The analytical ion accelerator: RBS instrumentation from a surface analyst's perspective

Abstract

In constructing Rutherford backscattering spectrometry (RBS) instrumentation for quantitative thin film analysis, particular attention has been given to the areas of: 1) sensitivity, 2) accuracy, 3) versatility, 4) ease of operation, and 5) ease of maintenance. Starting with a commercially available ion accelerator (1 MV tandem; General Ionex Corp.) the collection of backscattered He has been optimized by using four cooled surface barrier detectors placed at θ = 170°. Each detector has its own amplifier, ADC and memory subgroup in the MCA. Energy resolution is of the order of 12 keV. The custom-made, bakeable, all-metal beam line and sample chamber are differentially pumped to maintain 10 −9 Torr in the sample chamber during analysis. All samples (including those for channeling analysis) are introduced into the analysis chamber from a vacuum load-lock (base pressure 10 −8 Torr). For maximum accuracy in determining the amount of the He + falling on the samples, indirect charge integration is used. A rotating vane is placed in the He + beam and ions backscattering off the vane are detected with a separate solid state detector. He + backscattered off the vane is related to He + entering the sample chamber by measuring that current with a deep, suppressed Faraday cup which can be rotated out of the He + beam for actual sample analysis. Since a secondary electron suppression cage is not needed, there is space to mount an X-ray detector in the sample chamber, as well as grazing-exit-angle detectors and detectors for elastic recoil hydrogen detection. MCA and goniometer control, data transfer and data reduction are accomplished using a Hewlett-Packard computer (HP1000 with an A900 processor).