Real-time predictive control of photoresist film thickness uniformity

With the trends toward larger wafer size and the linewidth going below 100 nm, one of the challenges is to control the resist thickness and uniformity to a tight tolerance in order to minimize the thin-film interference effect on the critical dimension. In this paper, we propose a new approach to improve resist thickness control and uniformity through the softbake process. Using an array of thickness sensors, a multizone bakeplate, and advanced control strategy, the temperature distribution of the bakeplate is manipulated in real time to reduce resist thickness nonuniformity. The bake temperature is also constrained to prevent the decomposition of a photoactive compound in the resist. We have experimentally obtained a repeatable improvement in resist thickness uniformity from wafer-to-wafer and across individual wafers. Thickness nonuniformity of less than 10 Angstrom has been obtained. On average, there is 10 x improvement in the thickness uniformity as compared to conventional softbake process.