Extremely deep SIMS profiling: oxygen in FZ silicon

With the use of 18 keV Cs+ mass-filtered primary beam of 500 nA focused into a spot ∼30 μm, rastered over a nominal area of 50 μm×50 μm and a pressure in the analysis chamber lower than 2×10−8 Pa during analysis we were able to achieve an oxygen background (bkg) equivalent to 8×1015 cm−3. This means that, provided adequate reproducibility of comparative measurements of blank and O-implanted standards, the bkg subtraction allows to reliably determine 16O concentration down to ∼5×1015 cm−3—a value close to the natural abundance of oxygen in floating zone (FZ) silicon. However, for depths exceeding ∼50 μm, the depth resolution degrades; also the removal rate decreases. In order to explore these effects more systematically, a special test structure consisting of phosphorus markers 20 μm apart embedded in undoped Si was manufactured with a chemical vapour deposition technique. A precise rate vs. depth function was derived employing a polynomial fit. When a dynamic range of at least two orders of magnitude is required for large depths, beveling of the sample followed by lateral line-scan analysis is necessary.