Floating Potential Measurements in the Near Field of an ICRH Antenna

May 1, 1989

J. Vac. Sci. Technol., A, 7, 1092-1098 (1989).

Caughman, J. B. O., Ruzic, D. N., Hoffman, D. H.

Large variations in the plasma potential can lead to large sheath potentials at the surface of the Faraday shield of anion cyclotron resonance heating (ICRH) antenna. This sheath can accelerate ions to energies where sputtering is significant, resulting in impurity generation. The time-varying floating potential is being measured in the near field of an ICRH antenna by using the Radio Frequency Test Facility (RFTF) at Oak Ridge National Laboratory. The antenna used is a resonant loop antenna that has a two-tier Faraday shield with a layer of graphite on the outer tier. The electron cyclotron heated plasma in RFTF is produced by a 10.6-GHz klystron. The magnetic field at the antenna is ~2 kG, with a plasma density of ~1010 cm–3 with an electron temperature of ~6 eV. Ionization by rf power from the antenna produces a local plasma of ~1011 with an electron temperature of 12 to 20 eV. The rf floating potential is measured by using a capacitively coupled probe that is scanned in front of the antenna, parallel to the current strap. A stationary Langmuir probe is used to measure the time-averaged floating potential. Measurements indicate that the potential scales with the antenna current and increases with plasma density to values of up to 300 V peak-to-peak. The rf component of the floating potential seems to follow the near field pattern of the antenna, indicating that these fields may be responsible for the potential formation.

Measurements of Time Varying Plasma Potential, and Electron Temperature and Density in a 13.6 MHz RF Discharge

May 1, 1989

J. Vac. Sci. Technol., A, 7, 972-976 (1989).

Wilson, J. L., Caughman, J. B. O., Nguyen, P. L., Ruzic, D. N.

Argon plasma measurements were conducted in a commercial capacitively coupled etcher operating at 400 mTorr with 60-W coupled power at 13.56 MHz. Time varying floating potential was measured using a capacitively coupled probe which uses a capacitive voltage divider and a field effect transistor buffer amplifier. Average floating potential was obtained from a high-input-impedance Langmuir probe. The floating potential was found to be sinusoidal, [21 sin(wt)–4]V±1.5 V, with a maximum of 17±1.5 V and a minimum of –25±1.5 V. From these data and the use of a low-input-impedance Langmuir probe, a calibrated instantaneous IVcharacteristic is used to obtain plasma potential andelectron temperature. Plasma potential was found to be sinusoidal, [21 sin(wt)+30]V±1.6 V, with a maximum of 51±1.6 V and a minimum of 9±1.6 V. Electron temperatures were 6.58±0.19 eV at maximum plasma potential and 6.49±0.19 eV at minimum plasma potential. The electron density for this experiment was determined to be 1.48±0.83×1010 electrons/cm3 .