Bulletin of the American Physical Society
60th Gaseous Electronics Conference
Volume 52, Number 9
Tuesday–Friday, October 2–5, 2007; Arlington, Virginia
Session CT1: Capacitively Coupled Discharges |
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Chair: Zoran Petrovic, Institute of Physics, Belgrade Room: Doubletree Crystal City Crystal Ballroom A |
Tuesday, October 2, 2007 10:00AM - 10:30AM |
CT1.00001: Electron heating in dual-frequency capacitive discharges Invited Speaker: Capacitive discharges excited by a superposition of two frequencies are important tools in applications where precise control of the plasma conditions is important, as when the energy and flux of ions leaving the plasma must be controlled independently. Elementary discussion of these discharges assumes that the two frequencies control the ion flux and energy practically independently, but more detailed investigation shows that that this is true only to a limited extent. In this paper, we focus on electron heating in dual frequency discharges. Such heating can be attributed to three mechanisms: Stochastic heating, Ohmic heating and secondary emission from plasma facing surfaces. We consider the processes contributing to these three mechanisms and their relative importance. We will discuss recent work on collisionless or stochastic heating in dual-frequency discharges, the relative importance of Ohmic and collisionless heating, and the effect of secondary electron emission. We will show that stochastic and Ohmic heating typically depend on both the low- and high-frequency current densities, so that the net heating depends strongly on both. We will also show that recent phase-resolved optical emission spectroscopy measurements are difficult to understand without the assumption that secondary electron emission is an important process. These results show that the two-frequencies coupled in a complex fashion, which does not however necessarily preclude effective independent control of ion flux, ion energy, and other important process parameters [Preview Abstract] |
Tuesday, October 2, 2007 10:30AM - 10:45AM |
CT1.00002: Ionisation dynamics and frequency coupling in dual frequency capacitively coupled plasmas D. O'Connell, T. Gans, E. Semmler, P. Awakowicz, A.R. Ellingboe, M.M. Turner Multi-frequency discharges can provide additional process control for technological applications. Plasma ionization and sustaining mechanisms are investigated in an asymmetric capacitively coupled radio-frequency (rf) discharge operated with 2 MHz and 14 MHz applied simultaneously to the same electrode. Energetic electrons, key to ionization mechanisms, are probed using phase resolved optical emission spectroscopy (PROES) resolving both the low and high frequency rf cycles. The electron dynamics exhibits a complex spatio-temporal structure. The observed dynamics is compared to particle-in- cell (PIC) simulations. Pronounced coupling effects of the two frequencies can be observed, in particular, when they are phase locked. The relative phase between the two frequencies and the ratio of the applied voltages determines details of the electron dynamics. Plasma ionization mechanisms can be controlled and tailored through variations of the relative phase and the voltage ratio. Funding: SFB 591, ProInno II, GRK 1051, EPSRC. [Preview Abstract] |
Tuesday, October 2, 2007 10:45AM - 11:00AM |
CT1.00003: Student Excellence Award Finalist: Electron heating in asymmetric capacitively coupled RF discharges at low pressures Julian Schulze, Brian Heil, Dirk Luggenhoelscher, Uwe Czarnetzki Electron dynamics in asymmetric capacitively coupled discharges is investigated by applying a combination of various diagnostics: Laser electric field measurements for the sheath, phase resolved optical emission spectroscopy for the excitation, Langmuir probe measurements for the EEDF and electron density in the bulk, a SEERS sensor for current measurements and a high voltage probe for the determination of the applied voltage. During the sheath expansion beams of high energetic electrons are observed. At low pressures the RF current is not sinusoidal, but strong high frequency oscillations caused by the Plasma Series Resonance (PSR) effect are observed. These current oscillations increase the sheath velocity and enhance stochastic heating. The current is compared to emission measurements showing a direct correlation. The electric field measurements are compared to a fluid sheath model. The measured RF current is compared to an analytical PSR model. Another analytical model demonstrates the influence of electron beams on the time averaged isotropic EEDF as it is measured by probes. It clearly demonstrates, that high energetic electron beams lead to an enhanced high energy tail, that is usually attributed to stochastic heating. [Preview Abstract] |
Tuesday, October 2, 2007 11:00AM - 11:15AM |
