Ester compounds, polymers, resist compositions and patterning process

Abstract: An ester compound of the following formula (1) is provided. R 1 is H, methyl or CH 2 CO 2 R 3 , R 2 is H, methyl or CO 2 R 3 , R 3 is C 1 -C 15 alkyl, R 4 is branched or cyclic, tertiary C 5 -C 20 alkyl group, Z is a divalent C 1- C 10 hydrocarbon group, and k is 0 or 1. A resist composition comprising as the base resin a polymer resulting from the ester compound is sensitive to high-energy radiation, has excellent sensitivity, resolution, and etching resistance, and is suited for micropatterning using electron beams or deep-UV.

Description: The present invention relates to (i) an ester compound, (ii) a polymer comprising units of the ester compound which is blended as a base resin to formulate a resist composition having a high sensitivity and resolution, and in particular, suitable as micropatterning material for VLSI fabrication, (iii) a resist composition comprising the polymer, and (iv) a patterning process using the resist composition. While a number of recent efforts have been made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, deep-ultraviolet lithography is thought to hold particular promise as the next generation in microfabrication technology. In particular, photolithography using a KrF or ArF excimer laser as the light source is strongly desired to reach the practical level as the micropatterning technique capable of achieving a feature size of 0.3 µm or less. For resist materials for use with a KrF excimer lasers, polyhydroxystyrene having a practical level of transmittance and etching resistance is, in fact, a standard base resin. For resist materials for use with ArF excimer lasers, polyacrylic or polymethacrylic acid derivatives and polymers containing aliphatic cyclic compounds in the backbone are under investigation. All these polymers have advantages and disadvantages, and none of them have been established as the standard base resin. More particularly, resist compositions using derivatives of polyacrylic or polymethacrylic acid give relatively satisfactory results with respect to resolution, but have extremely low etching resistance and are impractical because the resin backbone is weak. On the other hand, resist compositions using polymers containing alicyclic compounds in their backbone have a practically acceptable level of etching resistance because the resin backbone is robust, but are very low in resolution because the reactivity of acid-decomposable protective groups is extremely low as compared with those on the acrylic polymers. They are impractical as well. While a finer pattern rule is being demanded, there is a need to have a highly transparent resin which has a high reactivity enough to provide a satisfactory resolution, and at the same time, a robustness enough to provide a sufficient etching resistance. EP-A-0 930 541 and WO 99/14635 disclose radiation-sensitive polymers and esters for use in producing them which differ from those of the present invention in the nature of the acid-labile carboxyl protective group. The EP document teaches tert-butyl. The WO document discloses various tertiary cycloalkyl groups. Preferred embodiments of the present invention may enable one to provide (i) an ester compound capable of forming an acid-decomposable, robust polymer, (ii) a polymer which is blended as a base resin to formulate a resist composition having a higher sensitivity, resolution and etching resistance than conventional resist compositions, (iii) a resist composition comprising the polymer as a base resin, and (iv) a patterning process using the resist composition. The inventor has found that preferred ester compounds of the following general formula (1) obtainable by the method to be described later may be useful in preparing an acid-decomposable, robust polymer; that a resist composition comprising as the base resin a polymer prepared from the ester compound to a weight average molecular weight of 1,000 to 500,000 may have a high sensitivity, resolution and etching resistance; and that this resist composition lends itself to precise micropatterning. In a first aspect, the invention provides an ester compound of the following general formula (1). Herein R 1 is hydrogen, methyl or CH 2 CO 2 R 3 , R 2 is hydrogen, methyl or CO 2 R 3 , R 3 is a straight, branched or cyclic alkyl group of 1 to 15 carbon atoms, Z is a divalent hydrocarbon group of 1 to 10 carbon atoms, k is 0 or 1 and R 4 is a cyclic tertiary alkyl group of up to 20 carbon atoms, of the formula (L3) or (L4) shown below. In a second aspect, the invention provides a polymer comprising recurring units of the following general formula (1a-1) or (1a-2) derived from an ester compound of the general formula (1) as defined above, and having a weight average molecular weight of 1,000 to 500,000. Herein R 1 , R 2 , R 4 , Z and k are as defined above. In a preferred embodiment, the polymer further comprises recurring units of at least one of the following formulas (2a) to (10a). Herein R 1 , R 2 and k are as defined above; R 5 is hydrogen or a carboxyl or hydroxyl-containing monovalent hydrocarbon group of 1 to 15 carbon atoms; at least one of R 6 to R 9 is a carboxyl or hydroxyl-containing monovalent hydrocarbon group of 1 to 15 carbon atoms, and the remainders are independently hydrogen or a straight, branched or cyclic alkyl group of 1 to 15 carbon atoms, or R 6 to R 9 , taken together, may form a ring, and when they form a ring, at least one of R 6 to R 9 is a carboxyl or hydroxyl-containing divalent hydrocarbon group of 1 to 15 carbon atoms, and the remainders are independently a single bond or a straight, branched or cyclic alkylene group of 1 to 15 carbon atoms; R 10 is a monovalent hydrocarbon group of 3 to 15 carbon atoms containing a -CO 2 - partial structure; at least one of R 11 to R 14 is a monovalent hydrocarbon group of 2 to 15 carbon atoms containing a -CO 2 - partial structure, and the remainders are independently hydrogen or a straight, branched or cyclic alkyl group of 1 to 15 carbon atoms, or R 11 to R 14 , taken together, may form a ring, and when they form a ring, at least one of R 11 to R 14 is a divalent hydrocarbon group of 1 to 15 carbon atoms containing a -CO 2 - partial structure, and the remainders are independently a single bond or a straight, branched or cyclic alkylene group of 1 to 15 carbon atoms; R 15 is a polycyclic hydrocarbon group of 7 to 15 carbon atoms or an alkyl group containing such a polycyclic hydrocarbon group; R 16 is an acid labile group; X is -CH 2 - or -O-; and Y is -O- or - (NR 17 ) - wherein R 17 is hydrogen or a straight, branched or cyclic alkyl group of 1 to 15 carbon atoms. In a third aspect, the invention provides a resist composition comprising the polymer defined above, and more specifically, a resist composition comprising the polymer defined above, a photoacid generator, and an organic solvent. In a fourth aspect, the invention provides a process for forming a resist pattern comprising the steps of applying the resist composition defined above onto a substrate to form a coating; heat treating the coating and then exposing it to high-energy radiation or electron beams through a photo mask; and optionally heat treating the exposed coating and developing it with a developer. The polymer comprising recurring units of formula (1a-1) or (1a-2) originating from the ester compound of formula (1) is robust since it contains a bridged aliphatic ring in its backbone. The polymer is significantly improved in reactivity as well for the following reason. As described above, the polymer containing an alicyclic compound in its backbone has the general tendency that the reactivity of acid-eliminatable protective groups is low. However, the inventors have found that when an acid-eliminatable site is located at a position spaced from the backbone of the polymer, the reactivity is outstandingly improved possibly because of the increased probability of contact with acid. Then the polymer comprising recurring units of formula (1a-1) or (1a-2) has an acid elimination reactivity surpassing prior art polymers comprising recurring units of formula (0a-1) or (0a-2) below. Then a resist composition comprising the polymer as a base resin possesses both a high etching resistance due to the robustness of the resin and a satisfactory resolution due to the high reactivity of acid-decomposable groups and is thus quite useful in micropatterning. Herein, R 1 , R 2 , R 4 and k are as defined above. BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a diagram showing the 1 H-NMR spectrum of Monomer 1 obtained in Synthesis Example 1-1. FIG. 2 is a diagram showing the FT-IR of Monomer 1. DESCRIPTION OF THE PREFERRED EMBODIMENT Ester compound The novel ester compounds according to the first aspect of the invention are of the following general formula (1). Herein, R 1 is hydrogen, methyl our CH 2 CO 2 R 3 . R 2 is hydrogen, methyl or CO 2 R 3 . R 3 stands for straight, branched or cyclic alkyl groups of 1 to 15 carbon atoms, such as, for example, methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, tert-butyl, tert-amyl, n-pentyl, n-hexyl, cyclopentyl, cyclohexyl, ethylcyclopentyl, butylcyclopentyl, ethylcyclohexyl, butylcyclohexyl, adamantyl, ethyladamantyl and butyladamantyl. R 4 stands for a cyclic, tertiary alkyl group of up to 20 carbon atoms, of the following formulas (L3) and (L4). In formulas (L3) and (L4) and throughout the specification, the broken line denotes a valence bond. R L05 is a straight, branched or cyclic alkyl group of 1 to 8 carbon atoms or a substituted or unsubstituted aryl group of 6 to 20 carbon atoms. Examples of the straight, branched or cyclic alkyl group include methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, tert-butyl, tert-amyl, n-pentyl and n-hexyl. Exemplary aryl groups are phenyl, methylphenyl, naphthyl, anthryl, phenanthryl, and pyrenyl. Letter m is equal to 0 or 1, n is equal to 0, 1, 2 or 3, and 2m+n is equal to 2 or 3. R L06 is a straight, branched or cyclic alkyl group of 1 to 8 carbon atoms or a substituted or unsubstituted aryl group of 6 to 20 carbon atoms. Examples of these groups are the same as exemplified for R L05 . R L07 to R L16 independently represent hydrogen or monovalent hydrocarbon groups of 1 to 15 carbon atoms which may contain a hetero atom. Exemplary hydrocarbon groups are straight, branched or cyclic alkyl groups such as methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, tert-butyl, tert-amyl, n-pentyl, n-hexyl, n-octyl, n-nonyl, n-decyl, cyclopentyl, cyclohexyl, cyclopentylmethyl, cyclopentylethyl, cyclopentylbutyl, cyclohexylmethyl, cyclohexylethyl and cyclohexylbutyl, and substituted ones of these groups in which some hydrogen atoms are replaced by hydroxyl, alkoxy, carboxy, alkoxycarbonyl, oxo, amino, alkylamino, cyano, mercapto, alkylthio, sulfo