The introduction of cluster sources such as C60 and argon giant cluster ion beams (GCIB) have remedied the excessive chemical damage generated by energetic ion bombardment to a surface. In addition to extending the achievable depth of analysis, these cluster sources offer enhanced desorption of large molecules, expanding SIMS to biological applications.
In SIMS, the desorption of molecules (and therefore “secondary ions”) occurs through energetic “primary” ion bombardment. This bombardment process causes a lot of chemical damage to the sample surface in the form of molecular fragmentation. If the dose is too high (> 1% of the number of surface molecules), a graphitic residue builds up on the surface and the signal disappears. The introduction of cluster sources such as SF6 and C60 have remedied this problem to an extent, where its enhanced (nonlinear enhancement) desorption can efficiently remove fragmented molecules, permitting the removal of material from the sample in a layer-by-layer fashion to determine the composition of each layer with nanometer depth resolution (Figure A). Cluster sources have become increasingly important for the characterization of complex organic multilayer structures, especially to the organic electronics industry where defects in layers less than 10 nm can be particularly difficult to determine. The recent development of Ar2000 clusters have allowed orders of magnitude improvement with regard to the depth of analysis, oftentimes allowing characterization of films that are tens and even hundreds of micrometers thick (Figure B).
The true advantage of a cluster source lies in its ability to desorb very high mass molecules, where yields of molecular ions in the 1000 Da to 3000 Da range often increase significantly over conventional atomic sources. This means that the technique could be very useful for bioimaging, as the increased number of molecules per image pixel creates a better contrast between adjacent pixels (and therefore a better chemical map), as well as an increased the number of large biomolecules such as proteins for improved identification and differentiation inside cells and tissues.