Merging Phase Shifting Interferometry with Confocal Chromatic Microscopy

Abstract

The implementation of the basic physical principle of Chromatic Confocal Microscopy in the field of Phase stepping interferometry (PSI) opens new opportunities for the development of an innovative surface metrology method specially dedicated to 3D nanotopography with subnanometric z axis resolution altogether with a very large measuring range: typically up to one hundred micrometers. The basic property of optical sectioning inherent to (chromatic) Confocal imaging is particularly well adapted to Phase stepping Interferometry since it automatically solves the critical and time consuming problem of phase unwrapping computation. The axial chromatic extension of the chromatic confocal setup offers a very fast and easy way to determine the height of the different elementary surfaces forming the measured object. It is then easy to carry out, for each one of those elementary surfaces, a measurement in phase shifting interferometry, at the wavelength corresponding to the altitude indicated by the confocal chromatic, in order to reach subnanometric axial resolutions. The four phases needed for implementing the phase stepping interferometric measuring procedure can be successively realized by adequate spectral shifts instead of the classical axial displacements of the reference mirror which then stands in a fixed position. Consequently this chromatic confocal phase stepping interferometer (CCPSI) has definitely no moving part, the spectral shifts being done by electrooptical means. Typical applications are MEMS and microoptics surface topography and/or roughness metrology. For this purpose we designed a new system incorporating confocal chromatic imaging and phase stepping interferometry. As a direct consequence of the optical sectioning property, this system allows measuring through any type of optical window (for example a cover glass).