In-situ real-time temperature control of baking systems in lithography

Abstract

We proposed an in-situ method to control the wafer spatial temperature uniformity during thermal cycling of silicon substrate in the lithography sequence. These thermal steps are usually conducted by the placement of the substrate on the heating plate for a given period of time. We have previously proposed an approach for controling the steady-state wafer temperature uniformity in steady-state. In this paper, we extend the approach by considering the dynamic properties of the system. A detailed physical model of the thermal system is first developed by considering energy balances on the system. Next, by monitoring the bake-plate temperature and fitting the data into the model, the temperature of the wafer can be estimated and controlled in real-time. This is useful as production wafers usually do not have temperature sensors embedded on it, these bake-plates are usually calibrated based on test wafers with embedded sensors. However, as processes are subjected to process drifts, disturbances, and wafer warpages, real-time correction of the bake-plate temperatures to achieve uniform wafer temperature is not possible in current baking systems. Any correction is done based on run-to-run control techniques which depends on the sampling frequency of the wafers. Our approach is real-time and can correct for any variations in the desired wafer temperature performance during both transient and steady state. Experimental results demonstrate the feasibility of the approach.