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Sputter deposition for thin films

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Other application notes
Sputter deposition of thin films

the sputtering process - ion bombardment with ejection of 'sputtered' atoms. The sputtering process is
  • hit a surface with an ion (energy more than say 100eV).
  • this ion creates a cascade of collisions in the surface.
  • multiple collisions eject (or 'sputter') atoms from the surface.
The number of atoms ejected per incoming ion is the sputter yield. A target material with a high sputter yield (say silver) will sputter much faster than a material with a low sputter yield (say Al2O3).

Sputter yields
(for 500 eV Ar ions)
material sputter yield
Silver 2.80
Copper 2.00
Gold 1.70
Al 1.00
Si 0.50
SiO2 0.23
Al2O3 0.05

The sputter yield depends on :-

  • The target atom species
  • The incoming ion
  • The energy of the incoming ion
The yield varies widely between different target atoms. As the table shows compounds tend to sputter much more slowly than elements.

Sputtering is a relatively high energy process, the sputtered atoms leave with several eV (much higher than evaporated atoms which have around 0.1eV). This high energy is very good for the thin film growth process. It leads to hard dense films.

In ion beam sputtering the process is just as we describe above, an ion beam hitting a target, these ions sputter atoms from the target, these sputtered atoms cross the vacuum and form a growing thin film.

Magnetron sputter sources showing plasma above the sputter target In magnetron sputtering the source of ions is a plasma above the target. A large negative voltage from the sputter power suppply is applied to the target surface. Positive ions from the plasma are accelerated into the target surface by this applied voltage. The ions hitting the surface :-

  1. sputter the target atoms out to form a thin film
  2. release secondary electrons to sustain the plasma
The trick in a magnetron is the use of a magnetic field to trap these secondary electrons. This allows the magnetron to
  1. run at lower pressure (keeping arrival energies high and giving good thin film microstructure).
  2. have a high plasma density and so give better sputter deposition rates.