Chemistry Reference
In-Depth Information
An example of generation of a large planar RF plasma in the frequency range
13.56-200 MHz for gas pressures near 5 Pa is reported by Kawai [73] using a ladder-
shaped antenna (422 mm
×
422 mm with 17 rods) for preparing a-Si:H films for
solar cells.
The capacitive coupling has the advantage of plasma generation including an
energetic ion current (100-1 000 eV) in direction of the powered electrode. This
method allows charge carrier densities of 10
9
-10
10
cm
−
3
. Higher plasma densities
require excitation mechanisms with higher ionization efficiency as well as minimized
diffusion losses by improved confinement and application of resonant effects in an
externally generated static magnetic field.
3.8.7 I
NDUCTIVELY
C
OUPLED
D
ISCHARGES
Inductive coupled discharges are excited by an RF electric field of a helical coil
surrounding the plasma volume or of a spiral coil in front of the plasma [74,75]
(Figure 3.44a and b). The last planar configuration is similar to a capacitively coupled
plasma reactor. The plasma generating voltage is induced by the varying magnetic
field generated by the RF current in the exciting coil. The coupling between the coil
and the plasma can be understood as a transformer with many primary windings
of the coil and the plasma as one winding. The inductive excited plasma has typical
high electron densities (10
11
-10
12
cm
−
3
) and low electric fields. The inductive coupled
plasma builds up, in contrast to the capacitively coupled one, only a small sheath volt-
age in front of the wall or of the substrate (20-40 V), which results in low ion energies.
The low kinetic energy of the positive ions makes such plasmas suitable for material
MN
RF
MN
RF
(a)
(b)
FIGURE 3.44
Scheme of an inductively coupled RF discharge with helical (a) and plane
spiral (b) coupler. RF, generator; MN, matching network.
