JP2003209098A - Plasma treatment device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a plasma treatment device in which a foreign material can be prevented from being stuck on a dielectric provided between a high frequency antenna and plasma by plasma treatment. <P>SOLUTION: A plasma etching device 100 is provided with a high frequency antenna 112, a high frequency power source 160 connected to the high frequency antenna, a susceptor 106 for installing an object to be treated, a dielectric cover 174 provided between the high frequency antenna and the susceptor, a conductor 170 provided between the high frequency antenna and the dielectric, a ground circuit 180 connected to the conductor, an inductance variable inductor 302 provided within the ground circuit and a capacitance variable capacitor 304. By adjusting the inductance or capacitance thereof, the sticking speed of the foreign material to the dielectric cover becomes approximately equal with the sputtering speed of the dielectric cover 174 such that the sticking of the foreign material can be prevented. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a plasma processing apparatus for preventing foreign matter from adhering to a dielectric provided between a high frequency antenna connected to a high frequency power source and a plasma generating section.

[0002]

2. Description of the Related Art Inductively coupled plasma processing apparatus (hereinafter referred to as ICP
The device is a device that supplies high-frequency power to a spiral, coil, or spiral high-frequency antenna arranged outside the processing container through a dielectric such as quartz, which is a part of the processing container. This is a plasma processing apparatus that generates a plasma of a processing gas by the induction electric field formed in the.

This ICP device is excellent in that high density plasma can be obtained mainly because plasma is generated by an induction electric field, and it is used for manufacturing a semiconductor device, a substrate for a liquid crystal display device (hereinafter referred to as an LCD substrate) and the like. Used in etching and film formation processes.

However, in the ICP apparatus, foreign matter adheres to the surface of the dielectric provided between the high-frequency antenna and the plasma generation unit on the side of the plasma generation unit.
It may cause the generation of particles.

Further, in an ICP device using a high frequency antenna having a horizontal component (for example, a plurality of high frequency antennas when viewed from above), the amount of foreign matter attached varies depending on the in-plane position of the dielectric. This is because the potential difference at each position of the high-frequency antenna causes the difference in capacitive coupling strength between the high-frequency antenna and the plasma, and the sputtering of the dielectric material is superior to the adhesion of foreign matter to the dielectric material in the portion where the capacitive coupling is strong. On the other hand, it is considered that foreign matter adheres to the dielectric more predominantly than the sputtering of the dielectric in the portion where the capacitive coupling is weak. In addition to the above problems, the difference in the amount of foreign matter attached depending on the in-plane position of the dielectric also causes the problem of non-uniformity of plasma processing within the surface of the substrate to be processed. This problem becomes more remarkable as the device becomes larger. (For example, see Patent Document 1)

[0006]

[Patent Document 1] Japanese Patent Laid-Open No. 10-275694 (No. 3-4)
(Page, Fig. 1)

[0007]

In order to solve this problem, there is also a method of heating the dielectric to prevent foreign matter from adhering. However, in such a method, the heating means itself becomes large in size with the recent increase in the size of the device, which leads to an increase in the cost of the device. In addition, since the substrate to be processed must be cooled and controlled during processing, the dielectric cannot be heated to a very high temperature.

The present invention has been made in view of the above problems of the conventional plasma processing apparatus, and its purpose is to:
It is an object of the present invention to provide a plasma processing apparatus that prevents foreign matter from adhering to a dielectric provided between a high frequency antenna and a plasma generation unit.

[0009]

In order to solve the above problems, according to a first aspect of the present invention, a susceptor on which an object to be processed is installed, a high frequency antenna, and a high frequency power source connected to the high frequency antenna. A dielectric provided between the high frequency antenna and the susceptor, a conductor provided between the high frequency antenna and the dielectric, a ground circuit connected to the conductor, and the ground circuit Provided is a plasma processing apparatus including an inductor provided therein.

According to the above structure, it is possible to prevent foreign matter from adhering to the dielectric provided between the high frequency antenna and the plasma generating section, and manufacture a high quality product.
Incidentally, when an electric resistance is used instead of the inductor as in the above-mentioned Patent Document 1, foreign matter adheres to the dielectric at a minimum when the conductor is electrically floated by an open / close switch or the like. Even with this, it is not possible to deal with a device having a large amount of adhesion. On the other hand, when an inductor is used, the amount of sputtering of the dielectric can be increased depending on the set value of the inductance, and foreign substances can be prevented from adhering to the dielectric in various devices.

