Papers

AVS2004 Session NS-FrM: Nanometer-scale Structures

Friday, November 19, 2004 8:20 AM in Room 213D

Friday Morning

Start	Invited?	Item
8:20 AM		NS-FrM-1 Peter Mark Memorial Award Presentation: Nanometer-Scale Fabrication Techniques for Building Semiconductor Devices K. Guarini (IBM T. J. Watson Research Center) Integrated circuit performance improvements have been achieved largely by aggressive shrinking of the silicon transistor and metal interconnect dimensions. Future technology generations will require even greater innovation to continue the performance trend. New materials, device structures, and integration schemes will all play roles in advancing CMOS technology. Novel nanometer-scale patterning techniques will be critical to this success. This talk will give an overview of nanometer-scale patterning needs of the semiconductor industry and highlight several novel solutions. Direct write scanning probe lithography (SPL) is a high-resolution patterning technique that uses a sharp tip in close proximity to a sample to pattern nanometer-scale features. Low energy electrons field emitted from a probe tip can be used to expose polymer resists with sub-30-nm resolution and nanometer-scale alignment registration. In comparison to electron beam lithography, SPL has wider exposure latitude, improved linearity, and reduced proximity effects. Material self assembly provides an alternative means for pattern formation at the nanometer-scale. With feature sizes defined by fundamental molecular properties, self assembly can access dimensions and densities beyond the capabilities of conventional patterning techniques. Our work has focused on identifying and demonstrating key applications of self assembly. In one example, we enhanced the capacity of thin film metal-oxide-semiconductor devices using nanostructured electrodes patterned by self assembly. We have also demonstrated the use of material self assembly in facilitating continued scaling of non-volatile FLASH memories. These high resolution patterning processes offer innovative solutions to existing challenges in microelectronics and are well suited to enabling enhanced device performance and functionality by augmenting the available tool kit for manufacturing.
9:00 AM		NS-FrM-3 Self-assembled Multilayers Creating Tailored Resists for Nanostructure Fabrication M.E. Anderson, E.M. Carter, A.R. Kurland, C. Srinivasan, M.W. Horn, P.S. Weiss (The Pennsylvania State University) Designing and patterning complex hierarchical assemblies by exploiting methods of directed self-assembly in combination with a variety of lithographic techniques has been an active area of research for patterning in the sub-100 nm regime. We have used self-assembled multilayers to create molecular ruler resists to define nanostructures with precise spacing and edge resolution reaching the nanometer-scale.^1-3 A molecular ruler resist of self-assembled multilayers, composed of alternating layers of α,ω-mercaptoalkanoic acids and coordinated metal ions, is selectively deposited on initial lithographically defined gold structures. This resist can be tuned based on the number of layers deposited to a desired thickness (routinely between 10-100 nm). Then, metal is deposited on the sample and the resist is removed, yielding spacings between metal structures dependent on the dimensions of the tailored resist. Work is underway to build molecular ruler resists independently either by capping selected regions of growth or by orthogonal growth of two different multilayer systems. A scheme in development is electroless metal deposition of the secondary metal, where the ruler resist both defines the structure spacing and inhibits deposition for selective metal placement. Molecular ruler resists can withstand the rigors of lithographic processing and are being developed to advance this method toward device fabrication. ¹ A. Hatzor and P.S. Weiss, Science 291, 1019 (2001). ² M. E. Anderson et. al., Journal of Vacuum Science and Technology B 20, 2739 (2002). ³ M. E. Anderson et. al., Journal of Vacuum Science and Technology B 21, 3116 (2003).
9:20 AM		NS-FrM-4 Studies of Self-Assembling Bilayers for Layered Nanofabrication (LNF) T.-Y. Shih, A.A.G. Requicha, M.E. Thompson, B.E. Koel (University of Southern California) Designed fabrication of structures on a nanometer scale often requires progress in the efficiency and control in deposition of self-assembled monolayers, especially in an approach we have called layered nanofabrication (LNF). We report on the embedding of gold nanoparticles in several layers of octadecyltrichlorosilane (OTS) deposited on a SiO₂ surface. Atomic force microscopy (AFM) was used in ex-situ studies of the formation of self-replicating bilayers with hydrophobic-hydrophilic properties and hydrogen bonding as triggered by treatments using acetone. Analysis of the Au nanoparticles after several bilayer-by-bilayer growth cycles showed that they decreased in apparent height and roughness according to the number of deposited layers. Ellipsometry was used to monitor the OTS film thickness and characterize the film growth mode. We were able to achieve a controllable, stepwise linear growth of a flat, multilayer film that eventually produced a high-quality, planarized nanoparticle-containing surface.
