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For a long time, microwave plasma sources without magnetic field have not
been considered suitable for the generation of high density plasmas.
Electromagnetic waves cannot propagate in over-dense plasmas. The wave is
reflected at the plasma surface due to the skin effect
and becomes an evanescent wave. Its penetration depth corresponds to the
skin depth <math>\delta<math>, which can be approximated by
- <math>\delta \simeq
c\,\big/\sqrt{\omega_{p_e}^2 - \omega^2}. <math>
The non-vanishing penetration depth of an evanescent wave opens an alternative
way of heating a plasma: Instead of traversing the
plasma, the conductivity of the plasma enables the wave to propagate along
the plasma surface. The wave energy is then transferred to the plasma by an
evanescent wave which enters the plasma perpendicular to its surface
and decays exponentially with the skin depth.
This transfer mechanism allows
to generate over-dense plasmas with electron densities
beyond the critical density.
Plasmas that are excited by propagation of
electromagnetic surface waves are called surface-wave-sustained.
Surface wave plasma sources can
be divided into two groups depending upon whether
the plasma generates part of its own waveguide by ionisation or not. The
former is called a self-guided plasma.
The surface wave mode
allows to generate uniform high-frequency-excited plasmas in volumes whose lateral
dimensions extends over several wavelengths of the
electromagnetic wave, e.g. for microwaves of 2.45 GHz in vacuum the wavelength amounts to 12.2 cm.
Surface-wave-sustained plasmas (SWP) can be operated in a large variety
of recipient geometries. The pressure range
accessible for surface-wave-excited plasmas depends on the process gas and the diameter of the
recipient. The larger the chamber
diameter, the lower the minimal pressure necessary for the SWP mode.
Analogously, the maximal pressure where a stable SWP can be operated decreases
with increasing diameter. The numerical modelling of SWPs is quite involved.
The plasma is created by the electromagnetic wave, but it also reflects and guides this same
wave. Therefore, a truly self-consistent description is necessary.
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