JP2003086581A - Antenna for generating large-area plasma - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an antenna for generating large-area plasma which can be applied to the film production of a large-area solar battery, liquid crystal, etc., or etching of a semiconductor and others by generating high-density and uniformizing plasma across a large area. SOLUTION: The antenna is composed of an array antenna 2 obtained by arranging a plurality of antenna elements 6, composed of columnar conductors in parallel and in a planar state with power feed directions set alternately opposite. All of the antenna elements 6 have their surfaces covered with dielectrics.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a large area plasma generating antenna, and more specifically, for example, 1 m × 1 m.
Plasma is generated uniformly and with high efficiency over a large area such as
D), a plasma generation antenna applicable to liquid crystal film formation, semiconductor etching, and the like.

[0002]

2. Description of the Prior Art Amorphous type and crystalline type thin film solar cells have already been used in various fields, but it is well known that early application as a clean energy source for power supply is desired. is there. The thin film solar cell for electric power is required to have a large area of at least 1 m × 1 m. In order to generate such a large-area plasma using an antenna, conventionally, a conductor columnar antenna was directly inserted into gas, but it is difficult to generate spatially uniform plasma. It is necessary to develop a new plasma generation antenna that can be manufactured at high speed and with high quality.

[0003]

By the way, as an apparatus for manufacturing a large-sized thin film solar cell, for example, an ECR (erectron) is used.
cyclotron reasonance) It is possible to use a plasma CVD apparatus. However, in order to generate plasma for obtaining a vapor deposition surface having a large area, there is a problem that the arrangement of the magnetic field generating coil used in the cyclotron and the arrangement of the radiated radio wave antenna interfere with each other, which is difficult to realize.

As another means for generating plasma, it is possible to generate plasma only by the antenna. However, in this method, since plasma is a good conductor of electricity, even if high frequency power is fed to the antenna, a phenomenon (shielding effect) that energy does not propagate from the power feed port first occurs. Moreover, in order to increase the film forming speed in order to reduce the manufacturing cost, it is necessary to increase the plasma density, and then the shielding effect is further increased.

Further, in order to obtain a large-sized vapor deposition surface or etching surface as described above, the operating frequency is about 13 MHz which is used in the conventional ECR plasma CVD apparatus and 1 m ×
If the area is about 1 m, it needs to be as high as about 100 MHz. Since such a high frequency has a wavelength equal to or smaller than the chamber size, it becomes more difficult than before to obtain a uniform radio wave intensity.

The present invention has been made by paying attention to the above problems and prevents the occurrence of the shielding effect and generates a high density and more uniform plasma over a large area. For example, a large area solar cell or liquid crystal. It is an object of the present invention to provide a large-area plasma generation antenna that can be applied to throat film formation, etching of semiconductors and the like.

[0007]

In order to achieve the above object, a large-area plasma generating antenna according to the present invention has a plurality of antenna elements made of columnar conductors, which are arranged in parallel with the feed directions reversed. Each of the antenna elements has a surface covered with a dielectric material and is composed of an array antenna arranged in a plane.

The dielectric material is, for example, quartz or ceramics. However, the present invention is not limited to these exemplified materials.

The antenna element will be described below with reference to the attached FIG. FIG. 1A shows the opposing conductor wall W.
The antenna elements R having the same length are separated from each other (both are grounded) by a predetermined distance, and the free ends of the antenna elements R are separated from the facing conductor wall W by a predetermined distance h.
The figure shows a case where the antenna is formed by opening the antenna to avoid interference with the wall surface of the conductor wall W.

The symbol w shown in FIG. 1 indicates the antenna element R.
It is a metal member (grounded) that covers the base of
It is a member for adjusting the length of the antenna element R, which corresponds to the above-mentioned predetermined interval h, but prevents radio waves from being radiated from the part. However, it is not essential to the present invention and can be omitted.

In the antenna shown in FIG. 1A, a high frequency current having a wavelength of 4/3 times the length is supplied to the antenna element R to form a standing wave.
The upper part of FIG. 1B shows a standing wave formed when the free end side of the antenna element R is open, and the lower part shows a conductor wall W that reflects radio waves on the free end side of the antenna element R. In some cases. Since film formation, etching, etc. are usually performed under reduced pressure, they are performed by the configuration shown in the lower stage of FIG. 1B, that is, FIG.

As can be understood from the above description, the length Z of the antenna element is given by the formula [1] with respect to the wavelength of the high frequency supplied to the array antenna.

[0013]

[Equation 1] In the formula [1], n represents zero or a positive integer.

The length of the antenna element given by the equation (1) may be determined in practice so that a high-frequency standing wave can be obtained on the antenna element based on this length. It does not specify a strictly strict length, and it suffices to satisfy this value substantially.

When the antenna element is arranged in a planar shape, it is usually a planar shape, but the present invention is not limited to this.

The antenna element needs to be a conductor, and generally copper, aluminum, platinum or the like can be used. However, the present invention is not limited to these exemplary metals.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION Plasma CV with reference to the drawings.
The present invention will be specifically described by showing an embodiment implemented by the D device.

A CVD apparatus (hereinafter simply referred to as an apparatus) 1 shown in FIGS. 2 and 3 has a large area plasma generating antenna (hereinafter simply referred to as an antenna) 2 of the present embodiment arranged in a chamber 3.
The vapor deposition substrates 4 such as glass are attached to the substrate pedestals 3b arranged on the metal wall surface 3a of the chamber 3 on both sides thereof, and the vapor deposition gas is introduced into the apparatus 1 through the gas supply pipe 5 opening to the wall surface 3a. It is a thing. The vapor deposition gas used is not constant depending on the vapor deposition purpose, but silane gas or the like can be used for solar cells.