9:40 AM		NS-FrM-5 Cross-sectional Ballistic Electron Emission Microscopy Studies of Molecular Beam Epitaxy Grown Quantum Wells C. Tivarus, J.P. Pelz, M.K. Hudait, S.A. Ringel (The Ohio State University) Schottky diodes formed on GaAs quantum wells grown by Molecular Beam Epitaxy are studied using Cross-sectional Ballistic Electron Emission Microscopy (XBEEM), in order to determine the influence of quantum confinement and pinning effects on Schottky barrier (SB) formation. The diodes were formed by both in-situ and ex-situ gold deposition on cleaved sides of heterostructures composed of a sequence of GaAs quantum wells (QW) separated by AlGaAs barrier layers. Using this technique, we were able to determine the local SB height for each QW. We found that the SB height increases with decreasing the QW thickness d_QW , varying from ~0.91 eV for d_QW ≥ 9 nm to ~1.04eV for d_QW=1 nm. This dependence will be discussed in terms of the QW confinement energy as well as reduced pinning effects at the QW/metal interface. We will also compare XBEEM samples made by in-situ vs. ex-situ metal deposition to clarify the role of chemical treatment and of an interfacial oxide layer on the SB formation. Additionally, the cross sectional configuration offered a direct measure of electron beam spreading due to scattering inside the metal film, as a function of film thickness. This spreading was surprisingly large, with full width at half maximum spreading of ~16 nm (~23 nm) for a 4 nm (7nm) thick Au film. The measurements will be compared with model simulations to quantify inelastic and elastic hot-electron scattering processes in the bulk and at the interfaces of the metal film, and to gain insight into the controversial issue of whether lateral momentum is conserved during hot electron transport over a metal/semiconductor interface. Work was supported by NSF and Office of Naval Research.
10:00 AM		NS-FrM-6 Defect Mediated Transport in Nanostructures by Scanning Impedance Spectroscopy R. Shao, M.P. Nikiforov, J. Vavro, D.A. Bonnell (The University of Pennsylvania) Defects and interfaces can dictate the transport properties of macroscopic structures and completely dominate the behavior of nanostructures. We have shown that Scanning Impedance Microscopy (SIM) can be used to determine local electronic structures of scattering centers in nanotubes/wires and the perturbing effect of atomic interfaces on local continuum properties. This talk will combine SIM and low temperature transport measurements to quantify the effect of defects on transport in individual nanofibers and at atomically abrupt oxide interfaces in SrTiO₃ bicrystals. In the latter case we have found a defect induced phase transition at low temperature.
10:20 AM		NS-FrM-7 Supramolecular [60]Fullerene/Porphyrin Assemblies on Metals H. Spillmann (University of Basel, Switzerland); D. Bonifazi (ETH Zürich, Switzerland); A. Kiebele, H.-J. Güntherodt (University of Basel, Switzerland); T.A. Jung (Paul Scherrer Institute, Switzerland); F. Diederich (ETH Zürich, Switzerland) The unique electrochemical and photophysical properties of porphyrin and [60]fullerene compounds makes them promising candidates for the construction of two- and three-dimensional organic-based materials. An important question is how pristine C60 and self-assembled monolayer of porphyrin arrays will organize on surfaces. Self-assembly of electron-rich flat aromatic molecules such as porphyrins shall enable the selective formation of electron donating monolayer, which can be covered by pristine C60. The chromophore interaction between the electron donor (porphyrin) and acceptor (C60) should enhance a supramolecular multilayer structure. Herein, we report first Scanning-Tunneling-Microscopy investigations of the behaviour of several porphyrin derivatives on metal surfaces wherein single porphyrin molecules are forcefully arranged in self-assembled monolayer. The specific properties of the single porphyrin derivatives make such molecularly modified surfaces suitable candidates for patterned surfaces to allocate fullerenes. Consequently, first nanostructures based on the interaction of the fabricated porphyrin-based assemblies with fullerene molecules will be presented.
10:40 AM		NS-FrM-8 Synthesis and Optical Characterization of Gold Nanoplates C.S. Ah, W.S. Yun, D.H. Ha, K.J. Kim (Korea Research Institute of Standards and Science, Korea) We report on a high-yield synthesis and optical characterization of gold nanoplates. The nanoplates were prepared by controlled reduction of hydrogen tetrachloroaurate with reduced amount of sodium citrate in the presence of poly(vinyl pyrrolidone). The plates have a shape of either triangle or truncated-triangle of around 10⁴ - 10⁵ nm² in area with a thickness of about 20 - 30 nm. High-resolution TEM analysis reveals that the individual plate is a single crystal with well-developed flat {111} facets. Optical spectra of the nanoplates show distinctive plasmon absorption bands which can be attributed to their highly-anisotropic shape. The optical characteristics are strongly dependent upon the size of the gold nanoplates. In particular, the absorption band in NIR region, which is believed to originate from the longitudinal dipole plasmon resonance, dramatically shifts to red when the planar size of the plate increases. We hope that this gold nanoplate should serve as an excellent platform for molecular self-assembly and as a useful building-block in developing new-materials and devices.
