Copolymer (methacrylonitrile–methacrylic acid), which has no significant deep UV absorption prior to prebake, starts to have a broad UV absorption with the maximum at 246 nm after prebake at 130 °C in air. Cyclized structures responsible for this absorption undergo photochemical reactions: (1) main chain scissions at deep‐UV exposure, initiated by removal of isocyanic acid, providing positive tone images; (2) hydrogen transfer reactions at mid‐UV exposures to result in conjugated structures or cross linkages, providing negative tone images. With relatively high sensitivity, 50 mJ/cm2for positive and 100 mJ/cm2for negative tone images, polymer patterns with vertical wall profiles are obtained. Postbake of these resist patterns increases dry etch resistance significantly.