Abstract:
Copolymers prepared by radical polymerization of a substituted norbornene monomer and a fluoromethacrylic acid, fluoromethacrylonitrile, or fluoromethacrylate comonomer are provided. The polymers are useful in lithographic phtoresist compositions, particularly chemical amplification resists. In a preferred embodiment, the polymers are substantially transparent to deep ultraviolet (DUV) radiation, i.e., radiation of a wavelength less than 250 nm, including 157 nm, 193 nm and 248 nm radiation, and are thus useful in DUV lithographic photoresist compositions. A process for using the composition to generate resist images on a substrate is also provided, i.e., in the manufacture of integrated circuits or the like.

Description:
TECHNICAL FIELD 
     This invention relates generally to the fields of polymer chemistry, lithography, and semiconductor fabrication. More specifically, the invention relates to novel alicyclic polymers, particularly substituted norbornene fluoroacrylate copolymers, which are useful in lithographic photoresist compositions, particularly chemical amplification photoresist compositions including ultraviolet, electron-beam, and x-ray photoresists. 
     BACKGROUND 
     There is a desire in the industry for higher circuit density in microelectronic devices made using lithographic techniques. One method of increasing the number of components per chip is to decrease the minimum feature size on the chip, which requires higher lithographic resolution. This has been accomplished over the past twenty years by reducing the wavelength of the imaging radiation from the visible (436 nm) down through the ultraviolet (365 mn) to the deep ultraviolet (DUV) at 248 nm. Development of commercial lithographic processes using ultra-deep ultraviolet radiation, particularly 193 nm, is now becoming of interest. See, for example, Allen et al. (1995), “Resolution and Etch Resistance of a Family of 193 nm Positive Resists,”  J. Photopolym. Sci. and Tech.  8(4):623-636, and Abe et al. (1995), “Study of ArF Resist Material in Terms of Transparency and Dry Etch Resistance,”  J. Photopolym. Sci. and Tech.  8(4):637-642. The resists proposed for use with 193 nm imaging radiation do not appear suitable for use with 157 nm radiation due to their poor transparency at the 157 nm wavelength. 
     Certain attempts have been made to develop 157 nm resists, for example using heavily fluorinated materials such as polytetrafluoroethylene (e.g., Teflon AF®; see Endert et al. (1999)  Proc. SPIE - Int. Soc. Opt. Eng,  3618:413-417) or hydridosilsesquioxanes (see U.S. Pat. No. 6,087,064 to Lin et al.). These materials do not, however, have the requisite reactivity or solubility characteristics. The challenge in developing chemically amplified resists for 157 nm lithography is in achieving suitable transparency in polymers that have acid-labile functionalities and developed with industry standard developers in either exposed or unexposed areas depending on whether the resist is positive or negative. 
     Polymers prepared from trifluoromethyl-substituted acrylates have been described. See, for example, Ito et al. (1981), “Methyl Alpha-Trifluoromethylacrylate, an E-Beam and UV Resist,”  IBM Technical Disclosure Bulletin  24(4):991, Ito et al. (1982)  Macromolecules  15:915-920 which describes preparation of poly(methylα-trifluoromethylacrylate) and poly(α-trifluoromethylacrylonitrile) from their respective monomers, and Ito et al. (1987), “Anionic Polymerization of α-(Trifluoromethyl)Acrylate,” in  Recent Advances in Anionic Polymerization , T. E. Hogen-Esch and J. Smid, Eds. (Elsevier Science Publishing Co., Inc.), which describes an anionic polymerization method for preparing polymers of trifluoromethylacrylate. Willson et al.,  Polymer Engineering and Science  23(18):1000-1003, also discuss poly(methyl α-trifluoromethylacrylate) and use thereof in a positive electron beam resist. However, none of these references discloses the utility of trifluoromethyl-substituted acrylate polymers in chemical amplification resists. 
     Alicyclic polymers have also attracted a great deal of attention for their potential utility in advanced microelectronics technologies. The interest stems from their low dielectric constants and low UV absorption. Polymers of alicyclic monomers such as norbornene are typically prepared by metal-mediated addition or ring-opening metathesis polymerization (ROMP), which suffers from the drawbacks of high cost and possible metal contamination. Alternatively, the electron-rich norbornene monomers can be radically copolymerized with electron-deficient maleic anhydride to produce alternating copolymers, which have been heavily evaluated as 193 nm (ArF excimer laser) resist polymers. The norbornene-maleic anhydride co- and terpolymers can be readily prepared and tend to offer high performance lithographic imaging. However, these polymers tend to exhibit relatively high absorption at 157 nm. Furthermore, the maleic anhydride unit is of low functionality and therefore limits functionalization of such copolymers to only the norbornene monomer unit. It has been difficult to identify electron-deficient monomers that undergo radical copolymerization with norbornene derivatives. Such monomers have now been identified, and this invention now provides novel polymers that are synthesized by radical copolymerization of substituted norbornene derivatives—e.g., hexafluoroisopropanol-substituted norbornene—with comonomers such as α-trifluoromethylacrylic acid and derivatives thereof. The polymers provide a versatile foundation for designing chemical amplification resist systems, particularly deep UV resist systems (e.g., 157 nm, 193 nm and 248 nm resists) that combine high UV transparency, aqueous base development, good dry etch resistance, and chemical amplification imaging capability. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a primary object of the invention to address the above-described need in the art by providing novel substituted norbornene fluoroacrylate copolymers suitable for use in lithographic photoresist compositions. 
     It is another object of the invention to provide a lithographic photoresist composition containing a substituted norbornene fluoroacrylate copolymer. 
     It is still another object of the invention to provide such a composition wherein the substituted norbornene fluoroacrylate copolymer is transparent to deep ultraviolet radiation, i.e., radiation having a wavelength less than about 250 nm. 
     It is yet another object of the invention to provide such a composition wherein the substituted norbornene fluoroacrylate copolymer is a copolymer of a norbornene monomer substituted with a fluorine-containing moiety and a fluorinated methacrylic acid, a fluorinated methacrylonitrile or a fluorinated methacrylate. 
     It is still another object of the invention to provide a method for generating a resist image on a substrate using a photoresist composition as described herein. 
