Post-field ionization of Si clusters in atom probe tomography: A joint theoretical and experimental study

Abstract

A major challenge for atom probe tomography (APT) quantification is the inability to decouple ions that possess the same mass–charge (m/n) ratio but a different mass. For example, 75As+ and 75As22+ at ∼75 Da or 14N+ and 28Si2+ at ∼14 Da cannot be differentiated without the additional knowledge of their kinetic energy or a significant improvement of the mass resolving power. Such mass peak overlaps lead to ambiguities in peak assignment, resulting in compositional uncertainty and an incorrect labeling of the atoms in a reconstructed volume. In the absence of a practical technology for measuring the kinetic energy of the field-evaporated ions, we propose and then explore the applicability of a post-experimental analytical approach to resolve this problem based on the fundamental process that governs the production of multiply charged molecular ions/clusters in APT, i.e., post-field ionization (PFI). The ability to predict the PFI behavior of molecular ions as a function of operating conditions could offer the first step toward resolving peak overlap and minimizing compositional uncertainty. We explore this possibility by comparing the field dependence of the charge-state-ratio for Si clusters (Si2, Si3, and Si4) with theoretical predictions using the widely accepted Kingham PFI theory. We then discuss the model parameters that may affect the quality of the fit and the possible ways in which the PFI of molecular ions in APT can be better understood. Finally, we test the transferability of the proposed approach to different material systems and outline ways forward for achieving more reliable results.