cally, acetoxystyrene or acetoxyvinylnaphthalene is used instead of hydroxystyrene or hydroxyvinylnaphthalene, and after polymerization, the acetoxy group is deprotected by alkaline hydrolysis, for thereby converting the polymer product to hydroxystyrene or hydroxyvinylnaphthalene. For alkaline hydrolysis, a base such as aqueous ammonia or triethylamine may be used. Preferably the reaction tempera› ture is -20° C. to 100° C., more preferably 0° C. to 60° C., and the reaction time is 0.2 to 100 hours, more preferably 0.5 to 20 hours. 

The base polymer should preferably have a weight aver› age molecular weight (Mw) in the range of 1,000 to 500,000, and more preferably 2,000 to 30,000, as measured by GPC versus polystyrene standards using tetrahydrofuran (THF) solvent. With too low a Mw, the resist composition may become less heat resistant. A polymer with too high a Mw may lose alkaline solubility and give rise to a footing phenomenon after pattern formation. 

If a base polymer has a wide molecular weight distribu› tion or dispersity (Mw/Mn), which indicates the presence of lower and higher molecular weight polymer fractions, there is a possibility that foreign matter is left on the pattern or the pattern profile is degraded. The influences of molecular weight and dispersity become stronger as the pattern rule becomes finer. Therefore, the base polymer should prefer› ably have a narrow dispersity (Mw/Mn) of 1.0 to 2.0, especially 1.0 to 1.5, in order to provide a resist composition suitable for micropatterning to a small feature size. 

It is understood that a blend of two or more polymers 30 which differ in compositional ratio, Mw or Mw/Mn is acceptable. 

Other Components 

With the sulfonium salt having formula (1-1) or iodonium salt having formula (1-2) and the base polymer, defined above, other components such as an organic solvent, sur› factant, dissolution inhibitor, and crosslinker may be blended in any desired combination to formulate a chemi› cally amplified positive or negative resist composition. This positive or negative resist composition has a very high sensitivity in that the dissolution rate in developer of the base polymer in exposed areas is accelerated by catalytic reaction. In addition, the resist film has a high dissolution contrast, resolution, exposure latitude, and process adapt› ability, and provides a good pattern profile after exposure, and minimal proximity bias because of restrained acid diffusion. By virtue of these advantages, the composition is fully useful in commercial application and suited as a pattern-forming material for the fabrication of VLSis. Par› ticularly when a chemically amplified positive resist com› position capable of utilizing acid catalyzed reaction is for› mulated, the composition has a higher sensitivity and is further improved in the properties described above. 

The resist composition may contain an acid generator other than the sulfonium salt having formula (1-1) and iodonium salt having formula (1-2). The other acid generator is typically a compound (PAG) capable of generating an acid upon exposure to actinic ray or radiation. Although the PAG used herein may be any compound capable of generating an acid upon exposure to high-energy radiation, those com- pounds capable of generating sulfonic acid, imide acid (imidic acid) or methide acid are preferred. Suitable PAGs include sulfonium salts, iodonium salts, sulfonyldiazometh› ane, N-sulfonyloxyimide, and oxime-O-sulfonate acid gen› erators. Exemplary acid generators are described in JP-A 2008-111103, paragraphs [0122]-[0142] (U.S. Pat. No. 7,537,880). The other acid generator is preferably added in 

an amount of O to 200 parts, and more preferably 0.1 to 100 parts by weight per 100 parts by weight of the base polymer. 

In the case of positive resist compositions, inclusion of a dissolution inhibitor may lead to an increased difference in dissolution rate between exposed and unexposed areas and a further improvement in resolution. In the case of negative resist compositions, a negative pattern may be formed by adding a crosslinker to reduce the dissolution rate of exposed area. 

Examples of the organic solvent used herein are described in JP-A 2008-111103, paragraphs [0144]-[0145] (U.S. Pat. No. 7,537,880). Exemplary solvents include ketones such as cyclohexanone, cyclopentanone and methyl-2-n-pentyl ketone; alcohols such as 3-methoxybutanol, 3-methyl-3- methoxybutanol, l-methoxy-2-propanol, and l-ethoxy-2- propanol; ethers such as propylene glycol monomethyl ether, ethylene glycol monomethyl ether, propylene glycol monoethyl ether, ethylene glycol monoethyl ether, propyl› ene glycol dimethyl ether, and diethylene glycol dimethyl ether; esters such as propylene glycol monomethyl ether acetate (POMEA), propylene glycol monoethyl ether acetate, ethyl lactate, ethyl pyruvate, butyl acetate, methyl 3-methoxypropionate, ethyl 3-ethoxypropionate, t-butyl acetate, t-butyl propionate, and propylene glycol mono-t- butyl ether acetate; and lactones such as y-butyrolactone, which may be used alone or in admixture. 

The organic solvent is preferably added in an amount of 100 to 10,000 parts, and more preferably 200 to 8,000 parts by weight per 100 parts by weight of the base polymer. 

Exemplary surfactants are described in JP-A 2008- 111103, paragraphs [0165]-[0166]. Inclusion of a surfactant may improve or control the coating characteristics of the resist composition. The surfactant is preferably added in an amount of 0.0001 to 10 parts by weight per 100 parts by weight of the base polymer. 

The dissolution inhibitor which can be used herein is a compound having at least two phenolic hydroxyl groups on the molecule, in which an average of from O to 100 mo! % of all the hydrogen atoms on the phenolic hydroxyl groups are replaced by acid labile groups or a compound having at least one carboxyl group on the molecule, in which an average of 50 to 100 mo! % of all the hydrogen atoms on the carboxyl groups are replaced by acid labile groups, both the compounds having a molecular weight of 100 to 1,000, and preferably 150 to 800. Typical are bisphenol A, trisphenol, phenolphthalein, cresol novolac, naphthalenecarboxylic acid, adamantanecarboxylic acid, and cholic acid derivatives in which the hydrogen atom on the hydroxyl or carboxyl group is replaced by an acid labile group, as described in U.S. Pat. No. 7,771,914 (JP-A 2008-122932, paragraphs [0155]-[0178]). 

In the positive resist composition, the dissolution inhibitor is preferably added in an amount of O to 50 parts, more preferably 5 to 40 parts by weight per 100 parts by weight of the base polymer. 

Suitable crosslinkers which can be used herein include epoxy compounds, melamine compounds, guanamine com› pounds, glycoluril compounds and urea compounds having substituted thereon at least one group selected from among methylol, alkoxymethyl and acyloxymethyl groups, isocya› nate compounds, azide compounds, and compounds having a double bond such as an alkenyl ether group. These compounds may be used as an additive or introduced into a polymer side chain as a pendant. Hydroxy-containing com› pounds may also be used as the crosslinker. 

Of the foregoing crosslinkers, examples of suitable epoxy compounds include tris(2,3-epoxypropyl) isocyanurate, 

