Hairpin resonator probe measurements in RF plasmas

This work investigates the use of hairpin probes in plasma where RF plasma potential is present. The microwave resonance of the hairpin is used to determine electron density. Two types of hairpin probe were used. One type was dc coupled: its dc potential could be varied while monitoring its resonance frequency and collected current. The other probe was designed to be fully floating, being (dc) isolated from ground and able to float with RF variations in the plasma potential. Additional measurements of the RF plasma potential and its effect on the dc floating potential of the former probe were made using a wire loop probe. The resonant frequency of the dc coupled probe at zero current (nominal floating potential) was less than that determined from the fully floating probe. This is attributed to the wider sheath around the former caused by RF plasma potential across it. The presence of the electron-free sheath around the wires of the hairpin is included in the analysis that links the resonant frequency to the electron density in the bulk plasma. When the dc coupled probe was biased at the true floating potential (determined from independent loop probe measurements) its resonant frequency was closer to, though still consistently higher than, that of the floating probe. This work shows that RF potential across the probe sheath affects the resonance of a hairpin probe and should be accounted for when using hairpin probes in discharges where RF plasma potential variations are even as low as a few times the electron temperature (in volts)

ICP diagnostics with magnetic probes

Magnetic probe (B-dot) diagnostics of inductively coupled plasmas are reviewed in this lecture. The diagnostic fundamentals, the relationships between the measured B-dot signals and the rf electric fields and currents in an ICP, the design of B-dot experiment and the results of B-dot measurements carried out over a wide range of ICP parameters are discussed here. It is shown that commonly used B-dot probes enclosed in a dielectric shell significantly disturb the measured B-dot signals and may lead to erroneous results. Some new results on the electromagnetic field and rf current distribution measured in a cylindrical ICP with a flat inductor coil are discussed in terms of anomalous skin effect