SIMS Analysis of Nitrogen in Silicon Carbide Using Raster Change Technique
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0911-B05-08
SIMS Analysis of Nitrogen in Silicon Carbide Using Raster Change Technique Larry Wang, and Byoung-Suk Park SIMS Services, Evans Analytical Group, 810 Kifer Rd., Sunnyvale, CA, 94087-5203 Abstract. Today’s state of the art silicon carbide (SiC) growth can produce semi-insulating crystals with a background doping around 5×1015 atoms/cm3 or lower. It is essential to have an accurate measurement technique with low enough detection limit to measure low level nitrogen concentration. Current SIMS detection limit of low 1015 atoms/cm3 will provide accurate determination for nitrogen doping level of 5×1016 atom/cm3 or higher. In order to determine the lower nitrogen concentration, it is necessary to provide a better detection limit and to separate the contribution of background nitrogen properly. The “raster changing” method provides an accurate way to determine and remove contribution of background nitrogen to the signal, because secondary ion intensities and matrix ion intensities can be analyzed at the same location of the sample by changing the primary beam raster size during a profile. In this study we have succeeded in applying the raster changing method to (a) N in the SiC substrate located under an SiC epi layer, and (b) the detection of N as low as 3×1014 atom/ cm3 a bulk-doped SiC substrate.
Introduction Due to its unique capabilities of high detection sensitivity for a variety of elements under depth profiling mode, Secondary Ion Mass Spectrometry (SIMS) is an essential tool for characterization of dopants and impurities in SiC material. Nitrogen, which is a shallow donor in SiC, is present as a trace impurity in all SiC wafers and epi-layers. Depending on the design and its operation, a SiC growth system can contain significant amounts of nitrogen, which will lead to a fluctuating nitrogen residual doping in the grown material. A low and controlled nitrogen background is necessary for reproducible growth of semiinsulating wafers and low doped epi-layers. SIMS feedback of nitrogen in these crystals can provide understanding of the compensation mechanism and process conditions. For many years, SIMS has been used routinely to determine nitrogen concentration at level of 17 10 to 1019 atoms/cm3 with good precision. Today’s state of the art SiC growth can produce semiinsulating crystals with a background doping around 5×1015 atoms/cm3 or lower. It requires SIMS to have better precision to measure low level nitrogen concentration. With upgraded SIMS instrumentation (improved vacuum and better primary beam intensity) and improved analysis protocol, we can now achieve N detection limit of 2-5×1015 atoms/cm3 routinely while maintaining excellent depth resolution. While these detection limits are very good, they are not sufficient to provide a routine way for an accurate measurement of low level (
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