An atom probe for three-dimensional tomography

Abstract

ELECTRIC-field-induced evaporation of ions from a needle-like surface, and their subsequent identification by time-of-flight mass spectrometry, forms the basis of the atom-probe technique¹. This has proved to be a powerful analytical tool^2,3, permitting the quantitative determination of material composition in a small selected region of the surface (depths of 1–5 nm) with single-layer resolution. Positional information for the atoms within each layer is lost, however. In contrast, the field-ion microscope³ provides atomic-resolution images of surfaces, but without information about the nature of the atoms. Several attempts have been made to combine these two techniques by extending the time-of-flight measurement into two dimensions, but they have been limited by their inability to identify all chemical species⁴ or to combine spatial and temporal information for multiple events⁵, especially for ions with very similar mass-to-charge ratios⁶. Here we make use of a recently developed⁷ multiple-impact detector to construct a position-sensitive atom probe with sufficiently high temporal resolution (of the order of 10 ns) to avoid these earlier problems; thus, reliable composition and position data can be obtained at the same time. We illustrate the performance of this instrument by imaging the three-dimensional distribution of chemical heterogeneities in a nickel-based alloy on a near-atomic scale.