Implementation of an Actively Compensated RF-driven Probe Technique for Diagnostics of VHF RF Plasmas

Electrostatic probes are widely used for plasma discharge diagnostics. Key plasma parameters such as electron density, electron temperature, the energy probability function (EEPF), and the plasma potential are obtainable. In RF discharges, oscillations of the plasma potential and the sheath voltage near the probe result in the distortion of plasma parameters and necessitates compensation. The common techniques are the introduction of resonant filters, or active compensation of the sheath oscillations by applying a relevant RF signal to the probe. Introduction of the passive filters is limited by selected frequency of interest, furthermore the filters need to be near the probe tip, increasing the probe dimensions and complicating its design. Attempting to actively compensate the Langmuir probe via use of the reference signal from the power generator can be troublesome, as the plasma potential frequency and waveform may differentiate. We propose a "contactless" universal method for collection of the reference signal from the plasma volume by means of a separate "antenna" electrode. We demonstrate successful implementation of the method at frequencies above 100MHz in Ar and Ar/C4F8 CCP discharges. Detailed characterization of the improved EEPFs and plasma parameters will be discussed.