Optimization of secondary ion mass spectrometry detection limit for N in SiC

Abstract

Controlled changes in the Cs+ primary ion beam density and analytical expressions describing the sources (bulk concentration, memory effect, and adsorption of N from the residual vacuum) of secondary ion mass spectrometry analyte secondary ion intensities were used to determine the contributions to the N secondary ion intensity obtained during the analysis of trace levels of N in bulk SiC. This methodology allows the determination of N concentrations that can be substantially less than the apparent N secondary ion background intensity. It was shown that for the Cameca IMS-6F instrumental conditions used, memory effect is the main contributor to the N background signal. Taking into consideration the Cs+ beam diameter, the raster size, the diameter of the ion-extracted area, and the impurity secondary ion intensity, an optimized combination of primary ion beam current and raster size was determined that resulted in the best detection limit for N in bulk SiC. This detection limit for N in bulk SiC (∼6×1014 atoms/cm3) was obtained for a primary ion current of 100 nA, a raster size of 45 μm×45 μm, and a secondary ion extraction area diameter of 30 μm.