Electromagnetic sources of nonuniformity in large area capacitive reactors

Theory and experiment show that two electromagnetic modes are necessary and sufficient to determine the field nonuniformity within a parallel-plate rf capacitive plasma reactor. These two modes give rise to the standing wave effect and the telegraph effect. The standing wave effect is associated with high frequencies in large reactors where the reactor size is larger than about a tenth of the vacuum wavelength of the rf excitation. The telegraph effect is associated with asymmetric electrode areas, which necessitates the redistribution of rf current along the plasma to maintain rf current continuity. (c) 2006 Elsevier B.V. All rights reserved.

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Christoph Hollenstein, Alan Howling, Laurent Fabien Hubert Sansonnens
2007
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https://infoscience.epfl.ch/record/120433?ln=en

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Related publications

A voltage uniformity study in large-area reactors for RF plasma deposition

Christoph Hollenstein, Alan Howling, Laurent Fabien Hubert Sansonnens

Non-uniform voltage distribution across the electrode area results in inhomogeneous thin-film RF plasma deposition in large-area reactors. In this work, a two-dimensional analytic model for the calculation of the voltage distribution across the electrode area is presented. The results of this model are in good agreement with measurements performed without plasma at 13.56 MHz and 70 MHz in a large-area reactor. The principal voltage inhomogeneities are caused by logarithmic singularities in the vicinity of RF connections and not by standing waves. These singularities are only described by a two-dimensional model and cannot be intuitively predicted by analogy to a one-dimensional case. Plasma light emission measurements and thickness homogeneity studies of a-Si:H deposited films show that the plasma reproduces these voltage inhomogeneities. Improvement of the voltage uniformity is investigated by changing the number and position of the RF connections.

1997

Nonuniform radio-frequency plasma potential due to edge asymmetry in large-area radio-frequency reactors

Juliette Ballutaud, Christoph Hollenstein, Alan Howling, Laurent Fabien Hubert Sansonnens

In small area capacitive reactors, the rf and dc components of the plasma potential can be assumed to be uniform over all the plasma bulk because of the low plasma resistivity. In large area reactors, however, the rf plasma potential can vary over a long range across the reactor due to rf current flow and the nonzero plasma impedance. A perturbation in rf plasma potential, due to electrode edge asymmetry or the boundary of a dielectric substrate, propagates along the resistive plasma between capacitive sheaths. This is analogous to propagation along a lossy conductor in a transmission line and the damping length of the perturbation can be determined by the telegraph equation. Some consequences are the following: (i) The spatial variation in sheath rf amplitudes causes nonuniform rf power dissipation near to the reactor sidewalls. (ii) The surface charge and potential of a dielectric substrate can be negative and not only positive as for a uniform rf plasma potential. The variation of sheath dc potential across a dielectric substrate causes nonuniform ion energy bombardment. (iii) The self-bias voltage depends on the plasma parameters and on the reactor and substrate dimensions-not only on the ratio of electrode areas. (iv) The nonuniform rf plasma potential in presence of the uniform dc plasma potential leads to nonambipolar dc currents circulating along conducting surfaces and returning via the plasma. Electron current peaks can arise locally at the edge of electrodes and dielectric substrates. Perturbations to the plasma potential and currents due to the edge asymmetry of the electrodes are demonstrated by means of an analytical model and numerical simulations. (C) 2004 American Institute of Physics.

2004

Electromagnetic field nonuniformities in large area, high-frequency capacitive plasma reactors, including electrode asymmetry effects

Christoph Hollenstein, Alan Howling, Laurent Fabien Hubert Sansonnens

Electromagnetic wave propagation effects can give rise to important limitations for processing uniformity in large area, radio-frequency (rf) capacitive plasma reactors. The electromagnetic wavefield solution is derived for a capacitive, high-frequency, cylindrical reactor with symmetric or asymmetric electrode areas containing a uniform plasma slab. It is shown that only two distinct electromagnetic modes are necessary and sufficient to determine the electromagnetic fields everywhere within the reactor except close to the sidewalls. The first mode gives rise to the interelectrode rf voltage standing wave effect associated with high frequencies in large area reactors, and the second mode gives rise to the telegraph effect associated with asymmetric electrode areas, which necessitates the redistribution of rf current along the plasma to maintain rf current continuity. This work gives a unified treatment of both effects which have previously been studied separately, experimentally and theoretically, in the literature. The equivalent circuit of each mode is also derived from its respective dispersion relation. Examples of this electromagnetic wavefield solution show that both modes can cause nonuniformity of the plasma rf potential, depending on the reactor geometry, excitation frequency and plasma permittivity and sheath width, which has consequences for large-area plasma processing.

2006

Plasma (physics)

Plasma () is one of four fundamental states of matter, characterized by the presence of a significant portion of charged particles in any combination of ions or electrons. It is the most abundant form

Nuclear reactor

A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation

Standing wave

In physics, a standing wave, also known as a stationary wave, is a wave that oscillates in time but whose peak amplitude profile does not move in space. The peak amplitude of the wave oscillations