Large-scale NEMS Fabrication: Directed Self-Assembly...

NSF Summer Institute 2006Teri W. Odom

Large-scale NEMS Fabrication: Directed Self-Assembly or

Nanolithography

Teri W. OdomNorthwestern University

todom@northwestern.edu

7 August 2006

Goals of Nanofabrication• Goals

– Parallel processing of components over large areas

– Fabrication of structures 10 nm– Cost effectiveness

• Function– Nanoprocessors– Sub-wavelength optics– Biological assays

• Issues– Material properties

• ultra-small grains• amorphous materials

– Multilayer structures• registration• alignment

x 1/100

Making Nanostructures (< 100 nm)Pattern Electron beam, edge lithography, scanning probe

Master

Replica

Multi-level Device

Replicate

Patterntransfer

Conventional Lithography

Directed Assembly

Scanning Probe Lithography

Nanofabrication Techniques

Contact Nanolithography

Photolithography roadmap

W. Wakamiya (Selete)

Planar Fabrication Processes

Etch Grow Dope Lift-off

Expose

PATTERN TRANSFER

LITHOGRAPHY

Process Flow in Photolithography

Evaporation and Lift-off

T. Odom et al. Langmuir, 18, 5314 (2002)

Electron Beam Lithography

Paul Scherrer Inst

L. Sohn (Princeton)

E-beam Patterning of PMMA

250 nm

Whitesides group (Harvard)

Focused Ion Beam (FIB)

www.fibics.com

Next Generation Lithography

Bell labs

SCattering with Angular Limitation Projection Electron-Beam Lithography

100 keV

www1.bell-labs.com/project/SCALPEL/sot.html

Extreme Ultraviolet Lithography (EUV)

• Vacuum operation• 11-14 nm light• Reflective coatings on

optics and masks

www-als.lbl.gov/als/science/ sci_archive/53euv.html

Directed Assembly

Nano-Imprint Lithography

Nanonex

patterned resist

S. Chou (Princeton)

Laser-assisted Direct Imprinting

S. Chou (Princeton), Nature 417, 835 (2002)

Laser-assisted Embossing

Silicon nanowells

500 nm

100-nm sphere mold

500 nm

nanosphere mold

J.E. Barton and T.W. Odom, Nano Letters 4, 1525 (2004)

Soft Lithographic Techniques

Near field lithographyMicrotransfer molding(μTM)

Micromolding in capillaries (MIMIC)

Microcontact printing(μCP)

Micromolding Techniques

G.M. Whitesides, Angewandte Chemie 37, 550-575 (1998)

Replica Molding (REM)

Microtransfer Molding (μTM)

Micromolding in Capillaries (MIMIC)

MIMIC of Systems in Solvent

Extending Soft lithography to Sub-1000 nm

T. Odom et al. Langmuir, 18, 5314 (2002)

Patterning using Edge Techniques

Controlled overetchingTopographically directedetching

Topographically directed photolithography

Near field photolithography

hυ hυ

Near Field Photolithography

G. Whitesides, Appl. Phys. Lett. 70, 2658 (1997) T. Odom et al. Langmuir 18, 5314 (2002)

Topographically Directed Photolithography

elastomericmold

photoresist

solvent

embossedpattern

photoresist

Pattern transfer

G.M. Whitesides, Appl. Phys. Lett., 73, 2893 (1998)

Topographically Directed Etching

< 50 nm

~50 nm

G. Whitesides, JACS 121, 8356 (1999)

Controlled Overetching

T. Odom et al. JACS 124, 12112 (2002); G. Whitesides, Adv. Mat. 13, 604 (2001)

Directed Assembly

Intro to Scanning Probe Lithography• High Resolution Patterning Using Scanning Probes

– Chemical and molecular patterning (DPN)– Mechanical patterning

• Scratching • Nanoindentation

– Local heating– Voltage bias application

• Field Enhanced Oxidation (of silicon or metals) • Electron exposure of resist materials

– Manipulation of nanostructures • Factors Important in Scanning Probe Lithography

– Resolution– Alignment accuracy– Reliability – Throughput

AFM: General Overview

Optical Detection of Cantilever Changes

(A+B)-(C+D)

A BC D

(A+C)-(B+D)

Scanning Probe Lithography (AFM)Mechanical scratching

C. Lieber (Harvard), Science 257, 375 (1992); H. Dai (Stanford), APL 11, 1508 (1998) C.F. Quate (Stanford), J. of App. Phys. 70, 2725 (1991); www.nanoscience.de/group_r/mfm

Chemical modification

Electrostatic writing Magnetic writing

Molecular writing

Dip-Pen Nanolithography (DPN)

http://www.chem.northwestern.edu/~mkngrp/timeref.html#dpn

AFM Lithography Scratching• Material is removed from the substrate leaving deep

trenches with the characteristic shape of the plough used • The advantages of nanoscratching for lithography

– Precision of alignment– The absence of additional processing steps, such as etching the

substrate. • Depending on the applied load, AFM can characterize

micro-wear processes silicon for magnetic head sliders, polymers for electronic packaging and liquid crystals displays.

http://www.ntmdt.ru/SPM-Techniques/Lithographies/AFM_Lithography_-_Scratching_mode51.html

Nanoindentation and Scratching

Diamond-like Carbon (DLC)

15, 20, 25 μN

1.5 nm deep at 4.4 μN

Millipede: Data Storage in a Polymer• Tips are brought into contact

with a thin polymer film • Bits are written by heating a

resistor built into the cantilever to a temperature of ~ 400 C. The hot tip softens the polymer and briefly sinks into it, generating an indentation.

