Lithography
NANO 101
Introduction to Nanotechnology
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“Photoengraving”
- Transfer pattern into reactive polymer film (“resist”)
- Use resist to replicate pattern into thin film or substrate
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Photolithography
Electron beam lithography
X-ray lithography
Focused ion beam lithography
Lithography
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• “Printing”
• Transfer of the pattern using an optical technique
• Resist = photoresist; photoactive polymer, positive or
negative
1. Coat substrate with resist
2. Mask; expose with light
3. Develop (dissolve exposed
OR unexposed areas with
chemicals)
4. Etch unprotected areas or
deposit layer of metal
5. Strip resist
Photolithography
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• Maximum resolution (minimum size) of individual
features limited by diffraction
Diffraction:
• bending of light
• around an edge
• through a slit
• past an object (edges)
• direction changes
• wavelength, frequency stay
the same
Photolithography
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Photolithography
Maximum resolution:
2bmin
d

 3  s  
2

λ
b
s
d
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Three Modes of Photolithography
http://www.ece.gatech.edu/research/labs/vc/theory/photolith.html
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Photolithography: Shadow Printing
• Contact mode:
• s=0
• Best resolution; near 100% accuracy
• Maximum resolution rarely achieved
• Substrate and resist film rarely completely uniform
• Proximity mode:
• Small gap between mask and substrate
• Need extremely flat substrates and resist films
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Photolithography: Projection Printing
Resolution
• Worse resolution than shadow printing
• Lens imperfections
• Increased diffraction
• type of resist material
• optical system (apertures)
• exposure wavelength (λ)
• Best resolution ~ λ/2
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Radiation with smaller wavelengths
• Deep Ultra-Violet (DUV): λ < 300 nm
• Need to use special lasers
• Minimum pattern size ~ 100 nm
• Extreme UV (EUV): λ = 11-13 nm
• Minimum pattern size ~ 60 nm
• Strong absorption of light by lenses
• Low reflectance of light by mirrors
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http://www.lcse.umn.edu/specs/labs/images/spectrum.gif
Current State of EUV
• Sustained 100W average source power
• 1,000 wafers processed in 24 hrs
http://www.extremetech.com/computing/199782-tmsc-announces-lithographymilestone-as-euv-moves-closer-to-production
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Other Options:
• ArF laser (193
nm) is current
state of art
Diffraction limit
n = index of
refraction
n air : 1.0
n water: 1.3
– Immersion
(change
refractive
index)
– Double
Patterning
http://www.extremetech.com/computin
g/190845-intel-forges-ahead-to-7nm11
without-the-use-of-euv-lasers
Double Patterning
http://willson.cm.utexas.edu/Research/Sub_Files/DoubleExposure/index.php
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X-Ray Lithography
• X-Rays
• 0.04 nm < λ < 0.4 nm
• System
• Mask
• X-ray absorbing material
pattern on a thin X-ray
transparent material
• X-ray source
• Bright enough in necessary
wavelength range
• Expensive
• X-ray sensitive material
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http://www.camd.lsu.edu/microfabrication/latech.htm
LIGA
• Lithography, Electroplating, Molding
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http://sylmand.info/features/x-ray-lithography/
Electron Beam Lithography
E-beam:
– Finely focused beam of electrons (few nm dia.)
