Ty J. Prosa

Local Electrode Atom Probe Tomography

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Evolution of tip shape during field evaporation of complex multilayer structures.

Atom‐probe tomography analysis of complex multilayer structures is a promising avenue for studying interfacial properties. However, significant artefacts in the three‐dimensional reconstructed data arise due to the field evaporation process. To clarify the origin and impact of these artefacts for a FeCoB/FeCo/MgO/FeCo/IrMn multilayer, tip shapes were observed by transmission electron microscopy and compared to those obtained by finite difference modelling of electric fields and evaporation processes. It was found that the emitter shape is not spherical and its surface morphology evolves during successive evaporation of the different layers. This evolving morphology contributes to the artefacts generally observed in the reconstructed atom‐probe data for multilayer structures because algorithms for three‐dimensional reconstruction are based on the assumption that the shape of the emitter during field evaporation is spherical. Some proposed improvements to data reconstruction are proposed.

Wide-Field-of-View Atom Probe Reconstruction

In atom probe tomography, it is usually desirable to obtain the largest possible field of view (FOV) in the analysis and recent advances in instrumentation [1] have made significant increases in FOV. However, the most commonly used data reconstruction techniques were designed for much smaller FOV instruments and as such, the small-angle approximations employed are less accurate for the current generation of instruments. Prior to the advent of wide FOV instruments, the geometric assumptions described by Blavette [2], and later applied by Bas [3], were widely considered the standard global reconstruction technique [4]. This model incorporates a simple point projection to account for lateral magnification and uses geometric models of the global tip shape to reconstruct depth information. It also assumes that the original shape of acquired volumes is small enough in lateral extent to be considered cylindrical and the radius of the tip is determined atom-by-atom by the specimen voltage. In the early reconstructions [3], the actual shank angle is ignored and it is assumed to be zero in the calculation of the volume increment. In using the voltage as a proxy for the tip radius it will often be the case that the reconstructed geometry is not conical, and indeed may be extremely irregular. This can happen for instance in a multi-layer system where evaporation fields are rapidly changing. In this case the assumption of a fixed evaporation field is clearly erroneous and induces rapidly changing model geometry.

Improvements in planar feature reconstructions in atom probe tomography

Standard atom probe tomography spatial reconstruction techniques have been reasonably successful in reproducing single crystal datasets. However, artefacts persist in the reconstructions that can be attributed to the incorrect assumption of a spherical evaporation surface. Using simulated and experimental field evaporation, we examine the expected shape of the evaporating surface and propose the use of a variable point projection position to mitigate to some degree these reconstruction artefacts. We show initial results from an implementation of a variable projection position, illustrating the effect on simulated and experimental data, while still maintaining a spherical projection surface. Specimen shapes during evaporation of model structures with interfaces between regions of low‐ and high‐evaporation‐field material are presented. Use of two‐and three‐dimensional projection‐point maps in the reconstruction of more complicated datasets is discussed.

Toward atom probe tomography of microelectronic devices

Atom probe tomography and scanning transmission electron microscopy has been used to analyze a commercial microelectronics device prepared by depackaging and focused ion beam milling. Chemical and morphological data are presented from the source, drain and channel regions, and part of the gate oxide region of an Intel® i5-650 p-FET device demonstrating feasibility in using these techniques to investigate commercial chips.

Effect of analysis direction on the measurement of interfacial mixing in thin metal layers with atom probe tomography.

The accuracy and precision of thin-film interfacial mixing as measured with atom probe tomography (APT) are assessed by considering experimental and simulated field-evaporation of a Co/Cu/Co multilayer structure. Reconstructions were performed using constant shank angle and Z-scale reordering algorithms. Reconstruction of simulated data (zero intermixing) results in a 10-90% intermixing width of ~0.2 nm while experiential intermixing (measured from multiple runs) was 0.47 ± 0.19 and 0.49 ± 0.10nm for Co-on-Cu and Cu-on-Co interfaces, respectively. The experimental data were collected in analysis orientations both parallel and anti-parallel to film growth direction and the impact of this on the interfacial mixing measurements is discussed. It is proposed that the resolution of such APT measurements is limited by the combination of specimen shape and reconstruction algorithms rather than by an inherent instrumentation limit.

