Real-time estimation and control of CD uniformity in lithography

Critical dimension (CD) is one the most critical variable in the lithography process with the most direct impact on the device speed and performance of integrated circuit. In this paper, we first investigate the key parameters that affect CD in the lithography sequence: the photoresist thickness and wafer temperature. This paper presents an innovative approach to real-time spatial CD uniformity control by monitoring and controlling the photoresist film thickness and temperature during the various bake step in the lithography sequence. First, we demonstrate that with in-situ estimation of the wafer temperature and air-gap, we can control the CD uniformity for both flat and warped wafers. Next, by positioning an array of spectrometers positioned above a programmable multizone bakeplate to monitor the resist thickness in real-time. The resist thickness 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 a control algorithm. We have experimentally obtained a repeatable improvement in controlling the CD uniformity via the various baking process from wafer-to-wafer and within wafer.