake and or other resist hardening techniques and or pattern generation. In any event the temperature of stripping should be less than the flash point of the stripper composition. The flash point of the compositions of the invention is, unlike most prior art strippers, relatively high and of the order of at least about 88°C depending upon the exact formulation employed in the stripper composition. The mode of contacting the stripper composition with the resist to be stripped is not critical. Advantag- eously the resist and substrate is sprayed with or immersed in a bath of the stripper composition which is maintained at the appropriate temperature. Completion of stripping can be readily determined by visual inspection of the substrate if necessary using a microscope or like means. When the stripping operation has been completed the substrate is washed with water or dilute aqueous solu¬ tions of surfactant in order to remove any residual stripper composition, the latter being soluble in, or miscible with, water. Illustrative of the various types of resist which can be stripped in accordance with the invention are positive resists which generally comprise a novolak resin, polyacrylamide or acrylic copolymer resin in association with photosensitizer such as an ester of l-oxo-2-diazo-naphthoquinone-5-sulfonic acid; negative resists which generally comprise polyvinylcinnamates, styrene-maleic anhydride and like resins in association with a free radical generating photoinitiator and a photosensitizer; and other types of resist such as poly- olefinsulfone based resins of which those described in U.S. Patent 4,513,077 are typical. The stripper compositions of the invention are particularly well adapted to the stripping of positive photoresists which latter are the type of photoresist most commonly employed in fabricating high resolution geometries in microelectronic applications. The stripping compositions possess a number of advantages over those hitherto employed in the art. Thus they are capable of stripping a wide variety of resists from metallic and other substrates without attacking the substrate or any circuitry or the like which may have been generated on the substrate while the resist was in place " thereon. The compositions pose no significant toxicity hazard to qualified personnel handling the same. The compositions are miscible with water and therefore readily washed from the substrate after the stripping operation has been completed. The compositions possess a relatively high flash point of the order of at least about 88°C which is generally in excess of the temperatures to which the compositions may have to be heated in order to strip the resist. Many resists can be stripped in a very short time, of the order of several minutes, using bath operating temperatures of the order of 20-25°C, i.e., without the need to employ elevated temperatures. The stripping bath is stable and can be re-used many times over a prolonged period without losing its efficacy. Further the two principal components can be readily recovered by distillation and recycled if and when the bath is finally retired from use. Further, because the stripper compositions of the invention are effective in most cases at temperatures at, or close to, room temperature, it is possible to use a wide range of engineering materials such as fiberglass reinforced resins, low thermal flow plastics and the like in fabricating the stripping bath and accessories such as ductwork and like installations. Stripping compositions hitherto employed in the art required higher operating temperatures which involved vaporization of solvents and led in many instances to corrosion of ductwork and other equipment with consequent and possibly undesirable contamination of the stripping bath. Operation at temperatures which led to vaporization of stripping elements also caused change in composition of the stripping bath where the latter contained two or more components of different volatility. The following examples illustrate specific embodi¬ ments of the compositions and process of the invention and the best mode currently known to the inventors for carrying out the same but are not to be construed as limiting. EXAMPLE 1 A silicon wafer with a silicon dioxide coating was spuncoated at 5000 rpm with a high resolution, high contrast, high aspect ratio positive photoresist system comprising a solvent blend solution of a novolak resin and a trihydroxybenzophenone ester of 2-diazo-l-oxo- naphthoquinoneδ-sulfonic acid [ULTRAMACtrø PR914: MacDermid, Inc., Waterbury, Ct.]