The present paper describes a novel class of norbornene-based copolymers containing 7,7-dimethyloxepan-2-one acid labile groups. Poly(3-(bicyclo[2.2.1]hept-5-en-2-ylhydroxymethyl)-7,7-dimethyloxepan-2-one- co-5-((2-decahydronaphthyl)oxycarbonyl)-norbornene-co-5-norbornene-2-carboxy lic acid-co-maleic anhydride) was synthesized and evaluated as a potential chemically amplified resist for ArF lithography. The 7,7-dimethyloxepan-2-one group of the matrix polymer was readily cleaved and the carboxylic acid functionality was formed by acid-catalyzed ring-opening reaction in the exposed region after post-exposure bake. The resist patterns of 0.12μm feature size were obtained at a dose of 10mJcm - 2 with a conventional developer, 2.38wt% tetramethylammonium hydroxide aqueous solution, using an ArF excimer laser stepper.