Secondary Ion Mass Spectrometry (SIMS) is a method of mass spectrometry with secondary ions ejected from a solid that is bombarded by high-energy (> 1 keV) primary ions.
The bombardment with high-energy primary ions triggers a collision cascade (similar to pool or pocket billard) in the solid. If the cascade returns to the surface of the sample, particles are ejected. This requires that the mass of the primary ions is similar to the mass of the atoms in the solid (that's why electrons won't work) and that the cinetic energy of the primary ions is much higher than the energy of the chemical bonds in the solid. Secondary particles are:
- neutral atoms or molecules
- atomic oder molecular ions
- singly or multiply charged
- positively oder negatively charged
At very low primary ion doses of < 1012/cm2 no spot on the sample surface is hit twice. This is the definition of Static SIMS, where only the topmost surface layer of the sample is sputtered and removed by the primary ions. Therefore only the sample's surface is analysed. The modification of the sample surface by primary ions does not influence the analysis because no spot of the surface is hit twice by primary ions.
At primary ion doses of >> 1012/cm2 each spot on the sample surface is hit several times. In the course of an analysis material is continually removed reaching typical final depths of 0.1–10 μm. This is the definition of Dynamic SIMS, where a small volume at the surface of the sample is analysed. By plotting the secondary ion intensity vs sputtering time one can obtain a depth profile with a depth resolution down to 1 nm.
In Dynamic SIMS mode primary ions are implanted into the sample in the course of the analysis and therefore the sample is changed chemically and structurally. After a short while an equilibrium between implantation of primary ions and sputtering of the sample (including the implanted primary ions) is reached. This modification of the sample can be used to increase the yield of secondary ions: Oxygen primary ions (high electronegativity), for example, increase the yield of positive secondary ions while cesium (low electronegativity) supports the formation of negative secondary ions.