Secondary ion mass spectrometry of MCsn+ molecular ion complexes

Excellent detection sensitivity, high dynamic range and good depth resolution make the SIMS technique extremely powerful for the analysis of surfaces and interfaces. However, a serious problem in SIMS analysis is its “matrix effect” that hinders the quantification of a certain species in a sample and consequently, probing the composition of surfaces or interfaces by SIMS is greatly hindered. Appropriate corrective measures are therefore, needed to calibrate the secondary ion currents into respective concentrations for accurate compositional analysis. Working in the MCs+-SIMS mode (M - element to be analyzed, Cs+ - bombarding ions) can circumvent the matrix effect. The quantitative potential of the MCs+-SIMS method is understood by assuming that an MCs+ ion is generated by the combination of a secondary neutral M0 atom with a re-sputtered Cs+ ion in the near-surface region. The emission process for the species M0 is thus decoupled from the subsequent MCs+ ion formation process, in analogy with the ion formation in secondary neutral mass spectrometry (SNMS), resulting in a drastic decrease in matrix effect. Although this technique has found its applicability in direct quantification, it generally suffers from a low useful yield. In such cases, detection of MCsn+ (n = 2, 3, …) molecular ions offers a better sensitivity as the yields of such molecular ion complexes have often been found higher than that of MCs+ ions. This is true in most of the cases where the elements are strongly electronegative with respect to cesium. Several works have been reported on the emission of MCsn+ molecular ions in the SIMS process, but a complete understanding on the formation mechanism of these ion complexes is still lacking. The kinetic energy distributions of secondary MCsn+ molecular ion complexes has been found to be an effective approach to estimate the local instantaneous surface work function changes under various surface exposure conditions, thereby enabling one to elucidate on the probable formation mechanisms of these molecular complexes from their emission dynamics. The present paper addresses a brief review on the MCsn+ molecular ion complexes that are emitted in the SIMS process, including various phenomenological approaches on the subject.