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Abstract

Interferometer system and method for use in a level sensor of an exposure apparatus and autofocus system employing same. Operating either at a single or multiple wavelengths, the interferometric system employs two diffraction orders, formed by diffraction grating of the system, as reference and sample beams and is structured to ensure that only light contained in one of the two orders interacts with a wafer under test, thereby ensuring that no interference fringes are projected onto the sample. The diffraction grating is positioned such that its grooves are nominally perpendicular to the direction of wafer scan. Based on measurement data representing interference between reference and sample beams at the detector, a determination of change in position of the wafer in the sample arm is made with increased sensitivity and/or resolution.

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First Claim