Plasma processing device and plasma processing method

Abstract

The present invention improves circumferential uniformity of plasma processing by minimizing circumferential variation of microwave electric field intensity in a wafer outer peripheral part. In a plasma processing device in which a TE11 mode microwave is introduced coaxially into a processing chamber via a circular waveguide, a 90° phase difference plate of a microwave is installed from above in the circular waveguide, and a 180° phase difference plate is installed below the 90° phase difference plate. The plasma processing device further has a rotation drive mechanism connected to the 180° phase difference plate. The plasma processing device further has a detector for detecting variations in the electric field intensity in the circumferential direction connected below the 180° phase difference plate, and has a control means for adjusting the rotation angle of the 180° phase difference plate so as to minimize the deterioration of the axial symmetry of the electric field intensity detected by the detector.

Description

Plasma processing apparatus and plasma processing method

[0001] The present invention relates to a plasma processing apparatus and a plasma processing method for processing a substrate-like sample, such as a semiconductor wafer, placed in a processing chamber in a vacuum vessel using plasma generated in the processing chamber, such as etching.

[0002] The structures of semiconductor devices are becoming increasingly miniaturized and complex in order to simultaneously improve their computing power and reduce power consumption. As a result of the increasing complexity of manufacturing processes and the need for more difficult processing, the manufacturing costs of semiconductor devices are rising. To reduce manufacturing costs, it is necessary to improve mass productivity by increasing the number of high-quality semiconductor devices manufactured from a single semiconductor wafer (hereinafter referred to as a unit or wafer). In order to increase the number of semiconductor devices obtained from a single wafer, it is necessary to ensure good process uniformity across the wafer surface. In particular, since the number of semiconductor devices that can be obtained is greater at the wafer periphery, it is important to ensure good process uniformity in the circumferential direction in order to process the wafer periphery uniformly.

[0003] In the manufacture of semiconductor devices, plasma processing such as plasma etching, plasma CVD (Chemical Vapor Deposition), plasma ashing, etc. Various methods are known for generating plasma, including a method of applying a DC voltage between electrodes and a method of generating plasma using high-frequency power, such as ICP (Inductively Coupled Plasma), CCP (Capacitively Coupled Plasma), ECR (Electron Cyclotron Resonance), and surface wave excited plasma.

[0004] In plasma processing apparatuses that use microwaves, microwaves are often introduced from the central axis of the wafer toward the surface of the wafer to be processed in order to generate plasma that is uniform in the circumferential direction relative to the central axis of the wafer. However, depending on the mode in which the microwave propagates within the waveguide, the electric field distribution is not necessarily axially symmetric. For example, in the TE11 mode, which is the fundamental mode of microwaves propagating within a circular waveguide, the electric field distribution of the microwave is not axially symmetric. In such cases, rotating the polarization plane to form a circularly polarized wave is effective in making the electric field distribution axially symmetric.

[0005] In the configuration of Patent Document 1, a microwave rotation generator (dielectric plate) is installed inside the circular waveguide in order to make the electric field distribution in the circular waveguide symmetrical about an axis.

[0006] Patent Document 2 discloses a structure in which a circular waveguide has multiple stubs inside and the amount of stub insertion is controlled. The axial symmetry of the electric field distribution inside the circular waveguide can be improved by adjusting the amount of stub insertion. By adjusting the amount of stub insertion, it is possible to ensure the axial symmetry of the electric field distribution.

[0007] JP 2010-50046 A JP 2003-110312 A

[0008] Microwave Circuits (Kunihiro Suetake and Shuichi Hayashi, Ohmsha, 1958)

[0009] In plasma processing apparatuses that generate plasma using high-frequency electric fields such as microwaves, as described in Patent Document 1 and Non-Patent Document 1, a microwave rotating generator or a 90-degree phase retarder made of a dielectric plate is proposed as a means for forming a circularly polarized wave. With this method, an axially symmetric electric field distribution can be expected when the reflected wave from the exit of the waveguide is small. However, when the reflected wave returning from the processing chamber is large, the electric field oscillation direction of the microwave incident on the microwave rotating generator or the 90-degree phase retarder may deviate from the direction assumed at the time of design, resulting in a loss of axial symmetry of the electric field distribution. For example, in plasma processing apparatuses, depending on the processing conditions, the microwave may not be fully absorbed by the plasma and may return to the waveguide. To ensure axial symmetry of the electric field distribution under various discharge conditions, dynamic control of the electric field distribution is required.

