Interactions of energetic particles

The creation of shallow junctions in the semiconductor industry requires the manipulation of very low energy (< 1keV) and very high current (>1mA) ion beams.

It is very difficult to transport an ion beam of high current and low velocity as the beam tends to space-charge neutralise during transport and will become severely divergent. For the currents and energies required for this application beams can only be transported a few cms. One method of circumventing this problem is to use a de-acceleration stage in the target chamber to transport the beam at a higher velocity and then slow it down just in front of the target.

Another way is to use a molecular species or cluster with a single charge, allowing a substantially reduced current to be used for the same implant - one charge many atoms implanted. One candidate is the decaborane molecule (B10H14).

Some preliminary studies show that it might penetrate more deeply than a conventional single boron ion for the same initial velocity.

Publication

Ultrashallow Junctions in Si Using Decaborane? A Molecular Dynamics Simulation Study. R.Smith, M.Shaw, R.P.Webb, M.A.Foad, Journal of Applied Physics, 83(6), (1998) 3148-3152.

Due to non-linear cascade effects from cluster impacts the sputtering yields can be very high, the highest experimentally recorded is in the tens of thousands of atoms per cluster.

A cluster tends to deposit its energy close to the surface of a target and so will tend to be more surface sensitive than a single atom sputtering event. Cluster impact can produce coordinated motion in a surface after impact and this is a more gentle process via which a large molecule maybe desorbed from a surface with the consequence that the yield of intact species is likely to be enhanced.

The cascade produced from a single atom impact is often more discrete and consequently results in fragmentation of large molecules.

Publication

Molecular Dynamics Simulation of the Cluster Impact Induced Molecular Desorption Process. R.P.Webb, M.Kerford, E.Ali, M.Dunn, L.Knowles, K.Lee, J.Mistry, F.Whitefoot, Surface and Interface Analysis, 31, (2001), 297-301.

The simulations use a Molecular Dynamics scheme with a fixed time step of 0.2fs. The simulations run typically for about 2ps (which takes about 4 hours of real time). The energy stability is good over this time scale (better than 99 per cent).

The scheme is very simple:

• Solve Newton's Laws of Motion for many bodies simultaneously.
• Strength comes from the use of realistic interaction potentials.
• Tersoff and Brenner potentials describe the bonding and many body terms.
• Ziegler-Biersack-Littmark potentials are used at close separations.
• Long range terms used (to model inter planar bonding in graphite particularly).
• Between 100,000 and 1,200,000 atoms are used in the simulations.
• Free and periodic boundaries used - best to make system large enough to avoid boundaries rather than employ complex boundary conditions.

Publication

A good general description of the techniques can be found in Atomic and Ion Collisions in Solids and at Surfaces R.Smith, M.Jakas, D.Ashworth, R.Oven, M.Bowyer, I.Chakarov, R.P.Webb, (Ed. R. Smith, Pub. Cambridge University Press) (1997).