Moving boundary transport model for acid diffusion in chemically amplified resists

Abstract

We propose a general model for latent image formation in chemically amplified resists. The model is based on a moving boundary acid transport concept that incorporates transient free volume generation and densification. It is based on experimental observation of negligible acid diffusion in polyhydroxysterene below Tg. The model offers insight into the post exposure bake (PEB) reaction mechanism that governs the relief image formation in chemically amplified resists. During PEB, there is a thermally induced deprotection catalyzed by the photogenerated acid that produces volatile by-products thereby generating free volume in the resist polymer. The free volume enhances local diffusivity of the acid. The rapid loss of the volatile products is followed by relaxation of the polymer matrix which eliminates the transient free volume and densifies the polymer. The densified polymer inhibits the diffusion of any acid trapped in the deprotected sites. We present cases where the model reduces to Fickean and case II type reaction driven diffusion models under some simplifying assumptions. The model was implemented in simulation tools for resist models to simulate one-dimensional and two-dimensional profiles. The results imply that the relief image formation depends strongly on both the mechanical the chemical properties of the resist. This model provides new directions for resist process optimization.