Bulletin of the American Physical Society
66th Annual Gaseous Electronics Conference
Volume 58, Number 8
Monday–Friday, September 30–October 4 2013; Princeton, New Jersey
Session QR2: Capacitively Coupled Plasmas II |
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Chair: Ikuo Sawada, Tokyo Electron U.S. Room: Ballroom II |
Thursday, October 3, 2013 3:30PM - 4:00PM |
QR2.00001: Electron heating in capacitively coupled plasmas revisited: single and multi-frequency discharges Invited Speaker: Trevor Lafleur Using particle-in-cell (PIC) simulations, we re-analyse the mechanism of electron heating in low pressure capacitively coupled plasmas (CCPs). After equilibrium has been reached in the simulations, spatio-temporal moments of the electron distribution function are taken within the rf cycle, and from this the density, current, pressure, and momentum loss due to collisions, of the electrons is found in the discharge. With these moments we then reconstruct each term in the electron fluid mechanical energy conservation equation, so as to explicitly analyse the power deposition process. We perform simulations for both single frequency sinusoidal discharges, and also for more recent multi-frequency, or ``tailored voltage waveform'' driven discharges. The single frequency (13.56 MHz) simulations are modelled on the original experiments in Argon performed by Godyak, showing the transition from a bi-Maxwellian distribution function at low pressure (below 200 mTorr) to a Druyvestyen-type distribution at high pressures (above 400 mTorr). The results of the PIC moment analysis shows that only two terms in the fluid conservation equation contribute a net power deposition: a term accounting for collisional power absorption, and a term accounting for pressure heating. The latter term is dominant at low pressures, while the former is dominant at higher pressures. We find however that the collisional heating is almost always significant, and even at the lowest pressure, accounts for about 40\% of the total power absorption. By comparing the electron momentum loss due to collisions with that usually used in analytical sheath models, we find a significant difference at low pressures, which cannot be explained by conventional local kinetic theories based on the two-term expansion of the Boltzmann equation. The moment analysis is repeated for the multi-frequency discharges, where we obtain similar results: collisional power absorption is always observed to be significant, even at the lowest pressures simulated (20 mTorr). However the generation of a bias voltage due to the electrical asymmetry effect, and consequently the unequal division of the sheath voltages, causes high frequency oscillations to develop in the plasma at frequencies more than an order of magnitude higher than the applied rf frequencies. These so-called nonlinear plasma series resonance oscillations are found to enhance both the collisional and pressure heating, and for sufficiently large applied voltages, an additional heating mechanism is identified associated with the electron inertial terms in the conservation equation. [Preview Abstract] |
Thursday, October 3, 2013 4:00PM - 4:15PM |
QR2.00002: Electron heating and control of ion properties in capacitive discharges driven by customized voltage waveforms Julian Schulze, Aranka Derzsi, Ihor Korolov, Edmund Schuengel, Zoltan Donko We investigate the electron heating dynamics in capacitive radio frequency plasmas driven by customized voltage waveforms and study the effects of modifying this waveform on the DC self-bias, $\eta $, the ion flux, $\Gamma _{i}$, and the mean ion energy, E$_{i}$, at the electrodes by PIC simulations. The driving voltage waveform is customized by adding N consecutive harmonics (N $\le $ 4) of 13.56 MHz with specific harmonics' amplitudes and phases. In an argon plasma, we find $\eta $ to be generated via the Electrical Asymmetry Effect for N $\ge $ 2. $\eta $ can be controlled by adjusting the harmonics' phases and is enhanced by adding more consecutive harmonics. At 3 Pa, the discharge is operated in the $\alpha $-mode and E$_{i}$ can be controlled by adjusting the phases at constant $\Gamma_{i}$. The ion flux can be increased by adding more harmonics due to the enhanced electron sheath heating. However, we find E$_{i}$ not to remain constant as a function of N at both electrodes due to a change of $\eta $ as a function of N. At 100 Pa and using a high secondary electron emission coefficient of $\gamma =$ 0.4, the discharge is operated in the $\gamma $-mode. Due to this mode transition and the specific ionization dynamics in the $\gamma $-mode, $\Gamma_{i}$ is no longer constant as a function of the harmonics' phases and decreases with increasing N. [Preview Abstract] |