     It is a further object of the invention to provide a method for forming a patterned structure on a substrate by transferring the aforementioned resist image to the underlying substrate material, e.g., by etching. 
     Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. 
     In one aspect, then, the present invention relates to a substituted norbornene fluoroacrylate copolymer prepared by copolymerization of a first monomer having the structure                           
     wherein m is zero or 1, r is zero or 1, L is a linking group, R 1  is linear or branched fluoroalkyl, R 2  is linear or branched alkyl or fluoroalkyl, and R 3  is hydrogen, alkyl (preferably lower alkyl), —C(O)R, —CH 2 —C(O)OR, C(O)OR, or SiR 3  wherein R is alkyl, preferably lower alkyl, and at least one additional monomer having the structure (II)                           
     wherein R 4  is —COOH, —CN, an amide, an acid-inert ester, or an acid-cleavable functionality. The polymer may serve as either the base-soluble component of an unexposed resist or as an acid-labile material that releases acid following irradiation. 
     In another aspect, the invention relates to a lithographic photoresist composition comprising a substituted norbornene fluoroacrylate copolymer as described above and a photosensitive acid generator (also referred to herein as a “photoacid generator,” a “PAG,” or a “radiation-sensitive acid generator”). 
     The present invention also relates to the use of the resist composition in a lithography method. The process involves the steps of (a) coating a substrate (e.g., a ceramic, metal or semiconductor substrate) with a film comprising a radiation-sensitive acid generator and a copolymer as provided herein; (b) exposing the film selectively to a predetermined pattern of radiation to form a latent image therein; and (c) developing the image using a suitable developer composition. The radiation may be ultraviolet, electron beam or x-ray. Ultraviolet radiation is preferred, particularly deep ultraviolet radiation having a wavelength of less than about 250 nm, e.g., 157 nm, 193 nm, or 248 nm. The pattern from the resist structure may then be transferred to the underlying substrate. Typically, the transfer is achieved by reactive ion etching or some other etching technique. Thus, the compositions of the invention and resulting resist structures can be used to create patterned material layer structures such as metal wiring lines, holes for contacts or vias, insulation sections (e.g., damascene trenches or shallow trench isolation), trenches for capacitor structures, etc. as might be used in the design of integrated circuit devices. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Overveiw and Definitions 
     Before describing the present invention in detail, it is to be understood that this invention is not limited to specific compositions, components or process steps, as such may vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting. 
     It must be noted that, as used in this specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a monomer” includes a combination of two or more monomers that may or may not be the same, a “photoacid generator” includes a mixture of two or more photoacid generators, and the like. 
     In describing and claiming the present invention, the following terminology will be used in accordance with the definitions set out below. 
     The term “alkyl” as used herein refers to a branched or unbranched saturated hydrocarbon group of 1 to 24 carbon atoms, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, t-butyl, octyl, decyl, tetradecyl, hexadecyl, eicosyl, tetracosyl and the like, as well as cycloalkyl groups such as cyclopentyl, cyclohexyl and the like. The term “lower alkyl” intends an alkyl group of 1 to 6 carbon atoms, and the term “lower alkyl ester” refers to an ester functionality —C(O)O—R wherein R is lower alkyl. 
     The term “alkylene” as used herein refers to a difunctional branched or unbranched saturated hydrocarbon group of 1 to 24 carbon atoms, such as methylene, ethylene, n-propylene, n-butylene, n-hexylene, decylene, tetradecylene, hexadecylene, and the like. The term “lower alkylene” refers to an alkylene group of one to six carbon atoms. 
     The term “fluorinated” refers to replacement of a hydrogen atom in a molecule or molecular segment with a fluorine atom. The term “perfluorinated” is also used in its conventional sense to refer to a molecule or molecular segment wherein all hydrogen atoms are replaced with fluorine atoms. Thus, a “fluorinated” methyl group encompasses —CH 2 F and —CHF 2  as well as the “perfluorinated” methyl group —CF 3 . 
     “Optional” or “optionally” means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where said event or circumstance occurs and instances where it does not. For example, the phrase “optionally substituted lower alkyl” means that a lower alkyl moiety may or may not be substituted and that the description includes both unsubstituted lower alkyl and lower alkyl where there is substitution. 
     The term “polymer” is used to refer to a chemical compound that comprises linked monomers, and that may be linear, branched, or crosslinked. 
     The terms “photogenerated acid” and “photoacid” are used interchangeably herein to refer to the acid that is created upon exposure of the present compositions to radiation, i.e., as a result of the radiation-sensitive acid generator in the compositions. 
     The term “substantially transparent” as used to describe a polymer that is “substantially transparent” to radiation of a particular wavelength refers to a polymer that has an absorbance of less than about 5.0/micron, preferably less than about 4.0/micron, most preferably less than about 3.5/micron, at a selected wavelength. 
     For additional information concerning terms used in the field of lithography and lithographic compositions, reference may be had to  Introduction to Microlithography , Eds. Thompson et al. (Washington, D.C.: American Chemical Society, 1994). 
     The Fluorinated Norbornene Copolymer 
     The fluorinated norbornene copolymer is prepared by copolymerization of a monomer having the structure (I)                           
     wherein m is zero or 1, r is zero or 1, L is an alkylene or oxyalkylene linking group, R 1  is linear or branched fluoroalkyl, R 2  is linear or branched alkyl or fluoroalkyl, and R 3  is hydrogen, alkyl (preferably lower alkyl), —C(O)—R, —CH 2 —C(O)OR, —C(O)OR or Si(R) 3  wherein R is alkyl, preferably lower alkyl (e.g., R 3  may be —C(O)OC(CH 3 ) 3 , —CH 2 C(O)OC(CH 3 ) 3 , —C(O)CH 3 , or —Si(CH 3 ) 3 ), and at least one additional monomer having the structure (II)                           
     wherein R 4  is —COOH, —CN, an amide, an acid-inert moiety such as an acid-inert ester, or an acid-cleavable functionality. The polymer may serve as either the base-soluble component of an unexposed resist or as an acid-labile material that releases acid. 
