Session SR52: Plasma Diagnostic: Electrical Diagnostics II

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.   

Low-Noise Plasma Antennas - Practical Noise Power Measurements

Plasma antennas [1] have been of enduring interest to plasma scientists and RF engineers since the first US patent was issued in 1919 and early investigations in the 1960s. Plasma antenna advantages include the ability to turn it on and off, frequency and radiation pattern tunability, and reduced cross-coupling among array elements. Potential disadvantages include lower gain and efficiency due to the high ohmic resistance and high Johnson-Nyquist thermal noise [2] due to the high electron temperature in steady-state plasmas.

Recently the gain of a tunable plasma antenna was measured, and a regime where the gain is only slightly below that of a reference metallic antenna was demonstrated [3]. In this work, a concept of reducing the thermal noise of plasma antennas by sustaining the plasma with repetitive RF pulses is discussed. We also present the application of measurements borrowed from RF practice to characterize the noise at the terminals of a plasma antenna. The methods, challenges, and initial results are discussed.

[1] T. Anderson, Plasma Antennas. Artech House, 2011.

[2] T. R. Anderson, IEEE Int. Symp. Electromagn. Compat., vol. 1, pp. 498–501, 2002.

[3] V. Podolsky, A. Semnani, and S.O. Macheret, IEEE Transactions on Plasma Science, Vol. 48, No. 10, 2020, pp. 3524 – 3534   

Recent progress on cutoff probe development for real time processing monitoring

The cut-off probe is the simplest of the plasma diagnostic tools made out of a simple intuition about the cut-off phenomenon of plasma waves, but it has had many problems to solve. Recently, the modeling development of the probe has revealed the physics behind the probe spectrum (S21), and the accuracy of probe measurements has been significantly improved. Now we believe it is time to move to semiconductor processing applications such as real-time processing monitoring for cut-off probes. In this talk, I would like to describe the recent progress in the development of cut-off probes for real-time plasma processing, such as planar cut-off probes, surface wave probes, including a brief historical review of cut-off probe research conducted over nearly two decades.   

Spectra of a geometry improved ideal model of the pMRP

The planar multipole resonance probe (pMRP) is a diagnostic-tool based on the concept of active plasma resonance spectroscopy (APRS), which excites the plasma in the GHz range and records the response to detect resonances. Due to its planar design the pMRP is especially suited to monitor plasma processes without perturbing them. To determine plasma parameter from measured resonances, a model for the relation between plasma and resonance parameter is required. Previous work showed a deviation of the analytical solution for the ideal model to the full 3D electromagnetic simulations of the real pMRP. This difference is mainly caused by the idealized geometry instead of the electrostatic approximation in the analytical model. To allow for a better agreement of the spectra the model of the pMRP can be improved with a more realistic geometry, but requires partially numerical calculations which can still be applied in the functional analytic approach. The resulting spectra will be compared to full electromagnetic simulations and former analytical results.   

Kinetic modeling of the planar Multipole Resonance Probe

Active Plasma Resonance Spectroscopy (APRS) denotes a class of plasma diagnostic methods that utilize the ability of the plasma to resonate at or near the electron plasma frequency. Many APRS probes are invasive and perturb the plasma by their physical presence. The planar Multipole Resonance Probe (pMRP) solves this problem: it can be integrated into the chamber wall and minimizes the perturbation. In this work, a kinetic model is developed to investigate the behavior of the pMRP. This model consists of the Boltzmann equation, which is coupled with the Poisson equation under the electrostatic approximation. The spectral response of the probe-plasma system is found by calculating the complex admittance. Both collision-less damping and collisional damping appear in the kinetic spectra. This model provides the possibility to calculate the electron density, electron temperature, and electron-neutral collision frequency from the measurements.   

Measuring Electron Dynamics in the Magnetic Nozzle of an ECR Thruster

The diagnosis of plasma parameters, such as electron density and temperature, is a key issue in understanding and controlling electric propulsion systems. To date, electron density is mostly measured with electrostatic probes such as Langmuir probes, which can be quite invasive, or with spectroscopy methods. In this frame, we assess the performances of a novel resonant probe for the characterization of an ECR plasma thruster. The curling probe is a microwave resonant probe firstly proposed in 2011 by Liang et al., which has several advantages with respect to other diagnostics: compactness, robustness, embeddable in a reactor/thruster wall, and the possibility to perform density measurements through a dielectric wall. We present here some recent improvement of the curling probe diagnostic that includes a calibration procedure and a correction method capable of accounting for the electron-depleted plasma sheath that forms at the probe surface. We propose here the first totally non-intrusive time-resolved electron density measurement inside an ECR thruster source, and a 2D electron density mapping of the ECR magnetic nozzle. Measurements are used to investigate plasma expansion in the magnetic nozzle.