Papers

ICMCTF2003 Session H1-1: Nanostructuring Film Surfaces and Interfaces

Tuesday, April 29, 2003 8:30 AM in Room San Diego

Tuesday Morning

Start	Invited?	Item
8:30 AM		H1-1-1 Theory of Charged Clusters Linking Nano Science and Technology to Thin Films Nong-Moon Hwang (Seoul National University, South Korea) Based on experimental and theoretical analyses, we suggested a new possibility that the CVD diamond films grow not by the atomic unit but by the charged clusters containing a few hundreds of carbon atoms, which form spontaneously in the gas phase. These hypothetical negatively-charged clusters were experimentally confirmed under a typical hot-filament diamond CVD process. Thin film growth by charged clusters or gas phase colloids of a few nanometers was also confirmed in Si and ZrO₂ CVD and appears to be general in many other CVD processes. Many puzzling phenomena in the CVD process such as selective deposition and nanowire growth could be explained by the deposition behavior of charged clusters. Charged clusters were shown to generate and contribute at least partially to the film deposition by thermal evaporation. Origin of charging at the relatively low temperature was explained by the surface ionization described by Saha-Langmuir equation. The hot surface with a high work function favors positive charging of clusters while that of a low work function favors negative charging.
9:10 AM		H1-1-3 Nanopipes in Epitaxial CrN, ScN, TiN, and TaN Layers D. Gall (Rensselaer Polytechnic Institute); C.-S. Shin, M.A. Wall, S. Kodambaka, K. Ohmori, I. Petrov, J.E. Greene (University of Illinois) Epitaxial layers of NaCl-structure transition-metal nitrides were grown on MgO(001) at 600-1000°C by ultra-high-vacuum magnetically-unbalanced magnetron sputter deposition in pure N₂ and N₂+Ar discharges at 3-20 mTorr. The layers are single crystalline with a cube-on-cube epitaxial relationship with the substrate and exhibit a periodic surface mound structure with square shaped mounds bounded by edges along low energy <100> directions. Surface cusps develop during growth under limited adatom mobility conditions which lead, due to shadowing of atoms impinging at oblique angles, to the formation of nanopipes: 1-nm-wide open structures, elongated along the [001] growth direction. The nanopipes have rectangular cross-sections and form self-organized arrays aligned in orthogonal [100] and [010] directions, precisely replicating the in-plane correlation of the surface morphology. The nanopipe width and direction is controlled by the deposition angle. Non-normal deposition increases the level of atomic shadowing and introduces a controlled tilt to the nanopipes. Increasing the N₂⁺-ion irradiation flux or decreasing the N₂ partial pressure (and, hence, the steady-state N coverage) during growth provides a corresponding increase in cation surface mobilities leading to smoother surfaces with islands that exhibit dendritic edges elongated along <110> directions due to a Bales-Zangwill-type instability in the presence of step edges. This decrease in rms surface roughness reduces the level of atomic shadowing and, combined with the higher adatom mobility, results in partial or full suppression of nanopipe formation.
9:30 AM		H1-1-4 Interface Effects of Energetic Particles on DC Sputter Deposition of W/B₄C X-ray Multilayer Mirrors F. Eriksson, J. Birch (Linköping University, Sweden); G.A. Johansson, H.M. Hertz (Royal Institute of Technology, Sweden); I.V. Katardjiev (Uppsala University, Sweden) Ion-assisted magnetron sputtering have been used to grow W/B₄C multilayers (Λ=3.4 nm) as normal-incidence soft x-ray mirrors. For maximum reflectivity, abrupt and flat interfaces are essential. A low substrate temperature and no energetic particle irradiation during growth minimizes bulkdiffusion and interface mixing. However, such conditions may lead to a kinematically limited growth with an increased and accumulated roughness as a consequence. In this work varying sputtering pressures (3-20 mTorr), sputtering gases (Ar, Kr), substrate temperatures (20°C and 150°C), and ion irradiation energies (0-100 eV) have been used to study the effects of energetic particle radiation. Soft x-ray measurements were carried out using a reflectometer with a soft x-ray laser-plasma source. The maximal reflectivity was obtained using a substrate bias of -85 V, an Ar sputtering pressure of 3 mTorr and substrate temperature of 20°C. Individual layer thicknesses were determined by simulations of experimental hard x-ray (Cu-Kα) reflectivity curves. Large non-linearities of the deposition rates as a function of the layer thicknesses were evident. The deposition rate of B₄C was reduced up to 20% during deposition of the first 1.5 nm. This is explained by resputtering of the growing film by highly energetic backscattered neutrals from the W-target in combination with the sputter yield amplification effect. TRIM simulations combined with calculations of the transport through the gas-filled chamber shows that backscattered Ar-atoms have energies ranging up to 200 eV at 3 mTorr sputtering pressure. The experimental results are discussed and compared with dynamic simulations using the TRIM code.
