Exploring the fundamental limit of CD control: shot noise and CD uniformity improvement through resist thickness

We have observed that increasing the thickness of the resist can improve the critical dimension uniformity (CDU) in electron beam lithography. This is our first experimental demonstration that increasing the acid generation in the resist by incident electrons is a pathway to reduce the effect of shot noise on CDU. The measurements were made with our Quadra raster shaped beam lithography system. The resist was REAP 200, a chemically amplified resist. The thicknesses were 200, 300 and 600 nm. The phenomenon is consistent with our model prediction that there would be a reduction of the shot-noise-induced CDU as the number of acid molecules generated in the chemically amplified resist increased with the resist thickness. We used the model to estimate the acid generation efficiency and the resist blur. We have also observed deviations from this trend in the thick resist (600 nm) suggesting complexity that may not be explained by the model. We are continuing our investigation to confirm these preliminary results.