Bulletin of the American Physical Society
74th Annual Gaseous Electronics Conference
Volume 66, Number 7
Monday–Friday, October 4–8, 2021;
Virtual: GEC Platform
Time Zone: Central Daylight Time, USA
Session CT11: Plasma Diagnostics: Electrical Diagnostics I |
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Chair: Gabe Xu, University of Alabama in Huntsville Room: Virtual GEC platform |
Tuesday, October 5, 2021 8:00AM - 8:30AM |
CT11.00001: Modern Langmuir probe diagnostics Invited Speaker: Valery A Godyak During the last decades, it has become evident that many fundamental concepts of low-temperature plasma physics must be revised considering its highly non-equilibrium nature. Most definitions of many plasma parameters and their diagnostics are based on the assumption of a Maxwellian Electron Distribution Function (EEDF. However, it is known for a long time that the assumption of Maxwellian EEDF is not the case for typical conditions in gas discharges and space plasmas. For non-Maxwellian EEDF, the electron temperature Te is losing its usual meaning as a parameter of electron distribution but usually implies a mean electron energy Teff. The key plasma parameters and rates of electron-induced chemical reactions calculated with Te found from a narrow portion of the real EEDF (with probes or spectroscopy) may strongly disagree with those calculated with realistic EEDF. It is known that Teff, plasma conductivity, Debye length, and the ion sound speed are mainly defined by the low-energy part of the EEDF, while inelastic collision rates and floating wall potential (responsible for electron energy loss to the wall) are extremely sensitive to the EEDF shape at high electron energies. The errors in the classical probe diagnostics, based on electron or/and ion probe currents, due to non-Maxwellian EEDF and others non-accounted in classical probe theories effects are discussed in this presentation. It is shown that measurement of EEDF according to Druyvestein procedure is the only reliable probe technique for diagnostics of low-temperature plasma. The rest of the presentation is devoted to errors in EEDF measurement, their origination, and ways of their mitigation. Examples of professionally executed EEDF measurement performed by different authors in laboratory setups and in plasma processing reactors demonstrate practical feasibility for accurate EEDF measurement in a wide range of plasma devices, providing the fundamental requirement for classical probe diagnostics are satisfied. |
Tuesday, October 5, 2021 8:30AM - 8:45AM |
CT11.00002: Analysis and Comparison of the Electron Temperatures and the Number Densities using a Single Langmuir Probe and Optical Emission Spectroscopic Diagnostic System Kirk Boehm, James Rogers, Richard Branam In order to best characterize a radiofrequency (RF) plasma source, we identified the need for an intrusive probe able to survive high temperatures with very reactive plasma. The optimal hollow cathode materials for space propulsion systems can use argon as a source gas. The objective is to compare the intrusive Langmuir probe system and the non-intrusive optical emission spectroscopic system with each other with a focus on the electron temperatures and densities. The results can be used to relate the characteristics of a hollow cathode's plasma. The Langmuir probe analysis is based on the floating potential method which showed smaller errors compared to other available methods. The analysis of the emission spectrum (optical emission spectroscopy – branching fraction method) is based on a collisional radiative model (CRM). CRM determines possible collisional and radiative processes of atoms and ions. Literature research showed differences of ± 20% in electron temperatures and densities using RF generated plasmas between the two before mentioned diagnostic systems. The RF plasma source operated at 1000, 1250, 1500, 1750, and 2000 W with a constant pressure of 1.0 Torr. Langmuir probe and optical emission measurements have been performed at the same location inside of the quartz tube. The non-intrusive plasma measurements can reliably characterize the plasma instead of the classical intrusive measurements in hollow cathodes and other plasma-based propulsion systems. |
Tuesday, October 5, 2021 8:45AM - 9:00AM |
