Automated Atom-By-Atom 3D FIM Reconstruction 257


However, as the focus of this paper is to illustrate the 3D reconstruction algorithm, the majority of results presented in this study characterize an undamaged region of this sample. For the second reconstruction demonstration, samples


were electropolished from a M50 bearing steel with a composition of: 3.84 at% C, 4.38 at% Cr, 2.4 at% Mo, 0.003 at % W, 87.48 at% Fe, and Co content <0.0004 at%. Electro- polishing was performed in a two-step process, with 25% perchloric acid in acetic acid used for the first stage, and 2% perchloric acid in butoxyethanol used for the second stage. FIM experiments were performed on a 3DAP-LAR instru-


ment (Oxford Nanoscience, Oxford, UK), where a phosphor screen dedicated to FIM mode is installed, and images are taken digitally with a AVT stingray camera (Allied Vision Techno- logies, Stadtroda, Germany). Experiments were performed at a temperature of 50 K, using voltage pulsing at a repetition rate of 100kHz for the controlled evaporation of the specimen surface. Ultrahigh purity helium was used as the imaging gas for the


tungsten case, whereas neon was used in the steel case. Constant pressure of 3×10−5mbar was maintained in both cases. The DC voltage in both cases was determined according


to the best imaging conditions, and was raised throughout the measurement as atoms were removed and the radius of the specimen increased. The voltage pulser was set to maintain controlled evaporation, with an average value of 25% of the applied DC voltage at each point in time. Images were taken at a rate of 15 fps, in signal-to-noise ratio (SNR) mode of eight images (in this mode the camera takes and averages a set number of images to optimize the signal-to- noise ratio), and continuously recorded the arrangement of atoms on the evolving surface of the samples.


FIM Images


In Figure 1a, a typical FIM image of the tungsten tip is shown. Each bright spot on the image is representative of the


Figure 1. a: Schematic illustration of the field ion microscopy (FIM) set-up. Imaging gas atoms are absorbed on the tip of the sample on the left, and are ionized by the electric field toward the phosphor screen on the right. The illustration does not represent the real dimensional scale between the sample and the screen. On the right, FIM image of a tung- sten tip with the BCC symmetry evident, and several of the poles indexed. The analyzed [2,2,2] pole is marked on the FIM image. b: Close-up of the [2,2,2] pole. 1–4 are time-ordered images describing the evaporation process of a crystal- lographic plane, beginning from the outside and moving inwards. The square in (1) marks the center of the pole where the integrated intensity in (c) is measured. The plane imaged in (1–3) had been fully evaporated in (4), and the layer below is now exposed. (4) Corresponds to a sharp decrease in integrated intensity as can be seen in (c). c: Integrated intensity from the center of the [2,2,2] pole across 500 FIM images. Each intensity interval of the plot represents the evaporation of one crystallographic layer as a function of image number. Points 1–4 correspond to the contributions from points in time 1–4 marked in (c).


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