Bulletin of the American Physical Society
62nd Annual Gaseous Electronics Conference
Volume 54, Number 12
Tuesday–Friday, October 20–23, 2009; Saratoga Springs, New York
Session LW1: Dusty and Negative Ion Plasmas |
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Chair: Lawrence Overzet, University of Texas at Dallas Room: Saratoga Hilton Ballroom 1 |
Wednesday, October 21, 2009 8:00AM - 8:15AM |
LW1.00001: Numerical simulations of a nanodusty RF plasma Steven Girshick, Pulkit Agarwal The nucleation, growth, charging and transport of nanoparticles in a low-pressure RF plasma have profound effects on the plasma. In previous work we developed a numerical model for the spatiotemporal evolution of the plasma-nanoparticle system. The model simulates a one-dimensional parallel-plate capacitively-coupled plasma. It includes solutions to electron and ion population balance equations, and Poisson's equation for the electric field, models stochastic particle charging, and self-consistently solves the aerosol general dynamic equation, including particle size- and charge-dependent coagulation (including the effect of image potentials), and particle transport by electrostatic forces, Brownian diffusion, ion drag and thermophoresis. In the present work this model is used to explore the effect of system parameters on the nanoparticle and plasma behavior. Parameters studied include pressure, temperature and temperature distribution, applied voltage and gas flow rate. In addition we examine the behavior of the system at long times, when particles are pushed by ion drag out of the center of the plasma, opening a void that allows fresh nucleation. [Preview Abstract] |
Wednesday, October 21, 2009 8:15AM - 8:30AM |
LW1.00002: Model and experimental evidence of heating of nanoparticles in low-pressure silane plasmas Federico Galli, Uwe Kortshagen Recently we developed a nanoparticle-plasma charging-heating model which included the effect of collisions between ions and neutrals in proximity of the particles and showed that for pressures of a few Torr a charge distribution that is less negative than collisionless orbital motion limited theory is obtained. The model also predicted the nanoparticle temperature distribution and morphology (amorphous or crystalline) to be a function of ion density. To support the theory a batch plasma reactor was used to nucleate, grow and crystallize silicon nanoparticles. The nanoparticle size distribution and morphology were characterized using transmission electron microscopy, x ray diffraction and Raman light scattering. Experimental results indeed show a strong correlation between crystallinity and ion density, here measured with time and space resolution using a capacitive probe technique. The use of a simple floating potential probe method is also presented. The probe traces show distinctive features belonging to a nucleation and growth phase and a successive diffusional loss phase. A simple theoretical model is proposed to explain probe measurements as a function of the other plasma properties. [Preview Abstract] |
Wednesday, October 21, 2009 8:30AM - 8:45AM |
LW1.00003: Formation of ion-ion plasmas and ion beams in continuously rf generated plasmas Ane Aanesland, Lara Popelier, Pascal Chabert Ion-ion plasmas can offer useful advantages in a variety of applications where neutral or quasi-neutral beams are used, some examples are charge-free etching, neutral beam injections for fusion or for electric propulsion. For many electronegative gases the cross sections are such that ionizing collisions (creating positive ions) are dominating at high electron temperatures while electron attachment dominates at low Te (creating negative ions). Hence, ion-ion plasmas are therefore generally formed in the afterglow of electronegative plasmas or in the periphery of magnetized electronegative plasmas; in both cases the ion-ion plasma is formed where or when the electron temperature drops and efficient electron attachment can occur. The ion density in the ion-ion plasma region drops generally by a factor of ten or more, compared to the regions where electrons are present. This density decrease is catastrophic for the efficiency of any application using ion-ion plasmas. We demonstrate here that by tailoring Te and by injecting the electronegative gas in both a high and a low Te region can efficiently produce ion-ion plasmas. Grid-less extraction and acceleration of ions from an ion-ion plasma, by biasing the bulk plasma to large positive or negative voltages, is being investigated. Results from this experimental investigation will be reported here. [Preview Abstract] |
