PURPOSE:To enhance reproducibility of a measured plasma parameter by weighting, to a plasma parameter measuring circuit, an ac voltage detected by a plasma potentiometer connected to a plasma parameter measuring means. CONSTITUTION:A high frequency voltage detecting circuit 9 connected to a capacitive coupling probe measures a plasma potential, and an amplifier 11 amplifies only an ac component to weight it to a plasma parameter measuring circuit. An ammeter 7 measures a current value flowing into the probe 5 at the time of application of a bias voltage between the plasma and the probe 5. The resultant voltage/current characteristic shows an ideal curve without any roughness. When the bias voltage applied between the plasma and the probe 5 is zero, the current value flowing into the probe 5 becomes theoretically zero.
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