Phase shift interferometry

Since the small dimension of the structures the accurate roughness measurement is a challenge and requires a high control of the environment surrounding the instruments in terms of temperature, pressure and vibration stability. Ones of the main  instruments devoted to measure such small heigh variations are Phase Shift Interferometers. An interferometer is an optical device which utilizes the effect of interference. Typically, it starts with some input beam, splits it into two separate beams with some kind of beam splitter (a partially transmissive mirror), possibly exposes some of these beams to some external influences (e.g. some length changes or refractive index changes in a transparent medium), and recombines the beams. The power or the spatial shape of the resulting beam can then be used for a measuring the optical surfaces quality in terms of roughness and flatness. When a light source with low optical bandwidth is used (even a single-frequency laser), the detector signal varies periodically when the difference in arm lengths is changed. Such a signal makes a possibility to do profile/roughness measurements with a depth resolution well below the wavelength, but there is an ambiguity. For example, the fringe pattern does not reveal if the surface is approaching or moving farther. This problem may be solved by modulating the arm length difference with a vibrating mirror (or with an optical modulator) and by monitoring the resulting modulation on the detector in addition to the average signal power. Simultaneous operation of an interferometer with two wavelengths is another way of removing the ambiguity. If the detector is a kind of camera, like a Charge Coupled Device (CCD), and the surfaces monitored are fairly smooth, the phase profile (and thus the profile of optical path length) can be reconstructed by recording several images with different overall phase shifts. A phase-unwrapping algorithm can be used to retrieve unambiguously surface maps extending over more than a wavelength. However, such methods may not work for rough surfaces or for surfaces with steep steps. Figures 1-2 show some examples of measurements obtained by the fizeau  interferometer (WYKO) available at the Optical Metrology Lab.

Figure 1: Example of Phase Shift Interferometric measurement using WYKO Fizeau interferometer available at the Optical Metrology Lab on an X-ray flat mirror.

Figure 2: Example of Phase Shift Interferometric measurement using WYKO Fizeau interferometer available at the Optical Metrology Lab, magnification to short spatial wavelenghts.

Last Updated on Monday, 18 January 2021 08:20