In Situ Monitoring of Photoresist Thickness Uniformity of a Rotating Wafer in Lithography

The coating of the photoresist on the semiconductor substrate is a common process in lithography sequence. It is important to monitor the uniformity of the photoresist thickness across the substrate as the nonuniformity in photoresist thickness leads to variations in the linewidth/critical dimension (CD). In this paper, we propose a simple in situ photoresist thickness monitoring system. Our approach involves the integration of a single spectrometer to measure the photoresist thickness contour on the wafer during the spin-coating step or the edge-bead removal step. We note that the existing approaches in the monitoring of photoresist thickness are for the cases of nonrotating wafers. Our proposed approach also does not require extra processing steps compared with offline tools, which require the wafer to be moved from the processing equipment to the metrology tool. The experimental results are compared with an offline ellipsometer: the worst-case error is found to be less than 1%.