Interaction of biased electrodes and plasmas: sheaths, double layers, and fireballs

Baalrud, Scott; Scheiner, Brett; Yee, Benjamin; Hopkins, Matthew M.; Barnat, Edward V.

doi:10.1088/1361-6595/ab8177

Cited by 61 publications

(59 citation statements)

References 369 publications

(939 reference statements)

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“…After passing its peak, the local electric fields enhance the ionizing collisions close to the target by one order of magnitude, allowing for a space-charge inversion to a net positive charge and corresponding local rise of the plasma potential. Such features are likewise observed in double-layers and anode spots in the absence of magnetic fields [27,28], where oscillations similar to those herein have been measured and explained as the double layer stabilization in the electron sheath. We also note that the double layer potential drop between the region close to the target and the bulk plasma is slightly higher than the ionization potential of Ar [29].…”

Section: Discussionsupporting

confidence: 87%

Afterglow dynamics of plasma potential in bipolar HiPIMS discharges

Avino

Manke

Richard

et al. 2021

Plasma Sources Sci. Technol.

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In bipolar magnetron sputtering, the plasma afterglow is initiated by switching the target bias from a negative to positive voltage. In the following, the plasma potential evolution in this configuration is characterized, being responsible for the ion acceleration at the substrate sheath potential fall, in particular in high power impulse magnetron sputtering (HiPIMS). A mass-energy analyzer and a Langmuir probe respectively measure the ion energies and the plasma/floating potential at different positions within HiPIMS discharges. A plasma potential drop and rise in the first 45 μs of the afterglow is observed, settling in the plasma bulk towards values below the applied positive bias. The measured ion energies agree with the plasma potential values before and after the drop-rise. To gain more comprehensive insights into the mechanisms responsible for such a potential evolution, particle-in-cell Monte Carlo 3D simulations of bipolar direct current magnetron sputtering discharges are explored in equivalent geometries. Despite their average power being orders of magnitude lower compared to the HiPIMS configuration, a similar afterglow behavior is observed. This indicates that the measured dynamics are not specific to HiPIMS, but rather a feature of bipolar magnetron sputtering. The responsible mechanisms are studied further: the effects of various system parameters are decoupled, with the magnetic field configuration emerging as crucial for the plasma potential drop-rise dynamics and the associated re-ionization close to the target.

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Section: Discussionsupporting

confidence: 87%

Afterglow dynamics of plasma potential in bipolar HiPIMS discharges

Avino

Manke

Richard

et al. 2021

Plasma Sources Sci. Technol.

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“…The double layer potential scales pro-portional to the ion beam energy, implying that the DL is due to ion beam stagnation. This is fundamentally different from a sheath expansion due to ionization of neutrals, called a fireball [18]. The potential drop of fireball double layers is close to the ionization energy level necessary to balance the particle inflow and outflow.…”

Section: Ion Injection Into Electron Rich Sheath Producing Sheath mentioning

confidence: 95%

“…The size, location and stability of the double layer depends on the balance between production and losses. For plane electrodes the shape of the fireball is usually spherical [16,18].…”