[0010] One dynamic control method is the use of stubs, as described in Patent Document 2. The problem with the method of varying the insertion amount of stubs is that a drive mechanism is required depending on the number of stubs, making the control method complicated. Furthermore, installing stubs and a drive mechanism means that the structure protrudes outside the circular waveguide by at least the length of the stubs, which increases the footprint of the device.

[0011] An object of the present invention is to provide a technique for minimizing the circumferential variation in microwave electric field strength at the outer periphery of a wafer and improving the circumferential uniformity of plasma processing.

[0012] Other objects and novel features will become apparent from the description of this specification and the accompanying drawings.

[0013] A brief summary of the representative aspects of the present invention is as follows.

[0014] According to one embodiment, in a plasma processing apparatus in which TE11 mode microwaves are coaxially introduced into a processing chamber via a circular waveguide, a 90° microwave phase plate is installed from above within the circular waveguide, and a 180° phase plate is installed below the 90° microwave phase plate. A rotation drive mechanism is connected to the 180° phase plate. A detector (measurement means) is connected below the 180° phase plate to detect variations in electric field strength in the circumferential direction, and control means is provided to adjust the rotation angle of the 180° phase plate so as to minimize deterioration of the axial symmetry of the electric field strength detected by the detector.

[0015] The circumferential variation in microwave electric field strength at the wafer periphery is minimized, improving the circumferential uniformity of plasma processing, thereby increasing the number of high-quality semiconductor devices manufactured from a single semiconductor wafer and improving mass productivity of semiconductor devices.

[0016] FIG. 1 is a cross-sectional view of an etching apparatus according to a first embodiment of the present invention. FIG. 2 is a cross-sectional view of a 90° phase difference plate and a circular waveguide. FIG. 3 is a diagram showing the effect of a 90° phase difference plate. FIG. 4 is a cross-sectional view of a 180° phase difference plate and a circular waveguide. FIG. 5 is a diagram showing the effect of a 180° phase difference plate. FIG. 6 is an enlarged cross-sectional view from an automatic matching box to a circular waveguide. FIG. 7 is a cross-sectional view of a plasma processing apparatus equipped with a circular polarization detector according to a second embodiment of the present invention. FIG. 8 is an enlarged cross-sectional view of the circular polarization detector.

[0017] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. However, in the following description, the same components are designated by the same reference numerals, and repeated explanations may be omitted. Note that the drawings may be more schematic than the actual embodiment to make the description clearer, but they are merely examples and do not limit the interpretation of the present invention.

[0018] FIG. 1 shows a cross-sectional view of an etching apparatus according to a first embodiment of the present invention. In the plasma processing apparatus 100 shown in FIG. 1 , microwaves are generated from a microwave source 1 and transmitted through an isolator 2, an automatic matching box 3, a rectangular waveguide 4, and a circular-to-rectangle converter 6 to a circular waveguide 5. In this embodiment, a 2.45 GHz microwave, which is commonly used industrially, is used. The isolator 2 is used to protect the microwave source 1 from reflected microwaves. The automatic matching box 3 is used to adjust the load impedance and suppress reflected waves to efficiently supply microwaves. To facilitate the handling of microwave propagation phenomena, the cross-sectional dimensions of the waveguides are specified so that only the fundamental TE10 mode propagates in the rectangular waveguide 4, and only the fundamental TE11 mode propagates in the circular waveguide 5. The circular waveguide 5 is equipped with a 90-degree retardation plate 7 and a 180-degree retardation plate 9 connected to a rotary drive mechanism 8. The microwaves introduced into the circular waveguide 5 propagate through the 90° phase difference plate 7, the 180° phase difference plate 9, and the cavity 10, and are introduced into a substantially cylindrical plasma processing chamber 13 (also referred to as a processing vessel) through a microwave introduction window 11 and a shower plate 12. The circular waveguide 5 is installed axially symmetrically with respect to the plasma processing chamber 13.

[0019] The cavity 10 uses a conductor as a material that reflects microwaves, and is made of, for example, aluminum.

[0020] To protect the sidewall of the plasma processing chamber 13 from plasma, an inner cylinder 14 is installed inside the sidewall of the plasma processing chamber 13. Quartz is used as a highly plasma-resistant material for the inner cylinder 14 located near the plasma. Alternatively, yttria, alumina, yttrium fluoride, aluminum fluoride, aluminum nitride, or the like may be used as a highly plasma-resistant material.

