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Vacuum for thin film deposition

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Other application notes

Vacuum for thin film deposition

There are two reasons for using a vacuum when depositing thin films

  • keeping reactive gases out of the growing film
  • keeping a high arrival energy of the depositing species
The two numbers typically used to quantify these requirements are
  • monolayer formation time
  • mean free path

monolayer formation time

This is a slightly idealized number but gives a very useful way of thinking about contamination of the growing thin film. To get the monolayer formation time we assume all of the arriving gas sticks to a surface and that it forms a neat monolayer of stuck atoms. The number we get will depend on the gas species, the pressure and the gas temperature. For Nitrogen at around 20oC the monolayer formation time is shown in the figure.

If we are growing our thin film at 1 nm/s, a typical atom is say 0.2nm diameter, so our film is growing at say 5 monolayers per second. The time taken to form a monolayer of thin film is then 0.2 seconds. To get a pure film we need to make sure that a monolayer of reactive gas takes much longer than 0.2 seconds to form. So looking at the graph a vacuum of 1 x 10-6mBar seems reasonable (monolayer formation time of 1 second). This calculation is obviously much more important for a reactive thin film material (say Ti, Al, Cr), than it is for an unreactive one (say Gold).

mean free path

The thin film microstructure depends on the arrival energy of the species forming the film. It doesn't matter if this energy is coming with the depositing species from a sputter source or being added by ion bombardment. The energetic species have to make it from the sputter source or ion gun to the substrate without losing energy. Collisions with gas atoms will cause these energetic species to lose energy. The distance between collisions will again depend on the species involved, the pressure and the gas temperature. The easiest way to handle these collisions is to use the average distance between collisions (the mean free path). For Nitrogen at around 20oC the mean free path is shown in the figure.

The pressure required will depend on how far we are trying to transport the energy. In sputtering it might only be 0.1 to 0.2m, in evaporation it might be as much as 1m. We can then see from the figure of mean free path versus pressure that to transport the flux that forms our thin film with good energy transfer we need :-

  • for sputtered thin films with a source to substrate distance of 0.1m then 1 x 10-3 mBar is sufficient,
  • for evaporated thin films with a source to substrate distance of 1m then 1 x 10-4 mBar is required.