Real-Time Control of Photoresist Extinction Coefficient Uniformity in the Microlithography Process

Critical dimension (CD) or linewidth is one of the most critical variables in the lithography process with the most direct impact on device speed and performance of integrated circuits. The extinction coefficient is a photoresist property that can have an impact on the CD uniformity. The extinction coefficient is a measure of the absorption of the photoresist and determines the required exposure dose for printing the features. Hence, nonuniformity in extinction coefficients across the wafer will lead to nonuniformity in the linewidth. This paper presents an innovative approach to the control within wafer photoresist extinction coefficient uniformity. Previous works in this literature can only control the average uniformity of the extinction coefficient. Our approach uses an array of spectrometers positioned above a multizone bakeplate to monitor the extinction coefficient. The extinction coefficient can be extracted from the spectrometer data using standard optimization algorithms. With these insitu measurements, the temperature profile of the bakeplate is controlled in real-time by manipulating the heater power distribution using the 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 uniformity is achieved