JP2013134835A - Plasma processing apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the plasma potential by reducing the effective inductance of an antenna in an induction coupling plasma processing apparatus, and to generate plasma of larger area having two-dimensional plasma density distribution of high uniformity.SOLUTION: An antenna 30 having a straight planar shape comprises reciprocating conductors 31, 32 which are arranged to be close each other in a vertical direction Z. High frequency currents Iflow in the reciprocating conductors 31, 32 in a reverse direction with each other. An interval D in the vertical direction Z between the reciprocating conductors 31, 32 is made small in the central part of the antenna in the longitudinal direction X, and made large at both ends. A plurality of such antennas 30 are arranged in parallel. A length N of a region Ahaving a small interval D in the central part of each antenna is made large in the central part of the arrangement direction Y, and made small at both ends.

Description

  The present invention relates to a plasma processing apparatus that performs processing such as film formation by plasma CVD, etching, ashing, sputtering, etc. on a substrate using plasma. More specifically, the present invention is generated by applying a high-frequency current to an antenna. The present invention relates to an inductively coupled plasma processing apparatus that generates plasma by an induced electric field and performs processing on a substrate using the plasma.

  The present application relates to a further improved invention of Japanese Patent No. 4844697 (plasma processing apparatus).

  As belonging to a plasma processing apparatus that generates high-frequency plasma, a capacitively coupled plasma processing apparatus that generates capacitively coupled plasma (abbreviated as CCP) and an inductively coupled type that generates inductively coupled plasma (abbreviated as ICP) Plasma processing apparatus.

  In brief, the capacitively coupled plasma processing apparatus applies a high-frequency voltage between two parallel electrodes and generates plasma using a high-frequency electric field generated between the two electrodes.

  In this capacitively coupled plasma processing apparatus, a high voltage is applied to the plasma to increase the plasma potential, and charged particles (for example, ions) in the plasma impinge on and collide with the substrate with high energy, so that they are formed on the substrate. There are problems such as increased damage to the film and deterioration of the film quality.

  On the other hand, an inductively coupled plasma processing apparatus, in simple terms, generates plasma by an induced electric field generated by flowing a high-frequency current through an antenna. There is an advantage that it can be lowered.

  As an example of such an inductively coupled plasma processing apparatus, Patent Document 1 discloses that a flat antenna is attached to an opening of a vacuum vessel via an insulating frame, and a high-frequency power source is connected between one end and the other end of the antenna. A plasma processing apparatus is described in which a high-frequency power is supplied to flow a high-frequency current, plasma is generated by an induced electric field generated thereby, and a substrate is processed using the plasma.

International Publication No. WO 2009/142016 (paragraphs 0024-0026, FIG. 1)

  Even in an inductively coupled plasma processing apparatus, when an antenna is lengthened to cope with a large substrate, the impedance (particularly inductance) of the antenna increases, thereby generating a large potential difference between both ends of the antenna.

  Since the antenna potential is reflected in the plasma potential via the electrostatic capacitance with the plasma, the plasma potential increases as the antenna potential increases. As a result, charged particles (for example, ions) in the plasma impinge and collide with the substrate with high energy, so that damage to the film formed on the substrate is increased and the film quality is deteriorated.

  Therefore, the present invention is an inductively coupled device that can reduce the effective inductance of the antenna to suppress the plasma potential, and can control the plasma density distribution in the longitudinal direction by the antenna. The main purpose is to provide a device. Another object is to make it possible to generate a plasma with a larger area and more uniform plasma density distribution in two dimensions.

  One of the plasma processing apparatuses according to the present invention generates plasma by generating an induction electric field in a vacuum vessel by flowing a high-frequency current through an antenna having a substantially straight planar shape, and uses the plasma to form a substrate. A reciprocating plasma processing apparatus for performing a treatment, wherein the antenna is disposed close to each other in a direction in which a perpendicular standing on the surface of the substrate is expanded and contracted, and the high-frequency currents flow in opposite directions. The distance between the reciprocating conductors in the direction in which the perpendicular is expanded and contracted is relatively small at the center in the longitudinal direction of the antenna and relatively large at both ends. These are arranged in parallel with each other, and high frequency power is supplied in parallel from a common high frequency power supply to the plurality of antennas. And the length in the antenna longitudinal direction of the region with a small conductor interval in the central portion of each antenna is relatively large in the central antenna in the parallel arrangement direction of the plurality of antennas, The antennas at both ends are relatively small.

  In the following, in order to simplify the expression, the direction in which the vertical line standing on the surface of the substrate is expanded and contracted is referred to as the vertical direction, and the direction intersecting the vertical line is referred to as the horizontal direction. Accordingly, the vertical direction is not necessarily the vertical direction.

  In this plasma processing apparatus, as for each antenna, the antennas are constituted by reciprocating conductors that are arranged close to each other in the vertical direction and in which high-frequency currents flow in opposite directions. The effective inductance of the antenna becomes smaller due to the inductance. Therefore, the potential of the antenna can be kept low, and the plasma potential can be kept low.

  Moreover, by changing the vertical spacing between the reciprocating conductors in the longitudinal direction of the antenna, the mutual inductance between the reciprocating conductors can be varied in the longitudinal direction of the antenna, so that the electromagnetic energy supplied from the antenna to the plasma is It can be changed in the longitudinal direction of the antenna. Therefore, the plasma density distribution in the longitudinal direction can be controlled by this antenna.

  In addition, since a plurality of antennas arranged in parallel with each other and supplied with high-frequency power in parallel are provided, plasma having a larger area can be generated.

  Furthermore, the electromagnetic energy supplied to the plasma from each antenna by making the vertical spacing between the reciprocating conductors of each antenna relatively small at the center in the antenna longitudinal direction and relatively large at both ends, Since it can be relatively larger in the vicinity of both ends than in the vicinity of the central portion in the antenna longitudinal direction, the uniformity of the plasma density distribution in the longitudinal direction of each antenna can be improved.

  In addition, the length in the antenna longitudinal direction of the region having a small conductor interval in the central portion of each antenna is relatively large in the central antenna in the parallel arrangement direction of the plurality of antennas and relatively small in the antennas at both ends. Therefore, the electromagnetic energy supplied to the plasma from multiple antennas can be made relatively larger near both ends than near the center in the parallel arrangement direction of the antennas, so that the plasma density distribution in the parallel arrangement direction of multiple antennas The uniformity can be improved.

  By both the above actions, the uniformity of the plasma density distribution in the longitudinal direction and the parallel arrangement direction of the antenna, that is, the uniformity of the plasma density distribution in two dimensions can be improved.

  Instead of, or in combination with, the length in the antenna longitudinal direction of the region having a small conductor interval at the center of each antenna as described above, the conductor intervals of the corresponding portions of each antenna can be changed to a plurality of antennas. May be relatively small in the central antenna in the parallel arrangement direction, and relatively large in the antennas at both ends.

  From the high frequency power source, one of the first and second flat plate conductors arranged in parallel and close to each other is used as a forward path for high frequency current, and the other flat plate conductor is used as a return path for high frequency current. You may make it supply electric power in parallel.

  The invention according to claim 1 has the following effects.

  (A) Speaking of each antenna, the antenna is constituted by reciprocating conductors that are arranged close to each other in the vertical direction and in which high-frequency currents flow in opposite directions. The effective inductance of the antenna is reduced. Therefore, the potential of the antenna can be kept low, and the plasma potential can be kept low. As a result, the energy of charged particles incident on the substrate from the plasma can be reduced. Thereby, for example, damage to the film formed on the substrate can be suppressed to a small level, and the film quality can be improved. Further, even when the antenna is lengthened, the plasma potential can be kept low by keeping the antenna potential low for the above reasons, so that it becomes easy to cope with the increase in size of the substrate by lengthening the antenna.

  (B) For each antenna, the mutual inductance between the reciprocating conductors can be changed in the longitudinal direction of the antenna by changing the vertical distance between the reciprocating conductors in the longitudinal direction of the antenna. The electromagnetic energy supplied to can be varied in the longitudinal direction of the antenna. Therefore, the plasma density distribution in the longitudinal direction can be controlled by this antenna. As a result, the processing state of the substrate in the longitudinal direction of the antenna can be controlled. For example, the film thickness distribution in the longitudinal direction of the antenna can be controlled.

  (C) Speaking of each antenna, the antenna is constituted by reciprocating conductors that are arranged close to each other in the vertical direction and in which high-frequency currents flow in opposite directions, and the vertical spacing between the reciprocating conductors is determined by the antenna. As compared with the case where the distance between the reciprocating conductors arranged close to each other in the left-right direction is changed, the magnetic flux density in the vicinity of the lower part of the antenna due to the change in the distance between the reciprocating conductors is changed. Good controllability. Therefore, the controllability of electromagnetic energy supplied from the antenna to the plasma by changing the interval between the reciprocating conductors, and hence the controllability of the plasma density distribution are good.

  (D) Since a plurality of antennas arranged in parallel with each other and supplied with high-frequency power in parallel are provided, a plasma having a larger area can be generated.

