Leveraging small scale electron density oscillations in RF plasmas to simplify hairpin resonator probe measurements

Summary form only given. Hairpin resonator probes have several design criteria that must be met in order for them to be an effective plasma diagnostic; one of the most important criteria is that they have a very high quality factor, typically Q > 100 or better.1 This criteria is so that the measured resonance can be resolved despite the presence of other chamber resonances and is able to be resolved when loading effects from secondary resonators, electron collisions, and transmission line poles contribute to the electrical characteristics of the probe. Even at these high Q-factors extraction of the resonant frequency can be difficult, particularly for larger industrial systems. An analysis method for low pressure RF driven plasmas will be presented that can greatly simplify the extraction of resonant frequency data for a modestly high Q resonator. By leveraging the large δZ/δω of the high Q circuit compared to the lower δZ/δω characteristics of interfering influences such as transmission line effects and chamber loading, the small oscillations in resonance frequency generated by density oscillations on the RF time scale are used to more easily differentiate resonance probe characteristics from other masking electrical characteristics. Results will be presented for experiments conducted on a 300mm inductively coupled plasma reactor used for semiconductor processing. Both parametric study of electron density as a function of process parameters and comparison to other diagnostics such as ion flux probes and field measurements will be presented. Additionally, standard reflection and transmission measurement techniques will be compared side by side with the δZ/δω measurement technique outlined here to demonstrate the improvement in resonant frequency identificaton and extraction over a large process window.