PHOTORESIST STRIPPER AND STRIPPING METHOD

Abstract: A composition for stripping a photoresist from a sub­ strate comprising a mixture of lactams where a first Lactam comprises at least 60% of the stripping solution and a second, different lactam comprises the balance. The use of the mixed lactam solution enhances the rate of removal of the resist from a substrate. The preferred lactams are pyrrolidones.

Description: PHOTORESIST STRIPPER Background of¯the Invention 1. Field of the Invention This invention relates to a novel stripping solution and its use for removing developed positive photoresist from the surface of a substrate. 2. Brief Description of the Prior Art Where a pattern is to be formed on a substrate, such as metallic circuitry in the manufacture of printed circuit boards and integrated circuits, it can either be put down selectively in the appropriate areas such as by plating, or it can be put down over the entire substrate and selectively removed by chemical etching. A method well known in the art for forming a selective pattern involves forming a layer of resist over a substrate that may be uncoated or coated with a metal, (hereafter collectively referred to as wsubstrate"), in those areas to be shielded from metal deposition or metal removal, dependent upon the process of manufacture employed. Typically, the resist layer is a polymer that is substantially unaffected by the metal deposition or metal removal process, and thus is able to protect the underlying areas of the substrate. For this purpose, various photoresists are known, that is, substances which form the desired masking pattern on the substrate upon exposure to activating light through a photographic image. Photoresists are generally of two types, positive photoresists and negative photoresists. Prior to exposure o activating iignt, x-ray i iectre am radiation, a ositive resist is insoluble in a developer solution. Upon xposure, the light struck portion of the photoresist becomes oluble in the developer while the unexposed areas remain nsoluble. Upon development, usually in aqueous basic solution, the solubilized portions are removed, leaving the insoluble ortions of the resist layer on the substrate in the pattern of he photographic image. Negative acting photoresists are chemicaly distinct from ositive resists, and become insolubilized upon exposure to ctivating light. Negative photoresists are, for example, yclicized rubber-based resists, while positive photoresists typically comprise, for example, a novolak resin rendered insoluble in base solution by addition of a light-sensitive arterial to the novolak known in the art as an inhibitor. Upon exposure to actinic radiation, the light-sensitive material is hemically altered, and is converted by exposure from a dissolution inhibitor to a dissolution accellerator. Other types of both positive and negative photoresists are also known. Subsequent to depositing or removing metal or oxide layers nthe exposed portions of the substrate, the resist layer must be removed or stripped from the surface. The removal process ust not adversely affect either the substrate nor the metal layer. The substrate can be a silicon dioxide coated silicon afer, as in the manufacture of semiconductor microcircuitry where the metallic microcircuitry is typically aluminum or an aluminum alloy. Other materials used in I.C. manufacture, or used for example in the manufacture of printed circuit boards, include a variety of inorganic substrate materials such as carnet crystal, L.E.D. and L.C.D. surfaces; e.g., indium oxide, iron oxide, chrome and virtually every other metal. The means employed for stripping the resist must act without corroding, dissolving, or dulling the surface of such materials, and without chemically altering the material of the substrate. Prior art methods for removing positive photoresists include the use of organic stripping compositions comprising phenol and linear alkyl benzene sulfonic acids, as exemplified in U.S. Patent No. 3,871,929. Other known organic stripping compositions contain organic sulfonic acids of the formula R-SO3E, where R is an organic radical; an organic solvent; fluoride ion; and optionally, phenol, as disclosed in U.S. Patent No. 4,165,295. Prior art inorganic stripping compositions also include aqueous sulfuric acid compositions containing fluoride ion, as exemplified in U.S. Patent No. 3,932,130, U.S. Patent No. 3,654,001 and U.S. Patent No. 3,080,071. The above-mentioned stripping compositions create problems during use because of their tendency to etch silicon dioxide substrates, which effect is likely caused by the acidity or basicity of the composition. The organic stripping compositions cause undesirable dulling of the surface of aluminum circuitry over the substrate, or undesirable etching of a silicon dioxide substrate. Stripping compositions containing phenolics are undesirable in that phénolics are toxic, poorly waste-treatable, and can actually lower stripping efficiency when employed in compositions for stripping positive-type photoresists. The use of phenol-type strippers is rapidly becoming untenable due to environmental considerations LL.H. Kaplan and BIK, Bergin, pL Electrochemical Sac., 127,386 (1980)1. Many known stripping compositions are