     The aforementioned copolymer will thus comprise monomer units having the structure of formula (III)                           
     wherein m, r, L, R 1 , R 2  and R 3  are as defined above, and at least one additional monomer unit having the structure (IV)                           
     In the second monomer, R 4  is —COOH, —CN, an amide, an acid-inert moiety such as an acid-inert ester, or an acid-cleavable functionality, i.e., a molecular moiety that is cleavable with acid, particularly photogenerated acid. Suitable acid-cleavable functionalities include, but are not limited to, esters, carbonates, ketals and acetals, exemplified below:                           
     wherein m is in the range of 1 to 3, n is zero or 1, R 6  is hydrogen, lower alkyl (e.g., methyl, ethyl, isopropyl, etc.), fluorinated lower alkyl (e.g., trifluoromethyl, pentafluoroethyl, 1,1,1,3,3,3-hexafluoroisopropyl), or lower alkyl substituted with a tri(lower alkyl)silyl group (e.g., trimethylsilyl-substituted ethyl, or —CH 2 CH 2 Si(CH 3 ) 3 ), or is selected so as to provide an acid-cleavable substituent, R 7  and R 8  are lower alkyl or are linked to form a five-or six-membered heterocyclic ring that may or may not contain an additional heteroatom and/or a carbonyl moiety, R 9  and R 10  are hydrogen, lower alkyl, or fluorinated lower alkyl, n is zero or 1, and L is a linking group, and R 11  is lower alkyl, fluorinated lower alkyl, or lower alkyl substituted with a tri(lower alkyl)silyl group, or is selected so as to provide an acid-cleavable substituent. Preferably, R 4  has the structure of formula (V), and R 6  is selected so as to provide an acid-cleavable functionality, i.e., a molecular moiety that is cleavable with acid, particularly photogenerated acid. 
     Preferred acid-cleavable pendant groups are organic ester groups that undergo a cleavage reaction in the presence of photogenerated acid to generate a carboxylic acid group. Typically, the reaction of acid-cleavable functional groups with photogenerated acid occurs only, or is promoted greatly by, the application of heat to the film. Those skilled in the art will recognize the various factors that influence the rate and ultimate degree of cleavage of acid-cleavable functional groups as well as the issues surrounding integration of the cleavage step into a viable manufacturing process. The product of the cleavage reaction is a polymer-bound acid group, which, when present in sufficient quantities along the polymer backbone, imparts solubility to the polymer in basic aqueous solutions. 
     The copolymer may comprise different monomer units each having the structure (III), different monomer units each having the structure (IV) and/or one or more other monomer units, typically formed from addition polymerizable monomers, preferably vinyl monomers, for example to enhance the performance of the photoresist. Thus, the polymer may comprise minor amounts of acrylic acid or methacrylic acid monomer (e.g., 5-30%) to enhance development. The polymer may also comprise other suitable monomer units such as hydroxystyrene to enhance development and etch resistance or a silicon-containing monomer unit (e.g., a silicon-containing acrylate, methacrylate or styrene) to enhance oxygen plasma etch resistance for bilayer applications. In general, suitable comonomers include, but are not limited to, the following ethylenically unsaturated polymerizable monomers: acrylic and methacrylic acid esters and amides, including alkyl acrylates, aryl acrylates, alkyl methacrylates and aryl methacrylates (for example, methyl acrylate, methyl methacrylate, n-butyl acrylate, n-butyl methacrylate, t-butyl acrylate, t-butyl methacrylate, 2-ethylhexyl methacrylate, benzyl acrylate and N-phenylacrylamide); vinyl aromatics, including unsubstituted styrene and styrene substituted with one or two lower alkyl, halogen or hydroxyl groups (for example, styrene derivatives such as 4-vinyltoluene, 4-vinylphenol, α-methylstyrene, 2,5-dimethylstyrene, 4-t-butylstyrene and 2-chlorostyrene); butadiene; vinyl acetate; vinyl bromide; vinylidene chloride; and C 5 -C 20 , generally C 7 -C 15 , cyclic olefin monomers such as norbornene and tetracyclododecane; fluorinated analogs of any of the foregoing, e.g., fluorinated acrylic and methacrylic acid esters (e.g., fluorinated alkyl acrylates, fluorinated aryl acrylates, fluorinated alkyl methacrylates and fluorinated aryl methacrylates); and others readily apparent to one skilled in the art. The structure (III) and structure (IV) monomer units will generally be present in a ratio of approximately 1:2; that is, the copolymer will generally comprise approximately 30-40% structure (III) monomer units and, correspondingly, approximately 70%-60% structure (IV) monomer units. This is not necessarily the case, however, if additional types of monomer units are present. 
     Monomer Synthesis and Polymerization 
     The present copolymers may be readily synthesized using methods described in the pertinent texts and literature, or as known to those of ordinary skill in the art. Methods for synthesizing representative monomers are described in the examples, as are methods for preparing the fluorinated norbornene copolymers. 
     The copolymers of the invention can be prepared by radical copolymerization, using a suitable free radical initiator. The initiator may be any conventional free radical-generating polymerization initiator. Examples of suitable initiators include peroxides such as O-t-amyl-O-(2ethylhexyl)monoperoxycarbonate, dipropylperoxydicarbonate, and benzoyl peroxide (BPO) as well as azo compounds such as azobisisobutyronitrile (AIBN), 2,2′-azobis(2-amidino-propane)dihydrochloride, and 2,2′-azobis(isobutyramide)dihydrate. The initiator is generally present in the polymerization mixture in an amount of from about 0.2 to 5% by weight of the monomers. The resulting copolymer typically as a number average molecular weight in the range of approximately 500 to 50,000, generally in the range of approximately 1,000 to 15,000. 
