EBID for APT Specimen Capping 323
Table 2. Compositions of the Electron Beam-Induced Deposition Layers as Measured by X-Ray Spectroscopy (EDX) and Atom Probe Tomography (APT).
Depositions Au
Au+O2 Co Fe Pd Pt
EDX Composition (at%)
Au27, C62, O10 >95% Au
Co80, C10, O10 Fe58, C3, O39
Pd28, C57, O8, F7 Pt18, C77, O5
depositions did not have sufficient or reliable statistics for confident assignment of an uncertainty. For the Au, Pd, and Pt depositions, the measured metal content by both EDX and APT was around 20–30 at% in all cases. The balance of those layers were carbon and oxygen. EDX and APT differed on the relative concentrations of those, with EDX measure- ments indicating more C and APTmeasurements indicating more O. These discrepancies could result from electron beam-assisted carbon deposition during the EDX scans, residual oxygen-containing species in the APT chamber, and/or undercounting of carbon in the atomprobe as a result of multiple ion events (Thuvander et al., 2011). As is discussed more in the next section, the APT measurements on these depositions collected relatively few ions and they each exhibited a two-phase structure. The Au+O2 deposition was measured as >95 at% gold
by EDX. All APT measurements of this deposition indicated not only gold but also displayed a large amount of silicon in the mass spectra, which precluded a reliable determination of the composition of just the Au layer. The reasons for this result are discussed more later. Composition of the Co and Fe layers asmeasured by the
sufficient contrast in the SEM between the deposition and the surface region of interest (ROI) for ease of targeting the endpoint and uniform milling. Figure 1 shows images of a sharpened tip for each deposition. In each case both of these criteria appear to be met. The additional criteria of adhesion of the layer to the specimen surface, potential peak inter- ferences in the mass spectrum, evaporation behavior and evaporation field of the deposition, and range of Ga+
two techniques agreed quite nicely. The Co deposition was shown to be 80 at% Co and roughly 10 at% each of C and O, which is in the range of values seen by others for this precursor (Utke et al., 2005; Fernández-Pacheco et al., 2009; Córdoba et al., 2010; Mulders et al., 2011; Pfeiffer et al., 2015). The Fe deposition was ~60 at% Fe and 40 at% O. These values are quite a bit different than previous reports using the same precursor, where ~15 at%C, 15 at%O, and 70 at%Fe was seen for deposition on a SiO2 substrate (Lavrijsen et al., 2011) and~2 at% C, 2 at% O, and 96 at% Fe was seen for deposition on gold (Pfeiffer et al., 2015). This variability suggests that deposition conditions, particularly the substratematerial, may exert a large influence over the final composition. Two key criteria for this application of EBID are having
APT Composition (at%)
Au23, C42, O35 x
Co82, C10, O8 Fe60, C0, O40
Pd20, C35, O44, F0 Pt31, C39, O30
APT Uncertainty (at%)
x x
Co3, C1, O3 Fe1, C0, O1
Pd7, C1, O6, F1 x
ion penetration into the deposition are considered for the individual depositions in the following discussion.
APT of Au, Pd, and Pt
The Au, Pd, and Pt depositions had similar APT results and, therefore, their results have been aggregated for discussion. These were analyzed using laser energies of 15 and 30pJ over a voltage range of 0.7–3.2 kV. The analyses acquired 40–430 k ions with an average of about 230 k ions. This corresponds roughly to a removed depth of 5–50nm.Eachanalysisincluded an abrupt transition from the deposition material to the silicon post as evidenced by a change in the mass spectrum to one consisting of only silicon. In each case, the transition occurred at applied biases slightly>3 kV. Shortly after this transition, the APT data collection was manually halted. From pre- and post- APT imaging in the SEM, it was determined that these transi- tions were indicative of microfracturing of the specimen. That is, the remainder of the deposited cap and some portion of the underlying Si post were lost during such an event. This is illu- strated by overlay of the before and after SEM images for the Au deposition in Figure 2. Consistent with TEM observations from EBID Pt (Rotkina et al., 2005) and with APT observations from ion beam deposited Pt (Gerstl et al., 2006), the Pt deposition reconstructions indicated 0.6–7-nm-sized Pt-rich regionswithin a C-rich matrix. This composite structure is the likely source for premature failure of these capping layers, with at least two plausible explanations. (1) It could be failure at the interface between the deposition and the Si post. No pretreatment of the Si was performed, therefore a thin oxide layer is expected on the surface. The Pt–C deposition on the oxidemay lead to poor adhesion as has been suggested by the work of others (Rykaczewski et al., 2011). (2) The failures could also occur within the depositions. The metal particles and carbon matrix are expected to have very different eva- poration fields (Tsong, 1978).Once a sufficient bias is reached, localized high fields at the metal particle—carbon matrix interfaces may result in decohesion and premature failure.
APT of Au+O2 APT analyses on this deposition were attempted using a range of conditions: laser energies from 30–240 pJ, corresponding to biases of 5.2–1.1 kV. In each case the mass
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