Laser-Pulsed Atom Probe Analysis Condition 439
Figure 7. Thefractionofmultipledetectioneventsasafunctionofcharge-stateratio of tungsten ions [W4+/(W2++W3+)] plottedusing APTdataacquiredfrom(a) LEAP 3000X HR (3000PE and 3000BT test series), (b) LEAP 5000 XS (5000PEA and 5000PRR test series). Here A and D indicate that the variable was changed in ascending and descending sequence, respectively. The detailed experimental conditions are listed in Table 2.
observed in this work, a larger specimen radius can improve mass resolution. This difference is attributed to the different thermal diffusivity of these two materials, which is the intrinsic factor controlling the cooling rate. The impact of the laser repetition rate on the mass resolution is also closely related to the thermal diffusivity of the specimen. For materials with good thermal diffusivity, after the illumina- tion with the ultra-fast laser pulse, the specimen can cool down within 1 ns (Vurpillot et al., 2009), but for the samples of poor thermal diffusivity, the cooling time will be longer. In this study, we did not observe a strong dependence of mass resolution on pulse repetition rate. The highest laser pulse repetition rate applied is 1MHz, where the time interval between two pulses is 1 μs, indicating that the time needed by the specimen to cool back to the base temperature is <1 μs. Actually, the cooling down time of the specimen can be roughly estimated from the length of the peak tails in the correctedTOF spectrum, which is around 6 ns, much shorter than 1 μs. If the cooling time is longer than the pulse interval, the effects of the laser pulse will accumulate and the specimen will not cool back to the base temperature. Con- sequently, the mass resolution will increasingly degrade, which is not observed here. In addition, as specimen geometry can also influence the cooling time, the impact of laser pulse repetition rate on mass resolution is also affected by specimen geometry. In contrast to the mass resolution, the NSR is not so
sensitive to pulse energy and specimen base temperature. Even though different analysis parameters and specimens were used in the 18 measurements of the 3000PE and the 3000BT series, the resulting NSR values only show small variations (Fig. 3a, 3c), calculated to be 0.14±0.042. The specimen radius and the laser pulse repetition rate have influences on the NSR. When the specimen radius is large and the pulse repetition rate is low, the NSR will increase. There are several origins of the background noise. First, residual-gas atoms, or other types of atoms from the chamber or contamination adsorbed on the specimen
surface, can be field evaporated by the pulse or the intense DC electric field and contribute to the background. Figure 3 shows the average pressure in the analysis chamber of each measurement. Normally it is in the range of 10−9 Pa (10−11 torr). Although decreasing the specimen base temperature can lead to a drop in the pressure, due to the condensation of some gas species such as CO and CO2, the background level is only slightly affected. Second, the micro-channel plate (MCP) of the detector will also generate noise. Third, some heavy ionswhich are too slow to reach the detector within an individual detection window, as well as ions with substantial delays in evaporation and largely shif- ted TOF, will also be treated as background noise in the mass spectrum. In this study, the contributions from the third aspect are thought to be limited and the vast majority of noise is supposed to come from residual gas and the MCP. These background ions are normally evenly distributed in time. With a higher laser repetition rate or detection efficiency, the time needed to collect the same amount of signals is shorter. Therefore, the amount of noise is reduced. Larger specimen radii require higher standing voltages and the TOF of noise ions becomes shorter, hence, more noise ions can reach the detector, resulting in a higher level of background. On the other hand, a larger specimen radius means a larger surface area for residual-gas atom absorption. Besides, DC evaporationmay also have a contribution to the tremendous increase in the NSR of the 100 kHz test in the 5000PRRD series. When a low pulse repetition rate and a specimen with large radius are adopted, the temperature rise of the specimen is smaller, leading to an increase in the evaporation field and, as a result, DC evaporation could take place. The accuracy of the resulting chemical composition is linked to the fraction of multiple events. Generally, there are
two main mechanisms giving rise to the multiple events. The first one is termed as correlated field evaporation. When one atom is field evaporated, atoms in the vicinity sites are prone to field evaporate successively due to the increase in the
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