Investigation of the quality of an As35S65 grating by spectroscopic ellipsometry

The quality of an As35S65 chalcogenide glass (ChG) grating fabricated by electron beam lithography (EBL) was characterized by optical scatterometry based on spectroscopic ellipsometry (SE) in the visible and near infrared spectral range and complementary techniques providing direct images, especially atomic force microscopy (AFM). The geometric dimensions and the shape of patterned grating lines were determined by fitting modeled values (calculated by the Fourier modal method) to SE experimental data. A simple power-dependent function with only one variable parameter was successfully used to describe the shape of the patterned lines. The result yielded by SE is shown to correspond to AFM measurement with high accuracy, provided that optical constants of ChG modified by EBL were used in the fitting procedure. The line edge roughness (LER) of the grating was also investigated by further fitting the SE data to find out that no LER is optically detectable in the spectral range used, which is essential for the functionality of optical tools fabricated by EBL.