or other groups. Alternatively, R L07 to R L16 , taken together, form a ring, in which case each of said two of R L07 to R L16 represents a divalent C 1 -C 15 hydrocarbon group which may contain a hetero atom, when they form a ring. Examples include the ones exemplified above for the monovalent hydrocarbon groups, with one hydrogen atom being eliminated. Two of R L07 to R L16 which are attached to adjoining carbon atoms (for example, a pair of R L07 and R L09 , R L09 and R L15 , R L13 and R L15 , or a similar pair) may bond together directly to form a double bond. Examples of the acid labile groups of formula (L3) include 1-methylcyclopentyl, 1-ethylcyclopentyl, 1-n-propylcyclopentyl, 1-isopropylcyclopentyl, 1-n-butylcyclopentyl, 1-sec-butylcyclopentyl, 1-cyclopentylcyclopentyl, 1-cyclohexylcyclopentyl, 1-(2-bicyclo[2.2.1]-heptyl)cyclopentyl, 1-methylcyclohexyl, 1-ethylcyclohexyl, 3-methyl-1-cyclopenten-3-yl, 3-ethyl-1-cyclopenten-3-yl, 3-methyl-1-cyclohexen-3-yl, and 3-ethyl-1-cyclohexen-3-yl groups. The acid labile groups of formula (L4) are exemplified by the following groups. Z stands for divalent hydrocarbon groups of 1 to 10 carbon atoms, preferably straight or branched alkylene groups, such as, for example, methylene, ethylidene, ethylene, propylidene, propylene, trimethylene, tetramethylene, pentamethylene, hexamethylene, and heptamethylene. The letter k is 0 or 1. Illustrative, non-limiting, examples of the ester compounds of the invention are given below. The ester compound of the invention is prepared, for example, by process A entailing esterification reaction of a carboxylic acid of formula (11) with a halogenated alkyl carboxylate of formula (12) or process B entailing esterification reaction of a carboxylic halide of formula (13) with a hydroxyalkyl carboxylate of formula (14), although the preparation is not limited thereto. Herein, R 1 , R 2 , R 4 , Z and k are as defined above, and X is bromine or chlorine. The esterification reaction of process A readily takes place under well-known conditions, preferably by mixing the reactants in a solvent such as tetrahydrofuran, N,N-dimethylformamide or dimethyl sulfoxide in the presence of a base such as potassium carbonate or potassium tert-butoxide, while heating the reaction mixture if desired. The esterification reaction of process B readily takes place under well-known conditions, preferably by mixing the reactants in a solvent such as benzene in the presence of a dehydrating agent such as dicyclohexylcarbodiimide, while heating the reaction mixture if desired. It is noted that the halogenated alkyl carboxylate of formula (12) and the hydroxyalkyl carboxylate of formula (14) can be prepared by combinations of well-known reactions although the optimum method used differs depending on the structure of Z. For example, the halogenated alkyl carboxylate of formula (12-1) wherein Z is -CH 2 - and the hydroxyalkyl carboxylate of formula (14-1) wherein Z is -CH 2 CH(CH 3 )- can be prepared according to the following schemes. Polymer In the second aspect, the invention provides a polymer or high molecular weight compound comprising recurring units of the following general formula (1a-1) or (1a-2) derived from the ester compound of the general formula (1), and having a weight average molecular weight of 1,000 to 500,000, and preferably 5,000 to 100,000. Herein R 1 , R 2 , R 4 , Z and k are as defined above. The polymer of the invention may further comprise recurring units of at least one type selected from recurring units of the following formulas (2a) to (10a) which are derived from monomers of the following general formulas (2) to (10). In the above formulae, X is -CH 2 - or -O-; Y is -O- or -(NR 17 )- wherein R 17 is hydrogen or a straight, branched or cyclic alkyl group of 1 to 15 carbon atoms; and k is equal to 0 or 1. Then formulae (6a-1) to (9a-2) may also be represented by the following formulae (6a-1-1) to (9a-2-2). Herein R 1 and R 2 are as defined above. R 5 stands for hydrogen or carboxyl or hydroxyl-containing monovalent hydrocarbon groups of 1 to 15 carbon atoms, for example, carboxyethyl, carboxybutyl, carboxycyclopentyl, carboxycyclohexyl, carboxynorbornyl, carboxyadamantyl, hydroxyethyl, hydroxybutyl, hydroxycyclopentyl, hydroxycyclohexyl, hydroxynorbornyl, and hydroxyadamantyl. At least one of R 6 to R 9 is a carboxyl or hydroxyl-containing monovalent hydrocarbon group of 1 to 15 carbon atoms, and the remaining R's are independently hydrogen or a straight, branched or cyclic alkyl group of 1 to 15 carbon atoms. Examples of the carboxyl or hydroxyl-bearing monovalent hydrocarbon group of 1 to 15 carbon atoms include carboxy, carboxymethyl, carboxyethyl, carboxybutyl, hydroxymethyl, hydroxyethyl, hydroxybutyl, 2-carboxyethoxycarbonyl, 4-carboxybutoxycarbonyl, 2-hydroxyethoxycarbonyl, 4-hydroxybutoxycarbonyl, carboxycyclopentyloxycarbonyl, carboxycyclohexyloxycarbonyl, carboxynorbornyloxycarbonyl, carboxyadamantyloxycarbonyl, hydroxycyclopentyloxycarbonyl, hydroxycyclohexyloxycarbonyl, hydroxynorbornyloxycarbonyl, and hydroxyadamantyloxycarbonyl. Examples of the straight, branched or cyclic alkyl group of 1 to 15 carbon atoms are the same as exemplified for R 3 . R 6 to R 9 , taken together, may form a ring, and in that event, at least one of R 6 to R 9 is a divalent hydrocarbon group of 1 to 15 carbon atoms having a carboxyl or hydroxyl group, while the remaining R's are independently single bonds or straight, branched or cyclic alkylene groups of 1 to 15 carbon atoms. Examples of the carboxyl or hydroxyl-bearing divalent hydrocarbon group of 1 to 15 carbon atoms include the groups exemplified as the carboxyl or hydroxyl-bearing monovalent hydrocarbon group, with one hydrogen atom eliminated therefrom. Examples of the straight, branched or cyclic alkylene groups of 1 to 15 carbon atoms include the groups exemplified for R 3 , with one hydrogen atom eliminated therefrom. R 10 is a monovalent hydrocarbon group of 3 to 15 carbon atoms containing a -CO 2 - partial structure, for example, 2-oxooxolan-3-yl, 4,4-dimethyl-2-oxooxolan-3-yl, 4-methyl-2-oxooxan-4-yl, 2-oxo-1,3-dioxolan-4-ylmethyl, and 5-methyl-2-oxooxolan-5-yl. At least one of R 11 to R 14 is a monovalent hydrocarbon group of 2 to 15 carbon atoms containing a -CO 2 - partial structure, while the remaining R's are independently hydrogen or straight, branched or cyclic alkyl groups of 1 to 15 carbon atoms. Examples of the monovalent hydrocarbon group of 2 to 15 carbon atoms containing a -CO 2 - partial structure include 2-oxooxolan-3-yloxycarbonyl, 4,4-dimethyl-2-oxooxolan-3-yloxycarbonyl, 4-methyl-2-oxooxan-4-yloxycarbonyl, 2-oxo-1,3-dioxolan-4-ylmethyloxycarbonyl, and 5-methyl-2-oxooxolan-5-yloxycarbonyl. Examples of the straight, branched or cyclic alkyl groups of 1 to 15 carbon atoms are the same as exemplified for R 3 . R 11 to R 14 , taken together, may form a ring, and in that event, at least one of R 11 to R 14 is a divalent hydrocarbon group of 1 to 15 carbon atoms containing a -CO 2 - partial structure, while the remaining R's are independently single bonds or straight, branched or cyclic alkylene groups of 1 to 15 carbon atoms. Examples of the divalent hydrocarbon group of 1 to 15 carbon atoms containing a -CO 2 - partial structure include 1-oxo-2-oxapropane-1,3-diyl, 1,3-dioxo-2-oxapropane-1,3-diyl, 1-oxo-2-oxabutane-1,4-diyl, and 1,3-dioxo-2-oxabutane-1,4-diyl, as well as the groups exemplified as the monovalent hydrocarbon group containing a -CO 2 - partial structure, with one hydrogen atom eliminated therefrom. Examples of the straight, branched or cyclic alkylene groups of 1 to 15 carbon atoms include the groups exemplified for R 3 , with one hydrogen atom eliminated therefrom. R 15 is a polycyclic hydrocarbon group having 7 to 15 carbon atoms or an alkyl group containing a polycyclic hydrocarbon group, for example, norbornyl, bicyclo[3.3.1]-nonyl, tricyclo[5.2.1.0 2,6 ]decyl, adamantyl, ethyladamantyl, butyladamantyl, norbornylmethyl, and adamantylmethyl. R 16 is an acid labile group, and k is 0 or 1. X is -CH 2 - or -O-. Y is -O- or -(NR 17 )- wherein R 17 is hydrogen or a straight, branched or cyclic alkyl group of 1 to 15 carbon atoms, examples of which are as described for R 3 . The acid labile groups represented by R 16 may be selected from a variety of such groups. Examples of the acid labile group are groups of the following general formulae (L1) to (L4), tertiary alkyl groups of 4 to 20 carbon atoms, preferably 4 to 15 carbon atoms, trialkylsilyl groups in which each alkyl moiety has 1 to 6 carbon atoms, and oxoalkyl groups of 4 to 20 carbon atoms. R L01 and R L02 are hydrogen or straight, branched or cyclic alkyl groups of 1 to 18 carbon atoms, preferably 1 to 10 carbon atoms. Exemplary alkyl groups include methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, tert-butyl, cyclopentyl, cyclohexyl, 2-ethylhexyl, and n-octyl. R L03 is a monovalent hydrocarbon group of 1 to 18 carbon atoms, preferably 1 to 10 carbon atoms, which may contain a hetero atom such as oxygen, examples of which include unsubstituted straight, branched or cyclic alkyl groups and straight, branched or cyclic alkyl groups in which some hydrogen atoms are replaced by hydroxyl, alkoxy, oxo, amino, alkylamino or the like. Illustrative examples are the substituted alkyl groups shown below. A pair of R L01 and R L02 , R L01 and R L03 , or R L02 and R L03 may form a ring. Each of R L01 , R L02 and R L03 is a straight or branched alkylene group of 1 to 18 carbon atoms, preferably 1 to 10 carbon atoms when they form a ring. R L04 is a tertiary alkyl group of 4 to 20 carbon atoms, preferably 4 to 15 carbon atoms, a trialkylsilyl group in which each alkyl moiety has 1 to 6 carbon atoms, an oxoalkyl group of 4 to 20 carbon atoms, or a group of formula (L1). Exemplary tertiary alkyl groups are tert-butyl, tert-amyl, 1,1-diethylpropyl, 1-ethylcyclopentyl, 1-butylcyclopentyl, 1-ethylcyclohexyl, 1-butylcyclohexyl, 1-ethyl-2-cyclopentenyl, 1-ethyl-2-cyclohexenyl, and 2-methyl-2-adamantyl. Exemplary trialkylsilyl groups are trimethylsilyl, triethylsilyl, and dimethyl-tert-butylsilyl. Exemplary oxoalkyl groups are 3-oxocyclohexyl, 4-methyl-2-oxooxan-4-yl, and 5-methyl-5-oxooxolan-4-yl. Letter y is an integer of 0 to 6. R L05 is a straight or branched alkyl group of 1 to 8 carbon atoms or a substituted or unsubstituted aryl group of 6 to 20 carbon atoms. Examples of the straight, branched or cyclic alkyl group include methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, tert-butyl, tert-amyl, n-pentyl and n-hexyl. Exemplary aryl groups are phenyl, methylphenyl, naphthyl, anthryl, phenanthryl, and pyrenyl. Letter m is equal to 0 or 1, n is equal to 0, 1, 2 or 3, and 2m+n is equal to 2 or 3. R L06 is a straight, branched or cyclic alkyl group of 1 to 8 carbon atoms or a substituted or unsubstituted aryl