The technique of Secondary Ion Mass Spectrometry (SIMS) is the most sensitive of all the commonly-employed surface analytical techniques - capable of detecting impurity elements present in a surface layer at < 1 ppm concentration, and bulk concentrations of impurities of around 1 ppb (part-per-billion) in favourable cases. This is because of the inherent high 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 - SIMS Imaging and Depth Profiling.
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
SECONDARY ION TRANSFER
5. Secondary Ion Transfer.
After the secondary ions have been extracted from the sample surface by the immersion lens
they are transferred by a second electrostatic (transfer) lens into the mass spectrometer. The purpose
of this transfer lens is to form a real magnified image of the sample surface at the position of the
field aperture and to focus the secondary ion beam onto the entrance slit of the spectrometer.
At the same position as the entrance slit is the contrast aperture. Smaller contrast apertures
intercept ions with off-axis components, resulting in greater spatial resolution but reduced ion
intensities.
The immersion lens and the transfer lens together form the Cameca ion microscope, which
enables an image to be viewed by an appropriate detector at the position of the field aperture. The
Cameca has three transfer lenses, but only a single lens is used at any one time, and is user selected.
Each lens produces a different magnification of the sample surface at the position of the field
aperture.