A COMPARISON OF A PASSIVE (FILTERED) AND AN ACTIVE (DRIVEN) PROBE FOR RF PLASMA DIAGNOSTICS

Two different electrostatic probes have been tested simultaneously in the same plasma. The passive design uses a distributed inductive chain to block RF signals from the probe tip. The tip itself is capacitively coupled to a secondary ring electrode of large area, which effectively drives the probe tip potential in phase with the plasma space potential. The active design substitutes an externally generated sinusoidal signal (synchronous with the plasma excitation) of controllable phase and amplitude for the signal from the ring. Both probes were located on the mid-plane of a capacitively coupled RF plasma, but sufficiently spaced to prevent interference with one by the other. In most cases the passive probe is found to float more positive than the active one. Both probes give results in close agreement when the plasma excitation is predominantly sinusoidal. The passive probe has a broad-band response which enables its tip to follow more closely non-sinusoidal RF potentials in the plasma. The active probe is more expensive in terms of components external to the plasma chamber but is considerably simpler to construct.