Atom Probe Sample Preparation
Nanotechnik) as an alternative bonding material. SemGlu is a high-vacuum-compatible polymer adhesive that hardens under intense electron beam irradiation and is also a good electrical conductor [ 19 ].
To illustrate this new methodology, we present in this article an example of the preparation method used for a specimen of Bristol iron meteorite and two steel specimens. We recently published a detailed (S)TEM and APT study of the kamacite (ferrite)-taenite(austenite) mineral interface in the Bristol iron meteorite, three analytical steel standards, and the North Chile meteorite using the SemGlu method [ 13 , 14 ].
Materials and Methods Alloy specimens . To demonstrate this new procedure, we used a specimen of Bristol iron meteorite (ME 2248; Figure 1a ) from the Field Museum of Natural History in Chicago and two well-characterized steel specimens from Analytical Reference Materials International (ARMI): CRM 182C and IARM 341A. The CRM 182C standard contains 2.80±0.03 wt% Ni and 0.171±0.004 wt% Co; the IARM 341A standard contains 7.16±0.03 wt% Ni and 4.44±0.02 wt% Co. For detailed compositional information, please refer to [ 14 ]. Initial FIB lift-outs were extracted from the kamacite (ferrite) region of the meteorite sample ( Figure 1b ), and the steel specimens were produced using the site-specific lift-out procedure [ 16 ] as routinely done for conventional TEM lamella preparations [ 19 ]. A Zeiss 1540 XB FIB-SEM at the Center for Nanoscale Materials at Argonne National Laboratory (ANL) was used for this step. The composition of the standard-steel specimens were acquired from the ARMI database and were also measured with wavelength-dispersive X-ray spectrometry (WDS) using an INCAWave spectrometer on a TESCAN LYRA3 FIB-SEM at the University of Chicago. An operating voltage of 20 kV and a beam current of ~9 nA were used. Pure Fe, Ni, and Co metals and a synthetic calcium phosphate were used as standards, and counting times for peak and backgrounds measurements were 30 seconds for Fe and Ni, 40 seconds for Co, and 60 seconds for P. Half-grid preps . Independent of the above, copper TEM lift-out half grids with 5 posts (Omniprobe Lift-Out Cu 5 posts) were electropo- lished using 10% perchloric acid for 20–30 sec, until a 20–30 µm apex
26
radius was achieved [ 9 ]. T e electropolished half grids were attached to half grid holders ( Figure 2a ) from Hummingbird Scientifi c Instruments. T ese grid holder tips were fabricated to facilitate loading into a FIB-SEM stage, a TEM holder, and the atom probe tomograph without the need for removal of the grids from the tip. T is lowers the risk of specimen damage considerably as has been reported in other studies [ 12 , 17 , 18 ]. Adhesive bonding . Once the lamella was liſt ed out of the bulk meteorite, using an in situ micromanipulator, it was attached to the electropolished Cu grid using SemGlu ( Figure 2 ). SemGlu is a high-vacuum-compatible adhesive that has a conductivity similar to that of carbon tapes used for grounding of routine SEM samples [ 20 – 22 ]. T e specifi c advantage of SemGlu in this situation is that its hardening/polymerization can be greatly accelerated under intense electron beam irradi- ation (1–1.5 nA, 20 keV). Electron imaging in the FIB-SEM under conditions of low beam current (<100 pA, 5 keV) does
Figure 2 : SEM images showing the sample preparation steps for making a nanotip for APT. (a) SEM image of the fi ve presharpened copper microposts on a TEM half grid attached to a Hummingbird Scientifi c tomographic holder. (b) SEM image showing an extracted lamella attached to a copper post when the copper half grid is placed vertically inside the FIB-SEM and after the SemGlu was cured by scanning with a 5 keV electron beam. (c) SEM image showing lamella attachment when the copper half grid is placed horizontally inside the FIB-SEM. (d) Attached lamella after curing the SemGlu using 5 and 20 keV electron beams. (e) Backscattered electron SEM image showing fi nal sample shape after high-keV and low-keV annular milling. (f) Bright-fi eld TEM image of the nanotip showing the fi nal tip and the base of the lamella with SemGlu.
www.microscopy-today.com • 2018 March
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