. The resulting coating had an average thickness of 1.2 microns. The coated wafer was baked at 100°C for 30 minutes in a convection oven to evaporate the solvents from the coating before being exposed through a mask to UV light in a broad band contact exposure mode using an Oriel printer. The exposed photoresist was developed using an alkaline developer [ULTRAMAC MF-28: MacDermid Inc.] to give an image of high resolution with walls approaching 90 degrees. The imaged wafer was then baked at 150°C for 30 minutes in a forced air convection oven. The resulting wafer was then immersed in a stripper composition of the invention consisting of a 1:1 mixture by weight of morpholine and N-methylpyrrolidone and having a flash point of 38 β C. The temperature of the bath was circa 25 β C. It was found by visual inspection that the photoresist image had been completely stripped from the substrate within 5 minutes after immersion. No damage was found in the substrate itself. The above experiment was repeated but varying the temperature at which the post-bake of the photoresist image was carried out. The same stripping composition was employed. The following table summarizes the post- bake - temperature and time employed in the various repeat runs together with the stripping bath temperature and time of immersion necessary to achieve complete removal of the photoresist in each instance. TABLE I Post-bake Temperature Stripping Bath and Time Temperature and Time 150°C 30 minutes 25°C 5 minutes 160°C 30 minutes 35°C 5 minutes 170°C 30 minutes 50-75°C 10 minutes 180°C 30 minutes 100°C 20 minutes EXAMPLE 2 Stripper compositions were prepared by admixture of the components shown below in the proportions stated (all parts are percent by volume) . Stripper Composition A B Morpholine 25 • 10 N-methylpyrrolidone 25 10 Dipropylene glycol monomethyl ether 50 70 Surfactant^ 0 10 Flash Point °C 38 38 1: Surfynol 440: Air Products. EXAMPLE 3 - The process set forth in Example 1 for preparation and post-baking of a photoresist image on a substrate was repeated exactly as described. The photoresist was com- pletely stripped from the substrate by immersion for 5 minutes at 25°C in a stripper composition of the inven¬ tion consisting of a 1:1 by weight admixture of 4-(2-hydr- oxyethyl) morpholine and N-methylpyrrolidone having a flash point of 88°C. The substrate suffered no damage during the stripping. EXAMPLE 4 An arsenic implant wafer was prepared as follows. A silicon wafer coated with silicon oxide was spun-coated with a novolak-based positive photoresist (Eastman Kodak EK-820 resist) and soft-baked at 100 β C before being exposed to actinic radiation via a mask. The image was developed using a proprietary developer [Eastman Kodak EK-809] and post-baked at approximately 150°C. Arsenic implantation was then effected using a beam energy of 140 Kev at 10" 7 Torr with a dosage rate of IO 15 atoms/cm^. The temperature to which the resist was exposed during this treatment is estimated to be in excess of 200°C making the resist extremely resistant to solvent stripping. However, complete stripping was achieved by immersing the wafer for 20 minutes in a bath of a stripper composition of the invention containing 50 percent by weight of N-methylpyrrolidone and 50 percent by weight of N-(2-hydroxyethyl) morpholine. The bath temperature was 85-87°C. A second wafer, prepared in exactly the same man¬ ner, was immersed for 20 minutes in a bath of a stripper composition of the invention containing 50 percent by weight of N-methylpyrrolidone and 50 percent by weight of morpholine. The bath temperature was only 30-31 β C and the extent of stripping under these conditions was. approximately 50 percent. EXAMPLE 5 A chromium mask, coated with a layer of a poly(butene-1-sulfone) and thereafter baked at 100°C for 30 minutes, was imaged using an electron beam source. The resulting image was developed using methyl isoamyl ketone with an ispropyl alcohol rinse. The exposed chromium was etched away using ceric ammonium sulfate etchant. Thereafter the mask was immersed in a mixture of equal parts by weight of morpholine and N-methyl- pyrrolidone at room temperature (circa 25°C). Complete removal of the resist occurred in 60 seconds.