     The Photoacid Generator 
     The second component of the resist composition is a photoacid generator. Upon exposure to radiation, the photoacid generator generates a strong acid. A variety of photoacid generators can be used in the composition of the present invention. The photosensitive acid generators used in the photoresist compositions of the invention may be any suitable photosensitive acid generator known in the art which is compatible with the other components of the photoresist composition. Examples of preferred photoresist acid generators (PAGs) include, but are not limited to, α-(trifluoromethylsulfonyloxy)-bicyclo[2.2.1]hept-5-ene-2,3-dicarboximide (MDT), onium salts, aromatic diazonium salts, sulfonium salts, diaryliodonium salts and sulfonic acid esters of N-hydroxyamides or -imides, as disclosed in U.S. Pat. No. 4,731,605. Also, a PAG that produces a weaker acid such as dodecane sulfonate of N-hydroxy-naphthalimide (DDSN) may be used. Combinations of PAGs may be used. Generally, suitable acid generators have a high thermal stability (preferably to temperatures greater than 140° C.) so they are not degraded during pre-exposure processing. In addition to MDT and DDSN, suitable sulfonate compounds are sulfonate salts, but other suitable sulfonate PAGs include sulfonated esters and sulfonyloxy ketones. See U.S. Pat. No. 5,344,742 to Sinta et al., and  J. Photopolymer Science and Technology,  4:337-340 (1991), for disclosure of suitable sulfonate PAGs, including benzoin tosylate, t-butylphenyl α-(p-toluenesulfonyloxy)-acetate and t-butyl α-(p-toluenesulfonyloxy)-acetate. Onium salts are also generally preferred acid generators of compositions of the invention. Onium salts that contain weakly nucleophilic anions have been found to be particularly suitable. Examples of such anions are the halogen complex anions of divalent to heptavalent metals or non-metals, for example, Sb, B, P, and As. Examples of suitable onium salts are aryl-diazonium salts, halonium salts, aromatic sulfonium salts and sulfoxonium salts or selenium salts, (e.g., triarylsulfonium and diaryliodonium hexafluoroantimonates, hexafluoroarsenates, trifluoromethanesulfonates and others). Examples of suitable preferred onium salts can be found in U.S. Pat. Nos. 4,442,197, 4,603,101, and 4,624,912. Other useful acid generators include the family of nitrobenzyl esters, and the s-triazine derivatives. Suitable s-triazine acid generators are disclosed, for example, in U.S. Pat. No. 4,189,323. 
     Still other suitable acid generators include N-camphorsulfonyloxynaphthalimide, N-pentafluorophenylsulfonyloxynaphthalimide, ionic iodonium sulfonates, e.g., diaryl iodonium (alkyl or aryl) sulfonate and bis-(di-t-butylphenyl)iodonium camphanylsulfonate, perfluoroalkanesulfonates, such as perfluoropentanesulfonate, perfluoromethanesulfonate; aryl (e.g., phenyl or benzyl) triflates and derivatives and analogs thereof, e.g., triphenylsulfonium triflate or bis-(t-butylphenyl)iodonium triflate; pyrogallol derivatives (e.g., trimesylate of pyrogallol); trifluoromethanesulfonate esters of hydroxyimides, α,α′-bis-sulfonyl-diazomethanes; sulfonate esters of nitro-substituted benzyl alcohols; naphthoquinone-4-diazides; and alkyl disulfones. Other suitable photoacid generators are disclosed in Reichmanis et al. (1991),  Chemistry of Materials  3:395, and in U.S. Pat. No. 5,679,495 to Yamachika et al. Additional suitable acid generators useful in conjunction with the compositions and methods of the invention will be known to those skilled in the art and/or are described in the pertinent literature. 
     The Resist Composition 
     The photoresist composition herein comprises both a fluorinated norbornene copolymer as described in detail above and an acid generator, with the polymer representing up to about 99 wt. % of the solids included in the composition, and the photoacid generator representing approximately 0.5-10 wt. % of the solids contained in the composition. Other components and additives may also be present. 
     For example, a positive photoresist composition may include a dissolution inhibitor, and a negative photoresist composition will include a crosslinking agent. If dissolution inhibitors and crosslinking agents are present, they will typically represent in the range of about 1 wt. % to 40 wt. %, preferably about 5 wt. % to 30 wt. %, of the total solids. 
     Suitable dissolution inhibitors will be known to those skilled in the art and/or described in the pertinent literature. Preferred dissolution inhibitors have high solubility in the resist composition and the solvent used to prepare solutions of the resist composition (e.g., propylene glycol methyl ether acetate, or “PGMEA”), exhibit strong dissolution inhibition, have a high exposed dissolution rate, are substantially transparent at the wavelength of interest, exhibit a moderating influence on T g , strong etch resistance, and display good thermal stability (i.e., stability at temperatures of about 140° C. or greater). Suitable dissolution inhibitors include, but are not limited to, bisphenol A derivatives and carbonate derivatives, for example bisphenol A derivatives wherein one or both hydroxyl moieties are converted to a t-butoxy substituent or a derivative thereof such as a t-butoxycarbonyl or t-butoxycarbonylmethyl group; fluorinated bisphenol A derivatives such as CF 3 -bisphenol A—OCH 2 (CO)—O-tBu (6F-bisphenol A protected with a t-butoxycarbonylmethyl group); normal or branched chain acetal groups such as 1-ethoxyethyl, 1-propoxyethyl, 1-n-butoxyethyl, 1-isobutoxy-ethyl, 1-t-butyloxyethyl, and 1-t-amyloxyethyl groups; and cyclic acetal groups such as tetrahydrofuranyl, tetrahydropyranyl, and 2-methoxytetrahydropyranyl groups; androstane-17-alkylcarboxylates and analogs thereof, wherein the 17-alkylcarboxylate at the 17-position is typically lower alkyl. Examples of such compounds include lower alkyl esters of cholic, ursocholic and lithocholic acid, including methyl cholate, methyl lithocholate, methyl ursocholate, t-butyl cholate, t-butyl lithocholate, t-butyl ursocholate, and the like (see, e.g., Allen et al. (1995)  J. Photopolym. Sci. Technol ., cited supra); hydroxyl-substituted analogs of such compounds (ibid.); and androstane-17-alkylcarboxylates substituted with 1 to 3 C 1 -C 4  fluoroalkyl carbonyloxy substituents, such as t-butyl trifluoroacetyllithocholate (see, e.g., U.S. Pat. No. 5,580,694 to Allen et al.). 
     The crosslinking agent used in the photoresist compositions of the invention may be any suitable crosslinking agent known in the negative photoresist art which is otherwise compatible with the other selected components of the photoresist composition. The crosslinking agents preferably act to crosslink the polymer component in the presence of a generated acid. Preferred crosslinking agents are glycoluril compounds such as tetramethoxymethyl glycoluril, methylpropyltetramethoxymethyl glycoluril, and methylphenyltetramethoxymethyl glycoluril, available under the POWDERLINK trademark from American Cyanamid Company. Other possible crosslinking agents include: 2,6-bis(hydroxymethyl)-p-cresol and compounds having the following structures:                           
     including their analogs and derivatives, such as those found in Japanese Laid-Open Patent Application (Kokai) No. 1-293339, as well as etherified amino resins, for example methylated or butylated melamine resins (N-methoxymethyl- or N-butoxymethyl-melamine respectively) or methylated/butylated glycolurils, for example as can be found in Canadian Patent No. 1 204 547. Combinations of crosslinking agents may be used. 