Further, it is preferable that the inductance of the inductor is variable. Further, the ground circuit may have a capacitor or circuit opening / closing means. The capacitance of the capacitor is preferably variable.

According to the above configuration, the potential of the high-frequency antenna, the material and thickness of the dielectric provided between the high-frequency antenna and the plasma generator, and the change in the distance between the high-frequency antenna and the object to be processed can be dealt with. , Fine adjustment is possible to prevent foreign matter from adhering to the dielectric.

Further, in this device, a measuring means for measuring the electric potential of the electric conductor, and one or more of the inductance of the inductor and the capacitance of the capacitor are changed based on the electric potential of the electric conductor measured by the measuring means. With the configuration including the control means, it becomes easy to change the inductance of the inductor and the capacitance of the capacitor according to the change of the processing condition, so that it is possible to provide a plasma processing apparatus capable of more accurate plasma processing.

In order to solve the above problems, according to a second aspect of the present invention, a susceptor on which an object to be processed is installed and a susceptor which is disposed so as to face the susceptor and is parallel to a processing surface of the object to be processed. A high-frequency antenna having a component, a high-frequency power source connected to the high-frequency antenna, a dielectric provided between the high-frequency antenna and the susceptor, and a conductive material provided between the high-frequency antenna and the dielectric. There is provided a plasma processing apparatus including a body, a ground circuit connected to the conductor, and an impedance element provided in the ground circuit.

Here, the high frequency antenna having a component parallel to the processing surface of the object to be processed means a high frequency antenna having a horizontal component when the object to be processed is arranged horizontally. A high-frequency antenna having a horizontal component is, for example, a high-frequency antenna that is wound in a spiral shape and that looks like a plurality of coils when viewed from the vertical direction (for example, when viewed from above). Say.
Note that a high-frequency antenna having a horizontal component appears to be wound only once when viewed from the vertical direction (for example, when viewed from above) like a high-frequency antenna wound in a coil in the vertical direction. The shape of the high frequency antenna is not included. Further, when a high-frequency antenna having a component parallel to the processing surface of the object is processed, for example, when the substrate to be processed is erected vertically, it is not a high-frequency antenna having a horizontal component but a vertical component. It refers to the high-frequency antenna that it has.

According to the above structure, it is possible to reduce the difference in the amount of foreign matter attached depending on the in-plane position of the dielectric, and to improve the in-plane processing uniformity of the object to be processed. Particularly, in a device in which the high-frequency antenna is provided such that at least a part of the orthogonal projection of the high-frequency antenna onto the surface to be processed of the object to be processed overlaps the processing portion of the object to be processed, the influence of the high-frequency antenna In addition, the difference in the amount of adhered foreign matter due to the in-plane position of the dielectric is large, but with this configuration, this difference can be eliminated, and the in-plane uniform processing of the object to be processed becomes possible.

In order to solve the above problems, according to a third aspect of the present invention, the susceptor for mounting the object to be processed and at least a part of the antenna are directly above the object to be processed during processing. A high-frequency antenna provided, a high-frequency power source connected to the high-frequency antenna, a dielectric provided between the high-frequency antenna and the susceptor, and a conductive material provided between the high-frequency antenna and the dielectric. There is provided a plasma processing apparatus including a body, a ground circuit connected to the conductor, and an impedance element provided in the ground circuit.

In an apparatus in which at least a part of the high-frequency antenna is directly above the object to be processed during processing, the influence of the high-frequency antenna increases the difference in the amount of foreign matter attached depending on the in-plane position of the dielectric. According to the above configuration, this difference can be eliminated, and the in-plane uniform processing of the object to be processed becomes possible.

