An interferometric method for simultaneous measurement of thickness, refractive index, and surface profile of a silicon wafer

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We propose an optical system capable of simultaneously measuring physical thickness, group refractive index, and surface profile of a single-layer substrate based on a spectral domain interferometer. Specifically, the proposed method can be functionally divided into two parts; one is the Mach-Zehnder configuration for thickness and refractive index measurements, the other is the Michelson configuration for surface profile measurement. To integrate two different configurations into a single system, two fiber components of an optical circulator and a 2-by-1 optical coupler were installed for the purpose of acquiring both signals reflected from and transmitted through the sample. In addition, the Michelson configuration was realized by replacing a right-angle turning mirror with a beamsplitter and adding a reference mirror in the Mach-Zehnder layout. For feasibility test of the proposed method, a 100-mm-diameter silicon wafer was laterally scanned within a square area of 50 mm2 using a two-axis motorized linear stage. The reference mirror for surface profile measurement was suitably positioned along the optical axis to prevent the overlap between the optical path differences. As a result, the distribution maps of physical thickness, group refractive index, and surface profile were successfully measured over the target area of the silicon wafer. In the proposed setup, the measured surface profile of a plane-parallel substrate like a silicon wafer represents the bending information in its natural state. The proposed method is highly applicable to the semiconductor or display devices inspection where thickness and surface profile measurement should be monitored simultaneously.