Laser-Tip Interaction Part Two: Light Absorption by Nanoscale Semiconducting Tips in Laser-Assisted Atom Probe Tomography

摘要

Atom Probe Tomography [1,2] (APT) is an analysis technique based on the emission of ionized species from a nanoscale conical tip with a sub-100-nm apex radius. Due to the high constant voltage applied to the sample, a strong electric field just below the field-evaporation threshold (～10-50 V/nm) is generated at the tip apex [3]. An electric pulse (voltage-pulsed APT, typically used for good electrical conductors) or a sub-picosecond laser pulse (laser-assisted APT, typically for poor conductors) is then used to trigger the emission. The evaporated ions, after being accelerated in the electric field onto a position-sensitive detector, are chemically identified by time of flight mass spectrometry. This allows for the full compositional analysis of three-dimensional volumes with sub-nanometer resolution. While the evaporation mechanism of voltage- pulsed APT is well understood [4], the impact of the laser on a nanoscale tip during a laser-assisted APT experiment is still under intense investigation [5-17]. The latter is indeed considerably influenced by the light absorption properties of the conical tip as well as by the nonlinear dynamics of the carriers/heat generated in the tip by the laser pulse. These complex effects control the evolution of the surface of the nanoscale object and, hence, the time dependence (i.e. final mass resolution] and nature (single or multiply charged ions, clusters,...) of the emitted species. A clear and quantitative understanding of these phenomena is therefore required in order to fully exploit the power of the laser-assisted APT technique and to transition from an experimental, intuition-based operation and interpretation towards an optimized and quantitative analysis.