In the above second and third aspects of the present invention, the impedance element is preferably an inductor. This is because it can be widely applied to devices in which the potential of the high frequency antenna, the material and thickness of the dielectric material installed between the high frequency antenna and the plasma, and the distance between the high frequency antenna and the object to be processed are different. It is preferable that the inductance of the inductor is variable. Further, the ground circuit may have a capacitor or circuit opening / closing means. In this case, the capacitance of the capacitor is preferably variable.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A plasma etching apparatus will be described as an example of a plasma processing apparatus to which the present invention is applied. FIG. 1 is a schematic sectional view of a plasma etching apparatus 100 according to an embodiment of the present invention, FIGS.
Is a diagram for explaining the shape of an antenna having a horizontal component, FIG. 5 is a diagram for explaining a shape that does not correspond to an antenna having a horizontal component, and FIG. 6 is a diagram of a plasma processing apparatus for vertically processing a non-processing object. FIG. 7 and FIG. 7 are schematic views of an apparatus in which at least a part of the orthogonal projection of the high-frequency antenna onto the surface to be processed overlaps the surface to be processed.

As shown in FIG. 1, the plasma etching apparatus 100 has a rectangular cylindrical processing container 102 made of a conductive material such as aluminum. Processing container 102
Is grounded, and the etching process is performed in this processing container 10
It is done within 2.

At the bottom of the processing container 102, the insulating member 10
A susceptor 106 having a substantially rectangular shape is provided via the terminal 7. The susceptor 106 has an object to be processed, for example, an LCD substrate L.
Is placed. The susceptor 106 includes an electrode portion 106a,
It is composed of an electrode protection part 106b.

The electrode portion 106a is made of a conductive material such as aluminum or stainless steel whose surface is anodized. The electrode protection part 106b is the electrode part 106a.
It covers parts other than the mounting surface and is made of an insulating material such as ceramics.

A high frequency power supply 113 is electrically connected to the electrode portion 106a of the susceptor 106 via a matching circuit 111. During plasma processing, a high frequency power of 113 MHz is applied from the high frequency power supply 113 to generate a bias potential, and plasma excited in the processing unit 103 is effectively applied to the processing surface of the LCD substrate L. It is possible to pull in. Here, the electrode portion 106a of the susceptor 106 may be simply grounded.

Above the susceptor 106 is the antenna chamber 11
0 is provided. A high frequency antenna 112 is provided inside the antenna chamber 110. High frequency antenna 11
Reference numeral 2 denotes a conductor such as copper, aluminum, and stainless formed in a spiral shape, a coil shape, or a loop shape, and is provided on the upper portion of the processing container 102. A matching circuit 162 is provided between both terminals of the high frequency antenna 112.
A high frequency power supply 160 for plasma generation is connected via the. If the antenna impedance is high, high frequency antennas may be multiplexed.

Here, the high frequency antenna is preferably a high frequency antenna having a component parallel to the processing surface of the object to be processed. Especially when the object to be processed is placed horizontally,
As shown in FIG. 4, a high frequency antenna having a horizontal component is preferable. The high-frequency antenna having a horizontal component is a high-frequency antenna having a shape that looks like a plurality of coils when viewed in the vertical direction (for example, when viewed from above). In the example shown in FIG. 2, the high frequency antenna 112 is formed in a spiral shape on the flat plate-shaped dielectric body 120. Between the terminals 112a and 112b of the high frequency antenna 112,
A high frequency power supply 160 for plasma generation is connected via a matching circuit 162.

In the example shown in FIG. 3, the high frequency antenna 212
Is spirally formed on the dielectric 220 having a substantially trapezoidal cross section. Both terminals 212a, 2 of the high frequency antenna 212
A high frequency power supply 160 for plasma generation is connected between the 12b via a matching circuit 162. Further, in the example shown in FIG. 4, the substrate mounted on the mounting table 306 is
A circular semiconductor wafer W, for example, is used as the high frequency antenna 31.
2 is spirally formed on a dome-shaped dielectric body 320 having a substantially semicircular cross section. A high frequency power supply 160 for plasma generation is connected via a matching circuit 162 between both terminals 312a and 312b of the high frequency antenna 312. The dome-shaped dielectric 320 is preferably a dome-shaped bottom surface when processing an LCD substrate. When projected onto the surface to be processed of the object to be processed, the high frequency antennas 212 and 312 in FIGS. 3 and 4 are both spiral like the high frequency antenna 112.

