Bulletin of the American Physical Society
64th Annual Gaseous Electronics Conference
Volume 56, Number 15
Monday–Friday, November 14–18, 2011; Salt Lake City, Utah
Session AM1: Workshop: Control of Distribution Functions in Low Temperature Plasmas |
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Chair: Igor Kaganovich and Yevgeny Raitses, Princeton Plasma Physics Lab, David Graves, UC Berkeley, and Gottlieb Oehrlein, University of Maryland Room: 255A |
Monday, November 14, 2011 7:45AM - 8:00AM |
AM1.00001: INTRODUCTION Igor Kaganovich . [Preview Abstract] |
Monday, November 14, 2011 8:00AM - 8:30AM |
AM1.00002: Electron Temperature Modification in Gas Discharge Plasmas Valery Godyak In gas discharge plasma with a Maxwellian electron energy distribution function (EEDF), the ionization balance results in the electron temperature Te being solely a function of the product of gas pressure p and plasma characteristic size d, Te = Te(pd), independently on plasma density and electron heating mechanism. This common feature of gas discharge plasma takes place in self-sustained discharges where ionization is locally coupled with electron heating, usually in a uniform heating electric field. At such condition, there is no room for electron temperature change at fixed pd. Variety of non-equilibrium phenomena observed in self-organized dc and rf discharge structures, and in relaxation process therein suggests the way to EEDF and Te modification. At such conditions, the electron heating can be separated (in space or/and in time) from the ionization. Few examples of such discharge structures in well know stationary dc and rf discharges and in plasma transient processes, leading to considerable mean electron energy reduction, will be considered in the presentation together with abbreviated review of existing methods and experimental results on EEDF control in laboratory plasmas. [Preview Abstract] |
Monday, November 14, 2011 8:30AM - 9:00AM |
AM1.00003: Plasma-surface interactions in the control of plasma distribution functions: $\alpha $-C:H surfaces impacted by Ar/H2 plasmas David Graves, Ning Ning, Nick Fox-Lyon, Gottlieb Oehrlein The purpose of the study is to establish the role of surface processes in influencing and controlling characteristic plasma distributions functions during the erosion of thin $\alpha $-C:H films. The dominant effect following Ar$^{+}$ impacts on a $\alpha $-C:H film is near-surface H depletion, which results in a carbon rich near-surface-region (modified layer). The modified layer thickness increases with increasing ion energy; predicted thickness is in agreement with measurements. Energetic H$_{2}^{+}$ impacts resulted in H insertion and hydrocarbon cluster erosion. Near-surface film structure and composition result from competition between these processes. The effect of H$^{+}$ impacts and possible synergetic effect of ions, additional neutrals and VUV photons will be discussed. In addition, the species ejected into the plasma following ion and neutral impacts will alter the plasma distribution functions; these effects will be highlighted as well. [Preview Abstract] |
Monday, November 14, 2011 9:00AM - 9:30AM |
AM1.00004: BREAK |
Monday, November 14, 2011 9:30AM - 9:55AM |
AM1.00005: Ion energy distribution functions of ions hitting nanoparticles in plasmas Uwe Kortshagen, Federico Galli, Meenakshi Mamunuru Nonthermal plasmas are interesting sources for the synthesis of nanoparticles. Among the unique features of nonthermal plasmas is that high quality nanocrystals, even of high melting point materials, can be produced in a low temperature environment. It is believed that the energy distribution of ions hitting the nanoparticles during nanoparticle growth is an essential parameter for the formation of high quality nanocrystals. In this presentation, we discuss results of a molecular dynamics-Monte Carlo simulation, which simulates the motion of ions around a nanoparticle and their impact on the nanocrystal surface. We are studying the cases of both a pure argon plasma and an argon-hydrogen plasma. An interesting interplay between different ion species in the plasma is found. We show that in certain ranges of discharge pressure, nanoparticles are exposed to a strong flux of low energy ions, which may be responsible for the restructuring of the nanoparticle surfaces during growth and for the formation of rather defect free nanocrystals. [Preview Abstract] |
Monday, November 14, 2011 9:55AM - 10:20AM |
