Chemistry Reference
In-Depth Information
The reactants are gaseous. They will provide directly in the recipient as inert gas,
reactive gas, precursor gas, or produced in the reaction chamber by sputtering process.
The electrical source to form the plasma is an electric field (DC, MW, RF), which
can act directly on the charged particles only. The plasma state can be influenced by
the power of external energy source, by working pressure and auxiliary discharges,
or additional ion sources. The plasma-activated material arrives at the substrate to
form the films or to modify the surface layers of the substrate material.
Production and modification of thin inorganic films include PECVD and
PEPVD (often used as sputtering deposition), implantation, and surface modification
processes [370].
In case of PECVD, all reactants are gases or liquids. Precursor compounds or
reactive species from such volatile, chemically stable feed compounds will be formed
by energy transport of the electrons in the plasma, i.e., electron impact decompo-
sition and electron impact ionization initialize the chemical reactions. The reactive
chemicals can lead to deposition, which finally rules the layer growth. Because
PECVD often requires reactions between gas-phase precursor components, the gas
pressure in the PECVD discharge cannot be too low. Usually the working pressures
in these systems are 0.1-10 mbar, which is considerably higher than the pressure
in reactive sputter deposition processes. The surface temperature in PECVD is low,
about 500
◦
C, whereas the nonplasma CVD activated thermally requires usually much
higher temperatures. In PECVD, typical values for plasma density are in the range
of 10
9
-10
12
cm
−
3
and the ionization degrees are 10
−
7
-10
−
4
.
In PEPVD as well as in PVD, at least one chemical agent is solid and has to be
transformed into the gas phase. This can be done by evaporation or by sputtering.
Physical sputtering implies that atoms sputtered from the target material are
directly transported and deposited without chemical reactions to a substrate. In
reactive sputter deposition, a feed gas is supplied to the plasma deposition system.
Molecules of the feed gas form active species in the gas phase, producing chemically
active species, which react with the target material during ion bombardment. The film
deposited on the substrate includes not only material of the ion bombarded target but
also compounds formed due to the reactive gases.
8.2.4.2.2 Types of Plasma Chemical Reactions
Chemical reactions in plasma-enhanced vapor deposition processes can take place in
the plasma volume (reaction type 1), directly on the substrate surface (reaction type
2) or by complex chemical transformation reactions on the substrate surface (reaction
type 3) (see Figure 8.54).
Unlike the chemical reactions of type 1 that are well described for selected
plasmas [370-372], chemical reactions of type 2 are not well known yet. One reason
for this is the difficulty to perform direct experimental verifications for such plasma
surface processes. It is assumed that deposition material arrives at the substrate
mostly in atomic or molecular form (see Figure 8.54). The atoms or molecules
diffuse around the substrate with a motion determined by its binding energy to the
substrate and is influenced by the nature as well as the temperature of the substrate.
After a certain time, the particles will either evaporate from the surface or will join
with another diffusing particle to form a doublet, which is less mobile but more stable
