Measurement of transparent coating thickness by the use of white light interferometry

Abstract

White light interferometry is widely used to measure surface profiles in order to overcome the phase ambiguity problem, which is inevitable in the monochromatic light interferometry. In this paper, we describe a white light interferometric technique to determine the thickness of transparent coating on a silicon substrate. The layer thickness is obtained by a straightforward subtraction of two surfaces in the coated layer. The surface information is extracted by the white light interferometric surface profiler developed in the proposed system. In the surface extraction, a digital filtering technique based on fast Fourier transform (FFT) is used to retrieve the envelopes of the interferograms, and then differential operation is implemented to determine the peaks of the envelopes. Experimental work conducted on a flat mirror surface demonstrates the feasibility of the proposed method with a measuring accuracy in the order of nanometers. Results also show that by the use of the proposed method the coating thickness in the order of micrometers can be extracted satisfactorily without phase ambiguity problem.