Ultra Low Energy Secondary Ion Mass Spectrometry and some Current Applications.

From its development in the early 1960s until 1990 or so, dynamic secondary ion mass spectrometry (SIMS) routinely employed primary beam energies of 5-20 keV. In the late 1980s, demand for depth profile analysis of semiconductor heterostructures with sharp interfaces grew rapidly. This was followed closely by the requirements of emerging semiconductor technologies dependent on shallow implantation. The beam energies then available were too high to provide adequate depth resolution in the former case, or quantitative near-surface analysis in the latter, and this drove the development of SIMS instrumentation able to work routinely with beam energies down to 250 eV. The earliest such development was the floating low energy ion gun (FLIG™). The FLIG™ allowed primary beam energies of 0.2-1.0 keV to be used and led to the coining of the term ‘ultra low energy SIMS’ (uleSIMS). Dynamic uleSIMS profiling enables sub-nanometre depth resolution to be obtained, whilst retaining the large dynamic range and detection levels (ppm/ppb) essential in semiconductor analysis. Nevertheless, profiling at such low energies still has limitations, for example, transient behaviour at the surface and at hetero-interfaces, and the development of nanotopography if the analytical conditions are not chosen properly. A new way of using uleSIMS is in dynamic mass spectral analysis. The low primary beam energies combined with a low beam current allow for only the uppermost few nanometres of the material’s surface to be investigated, preserving chemical information, but allowing for the removal of monolayers of contamination which may be irrelevant to the problem at hand (in contrast to static SIMS). Correct interpretation of mass spectra can lead to a better understanding of surface chemistry. Examples of both dynamic and mass spectra uleSIMS will be given. Although uleSIMS has been extensively used in the semiconductor field, other material science disciplines are embracing it. Examples of these are as diverse as glass technology and cultural heritage research.