     The remainder of the resist composition is composed of a solvent and may additionally, if necessary or desirable, include customary additives such as dyes, sensitizers, additives used as stabilizers and acid-diffusion controlling agents, coating aids such as surfactants or anti-foaming agents, adhesion promoters and plasticizers. 
     The choice of solvent is governed by many factors not limited to the solubility and miscibility of resist components, the coating process, and safety and environmental regulations. Additionally, inertness to other resist components is desirable. It is also desirable that the solvent possess the appropriate volatility to allow uniform coating of films yet also allow significant reduction or complete removal of residual solvent during the post-application bake process. See, e.g.,  Introduction to Microlithography , Eds. Thompson et al., cited previously. In addition to the above components, the photoresist compositions of the invention generally include a casting solvent to dissolve the other components so that the overall composition may be applied evenly on the substrate surface to provide a defect-free coating. Where the photoresist composition is used in a multilayer imaging process, the solvent used in the imaging layer photoresist is preferably not a solvent to the underlayer materials, otherwise the unwanted intermixing may occur. Examples of suitable casting solvents include: ethoxyethylpropionate (“EEP”), a combination of EEP and γ-butyrolactone (“GBL”), PGMEA, and ethyl lactate. The invention is not limited to selection of any particular solvent. Solvents may generally be chosen from ether, ester-, hydroxyl-, and ketone-containing compounds, or mixtures of these compounds. Examples of appropriate solvents include cyclopentanone, cyclohexane, lactate esters such as ethyl lactate, alkylene glycol alkyl ether esters such as propylene glycol methyl ether acetate, alkylene glycol monoalkyl eters such as methyl cellosolve, butyl acetate, 2-ethoxyethanol, and ethyl 3-ethoxypropionate. Preferred solvents include ethyl lactate, propylene glycol methyl ether acetate, ethyl 3-ethoxypropionate and their mixtures. The above list of solvents is for illustrative purposes only and should not be viewed as being comprehensive nor should the choice of solvent be viewed as limiting the invention in any way. Those skilled in the art will recognize that any number of solvents or solvent mixtures may be used. 
     Greater than 50 percent of the total mass of the resist formulation is typically composed of the solvent, preferably greater than 80 percent. 
     Other customary additives include dyes that may be used to adjust the optical density of the formulated resist and sensitizers which enhance the activity of photoacid generators by to absorbing radiation and transferring it to the photoacid generator. Examples include aromatics such as functionalized benzenes, pyridines, pyrimidines, biphenylenes, indenes, naphthalenes, anthracenes, coumarins, anthraquinones, other aromatic ketones, and derivatives and analogs of any of the foregoing. 
     A wide variety of compounds with varying basicity may be used as stabilizers and acid-diffusion controlling additives. They may include nitrogenous compounds such as aliphatic primary, secondary, and tertiary amines, cyclic amines such as piperidines, pyrimidines, morpholines, aromatic heterocycles such as pyridines, pyrimidines, purines, imines such as diazabicycloundecene, guanidines, imides, amides, and others. Ammonium salts may also be used, including ammonium, primary, secondary, tertiary, and quaternary alkyl- and arylammnonium salts of alkoxides including hydroxide, phenolates, carboxylates, aryl and alkyl sulfonates, sulfonamides, and others. Other cationic nitrogenous compounds including pyridinium salts and salts of other heterocyclic nitrogenous compounds with anions such as alkoxides including hydroxide, phenolates, carboxylates, aryl and alkyl sulfonates, sulfonamides, and the like may also be employed. Surfactants may be used to improve coating uniformity, and include a wide variety of ionic and non-ionic, monomeric, oligomeric, and polymeric species. Likewise, a wide variety of anti-foaming agents may be employed to suppress coating defects. Adhesion promoters may be used as well; again, a wide variety of compounds may be employed to serve this function. A wide variety of monomeric, oligomeric, and polymeric plasticizers such as oligo- and polyethyleneglycol ethers, cycloaliphatic esters, and non-acid reactive steroidally-derived materials may be used as plasticizers, if desired. However, neither the classes of compounds nor the specific compounds mentioned above, are intended to be comprehensive and/or limiting. One versed in the art will recognize the wide spectrum of commercially available products that may be used to carry out the types of functions that these customary additives perform. 
     Typically, the sum of all customary additives will comprise less than 20 percent of the solids included in the resist formulation, preferably, less than 5 percent. 
     Use in Generation of Resist Images on a Substrate 
     The present invention also relates to a process for generating a resist image on a substrate comprising the steps of: (a) coating a substrate with a film comprising the resist composition of the present invention; (b) imagewise exposing the film to radiation; and (c) developing the image. The first step involves coating the substrate with a film comprising the resist composition dissolved in a suitable solvent. Suitable substrates are ceramic, metallic or semiconductive, and preferred substrates are silicon-containing, including, for example, silicon dioxide, silicon nitride, and silicon oxynitride. The substrate may or may not be coated with an organic anti-reflective layer prior to deposition of the resist composition. Alternatively, a bilayer resist may be employed wherein a resist composition of the invention forms an upper resist layer (i.e., the imaging layer), and the underlayer is comprised of a material that is highly absorbing at the imaging wavelength and compatible with the imaging layer. Conventional underlayers include diazonapthoquinone (DNQ)/novolak resist material. 
