5.5 Secondary Ion Mass Spectrometry

The technique of Secondary Ion Mass Spectrometry (SIMS) is the most sensitive of all the commonly-employed surface analytical techniques - this is because of the inherent sensitivity associated with mass spectrometric-based techniques.

There are a number of different variants of the technique :

Statics SIMS :	used for sub-monolayer elemental analysis
Dynamic SIMS :	used for obtaining compositional information as a function of depth below the surface
Imaging SIMS :	used for spatially-resolved elemental analysis

All of these variations on the technique are based on the same basic physical process and it is this process which is discussed here, together with a brief introduction to the field of static SIMS. Further notes on dynamic and imaging SIMS can be obtained in Section 7.4 .

In SIMS the surface of the sample is subjected to bombardment by high energy ions - this leads to the ejection (or sputtering) of both neutral and charged (+/-) species from the surface. The ejected species may include atoms, clusters of atoms and molecular fragments.

In traditional SIMS it is only the positive ions that are mass analysed - this is primarily for practical ease but it does lead to problems with quantifying the compositional data since the positive ions are but a small, non-representative fraction of the total sputtered species. It should be further noted that the displaced ions have to be energy filtered before they are mass analysed (i.e. only ions with kinetic energies within a limited range are mass analysed).

The most commonly employed incident ions (denoted by I⁺ in the above diagram) used for bombarding the sample are argon ions ( Ar⁺ ) but other ions (e.g. alkali metal ions, Ga⁺ ) are preferred for some applications.

The mass analyser is typically a quadrupole MS analyser with unit mass resolution, but high specification time-of-flight (TOF) analysers are also used and provide substantially higher sensitivity and a much greater mass range (albeit at a higher cost).

Static SIMS

In static SIMS (SSIMS) the aim is to obtain sufficient signal to provide compositional analysis of the surface layer without actually removing a significant fraction of a monolayer - i.e. to be able to analyse less than ca. 10¹⁴ atoms/molecules (about 10% of a monolayer for a 1cm² sample). The technique is then capable of providing information about the topmost single atomic layer of the surface.

Example : static SIMS spectra from the surface of PTFE (polytetrafluoroethylene)

	Note the substantial differences between the positive and negative ion spectra. In each case the most intense peaks correspond to relatively stable positive and negative ions respectively.