Atom Probe Sample Preparation

Figure 3 : (a) Schematic diagram illustrating the APT nanotip sample preparation steps in a FIB-SEM using SemGlu when the sample lamella is attached horizontally onto a presharpened Cu post of a half grid. The presharpened Cu posts are shaped using Ga-ions prior to deposition of SemGlu. Once the lamella is attached, the contact region of SemGlu and lamella is scanned with a 5 keV electron beam. After 1–2 minutes of scanning, the lamella can be cut off to attach on another post. Finally, the SemGlu is completely cured by scanning around the base of each attached lamella using a 20 keV electron beam. (b) Schematic diagram illustrating the APT nanotip sample preparation steps in a FIB-SEM using SemGlu when the lamella is attached vertically on a presharpened Cu post of a half grid. The top of each post is milled with Ga-ions prior to deposition of SemGlu. Procedures for attachment of lamella and curing of SemGlu are as explained in (a).

not induce polymerization. T is enables routine manipulation of the specimen to the target area on the grid using in situ micromanipulators prior to complete hardening/polymer- ization [ 20 – 22 ]. For APT specimen preparation, the glue was fi rst applied to the electropolished posts of the half grid (or fl at-top Si microtip arrays) with a sharp needle. T is procedure was done under a light microscope manually or by using a micromanipulation setup ( Figure 3 ) external to the FIB-SEM. Care must be taken to only apply a very minimal amount of glue onto the post (for example, as a thin surface fi lm) because excess glue can adversely aff ect later steps in the FIB-SEM and (S)TEM (see below).

Attaching the lamella to the post . T e grid and holder was then placed either horizontally or vertically into the FIB-SEM ( Figures 2 and 3 ). We have found that it is typically easier to orient an interface or grain boundary parallel to the vertical axis of the fl at-top or electropolished grid post if the lamella is attached horizontally to the grid posts. In addition, prior to careful application of SemGlu onto the Cu posts, the Cu posts were cut using the FIB to produce a fl attened top as illustrated in Figure 3 . Once inserted into the FIB-SEM, the instrument’s micromanipulator was used to position the liſt -out lamella close to the electropolished posts ( Figures 2 and 3 a). T is step is done at relatively low magnifi cation, low acceleration voltage (that is, 5 kV), and low electron beam current (<0.1 nA) in electron imaging mode, so as not to polymerize the SemGlu. Horizontal attachment of the lamella ( Figures 2 c–d and 3 a) makes it easier to irradiate the SemGlu region with the electron beam. Curing the adhesive . Once the lamella was in contact with the electropolished posts, the glue was cured in multiple steps by fi rst irradiating only the region where the lamella is attached to the post using a rectangular scanning window at high magnifi cation (50,000×), a short beam dwell time (0.5 µs/ pixel), 5 kV, and beam currents of 1–1.5 nA. T e specifi c

2018 March •

rectangular scanning area and magnifi - cation employed depends on the size of the specimen. We have found that rastering the beam for 1–2 minutes with 1 nA current generally results in a strong bond. T e lamella can then be subsequently cut with the ion beam ( Figures 2 c and 3 ). Other slices from the lamella may be attached to the remaining free posts. In a second curing step, a high-beam-current raster was scanned over the interface between the specimen and the SemGlu for 1–2 minutes at similar magnifi cation and dwell time but at high acceleration voltage (20 kV) and higher beam current (1–1.5 nA) to complete the curing of the SemGlu. Aſt er the second curing step, the sample was rotated 180 degrees to cure the SemGlu-sample interface that is on the reverse side. T is is an important step to ensure homogeneous curing of the glue and

increased stability of the lamella. Aſt er fi nal curing of the SemGlu in the above manner, the bond strength should be similar to regular epoxy-based glues.

Alternatively, if the lamella is attached vertically to the top of the Cu grid posts ( Figures 2 b, 3 b), the above procedure needs minor modifi cations to achieve optimized sample attachment. In this case, prior to applying the SemGlu onto the Cu posts, the posts are milled using the FIB as illustrated in Figure 3b . Here, the base of the liſt -out lamella has a similar confi guration as the top of the milled posts, providing the ideal geometry for the attachment ( Figure 2 b, 3 b). Again, once the lamella touches the top of the post, the electron beam should be selectively rastered over the specimen-glue interface using a rectangular scanning window at high magnifi cation and short dwell times (0.5 µs/pixel). T e fi rst step of curing the glue is done by scanning a 5 keV electron beam at high current (1–1.5nA). At times in this orientation, it may be diffi cult to see the interface of the specimen and the glue in the SEM scanning window. In this situation, the stage and microma- nipulator should be tilted a few degrees prior to attachment of the lamella to the post for a better view and improved curing. Once the specimen is attached to the post, the same procedure of curing the SemGlu may be used as described above. Curing should be done on all sides at the base of the lamella for strong and homogeneous bonding. Final shape of the APT nanotip . T e next step of shaping is accomplished by cutting the attached specimen from the sides to shape it into a square prism (cross sections of 1 µm×1 µm) using a 500 pA Ga-beam current and milling into a pyramid shape (height of 1 µm) with an apex cross section of 300 nm×300 nm using a 50 pA current [ 18 ]. T e fi nal two steps of annular milling were performed with a 5–10 pA/10 kV Ga-beam to bring the sample to its fi nal needle shape, followed by cleaning with 50 pA/5 kV Ga ions ( Figures 2e–f ).


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