  (E) The electromagnetic energy supplied to the plasma from each antenna is reduced by making the vertical spacing between the reciprocating conductors of each antenna relatively small at the center in the antenna longitudinal direction and relatively large at both ends. Since it can be made relatively larger in the vicinity of both end portions than in the vicinity of the central portion in the antenna longitudinal direction, the uniformity of the plasma density distribution in the longitudinal direction of each antenna can be improved. In addition, the length in the antenna longitudinal direction of the region having a small conductor interval in the central portion of each antenna is relatively large in the central antenna in the parallel arrangement direction of the plurality of antennas and relatively small in the antennas at both ends. Therefore, the electromagnetic energy supplied to the plasma from multiple antennas can be made relatively larger near both ends than near the center in the parallel arrangement direction of the antennas, so that the plasma density distribution in the parallel arrangement direction of multiple antennas The uniformity can be improved. By both the above actions, the uniformity of the plasma density distribution in the longitudinal direction and the parallel arrangement direction of the antenna, that is, the uniformity of the plasma density distribution in two dimensions can be improved. As a result, since the uniformity of substrate processing in two dimensions can be improved, processing with good uniformity can be performed on a larger area substrate. For example, a film having a good uniformity of film thickness distribution in the substrate surface can be formed on a substrate having a larger area.

  (F) The length in the antenna longitudinal direction of the region having a small conductor interval in the central portion of each antenna is relatively large in the central antenna in the parallel arrangement direction of the plurality of antennas, and relatively long in the antennas at both ends. By reducing the size, it is possible to improve the uniformity of the plasma density distribution in the parallel arrangement direction of the plurality of antennas without inserting an impedance element for adjusting the high frequency current between each antenna and the high frequency power source. Therefore, the circuit configuration can be simplified. In addition, since there is no power loss in the impedance element, it is possible to efficiently supply high-frequency power to each antenna from a high-frequency power source and efficiently input high-frequency power to plasma generation. As a result, it is possible to increase the plasma generation efficiency and thus the utilization efficiency of the high frequency power.

  According to invention of Claim 2, in addition to the effect similar to the effect of said (a)-(d) of invention of Claim 1, there exists the following effect.

  (G) The electromagnetic energy supplied to the plasma from each antenna is reduced by making the vertical spacing between the reciprocating conductors of each antenna relatively small at the center in the antenna longitudinal direction and relatively large at both ends. Since it can be made relatively larger in the vicinity of both end portions than in the vicinity of the central portion in the antenna longitudinal direction, the uniformity of the plasma density distribution in the longitudinal direction of each antenna can be improved. In addition, the distance between the conductors of each antenna corresponding to each other is made relatively small at the central antenna in the parallel arrangement direction of the plurality of antennas and relatively large at the antennas at both ends, so that the plasma from the plurality of antennas The electromagnetic energy supplied to the antenna can be made relatively greater near both ends than near the center in the antenna parallel arrangement direction, so that the uniformity of the plasma density distribution in the parallel arrangement direction of the plurality of antennas can be improved. it can. By both the above actions, the uniformity of the plasma density distribution in the longitudinal direction and the parallel arrangement direction of the antenna, that is, the uniformity of the plasma density distribution in two dimensions can be improved. As a result, since the uniformity of substrate processing in two dimensions can be improved, processing with good uniformity can be performed on a larger area substrate. For example, a film having a good uniformity of film thickness distribution in the substrate surface can be formed on a substrate having a larger area.

  (H) The distance between the conductors of each antenna corresponding to each other is made relatively small in the central antenna in the parallel arrangement direction of the plurality of antennas and relatively large in the antennas at both ends, thereby Even if an impedance element for adjusting a high-frequency current is not inserted between the high-frequency power source and the plasma density distribution in the direction in which the plurality of antennas are arranged in parallel can be improved. Therefore, the circuit configuration can be simplified. In addition, since there is no power loss in the impedance element, it is possible to efficiently supply high-frequency power to each antenna from a high-frequency power source and efficiently input high-frequency power to plasma generation. As a result, it is possible to increase the plasma generation efficiency and thus the utilization efficiency of the high frequency power.

  According to invention of Claim 3, there exists the following further effect. That is, from the high-frequency power source, one of the first and second flat-plate conductors arranged in parallel and close to each other is used as a high-frequency current outgoing path, and the other flat-plate conductor is used as a high-frequency current return path, and the plurality of antennas Since the high-frequency power is supplied in parallel to each other, the high-frequency currents flowing through the first and second flat plate conductors flow in opposite directions while being close to each other. As a result, the mutual impedance between the first and second plate conductors decreases the inductance of both plate conductors, and therefore the high-frequency power loss in the first and second plate conductors decreases. Therefore, it is possible to efficiently supply high-frequency power from a common high-frequency power supply to a plurality of antennas and to efficiently input high-frequency power by plasma generation, so that the plasma generation efficiency and thus the utilization efficiency of high-frequency power can be further increased. it can.

  According to invention of Claim 4, there exists the following further effect. That is, the other end of the upper conductor constituting each antenna is connected to the high-frequency power source via the matching circuit through the first and second flat plate conductors, and the other end of the lower conductor is directly or directly connected to the capacitor. Therefore, the potential of the lower conductor is lower than the potential of the upper conductor. Since the lower conductor is located on the plasma side, the potential of the lower conductor is reflected more strongly in the plasma potential than the potential of the upper conductor, so that the potential of the lower conductor becomes lower as described above. As a result, the plasma potential can be kept lower. As a result, the effect of the above (a) of claim 1 or 2 by suppressing the plasma potential (that is, the energy of charged particles incident on the substrate from the plasma can be suppressed low, even when the antenna is lengthened. ) Can be further reduced.

It is sectional drawing which shows one Embodiment of the plasma processing apparatus which concerns on this invention. It is a figure which shows the antenna in FIG. 1, (A) is a top view, (B) is a side view, (C) is CC sectional drawing. It is a figure which shows the other example of the cross-sectional shape of the plate-shaped reciprocating conductor arrange | positioned at an up-down direction. It is a figure which shows the schematic example of the plasma density distribution in the longitudinal direction at the time of using a well-known simple planar antenna. It is a figure for demonstrating the impedance etc. of the reciprocating conductor which has mutually approached. It is a schematic side view which shows an example of the antenna which has changed the space | interval of the up-down direction between the reciprocating conductors arrange | positioned at an up-down direction in the longitudinal direction of an antenna. It is a figure which shows the model used when changing the space | interval of the up-down direction between the plate-shaped reciprocating conductors arrange | positioned at an up-down direction, and simulating the change of the magnetic flux density near the center lower part of an antenna. It is a schematic side view which shows the other example of the antenna which has changed the space | interval of the up-down direction between the reciprocating conductors arrange | positioned at an up-down direction in the longitudinal direction of an antenna. It is a schematic side view which shows the further another example of the antenna which has changed the space | interval of the up-down direction between the reciprocating conductors arrange | positioned at an up-down direction in the longitudinal direction of an antenna. It is a figure which shows an example of the cross-sectional shape of the rod-shaped reciprocating conductor arrange | positioned at an up-down direction. It is a figure which shows an example of the cross-sectional shape of a reciprocating conductor with one conductor plate-shaped and the other conductor rod-shaped. It is a figure which shows the other example of the cross-sectional shape of a reciprocating conductor with one conductor plate-shaped and the other conductor rod-shaped. It is a figure which shows the model used when changing the space | interval of the left-right direction between the plate-shaped reciprocating conductors arrange | positioned at the left-right direction, and simulating the change of the magnetic flux density in the lower center vicinity of an antenna. It is a figure which shows the model used when changing the space | interval of the up-down direction between the rod-shaped reciprocating conductors arrange | positioned at an up-down direction, and simulating the change of the magnetic flux density in the lower center vicinity of an antenna. It is a figure which shows the model used when changing the space | interval of the left-right direction between the rod-shaped reciprocating conductors arrange | positioned at the left-right direction, and simulating the change of the magnetic flux density in the lower center vicinity of an antenna. It is a graph which shows an example of the magnetic flux density change with respect to a space | interval change in the case of the model shown in FIG. 7 (Example) and the case of the model shown in FIG. 13 (comparative example). 15 is a graph showing an example of a change in magnetic flux density with respect to a change in spacing in the case of the model shown in FIG. 14 (Example) and in the case of the model shown in FIG. 15 (Comparative Example). It is a schematic plan view which shows an example which arranged the some antenna in parallel. It is a schematic plan view which shows the other example which has arrange | positioned the several antenna in parallel. It is a cross section along the Y direction of the antenna shown in FIG. 19, (A) shows an AA cross section and a CC cross section, and (B) shows a BB cross section. FIG. 10 is a schematic plan view showing still another example in which a plurality of antennas are arranged in parallel. It is a cross section along the Y direction of the antenna shown in FIG. 21, (A) shows an AA cross section and a CC cross section, and (B) shows a BB cross section. FIG. It is a figure which shows the schematic example of two-dimensional plasma density distribution obtained when the well-known simple planar antenna long in X direction is arranged in multiple numbers in Y direction. It is a figure which shows the schematic example of the two-dimensional magnetic flux density distribution in the antenna group lower vicinity obtained in the case of the example shown in FIG. 19-20 or FIG. 21-22. It is a figure which shows the schematic example of the two-dimensional plasma density distribution obtained in the case of the example shown in FIGS. 19-20 or 21-22. It is a schematic plan view which shows an example in the case of supplying a high frequency electric power to several antennas arranged in parallel through a parallel plate conductor. It is a perspective view which expands and shows the parallel plate conductor in FIG. It is a figure which shows the example (A) and the schematic example (B) of the electric potential distribution of the antenna in that case which connect the high frequency power supply to the lower conductor which comprises an antenna via a matching circuit, and earth | ground the upper conductor. It is a figure which shows the schematic example (B) of the example (A) which has connected the high frequency power supply to the upper side conductor which comprises an antenna via a matching circuit, and has earthed the lower side conductor, and the antenna potential in that case. It is a figure which shows an example in the case of changing the position of the upper side conductor which comprises an antenna to an up-down direction. It is a figure which shows an example of the more concrete structure around the position adjustment part shown in FIG. It is a schematic perspective view which shows an example of the circumference | surroundings of several antennas which employ | adopted the technique demonstrated with reference to FIGS. 19-20, 26, 29, and 30 comprehensively, and a parallel plate conductor is shifted upwards. Are shown. FIG. 31 is a schematic perspective view showing an example around a plurality of antennas that comprehensively employ the technology described with reference to FIGS. 21 to 22, 26, 29, and 30, and the parallel plate conductors are shifted upward; Are shown.

