We propose and realize a modified spectral-domain interferometer to measure the physical thickness profile and group refractive index distribution of a large glass substrate simultaneously. The optical layout was modified based on a Mach-Zehnder type interferometer, which was specially adopted to be insensitive to mechanical vibration. According to the measurement results of repeated experiments at a length of 820 mm along the horizontal axis, the standard deviations of the physical thickness and group refractive index were calculated to be 0.173 μm and 3.4 × 10−4, respectively. To verify the insensitivity to vibration, the physical thickness values were monitored at a stationary point while the glass panel was swung at an amplitude exceeding 20 mm. The uncertainty components were evaluated, and the combined measurement uncertainty became 161 nm (k = 1) for a glass panel with a nominal thickness of 0.7 mm.
We have proposed and demonstrated a novel method that can determine both the geometrical thickness and refractive index of a silicon wafer at the same time using an optical comb. The geometrical thickness and refractive index of a silicon wafer was determined from the optical thickness using phase information obtained in the spectral domain. In a feasibility test, the geometrical thickness and refractive index of a wafer were measured to be 334.85 μm and 3.50, respectively. The measurement uncertainty for the geometrical thickness was evaluated as 0.95 μm (k = 1) using a preliminary setup.