Copolymers of Methyl Methacrylate and Methacrylic Acid and Their Metal Salts as Radiation Sensitive Resists

Abstract

Copolymers of methyl methacrylate and up to 40 mole percent methacrylic acid were investigated as positive resists for electron beam and x‐ray lithography. With electron beam irradiation the sensitivity of copolymer is about four times that of the methyl methacrylate homopolymer. No deterioration in resolution or in handling properties is observed, and an improvement in adhesion and in pattern thermal stability is obtained. Preferential swelling of the copolymers at very low electron doses followed by ultrasonic dispersal of the swollen areas allows a further increase in sensitivity by an order of magnitude. With an x‐ray exposure, the sensitivity improvement of the copolymers over PMMA varies between 3 and 10 depending on the contrast required. Incorporation of heavy metals, in particular thallium (I), as salts of the copolymer acids results in a substantial increase in linear x‐ray absorption coefficient, with a corresponding increase in lithographic sensitivity.