     Preferably, the surface of the substrate is cleaned by standard procedures before the film is deposited thereon. Suitable solvents for the composition are as described in the preceding section, and include, for example, cyclohexanone, ethyl lactate, and propylene glycol methyl ether acetate. The film can be coated on the substrate using art-known techniques such as spin or spray coating, or doctor blading. Preferably, before the film has been exposed to radiation, the film is heated to an elevated temperature of about 90-150° C. for a short period of time, typically on the order of about 1 minute. The dried film has a thickness of about 0.02-5.0 microns, preferably about 0.05-2.5 microns, most preferably about 0.10 to 1.0 microns. The radiation may be ultraviolet, electron beam or x-ray. Ultraviolet radiation is preferred, particularly deep ultraviolet radiation having a wavelength of less than about 250 nm, e.g., 157 nm using an F 2  excimer laser. The radiation is absorbed by the radiation-sensitive acid generator to generate free acid which with heating causes cleavage of the acid-cleavable pendant groups and formation of the corresponding acid. After the film has been exposed to radiation, the film may again be heated to an elevated temperature of about 90-150° C. for a short period of time, on the order of about 1 minute. It will be appreciated by those skilled in the art that the aforementioned description applies to a positive resist, and with a negative resist the exposed regions would typically be crosslinked by acid. 
     The third step involves development of the image with a suitable solvent. Suitable solvents include an aqueous base, preferably an aqueous base without metal ions such as the industry standard developer tetramethylammonium hydroxide or choline. Because the fluorinated norbornene copolymer of the resist composition is substantially transparent at 157 nm, the resist composition is uniquely suitable for use at that wavelength. However, the resist may also be used with wavelengths of 193 nm and 248 nm, or with electron beam or x-ray radiation. 
     The pattern from the resist structure may then be transferred to the material of the underlying substrate. Typically, the transfer is achieved by reactive ion etching or some other etching technique. Thus, the compositions of the invention and resulting resist structures can be used to create patterned material layer structures such as metal wiring lines, holes for contacts or vias, insulation sections (e.g., damascene trenches or shallow trench isolation), trenches for capacitor structures, etc. as might be used in the design of integrated circuit devices. Accordingly, the processes for making these features involves, after development with a suitable developer as above, etching the layer(s) underlying the resist layer at spaces in the pattern whereby a patterned material layer or substrate section is formed, and removing any remaining resist from the substrate. In some instances, a hard mask may be used below the resist layer to facilitate transfer of the pattern to a further underlying material layer or section. In the manufacture of integrated circuits, circuit patterns can be formed in the exposed areas after resist development by coating the substrate with a conductive material, e.g., a metallic material, using known techniques such as evaporation, sputtering, plating, chemical vapor deposition, or laser-induced deposition. Dielectric materials may also be deposited by similar means during the process of making circuits. Inorganic ions such as boron, phosphorous, or arsenic can be implanted in the substrate in the process for making p-doped or n-doped circuit transistors. Examples of such processes are disclosed in U.S. Pat. Nos. 4,855,017, 5,362,663, 5,429,710, 5,562,801, 5,618,751, 5,744,376, 5,801,094, and 5,821,469. Other examples of pattern transfer processes are described in Chapters 12 and 13 of Moreau,  Semiconductor Lithography, Principles, Practices, and Materials  (Plenum Press, 1988). It should be understood that the invention is not limited to any specific lithographic technique or device structure. 
     It is to be understood that while the invention has been described in conjunction with the preferred specific embodiments thereof, that the foregoing description as well as the examples which follow are intended to illustrate and not limit the scope of the invention. Other aspects, advantages and modifications within the scope of the invention will be apparent to those skilled in the art to which the invention pertains. 
     All patents, patent applications, and publications mentioned herein are hereby incorporated by reference in their entirety. 
    
    
     EXPERIMENTAL 
     The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how to prepare and use the compositions disclosed and claimed herein. Efforts have been made to ensure accuracy with respect to numbers (e.g., amounts, temperature, etc.) but some errors and deviations should be accounted for. Unless indicated otherwise, parts are parts by weight, temperature is in °C. and pressure is at or near atmospheric. Additionally, all starting materials were obtained commercially or synthesized using known procedures. Measurements: NMR spectra were recorded on Varian T-60 ( 1 H), Varian CFT-20 ( 1 H and  13 C), IBM NR-80 ( 19 F), and Bruker AF250 ( 1 H and  13 C) spectrometers. Gel permeation chromatography (GPC) was performed with a Waters Model 150 chromatograph equipped with six μ-Styragel columns. Measurements were made at 30 and 40° C. in tetrahydroforan (THF). Combustion analyses were performed by Childers Laboratories, Milford, N.J., and by Chemical Analytical Services, University of California, Berkeley, Calif. 
     EXAMPLE 1 
     Preparation of α-(Trifluoromethyl)acrylonitrile (TFMAN) 
     (a) 1,1,1-Trifluoroacetone cyanohydrin: To a stirred and cooled solution of 127.2 grams (2.6 mol) sodium cyanide in 544 mL water was added dropwise 300 grams (2.68 mol) of 1,1,1-trifluoroacetone. This solution was cooled to 4° C. and a cooled solution of 6N sulfuric acid (910 g) was added slowly with mechanical stirring keeping the temperature below 10° C. After stirring overnight at room temperature the reaction was extracted with diethyl ether, the extracts dried with sodium sulfate, filtered, and evaporated. The residue was distilled under reduced pressure (50 mm Hg) and the fractions boiling between 40-80° C. were collected and mixed with an equal volume of diethyl ether then stirred with sodium sulfate overnight. Filtration, evaporation, distillation, and sodium sulfate treatment of the residue was repeated twice to obtain 270 grams (72%) of cyanohydrin (b.p.: 76-79° C./50 mm). 
     (b) 1,1,1-Trifluoroacetone cyanohydrin acetate: Triethylamine (194.3 g, 1.924 mol) was added dropwise to a stirred solution of 269 grams (1.935 mol) of the cyanohydrin prepared in part (a), 202.7 grams (1.987 mol) of acetic anhydride, 320 mL of methylene chloride, and 630 milligrams of 4-dimethylaminopyridine cooled to 4° C. After stirring overnight at room temperature the reaction was diluted with 200 mL water and extracted with methylene chloride. The methylene chloride extract was dried with sodium sulfate, filtered, concentrated, and distilled. The product acetate (321 grams, 91%) was collected at 81-82° C./50 mm Hg. 
