Joseph H. Bunton

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.

Advances in Pulsed-Laser Atom Probe: Instrument and Specimen Design for Optimum Performance

The performance of the pulsed-laser atom probe can be limited by both instrument and specimen factors. The experiments described in this article were designed to identify these factors so as to provide direction for further instrument and specimen development. Good agreement between voltage-pulsed and laser-pulsed data is found when the effective pulse fraction is less than 0.2 for pulsed-laser mode. Under the conditions reported in this article, the thermal tails of the peaks in the mass spectra did not show any significant change when produced with either a 10-ps or a 120-fs pulsed-laser source. Mass resolving power generally improves as the laser spot size and laser wavelength are decreased and as the specimen tip radius, specimen taper angle, and thermal diffusivity of the specimen material are increased. However, it is shown that two of the materials used in this study, aluminum and stainless steel, depend on these factors differently. A one-dimensional heat flow model is explored to explain these differences. The model correctly predicts the behavior of the aluminum samples, but breaks down for the stainless steel samples when the tip radius is large. A more accurate three-dimensional model is needed to overcome these discrepancies.

Characterization of ultralow-energy implants and towards the analysis of three-dimensional dopant distributions using three-dimensional atom-probe tomography

The addition of a local electrode geometry has transformed the conventional atom probe into a high-speed, high sensitivity tool capable of mapping three-dimensional (3D) dopant atom distributions in nanoscale volumes of Si. Fields of view exceeding 100nm in diameter and collection rates exceeding 18×106at.∕h are possible with the local electrode geometry. The 3D evolution of dopants, specifically dopant clustering, grain-boundary segregation, shallow-doped B layers, Ni–Si layers, and preamorphization regions, was analyzed. A 200eV B11 implant in Ge-amorphized Si was mapped. The native surface oxide, 8-nm-deep B-doped layer, and Ge distribution were simultaneously mapped in 3D space. A subsequent Ni silicide process was analyzed to show Ni penetration through the doped layer. In a heavily doped poly-Si sample, a cluster of dimensions 2×7×8nm3 and containing 264 B atoms was identified at the intersection of three grains. This shows that annealing highly overdoped thin poly-Si layers does not facilitate unif...

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.

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.

Atom Probe Tomography Analysis of Thick Film SiO 2 and Oxide Interfaces: Conditions Leading to Improved Analysis Yield

Atom Probe Tomography (APT) is a time-of-flight mass spectrometry imaging technique that establishes extreme surface electric fields (~20 V/nm) on a specimen to initiate field-evaporation of surface material [1]. Consequently, it is somewhat surprising that a number of recent results have established that large band-gap dielectric materials (electrical insulators) can be analyzed by laserpulsed APT, especially when the bulk band-gap of the material readily exceeds the photon energy of the laser-pulse used to heat the sample. These dielectric materials include but are not limited to Al2O3, SiO2, ZnO, MgO, and olivine [2-6].

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.