322 David R. Diercks et al.
surface or near-surface site-specific regions for FIB preparation include discontinuous thin films and interfaces, device stacks, grain boundaries, phase boundaries, and defects. This can also be applied to nanoparticles or nanowires dispersed on a surface. Although there are advantages of in situ EBID for
depositing the protective layer, the most commonly used methyl cyclopentadienyl platinum trimethyl (MeCpPtMe3) source may not be optimal in many instances. EBID and ion beam-induced deposition using this source results in Pt nanoparticles within a carbon and oxygen matrix (Rotkina et al., 2005; Botman et al., 2006; Gerstl et al., 2006). Such a structure does not always achieve great adhesion, which is especially problematic for APT analysis where mechanically weak interfaces are prone to delaminate or fracture under the high applied
fields.An additional challenge forAPT is that this deposited material field evaporates unevenly, likely due to it being a nanocomposite with different evaporation field char- acteristics for the two phases. Furthermore, for somematerials
of interest this deposition has an incompatible evaporation field, mills very differently than the underlying material, and/ or produces interfering peaks in the mass spectrum. If there are multiple possible capping materials readily
available, then researchers can better optimize their choice of protective layer based on the previously mentioned criteria, such as evaporation field matching, mass interference avoidance, and adhesion improvement. This will in turn provide greater flexibility in trying to analyze near-surface features by APT. Therefore, several commercially available EBID precursors were evaluated for use in the FIB-based fabrication and subsequent APT analysis of silicon specimens. Silicon was chosen both because of its ease of APT analysis and its moderate evaporation field.
MATERIALS ANDMETHODS
Depositions of MeCpPtMe3, palladium hexa- fluoroacetylacetonate, dimethyl-gold-acetylacetonate [Me2Au (acac)], Me2Au(acac) plus flowing O2, dicobalt octacarbonyl, and diiron nonacarbonyl were made on top of reactive-ion- etched tapered silicon posts each having a 2 µmdiameter flat top (Thompson et al., 2005). For simplicity, these depositions will subsequently be called Pt, Pd, Au, Au+O2,Co, andFe, respectively. All depositions were made in either an FEIHelios 650 or FEI Helios 600 (FEI Co., Eindhoven, Netherlands) instrument using a focused electron beam operating at 5 kV and 6 nA. Some details regarding deposition times and thick- nesses are shown in Table 1. It is noted that growth rates of the depositions on the
tops of these posts is significantly slower than what has been observed for flat surfaces. This is probably due to different surface diffusion and retention behavior of the precursor molecules for the two cases resulting from the height differences and precursor shading effects at the Si posts. The more typical scenario for lift-out APT specimen pre- paration is a flat surface, and, therefore, the deposition yield obtained here is likely not indicative of the times required for achieving a given thickness for that geometry. For this
Table 1. Some Deposition Times and Thicknesses.
Depositions Deposition Time (min) Deposition Thickness (µm) Au
Au+O2 Co Fe Pd Pt
60 80 40 22 80 8
0.5 0.2 3.5
0.24 3.1 1.1
investigation, the deposition directly onto preformed posts was done for ease and uniformity of specimens. It is not anticipated that this geometry significantly altered the results other than the deposition rate. Previouswork on EBIDgrowth of freestanding nanowires has indicated a similar composi- tion, morphology, and structure as EBID films grown on flat substrates from the same precursor (Frabboni et al., 2006). After making the depositions and before subsequent
processing, energy dispersive X-ray spectroscopy (EDX) was performed on each of the depositions using either an EDAX Genesis or EDAX Octane Super detector (EDAX Inc., Mahwah, NJ, USA). These analyses were performed for determination of composition. In preparation for APT analysis, these posts were shar-
pened by annular milling patterns with a 30 kV Ga ion beam to dimensions appropriate for APT specimens (radius of curvature≤70 nm) (Larson et al., 1998).A2kV Ga ion beam was then used to remove approximately an additional 50nm of material to eliminate the region damaged by the 30kV beam, as is now common practice in APT specimen pre- paration (Thompson et al., 2007). For all specimens, this left at least 100nm of deposited material on top of the silicon post, except for the Au+O2 where only 70nm was left due to the smaller initial thickness of the deposition. APT data collection was performed using a Cameca
LEAP4000XSi system(Cameca,Madison,WI, USA) at a base temperature of 49.7K, a detection rate of one event per 200 pulses (0.5%), and a laser pulse frequency of 250 kHz. Over a series of analyses, various laser energies, resulting in a range of applied biases, were used to explore the evaporation behavior of each deposition. Attempts were also made to perform analyses which transitioned from the deposition into the underlying silicon post. The particulars of these are discussed in the results subsections for each deposition. Analysis of the APT data was done using Cameca’s IVAS 3.6.6 software. Reconstructions were generated from SEMimages of the tips using the tip profile method.
RESULTS AND DISCUSSION
Compositional Measurement The compositions of the EBID layers were measured both by EDX and APT analyses. These are compared in Table 2. The APT uncertainties shown in the table are based on standard deviations of values from multiple analyses. Some
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