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Highly sensitive wavefront sensor for characterization of micro- to nanometer-scale surface flatness deviations

Wave front sensing is an optical method allowing non-contacting topography measurements of flat surfaces. Applications of the method are, for instance, the characterization of optical components, semiconductor surfaces, or subcomponents used in semiconductor manufacturing equipment. The method developed here is covering the characterization of flatness on mirror-like surfaces within three orders of magnitude from micro- to nanometer scale. This is due to the high range of detectable surface slopes from very low to relatively high values. Therefore, the method is applicable to both, micro- and nanometer scale height deviations on surfaces. The wave front sensing is capable of studying the topography in a real-time operating mode. The technique enables vertical resolution of approximately 10 nm at a lateral resolution of 0.6 mm on bare silicon wafer surfaces. Moreover, highly reflective surfaces with height deviations of 10-15 μm could be easily resolved at a lateral resolution of 2.4 mm. In this study, we focused on the application in semiconductor surfaces and manufacturing equipment: measurements were performed on bare wafers as well as on the mirror-like surface of a wafer holder used for wafer polishing (a 'polishing head'). An obstacle for measurements is a low reflectivity of surfaces. Both, metallic surfaces and silicon wafers, however, show high surface reflectivity.