insufficiently effective against certain resists because their rate of attack or dissolution on the resist is too slow. In particular, known stripping compositions are insufficiently effective against those resists which, during the course of device manufacture, have been subjected to elevated temperatures, such as during an exceptionally high temperature post-bake, to high energy processing, such as during ion implantation , or to exceptionally harsh chemical environments. This decreases their usefulness generally, and restricts the flexibility of processing lines in which they are used. Summary of the Insrenti on The subject invention is based upon the discovery that compositions having a lactam structure are effective strippers for positive resists and do not possess the disadvantages of prior art strippers as described above. Moreover, it is a discovery of the invention that though lactams strip photoresist from a substrate at a reasonable rate comparable to strippers of the prior art, the rate of removal can be significantly increased by use of a solution containing more than one lactam where the several lactams in solution differ from each other. The strippers of the invention are especially useful for tripping exposed resists from a substrate coated with a ositive working, alkali soluble resist, and are believed to be seful for stripping other resists as well such as E beam esists based upon such polymers as polymethyl methacrylate; eeswax based resists; and polyimide type resists. The strippers of the invention are also especially useful or stripping resist from a silicon dioxide substrate as the trippers are essentially neutral and consequently, do not etch the silicon dioxide while stripping the resist. Description of the Preferred Embodiments The strippers of the invention are used to remove positive irking resists from a substrate. The positive working resists comprise a light sensitive compound in a film-forming polymer diner. The light sensitive compounds, or inhibitors or sensitizers as they are often called, most frequently used are esters and amides formed from o-quinone diazide sulfonic acids. These esters and amides are well known in the art and are described by W. S. DeForest, Photoresist Materials and McGraw McGraw Kill, Inc, New York, pages 47-55, (1975), incorporated herein by reference. These light sensitive compounds, and the methods used to make the same, are all well documented in prior patents including German Patent No. 865,140 granted February 2,1953 and U.S. Patents Nos. 2,767,092; 3,046,110; 3,046,112; 3,046;119; 3,046,121; 3,046,122 and 3,106,465, all incorporated herein by reference. Additional sulfonic amide sensitizers that have been used in the formulation of positive acting ph#to#t-sists are shown in U.S. Patent No. 3,637,384, also incorporated herein by reference. These materials are formed by the reaction of a suitable diazide of an aromatic sulfonyl chloride with an appropriate rosin amine. Methods for the manufacture of these sensitizers and examples of the same are shown in U.S. Patent No. 2,797,213 incorporated herein by reference. Other positive working diazo compounds have been used for specific purposes. For example, a diazo compound used as a positive working photoresist for deep U.V. lithography is Meldrum's diazo and its analogs as described by Clecak et al., Technical Disclosure Bulletin, Volume 24, Number 4, September 1981, IBM Corporation, pp. 1907 and 1908. An o-quinone diazide compound suitable for laser imaging is shown in U.S. Patent No. 4,207,107. The aforesaid references are also incorporated herein by reference. The resin binders most frequently used with the o-quinone diazides in commercial practice are the alkali soluble phenol formaldehyde resins known as the Novolak resins. Photoresists using such polymers are illustrated in U.K. Patent No. 1,110,017 incorporated herein by reference. These materials are the product of reaction of a phenol with formaldehyde under conditions whereby a thermoplastic polymer is formed with a melting point of about 1200 C. Novolaks with melting points in excess of 1200C are known but are not generally used in photoresist formulations because they are often brittle or have other properties limiting their use. Another class of binders used with both the negative acting romatic azides and the positive acting o-quinone diazides are he homopolymers and copolymers of vinyl phenol. Photoresists f this nature are disclosed in U.S. Patent No. 3,869,292# upra. It is believed that photoresists using binders of olymers formed from vinyl phenols have not found extensive use in commerce. In the prior art, the above described positive resists using novolak resins as a binder are most often used as masks to rotect substrates from chemical etching in photo-engraving rocesses. For example, in a conventional process for the anufacture of a printed circuit board, a copper-clad substrate is coated with a layer of a positive working photoresist, is exposed to actinic radiation to form a latent circuit image in the photoresist coating, developed with a liquid developer to form a relief image and etched with a chemical etchant whereby unwanted copper is removed and copper protected by the photoresist mask is left behind in a circuit