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 FT1: Capacitively-Coupled Plasmas I |
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Chair: Amy Wendt, University of Wisconsin Room: Saratoga Hilton Ballroom 1 |
Tuesday, October 20, 2009 8:00AM - 8:30AM |
FT1.00001: The electrical asymmetry effect in capacitive discharges Invited Speaker: One of the major demands in plasma processing has always been the independent control of ion energy and ion flux. Dual-frequency capacitive discharges with one low and one typically an order of magnitude higher frequency are one of the concepts presently applied in industry. However, recent investigations have shown that there is in fact a coupling between the two frequency components that limits independent control by the two RF powers. Here, a novel concept is introduced based on the electrical asymmetry effect (EAE) that provides simple and stable control of ion energy and flux in an almost ideally independent way [1]. Also here two RF frequencies are applied but with the second frequency being exactly the second harmonic of the first and with a fixed but controllable phase. This phase is the control parameter for the ion energy that changes approximately linearly with the phase. Geometrically symmetric discharges can be made effectively asymmetric with one electrode showing a higher sheath potential than the other. Choosing the proper phase allows then to reverse the situation or to make the discharge symmetric. In geometrically asymmetric discharges the wall potential can be raised or lowered. When tuning the phase, the flux stays approximately constant and its absolute value can be set with the RF amplitudes. The concept of the EAE is developed and analyzed by 1) an analytical model, 2) a hydrodynamic and Monte-Carlo (MC) simulation, 3) a self consistent PIC/MC simulation, and 4) an experimental verification in a laboratory experiment. All four approaches show excellent agreement and confirm the above advantages. The technique has found successful application already in an industrial reactor for large area solar cell production (Leybold Optics). Compared to the standard single frequency case at 13.56 MHz the silicon deposition rate was easily more than doubled and the homogeneity improved. \\[4pt] [1] Brian G. Heil, U. Czarnetzki, R. P. Brinkmann, T. Mussenbrock, JPhysD: Appl. Phys. 42, 165202 (2008) [Preview Abstract] |
Tuesday, October 20, 2009 8:30AM - 8:45AM |
FT1.00002: Experimental investigations of the Electrical Asymmetry Effect Julian Schulze, Edmund Schuengel, Dirk Luggenhoelscher, Uwe Czarnetzki, Zoltan Donko In 2008 a method to generate a variable DC self bias even in geometrically symmetric capacitively coupled radio frequency (CCRF) discharges was proposed theoretically [1]. If the discharge is operated at a fundamental frequency and its second harmonic, it has been predicted that the resulting DC self bias can be adjusted by the phase angle between the applied voltage harmonics. A PIC simulation demonstrated that this Electrical Asymmetry Effect (EAE) allows separate control of the energy and flux of ions at the electrode surfaces. Here the EAE and the related separate control of ion energy and flux is investigated experimentally in a geometrically symmetric CCRF discharge operated at 13.56 MHz and 27.12 MHz with variable phase shift between the harmonics in argon at different pressures. The DC self bias, the energy as well as the flux of ions at the grounded electrode are measured. The experimental results verify the theoretical predictions and show that ion energy and flux can be controlled separately. The EAE is optimized by choosing low and high frequency voltage amplitudes that yield the strongest relative DC self bias. \\[4pt] [1] Heil B G et al. 2008 J. Phys. D 41 165202 [Preview Abstract] |
Tuesday, October 20, 2009 8:45AM - 9:00AM |
FT1.00003: Dual-frequency capacitive radiofrequency discharges: Effect of low-frequency power on electron density and flux Jean-Paul Booth, Garrett Curley, Dragana Maric, Jerome Bredin, Pascal Chabert Dual-frequency capacitively-coupled etch reactors using Ar/fluorocarbon/O2 mixtures are widely employed for etching of dielectric films for integrated circuit manufacture. We have measured the ion flux to the wall and the center electron density (using a microwave hairpin resonator) as a function of 2 and 27 MHz power (W2 and W27) in a modified industrial etch reactor. In Ar/O2 discharges both flux and density increase progressively with both W2 and W27, and the flux/density ratio remains constant, in accordance with simple electropositive transport theory. The high plasma densities observed can be attributed to the large secondary electron emission coefficient of oxidized Si. In Ar/C4F8/O2 mixtures flux and density are again increased by both W2 and W27. However, the electron density is much lower, and the ratio flux/density is not constant, reaching very high values for high W2/W27 ratios. The reasons for this will be discussed in terms of negative ion production and plasma chemistry. [Preview Abstract] |
Tuesday, October 20, 2009 9:00AM - 9:15AM |
FT1.00004: Non-linear frequency coupling in dual radio-frequency atmospheric pressure plasmas Jochen Waskoenig, Timo Gans Dual frequency operation provides additional control over power coupling and ionization mechanisms in radio-frequency driven atmospheric pressure plasmas. The tailored electron dynamics allows manipulation of mode transitions and plasma chemistry. Numerical simulations, benchmarked against experiments using phase resolved optical emission spectroscopy, reveal that plasma ionization, and associated mode transitions, are governed through frequency coupling in the dynamics of the plasma boundary sheath. Ionization in low-power mode is determined by the non-linear coupling of electron heating and the momentary local plasma density. Ionization in high-power mode is driven by electron avalanches during phases of transient high electric fields within the boundary sheath. The transition between these distinctly different modes is controlled by the total voltage of both frequency components. Under certain conditions it is observed that plasmas operated in helium with small admixtures of oxygen can contain significant densities of negative ions influencing the sheath dynamics and creating transient double layers. [Preview Abstract] |
Tuesday, October 20, 2009 9:15AM - 9:30AM |
FT1.00005: EED$f$ of the DC+RF Hybrid Etcher: Simulation and Measurement Lee Chen, Lin Xu, Merritt Funk The DC+RF Hybrid is a RF-capacitively coupled plasma etcher with RF applied to the wafer electrode and a high-negative DC voltage on the opposite electrode 3cm away. Secondary electrons from the DC electrode are accelerated by sheath and form ballistic electrons. Gridded energy analyzers are placed behind the RF electrode for EED$f$ measurements. Experiment's pressure-range varies from 30mt to 70mt with DC-voltage up to --1kV. EED$f$ reveals, (1) Maxwellian bulk, (2) ballistic electrons with energy corresponding to the applied DC-voltage, (3) a continuum from Maxwellian to the ballistic electron peak, (4) middle-energy electrons with distinct energy-peak. Measured EED$f$ qualitatively agree with PIC numerical experiments. The energy of the distinct middle-energy peak seems to depend on the sheath thickness and varies from $\sim $ 40eV to 300eV. While ballistic electrons' finite collisions contribute to the continuum, other non-negligible channel such as Landau-damped e$^{-}$-beam plasma waves, should be considered. The distinct middle-energy peak could be resulted from Landau damping of a strong plasma wave of a specific wave number. The energy range of middle-energy peak is favorable in sustaining ionization, rendering the necessity of heating the Maxwellian bulk for a similar level of ionization. [Preview Abstract] |
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