  One embodiment of the plasma processing apparatus according to the present invention is shown in FIG. 1, and the antenna 30 is extracted and shown in FIG. In order to indicate the orientation of the antenna 30 and the like, the X direction, the Y direction, and the Z direction that are orthogonal to each other at one point are shown in the drawing. The Z direction is a direction in which the perpendicular 3 standing on the surface of the substrate 2 is expanded and contracted, and the Y direction is a direction intersecting (for example, orthogonal to) the perpendicular 3, and these are for simplifying the expression as described above. These are referred to as the vertical direction Z and the horizontal direction Y, respectively. The X direction is a direction that intersects (for example, is orthogonal to) the perpendicular 3 and is a longitudinal direction of the antenna 30. For example, the X direction and the Y direction are horizontal directions, but are not limited thereto. The same applies to the other drawings.

This apparatus, by generating an induced electric field in the vacuum vessel 4 to produce a plasma 50 by the induced electric field by the planar shape frequency current I _R from the high frequency power source 42 in a substantially straight antenna 30, the plasma 50 is an inductively coupled plasma processing apparatus for processing the substrate 2 using 50.

  “Substantially straight” means not only literally a straight state but also a state that is almost straight (substantially straight).

  The substrate 2 is, for example, a substrate for a flat panel display (FPD) such as a liquid crystal display or an organic EL display, a flexible substrate for a flexible display, a substrate for a semiconductor device such as a solar cell, or the like. is not.

  The planar shape of the substrate 2 is, for example, a circle or a rectangle, and is not limited to a specific shape.

  The treatment applied to the substrate 2 is, for example, film formation by plasma CVD, etching, ashing, sputtering, or the like.

  This plasma processing apparatus is also called a plasma CVD apparatus when a film is formed by plasma CVD, a plasma etching apparatus when etching is performed, a plasma ashing apparatus when ashing is performed, and a plasma sputtering apparatus when sputtering is performed.

  The plasma processing apparatus includes, for example, a metal vacuum vessel 4, and the inside is evacuated by a vacuum evacuation device 8.

A gas 24 is introduced into the vacuum vessel 4 through a gas introduction pipe 22. The gas 24 may be set according to the processing content applied to the substrate 2. For example, when forming a film on the substrate 2 by the plasma CVD method, the gas 24 is a source gas or a gas obtained by diluting it with a diluent gas (for example, H ₂ ). More specifically, an Si film is formed on the surface of the substrate 2 when the source gas is SiH _4, an SiN film is formed when SiH ₄ + NH ₃ is used, and an SiO ₂ film is formed when SiH ₄ + O ₂ is used. be able to.

  A holder 10 that holds the substrate 2 is provided in the vacuum container 4. In this example, the holder 10 is supported by the shaft 16. A bearing portion 18 having an electrical insulation function and a vacuum sealing function is provided at a portion where the shaft 16 penetrates the vacuum container 4. As in this example, a negative bias voltage may be applied to the holder 10 from the bias power source 20 via the shaft 16. The bias voltage may be a negative pulse voltage. With such a bias voltage, for example, the energy when positive ions in the plasma 50 are incident on the substrate 2 can be controlled to control the crystallinity of the film formed on the surface of the substrate 2.

  An antenna 30 is provided in the opening 7 of the ceiling surface 6 of the vacuum vessel 4 with an insulating frame 38 interposed therebetween. Between these elements, a packing 40 for vacuum sealing is provided. The antenna 30 is composed of reciprocating conductors 31 and 32 arranged close to each other in the vertical direction Z. In this example, the planar shape of the antenna 30 (more specifically, the reciprocating conductors 31 and 32 constituting the antenna 30) is planar. More specifically, the planar shape is rectangular in this embodiment, but is not limited thereto. The antenna 30 will be described in detail later.

  The material of the antenna 30 is, for example, copper (more specifically, oxygen-free copper), aluminum, or the like, but is not limited thereto.

More specifically, high-frequency power is supplied to the antenna 30 from the high-frequency power source 42 via the matching circuit 44 to the reciprocating conductors 31 and 32, whereby a high-frequency current I _R flows through the antenna 30. That is, the reciprocating conductors 31 and 32 constituting the antenna 30 opposite the high-frequency current (return current) I _R is passed through each other (because the high frequency, the direction of the high-frequency current I _R is inverted by the time. Hereinafter the same) . The high frequency current I _R generates a high frequency magnetic field around the antenna 30, thereby generating an induction electric field in a direction opposite to the high frequency current I _R. Due to this induced electric field, electrons are accelerated in the vacuum container 4 to ionize the gas 24 in the vicinity of the antenna 30 and generate a plasma 50 in the vicinity of the antenna 30. The plasma 50 diffuses to the vicinity of the substrate 2, and the above-described processing can be performed on the substrate 2 by the plasma 50.

  The high frequency power source 42 has an output end 42 a and a ground end 42 b, and the output end 42 a is connected to one end portion of the conductor 31 through a matching circuit 44. The ground end 42b is grounded. One end of the conductor 32 is also grounded.

  The frequency of the high-frequency power output from the high-frequency power source 42 is, for example, a general 13.56 MHz, but is not limited to this.

  The matching circuit 44 includes a capacitor 64 in series with a line and a capacitor 65 in parallel. Both capacitors 64 and 65 are variable in capacity in this example.

The antenna 30 will be described in detail. As shown in FIG. 5, the total impedance Z _T of the parallel reciprocating conductors 61 and 62 that are close to each other is expressed by the following equation as described in the book of electrical theory as a differential connection. . Here, R ₁ and L ₁ are the resistance and self-inductance of one conductor 61, respectively, R ₂ and L ₂ are the resistance and self-inductance of the other conductor 62, and M is between the two conductors 61 and 62, respectively. Mutual inductance.

[Equation 1]
Z _T = (R ₁ + R ₂ ) + j (L ₁ + L ₂ −2M)

Here, to simplify the _{description, R 1 = R 2 = R} , When L ₁ = L ₂ = L, total impedance Z _T is represented by the number 2, in the inductance L _T is the number 3 of which expressed. As in the inductance L _T, a material obtained by combining the self and mutual inductances, in this specification will be referred to as effective inductance.

[Equation 2]
Z _T = 2R + j2 (LM)

[Equation 3]
L _T = 2 (LM)

As can be seen from the above equation, the mutual inductance M between the reciprocating conductors 61 and 62 is increased, total impedance Z _T and the effective inductance L _T is reduced. The electromagnetic energy G generated by flowing a high-frequency current I _R from the high-frequency power source 42 to the reciprocating conductors 61 and 62 is expressed by the following equation. Therefore, when the mutual inductance M increases, the electromagnetic energy G decreases, The acting magnetic effect is reduced. In the case of plasma generation, the electromagnetic energy that can be supplied to the plasma decreases and the plasma density decreases. The reverse case is the opposite.

[Equation 4]
_{G = (1/2) L T I} R 2
_{= (L-M) I R} 2

  When the mutual inductance M is not uniform in the longitudinal direction of the reciprocating conductors 61 and 62, that is, when the mutual inductance M is changed (in other words, changed), if each region is viewed, Depending on the mutual inductance M, the effective inductance and electromagnetic energy are determined.

The antenna 30 constituting the present invention applies the above principle. For example, as in the example shown in FIG. 6, the antenna 30 is constituted by reciprocating conductors 31 and 32 arranged close to each other in the vertical direction Z, and the distance D in the vertical direction Z between the reciprocating conductors 31 and 32. and if is varied in two stages in the longitudinal direction X of the antenna 30, than the mutual inductance M ₂ area a ₂ of the small distance D _2, the direction of the mutual inductance M ₁ area a ₁ of the large distance D ₁ (Ie, M ₁ <M ₂ ). Therefore, the effective inductance of the area A ₁ is larger than the area A ₂ as can be seen by referring to the above equation 3, and the antenna 30 (more specifically, as can be seen from the description referring to the above equation 4). the magnetic flux density of the space at a distance H ₁ from the conductors 31), than the magnetic flux density B ₂ in the area a _2, towards the magnetic flux density B ₁ in the area a ₁ is greater (i.e. B _1> B ₂₎ . Therefore, the plasma density in the region A ₁ can be made larger than the plasma density in the region A ₂ .

  In FIG. 6 and other examples to be described later, the matching circuit 44 is omitted for the sake of simplification, but normally, as in the example shown in FIG. A matching circuit 44 is provided therebetween.

The result of simulating the change in magnetic flux density near the center lower part of the antenna 30 by changing the distance D in the vertical direction Z between the plate-like reciprocating conductors 31 and 32 arranged in the vertical direction Z will be described. The model used for this simulation is shown in FIG. A constant (peak value is 100 A) high-frequency current I _R was passed with the thickness T of both conductors 31 and 32 being 3 mm, the width W in the left-right direction Y being 70 mm, and the length in the X direction being sufficiently long. The magnetic flux density B at a point P separated by a distance H ₁ (fixed at 5 mm) below the center in the left-right direction Y of the conductor 31 was measured while changing the interval D. The results are shown in Table 1.

