28  US 8,968,979 B2  27  Examples of the repeat units of formula (2) are shown  below.  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.  (3)  Me  Me  ~  ~  -00  ~  Me  Me  ~  ~  <to  co  Me  Me  ~  ~  lX)  ~  ’X  ’X  -00  ~  ’X  ’X  -continued  -continued 