Making nanostructures Top down Approach Photolithography Electron beam
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: Oxidation place a protective layer (100 -2000 nm) on the surface • Masking: Masking features are open in the layer window by light • Implantation: Implantation doping step of the exposed sites • Etching: Etching remove the protective layer • Metalization: Metalization contacting by metal deposition • Lift-off: 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 Incident wavelength Resolution 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: 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 k. V 20 k. V 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 e. Vs. 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: RMS speed in cm/s from the equipartition theorem Number of molecules hitting a surface per unit time For O 2 at 300 K 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 2 D-growth favourite with respect to 3 D-growth favourite with respect to 2 D-growth. Epitaxial growth: 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) CVD Creation of reactive chemical species close to the surface. Ex. Si. H 4 Si + 2 H 2
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 Hg. Te/Hg. Cd. Te 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 k. V.
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 60 nm 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
- Slides: 30