The magnetic flux density B ₀ in Table 1 is the magnetic flux density at the point P in the case of a known planar antenna having only the conductor 31 without the conductor 32. Table 1 shows the ratio B / B ₀ and the square ratio (B / B ₀ ) ² between the magnetic flux density B _{0 in} that case and the magnetic flux density in the case of the reciprocating conductors 31 and 32. The reason for paying attention to the square ratio (B / B ₀ ) ² is that the supply of electromagnetic energy to the plasma is approximately proportional to the square of the magnetic flux density B, like eddy current loss. As can be seen from this table, by changing the distance D, the magnetic flux density B and the square ratio (B / B ₀ ) ² can be greatly changed. For example, when the distance D is 1 mm and 50 mm, a difference of about 60 times can be added to the square ratio (B / B ₀ ) ² .

FIG. 8 shows that the distance D in the vertical direction Z between the reciprocating conductors 31 and 32 arranged close to each other in the vertical direction Z is larger than the distance D ₂ in the central region A ₂ in the longitudinal direction X of the antenna 30. In this example, the distances D ₁ and D ₃ (D ₁ = D _{3 in} this example) between the regions A ₁ and A ₃ at both ends are increased stepwise. In other words, the distance D in the vertical direction Z between the reciprocating conductors 31 and 32 is relatively small in the central area A ₂ in the longitudinal direction X of the antenna 30 (the distance is D ₂ ). A ₁ and A ₃ are relatively large (the intervals are D ₁ and D ₃ ), and the intervals D ₁ and D ₃ (D ₁ = D _{3 in} this example) are increased stepwise (stepwise). This is an example. In this case, the mutual inductances M _{1 to} M ₃ of the areas A _{1 to} A ₃ are M ₁ = M ₃ <M ₂ , and the effective inductances of the areas A ₁ and A ₃ are larger than the area A ₂ . The magnetic flux densities B _{1 to} B ₃ are B ₁ = B ₃ > B ₂ . As a result, the electromagnetic energy supplied to the plasma from the antenna 30 can be made relatively larger in the regions A ₁ and A _{3 at} both ends than in the region A ₂ at the center.

  The antenna 30 shown in FIGS. 1 and 2 corresponds to the antenna 30 shown in FIG. 8 and is a more specific example. The antenna 30 shown in FIGS. 1 and 2 will be described in detail. The antenna 30 has reciprocating conductors 31 and 32 arranged close to each other with a distance D in the vertical direction Z. Both the conductors 31 and 32 are plate-shaped, and the planar shape thereof is a rectangle as described above. The lower surface of the conductor 31 on the lower side (plasma 50 side) is located in the vacuum atmosphere in the vacuum vessel 4, and the upper conductor 32 is located in the atmosphere. One end of each of the conductors 31 and 32 is electrically open, and an insulator 36 is provided there in this example. The other end portions are electrically connected to each other at the connection portion 33. High-frequency power is supplied from one high-frequency power source 42 via a matching circuit 44 between one end portions of both the conductors 31 and 32.

  In this example, both the conductors 31 and 32 have a plate shape (specifically, the conductor 31 has a flat plate shape and the conductor 32 has a bent plate shape). In this case, for example, the thickness of the lower conductor 31 may be increased as in the example shown in FIGS. 1 and 2C, or the thickness of both the conductors 31 and 32 as in the example shown in FIG. The sizes may be similar to each other.

The upper conductor 32 is bent in a stepped manner, whereby the distances D ₁ and D ₃ between the regions at both ends rather than the distance D ₂ between the regions at the center in the longitudinal direction X of the antenna 30 (D in this example) ₁ = D ₃ ) is gradually increased. In this way, as can be seen from the above description, in the longitudinal direction X of the antenna 30, the mutual inductance M at both ends is made smaller in stages than in the center. That is, in this example, M ₁ = M ₃ <M ₂ . As a result, similarly to the case of FIG. 8, the electromagnetic energy supplied from the antenna 30 to the plasma 50 can be made relatively larger at both end portions than at the central portion in the longitudinal direction X of the antenna 30.

  The distance D and the mutual inductance M of the antenna 30 may be changed stepwise in the longitudinal direction X of the antenna 30 as in the examples shown in FIGS. 1 and 8 or may be changed continuously. . The same applies to other examples described later. Even if the distance D and the like are changed stepwise, the plasma density can be smoothly changed because the plasma has a diffusing action. For example, the upper conductor 32 shown in FIGS. 1 and 2 is formed in a gentle valley shape with a depressed central portion, and the distance D and the mutual inductance M of the antenna 30 are continuously changed in the longitudinal direction X of the antenna 30. You may let them.

  Instead of the antenna 30 shown in FIGS. 1, 2, and 8, the antenna 30 having the structure shown in FIG. 6 may be provided. Furthermore, you may provide the antenna 30 of the other structure shown in FIG.

  The example shown in FIG. 9 is a modification of the example shown in FIGS. That is, instead of supplying high-frequency power from the end of the antenna 30 as shown in FIGS. 1 and 8 (end feeding), high-frequency power is supplied from the center of the antenna 30 as shown in FIG. (Central feeding) may be used. Similarly, in the antenna 30 of another example, the central feeding may be used.

  The reciprocating conductors 31 and 32 constituting the antenna 30 may have a plate shape as in the above-described example, a rod shape as in the example shown in FIG. 10, or a combination of a plate shape and a rod shape, for example. good. For example, as in the example shown in FIG. 11, the lower conductor 31 may be plate-shaped and the upper conductor 32 may be rod-shaped. In that case, as shown in the example shown in FIG. 12, the upper bar-shaped conductors 32 may be a plurality (two in FIG. 12) electrically parallel to each other. Also in these cases, the reciprocating conductors 31 and 32 are electrically connected to each other at the end portion in the longitudinal direction X at the connection portion (see the connection portion 33 in FIG. 1).

  The cross-sectional shape of each of the conductors 31 and 32 is not limited to the illustrated example, and may be a circle, an ellipse, a rectangle, or the like. Alternatively, a structure may be adopted in which the conductors 31 and 32 are made hollow and a coolant such as cooling water is passed therethrough to forcibly cool the conductors 31 and 32.

In the plasma processing apparatus according to the present invention, the antenna 30 is constituted by the reciprocating conductors 31 and 32 that are arranged close to each other in the vertical direction Z and in which the high-frequency currents I _R flow in opposite directions. As can be seen from Equation 3, the effective inductance of the antenna 30 is reduced by the mutual inductance between the reciprocating conductors 31 and 32. Since the impedance of the antenna 30 is mostly an inductance in the high frequency region, the effective inductance is reduced, so that the potential difference generated in the antenna 30 is reduced, the potential of the antenna 30 is reduced, and the potential of the plasma 50 is reduced. Can be suppressed.

  As a result, the energy of charged particles (for example, ions) incident on the substrate 2 from the plasma 50 can be kept small. Thereby, for example, when a film is formed on the substrate 2 by the plasma 50, damage to the film can be suppressed to be small, and the film quality can be improved. Further, even when the antenna 30 is lengthened, the plasma potential can be kept low by keeping the potential of the antenna 30 low for the above reasons. Therefore, it is easy to cope with the increase in size of the substrate 2 by lengthening the antenna 30. Become.

  Moreover, the mutual inductance M between the reciprocating conductors 31 and 32 can be changed in the longitudinal direction X of the antenna 30 by changing the distance D in the vertical direction Z between the reciprocating conductors 31 and 32 in the longitudinal direction X of the antenna. Therefore, the electromagnetic energy supplied from the antenna 30 to the plasma 50 can be changed in the longitudinal direction X of the antenna 30. Therefore, the plasma density distribution in the longitudinal direction X can be controlled by the antenna 30. As a result, the processing state of the substrate in the longitudinal direction X of the antenna 30 can be controlled. For example, when a film is formed on the substrate 2 by the plasma 50, the film thickness distribution in the longitudinal direction X of the antenna 30 can be controlled.

Further, the antenna 30 is configured by reciprocating conductors 31 and 32 that are arranged close to each other in the vertical direction Z and in which the high-frequency currents I _R flow in opposite directions, and the vertical direction Z between the reciprocating conductors 31 and 32. Since the distance D between the reciprocating conductors 31 and 32 is changed in the longitudinal direction X of the antenna 30 as compared with the case where the distance in the left and right direction Y between the reciprocating conductors arranged close to each other in the left and right direction is changed. The controllability of the magnetic flux density in the vicinity of the lower part of the antenna 30 due to the change in the distance D is good. Therefore, the controllability of the electromagnetic energy supplied from the antenna 30 to the plasma 50 and the controllability of the plasma density distribution by changing the distance D between the reciprocating conductors 31 and 32 are good. This will be described below based on the simulation result.

  In the simulation, the magnetic flux density at a point P (see FIGS. 7 and 13 to 15) below the center in the left-right direction Y of the antenna 30 when the distance D between the reciprocating conductors 31 and 32 constituting the antenna 30 is changed. The change in B was calculated.

  First, as one embodiment of the present invention, as shown in FIG. 7, when the plate-like reciprocating conductors 31 and 32 constituting the antenna 30 are arranged close to each other in the vertical direction Z. FIG. 16 shows an example of the change in the magnetic flux density B at the point P when the distance D in the vertical direction Z between the reciprocating conductors 31 and 32 is changed. The calculation conditions at this time are the same as those explained in the explanation part of FIG. 7, and this embodiment is the same as the magnetic flux density B in Table 1.

