A precise nondestructive optical contour mapping method with adjustable sensitivity for noncontact testing of surface deformations up to 30 microm/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.
In the photolithographic process, presently employed for the production of integrated circuits, sets of correlated masks are used for exposing the photoresist on silicon wafers. Various sets of masks which are printed in different printing tools must be aligned correctly with respect to the structures produced on the wafer in previous process steps. Even when perfect alignment is considered, displacements and distortions of the printed wafer patterns occur. They are caused by imperfections of the printing tools or/and wafer deformations resulting from high temperature processes. Since the electrical properties of the final integrated circuits and therefore the manufacturing yield depend to a great extent on the precision at which such patterns are superimposed, simple and fast overlay measurements and flatness measurements as well are very important in IC-manufacturing. A simple optical interference method for flatness measurements will be described which can be used under manufacturing conditions. This method permits testing of surface height variations by nearly grazing light incidence by absence of a physical reference plane. It can be applied to polished surfaces and rough surfaces as well.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.