Performance of the new high mass resolution time of flight atom probe

Abstract

Detectability and mass resolution of a newly built high mass resolution time-of-flight atom probe were examined. In order to maximize the detectability, the specimen tip apex, the probe hole, and the entering side of the energy-compensating deflector were aligned by a laser beam. Transmittance of ions through the deflector and mass resolution were optimized varying the combinations of deflector electrode voltages. The detectability, defined as the ratio of the number of all atoms in the area aimed by the probe hole to the number of field evaporated ions arriving at the detector, was found to be dependent on the crystallographic plane and its aimed area, possibly due to the rolling motion of the field evaporating atoms in a certain preferable direction. The observed detectability ranges from only 10% at the central area of the W (011) plane, which is dark in a multiatom-layer desorption image, to 91% at the (011) plane side of the W(114) plane, which is significantly higher than the maximum detectability limited by the effective area of a chevron detector, 55%. Thus, the overall detectability might be as high as 50% and the transmittance would be nearly 100%. The mass resolution Δm/m was 1/1100 at half peak height which was comparable to the theoretically expected mass resolution limited by the pulse width and the time resolution of the timer, 5 ns, and each of the detected W ions was clearly discriminated corresponding to each W isotope. The atom probe also successfully demonstrated its unique capability of the atomic layer-by-layer mass analysis providing the ultimately fine depth profile.