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Making nanostructures:
Top down Approach
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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: place a protective layer (100-2000 nm) on the
surface
• Masking: features are open in the layer window by light
• Implantation: doping step of the exposed sites
• Etching: remove the protective layer
• Metalization: contacting by metal deposition
• 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
r
Resolution

Incident wavelength
2 NA
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: 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 kV
20 kV
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 eVs.
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:
v
2
Ns 
 1.58 10
1
2 3

4
v2
T
M
RMS speed in cm/s from the
equipartition theorem
Number of molecules hitting
a surface per unit time
For O2 at 300K 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
2D-growth favourite with respect
to 3D-growth.
3D-growth favourite with respect
to 2D-growth.
Epitaxial 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)
Creation of reactive chemical species close to
the surface.
Ex. SiH4  Si + 2H2
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 HgTe/HgCdTe 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 kV.
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 60nm 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