[0021] Quartz is used as a material that transmits microwaves for the microwave introduction window 11 and the shower plate 12. Alternatively, other dielectric materials that transmit microwaves may be used. Alternatively, materials with high plasma resistance, such as yttria, alumina, yttrium fluoride, aluminum fluoride, and aluminum nitride, may be used.

[0022] Gas is supplied from a gas supply means 15 between the microwave introduction window 11 and the shower plate 12. The gas supply means 15 has a function of supplying a desired flow rate using a mass flow controller. The type of gas to be used is appropriately selected depending on the film to be treated, and a combination of multiple gases is supplied at a predetermined flow rate.

[0023] The shower plate 12 is provided with a plurality of gas supply holes, through which gas is supplied to the plasma processing chamber 13. The supplied gas is evacuated to a vacuum by a turbo molecular pump 17 via a conductance control valve 16.

[0024] The lower part of the plasma processing chamber 13 is provided with a substrate stage / radio-frequency electrode 19 on which a substrate 18 to be processed is placed, and an insulating plate 29 is provided below the substrate stage / radio-frequency electrode 19. Bias power is supplied to the substrate stage / radio-frequency electrode 19 from a bias power supply 20 via an automatic matching box 21. The substrate 18 to be processed is a semiconductor wafer serving as a substrate-like sample. The central axis of the circular waveguide 5 coincides with the central axis of the plasma processing chamber 13. Furthermore, the central axis of the plasma processing chamber 13 coincides with the central axis of the substrate stage / radio-frequency electrode 19, and is installed axially symmetrically so as to coincide with the central axis of the semiconductor wafer placed on the substrate stage / radio-frequency electrode 19.

[0025] To achieve the desired etching, the bias power is adjusted to control the energy of the ions incident on the substrate 18. The substrate stage / high-frequency electrode 19 is equipped with a suction mechanism and temperature control means for the substrate 18, not shown, and the temperature of the substrate 18 is adjusted as necessary to achieve the desired etching.

[0026] A susceptor 22 and a stage cover 23 are installed to protect the outer periphery of the substrate stage / high-frequency electrode 19 from plasma. Quartz is used for the susceptor 22 and the stage cover 23 as a highly plasma-resistant material. Etching is performed by generating plasma 24 in the plasma processing chamber 13 using microwaves supplied from the microwave source 1, and irradiating the substrate 18 to be processed with the ions and radicals generated there.

[0027] Next, the detailed internal structure of the circular waveguide 5 and its effects will be described. First, the detailed structure of the 90° phase difference plate 7 will be described using Figure 2. Figure 2 is a top cross-sectional view of the circular waveguide 5 and the 90° phase difference plate 7. Consider the case where a microwave in TE11 mode is introduced with the electric field oscillation direction 30 in the y-axis direction in the figure and propagates from the front to the back of the page. The 90° phase difference plate 7 is installed at an angle rotated 45° with respect to the electric field oscillation direction. Here, the y' axis is defined as the axis in the direction of the 90° phase difference plate 7, and the x' axis is defined as the axis perpendicular to the y' axis. The 90° phase difference plate 7 uses quartz as a dielectric material that transmits microwaves.

[0028] FIG. 3 shows the effect of the 90° retardation plate. FIG. 3A is a cross-sectional view of a circular waveguide in the x'-z plane, and FIG. 3B is a cross-sectional view of a circular waveguide in the y'-z plane. In FIGS. 3A and 3B, the curves in the circular waveguide 5 represent waves, with FIG. 3A showing a wave with amplitude in the x' direction and FIG. 3B showing a wave with amplitude in the y' direction. A microwave with amplitude in the y-axis direction can be expressed as a superposition of waves introduced with equal amplitude and phase in the x' and y' directions. When a microwave passes through the 90° retardation plate 7, the phase difference between the wave with amplitude in the x' direction and the wave with amplitude in the y' direction becomes 90°. This is because the phase of the wave with amplitude in the y' direction is delayed by 90° when passing through the 90° retardation plate 7. This means that a circularly polarized wave is formed. Note that the height H of the 90° retardation plate 7 is １ It is necessary to set the height H of the 90° retardation plate 7 to an appropriate value so that the phase difference between the x' component wave and the y' component wave is 90°. １ is expressed as the following equation (1).

