Secondary Ion Mass Spectrometry

Static SIMS mode

Under the ion bombardment, fragment ions or even intact molecular ions are emitted from the top monolayer. One important point is that after each ion impact, all molecules within a 5-10 nm diameter are damaged and no further molecular information can be obtained from this zone. Thus, the number of molecule available on a given top surface area is finite and limited. If the primary ion dose is limited to a level at which every primary ion should (statistically) always hit a fresh area, the (static) SIMS spectrum reveals molecular information through its peaks and its peak pattern (relative peak intensities). Progressively, as the ion dose increases, the molecular signal decreases then vanishes when the whole area has been damaged. To stay in static SIMS mode, the primary ion dose must be kept below 1E12 Primary Ions per cm2.

Analysis of bulk sample surface in static SIMS gives rise to a fingerprint mass spectrum that contains "low mass" (< 500 amu) ion fragments and thus helps identifying organic surface composition. Due to the complexity of the static SIMS mass spectrum, this technique is mostly used as a qualitative characterization of the molecular composition of the top surface. Access to the oligomeric distribution and "intact" molecular ions is possible by preparing samples as a monolayer on noble metal substrate, at the cost of the loss of the lateral spatial distribution information. Cationization by such metal substrate allows access to mass range of up to a few thousands amu. For some materials, using poly-atomic primary ions (i.e. 60C+) allows to go beyond the traditional static SIMS limit and keep some molecular signal.

By focusing and scanning the primary ion beam, molecular information can be obtained with sub-micron lateral resolution, and molecular surface distribution can be imaged.

More on SIMS:   Introduction   -   Dynamic Mode