Very Precise Thickness Measurement of Thin Films

Abstract

IT has already been pointed out¹ that the Smith type of interference microscope, using a Wollaston prism in the rear focal plane of the objective, is a suitable instrument for measuring thin films. The film is deposited, with a reasonably sharp edge, on a flat surface and covered with a reflecting metallic film, using, for example, silver or aluminium. The image of an illuminated slit, doubled by the Wollaston prism, is projected by the objective, via a prism type of vertical illuminator, normally on to the specimen. The reflected beams are polarized mutually at right angles and the path-difference between them is zero if both slit images fall either on to the substrate only or on to the film only. If one image falls on to the film and the other on to the substrate the path-difference is equal to twice the film thickness, and the problem becomes the polarimetric one of measuring this path-difference.