As a comparative example with respect to the above, when the plate-like reciprocating conductors 31 and 32 constituting the antenna 30 are arranged close to each other in the left-right direction Y as shown in FIG. FIG. 16 shows a change of the magnetic flux density B at the point P when the distance D in the left-right direction Y is changed. The calculation conditions at this time are such that the thickness T of both conductors 31 and 32 is 70 mm, the width W in the left-right direction Y is 3 mm, and other distances H ₁ and the magnitude of the high-frequency current I _R are the same as in FIG. I made it.

  As can be seen from FIG. 16, when the distance D is changed, in the case of the comparative example, the change in the magnetic flux density B is saturated immediately, whereas in the case of the embodiment, the magnetic flux density B is gentle. It is changing in a state close to proportional to, and the change width is also large. Therefore, the control of the magnetic flux density B is better in the example than in the comparative example. That is, by changing the distance D, the magnetic flux density B is controlled, thereby controlling the electromagnetic energy supplied from the antenna 30 to the plasma, and thus controlling the plasma density distribution.

Next, as one embodiment of the present invention, as shown in FIG. 14, when the rod-like reciprocating conductors 31 and 32 constituting the antenna 30 are arranged close to each other in the vertical direction Z, FIG. 17 shows an example of a change in the magnetic flux density B at the point P when the distance D in the vertical direction Z between the reciprocating conductors 31 and 32 is changed. The calculation conditions at this time were such that the diameter d of both the conductors 31 and 32 was 6 mm, and the other distance H ₁ and the magnitude of the high-frequency current I _R were the same as those in FIG.

  Further, as a comparative example with respect to the above, as shown in FIG. 15, when rod-like reciprocating conductors 31, 32 constituting the antenna 30 are arranged close to each other in the left-right direction Y, between the reciprocating conductors 31, 32. FIG. 17 shows a change in the magnetic flux density B at the point P when the distance D in the left-right direction Y is changed. The calculation conditions at this time were the same as those in FIG. 14 except that the arrangement of the two conductors 31 and 32 was changed as described above.

  As can be seen from FIG. 17, when the distance D is changed, in the case of the comparative example, the magnetic flux density B changes once and then decreases, whereas in the case of the example, The magnetic flux density B changes gently in a nearly proportional state. Therefore, also in this case, the control of the magnetic flux density B is better in the example than in the comparative example. That is, by changing the distance D, the magnetic flux density B is controlled, thereby controlling the electromagnetic energy supplied from the antenna 30 to the plasma, and thus controlling the plasma density distribution.

  By the way, normally, that is, when a known simple planar antenna is used, the plasma density distribution in the longitudinal direction X is smaller than the plasma density at the center part as shown in FIG. 4, for example. It becomes a mountain-shaped distribution. The reason for this will be briefly explained. The plasma diffuses from the left and right sides in the central portion, whereas the plasma diffuses only from one side at both ends.

  On the other hand, as in the example shown in FIGS. 1, 2, 8, etc., the distance D in the vertical direction Z between the reciprocating conductors 31, 32 is larger than the distance between the central portions in the longitudinal direction X of the antenna 30. In other words, by increasing the distance between both ends, in other words, the distance D in the vertical direction Z between the reciprocating conductors 31 and 32 is relatively small at the center in the longitudinal direction X of the antenna 30 and relatively at both ends. By increasing the size, the mutual inductance at both ends can be made smaller than the center in the longitudinal direction X of the antenna 30, so that the effective inductances at both ends relative to the center of the antenna 30 are relative to each other as described above. Become bigger. As a result, the electromagnetic energy supplied from the antenna 30 to the plasma 50 is made relatively larger near both ends than near the center in the longitudinal direction X of the antenna 30, contrary to the mountain shape, and more than near the center. Since the plasma 50 can be generated more strongly in the vicinity of both ends, the above-mentioned peak-shaped plasma density distribution can be corrected, and the uniformity of the plasma density distribution in the longitudinal direction X of the antenna 30 can be improved. As a result, the uniformity of substrate processing in the longitudinal direction of the antenna 30 can be improved. For example, when a film is formed on the substrate 2 by the plasma 50, the uniformity of the film thickness distribution in the longitudinal direction X of the antenna 30 can be improved.

  In addition, you may provide the shielding board 46 which shields the surface inside the vacuum vessel 4 of the antenna 30 from the plasma 50 like embodiment shown in FIG. The shielding plate 46 is made of an insulating material. The shielding plate 46 may be attached directly near the entrance of the opening 7 of the ceiling surface 6 of the vacuum vessel 4 or may be attached using a frame-like support plate 48 as in this embodiment. Even when the antenna 30 other than the example shown in FIG. 1 is used, such a shielding plate 46 may be provided.

  The material of the shielding plate 46 is, for example, quartz, alumina, silicon carbide, silicon or the like. If it is difficult to reduce oxygen by hydrogen plasma and release oxygen from the shielding plate 46, a non-oxide material such as silicon or silicon carbide may be used. For example, it is easy to use a silicon plate.

  If the shielding plate 46 is provided, the surface of the antenna 30 or the like is sputtered by charged particles (mainly ions) in the plasma 50, and metal contamination (metal contamination) occurs in the plasma 50 and the substrate 2. Occurrence can be prevented.

  Even if the shielding plate 46 is provided, the shielding plate is made of an insulating material and cannot prevent the potential of the antenna 30 from reaching the plasma 50. Therefore, as described above, the effective inductance of the antenna 30 is reduced to reduce the antenna 30. It is effective to keep the potential of

  As in the example shown in FIG. 18, a plurality of the antennas 30 having the above-described configuration may be arranged in parallel in the Y direction, and high frequency power may be supplied in parallel to each antenna 30 from a common high frequency power source 42. . Further, as shown in the figure, high frequency power may be supplied in parallel from a common high frequency power source 42 to the plurality of antennas 30 via variable impedances 52 connected in series to the respective antennas 30.

  Each antenna 30 may have any configuration described above with reference to FIGS. 1, 2, 6, 8, 9, and the like. The antenna 30 of each example described later may be used.

  The variable impedance 52 may be a variable inductance as shown in FIG. 18, a variable capacitor (variable capacitance), or a mixture of both. By inserting the variable inductance, it is possible to increase the impedance of the power feeding circuit, and thus it is possible to suppress the current of the antenna 30 through which a high-frequency current flows excessively. By inserting a variable capacitor, when the inductive reactance is large, the capacitive reactance can be increased and the impedance of the power feeding circuit can be decreased. Therefore, the current of the antenna 30 in which high-frequency current hardly flows can be increased. .

  In the case of the example shown in FIG. 18, since a plurality of antennas 30 are arranged in parallel with each other and high-frequency power is supplied in parallel, plasma with a larger area can be generated. In addition, the potential of each antenna 30 can be kept low by the above action, and the plasma density distribution in the longitudinal direction X of each antenna 30 can be controlled.

  Furthermore, since the variable impedance 52 is interposed in each antenna 30 and the balance of the high-frequency current flowing through the plurality of antennas 30 can be adjusted by the variable impedance 52, the plasma density distribution in the parallel direction Y of the plurality of antennas 30 can be adjusted. Can also be controlled. As a result, the plasma potential can be kept low, and it is possible to generate a plasma with a larger area and a better plasma density distribution.

  Next, still another embodiment of the present invention will be described. Portions that are the same as or correspond to those in the above-described example are denoted by the same reference numerals, and differences from the above-described example will be mainly described below.

  In the following, in order to easily distinguish the two conductors 31 and 32 constituting the reciprocating conductors 31 and 32 of the antenna 30, the plasma 50 (see FIG. The conductor 31 is sometimes referred to as the lower conductor 31, and the conductor 32 on the opposite side (in brief, the upper side) of the plasma 50 may be referred to as the upper conductor 32.

  The example shown in FIGS. 19 and 20 includes a plurality of antennas 30 that are arranged in parallel to each other in the Y direction. That is, they are arranged side by side in the Y direction. The number of antennas 30 is five in the illustrated example, but is not limited to this. In addition, since the example which employ | adopted the technique as described in FIG.19, FIG.20 etc. comprehensively is an example of FIG. 32 mentioned later, when this figure is referred, the whole is easy to understand.

Each antenna 30 has the same structure as the antenna 30 of the example shown in FIGS. 1, 2, and 8 except that the length N of the region A _{2 in} the antenna longitudinal direction X is large or small. That is, the interval D in the vertical direction Z of the reciprocating conductors 31 and 32 constituting each antenna 30 is relatively small at the center in the longitudinal direction X of the antenna 30 and relatively large at both ends. More specifically, the intervals D ₁ and D ₃ (D ₁ = D _{3 in} this example) between the regions A ₁ and A ₃ at both ends are stepped (stepped) rather than the interval D ₂ between the central regions A _2. ) Is bigger. In this example, the lower conductor 31 is a flat conductor and corresponds to the conductor 31 shown in FIG. The same applies to other examples described later.

  High frequency power is supplied in parallel to the plurality of antennas 30 from a common high frequency power supply 42 via a matching circuit 44. More specifically, in this example, the high frequency power supply 42 is connected to the end portion 32a (the end portion opposite to the connection portion 33 shown in FIG. 1) of the upper conductor 32 constituting each antenna 30 via the matching circuit 44. The end portion 31a of the lower conductor 31 (the end portion opposite to the connection portion 33) is grounded (direct grounding in this example). That is, the end portion 32a of each upper conductor 32 is used as a supply end for high-frequency power, and the end portion 31a of each lower conductor 31 is used as a termination point.