group of 6 to 20 carbon atoms. Examples of these groups are the same as exemplified for R L05 . R L07 to R L16 independently represent hydrogen or monovalent hydrocarbon groups of 1 to 15 carbon atoms which may contain a hetero atom. Exemplary hydrocarbon groups are straight, branched or cyclic alkyl groups such as methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, tert-butyl, tert-amyl, n-pentyl, n-hexyl, n-octyl, n-nonyl, n-decyl, cyclopentyl, cyclohexyl, cyclopentylmethyl, cyclopentylethyl, cyclopentylbutyl, cyclohexylmethyl, cyclohexylethyl and cyclohexylbutyl, and substituted ones of these groups in which some hydrogen atoms are replaced by hydroxyl, alkoxy, carboxy, alkoxycarbonyl, oxo, amino, alkylamino, cyano, mercapto, alkylthio, sulfo or other groups. Alternatively, R L07 to R L16 , taken together, form a ring (for example, a pair of R L07 and R L08 , R L07 and R L09 , R L08 and R L10 , R L09 and R L10 , R L11 and R L12 , R L13 and R L14 , or a similar pair form a ring). Each of R L07 to R L16 represents a divalent C 1 -C 15 hydrocarbon group which may contain a hetero atom, when they form a ring, examples of which are the ones exemplified above for the monovalent hydrocarbon groups, with one hydrogen atom being eliminated. Two of R L07 to R L16 which are attached to adjoining carbon atoms (for example, a pair of R L07 and R L09 , R L09 and R L15 , R L13 and R L15 , or a similar pair) may bond together directly to form a double bond. Of the acid labile groups of formula (L1), the straight and branched ones are exemplified by the following groups. Of the acid labile groups of formula (L1), the cyclic ones are, for example, tetrahydrofuran-2-yl, 2-methyltetrahydrofuran-2-yl, tetrahydropyran-2-yl, and 2-methyltetrahydropyran-2-yl. Examples of the acid labile groups of formula (L2) include tert-butoxycarbonyl, tert-butoxycarbonylmethyl, tert-amyloxycarbonyl, tert-amyloxycarbonylmethyl, 1,1-diethylpropyloxycarbonyl, 1,1-diethylpropyloxycarbonylmethyl, 1-ethylcyclopentyloxycarbonyl, 1-ethylcyclopentyloxycarbonylmethyl, 1-ethyl-2-cyclopentenyloxycarbonyl, 1-ethyl-2-cyclopentenyloxycarbonylmethyl, 1-ethoxyethoxycarbonylmethyl, 2-tetrahydropyranyloxycarbonylmethyl, and 2-tetrahydrofuranyloxycarbonylmethyl groups. Examples of the acid labile groups of formula (L3) include 1-methylcyclopentyl, 1-ethylcyclopentyl, 1-n-propylcyclopentyl, 1-isopropylcyclopentyl, 1-n-butylcyclopentyl, 1-sec-butylcyclopentyl, 1-methylcyclohexyl, 1-ethylcyclohexyl, 3-methyl-1-cyclopenten-3-yl, 3-ethyl-1-cyclopenten-3-yl, 3-methyl-1-cyclohexen-3-yl, and 3-ethyl-1-cyclohexen-3-yl groups. The acid labile groups of formula (L4) are exemplified by the following groups. The polymer of the invention may be prepared by polymerizing an ester compound of formula (1) or by copolymerizing a first monomer in the form of an ester compound of formula (1) with a second monomer in the form of at least one compound of formulas (2) to (10). By properly adjusting the proportion of the respective monomers used in the copolymerization reaction, the polymer can be tailored so that it may exert the preferred performance when blended in resist compositions. In addition to (i) the monomer of formula (1) and (ii) the monomer or monomers of formulas (2) to (10), the polymer of the invention may have copolymerized therewith (iii) another monomer having a carbon-to-carbon double bond other than (i) and (ii). Examples of the additional monomer (iii) include substituted acrylic acid esters such as methyl methacrylate, methyl crotonate, dimethyl maleate, and dimethyl itaconate, unsaturated carboxylic acids such as maleic acid, fumaric acid and itaconic acid, substituted or unsubstituted norbornenes such as norbornene and methyl norbornene-5-carboxylate, and unsaturated acid anhydrides such as itaconic anhydride. The polymers of the invention may contain (I) more than 0 mol% to 100 mol%, preferably 20 to 90 mol%, more preferably 30 to 80 mol% of units of formula (1a-1) or (1a-2) derived from the monomer of formula (1), (II) 0 mol% to less than 100 mol%, preferably 1 to 95 mol%, more preferably 5 to 90 mol% of units of one or more types of formulae (2a) to (10a) derived from the monomers of formulae (2) to (10), and optionally, (III) 0 to 80 mol%, preferably 0 to 70 mol%, more preferably 0 to 50 mol% of units of one or more types derived from the additional monomers (iii). The polymers of the invention have a weight average molecular weight of 1,000 to 500,000, preferably 3,000 to 100,000. Outside the range, the etching resistance may become extremely low and the resolution may become low because a substantial difference in rate of dissolution before and after exposure is lost. Preparation of the polymer according to the invention is conducted by effecting polymerization in a well-known way between an ester compound of formula (1) and another compound having a carbon-to-carbon double bond, which is typically selected from the above-described monomers (ii) and (iii). The preferred polymerization reaction is radical polymerization, anionic polymerization or coordination polymerization although various other reactions are possible. For radical polymerization, preferred reaction conditions include (a) a solvent selected from among hydrocarbons such as benzene, ethers such as tetrahydrofuran, alcohols such as ethanol, and ketones such as methyl isobutyl ketone, (b) a polymerization initiator selected from azo compounds such as 2,2'-azobisisobutyronitrile and peroxides such as benzoyl peroxide and lauroyl peroxide, (c) a temperature of about 0°C to about 100°C, and (d) a time of about 1/2 hour to about 48 hours. Reaction conditions outside the described range may be employed if desired. For anionic polymerization, preferred reaction conditions include (a) a solvent selected from among hydrocarbons such as benzene, ethers such as tetrahydrofuran, and liquid ammonia, (b) a polymerization initiator selected from metals such as sodium and potassium, alkyl metals such as n-butyllithium and sec-butyllithium, ketyl, and Grignard reagents, (c) a temperature of about -78°C to about 0°C, (d) a time of about 1/2 hour to about 48 hours, and (e) a stopper selected from among proton-donative compounds such as methanol, halides such as methyl iodide, and electrophilic compounds. Reaction conditions outside the described range may be employed if desired. For coordination polymerization, preferred reaction conditions include (a) a solvent selected from among hydrocarbons such as n-heptane and toluene, (b) a catalyst selected from Ziegler-Natta catalysts comprising a transition metal (e.g., titanium) and alkylaluminum, Phillips catalysts of metal oxides having chromium or nickel compounds carried thereon, and olefin-metathesis mixed catalysts as typified by tungsten and rhenium mixed catalysts, (c) a temperature of about 0°C to about 100°C, and (d) a time of about 1/2 hour to about 48 hours. Reaction conditions outside the described range may be employed if desired. Resist composition Since the polymer of the invention is useful as the base polymer of a resist composition, especially a chemically amplified positive resist composition, the other aspect of the invention provides a resist composition comprising the polymer. Preferably, the resist composition is defined as comprising the polymer, a photoacid generator, and an organic solvent. Photoacid generator The photoacid generator is a compound capable of generating an acid upon exposure to high energy radiation or electron beams and includes the following: (i) onium salts of the formula (P1a-1), (P1a-2) or (P1b), (ii) diazomethane derivatives of the formula (P2), (iii) glyoxime derivatives of the formula (P3), (iv) bissulfone derivatives of the formula (P4), (v) sulfonic acid esters of N-hydroxyimide compounds of the formula (P5), (vi) β-ketosulfonic acid derivatives, (vii) disulfone derivatives, (viii) nitrobenzylsulfonate derivatives, and (ix) sulfonate derivatives. These photoacid generators are described in detail. (i) Onium salts of formula (P1a-1), (P1a-2) or (P1b): Herein, R 101a , R 101b , and R 101c independently represent straight, branched or cyclic alkyl, alkenyl, oxoalkyl or oxoalkenyl groups of 1 to 12 carbon atoms, aryl groups of 6 to 20 carbon atoms, or aralkyl or aryloxoalkyl groups of 7 to 12 carbon atoms, wherein some or all of the hydrogen atoms may be replaced by alkoxy or other groups. Also, R 101b and R 101c , taken together, may form a ring. R 101b and R 101c each are alkylene groups of 1 to 6 carbon atoms when they form a ring. K - is a non-nucleophilic counter ion. R 101a , R 101b , and R 101c may be the same or different and are illustrated below. Exemplary alkyl groups include methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, tert-butyl, pentyl, hexyl, heptyl, octyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclopropylmethyl, 4-methylcyclohexyl, cyclohexylmethyl, norbornyl, and adamantyl. Exemplary alkenyl groups include vinyl, allyl, propenyl, butenyl, hexenyl, and cyclohexenyl. Exemplary oxoalkyl groups include 2-oxocyclopentyl and 2-oxocyclohexyl as well as 2-oxopropyl, 2-cyclopentyl-2-oxoethyl, 2-cyclohexyl-2-oxoethyl, and 2-(4-methylcyclohexyl)-2-oxoethyl. Exemplary aryl groups include phenyl and naphthyl; alkoxyphenyl groups such as p-methoxyphenyl, m-methoxyphenyl, o-methoxyphenyl, ethoxyphenyl, p-tert-butoxyphenyl, and m-tert-butoxyphenyl; alkylphenyl groups such as 2-methylphenyl, 3-methylphenyl, 4-methylphenyl, ethylphenyl, 4-tert-butylphenyl, 4-butylphenyl, and dimethylphenyl; alkylnaphthyl groups such as methylnaphthyl and ethylnaphthyl; alkoxynaphthyl groups such as methoxynaphthyl and ethoxynaphthyl; dialkylnaphthyl groups such as dimethylnaphthyl and diethylnaphthyl; and dialkoxynaphthyl groups such as dimethoxynaphthyl and diethoxynaphthyl. Exemplary aralkyl groups include benzyl, phenylethyl, and phenethyl. Exemplary aryloxoalkyl groups are 2-aryl-2-oxoethyl groups such as 2-phenyl-2-oxoethyl, 2-(1-naphthyl)-2-oxoethyl, and 2-(2-naphthyl)-2-oxoethyl. Examples of the non-nucleophilic counter ion represented by K - include halide ions such as chloride and bromide ions, fluoroalkylsulfonate ions such as triflate, 1,1,1-trifluoroethanesulfonate, and nonafluorobutanesulfonate, arylsulfonate ions such as tosylate, benzenesulfonate, 4-fluorobenzenesulfonate, and 1,2,3,4,5-pentafluorobenzenesulfonate, and alkylsulfonate ions such as mesylate and butanesulfonate. Herein, R 102a and R 102b independently represent straight, branched or cyclic alkyl groups of 1 to 8 carbon atoms. R 103 represents a straight, branched or cyclic alkylene groups of 1 to 10 carbon atoms. R 104a and R 104b independently represent 2-oxoalkyl groups of 3 to 7 carbon atoms. K - is a non-nucleophilic counter ion. Illustrative of the groups represented by R 102a and R 102b are methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, tert-butyl, pentyl, hexyl, heptyl, octyl, cyclopentyl, cyclohexyl, cyclopropylmethyl, 4-methylcyclohexyl, and cyclohexylmethyl. Illustrative of the groups represented by R 103 are methylene, ethylene, propylene, butylene, pentylene, hexylene, heptylene, octylene, nonylene, 1,4-cyclohexylene, 1,2-cyclohexylene, 1,3-cyclopentylene, 1,4-cyclooctylene, and 1,4-cyclohexanedimethylene. Illustrative of the groups represented by R 104a and R 104b are 2-oxopropyl, 2-oxocyclopentyl, 2-oxocyclohexyl, and 2-oxocycloheptyl. Illustrative examples of the counter ion represented by K - are the same as exemplified for formulae (P1a-1) and (P1a-2). (ii) Diazomethane derivatives of formula (P2) Herein, R 105 and R 106 independently represent straight, branched or cyclic alkyl or halogenated alkyl groups of 1 to 12 carbon atoms, aryl or halogenated aryl groups of 6 to 20 carbon atoms, or aralkyl groups of 7 to 12 carbon atoms. Of the groups represented by R 105 and R 106 , exemplary alkyl groups include methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, tert-butyl, pentyl, hexyl, heptyl, octyl, amyl, cyclopentyl, cyclohexyl, cycloheptyl, norbornyl, and adamantyl. Exemplary halogenated alkyl groups include trifluoromethyl, 1,1,1-trifluoroethyl, 1,1,1-trichloroethyl, and nonafluorobutyl. Exemplary aryl groups include phenyl; alkoxyphenyl groups such as p-methoxyphenyl, m-methoxyphenyl, o-methoxyphenyl, ethoxyphenyl, p-tert-butoxyphenyl, and m-tert-butoxyphenyl; and alkylphenyl groups such as 2-methylphenyl, 3-methylphenyl, 4-methylphenyl, ethylphenyl, 4-tert-butylphenyl, 4-butylphenyl, and dimethylphenyl. Exemplary halogenated aryl groups include fluorophenyl, chlorophenyl, and 1,2,3,4,5-pentafluorophenyl. Exemplary aralkyl groups include benzyl and phenethyl. (iii) Glyoxime derivatives of formula (P3) Herein, R 107 , R 108 , and R 109 independently represent straight, branched or cyclic alkyl or halogenated alkyl groups of 1 to 12 carbon atoms, aryl or halogenated aryl groups of 6 to 20 carbon atoms, or aralkyl groups of 7 to 12 carbon atoms. Also, R 108 and R 109 , taken together, may form a ring. R 108 and R 109 each are straight or branched alkylene groups of 1 to 6 carbon atoms when they form a ring. Illustrative examples of the alkyl, halogenated alkyl, aryl, halogenated aryl, and aralkyl groups represented by R 107 , R 108 , and R 109 are the same as exemplified for R 105 and R 106 . Examples of the alkylene groups represented by R 108 and R 109 include methylene, ethylene, propylene, butylene, and hexylene. (iv) Bissulfone derivatives of formula (P4) Herein, R 101a and R 101b are as defined above. (v) Sulfonic acid esters of N-hydroxyimide compounds of formula (P5) Herein, R 110 is an arylene group of 6 to 10 carbon atoms, alkylene group of 1 to 6 carbon atoms, or alkenylene group of 2 to 6 carbon atoms wherein some or all of the hydrogen atoms may be replaced by straight or branched alkyl or alkoxy groups of 1 to 4 carbon atoms, nitro, acetyl, or phenyl groups. R 111 is a straight, branched or cyclic alkyl group of 1 to 8 carbon atoms, alkenyl, alkoxyalkyl, phenyl or naphthyl group wherein some or all of the hydrogen atoms may be replaced by alkyl or alkoxy groups of 1 to 4 carbon atoms, phenyl groups (which may have substituted thereon an alkyl or alkoxy of 1 to 4 carbon atoms, nitro, or acetyl group), hetero-aromatic groups of 3 to 5 carbon atoms, or chlorine or fluorine atoms. Of the groups represented by R 110 , exemplary arylene groups include 1,2-phenylene and 1,8-naphthylene; exemplary alkylene groups include methylene, 1,2-ethylene, 1,3-propylene, 1,4-butylene, 1-phenyl-1,2-ethylene, and norbornane-2,3-diyl; and exemplary alkenylene groups include 1,2-vinylene, 1-phenyl-1,2-vinylene, and 5-norbornene-2,3-diyl. Of the groups represented by R 11 , exemplary alkyl groups are as exemplified for R 101a to R 101c ; exemplary alkenyl groups include vinyl, 1-propenyl, allyl, 1-butenyl, 3-butenyl, isoprenyl, 1-pentenyl, 3-pentenyl, 4-pentenyl, dimethylallyl, 1-hexenyl, 3-hexenyl, 5-hexenyl, 1-heptenyl, 3-heptenyl, 6-heptenyl, and 7-octenyl; and exemplary alkoxyalkyl groups include methoxymethyl, ethoxymethyl, propoxymethyl, butoxymethyl, pentyloxymethyl, hexyloxymethyl, heptyloxymethyl, methoxyethyl, ethoxyethyl, propoxyethyl, butoxyethyl, pentyloxyethyl, hexyloxyethyl, methoxypropyl, ethoxypropyl, propoxypropyl, butoxypropyl, methoxybutyl, ethoxybutyl, propoxybutyl, methoxypentyl, ethoxypentyl, methoxyhexyl, and methoxyheptyl. Of the substituents on these groups, the alkyl groups of 1 to 4 carbon atoms include methyl, ethyl, propyl, isopropyl, n-butyl, isobutyl and tert-butyl; the alkoxy groups of 1 to 4 carbon atoms include methoxy, ethoxy, propoxy, isopropoxy, n-butoxy, isobutoxy, and tert-butoxy; the phenyl groups which may have substituted thereon an alkyl or alkoxy of 1 to 4 carbon atoms, nitro, or acetyl group include phenyl, tolyl, p-tert-butoxyphenyl, p-acetylphenyl and p-nitrophenyl; the hetero-aromatic groups of 3 to 5 carbon atoms include pyridyl and furyl. Illustrative examples of the photoacid generator include: onium salts such as diphenyliodonium trifluoromethanesulfonate, (p-tert-butoxyphenyl)phenyliodonium trifluoromethanesulfonate, diphenyliodonium p-toluenesulfonate, (p-tert-butoxyphenyl)phenyliodonium p-toluenesulfonate, triphenylsulfonium trifluoromethanesulfonate, (p-tert-butoxyphenyl)diphenylsulfonium trifluoromethanesulfonate, bis(p-tert-butoxyphenyl)phenylsulfonium trifluoromethanesulfonate, tris(p-tert-butoxyphenyl)-sulfonium trifluoromethanesulfonate, triphenylsulfonium p-toluenesulfonate, (p-tert-butoxyphenyl)diphenylsulfonium p-toluenesulfonate, bis(p-tert-butoxyphenyl)phenylsulfonium p-toluenesulfonate, tris(p-tert-butoxyphenyl)sulfonium p-toluenesulfonate, triphenylsulfonium nonafluorobutanesulfonate, triphenylsulfonium butanesulfonate, trimethylsulfonium trifluoromethanesulfonate, trimethylsulfonium p-toluenesulfonate, cyclohexylmethyl(2-oxocyclohexyl)sulfonium trifluoromethanesulfonate, cyclohexylmethyl(2-oxocyclohexyl)sulfonium p-toluenesulfonate, dimethylphenylsulfonium trifluoromethanesulfonate, dimethylphenylsulfonium p-toluenesulfonate, dicyclohexylphenylsulfonium trifluoromethanesulfonate, dicyclohexylphenylsulfonium p-toluenesulfonate, trinaphthylsulfonium trifluoromethanesulfonate, cyclohexylmethyl(2-oxocyclohexyl)sulfonium trifluoromethanesulfonate, (2-norbornyl)methyl(2-oxocyclohexyl)sulfonium trifluoromethanesulfonate, ethylenebis-[methyl(2-oxocyclopentyl)sulfonium trifluoromethanesulfonate], and 1,2'-naphthylcarbonylmethyltetrahydrothiophenium triflate; diazomethane derivatives such as bis(benzenesulfonyl) diazomethane, bis(p-toluenesulfonyl)diazomethane, bis(xylenesulfonyl)diazomethane, bis(cyclohexylsulfonyl)-diazomethane, bis(cyclopentylsulfonyl)diazomethane, bis(n-butylsulfonyl)diazomethane, bis(isobutylsulfonyl)-diazomethane, bis(sec-butylsulfonyl)diazomethane, bis(n-propylsulfonyl)diazomethane, bis(isopropylsulfonyl)-diazomethane, bis(tert-butylsulfonyl)diazomethane, bis(n-amylsulfonyl)diazomethane, bis(isoamylsulfonyl)diazomethane, bis(sec-amylsulfonyl)diazomethane, bis(tert-amylsulfonyl)diazomethane, 1-cyclohexylsulfonyl-1-(tert-butylsulfonyl)diazomethane, 1-cyclohexylsulfonyl-1-(tert-amylsulfonyl)diazomethane, and 1-tert-amylsulfonyl-1-(tert-butylsulfonyl)diazomethane; glyoxime derivatives such as bis-o-(p-toluenesulfonyl)-α-dimethylglyoxime, bis-o-(p-toluenesulfonyl)-α-diphenylglyoxime, bis-o-(p-toluenesulfonyl)-a-dicyclohexylglyoxime, bis-o-(p-toluenesulfonyl)-2,3-pentanedioneglyoxime, bis-o-(p-toluenesulfonyl)-2-methyl-3,4-pentanedioneglyoxime, bis-o-(n-butanesulfonyl)-α-dimethylglyoxime, bis-o-(n-butanesulfonyl)-α-diphenylglyoxime, bis-o-(n-butanesulfonyl)-α-dicyclohexylglyoxime, bis-o-(n-butanesulfonyl)-2,3-pentanedioneglyoxime, bis-o-(n-butanesulfonyl)-2-methyl-3,4-pentanedioneglyoxime, bis-o-(methanesulfonyl)-α-dimethylglyoxime, bis-o-(trifluoromethanesulfonyl)-α-dimethylglyoxime, bis-o-(1,1,1-trifluoroethanesulfonyl)-α-dimethylglyoxime, bis-o-(tert-butanesulfonyl)-α-dimethylglyoxime, bis-o-(perfluorooctanesulfonyl)-α-dimethylglyoxime, bis-o-(cyclohexanesulfonyl)-α-dimethylglyoxime, bis-o-(benzenesulfonyl)-α-dimethylglyoxime, bis-o-(p-fluorobenzenesulfonyl)-α-dimethylglyoxime, bis-o-(p-tert-butylbenzenesulfonyl)-α-dimethylglyoxime, bis-o-(xylenesulfonyl)-α-dimethylglyoxime, and bis-o-(camphorsulfonyl)-α-dimethylglyoxime; bissulfone derivatives such as bisnaphthylsulfonylmethane, bistrifluoromethylsulfonylmethane, bismethylsulfonylmethane, bisethylsulfonylmethane, bispropylsulfonylmethane, bisisopropylsulfonylmethane, bis-p-toluenesulfonylmethane, and bisbenzenesulfonylmethane; β-ketosulfone derivatives such as 2-cyclohexylcarbonyl-2-(p-toluenesulfonyl)propane and 2-isopropylcarbonyl-2-(p-toluenesulfonyl)propane; disulfone derivatives such as diphenyl disulfone and dicyclohexyl disulfone; nitrobenzyl sulfonate derivatives such as 2,6-dinitrobenzyl p-toluenesulfonate and 2,4-dinitrobenzyl p-toluenesulfonate; sulfonic acid ester derivatives such as 1,2,3-tris (methanesulfonyloxy)benzene, 1,2,3-tris(trifluoromethanesulfonyloxy)benzene, and 1,2,3-tris(p-toluenesulfonyloxy)-benzene; and sulfonic acid esters of N-hydroxyimides such as N-hydroxysuccinimide methanesulfonate, N-hydroxysuccinimide trifluoromethanesulfonate, N-hydroxysuccinimide ethanesulfonate, N-hydroxysuccinimide 1-propanesulfonate, N-hydroxysuccinimide 2-propanesulfonate, N-hydroxysuccinimide 1-pentanesulfonate, N-hydroxysuccinimide 1-octanesulfonate, N-hydroxysuccinimide p-toluenesulfonate, N-hydroxysuccinimide p-methoxybenzenesulfonate, N-hydroxysuccinimide 2-chloroethanesulfonate, N-hydroxysuccinimide benzenesulfonate, N-hydroxysuccinimide 2,4,6-trimethylbenzenesulfonate, N-hydroxysuccinimide 1-naphthalenesulfonate, N-hydroxysuccinimide 2-naphthalenesulfonate, N-hydroxy-2-phenylsuccinimide methanesulfonate, N-hydroxymaleimide methanesulfonate, N-hydroxymaleimide ethanesulfonate, N-hydroxy-2-phenylmaleimide methanesulfonate, N-hydroxyglutarimide methanesulfonate, N-hydroxyglutarimide benzenesulfonate, N-hydroxyphthalimide methanesulfonate, N-hydroxyphthalimide benzenesulfonate, N-hydroxyphthalimide trifluoromethanesulfonate, N-hydroxyphthalimide p-toluenesulfonate, N-hydroxynaphthalimide methanesulfonate, N-hydroxynaphthalimide benzenesulfonate, N-hydroxy-5-norbornene-2,3-dicarboxyimide methanesulfonate, N-hydroxy-5-norbornene-2,3-dicarboxyimide trifluoromethanesulfonate, and N-hydroxy-5-norbornene-2,3-dicarboxyimide p-toluenesulfonate. Preferred among these photoacid generators are onium salts such as triphenylsulfonium trifluoromethanesulfonate, (p-tert-butoxyphenyl)diphenylsulfonium trifluoromethanesulfonate, tris(p-tert-butoxyphenyl)sulfonium trifluoromethanesulfonate, triphenylsulfonium p-toluenesulfonate, (p-tert-butoxyphenyl)diphenylsulfonium p-toluenesulfonate, tris(p-tert-butoxyphenyl)sulfonium