– Electrons deflected accurately and precisely to
“write” pattern without mask
Resolution
– Diffraction not an issue
• λ < 1 Å (0.1 nm)
– Scattering
• Forward (in resist layer)
• Backwards (substrate)
http://nanotechweb.org/cws/article/tech/18642
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Electron Beam Lithography
System
• Electron source (gun)
• Electron column (forms beam)
• Mechanical stage
• Control Computer
https://smif.lab.duke.edu/pict.htm
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Focused Ion Beam (FIB) Lithography
• Ions scatter much less than electrons
• Sources:
– Liquid metal ions (Ga; Au-Si-Be alloys)
– Long lifetime, high stability
• Resolution
– sub-µm dimensions (~250 nm)
– High resist exposure sensitivity
– Negligible ion scattering in resist
– Low back scattering from substrate
• Extensive substrate damage
• Also used for etching, deposition, and doping
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Focused Ion Beam (FIB) Applications
• Etching
– Physical sputtering etching
• Bombard areas to be etched with energetic ion beams
• Simple, applicable to any sample material
– Chemical etching
• Chemical reactions between substrate surface and gas
molecules adsorbed onto surface
• Increased etching rate, little residual damage
• Deposition
– Direction deposition (low energy ions)
– Chemical-assisted deposition
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FIB – etching/deposition
Nano Factory Achieved by Focused Ion Beam
Toshiaki Fujii, and Takashi Kaito, Microsc Microanal 11(Suppl 2), 2005 - See more at:
http://glia.ca/meanderings-wordpress/focus#sthash.06BIxsK8.dpuf
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FIB - Deposition
• Deposition of Pt on Al substrate to form micrograting for measuring material deformation
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Soft Lithography
• Alternative to photolithography
• Cheaper / more flexible
• Printing of Self-Assembled Monolayers (SAMs)
• Molding of liquid precursors
Techniques:
• Microcontact printing
• Nanoimprint
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Microcontact Printing
• “Stamp” made
– Pour liquid polymer into a mold to make a “stamp”
– Mold often made by photolithography
• “Ink” the stamp
– Dip into solution so SAM formed on surface of stamp
• Stamp the substrate
– Place the inked stamp on a substrate
– SAM transferred to substrate in specific pattern
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Microcontact Printing
Voskuhl, J., Wendeln, C., Versluis, F., Fritz, E.-C., Roling, O., Zope,
H., Schulz, C., Rinnen, S., Arlinghaus, H. F., Ravoo, B. J. and Kros,
A. (2012), Immobilization of Liposomes and Vesicles on Patterned
Surfaces by a Peptide Coiled-Coil Binding Motif . Angew. Chem.
Int. Ed., 51: 12616–12620. doi: 10.1002/anie.201204836
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Nanoimprint
1. Make template
2. Coat substrate with polymer
3. Press stamp into polymer at high temperature; polymer deforms
4. Cool polymer and pull stamp away
5. Polymer can be then be etched or used as is
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D.R. Hines et al., Appl. Phys. Lett. 86 (16), 163101 (2005).
Nanomanipulation and
Nanolithography
Based on Scanning Probe Microscopy (SPM) techniques
- can be used for molecular manipulation
Types of Scanning Probe Microscopy (SPM)
• Scanning Tunneling Microscopy (STM)
– Electrically conducting materials
• Atomic Force Microscopy (AFM)
– Dielectric (insulating) materials
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• STM with Tungsten tip
• Placed Xenon atoms on surface
• UHV and low temperature
– Clean environment and surface
– Absence of thermal diffusion on surface
Nanomanipulation
D.M. Eigler and E.K. Schweizer, Nature 344, 524 (1990).
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The World’s Smallest Movie
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Nanomanipulation with AFM
• Perpendicular Processes: atoms lifted, then dropped
• Parallel Processes: atoms dragged along surface
• Pushing
• Pulling
• Sliding
C. Baur, et al., Nanotechnology 9, 360 (1998).
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Advantages
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Nanoscale control in three dimensions, necessary for
atomic manipulation
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Manipulation and characterization
Disadvantages
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Small scanning area
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Slow scanning
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Tips must be high quality and consistent
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Surface must be flat and smooth
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UHV and low temperatures
SPM Nanofabrication
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• Works under ambient conditions
• Scan tip across substrate, atoms or molecules move
from AFM tip to substrate
Dip-Pen Nanolithography (DPN)
C. Mirkin, Northwestern Univ.
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Polymer Pen Lithography
• DPN + µCP
11 million pen array
http://cen.acs.org/articles/87/i12/BoronDreams.html
Science 19 September 2008:vol. 321 no. 5896 1658-1660
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Summary
• Photolithography
– Reaching size limits (diffraction, etc.)
• Soft Lithography
– Relatively new
– Fabrication of nanostructrures and nanodevices
• SPM-based techniques
– Relatively new
– Promise for using atoms and molecules as building
blocks
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