[0029] H １ = λg / (4(√ε ｒ -1)) (1) where λg represents the microwave wavelength in a circular waveguide in a vacuum, and ε ｒ is the dielectric constant of the 90° retardation plate 7. For example, the wavelength in the guide for a microwave with a frequency of 2.45 GHz is 203 mm. If the dielectric constant of quartz is 3.8, then H １ becomes 53.5 mm.

[0030] To summarize the above, when there is no reflection from the plasma, the use of the 90° retarder 7 allows for the formation of circularly polarized waves and the introduction of microwaves uniformly in the circumferential direction. In this example, a rectangular parallelepiped retarder was used as the 90° retarder 7, but the 90° retarder does not necessarily have to be rectangular as long as the purpose is to form a 90° phase difference. Furthermore, the 90° retarder 7 may partially reflect the microwaves at its end faces, causing the angle at which the 90° retarder 7 is formed to deviate from 45°. In other words, the angle at which the 90° retarder 7 is installed with respect to the electric field oscillation direction is not necessarily limited to 45°, but may be set to an optimal angle as appropriate depending on the microwave reflection coefficient of the 90° retarder 7.

[0031] When the 90° retardation plate 7 is disposed at an angle of 45° counterclockwise with respect to the vibration direction of the electric field as in this embodiment (see FIG. 2), a counterclockwise circularly polarized wave is generated.

[0032] Next, the detailed structure of the 180° phase difference plate 9 connected to the rotary drive mechanism 8 will be described with reference to FIG. 4. FIG. 4 is a top cross-sectional view of the circular waveguide around the 180° phase difference plate 9. The circular waveguide 5 has a double structure (5-1, 5-2), consisting of an inner waveguide 5-1 and an outer waveguide 5-2 that covers its outer periphery. The inner waveguide 5-1 supports the 180° phase difference plate 9, and the inner waveguide 5-1 and the 180° phase difference plate 9 can be rotated together in the θ direction by the actuator 25 of the rotary drive mechanism 8. The rotation angle of the 180° phase difference plate 9 is constantly monitored by an encoder (not shown). Quartz, a material that transmits microwaves, is used as the material for the 180° phase difference plate 9, for example.

[0033] For example, the actuator 25 of the rotary drive mechanism 8 is an electromagnetic motor, and the connection with the inner waveguide 5-1 is made up of gears or a belt. Alternatively, the actuator 25 may be an ultrasonic motor.

[0034] As shown in Figure 4, the 180° retardation plate 9 is inserted at an angle of θ with respect to the x-axis. Here, the axis in the θ direction is defined as the x"-axis, and the axis perpendicular to the x"-axis is defined as the y"-axis. Using Figure 5, we will describe the effect when a counterclockwise circularly polarized wave generated by the 90° retardation plate 7 is incident on the 180° retardation plate 9.

[0035] 5A is a cross-sectional view of the circular waveguide in the y''-z plane, and FIG. 5B is a cross-sectional view of the circular waveguide in the x''-z plane. As a result of the microwave having an amplitude on the x'' axis being delayed in phase by 180°, a phase difference of 180° occurs between the x'' component wave and the y'' component wave at the exit of the 180° retardation plate 9. Since the incident wave on the 180° retardation plate 9 is a circularly polarized wave having a phase difference of 90°, as a result, the phase difference between the wave having electric field components on the x'' axis and the y'' axis is 90° + 180° = 270°. This means that the wave is converted into a right-handed circularly polarized wave. The height H2 of the 180° retardation plate needs to be set to an appropriate value so that the phase difference between the x'' component wave and the y'' component wave is 180°. The height H of the 180° retardation plate 9 ２ is expressed as the following equation (2).

[0036] H ２ = λg / (2(√ε ｒ -1)) (2) The 180° retardation plate 9 has the effect of reversing the polarization direction of the circularly polarized wave.

[0037] Furthermore, the effect of the 180° retarder 9 of rotating the polarization plane of linearly polarized wave will be described with reference to Figure 6. As shown in Figure 6, consider the case where the insertion direction of the 180° retarder 9 is the x-axis, and linearly polarized wave having an electric field oscillation direction 34 oriented at an angle α with respect to the x-axis is incident. In this case, the 180° retarder 9 delays the phase of the wave component in the x-axis direction, and the microwave that passes through the 180° retarder 9 is converted into linearly polarized wave having an electric field oscillation direction 35 oriented at an angle -α with respect to the x-axis. This means that adjusting the rotation angle of the 180° retarder 9 makes it possible to adjust the angle of the polarization plane of linearly polarized wave.

