Control of semiconductor substrate temperature uniformity during photoresist processing in lithography

Current photoresist processes in advanced lithography system are especially sensitive to temperature. In this manuscript, we proposed an in situ method to control the wafer spatial temperature uniformity during thermal cycling of silicon wafer in the lithography sequence. These thermal steps are usually conducted by the placement of the substrate on the heating or bake-plate for a given period of time. Our approach to improve these systems/processes involved both a re-design of the thermal processing system as well as control design. A programmable multizone thermal processing module together with a model-based feedback control method are developed to achieve temperature uniformity of a silicon wafer throughout the processing temperature cycle of ramp, hold and quench in post-exposure bake (PEB) step of lithography. Based on a detailed model of the system and process monitoring, the wafer temperature 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 depend on the sampling frequency of the wafers. Our approach is realtime and can correct for any variations in the desired wafer temperature performance during both transient and steady state. The impact if even more significant under conditions where the wafer is warped resulting in non-uniform air-gap between the wafer and bake-plate and hence non-uniform heating. Experimental results demonstrate the feasibility of the approach.