Bulletin of the American Physical Society
65th Annual Gaseous Electronics Conference
Volume 57, Number 8
Monday–Friday, October 22–26, 2012; Austin, Texas
Session DT4: Capacitively Coupled Plasmas |
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Chair: Pascal Chabert, Ecole Polytechnique - CNRS Room: Salon DE |
Tuesday, October 23, 2012 10:00AM - 10:15AM |
DT4.00001: Plasma characteristics in non-sinusoidally excited CCP discharges Trevor Lafleur, Jean-Paul Booth Using particle-in-cell (PIC) simulations we perform a characterization of the plasma response to positive pulse-type voltage excitations (with a repetition frequency of 13.56 MHz) in a geometrically symmetric CCP reactor (with a gap length of 2 cm) operated with argon (for pressures between 20-500 mTorr). Use of these non-sinusoidal waveforms generates an electrical asymmetry effect in the system, which necessitates the formation of a DC bias. This DC bias, together with the shape of the voltage waveforms used, produces a number of new phenomena that are not present in typical sinusoidal discharges: (1) the plasma density and ion flux can be increased as the pulse width is reduced, (2) a significant asymmetry in the ion fluxes to the powered and grounded electrodes develops as the pressure increases, (3) the average ion energy striking the grounded electrode remains low and approximately constant as the pulse width decreases, and (4) the sheath at the grounded electrode never fully collapses; electrons are no longer lost in sharp pulses, but escape essentially throughout the rf cycle. Effects (1) and (3) above offer the possibility for a new form of control in these types of discharges, where the ion flux can be increased while the ion energy on the grounded electrode can be kept small and essentially constant. This effect has recently been exploited to control the crystallinity of silicon thin films [1], where the low ion bombarding energy was found to improve the quality of films grown. \\[4pt] [1] Johnson E V, Pouliquen S, Delattre P A, and Booth J P, \textit{J. Non-Cryst. Solids} 2012, in press. [Preview Abstract] |
Tuesday, October 23, 2012 10:15AM - 10:30AM |
DT4.00002: Tailored Voltage Waveform Capacitively-Coupled Plasmas Jean-Paul Booth, Trevor Lafleur, Pierre-Alexandre Delattre, Erik Johnson A major limitation of large-area capacitively-coupled plasmas for materials processing is the inability to increase plasma density without increasing ion bombardment energy. Heil et al. (J. Phys. D 41. 165202, (2008)) demonstrated that for a driving voltage comprising one frequency, f, and it's harmonic 2f, the symmetry of the sheaths can be broken (the Electrical Asymmetry Effect, EAE). We have investigated large-area plasmas (50cm dia) in Ar driven by arbitrary voltage waveforms. Specifically we studied waveforms comprising sharp positive pulses (10-20ns wide, 15MHz repetition frequency). The voltage waveform was measured by an HV probe close to the powered electrode edge, the electron density was measured with a microwave hairpin resonator, the ion flux was measured by an array of planar ion flux probes in the grounded counter-electrode, and the power absorbed was determined from the current and voltage waveforms measured by a derivative probe. As well as the expected EAE observed in the electrode self-bias, we were able to demonstrate a dramatic increase in electron density (and concomitant increased power absorption) with reduced pulse-width at constant amplitude, in qualitative agreement with recent PIC simulations (Lafleur et al, APL 100, 194101(2012)). [Preview Abstract] |
Tuesday, October 23, 2012 10:30AM - 10:45AM |
DT4.00003: A numerical analysis of plasma non-uniformity in the parallel plate VHF-CCP and the comparison among various model Ikuo Sawada We measured the radial distribution of electron density in a 200 mm parallel plate CCP and compared it with results from numerical simulations. The experiments were conducted with pure Ar gas with pressures ranging from 15 to 100 mTorr and 60 MHz applied at the top electrode with powers from 500 to 2000W. The measured electron profile is peaked in the center, and the relative non-uniformity is higher at 100 mTorr than at 15 mTorr. We compare the experimental results with simulations with both HPEM and Monte-Carlo/PIC codes. In HPEM simulations, we used either fluid or electron Monte-Carlo module, and the Poisson or the Electromagnetic solver. None of the models were able to duplicate the experimental results quantitatively. However, HPEM with the electron Monte-Carlo module and PIC qualitatively matched the experimental results. We will discuss the results from these models and how they illuminate the mechanism of enhanced electron central peak.\\[4pt] [1] T. Oshita, M. Matsukuma, S.Y. Kang, I. Sawada: \textit{The effect of non-uniform RF voltage in a CCP discharge}, The 57$^{th}$ JSAP Spring Meeting 2010\\[4pt] [2] I. Sawada, K. Matsuzaki, S.Y. Kang, T. Ohshita, M. Kawakami, S. Segawa: 1-st IC-PLANTS, 2008 [Preview Abstract] |
Tuesday, October 23, 2012 10:45AM - 11:00AM |
DT4.00004: Extension of CCP DC Self Bias Models To The Case of Fractional Diversion of Powered Electrode Current to Ground D.L. Keil, E. Augustyniak, E. Dorai, F. Galli In many commercial CCP plasma process systems the DC-self bias is available as a reported process parameter. Since commercial systems typically limit the number of onboard diagnostics, there is great incentive to understand how DC-self bias can be expected to respond to various system perturbations. This work examines the effect on DC-self bias due ``bleeding'' DC current to ground through an RF filter and resistor. By extending the work of Y.P. Song et.al. [1] a relationship between this bleed current and DC bias is developed that predicts DC-self bias change in terms of electrode areas and the Bohm currents to each electrode. Additionally, a circuit model is also presented which gives similar results. These models are then compared to experimental results, with model fit values providing an experimental measure of electrode areas and Bohm currents. \\[4pt] [1] Y.P. Song, D. Field and D.F. Klemperer, J. Phys. D Appl. Phys 23 (1990) pgs 673-681 [Preview Abstract] |
