Microsc. Microanal. 23, 247–254, 2017 doi:10.1017/S1431927617000253
© MICROSCOPY SOCIETY OF AMERICA 2017
New Atom Probe Tomography Reconstruction Algorithm for Multilayered Samples: Beyond the Hemispherical Constraint
Nicolas Rolland,1* François Vurpillot,1 Sébastien Duguay,1 Baishakhi Mazumder,2 James S. Speck,3 and Didier Blavette1
1Groupe de Physique des Matériaux, Université et INSA de Rouen-UMR CNRS 6634-Normandie Université,
76801 St Etienne du Rouvray, France 2Department of Material Design and Innovation, University at Buffalo, Buffalo, NY 14260, USA 3Materials Department, University of California, Santa Barbara, CA 93106, USA
Abstract: Accuracy of atom probe tomography measurements is strongly degraded by the presence of phases that have different evaporation fields. In particular, when there are perpendicular interfaces to the tip axis in the specimen, layers thicknesses are systematically biased and the resolution is degraded near the interfaces. Based on an analytical model of field evaporated emitter end-form, a new algorithm dedicated to the 3D reconstruction of multilayered samples was developed. Simulations of field evaporation of bilayer were performed to evaluate the effectiveness of the new algorithm. Compared to the standard state-of-the-art reconstruction methods, the present approach provides much more accurate analyzed volume, and the resolution is clearly improved near the interface. The ability of the algorithm to handle experimental data was also demonstrated. It is shown that the standard algorithm applied to the same data can commit an error on the layers thicknesses up to a factor 2. This new method is not constrained by the classical hemispherical specimen shape assumption.
Key words: atom probe tomography, 3D reconstruction, multilayers, field evaporation, simulation
INTRODUCTION Among the various material architectures that can be analyzed with atom probe tomography, multilayered tips are one of the major architectures. Indeed, due to the outstanding depth resolution of the technique, one should theoretically be able to characterize interface chemistry with atomic resolution. Applications cover a wide range of fields such as magnetism, nano-electronics, or photonics. Nevertheless, in practical the effectiveness of the technique is limited by artifacts emerging from the algorithm used to perform three-dimensional (3D) reconstructions. Those artifacts are well understood by mean of numerical simulations and correlative microscopy (Vurpillot et al., 2004, 2013a; Larson et al., 2011; Marquis et al., 2011; Haley et al., 2013; Oberdorfer et al., 2013, 2015). Indeed, it appears that during the evaporation of multilayered samples, the evaporation field difference between the materials induces important variations of the tip surface curvature. This in turn affects the ion projection toward the detector. Since the first reconstruction algorithm proposed about 20 years ago (Bas et al., 1995), the assumption of a hemispherical tip apex during evaporation is at the core of every reconstruction method, hereinafter referred to as standard reconstruction method (Blavette et al., 1982; Geiser et al., 2009; Gault et al., 2011a, 2011b; Suram & Rajan, 2013; Vurpillot et al., 2013b). This constraint prevents the obtainment of accurate 3D reconstructions.
*Corresponding author.
Nicolas.rolland1@
univ-rouen.fr Received June 29, 2016; accepted January 21, 2017
Some promising roads have been explored to release this
hemispherical constraint with the help of the numerical simula- tion of field evaporation (Larson et al., 2012; Beinke et al., 2016). However, at this time those methods are hardly applicable to real experimental data set due to the high computational require- ments of the simulations. We recently developed an analytical model to describe the evaporation of a multilayered sample (Rolland et al., 2015a). The model produces an output almost equivalent to numerical simulations, but is only few seconds to run. In the current work, we propose to use the information provided by this model to build an improved 3D reconstruction model dedicated to multilayered emitters. The physical basis of the analytical model describing the evolution of a multilayered sample when field evaporated will be recalled first. The new reconstruction method will then be presented. Two versions of the reconstruction algorithm will be introduced, corresponding to extensions of the standard cone angle and voltage curve algorithms, respectively. The effectiveness of the method will be demonstrated both on simulated and experimental data sets.
MODEL
The model can be seen as an extension of conventional models included in standard reconstruction algorithms as it relies on exactly the same hypothesis. Indeed, it is assumed that the initial tip shape is a hemisphere cap seated on a truncated cone with a continuous derivative all over the surface. In addition, the common relationship between the surface electric field F and the radius of curvature R holds
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