Looking for single ion impacts in PMMA

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Looking for single ion impacts in PMMA Victoria
Millar David Hoxley Chris Pakes Steven
Prawer

• Why?
• Top down process utilises ion implantation for
  the placement of P ions in the
  silicon substrate. Ion implantation is
  essentially random.
• A single ion resist would be useful for locating
  the impact sites of a random implant.
• OR
• A single ion resist would allow electrical
  registration to be tested.
• verification of step and repeat fabrication of a
  qubit array.

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• Candidate Resists
• Ion irradiation is an established tool for the
  modification of the chemical structure and
  physical properties of materials.
• Need a resist with sufficient sensitivity to
  demonstrate a measurable modification due to the
  passage of a single ion.
• Candidate resists are diamond-like carbon,
  polymethylmethacrylate (PMMA), monolayer films of
  C60 and self assembled monolayers of
  Cyclooctatetraene (COT) and octadecyltrichlorosila
  ne (OTS).
• Studies tend to be restricted either to the use
  of high energy heavy ions or require particular
  environmental conditions.
• PMMA appears to be the most promising.

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• PMMA
• Ion beam modification of PMMA takes the form of
  latent chemical damage caused by secondary
  electrons.
• This damage results in an increase in solubility
  of the irradiated areas.
• Subsequent development of the irradiated sites
  results in a hole in the PMMA situated above the
  implanted ion.

50nm
PMMA
substrate

• We implanted 50 nm thick PMMA with 4 MeV 4He ions
  in an array of spots with varying ion fluence
  from 109 to 1014 ions.cm-2.

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• Imaging of the samples was done with an AFM in
  intermittent-
• contact mode.

AFM image (1?1 mm), obtained using a
conventional Si cantilever, showing a region of
PMMA irradiated with an ion fluence of 109
ions.cm-2. b) topographic profile taken from
an individual etched feature

• Difficulties in observing single ion features
  arise from the shape of the conventional
  cantilever determining the lateral resolution and
  capability of imaging high aspect ratio features.

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• Probes consisting of multi-walled nanotubes have
  been fabricated by mechanically attaching a
  bundle or a single walled nanotube to a
  conventional AFM probe.
• The geometry of a carbon nanotube with diameter
  10 nm and length of 500 nm offers extremely good
  lateral resolution in imaging features of high
  aspect ratio.

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Intermittent-contact AFM image (1?1 mm), obtained
using a carbon nanotube probe, showing a region
of PMMA irradiated with an ion fluence of 109
ions.cm-2 and a topographic profile taken from an
individual etched feature

• SUMMARY
• AFM images clearly demonstrate that the material
  properties of PMMA can be modified in a
  controlled way by ion irradiation.
• Identification of features that we ascribe to
  single ion impacts is facilitated by the use of
  carbon nanotube based images.

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• FUTURE WORK
• Use PMMA to create and array of P ions
• PMMA allows us to potentially examine the
  possibility of subsequently metalising the
  developed films to form self-aligned contacts
  above a single implanted ion.

Au gates
PMMA
Si
Si