The effect of intensity distribution in the reflected beam on the detection error of monochromatic optical autofocus systems

The measurement error induced by the intensity distribution in the reflected beam from the surface of a substrate was studied to improve the focal accuracy of an autofocus system. It is shown that scanning of the probe beam across the wafer surface reduces the effect of nonuniform reflectivity. An accuracy of 0.6 μm can be obtained. A signal processing circuit which accommodates 40 dB of incident intensity range is developed.