pattern. For the manufacture of printed circuit boards, the photoresist must possess chemical resistance, must adhere to the circuit board substrate and for high density circuits, must be capable of fine line image resolution. Similar photoresists are also used in the fabrication of semiconductors. As in the manufacture of printed circuits, the photoresist is coated onto the surface of a semiconductor wafer and then imaged and developed. Following development, the wafer is typically etched with an etchant whereby the portions of the wafer bared bared by the development of the photoresist are dissolved while the portions of the wafer coated with photoresist are protected, thereby defining a circuit pattern. For use in the manufacture of a semiconductor, the photoresist must possess resistance to chemical etchants, must adhere to the surface of the semiconductor wafer and must be capable of very fine line image resolution. Recent developments in photoresist technology involve processes where high temperatures are encountered. For example, a recent development in the fabrication of semiconductors substitutes dry plasma etching for wet chemical etching to define a circuit. Plasma etching provides advantages over wet chemical etching in that it offers process simplification and improves dimensional resolution and tolerance. However, the demands on the resist are significantly greater when using plasma etching. For both wet etching and plasma etching, the resist must adhere to the substrate and must be capable of fine line image resolution. For plasma etching, in addition to these properties, the resist must often be capable of withstanding high temperatures without image deformation and without eroding as plasma etching generates high temperatures at the wafer surface. When photoresists are heated to elevated temperatures to improve their adhesion to substrates and their resistance to chemical etchants, they become increasingly more difficult to remove from a substrate Consequently, the time to remove or strip the photoresist from the substrate is increased resulting in an overall increase in the processing time to manufacture an electronic device. The strippers of the invention are capable of stripping photoresists at a more rapid rate than prior art trippers and consequently, the processing time required to manufacture an electronic device is reduced. The strippers of the invention comprise a solution containing a mixture of lactams where each lactam in the mixture conforms to the structural formula: EMI9.1 where n is a whole integer of from 1 through 3, and the R on each methylene group, individually, is selected from the group of hydrogen, a polar group such as hydroxy, carboxyl, sulfoxy, etc., lower alkyl or lower alkyl substituted with a polar group; and R1 is selected from the group of hydrogen, lower alkyl, including cycloalkyl, either unsubstituted or substituted with a polar group and aryl. The preferred lactams in accordance with the invention are pyrrolidones - i.e., where in the above formula, n is 1 and R is hydrogen. Examples of suitable lactams include the following: butyrolactams EMI9.2 EMI10.1 valerolactams EMI10.2 EMI10.3 caprolactams EMI10.4 To enhance the rate of removal of photoresist from a ubstrate, the lactam solution contains a mixture of lactams ,g., two or more lactams where each conforms to one of the bove general formulas, with one of the lactams comprising at east 60% by volume of the total solution and the remaining actams, differing from the first, comprise the balance. Preferably, the first lactam is present in an amount of from 90 to 99% by volume of the solution and the remaining lactams omprise the balance. In this respect, it has been found that the rate of removal of photoresist increases rapidly from about % of the second lactam in solution up to a maximum at between bout 3 to 8%, dependent upon the specific mixture and photoresist used. Thereafter, as the concentration of the second lactam increases above about 10%, the rate photoresist removal drops off and decreases down to a rate equivalent to the rate obtainable with a single lactam. Typically, the rate of removal of photoresist using the mixed lactam solution is at least one and one half times the rate obtainable with either lactam used alone. In addition to an increased rate of removal, the stripping compositions of the invention also provide a significant advantage in waste treatment since the preferred compositions do not contain phenolic compounds. In addition, the preferred lactams employed in the stripping composition of the invention are liquid at room temperature, thereby facilitating handling, and they are miscible with numerous solvents including, for example, water, alcohol, xylene, acetone, etc. In addition to the above advantages, the absence of metal ions in the lactam mixture precludes metalion contamination of substrates during the stripping process, and also renders the tripping compositions more easily waste treatable. Another advantage of the lactam mixture of the invention is that the solutions are essentially neutral in pH whereby attack on acid or alkali sensitive materials is avoided. The stripping solutions of the invention effectively remove positive photoresists faster and at lower temperatures