Presentation on theme: "Nanofabrication nanoLithography nano + bio Directed Assembly nano + bio + info Self-assembly."— Presentation transcript:
Nanofabrication nanoLithography nano + bio Directed Assembly nano + bio + info Self-assembly
Lithography Precise, but expensive and difficult at small sizes (< 50 nm) Photolithography: Widely used for microchip mass production Electron-Beam Lithography: High resolution, individual research devices Ion Beam Lithography: Special purpose (milling, direct deposition)
Resolution limit /2 Large object: Optical ruler counts /2 interference fringes /2 limit Smaller objects need shorter
Going to Shorter Wavelength (DUV) Can’t go farther: There is one more excimer laser line at 157 nm (the F 2 laser). However, one cannot produce good enough optics with CaF 2 (or any other material that remains transparent at such a short wavelength).
Trick 1 to Push beyond /2 : Immersion Lithography The higher refractive index of water reduces the wavelength (n = 1.44 at 193 nm).
Trick 2 to Push beyond /2 : Phase Shift Mask + Enhanced Resist Contrast Absorbing Mask Phase Mask Enhanced Contrast In contrast to the traditional absorbing masks, a phase shift mass contains regions of transparent material with high refractive index for shifting the phase. Thereby the oscillations originating from diffraction are converted to a damped decay. A photoresist with a high contrast narrows the decay width. This requires very good control of the exposure and the resist development.
Leapfrog to 13 nm (EUV) Need to go to mirror optics, since all materials absorb. Regular mirrors only reflect at oblique incidence, leading to asymmetric optics that are difficult to control. Use multilayer mirrors, where interference of multiple layers enhances the reflectivity. 13 nm is preferred, because it allows the use of silicon-based multilayer mirrors. (Si begins to absorb below 13 nm due to the Si 2p core level at about 100 eV.) Use synchrotron radiation for testing. Need lab-based light source for mass production.
By interference of the 1 st orders one can cut the mask period in half. Two, three, or four diffracted beams interfere to yield dense lines and spaces, or cubic or hexagonal arrays of dots 1:1 Lines, 55 nm Pitch PMMA Cubic Array of Holes, 57 nm pitch EUV Interference Lithography Paul Nealey (Madison), Harun Solak (Switzerland)
Self-assembly Cheap, atomically-precise at small sizes ( 50 nm)
Nanocrystals These are surprisingly simple to make
Synthesis of Nanocrystals in Inverse Micelles I Surfactant: Hydrophilic HeadExample: Phospholipid + Hydrophobic Tail Micelle: Inverse Micelle: Heads outside, Water outsideHeads inside, Water inside A nanoscale chemical beaker with aqueous solution inside
Synthesis of Nanocrystals in Inverse Micelles II Recipe: 1)Fill inverse micelles with an ionic solution of the desired material. 2)Add a reducing agent to precipitate the neutral material. 3)Narrow the size distribution further by additional tricks.
