Chemistry Reference
In-Depth Information
are characterized by a separation of the active plasma zone and the work piece. High
fluxes of charged particles combined with activated neutral particles interact with the
substrate. The uniformity of the particle flux is less dependent on the power coupling.
Closed-coupled sources maximize the useful particle fluxes to the substrate.
The homogeneity of the particle flux depends strongly on the position of the power
coupling. The surface treatment of the substrate is also controlled by its temperature
and bias voltage.
Fabrication processes include often some different production steps like surface
cleaning, deposition of different materials with diverse feed gases, and also different
plasma conditions. For this purpose the installation of clusters of different plasma
reactors each with its specific task is used. The reactors are connected by a suitable
transport system.
Plasma chemical reactors operating with various feed gases, especially for depo-
sition processes, show not only deposition on the target surface, but also at the walls
and on the installations inside the reactor vessel. These deposits are contaminants
and the source of, e.g., gaseous decomposition products, adsorbed and later desorbed
gases. Also the electrical conductivity can be changed. This will influence the plasma
processes. Therefore, possibilities for surface cleaning and conditioning of the reactor
are important for the reproducibility of the plasma chemical process.
The feed gas inlet system is an important part of the reactor. The source of the
feed gas may be different in dependence of its properties. Permanent gases can be
controlled by gas flow controllers, vapors of liquids demand a heating of the whole
inlet installation to avoid condensation of the vapor along the feeding line. Thermal
evaporators have been proved for the production of sufficient vapor flows.
The results of the plasma chemical treatment of gaseous or solid state materials
can be evaluated on the one hand by the quality of the product, on the other hand for
gaseous compounds by the rate of formation or transformation, for surface processes
by the activation, etch and deposition rate. The energetic efficiency is an important
parameter for gaseous processes.
Goodreproducibilityandlowsensitivityagainstvariationsoftheinputparameters
are essential for each commercial application of a plasma chemical process.
3.8.15 C
ONCLUSIONS
The presented plasma sources cover a broad field of parameters like gas pressure,
charge carrier and radical density, electron and ion energy, ion energy at the substrate
surface, and also density of UV radiation. Their scalability and timescales are differ-
ent. The adjustment for the required plasma chemical application remains a challenge
for scientific and engineering work. Furthermore, the history of the development of
plasma sources shows the possibility of appearance of new source principles also in
the future.
