Planar gas introducing unit of a CCP reactor

Details Planar gas introducing unit of a CCP reactor Planar gas introducing unit of a CCP reactor

2000-03-28

2001-12-25

Wong, Don (Department: 2821)

Electric lamp and discharge devices: systems

Discharge device load with fluent material supply to the...

Plasma generating

C118S7230MR, C156S345420

Reexamination Certificate

active

06333601

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
Referenced-applications
The present application corresponds to and claims priority of Japanese Patent Application No. 11-139325, filed in Japan on May 19, 1999, the entire contents of which are incorporated herein by reference.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
Not Applicable
REFERENCE TO A MICROFICHE APPENDIX
Not Applicable
BACKGROUND OF THE INVENTION
Description of Related Art
Capacitively coupled plasma (CCP) reactors arc utilized in a wide range of applications in the semiconductor device fabrication industry. CCP reactors have attracted great interest owing to their numerous advantages such as (1) a low aspect ratio of the plasma reactor due to a narrow gap between the cathode and the anode electrodes, (2) better radial uniformity of the plasma, (3) ease of plasma ignition, and (4) ability to use a shower-head-type planar gas introducing device to yield a better gas distribution.
One of the problems with CCP reactors concerns the nonuniform erosion of a gas inlet plate of the shower-head-type planar gas introducing unit. The gas inlet plate has a plurality of gas inlet holes which are subjected to a higher erosion rate by the plasma compared to other areas of the gas inlet plate. The erosion of the gas inlet holes occurs from a lower side as well as from an upper side thereof. The erosion process is explained in detail in accordance with
FIGS. 4 and 5
below.
FIG. 4
shows a simplified diagram of one example of a CCP reactor
100
. This CCP reactor
100
includes a top plate
51
. a bottom plate
52
, a cylindrical side wall
53
, a top electrode
54
and a wafer holder
55
. The top electrode
54
is arranged by a ring insulator
56
at the upper side of the CCP reactor
100
and the wafer holder
55
is arranged on the bottom plate
52
and supported by a planar insulator
57
. Further, the insulator plate
58
is arranged between the top plate
51
and the top electrode
54
. The top electrode
54
is made of a metal, for example, aluminum. Below the top electrode
54
there is a gas inlet plate
59
. Between the top electrode
54
and the gas inlet plate
59
, there is a narrow space
60
forming a gas reservoir. The purpose of the gas reservoir
60
is to provide a uniform gas distribution over the gas inlet plate
59
. The material of the gas inlet plate
59
depends on the type of plasma application, for example, in dry etching applications, carbon or Si is usually used. In other applications, dielectric materials, for example, quartz or ceramic, are usually used. There is a large number of gas inlet holes
59
a
in the gas inlet plate
59
for the introduction of a process gas from the gas reservoir
60
to a plasma generation region. The diameter of the gas inlet holes
59
a
is approximately 0.5 mm. The separation between each of the gas inlet holes
59
a
may vary from 5 mm to 20 mm in an ordinary plasma source. However, regardless of the separation between the gas inlet holes
59
a
, an equal distance between the gas inlet holes
59
a
is usually kept. That is, gas inlet holes
59
a
are formed at the corners of identical squares drawn on the gas inlet plate
59
. A wafer
61
to be processed is situated on the wafer holder
55
. The wafer
61
faces the gas inlet plate
59
in a parallel fashion.
An RF power source
62
is connected to the top electrode
54
via a matching circuit
63
. The RF power source
62
usually operates at a frequency in range of 10 MHz to 100 MHZ. When the RF electric power is supplied to the top electrode
54
, a plasma is generated between the gas inlet plate
59
and the wafer holder
55
by a capacitively coupled mechanism. The plasma generation region, however, lies in the vicinity of the gas inlet plate
59
, since an electron heating process primarily occurs by an oscillation of sheath voltage that lies just below the gas inlet plate
59
. Therefore, a plasma density is higher, closer to the gas inlet plate
59
and gradually decreases in a downstream direction because of gas phase recombination and a ambipolar diffusion.
As mentioned above, a major problem associated with the gas inlet plate
59
is the high erosion rate of the gas inlet holes
59
a
by the plasma. This erosion results in a lower utilization efficiency of the gas inlet plate
59
. The erosion process is explained below.
Plasma is usually generated at a low pressure, for example, in the range of 10 mTorr to 100 mTorr, in most present industrial applications. However, referring to
FIG. 5
, at an end
59
a
-
1
of the gas inlet holes
59
a
there is a slightly higher gas pressure, and inside the gas reservoir
60
there is an even higher gas pressure. The plasma density changes depending on the pressure. At higher pressures, the plasma density becomes higher. In a capacitively coupled plasma, an RF electrode generally has a self bias voltage. In the situation explained above, the gas inlet plate
59
functions as the RF electrode and therefore, has a self bias voltage. The value and polarity of the self bias voltage generated in the gas inlet plate
59
depends on many parameters, for example, the surface area ratio of the cathode (the gas inlet plate) and the anode (all grounded surfaces where the plasma is in contact with), the operating frequency of the RF power source
62
, and the plasma density, etc. In most of the plasma sources used in practical applications, the RF electrode attains a negative self bias voltage. Owing to this negative self bias voltage, the positive ions in the plasma accelerate towards the gas inlet plate
59
and bombard its surface. These ions gain a high energy by the acceleration process, thus the bombardment of ions on the gas inlet plate
59
causes a sputtering of the gas inlet plate
59
. As explained above, the sputtering damage is higher at the gas inlet holes
59
a
, since there is a higher plasma density at these places. This process causes an extruded erosion of the gas inlet holes
59
a
compared to the other areas of the gas inlet plate
59
, resulting in an enlargement of the diameter of the gas inlet hole
59
a
. With the increase of the gas inlet hole diameter, the plasma tends to confine in the gas inlet hole
59
a
by multireflections of electrons on the walls of gas inlet hole
59
a
. Accordingly, the erosion rate in the gas inlet hole
59
a
accelerates with the plasma-on time. This process yields a conical shaped gas inlet hole
64
at the lower end
59
a
-
1
thereof, as shown in
FIG. 5
, after several hours of operation.
Similarly, micro-plasmas generated at an upper end
59
a
-
2
of the gas inlet hole
59
a
causes an erosion of the upper side of the gas inlet hole
59
a
. Because of these erosion processes, a conical shaped gas inlet hole
65
is formed at the upper end
59
a
-
2
thereof, as shown in FIG.
5
. The service life of the gas inlet plate
59
is limited by the condition that the eroded upper and lower ends of the gas inlet hole
59
a
are joined. As a result, usually thicker, approximately 10 mm, gas inlet plates are used with CCP reactors
100
in order to increase their service life. However, the gas inlet plate
59
has a very low utilization efficiency since the erosion of gas inlet holes
59
a
determines its service life.
Moreover, if a polymer deposition gas chemistry is used in generating the plasma, a polymer deposition
66
can be observed in an area on the lower surface of the top electrode
54
, which is just above the gas inlet hole
59
a
as shown in FIG.
5
. The polymer deposition
66
can be attributed to the following two reasons. (1) Due to the micro-plasma generated at the upper end
59
a
-
2
of gas inlet hole
59
a
, polymer depositing radicals are formed. (2) The polymer depositing radicals generated in the main plasma below the gas inlet plate
59
can diffuse through the gas inlet hole
59
a
. This diffusion process increases with an increase of the gas inlet hole diameter with plasma-on time. Before the thickness of the layer formed by the polymer deposition
66
becomes thick enough to peel off and release into

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