Investigation of the scattering behaviour of sputtered optical coatings

We investigated RF-magnetron sputtering of Ta2O5 and SiO2 as a potential production technique for high quality optical coatings such as gyro laser mirrors. Our primary concern was to reduce the light scattering of the sputtered layers as much as possible because scattering is one of the key problems of such mirrors. For the scattering measurements we used substrates of BK7, fused silica and silicon wafers. The silicon wafers proved to have the best and most reproducible surface quality. Therefore they were mostly used for the basic scattering investigations. Starting from very high scattering levels (approx. 1000 ppm) due to flaking inside the deposition chamber we could reduce this value to the few ppm-range. This value is comparable to that of the best gyro mirrors produced by secondary ion beam sputtering, making the magnetron sputtering process an attractive alternative to the highly sophisticated ion beam sputtering. (author)