Ellipsometric measurement of subsurface damaged layer thickness and refractive index of sapphire substrates

To quantitatively measure the thickness d and refractive index n of the damage layer on sapphire substrate produced during chemical mechanical polishing, a spectral ellipsometric method is proposed. First, the change of spectral polarization state (i. e., amplitudes ratio and phases difference) of light (from 250 to 1 650 nm) reflected on sapphire substrate is measured. Then, the thickness and refractive index of damaged layer are extracted by optical modeling and inversion of measured data. The d and n of substrates polished by Al2O3 and SiO2 abrasives are measured. The d of the former fluctuates with the better result about 1. 4 nm after 40 min, and the latter continues decreasing with the better result 1 nm after 20 min. The n produced by two abrasives is both smaller than that of sapphire crystal. Meanwhile, the experiment and simulation analysis indicate that the change of phase difference is similar with d, which can be used to quickly evaluate the change of damage layer thickness because of no requirements for modelling and inversion. Therefore, the proposed method can be utilized to monitor the machining process as a nondestructive optical way. © 2023 Science Press. All rights reserved.