High Dynamic Range SIMS Depth Profiles for Aluminium in Silicon-on-Sapphire

Silicon-on-Sapphire (SOS) is the most advanced silicon-on-insulator technology for CMOS-VLSI. We have described techniques for extracting accurate boron profiles from SOS [1] and, elsewhere in these proceedings, methods for obtaining accurate depth profiles from resistive multi-layer structures like SOS [2]. The substrates used for SOS form a potential source of aluminium doping which could lead to poor mobilities and unpredictable device behaviour. Many of the processing techniques for SOS (especially self-implantation and regrowth [3]) are very likely to remove Al atoms from the substrate and redistribute them in the silicon. This SIMS study of the behaviour of Al in SOS is part of a programme to define acceptable processing schedules. Three problems may be anticipated: Firstly, we wish to obtain accurate epi-layer and interface profiles from a resistive structure on an insulator. Secondly, we require quantification from impurity (i.e. < 1%) to matrix concentrations, across two different matrices, so calibration standards must be chosen with care, and the interpretation at the interface may be semi-quantitative. Thirdly, the Al+ signal is only 1 amu away from the intense Si+ matrix emission. Thus, the dynamic range will be dependenI on the peak-shape of the mass spectrometer, and the relative ion yields of Al+ and Si+.