Computer Simulation of Sputtering

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Computer Simulation of Sputtering
Collision Cascadesin Ionic Materials
D Ramasawmy, S D Kenny and Roger
Smith Department of Mathematical Sciences
Email madr_at_lboro.ac.uk
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Outline

• Sputtering Definition, History Applications.
• Computer Simulation of Sputtering of NaCl by
  impact with a Na ion.
• Use of DPMTA code Order (N) to evaluate
  Coulombic interactions.
• Collision Cascades in NaCl.

DPMTA http//www.ee.duke.edu/Research/SciComp
/Docs/Dpmta/dpmta.html
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Sputtering
Definition

• Sputtering is the removal of surface atoms due to
  energetic particle bombardment.
• This is caused by collisions between the incoming
  particles and the atoms in the near surface
  layers of a solid.

History

• The first recorded observation of sputtering was
  made by W R Grove 150 years ago.
  

Applications

• Sputtering is not just an unwanted effect which
  destroys cathodes and contaminates plasmas.
• It is used in many modern industrial processes
  including surface cleaning and etching, thin film
  deposition, surface and surface layer analysis.

W R Grove, Philosophical Transactions, vol
142, page 87, 1852.
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MD Methodology

• Target a NaCl lattice
• System size 1944 particles
• Impact particle a Na ion at normal incidence
  with energy of 1 KeV
• Fixed boundary conditions were taken along the
  sides while the top and bottom surfaces were
  free.
• Several hundreds of trajectories with run-time
  of 2.0 ps were carried out for different impact
  positions to yield a good statistics.
• A particle is considered sputtered if it is
  moving away from the surface and it has
  sufficient KE to overcome the electrostatic
  attraction.

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MD Methodology

• The electrostatic interactions were evaluated
  using a brute force method
• The potential used was that of the Buckingham
  form as given by Catlow et al.
• This potential as given was not suitable for
  modelling collisional phenomena.
• The Na - Cl- potential was hardened using a
  screened coulomb potential to overcome the
  over attractive forces for small separation.

C.R.A. Catlow, K.M. Diller and M.J.Norgett, J.
Phys. C Solid State Phys., 10, 1394 (1977).
D. Ramasawmy, S.D. Kenny, Roger Smith, NIMB
(2002).
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Simulation

• Due to the symmetry of the (100) surface of the
  NaCl lattice, we have considered only one quarter
  of the area of the surface unit cell.

Cl- ion
From the results, sputtering was observed to
occur only for impacts concentrated around the
Na ion and the Cl- ion. Furthermore, for the
majority of impacts outside these regions,
channelling was observed.
Reduced Impact Zone
Na ion
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Results

• The overall sputtering yield was determined to
  be 0.36 with a variance of 0.01.
• The total no of sputtered particles was almost
  evenly distributed between the 2 species 51
  Na 49 Cl-.
• A lower yield of sputtered particles was observed
  compared to similar impacts on metals.
• The sputtered particles were classified into
  groups according to their kinetic energies and
  atomic types.
• More low energy particles were ejected compared
  from metals and semi-conductors.
• The origin of the ejected particles is summarised
  in the table. It shows a substantial contribution
  from subsurface layers.

76.5
6.5
3.3
3.5
4th layer
4.1
6.0
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Further Results

• The angular distributions show less structure
  representative than is typical for sputtering
  from metals and semi-conductors.
• The majority of trajectories lead to only a small
  number of sputtered particles.
• The ions are often seen to come off as NaCl
  dimers.

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Movie of Sputtering

• Example of a Computer Simulation of the
  Sputtering of NaCl by impact with a Na ion with
  1 KeV at normal incidence.

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Discussion

• We have observed that ionic materials show a
  number of characteristic differences from metals
  and semi-conductors. They are as follows
• a) Lower ejection yields
• b) Larger contribution from subsurface layers
• c) Less well-defined angular distributions
• d) Large number of low energy ejected
  particles.
• There is a number of features that warrant
  further investigation, namely the effect of
  bombarding species, the crystal size and cluster
  formation.

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DPMTA

• DPMTA (a Distributed Implementation of the
  Parallel Multipole Tree Algorithm) code developed
  at Duke University was implemented within our MD
  code.
• DPMTA is based on the FMM (the Fast Multipole
  Method) and was originally developed by Greengard
  and Rokhlin.
• This method is O(N) meaning that it is faster
  compared with the crude method which is O(N2)
  and which we used in our initial study.
• We are now simulating bigger system sizes and
  this will enable us to study sputtering,
  collision cascades and other effects in more
  detail.

DPMTA http//www.ee.duke.edu/Research/SciComp/
Docs/Dpmta/dpmta.html L. Greengard, V.
Rokhlin, J. Comp. Phys. 82 (1997) 135
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Collision Cascades
We are currently doing some test simulations on
Collision Cascades. Below is one example in which
a Cl- ion about the centre of a NaCl lattice is
given 250 eV along a certain direction.
Temperature 0 K System Size 5832 particles.

• Legend
• Colours of Spheres
• Blue / Purple Interstitial (Cl- ion)
• Red Interstitial (Na ion)
• Brown Vacancy (Cl- ion)
• Grey Vacancy (Na ion)

Acknowledgements H Hurchand ( Collision Cascades
)