Patent ID: 7369251

Claim:
A method, comprising: providing a flat panel, substrate or wafer having opposing front and back surfaces, the surfaces having corresponding positions; using an optical probe beam with a substantially uniform wavefront to illuminate at least the back surface to produce a reflected probe beam with a reflected wavefront that carries distortions caused by an illuminated area on at least the back surface; directing the reflected probe beam through an optical shearing interferometer device to obtain an optical interference pattern between the reflected wavefront and another replica of the reflected wavefront that is spatially shifted by a shearing distance; adjusting a phase shift between the reflected wavefront and the replica of the reflected wavefront to obtain a plurality of phase-shifted interference patterns of different phase shifts from the optical shearing interferometer; and processing the interference patterns to obtain information on surface slopes of at least one portion of the front surface, the at least one portion of the front surface corresponding to the illuminated area of at least the back surface.