On the other hand, in the example shown in FIG. 5, the high frequency antenna 412 has a coil shape in which the same diameter is vertically wound on the side surface of a cylindrical dielectric body 420 such as a hollow quadrangular prism. It is configured and has a shape that appears to be wound once when viewed from the vertical direction (for example, when viewed from above). Such a high-frequency antenna has a quadrangular shape when projected onto the surface of the object to be processed, and is not included in the high-frequency antenna having a horizontal component.

However, in the case of an apparatus for processing while holding the substrate L to be processed on the side surface of the susceptor 206 which stands vertically as shown in FIG. 6, for example, a high frequency antenna having a component parallel to the processing surface of the object to be processed. Is not a high-frequency antenna having a horizontal component, but like the high-frequency antenna 512, is formed in a spiral shape by being installed flatly on the side surface of a plate-shaped dielectric 520 provided upright. This is a high frequency antenna having a directional component.

Although FIG. 6 shows the case where the high frequency antenna 512 is provided on the side surface of the susceptor 206 of the dielectric 520, the present invention is not necessarily limited to this, and the high frequency antenna is provided on the side surface opposite to the susceptor 206. 512 may be provided. Further, in the above-described dielectric 220 shown in FIG. 3, at least a part of the orthogonal projection of the high frequency antenna 212 onto the surface to be processed of the object to be processed as shown in FIG. 7 (point h in FIG. 7).
May be provided so as to overlap the processing part of the object to be processed. The same applies to the dielectric 320 shown in FIG.

By the way, in the lower part of the dielectric 120,
As shown in FIG. 1, an inductor 30 having a conductor 170, which is a feature of the present invention, has a variable inductance.
2 and a ground circuit 180 having a capacitor 304 having a variable capacitance are connected. The ground circuit 180 may have circuit opening / closing means (not shown). Furthermore, in a portion facing the LCD substrate L below the conductor 170,
A dielectric cover 174 is provided. Dielectric cover 174
Is made of a dielectric material thinner than the dielectric material 120, such as quartz or ceramics, and the cost is suppressed by replacing only the dielectric cover 174 at the time of replacement due to adhesion of foreign matter.

The dielectric cover 174 is provided with a shower head (not shown), and a predetermined processing gas such as fluorocarbon gas or Ar gas is supplied from the processing gas source 130 via the flow rate control unit (MFC) 132. , Processing unit 103
To introduce.

The exhaust pipe 1 is provided at the bottom of the processing container 102.
52 is connected so that the atmosphere inside the processing container 102 can be exhausted by an exhaust means (not shown), for example, a vacuum pump, and the atmosphere of the processing unit 103 can be set to an arbitrary degree of reduced pressure. .

A gate valve 154 is provided on the side of the processing container 102, and an unprocessed LCD substrate L is transferred from a load lock chamber installed adjacent to the processing container 102 by a transfer mechanism including a transfer arm. Can be loaded into the processing unit 103.

The operation of the plasma etching apparatus 100 configured as above will be described. First, the gate valve 154 is opened and the LCD substrate L is transported to the processing unit 103 via the gate 150 by a transport arm (not shown). Then, the lifter pin (not shown) rises from the susceptor 106. The LCD substrate L is placed on the lifter pins, and when the lifter pins descend, the LCD substrate L is placed on the susceptor 106.

A predetermined processing gas is introduced into the processing section 103,
A vacuum pump (not shown) connected to the exhaust pipe 152 is evacuated to a predetermined degree of vacuum, for example, 30 mTorr.
Is adjusted to the degree of vacuum.

Subsequently, the high frequency power supply 160 supplies high frequency power of, for example, 13.56 MHz to the high frequency antenna 112 in the antenna chamber 110 via the matching circuit 162. At this time, plasma is generated in the processing unit 103 by the inductive action of the high frequency antenna 112.

The plasma of the processing unit 103 thus generated moves toward the LCD substrate L on the susceptor 106 by the bias potential applied to the susceptor 106, and the surface to be processed is subjected to a desired etching process. You can After the etching process is completed, the processed LCD substrate L is carried out to the load lock chamber through the gate 150.

Next, the effect of preventing foreign matter from adhering to the processing portion 103 side of the dielectric cover 174 by the conductor 170 when performing the etching process using the plasma etching apparatus 100 as described above will be described.