p-toluenesulfonate, trinaphthylsulfonium trifluoromethanesulfonate, cyclohexylmethyl(2-oxocyclohexyl)sulfonium trifluoromethanesulfonate, (2-norbornyl)methyl(2-oxocylohexyl)sulfonium trifluoromethanesulfonate, and 1,2'-naphthylcarbonylmethyltetrahydrothiophenium triflate; diazomethane derivatives such as bis(benzenesulfonyl)diazomethane, bis(p-toluenesulfonyl)diazomethane, bis(cyclohexylsulfonyl)-diazomethane, bis(n-butylsulfonyl)diazomethane, bis(isobutylsulfonyl)diazomethane, bis(sec-butylsulfonyl)-diazomethane, bis(n-propylsulfonyl)diazomethane, bis(isopropylsulfonyl)diazomethane, and bis(tert-butylsulfonyl)diazomethane; glyoxime derivatives such as bis-o-(p-toluenesulfonyl)-α-dimethylglyoxime and bis-o-(n-butanesulfonyl)-α-dimethylglyoxime; bissulfone derivatives such as bisnaphthylsulfonylmethane; and sulfonic acid esters of N-hydroxyimide compounds such as N-hydroxysuccinimide methanesulfonate, N-hydroxysuccinimide trifluoromethanesulfonate, N-hydroxysuccinimide 1-propanesulfonate, N-hydroxysuccinimide 2-propanesulfonate, N-hydroxysuccinimide 1-pentanesulfonate, N-hydroxysuccinimide p-toluenesulfonate, N-hydroxynaphthalimide methanesulfonate, and N-hydroxynaphthalimide benzenesulfonate. These photoacid generators may be used singly or in combinations of two or more thereof. Onium salts are effective for improving rectangularity, while diazomethane derivatives and glyoxime derivatives are effective for reducing standing waves. The combination of an onium salt with a diazomethane or a glyoxime derivative allows for fine adjustment of the profile. The photoacid generator is added in an amount of 0.1 to 15 parts, and especially 0.5 to 8 parts by weight, per 100 parts by weight of the base resin (all parts are by weight, hereinafter). Less than 0.1 part of the photoacid generator would provide a poor sensitivity whereas more than 15 parts of the photoacid generator would adversely affect transparency and resolution. Organic solvent The organic solvent used herein may be any organic solvent in which the base resin, photoacid generator, and other components are soluble. Illustrative, non-limiting, examples of the organic solvent include ketones such as cyclohexanone and methyl-2-n-amylketone; alcohols such as 3-methoxybutanol, 3-methyl-3-methoxybutanol, 1-methoxy-2-propanol, and 1-ethoxy-2-propanol; ethers such as propylene glycol monomethyl ether, ethylene glycol monomethyl ether, propylene glycol monoethyl ether, ethylene glycol monoethyl ether, propylene glycol dimethyl ether, and diethylene glycol dimethyl ether; and esters such as propylene glycol monomethyl ether acetate, propylene glycol monoethyl ether acetate, ethyl lactate, ethyl pyruvate, butyl acetate, methyl 3-methoxypropionate, ethyl 3-ethoxypropionate, tert-butyl acetate, tert-butyl propionate, and propylene glycol mono-tert-butyl ether acetate. These solvents may be used alone or in combinations of two or more thereof. Of the above organic solvents, it is recommended to use diethylene glycol dimethyl ether and 1-ethoxy-2-propanol because the photoacid generator serving as one of the resist components is most soluble therein, propylene glycol monomethyl ether acetate because it is a safe solvent, or a mixture thereof. An appropriate amount of the organic solvent used is about 200 to 1,000 parts, especially about 400 to 800 parts by weight per 100 parts by weight of the base resin. Other polymer To the resist composition of the invention, another polymer other than the inventive polymer comprising recurring units of formula (1a-1) or (1a-2) originating from the ester compound of formula (1) may also be added. The other polymers that can be added to the resist composition are, for example, those polymers comprising units of the following formula (R1) or (R2) and having a weight average molecular weight of about 1,000 to about 500,000, especially about 5,000 to about 100,000 although the other polymers are not limited thereto. Herein, R 001 is hydrogen, methyl or CH 2 CO 2 R 003 . R 002 is hydrogen, methyl or CO 2 R 003 . R 003 is a straight, branched or cyclic alkyl group of 1 to 15 carbon atoms. R 004 is hydrogen or a monovalent hydrocarbon group of 1 to 15 carbon atoms having a carboxyl or hydroxyl group. At least one of R 005 to R 008 represents a monovalent hydrocarbon group of 1 to 15 carbon atoms having a carboxyl or hydroxyl group while the remaining R's independently represent hydrogen or a straight, branched or cyclic alkyl group of 1 to 15 carbon atoms. Alternatively, R 005 to R 008 , taken together, may form a ring, and in that event, at least one of R 005 to R 008 is a divalent hydrocarbon group of 1 to 15 carbon atoms having a carboxyl or hydroxyl group, while the remaining R's are independently single bonds or straight, branched or cyclic alkylene groups of 1 to 15 carbon atoms. R 009 is a monovalent hydrocarbon group of 3 to 15 carbon atoms containing a -CO 2 - partial structure. At least one of R 010 to R 013 is a monovalent hydrocarbon group of 2 to 15 carbon atoms containing a -CO 2 - partial structure, while the remaining R's are independently hydrogen or straight, branched or cyclic alkyl groups of 1 to 15 carbon atoms. R 010 to R 013 , taken together, may form a ring, and in that event, at least one of R 010 to R 013 is a divalent hydrocarbon group of 1 to 15 carbon atoms containing a -CO 2 - partial structure, while the remaining R's are independently single bonds or straight, branched or cyclic alkylene groups of 1 to 15 carbon atoms. R 014 is a polycyclic hydrocarbon group having 7 to 15 carbon atoms or an alkyl group containing a polycyclic hydrocarbon group. R 015 is an acid labile group. R 016 is hydrogen or methyl. R 017 is a straight, branched or cyclic alkyl group of 1 to 8 carbon atoms. X is CH 2 or an oxygen atom. Letter k' is equal to 0 or 1; a1', a2', a3', b1', b2', b3', c1', c2', c3', d1', d2', d3', and e' are numbers from 0 to less than 1, satisfying a1'+a2'+a3'+b1'+b2'+b3'+c1'+c2'+c3'+d1'+d2'+d3'+e' = 1; f', g', h', i', and j' are numbers from 0 to less than 1, satisfying f'+g'+h'+i'+j' = 1. Exemplary groups of these R's are as exemplified above for R 1 to R 16 . The inventive polymer (comprising recurring units of formula (1a-1) or (1a-2)) and the other polymer are preferably blended in a weight ratio from 10:90 to 90:10, more preferably from 20:80 to 80:20. If the blend ratio of the inventive polymer is below this range, the resist composition would become poor in some of the desired properties. The properties of the resist composition can be adjusted by properly changing the blend ratio of the inventive polymer. The other polymer is not limited to one type and a mixture of two or more other polymers may be added. The use of plural polymers allows for easy adjustment of resist properties. Dissolution regulator To the resist composition, a dissolution regulator may be added. The dissolution regulator is a compound having on the molecule at least two phenolic hydroxyl groups, in which an average of from 0 to 100 mol% of all the hydrogen atoms on the phenolic hydroxyl groups are replaced with acid labile groups or a compound having on the molecule at least one carboxyl group, in which an average of 50 to 100 mol% of all the hydrogen atoms on the carboxyl groups are replaced with acid labile groups, both the compounds having an average molecular weight within a range of 100 to 1,000, and preferably 150 to 800. The degree of substitution of the hydrogen atoms on the phenolic hydroxyl groups with acid labile groups is on average at least 0 mol%, and preferably at least 30 mol%, of all the phenolic hydroxyl groups. The upper limit is 100 mol%, and preferably 80 mol%. The degree of substitution of the hydrogen atoms on the carboxyl groups with acid labile groups is on average at least 50 mol%, and preferably at least 70 mol%, of all the carboxyl groups, with the upper limit being 100 mol%. Preferable examples of such compounds having two or more phenolic hydroxyl groups or compounds having at least one carboxyl group include those of formulas (D1) to (D14) below. In these formulas, R 201 and R 202 are each hydrogen or a straight or branched alkyl or alkenyl of 1 to 8 carbon atoms; R 203 is hydrogen, a straight or branched alkyl or alkenyl of 1 to 8 carbon atoms, or -(R 207 ) h -COOH; R 204 is -(CH 2 ) i - (where i = 2 to 10), an arylene of 6 to 10 carbon atoms, carbonyl, sulfonyl, an oxygen atom, or a sulfur atom; R 205 is an alkylene of 1 to 10 carbon atoms, an arylene of 6 to 10 carbon atoms, carbonyl, sulfonyl, an oxygen atom, or a sulfur atom; R 206 is hydrogen, a straight or branched alkyl or alkenyl of 1 to 8 carbon atoms, or a hydroxyl-substituted phenyl or naphthyl; R 207 is a straight or branched alkylene of 1 to 10 carbon atoms; R 208 is hydrogen or hydroxyl; the letter j is an integer from 0 to 5; u and h are each 0 or 1; s, t, s', t', s", and t" are each numbers which satisfy s+t = 8, s'+t' = 5, and s"+t" = 4, and are such that each phenyl skeleton has at least one hydroxyl group; and α is a number such that the compounds of formula (D8) or (D9) have a molecular weight of from 100 to 1,000. In the above formulas, suitable examples of R 201 and R 202 include hydrogen, methyl, ethyl, butyl, propyl, ethynyl, and cyclohexyl; suitable examples of R 203 include the same groups as for R 201 and R 202 , as well as -COOH and -CH 2 COOH; suitable examples of R 204 include ethylene, phenylene, carbonyl, sulfonyl, oxygen, and sulfur; suitable examples of R 205 include methylene as well as the same groups as for R 204 ; and suitable examples of R 206 include hydrogen, methyl, ethyl, butyl, propyl, ethynyl, cyclohexyl, and hydroxyl-substituted phenyl or naphthyl. Exemplary acid labile groups on the dissolution regulator include groups of the following general formulae (L1) to (L4), tertiary alkyl groups of 4 to 20 carbon atoms, trialkylsilyl groups in which each of the alkyls has 1 to 6 carbon atoms, and oxoalkyl groups of 4 to 20 carbon atoms. In these formulas, R L01 and R L02 are each hydrogen or a straight, branched or cyclic alkyl having 1 to 18 carbon atoms; and R L03 is a monovalent hydrocarbon group of 1 to 18 carbon atoms which may contain a heteroatom (e.g., oxygen). A pair of R L01 and R L02 , a pair of R L01 and R L03 , or a pair of R L02 and R L03 may together form a ring, with the proviso that R L01 , R L02 , and R L03 are each a straight or branched alkylene of 1 to 18 carbon atoms when they form a ring. R L04 is a tertiary alkyl group of 4 to 20 carbon atoms, a trialkysilyl group in which each of the alkyls has 1 to 6 carbon atoms, an oxoalkyl group of 4 to 20 carbon atoms, or a group of the formula (L1). R L05 is a straight, branched or cyclic alkyl group of 1 to 8 carbon atoms or a substituted or unsubstituted aryl group of 6 to 20 carbon atoms. R L06 is a straight, branched or cyclic alkyl group of 