New wave front phase sensor used for 3D shape measurements of silicon wafers

Wave Front Phase Imaging (WFPI) is a new wafer shape measurement technique that acquires millions of data points in just seconds or less, on a full 300mm silicon wafer. This provides lateral resolution well below 100 mu m with the possibility of reaching the lens' optical resolution limitation between 3-4 mu m. The system has high repeatability with root-mean-square (RMS) standard deviation (sigma(RMS)) in the single digit nm for the global wafer shape geometry and for nanotopography it reaches in the sub angstrom (angstrom = 10(-10) m) range. WFPI can collect data on the entire wafer to within a single pixel away from the wafer edge roll off(1). The flatness of the silicon wafers used to manufacture integrated circuits (IC) is controlled to tight tolerances to help ensure that the full wafer is sufficiently flat for lithographic processing. Advanced lithographic patterning processes require a detailed map of the wafer shape to avoid overlay errors caused by depth-of-focus issues(2). In this paper we go deep into the theoretical explanation as to how the wave front phase sensor works.