Monitoring of Ion Purity in High-energy Implant via RBS

The UAlbany Dynamitron is used for high-energy ion implantation as well as for routine materials analysis. Its ion source can be run using any one of fourteen different gases, leading to concerns of contamination during an implantation. The system has the usual well-calibrated mass-separation using a magnetic analyzer. A pre- or post-implant mass spectrum through this analyzer can give a useful understanding of unintended ions within the source beam, but it does not provide direct identification for such ions as CO or diatomic nitrogen-14 when implanting silicon-28. Since these possible components have the same momentum and charge (i.e. +1), the beamline mass separator will transmit them all. Because backscattered ions from the mass-separated beam will have only atomic scattering, this allows for element detection following the breakup of any molecular ion components. The verification system consists of a back-angle particle detector along with a movable temporary target consisting of a very thin film of gold on a carbon or silicon substrate. The backscattered spectrum can then be analyzed for the presence of unwanted elements. While this does not provide for removal of the unwanted components, it does provide for the identification and measurement of the problem. We show the physical layout, software and extra details necessary for successful use of the technique.