If the conductor 170 of this embodiment is not provided,
Since the central portion of the dielectric cover 174 is near the power supply point, the potential is very high, and the dielectric cover 174 is sputtered due to capacitive coupling between the high frequency antenna 112 and plasma, so that foreign matter is unlikely to adhere. On the other hand, high frequency antenna 1
At the end portion of 12 or a place away from the power feeding portion of the high frequency antenna 112, since the electric field perpendicular to the dielectric cover 174 is small and the sputtering rate is small, a lot of foreign matter adheres.

Therefore, as in this embodiment, the conductor 170
Installed between the dielectric 120 and the dielectric cover 174,
When grounded via the inductor 302 and the capacitor 304, a closed circuit including the inductor 302, the capacitor 304, the conductor 170, the dielectric cover 174, and plasma is formed, and the conductor 170 and plasma are capacitively coupled.

As a result, the outer edge of the dielectric cover 174 is also sputtered, and the adhesion of foreign matter is reduced. However, if the speed of spattering is higher than the speed of adhesion of the foreign matter, the dielectric cover 174 may be scraped, and impurities may be mixed into the object to be processed.
It is necessary to adjust the inductance of 02 and the capacitance of the capacitor 304 so that the foreign matter deposition rate and the sputter rate are approximately equal.

With respect to a device without the conductor 170 and a device in which the capacitance of the capacitor 304 is changed, small glass pieces are arranged at the central portion, the middle portion, and the outer edge portion of the dielectric cover 174, and the treated object is treated. The etching process was performed and the change of the surface was measured. At this time, however, there was no significant difference in the etching rate and distribution of the object to be processed. Hereinafter, the positive value represents the amount of foreign matter attached, and the negative value represents the scraped amount of the glass piece.

As a result, in the device without the conductor 170,
The center portion was -0.7 μm, the middle portion was +0.2 μm, and the outer edge portion was +0.9 μm. In a device with conductor 170, the capacitance of capacitor 304 is 1000
In the case of pF, the central part is +0.5 μm and the middle part is +0.
When the capacitance was 1500 pF, the central portion was -0.3 µm, the middle portion was -0.8 µm, and the outer edge portion was -0.3 µm.

In the device having the conductor 170 used for this evaluation, the capacitance of the capacitor 304 is 1000
It can be seen that there is an optimum value between the pF and the 1500 pF at which the foreign matter deposition rate and the sputter rate are almost the same. It was also found that the in-plane non-uniformity of the adhesion of foreign matter to the dielectric cover 174 was significantly alleviated.

On the other hand, in the device having the conductor 170, the inductor is not provided in the ground circuit 180 and the capacitor 3
04 is used, the equivalent circuit of the capacitor 304 is used to set the capacitance of the capacitor 304 within the range of 0 to 4000 pF.
After changing 7 points, the deposition rate of foreign matter and the sputter rate did not become almost equal.

Further, the equivalent circuit is provided with an inductor, and the resistance between the conductor and the plasma is significantly increased and evaluated. By adjusting the inductance of the inductor, the adhesion speed of foreign matter and spattering can be improved. There were places where the speeds were almost the same.

As described in detail above, according to the plasma etching apparatus 100 of this embodiment, the conductor 170 is installed between the dielectric 120 and the dielectric cover 174, and the inductor 302 and the capacitor 304 are connected. By grounding through and adjusting the inductance of the inductor 302 and the capacitance of the capacitor 304, it is possible to prevent the foreign matter from adhering by making the foreign matter deposition rate and the sputter rate substantially equal. Therefore, it can be said that the present invention can be applied to various devices in which the potential of the high frequency antenna, the material and thickness of the dielectric material installed between the high frequency antenna and the plasma, and the distance between the high frequency antenna and the object to be processed are different.

[0049]

According to the present invention, in an inductively coupled plasma processing apparatus, a conductor is provided between the high frequency antenna and the inductive plasma, and the conductor is grounded via an inductor and a capacitor, so that the antenna It is possible to prevent foreign matter from adhering to the dielectric provided between the plasma and the plasma, and to alleviate in-plane non-uniformity of foreign matter adherence, thereby providing a plasma processing apparatus capable of high quality plasma processing. .

[Brief description of drawings]

FIG. 1 is a schematic sectional view of a plasma etching apparatus 100 according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating an example of a high frequency antenna having a horizontal component according to the same embodiment.

