Analysis of thermally activated kinetics and uniformity of photoresist ashing process on 300 mm wafers

Abstract

Ashing rate and nonuniformity are the usual parameters used for the evaluation of industrial ashers. A detailed study of the wafer temperature and of its effect on the two contributions to the diminution of photoresist thickness, namely, shrinkage and chemical reactions, gives a solid ground for understanding the variations of ashing rate with process conditions. It is in particular shown how variations of shrink with temperature affect the measured ashing rate. This analysis serves as a basis for proposing tests that could be used as “standards” and ease the objective comparison of industrial ashers, based on real performances. They include a determination of ashing rate on hard-baked wafers for characterization of radical density, a demonstration of the ion-free nature of the process, and a determination of wafer heat-up rate. Due to the very reproducible nature of the distribution of ashing rates over a wafer, it is argued that the “range” parameter should be used over standard deviation for the measurement of nonuniformity within a wafer.