Interferometric method and apparatus to measure surface topography

Abstract

A non-contact method for measuring the composition-dependent phase change on reflection that occurs using profile measuring interferometry. A profile measuring interference microscope utilizing an extended, narrow bandwidth illumination source is used to generate a two-beam interference intensity pattern at a given field position on a detector array. A reference surface of known surface characteristics and an unknown test surface being profiled are axially translated relative to each other while the interference intensity pattern impinging on the detector array is sampled, digitized, stored and then utilized to produce a digitized two-beam interference intensity pattern, the shape of which is characteristic of the particular interferometer configuration. The axial position of maximum interference contrast and the phase of the digitized two-beam interference intensity pattern at this position of maximum interference contrast are determined by mathematical analysis of the intensity data. The phase at the maximum interference contrast position is divided by an analytically-derived constant to determine the phase change on reflection from the test surface, without regard to the compositional characteristics of the test surface material. The ability to measure and quantify the phase change on reflection for materials with complex indices of refraction notably improves the accuracy of heretofore-known profile measuring interferometric techniques when profiling test surfaces formed of two or more dissimilar materials with at least one having a complex index of refraction.