9:50 AM		H1-1-5 Oxidation of Zirconium and Zirconium Nitride Thin Films: Comparison of Thermal, Electron Bombardment, and Local Oxidation Processes N. Farkas, G. Zhang, E.A. Evans, R.D. Ramsier (The University of Akron); J.A. Dagata (National Institute of Standards and Technology) The corrosion resistant behavior of the zirconium materials system in nuclear and chemical applications is well known, and is normally attributed to the presence of a passivating oxide layer on the exposed surface. Modification of this surface layer with species such as nitrogen and its resulting environmental stability are of importance not only for corrosion mitigation, but for the development of thin nitride and oxide films for dielectrics. Atomic force microscope (AFM)-induced local oxide patterning, previously developed for nanolithography, is employed here as a means of investigating oxidation mechanisms on sputter-deposited Zr and ZrN thin films. Specifically, we examine the role of film thickness and nitridation in altering local oxide growth kinetics under conditions that extend the range of previous studies carried out at thermal and low-energy electron beam energies. These experiments allow us to probe rapid surface-subsurface diffusion kinetics of reactants H, N, and O. As the anodization time increases, very thin films show an enhancement in oxide-feature height whereas the behavior of the other films is unchanged, demonstrating the role of the film/substrate interface. Under the same conditions, the height of features grown on ZrN films is greater than for those grown on Zr films, indicating that nitrogen plays a role in the oxidation process.
10:10 AM		H1-1-6 Structure and Properties of Co Nanoparticle Thin Films X. Nie (Deparment of Mechanical Engineering, Louisiana State University, Baton Rouge, LA 70803); J.C. Jiang, E.I. Meletis (Louisiana State University); H. Laurence (Southern University) Magnetic nanoparticle thin films with a particle size less than 10 nm have potential for applications of high-density recording media and magnetic sensors. The most popular method for nanoparticle synthesis is a wet-chemical route. In the present work, a vacuum evaporation approach via co-deposition of two immiscible elements (i.e., Co and carbon) was used to synthesize nanocrystalline (Co) thin films in which a new, so-called epsilon-Co phase was found. Microstructure of the epsilon-Co was analyzed using high-resolution transmission electron microscopy, electron diffraction, and X-ray absorption spectroscopy. A Superconducting Quantum Interference Device (SQUID) magnetometer was employed to study magnetic properties. For study of thermal stability of microstructure, the as-deposited films have been annealed at different temperatures; and their structure transitions and magnetism changes after annealing have been investigated. The results showed that the Co structure changed from epsilon-Co into hcp-Co or fcc-Co depending on the original nanoscale size. Interesting changes in the magnetic behavior as a function of the nanocrystalline size were also observed.
10:30 AM		H1-1-7 Thin Metal Layers on Oxide Substrates: Can we Predict the Thermal Stability? T. Wagner, Q. Fu, C. Winde (MPI for Metals Research, Germany) Future developments are critical to overcoming obstacles to miniaturization as feature sizes in devices reach the nanoscale. For technological applications for example, the thermal and environmental stability of metal films on oxides substrates is of great importance. For quantitative investigations of the relevant and fundamental processes special model systems are needed. In this context, we present recent results concerning the growth and thermal stability of very thin (< 1 nm) metal films (e.g. Pd, Fe, Mo, Cr, Al on single crystal oxide substrates (e.g. SrTiO₃, TiO₂, Al₂O₃). The nucleation and thermal stability of the metal/oxide systems were investigated with surface science tools, i.e. XPS, STM, RHEED. In addition, interfaces between thicker (50 nm) metal films and oxide substrates were characterized with different TEM techniques. General trends will be discussed which allow the prediction of interfacial reactions between thin metal films and oxide substrates.