AM1.00006: Electron energy distribution functions modified by MacKenzie Maxwell Demons Noah Hershkowitz, Chi-Shung Yip MacKenzie's ``Maxwell Demon'' is an array of positively biased thin wires mounted in a plasma. The original version\footnote{K. R. MacKenzie, R. J. Taylor, D. Cohn, E. Ault, and H. Ikezi, App. Phys. Lett. {\bf 18}, 529 (1971)} consisted of a 60cm x 60cm grid of 400 0.025mm diameter tungsten wires. Electrons accelerate radially towards the positively biased wires but only cold electrons with low angular momentum are collected. Removal of the cold electrons heats the plasma. Our experiments explore much smaller alternative demon geometries and their effects on argon and xenon plasmas in a multi-dipole plasma device. Using this device the electron temperature could be increased by factors of two or more. Effects on the plasma are compared to those obtained by applying a positive voltage to larger electrodes. At high positive voltage, a relaxation instability in the kHz range is induced which limits the application of this technique. [Preview Abstract] |
Monday, November 14, 2011 10:20AM - 10:45AM |
AM1.00007: Multiple electron distributions in multidipolar plasma devices Scott Robertson, Scott Knappmiller Hot filament discharge devices with multidipolar surface magnetic fields have densities and temperatures higher than in these devices without multidipolar fields. We compare Langmuir probe data from a device operated with and without surface multidipolar fields. The data show that for both configurations the electrons with energy below about 10 eV consist of two populations: the bulk plasma electrons electrostatically confined by the plasma potential and the secondary electrons from the wall. For the device with the multidipolar fields, the secondary electrons from the walls are increased in density by about two orders of magnitude, indicating that these electrons are mirror confined by the multidipolar fields. This mirror confinement is aided by the electrostatic potential drop in the sheath at the wall. A relatively simple mathematical model for energy balance shows that the heating by the secondaries accounts for the increased temperature of the bulk plasma electrons. [Preview Abstract] |
Monday, November 14, 2011 10:45AM - 11:10AM |
AM1.00008: Volumetric Control of Anisotropic Electron Distribution Functions in Plasmas with Langmuir Oscillations A.S. Mustafaev, I.D. Kaganovich, V.I. Demidov, S.F. Adams Given that the development of future microelectronics depends on the application and control of anisotropic electron distribution functions (AEDFs) in plasmas, the development of methods to measure and control AEDFs is of great importance. This talk presents the technique of AEDF measurements with flat one-sided electric probes in low pressure plasmas and demonstrates how volumetric excitation of Langmuir oscillations can modify and control AEDFs. It will be demonstrated that Langmuir oscillations can provide independent tailoring of the AEDF energetic part with respect to the angular velocity distribution and electron energy relaxation. Specific criteria for excitation of Langmuir oscillations will be provided and have been experimentally tested. The cross section for quasi-elastic collisions of the beam electrons with Langmuir plasmons has been estimated. The beam instability changes the shape of the AEDF with respect to its energies and hence changes the properties of the plasma. This circumstance should be taken into account in technologies that use plasmas with fast electrons. This work was supported by the DOE (DE-SC0001939), NSF CBET-0903635, AFOSR and the Ministry of Education of the Russian Federation. [Preview Abstract] |
Monday, November 14, 2011 11:10AM - 11:40AM |
AM1.00009: The Langmuir Paradox: can the ion acoustic instability at the sheath edge thermalize the ions too? Chi-Shung Yip, Noah Hershkowitz, Greg Severn Recently there was a theoretical prediction\footnote{S.D. Baalrud, et al., Phys. Rev. Lett. {\bf 102} 245005 (2009).} that in single-species plasmas, ion collisional friction in the plasma will be enhanced by ion acoustic instability. The theory predicted that instability will not only enhance the thermalisation of the the electrons, but will also, near the sheath-edge, thermalize the non- Maxwellian of an ion velocity distribution function (IVDF), caused by charge exchange in the presheath. The theory also predicted that this instability will disappear through collisional damping as neutral pressure of the plasma increases. This experiment aims to verify this theory by measuring the IVDFs near the sheath edge in a multi-dipole chamber discharge in Xenon gas at a variety of neutral pressures. The IVDFs are determined by Laser-Induced Florescence, the electron temperature is measured by a Langmuir probe and the plasma potential towards the boundary is measured by an emissive probe. [Preview Abstract] |
