Transcript Colloquium - UW-Madison Department of Physics

Nanofabrication
Lithography
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Directed Assembly
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)
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
PMMA
EUV
Transmission
Grating Mask
500 nm
+1 Diffraction
-1 Diffraction
Cubic Array of Holes, 57 nm pitch
Sample
By interference of the 1st orders
one can cut the mask period in half.
Paul Nealey (Madison), Harun Solak (Switzerland)
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
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.
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 adjusts 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 TiO2 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
(eclipsed)
fcc (diamond)
(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
Unexpected
Structures :
Gold at the center,
not the edge !
Graphitic silicon
ribbon !
First Principles
Calculations:
Graphitic Gold
Silicon chain
Sanchez-Portal et al.,
PRB 65, 081401 (2002)
Crain, Erwin, et al.,
PRB 69, 125401 (2004)
X-Ray Diffraction:
Si(557) - Au
Robinson et al.,
PRL 88, 096104 (2002)
Free-standing Nanowires
Carbon Nanowire
inside a Nanotube
Zhao et al., PRL 90, 187401 (2003)
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 nanoobjects (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:
Shear via PDMS:
Thomas, Smith (MIT)
Chaikin (Princeton)
Naito et al. (Toshiba)
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