Robert M. Ulfig

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.

Local Electrode Atom Probe Tomography

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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.

Three-dimensional chemical imaging of embedded nanoparticles using atom probe tomography

Analysis of nanoparticles is often challenging especially when they are embedded in a matrix. Hence, we have used laser-assisted atom probe tomography (APT) to analyze the Au nanoclusters synthesized in situ using ion-beam implantation in a single crystal MgO matrix. APT analysis along with scanning transmission electron microscopy and energy dispersive spectroscopy (STEM-EDX) indicated that the nanoparticles have an average size ~8-12 nm. While it is difficult to analyze the composition of individual nanoparticles using STEM, APT analysis can give three-dimensional compositions of the same. It was shown that the maximum Au concentration in the nanoparticles increases with increasing particle size, with a maximum Au concentration of up to 50%.

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.

Intermixing and phase separation at the atomic scale in Co-rich (Co,Fe) and Cu multilayered nanostructures

Despite the fact that Co-rich (Co,Fe) alloys and Cu are immiscible materials in bulk form, evidence of thermally induced mixing at the atomic scale has been observed in thin-film multilayers of (Co,Fe) and Cu. However, long term anneals at lower temperatures produced a breakup of the multilayers into a two-phase mixture of (Co,Fe) and Cu particles. The observations were made with the use of the three-dimensional atom probe technique, with supporting evidence from differential scanning calorimetry and x-ray diffraction. Besides their scientific importance, these results are of interest where these (Co,Fe) and Cu thin films are used to produce the giant magnetoresistive effect.

Promoting Standards in Quantitative Atom Probe Tomography Analysis

Atom probe tomography (APT) is a maturing three-dimensional compositional technique with nanoscale spatial resolution. The instrument is a combination of a field ion microscope and a mass spectrometer and has been described in great detail previously [1]. Recent advancements in sample preparation techniques, the introduction of a commercially available laser-pulsed system, ease and speed of data collection, and advances in data analysis software have expanded the role of the technique from primarily metal systems to a wide variety of semiconductor and data storage applications [1-3].

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.

Applications of the Local Electrode Atom Probe

Over the past decade there has been a substantial expansion of the applicability of atom probe tomography (APT) to materials of all types. This expansion has been spurred on by major instrumental developments resulting in the achievement of high data collection rates, large fields of view, and renewed utilization of laser pulsing (see Chap. 3). Furthermore, the advent of focused ion beam (FIB) methods for specimen preparation (see Chap. 2) in APT has had an equally profound impact. FIB-based methods have not only made it possible to create a LEAP specimen from nearly any bulk material type, but also provided the capability to create specimens from specified regions of a sample and in nearly any orientation. At the turn of the century, random site specimen creation was the norm and site-specific specimen creation was a time-consuming process involving many handling steps in conjunction with electropolishing and electron microscopy. Today, FIB-based site-specific specimen creation is routine and electropolishing is often only used when it is more practical. While the use of thermal pulsing is necessary for materials that cannot be evaporated successfully with field pulsing, thermal pulsing also has been found to improve yield for a range of metal applications as well. Certain metal specimens, even high-strength steels, that will not run with adequate yield in voltage pulsing have been found to run with much higher success rate in laser pulsing. With these major advances, applications of APT have blossomed.

Selected Analysis Topics

LEAP data analysis is performed using the CAMECA Integrated Visualization and Analysis Software (IVAS) package. IVAS was introduced in 2004 on a Windows XP® software platform based on Java™ and Netbeans and continues to require a Windows operating system environment. CAMECA offers several levels of licensing from free and short term use to permanent full versions. Through the years, IVAS development has closely followed a guiding principle that prioritizes ease of use and availability of unencrypted file formats so that analysis can be performed both with IVAS as well as user-developed software. Discussing all the features and concepts pertaining to IVAS would require a separate book (see the IVAS User Guide). Consequently, this chapter describes only some key IVAS topics and features useful for atom probe tomography (APT) practitioners. The features and topics addressed are in no particular order, but each should have some relevance for experienced and novice analysts alike.