Further, the length N of the antenna A in the antenna longitudinal direction X of the region A _{2 having} a small conductor interval at the center of each antenna 30 is relatively increased in the antenna 30 at the center in the parallel arrangement direction Y of the plurality of antennas 30. The antennas 30 at both ends are relatively small.

  The examples shown in FIGS. 19 and 20 have the same effects as those described above for the examples shown in FIGS.

  Further, the examples shown in FIGS. 19 and 20 have the following effects.

  Since a plurality of antennas 30 arranged in parallel with each other and supplied with high-frequency power in parallel are provided, a plasma having a larger area can be generated. A larger area means a larger area than when one antenna 30 is used.

  The plasma density distribution is as follows.

  FIG. 23 shows a schematic example of a two-dimensional plasma density distribution obtained when a plurality of known simple planar antennas long in the X direction are arranged in parallel in the Y direction. 23 to 25, the two-dimensional plasma density distribution, the film thickness distribution, and the magnetic flux density distribution are shown in a simplified manner as concentric circles or circles for the sake of convenience. Is not limited to concentric circles or circles. For example, the shape may be broken from a concentric circle or a circle, or it may be an ellipse or a rounded rectangle.

  Looking at the antenna longitudinal direction X, in the case of a known simple planar antenna that is long in the X direction, as described above with reference to FIG. 4, the plasma density distribution in the antenna longitudinal direction X is the plasma density at the center. It becomes a mountain-shaped distribution with a smaller plasma density at both ends. The reason for this will be briefly explained. The plasma diffuses from the left and right sides in the X direction in the central portion, whereas the plasma diffuses only from one side at both ends.

  Next, looking at the antenna parallel arrangement direction Y, when a plurality of known simple planar antennas are arranged, the plasma density distribution in the antenna parallel arrangement direction Y is also a peak in which the plasma density at both ends is smaller than the plasma density at the center. It becomes a type distribution. The reason for this will be briefly described. The plasma diffuses from the left and right sides in the Y direction in the central portion, whereas the plasma diffuses only from one side at both ends.

  Therefore, looking at the two dimensions in the XY direction, the two-dimensional plasma density distribution obtained when a plurality of known simple planar antennas that are long in the X direction are arranged in parallel in the Y direction is as shown in FIG. It becomes a mountain-shaped distribution that rises near the center.

  When a film is formed on the substrate 2 (see FIG. 1 and FIG. 23) using plasma having such a density distribution, the film thickness distribution is also a mountain-shaped distribution in which the vicinity of the center is raised as in the case of the plasma density distribution. become.

On the other hand, in the case of the example shown in FIGS. 19 and 20, the distance D in the vertical direction Z between the reciprocating conductors 31 and 32 of each antenna 30 is relatively small in the central portion in the antenna longitudinal direction X. By making the both ends relatively large, the plasma is generated from each antenna 30 for the reason described above using Equations 3 and 4 as in the case of the example shown in FIGS. 50 electromagnetic energy supplied (see FIG. 1. hereinafter the same), it is possible to increase relatively near both ends than near the center of the antenna longitudinal direction X, i.e. a high-frequency current I _R to each antenna 30 Since the magnetic flux density generated near the lower part of each antenna 30 by flowing can be made relatively larger near both ends than near the center in the antenna longitudinal direction X. It is possible to improve the uniformity of the plasma density distribution in the longitudinal direction X of the antenna 30.

Furthermore, the length N in the antenna longitudinal direction X of the region A _{2 having} a small conductor interval D at the center of each antenna 30 is relatively increased in the antenna 30 at the center in the parallel arrangement direction Y of the plurality of antennas. The electromagnetic energy supplied from the plurality of antennas 30 to the plasma 50 is centered in the antenna parallel arrangement direction Y for the reason described above using Equations 3 and 4 by making the antenna 30 relatively small. It can be made relatively larger in the vicinity of both ends than in the vicinity of the portion. That is, the magnetic flux density generated in the vicinity below the antenna group by passing a high frequency current I _R in parallel to a plurality of antennas 30, relatively large near both end portions than near the center of the antenna parallel arrangement direction Y Can do. In short, the magnetic flux density is small in the region A _{2 where the} distance D is small for the reason described above, but the length N of the region A ₂ is small at both ends in the antenna parallel arrangement direction Y, so that the magnetic flux density is relative. to increase, in the central portion is from the magnetic flux density and the length N of the region a ₂ is large becomes relatively small. As a result, the uniformity of the plasma density distribution in the antenna parallel arrangement direction Y can be improved.

  By the action in the XY directions, the uniformity of the plasma density distribution in the longitudinal direction X and the parallel arrangement direction Y of the antenna 30 can be improved, that is, the uniformity of the plasma density distribution in two dimensions.

  The above will be further described with reference to FIGS. In these drawings, the planar shape of the substrate 2 is illustrated as a quadrangle, but as described above, the planar shape of the substrate 2 is not limited to this, and may be a circle or the like.

FIG. 24 shows a schematic example of a two-dimensional magnetic flux density distribution near the lower part of the antenna group obtained in the case of the examples shown in FIGS. For the above reason, the magnetic flux density distribution is a distribution in which the vicinity of the central portion is depressed as shown in FIG. Thus, plasma generation effects produced by passing a high frequency current I _R also becomes recessed is near the center as well distributed as Figure 24 to the antenna group. Since the plasma density distribution shown in FIG. 23 can be corrected by the plasma generation operation having such a distribution, the uniformity of the plasma density distribution in two dimensions in the XY directions is improved as in the example shown in FIG. Can do. The degree of the correction is, for example, adjusting the distances D _{1 to} D ₃ in the areas A _{1 to} A ₃ of the antennas 30 and / or the length N in the antenna longitudinal direction X of the areas A _{1 to} A _3. Can be adjusted.

  As a result, the uniformity of substrate processing in two dimensions can be improved, so that processing with good uniformity can be performed on the substrate 2 having a larger area. For example, when a film is formed on the substrate 2, a film having a uniform film thickness distribution in the substrate plane can be formed on the substrate 2 having a larger area, for example, as shown in FIG. it can.

Further, as described above, the length N in the antenna longitudinal direction X of the region A _{2 having} a small conductor interval D in the central portion of each antenna 30 is relatively set in the central antenna 30 in the parallel arrangement direction Y of the plurality of antennas. For example, FIG. 18 and its explanation can be obtained by increasing the size and relatively reducing the size of the antenna 30 at both ends without inserting an impedance element for adjusting the high-frequency current between each antenna 30 and the high-frequency power source 42. The uniformity of the plasma density distribution in the parallel arrangement direction Y of the plurality of antennas 30 can be improved without inserting an inductance, a capacitance, or the like as described above. Therefore, the circuit configuration can be simplified. In addition, since there is no power loss in the impedance element, it is possible to efficiently supply high-frequency power to each antenna 30 from the high-frequency power source 42 and efficiently input the high-frequency power to plasma generation. As a result, it is possible to increase the plasma generation efficiency and thus the utilization efficiency of the high frequency power.

  The plurality of antennas 30 arranged in parallel in the Y direction may be as shown in the examples shown in FIGS. Note that an example in which the techniques described in FIG. 21, FIG. 22 and the like are comprehensively adopted is an example of FIG. 33 to be described later.

The example shown in FIGS. 21 and 22 will be described mainly with respect to the differences from the examples shown in FIGS. 19 and 20. In this example, the region A _{2 having} a small conductor interval D at the center of each antenna 30 is described. Instead of making the difference in the length N in the antenna longitudinal direction X as described above (that is, the length N of the area A ₂ of each antenna 30 is the same as each other), as shown in FIG. The conductor spacing D of the corresponding portions of 30 is relatively small in the central antenna in the parallel arrangement direction Y of the plurality of antennas 30 and relatively large in the antennas at both ends. That is, the conductor spacing D ₁ of the area A ₁ of each antenna 30 (see FIG. 22 (A)), (see FIG. 22 (B)) conductor spacing D ₂ of the region A _2, conductor spacing D ₃ regions A ₃ (FIG. 22 (A)), the conductor spacing of the antenna 30 at the center in the antenna parallel arrangement direction Y is relatively small, and the conductor spacing of the antenna 30 at both ends is relatively large. In order to do this, for example, it is easy to change the position in the vertical direction Z of the upper conductors 32 of the same shape constituting each antenna 30 little by little.

  In the case of the example shown in FIGS. 21 and 22, the same operational effects as the examples shown in FIGS. 19 and 20 can be obtained.

  The plasma density distribution will be described. By making the distance D in the vertical direction Z between the reciprocating conductors 31 and 32 of each antenna 30 relatively small at the center in the antenna longitudinal direction X and relatively large at both ends. Since the electromagnetic energy supplied from each antenna 30 to the plasma 50 can be made relatively greater near both ends than near the center in the antenna longitudinal direction X, the plasma density distribution in the longitudinal direction X of each antenna 30 can be increased. Uniformity can be improved. This is the same as the case of the example of FIGS.

