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Nanolithography

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To go far beyond the Raleigh limit of optical processes, other probes must be applied. By definition a nanostructure is one with nanometer dimensions, but in practice ... – PowerPoint PPT presentation

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Title: Nanolithography


1
Nanolithography
  • Lecture 15
  • G.J. Mankey
  • gmankey_at_mint.ua.edu

2
Writing and Printing
  • To go far beyond the Raleigh limit of optical
    processes, other probes must be applied.
  • By definition a nanostructure is one with
    nanometer dimensions, but in practice true
    nanoscience implies a paradigm shift of the
    lithography process to below 100 nm.
  • Sub 100-nanometer writing and printing requires
    the use of x-rays, electron beams, focused ion
    beams, atomic force probe tips, or a nanoimprint
    mask.
  • Printing processes are x-ray lithography and
    nanoimprint lithography.
  • The writing processes all involve scanning the
    probe across the surface writing a single spot at
    a time and are thus time consuming and expensive.

3
Why Nano?
  • Semiconductors and magnetic storage devices will
    get smaller.
  • A current bit on a hard drive is 300 nm by 100
    nm.
  • The future evolution of this technology will
    require nanoscale readers, writers and possible
    media.
  • To be marketable, these devices must be
    inexpensive and reliable.
  • The challenge for physics is not only to make it
    possible, but routine!

ref W. Eberhardt in Frontiers in Surface and
Interface Science (2002).
4
Resolution Limit of e-beams
  • The wavelength of an electron is
  • The diffraction limited beam has a diameter d
    0.6l/a, where a is the beam divergence (0.01).
  • A 10 kV beam has l 0.01 nm and this simple
    analysis gives a 1 nm spot.
  • In general, the spot sizes are added in
    quadrature, as shown in the diagram.

ref Handbook of Microlithography, Micromachining
and Microfabrication, SPIE (1997)
5
Electron-Solid Interactions
  • Increasing electron beam energy arbitrarily will
    not result in a smaller spot in the resist.
  • Higher energy electrons produce more secondary
    electrons which travel through the solid and
    expose a larger volume of resist.
  • This is similar to photoelectron blur in x-ray
    lithography which ultimately puts a limit on the
    smallest achievable feature.
  • Features 10 nm are theoretically possible, but
    rarely demonstrated.

ref Handbook of Microlithography, Micromachining
and Microfabrication, SPIE (1997)
6
Two Layer Resist Processing
  • The best results of e-beam have been achieved for
    making a pattern of a single layer of evaporated
    or sputtered material.
  • The two-layer process produces an undercut
    structure which functions as a shadow mask for
    the deposition process.
  • After deposition from a collimated source, the
    resist is removed with a solvent.

ref Handbook of Microlithography, Micromachining
and Microfabrication, SPIE (1997)
7
Resist Contrast
High Contrast
Low Contrast
  • Resist contrast determines sensitivity to process
    parameters such as exposure time and resist
    thickness.
  • Higher contrast resist usually has a more
    vertical sidewall angle.
  • The contrast is defined as

D1
Film Retention ()
D2
Log (Electron Dose)
ref Handbook of Microlithography, Micromachining
and Microfabrication, SPIE (1997)
8
Dot Arrays Fabricated by EBL
  • AFM images of dot arrays of different period
    fabricated by e-beam lithography, followed by
    thermal evaporation and lift-off.
  • The pitch, P, is varied systematically while the
    raster rate and beam current are constant.
  • Under the experimental conditions, uniform arrays
    are obtained when the period is greater than
    45nm. The density of the dots is up to 300
    Gdot/in2.
  • Image size is one square micrometer.

9
X-Ray Lithography
  • X-rays are produced by synchrotron radiation from
    a bending magnet in a high energy electron
    storage ring.
  • Resist, usually PMMA, is exposed through an x-ray
    mask in proximity or in contact with the wafer.
  • This technique has the advantage that very high
    aspect ratios can be achieved, since the x-rays
    are highly penetrating.

ref Handbook of Microlithography, Micromachining
and Microfabrication, SPIE (1997)
10
Resolution Limit of XRL
  • The wavelength of an x-ray is given by l
    1.2/E(keV), so one would expect higher energies
    to be more desirable.
  • Blur, caused by the excitation of photoelectrons
    and the associated shower of secondary electrons
    limits the patterning ability to about 20 nm.
  • XRL has been applied to MEMS, where the high
    aspect ratio is a distinct advantage.

ref Handbook of Microlithography, Micromachining
and Microfabrication, SPIE (1997)
11
AFM Lithography
  • The AFM tip is used to "bulldoze" through a top
    layer of resist.
  • Submersion in developer produces an undercut in
    the lower layer of resist.
  • The top layer then serves as a shadow mask for
    deposition.
  • After removal of resist, the deposited
    nanostructure remains.
  • For nanocontact printing, the initial structuring
    is performed in parallel with a contact mask.

Undercut
Developing
AFM Bulldozing
Depositing
Lift Off
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