Optical profiling using white light interference in spectral domain

The rapidly developing fields of Micro-Electro Mechanical System (MEMS) and micro optics require non contact and high precision measurements. Optical profilers are ideally suited for this. Monochromatic Phase Shifting Interferometry(PSI) can be used to measure slowly varying profiles and step height of less than half a wavelength. White light interferometry is used when large step height are to be measured. White Light interferogram may be considered as an incoherent superposition of a large number of monochromatic interferograms. These individual interferogram may be accessed at the output of a spectroscope when the white light interferogram is imaged at its entrance slit. For a given optical path difference(OPD) in the interferometer, the phases of these interferograms are different and are linearly related to the wave number of the constituent spectral component. If the phases of all the constituent interferograms are determined, the OPD can be obtained as the slope of the phase versus wave number line. We recently proposed that conventional PZT phase shifting can be used for this purpose. The OPD is related to the height of the test object at a point. If OPD is measured along the entire entrance slit of the spectroscope, a line profile of the object can be determined. In this paper we show that the profile obtained from the slope can be improved by using the monochromatic phase data which is already available from the measurement. Further this paper will also focus on problems as the measurement range is increased.