Proceedings PaperStepper overlay performance measurements using the air-turbulence-compensated interferometer
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Air turbulence affects the performance of the helium-neon interferometer used to control the wafer stage of stepper or step and scan lithography systems. In this paper we describe the principles of operation and in-stepper performance of an air turbulence compensated interferometer designed to address these problems. Collinear combination of a two-wavelength compensation system using second harmonic interferometry, with the existing HeNe interferometer used for length measurement, provides a highly accurate system with real-time compensation for air turbulence. This paper reports on the hardware configuration and preliminary performance evaluation of an ATCI system which has been installed on a semiconductor wafer stepper. A brief overview of the signal processing algorithms is provided, showing the automatic compensation features and noise insensitivity of the algorithm.