Thursday, October 3, 2013 4:15PM - 4:30PM |
QR2.00003: Student Award Finalist - The role of surface properties in the dynamics of radio-frequency plasma sheaths: measurements and simulations Arthur Greb, Andrew Gibson, Kari Niemi, Deborah O'Connell, Timo Gans Plasma processing on an industrial scale is becoming increasingly complex and now demands new strategies for process metrology. Of particular interest is the energy transport in the interface region between non-equilibrium low-pressure plasma and the surface. Experimental measurements are coupled to a benchmarked 1D fluid model, with improved energy dependent treatment of ion mobilities, for a geometrically asymmetric capacitively coupled oxygen rf discharge~[1]. Within a pressure range of $10-100$~Pa the simulations predict that changing surface conditions have a significant effect on the surface loss probability and lifetime of metastable oxygen, and consequently electronegativity, as well as the secondary electron emission coefficient. These substantially influence the plasma sheath dynamics on a nanosecond timescale. For different surface materials, we confirm our findings by comparing excitation features obtained from simulations with phase resolved optical emission spectroscopy measurements. This allows us to develop new metrology concepts to monitor and control plasma-surface interaction processes in real-time. The authors thank Intel Ireland, Ltd. for supporting this research.\\[4pt] [1] A. Greb et al., Phys. Plasmas 20 (2013) [Preview Abstract] |
Thursday, October 3, 2013 4:30PM - 4:45PM |
QR2.00004: An Analytical Model for the Radio-Frequency Sheath Uwe Czarnetzki An analytical model for the planar radio frequency (RF) sheath in capacitive discharges is developed based on the applied RF voltage as the boundary condition. In a first step, the individual sheath voltages and the self-bias are calculated using a cubic-charge voltage relation. In the second step, a single integro-differential equation is derived to describe the ion flow velocity in the sheath under all conditions of collisionality. Central to the model is the screening function that describes the screening of the ion density by the mean electron density in the sheath. Numerical integration of the sheath equation is straight forward. However, for the collisionless as well as the collisional case explicit, simple, and precise analytical approximations can be found. Drift velocities, densities, fields, currents, and charge-voltage relations are calculated. Further, the Child-Langmuir laws for both cases of collisonality are derived. These solutions are in very good agreement with experimental data from the literature based on laser electric field measurements, the Brinkmann sheath model, and PIC simulations. The technique works well also for other waveforms, e.g. the electrical asymmetry effect or tailored pulse waveforms. [Preview Abstract] |
Thursday, October 3, 2013 4:45PM - 5:00PM |
QR2.00005: Transient plasma parameters in pulsed RF CCP discharges Theresa Kummerer, David Coumou, Steven Shannon Low pressure plasmas driven by kHz pulsed RF power sources provide very unique conditions for materials processing. The electrical transients generated by these rapid pulses present unique challenges for efficient power delivery and control; these challenges are compounded further in systems with multiple power sources. To advance closed loop control of power delivery through these transients and further advance pulsed RF power delivery, a detailed study of plasma conditions during the on and off transitions in the pulse cycle has been carried out. Time resolved Langmuir probe, OES, and gated CCD images are combined with high time resolution in-line RF metrology to study changes in plasma conditions and their impact on discharge impedance and power delivery for both single and multiple independent RF source configurations. By correlating plasma parameters with electrical measurement, an extension of existing empirical models that measure plasma parameters through RF measurement is made. This extended model will provide time resolved plasma parameters within the pulse cycle, enabling pulse parameter optimization (pulse frequency, duty cycle, etc.) for critical processes using in-situ diagnosis. [Preview Abstract] |
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