TOF‐SIMS: G‐SIMS - mikee9265/SIMS-Wiki GitHub Wiki
The spectra produced in a TOF-SIMS instrument from a given sample will vary substantially depending on the primary ion source used. Light monatomic ions produce spectra with relatively more fragmentation. Cluster ion sources produce spectra with relatively more intense molecular ions and higher mass fragment ions. The variation in relative ion intensities itself provides information about the sample.
The Gentle SIMS (G-SIMS) methodology exploits the differences between spectra obtained with different ion sources (Gilmore and Seah 2000). G-SIMS is based on the idea that the trend in spectra from ion sources producing more fragmentation to those producing less can be extrapolated to produce the spectra that would be produced in the scenario in which the sputter event was so gentle that molecular ions would be emitted from the sample with virtually no fragmentation at all. G-SIMS is thus intended to simplify the spectra and indicate the ions that are most indicative of the original state of the sample.
The G-SIMS methodology magnifies differences in the two spectra obtained by two different ion sources to a large degree. The primary ions need to be quite different, preferably a low mass atomic ion coupled with a cluster ion. For example, Au1+ and Au3+ are not an ideal pair for G-SIMS analysis, nor are Bi1+ and Bi3+.
The G-SIMS spectrum is generated most successfully when the sample surface is so homogeneous that spectra taken in different locations are identical, and therefore the G-SIMS calculation will not be accidently magnifying differences in the sample itself, rather than the intended ion yield differences of the two sources. For a less homogeneous sample, the two spectra that will be the basis of the G-SIMS spectrum would need to be taken from the exact same location on the sample. This latter experiment is difficult when two different ion sources are used for the analysis.
A single source that can produce both a lower mass atomic ion and a cluster ion is the BiMn source, used in an LMIG (Green et al. 2008). Mn+ is the lower mass atomic ion emitted by this source, and the ever-popular Bi3+ the cluster source. Switching between the two requires only a change in timing within the gun, therefore the two sources will produce identical raster scans on the sample with ease. Not only does this allow the G-SIMS analysis on less than homogeneous samples but it also allows for the calculation of G-SIMS images. The analyst still needs to make the decision in advance to use some of the limited ion flux available in the analysis before the Static SIMS limit is reached for a less informative Mn+ spectrum, so the technique is more likely to be used when larger sample areas, where the static SIMS limit presents less of an issue, are analyzed.