Sputter Deposition

When an energetic particle strikes a surface (the target), a plume of material is released, like the shower of sand when a golf ball lands in the bunker. This effect is known as 'sputtering' and is used to produce films of materials as thin as just a few millionths of a millimetre. The source of the ions might either be a local plasma (diode or planar magnetron sputtering) or a separate ion beam source (ion beam deposition).

Diode Sputtering

If a high negative potential difference (~1000V) is applied between the target and the substrate in a rarefied Argon environment, electrons released from the target collide with Argon atoms and ionise them, giving them a positive charge. These are then accelerated towards the target and strike it at high energy, releasing target material. However, diode sputtering requires a relatively high process pressure because the electrons follow a short, direct route to the anode (the substrate) so the probability of any given electron striking a given Argon atom (the collisional cross section) is relatively low, and many gas atoms are needed to increase this. This in turn means that sputtered material goes through many collisions with gas atoms, greatly reducing the amount actually reaching the substrate - deposition rates are low and much of the target material coats the system rather than the substrate but coverage of uneven surfaces can be highly uniform.

Planar Magnetron Sputtering

If a strong mangetic field is applied the electrons follow a spiral path around the field lines giving them a much longer path length before being absorbed into a surface. This greatly increases their chance of striking and ionising an Argon atom and therefore gives the same ion density at a much lower pressure than for diode sputtering. If the ends of the magnetic field lines are at the cathode then the electrons will continue to bounce back and forward almost indefinitely. In planar magnetron sputtering a strong toroidal magnetic field confines the electrons in this way (leading to the distinctive toroidal plasma 'halo', right) and deposition at pressures as low as one millionth of an atmosphere. This allows for much higher deposition rates and more efficient usage of released material, but the toroidal plasma only erodes a 'race-track' in the target and little of the material near the centre or around the circumference can be used.

RF, MF and Pulsed Sputter Deposition

When the Argon ion strikes the target an electron is released from the surface and combines with the ion to neutralise it, returning it to the vacuum as an Argon atom. If the target material is dielectric this process rapidly casues a charge build-up at the surface until Argon ions are no longer attracted, electrons are no longer released and the plasma extinguishes. To sputter non-conducting materials it is therefore necessary to apply AC or pulsed power to the target whereby the ion charge, which has built up on the surface, is expelled during the positive or neutral phase. Because the power supply is only negative half the time, rates are lower than for DC sputtering whilst power supplies are more complex and therefore more expensive.

Ion Beam Deposition

Alternatively, if the source of ions is not local to the target but is rather a neutralised beam, the electrical characteristics of the target are no longer of importance. In this technique, known as ion beam deposition (IBD) ions are extracted from a separate ion source to which the Argon is fed directly. The vacuum chamber pressure is therefore lower than that in the ion source allowing very low deposition pressures. IBD is often used in UHV systems and for ultra-clean applications. It also has the advantage that the energy with which the ions impact the target can be varied independently of other system characteristics such as ion density - both energy and rate can affect the characteristics of the film grown.

Oxford Vacuum Science manufactures bench-top research and light industrial thin film deposition systems incorporating sputter deposition. For more details contact technology@oxford-vacuum.com or click on the contact link.


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