     (c) 2-(Trifluoromethyl)acrylonitrile: A total of 320 grams (1.7 mol) of the cyanohydrin acetate prepared in part (b) was passed in a slow stream of nitrogen through a heated (500-550° C.) glass tube packed with glass tubing pieces. The pyrolysis product was collected via trap cooled in liquid nitrogen. The crude pyrolysis product was purified by fractional distillation to yield 139 grams (65%) of pure 2-(trifluoromethyl)acrylonitrile (TFMAN). Fractions with purity greater than 99.5% (GC) were used for polymerization: bp 75-77° C. [lit. bp 75.9-76.2° C. (759 mm)];  1 H NMR (60 MHz, neat) δ 6.62 and 6.70 (CH 2 );  13 C NMR (20 MHz, CDCl 3 ) δ 112.2 (CN), 115.4 (αC) (J CF ≅40 Hz), 120.3 (CF 3 ) (J CF =272.5 Hz), 138.4 (CH 2 ) (J CF =3.8 Hz). 
     EXAMPLE 2 
     Preparation of 2-(Trifluoromethyl)acrylic acid (TFMAA) 
     (a) 3-Hydroxy-2-(trifluoromethyl)propionic acid: 2-(Trifluoromethyl)acrylonitrile (60 grams, 0.496 mol) was added dropwise to 169 grams of sulfuric acid (1.725 mol) which was stirred mechanically and heated to 120-130° C. After the addition was complete the mixture was stirred for an additional 30 minutes at 120-130° C. The mixture was allowed to cool slightly, 170 mL of water was added, and the reaction heated for 5 hours at 120° C. The cooled reaction mixture was extracted with several portions of diethyl ether, the ether extracts washed with brine, dried with sodium sulfate, filtered and, evaporated to yield 68.5 grams (86%) of the hydroxy-acid as a colorless syrup. 
     (b) 2-(Trifluoromethyl)acrylic acid (TFMAA): Syrupy 3-hydroxy-2-(trifluoromethyl) propionic acid (73 grams, 0.459 mol) was slowly added to 24 grams of phosphorus pentoxide (0.085 mol) while avoiding excessive heat evolution. The mixture was cautiously heated to 160-200° C. while collecting the product with an air-cooled short path distillation apparatus as it disilled from the mixture. The solid product was redistilled, collecting pure product fractions (b. p. 157-160° C.) totaling 50.5 grams (79%). 
     EXAMPLE 3 
     Preparation of Methyl α-(Trifluoromethyl)acrylate (MTFMA) 
     Methyl α-(trifluoromethyl)acrylate was synthesized by a modification of the method described by Buxton (1954)  J. Chem. Soc ., p. 366, as follows: Direct methanolysis of the cyanohydrin of 1,1,1-trifluoroacetone (prepared in Example 1, part (a)) gave methyl α-hydroxy-α-(trifluoromethyl)propionate in ca. 70% yield. The α-hydroxyl group was esterified by treating the propionate with acetic anhydride, triethylamine, and 4-(dimethylamino)pyridine in methylene chloride to give methyl α-acetoxy-α-(trifluoromethyl)propionate in ca. 75% yield. Pyrolysis of the ester acetate yielded MTFMA in ca. 62% yield. After ether extraction of the crude pyrolysis product, MTFMA was purified by fractional distillation and purity assayed by gas chromatography (GC). Fractions purer than 99.5% were used for all polymerizations: bp 104-105° C. (lit. bp 103.8-105° C.;  1 H NMR (60 MHz, CDCl 3 ) δ 3.80 (CH 3 O), 6.40 and 6.67 (CH 2 );  13 C NMR (20 MHz, CDCl 3 ) δ 53.2 (CH 3 O); 122.2 (CF 3 ) (J CF =274 Hz), 133.2 (CH 2 ) (J CF ≅5 Hz), 133.2 (δC), (J CF ≅50 Hz), 162.7 (C=O). 
     EXAMPLE 4 
     Preparation of t-Butyl 2-(trifluoromethyl)acrylate (TBTFMA) 
     Oxalyl chloride (87.3 g, 0.68 mol) was added to a solution of 2-(trifluoromethyl)-acrylic acid (TFMAA) (15.0 g, 0.11 mol) in methylene chloride (90 mL) in a dropwise manner at room temperature under an nitrogen atmosphere. The solution was heated to reflux for 5 hours, and the solvent and excess reagent were removed on a rotary evaporator. The resulting acid chloride was added to a stirred solution consisting of t-butanol (44 g, 0.59 mmol) and pyridine (33.6 g, 0.42 mol) in methylene chloride at 0° C. over a 10 min period. After stirring overnight at room temperature, the solution was neutralized with diluted aqueous HCl and washed with saturated aqueous sodium bicarbonate. The organic phase was removed, and the aqueous phase was extracted with pentane. The combined organic solvents were reduced to a small volume by distillation and then the residue was chromatographed on a column of silica gel. The product was re-purified by distillation to give 14.0 g of TBTFMA as a colorless liquid (70%). 
     EXAMPLE 5 
     
       
                 
         
             
             
         
      
     
     Preparation of Norbornene-BOCHF 
     Norbornene-5-(1,1,1-trifluoro-2-trifluoromethyl-2-t-butoxycarbonyloxypropane) (“norbornene-BOCHF” or “NB-BOCHF”) was synthesized as illustrated in Scheme 1. 
     In a 500 mL steel autoclave were placed cyclopentadiene (22.00 g, 0.33 mol), 1 (109.00 g, 0.389 mol), and hydroquinone (450 mg) under a slight argon pressure. The reactor was heated at 170° C. for 17 hrs. The unreacted fluoro compound was distilled out and the desired product 2 was isolated and purified by distillation under a reduced pressure. Yield of 2 (exo/endo=1/4): 28.60 g (31%). bp: 105-106° C./55 torr. Anal. Calcd. for C 11 H 12 F 6 O: C, 48.18; H, 4.41; F, 41.50. Found: C, 48.11; H, 4.53; F, 41.47.  13 C NMR (62.9 MHz, CD 2 Cl 2 ): 32.9 (exo CH), 33.0 (endo CH), 34.5 (endo CH 2 ), 35.1 (exo CH 2 ), 35.4 (endo CH 2 ), 37.1 (exo CH 2 ), 77.48 (exo COH, J CF =28.5 Hz), 77.56 (endo COH, J CF =28.5 Hz), 124.3 (CF 3 , J CF =286.9 Hz), 132.6 (endo CH═), 137.0 (exo CH═), 137.5 (exo CH═), 138.9 (endo CH═). 