11:00 AM		NS-FrM-9 Non-Spherical Metal Nanoparticles: Tuning Optical Properties by Controlling Structure J.S. Shumaker-Parry, H. Rochholz, F. Stefani, W. Knoll, M. Kreiter (Max Planck Institute for Polymer Research, Germany) Gold colloids have been studied extensively due to the dependence of their optical properties on colloid size, interparticle spacing, and local dielectric environment. Recently there have been efforts to prepare non-spherical nanoparticles because of their unique electronic, optical and other physical properties. We have fabricated novel non-spherical metal nanoparticles by combining colloidal lithography, metal film evaporation and ion beam milling. This process produces a large number of metallic nanoparticles on a surface with uniform size and identical orientation in parallel, a major advantage compared to electron beam lithography methods. Gold and silver quartermoon and c-shaped nanoparticles have been fabricated. A finite element method was applied to classical electrodynamics for two-dimensional models of these nanoparticles in order to understand the optical characteristics. These calculations predict large electromagnetic field enhancements that are localized or uniformly distributed, depending on the nanoparticle structure. We have characterized the optical properties of the nanoparticles using extinction scattering cross section spectroscopy and single-particle light scattering spectroscopy. We show that the optical properties of the nanoparticles can be tuned by changing the size of the colloid mask and by controlling the opening of the c-structure.
11:20 AM		NS-FrM-10 Elucidation of the Electronic Properties of Immobilized Alkanethiolate-Stabilized Gold Clusters and Nanoparticles Using Scanning Tunneling Microscopy P.S. Weiss, R.K. Smith, S.U. Nanayakkara, B.A. Mantooth, G. Woehrle (The Pennsylvania State University); J.E. Hutchinson (University of Oregon) The single electron transport properties of metal nanoparticles have led to great interest in their potential integration into nanoscale electronics. Here, we discuss and compare the electronic characteristics of isolated, solution-derived, and ligand-stabilized gold clusters (Au₁₁L₁₀) and nanoparticles (Au₁₀₁L₄₃), taken in both cryogenic (4 K, UHV) and ambient conditions using scanning tunneling microscopy and spectroscopy. The clusters and particles (d_CORE = 0.8 nm and 1.5 Â± 0.5 nm, respectively) are immobilized on alkanethiolate self-assembled monolayers with inserted dithiol molecules. We thoroughly characterize the self-assembled monolayer surface to which the nanostructures are attached with both local probes and ensemble measurements. At low temperature, the Au₁₁ clusters demonstrate Coulomb blockade behavior, with zero-conductance gaps resulting from quantum size effects.
11:40 AM		NS-FrM-11 Optical Properties of Ordered Arrays of Vanadium Dioxide Nanoparticles R. Lopez, J.Y. Suh, R.F. Haglund Jr., L.C. Feldman (Vanderbilt University) Arrays of vanadium oxide nanoparticles with long-range order have been fabricated by pulsed laser deposition in an arbitrary pattern defined by focused ion-beam lithography. Interaction of light with the nanoparticles is controlled by the geometrical arrangement as well as by the differing optical properties displayed by the metallic and semiconducting phases of VO₂. The particle arrays present a previously unknown scattering resonance in the blue region of the spectrum. The scattering is pronounced in the semiconductor phase, but is less intense and slightly blue-shifted in metallic state. This phenomenon is intrinsically interesting, since previous studies of VO₂ switching have been confined to the IR region, whereas this configuration produces the relevant optical effects of the transition in the visible spectrum. In addition, the transition to the metallic state does not exhibit the usual step function at the critical temperature. Instead, the scattered light begins to increase in intensity at 68°C, the temperature of the bulk phase transition, and reaches a maximum before falling to a lower value in the metallic state resulting in a anomalous double loop hysteresis. This transient scattering enhancement results from the order-disorder transition that occurs as the nanoparticles are transformed by random fluctuations. The disorder or inhomogeneity present during this process enhances the scattering by coherent contributions of length scales longer than the array lattice constant, the necessary additional Fourier components to describe a disordered system. Arrays such as this open up new opportunities to study surface plasmon interactions for nanoparticles in close proximity, with the added advantage that the interaction can be switched on by the thermally driven metal-semiconductor phase transition in VO₂.