FIG. 3 is a diagram illustrating another example of a high frequency antenna having a horizontal component according to the same embodiment.

FIG. 4 is a diagram illustrating another example of a high frequency antenna having a horizontal component according to the same embodiment.

FIG. 5 is a diagram illustrating an example that does not correspond to a high frequency antenna having a horizontal component in the same embodiment.

FIG. 6 is a schematic view of an apparatus for vertically standing and processing a substrate to be processed in the same embodiment.

FIG. 7 is a schematic diagram of an apparatus provided so that at least a part of the orthogonal projection of the high-frequency antenna on the surface to be processed of the high frequency antenna in the same embodiment overlaps the surface to be processed.

[Explanation of symbols]

100 plasma etching equipment 102 processing container 103 processing unit 106 susceptor 110 antenna room 112 high frequency antenna 120 dielectric 160 high frequency power supply 170 conductor 174 Dielectric cover 180 ground circuit 302 inductor 304 capacitor

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Tsutomu Satoyoshi             TBS release, 5-3-6 Akasaka, Minato-ku, Tokyo             Sending Center Tokyo Electron Limited (72) Inventor Michio Nishimura             TBS release, 5-3-6 Akasaka, Minato-ku, Tokyo             Sending Center Tokyo Electron Limited F term (reference) 4G075 AA24 AA30 AA52 BC04 BC06                       CA25 DA02 EB01 EB42 EC25                       FC11 FC15                 5F004 AA15 BA20 BB18 BB32 BD04

Claims (15)

[Claims] 1. A susceptor on which an object to be processed is installed, a high-frequency antenna, a high-frequency power source connected to the high-frequency antenna, a dielectric provided between the high-frequency antenna and the susceptor, and the high-frequency antenna. A plasma processing apparatus, comprising: a conductor provided between the dielectric, a ground circuit connected to the conductor, and an inductor provided in the ground circuit. 2. The plasma processing apparatus according to claim 1, wherein the inductance of the inductor is variable. 3. The plasma processing apparatus according to claim 1, wherein the ground circuit has a capacitor. 4. The plasma processing apparatus according to claim 3, wherein the capacitance of the capacitor is variable. 5. The plasma processing apparatus according to claim 1, wherein the ground circuit has a circuit opening / closing means. 6. A measuring means for measuring the electric potential of the conductor, and at least one of an inductance of the inductor and a capacitance of the capacitor is changed based on the electric potential of the conductor measured by the measuring means. The plasma processing apparatus according to claim 3, further comprising a control means. 7. A susceptor on which an object to be processed is installed, a high-frequency antenna having a component parallel to a processing surface of the object to be processed, the high-frequency power source connected to the high-frequency antenna, the susceptor being opposed to the susceptor. A dielectric provided between the high frequency antenna and the susceptor; a conductor provided between the high frequency antenna and the dielectric; a ground circuit connected to the conductor; and the ground. A plasma processing apparatus comprising: an impedance element provided in a circuit. 8. The high-frequency antenna is provided such that at least a part of an orthogonal projection of the high-frequency antenna onto a surface to be processed of the object to be processed overlaps with a processing part of the object to be processed. The plasma processing apparatus according to claim 7. 9. A susceptor for mounting an object to be processed, a high-frequency antenna provided so that at least a part of the antenna is directly above the object to be processed during processing, and a high-frequency power source connected to the high-frequency antenna. A dielectric provided between the high frequency antenna and the susceptor; a conductor provided between the high frequency antenna and the dielectric; a ground circuit connected to the conductor; and the ground. A plasma processing apparatus comprising: an impedance element provided in a circuit. 10. The plasma processing apparatus according to claim 7, wherein the impedance element is an inductor. 11. The plasma processing apparatus according to claim 10, wherein the inductor has a variable inductance. 12. The high frequency power source is connected so that high frequency power is fed from near the inner end of the high frequency antenna. Plasma processing equipment. 13. The ground circuit according to claim 7, wherein the ground circuit has a capacitor.
The plasma processing apparatus according to the item. 14. The plasma processing apparatus of claim 13, wherein the capacitance of the capacitor is variable. 15. The plasma processing apparatus according to claim 7, wherein the ground circuit has a circuit opening / closing means.