than other known organic stripping compositions. The rate of attack on metals such as aluminum, oxides and other inorganic substrate materials, including silicon and silicon dioxide, is significantly less than that of other stripping compositions. As would be apparent to those skilled in the art, other ingredients may be added to the stripping solutions to adapt them to particular applications, such as dyes or colorants, or a wetting agent as would be known to the art. For most applications however, such as in the manufacture of integrated circuits employing aluminum over silicon and silicon dioxide substrates, and in the manufacture of printed circuit boards, the addition of a wetting agent has not been found to be necessary and has not been observed to significantly effect the efficacy of the stripping solution. Where a positive photoresist is soft-baked after development, that is where the casting solvents are driven off at temperatures of about 900C, but the resist resin components remain otherwise substantially unaltered, the present stripping compositions can be used effectively even at temperatures as low as room temperature. Where the resist is hard-baked (120 C to 2000C), where thermal cross-linking may occur, the stripping solution can be used at temperatures of about 500C to 1500C, and preferably at temperatures varying between about 800C and 90 C. Lower temperatures can be used with longer stripping times. It is believed that the stripping compositions of the invention may be useful for stripping positive photoresists even when used in conjunction with ion-implanting techniques in the manufacture of integrated circuits. Other known stripping compositions require much longer, often many hours longer, to strip such resists and often do not remove the resist at all. Using the present stripping composition, the resist is removed cleanly and without residue, and a simple water rinse completely removes the stripping solution from the work piece. In addition to novel stripping solutions, the present invention provides a method for using the solutions to remove a positive photoresist from a substrate. In a preferred mode of the invention, a device to be stripped of positive photoresist is immersed in the novel stripping solution of the invention, at a temperature as described above, preferably, 800C to 900C. The solution is stirred or otherwise agitated. The device is contacted with the stripping solution either continuously, which is preferred, or intermitently, typically for sufficient time to completely remove the resist. In an alternative mode of Operation, the device to be stripped of resist can be contacted ith the stripping composition as a spray. It should be noted that the stripping method and stripping composition of the invention are not limited to the foregoing temperature or concentration ranges, but rather will be effective, though perhaps less so, outside of the preferred ranges. The stripping solutions of the invention provide another advantage since, in contrast to its excellent efficacy in the removal of positive photoresist, it does not significantly remove certain materials such as cured or partially cured polyimides and cyclized rubber resist. Accordingly, the stripping solutions of the invention can be used to remove one or more layers of positive resist while leaving one or more layers of such materials intact over the substrate. Thus, the stripping solutions can be used in conjunction with bi-layer and ulti-layer masking techniques to simplify and improve the manufacture of, for example, integrated circuits and printed circuit boards. It should be understood that while the novel stripping solutions of the invention are most notable for their surprising efficacy in stripping of positive photoresists, there is no intent to exclude from the scope of the invention its use-to strip any negative photoresist or other masking layer strippable by the stripping composition of the invention. The following examples will better illustrate the invention and are not to be considered as limitations on the scope thereof. Example 1 Silicon wafers, three inches in diameter, were coated with FIICROPOSIT-1450J Photoresist (Shipley Company Inc., Newton, .), using conventional techniques. Two solutions were prepared, the first comprising N-methyl-2-pyrrolidone (NMP) and the second consisting of a mixture of 95 parts NMP and 5 parts -(2-hydroxyethyl)- 2-pyrrolidone (EEP). Both solutions were naintained at 800C with magnetic stir bar agitation at slow speed. The silicon wafers were coated and baked at 150 C for 30 minutes to yield a final film thickness of 1.1 um . The so formed films were stripped of resist by immersion in the above stripping solutions, film side down until all resist was removed from the wafer. The minimum stripping time (MST in inutes:seconds) and rate of removal (in microns per minute) for each was observed to be as follows: NMP NMP + m EP MST 5.0 1.0 1.0:40 Rate 0.2 1.0 0.6 Example 2 A series of stripping solutions were prepared according to the present invention consisting of N-(2-hydroxyethyl)-2pyrrolidone (NILE) in N-methyl-2-pyrrolidone (NMP) where, for each test, the NMP was present in a different concentration. Each solution was maintained at approximately 800C during