The relationship between critical dimension shift and diffusion in negative chemically amplified resist systems

Abstract

Reports of latent image instability in several positive acid catalyzed resists have stimulated interest in the image stability of advanced negative resists (ANR). The critical dimension stability of the chemically amplified ANR resist SAL603 with respect to delay time between the exposure and post-exposure bake (PEB) steps was determined. Both dense and isolated 1.0 μm features were stable during a 24 h delay period and delay times from 24 h to 48 h resulted in a decrease in linewidth. The method of loss in linewidth as a function of delay time was not due to diffusion of the acid within the film. The threshold crosslink density model was introduced based on several simple assumptions concerning the effect of crosslink density (Θ) on image formation and the relationship of the crosslink density to resist acid concentrations. A method was described whereby the threshold crosslink density model can be used to determine the relative crosslink density of the resist as a function of distance along the feature edge. An analysis of the relative values of crosslink density within the resist was used to show that acid loss and not diffusion was responsible for the change in resist linewidth as a function of delay time.