Angle-Resolved Ellipsometric Analysis of Polishing-Induced Subsurface Damages in Calcium Fluoride Crystals for Ultra-Precision Manufacturing

Abstract

Calcium fluoride (CaF2), renowned for its exceptional optical and mechanical properties, is a critical material in advanced semiconductor and optical manufacturing. However, tribological processes such as polishing introduce subsurface damages (SSDs), including micro-cracks and lattice dislocation, which degrade functional performance and reduce service life. This study presents a novel, nondestructive method for SSD inspection using single-shot angle-resolved ellipsometric spectra (ARES) acquisition and analysis. The effects of SSD layer thickness and surface roughness on ARES are systematically evaluated through the effective medium approximation. Finite-difference time-domain simulations demonstrate the high sensitivity of Ψ-ARES (p-/s-amplitude ratio) to nanoscale SSD variations. An ARES measurement system employing back-focal-plane imaging was developed to validate the approach on CaF2 samples subjected to varying polishing conditions. The results showed good agreement with that of atomic force microscopy, white light interferometry and transmission electron microscopy. This work establishes angle-resolved ellipsometry as a powerful tool for nondestructive subsurface damage evaluation, enabling improved polishing processes and enhanced performance of ultra-precision manufacturing.

Keywords: Angle-resolved ellipsometrySubsurface damagesFinite-difference time-domainOptical crystalsNon-destructive test