• For reading, the resistor is operated at lower temperature, ~300 C. When the tip drops into an indentation, the resistor is cooled by the resulting better heat transport, and a measurable change in resistance occurs.

• The 1,024-tip experiment achieved an areal density of 200 Gb/in2

http://www.research.ibm.com/resources/news/20020611_millipede.shtml

Electric Field Enhanced Oxidation

• Voltage bias between a sharp probe tip and a sample generates an intense electric field at the tip– Oxidization of silicon– Anodization of metals

• The high field desorbs the hydrogen on the silicon surface and enables exposed silicon to oxidize in air

• Oxidation depends on humidity• Can achieve sub-50 nm feature sizes

http://www.ntmdt.ru/SPM-Techniques/Lithographies/AFM_Oxidation_Lithography_mode37.html

AFM Nanolithography

Digital Instruments

Parallel Field Enhanced Oxidation

Oxidation of silicon with 50 probe tips. Probes are spaced by 200 μm.http://www.stanford.edu/group/quate_group/LithoFrame.html

Typical scan area of commercial AFMs.

Electron Exposure of Organic Polymers

• Electron Exposure of Resist– When a conducting tip is biased negatively with respect to a

sample, electrons are field-emitted from the tip• Exposure Using Scanning Probes

– Polymers have low threshold voltage, high sensitivity, sub-100-nm resolution, and good dry etch resistance.

– Resist can be easily deposited on substrates– The polymer surface is soft and pliable which minimizes the

tip wear

http://www.stanford.edu/group/quate_group/LithoFrame.html

Patterns in Resist Transferred to Si• Feature sizes of patterns written determined by the

exposure dose • Can be accurately controlled down to about 25 nm in

50-100 nm thick resist

Non Contact AFM Lithography• Silicon probe tip acts as a source of electrons• The field emission current from the tip is used as the

feedback signal to control the tip-sample spacing

AFM Manipulation of Polystyrene

Digital Instruments

Tip Direction

STM Lithography• Application of voltage pulse between tip and sample• “Pushing” atoms• Advantages of STM Lithography

– Information storage devices– Nanometer patterning technique– Manipulations of big molecules and individual atoms

• Example of STM Lithography: local exposure of LB film

http://www.ntmdt.ru/SPM-Techniques/Lithographies/STM_Lithography_mode50.html

• Tunneling through a rectangular barrier

• Elastic tunneling versus inelastic tunneling– Elastic tunneling: energy of tunneling electrons conserved– Inelastic tunneling: the electron loses a quantum of energy within the tunneling

barrier

One dimensional (1D) tunneling

( )202

2m V Eκ = −

Incident wave Transmitted wave

Exponential decay

ikzI Ae−Ψ =

zII Be κ−Ψ =

'ik zIII Ce−Ψ =

STM: General Overview

CurrentAmplifier

FeedbackControl

PositionControl

Tip Atoms

~30 nm

Surface AtomsBias Voltage

Tip Path

PiezoelectricTransducer

Constant Current ModeSI

• Δ Z(x,y): Constant integrated DOS• If surface atoms have identical p(E),

contour of atomic corrugation• Spatial resolution depends on

• Status of tip• Electronic properties of sample• Applied bias voltage

Constant Height ModeSI

• Δ I(x,y): Variation of DOS at fixed height• High contrast and fast scanning• Improved performance

• Insensitive to low frequency mechanical vibrations and electronic noise

• Piezoelectric hysteresis reduced

STM: Manipulation of Atoms

STM Manipulation of Atoms

M. Crommie (UC Berkeley), Science 262, 218 (1993)

Feedback Controlled Lithography (FCL)• FCL monitors the STM feedback signal and the tunneling

current during patterning– Terminates the patterning process when a bond is broken– Controlled doses of electrons can be written over an area to

remove hydrogen atoms and create templates of individual dangling bonds

• Examples of organic molecules patterned on Si:H (100) surfaces: norbornadiene, copper phthalocyanine and C60

Mark C. Hersam, Northwestern University

Directed Assembly

Nanosphere Lithography

Ag Nanoparticles

Colloidal crystal mask

R. P. van Duyne (Northwestern)

Organic Multi-layers as Molecular Rulers

P. Weiss (Penn State), Science 291, 1019 (2001)

Size Reduction Lithography

G.A. Somorjai (UC Berkeley), J. Phys. Chem. B 107, 3340 (2003)

Summary of Nanolithography Strategies• Photons

– UV, DUV, EUV, x-rays– Diffraction, depth of focus

• Particles– Electrons and ions– Writing is serial, writing area is small

• Physical contact– Printing, molding, embossing– Adhesion of mold and replica, pattern transfer element

• Edge-based technologies– Near field phase shifting lithography, topographic methods– Diffraction

• Deposition– Shadow evaporation– Low flexibility in fabricating masks

• Self-assembly– Surfactant systems, block copolymers– Control over order, density of defects G. Whitesides (Harvard), Chem. Rev. 99, 1823 (1999)

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