324 David R. Diercks et al.


Figure 1. Scanning electron microscopic images of milled tips for each of the depositions with lines indicating the inter- faces between the depositions and the silicon. The depositions are (a)Au, (b)Au+O2,(c)Co, (d) Fe, (e)Pd, and(f)Pt.


spectrum included peaks from both gold and silicon. The reason for this can be seen in comparing the evaporation fields of the twomaterials. In voltage pulsing, the evaporation field of gold is 60% greater than that of silicon (Tsong, 1978; Kellogg, 1983). Though the evaporation fields with the laser pulsing used here are not known, a similarly large disparity is expected. With a high evaporation field material on top of a low eva- poration fieldmaterial, it is possible to generate sufficient field for observing evaporation of the bottom material from the surface further down the tip before complete removal of the top layer (Marquis et al., 2011; Lee et al., 2014), especially with theseveredifferencein fields for these twomaterials.At higher laser energies, the relative contribution of the field to the eva- poration is decreased. However, even the highest laser energy and lowest applied bias attempted here did not prevent the silicon from also being removed during evaporation of the gold. This, along with the EDX data, suggests that this is a high purity deposition with a high evaporation field, likely approaching that of pure gold. As such, this deposition could be used as a capping layer for making APT specimens of very high evaporation field materials. Yet, the usefulness of this is somewhat mitigated by the need for flowing oxygen during deposition and a relatively slow deposition rate.


APT of Co


The APT analysis of the Co deposition was performed using laser energies from 7–60 pJ, which resulted in applied biases


of 4.6–1.4 kV. As shown in Table 2, there was little variation in the measured composition of the deposition over these analysis conditions. Some analyses were allowed to proceed all the way through the Co deposition and into the silicon with a smooth transition between the two phases as seen in Figure 3a. The mass spectrum from the bulk of a Co deposition run at 4.2–4.6kV and 7 pJ is shown in Figure 3b. The predominant species is Co++. In addition, the APT reconstructions indicate a homogeneous deposition with local Co variations of at most 5 at%. The evaporation field for this deposition was estimated


based on the charge state ratios (CSRs) observed for the Co+ and Co++ peaks from the different analyses. The different applied biases/laser energies provide a range of fields. Turning the applied bias into an estimated field requires adjustments based on the tip radius at each condition. Using the SEM images of the initial and final radii, the radius at each intermediate point can be estimated, similar to what was done elsewhere using TEM images (Diercks & Gorman, 2015). There are small changes in the flux and possibly the apex tangential discontinuity (sphere-to-cone ratio) (Larson et al., 2011) as the radius changes, which may affect the estimate of the field; however, the effects of these over the range of analysis parameters used is expected to be relatively small (Diercks & Gorman, 2015). The plot of relative fields shown in Figure 4a can then be compared with the expected Co ionization state curves (Kingham, 1982) to generate the absolute field values. Specifically, the point where an


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