Real-time estimation and control of photoresist properties in microlithography

In semiconductor manufacturing process, the thickness and extinction coefficient are two important properties of photoresist which need careful estimation and control. Wafer warpage is common in microelectronics processing due to stress induced. In this paper, the effect of warpage on the accuracy of resist properties estimation is investigated and an in-situ calibration method is proposed. We will also propose a new application of the conventional reflectometer which can be used for in-situ warpage detection and theoretical analysis is made by using inverse square law. Non-uniformity in extinction coefficient across the wafer will lead to non-uniformity in the linewidth. With the availability of accurate estimation of extinction coefficient, an innovative approach to control the within wafer photoresist extinction coefficient uniformity is also proposed. Our approach uses an array of spectrometers positioned above a multizone bakeplate to monitor the extinction coefficient. With these in-situ measurements, the temperature profile of the bakeplate is controlled in real time by manipulating the heater power distribution using conventional proportional-integral (PI) control algorithm. We have experimentally obtained a repeatable improvement in the extinction coefficient uniformity from wafer-to-wafer and within wafer. A 70% improvement in extinction coefficient average uniformity is achieved.