A precise nondestructive optical contour mapping method with adjustable sensitivity for noncontact testing of surface deformations up to 30 µm/cm is described. The method employs an optical grating that is placed in front of the test surface. Illumination of the grating by a monochromatic plane wave generates an interference pattern between the beam components of two different diffraction orders. Reflection at the test surface and superposition with the fixed grating generate a fringe pattern that resembles the surface contours of the test object. This fringe pattern consists of a superposition of an interference line system and a moiré line system. Whereas the distance between two adjacent contour lines of the interference pattern corresponds to a surface deformation of a half-wavelength of the illuminating light the distance between two lines of the moiré system is determined by the grid constant and the direction of the grid illumination. Therefore, the scale of measurement can be chosen according to the problem. Applications for surface flatness testing of semiconductor wafers and photomasks are presented.
W. Jaerisch and G. Makosch, "Optical Contour Mapping of Surfaces," Appl. Opt. 12, 1552-1557 (1973)