2. Advanced Launch Corporative Innovation Center, Nanjing University of Science and Technology, Nanjing, Jiangsu 210094, China
Overview: An oblique incidence dynamic phase-shifting interferometer based on inclination angle deflection is proposed to quickly obtain the surface distribution of optical surface with flatness of micron dimension. The measurement is based on the Michelson interference system. The He-Ne laser which operates at 632.8 nm is used as the light source. In combination with a phase mask grating, the four (±1, ±1) diffraction orders is selected to generate a 2×2 point source array which is introduced into the interference system as light source configuration. Adjust the distance of each point source off the optical axis precisely, the incidence angle of each point source on the interferometer cavity is changed to induce phase shift. In this work, we introduce equal phase shift interval of π/2 in the interferograms. We choose double telecentric imaging system to ensure the entire test part can be clearly imaged. By inducing a lens array into the imaging system, the four interferograms can be separated spatially on a single CCD. Then the four phase-shifting interferograms are captured simultaneously on the CCD target, and the initial phase will be retrieved exactly by employing the four-bucket algorithm, thereby realizing dynamic measurement. To analyze the measurement error which is introduced by the offset amounts of each point source, we establish a simulation model to study the effect of the collimated wavefront quality on the measurement. It is necessary to ensure that the aberrations which collimated wavefront contains smaller than λ/4, and the F-number of the collimating objective lens should be greater than 8.5. We also design the imaging system to ensure image quality through Zemax software. We build up the experiment device and choose a silicon wafer with aperture of 35 mm as the test part. The flatness of silicon wafer is about 10 μm dimension, and the measurement of the thin wafer is susceptible to vibration. Furthermore, to study the influence of the oblique incidence angle on the sensitivity factor and the range of measurement, a mathematical model based on the oblique incidence measurement is established to select the best incidence angle. Considering the sensitivity factor, the aperture of the test part, the surface reflectance and the spatial resolution, the silicon wafer is measured at oblique incidence angle of 68°, the root mean square (RMS) is 1.631 μm and peak-to-valley (PV) is 9.082 μm. The experimental results indicate that the proposed interferometer overcomes the disturbance of vibration environment and extends the measurement range of interferometer with high precision by introducing the simultaneous phase-shifting interferometry based on inclination angle deflection into the oblique incidence interference system.