Chemistry Reference
In-Depth Information
TABLE 3.13
Selected Radio Frequency Plasma Reactors
Type
Pressure [Pa]
n
e
[cm
3
]
T
e
[eV]
Applications
Capacitive
Low pressure
0.1-10
10
10
1-10
Sputtering
Moderate pressure
10-1000
10
10
1-10
Deposition
Dual frequency
1
10
10
Etching, separate control ion
density (ion flux) and ion energy
Inductive
0.1-1000
10
12
1
Etching
Neutral loop discharge
0.01-1
10
11
5
Etching
Helicon discharge
0.01-1
10
13
1
Etching
treatment with reduced surface damage in semiconductor industry and surface treat-
ment of various heat sensitive materials. The application of a spiral coil in front of
the plasma volume generate plasmas for homogeneous treatment of a planar object.
For the treatment of 3D objects double inductively coupled plasma reactors with two
flat RF-coils are developed, one on the top and another on the bottom of the reactor
vessel [76]. The plasma is of reasonable homogeneity in radial and axial direction.
An interesting variant of inductively coupled plasma sources for low processing
pressures (
0.1 Pa) is the neutral loop discharge source [77,78]. The plasma is
generated along a magnetic neutral loop (
B
∼
0) by an RF electric field excited by an
one turn antenna. The magnetic neutral loop is produced by a set of three DC magnetic
coils, in which the inner coil produces a magnetic field in opposite direction to the
outer coils. In this configuration plasma is generated with high electron density and
relatively low electron temperature. The position of the plasma can be controlled by
the magnetic field and therefore a uniform processing can be achieved over large
wafers.
The helicon plasma source operates usually with a static longitudinal homoge-
neous magnetic field (0.005-0.03 T) in a cylindrical vacuum vessel (1
=
10
−
2
-10 Pa).
Helicon waves are generated by specially shaped antennas in the frequency range
between 1 and 50 MHz, usually 13.56 MHz for processing plasmas. In Ar peak elec-
tron densities of 10
12
-10
13
cm
−
3
are achieved [79,80]. The high plasma density is
promoted by the (collision-free) Landau damping of the helicon waves.
Capacitively and inductively coupled radio frequency plasma reactors are espe-
cially improved in microelectronic industry for wafer manufacturing with its broad
application spectrum for thin film deposition and plasma etching. Some examples
of RF plasma reactors with typical data of plasma parameters and applications are
shown in Table 3.13.
·
3.8.8 M
ICROWAVE
D
ISCHARGES
The plasma generation by microwaves has the advantage of an electrodeless discharge
with an effective power absorption in a broad pressure range (1-10
5
Pa), with high
electron densities (up to 10
13
cm
−
3
) and low sheath potentials and therefore low ion
