A SIMS Primer - Dynamic SIMS
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As the ion dose is increased, and more material is ejected, the surface begins to recede. If specific masses are chosen and their signals followed as a function of sputter time, a depth profile of their concentration will be produced. Dynamic SIMS is unsurpassed by other surface analytical techniques both in terms of the sensitivity it can achieve and the fact that it can analyse the entire periodic table.
The primary ion beam is raster scanned, usually in a square, across the sample surface so that a well defined, flat bottomed, crater is produced. As the ion beam has a generally Gaussian distribution the crater edges are somewhat curved and when the beam is in their vicinity ions characteristic of crater wall will be emitted. To overcome this, ions are only recorded only when the beam is within a central area on the crater floor, a region termed the gate.
Dynamic SIMS is an extremely sensitive analytical tool and is often the only technique that can detect interface contamination in layered structures, or doping in semiconductors. The highest sensitivity requires the largest possible fraction of the emitted material to be ionised. The possibility of an ejected atom or molecule being emitted in the ionised state depends strongly on the chemical environment from which it arose and can vary by orders of magnitude. The reactive ion beams oxygen and caesium maximise the yield of electropositive and electronegative species respectively by modifying the chemistry of the altered layer. In the case of oxygen bombardment, a thin (1-20nm) stoichiometric oxide may be formed.
When bombardment first begins primary ions are implanted into the target and only target material is sputtered (Static SIMS). As more primary ions are implanted, and the altered layer begins to form, the surface chemistry is modified, becoming a mixture of both target and primary. Eventually, the number of previously implanted primary ions being ejected and the number of new ones arriving becomes equal and a steady state condition arises. A SIMS depth profile can thus be divided into two parts, the near surface (typically 2 - 20nm) pre-equilibrium and the deeper equilibrium regions.
In the pre-equilibrium region the surface chemistry is rapidly changing and thus the ion yield is unstable. Similarly, because initially only target material is ejected the erosion rate of the surface is higher than at equilibrium, when both target and primary beam material are sputtered.