CT11.00003: In-situ measurement of electron emission yield at silicon surfaces in Ar/CF4 plasmas Mark Sobolewski Plasma simulations require accurate data for the ion-induced electron emission yield at plasma-exposed surfaces. For industrially relevant plasmas, however, direct measurement of yields using ion beams is impractical. In contrast, measurements made in situ, during plasma exposure, provide useful values for the total or effective yield produced by all incident ions. Here, in-situ measurements were performed in an icp system in Ar/CF4 mixtures at 1.3 Pa. The current and voltage across the sheath adjacent to the rf-biased substrate electrode were measured, along with Langmuir probe measurements of ion current density and electron temperature. The measurements are input into a numerical sheath model, which allows the emitted electron current to be distinguished from other currents. The effective yield was determined for thermally oxidized, in-situ etched, and sputter-cleaned silicon surfaces. For thermal oxides in pure Ar, yields agreed with previous measurements [1] on sputtered oxides. By combining measurements made for several mixtures with mass spectrometer data for the relative flux of each ionic species, estimates or bounds were obtained for the individual electron emission yields of the most prevalent ions. |
Tuesday, October 5, 2021 9:00AM - 9:15AM |
CT11.00004: Wireless Retarding Field Analyzer for Ion Energy Distribution Measurements in Plasma Processes David Gahan, Paul Scullin, James Doyle, JJ Lennon, Andre Lochner Retarding field analyzers (RFAs) have been used for decades to measure the ion energy distribution at surfaces during plasma processing. At first, RFAs were used mostly in grounded situations. In more recent times, RFAs suitable for radio-frequency (RF) biasing were developed. A typical design consists of the RFA grid stack embedded in a substrate-like carrier. The grid signals are supplied to the RFA (installed in the plasma chamber) from the airside through RF chokes and vacuum feedthrough located at a vacuum port. The RFA carrier is wired to the vacuum feedthrough on the vacuum side. While this type of solution works well for R&D applications, it is not ideal for a production environment (even for off-line maintenance work) due to the wiring and the need to open the chamber for installation. |
Tuesday, October 5, 2021 9:15AM - 9:30AM |
CT11.00005: Relative calibration of a retarding field energy analyzer sensor array Manuel Schröder, Ihor Korolov, Stefan Ries, David A. A Schulenberg, Peter Awakowicz, Julian Schulze Arrays of Retarding Field Energy Analyzer (RFEA) sensors distributed over a planar electrode surface are frequently used to measure ion distribution functions spatially resolved across a substrate surface. Typically, no sensor-to-sensor calibration is performed. To obtain realistic information on the spatial distribution of the ion flux, we developed a procedure to calibrate the ion flux measured by each sensor relative to a global reference sensor. To achieve this, we systematically changed the positions of all sensors of such an array to determine sensor specific calibration factors. The effects of the neutral gas pressure, RF power, gas mixture and time on these calibration factors are investigated in a large area low pressure CCP. We find that such a relative calibration is essential for obtaining accurate ion flux profiles, i.e. without such a calibration unrealistic results are measured. The calibration factors are almost independent of external control parameters, but change over time, i.e. such a calibration must be repeated periodically. |
Tuesday, October 5, 2021 9:30AM - 9:45AM |
CT11.00006: Experimental Diagnostics for Electrons and Atoms in Low Pressure Iodine Plasmas Benjamin Esteves, Anne Bourdon, Alejandro Alvarez Laguna, Pascal Chabert, Cyril Drag
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Tuesday, October 5, 2021 9:45AM - 10:00AM |
CT11.00007: Fast-Sweeping Langmuir probes: What happens to the I-V trace when sweeping frequency is higher than the ion plasma frequency? Chenyao Jin, Chi-Shung Yip, Wei Zhang, Di Jiang, Guosheng Xu Limited particle transit time is one of several limiting factors which determines the maximum temporal resolution of a Langmuir probe. In this work, we have revisited known fast sweep Langmuir probe techniques in a uniform, quiescent multi-dipole confined hot cathode discharge with two operation scenarios in which the probe sweeping frequencies is much lower and higher than the ion plasma frequencies respectively. This allows the investigation of the effect of limited ion-motion on I-V traces. Distortions of I-V traces at high frequencies, previously claimed to be ion-motion limitation effect, was not found unless shunt resistance is sufficiently high, despite achieving a ratio of ~ 3 between the probe sweeping frequency and the ion plasma frequency. This result essentially is a manifestation of the Langmuir probe as an electron collecting probe. Additionally, techniques in fast sweep Langmuir probe are briefly discussed. The comparison between the HDLP and the single probe setup shows that the capacitive response can be removed via subtracting a load line for the single probe setup almost as effective as using an HDLP setup, but the HDLP setup does remain advantageous in its facilitation of better recovery of weak current signal common in low plasma density situations. |
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