A new structure for RF-compensated Langmuir probes with external filters tunable in the absence of plasma

Abstract

A new Langmuir probe structure using externally placed filters that can be tuned in the absence of plasma is proposed. The probe design and tuning procedure take into account especially the change in the probe's environment when plasma is turned on, thereby ensuring that the filters do not become detuned in the presence of plasma. Measurement of the RF voltage amplitudes in RF plasma using a calibrated capacitive probe gave, respectively, ≈15.7 V, ≈4.1 V, ≈2.1 V and ≈0.5 V at the fundamental frequency (≈13.56 MHz), the second, third and the fourth harmonics; based on these values a three-stage filter was built at the fundamental and the second and third harmonics. A complete analysis of the probe including its stray capacitance, RF equivalent circuit, filter and plasma impedance has been carried out, from which the maximum RF sheath voltage drop could be estimated as ≈0.27 V, ≈0.34 V and ≈1.30 V at the fundamental, the second harmonic and the third harmonic, respectively; the drop for the latter is somewhat large because of unexpectedly high loss in the filter components at the higher frequencies. I–V characteristics presented show that the floating potential of the probe decreases by ≈60 V, as the probe is detuned progressively from its tuned condition; also, the electron temperature increases from ≈1.7 to 3.5 eV with progressive detuning. It is worth noting here that although the method of calibrating the capacitive probe in this work is accurate for moderately high plasma densities (RF skin depth in plasma (= δs) small in comparison with the plasma dimension, L) its accuracy for lower densities (δs ∼ L) is not too certain. Therefore, although the probe itself can be used at low plasma densities, verifying its efficacy for such cases could be difficult.