Sputtering fabrication of silicon nitride and silicon oxide based dichroic mirrors

Abstract

Thin films in optical materials utilize the properties of multiple materials to obtain specific and fine-tuned transmission, absorption and reflectance at wavelengths. Dichroic mirrors exhibit very different reflectance and transmission rates at certain cut-off wavelengths, which can be adjusted using changes in layer materials and thickness. This is due to constructive optical interference between alternating layers of two thin films of different refractive indices. This study explored the sputtering methods of thin-film multilayers to form dichroic mirrors in the visible spectrum for future solar-cell applications. Silicon oxide and silicon nitride targets were selected as materials used in the sputtering process. The sputtered multilayers and films were then characterized and analyzed using spectrophotometry. The transmission spectrum of the initial multilayer depicted failure in transmission at wavelengths under 500nm. The components of the multilayer were then sputtered and analyzed to troubleshoot the problematic nitride films. Transmission spectra were utilized to select each following process, and both reactive sputtering and cosputtering were explored as means of creating nitride films with functional properties. Transmission spectra were analyzed using the Swanepoel method to quantify optical characteristics to assure reactive sputtering of the targets in a nitrogen environment as a viable direction of mirror construction. Possible further work include the use of other targets such as titanium oxide, and different chamber gas mixtures for finer control in the composition of the film layers.