Comparison Of Wyko And Tis Measurements Of Surface Finish

E. L. Church; G. M. Sanger; P. Z. Takacs

doi:10.1117/12.939843

20 April 1987 Comparison Of Wyko And Tis Measurements Of Surface Finish

E. L. Church, G. M. Sanger, P. Z. Takacs

Proceedings Volume 0749, Metrology: Figure and Finish; (1987) https://doi.org/10.1117/12.939843
Event: OE LASE'87 and EO Imaging Symposium, 1987, Los Angeles, CA, United States

Abstract

Profile and area measurements of the roughness of a given surface will generally be different since the two measurement techniques are sensitive to different areas of surface-frequency space. We explore the magni-tudes of these differences by calculating the ratio of rms roughness values, σ(TIS)/ σ(Wyko), using strawman models of a Wyko profiling microscope and a Talandic integrating scatterometer applied to surfaces having different roughness power spectra. As expected, the results show that this ratio can vary widely about unity, with values depending on the magnification of the objective used in the Wyko microscope and the "color" of the surface spectrum. An amazing counter example appears to occur for surfaces having an approximately "1/82" BRDF, or equivalently, a 1/f profile power spectrum -- shapes which are frequently observed for non-metallic mirror surfaces. In this case the predicted TIS and Wyko roughness values are essentially identical and inde-pendent of the Wyko magnification. This equality, however, comes from a curious mathematical-numerical coinci-dence and does not mean that these apparently "universal" values represent any intrinsic finish parameters of the surface being measured. In fact, if the Wyko data are filtered to remove the contributions from surface wavelengths longer than those included in the TIS measurements in order to more nearly match the instrumental bandwidths, the calculated ratio of measured rms roughness values increases to 1.5 to 5, depending on the Wyko parameters used. These results illustrate the fact that any realistic comparison of profile and area measure-ments of surface finish requires a knowledge of both the instrumental transfer functions and the form of the power spectrum of the surface being measured. The present paper discusses these issues and provides analytic machinery for the detailed quantitative comparison of profile, TIS and BRDF measurements.

Citation Download Citation

E. L. Church, G. M. Sanger, and P. Z. Takacs "Comparison Of Wyko And Tis Measurements Of Surface Finish", Proc. SPIE 0749, Metrology: Figure and Finish, (20 April 1987); https://doi.org/10.1117/12.939843

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