438 Zirong Peng et al.


Figure 6. Schematic drawing showing the relationship between the evaporation field and temperature for two different cases. The red line represents the APT experiments done in this work with cemented tungsten carbide sample and 0.002 ion/pulse target detection rate. The blue line represents the study of Kolli & Meisenkothen (2014) using copper sample and 0.01 ion/pulse nominal detection rate.


and 700 pJ energies in the green laser system, indicating that ~12 times higher laser energy is required for the large-spot green laser in the 3000X HR instrument to achieve a similar evaporation field as the more focused UV laser in the 5000 XS instrument. Here, the Kingham curves of tungsten (Kingham, 1982) were used to assess the strength of the electric field induced by varying the laser pulse energy in this work. The evaporation field of any material depends on its work function (Podchernyaeva et al., 1969). The work function of tungsten carbide is around 3.6 eV, whereas the work function of tungsten is around 4.5 eV (Fomenko, 1966). This difference will result in a shift of the Kingham curves, but a rough estimation of the filed strength is still possible using the existing curves for tungsten. As demonstrated in Figure 5b, due to the difference in laser pulse energy, the evaporation field changed in the range of 46–49 V/nm.


DISCUSSION


Comparing among Figure 2, Figure 3 and Figure 4, it can be seen that the mass resolution, the background level, the chemical composition, and the multiple fraction are affected differently by APT analysis conditions and specimen geometry. The mass resolution reflects the spread of m/n of each ion type. As m/n is calculated from the acceleration voltage, flight path length and TOF, any fluctuations in any of these three parts will contribute to deviations in m/n. Modern APT instruments, such as the ones utilized in this study, have quite stable performance. Substantial systematic fluctuations in the measurement of voltage, ion impact position and time on the detector are expected to be very low. Intrinsically, the differences in the ion trajectory will lead to variations in the flight distance. To decrease these differences, during the processing of raw data, a “correction” procedure, referred to as bowl correction, is usually applied to the data. Even if there might be some errors in the estimation of the real flight distance due to the limitations of the applied ion projection theory, this type of error should not be strongly influenced by the


analysis conditions considering that the studied material in this work is single phase and the analyzed specimens exhibited similar geometries. In APT, theTOFof an ion is calculated under the assumption that it is generated at the time of the evaporation pulse. However, in the laser-pulsingmode, the field evaporation is usually a thermally assisted process. Atoms can be field evaporated during the decay of the specimen temperature after the laser pulse (Kellogg & Tsong, 1980; Vurpillot et al., 2006, 2009), resulting in inaccurate TOF values and deteriorated mass resolution. Increasing the specimen shank angle enables faster cooling of the specimen surface, mitigating the delays in evaporation of ions and hence mass resolution will be improved. This effect explains why much better mass resolution was achieved in the 3000BTD series than in the 3000BTA series. This shank-angle-dependent mass resolution phenomenon has also been observed in several previous studies (Bunton et al., 2007; Cerezo et al., 2007; Perea et al., 2008; Tang et al., 2010). On the other hand, with respect to the same degree of error, increasing the TOF itself can also help improve the mass reso- lution. When other conditions are fixed, a higher laser pulse energy provides higher thermal input, raising the peak temperature of the specimen apex (Liu & Tsong, 1984) and correspondingly, the required base field, i.e. the standing DC voltage, will decrease. As a result, the TOF of field-evaporated ions will become relatively longer and have a wider error toler- ance. For the same reason, elevating the specimen base tem- perature can also increase mass resolution, but the effect is less distinct. Normally the specimen base temperature only varies within 100K, however after the interaction with the laser beam, the temperature of the tip apex can increase by a few hundred Kelvin (Lee et al., 1980; Kellogg, 1981; Marquis & Gault, 2008; Diercks&Gorman, 2015). In contrast to our results, earlier work in copper showed that mass resolution is degraded by increasing the pulse energy or specimen base temperature (Kolli & Meisenkothen, 2014). A possible reason is that, in that work, a much higher detection rate has been adopted (10 ions detected every 1,000 pulse). As illustrated in Figure 6, under such cir- cumstances, the impact of temperature increase on the base field is weaker. Even with a high temperature rise, the standing voltage will only decrease slightly, and thus the improvement in mass resolution therefrom will be small. However, due to this large temperature increase, the cooling of the specimen takes much longer time, resulting in pronounced delayed field evaporation of ions and substantially deteriorating mass resolution. The change in mass resolution is actually a combination


of the effects from these two above-mentioned aspects, i.e. the TOF itself (standing voltage) and the cooling of the tip apex. Comprehensive considerations are required when evaluating the impact of experimental conditions and spe- cimen geometry on the final mass resolution. Increasing the specimen radius, on the one hand, will have the negative effect that the standing voltage will be higher, whereas on the other hand, as the cooling rate is proportional to the cross- sectional area, a larger specimen radius enables faster cooling. Bunton et al. (2007) showed that for aluminum specimens, the specimen radius does not affect the mass resolution, but for the stainless steel samples, as we have


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