Making nanostructures: Top down Approach

• Photolithography• Electron beam lithography• Micromechanical structures• Thin films, including MBE• Self-assembled masks• Focused Ion Beam milling• Stamp technology• Nanojunctions

PhotolithographyPhotolithography

Copyright Stuart Lindsay (2008)

Ex. PPMA

Ex. HF

• OxidationOxidation: place a protective layer (100-2000 nm) on the surface

• MaskingMasking: features are open in the layer window by light

• ImplantationImplantation: doping step of the exposed sites

• EtchingEtching: remove the protective layer

• MetalizationMetalization: contacting by metal deposition

• Lift-offLift-off: complement of etching. Deposition of layers on a patterned photoresist

Copyright Stuart Lindsay (2008)

NAr

2

Photolithography with micron-scale resolution is a useful precursor tool for generating nanostructures by other methods.

Optical lenses resolution: 0.5 μ

Current top resolution of photolithography: ≈ 50 nm

Numerical Aperture of the optical lens

Incident wavelength

Resolution

Evolution of ElectronicsEvolution of Electronics

Bell LabsFirst

Transistor

1959 Texas Instrs.First Integrated Circuit

(Intel)

1947

65 nm

Excimer Laser StepperExcimer Laser Stepper

248-157 nm

(Reprinted with permission of ASML Corporate Communications)

Stepper Motor: Scanning the wafer with nanometer scale accuracy

Electronics made by LithographyElectronics made by Lithography

(Reprinted with permission John Wiley and Sons)

CMOSCMOS: Complementary Metal Oxide on Silicon

Diffusion through holes /masking/metal coating

E-beam LithographyE-beam Lithography

Copyright Stuart Lindsay (2008)

The E-beam is turned on/off and directed in a prearranged pattern over the surface of the resist.

Copyright Stuart Lindsay (2008)

Monte Carlo simulation of spatially distributed beams in electron-beam lithography, D.F. Keyser, N.S. Viswanathan, J. Vac. Sci. Technol. Vol. 12, 1975

Copyright Stuart Lindsay (2008)

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.

10 kV 20 kV

Micro-electro-mechanical structuresMicro-electro-mechanical structures(MEMS)(MEMS)

• Micron-scale free standing structures made by undercutting

Ex. AFM Probes

Copyright Stuart Lindsay (2008)

Complete Cantilever FabricationComplete Cantilever Fabrication

Copyright Stuart Lindsay (2008)

(Reprinted with permission from IOP Publishing Ltd.,And courtesy of Professor Anja Boisen)

MEMS mirror projection array

Optical switch made from a silicon mirror, composed by 800,000 electronically tiltable mirrors. Electronics and transducers are located under each mirror.

Each mirror is separated by 0.5μ

Thin Film TechnologiesThin Film Technologies

• From the kinetic theory of gases:

M

Tv 42 1058.1 RMS speed in cm/s from the

equipartition theorem

2

32

1vN s

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.

Number of molecules hitting a surface per unit time

Modes of epitaxial growthModes of epitaxial growth

Epitaxial growthEpitaxial growth: in a homogeneous system, element x is deposited onto a surface of a single crystal of the same element.

Layer-by-layer uniform growth

2D-growth favourite with respect to 3D-growth.

3D-growth favourite with respect to 2D-growth.

Vacuum depositionVacuum deposition

• SputteringSputtering Bombardment of the material by an energetic ion

beam

• Thermal evaporationThermal evaporation

• Chemical Vapour DepositionChemical Vapour Deposition (CVDCVD) Creation of reactive chemical species close to the

surface.

Ex. SiH4 Si + 2H2

UHV Thin Film Deposition SystemUHV Thin Film Deposition System

(Courtesy of Professor Robert Lad, Laboratory for Surface Science and Technology, University of Maine)

Molecular Beam Epitaxy (MBE)Molecular Beam Epitaxy (MBE)MBE: Epitaxial growth of atomic layers on a substrate

Strain energy limits thickness Kinetic factors

Copyright Stuart Lindsay (2008)

(Courtesy of Professor Jeff Drucker, Department and School of Materials, Arizona State University)

• trapping of adatoms at special sites• diffusion on the surface• association/dissociation rate of small clusters• formation rate of stable clusters

Semiconductor superlattice

Copyright Stuart Lindsay (2008)

(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. )

Block copolymer masksBlock copolymer masks• Phase separation of incompatible block copolymers

Immiscible polymers phase-separate into a quite ordered domain structure

Copyright Stuart Lindsay (2008)

Self-assembled masksSelf-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 masksStructures 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 BeamFocused Ion Beam

Focused Ion BeamFocused Ion Beam

Gallium liquid metal ion source. Typical energies of ion beams are 5-30 kV.

Copyright Stuart Lindsay (2008)

• collection of the scattered ions (ion beam imaging)

• collection of secondary electrons

• implantation of Gallium ions

Ions are thousands of times heavier than electrons:Electrostatic fields are more efficient than magnetic fields(electrostatic focusing)

Focused Ion BeamFocused Ion Beam

Resolution: few tens of nanometers

Ion beam irradiation of a gold film

SEM image of an insulator defect.The sample was prepared by a FIB.

Stamp TechnologyStamp Technology

Chemical patterning by soft imprint lithography.

Thermoplastic

““Stamped” MOSFET with 60nm gateStamped” MOSFET with 60nm gate

Fabrication of 60 nm transistors on 4-in wager using nanoimprint at all lithography levels

Nanoscale JunctionsNanoscale Junctions

Strachan D.R. et al., 2008 Phys. Rev. Lett., 100, 056805

Gold nanowire broken by conventional nanolitography.

High current densities lead to substantial local heating, causing electromigration.