Reactive sputtering for compound thin films

The sputtering process is often used to deposit metal thin films. To make sputtered metal thin films we just run the sputtering process in an inert gas (usually Argon). To make a compound thin film (such as SiO₂, AlN, TiC) by reactive sputtering we add the appropriate reactive gas to the sputtering process. In a perfect world the reactive gas would react only with the growing film to make a thin film of the required compound. Unfortunately the reactions occur with the sputter target surface as well. This 'target poisoning' complicates reactive sputtering and reduces the thin film growth rate.

There are two key points in this reactive sputtering process :-

• Part of the reactive gas is going down the pumps, the rest is going into the growing thin film.
• Compounds tend to sputter more slowly than metals.

This positive feedback is seen as 'target hysteresis' - sudden jumps in the thin film deposition rate, target voltage or oxygen pressure. To successfully make compound thin films by reactive sputtering we must stay away from these sudden jumps in the process. Best performance is acheived by careful design of the system furniture, the pumping system, the magnetron and the gas delivery. The target hysteresis can be completely eliminated by the use of a feedback system controlling the reactive gas flow, the control signal can come from emission lines from the plasma, oxygen pressure measurements or target voltage.