Bulletin of the American Physical Society
2006 59th Annual Gaseous Electronics Conference
Tuesday–Friday, October 10–13, 2006; Columbus, Ohio
Session WF1: Capacitively Coupled Plasmas |
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Chair: Miles Turner, Dublin City University, Ireland Room: Holiday Inn Salon CD |
Friday, October 13, 2006 10:00AM - 10:30AM |
WF1.00001: Negative Ions in Dual-Frequency Capacitively Coupled Fluorocarbon Plasmas Invited Speaker: Dual-frequency capacitively coupled plasmas in fluorocarbon-based gases are widely used for etching SiO$_{2}$-based dielectric films in integrated circuit manufacture. We have studied a customized 2 + 27 MHz industrial etch reactor, running in Ar/O$_{2}$ with c-C$_{4}$F$_{8}$ or CF$_{4}$ gas mixtures at pressures in the region of 50 mTorr (6.6 Pa). Negative ions could play an important role in this type of plasma. The presence of negative ions will modify the positive ion flux arriving at a surface, and may even reach the surface and participate in etching. We have measured the electron density using a microwave hairpin resonator [1] and the positive ion flux with an ion flux probe [2]: the ratio of these two quantities varies strongly with gas chemistry and gives evidence for the presence of negative ions [3]. For example, by varying the flow of c-C$_{4}$F$_{8}$ in an Ar/O$_{2}$ mixture this ratio shows evidence of high electronegativity for high c-C$_{4}$F$_{8}$ flowrates. We have also measured the negative fluorine ion, F$^{-}$, density directly by high-sensitivity cavity ring-down absorption spectroscopy [4] in the wavelength range 340 to 360 nm to determine the density of absorbing F$^{-}$ ions from the known photo-detachment cross-section. The F$^{-}$ densities were seen to reach values in the 10$^{11}$cm$^{-3}$ range, giving electronegative fractions, $\alpha $ = n$_{-}$/n$_{e}$, of up to $\approx $ 15 when used in conjunction with the hairpin probe measurements. We acknowledge financial assistance from Lam Research Corporation. \newline \newline [1] Piejak et al, J. Appl. Phys. \textbf{95} (2004), 3785-3791 \newline [2] Braithwaite et al, Plasma Sources Sci. Technol., \textbf{5} (1996), 677-684 \newline [3] Chabert et al, Plasma Sources Sci. Technol., \textbf{8} (1999), 561-566 \newline [4] Booth et al, Appl. Phys. Lett., \textbf{88} (2006), 151502 [Preview Abstract] |
Friday, October 13, 2006 10:30AM - 10:45AM |
WF1.00002: New insights into electron heating and ionisation mechanisms in CCP discharges at low pressures Deborah O'Connell, Timo Gans, Uwe Czarnetzki, David Vender, Rod Boswell Details on plasma sustainment of capacitively coupled plasmas (CCPs), at relatively low pressures when regular ohmic heating is not efficient, are an open question for decades. The main difficulty has been the extreme diagnostics challenge. Recent advances in phase resolved optical emission spectroscopy (PROES) has allowed detailed spatio-temporal investigations of the electron dynamics, on a nano-second time scale, within the RF cycle. PROES and particle-in-cell (PIC) simulation results show that at comparatively low pressures ($<$ 10 Pa) the main ionisation channel in CCPs is via a large amplitude electron beam plasma interaction powered by the electric field of the sheath expansion into the plasma. Following this interaction and its associated waves and plasma ionisation, a resonance at the sheath edge is observed in both the PIC simulation and PROES measurements. [Preview Abstract] |
Friday, October 13, 2006 10:45AM - 11:00AM |
WF1.00003: Electron heating mechanisms in dual frequency capacitive discharges M.M. Turner, P. Chabert We discuss electron heating mechanisms in the sheath regions of dual-frequency capacitive discharges, with the twin aims of identifying the dominant mechanisms and supplying closed-form expressions from which the heating power can be estimated. We show that the heating effect produced by either Ohmic or collisionless heating is much larger when the discharge is excited by a superposition of currents at two frequencies than if either current had acted alone. This coupling effect occurs because the lower frequency current, while not directly heating the electrons to any great extent, strongly affects the spatial structure of the discharge in the sheath regions. [Preview Abstract] |
Friday, October 13, 2006 11:00AM - 11:15AM |
