Transcript Chapter 5
Making nanostructures: Top down Approach • • • • • • • • Photolithography Electron beam lithography Micromechanical structures Thin films, including MBE Self-assembled masks Focused Ion Beam milling Stamp technology Nanojunctions Photolithography Ex. PPMA Ex. HF Copyright Stuart Lindsay (2008) • Oxidation: place a protective layer (100-2000 nm) on the surface • Masking: features are open in the layer window by light • Implantation: doping step of the exposed sites • Etching: remove the protective layer • Metalization: contacting by metal deposition • Lift-off: complement of etching. Deposition of layers on a patterned photoresist Photolithography with micron-scale resolution is a useful precursor tool for generating nanostructures by other methods. Optical lenses resolution: 0.5 μ r Resolution Incident wavelength 2 NA Numerical Aperture of the optical lens Current top resolution of photolithography: ≈ 50 nm Copyright Stuart Lindsay (2008) Evolution of Electronics 1947 1959 Texas Instrs. First Integrated Circuit Bell Labs First Transistor (Intel) 65 nm Excimer Laser Stepper 248-157 nm (Reprinted with permission of ASML Corporate Communications) Stepper Motor: Scanning the wafer with nanometer scale accuracy Electronics made by Lithography Diffusion through holes /masking/metal coating (Reprinted with permission John Wiley and Sons) CMOS: Complementary Metal Oxide on Silicon E-beam Lithography The E-beam is turned on/off and directed in a prearranged pattern over the surface of the resist. Copyright Stuart Lindsay (2008) Copyright Stuart Lindsay (2008) 10 kV 20 kV Monte Carlo simulation of spatially distributed beams in electron-beam lithography, D.F. Keyser, N.S. Viswanathan, J. Vac. Sci. Technol. Vol. 12, 1975 The resolution is limited by the scattering of secondary electrons, that cause damage of the photoresist even at energies as low as a few eVs. Copyright Stuart Lindsay (2008) Micro-electro-mechanical structures (MEMS) • Micron-scale free standing structures made by undercutting Ex. AFM Probes Copyright Stuart Lindsay (2008) Complete Cantilever Fabrication (Reprinted with permission from IOP Publishing Ltd., And courtesy of Professor Anja Boisen) Copyright Stuart Lindsay (2008) MEMS mirror projection array Each mirror is separated by 0.5μ Optical switch made from a silicon mirror, composed by 800,000 electronically tiltable mirrors. Electronics and transducers are located under each mirror. Thin Film Technologies • From the kinetic theory of gases: v 2 Ns 1.58 10 1 2 3 4 v2 T M RMS speed in cm/s from the equipartition theorem Number of molecules hitting a surface per unit time For O2 at 300K this is ca. 1015 molecules·cm-2 at 10-6 Torr: ≈ a monolayer of adsorbed molecule per second. A vacuum of 10-9 torr is required. Modes of epitaxial growth Layer-by-layer uniform growth 2D-growth favourite with respect to 3D-growth. 3D-growth favourite with respect to 2D-growth. Epitaxial growth: in a homogeneous system, element x is deposited onto a surface of a single crystal of the same element. Vacuum deposition • Sputtering Bombardment of the material by an energetic ion beam • Thermal evaporation • Chemical Vapour Deposition (CVD) Creation of reactive chemical species close to the surface. Ex. SiH4 Si + 2H2 UHV Thin Film Deposition System (Courtesy of Professor Robert Lad, Laboratory for Surface Science and Technology, University of Maine) Molecular Beam Epitaxy (MBE) MBE: Epitaxial growth of atomic layers on a substrate • trapping of adatoms at special sites • diffusion on the surface • association/dissociation rate of small clusters • formation rate of stable clusters (Courtesy of Professor Jeff Drucker, Department and School of Materials, Arizona State University) Strain energy limits thickness Kinetic factors Copyright Stuart Lindsay (2008) Semiconductor superlattice (Reprinted from Journal of Crystal Growth, Volume 271, T. Aoki, M. Takeguchi, P. Boieriu, R. Singh, C. Grein, Y. Chang, S. Sivananthan and David J. Smith, "Microstructural characterization of HgTe/HgCdTe superlattices" Pages 29-36, Copyright 2004, with permission from Elsevier. ) Copyright Stuart Lindsay (2008) Block copolymer masks • Phase separation of incompatible block copolymers Immiscible polymers phase-separate into a quite ordered domain structure Copyright Stuart Lindsay (2008) Self-assembled masks Polystyrene/polybutadiene 36/11 Spontaneously forms nanometer scale phase-separated domains. Polybutadiene is selectively etched by ozone treatment. Structures made with block-copolymer masks TEM images showing (A) a spherical micro-domain monolayer film after removal of poly butadiene by ozone treatment, (B) the resulting array of holes in silicon nitride after RIE, (C) cylindrical microdomains in which the darker regions are osmium stained poly butadiene domains and (D) the resulting cylindrical pattern etched into the silicon nitride surface. Focused Ion Beam Focused Ion Beam Gallium liquid metal ion source. Typical energies of ion beams are 5-30 kV. Ions are thousands of times heavier than electrons: Electrostatic fields are more efficient than magnetic fields (electrostatic focusing) • collection of the scattered ions (ion beam imaging) • collection of secondary electrons • implantation of Gallium ions Copyright Stuart Lindsay (2008) Focused Ion Beam Ion beam irradiation of a gold film SEM image of an insulator defect. The sample was prepared by a FIB. Resolution: few tens of nanometers Stamp Technology Thermoplastic Chemical patterning by soft imprint lithography. “Stamped” MOSFET with 60nm gate Fabrication of 60 nm transistors on 4-in wager using nanoimprint at all lithography levels Nanoscale Junctions Gold nanowire broken by conventional nanolitography. High current densities lead to substantial local heating, causing electromigration. Strachan D.R. et al., 2008 Phys. Rev. Lett., 100, 056805