Fabrication of silicon-based optical components for an ultraclean accelerator mass spectrometry negative ion source

Abstract

An ultraclean accelerator mass spectrometry negative ion source for semiconductor material mass analysis has been built and is in operation at the University of North Texas’ Ion Beam Modification and Analysis Laboratory (IBMAL). The source is unique in that the active surfaces and apertures of the optical components in the ion source have been fabricated from high-purity single crystal silicon. This prevents both the 133Cs+ beam incident on the semiconductor samples and the negative ions from the sample surfaces from ‘‘seeing’’ and sputtering any metal surfaces (mostly stainless steel) in the beamline. The Cs+ beam can be rastered across the sample surface and the impact energy is adjustable to control depth-profiling rates. An ultraclean ion source of this type is necessary to prevent the injection of Fe and other beamline elements onto the sample or into the tandem accelerator, which is equivalent to putting an impurity signal into the mass analysis of the semiconductor sample. Suppression of these elements increases the sensitivity of the analysis to one part in 1012 for many masses. The fabrication and alignment of the optical components—einzel lenses, steerers, raster/scanners, and Faraday cups—in the negative ion source will be presented. Also, simulation trajectories are presented to show (1) the interaction of the incident Cs+ ion beam, the semiconductor sample, and holder (biased up to −30 kV), the extracted beam in the sample chamber, and associated potential field lines; and (2) the potential field lines of the octupole steerers.