Effect of energy selection on quantitative analysis in secondary ion microanalysis

Abstract

Systematic change of voltage on the components of the secondary ion (SI) extraction system of our AEI-IM20 ion microprobe produced major changes of relative intensities of secondary ions passing through the mass spectrometer. The repeller, which bends the SI beam through about 60°, has the greatest effect, and can be used to plot the energy distribution. The extractor and the deflecting and focusing components have smaller but significant effects. Because low-energy secondary ions have a near-symmetrical distribution, whereas high-energy ones have an assymetric distribution favoring high energies, tuning of the acceptance band to higher energy reduces interference from low-energy ions, which tend to be unwanted molecular ions, at the expense of reduced transmission. Tuning to lower energy increases interference but gives higher transmission. The former condition is desirable for instruments restricted to low mass resolution, whereas both conditions are valuable for instruments adjustable for both high and low mass resolution. Other important factors are (a) sensitivity to surface irregularities which perturb SI energy collection, and (b) change in derived “temperatures” from thermodynamic sputtering models merely from tuning the energy acceptance band. Careful attention to the above factors yielded reproducible SI ratios for the binary series of plagioclase feldspars.