Section: Electron-rich Sheaths Developing Into Fireballsmentioning

confidence: 99%

Untitled

2021

J. Technol. Space Plasmas

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The properties of sheaths and associated potential structures and instabilities cover a broad field which even a review cannot cover everything. Thus, the focus will be on about a dozen examples, describe their observations and focus on the basic physical explanations for the effects, while further details are found in the references. Due to familiarity the review focuses mainly on the authors work but compared and referenced related work. The topics start with a high frequency oscillation near the electron plasma frequency. Low frequency instabilities also occur at the ion plasma frequency. The injection of ions into an electron-rich sheath widens the sheath and forms a double layer. Likewise, the injection of electrons into an ion rich sheath widens and establishes a double layer which occurs in free plasma injection into vacuum. The sheath widens and forms a double layer by ionization in an electron rich sheath. When particle fluxes in "fireballs" gets out of balance the double layer performs relaxation instabilities which has been studied extensively. Fireballs inside spherical electrodes create a new instability due to the transit time of trapped electrons. On cylindrical and spherical electrodes the electron rich sheath rotates in magnetized plasmas. Electrons rotate due to E × B 0 which excites electron drift waves with azimuthal eigenmodes. Conversely a permanent magnetic dipole has been used as a negative electrode. The impact of energetic ions produces secondary electron emission, forming a ring of plasma around the magnetic equator. Such "magnetrons" are subject to various instabilities. Finally, the current to a positively biased electrode in a uniformly magnetized plasma is unstable to relaxation oscillations, which shows an example of global effects. The sheath at the electrode raises the potential in the flux tube of the electrode thereby creating a radial sheath which moves unmagnetized ions radially. The ion motion creates a density perturbation which affects the electrode current. If the electrode draws large currents the current disruptions create large inductive voltages on the electrode, which again produce double layers. This phenomenon has been seen in reconnection currents. Many examples of sheath properties will be explained. Although the focus is on the physics some examples of applications will be suggested such as neutral gas heating and accelerating, sputtering of plasma magnetrons and rf oscillators.

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“…The ion creation results in broadening the potential profile until forming a double layer at the boundary as a visible "fireball" or "anode glow" [13,14,15,16]. Various instabilities arise, such as fireball relaxation instabilities and electron transit time instabilities [17,18]. In magnetized plasmas electron rich sheaths can rotate due to E × B 0 drifts which forms electron drift wave eigenmodes [19].…”

Section: Introductionmentioning

confidence: 99%

Sheaths and Double Layers with Instabilities

Stenzel

Grünwald²,

Ioniţă³

et al. 2021

J. Technol. Space Plasmas

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The properties of sheaths and associated potential structures and instabilities cover a broad field which even a review cannot cover everything. Thus, the focus will be on about a dozen examples, describe their observations and focus on the basic physical explanations for the effects, while further details are found in the references. Due to familiarity the review focuses mainly on the authors work but compared and referenced related work. The topics start with a high frequency oscillations near the electron plasma frequency. Low frequency instabilities also occur at the ion plasma frequency.The injection of ions into an electron-rich sheath widens the sheath and forms a double layer. Likewise, the injection of electrons into an ion rich sheath widens and establishes a double layer which occurs in free plasma injection into vacuum. The sheath widens and forms a double layer by ionization in an electron rich sheath. When particle fluxes in "fireballs" gets out of balance the double layer performs relaxation instabilities which has been studied extensively. Fireballs inside spherical electrodes create a new instability due to the transit time of trapped electrons. On cylindrical and spherical electrodes the electron rich sheath rotates in magnetized plasmas. Electrons rotate due to $\mathbf E \times \mathbf B_0$ which excites electron drift waves with azimuthal eigenmodes. Conversely a permanent magnetic dipole has been used as a negative electrode. The impact of energetic ions produces secondary electron emission, forming a ring of plasma around the magnetic equator. Such "magnetrons" are subject to various instabilities. Finally, the current to a positively biased electrode in a uniformly magnetized plasma is unstable to relaxation oscillations, which shows an example of global effects. The sheath at the electrode raises the potential in the flux tube of the electrode thereby creating a radial sheath which moves unmagnetized ions radially. The ion motion creates a density perturbation which affects the electrode current. If the electrode draws large currents the current disruptions create large inductive voltages on the electrode, which again produce double layers. This phenomenon has been seen in reconnection currents. Many examples of sheath properties will be explained. Although the focus is on the physics some examples of applications will be suggested such as neutral gas heating and accelerating, sputtering of plasma magnetrons and rf oscillators.

show abstract

Interaction of biased electrodes and plasmas: sheaths, double layers, and fireballs

Cited by 61 publications

References 369 publications

Afterglow dynamics of plasma potential in bipolar HiPIMS discharges

Afterglow dynamics of plasma potential in bipolar HiPIMS discharges

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Sheaths and Double Layers with Instabilities

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