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Examples of the repeat units of formula (2) are shown below. 

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Using a polymer comprising acid labile group-containing repeat units represented by formula (1) or (2), a chemically amplified positive resist composition having a high resolution and sensitivity can be formulated. For use in EB image writ› ing for mask production, the resist composition should pref› erably have higher etching resistance. In this embodiment, the polymer may comprise aromatic ring-containing repeat units represented by the general formula (3) or (4) or the general formula (5) or (6). More preferably the polymer may com› prise indene or acenaphthylene groups, whereby dissolution contrast and line edge roughness are improved. 

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