     The OH functionality of 2 was protected with a t-butoxycarbonyl (tBOC) group. To a suspension of 0.80 g (0.033 mol) sodium hydride (60% in mineral oil) in 40 mL anhydrous tetrahydrofuran (THF) cooled with an ice bath was slowly added a solution of 1a (9.00 g, 0.033 mol) in 10 mL of THF. A vigorous hydrogen evolution occurred. After the addition was completed and the gas ceased to evolve, the cooling bath was removed and a solution of 7.40 g (0.033 mol) of di-t-butyl dicarbonate in 10 mL of THF was added dropwise to the alcoholate solution. Stirring was continued for 18 hrs. The reaction mixture was washed with water until the aqueous layer became neutral and then twice with a brine solution. The organic layer was dried over magnesium sulfate, the solvent removed, and the colorless oil distilled in vacuo to give norbornene-5-(1,1,1-trifluoro-2-trifluoromethyl-2-t-butoxycarbonyloxypropane) 3, NB-BOCHF. Yield: 7.8 g (63%). bp: 63-64° C./4 torr. Anal. Calcd. for C 16 H 20 F 6 O 3 : C, 51.34; H, 5.39; F, 30.45. Found: C, 51.76; H, 5.46; F, 34.41.  13 C NMR (62.9 MHz, CD 2 Cl 2 ): 27.7 (CH 3 ), 32.2 (endo CH 2 ), 32.7 (endo CH), 33.2 (exo CH 2 ), 32.85 (exo CH), 33.2 (exo CH 2 ), 34.9 (endo CH 2 ), 35.3 (exo CH 2 ), 42.4 (exo CH), 43.3 (endo CH), 45.9 (exo CH 2 ), 47.5 (endo CH), 48.25 (exo CH), 50.0 (endo CH 2 ), 83.67 (exo  C (CF 3 ) 2 , J CF =28.5 Hz), 83.79 (endo  C (CF 3 ) 2 , J CF 28.5 Hz), 85.3 ( C (CH 3 ) 3 ), 122.6 (CF 3 , J CF =289.9 Hz), 132.3 (endo CH═), 136.7 (exo CH═), 137.5 (endo CH═), 139.1 (endo CH═), 149.47 (endo C═O), 149.52 (exo C═O). 
     EXAMPLE 6 
     Synthesis of Norbornene-5-(2-trifluoromethyl)propan-2-ol (NB-TFOH) 
     (a) 2-(Trifluoromethyl)pent-4-en-2-ol/tetrahydrofuran complex: 1,1,1-Trifluoroacetone was added dropwise to 500 mL of a mechanically stirred 2 M solution of allylmagnesium chloride in THF while maintaining the temperature below 15° C. The mixture was warmed to room temperature and stirred overnight. The mixture was then cooled to 4° C. as 250 mL of 6 N hydrochloric acid was added dropwise along with a 250 mL portion of diethyl ether added as the mixture began to solidify. The resulting slurry was filtered and the solid washed with three 100 mL portions of diethyl ether. The ether was removed under reduced pressure (100 mm Hg) and the residue distilled. The product distilled as a THF complex (b.p.: 105-115° C.) yielding 143 grams (ca 75%) of the alcohol/THF complex. 
     (b) Norbornene-5-(2-trifluoromethyl)propan-2-ol: In a 500 mL steel autoclave were placed 33.6 grams (0.508 mol) of freshly distilled cyclopentadiene, 113.9 grams (0.594 mol) 2-(trifluoromethyl) pent-4-en-2-ol/tetrahydrofuran complex, and 600 milligrams of hydroquinone under a slight overpressure of nitrogen. The reactor was heated 17 hours at an internal temperature of 170° C. The unreacted starting alcohol/THF was removed by distillation at atmospheric pressure and the residue distilled under high vacuum(b.p.:60-65° C./150-200 milliTorr) to yield 40.9 grams (37%) of product. The recovered alcohol/THF complex could be retreated with cyclopentadiene as above to yield additional product. 
     EXAMPLE 7 
     Free Radical Copolymerization of TFMAA with Norbornene (NB) 
     TFMAA (0.056 g, 0.46 mmol) and NB (0.037 g, 0.45 mmol) were dissolved in dioxane (0.5215 g) along with AIBN (0.0071 g, 0.043 mmol) as initiator. The mixture was deaerated by repeating freeze-thaw cycles and heated at 70° C. under nitrogen for about 24 hours. The polymer was isolated by precipitation in hexanes, providing 72% yield. The copolymer contained TFMAA and NB at a ratio of 65:35, had a number average molecular weight of 2,200 and a weight average molecular weight of 2,900. 
     EXAMPLE 8 
     Radical Polymerization of TBTFMA with Norbornene and NB-HFOH 
     Radical polymerization of TBTFMA (8.71 g, 52.6 mmol), norbornene (NB) (1.98 g, 25 mmol), and acetyl-protected NB-HFOH (7.92 g, 25 mmol) were carried out in ethyl acetate (3 mL) with AIBN (400 mg) as an initiator at 75° C. under nitrogen atmosphere. Additional AIBN was added periodically during polymerization. After 10 days, the viscous reaction mixture was poured into cold methanol, and the precipitates were filtered, redissolved in ethyl acetate and re-precipitated in cold methanol. The polymer was dried at 90° C. overnight under vacuum (9.20 g, 45%). 
     A solution of 5% of KOH in THF/water (30 mL/5 mL) was added to a solution of the polymer (3.50 g) in THF (10 mL). The mixture was heated at 50° C. under nitrogen atmosphere for 3 hours. The reaction mixture was cooled to room temperature and diluted with saturated aqueous NaCl. The organic phase was removed. The aqueous phase was extracted with methylene chloride, and the combined organic phase was concentrated to a small volume, which was dropwise added to pentane. The precipitates were filtered and dried at 90° C. overnight under vacuum (1.8 g). 
     EXAMPLE 9 
     Free Radical Copolymerization of Other Monomers with Norbornene and Norbornene Analogs 
     The polymerization method of Example 6 was repeated using MTFMA and NB-BOCHF at a feed ratio of 53/47, resulting in a copolymer containing the two nonomers at a ration of 69/31. Yield: 20%.