On the Voltage and Bowl Correction of Trigger-Uncorrelated Multihit Events

Modern atom probe tomography is based on time-of-flight (ToF) mass spectrometry. In this method, the detector precisely measures the time difference between when an ion evaporates and the time it strikes a detector located a known distance away. Combining this ToF measurement with knowledge of the detector geometry and the electric fields applied to the sample the mass-to-charge ratio (m/z) of the ion can be determined accurately. Ideally for any such measurement scheme there is a well-defined trigger pulse (voltage or light) that causes field ion evaporation, and evaporation does not occur in the absence of a trigger pulse. Realistically however, this is not the case -field evaporation occurs in the absence of the trigger pulse and contributes to the background signal. For the case of ion evaporation events that are truly random (i.e. uncorrelated with respect to the trigger pulse and the other ions), there is no current method to extract accurate m/z information and if these ions originated from the sample then information is permanently lost. For a `singles’ event where only one ion strikes the detector following a trigger pulse there are two pathways shown in Figure 1a. The pathway denoted `i’ shows the ideal behavior -a single ion leaves the sample coincident with the trigger pulse and thus the recorded time duration between trigger and detection is the actual ToF for the ion. In contrast, the pathway denoted `ii’ shows the background signal -here a single ion leaves the tip uncorrelated with the trigger pulse and so the recorded ToF differs from the actual ToF and renders the ion’s measured m/z incorrect. Because signal is lost in pathway `ii’ events, experimental conditions are chosen to minimize this behavior (e.g. increasing the pulse fraction, reducing the temperature, minimizing the standing DC field).