Multiple wavelength interferometry for surface profiling

摘要

Interferometry is a well established technique for surface profiling. The conventional interferometric surfaceprofilers using a single wavelength offer excellent vertical resolution, but a serious limitation to their use is thatthey can only handle smooth profiles and step heights less than half a wavelength. In the situation where thesurface profile is discontinuous, white light interferometry has been applied with great success. However thescanning white light interferometry requires large number of frames to be recorded, whereas in spectrallyresolved white light interferometry only a line profile of the object is obtained, although the requirement onnumber of frames is similar to the single wavelength phase shifting interferometry. In this paper we discuss threewavelength interferometry in which a limited number of frames suitable for phase shifting technique are recordedat three laser wavelengths. The phase evaluation at the three wavelengths gives wrapped phase at any pixelcorresponding to these wavelengths. The fringe order is obtained considering the fact the variation of phase withwavenumber for a given profile height is linear. The slope of the phase verses wavenumber line gives the absolutevalue of the profile height and is used to ascertain the fringe order. The fringe order along with the wrapped phasegives the profile height with a resolution given by phase shifting technique. Experimental results on etched siliconsamples are presented.