denotes a C$_{3}$ -C$_{12 }$ cyclic hydrocarbon group or a bridged cyclic hydrocarbon group, which may contain hydroxyl, -0- or -S-, R 11 , R $^{12 }$and R $^{13 }$are each independently hydrogen, fluorine or trifluoromethyl, at least either one of R 11 , R $^{12 }$and R 13 contains at least one fluorine atom. The subscript a is 1 or 2, r, s, t, u and v are numbers in the range: 0<r<l .0, 0<ss0.8, 0<ts0.8, 0<us0.2, and 0<vs0.2. Also provided is a chemically amplified positive resist composition comprising (A) an organic solvent, (B) a base resin comprising the polymer of any one of the foregoing embodiments, (C) a photoacid generator, and (D) a basic compound. 

In another aspect, the invention provides a process for forming a pattern, comprising the steps of applying the posi› tive resist composition of any one of the foregoing embodi› ments onto a substrate to form a resist coating; heat treating the resist coating, then exposing it to high-energy radiation, EUV or electron beam through a photomask; and optionally heat treating the exposed resist coating, then developing with a developer. 

In a further aspect, the invention provides a process for forming a pattern, comprising the steps of applying the posi› tive resist composition of any one of the foregoing embodi› ments onto a substrate to form a resist coating; heat treating the resist coating, then exposing it to high-energy radiation or electron beam through a photomask; and heat treating the exposed resist coating, then developing with a developer. In one embodiment, the exposing step is performed by immer› sion lithography including holding a high refractive index liquid having a refractive index of at least 1.0 between the resist coating and a projection lens. In another embodiment, a protective film is formed on the resist coating, and the expos› ing step is performed by immersion lithography including holding a high refractive index liquid having a refractive index of at least 1.0 between the protective film and a projec› tion lens. 

In a still further aspect, the invention provides a photomask blank comprising a chromium compound film and a coating of the resist composition ofany one of the foregoing embodi› ment. A pattern is formed by heat treating the photomask blank defined above, subjecting the resist coating to pattern› wise exposure to high-energy radiation through a photomask or patternwise exposure to high-energy beam, optionally heat treating the exposed resist coating, and developing it with a developer. 

When used in the micropatterning technology, especially ArF lithography, the resist composition of the invention exhibits a very high resolution capability. The composition is thus useful in precise micropatterning. When used in the EB image writing for processing of masks, the resist composition exhibits a high resolution of fine size pattern and sensitivity sufficient to comply with high-accelerating-voltage EB irra› diation, and high etch resistance. The composition is also useful in mask production. 

In the disclosure, the singular forms "a," "an" and "the" include plural referents unless the context clearly dictates otherwise. The notation (Cn-Cm) means a group containing from n tom carbon atoms per group. The abbreviation Me stands for methyl. 

In the prior art, chemically amplified positive resist com› positions use a base resin comprising repeat units having a 

## Advantageous Effects oflnvention 

## DESCRIPTION OF EMBODIMENTS 

potentially alkaline-soluble acidic substituent group which is protected with a protective group ( or acid labile group) which will be eliminated under the action ofacid. Exemplary poten› tially alkaline-soluble acidic substituent groups include 5 hydroxyphenyl groups and carboxylic acids. The acid labile groups for protecting the former include acetal and tert-bu› toxycarbonyl groups, and the acid labile groups for protecting the latter are often tertiary alkyl groups. A typical base resin comprising repeat units containing hydroxyphenyl is polyhy- lO droxystyrene. Typical base resins comprising repeat units containing carboxylic acid include poly(methacrylic acid) and poly(acrylic acid). These resins in which some or all acidic substituent groups are protected with the above-men- 15 tioned acid labile groups are used as the base resin in chemi› cally amplified positive resist compositions. 

The potentially alkaline-soluble acidic substituent group which is protected with an acid labile group reacts with the acid which is generated by an acid generator upon exposure to high-energy radiation such as ArF excimer laser radiation, EUV or EB whereby the substituent group turns into an alkaline-soluble substituent group. In this way, the exposed regions are dissolved in an alkaline developer, yielding a positive resist pattern. It is known that in the microlithogra- phy of a positive resist pattern, carboxylic acid surpasses hydroxyphenyl as the acidic substituent group protected with an acid labile group. This is because the carboxylic acid resulting from deprotection by reaction with acid has a higher acidity than hydroxyphenyl, offering a higher alkali solubility to enable micropatterning. 

However, it is also known that the reactivity with acid of the acid labile group ( e.g., t-alkyl) for protecting carboxylic acid is very low as compared with the reactivity with acid of the acid labile group ( e.g., acetal) for protecting hydroxyphenyl. That is, the reaction with acid for deprotection oft-alkyl› protected polymethacrylic acid is very weak as compared with the reaction with acid for deprotection of acetal-pro› tected polyhydroxystyrene. As a result, a chemically ampli- fled positive resist composition using t-alkyl-protected poly› methacrylate has a very low sensitivity upon patterning. 

For example, the chemically amplified resist composition using t-alkyl-protected polymethacrylate has a lower sensi› tivity than the composition using acetal-protected polyhy- droxystyrene. Since the chemically amplified resist compo› sition using t-alkyl-protected polymethacrylate is low reactive, the post-exposure baking (PEB) step for promoting the reaction with acid to facilitate deprotection must be at a higher temperature. Differently stated, for the chemically amplified resist composition using t-alkyl-protected poly› methacrylate, the acid generated by the acid generator must be a very strong acid such as perfluoroalkanesulfonic acid. 

In the event where the acidic substituent group protected with an acid labile group is carboxylic acid, a positive resist composition using the same is favorable in processing of a fine size resist pattern, but has a drawback of low sensitivity because oflow reactivity with acid of the acid labile group. 

Then the inventors sought for a resin comprising repeat units containing carboxylic acid protected with an acid labile group which can provide a sufficiently high resolution to enable micropatterning and also impart a high sensitivity because of very high reactivity of the acid labile group with the acid generated by the acid generator. In a first aspect, the invention provides a positive resist composition comprising a polymer comprising repeat units having the general formula (1) or (2). 

