Interferometric thickness analyzer and measuring method

Abstract

Flat objects (6), e.g. non-transparent wafers, are tested over their entire surface for thickness variations in an interferometric set-up (FIG. 1) where symmetrical opposite points on the front and rear surface, respectively are sensed by the same light ray (10). A laser beam (3) impinges under oblique angle of incidence on a beam splitter (e.g. a grating 4) to generate a reference beam (3a) and a measurement beam (3b). The latter is directed by a first of a pair of plane mirrors (5a) to the front surface of sample (6) from which it is reflected again to plane mirror (5a) to reach blazed grating (7) which deflects it symmetrically to plane mirror (5b). The rear surface of sample (6) is thus illuminated under the same angle of incidence in a way that each ray (10) is directed to the point on the rear surface that is exactly opposite to its reflection point at the front surface. The beam reflected at the rear surface passes after further reflection at plane mirror (5b) through beam splitter (4) in the same direction as reference beam (3a) thus generating an interference pattern on screen (8). The fringe separation in the interference pattern is representative of the local thickness of sample (6) and can be evaluated electronically or by pattern recondition methods to thickness readings with an accuracy of .lambda./200. A second embodiment of the invention uses a folded mirror instead of blazed grating (7) for the deflection.