Wednesday, October 21, 2009 8:45AM - 9:00AM |
LW1.00004: A global (volume averaged) model of the chlorine discharge Jon Tomas Gudmundsson, Eythor Gisli Thorsteinsson A global (volume averaged) model is developed for the chlorine discharge using a revised reaction set [1]. The model is applied to explore both a steady state and pulse modulated discharge. Various calculated plasma parameters are compared to measurements found in the literature, showing a good overall agreement. The reaction rates for the various reactions are evaluated in the pressure range 1 -- 100 mTorr. In particular we explore the dissociation process as well as the creation and destruction of the negative ions Cl$^-$. The mechanism for Cl creation is complex, although electron impact dissociation dominates with roughly 50 -- 60 \% contribution, dissociative electron attachment is also of importance and mutual neutralization is an important contributor to the production of Cl atoms at higher pressures. The electronegativity increases rapidly with decreasing dissociation fraction since the Cl$^-$ ions are created entirely by dissociative electron attachment, predominantly from Cl$_2(v=0)$ but Cl$_2(v>0)$ has upto 14 \% contribution at 100 mTorr. The negative ion Cl$^-$ is lost almost entirely through mutual neutralization with Cl$^+_2$, but Cl$^+$ has a significant contribution at low pressure. Furthermore, dilution by argon was explored. Dilution by argon increases the electron temperature and the density of Cl$^+$ ions significantly. [1] E. G . Thorsteinsson and J. T. Gudmundsson, Plasma Sources Science and Technology, submitted 2009 [Preview Abstract] |
Wednesday, October 21, 2009 9:00AM - 9:15AM |
LW1.00005: Negative ion temperature in a low-pressure oxygen ICP discharge Derek Monahan, Miles Turner The primary reaction mechanism leading to the formation of negative ions under typical oxygen discharge conditions is dissociative attachment of molecular $\mathrm{O}_2$.Though unable to absorb significant energy from the applied rf field these ions are formed ``hot'', acquiring kinetic energy from the local ambipolar field and via the process of chemical dissociation. However, their negative charge and large mass (relative to the electrons) means they are confined to the discharge bulk and ``cooled'' efficiently in elastic collisions with the background gas. Thus, one commonly assumes that the effective negative ion temperature, $T_-$, is in approximate equilibrium with the background gas temperature, $T_{\mathrm{g}}$. In our ICP-like oxygen plasma simulations, we observe a notable increase in $T_-$ as the $O_2(a^1\Delta_g)$ metastable density is increased. We attribute this increase in effective temperature to an increase in the destruction rate of the negative ions and conclude that, in a sufficiently destructive plasma environment, $T_-$ may exceed $T_g$ considerably. [Preview Abstract] |
Wednesday, October 21, 2009 9:15AM - 9:30AM |
LW1.00006: Time averaged negative ion density measurements in a reactive pulsed DC magnetron using the eclipse photodetachment method Robert Dodd, ShaoDon You, Paul Bryant, James Bradley Using Langmuir probe assisted laser photodetachment technique time averaged negative ion density in the bulk plasma of a reactive pulsed DC magnetron has been determined. Experimental results are shown for various oxygen / argon gas mixtures (0 - 100{\%}), applied power (300 - 600 W), magnetron frequency (5 -- 100 kHz) and total discharge pressure (2 - 25 mTorr). Laser photodetachment measurements showed the O$^{-}$ ion to dominate over O$_{2}^{-}$. The maximum ratio of the negative ion to electron density, $\alpha $, was found to be 0.2, weakly electronegative plasma ($\alpha <$ 1). Variation of the operating parameters showed clear trends in the negative ion densities with maximums observed with increasing power (at 318 W) and oxygen partial pressure (30{\%} O$_{2})$. With variation of operating parameters clear trends in $\alpha $ were seen. For instance, with increasing magnetron power $\alpha$ was found to decrease from 0.2 to 0.01. This trend was accompanied by a steady rise in the electron density. This observation was attributed to the enhanced detachment (destruction) rate of O$^{-}$ ions as the energy of secondary electrons from the cathode increased with increased target voltage. These new results show significant concentrations of negative ions are present in the bulk magnetron plasma when operated in argon / oxygen gas mixtures during pulsed DC sputtering. [Preview Abstract] |
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