Radiofrequency phase resolved measurements with the hairpin resonator probe: challenges and potential benefits*

Microwave probes have long been shown to be capable of excellent time resolution, with a couple works exploring measurements within the radiofrequency (rf) cycle of a capacitively coupled plasma using a hairpin resonator probe. Interpretation of results is not trivial due to complications arising from the possible formation of a rf sheath across the probe surface and probe spatial resolution. It is shown that both issues can be addressed by modeling the probe isolation from ground and probe electric field spatial distribution, respectively. Several recent works have explored the possibility of measuring electron temperature with microwave probes using various approaches. One approach relies the relation between effective electron neutral collision frequency and electron temperature. Results from this approach in a pulsed argon capacitively coupled plasma at 400 mTorr using a hairpin resonator probe are discussed. Limitations to this approach including electron neutral collision cross section dependence, limited ranges of applicability for certain neutral gases, and potential ambiguity with respect to electron energy distribution functions are also discussed. The time resolution capabilities for this class of probes, coupled with the ability to measure electron temperature, immediately suggests that electron heating dynamics may be investigated within the rf cycle, adding a valuable path towards understanding rf discharges at these fundamental timescales.