Optimal field pulsing for atom probes with counter electrodes

We have calculated energy shifts and energy deficits arising from voltage-pulsing in both conventional atom probe (CAP) (electrode at 5 mm) and local-electrode atom probe (LEAP) (electrode at 1 μm). The effects of voltage pulse duration and rise time for pulsing of both the specimen (positive) and the electrode (negative) have been considered. For the negatively pulsed CAP case there is an optimum in the pulse duration that minimizes the energy shift which corresponds to the time required for the ions to reach the electrode. For pulse durations of less than this value, the energy shift increases with increasing mass-to-charge ratios. For pulse durations of greater than this value, the energy shift increases with decreasing mass-to-charge ratios. The energy deficit remains relatively constant with pulse duration. For the negatively pulsed electrode case in the LEAP, the energy shift increases with pulse duration due to the deceleration of the ions. This suggests that fabrication of an electrode with sufficient thickness to create a small field-free region is required if negative pulsing is to be employed. This scenario was investigated as a function of pulse rise time. Energy deficits for this case show that the shortest possible rise time is beneficial when voltage pulsing in the LEAP.