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Metrology of undoped double-sided polished silicon wafer: surface, thickness and refractive index profile measurements

In this investigation, we describe a technique to obtain the 3D profile of surface, thickness and refractive index of an undoped double-side polished Si wafer at once. This technique is based on low coherence scanning interferometry (LCSI) and spectrally-resolved interferometry (SRI) using a NIR light, which is around 1 μm, for which transmission is non-zero for undoped silicon and also detectable by the typical visible CCD camera. LCSI allows for the measurements of surface, thickness and refractive index profiles of the Si wafer while SRI can determine their nominal values. For group refractive index measurements, the target which consists of a Si wafer segment and a mirror was designed. Consequently, the combination of these two techniques with the target enables to measure surface, thickness and refractive index profiles simultaneously and accurately. In the experiments, an undoped double sided polished (DSP) Si wafer with 475 μm thickness was measured and the 3D profiles of optical thickness, geometrical thickness, group refractive index were successfully obtained. Because of not using an expensive IR CCD camera and an optical source, the proposed technique is cost-effective.