my group has a small vacuum chamber built for thin film co sputtering, the two guns were initially coupling but we solved that issue by spacing the guns farther apart. However, the issue with the plasma is still there. When we fire both guns, the plasma starts flickering on and off at a high frequency. We are fairly sure that it’s not due to contaminants in the chamber.
The plasma starts flickering because the reflexion coefficient is too high and the RF power goes back into the coaxial cable.
Modify the coupling between incoming RF coaxial cable and the vacuum system (use a variable capacitor with air as dielectric) to have a lower reflection coefficient.
You can also try to decrease the incoming RF power
In addition to the comment by Valentin Bogatu , how does the matching for your RF source work? Is it automatic or manually?
If it's manually, it could make sense to check if the flickering can be decreased by retuning, if it's automatic, check if the capacitors don't come to rest, maybe their positions might be correlatable to the flickering frequency.
@Jurgen Weippert The magnetron sputtering that I worked on had a variable capacitor with air as dielectric and a small coil to match the input RF impedance to the instalation impedance. This was done manually. I started RF power (3 to 5 W/ cm^2) the pressure was 10^(-1) - 10^(-3) Torr, then at some power the RF plasma was starting to flicker (to return into the feeder). There were 2 indicators, 1 indicator for incoming power, 1 indicator for the reflected power. When the plasma was flickering the reflected power was very high. Modifying the impedance of the matching coil was making a good plasma into the instalation.
1. We are seeing 0 reflected power
2. The matching is automatic and it is stable
We are now thinking that it is something with the gas flow rate. When we let the system run for an extended time, we had forgotten to open the Ar tank and the line was slowly bled dry. At some point, the system and the flowrate/pressure reached equilibrium and the flickering stopped. We are going to start testing how different gas settings alters the system when we have the lab available at the end of November.
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