Multiple wavelength interferometry for surface profiling

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