Accuracy improvement of focusing and leveling sensors based on pupil phase modulation

The focusing and leveling sensor (FLS) is crucial for achieving high-accuracy vertical measurements in lithography systems. To improve the vertical measurement accuracy of FLS in multilayer processes, this study explores the feasibility of reducing vertical measurement errors through pupil phase modulation, leveraging the Abbe imaging model and point spread function (PSF) engineering. Two approaches are proposed: installing phase-complementary optical path modulators (OPMs) or spiral phase plates (SPPs) into the optical path. A simulation model is built and experimentally validated, showing consistency in vertical measurement errors between simulations and experiments. The results indicate that when the spatial frequency of wafer surface reflectance due to process variation is 2.5 cycles/mm, introducing complementary OPMs or SSPs can reduce vertical measurement errors by approximately 92.3% and 81.2%, respectively. This demonstrates that both modulation methods effectively reduce vertical measurement errors, thereby improving the measurement accuracy of FLS. These methods offer significant insights for increasing process tolerance and improving the measurement accuracy of FLS in lithographic processes.