Optical constants of SiO2 from 196 to 1688 nm (0.735–6.33 eV) from 20, 40, and 60 nm films of reactively sputtered SiO2 on Eagle XG® glass by spectroscopic ellipsometry

Abstract

Three thicknesses of reactively sputtered SiO2, nominally 20, 40, and 60 nm, deposited on Corning Eagle XG® (EXG), an important display glass, were analyzed by spectroscopic ellipsometry. Reflection ellipsometry data from the samples were analyzed from 196 to 1688 nm at angles of 55°–60°, inclusive, in 1° increments. These angles were chosen because they bracket the Brewster angles of both EXG glass and SiO2. The backsides of the samples were roughened by sand blasting to suppress backside reflections from the substrates. A total of nine datasets were collected from nine different samples (three for each nominal thickness of SiO2), each at six different angles of incidence (54 spectra). The optical constants for each thickness of SiO2 was determined, as well as a set of constants for all the films (the material in general) via a multisample analysis (MSA). The optical constants of the SiO2 films were modeled using two poles with transparency assumed over the entire spectral range (a Sellmeier model). A Bruggeman effective medium approximation (BEMA) roughness layer was included in the model, which assumed 50/50 volume fractions that corresponded to the modeled SiO2 optical constants and void. The fit did not substantially improve when an interface layer between the Eagle XG® and the sputtered film was included, so it was omitted. Three sets of previously reported optical constants for the substrate over two wavelength ranges were considered in the modeling. The thin SiO2 films analyzed herein have very similar optical properties to those of their EXG substrate—this is a challenging analytical problem involving a transparent film on a transparent substrate. Accordingly, analysis of multiple samples, an MSA that included multiple film thicknesses analyzed at multiple angles, and an exploration of multiple modeling approaches helped ensure that the optical constants reported herein are accurate and the modeling robust. In particular, these measures helped avoid correlation between the optical constants, the layer thicknesses, and the thicknesses of the BEMA layers. Atomic force microscopy roughness measurements were made on the SiO2 films and compared to the roughness values obtained by spectroscopic ellipsometry. In summary, the authors provide here the optical constants and some accompanying physical characterization of sputtered 20–60 nm SiO2 films from 196 to 1688 nm.