SIMS TECHNIQUE

The Ion Microprobe employs Secondary Ion Mass Spectrometry (SIMS) for the chemical analysis of small volumes of material. In SIMS the surface of the sample is bombarded under vacuum with a finely focused beam of primary ions (Cs+, O+, O- or Ar+). The collision cascade results in the ejection and ionisation of atoms and molecules from the surface layers of the sample. These secondary ions are accelerated into a double focusing mass spectrometer where they are separated according to their energy and mass/charge ratio before being detected.

There are three basic modes of operation;

• Point Analysis: A focused stationary beam is used to determine the composition at a point. The spot size is normally between 1 and 25um but is dependent on the application.
• Depth profiling: Scanning the primary ions over the surface of the sample causes the surface layers to be slowly eroded, revealing the compositional change with depth.
• Imaging: The distribution of elements or isotopes may be imaged with a spatial resolution of about ~1um over areas up to 500um2.

The Ion Microprobe can quantitatively analyse nearly all elements in the periodic table from H to U, but there are differences in sensitivity (Relative Ion Yields) exceeding four orders of magnitude. However, for many elements the detection limits are in the p.p.b. range. The instrument may also be used to determine the isotopic ratios of elements (e.g. C, O, Si, B, Li, S) to a precision of <0.05% in suitable material

The only sample preparation required is the production of a flat surface. The sample must be less than 1" in diameter and less than 0.5" thick. Any vacuum compatible material can be analysed including conductors (e.g. metals, alloys, sulphides), non-conductors (e.g. silicate minerals, ceramics, glasses) and biological material.