Optical thickness measurement of substrates using a transmitted wavefront test at two wavelengths to average out multiple reflection errors

摘要

I measure the optical thickness of thin substrates using a transmitted wavefront test in which the object is placed inside a Fizeau cavity comprised of a reference flat and a mechanically actuated transmission flat for phase shifting interferometry (PSI). Traditionally, this test had been complicated by the unwanted secondary reflections between the object surfaces even when the object is tilted. These reflections generate errors that are increasingly difficult to suppress as the substrate thickness decreases. The new technique involves two successive PSI measurements of the optical profile separated by a discrete change in source wavelength. The change in source wavelength is calculated so as to invert the error contributions from multiple surface reflections. Thus the average of the two measurements is relatively free of these error contributions.