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Title: Electrical characterization of a capacitive rf plasma sheath

Abstract

The authors report on an experimental system designed to investigate and characterize capacitive radio frequency (rf) sheaths. An electrode mounted in an inductive plasma reactor and driven with separate rf and direct current (dc) power sources is used. The advantage of this design is that the electrode sheath is decoupled from the plasma parameters. This allows detailed investigation of the sheath with different bias conditions without perturbing the bulk plasma parameters. Power coupled to ions and electrons through the sheath, at low pressure, is investigated and a method to determine the electron conduction current to the electrode, using the external dc bias, is presented.

Authors:
;  [1]
  1. National Center for Plasma Science and Technology, School of Physical Sciences, Dublin City University, Glasnevin, Dublin 9 (Ireland)
Publication Date:
OSTI Identifier:
20953248
Resource Type:
Journal Article
Resource Relation:
Journal Name: Review of Scientific Instruments; Journal Volume: 78; Journal Issue: 1; Other Information: DOI: 10.1063/1.2430679; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; DESIGN; DIRECT CURRENT; ELECTRODES; ELECTRONS; HIGH-FREQUENCY DISCHARGES; IONS; PLASMA; PLASMA DIAGNOSTICS; PLASMA SHEATH; RADIOWAVE RADIATION

Citation Formats

Gahan, D., and Hopkins, M. B. Electrical characterization of a capacitive rf plasma sheath. United States: N. p., 2007. Web. doi:10.1063/1.2430679.
Gahan, D., & Hopkins, M. B. Electrical characterization of a capacitive rf plasma sheath. United States. doi:10.1063/1.2430679.
Gahan, D., and Hopkins, M. B. Mon . "Electrical characterization of a capacitive rf plasma sheath". United States. doi:10.1063/1.2430679.
@article{osti_20953248,
title = {Electrical characterization of a capacitive rf plasma sheath},
author = {Gahan, D. and Hopkins, M. B.},
abstractNote = {The authors report on an experimental system designed to investigate and characterize capacitive radio frequency (rf) sheaths. An electrode mounted in an inductive plasma reactor and driven with separate rf and direct current (dc) power sources is used. The advantage of this design is that the electrode sheath is decoupled from the plasma parameters. This allows detailed investigation of the sheath with different bias conditions without perturbing the bulk plasma parameters. Power coupled to ions and electrons through the sheath, at low pressure, is investigated and a method to determine the electron conduction current to the electrode, using the external dc bias, is presented.},
doi = {10.1063/1.2430679},
journal = {Review of Scientific Instruments},
number = 1,
volume = 78,
place = {United States},
year = {Mon Jan 15 00:00:00 EST 2007},
month = {Mon Jan 15 00:00:00 EST 2007}
}
  • The electrical breakdown characteristics of the rf capacitive plasma source are investigated theoretically and experimentally. The plasma source is the electrode type consisting of the concentric cylinders for generating nonequilibrium plasma at atmospheric pressure. The theoretical model based on the diffusion-controlled breakdown mechanism is proposed to analyze the electrical breakdown phenomenon in this rf capacitive plasma source of the coaxial cylinders. The electron temperature at the electrical breakdown is calculated from the theoretical model, thereby evaluating the electrical breakdown voltages. The experimental data of the electrical breakdown voltage are measured with respect to the variation of the geometric parameters ofmore » plasma source, the gas temperature, and the concentration of the foreign reactive gases (oxygen and nitrogen) mixed in the helium gas. The theoretical results of the electrical breakdown voltage agree remarkably well with experimental data. This indicates that not only the electron temperature is important in determining the electrical breakdown voltage, but also the geometric variables, the gas temperature, and the scattering cross sections of molecules play significant roles.« less
  • A self-consistent solution for the dynamics of a high-voltage, capacitive RF sheath driven by a sinusoidal current source is obtained, under the assumptions of time-independent, collisional ion motion and inertialess electrons. The results are that the ion current density is 1.68epsilon/sub o/(2e/M)/sup 1/2/V-bar/sup 3/2/lambda/sub i//sup 1/2//s/sup 5/2//sub m/, where V-bar is the dc self-bias voltage, lambda/sub i/ is the ion mean free path, s/sub m/ is the sheath thickness, e/M is the ion charge-to-mass ratio, and epsilon/sub o/ is the free-space permittivity, the sheath capacitance per unit area for the fundamental voltage harmonic is 1.52epsilon/sub o//s/sub m/, the ratio ofmore » the dc to the peak value of the oscillating voltage is 0.40, the second and third voltage harmonics are, respectively, 19.3 and 5.3 percent of the fundamental, and the conductance per unit area for stochastic heating by the oscillating sheath is 2.17(e/sup 2/n/sub o//m..mu../sub e/)lambda/sup 2/3//sub D//lambda/sub i/s/sub m//sup 1/3/, where n/sub o/ is the ion density and lambda/sub D/ is the Debye length at the plasma-sheath edge, and ..mu../sub e/ = (8eT/sub e//..pi..m)/sup 1/2/ is the mean electron speed.« less
  • A self-consistent solution for the dynamics of a high voltage, capacitive RF sheath driven by a sinusoidal current source is obtained under the assumptions of time-independent, collisionless ion motion and inertialess electrons. Some results are that 1) the ion sheath thickness s/sub m/ is ..sqrt..50/27 larger than a Child's law sheath for the same dc voltage and ion current density; 2) the sheath capacitance per unit area for the fundamental voltage harmonic is 1.226 epsilon/sub O//s/sub m/, where epsilon/sub O/ is the free space permittivity; 3) the ratio of the dc to the peak value of the oscillating voltage ismore » 54/125; 4) the second and third voltage harmonics are, respectively, 12.3 and 4.2 percent of the fundamental; and 5) the conductance per unit area for stochastic heating by the oscillating sheath is 2.98 (lambda/sub D//s/sub m/)/sup 2/3/ (e/sup 2/n/sub O//m..mu../sub e/), where n/sub O/ is the ion density and lambda/sub D/ is the Debye length at the plasma-sheath edge, and ..mu../sub e/ = (8eT/sub e//..pi..m)/sup 1/2/ is the mean electron speed.« less
  • Plasma characterization and impedance matching are an integral part of any radio frequency (RF) based plasma source. In long pulse operation, particularly in high power operation where plasma load may vary due to different reasons (e.g. pressure and power), online tuning of impedance matching circuit and remote plasma density estimation are very useful. In some cases, due to remote interfaces, radio activation and, due to maintenance issues, power probes are not allowed to be incorporated in the ion source design for plasma characterization. Therefore, for characterization and impedance matching, more remote schemes are envisaged. Two such schemes by the samemore » authors are suggested in these regards, which are based on air core transformer model of inductive coupled plasma (ICP) [M. Bandyopadhyay et al., Nucl. Fusion 55, 033017 (2015); D. Sudhir et al., Rev. Sci. Instrum. 85, 013510 (2014)]. However, the influence of the RF field interaction with the plasma to determine its impedance, a physics code HELIC [D. Arnush, Phys. Plasmas 7, 3042 (2000)] is coupled with the transformer model. This model can be useful for both types of RF sources, i.e., ICP and helicon sources.« less