Ion probe mass spectrometry: Overview

The bombardment of a surface by 1–25 keV primary ions causes the upper atomic layers to be “sputtered” or stripped off. Most of the material leaves as neutral atoms or molecules, but a small fraction is ejected as positive or negative ions. These secondary ions can then be characterized by a mass spectrometer to provide an analysis of that portion of the material sampled by the primary ion beam. At any one instant, the excitation volume is bounded by the primary ion beam periphery (1–300 μm) and extends to a depth of 5–50 A depending on experimental conditions. By mass spectrometrically monitoring one or several masses, the analyst can obtain isotopic and concentration in-depth profiles of the sample constituents. Alternatively he can obtain a complete mass spectrum as the primary beam erodes the sample. Control and localization of the primary ion beam erosion permits chemical analysis with lateral x, y resolutions of approximately 1 μm, the examination of fractional surface monolayers, an in-depth analysis with better than 100 A resolution, and the acquisition of secondary ion images. The mass spectrometer provides elemental coverage from hydrogen to uranium, isotopic characterization and detection limits of 10-15−10-19g, depending on the element under consideration. This paper will discuss the production of secondary ions, the various types of instrumentation used, and applications of the ion probe technique.