Volume 89, Issue 11, 01 June 2001
Index of content:
- PLASMAS AND ELECTRICAL DISCHARGES (PACS 51-52)
89(2001); http://dx.doi.org/10.1063/1.1359754View Description Hide Description
Electron production rate and electron density in cold optically pumped CO–Ar and plasmas in the presence of small amounts of and NO have been measured using a Thomson discharge probe and microwaveattenuation. Nonequilibrium ionization in the plasmas is produced by an associative ionization mechanism in collisions of highly vibrationally excited CO molecules. It is shown that adding small amounts of or NO (50–100 mTorr) to the baseline gas mixtures at results in an increase of the electron density by up to a factor of 20–40 (from to This occurs while the electron production rate either decreases (as in the presence of or remains nearly constant within a factor of 2 (as in the presence of NO). It is also shown that the electron–ion recombination rates inferred from these measurements decrease by two to three orders of magnitude compared to their baseline values (with no additives in the cell), down to with 50–100 mTorr of oxygen or nitric oxide added to the baseline CO–Ar mixture, and with 75–100 mTorr of or NO added to the baseline mixture. The overall electron–ion removal rates in the presence of equal amounts of or NO additives turn out to be very close, which shows that the effect of electron attachment to oxygen at these conditions is negligible. These results suggest a novel method of electron density control in cold laser-sustained steady-state plasmas and open a possibility of sustaining stable high-pressure nonequilibrium plasmas at high electron densities and low plasma power budget.
89(2001); http://dx.doi.org/10.1063/1.1359755View Description Hide Description
This article presents an experimental demonstration of a high-pressure unconditionally stable nonequilibrium molecular plasma sustained by a combination of a continuous wave CO laser and a sub-breakdown radio frequency (rf) electric field. The plasma is sustained in a mixture containing trace amounts of NO or at pressures of The initial ionization of the gases is produced by an associative ionization mechanism in collisions of two CO molecules excited to high vibrational levels by resonance absorption of the CO laser radiation with subsequent vibration-vibration pumping. Further vibrational excitation of both CO and is produced by free electrons heated by the applied rf field, which in turn produces additional ionization of these species by the associative ionization mechanism. In the present experiments, the reduced electric field, is sufficiently low to preclude field-induced electron impact ionization. Unconditional stability of the resultant cold molecular plasma is enabled by the negative feedback between gas heating and the associative ionization rate. Trace amounts of nitric oxide or oxygen added to the baseline gas mixture considerably reduce the electron–ion dissociative recombination rate and thereby significantly increase the initial electron density. This allows triggering of the rf power coupling to the vibrational energy modes of the gas mixture. Vibrational level populations of CO and are monitored by infrared emission spectroscopy and spontaneous Raman spectroscopy. The experiments demonstrate that the use of a sub-breakdown rf field in addition to the CO laser allows an increase of the plasma volume by about an order of magnitude. Also, CO infrared emission spectra show that with the rf voltage turned on the number of vibrationally excited CO molecules along the line of sight increase by a factor of 3–7. Finally, spontaneous Raman spectra of show that with the rf voltage the vibrational temperature of nitrogen increases by up to 30%. This novel energy efficient approach allows sustaining large-volume high-pressure molecular plasmas without the use of a high-power CO laser. This opens a possibility of using the present technique for high-yield plasma chemical synthesis and plasma material processing.
89(2001); http://dx.doi.org/10.1063/1.1368397View Description Hide Description
Charging of micron-size particulates, often appearing in fluorocarbon plasma etching experiments, is considered. It is shown that in inductively coupled and microwave slot-excited plasmas of and Ar gas mixtures, the equilibrium particle charge and charge relaxation processes are controlled by a combination of microscopic electron, atomic and and molecular ion and currents. The impact of molecular ion currents on the particulate charging and charge relaxation processes is analyzed. It is revealed that in low-power (<0.5 kW) microwave slot-excited plasmas, the impact of the combined molecular ion current to the total positive microscopic current on the particle can be as high as 40%. The particulate charge relaxation rate in fluorocarbon plasmas appears to exceed which is almost one order of magnitude higher than that from purely argon plasmas. This can be attributed to the impact of positive currents of fluorocarbon molecular ions, as well as to the electron density fluctuations with particle charge, associated with electron capture and release by the particulates.
89(2001); http://dx.doi.org/10.1063/1.1364653View Description Hide Description
The prebreakdown phenomena and the formation process of the glow discharge in a low-pressure Ar gas were investigated under a uniform field gap. Prebreakdown phenomena were observed for (where p is pressure, d the gap distance) in Ar gas under conditions of a slowly increasing voltage. It was observed that the prebreakdown phenomena formed pulse discharges up to the transition to the glow discharge. The amplitudes of the photon and current pulses due to the pulse discharge increased with time, and then decreased as soon as the transition to a steady glow discharge occurred. When the overvoltage or external series resistance was increased, the pulse amplitudes increased with the applied voltage and decreased with the resistance. The characteristics of the prebreakdown phenomena were changed by the shape of the electrodes. The formation mechanism of the glow discharge can be qualitatively explained by that of the streamer in a high-pressuredischarge. The transient glow discharge was observed, and its duration increased with an increase in resistance. The instability of the glow discharge was controlled by three factors, namely, Kaufmann’s criterion, the Child–Langmuir law, and the density balance between the production and removal rates of electrons.
89(2001); http://dx.doi.org/10.1063/1.1365436View Description Hide Description
We propose a semianalytical ion dynamicsmodel for a collisionless radio-frequency biased sheath. The model uses bulk plasma and electrodeboundary conditions to predict the ion impact energy distribution and electrical properties of the sheath. The proposed model accounts for ion inertia and ion current modulation at bias frequencies that are of the same order of magnitude as the ion plasma frequency. A relaxation equation for ion currentoscillations is derived, which is coupled with a damped potential equation in order to model ion inertia effects. We find that inclusion of ion current modulation in the sheathmodel shows marked improvements in the predictions of the sheathelectrical properties and ion energy distribution function.