Medium energy backscattering spectrometry: a new tool for characterizing optical thin films

We have developed a new method of analyzing surfaces and thin films based on time-of-flight analysis of backscauered particles. The technique employs ions with incident energies from a few tens of keY up to about 500 keY. It combines the quantitative prowess of Rutherford backscattering spectrometry with increased surface sensitivity, increased depth resolution and reduced target damage. Several probe species have been used successfully for analyses with He and Li at 250 keY and C2 at 500 keY being particularly useful because of the range of cross sections and resolutions for depth profiling that they offer. This paper describes the technique and presents studies of the composition of beryllium surfaces for mirror applications and of He indiced erosion of CaF2 and MgF2 films on Be substrates, simulating the exposure of optical coatings to one type of radiation found in Earth orbit