How does SIMS work?
SIMS is a destructive surface analytical technique. A primary ion beam (O or Cs) bombards the surface, causing ions (anions and cations), atoms and molecules to be ejected ('sputtered') from the surface. The energy of the primary ions can be of the order of a few 100 eV to 10s of keV, more than enough to eject atoms and molecules from the surface layers. The actual depth of interaction is usually a few nm.
As the beam moves ('rasters') across the surface it sputters a crater deeper into the sample (see image below). Continuous detection of these sputtered secondary ions allows SIMS to create a depth profile of the surface and underlying layers to a depth of 10 microns with a depth resolution of 2-5nm.
Why is it called DYNAMIC?
ANSTO's Cameca ims (5f) SIMS instrument is referred to as "dynamic" as it can both sputter at high rates and analyse the liberated ionised species simultaneously.
ANSTO currently has the only Cameca ims (5f) dynamic SIMS instrument in Australia.In comparison, time-of-flight ?static? SIMS (TOF-SIMS) instruments are quite common. The main difference is that TOF-SIMS are generally restricted to shallow surface analysis due to their very low sputter rates. In most cases, this makes depth profiling below the surface and into the near surface region (i.e. micron depths) using TOF-SIMS extremely time-consuming and unrealistic.
Main Steps of SIMS Analysis Process·
- Primary ions are produced by a cesium (1) or oxygen (2) source and are accelerated along the primary column (3-4) and focused onto the sample surface (5). ·
- Sputtered secondary ions pass through a secondary column (6-8), where they are collected and separated by charge (9).·
- The secondary ions are then collected and focussed through the mass spectrometer (10-12). ·
- The analysed ions are then directed into detectors (13-14), and a signal is measured. The electron multiplier (17) amplifies the incoming signal for each element to be large enough to be measured.
The signal can be collected and displayed in a number of ways, outlined in the SIMS capabilities section.
Why use SIMS?
The ability to detect trace elements in the surface and near surface regions of solid materials as a function of depth down to 10 microns.
The ability to characterise doping homogeneity, segregation, trace level imputities or diffusion occuring at the interface or in the bulk of multilayered materials.
The SIMS primary ion beam (probe) can be realistically focused down to 5 mm in diameter. This small analysis area enables greater ability to control of the specific location of analysis on a sample, e.g. this can allow individual grains in geological samples to be analysed.
SIMS can also provide high quality, reliable quantitative analysis with the provision of reference standards.
Isotopic ratio analysis can be used to measure isotopic fractionation in biological samples