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Atom Probe Sample Preparation


body. During the formation of the Widmanstätten pattern, Ni diff uses out of kamacite into the taenite phase, and the diff usion of Ni is fast enough during the slow cooling process to produce a homogenous Ni concentration in the center of the kamacite plate. A detailed discussion about the formation of Widmanstätten pattern can be found in [ 29 ].


Discussion


One of the disadvantages of using SemGlu is the possibility of enhanced carbon contamination during (S)TEM work. However, the following precautions have been found to reduce the magnitude of carbon contamination. It is important that the analyst maximizes the distance between the apex of the nanotip and the base of the specimen, where the SemGlu bond is located. We have found that the distance between the nanotip and the attachment area should be greater than 5 µm to decrease the carbon contamination signifi cantly. Also, the SemGlu must be effi ciently cured, using a 20 keV electron beam, all around the base of the specimen where the SemGlu is present. If no (S)TEM analysis is required prior to APT, smaller specimens with lengths of 3–4 µm from base to apex can be used. But this is not recommended for non-conductive or dielectric samples, where laser heating must be used, because residual non-polymerized SemGlu at the base of the specimen can move to the apex of the nanotip during APT through surface diff usion. We also have observed higher peak tails in the APT mass spectra for smaller (lengths of 2–3 µm) dielectric specimens, which can be attributed to the low thermal conductivity of SemGlu. Another obvious, yet important, step is to minimize the volume of SemGlu on the Si posts and grids before attaching the specimen. Excessive SemGlu can adversely aff ect the specimen and sometimes leads to migration of uncured epoxy to the top of the sample by the capillary eff ect. Any minor carbon contami- nation that builds up during (S)TEM work can be removed by Ar + plasma cleaning [ 30 ]. However, for samples that are susceptible to oxidation, plasma cleaning can lead to increased rates of oxidation during subsequent storage. Care must be taken to store samples in high vacuum or in an inert atmosphere aſt er plasma cleaning.


Conclusion


Figure 6 : APT reconstruction of the nanotip (Tip C) prepared from a kamacite region in the Bristol IVA iron meteorite. Only 5% of the detected atoms are shown for clarity.


wt%, P=0.024±0.004 wt%, and Cr=0.009±0.004 wt% (2). For Tip C the measured composition was: Fe=94.3±0.4 wt%, Ni=5.14±0.50 wt%, Co=0.49±0.05 wt%, P=0.028±0.004 wt%, and Cr=0.020±0.004 wt% (2). T is is the expected composition for kamacite [ 28 ], and detailed information about the APT data processing and error estimation is discussed in [ 14 ]. T e composition of the kamacite nanotips were found to be homogeneous compared to the steel-standards used in our study. T is is because kamacite does not transform to any other phase at low temperatures (< 400°C) during millions of years of cooling within the iron meteorite parent


2018 March • www.microscopy-today.com


We used SemGlu as an alternative to beam-induced deposition of Pt, W, and C from gaseous organo-metallic precursors to attach the thinned lamellae onto sharpened Cu and Mo posts in Cu or Mo TEM half grids and onto fl at-top Si posts. All the nanotips in our study were prepared in the FIB-SEM using SemGlu and were successfully studied with (S) TEM and APT. No sample failure occurred with APT before ~4–25 million atoms were detected. Some of the samples were stable even at high extraction voltages (~13 kV). SemGlu can therefore be used as an alternative bonding material for preparing stable nanotips both for TEM and APT studies.


Acknowledgments T e authors thank the editor-in-chief, Dr. Charles Lyman, for very helpful suggestions and corrections that improved the manuscript signifi cantly. T e authors acknowledge Sung-Il Baik for providing electropolished grids and for helpful discus- sions, and Betty Strack for maintaining the SEM Laboratory at the Field Museum. T e local-electrode atom-probe (LEAP) at Northwestern University was purchased and upgraded with funding from NSF-MRI (DMR-0420532) and ONR-DURIP (N00014-0400798, N00014-0610539, N00014-0910781) grants.


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