Artifacts in low-energy depth profiling using oxygen primary ion beams: Dependence on impact angle and oxygen flooding conditions

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Abstract
The depth resolution and the accuracy of depth calibration in sputter profiling of boron delta distributions in silicon have been investigated using 1 keV O2+ ion bombardment at impact angles θ between 2° and 62° (to the surface normal) in combination with secondary ion mass spectrometry. The effect of jet-type oxygen flooding during sputter erosion was studied at 62°. For θ up to 34°, the depth resolution was essentially independent of θ (full width at half maximum, FWHM, 2.9 nm, decay length 1.4 nm) and also independent of depth (for deltas located between 40 and 190 nm). However, the well-known peak shift due to the initial short-term change in erosion rate was a factor of 2.7 larger at 34° than at 2° (3.5 vs 1.3 nm). As θ exceeded about 36°, two other types of artifacts were observed during bombardment in vacuum, (i) a severe profile broadening and (ii) a large long-term change in erosion rate which gave rise to crater-depth dependent peak shifts of the deltas (erosion rate reduced for 41°<θ<62°, but enhanced for 36°<θ<40°). Both effects became more severe as the sputtered depth became larger. In accordance with the previous surface topography investigations, the observed changes of the matrix ion signals provide evidence for a rapid onset of bombardment induced surface roughening, or ripple formation, starting at depths as low as about 15 nm. The artifacts were most severe at angles between 47° and 52°, in which case, at a depth of 190 nm, the FWHM increased to as much as 13 nm. In parallel, the erosion rate, averaged over depth intervals of 41–51 nm, decreased continuously by up to 23%. As a result, the 41 nm boron delta appeared to be shifted towards the surface by as much 6 nm. The very large changes in erosion rate invalidate depth calibration procedures based on final crater-depth measurements. At 62°, the depth profiling artifacts were significantly enlarged by oxygen flooding, notably at intermediate pressures, at which ripples grew so rapidly that the FWHM of boron deltas located at a depth of only 88 nm amounted to 13 nm. Even at the highest tolerable oxygen pressure (5×10−6 hPa), the FWHM was a factor of about 2 larger than at normal or near-normal beam incidence in vacuum. Furthermore, the decreasing erosion rate gave rise to an apparent 4 nm shift of the 41 nm delta towards the surface. We conclude that 1 keV O2+ bombardment at oblique incidence (36°<θ<62°) always gives rise to rapid ripple growth in vacuum and apparently also with oxygen flooding. Accurate depth profiles cannot be obtained under these conditions.

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