Furthermore, by making the conductor interval D of the portions corresponding to each antenna 30 relatively small in the central antenna 30 in the parallel arrangement direction Y of the plurality of antennas and relatively large in the antennas 30 at both ends, The electromagnetic energy supplied from the plurality of antennas 30 to the plasma 50 is made relatively larger near the both ends than near the center in the antenna parallel arrangement direction Y for the reasons described using Equations 3 and 4 above. be able to. That is, the magnetic flux density generated in the vicinity below the antenna group by passing a high frequency current I _R in parallel to a plurality of antennas 30, relatively large near both end portions than near the center of the antenna parallel arrangement direction Y Can do. To put it simply, the magnetic flux density is relatively large at both ends in the antenna parallel arrangement direction Y, and the magnetic flux density is relatively large. is there. As a result, the uniformity of the plasma density distribution in the antenna parallel arrangement direction Y can be improved.

  By the action in the XY directions, the uniformity of the plasma density distribution in the longitudinal direction X and the parallel arrangement direction Y of the antenna 30 can be improved, that is, the uniformity of the plasma density distribution in two dimensions.

That is, also in the example shown in FIGS. 21 and 22, the magnetic flux density distribution in the vicinity of the lower part of the antenna group is a distribution in which the vicinity of the central portion is depressed as in the example shown in FIG. Thus, plasma generation effects produced by passing a high frequency current I _R also becomes recessed is near the center as well distributed as Figure 24 to the antenna group. Since the plasma density distribution shown in FIG. 23 can be corrected by the plasma generation operation having such a distribution, the uniformity of the plasma density distribution in two dimensions in the XY directions is improved as in the example shown in FIG. Can do. The degree of the correction is, for example, adjusting the distances D _{1 to} D ₃ in the areas A _{1 to} A ₃ of the antennas 30 and / or the length N in the antenna longitudinal direction X of the areas A _{1 to} A _3. Can be adjusted.

  As a result, the uniformity of substrate processing in two dimensions can be improved, so that processing with good uniformity can be performed on the substrate 2 having a larger area. For example, when a film is formed on the substrate 2, a film having a uniform film thickness distribution in the substrate plane can be formed on the substrate 2 having a larger area, for example, as shown in FIG. it can.

  If necessary, the technique as shown in FIGS. 19 and 20 and the technique as shown in FIGS. 20 and 21 may be used in combination.

The example shown in FIG. 26 is arranged on one end side of the plurality of antennas 30 (on the side of the end portions 31a and 32a opposite to the connection portion 33 described above), and is close to each other and arranged in parallel. The flat plate conductor 84 and the second flat plate conductor 86 are further provided, and the plurality of antennas 30 are connected to the first and second flat plate conductors 84 and 86 in parallel with each other. The high-frequency power source 42 is connected to the plurality of antennas 30 by using one of the first and second plate conductors 84 and 86 as an outward path of the high-frequency current I _R and using the other plate conductor as a return path of the high-frequency current I _R. Supply high-frequency power in parallel.

  More specifically, in this example, the end 32 a of the upper conductor 32 of each antenna 30 is connected to the first flat conductor 84, and the end 31 a of the lower conductor 31 of each antenna 30 is connected to the second flat conductor 86. Is connected. Further, the high frequency power supply 42 is connected to the first flat plate conductor 84 via the matching circuit 44, and the second flat plate conductor 86 is grounded (in this example, directly grounded). The flat conductor 86 may be grounded via a capacitor. The reason will be described later.

  Both flat conductors 84 and 86 are enlarged and shown in FIG. These are parallel plate conductors extending along the Y direction. The flat conductors 84 and 86 are electrically insulated by, for example, sandwiching a thin insulator between them or providing an insulating spacer.

Although the direction of the high-frequency current I _R is reversed with time as described above, there is a relationship in which one of the plate conductors 84 and 86 is the forward path of the high-frequency current I _R and the other is the return path of the high-frequency current I _R. does not change.

In this example, the high frequency power supply 42, one of the flat conductor of the first and second flat conductor 84, 86 in parallel to close each other and forward of the high frequency current I _R, the high frequency current and the other flat conductor as return of I _R, since so as to supply high frequency power in parallel to a plurality of antennas 30, a high-frequency current I _R flowing through the first and second flat conductor 84, 86 opposite in a state of close proximity to one another Flowing into. As a result, as described above with reference to Equations 2 and 3, etc., the mutual impedance between the first and second flat conductors 84 and 86 and the inductance of both flat conductors 84 and 86 are reduced. The high-frequency power loss in the first and second flat conductors 84 and 86 is reduced. Accordingly, it is possible to more efficiently supply the high frequency power from the common high frequency power supply 42 to the plurality of antennas 30 and to efficiently input the high frequency power by plasma generation, thereby further improving the plasma generation efficiency and hence the utilization efficiency of the high frequency power. be able to.

Note that the positions of feeding power to both the flat conductors 84 and 86, that is, the connection point and the grounding point position of the matching circuit 44 are the longitudinal direction (Y direction) of both flat conductors 84 and 86 as in the example shown in FIG. It is preferable to be near the central part. This is because the high frequency current I _R can be supplied to each antenna 30 with a good balance.

  26, as shown in FIG. 26, the high frequency power source 42 is connected to the end portion 32a of the upper conductor 32 constituting each antenna 30 via the matching circuit 44, and the lower conductor is formed. The end 31a of 31 is preferably grounded (directly grounded or grounded via a capacitor). The reason will be described below. In the examples shown in FIGS. 28 and 29, the flat conductors 84 and 86 are omitted for the sake of simplicity, but there is no problem in the description of the electric circuit.

  The example shown in FIG. 28 corresponds to a simplified version of the antenna 30, the high frequency power source 42, and the matching circuit 44 shown in FIGS. In this example, the high-frequency power source 42 is connected to the end 31 a opposite to the connection portion 33 of the lower conductor 31 via the matching circuit 44, and the end 32 a opposite to the connection portion 33 of the upper conductor 32 is connected. Grounded (direct grounding in this example). That is, the end 31a of the lower conductor 31 is a supply end for high-frequency power, and the end 32a of the upper conductor 32 is a termination point.

Since the antenna 30 (more specifically, the same below 33. conductors 31 and 32 and the connecting portion constituting it) has an impedance, I a potential difference am generated when high frequency current I _R. In that case, the matching circuit 44 normally has an end portion on the side of the matching circuit 44 (that is, the high-frequency power source 42 side) (for example, in the example of FIG. 28), for example, because it has a series capacitor 64 as in the example shown in FIG. Then, the potential of the end portion 31a) is the highest.

  A schematic example of the potential distribution of the antenna 30 in this case is shown in FIG. For the above reason, the potential of the lower conductor 31 is higher than the potential of the upper conductor 32. The lower conductor 31 is located on the plasma 50 side, and the potential of the lower conductor 31 is reflected more strongly in the plasma potential than the potential of the upper conductor 32, so that the potential of the lower conductor 31 is as described above. The plasma potential also increases as the value increases. As the plasma potential increases, as described above, the energy of charged particles incident on the substrate 2 from the plasma 50 also increases, and damage to the film formed on the substrate 2 also increases. The example shown in FIG. 28 still has room for improvement in this respect.

  FIG. 29 shows an example in which the above is improved. In this example, a high-frequency power source 42 is connected to the end portion 32a of the upper conductor 32 via a matching circuit 44, and the end portion 31a of the lower conductor 31 is grounded (direct grounding in this example). That is, the end portion 32a of the upper conductor 32 is used as a supply end for high-frequency power, and the end portion 31a of the lower conductor 31 is used as a termination point.

  A schematic example of the potential distribution of the antenna 30 in this case is shown in FIG. For the above reason, the potential of the lower conductor 31 is lower than the potential of the upper conductor 32. The lower conductor 31 is located on the plasma 50 side, and the potential of the lower conductor 31 is reflected more strongly in the plasma potential than the potential of the upper conductor 32, so that the potential of the lower conductor 31 is as described above. The plasma potential also decreases as the temperature decreases. That is, in the example of FIG. 29, the plasma potential can be suppressed lower than in the example of FIG.

  As a result, the above-described effect by suppressing the plasma potential, that is, the energy of charged particles incident on the substrate 2 from the plasma 50 can be suppressed low, and the plasma potential can be suppressed even when the antenna 30 is lengthened. Etc. can be further enhanced.

  Note that, when the end portion 32a of the upper conductor 32 and the end portion 31a of the lower conductor 31 are grounded, they may be grounded via a capacitor (series capacitor) instead of being directly grounded as in the above example. In the case of grounding via a capacitor, simply speaking, although the potential of the antenna 30 is increased by the potential difference between both ends of the capacitor, the example shown in FIG. 29 is still better than the example shown in FIG. Since the plasma potential can be kept low, the above-described effects can be achieved.

  Since the example shown in FIG. 26 employs the power feeding method shown in FIG. 29, the above effects can be achieved. The same applies to the examples shown in FIGS.

  As in the example shown in FIG. 30, the upper conductor 32 and the lower conductor 31 constituting each antenna 30 are made different structures, and the position of the upper conductor 32 is changed in the vertical direction Z as indicated by an arrow J ( Position adjustment) may be possible. In this example, the method of feeding power to the antenna 30 is the same as the example shown in FIGS. In this example as well, the flat conductors 84 and 86 are not shown for the sake of simplicity, but the relationship with the flat conductors 84 and 86 will be described later with reference to FIGS. 32 and 33. .

  The end portions 31b and 32b on the same side of the two conductors 31 and 32 are electrically connected to each other by the connecting portion 33 and further in this example via the position adjusting portion 70.

  The position adjusting unit 70 enables the position adjustment of the upper conductor 32 in the vertical direction Z, and electrically connects the end 32b of the upper conductor 32 and the connecting portion 33 to each other. The position of the upper conductor 32 is fixed by a fixing member 72.