Tuesday, October 23, 2012 11:00AM - 11:15AM |
DT4.00005: Analysis of the phase controlled capacitively coupled plasma using triode circuit model Myung-Sun Choi, Seok-Hwan Lee, Gon-Ho Kim, Dougyong Sung Phase controlled CCP source is attracted recently for high density capacitively coupled VHF plasma source to enhance their poor plasma uniformity due to standing wave effect by controlling phase difference. And it also enhances process performance. But the phase controlled CCP has hardness of controlling plasma density due to the asymmetry of the curve for plasma density with changing phase and its non-linear drift. To understand these characteristics of phase controlled CCP, this work investigates the effect of wall electrode using triode circuit model and compares with experimental results in 100MHz phase controlled CCP source. Plasma density varies with changing ratio of current flowing through the chamber wall and current flowing between electrodes. The asymmetry of the curve for plasma density with phase is due to the effect of area difference of powered electrodes, and this asymmetry increases with the current flowing through the wall electrode. we are going to report the asymmetry effect of electrodes on the plasma density control of phase controlled CCP and discuss the difference of physical area ratio and effective area ratio of triode CCP source. [Preview Abstract] |
Tuesday, October 23, 2012 11:15AM - 11:30AM |
DT4.00006: The effect of the driving frequencies on the Electrical Asymmetry Effect in dual-frequency capacitive radio frequency plasmas Julian Schulze, Ihor Korolov, Uwe Czarnetzki, Zoltan Donko In capacitive radio frequency discharges driven by two consecutive phase locked harmonics, an electrical asymmetry is induced and a DC self bias is generated as a function of the phase shift between the driving frequencies. Until now, only dual-frequency discharges operated at a fundamental frequency of 13.56 MHz have been investigated. It was shown, that a maximum self bias of 25{\%} of the driving voltage amplitude can be generated electrically and that the mean ion energy at the electrodes can be controlled separately from the ion flux by adjusting the phase in a geometrically symmetric reactor. Here, we study the effect of changing this fundamental frequency between 0.5 MHz and 27.12 MHz on the Electrical Asymmetry Effect by Particle-in-Cell simulations and an analytical model for different $\gamma $-coefficients. We find the electrical generation of the DC self bias and the quality of the separate control of ion properties to be strongly reduced at lower frequencies. This is caused by the effect of the driving frequencies on the charge and electron heating dynamics. These effects are understood by the model. [Preview Abstract] |
Tuesday, October 23, 2012 11:30AM - 11:45AM |
DT4.00007: Electromagnetic Particle-in-Cell Simulation of Large Area High-Frequency Capacitive Coupled Plasmas for the Reduction of Standing Wave Effects Jin Seok Kim, In Cheol Song, Seok Won Hwang, Ho-Jun Lee, Hae June Lee A finite difference time domain (FDTD) method was adopted to investigate the electric-field distributions depending on the driving frequency of a 450-mm Capacitive Coupled Plasma (CCP) reactor. High frequency wave contains a short wavelength which induces standing wave effects in the CCP reactor. The induced standing waves make non-uniform electric field distributions which causes bad effects on the uniformity of plasma processing. This presentation shows the plasma density uniformity depending on electrode structures and other factors. The investigation of standing wave effect is coupled with the electromagnetic particle-in-cell (PIC) simulation. A PIC simulation shows very high accuracy compared with fluid simulations and gives more information for electron and ion energy distributions, but has a disadvantage of slow speed caused by individual calculation of lots of computational particles. A computation method using graphic processing units makes it possible to establish a low-cost and low-power personal super computer. In order to overcome the heavy computation problem of a PIC method, we have developed a two-dimensional parallelized PIC code utilizing GPUs. In this presentation, the coupled simulation is presented using FDTD electromagnetic wave solver and PIC method. [Preview Abstract] |
Tuesday, October 23, 2012 11:45AM - 12:00PM |
DT4.00008: Electric fields measurements in the collisional RF plasma pre-sheath during microgravity conditions Job Beckers, Dirk Trienekens, Gerrit Kroesen When a plasma comes into contact with a solid body, a space charge region is formed near that surface. This is due to the difference in mobility between the ions and the much lighter electrons. Electric fields in the plasma sheath have demonstrated to be key in almost every plasma application where the acceleration of ions at the border of the discharge is utilized. However, measuring these fields is extremely difficult. In the lower regions of the plasma sheath, where the electric fields are high, they have been measured by Stark Broadening and Stark shift. To gain higher spatial resolution we have recently developed a novel tool using microparticles under hyper-gravity conditions in a centrifuge. Consequently, measuring at positions closer to the plasma bulk than the equilibrium position of the microparticle under normal gravity conditions was impossible. In this paper we present a continuation of this research line towards positions closer to the plasma bulk. This is achieved a combination of measurements of the microparticle equilibrium position under microgravity conditions during parabolic flights and a collisional plasma sheath model. We have been able to measure the electric field throughout the plasma sheath and a part of the pre-sheath region. [Preview Abstract] |
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