222 Lu Zhao et al.
collected by a position-sensitive detector that determines the time and the location of the impact. Assuming that ions are instantaneously accelerated without an initial velocity and that the whole potential energy is converted into kinetic energy, the mass over charge ratioM(m/n) of the ions can be written as
M= m n =kV
t2 l2 ; f (3)
the specimen, l the ion total flight distance and tf thetimeof flight of the collected ion. Mass resolution is one of the key performance parameters
where k = 0.193m2/kV/µs−2, V is the total voltage applied to
of modern atom probe. For example, both quantitative composition measurements and three-dimensional reconstruc- tions rely on the mass spectrum quality and processing. It is well known that the conventional voltage pulsed atom probe is limited in mass resolution by the energy spread caused by voltage pulsing, the mass resolving power can be <100 in a straight flight-path atom probe (Krishnaswamy, 1974; Krishnaswamy & Muller, 1974; Kelly et al., 1996; Cerezo & Vaumousse, 2001). In fact, ions generally leave the sample at slightly different times relative to the pulse maximum (start-time), due to the stochastic nature of the evaporation process (Gault et al., 2012). This results in some ions of the same nature acquiring slightly smaller kinetic energies. In addition, the emitted ions experience a time-varying field during the accelerating region. The ions gain systematically less kinetic energy with respect to the full potential energy applied to the tip. The atom probe design generally includes a counter electrode,
positioned very close to the tip, which minimizes the effect of the finite acceleration time. Although this configuration reduces energy deficit, some deficit will still remain. One of the primary goals in atom probe tomography instrumentation is to improve
atom probe. It is clear that each new device always introduces unexpected shortcomings and complicates the instrument. An alternative method is to achieve FE by applying a
laser pulse to briefly increase the temperature of the apex of the sample (Kellogg & Tsong, 1980). The energy deficits are eliminated as the DC field due to the standing applied voltage promotes FE and accelerates the emitted ions. This improves mass resolution. Even so, significant degradations of mass resolution are experimentally observed in the analysis of some highly electrically resistive materials, for example, MgO and intrinsic silicon at cryogenic temperature (Arnoldi et al., 2014; Sévelin-Radiguet et al., 2015). In LAAP, the mass spectrum quality also depends on factors including the laser wavelength, laser power, and shank angle (Bunton et al., 2007; Houard, 2010; Houard et al., 2010). Many authors have reported that the peak temperature at the tip surface must be maintained sufficiently low to avoid surface diffusion which has an impact on the APT spatial resolution (Cerezo et al., 2007; Gruber et al., 2011). In addition, low thermal conductivity could reduce the mass resolving power to values as lowas 100–200 fullwidth at half maximum
(FWHM), for amass-to-charge-state ratio around 30, such as 56Fe2+(Houard et al., 2011).Decreasing the temperature could enhance the mass resolution, but a correct composition measurement for some materials, such as GaN and AlN, needs also an appropriate laser power/DC electric field ratio (Mancini et al., 2014). As a conclusion, the optimization in laser pulse mode is generally cumbersome and prevents the routine application of the laser pulsemode for some materials. Table 2 compares the performance of the two atom probe modeswith different configurations. It is clear that none of the instruments could satisfy all the critical requirements. Our theoretical study shows that the mass spectrum can be
mass resolution without sacrificing the spatial resolution, the field-of-view, and the detection efficiency. Several approaches have been proposed to compensate or reduce these spreads, such as the Poshenreider lens (Poschenrieder, 1971, 1972), the reflectron (Cornish & Cotter, 1993), and double-electrodes (Kelly et al., 1995; Cerezo et al., 2000), each have been applied to different generations of instruments to improve mass resolution. Table 1 compares the different devices combining with
improved by modifying the shape of the applied voltage pulsetohavea flat top, a sharp front, and leading edge associated with a short tip-to-electrode distance (Zhao et al., 2015). However, in order to optimize the energy spread, the ion must also be evaporated on a really flat and stable part of the pulse. Inspired by the proposition of synchronized laser pulse and voltage pulse in Kelly (2011) and the pump-probe method, we performed laser pulsing evaporation with a dynamic field andachievedansignificant improvement in mass resolution.
Table 1. Summary of the Solution for Optimizing the Performance of the Time-of-Flight Mass Spectrometer in Atom Probe, the Number of ‘+’ From One to Four Represents the Level of Improvement.
Solutions Reflectron Method
Local electrode Short tip-to-electrode distance (Kelly & Larson, 2000)
Advantage
Simple and no additional device
Curved field (Cornish & Cotter, 1993) Achieve time-focusing on the detector
Double electrode Post-acceleration (Cerezo et al., 2000) Simple Double electrode Post-deceleration (Kelly et al., 1996) Simple
Degradation of spatial resolution/reduction of the detection efficiency
Perturb tip field/limited to a few kV of post-acceleration
Valid only over a limited range of mass
Disadvantage Mass Resolution
+ (compensate dynamic effects)
++++ ++ +++
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