Computer simulation of sputtering collision cascades in ionic materials
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Computer Simulation of Sputtering & Collision Cascades in Ionic Materials. D Ramasawmy*, S D Kenny and Roger Smith Department of Mathematical Sciences. * Email: madr@lboro.ac.uk. Sputtering: Definition, History & Applications.

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Computer simulation of sputtering collision cascades in ionic materials l.jpg

Computer Simulation of Sputtering & Collision Cascadesin Ionic Materials

D Ramasawmy*, S D Kenny and Roger Smith

Department of Mathematical Sciences

* Email: madr@lboro.ac.uk


Outline l.jpg

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.

Outline

‡ DPMTA : http://www.ee.duke.edu/Research/SciComp/Docs/Dpmta/dpmta.html


Sputtering l.jpg
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.


Md methodology l.jpg
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.


Md methodology5 l.jpg
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).


Simulation l.jpg
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


Results l.jpg

%

1st layer

2nd layer

3rd layer

5th layer

Reflected

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


Further results l.jpg
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.


Movie of sputtering l.jpg
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.


Discussion l.jpg

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.

Discussion


Dpmta l.jpg
DPMTA characteristic differences from metals and semi-conductors. They are as follows:

  • 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


Collision cascades l.jpg
Collision Cascades characteristic differences from metals and semi-conductors. They are as follows:

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 )