E Oltman

First Data from a Commercial Local Electrode Atom Probe (LEAP)

The first dedicated local electrode atom probes (LEAP [a trademark of Imago Scientific Instruments Corporation]) have been built and tested as commercial prototypes. Several key performance parameters have been markedly improved relative to conventional three-dimensional atom probe (3DAP) designs. The Imago LEAP can operate at a sustained data collection rate of 1 million atoms/minute. This is some 600 times faster than the next fastest atom probe and large images can be collected in less than 1 h that otherwise would take many days. The field of view of the Imago LEAP is about 40 times larger than conventional 3DAPs. This makes it possible to analyze regions that are about 100 nm diameter by 100 nm deep containing on the order of 50 to 100 million atoms with this instrument. Several example applications that illustrate the advantages of the LEAP for materials analysis are presented.

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.

Atomic-scale phase composition through multivariate statistical analysis of atom probe tomography data.

We demonstrate for the first time that multivariate statistical analysis techniques can be applied to atom probe tomography data to estimate the chemical composition of a sample at the full spatial resolution of the atom probe in three dimensions. Whereas the raw atom probe data provide the specific identity of an atom at a precise location, the multivariate results can be interpreted in terms of the probabilities that an atom representing a particular chemical phase is situated there. When aggregated to the size scale of a single atom (∼0.2 nm), atom probe spectral-image datasets are huge and extremely sparse. In fact, the average spectrum will have somewhat less than one total count per spectrum due to imperfect detection efficiency. These conditions, under which the variance in the data is completely dominated by counting noise, test the limits of multivariate analysis, and an extensive discussion of how to extract the chemical information is presented. Efficient numerical approaches to performing principal component analysis (PCA) on these datasets, which may number hundreds of millions of individual spectra, are put forward, and it is shown that PCA can be computed in a few seconds on a typical laptop computer.

Improving Data Quality in Atom Probe Tomography

Figure 1 schematically illustrates several methods to improve APT specimen yield through decreasing stress by decreasing the evaporation field required during data collection: 1) decrease ion detection rate, 2) increase specimen base temperature, 3) use laser rather than voltage pulsing, and 4) increase laser pulse energy. The current work explores the use of thin coatings to modify the thermal and/or optical properties of 302 stainless steel, SiN, or a Si/SiO2/Si/Ni test structure (Fig. 2) in order to improve yield [7]. The APT data collection parameters were nominally: specimen temperature 30 K, laser pulse energy 30 pJ at 625 kHz, and a detection rate of one ion every 333-1000 pulses.

Hardware and Software Advances in Commercially Available Atom Probe Tomography Systems

Over the past 15 years, the number of peer reviewed publications referencing the use of atom probe tomography (APT) has grown by nearly a factor of five [1]. A number of factors are involved in this very rapid adoption of APT, a microscopy that has been in use since the early 1970s. The performance of the typical atom probe today is orders of magnitude better than the early systems in terms of data collection rate, field of view, mass resolving power, reliability and software. The availability of easy to use laser pulsed systems coupled with the changes in performance and availability of FIB-SEM sample preparation has dramatically opened the array of applications that can be analyzed. These advances, with the improved ease of use and availability of atom probe tomography microscopes, have changed the typical user from an expert in APT to a scientist looking for answers that other microscopies cannot provide.

Using Ma ss Resolving Power as a Performance Metric in the Atom Probe

The measure of the peak width, m, in a mass spectrum is one of the most important metrics used to assess mass spectrometer performance. The peak width is often expressed as the mass resolution by normalizing it to the mass of the peak, i.e. Δm/m. The reciprocal of the mass resolution, the mass resolving power (MRP), may also be used. Instruments with a higher mass resolving power can discern individual mass peaks better than instruments with a poorer mass resolving power [1]. The MRP of a spectrometer also impacts the signal-to-noise ratio for a peak and hence the detection limit for a given ion species [2]. To a first approximation, MRP in atom probe tomography will depend on the acquisition voltage (V), the flight path length (L), and the mass-to-charge state ratio (m/n). The MRP is normally calculated at fixed values of V, L, and m/n. For most time-of-flight mass spectrometry techniques, the (m/n), L, and V are constant throughout data acquisition. However, in an atom probe experiment, both the V and the L vary throughout the data acquisition process.

Performance Advances in LEAP systems

Performance advances in Atom Probe Tomography (APT) in recent years have driven a dramatic expansion in the published literature. This expansion is evidence that easier, faster, and better threedimensional nanoscale compositional information can enable a wide variety of research that was not reasonable to pursue even several years ago. Since the introduction of the commercially available laserpulsed atom probe in 2006, publications reporting APT results have tripled and the variety of applications continues to expand with each year [1].