Resist film uniformity in the microlithography process

With the trends toward smaller feature size, one of the challenge is to control the resist thickness and uniformity to a tight tolerance in order to minimize thin film interference effects on the critical dimensions. In this paper, we propose a new approach to improve resist thickness control and uniformity through the soft-bake process. Using an array of thickness sensors, a multizones bakeplate and a sliding mode control algorithm, the temperature distribution of the bakeplate is manipulated in real-time to reduce the resist thickness nonuniformity. The sliding mode control algorithm is implemented in a cascaded control structure so that the bake temperature is constrained. This is to prevent decomposition of the photoactive compound in the resist. We have experimentally demonstrated an improvement in the resist thickness uniformity across individual wafers and from wafer-to-wafer. The cascaded control structure using a sliding mode control algorithm provides a simpler and faster implementation of the thickness control strategy and makes it more suitable for real-time application. There is about 75 times reduction in the computation time and a resist thickness nonuniformity of less than 10 Å is achieved.