[0038] To summarize the above, the 180° phase difference plate 9 has the effect of reversing the direction of rotation of an incident circularly polarized wave. In addition, the 180° phase difference plate 9 has the effect of rotating the plane of polarization of an incident linearly polarized wave. When there is no reflected wave from the outlet of the circular waveguide 5, the linearly polarized wave incident on the 90° phase difference plate 7 is converted into a left-handed circularly polarized wave, and then converted into a right-handed circularly polarized wave by the 180° phase difference plate 9.

[0039] Next, a case where a reflected wave is generated from the outlet of the circular waveguide 5 will be described using FIG. 7 . FIG. 7 is an enlarged cross-sectional view from the automatic matching box 3 to the outlet of the circular waveguide 5. Possible examples of reflected waves include microwaves supplied to the plasma processing chamber 13 being reflected without being absorbed by the plasma 24, or being reflected by the wall of the plasma processing chamber 13. The reflected wave RW (RWx: x-axis component of the reflected wave RW, RWy: y-axis component of the reflected wave RW) propagates through the circular waveguide 5, the 180-degree retardation plate 9, the 90-degree retardation plate 7, and the circular-to-rectangle converter 6, in the opposite direction to the incident wave. The microwave electric field within the circular waveguide 5 can be expressed as a superposition of waves in the TE11 mode, with electric field oscillation directions along two independent axes (the x and y axes in FIG. 7 ). On the other hand, the rectangular waveguide 4 has an electric field oscillation direction along one axis (the z-axis in FIG. 7 ) corresponding to the TE10 mode. In other words, the wave propagating in the circular waveguide 5 loses one degree of freedom in the electric field oscillation direction in the process of being transmitted to the rectangular waveguide 4. The TE11 mode, whose electric field oscillation direction is in the y-axis direction, corresponds to the electric field oscillation direction 40 of the TE10 mode in the rectangular waveguide 4, and therefore can propagate toward the automatic matching box 3. On the other hand, in the circular waveguide 5, the electromagnetic wave in the TE11 mode, whose electric field oscillation direction is in the x-axis direction, cannot propagate through the rectangular portion of the circular-to-rectangle converter 6 and is reflected, re-entering the 90-degree retardation plate 7. In other words, the microwave incident on the 90-degree retardation plate 7 is a sum of the TE11 mode wave, whose electric field oscillation direction is in the y-axis direction, and the reflected wave RW (RWx) at the circular-to-rectangle converter 6. As a result, when the reflected wave RW is incident from the outlet of the circular waveguide 5, the axial symmetry of the circularly polarized wave is destroyed at the 90-degree retardation plate 7.

[0040] As described above, the rotation angle of the 180-degree retarder 9 can be used to adjust the angle of the polarization plane of linearly polarized waves, and therefore the angle of the polarization plane of the microwaves re-reflected at the circular-to-rectangle converter 6 can be adjusted. That is, the polarization plane of the waves incident on the 90-degree retarder 7 can be controlled, thereby minimizing the circumferential variation of the electric field. For example, rotating the 180-degree retarder 9 during plasma processing can minimize the circumferential variation of the time-averaged electric field. Alternatively, adjusting the angle of the 180-degree retarder 9 to an optimal angle depending on the processing conditions used in the plasma processing can reduce the circumferential variation of the electric field.

[0041] To control the angle to the optimum, it is necessary to measure the circumferential variation of the electric field. As an indirect measurement method, for example, the reflection coefficient and phase of the load may be monitored in the microwave automatic matching box 3, the reflection coefficient at the exit of the circular waveguide 5 may be estimated, and the rotation angle of the 180° retarder may be adjusted according to the reflection coefficient. As a direct measurement method, an electric field measurement means may be installed in the circular waveguide 5.

[0042] According to the plasma processing apparatus or plasma processing method of Example 1, the circumferential variation in microwave electric field strength can be minimized at the wafer periphery, improving the circumferential uniformity of plasma processing. This increases the number of high-quality semiconductor devices manufactured from a single semiconductor wafer, thereby improving the mass productivity of semiconductor devices.