Claims: - - What is claimed is: 1. A stripping composition comprising a mixture of (a) a compound of the formula and (b) a compound of the formula ^N^ O wherein R and R 1 are independently selected from the group consisting of hydrogen, alkyl from 1 to 3 carbon atoms, inclusive, and hydroxyalkyl from 1 to 3 carbon atoms, inclusive; the ratio by weight of (a) to (b) in said composition being in the range of from 4:1 to 0.25:1. 2. A stripping composition according to Claim 1 which also comprises a polar organic solvent diluent in an amount up to about 70 percent by weight, based on total weight of the stripping composition. 3. A stripping composition according to Claim 2 where¬ in said polar organic solvent diluent comprises a lower- alkyl ether of dipropylene glycol. 4. A stripping composition according to Claim 3 where¬ in said polar organic solvent diluent is dipropylene glycol mono-methyl ether. 5. A stripping composition according to Claim 1 where¬ in said component (a) is morpholine. 6. A stripping composition according to Claim 1 where¬ in said component (b) is N-methylpyrrolidone. 7. A stripping composition according to Claim 1 where¬ in said component (a) is N-(2-hydroxyethyl) morpholine. 8. A stripping composition comprising a mixture of (a) morpholine and (b) a compound of the formula: wherein R 1 is selected from the group consisting of hydrogen, alkyl from 1 to 3 carbon atoms, inclusive, and hydroxyalkyl from 1 to 3 carbon atoms, inclusive; the ratio by weight of (a) to (b) in said composition being in the range of from 4:1 to 0.25:1. 9. A stripping composition according to Claim 8 where¬ in said component (b) is N-methylpyrrolidone. 10. A stripping composition according to Claim 8 wherein said composition also comprises a polar organic solvent diluent in an amount up to about 70 percent by weight, based on total weight of the stripping composi¬ tion. 11. A stripping composition according to Claim 10 where¬ in said polar organic solvent diluent comprises a lower- alkyl ether of dipropylene glycol. 12. A stripping composition according to Claim 11 where¬ in said polar organic solvent diluent is dipropylene glycol mono-methyl ether. 13. A stripping composition comprising a mixture of (a) N-(2-hydroxyethyl) morpholine and (b) a compound of the formula: Rl wherein R^ is selected from the group consisting of hydrogen, alkyl from 1 to 3 carbon atoms, inclusive, and hydroxyalkyl from 1 to 3 carbon atoms, inclusive; the ratio by weight of (a) to (b) in said composi- tion being in the range of from 4:1 to 0.25:1. 14. A stripping composition according to Claim 13 where¬ in said component (b) is N-methylpyrrolidone. 15. A stripping composition according to Claim 13 wherein said composition also comprises a polar organic solvent diluent in an amount up to about 70 percent by weight, based on total weight of the stripping composi¬ tion. 16. A stripping composition according to Claim 15 wherein said polar organic solvent diluent comprises a lower-alkyl ether of dipropylene glycol. 17. A stripping composition according to Claim 16 wherein said polar organic solvent diluent is dipropylene glycol mono-methyl ether. 18. In a process for solvent stripping of a photoresist from a substrate after the photoresist has completed its protective function in respect of said substrate, the improvement which comprises employing the stripping composition of Claim 1 to effect said stripping. 19. A process according to Claim 18 wherein said stripping step is carried out at a temperature below the flash point of said stripping composition. 20. In a process for solvent stripping of a photoresist from a substrate after the photoresist has completed its protective function in respect of said substrate, the improvement which comprises employing the stripping composition of Claim 8 to effect said stripping. 21. A process according to Claim 20 wherein said stripping step is carried out at a temperature below the flash point of said composition. 22. In a process for solvent stripping of a photoresist from -a substrate after the photoresist has completed its protective function ' in respect of said substrate, the improvement which comprises employing the stripping composition of Claim 13 to effect said stripping. 23. A process according to Claim 22 wherein said stripping step is carried out at a temperature below the flash point of said composition. 24. A process according to any of Claims 18, 20 or 22 wherein said photoresist is a positive photoresist.

Inventors: TURNER EDWIN J, TURNER, EDWIN, J.
Applicants: MACDERMID INC, MACDERMID, INCORPORATED
Application year: 1987
Main classification: C09D9
IPC codes: C09D9, G03F7