Optical properties of thin amorphous silicon film on a phase shift mask for 157 nm lithography

Abstract

We introduced thin amorphous Si (a-Si) films to prevent the degradation of Si-O-N-F films that were fabricated for phase shift masks in 157 nm optical lithography. To investigate the optical properties of thin a-Si films, a spectroscopic ellipsometry (SE) analysis method was used. In SE analysis of thin a-Si films, the optical properties, such as absorbance and optical band gap, were analyzed using the two layer model of effective medium approximation (EMA) (a-Si+SiO)/a-Si/Cauchy model. To verify the validity of SE analysis, Si 2p peak and O 1s peak variations of a-Si films were analyzed by X-ray photoelectron spectroscopy (XPS) respect to sputtering time. By comparing the results of SE and XPS analyses, it was confirmed that SiO could represent the optical properties of an oxidized surface and that EMA(a-Si+SiO)/a-Si/Cauchy model was appropriated.