Refractive indices in thick photoresist films as a function of bake conditions and film exposure

Abstract

Prism coupling is applied to thick film DNQ/novolak photoresist materials in the 4.6 to 24 micrometer range. Refractive indices are obtained as a function of softbake temperature and softbake time for exposed and unexposed resists. The results for AZ<SUP>R</SUP> P4330 RS and AZ<SUP>R</SUP> EXP 9244 photoresists are compared at a film thickness of 4.6 micrometer. Each photoresist shows an increase in refractive index at 633 nm with softbake temperature and softbake time and a decrease in refractive index with an increase in exposure. The refractive index changes of the films are correlated to the solvent content as determined by radioactive labeling and by gas chromatography. In thicker films, solvent gradients profiles could be obtained by the application of an inverse WKB approximation to the effective indices. The lithographic performance of AZ<SUP>R</SUP>EXP 9244, a new generation thick film photoresist, is examined at various softbake temperatures and softbake times and interpreted in terms solvent content as obtained from the refractive index.