WF1.00004: Electron heating and ionisation mechanisms in dual-frequency capacitively coupled radio-frequency discharges Timo Gans, Julian Schulze, Deborah O'Connell, Uwe Czarnetzki, Bert Ellingboe, Miles Turner Despite its technological importance, the complexity of power coupling mechanisms in radio-frequency (rf) discharges is not yet fully understood. Insight into power dissipation requires temporal resolution on various time scales, in particular the dynamics within the rf cycle. Electron dynamics and ionisation mechanisms in dual-frequency capacitively coupled rf discharges is investigated using phase resolved optical emission spectroscopy (PROES), resolving both the high (27.12 MHz) and low (1.94 MHz) frequency rf cycles, and particle in cell (PIC) simulations. The electron dynamics exhibits a complex spatio- temporal structure. Excitation and ionisation, and, therefore, plasma sustainment is dominated through directed energetic electrons created through the dynamics of the plasma boundary sheath. These electrons propagate through the discharge volume with finite velocity, and are predominantly produced during contraction of the low frequency sheath - not during the sheath expansion when power dissipation is highest. [Preview Abstract] |
Friday, October 13, 2006 11:15AM - 11:30AM |
WF1.00005: Nonlinear electron resonance heating in dual frequency capacitive discharges D. Ziegler, T. Mussenbrock, R.P. Brinkmann Capacitively coupled plasmas (CCP's) play a major role in material processing. The drawback of conventional single frequency CCP's is that the ion bombardment energy and the ion flux to the substrate itself cannot be controlled independently. The problem can be remedied by the use of dual frequency CCP's. In such sources, the ratio of the applied frequencies is obviously an important control parameter. Rauf$^1$ found that at large ratios (e.g., $100\,\rm kHz$/$13.56\,\rm MHz$) the spectrum of the discharge current was just the superposition of two single-frequency spectra. For more comparable frequencies (e.g.,$6.78\,\rm MHz$/$13.56\,\rm MHz$), however, quite surprising nonlinear effects were observed. It is exactly this nonlinear behavior and its influence on the total energy budget that is discussed in this contribution - we present and analyze a nonlinear global model for a dual frequency CCP. Based on an exact analytical solution of the underlying equations we discuss the behavior of the model for various voltage ratios, frequency ratios, and gas pressures. We investigate in particular the heating at the plasma series resonance, either by direct excitation or via the nonlinear electron resonance heating mechanism$^2$.\par\noindent$^1$ S. Rauf and M.J. Kushner, IEEE Trans Plasma Sci. {\bf 27}, 1329 (1999)\\ $^2$ T. Mussenbrock and R.P. Brinkmann, Appl. Phys. Lett. {\bf 88}, 151503 (2006) [Preview Abstract] |
Friday, October 13, 2006 11:30AM - 11:45AM |
WF1.00006: Global model of a Dual frequency Capacitive Discharge Pierre Levif, Pascal Chabert, Miles Turner A major attraction of dual-frequency excitation is that it promises independent control of the ion flux and the ion energy. The electron heating mechanisms occurring within the dual-frequency sheath region were recently investigated by Turner and Chabert (Phys. Rev. Letters (2006) \textbf{96}, 205001). It was shown that the heating produced by the superposition of the two frequencies is much larger than the sum of the two frequency contributions. In the present paper, we use the heating models developed to construct a global model of a dual-frequency capacitive discharge operated in argon. For this, we must also discuss the dynamics of the sheath to obtain the equivalent of a dual-frequency Child law which relates the applied rf voltage, the electron density and the sheath size. By coupling the power and particle balance to the Child law mentioned above, one can obtain a self-consistent solution for all the plasma parameters. A major result of this model is that ion flux and ion energy are not decoupled since the low-frequency significantly contributes to plasma heating. [Preview Abstract] |
Friday, October 13, 2006 11:45AM - 12:00PM |
WF1.00007: Frequency dependent ion kinetics in a 300 mm dual-frequency capacitively coupled plasma reactor G.A. Hebner, E.V. Barnat, P.A. Miller, A.M. Paterson, J.P. Holland Argon ion kinetics were measured in a dual frequency, capacitively coupled 300 mm chamber. Laser induced fluorescence measurements of the argon ion metastable lineshape yield information on the ion temperature, density and drift velocity. The spatially-resolved LIF technique is a nonperturbative probe to investigate energy deposition mechanisms, ion energy distribution functions, charge exchange reactions, neutral heating, and plasma potential gradients within the plasma. This talk will discuss ion characteristics for a single rf frequency drive (13, 60 and 160 MHz), combinations of rf drive frequencies, as well as scaling with pressure (10 -- 70 mTorr) , rf power, and radial position. We find that the ion density increased linearly with rf power, as did the electron density, indicating the ion metastable state is formed from direct impact ionization. The ion temperature was on the order of 500 K. Radially resolved ion drift velocity measurements show the radial drift velocity can be lower at 60 MHZ than 13 MHz. Additional details will be discussed. This work was supported by Applied Materials and Sandia National Laboratories, a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
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