To bake or not to bake... : the role of prebake in the EUV resist process

Abstract

In photoresist processing a prebake is traditionally used after coating the photoresist on a wafer to drive off solvents in the resist, resulting in a more stable film. In comparison to other stages of the lithography process (e.g. the conditions of exposure and post-exposure events), limited attention is paid in the prebake optimization for the EUV application. In this work, investigation is done to clarify its role for the case of chemically amplified resists (CAR). Compared to the earlier DUV application, this resist is used at significantly smaller thicknesses and has a significantly different composition in terms of photo acid generator (PAG) and quencher types and concentration.

In a first screening, a commercial CAR material - coated on Si - was investigated towards contrast changes at different prebake temperatures. It was found that lower temperature can result in adhesion failure when substrate conditions are not optimized for adhesion. With proper adhesion promotion however, it was found that prebake temperature could be lowered significantly or even omitted, without clear change in contrast. Using model resists in combination with residual gas analysis (RGA), it was found that the use of photo-decomposable quencher could be responsible for maintaining contrast to lower bake temperatures. In a second investigation, an assessment towards outgas risk was done when using resists at lower prebake temperatures in EUV scanner environment. Finally, the printability of commercial CAR was tested on the NXE3400 EUV scanner at different prebake temperatures. This was done by coating the CAR on two available underlayer materials: spin-on-glass and deposited underlayer. Results show that the prebake temperature could be reduced or even omitted without a clear deterioration in process window, line edge roughness and defectivity. It was found that proper choice of underlayer material could even improve slightly the printing performance at lower prebake condition.