Reduction of multiple hits in atom probe tomography

Abstract

The accuracy of compositional measurements using atom probe tomography is often reduced because some ions are not recorded when several ions hit the detector in close proximity to each other and within a very short time span. In some cases, for example in analysis of carbides, the multiple hits result in a preferential loss of certain elements, namely those elements that frequently field evaporate in bursts or as dissociating molecules. In this paper a method of reducing the effect of multiple hits is explored. A fine metal grid was mounted a few millimeters behind the local electrode, effectively functioning as a filter. This resulted in a decrease in the overall detection efficiency, from 37% to about 5%, but also in a decrease in the fraction of multiple hits. In an analysis of tungsten carbide the fraction of ions originating from multiple hits decreased from 46% to 10%. As a result, the measured carbon concentration increased from 48.2 at.% to 49.8 at.%, very close to the expected 50.0 at.%. The characteristics of the multiple hits were compared for analyses with and without the grid filter.

Atom Probe Tomography: Development and Application to the Geosciences

“…However, while gross differences between elemental sensitivities are avoided, more subtle effects can still be present and may result in chemical or isotopic compositional biases. The position‐sensitive detector has some dead time associated with the impact of each ion, so that a high flux of multiple‐ion events will lead to a loss of data, which may preference particular mass peaks (Thuvander et al , , Meisenkothen et al , Thuvander et al ). The precise nature of the dead time, which may include spatial proximity as well as coincidence limits, can be complex and will depend on the specific construction of the detector hardware (Peng et al ).…”

Effects of detector dead-time on quantitative analyses involving boron and multi-hit detection events in atom probe tomography

“…Recent studies on the behavior of the ions that comprise multi-hit events indicate that they are closely correlated in space and time [15,22,37,38], while ions uncorrelated with the pulse tend to be single-hit events that contribute to the background noise signal [15,38]. Hence, filtering techniques can be used to extract the ion information for the high-multiplicity events, and thereby provide a mass spectrum that has an improved signal-to-noise ratio [15].…”

“…For example, for multiplicity-2 events, the ion hit positions on the detector can be used to calculate the relative separation distance between the two ions in the pair [37,38]. The distribution of the separation distances at the detector between ions in multiplicity-2 events for the nominally pure B is illustrated in Figure 6 and for the SRM2137 materials in Figure 7.…”

A Brief Comment on Atom Probe Tomography Applications

“…The main controllable factor governing the multiple events is detection efficiency of the APT system (limited capability of the detector). For example, the instrument of LEAP 3000X HR (formerly Imago, CAMECA, USA) whose detection efficiency is ~37% has 50 ps of timing resolution, but if ions hit close in space they have to be separated by more than 3 ns to be separated (Thuvander et al, 2012). It makes the accuracy of concentration measurements significantly reduce due to undetected ions hit the detector during dead time.…”