High-Resolution Wafer Surface Topology Measurement Using Phase-Shifting Shadow Moiré Technique

Abstract

The traditional shadow moiré technique has been employed, using the fringe pattern information and numerical interpolation, to construct the wafer surface topology. In this paper, phase-shifting shadow moiré technique is discussed and applied to the measurement of wafer surface topology with high resolution. The phase-shifting technique takes advantage of the gray level information to increase the physical resolution of the measurement. A series of fringe patterns are recorded while they are shifted by moving the wafer along the direction perpendicular to wafer surface. The phase is encoded in the variations in the intensity pattern of the recorded fringe images, and a simple point-by-point calculation recovers the phase. The needs to locate the fringe centers and interpolation are eliminated. Since the depth variation of wafer surface is very small, usually within the range of 0.1 to 20 microns, very fine grating is required in order to capture the small depth variation. However, very fine grating will introduce strong diffraction effect which blurs the fringe patterns. In this study, the Talbot distance is applied to obtain images with good contrast. The phase shift is realized by moving the wafer to change the distance between the wafer surface and the reference grating. A four-step phase wrapping algorithm is used to calculate the phase. The phase pattern recovered from four fringes patterns is presented. Future work such as how to reduce the noise, how to do phase unwrapping and calibration is also discussed.