2 November 2011 Measurement of absolute optical thickness by wavelength tuning interferometer
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Absolute optical thickness is a fundamental parameter for the design of optical elements. In semiconductor industry, it is necessary to measure the absolute optical thickness of the central part of the projection lenses with a high accuracy. However, even when the geometrical thickness is perfectly known, a typical refractive index of fused-silica has an ambiguity of 6 × 10-5 that gives an uncertainty of 180 nm in the optical thickness for a 3 mm-thick plate. Moreover, the optical thickness measured by white light interferometry and wavelength tuning interferometry is an optical thickness with respect to not the ordinary refractive index but the group refractive index. We measured the ordinary optical thickness of a fused silica plate of 6-inch square and 3 mm thickness by a wavelength tuning interferometer with a tunable phase shifting technique. We assumed the typical refractive index and dispersion of the fused silica as approximate values. The absolute interference order for the optical thickness was finally estimated, which gives a measurement resolution of typically 10 nm for the optical thickness.
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Kenichi Hibino, Kenichi Hibino, Yangjin Kim, Yangjin Kim, Youichi Bitou, Youichi Bitou, Mamoru Mitsuishi, Mamoru Mitsuishi, "Measurement of absolute optical thickness by wavelength tuning interferometer", Proc. SPIE 8011, 22nd Congress of the International Commission for Optics: Light for the Development of the World, 80110B (2 November 2011); doi: 10.1117/12.901637; https://doi.org/10.1117/12.901637

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