Bulletin of the American Physical Society
68th Annual Gaseous Electronics Conference/9th International Conference on Reactive Plasmas/33rd Symposium on Plasma Processing
Volume 60, Number 9
Monday–Friday, October 12–16, 2015; Honolulu, Hawaii
Session QR3: Negative Ion and Dust Particle Containing Plasmas |
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Chair: John Goree, University of Iowa Room: 305 AB |
Thursday, October 15, 2015 3:30PM - 3:45PM |
QR3.00001: Negative ion density in magnetically confined low-pressure argon-acetylene plasmas using laser-induced photodetachment Joelle Margot, Georges Al Makdessi, Ahmad Hamdan, Richard Clergereaux In plasmas generated in reactive gases such as silane and acetylene, dust particles can spontaneously form provided the residence time of the precursors is large enough for allowing volume interactions to dominate over surface interactions. In discharges at intermediate pressure (e.g. 100 mTorr), anions are considered to be the most likely precursors to dust particles formation. In the present work, we examine the negative ion density in very low pressure conditions, namely 1-10 mTorr. For this purpose, we investigate magnetized dusty plasmas produced in argon-acetylene mixtures in which dust particles have been observed. The negative ion density is measured using a laser photodetachment technique. It is is observed to increase with the magnetic field intensity and to slightly decrease with increasing C$_{2}$H$_{2}$ percentage in argon. In addition, it decreases with increasing gas pressure. The photodetachment cross section deduced from the photodetachment signal as a function of laser energy is found to be significantly higher than the value expected for the C$_{2}$H$^{-}$ ion, which may be explained by the presence in the plasma of negatively charged dust particles. [Preview Abstract] |
Thursday, October 15, 2015 3:45PM - 4:00PM |
QR3.00002: Controlled Fluxes of Silicon Nanoparticles By Extraction from a Pulsed RF Plasma Steven Girshick, Carlos Larriba-Andaluz Deposition of silicon nanoparticles onto substrates may be a means of growing monocrystalline silicon films at low substrate temperature if the nanoparticles' impact energy and size can be controlled to provide melting or amorphization of the nanoparticle without damaging the underlying film. In order to explore conditions that could produce such controlled fluxes of nanoparticles we numerically model a pulsed RF argon-silane plasma, with a positive DC bias applied during the afterglow phase of each pulse so as to extract and accelerate negatively charged silicon particles. Operating parameters studied include pulse on time, pulse off time, DC bias voltage, RF voltage and pressure. This set of parameters is tested to find conditions under which one can achieve a periodic steady state with repeatable pulse-to-pulse conditions that maximize silicon film growth rates while maintaining nanoparticle impact energies in the range 0.5-2.0 eV/atom. We utilize a previously developed 1-D dusty plasma numerical model, modified to consider pulsing and applied substrate bias. This model self-consistently solves for the coupled behavior of plasma, chemistry, and aerosol. Results show that it is possible by this method to produce nanoparticle fluxes that are tailored with respect to their distribution of impact energies and mass deposition rates. [Preview Abstract] |
Thursday, October 15, 2015 4:00PM - 4:15PM |
QR3.00003: Capacitively Coupled RF Plasmas for the Synthesis of Silicon Nanocrystals: Scaling and Mechanisms Aram H. Markosyan, Romain Le Picard, David H. Porter, Steven L. Girshick, Mark J. Kushner Silicon nanocrystals (SNCs) are of interest for light emitting electronics, photovoltaics, and biotechnology. SNCs are produced in low pressure capacitively coupled plasmas (CCPs) sustained in SiH$_{4}$ containing mixtures. To optimize these applications, it is necessary to control the size distribution of the SNCs. Particles 3-5 nm diameter are typically tailored by flow rates and power, however the fundamental processes responsible for this size control are not well understood. We developed a 2-d computer model for RF powered CCPs to predict the synthesis of SNCs. An aerosol sectional model was incorporated into the Hybrid Plasma Equipment Model. The reactor [1] is a quartz tube a few mm in diameter through which 100 sccm Ar and 15 sccm He/SiH$_{4}=$95/5 at 2 Torr are flowed. The SNC residence time is 1-2 ms in the dense plasma region near the electrodes. We found that the distribution of plasma potential is important in determining the growth and size distribution of the SNCs. The SNCs having long residence times in the plasma, thereby enabling growth, are usually negatively charged. To ultimately allow these SNCs to flow out of the plasma, the distribution of the plasma potential must enable the particles to be entrained in the neutral gas flow without a significant potential barrier. We also found that agglomeration of particles of \textless 1 nm is important in the rate of growth of SNCs. [1] L. Mangolini, et al. Nano Lett. \textbf{5}, 655 (2005). [Preview Abstract] |
Thursday, October 15, 2015 4:15PM - 4:30PM |
QR3.00004: Preliminary results of experimental measurements to determine microparticle charge in a complex plasma Eric Gillman, Bill Amatucci Microparticles in a dusty plasma typically collect many of the more mobile electrons as they charge up and therefore typically attain a net negative potential. The charge on these microparticles is typically estimated by calculating the charge on a spherical capacitor at the floating potential or by making measurements of particles levitating in the plasma sheath. However, secondary processes can alter the charging process and are significantly altered in the plasma sheath. Currently there is no reliable method to measure microparticle surface charge in the bulk region of complex or dusty plasmas. A novel, non-invasive, experimental method of measuring the charging of microparticles in the bulk region of a plasma will be presented. Ions impinging directly upon the microparticle surface and interacting electrostatically with the charged microparticle, known as collisional and electrostatic Coulomb ion drag, respectively, slows particle acceleration due to gravity as the particle falls through a plasma discharge. Since ion and neutral drag are commonly the dominant forces on microparticles in complex plasmas, the reduced acceleration is measured without a plasma to determine the neutral drag. By repeating the measurement with a plasma and subtracting the neutral drag, the ion drag is obtained. The microparticle net charge is then ascertained from the ion drag on isolated grains falling through a plasma discharge. * This work was supported by the Naval Research Laboratory Base Program. [Preview Abstract] |
