SIMS " An Overview


Introduction

Secondary Ion Mass Spectrometry is the most sensitive surface analysis technique providing quantifiable data to the ppb level and surface specificity to the uppermost monolayer. It can be used to analyse most solid vacuum compatible materials including metals, semiconductors, ceramics, polymers and biological material.
The sample is bombarded by a beam of ions, the primary ions, which sputter erode the surface. As a result of the energetic sputtering process, some of the ejected material (either atoms or molecules) is ionised, the secondary ions, and can thus be directed towards the mass spectrometer where detection is made.


Interaction of the primary beam with the target

When a primary ion impacts the specimen it gives up its energy through a series of collisions with target atoms which recoil and, in turn, collide with their neighbours. If they have sufficient energy, these will also go on to collide with other atoms, resulting in a collision cascade. At some point, the cascade may intersect the surface and the energetic particle, instead of colliding with a nearby atom, may escape from the material. If conditions are favourable the emitted particle may be in an ionised state. Generally, although the primary ion may penetrate many nanometres into the target, the ejected ions come from only the uppermost few atomic layers.
Although the volume of each cascade is small, 5-10nm in diameter, the effect of successive cascades over time is the formation of a mixed or altered layer at the expose surface of the specimen. This layer comprises a mixture of sample and primary beam species. The thickness of the altered layer varies considerably with species and energy, and can range from around one nanometre at low energy, to 10 nm or more at higher energies. The chemistry of the beam and target is also important as the ion yield (the fraction of the material that is ejected in an ionised state) varies significantly and non-linearly, with the local surface chemistry, thus if a suitable altered layer is created the ion yield will be enhanced with consequent improvement in sensitivity.