Monday, November 14, 2011 11:40AM - 12:05PM |
AM1.00010: DISCUSSION: Diagnostics Methods for Measurements of Plasma Distribution Functions |
Monday, November 14, 2011 12:05PM - 12:30PM |
AM1.00011: Physical Mechanisms of the Electron Energy Distribution Function Control in Inhomogeneous Non-stationary Plasma Anatoly Kudryavtsev, Lev Tsendin To predict the main scenarios of electron distribution function (EDF) control, first of all it is necessary to develop a kind of roadmap of formation a different modes of the EDF in the inhomogeneous unsteady plasma. The analysis shows that the time scales are determined by the ratio between the transient time tL (the characteristic time of electron transport through the plasma volume) and the relaxation times te of the EDF momentum tm (on velocity direction) and energy. Accordingly, for the spatial variable it is the ratio between the characteristic size of plasma L and an electron mean free path l (momentum relaxation) and a length of energy relaxation of electron energy le. A significant difference between the scale of momentum relaxation and energy te $>>$ tm, le $>>$ l (reaching two or more orders of magnitude), allows to predict the possible modes of the EDF formation, with various degrees of selectivity effects on different groups of electrons (from a local EDF when L $>>$ le and it is possible to affect only the entire ensemble of electrons) and the nonlocal EDF, when L $<<$ le and different groups of electrons behave independently of each other and it is possible to influence only on the interest profiles. [Preview Abstract] |
Monday, November 14, 2011 12:30PM - 2:00PM |
AM1.00012: Lunch Break and Industry Speakers Peter Ventzek, Robert J. Baker Lunch break and an informal discussion. [Preview Abstract] |
Monday, November 14, 2011 2:00PM - 2:15PM |
AM1.00013: Experiments, simulations and modelling of the electron energy distribution functions in low pressure rf excited non-local plasmas Rod Boswell, Christine Charles, Kazunori Takahashi Plasma systems having a critical dimension less than a mean free path for electron scattering from neutrals show large deviations from Maxwellian distributions in the electron energy distribution functions (eedf). The plasma excitation method, DC positive column and gamma, rf capacitive, inductive and wave all produce different effects on the eedf which must be unravelled to determine whether the boundaries or the ionisation mechanism dominate the details of the distribution. In particular, a magnetic field will serve to decouple the capacitive component of an inductive antenna from the purely ``inductive'' excitation. This talk will present experimental measurements of the electron energy probability function (eepf) using a compensated Langmuir probe and the Druyvesteyn method to determine the eepf. Problems associated with non-isotropic distributions will be discussed. The experimental results will be compared with 1D 3v self consistent PIC simulations of ``inductive'' heated discharges at different frequencies and lengths. An analytical model using a Druyvesteyn distribution of electrons shows very good agreement with experimental measurements. [Preview Abstract] |
Monday, November 14, 2011 2:15PM - 2:30PM |
AM1.00014: Control of electron energy distribution function by high frequency fields and by the effect of crossed magnetic field Zoran Petrovic, Sasa Dujko, Ronald White Ensembles of electrons can have collective transport properties even when there is no coupling between individual particles either through Coulomb force or through collisions with products of previous collisions. A number of these kinetic effects were identified and mostly explained recently. In particular combinations of time varying fields and cross sections present the opportunity to control the shape of the electron energy distribution functions (EEDF) and specific time dependences of collisional rates and transport coefficients. The energy coupling from external field to charged particles may thus be controlled and enhanced. For example, it is possible to control the transport of electrons, producing negative transient diffusion through a suitable time varying external magnetic field, and producing negative average velocity (flux drift velocity) through enhanced non-conservative processes. Time dependent electric and magnetic fields may be in particular useful in controlling the behavior of electrons giving new possibilities to modify energy coupling into collisional non-equilibrium plasma. [Preview Abstract] |
