Center for Atom Probe Tomography (NUCAPT)
Vist the NUCAPT website here.
Atom-probe tomography (APT) is a microanalytical instrument producing an atom-by-atom three-dimensional reconstruction of a sample, with sub-nanometer resolution with a typical analyzed volume of about 150 x 150 x 500 nm3. APT is particularly suitable to investigate nano-structured materials. Typical micro- and nanostructural features studied are: composition and morphology of second-phase precipitates or small clusters of solute atoms, compositional variation in modulated structures, multi-layer thin-film structures, dopant profiles of semiconductor structures (transistors), and analysis of the chemistry and topology of internal interfaces. Specimen preparation of almost any material is now possible employing a dual-beam focused-ion beam (FIB) microscope, which allows targeted sample preparation of a specific feature, such as a grain boundary or an individual transistor in a semiconductor device.
1. LEAP 4000XSI manufactured by Imago Scientific Instruments, Madison, Wisconsin: This instrument, a local electrode atom-probe (LEAP) tomograph, has an ultrafast detector capable of collecting up to 360 million ions per hour. Ions are evaporated from a sample’s surface either by voltage or ultraviolet (UV) laser pulses, which allows for the analysis of a broad spectrum of materials: metals, semiconductors, ceramics, biominerals, organic and biological samples, albeit with different degrees of success. A computer reconstructs a three-dimensional image of a sample with both the chemical identities and positions of individual atoms, with a depth resolution equal to the interplanar spacing, which can be as small as 0.1 nm: the lateral resolution in an atomic plane is between 0.3 to 0.5 nm. The microelectrode in the LEAP tomograph allows the analysis of microtips, prepared by FIB (ion-milling and/or the lift-out technique to target specific features), or wire microtips prepared by conventional electropolishing. Additionally, digital field-ion microscopy can be performed with this instrument..
2. A specimen preparation laboratory for preparing needle-shaped specimens for atom-probe tomography. Our lab features a high-speed precision saw to cut specimen blanks, an electropolishing station with a high-resolution stereo-microscope, and a commercial Simplex Electropointer automated electropolisher.
3. An Ion-beam sputter system (IBS/e) manufactured by South Bay Technologies. This system is utilized for depositing high-quality thin films for: (1) generating multi-layer structures; and (2) to assist with LEAP-tip preparation by FIB milling where the thin-film deposit marks and protects a sample’s surface when milling with gallium ions. The ion-beam sputter system does not use magnetron-based sputter guns and therefore is suitable for the deposition of magnetic materials, such as iron, nickel, and cobalt.
4. Arc Melter: Arc-melting is a fast and clean way of producing alloys of electrically conductive materials. The raw materials are placed on a water-cooled hearth in a vacuum chamber. After evacuation, the chamber is re-filled with Argon to be employed as an inert working gas. An electric arc is produced with a pointed electrode which heats the raw materials above their melting point, fusing them into an alloy droplet. The facility operates a MAM-1 arc melter manufactured by Edmund Bühler GmbH, Germany. The arc melter can process about 10-15 grams of material in one melting charge. A specially designed hearth allows for suction casting into a 3 mm diameter mold.
5. NUCAPT computing facility comprising
5.1 An 8 TB data server
5.2 Five individual high-end PC workstations for running IVAS for LEAP-data reconstruction and analysis
5.3 One workstation for Thermocalc, DICTra and MedeA simulations
5.4 Microway AMD Quad-core Opteron cluster with 31 nodes and 62 quad-core processors, with a total of 248 CPUs, 372 GB shared-memory and high-quality fiber- optical DDR InfiniBand Network for large-scale parallel calculations. This cluster is currently optimized for performing VASP DFT calculations and lattice kinetic Monte Carlo simulations.