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Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Recent Googling has resulted in the following information: (I've a much better idea of what I'm looking for now, thanks to the progress I've made here so far with this project)
Most systems operate between 1 and 50 millliTorr, with plasma being easier to light around 50 milliTorr, depending on gas used.
Apparently the real part of the load increases as power increases, and the imaginary part of the load decreases as power increases. A slight mis-match can increase the plasma density and this increase in power is delivered to the plasma (this needs some clarification, I think)
Also, someone claimed to have reverse engineered a system and discovered that the imaginary part of the load is negative for capacitatively coupled plasma and positive for inductively coupled plasmas. He also claims that the imaginary part of the load is around five times the real load, and quotes figures of 10 Ohms imaginary load for 2 Ohms of real load and 50 Ohms of imaginary load for 10 Ohms of real load.
This at first appears to contradict the point made earlier, but I appreciate these are generalisations based on specific systems.
This does give me some figures to start playing around with in QuickSmith, I'll probably get a chance to do this at some point over the Christmas period.
Hopefully be the new year I'll have a pretty good idea of where to start regarding values for the variable capacitors.
I seem to be making some real progress here. Thanks for all your help.
EDIT: I've also found reasonably reliable references for capacitatively coupled plasmas that suggest (I'm not sure about the pressures, but assume 10-15 millitorr, but may be lower) that before the plasma is lit resistance is several megohms, but after it is lit resistance drops to around 600 Ohms. These threads also refer to negative resistance which, I understand, only occurs in capacitatively coupled plasmas, not inductively coupled. I'm not sure how relevant this is to my own project. (I could run at higher pressures that would presumably involve lower resistance)
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Designing an Auto Match Unit for RF supply