Influence of acid generator structure on T-top formation in high-temperature bake processes for environmental stabilization

Abstract

The stabilization toward airborne contamination of chemical amplification resists can be achieved by annealing to reduce the free volume of resist films. Our new positive resist consisting of a thermally and hydrolytically stable polymer and acid generator can be baked above its glass transition temperature and is thus environmentally stable. However, the high temperature bake processes place a stringent demand on the selection of the acid generator (and of course on the resist polymer). In addition to the airborne contamination, evaporation of acid generators and/or photochemically generated acids during the bake processes can result in the T-top formation, especially when the acid generator is small in size and the bake temperatures are high. This paper describes the thermal stability of several acid generators and the influence of the acid generator structure on the T-top formation in our environmentally stable chemically amplified positive resist. The careful selection of a bulky acid generator provided excellent 0.25 μm lithography with the depth-of-focus of 1.2 μm and the exposure latitude of 20%.