Chemistry Reference
In-Depth Information
5
Elementary Processes on
Surfaces in Plasma-Wall
Interaction
CONTENTS
5.1 AdsorptionandDesorption................................................... 164
5.2 Sticking Coefficients and Surface Loss Probabilities ....................... 165
5.3 Surface Coverage, Residence Time, and Adsorption Isotherms............ 167
5.4 SurfaceDiffusion............................................................. 169
5.5 EnergyAccommodation...................................................... 169
5.6 ChemicalSurfaceReactions ................................................. 170
5.7 IonBombardmentinPlasmaProcessing .................................... 176
5.8 Particle Balance at Surfaces.................................................. 178
5.9 Energy Balance at Surfaces .................................................. 181
Plasma-surface interaction is one of the fastest growing branches in plasma physics
and has got an important issue in the field of applied surface science. Its basic question
includes the mastering of an old problem: the contact of different states of matter.
The investigation and application of plasma-surface interaction plays an essential
role in low-temperature plasma processing as etching, deposition, or modification of
surfaces as well as in fusion research.
The characterization of plasmas in contact with solid surfaces is incomparably
more complicated than that of unbounded plasmas. It requires the consideration of
marked anisotropies and inhomogeneities, deviations from quasi-neutrality, exchange
of energy and matter between plasma and solid, and may result in strongly non-
linear equations containing many unknown quantities. The aim of understanding
plasma-surface interaction is preferably an extensive description of the parameters
that characterize the plasma and the solid surface (substrate) by taking into consider-
ation particle and energy balances of the involved species. For example, the complex
processes during etching or deposition of thin surface layers can be described via
macroscopic rate equations depending on experimental conditions being realized in
the plasma reactor. Typical experimental process parameters are gas mixture, gas
flow, pressure, discharge power and frequency, reactor geometry, temperature, etc.
On the other hand, the overall rates involve a variety of elementary processes, which
have to be described by suitable models on a microphysical scale.
Elementary processes in plasmas—in particular during the interaction with
surfaces—are rather complex. An experimental distinction of single phases of the
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