2,978 research outputs found
The effect of ambipolar electric fields on the electron heating in capacitive RF plasmas
We investigate the electron heating dynamics in electropositive argon and
helium capacitively coupled RF discharges driven at 13.56 MHz by Particle in
Cell simulations and by an analytical model. The model allows to calculate the
electric field outside the electrode sheaths, space and time resolved within
the RF period. Electrons are found to be heated by strong ambipolar electric
fields outside the sheath during the phase of sheath expansion in addition to
classical sheath expansion heating. By tracing individual electrons we also
show that ionization is primarily caused by electrons that collide with the
expanding sheath edge multiple times during one phase of sheath expansion due
to backscattering towards the sheath by collisions. A synergistic combination
of these different heating events during one phase of sheath expansion is
required to accelerate an electron to energies above the threshold for
ionization. The ambipolar electric field outside the sheath is found to be time
modulated due to a time modulation of the electron mean energy caused by the
presence of sheath expansion heating only during one half of the RF period at a
given electrode. This time modulation results in more electron heating than
cooling inside the region of high electric field outside the sheath on time
average. If an electric field reversal is present during sheath collapse, this
time modulation and, thus, the asymmetry between the phases of sheath expansion
and collapse will be enhanced. We propose that the ambipolar electron heating
should be included in models describing electron heating in capacitive RF
plasmas
Customized ion flux-energy distribution functions in capacitively coupled plasmas by voltage waveform tailoring
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
Ionization by bulk heating of electrons in capacitive radio frequency atmospheric pressure microplasmas
Electron heating and ionization dynamics in capacitively coupled radio
frequency (RF) atmospheric pressure microplasmas operated in helium are
investigated by Particle in Cell simulations and semi-analytical modeling. A
strong heating of electrons and ionization in the plasma bulk due to high bulk
electric fields are observed at distinct times within the RF period. Based on
the model the electric field is identified to be a drift field caused by a low
electrical conductivity due to the high electron-neutral collision frequency at
atmospheric pressure. Thus, the ionization is mainly caused by ohmic heating in
this "Omega-mode". The phase of strongest bulk electric field and ionization is
affected by the driving voltage amplitude. At high amplitudes, the plasma
density is high, so that the sheath impedance is comparable to the bulk
resistance. Thus, voltage and current are about 45{\deg} out of phase and
maximum ionization is observed during sheath expansion with local maxima at the
sheath edges. At low driving voltages, the plasma density is low and the
discharge becomes more resistive resulting in a smaller phase shift of about
4{\deg}. Thus, maximum ionization occurs later within the RF period with a
maximum in the discharge center. Significant analogies to electronegative low
pressure macroscopic discharges operated in the Drift-Ambipolar mode are found,
where similar mechanisms induced by a high electronegativity instead of a high
collision frequency have been identified
Kinetic simulation of the sheath dynamics in the intermediate radio-frequency regime
The dynamics of temporally modulated plasma boundary sheaths is studied in
the intermediate radio frequency regime where the applied radio frequency and
the ion plasma frequency are comparable. Two kinetic simulation codes are
employed and their results are compared. The first code is a realization of the
well-known scheme, Particle-In-Cell with Monte Carlo collisions (PIC/MCC) and
simulates the entire discharge, a planar radio frequency capacitively coupled
plasma (RF-CCP) with an additional heating source. The second code is based on
the recently published scheme Ensemble-in-Spacetime (EST); it resolves only the
sheath and requires the time resolved voltage across and the ion flux into the
sheath as input. Ion inertia causes a temporal asymmetry (hysteresis) of the
sheath charge-voltage relation; also other ion transit time effects are found.
The two codes are in good agreement, both with respect to the spatial and
temporal dynamics of the sheath and with respect to the ion energy
distributions at the electrodes. It is concluded that the EST scheme may serve
as an efficient post-processor for fluid or global simulations and for
measurements: It can rapidly and accurately calculate ion distribution
functions even when no genuine kinetic information is available
Kinetic Interpretation of Resonance Phenomena in Low Pressure Capacitively Coupled Radio Frequency Plasmas
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
Effects of fast atoms and energy-dependent secondary electron emission yields in PIC/MCC simulations of capacitively coupled plasmas
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
Coherent electronic transport through a superconducting film
We study coherent quantum transport through a superconducting film connected
to normal-metal electrodes. Simple expressions for the differential conductance
and the local density of states are obtained in the clean limit and for
transparent interfaces. Quasiparticle interference causes periodic vanishing of
the Andreev reflection at the energies of geometrical resonances, subgap
transport, and gapless superconductivity near the interfaces. Application of
the results to spectroscopic measurements of the superconducting gap and the
Fermi velocity is analyzed.Comment: 5 pages, 4 figure
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