CT1.00004: Electron heating and ionization mechanisms in capacitively coupled dual frequency plasmas Egmont Semmler, Deborah O'Connell, Timo Gans, Peter Awakowicz, Achim von Keudell Capacitively coupled dual-frequency plasmas are increasingly used in various technological applications. They have been motivated for their separate control of plasma density and ion bombardment energy. It is known that the plasma density is mainly controlled by the high frequency component in the plasma current, whereas the ion bombardment energy can be tuned by the low frequency component. However the nonlinear nature of the plasma boundary sheath is the cause for critical frequency coupling effects that occur in these devices. Recent measurements by Langmuir probe,VI-probe and a rf-current sensor reveal strong resonant behavior at integer driving frequency ratios capable of enhancing the plasma production by a factor of two compared to non-integer ratios. This can be explained by indirectly heating the electrons at the plasma series resonance frequency. Additional measurements of ionisation mechanisms through energetic electrons by means of phase resolved optical emission spectroscopy (PROES) have unveiled complex coupling effects between the low and high frequency component. [Preview Abstract] |
Tuesday, October 2, 2007 11:15AM - 11:30AM |
CT1.00005: Numerical modelling of electron beams accelerated by the RF plasma boundary sheath B.G. Heil, J. Schulze, D. Luggenh\"olscher, U. Czarnetzki, T. Mussenbrock, R.P. Brinkmann The exact mechanism of electron heating by the RF plasma boundary sheath is a current research topic. Electron beams accelerated by the RF sheath and travelling through the plasma bulk have been observed using phase resolved measurements of plasma emissions and also with a Monte-Carlo simulation. In this work the acceleration of electron beams by the RF sheath is numerically investigated. At lower pressures, the RF current and also the expansion and contraction of the sheath are modulated by the plasma series resonance (PSR) effect. This modulation can lead to multiple electron beams being accelerated per RF cycle. It can also temporarily cause larger sheath velocities than would be the case if the current is sinusoidal as is commonly assumed, leading to a larger acceleration of the electron beams. The hypothesis is that at least a part of what is called stochastic heating is due to these electron beams. [Preview Abstract] |
Tuesday, October 2, 2007 11:30AM - 11:45AM |
CT1.00006: Discharge Electrode Impedance Effect on Nonlinear Wave Generation in Dual-Frequency Capacitively Coupled Plasma Yohei Yamazawa Resonantly growth of the wave originated from the plasma nonlinearity was observed in a capacitively coupled plasma reactor. We experimentally demonstrated the growth of the harmonics of the bias frequency by tuning a variable capacitor attached to the bottom electrode. We also observed the amplification of the wave having the frequency corresponding to the difference of the source and the bias frequency. A simple nonlinear equivalent circuit model can reproduce the experimental results. The results indicate that the electrode impedance should be taking into account in considering the resonance condition that dominates the amplification of the nonlinear wave. [Preview Abstract] |
Tuesday, October 2, 2007 11:45AM - 12:00PM |
CT1.00007: Analogy between electron heating mechanisms in symmetric dual and asymmetric single frequency capacitive RF discharges Julian Schulze, Brian Heil, Dirk Luggenhoelscher, Uwe Czarnetzki, Bert Ellingboe Electron heating mechanisms in single and dual frequency capacitively coupled RF discharges are a current research topic. Many theoretical, but only few experimental investigations on this topic exist. In this work electron heating is investigated experimentally in dual and single frequency discharges by Phase Resolved Optical Emission Spectroscopy. In both cases, the generation of beams of high energetic electrons during sheath expansion is identified to be the dominant cause of heating. In an asymmetric single frequency discharge the Plasma Series Resonance effect leads to high frequency modulations of RF current and sheath width similar to the sheath oscillation using two frequencies, which are caused by a second externally applied high frequency voltage. These modulations can be observed in terms of excitation in both cases. The reflection of electron beams at the opposite plasma boundary as well as a localised field reversal during the phases of sheath collapse is observed applying one and two frequencies. Electron heating is generally related to the sheath motion, which can be similar in dual and single frequency discharges under certain conditions. The physics of the beams and related excitation seems to be similar, although the rapid sheath oscillations have quite different causes. [Preview Abstract] |
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