Two floating sheath solutions with strong electron emission in planar geometry have been proposed, a “space-charge limited” (SCL) sheath and an “inverse” sheath. SCL and inverse models contain different assumptions about conditions outside the sheath (e.g., the velocity of ions entering the sheath). So it is not yet clear whether both sheaths are possible in practice, or only one. Here we treat the global presheath-sheath problem for a plasma produced volumetrically between two planar walls. We show that all equilibrium requirements (a) floating condition, (b) plasma shielding, and (c) presheath force balance, can indeed be satisfied in two different ways when the emission coefficient γ > 1. There is one solution with SCL sheaths and one with inverse sheaths, each with sharply different presheath distributions. As we show for the first time in 1D-1V simulations, a SCL and inverse equilibrium are both possible in plasmas with the same upstream properties (e.g., same N and Te). However, maintaining a true S...

Are two plasma equilibrium states possible when the emission coefficient exceeds unity

An independent control of the flux and energy of ions impacting on an object immersed in a plasma is often desirable for many industrial processes such as microelectronics manufacturing. We demonstrate that a simultaneous control of these quantities is possible by a suitable choice of a static magnetic field applied parallel to the plane electrodes in a standard single frequency capacitively coupled plasma device. Our particle-in-cell simulations show a 60% reduction in the sheath width (that improves control of ion energy) and a fourfold increase in the ion flux at the electrode as a consequence of the altered ion and electron dynamics due to the ambient magnetic field. A detailed analysis of the particle dynamics is presented, and the optimized operating parameters of the device are discussed. The present technique offers a simple and attractive alternative to conventional dual frequency based devices that often suffer from undesirable limitations arising from frequency coupling and electromagnetic effects.An independent control of the flux and energy of ions impacting on an object immersed in a plasma is often desirable for many industrial processes such as microelectronics manufacturing. We demonstrate that a simultaneous control of these quantities is possible by a suitable choice of a static magnetic field applied parallel to the plane electrodes in a standard single frequency capacitively coupled plasma device. Our particle-in-cell simulations show a 60% reduction in the sheath width (that improves control of ion energy) and a fourfold increase in the ion flux at the electrode as a consequence of the altered ion and electron dynamics due to the ambient magnetic field. A detailed analysis of the particle dynamics is presented, and the optimized operating parameters of the device are discussed. The present technique offers a simple and attractive alternative to conventional dual frequency based devices that often suffer from undesirable limitations arising from frequency coupling and electromagnetic effects.

Spatial symmetry breaking in single-frequency CCP discharge with transverse magnetic field

Kinetic effects of plasma-dielectric interaction are studied theoretically with respect to the mechanisms of electron extraction from solids in response to ion and electron bombardment, coupled with plasma dynamics of a weakly emissive sheath. Emission coefficients of incident beams are first calculated by quantum mechanical as well as semi-classical approaches involving Auger neutralization, energy-dependent ejection due to primary beam, and reflection of low-energy electrons, which are then incorporated into a 1D1V simulation and plasma kinetic theory. Presheath with sheath structures are derived using fluid and kinetic theory regarding ion-induced emission, respectively. The Bohm criterion considering surface emission is evaluated as well. For electron-induced emission, it is found that sheath potential is no longer collinear with plasma electron temperature in our integrated model. Additionally, reflection of low-energy electrons is justified to have a minor impact on the floating sheath for low temperature half-bounded plasma under low pressure. The combined effects of both incident ions and electrons in bounded plasma are analyzed with symmetrical/asymmetrical emission yields at two boundaries. It is then proved that wall potential is barely affected by bulk plasma influx if emission coefficients are symmetrical, while emission due to transiting beam can drastically modify the sheath solution. Electron reflection also becomes more influential if secondary electrons have low initial energy. Finally, we summarize different roles of ion and electron in sheath structure. It is shown that ion-induced emission mitigates sheath potential but cannot reach critical emission on contrast to that of electron flux.

Integrated modeling of plasma-dielectric interaction: kinetic boundary effects

Multipactor over a dielectric in vacuum inclines to engender interfacial gas desorption or evaporation, precipitating surface flashover and insulator failure. However, no consensus has been achieved regarding the exact mechanism during final breakdown stage, an expatiation of which therefore serves as our major motivation for this letter. By implementing the particle-in-cell simulation code, we investigate the microscopic evolution of the discharge development process and confirm the major component escalating the explosive space charge accumulation. The obtained current waveform validates the balance of charged particles between electrodes, corroborated by experimental results. A theoretical discharge model is then constructed to elucidate the physical reasoning of the previous phenomenon. Two distinct discharge modes are defined correspondingly, and the transition therein is found to be induced by rapid plasma density build-up.

Investigation of multipactor-induced surface plasma discharge and temporal mode transition

Effects of secondary electron emission (SEE) on the plasma density and electron excitation dynamics in dual-frequency (2 MHz and 14 MHz) capacitively coupled Ar plasmas are investigated. The plasma density np is measured with a hairpin probe, and the spatio-temporal distribution of electron excitation rate (ground state into Ar 2p1 state) is determined by phase resolved optical emission spectroscopy. It is generally found that as the low-frequency (LF) voltage ϕL increases, np first decreases at low ϕL, due to compressed the plasma bulk length by the LF source, and then increases slightly with ϕL, suggesting that the plasma is dominated by the α mode. When ϕL exceeds some critical value, np increases dramatically with ϕL, due to significantly enhanced ionization by secondary electrons, indicating a α–γ mode transition. An excitation pattern caused by SEE at the edge of the completely expanded LF sheath is observed at relatively high ϕL. Under various conditions, including the high-frequency voltage ϕH, the pressure p, the electrode gap L, and the electrode material, different dependences of np on ϕL are also discussed. It is found that the discharge turns into the γ mode at a lower ϕL when ϕH is higher. As p increases, the density peak moves axially towards the powered electrode, due to reduced sheath thickness, while its distance to the powered electrode is almost independent of other external conditions (ϕH, L, electrode material, etc). A higher p or higher L is favorable for the enhancement of np in γ mode, because secondary electrons can contribute more to the ionization. Due to the higher SEE coefficient of the aluminum electrode, a more significant increase in np in the γ mode can be seen than that with a stainless steel or copper electrode. Meanwhile, the spatio-temporal distributions of the electron excitation rate under the same conditions are analyzed to further understand the SEE effects.

https://iopscience.iop.org/article/10.1088/1361-6595/aaca8c/pdf

Effects of secondary electron emission on plasma density and electron excitation dynamics in dual-frequency asymmetric capacitively coupled argon plasmas

Emitted electrons are accelerated back into the plasma by the sheath. If their mean free path is large, they can propagate directly to another surface without suffering collisions. We analyze the effects of “transit” on plasma-surface interaction. When transit occurs, surfaces exchanging electrons are intricately coupled. All surfaces float more negatively than they would if the emission collisionally remixed with the bulk plasma. Asymmetries of the system drive a net “transit current” between the surfaces, which influences their potential difference. The larger the initial energy spread of the emitted electrons, the larger the potential difference.

/pdf/influence-of-emitted-electrons-transiting-between-surfaces-49oa2ke1ex.pdf

Influence of emitted electrons transiting between surfaces on plasma-surface interaction

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Influence of emitted electrons transiting between surfaces on plasma-surface interaction