Non-zero numerical aperture-corrected data analysis for imaging ellipsometry

Imaging ellipsometry (IE), which integrates single-point ellipsometry with optical microscopy, enables spatially resolved evaluation of thin-film thickness and/or optical constants across a sample surface. Conventional ellipsometry requires a highly collimated incident beam; however, strong collimation degrades spatial resolution in imaging measurements. Moreover, the incoherent light sources typically employed in imaging ellipsometers make it practically difficult to achieve perfect collimation. Consequently, IE operates with a finite convergence angle, i.e., a non-zero numerical aperture (NA). In this work, the influence of NA on ellipsometric measurement accuracy is investigated through numerical simulations using an air/SiO2 thin-film/silicon substrate structure. To mitigate this effect, an NA-corrected data-analysis method is proposed and experimentally validated using a laboratory-built rotating-compensator IE system.