1 to 8 carbon atoms or a substituted or unsubstituted aryl group of 6 to 20 carbon atoms. R L07 to R L16 independently represent hydrogen or monovalent hydrocarbon groups of 1 to 15 carbon atoms which may contain a hetero atom. Alternatively, R L07 to R L16 , taken together, may form a ring. Each of R L07 to R L16 represents a divalent C 1 -C 15 hydrocarbon group which may contain a hetero atom, when they form a ring. Two of R L07 to R L16 which are attached to adjoining carbon atoms may bond together directly to form a double bond. Letter y is an integer of 0 to 6. Letter m is equal to 0 or 1, n is equal to 0, 1, 2 or 3, and 2m+n is equal to 2 or 3. The dissolution regulator may be formulated in an amount of 0 to 50 parts, preferably 5 to 50 parts, and more preferably-10 to 30 parts, per 100 parts of the base resin, and may be used singly or as a mixture of two or more thereof. Less than 5 parts of the dissolution regulator may fail to yield an improved resolution, whereas the use of more than 50 parts would lead to slimming of the patterned film, and thus a decline in resolution. The dissolution regulator can be synthesized by introducing acid labile groups into a compound having phenolic hydroxyl or carboxyl groups in accordance with an organic chemical formulation. Basic compound In the resist composition of the invention, a basic compound may be blended. A suitable basic compound used herein is a compound capable of suppressing the rate of diffusion when the acid generated by the photoacid generator diffuses within the resist film. The inclusion of this type of basic compound holds down the rate of acid diffusion within the resist film, resulting in better resolution. In addition, it suppresses changes in sensitivity following exposure, thus reducing substrate and environment dependence, as well as improving the exposure latitude and the pattern profile. Examples of basic compounds include primary, secondary, and tertiary aliphatic amines, mixed amines, aromatic amines, heterocyclic amines, carboxyl group-bearing nitrogenous compounds, sulfonyl group-bearing nitrogenous compounds, hydroxyl group-bearing nitrogenous compounds, hydroxyphenyl group-bearing nitrogenous compounds, alcoholic nitrogenous compounds, amide derivatives, and imide derivatives. Examples of suitable primary aliphatic amines include ammonia, methylamine, ethylamine, n-propylamine, isopropylamine, n-butylamine, iso-butylamine, sec-butylamine, tert-butylamine, pentylamine, tert-amylamine, cyclopentylamine, hexylamine, cyclohexylamine, heptylamine, octylamine, nonylamine, decylamine, dodecylamine, cetylamine, methylenediamine, ethylenediamine, and tetraethylenepentamine. Examples of suitable secondary aliphatic amines include dimethylamine, diethylamine, di-n-propylamine, di-iso-propylamine, di-n-butylamine, di-iso-butylamine, di-sec-butylamine, dipentylamine, dicyclopentylamine, dihexylamine, dicyclohexylamine, diheptylamine, dioctylamine, dinonylamine, didecylamine, didodecylamine, dicetylamine, N,N-dimethylmethylenediamine, N,N-dimethylethylenediamine, and N,N-dimethyltetraethylenepentamine. Examples of suitable tertiary aliphatic amines include trimethylamine, triethylamine, tri-n-propylamine, tri-iso-propylamine, tri-n-butylamine, tri-iso-butylamine, tri-sec-butylamine, tripentylamine, tricyclopentylamine, trihexylamine, tricyclohexylamine, triheptylamine, trioctylamine, trinonylamine, tridecylamine, tridodecylamine, tricetylamine, N,N,N',N'-tetramethylmethylenediamine, N,N,N',N'-tetramethylethylenediamine, and N,N,N',N'-tetramethyltetraethylenepentamine. Examples of suitable mixed amines include dimethylethylamine, methylethylpropylamine, benzylamine, phenethylamine, and benzyldimethylamine. Examples of suitable aromatic and heterocyclic amines include aniline derivatives (e.g., aniline, N-methylaniline, N-ethylaniline, N-propylaniline, N,N-dimethylaniline, 2-methylaniline, 3-methylaniline, 4-methylaniline, ethylaniline, propylaniline, trimethylaniline, 2-nitroaniline, 3-nitroaniline, 4-nitroaniline, 2,4-dinitroaniline, 2,6-dinitroaniline, 3,5-dinitroaniline, and N,N-dimethyltoluidine), diphenyl(p-tolyl)amine, methyldiphenylamine, triphenylamine, phenylenediamine, naphthylamine, diaminonaphthalene, pyrrole derivatives (e.g., pyrrole, 2H-pyrrole, 1-methylpyrrole, 2,4-dimethylpyrrole, 2,5-dimethylpyrrole, and N-methylpyrrole), oxazole derivatives (e.g., oxazole and isooxazole), thiazole derivatives (e.g., thiazole and isothiazole), imidazole derivatives (e.g., imidazole, 4-methylimidazole, and 4-methyl-2-phenylimidazole), pyrazole derivatives, furazan derivatives, pyrroline derivatives (e.g., pyrroline and 2-methyl-1-pyrroline), pyrrolidine derivatives (e.g., pyrrolidine, N-methylpyrrolidine, pyrrolidinone, and N-methylpyrrolidone), imidazoline derivatives, imidazolidine derivatives, pyridine derivatives (e.g., pyridine, methylpyridine, ethylpyridine, propylpyridine, butylpyridine, 4-(1-butylpentyl)pyridine, dimethylpyridine, trimethylpyridine, triethylpyridine, phenylpyridine, 3-methyl-2-phenylpyridine, 4-tert-butylpyridine, diphenylpyridine, benzylpyridine, methoxypyridine, butoxypyridine, dimethoxypyridine, 1-methyl-2-pyridine, 4-pyrrolidinopyridine, 1-methyl-4-phenylpyridine, 2-(1-ethylpropyl)pyridine, aminopyridine, and dimethylaminopyridine), pyridazine derivatives, pyrimidine derivatives, pyrazine derivatives, pyrazoline derivatives, pyrazolidine derivatives, piperidine derivatives, piperazine derivatives, morpholine derivatives, indole derivatives, isoindole derivatives, 1H-indazole derivatives, indoline derivatives, quinoline derivatives (e.g., quinoline and 3-quinolinecarbonitrile), isoquinoline derivatives, cinnoline derivatives, quinazoline derivatives, quinoxaline derivatives, phthalazine derivatives, purine derivatives, pteridine derivatives, carbazole derivatives, phenanthridine derivatives, acridine derivatives, phenazine derivatives, 1,10-phenanthroline derivatives, adenine derivatives, adenosine derivatives, guanine derivatives, guanosine derivatives, uracil derivatives, and uridine derivatives. Examples of suitable carboxyl group-bearing nitrogenous compounds include aminobenzoic acid, indolecarboxylic acid, and amino acid derivatives (e.g. nicotinic acid, alanine, alginine, aspartic acid, glutamic acid, glycine, histidine, isoleucine, glycylleucine, leucine, methionine, phenylalanine, threonine, lysine, 3-aminopyrazine-2-carboxylic acid, and methoxyalanine). Examples of suitable sulfonyl group-bearing nitrogenous compounds include 3-pyridinesulfonic acid and pyridinium p-toluenesulfonate. Examples of suitable hydroxyl group-bearing nitrogenous compounds, hydroxyphenyl group-bearing nitrogenous compounds, and alcoholic nitrogenous compounds include 2-hydroxypyridine, aminocresol, 2,4-quinolinediol, 3-indolemethanol hydrate, monoethanolamine, diethanolamine, triethanolamine, N-ethyldiethanolamine, N,N-diethylethanolamine, triisopropanolamine, 2,2'-iminodiethanol, 2-aminoethanol, 3-amino-1-propanol, 4-amino-1-butanol, 4-(2-hydroxyethyl)morpholine, 2-(2-hydroxyethyl)pyridine, 1-(2-hydroxyethyl)piperazine, 1-[2-(2-hydroxyethoxy)ethyl]-piperazine, piperidine ethanol, 1-(2-hydroxyethyl)-pyrrolidine, 1-(2-hydroxyethyl)-2-pyrrolidinone, 3-piperidino-1,2-propanediol, 3-pyrrolidino-1,2-propanediol, 8-hydroxyjulolidine, 3-quinuclidinol, 3-tropanol, 1-methyl-2-pyrrolidine ethanol, 1-aziridine ethanol, N-(2-hydroxyethyl)phthalimide, and N-(2-hydroxyethyl)-isonicotinamide. Examples of suitable amide derivatives include formamide, N-methylformamide, N,N-dimethylformamide, acetamide, N-methylacetamide, N,N-dimethylacetamide, propionamide, and benzamide. Suitable imide derivatives include phthalimide, succinimide, and maleimide. In addition, basic compounds of the following general formula (B1) may also be included alone or in admixture. N(X) n (Y) 3-n B1 In the formula, n is equal to 1, 2 or 3; Y is independently hydrogen or a straight, branched or cyclic alkyl group of 1 to 20 carbon atoms which may contain a hydroxyl group or ether; and X is independently selected from groups of the following general formulas (X1) to (X3), and two or three X's may bond together to form a ring. In the formulas, R 300 , R 302 and R 305 are independently straight or branched alkylene groups of 1 to 4 carbon atoms; R 301 and R 304 are independently hydrogen, straight, branched or cyclic alkyl groups of 1 to 20 carbon atoms, which may contain at least one hydroxyl group, ether, ester or lactone ring; and R 303 is a single bond or a straight or branched alkylene group of 1 to 4 carbon atoms. Illustrative examples of the compounds of formula (B1) include tris(2-methoxymethoxyethyl)amine, tris{2-(methoxyethoxy)ethyl}amine, tris{2-(2-methoxyethoxymethoxy)ethyl}amine, tris{2-(1-methoxyethoxy)ethyl}amine, tris{2-(1-ethoxyethoxy)ethyl}amine, tris{2-(1-ethoxypropoxy)ethyl}amine, tris[2-{2-(2-hydroxyethoxy)ethoxy} ethyl]amine, 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo-[8.8.8]hexacosane, 4,7,13,18-tetraoxa-1,10-diazabicyclo-[8.5.5]eicosane, 1,4,10,13-tetraoxa-7,16-diazabicyclooctadecane, 1-aza-12-crown-4, 1-aza-15-crown-5, 1-aza-18-crown-6, tris(2-formyloxyethyl)amine, tris(2-acetoxyethyl)-amine, tris(2-propionyloxyethyl)amine, tris(2-butyryloxyethyl)amine, tris(2-isobutyryloxyethyl)amine, tris(2-valeryloxyethyl)amine, tris(2-pivaloyloxyethyl)amine, N,N-bis(2-acetoxyethyl)-2-(acetoxyacetoxy)ethylamine, tris(2-methoxycarbonyloxyethyl)amine, tris(2-tert-butoxycarbonyloxyethyl)amine, tris[2-(2-oxopropoxy)-ethyl]amine, tris[2-(methoxycarbonylmethyl)oxyethyl]amine, tris[2-(tert-butoxycarbonylmethyloxy)ethyl]amine, tris[2-(cyclohexyloxycarbonylmethyloxy)ethyl]amine, tris(2-methoxycarbonylethyl)amine, tris(2-ethoxycarbonylethyl)amine, N,N-bis(2-hydroxyethyl)-2-(methoxycarbonyl)ethylamine, N,N-bis(2-acetoxyethyl)-2-(methoxycarbonyl)ethylamine, N,N-bis(2-hydroxyethyl)-2-(ethoxycarbonyl)ethylamine, N,N-bis(2-acetoxyethyl)-2-(ethoxycarbonyl)ethylamine, N,N-bis(2-hydroxyethyl)-2-(2-methoxyethoxycarbonyl)ethylamine, N,N-bis(2-acetoxyethyl)-2-(2-methoxyethoxycarbonyl)ethylamine, N,N-bis(2-hydroxyethyl)-2-(2-hydroxyethoxycarbonyl)-ethylamine, N,N-bis(2-acetoxyethyl)-2-(2-acetoxyethoxycarbonyl)ethylamine, N,N-bis(2-hydroxyethyl)-2-[(methoxycarbonyl)methoxycarbonyl]ethylamine, N,N-bis(2-acetoxyethyl)-2-[(methoxycarbonyl)methoxycarbonyl]ethylamine, N,N-bis(2-hydroxyethyl)-2-(2-oxopropoxycarbonyl)ethylamine, N,N-bis(2-acetoxyethyl)-2-(2-oxopropoxycarbonyl)ethylamine, N,N-bis(2-hydroxyethyl)-2-(tetrahydrofurfuryloxycarbonyl)-ethylamine, N,N-bis(2-acetoxyethyl)-2-(tetrahydrofurfuryloxycarbonyl)ethylamine, N,N-bis(2-hydroxyethyl)-2-[(2-oxotetrahydrofuran-3-yl)oxycarbonyl]ethylamine, N,N-bis(2-acetoxyethyl)-2-[(2-oxotetrahydrofuran-3-yl)oxycarbonyl]-ethylamine, N,N-bis(2-hydroxyethyl)-2-(4-hydroxybutoxycarbonyl)ethylamine, N,N-bis(2-formyloxyethyl)-2-(4-formyloxybutoxycarbonyl)ethylamine, N,N-bis(2-formyloxyethyl)-2-(2-formyloxyethoxycarbonyl)ethylamine, N,N-bis(2-methoxyethyl)-2-(methoxycarbonyl)ethylamine, N-(2-hydroxyethyl)-bis[2-(methoxycarbonyl)ethyl]amine, N-(2-acetoxyethyl)-bis[2-(methoxycarbonyl)ethyl]amine, N-(2-hydroxyethyl)-bis[2-(ethoxycarbonyl)ethyl]amine, N-(2-acetoxyethyl)-bis[2-(ethoxycarbonyl)ethyl]amine, N-(3-hydroxy-1-propyl)-bis[2-(methoxycarbonyl)ethyl]amine, N-(3-acetoxy-1-propyl)-bis[2-(methoxycarbonyl)ethyl]amine, N-(2-methoxyethyl)-bis[2-(methoxycarbonyl)ethyl]amine, N-butylbis[2-(methoxycarbonyl)ethyl]amine, N-butyl-bis[2-(2-methoxyethoxycarbonyl)ethyl]amine, N-methyl-bis(2-acetoxyethyl)amine, N-ethyl-bis(2-acetoxyethyl)amine, N-methyl-bis(2-pivaloyloxyethyl)amine, N-ethyl-bis[2-(methoxycarbonyloxy)ethyl]amine, N-ethyl-bis[2-(tert-butoxycarbonyloxy)ethyl]amine, tris(methoxycarbonylmethyl)amine, tris(ethoxycarbonylmethyl)amine, N-butyl-bis(methoxycarbonylmethyl)amine, N-hexyl-bis(methoxycarbonylmethyl)amine, and