Monday, November 14, 2011 2:30PM - 2:55PM |
AM1.00015: Nonlocal Control of Electron Energy Distribution Functions in Plasmas with Active Boundaries V.I. Demidov, I.D. Kaganovich, A.S. Mustafaev, M.E. Koepke, Y. Raitses, S.F. Adams, I. Schweigert Nonlocal effects, associated with the presence of energetic electrons arising in volumetric processes and/or ejected into the plasma from the plasma boundaries have been studied. Presence of energetic electrons can significantly influence charged particle transport and plasma structure and, thereby, can serve to control plasma properties for plasma processing, plasma medicine, and other applications. Methods of controlling the electron energy distribution functions (EEDFs) are reviewed. These methods include designing the device geometry in such a way to separate the thermal electrons from the energetic electrons. This approach can be applied over a wide range of gas pressures. Another method involves applying additional voltage to conducting active boundaries. Both methods have been developed and tested for practical use in dc discharges, making use of both heated or cold cathodes. Nonlocal effects work best in short discharges without a positive column. Such discharges with a cold cathode can be especially advantageous for the practical development of micro-discharges at atmospheric pressure. This work was supported by the DOE (DE-SC0001939), NSF CBET-0903635, and AFOSR. [Preview Abstract] |
Monday, November 14, 2011 2:55PM - 3:20PM |
AM1.00016: Control of Ion and Electron Distribution Functions by the Electrical Asymmetry Effect Uwe Czarnetzki In capacitively coupled RF discharges the ion energy distribution functions at the electrodes and the electron energy distribution function in the bulk can be controlled by the electrically asymmetry effect (EAE). A fundamental RF frequency and its second harmonic are applied in parallel with a fixed but controllable phase. Even in a geometrically symmetric discharge a DC bias is established which can be tuned from negative to positive with the phase as the control parameter. Accordingly the ion energy distribution functions at both electrodes can be controlled. This allows maximizing the ion energy at one electrode while at the same time minimizing it at the counter electrode or vice versa. Also the sheath dynamics and the spatial and temporal Ohmic and stochastic heating of electrons are strongly influenced by the phase. However, the volume and period average of the power input is effectively constant which leads to a similar constant ion flux. The dynamics of these processes has been investigated by experiment, model, and simulation and very good agreement is found throughout. Based on these results the fundamental principles of distribution function control via the EAE will be explained. Further, advantages and limits will be discussed. [Preview Abstract] |
Monday, November 14, 2011 3:20PM - 3:50PM |
AM1.00017: DISCUSSION: Numerical Methods for Measurements of Plasma Distribution Functions |
Monday, November 14, 2011 3:50PM - 4:05PM |
AM1.00018: Plasma-Surface Interactions and Impact on Electron Energy Distribution Function N.A. Fox-Lyon, G.S. Oehrlein, N. Ning, D.B. Graves, V. Godyak The goal of this work is to explore the role of surface processes in influencing characteristic electron energy distribution functions (EEDF).As a model system, we use a well characterized, inductively coupled plasma system to examine Ar/H$_{2}$ (or D$_{2})$ discharges interacting with a-C:H films. The modification/erosion of a-C:H surfaces is monitored in real time by ellipsometry and the effects of gas mixtures and surface generated carbon on plasma parameters (T$_{e}$, plasma density, EEDF) are probed with Langmuir probe measurements. We find that plasma density decreased greatly (from 10$^{11}$ to 10$^{9}$ per cm$^{3})$ with small H$_{2}$ additions to Ar plasma (conditions: 10-30 mTorr, 300-600 W source power). The electron temperature was shown to increase with H$_{2}$ flow. At high H$_{2}$ flows, the electron energy distribution transitions from Maxwellian distribution to a two-temperature distribution. The addition of 1-20 {\%} CH$_{4}$ into H$_{2}$ plasma shows an increase in plasma density and a change in the electron temperature. The hydrocarbon erosion products of a-C:H films in H$_{2}$ plasma are found to cause a similar effect on plasma properties as CH$_{4}$ addition. These observations indicate that prediction/control of EEDF for plasmas interacting with reactive bounding surfaces requires an understanding of the consequences of the plasma-surface interactions. [Preview Abstract] |
