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The shrinking depth of focus of high numerical aperture immersion microlithography optics requires a tight wafer flatness budget. Bare wafer surface topography variation is a significant part of the focus budget for microlithography. Thus, as the wafer surface quality becomes increasingly important, the metrology to control the surface quality is increasingly challenged1. Advanced lithographic patterning processes require a detailed map of the free, non-gravitational, wafer shape, to avoid overlay errors caused by depth-of-focus issues2. The semiconductor industry has been using interferometry-based techniques for measuring the free form wafer shape of blank silicon wafers for several years1. In this paper we introduce a new measurement technique, Wave Front Phase Imaging (WFPI), that can measure the free form wafer shape of a silicon wafer by acquiring only the intensity of the reflected light. In the WFPI system, the wafer is held vertically to avoid the effects of gravity during measurements. The wave front phase is then measured by acquiring the 2-dimensional intensity distribution of the reflected, non-coherent, light at two or more distances along the optical path. This method allows for very high data acquisition speed and a high number of data points equal to the number of pixels available in the CMOS imaging sensor used. In the measurements presented in this paper, we acquired 16.3 million data points on the full 300mm blank silicon wafer, generating a lateral resolution of about 65μm per pixel. Blank silicon wafer manufacturers need to acquire such metrics as bow, warp, and flatness among other parameters, to provide with the silicon wafer when sent to the device maker fabs. These metrics are easily derived by generating a free form wafer shape map of both the front and the back surfaces.
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