Atom Probe Tomography: Seeing Millions of Atoms in 3D

In the world of microscopy and microanalysis of solid state nanostructures, a single atom or molecule defines the ultimate length scale. The challenge that we face is both imaging a single atom and being able to identify its chemistry; and this has been the driving force for the development of many techniques of microscopy and microanalysis (Figure 1). Some good examples of this, especially in materials science, include high resolution transmission electron microscopy and scanning tunneling microscopy (and its derivative atomic force microscopy).

Figure 1. A quantitative comparison of chemical sensitivity and spatial resolution among a variety of microanalytical techniques. APT also excels in its ability to capture different modalities of chemical and structural information.

Over the last thirty years, there has been a steady advancement in the development of transmission electron microscopy by fully exploiting the physics of electron optics ranging from reducing the aberrations associated the lenses to the aberrations associated with the electron probe itself. Due to these improvements, we can now achieve sub-angstrom resolution in atomic scale imaging of the internal structure of solids.

Similarly, scanning probe microscopy (or "near field' techniques where the probe is close to the specimen) can provide atomic scale images of the surface of a solid. Atom Probe Tomography (APT) represents a revolutionary characterization tool for materials that can image individual atoms in three dimensions, a major advancement over the two dimensional images of other microscopy techniques^1-8.

In fact APT is also a chemical analysis tool that allows us to detect the chemical identity of each atom. The uniqueness of APT is further enhanced by the fact that one can image hundreds of millions of atoms, and not just a few tens or at most a few hundred or so atoms in STM or TEM (Figure 2)

Figure 2. APT image of Sc clustering (red atoms) in an aluminum (blue atoms) alloy

Principles of Atom Probe Tomography

The APT has its origins in the field-ion microscope (FIM), wherethe surface atoms on a sharp needle like specimen (the radius of the tip of the sample is less than 50 nm) are imaged by field ionized gas atoms projected onto an imaging screen. FIM is a lensless point projection microscope that resolves individual atoms on the surface of the needle like specimen at over a million times.

With the integration of a mass spectrometer into the system, the FIM has evolved into what is now generically termed as atom probe tomography (APT), that produces 3-D compositional images at the atomic scale with very high analytical sensitivity (10 atomic parts per million). It involves the controlled removal of atoms from a specimen's surface by field evaporation or laser pulsing, and then sequentially imaging and analyzing them with a time-of-flight (TOF) mass spectrometer.

The extracted ions are projected onto a position-sensitive detector for recording their location. Time-of-flight measurements on the ions provide their chemical identity as a mass-to-charge ratio of the ion (Figure 3). While FIM was originally developed by Erwin W. Müller over half a century ago and the atom probe microscope itself dates back to ca. 1968, it is only fairly recently that highly sophisticated and reliable instruments have become commercially available. The recent commercialization of the local electrode atom probe (LEAP) further increases the maximum number of atoms in an analysis hundreds of millions of atoms and reduces the analysis time to minutes rather than days^4-6.

Figure 3. FIM image from laser pulsed W tip showing low density of atoms near major crystallographic orientations

Applications

Many excellent overviews of the historical evolution and applications of this technique exist^1-8. When combined with depth resolution of one inter-planar atomic layer for depth profiling, an APT provides the highest spatial resolution of any microanalysis technique. This capability provides a unique opportunity to study experimentally with atomic resolution, chemical clustering and 3-D distributions of atoms. The ability to resolve such structural and chemical detail with atomic scale resolution permits one to track spatial variations in chemistry (Figure 4)

Figure 4. In figure (a) one can observe the lattice planes in copper running vertically. The apparent curvature of these planes is due to instrumental distortions. Each dot is associated with one atom. The same specimen in figure b showing the 3 dimensional distribution of copper (orange), oxygen (red) and CuO (blue) ions associated with an ultra thin layer of oxide on the surface of the copper. Figure (c) shows a compositional profile with sub-nanometer resolution.