Monday, November 14, 2011 4:05PM - 4:30PM |
AM1.00019: Is particle distribution control necessary in plasma etching processes? M.M. Turner, B.J. Keville, S. Daniels, Y. Zhang, A.M. Holohan The performance of plasma processes commonly depends on a small number of critical variables. For example, in etch processes employed for semiconductor manufacturing, the crucial variables are often the flux and energy of positive ions and the fluxes of two neutral radicals, for instance fluorine and oxygen. Neutral radical fluxes, in particular, tend to drift, due to factors such as wall condition and disturbance of the process by etch products. Such drifts are highly undesirable and can seriously limit the performance of plasma processes. In principle, process drift of this kind can be mitigated by a closed-loop control scheme, which in practice means that, {\em inter alia}, the feedstock gas composition is manipulated to compensate for, {\em e.g.} variations in wall state. However, such a scheme does not control either the electron energy distribution function, the composition of the positive ion flux, or the ion energy distribution function, and all of these things may change when the neutral gas composition changes. In this work we show that these effects may be surprisingly small, and therefore that explicit measures to control these parameters may not be required. [Preview Abstract] |
Monday, November 14, 2011 4:30PM - 4:55PM |
AM1.00020: Fast Analytical-Numerical Model of Atmospheric Pressure Radio-Frequency Capacitive Discharges M.A. Lieberman, A.J. Lichtenberg, C. Lazzaroni, P. Chabert, A. Leblanc, Jing Zhang A fast one-dimensional analytical-numerical hybrid model of atmospheric pressure, radio-frequency (rf) driven discharges is developed. The feed gas is assumed to be helium or argon with small admixtures of oxygen, nitrogen, or other gases. The electrical characteristics are determined analytically from a current-driven homogeneous discharge model. The electron power balance is solved analytically to determine the time-varying electron temperature, which oscillates on the rf timescale. Averaging over the rf period yields enhanced rate coefficients for gas phase activated processes, an effect not usually considered in global models. The particle balance relations for all species are then integrated numerically, with assumed Maxwellian rate coefficients, to determine the equilibrium discharge parameters. The coupling of analytical solutions of the time-varying discharge and electron temperature dynamics, with the numerical solutions of the discharge chemistry, allows for a fast solution of the discharge equilibrium. Supported by Dept Energy Fusion Energy Science Contract DE-SC000193. [Preview Abstract] |
Monday, November 14, 2011 4:55PM - 5:02PM |
AM1.00021: Kr ion Laser-Induced Fluorescence using a tunable diode laser near 729nm Greg Severn |
Monday, November 14, 2011 5:02PM - 5:09PM |
AM1.00022: Simulating Electron Scattering in Cold-Cathode Discharges Alexander V. Khrabrov |
Monday, November 14, 2011 5:09PM - 5:34PM |
AM1.00023: Measurement of the perturbed distributions of electrons and ions Fred Skiff From the point of view of dynamics, plasmas are complex electromechanical systems with many degrees of freedom. These degrees of freedom are usually represented by the various forms of wave which can propagate in the plasma. Often the best way to sort out the plasma degrees of freedom is by observing the effects of waves on particle distribution functions. We will look at two recent examples. The first is the example of observing Alfv\'{e}n waves through their effects on the electron distribution function. The perturbation on the electrons is observed by looking at the effect of an Alfv\'{e}n wave on the propagation and damping of whistler mode waves (in the BAPSF at UCLA). A second example is the perturbation of the ion distribution function by kinetic waves in the drift-wave frequency range. These modes may appear to be part of the drift-wave spectrum, but are very different and can be distinguished by the perturbation they produce on the ion velocity distribution measured using laser-induced fluorescence (on a linear magnetized plasma column). [Preview Abstract] |
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