Chemistry Reference
In-Depth Information
BC
RF
MN
FIGURE 3.43
Scheme of a capacitive coupled RF discharge with plane electrodes and
coupling capacitor. RF, generator; MN, matching network; BC, coupling capacitor.
V
2
on the two electrodes depend on the electrode areas
A
1
,
A
2
as
A
2
A
1
n
V
1
V
2
=
,
(3.312)
where
n
varies in the range from
1 to 4 for different conditions [36,69,70]. Ion
energies at the powered electrode can reach values up to some hundred electron volts
due to the self-bias.
The application of two different frequencies for plasma excitation allows an
approximatecontrol of theplasmadensityandthereforeof theionfluxtotheelectrode
by the higher frequency (e.g., 27.12 MHz) as well as the ion energy by the low
frequency (e.g., 1.94 MHz) [77]. In a simplified picture this behavior can be explained
by the low impedance of the sheath capacitance for the high frequency and therefore
high power absorption and effective electron heating inside the plasma bulk. This
increases the ionization and the ion flux. The low frequency acts across the sheath
and controls the ion energy [3,72].
For the capacitively coupled discharges two different modes are observed, the α
and γ mode [3,72].
Various types of capacitively coupled reactors are used in applications. The barrel
reactor is cylindrical with outer electrodes fixed onto the dielectric wall. This reactor
was used in microelectronic industry to remove the photoresist by plasma etching or
plasma stripping in an oxygen plasma. The targets are floating at a potential against
the plasma of about 10-20 eV. Therefore the etch process occurs by neutral activated
species and a damage of the target material by energetic ions is avoided. This etching
is a purely chemical process. The action of the ions is utilized in the planar and
cylindrical diode reactors. Planar reactors consists of a planar electrode, the second
electrode is the reactor wall or a second planar electrode in front of the first one.
In a triode system the two planar electrodes are insulated with respect to the reactor
wall, which serves as a third electrode. A coaxial cylindrical diode system consists
of a centrally mounted hexagonal electrode which supports the six substrates, e.g.,
wafers. This electrode is powered, the reactor wall grounded. Today, the development
in microelectronic industry leaves the simultaneously treatment of several wafers and
goes in the direction of processing of larger wafers (300 mm).
∼
