In a inductively or capacitively coupled radio frequency discharge by a single frequency, the transition voltage
Vc or power
Pc from glow to arc discharge depends on the gas pressure
p. When superposing two applied fields with different frequencies at the same time, if
Vc decreases comparing with the value of single-frequency discharge, r.f. discharge plasma with high ionization state can be easily generated.
In a medium vacuum capacitively coupled r.f. discharge, effects of superposing two applied fields with different frequencies, that is, 2 MHz and 13.5 MHz have been studied. As a result,
Vc of 2 MHz side has decreased by the superposed effects in the low pressure region (ω>ν), but
Vc has not decreased in the high pressure region (ω<ν). Variations of
Pc by the superposed effects in the discharge space of 13.5 MHz side have not been able to observe in all pressure region.
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