Two Approaches For Scanning Surface Topography Using A Reflected Beam

Robert A. Bruce

doi:10.1117/12.939837

20 April 1987 Two Approaches For Scanning Surface Topography Using A Reflected Beam

Robert A. Bruce

Proceedings Volume 0749, Metrology: Figure and Finish; (1987) https://doi.org/10.1117/12.939837
Event: OE LASE'87 and EO Imaging Symposium, 1987, Los Angeles, CA, United States

Abstract

Small topographic variations of a spinning surface (e.g. disc, wafer) cause a reflected laser beam to change in two fundamentally distinct ways, in pathlength and in angle. This paper describes an experiment which compares surface topography scanning using each of these aspects. A laser beam heterodyne scanner was built to profile pathlength variations along a track. This instrument was modified to incorporate a beam position sensing capability using the same beam that is projected to and reflected from the surface. This per-mitted comparison of the two approaches using the same track topography. The topographic velocity of a beam spot on a spinning surface is given by pathlength variations as a function of time. The signal processor of the laser beam heterodyne interferometer has an output that is directly proportional to this velocity. This voltage relates directly to the circumferential slope along the track for a given radius. A quadrant sensor, indicating position of this same reflected beam, outputs a pair of voltages which indicate the orthogonal components of absolute slope. The quadrant sensor is oriented so that one of these components is circumferentially aligned with the track and can be directly compared with the heterodyne slope scan. Both approaches demonstrate excellent sensitivity for the surface waviness encountered, and within certain limits are not affected by reflectivity variations of the surfaces tested. Each approach has several features and advantages. The heterodyne approach is calibrated both to the laser wavelength and the flat electronic transfer function of the frequency-to-voltage converter used. It is capable of sensing topographic velocity over a broad dynamic range and wide frequency span. The reflected beam deflection approach has a simple optical design, has a second channel for radial slope, is immune to vibration, and is independent of the traversing speed.

Citation Download Citation

Robert A. Bruce "Two Approaches For Scanning Surface Topography Using A Reflected Beam", Proc. SPIE 0749, Metrology: Figure and Finish, (20 April 1987); https://doi.org/10.1117/12.939837

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