AN ATOM-PROBE FOR 3-DIMENSIONAL TOMOGRAPHY

ELECTRIC-field-induced evaporation of ions from a needle-like surface, and their subsequent identification by time-of-flight mass spectromet ry, forms the basis of the atom-probe technique1. This has proved to b e a powerful analytical tool2,3, permitting the quantitative determina tion of material composition in a small selected region of the surface (depths of 1-5 nm) with single-layer resolution. Positional informati on for the atoms within each layer is lost, however. In contrast, the field-ion microscope3 provides atomic-resolution images of surfaces, b ut 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 spa tial and temporal information for multiple events5, especially for ion s with very similar mass-to-charge ratios6. Here we make use of a rece ntly developed7 multiple-impact detector to construct a position-sensi tive atom probe with sufficiently high temporal resolution (of the ord er of 10 ns) to avoid these earlier problems; thus, reliable compositi on and position data can be obtained at the same time. We illustrate t he performance of this instrument by imaging the three-dimensional dis tribution of chemical heterogeneities in a nickel-based alloy on a nea r-atomic scale.