A pattern having a good balance of sensitivity, resolution and LWR is formed by providing a resist film comprising a base resin comprising recurring units having a C4-C6 tertiary alkoxy or alkoxycarbonyl group as an acid labile group and recurring units capable of generating a backbone-bound acid upon exposure, exposing the resist film to radiation, optionally post-exposure baking the resist film at a low temperature of 30-70° C., and developing the film.