Rapid automatic compensation for charging effects during the SIMS depth profiling of semiconductors

Abstract Quadrupole mass spectrometer SIMS instruments have an energy bandwith of approximately 10 eV, sufficient to sample the richest portion of the secondary ion emission, provided that the instrumental energy window is correctly placed. Changes > 1 V in the potential of the starting surface can change the instrumental sensitivity due to a shift and shape change in the energy spectrum of emitted ions. We show that non-uniform surface potential changes occur during the depth profiling of semiconductors, which can exceed 10 V even in a simple implant. Serious errors occur in the profiles obtained, which are not necessarily apparent unless specific tests are made whilst profiling. We describe a method for unambiguously identifying the occurrence of surface charging, sensing its magnitude, and applying a compensation potential to the sample under computer control. The method is based on (i) the inclusion in the profile of two channels tuned to the same molecular ion (e.g. Si 2 O + in silicon) but set up close to the high and low energy half-maximum points in the energy spectrum and (ii) the experimental observation that the change in instrumental transmission with sample potential in the vicinity of these points is approximately linear. By the application of a simple model for the energy spectrum shape the surface potential can be computed in real time and voltage feedback applied to compensate for any changes.