  An example of a more specific structure around the position adjusting unit 70 is shown in FIG.

  In this example, the fixing member 72 includes a bolt 73 and a nut 74. The number of both is two in the illustrated example, but is not limited to this.

  An extension portion 31c obtained by extending the end portion 31b of the lower conductor 31 is provided, and the connection portion 33 is formed by bending the extension portion 31c upward. The connection portion 33 (in other words, the extension portion 31c; the same applies hereinafter) has a hole 76 through which the bolt 73 is passed.

  An extension portion 32c obtained by extending the end portion 32b of the upper conductor 32 is provided, and the extension portion 32c is bent upward so as to be close to the connection portion 33 substantially in parallel. The extension portion 32 c can move (slide) in the up-down direction Z while being close to the connection portion 33. The extension 32c has a long hole (a hole long in the vertical direction Z) 78 through which the bolts 73 are passed. The long hole 78 is more preferable because the position of the upper conductor 32 in the vertical direction Z can be adjusted continuously (steplessly), but a plurality of holes are provided in the vertical direction Z instead of the long hole 78. May be.

  The position of the upper conductor 32 in the vertical direction Z can be changed (adjusted) by loosening the fixing member 72 and moving the upper conductor 32 in the vertical direction Z. By placing the upper conductor 32 in a desired position and tightening the fixing member 72, the position of the upper conductor 32 can be fixed. In addition, the end portion 32 b of the upper conductor 32 and the connection portion 33 can be electrically connected. Further, since the upper conductor 32 can be attached / detached by the above structure, that is, the upper conductor 32 can be attached / detached by removing the fixing member 72, the upper conductor 32 can be exchanged.

  Instead of providing the position adjusting portion 70 as described above, the connecting portion 33 may be a flexible conductor that allows the upper conductor 32 to move in the vertical direction Z. The position in the vertical direction Z can be adjusted. However, it is easier to adjust the position of the upper conductor 32 if the position adjusting unit 70 is provided.

  In the example shown in FIG. 30, as described above, the upper conductor 32 and the lower conductor 31 constituting the antenna 30 are different structures, and the position of the upper conductor 32 can be changed in the vertical direction Z. The distance D in the vertical direction Z between the reciprocating conductors 31 and 32 can be easily adjusted. For example, the distance D can be adjusted after the device is assembled. As a result, the magnetic field distribution of the antenna 30 can be adjusted, and consequently the plasma density distribution can be adjusted, so that it becomes easier to cope with the uniform plasma density distribution.

  Since the upper conductor 32 is detachable as described above, the upper conductor 32 can be replaced with one having the same structure or another structure.

  Therefore, by adopting the techniques shown in FIGS. 30 and 31 for the antennas 30 shown in FIGS. 19 to 20 or 21 to 22, the adjustment of the plasma density distribution in two dimensions in the XY directions can be easily performed. It becomes easy to cope with the uniformity of the two-dimensional plasma density distribution.

  FIG. 32 shows an example around a plurality of antennas that comprehensively employ the techniques described with reference to FIGS. 19 to 20, 26, 29, and 30. FIG. 33 shows an example around a plurality of antennas that comprehensively employ the techniques described with reference to FIGS. 21 to 22, 26, 29, and 30. In both figures, the flat conductors 84 and 86 are shifted upward.

  In both figures, the matters that have not been described so far will be described. The positions of the antennas 30 on the side opposite to the position adjusting unit 70, that is, the ends on the side where the flat conductors 84 and 86 are provided are positioned. A position adjustment unit 70a having a structure similar to that of the adjustment unit 70 is provided. Each position adjustment unit 70 a enables the position adjustment of the upper conductor 32 in the vertical direction Z and electrically connects the end 32 a of the upper conductor 32 to the flat conductor 84.

  More specifically, the structure in the vicinity of each position adjustment portion 70a is provided with an extension portion 31d obtained by extending the end portion 31a of each lower conductor 31 and bent upward. An extension part 32d is provided by extending the end part 32a of each upper conductor 32 and is bent upward. Flat conductors 84 and 86 are inserted between the extension portions 31d and 32d as indicated by an arrow K. The two flat conductors 84 and 86 are fixed so as to be close to each other and electrically insulated, for example, by sandwiching an insulating plate therebetween.

  The flat conductor 86 is fixed and electrically connected to an extension 31d of the lower conductor 31 by a fixing member (not shown) (for example, a bolt and a nut similar to the fixing member 72; the same applies hereinafter). As a result, the end 31 a of the lower conductor 31 is electrically connected to the flat conductor 86. The extension part 32d of each upper conductor 32 has a long hole (a hole long in the vertical direction Z) 78a similar to the long hole 78 shown in FIG. 31, and the position of the antenna 30 in the vertical direction Z can be adjusted. In addition, it is fixed to and electrically connected to the flat conductor 84 by a fixing member (not shown).

  Therefore, the position in the vertical direction Z of the upper conductor 32 of each antenna 30 can be adjusted. For example, as in the example shown in FIG. 32, the positions of the upper conductors 32 of all the antennas 30 can be aligned in the vertical direction Z, and as shown in FIG. The position of Z can also be shifted. Moreover, since the upper conductor 32 of each antenna 30 is detachable, it can also be replaced.

  The other operational effects of the example shown in FIGS. 32 and 33 are obtained by integrating the operational effects described above with reference to FIGS.

  In any case of the antenna 30 of any of the above examples, the upper conductor 32 is preferably positioned in the atmosphere. In the example shown in FIG. 1 as well, as described above, the upper conductor 32 is positioned in the atmosphere.

  If the upper conductor 32 is positioned in the atmosphere, even if the inside of the vacuum vessel 4 is in a vacuum state, the position of the upper conductor 32 can be adjusted and the upper conductor 32 can be separated without returning the inside of the vacuum vessel 4 to the atmospheric pressure state. Since it is possible to change to a shape, etc., the plasma density distribution described above can be easily adjusted. The time required for adjustment can also be shortened.

  Further, since the heat conduction in the space is larger in the atmosphere than in the reduced-pressure atmosphere, the cooling effect of the upper conductor 32 can be improved by positioning the upper conductor 32 in the atmosphere. It is also possible to forcibly cool the upper conductor 32 by blowing air from the fan. Therefore, the cooling efficiency of the upper conductor 32 can be increased.

2 Substrate 4 Vacuum container 24 Gas 30 Antenna 31, 32 Reciprocating conductor 31 Lower conductor 32 Upper conductor 42 High frequency power supply 44 Matching circuit 50 Plasma 84, 86 Flat conductor

Claims (4)

This is an inductively coupled plasma processing apparatus for generating a plasma by generating an induction electric field in a vacuum vessel by flowing a high-frequency current through an antenna having a substantially straight planar shape, and processing the substrate using the plasma. And
The antenna is constituted by a reciprocating conductor that is arranged close to each other in a direction in which a vertical line standing on the surface of the substrate is expanded and contracted and in which the high-frequency currents flow in opposite directions,
And the interval in the direction in which the perpendicular line between the reciprocating conductors is expanded and contracted is relatively small at the center in the longitudinal direction of the antenna, and relatively large at both ends.
Furthermore, a plurality of the antennas are provided, and these are arranged in parallel with each other
It is configured to supply high-frequency power in parallel from a common high-frequency power supply to the plurality of antennas,
In addition, the length in the antenna longitudinal direction of the region with a small conductor interval in the central portion of each antenna is relatively large in the central antenna in the parallel arrangement direction of the plurality of antennas, and relatively long in the antennas at both ends. The plasma processing apparatus is characterized by being made small. This is an inductively coupled plasma processing apparatus for generating a plasma by generating an induction electric field in a vacuum vessel by flowing a high-frequency current through an antenna having a substantially straight planar shape, and processing the substrate using the plasma. And
The antenna is constituted by a reciprocating conductor that is arranged close to each other in a direction in which a vertical line standing on the surface of the substrate is expanded and contracted and in which the high-frequency currents flow in opposite directions,
And the interval in the direction in which the perpendicular line between the reciprocating conductors is expanded and contracted is relatively small at the center in the longitudinal direction of the antenna, and relatively large at both ends.
Furthermore, it is provided with a plurality of the antennas, and these are arranged in parallel with each other,
It is configured to supply high-frequency power in parallel from a common high-frequency power supply to the plurality of antennas,
In addition, the conductor spacing of the corresponding portions of each antenna is relatively small in the central antenna in the parallel arrangement direction of the plurality of antennas and relatively large in the antennas at both ends. A plasma processing apparatus. The first and second flat plate conductors are further provided with first and second flat plate conductors disposed on one end side of the plurality of antennas and arranged close to each other in parallel. The antennas are connected to each other in parallel,
The high-frequency power source has one of the first and second flat-plate conductors as a high-frequency current forward path and the other flat-plate conductor as a high-frequency current return path, and the high-frequency power is parallel to the plurality of antennas. The plasma processing apparatus according to claim 1 or 2, which is supplied. The reciprocating conductor constituting each antenna has a lower conductor on the plasma side and an upper conductor on the opposite side of the plasma, and one end portions on the same side of both conductors are electrically connected to each other. And
The other end of the upper conductor of each antenna is connected to the first flat conductor,
The other end of the lower conductor of each antenna is connected to the second flat conductor,
Connecting the high-frequency power source to the first plate conductor via a matching circuit;
The plasma processing apparatus according to claim 3, wherein the second flat plate conductor is grounded directly or via a capacitor.

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