Phase-Shifting Interferometry for Surface Roughness Measurement on Glass Substrates

Phase shifting interferometry based on Michelson interferometer is purposed to measure surface roughness of glass slides which are widely used as substrates for thin-film coating processes. For the interferometry system, an optical flat with a flatness of λ/20 is used as a reference mirror. The high accuracy of the phase shifting is achieved by a piezoelectric-driven linear translation stage of the reference mirror. The reference-phase difference between the two-interfering beams is shifted by every π/2 in phase shifting. Five frames of interferograms under various phase differences of 0, π/2, π, 3π/2 and 2π are recorded by a CCD camera. Each image pixel of these frames is interpreted to access interference intensity information by five-step phase shifting algorithm for phase calculation, which in turn relates to surface height. The purpose-built interferometry is tested by the surfaces of two optical flats with flatness of λ/10 and λ/4, which are used as the test surfaces. Our measured flatness results are consistent with those of the commercial optical profilometer. The usefulness of the purpose-built interferometry is demonstrated on two types of the glass slides. Aluminum thin films are also deposited on these glass slides by the RF magnetron sputtering method to enhance reflectivity of the glass slide surfaces. The surface profiles and flatness parameters of these glass slides have been reported.