Lin, Jaeger, Sorensen, Klabunde, J. Phys. Chem B105, 3353 (2001) Nanocrystals with equal size form perfect arrays
"Perfect" Magnetic Particles: FePt (4nm) Sun, Murray, Weller, Folks, Moser, Science 287, 1989 (2000) Oleic acid spacer ad- justs the distance 3D array 2D array
Shape control of nanocrystals via selective surface passivation by adsorbed molecules. Only the clean surface facets will grow. Manna, Scher, Alivisatos, JACS 122, 12700 (2000)
Supported Catalysts Rhodium nanoparticles on a TiO 2 support
Zeolites Channels for incorporating catalysts or filtering ions O Si,Al Tetrahedra
Self-assembled Nanostructures at Surfaces Push Nanostructures to the Atomic Limit Reach Atomic Precision
> 100 atoms rearrange themselves to minimize broken bonds. Hexagonal fcc (diamond) (eclipsed)(staggered) Si(111)7x7 Most stable silicon surface
Si(111)7x7 as 2D Template One of the two 7x7 triangles is more reactive. Aluminum sticks there. Jia et al., APL 80, 3186 (2002)
1 kink in 20 000 atoms Straight steps because of the large 7x7 cell. Wide kinks cost energy. 15 nm Stepped Si(111)7x7 Viernow et al., APL 72, 948 (1998)
The 7x7 unit cell provides a precise 2.3 nm building block Step x-derivative of the topography “ illumination from the left ” Stepped Si(111)7x7 as 1D Template
Atomic Perfection by Self-Assembly Works up to 10 nm One 7x7 unit cell per terrace Kirakosian et al., APL 79, 1608 (2001) 5.731 592 8 nm
Sweep out Kinks into Bunches by Electromigration Yoshida et al., APL 87, 032903 (2005)
Clean Triple step + 7x7 facet "Decoration" of Steps 1D Atomic Chains With Gold 1/5 monolayer Si chain Si dopant
Clean 7 7 0.02 monolayer below optimum Au coverage Chains One-Dimensional Growth of Atom Chains
Gold chain Graphitic Silicon First Principles Calculations: Sanchez-Portal et al., PRB 65, 081401 (2002) Crain, Erwin, et al., PRB 69, 125401 (2004) X-Ray Diffraction: Robinson et al., PRL 88, 096104 (2002) Unexpected Structures : Gold at the center, not the edge ! Graphitic silicon ribbon ! Si(557) - Au
Zhao et al., PRL 90, 187401 (2003) Carbon Nanowire inside a Nanotube
Wu et al., Chem. Eur. J. 8, 1261 (2002) Silicon Nanowire Growth Works also for carbon nanotubes with Co, Ni as catalytic metal clusters.
Wu and Yang, JACS 123, 3165 (2001) Catalytic Nanowire Growth of Ge by Precipitation from Solution in Au Phase diagram for immiscible solids : The melting temperature of a mixture is lower than for the pure elements. (L = liquid region)
Peidong Yang et al., Science 292, 1897 (2001) and Int. J. of Nanoscience 1, 1 (2002) ZnO Nanowires Grown by Precipitation from a Solution SEM images of ZnO nanowire arrays grown on sapphire substrates. A top view of the well-faceted hexagonal nanowire tips is shown in (E). (F) High-resolution TEM image of an individual ZnO nanowire showing its growth direction. For the nanowire growth, clean (110) sapphire substrates were coated with a 10 to 35 Å thick layer of Au, with or without using TEM grids as shadow masks.
ZnO Nanowires for Solar Cells Leschkies et al., Nano Letters 7, 1793 (2007) Need to collect the electrons quickly in a solar cell to prevent losses. This can be achieved by running many nanowires to the places where electrons are created (here in CdSe dots which coat the ZnO wires).
Ohgai, …, Ansermet, Nanotechnology 14, 978 (2003) Striped Cu/Co Nanowires Grown by Electroplating into Etched Pores (Superlattices for efficient sensors)
Directed Assembly The best of both worlds Use lithography to define a grid. Then attach self-assembled nano- objects (dots, wires, diodes, … ).
Unpatterned Surface Patterned Surface (48 nm pitch) Assembly of Block Copolymers on Lithographically-Defined Lines S. O. Kim, H. H. Solak, M. P. Stoykovich, N. J. Ferrier, J. J. de Pablo, P. F. Nealey, Nature 411, 424 (2003). Perfect positioning over large distances Perfect line width, defined by the size of a molecule
Park, Chaikin, Register,... Transfer dot patterns from a block copolymer into a metal
Guided Self-Assembly of Block-Copolymers: From a random “fingerprint” patterns to an ordered lattice Polymer in groove: Thomas, Smith (MIT) Naito et al. (Toshiba) Shear via PDMS: Chaikin (Princeton) On a chemical pattern: Kim et al. (Madison) shear
Patterned Magnetic Storage Media for Perfect Bits
Co-polymers as etch masks Spiral grooves as guide for dots Naito et al. (Toshiba) IEEE Trans. Magn. 38, 1949 (2002)
A single magnetic dot for storing one bit. Side view
Magnetic force microscope dark: spin light: spin Normal microscope Dot pattern