Sputtering

If ions or atoms with energy several kiloelectronvolts bombard a solid surface, single atoms, molecules, or clusters are produced which are deposited on the surrounding wall of the vacuum chamber [36, 37]. The ions of a noble gas are mostly applied to bombard the solid surface. In standard sputtering equipment, the well-known glow discharge is used as the ion source. It forms if one puts an electric field of several hundreds or a thousand volts between two electrodes which stand separately in a vessel evacuated to about 10-1 mbar. In the glow discharge, not only neutral atoms but also ions and electron exist. The electrons are accelerated against the anode by the applied electric field and produce additional ions on the way of impact ionization. The ions are accelerated against the cathode and produce these secondary electrons which produce again so many new ions in the gas by collisional ionization that the ion loss on the cathode is compensated. This means that the discharge is self-sustaining. By the impact of positive gas ions on the cathode, the atoms of the cathode material are ejected and condense on the substrates forming the film. The particles sputtered from the cathode are more than 95% neutral and their average energy is considerably higher (5-20 eV) than that of thermally evaporated atoms (<1 eV). When applying the sputtering technique, one should bear in mind that some further processes take place:

1. the growing film is bombarded by neutral atoms of the inert gas;

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