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Register a new userDim and bright void regimes in capacitively-coupled RF complex plasmas
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A. Pikalev
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We demonstrate experimentally that the void in capacitively-coupled RF complex plasmas can exist in two qualitative different regimes. The bright void is characterized by bright plasma emission associated with the void, whereas the dim void possesses no detectable emission feature. The transition from the dim to the bright regime occurs with an increase of the discharge power and has a discontinuous character. The discontinuity is manifested by a kink in the void size power dependencies. We reproduce the bright void (mechanically stabilized due to the balance of ion drag and electrostatic forces) by a simplified time-averaged 1D fluid model. To reproduce the dim void, we artificially include the radial ion diffusion into the continuity equation for ions, which allows to mechanically stabilize the void boundary due to very weak electrostatic forces. The electric field at the void boundary occurs to be so small that it, in accordance with the experimental observation, causes no void-related emission feature.
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In this work, we analyze the creation of the discharge asymmetry and the concomitant formation of the DC self-bias voltage in capacitively coupled radio frequency plasmas driven by multi-frequency waveforms, as a function of the electrode surface characteristics. For this latter, we consider and vary the coefficients that characterize the elastic reflection of the electrons from the surfaces and the ion-induced secondary electron yield. Our investigations are based on Particle-in-Cell/Monte Carlo Collision simulations of the plasma and on a model that aids the understanding of the computational results. Electron reflection from the electrodes is found to affect slightly the discharge asymmetry in the presence of multi-frequency excitation, whereas secondary electrons cause distinct changes to the asymmetry of the plasma as a function of the phase angle between the harmonics of the driving voltage waveform and as a function the number of these harmonics.
The kinetic origin of resonance phenomena in capacitively coupled radio frequency plasmas is discovered based on particle-based numerical simulations. The analysis of the spatio-temporal distributions of plasma parameters such as the densities of hot and cold electrons, as well as the conduction and displacement currents reveals the mechanism of the formation of multiple electron beams during sheath expansion. The interplay between highly energetic beam electrons and low energetic bulk electrons is identified as the physical origin of the excitation of harmonics in the current.
We propose a method to generate a single peak at a distinct energy in the ion flux-energy distribution function (IDF) at the electrode surfaces in capacitively coupled plasmas. The technique is based on the tailoring of the driving voltage waveform, i.e. adjusting the phases and amplitudes of the applied harmonics, to optimize the accumulation of ions created by charge exchange collisions and their subsequent acceleration by the sheath electric field. The position of the peak (i.e. the ion energy) and the flux of the ions within the peak of the IDF can be controlled in a wide domain by tuning the parameters of the applied RF voltage waveform, allowing optimization of various applications where surface reactions are induced at particular ion energies.
In most PIC/MCC simulations of radio frequency capacitively coupled plasmas (CCPs) several simplifications are made: (i) fast neutrals are not traced, (ii) heavy particle induced excitation and ionization are neglected, (iii) secondary electron emission from boundary surfaces due to neutral particle impact is not taken into account, and (iv) the secondary electron emission coefficient is assumed to be constant, i.e. independent of the incident particle energy and the surface conditions. Here we question the validity of these simplifications under conditions typical for plasma processing applications. We study the effects of including fast neutrals and using realistic energy-dependent secondary electron emission coefficients for ions and fast neutrals in simulations of CCPs operated in argon at 13.56 MHz and at neutral gas pressures between 3 Pa and 100 Pa. We find a strong increase of the plasma density and the ion flux to the electrodes under most conditions, if these processes are included realistically in the simulation. The sheath widths are found to be significantly smaller and the simulation is found to diverge at high pressures for high voltage amplitudes in qualitative agreement with experimental findings. By switching individual processes on and off in the simulation we identify their individual effects on the ionization dynamics and plasma parameters. We conclude that fast neutrals and energy-dependent secondary electron emission coefficients must be included in simulations of CCPs in order to yield realistic results.
An 18-level argon collisional radiative model (CRM) suitable for low pressure was established. The model can be solved by combining the optical emission spectroscopy (OES) with Langmuir probe calibration. In the capacitively coupled plasmas (CCPs) with different frequency and power, the electron temperature and density obtained by the model were compared with those measured by Langmuir probe. It is found that the calibration point at any frequency or power is suitable for the fixed pressure. This method was then applied to the diagnosis of triple-frequency (TF) CCPs, it is shown that the high frequency (HF) power mainly controls the electron density, the low frequency (LF) power mainly controls the electron temperature, and the intermediate frequency (IF) power was between the two. Compared with the dual-frequency (DF) CCPs, it is found that with the increase of IF power, the HF power can control the electron density more independently with less influence on the electron temperature.
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