Nanofabrication

nano Lithography

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 F2

laser). However, one cannot produce good enough optics with CaF2

(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.

-1 Diffraction +1 Diffraction

Sample

Transmission Grating Mask

EUV

By interference of the 1st 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

500 nm500 nm500 nm

Paul Nealey (Madison), Harun Solak (Switzerland)

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 Example: Phospholipid

+ Hydrophobic Tail

Micelle: Inverse Micelle:

Heads outside, Water outside 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.

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 TiO2 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 Stepx-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

Electromigratio

n

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 77

0.02 monolayer

below

optimum Au

coverage

Chains

One-Dimensional Growth

of Atom Chains

Gold chain

GraphiticSilicon

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

Free-standing Nanowires

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 <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.

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