Optical interference method for the approximate determination of refractive index and thickness of a transparent layer

Abstract

Optical interference fringe measurements of the thickness of transparent layers can be rapid, accurate, and nondestructive. If the refractive index n of the layer being measured is known, it may be combined directly with interference fringe information to yield the layer thickness t. If, however, n is unknown, the measurement procedure necessarily becomes more complicated. In this paper, a new and simpler optical interference method is presented for the approximate determination of both n and t of a transparent layer on a transparent substrate. The required experimental information is obtained from a single spectrophotometric recording of either the reflectance or transmittance of the layer and its substrate. The theory of the method is presented, and an application of the method to measurements of layers of SIPOS (Semi-Insulating POlycrystalline Silicon) is described. The method requires that the layer being measured must be uniformly deposited on a flat substrate, and it must neither absorb nor scatter the light passing through it. The major approximation inherent in the method is that both the layer and the substrate are assumed to be nondispersive over the wavelength region of interest.