edfas.org 15 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 26 NO. 1 sample, typically of diameter less than 100 nm at the apex. To encourage field ion evaporation, an additional external short high-voltage or laser pulse is used to emit the atoms already under the applied field.[6-8] The externally triggered pulse is used to selectively evaporate surface material from the tip on an atom-by-atom basis. By removing single layers of a sample, ideally one atom at a time, and collecting the evaporated ions on a positionsensitive time-of-flight (TOF) detector (Fig. 1), a mass spectrum of each pulsed event can be obtained, and the x,y coordinates where each ion was intercepted by the detector are recorded. Current APT instruments have both straight flight path and reflectron configurations, with the latter employing electrostatic lenses to increase the mass resolution. The averaged TOF mass spectrum and the ion coordinate information are used to create a 3D virtual “model,” with each voxel representing a sub-nm spatially resolved element (or isotope) from the original specimen. APT is based on a thermal process that follows an Arrhenius rate equation (Eq. 1). The rate of evaporation, k, is increased by either decreasing the potential energy barrier, Ea, of atoms on the surface by increasing the applied field using a voltage pulse, or by increasing the temperature, T, of the material using a thermal laser pulse. Eq. 1 Cryogenic temperatures are used to suppress element diffusion on the surface of the specimen needle and confine the evaporation event to coordinate with the trigger pulse, thus increasing the signal-to-noise ratio. To that end, APT samples are mounted on a specimen stage that is cryogenically cooled to temperatures as low as ~20 K using liquid He. ATOM PROBE SAMPLE PREPARATION To obtain the high field required to ideally liberate an individual atom from the specimen surface, needleshaped samples are employed. The required fields of 10 to 50 V/nm can be obtained by applying a 5 kV standing voltage, for example, to a 100 nm diameter (or less) needle-shaped specimen. To prepare APT specimens with such geometry, a focused ion beam (FIB) lift-out method is often used.[10] Figure 2 shows an example of APT tip preparation from a bulk substrate using this approach. First, the area of interest is identified, and a wedge is cut in the bulk material (Fig. 2a). The wedge is lifted out and a small section is welded to the tip of a sacrificial post (Fig. 2b) that is typically made from W, Si, or a TEM grid bar.[11] The sample welded to the post is then sculpted to the final needle-shaped geometry using the Ga+ ion beam and an annular milling scheme (Fig. 2c-e). Due to the numerous milling and imaging steps, a protective metal cap (often Ni, Cr, or FIB-deposited Pt) can be deposited first, over the sample surface to prevent Ga+ ion implantation in the top layers. FIB-scanning electron microscopy (SEM) methods also enable precise identification and extraction of site-specific Fig. 2 A summary of APT tip preparation from a bulk sample showing SEM images of the (a) FIB-prepared sample bar, (b) welded section transferred to an external post, and (c-e) the gradual sharpening of the sample tip by annular milling. Reproduced from Ref 11 under the terms of the Creative Commons Attribution License. (a) (c) (b) (d) (e)
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