10:50 AM		H1-1-8 Templateless Self-assembly of Nanowire Cages A.V. Ellis, J. D'Arcy-Gall, R. Goswami, G. Ramanath (Rensselaer Polytechnic Institute) One-dimensional nanostructures have attracted considerable interest due to their unusual properties arising from quantum confinement effects. In order to harness these properties for potential applications such as device interconnects or active elements in catalysts and sensors, it is essential to create mesoscale network assemblies using nano-scale building blocks. Here, we report a simple, low-temperature, templateless approach to synthesize cocoon-shaped 3D cages comprised of interwoven Au nanowires. The synthesis route is an adaptation of the Brust method used to create protected-nanoclusters by the reduction of gold salt in solution mixtures. The nanocage structure was characterized by conventional and high-resolution transmission electron microscopy (TEM and HRTEM), and electron diffraction. UV-visible spectroscopy was used to quantify the nanocage yields from different solution mixtures. Our TEM measurements show that nanocage dimensions are uniform, and range from ~200 nm to a few microns depending upon the synthesis parameters and post-treatments. High-resolution TEM and diffraction measurements reveal that the constituent nanowires have a uniform diameter of ~5-10 nm, and consist of fcc polycrystals. By adjusting the solution chemistry during synthesis, we can not only control the nanocage yield, but also can tune the morphology of the nanostructures from isolated nanoparticles to nanowires to interwoven nanowire-networks. Based upon our results, we propose a phenomenological model involving solution chemistry and nanocluster surface charges to explain nanowire formation and nanocage assembly.
11:10 AM		H1-1-9 Recent Developments in Electrically Conducting Polyaniline Micro/Nanotube Blends A.R. Hopkins, R.A. Lipeles, W.H. Kao (The Aerospace Corporation) Since the discovery of conducting polymers in 1977, there has been substantial interest in the scientific and engineering communities in understanding their intrinsic properties and finding applications for their unique properties. Recently, one such polymer that has shown to be both processable and conducting when cast from solution is polyaniline (PANI). This electrically conducting polymer has shown significant promise for a wide range of electronic, optoelectronic and charge dissipation applications. The flexibility of its chemical design and synthesis makes it possible to tune PANI's end chemical and physical properties. Moreover, PANI has recently been fabricated into micro and nanotubes that offer very interesting conducting properties. The natural processing advantages of a polymer, together with its tube geometry, make these conductive polymer tubes an attractive material for charge dissipation applications. In this talk we investigate the synthesis and characterization of these polyaniline tubes which were fabricated by a template free synthesis. Futhermore, the processing techniques used to blend these conductive tubes with a series of insulating host materials will be presented. Ultimately these concepts will be used to build lighter, stronger and higher performance deployable space structures.
11:30 AM		H1-1-10 Design and Performance of an Electrothermal MEMS Microengine That is Capable of Bi-directional Motion E.S. Kolesar (Texas Christian University) Several technologies have been investigated for positioning individual elements in large-scale microelectromechanical systems (MEMS). This research focuses on the design and performance evaluation of asymmetrical electrothermal actuators. The MEMS surface micromachined electrothermal actuator uses resistive (Joule) heating to generate thermal expansion and movement. In the traditional asymmetrical electrothermal actuator design, the single-hot arm is narrower than the cold arm, and thus, the electrical resistance of the hot arm is greater. When an electrical current passes through the device, the hot arm is heated to a higher temperature than the cold arm. This temperature differential causes the hot arm to expand along its length, thus forcing the tip of the device to rotate about the flexure. Another variant of the asymmetrical design features a double-hot arm arrangement that eliminates the parasitic electrical resistance of the cold arm. Furthermore, the second hot arm improves electromechanical efficiency by providing a return current conductor that is also mechanically-active. In this design, the rotating cold arm can have a narrower flexure compared to the flexure in the traditional single-hot arm device because it no longer needs to conduct an electrical current. The narrower flexure results in an improvement in mechanical efficiency. This research compares the tip deflection and tip force performance of the asymmetrical single- and double-hot arm electrothermal actuator designs. As a practical application of the electrothermal actuator, the recent realization of MEMS microengine will be described, and a video of its bi-directional motion will be shown. The electrothermal actuator and microengine designs were accomplished with the MEMSPro CAD software program, and they were fabricated using the JDS Uniphase Integrated Microsystems Multi-User Microelectromechanical Systems (MEMS) Process (MUMPs) foundry.