A System for Simulation of Tip Evolution Under Field Evaporation

An improved method to numerically simulate the endform evolution of atom probe specimens has been developed in order to support reconstruction algorithm research. Similar to work described in [1], the system consists of a three-dimensional Poisson simulation and includes methods for choosing which atoms of the specimen to evaporate and a trajectory integrator to produce data that can be postevaporation processed, for example, by the IVAS system [2]. The current simulation uses a finite difference algorithm which specifies that atoms assigned to the tip are treated as part of the applied boundary conditions. The iteration used to solve for the potential is a standard multi-grid technique [3]. The simulation supports a wide range of tip geometries with included planar, spherical and cylindrical sub-structures. All components can be simulated with independently controllable compositions, evaporation fields and lattice structures.

Local electrode atom probe analysis of silicon nanowires grown with an aluminum catalyst

Local electrode atom probe (LEAP) tomography of Al-catalyzed silicon nanowires synthesized by the vapor–liquid–solid method is presented. The concentration of Al within the Al-catalyzed nanowire was found to be 2 × 10(20) cm(-3), which is higher than the expected solubility limit for Al in Si at the nanowire growth temperature of 550°C. Reconstructions of the Al contained within the nanowire indicate a denuded region adjacent to the Al catalyst/Si nanowire interface, while Al clusters are distributed throughout the rest of the silicon nanowire.

Effects of detector dead-time on quantitative analyses involving boron and multi-hit detection events in atom probe tomography.

In atom probe tomography (APT), some elements tend to field evaporate preferentially in multi-hit detection events. Boron (B) is one such element. It is thought that a large fraction of the B signal may be lost during data acquisition and is not reported in the mass spectrum or in the 3-D APT reconstruction. Understanding the relationship between the field evaporation behavior of B and the limitations for detecting multi-hit events can provide insight into the signal loss mechanism for B and may suggest ways to improve B detection accuracy. The present work reports data for nominally pure B and for B-implanted silicon (Si) (NIST-SRM2137) at dose levels two-orders of magnitude lower than previously studied by Da Costa, et al. in 2012. Boron concentration profiles collected from SRM2137 specimens qualitatively confirmed a signal loss mechanism is at work in laser pulsed atom probe measurements of B in Si. Ion correlation analysis was used to graphically demonstrate that the detector dead-time results in few same isotope, same charge-state (SISCS) ion pairs being properly recorded in the multi-hit data, explaining why B is consistently under-represented in quantitative analyses. Given the important role of detector dead-time as a signal loss mechanism, the results from three different methods of estimating the detector dead-time are presented. The findings of this study apply to all quantitative analyses that involve multi-hit data, but the dead-time will have the greatest effect on the elements that have a significant quantity of ions detected in multi-hit events.

Chemical mapping of mammalian cells by atom probe tomography

In atom probe tomography (APT), a technique that has been used to determine 3D maps of ion compositions of metals and semiconductors at sub-nanometer resolutions, controlled emissions of ions can be induced from needle-shaped specimens in the vicinity of a strong electric field. Detection of these ions in the plane of a position sensitive detector provides two-dimensional compositional information while the sequence of ion arrival at the detector provides information in the third dimension. Here we explore the use of APT technology for imaging biological specimens. We demonstrate that it is possible to obtain 3D spatial distributions of cellular ions and metabolites from unstained, freeze-dried mammalian cells. Multiple peaks were reliably obtained in the mass spectrum from tips with diameters of ~50 nm and heights of ~200 nm, with mass-to-charge ratios (m/z) ranging from 1 to 80. Peaks at m/z 12, 23, 28 and 39, corresponding to carbon, sodium, carbonyl and potassium ions respectively, showed distinct patterns of spatial distribution within the cell. Our studies establish that APT could become a powerful tool for mapping the sub-cellular distribution of atomic species, such as labeled metabolites, at 3D spatial resolutions as high as ~1 nm.