The extraordinary impact of the APT as a tool in nanoscience is hence governed by its collective capability to:

• image at the atomic level,
• analyze chemistry at the atomic level and
• gather both these classes of information in three dimensions and with spatial selectivity in the specimen with hundreds of millions of atoms

With these capabilities, APT is rapidly being applied to address a wide array of nanoscale science problems such as dopant mapping in semiconductors and solute clustering in complex alloys. Such fundamental studies are impacting a wide array of engineering technologies including developments in electronic materials such as photovoltaics and data storage. The ability to spatially resolve chemical clustering in complex alloys has important ramifications in the design of next generation of high temperature materials such as aerospace materials.

The Future for APT

The future of APT in the field of nanoscience and technology is both exciting as well as challenging. Two such applications are in the field of in-situ studies to study solid-gas reactions and the other is the study of biological materials

Researchers have for instance developed a gas reaction chamber for an atom probe to study catalytic reactions. As noted by Cerezo et.al., the needle shape of the specimen is of advantage in the study of reactions on the surface of catalyst materials. In this case, the apex of the needle-shaped specimen is a good analog for half a catalyst/nanoparticle, where multiple crystal planes are exposed in very close proximity to one another. This makes it possible to study effects due to the interplay between regions of different crystallography, unlike in the majority of surface science experiments that use flat single-crystal specimens. This is sometimes referred to as the "materials gap" in surface science. Interfacing a gas reaction chamber with the APT can help address issues such as which atomic sites are the most reactive, and how stable such catalyst surfaces are in the face of changing compositions/ flow rates of reactive gas molecules

The study of biological samples via APT offers the exciting potential for a new approach to structural biology. As noted by Greene et.al.¹⁰, to effectively analyze biological and organic specimens using APT, several experimental parameters-such as the choice of substrate, electric field strength, temperature, and laser pulse characteristics-must be optimized to yield mass fragments that can be repeatably evaporated and reliably identified from the typical organic background.

Finally, it should be recognized that an atom probe is in a sense the "Hubble telescope" for materials, with the unprecedented capacity to detect massive numbers of atoms at an extremely fast rate. This experimental capability in turn creates a computational challenge of dealing with the data deluge afforded by massive amounts of information that needs to be processed to quantify the image and chemistry¹¹.

References

1. A Cerezo, P.H. Clifton, S. Lozano-Perez, P. Panayi , G.Sha and GDW Smith; Overview- Recent Progress in three dimensional atom probe instruments and applications; Micro. Microanalyis 13 408-417 (2007)
2. MK Miller, A Cerezo, MG Hetherington, & GDW Smith, Atom Probe Field-Ion Microscopy, pp. 447-465. Oxford, UK: Oxford University Press. (1996)
3. M.K. Miller and E.A. Kenik, Atom Probe Tomography: A Technique for Nanoscale Characterization, Microsc. Microanal., 10:336-341, (2004)
4. T. F. Kelly, D. J. Larson, K. Thompson, J. D. Olson R. L. Alvis and J. H. Bunton, B. P. Gorman, Atom probe tomography of electronic materials; Annual Review of Materials Research, 37:681-727, (2007)
5. T. F. Kelly and M. K. Miller. Invited review article: atom probe tomography. Review of Scientific Instruments, 78(03):1101 ( 2007)
6. David N. Seidman Three-Dimensional Atom-Probe Tomography: Advances and Applications Annu. Rev. Mater. Res. 2007. 37:127-58
7. M. K. Miller. Atom-probe tomography. Kluwer Academic/Plenum Publishers, New York, (2000).
8. M. K. Miller, A. Cerezo, M. G. Heatherington, and G. D. W. Smith, Atom-probe field-ion microscopy, Clarendon Press, Oxford, (1996)
9. P.A.J. Bagot, , T Visart de Bocarme, A. Cerezo and G.D.W. Smith GDW 3D atom probe study of gas adsorption and reaction on alloy catalyst surfaces: I- Instrumentation Surface Science 600 3028-3035 (2006)
10. Mark E. Greene, Ty J. Prosa, John A. Panitz, David J. Larson, and Thomas F. Kelly Development of Atom Probe Tomography for Biological Materials; Microsc Microanal 15(Suppl 2), (2009) 582-583
11. S. Seal, M. Moody, A. Ceguerra, S. Ringer, K. Rajan, and S. Aluru. Tracking nanostructural evolution in alloys: Large-scale analysis of atom probe tomography data on Blue Gene/L. In Proc. 37th International Conference on Parallel Processing (ICPP), 338-345, (2008)