Thursday, October 15, 2015 4:30PM - 4:45PM |
QR3.00005: An expression for the $h_l$ factor in low-pressure electronegative plasma discharges Pascal Chabert The positive ion flux exiting a low-pressure plasma discharge is a crucial quantity in global (volume-averaged) models. In discharges containing only electrons and positive ions (electropositive discharges), it is common to write this flux $\Gamma_{\rm wall}=h_ln_{\rm i0}u_B$, where $n_{\rm i0}$ is the central positive ion density, $u_B$ is the positive ion fluid speed at the sheath edge (the Bohm speed), and $h_l$ is the positive ion edge-to-centre density ratio. There are well established formulae for $h_l$ in electropositive discharges, but for discharges containing negative ions (electronegative discharges), the analysis is more complicated. The purpose of this paper is to propose a formula for the $h_l$ factor in electronegative discharges valid in a large parameter space of practical interest. We use the numerical solution of fluid equations including Poisson's equation as a guide to derive an analytical expression that can easily be incorporated in global models. [Preview Abstract] |
Thursday, October 15, 2015 4:45PM - 5:00PM |
QR3.00006: Nanoparticle heating in atmospheric pressure plasmas Nicolaas Kramer, Eray Aydil, Uwe Kortshagen The plasma environment offers a number of attractive properties that allow for the generation of nanoparticle materials that are otherwise hard to produce by other means. Among these are the generally high temperatures that nanoparticles can attain within plasmas, enabling the generation of nanocrystals of high melting point materials. In low pressure discharges, these high temperatures are the result of energetic surface reactions that strongly heat the small nanoparticles combined with the relatively slow heat transfer to the neutral gas. At atmospheric pressure, the nanoparticle intrinsic temperature is much more closely coupled to the neutral gas temperature. We study the heating of nanoparticles in atmospheric pressure plasmas based on a Monte Carlo simulation that takes into account the most important plasma-surface reactions as well as the conductive cooling of nanoparticles through the neutral gas. We find that, compared to low pressure plasmas, significantly higher plasma densities and densities of reactive species are required in order to achieve nanoparticle temperatures comparable to those in low pressure plasmas. These findings have important implications for the application of atmospheric pressure plasmas for the synthesis of nanoparticle materials. [Preview Abstract] |
Thursday, October 15, 2015 5:00PM - 5:15PM |
QR3.00007: Behavior of Negative Hydrogen Ion and its Beam by Bias and Beam Extraction Voltages Haruhisa Nakano, Katsuyoshi Tsumori, Masashi Kisaki, Katsunori Ikeda, Shaofei Geng, Kenichi Nagaoka, Masaki Osakabe, Yasuhiko Takeiri, Osamu Kaneko, Gianluigi Serianni, Piero Agostinetti, Emanuele Sartori, Matteo Brombin, Christian Wimmer Negative hydrogen ion (H-) dynamics from production to beam extraction in H- source for fusion have not been enough understood in cesium-seeded negative-hydrogen-ion sources. This dynamics understanding contributes constructions of higher performance ion sources. The H- is produced on and emitted from plasma grid electrode (PG) which is boundary electrode between source plasma and beam. The H- density in the vicinity of the PG decreased with bias voltage (between PG and arc chamber) by suppression of H- emission and/or yield. The H- density decrement was observed in H- beam extraction phase and penetrated to 30 mm depth from PG. The depth and H- beam current decreased with bias voltage. One of the possibilities which explain it is extracted H- coming from space in the vicinity of the PG. An object made of ceramic was inserted above the PG aperture. The H- beam intensity decreased if the object was set 9 mm from PG. This does not conflict with the possibility. [Preview Abstract] |
Thursday, October 15, 2015 5:15PM - 5:30PM |
QR3.00008: Plasma Particle Lofting Lucas Heijmans, Sander Nijdam In plasma particle lofting, macroscopic particles are picked up from a surface by an electric force. This force originates from a plasma that charges both the surface and any particle on it, leading to an electric force that pushes particles off the surface. This process has been suggested as a novel cleaning technique in modern high-tech applications, because it has intrinsic advantages over more traditional methods. Its development is, however, limited by a lack of knowledge of the underlying physics. Although the lofting has been demonstrated before, there are neither numerical nor experimental quantitative measures of it. Especially determining the charge deposited by a plasma on a particle on a surface proves difficult. We have developed a novel experimental method using a ``probe force.'' This allows us to, for the first time, quantitatively measure the plasma lofting force. By applying this method to different plasma conditions we can identify the important plasma parameters, allowing us to tailor a plasma for specific cleaning applications. Additionally, the quantitative result can help in the development of new models for the electron and ion currents through a plasma sheath. [Preview Abstract] |
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