New dimensions in atom probe analysis

A new class of atom probe instruments has recently been developed which combine single atom sensitivity mass spectrometry with position sensing, and have the potential of reconstructing nanometer-scale composition variations in 3-D. These techniques will be very powerful in the study of the early stages of phase transformations, and surface and interface segregation. Some recent results obtained with the first of the instruments, the position-sensitive atom probe, are described. Difficulties arise in this instrument when more than one ion is evaporated from the analysis area on a single field evaporation pulse, which can limit the accuracy of analysis performed on the resultant data. Other 3-D atom probe instruments currently under development are described, and their expected performance in this respect is discussed. The significance of defining composition on the sub-nanometer scale is addressed, and the importance of atomic-scale modelling of the phenomena under investigation is indicated.