OBSERVATION OF RADIOFREQUENCY DISCHARGES AT VARIOUS DRIVING FREQUENCIES

Radio-frequency discharges at various driving frequencies f in the range of 1.9-28 MHz are observed, especially from the viewpoint of the sheath structure. The results reveal that for f > 10 MHz, the sheath thickness Z(s) measured by an emissive probe varies directly as f-1, while for f < 10 MHz, Z(s) is-proportional-to f-1/2. The electron density estimated from the ion saturation current of a needle probe is almost proportional to f2 in the former case. High-energy electrons accelerated by the expanding rf sheath can contribute to sustaining rf plasmas in the high-frequency region, whereas in the low-frequency region, secondary electrons emitted from an electrode can contribute to it in a similar manner to dc and ac discharges.