Plasma processing apparatus

Details Plasma processing apparatus Plasma processing apparatus

1999-02-16

2001-03-13

Mills, Gregory (Department: 1763)

Coating apparatus

Gas or vapor deposition

With treating means

C118S7230AN, C118S7230IR, C156S345420, C156S922000, C315S111210

Reexamination Certificate

active

06199505

ABSTRACT:

BACKGROUND OF THE INVENTION
CROSS-REFERENCE TO RELATED APPLICATION
This application claims the priority of Japanese Patent Application No. 10-111401, filed on Apr. 8, 1998, the contents of which are hereby incorporated herein by reference.
1. Field of the Invention
The invention concerns apparatus in which CVD (chemical vapor deposition) or etching, for example, is executed using plasma, and in particular it concerns plasma processing apparatus which is characterized by the construction of the transmission path by which the high frequency power is supplied to the plasma.
2. Discussion of Related Art
Surface processing such as CVD and etching is carried out widely using plasma at the present time, and LSI (large scale integrated circuits) and FPD (flat panel devices) are being manufactured using such techniques. Various systems are known for generating the plasma, but the plasma generating systems in which a high frequency discharge is used are widely employed because they enable a stable plasma to be obtained over a wide area. Plasma generating systems in which a high frequency discharge is used can be broadly classified into the capacitively coupled type systems and the inductively coupled type systems, and the invention of this present application concerns primarily plasma generation systems of the capacitively coupled type.
Plasma generating apparatus of the capacitively coupled type which is suitable for use with high frequencies (in the VHF/UHF band) has been disclosed in the specification of U.S. Pat. No. 5,210,466. A front cross sectional drawing of such conventional plasma processing apparatus is shown in
FIG. 7
of the present application. The vacuum chamber
10
is constructed from a cylindrical side wall
12
, a top-plate
14
and the bottom-plate
16
. The cylindrical cathode
18
is present in the middle of the interior of the vacuum chamber
10
, and is surrounded via the insulator
20
by the annular conductor
22
. Thus, the transmission path inside the chamber comprises the cathode
18
, the insulator
20
and the annular conductor
22
. The vacuum chamber
10
is pumped out with the pumping apparatus
34
. The discharge gas is delivered into the vacuum chamber
10
at an appropriate mass flow rate through the gas distribution plate
32
of the gas delivery apparatus and maintained at the prescribed pressure. High frequency energy from the high frequency power supply
26
is conducted into the abovementioned transmission path
24
within the vacuum chamber via the matching circuit
28
and plasma is generated between the cathode
18
and the anode (principally the gas distribution plate
32
). Thus, in an etching process, for example, the etching is achieved by the action of the ions in the plasma on the substrate
36
on the cathode
18
.
By adopting a transmission path of the type described above, the plasma processing apparatus shown in
FIG. 7
enables a plasma to be generated at high frequencies of from 50 to 800 MHZ. The electrode sheath voltage is reduced by using such a high frequency and electrical damage to the substrate is minimized, and deposition rates or etch rates which are adequate for industrial purposes can be attained.
In the conventional apparatus shown in
FIG. 7
, the transmission path
24
within the chamber is a single coaxial line, and this coaxial line has been designed along the following lines. If the characteristic impedance of the coaxial line is Z
0
, its length is L and the phase constant is &bgr;, then the input impedance Z
in
as seen from the matching circuit
28
of a coaxial line with a plasma providing a load impedance (Z
L
) is given by the following equation (1):
Z
in
=Z
0
[Z
L
+j
Z
0
tan(&bgr;L)]/[Z
0
+j
Z
L
tan(&bgr;L)]  (1)
It is clear from this equation that when Z
0
=Z
L
, then Z
in
=Z
0
irrespective of L. That is to say, by establishing a coaxial line which has a characteristic impedance Z
0
which is equal to the plasma impedance Z
L
, the impedance of the coaxial line as seen by the matching circuit
28
has a constant value (equal to the plasma impedance), and the matching conditions are ideal. However, in practice the plasma impedance varies depending on the state of the plasma and it has a certain width, and so the characteristic impedance Z
0
of the coaxial line is set roughly to the mid-value of the width of this variation. In this case, rigorously, Z
0
is not equal to Z
L
and the line length L has an effect, and the matching conditions of the matching circuit become poor. It is necessary to make the line length L much shorter than a quarter wavelength to minimize this extent of this effect.
In the conventional apparatus shown in
FIG. 7
of the '466 patent, the transmission path
24
within the chamber is a coaxial line with the cathode
18
as the internal conductor and the annular conductor
22
as the external conductor. The substrate
30
must be located on the cathode
18
and so the diameter of the cathode
18
must always be larger than the diameter of the substrate. The annular conductor
22
is on the outside of the cathode
18
and the outer wall
12
is on the outside of this. With apparatus which has such a construction the external dimensions of the outer wall
12
of the vacuum chamber become very large as the substrates become larger, and the weight of the apparatus is increased. As a result of the latest technical developments, a need for the size of the wafers which are being used for LSI to be increased to a diameter of 300 mm has arisen. Furthermore, in the case of FPD there is a need for an increase in the size of the glass substrates to 550×650 mm or to 1 meter square. Hence, the diameter of the cathode
18
has to be increased and, as a result, the weight of the apparatus is increased and there is a further problem in that the manufacturing costs are also increased.
OBJECTS AND SUMMARY
An aim of the invention is to provide plasma processing apparatus with which the weight of the apparatus is not much increased even if the size of the substrates is increased.
A further aim of the invention is to provide comparatively small, low-cost plasma processing apparatus in which the high frequency energy is coupled effectively to the plasma.
A plasma processing apparatus of the present invention makes use of high frequency power of from 30 to 300 MHZ (the VHF band) and is distinguished by the construction of the transmission path within the chamber. The transmission path within the chamber in this invention comprises a first coaxial line, a second coaxial line which has a smaller diameter than the first coaxial line and a radial line which is connected to the first coaxial line and the second coaxial line. Here, the “radial line” which is connected to the two coaxial lines, which are of different diameters signifies a transmission path in which the high frequency power is propagated not in the axial direction but in the radial direction.
The vacuum chamber walls are used for both the external conductor of the first coaxial line and the external conductor of the second coaxial line. The end surface of the first coaxial line faces the plasma forming space and the second coaxial line is connected to the aforementioned matching circuit. The high frequency power which is output from a high frequency power source is transmitted sequentially by the matching circuit, the second coaxial line, the radial line and the first coaxial line and supplied to the plasma. The substrate which is to be processed is located on the end surface of the first coaxial line which faces the plasma (or the end surface of the opposed electrode which faces the plasma).
The transmission path within the chamber should be impedance matched between the various structural parts. First of all, fundamentally, the design is such that the characteristic impedance of the first coaxial line is more or less equal to the plasma impedance. From this point of view, in this invention the characteristic impedance at the outer end of the radial line is preferably set

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