β-(diethylamino)-δ-valerolactone. The basic compound is preferably formulated in an amount of 0.001 to 10 parts, and especially 0.01 to 1 part, per part of the photoacid generator. Less than 0.001 part of the basic compound fails to achieve the desired effects thereof, while the use of more than 10 parts would result in too low a sensitivity and resolution. Other components In the resist composition, a compound bearing a ≡C-COOH group in a molecule may be blended. Exemplary, non-limiting compounds bearing a ≡C-COOH group include one or more compounds selected from Groups I and II below. Including this compound improves the PED stability of the resist and ameliorates edge roughness on nitride film substrates. Group I: Compounds in which some or all of the hydrogen atoms on the phenolic hydroxyl groups of the compounds of general formulas (A1) to (A10) below have been replaced with -R 401 -COOH (wherein R 401 is a straight or branched alkylene of 1 to 10 carbon atoms), and in which the molar ratio C/(C+D) of phenolic hydroxyl groups (C) to ≡C-COOH groups (D) in the molecule is from 0.1 to 1.0. In these formulas, R 408 is hydrogen or methyl; R 402 and R 403 are each hydrogen or a straight or branched alkyl or alkenyl of 1 to 8 carbon atoms; R 404 is hydrogen, a straight or branched alkyl or alkenyl of 1 to 8 carbon atoms, or a -(R 409 ) h -COOR' group (R' being hydrogen or -R 409 -COOH); R 405 is -(CH 2 ) i - (wherein i is 2 to 10), an arylene of 6 to 10 carbon atoms, carbonyl, sulfonyl, an oxygen atom, or a sulfur atom; R 406 is an alkylene of 1 to 10 carbon atoms, an arylene of 6 to 10 carbon atoms, carbonyl, sulfonyl, an oxygen atom, or a sulfur atom; R 407 is hydrogen, a straight or branched alkyl or alkenyl of 1 to 8 carbon atoms, or a hydroxyl-substituted phenyl or naphthyl; R 409 is a straight or branched alkylene of 1 to 10 carbon atoms; R 410 is hydrogen, a straight or branched alkyl or alkenyl of 1 to 8 carbon atoms, or a -R 411 -COOH group; R 411 is a straight or branched alkylene of 1 to 10 carbon atoms; the letter j is an integer from 0 to 5; u and h are each 0 or 1; s1, t1, s2, t2, s3, t3, s4, and t4 are each numbers which satisfy s1+t1 = 8, s2+t2 = 5, s3+t3 = 4, and s4+t4 = 6, and are such that each phenyl skeleton has at least one hydroxyl group; κ is a number such that the compound of formula (A6) may have a weight average molecular weight of 1,000 to 5,000; and λ is a number such that the compound of formula (A7) may have a weight average molecular weight of 1,000 to 10,000. Group II: Compounds of general formulas (A11) to (A15) below. In these formulas, R 402 , R 403 , and R 411 are as defined above; R 412 is hydrogen or hydroxyl; s5 and t5 are numbers which satisfy s5 ≥ 0, t5 ≥ 0, and s5+t5 = 5; and h' is equal to 0 or 1. Illustrative, non-limiting examples of the compound bearing a ≡C-COOH group include compounds of the general formulas AI-1 to AI-14 and AII-1 to AII-10 below. In the above formulas, R" is hydrogen or a CH 2 COOH group such that the CH 2 COOH group accounts for 10 to 100 mol% of R" in each compound, α and κ are as defined above. The compound bearing a ≡C-COOH group within the molecule may be used singly or as combinations of two or more thereof. The compound bearing a ≡C-COOH group within the molecule is added in an amount ranging from 0 to 5 parts, preferably 0.1 to 5 parts, more preferably 0.1 to 3 parts, further preferably 0.1 to 2 parts, per 100 parts of the base resin. More than 5 parts of the compound can reduce the resolution of the resist composition. The resist composition of the invention may additionally include an acetylene alcohol derivative for the purpose of enhancing the shelf stability. Preferred acetylene alcohol derivatives are those having the general formula (S1) or (S2) below. In the formulas, R 501 , R 502 , R 503 , R 504 , and R 505 are each hydrogen or a straight, branched, or cyclic alkyl of 1 to 8 carbon atoms; and X and Y are each 0 or a positive number, satisfying 0 ≤ X ≤ 30, 0 ≤ Y ≤ 30, and 0 ≤ X+Y ≤ 40. Preferable examples of the acetylene alcohol derivative include Surfynol 61, Surfynol 82, Surfynol 104, Surfynol 104E, Surfynol 104H, Surfynol 104A, Surfynol TG, Surfynol PC, Surfynol 440, Surfynol 465, and Surfynol 485 from Air Products and Chemicals Inc., and Surfynol E1004 from Nisshin Chemical Industry K.K. The acetylene alcohol derivative is preferably added in an amount of 0.01 to 2% by weight, and more preferably 0.02 to 1% by weight, per 100% by weight of the resist composition. Less than 0.01% by weight would be ineffective for improving coating characteristics and shelf stability, whereas more than 2% by weight would result in a resist having a low resolution. The resist composition of the invention may include, as an optional ingredient, a surfactant which is commonly used for improving the coating characteristics. Optional ingredients may be added in conventional amounts so long as this does not compromise the objects of the invention. Nonionic surfactants are preferred, examples of which include perfluoroalkylpolyoxyethylene ethanols, fluorinated alkyl esters, perfluoroalkylamine oxides, perfluoroalkyl EO-addition products, and fluorinated organosiloxane compounds. Useful surfactants are commercially available under the trade names Florade FC-430 and FC-431 from Sumitomo 3M K.K., Surflon S-141 and S-145 from Asahi Glass K.K., Unidine DS-401, DS-403 and DS-451 from Daikin Industry K.K., Megaface F-8151 from Dai-Nippon Ink & Chemicals K.K., and X-70-092 and X-70-093 from Shin-Etsu Chemical Co., Ltd. Preferred surfactants are Florade FC-430 from Sumitomo 3M K.K. and X-70-093 from Shin-Etsu Chemical Co., Ltd. Pattern formation using the resist composition of the invention may be carried out by a known lithographic technique. For example, the resist composition is applied onto a substrate such as a silicon wafer by spin coating or the like to form a resist film having a thickness of 0.3 to 2.0 µm, which is then pre-baked on a hot plate at 60 to 150°C for 1 to 10 minutes, and preferably at 80 to 130°C for 1 to 5 minutes. A patterning mask having the desired pattern is then placed over the resist film, and the film exposed through the mask to an electron beam or to high-energy radiation such as deep-UV rays, an excimer laser, or x-rays in a dose of about 1 to 200 mJ/cm 2 , and preferably about 10 to 100 mJ/cm 2 , then post-exposure baked (PEB) on a hot plate at 60 to 150°C for 1 to 5 minutes, and preferably at 80 to 130°C for 1 to 3 minutes. Finally, development is carried out using as the developer an aqueous alkali solution, such as a 0.1 to 5% (preferably 2 to 3%) aqueous solution of tetramethylammonium hydroxide (TMAH), this being done by a conventional method such as dipping, puddling, or spraying for a period of 0.1 to 3 minutes, and preferably 0.5 to 2 minutes. These steps result in the formation of the desired pattern on the substrate. Of the various types of high-energy radiation that may be used, the resist composition of the invention is best suited to fine pattern formation with, in particular, deep-UV rays having a wavelength of 248 to 193 nm, an excimer laser, x-rays, or an electron beam. The desired pattern may not be obtainable outside the upper and lower limits of the above range. The resist composition comprising the polymer as a base resin lends itself to micropatterning with electron beams or deep-UV rays since it is sensitive to high-energy radiation and has excellent sensitivity, resolution, and etching resistance. Especially because of the minimized absorption at the exposure wavelength of an ArF or KrF excimer laser, a finely defined pattern having sidewalls perpendicular to the substrate can easily be formed. EXAMPLE Synthesis Examples and Examples are given below by way of illustration and not by way of limitation. Synthesis Examples Ester compounds and polymers containing the same were synthesized according to the following formulation. Synthesis Example 1-1 Synthesis of Monomer 1 In 350 ml of methylene chloride was dissolved 74.4 g of 2-ethyl-2-bicyclo[2.2.1]heptyl 3-hydroxybutanoate (which was synthesized by reacting 2-ethyl-2-bicyclo[2.2.1]heptyl acetate with acetoaldehyde in the presence of lithium hexamethyldisilazide). Then 39.0 g of pyridine and a catalytic amount of 4-(N,N-dimethylamino)pyridine were added to the solution, which was stirred under ice cooling. To this reaction mixture below 10°C, 66.8 g of 2-norbornene-5-carboxylic chloride was added dropwise over one hour. After agitation was continued for 3 hours, the reaction solution was worked up in a conventional manner. The resulting oily substance was purified by silica gel column chromatography, obtaining 96.7 g of 2-(2-ethyl-2-bicyclo[2.2.1]-heptyloxycarbonyl)propyl 2-norbornene-5-carboxylate, designated Monomer 1. The yield was 84.9%. FIG. 1 shows the spectrum of Monomer 1 by 1 H-NMR (CDCl 3 , 270 MHz) and FIG. 2 shows the FT-IR diagram of Monomer 1. Synthesis Examples 1-2 to 1-4 Synthesis of Monomers 2 to 4 Monomers 2 to 4 were synthesized by the same procedure as above or a well-known procedure. Synthesis Example 2-1 Synthesis of Polymer 1 In 1 liter of tetrahydrofuran, 86.5 g of Monomer 1 and 24.5 g of maleic anhydride were dissolved, and 1.8 g of 2,2'-azobisisobutyronitrile added. After agitation was continued for 15 hours at 60°C, the reaction solution was concentrated in vacuum. The residue was dissolved in 400 ml of tetrahydrofuran, which was, in turn, added dropwise to 10 liters of n-hexane. The resulting solids were collected by filtration, washed with 10 liters of n-hexane, and dried in vacuum at 40°C for 6 hours, obtaining 68.0 g of a polymer, designated Polymer 1. The yield was 61.3%. Synthesis Examples 2-2 to 2-10 Synthesis of Polymers 2 to 10 Polymers 2 to 10 were synthesized by the same procedure as above or a well-known procedure. (Polymer 1) (x=0.50, e=0.50, Mw=12,200) (Polymer 