Real-time control of photoresist development process

Abstract

Critical dimension (CD) or linewidth is one the most critical variable in the lithography process with the most direct impact on the device speed and performance of integrated circuit. The resist development step is one of the critical step in the lithography process that can have an impact on the CD uniformity. The development rate can have an impact on the CD uniformity from wafer-to-wafer and within-wafer. Non-uniformity in the time to reach endpoint is the result of non-uniformity in film thickness, exposure dosage and resist chemical compound. This can in turn lead to non-uniformity in the linewidth. Conventional approach to control this process include monitoring the end-point of the develop process and adjust the development time or concentration from wafer-to-wafer or run-to-run. This paper presents an innovative approach to control the photoresist development rate in real-time by monitoring the photoresist thickness. Our approach uses a spectrometer positioned above a bakeplate to monitor the development rate. The absorption coefficient can be extracted from the spectrometers data using standard optimization algorithms. 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 time to reach end-point for the develop process from wafer-to-wafer. Nonuniformity of less than 5% in the time to reach endpoint has been achieved.