stripping. Silicon wafers prepared as in Example 1, were stripped in each of these solutions. TRie-observed minimum stripping times for each solution are shown in the following table: Solution NHEP(%) NMP(%) Min. Stripping Time Number (Minutes Seconds) 1 0 100 5:00 2 5 95 1:00 3 10 90 1:05 4 15 85 1:10 5 20 80 1:10 6 30 70 1:20 7 40 60 1:25 8 60 40 1:30 9 80 20 1:35 10 100- O 1:40 Example 3 The procedure of Example 2 was repeated except that N-cyclohexyl-pyrrolidone (NCP) was substituted for the N-(2-hydroxyethyl)-2-pyrrolidone. The results are set forth in the following table: Solution NCP(%) NMP(%) Min. Stripping Time Number (Minutes:Seconds) 1 0 100 9:30 2 10 90 8:00 3 20 80 7:30 4 40 60 8:00 5 60 40 9:00 6 80 20 11:00 7 100 0 12:25 The stripping times set forth above are higher than those typically obtained. For example, compare the results using 100% NMP is these examples with those obtained using 100% NMP in the earlier examples. The difference in the results is a consequence of the the samples of these examples having a prolonged post bake,

Claims: THE CLAIMS 1. A stripping composition for stripping positive working photoresist from a substrate comprising a solution of a mixture f lactams where a first lactam is present in a concentration of t least 60 percent by weight of the stripping composition and ne or more additional lactams, differing from the first lactam, comprises the balance of the lactams. 2. The stripping solution of Claim 1 where the lactams conform to the structural formula: EMI18.1 here n is a whole integer ranging from I to 3, each R substituted on a methylene group is selected from the group of hydrogen, a polar group, lower alkyl including cycloalkyl, and lower alkyl including cycloalkyl having polar group substitution, and R1 is selected from the same group as R and acryl. 3. The stripping composition of Claim I where each lactam is a pyrrolidone 4. The stripping composition of Claim 1 at essentially neutral pH. 5. The stripping composition of Claim 3 where the first pyrrolidone is present in a volume concentration of from 90 to 99 percent of the pyrrolidones and the second pyrrolidone comprises the balance of the pyrrolidones. 6. The stripping composition of Claim 3 where the first pyrrolidone is N-methyl-2-pyrrolidone. 7. The stripping composition of Claim 6 where the second pyrrolidone is selected from the group of N-(2-hydroxyethyl)-2- pyrrolidone and N-isopro'pyl-2-pyrrolidone. 8. A stripping composition for stripping positive working photoresists from a substrate comprising a solution of a mixture of pyrrolidones where a first pyrrolidone is present in a concentration of at least 60% by weight of the stripping composition and one or more additional pyrrolidones, differing from the first pyrrolidone, comprises the balance of the pyrrolidones within the stripping solution. 9. The stripping composition of Claim 8 having an essentially neutral pH. 10. The stripping composition of Claim 8 where the first pyrrolidone comprises from 90 to 99% by weight of the pyrrolidones and the second pyrrolidone comprises the balance. 11. The stripping composition of Claim 9 where the first pyrrolidone is N-methyl-2-pyrrolidone. 12. The stripping composition of Claim 11 where the second pyrrolidone is a member selected from the group of N-(2-hydroxyethyl)-2-pyrrolidone and N-isopropyl-2-pyrrolidone. 13. A method of stripping a positive working photoresist from a substrate comprising contacting said resist with a stripping composition comprising a solution of a mixture of ladtams where a first lactam is present in a concentration of at least 60 percent by weight of the stripping composition and one or more additional lactams, differing from the first 1#am, cswrises the balance of the lactams. 14. The method of Claim 13 where the lactams conform to the structural formula: EMI20.1 where n is a whole integer ranging from 1 to 3, each R substituted on a methylene group is selected from the group of hydrogen, a polar group, lower alkyl including cycloalkyl, and lower alkyl including cycloalkyl having polar group substitution, and R1 is selected from the same group as R and aryl. 15. The method of Claim 13 where each lactam is a pyrrolidone. 16. The method of Claim 15 where the first pyrrolidone is present in a volume concentration of from 90 to 99 percent of the pyrrolidones and the second pyrrolidone comprises the balance of the pyrrolidones. 17 The method of Claim 13 at where the stripping solution is at essentially a neutral pH. 18. The method of Claim 16 where the first pyrrolidone is -methyl-2-pyrrolidone. 19. The method of Claim 16 where the second pyrrolidone is selected from the group of N-(2-hydroxyethyl)-2- pyrrolidone and -cyclohexyl-2-pyrrolidone. 20. The method of claim 17 where contact is by immersion of the photoresist in the stripping composition. 21. The method of claim 17 where contact is by spraying the photoresist with the stripping composition. 22. The method of claims 20 or 21 where the stripping composition is at room temperature. 23. The method of claims 20 or 21 where the stripping composition is at an elevated temperature.

Inventors: MARTIN, ROBERT, PERKINS, MAJORIE
Applicants: SHIPLEY COMPANY INC.
Application year: 1985
Main classification: G03F7
IPC codes: H01L21, G03F7