Fabry--Perot Interferometry for Ultraviolet Photolithography Masks: Principle, Proof of Concept, and Optical Design Analysis

The increasing complexity and cost of photomasks in semiconductor manufacturing have motivated the exploration of reprogrammable alternatives compatible with existing ultraviolet photolithography systems. This study presents a validation of Fabry–Perot interferometry for reconfigurable photomask applications, focusing on the optical design space of Fabry–Perot pixels composed of high-refractive-index reflective films separated by sub-micrometer air cavities. Optical simulations were used to map the transmittance behavior across film thicknesses and cavity lengths and identify optimal design conditions producing transmittance maxima near unity and minima around 0.15, corresponding to a modulation contrast of ~0.85. Fixed-cavity Fabry–Perot pixels were fabricated on fused silica substrates using sputtered films with a refractive index of ~1.98 and different sacrificial layer thicknesses to vary the air cavity. Lithography experiments performed at the i-line wavelength (365nm) confirmed the predicted transmission behavior, with full resist development for cavities ranging from 150 nm to 190nm and ~75–80% resist retention for 270 nm and 290 nm cavities, consistent with the constructive and destructive interference states separated by ~80 nm of air cavity thickness, a quarter of the incident wavelength. The measured resist-remaining percentage followed the simulated transmission modulation, validating the optical model. These results suggest that the Fabry–Perot principle with high-refractive-index materials is a viable option to create reprogrammable photomasks that can provide high contrast and reconfigurable optical control within stepper-compatible lithography platforms.[2026-0029]