2) (x=0.50, e=0.50, Mw=10,500) (Polymer 3) (x=0.50, e=0.50, Mw=11,700) (Polymer 4) (x=0.50, e=0.50, Mw=13,500) (Polymer 5) (x=0.50, a=0.10, e=0.40, Mw=13,000) (Polymer 6) (x=0.50, b=0.10, e=0.40, Mw=13,100) (Polymer 7) (x=0.40, d=0.20, e=0.40, Mw=17,100) (Polymer 8) (x=0.60, b=0.40, Mw=24,300) (Polymer 9) (x=0.60, b=0.40, Mw=20,200) (Polymer 10) (x=0.50, b=0.50, Mw=18,900) Example I Resist compositions were formulated using inventive polymers and examined for resolution upon KrF excimer laser exposure. Examples I-1 to I-22: Evaluation of resist resolution Resist compositions were prepared by using Polymers 1 to 10 as the base resin, and dissolving the polymer, a photoacid generator (designated as PAG 1 and 2), a dissolution regulator (designated as DRR 1 to 4), a basic compound, and a compound having a ≡C-COOH group in the molecule (ACC 1 and 2) in a solvent in accordance with the formulation shown in Table 1. These compositions were each filtered through a Teflon filter (pore diameter 0.2 µm), thereby giving resist solutions. The solvents and basic compounds used are as follows. It is noted that the solvents contained 0.01% by weight of surfactant Florade FC-430 (Sumitomo 3M). PGMEA: propylene glycol methyl ether acetate TBA: tributylamine TEA: triethanolamine TMMEA: trismethoxymethoxyethylamine TMEMEA: trismethoxyethoxymethoxyethylamine These resist solutions were spin-coated onto silicon wafers, then baked on a hot plate at 110°C for 90 seconds to give resist films having a thickness of 0.5 µm. The resist films were exposed using an KrF excimer laser stepper (Nikon Corporation; NA 0.5), then baked (PEB) at 110° C for 90 seconds, and developed with a solution of 2.38% TMAH in water, thereby giving positive patterns. The resulting resist patterns were evaluated as described below. First, the sensitivity (Eth, mJ/cm 2 ) was determined. Next, the optimal dose (Eop, mJ/cm 2 ) was defined as the dose which provides a 1:1 resolution at the top and bottom of a 0.30 µm line-and-space pattern, and the resolution of the resist under evaluation was defined as the minimum line width (µm) of the lines and spaces that separated at this dose. The shape of the resolved resist pattern was examined under a scanning electron microscope. The composition and test results of the resist materials are shown in Table 1. Table 1 Example Base resin (pbw) Photoacid generator (pbw) Dissolution regulator (pbw) Basic compound (pbw) Solvent (pbw) Optimum dose (mJ/cm 2 ) Resolution (µm) Shape I-1 Polymer 1 PAG 1 TBA PGMEA 20.0 0.22 rectangular (80) (1) (0.078) (480) I-2 Polymer 2 PAG 1 TBA PGMEA 22.0 0.22 rectangular (80) (1) (0.078) (480) I-3 Polymer 3 PAG 1 TBA PGMEA 22.5 0.22 rectangular (80) (1) (0.078) (480) I-4 Polymer 4 PAG 1 TBA PGHEA 22.0 0.22 rectangular (80) (1) (0.078) (480) I-5 Polymer 5 PAG 1 TBA PGMEA 19.0 0.22 rectangular (80) (1) (0.078) (480) I-6 Polymer 6 PAG 1 TBA PGMEA 18.5 0.22 rectangular (80) (1) (0.078) (480) I-7 Polymer 7 PAG 1 TBA PGMEA 20.5 0.22 rectangular (80) (1) (0.078) (480) I-8 Polymer 8 PAG 1 TBA PGMEA 18.0 0.22 rectangular (80) (1) (0.078) (560) I-9 Polymer 9 PAG 1 TBA PGMEA 17.0 0.22 rectangular (80) (1) (0.078) (560) I-10 Polymer 10 PAG 1 TBA PGMEA 19.0 0.22 rectangular (80) (1) (0.078) (640) I-11 Polymer 1 PAG 2 TBA PGMEA 20.5 0.22 rectangular (80) (1) (0.078) (480) I-12 Polymer 1 PAG 2 TEA PGMEA 21.0 0.20 rectangular (80) (1) (0.063) (480) I-13 Polymer 1 PAG 2 TMMEA PGMEA 21.0 0.20 rectangular (80) (1) (0.118) (480) I-14 Polymer 1 PAG 2 TMEMEA PGMEA 21.5 0.22 rectangular (80) (1) (0.173) (480) I-15 Polymer 1 PAG 2 DRR 1 TEA PGMEA 17.5 0.24 rectangular (70) (1) (10) (0.063) (480) I-16 Polymer 1 PAG 2 DRR 2 TEA PGMEA 19.0 0.22 rectangular (70) (1) (10) (0.063) (480) I-17 Polymer 1 PAG 2 DRR 3 TEA PGMEA 21.5 0.22 rectangular (70) (1) (10) (0.063) (480) I-18 Polymer 1 PAG. 2 DRR 4 TEA PGMEA 20.0 0.20 rectangular (70) (1) (10) (0.063) (480) I-19 Polymer 1 PAG 2 ACC 1 TEA PGMEA 20.5 0.22 rectangular (80) (1) (4) (0.063) (480) I-20 Polymer 1 PAG 2 ACC 2 TEA PGMEA 22.0 0.22 rectangular (80) (1) (4) (0.063) (480) It is seen from Table 1 that the resist compositions within the scope of the invention have a high sensitivity and resolution upon KrF excimer laser exposure. Example II Resist compositions were formulated using inventive polymers and examined for resolution upon ArF excimer laser exposure. Examples II-1 to II-2: Evaluation of resist resolution Resist compositions were prepared as in Example I in accordance with the formulation shown in Table 2. The resulting resist solutions were spin-coated onto silicon wafers, then baked on a hot plate at 110°C for 90 seconds to give resist films having a thickness of 0.5 µm. The resist films were exposed using an ArF excimer laser stepper (Nikon Corporation; NA 0.55), then baked (PEB) at 110°C for 90 seconds, and developed with a solution of 2.38% TMAH in water, thereby giving positive patterns. The resulting resist patterns were evaluated as described below. First, the sensitivity (Eth, mJ/cm 2 ) was determined. Next, the optimal dose (Eop, mJ/cm 2 ) was defined as the dose which provides a 1:1 resolution at the top and bottom of a 0.25 µm line-and-space pattern, and the resolution of the resist under evaluation was defined as the minimum line width (µm) of the lines and spaces that separated at this dose. The shape of the resolved resist pattern was examined under a scanning electron microscope. The composition and test results of the resist materials are shown in Table 2. Table 2 Example Base resin (pbw) Photoacid generator (pbw) Dissolution regulator (pbw) Basic compound (pbw) Solvent (pbw) Optimum dose(mJ/cm 2 ) Resolution (µm) Shape II-1 Polymer 1 PAG1 - TBA PGMEA 16.0 0.16 rectangular (80) (1) (0.078) (480) II-2 Polymer 1 PAG2 DRR4 TEA PGMEA 15.0 0.15 rectangular (70) (1) (10) (0.063) (480) It is seen from Table 2 that the resist compositions within the scope of the invention have a high sensitivity and resolution upon ArF excimer laser exposure.

Claims: An ester compound of the following general formula (1): wherein R 1 is hydrogen, methyl or CH 2 CO 2 R 3 , R 2 is hydrogen, methyl or CO 2 R 3 , R 3 is a straight, branched or cyclic alkyl group of 1 to 15 carbon atoms, Z is a divalent hydrocarbon group of 1 to 10 carbon atoms, and k is 0 or 1, and R 4 is a cyclic tertiary alkyl group of up to 20 carbon atoms, of the formula (L3) or (L4) : wherein, the broken line denotes a valence bond, R L05 is a straight, branched or cyclic alkyl group of 1 to 8 carbon atoms or a substituted or unsubstituted aryl group of 6 to 20 carbon atoms, m is equal to 0 or 1, n is equal to 0, 1, 2 our 3, and 2m+n is equal to 2 or 3. R L06 is a straight, branched or cyclic alkyl group of 1 to 8 carbon atoms or a substituted or unsubstituted aryl group of 6 to 20 carbon atoms; R L07 to R L16 independently represent hydrogen or monovalent hydrocarbon groups of 1 to 15 carbon atoms which may contain a hetero atom, or two of R L07 to R L16 , taken together, form a ring, in which case each of said two of R L07 to R L16 represents a divalent C 1 -C 15 hydrocarbon group which may contain a hetero atom, or two of R L07 to R L16 which are attached to adjoining carbon atoms, bond together directly to form a double bond. A polymer comprising recurring units of the following general formula (1a-1) or (1a-2) derived from an ester compound of the general formula (1) of claim 1 and having a weight average molecular weight of 1,000 to 500,000, wherein R 1 , R 2 , R 4 , Z and k are as defined above. The polymer of claim 2 further comprising recurring units of at least one of the following formulas (2a) to (10a): wherein R 1 , R 2 and k are as defined above, R 5 is hydrogen or a carboxyl or hydroxyl-containing monovalent hydrocarbon group of 1 to 15 carbon atoms, at least one of R 6 to R 9 is a carboxyl or hydroxyl-containing monovalent hydrocarbon group of 1 to 15 carbon atoms, and the remainders are independently hydrogen or a straight, branched or cyclic alkyl group of 1 to 15 carbon atoms, or R 6 to R 9 , taken together, may form a ring, and when they form a ring, at least one of R 6 to R 9 is a carboxyl or hydroxyl-containing divalent hydrocarbon group of 1 to 15 carbon atoms, and the remainders are independently a single bond or a straight, branched or cyclic alkylene group of 1 to 15 carbon atoms, R 10 is a monovalent hydrocarbon group of 3 to 15 carbon atoms containing a -CO 2 - partial structure, at least one of R 11 to R 14 is a monovalent hydrocarbon group of 2 to 15 carbon atoms containing a -CO 2 - partial structure, and the remainders are independently hydrogen or a straight, branched or cyclic alkyl group of 1 to 15 carbon atoms, or R 11 to R 14 , taken together, may form a ring, and when they form a ring, at least one of R 11 to R 14 is a divalent hydrocarbon group of 1 to 15 carbon atoms containing a -CO 2 - partial structure, and the remainders are independently a single bond or a straight, branched or cyclic alkylene group of 1 to 15 carbon atoms, R 15 is a polycyclic hydrocarbon group of 7 to 15 carbon atoms or an alkyl group containing such a polycyclic hydrocarbon group, R 16 is an acid labile group, X is -CH 2 - or -O-, and Y is -O- or -(NR 17 )- wherein R 17 is hydrogen or a straight, branched or cyclic alkyl group of 1 to 15 carbon atoms. A resist composition comprising the polymer of claim 2 or claim 3. A process for forming a resist pattern comprising the steps of: applying the resist composition of claim 4 onto a substrate to form a coating, heat treating the coating and then exposing it to high-energy radiation or electron beams through a photo mask, and optionally heat treating the exposed coating and developing it with a developer. A process for producing an ester compound of claim 1 comprising a process A entailing esterification reaction for a carboxylic acid of formula (11) with a halogenated alkyl carboxylate of formula (12) or process B entailing esterification reaction of a carboxylic halide of formula (13) with a hydroxyalkyl carboxylate of formula (14), where, R 1 , R 2 , R 4 , Z and k are as defined above, and X is bromine or chlorine.

Inventors: HASEGAWA KOJI, HATAKEYAMA JUN, KINSHO TAKESHI, NAKASHIMA MUTSUO, NISHI TSUNEHIRO, TACHIBANA SEIICHIRO, WATANABE TAKERU, HASEGAWA, KOJI, HATAKEYAMA, JUN, KINSHO, TAKESHI, NAKASHIMA, MUTSUO, NISHI, TSUNEHIRO, TACHIBANA, SEIICHIRO, WATANABE, TAKERU, Hasegawa, Koji, Shin-Etsu Chemical Co., Ltd., Hatakeyama, Jun, Shin-Etsu Chemical Co., Ltd., Kinsho, Takeshi, Shin-Etsu Chemical Co., Ltd., Nakashima, Mutsuo, Shin-Etsu Chemical Co., Ltd., Nishi, Tsunehiro, Shin-Etsu Chemical Co., Ltd., Tachibana, Seiichiro, Shin-Etsu Chemical Co., Ltd., Watanabe, Takeru, Shin-Etsu Chemical Co., Ltd
Applicants: SHINETSU CHEMICAL CO, SHIN-ETSU CHEMICAL CO., LTD.
Application year: 2001
Main classification: G03F7
IPC codes: C07C69, G03F7, C07C67, C08F20, C08G61