Sources of error in quantitative depth profiling of shallow doping distributions by secondary-ion-mass spectrometry in combination with oxygen flooding

Abstract

Using a magnetic sector field instrument we have performed a detailed study on profile artifacts produced when measuring very shallow boron doping distributions (0.5 keV 11B) in silicon by oblique oxygen bombardment (1.9 keV O2+ at 56°) in combination with oxygen flooding. The raw profiles were calibrated by standard procedures, i.e., by assuming a constant erosion rate and a constant B-to-Si sensitivity ratio. The 11B profiles derived from measurements at high oxygen pressures (rapid “saturation” of the Si+ reference signal) turned out to be shifted towards the surface by up to 4.5 nm compared to profiles recorded at base pressure. This shift is larger by a factor of 2 than the value previously reported for normally incident O2+ beams of the same energy without oxygen flooding. A particularly large artifact (7 nm shift) has been observed with profiles implanted into Si covered with a 6 nm surface oxide film. The profile shifts appear to be related to a significant (up to 40%) decrease of the matrix reference signals observed under high-pressure flooding conditions at (apparent) depths between about 10 and 50 nm. The ion yield reduction may be indicative of a rapid development of bombardment induced surface roughening, accelerated by oxygen flooding. An additional contribution to the profile shift may originate from the large change in erosion rate of the sample brought about by high-pressure oxygen flooding (reduction in erosion rate by up to a factor of 3.5). Last, but not least, evidence is presented that the concept of stable B-to-Si sensitivity ratios breaks down in the transient region. These findings imply that the oxygen flooding technique in combination with oblique oxygen bombardment is not suited for quantitative depth profiling of shallow doping distributions.