Extensive investigations into the mechanism of laser pulsing in atom probe tomography have shown that the dominant pulsing mechanism is thermal [1]. This was recently confirmed by independent study [2]. Optimization of thermal pulsing is therefore guided by the heat flow in the specimen. Heating times which are less than or equal to the electron thermalization time in materials (~10 ps) can be achieved with lasers. In order to achieve the optimal performance with this mechanism, it is imperative that the smallest possible volume of material be heated so that rapid cooling (~100 ps) may occur. Imago has used this design criterion to develop its second generation laser pulsing system: the LEAP 4000X.Key new features of this system are: a beam conditioning unit, targeting assembly and in-vacuum optics to focus the laser beam to a smaller, diffraction-limited spot, a more precise automated beam alignment and focusing system to maintain the beam on the specimen apex during field evaporation, and a large dynamic range (>10 6 ) of pulse energies, Fig 1. The laser light is a third harmonic at 355 nm wavelength with a pulse duration of 10 ps and adjustable pulse repetition rates up to 1 MHz. The diffraction-limited spot at the specimen has a 4σ diameter less than 5 micron. This geometry makes it possible to achieve very high mass resolving power (FWHM>1900, FW.1M>950, FW.01M>525 at m/n=27 and >100 nm field of view) on suitable specimens, Fig. 2.A diffraction-limited spot and shorter wavelength mean smaller irradiated tip area. Shorter wavelength also leads to smaller heated depth because the anomalous skin depth decreases with wavelength in most materials [3,4]. The result is smaller heated volumes, faster cooling, and improved mass resolution. With the lower overall thermal load, 1 MHz repetition rates are achieved with no rise in base temperature. This makes it possible to obtain 100 million atom datasets in 1 hr.The benefits to the user are manifold. If there are significant variations in thermal absorption properties along the heated shank (layered structures, second phases, Pt-C bond from FIB), thermallag peaks that can appear in the mass spectrum are eliminated. A small heated volume minimizes migration of contamination along a shank (e.g., water, platinum from FIB joint) that produces contamination peaks and noise from these materials. By minimizing the overall thermal input to the specimen, a broad spectrum of materials including stainless steel, ceramic coatings, and multilayer structures and lift-out specimens may be analyzed with good performance. With the low noise, 1 appm detection of B and As has been demonstrated. The specimen yield has also been found to improve such that greater than 70% yield on lift-out specimens of single bit devices has been achieved. The combination of small focused spot and shorter wavelength produces the best possible data for all material types.
It is common that atomic planes are visible in atom probe tomographs of crystalline materials when the atomic plane normals are close to the direction of analysis, Fig. 1a, and the visibility of these atomic planes is the principal evidence for high spatial resolution in APT. However, the spatial resolution may actually be better than the value inferred from the spacing of the visible planes. Fourier analysis of APT images has been used to show that multiple atomic planes may be present though not observed in an atom map [1-3]. Recent work with spatial distribution maps has shown that the real space lattice may be resolved with many higher-index atomic planes in APT images [4]. Furthermore, the spatial information in the analysis direction derives from a fundamentally different mechanism than that in the transverse direction and it is desirable to quantify spatial resolution separately in each direction. A common method for determining spatial resolution in opticallyformed images uses Fourier analysis of the (two-dimensional) image to assess and quantify the spatial frequencies that are present in an image. Though Fourier techniques have been used similarly in (three-dimensional) APT, they are limited by their low signal and by the high demands for computational resources for three-dimensional transforms [2,3]. Vurpillot et al. defined spatial resolution in APT as the reciprocal of the width of the Fourier-space damping function that is convoluted with the reciprocal space spectrum [3]. However, this parameter is a measure of the uncertainty of atom position determinations and does not directly address the physical spatial scales resolved.
Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.
The recent development of laser-pulsed Local Electrode Atom Probe (LEAP ®) has provided an avenue to move beyond the analysis of metals and expand into many new areas of materials research. The successful analysis of semiconductor devices, high-k dielectric materials, ceramics, and data storage materials have all been recently reported [1]. In this paper we report on advances in specimen preparation techniques that have enabled the analysis of organic and biological materials. We will limit our discussion to the analysis of self-assembled monolayers (SAMs) and the analysis of buckminsterfullerene (C60) embedded within a compatible polymer matrix of poly(3dodecylthiophene) (P3DDT). These simple examples serve to support general techniques that will enable more sophisticated specimen preparation of nanoparticles and biomaterials/molecules of interest.
Atom Probe Tomography (APT) is an emerging technology used to analyze the three-dimensional structure of a variety of substances at resolutions in the sub-nanometer range. A strong electric field (~20 V/nm) coupled with a sharp needle-shaped specimen (~100 nm diameter end-form) and a twodimensional detector create a point-projection microscope whereby ions are field-evaporated from the surface and diverge to the detector creating a highly-magnified projection of the specimen surface [1]. The specimen preparation challenge is to form prospective materials into a sharp-needle geometry that also allows them to maintain structural integrity while exposed to an extreme electric field. The development of Focused-Ion-Beam-based TEM lift-out techniques and subsequent transfer of these techniques to APT has made it possible to ion-mill microscopic regions from a variety of solid bulk materials into a geometry appropriate for APT [2]. Unfortunately, not all materials with the correct shape survive application of the required electric field without premature failure, and not all needle geometries provide the same results [3]. Nevertheless, evaluation of the potential to analyze new materials with APT must start with the manufacture of specimen tips followed by assessment of analysis yield and data quality. Variation of tip shape, tip composition, and analysis conditions ultimately affect the ability of these specimens to survive analysis and yield interpretable data.The desire to apply APT to biological systems has had a long history [4]. The advent of commercial Pulsed-Laser APT systems has enabled analysis of electrically insulating materials such as silicon dioxide and sapphire [5]. A broad range of biological entities such as proteins, viruses and cells have structural and chemical compositions that are potentially amenable to APT, but methods to prepare needle-shaped specimens suitable for analysis by APT have not yet been reported. Here, we have evaluated the feasibility of analyzing biological specimens preserved under near-native conditions using these principles. We report the successful biopsy and manufacture of APT specimens from lyophilized mammalian cells, allowing for the possibility of high-resolution three-dimensional mapping of cells in a near-native state. Because cell interface adhesion was a concern for specimen survivability, HeLa cells grown on silicon substrates were manufactured in top-down [2], backside [6], and cross-section [7] orientations for purposes of comparative analysis (Fig. 1). Mass spectra recorded from a typical specimen are presented in Fig. 2, demonstrating detection of ions including hydrogen, carbon, nitrogen and oxygen, and several peaks with higher masses. Our results show that biological specimens prepared in cross-section orientation allow for controlled field-evaporation, allowing us to demonstrate the first 3D chemical map of a mammalian cell at nanometer resolution obtained using APT.
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