[0043] FIG. 8 shows a cross-sectional view of a plasma processing apparatus (etching apparatus) according to a second embodiment of the present invention. FIG. 8 is a cross-sectional view of a plasma processing apparatus 100a equipped with a circular polarization detector 26. As shown in FIG. 8, the circular polarization detector 26 is installed downstream of the 180° retardation plate 9 (at the exit side of the circular waveguide 5). By monitoring the state of the circular polarization with the circular polarization detector 26, the optimal angle of the 180° retardation plate 9 can be controlled. The circular polarization detector 26 can be regarded as a measuring means for measuring the electric field in the circumferential direction within the circular waveguide 5.

[0044] A specific example of the circular polarization detector 26 will be described with reference to FIG. 9 . To configure the circular polarization detector 26, electric field probes 27 are installed at multiple locations on the circular waveguide 5, and the circumferential variation of the electric field is monitored by measuring and comparing the electric fields measured by the multiple electric field probes 27. For example, a crystal detector is used as the electric field probe 27. At least three electric field probes 27 are installed around the circumference of the circular waveguide 5. Furthermore, to detect circularly polarized waves, the positions of the electric field probes 27 must not be line-symmetric. The measurement values ​​of the electric field probes 27 are input to a control unit (also referred to as a control means) 28 and processed. Based on the processing, the control unit 28 transmits a control signal to the rotary drive mechanism 8 for controlling the rotation angle of the 180° retarder 9. In this way, the control unit 28 controls the rotation angle of the 180° retarder 9 to minimize the circumferential variation of the electric field. In addition, in the plasma processing method using the plasma processing apparatus of FIGS. 8 and 9, the angle of the 180-degree retardation plate 9 is adjusted by the control unit 28 so as to minimize the circumferential variation of the electric field measured by the circular polarization detector 26 (or the electric field probe 27) which is the measuring means.

[0045] The plasma processing apparatus or plasma processing method according to the second embodiment can also provide the same effects as those of the first embodiment.

[0046] The invention made by the inventor has been specifically described above based on examples, but it goes without saying that the present invention is not limited to the above-described embodiments and examples, and various modifications are possible.

[0047] The present invention is applicable to a plasma processing apparatus that processes a sample on a substrate such as a semiconductor wafer by etching or the like.

[0048] REFERENCE SIGNS LIST 1 Microwave source 2 Isolator 3 Automatic matching box 4 Rectangular waveguide 5 Circular waveguide 5-1 Inner waveguide 5-2 Outer waveguide 6 Circular-to-rectangular converter 7 90° phase difference plate 8 Rotation drive mechanism 9 180° phase difference plate 10 Cavity 11 Microwave introduction window 12 Shower plate 13 Plasma processing chamber 14 Inner cylinder 15 Gas supply means 16 Conductance control valve 17 Turbo-molecular pump 18 Substrate to be processed 19 Substrate stage / high-frequency electrode 20 Bias power supply 21 Automatic matching box 22 Susceptor 23 Stage cover 24 Plasma 25 Actuator 26 Circular polarization detector 27 Electric field probe 28 Control unit 29 Insulating plate 30 Electric field oscillation direction of TE11 mode 34 Oscillation direction of the electric field of the microwave incident on the 180° phase difference plate 35 Oscillation direction of the electric field of the microwave passing through the 180° phase difference plate 40 Oscillation direction of the electric field in the TE10 mode 100, 100a Plasma processing apparatus (etching apparatus)

Claims

1. A plasma processing apparatus comprising: a microwave matching unit, a rectangular waveguide, and a circular waveguide connected to an approximately cylindrical processing vessel; the circular waveguide is axially symmetric with respect to the processing vessel; and a 90-degree phase difference plate and a 180-degree phase difference plate are installed in the circular waveguide.

2. A plasma processing apparatus according to claim 1, wherein a rotation drive mechanism is connected to the 180-degree phase difference plate.

3. A plasma processing apparatus according to any one of claims 1 and 2, further comprising a measuring means, disposed below said 180-degree phase difference plate, for measuring the electric field in the circumferential direction within said circular waveguide.

4. A plasma processing apparatus according to claim 3, further comprising a control means for adjusting the angle of said 180 degree phase difference plate so as to minimize the circumferential variation of the electric field measured by said measuring means.

5. A plasma processing method using the plasma processing apparatus according to claim 3, comprising adjusting the angle of the 180-degree retardation plate so as to minimize the circumferential variation of the electric field measured by the measuring means.

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