Nanofabrication

nano nano + bio nano + bio + info Lithography Self-assembly Directed 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

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/2

Large object: Optical ruler counts interference fringes  /2  /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 However, one cannot produce good enough optics with CaF 2 laser). (or any other material that remains transparent at such a short wavelength).

Trick 1 to Push beyond

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/2 : Immersion Lithography

The higher refractive index of water reduces the wavelength (n = 1.44 at 193 nm).

Trick 2 to Push beyond

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/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)

Use synchrotron radiation for testing.

Need lab-based light source for mass production.

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.)

EUV Interference Lithography

Two, three, or four diffracted beams interfere to yield dense lines and spaces, or cubic or hexagonal arrays of dots

EUV PMMA Transmission Grating Mask -1 Diffraction +1 Diffraction Sample

By interference of the  1 st orders one can cut the mask period in half.

Paul Nealey (Madison), Harun Solak (Switzerland)

Cubic Array of Holes, 57 nm pitch 1:1 Lines, 55 nm Pitch

Self-assembly

Cheap, atomically-precise at small sizes (< 5 nm), but poor positioning at large distances (> 50 nm)

Nanocrystals These are surprisingly simple to make

Synthesis of Nanocrystals in Inverse Micelles I Surfactant: Hydrophilic Head + Hydrophobic Tail Example: Phospholipid Micelle: Heads outside, Water outside Inverse Micelle: Heads 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.

Nanocrystals with equal size form perfect arrays Lin, Jaeger, Sorensen, Klabunde, J. Phys. Chem B105, 3353 (2001)

"Perfect" Magnetic Particles:

FePt (4nm)

3D array 2D array Oleic acid spacer ad justs the distance Sun, Murray , Weller, Folks, Moser, Science 287, 1989 (2000)

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

O Si,Al Tetrahedra Channels for incorporating catalysts or filtering ions

Self-assembled Nanostructures at Surfaces Push Nanostructures to the Atomic Limit Reach Atomic Precision

Si(111)7x7 Most stable silicon surface Hexagonal fcc (diamond) (eclipsed) (staggered) > 100 atoms rearrange themselves to minimize broken bonds.

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)

Stepped Si(111)7x7 1 kink in 20 000 atoms Straight steps because of the large 7x7 cell. Wide kinks cost energy.

Viernow et al., APL 72, 948 (1998) 15 nm

Stepped Si(111)7x7 as 1D Template The 7x7 unit cell provides a precise 2.3 nm building block x-derivative of the topography “ illumination from the left ” Step Step

Atomic Perfection by Self-Assembly Works up to 10 nm 5.731 592 8 nm One 7x7 unit cell per terrace Kirakosian et al., APL 79, 1608 (2001)

Sweep out Kinks into Bunches by Electromigration Yoshida et al., APL 87, 032903 (2005)

"Decoration" of Steps  1D Atomic Chains Clean Triple step + 7x7 facet With Gold 1/5 monolayer Si chain Si dopant

One-Dimensional Growth of Atom Chains Chains Clean 7  7 0.02 monolayer below optimum Au coverage

Graphitic Silicon Gold chain Si(557) - Au Unexpected Structures : Gold at the center, not the edge !

Graphitic silicon ribbon !

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)

Free-standing Nanowires

Zhao et al., PRL 90, 187401 (2003) Carbon Nanowire inside a Nanotube

Silicon Nanowire Growth Works also for carbon nanotubes with Co, Ni as catalytic metal clusters.

Wu et al., Chem. Eur. J. 8, 1261 (2002)

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) Wu and Yang, JACS 123, 3165 (2001)

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 <0001> 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.

Peidong Yang et al., Science 292, 1897 (2001) and Int. J. of Nanoscience 1, 1 (2002)

ZnO Nanowires for Solar Cells 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).

Leschkies et al., Nano Letters 7, 1793 (2007)

Striped Cu/Co Nanowires Grown by Electroplating into Etched Pores (Superlattices for efficient sensors) Ohgai, … , Ansermet, Nanotechnology 14, 978 (2003)

Directed Assembly

The best of both worlds

Use lithography to define a grid. Then attach self-assembled nano objects (dots, wires, diodes, … ).

Assembly of Block Copolymers on Lithographically-Defined Lines

Unpatterned Surface Patterned Surface (48 nm pitch) • Perfect positioning over large distances • Perfect line width, defined by the size of a molecule S. O. Kim, H. H. Solak, M. P. Stoykovich, N. J. Ferrier, J. J. de Pablo, P. F. Nealey, Nature 411, 424 (2003).

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 shear

Polymer in groove:

Thomas, Smith (MIT) Naito et al. (Toshiba)

Shear via PDMS:

Chaikin (Princeton)

On a chemical pattern:

Kim et al. (Madison)

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)

Side view A single magnetic dot for storing one bit.

Magnetic force microscope dark: spin  light: spin  Normal microscope Dot pattern