Apparent depths of B and Ge deltas in Si as measured by secondary ion mass spectrometry

Abstract

The apparent depths of B and Ge deltas in Si were measured with secondary ion mass spectrometry using 1–4 keV O2+ beams at oblique incidence (40°–80° with respect to the surface normal). The real depths of the Ge deltas were obtained via calibration against transmission electron microscopy (TEM). The measured centroids of the Ge delta peaks were 0.5±0.4 nm shallower than the real (TEM) depths, independent of angle and energy. For B there was a clear angular dependency of the centroid position, but the energy dependency was virtually absent (viz., differential shift <0.2 nm/keV/ion). Repeated analyses during a 2.5 year period showed a reproducibility in Ge delta peak position of 0.04 nm. Surprisingly, the slow but continuous growth of the native surface oxide had no effect on the apparent Ge delta depths. The profile shift of B towards the surface was attributed to the high sputter rate during the surface transient and to B outdiffusion under oxygen bombardment. It was concluded that decreasing the beam energy did not reduce the B shift; the positive effect of a thinner surface transient layer was nullified by a larger (equilibrium) oxygen concentration. In contrast, indiffusion of Ge compensated at all energies and angles the (apparent) Ge shift due to the transient high sputter rate.