A heating element (not shown) for heating the substrate 4 is attached to the substrate 3b, and reference numeral 3c shown in FIGS. 2 and 3 is an exhaust pipe for generating a vacuum. The apparatus structure described above is an example for explaining the outline of the plasma deposition apparatus, and the present invention should not be limitedly interpreted thereby.

The antenna 2 is an array antenna composed of a plurality of antenna elements 6, and each antenna element 6 is shown in FIG.
As shown in FIG. 3, the feeding directions are alternately opposite to each other, and the feeding directions are arranged parallel to each other (FIG. 2) and planarly (FIG. 3), and the common-mode power distributor 7 is arranged on the antenna element 6 of each pole. From here, a high-frequency current is distributed to each antenna element 6.

As shown in FIG. 4, the antenna element 6 has a rod shape (may be a pipe) made of a good electric conductor, and has a length S of (2n + 1) / 4 times (n in the formula:
Is a zero or a positive integer), the surface of which is covered with a dielectric 8 is electrically insulated from the opening 3d opened in the chamber wall 3a, and the high-frequency current supply end 6a side is It is connected to the core wire 9a of the coaxial feeder 9. The chamber wall 3a is grounded as shown in FIGS.

In FIGS. 2 and 3, the vacuum pump (not shown) connected to the exhaust pipe 3b is actuated to operate normally at 1 mmTorr.
When the vapor deposition gas is fed into the chamber 3 which is evacuated to about 1 Torr and a high frequency current is supplied to the antenna element 6,
As shown in FIG. 5, an alternating magnetic field and an electric field are generated around the antenna element 6, radio waves are radiated from the antenna element 6 to the surroundings, and the gas supplied into the chamber 3 is ionized into plasma.

In this case, since the plasma is conductive, when the chamber 3 is filled with plasma and becomes entirely conductive, the radiated radio waves are reflected by the plasma, and the radio waves are confined around the antenna element 6. The plasma heating region 10 (FIG. 6) is limited to this portion.

By the way, when the thickness of the dielectric 8 is a certain value (a critical value described below) or more, the electric field and the magnetic field are in a plane perpendicular to the axial direction of the antenna element (hereinafter referred to as the z axis) (FIG. 6). In the TEM mode (tra including the x-axis and y-axis) of
nsverse electromagnetic mode radio waves propagate in the z-axis direction. The relationship between the radio wave radiation and the dielectric coating will be sequentially described below.

When the frequency of the high-frequency current that is the energy source for plasma heating is f, it can be assumed that it is lower than the plasma frequency fp in the case of a normal plasma CVD apparatus or etching apparatus. Note that fp is given by the formula [2].

[0026]

[Equation 2] Where n _e is the number of electrons in a unit volume, −e is the charge of the electron,
m _e represents the mass of the electron, and ε ₀ represents the dielectric constant of vacuum. Expression (2) equation using the obtain the relationship between the electron density n _e and the plasma frequency fp 7 will be obtained. In FIG. 7, since the electron density generally used in plasma CVD is in the range of 10 ¹⁵ to 10 ¹⁷ , it can be seen that the plasma frequency fp used in plasma CVD has a value in the range of 300 MHz to 5 GHz.

The axis along the antenna element 6 is the z-axis, the potential of the central conductor portion at the position z is V (z), and the current is I.
(z), and the time dependence of the system is assumed to be exp (iωt). Here, ω = 2πf, and i represents an imaginary number (square root of −1). At this time, the basic equation of the radio wave propagating along the antenna element 6 is the change in the voltage V (z) and I (z) in the z-axis direction using the inductance L (z) and the capacitance C (z) per unit length. Can be expressed as in [Equation 3] and [Equation 4].

[0028]

[Equation 3]

[0029]

[Equation 4] Where L (ω) is the inductance per unit length,
C (ω) represents the capacity per unit length.

The inductance L (ω) and the capacitance C (ω) can be further divided into a dielectric-related part and a plasma-related part. When the cross-sectional shape of the antenna element 6 main body (conductor portion) is a circle with a radius of a and the dielectric is also a circular cross section with an outer diameter of b, the inductance L (ω)
And the capacitance C (ω) are given by the equations [5] and [6], respectively.

[0031]

[Equation 5]

[0032]

[Equation 6]

Here, ε represents the dielectric constant of the dielectric 8 of the antenna element 6, μ ₀ represents the dielectric constant of vacuum, and In represents the natural logarithm. When the equation (5) is an addition equation of the first term and the second term, the second term represents the inductance per unit length contributed by the plasma, and the equation (6) is the first term. The second term (including the negative sign) when the subtraction formula with the second term is used represents the capacity per unit length of plasma that contributes.

Furthermore, the function F (x) in the expressions (5) and (6) is the function given by the expression (7).

[0035]

[Equation 7] Here, K ₀ (u) is a 0th-order Bessel function, and FIG. 8 shows the relationship between x and F (x) in the equation [7].

The fact that the sign of the second term in the equation (6) is negative will be described below.

Waves propagating along the z-axis can be expressed as in [Equation 8].

[0038]

[Equation 8] Where γ is a propagation constant. The propagation constant γ is [Equation 3],
It is given by the equation 9 derived from the equation 4.

[0039]

[Equation 9]

If the antenna element has no dielectric coating, the relation between the radius a of the central conductor (antenna element 6) and the outer diameter b of the dielectric 8 is a = b, and C (ω) is given. The first term of the denominator of the equation 6] becomes 0, and the capacitance C (ω) per unit length becomes negative.

On the other hand, the "inductance per unit length" is always positive. Therefore, γ is a positive number from the formula [9]. At this time, the radio wave propagating along the longitudinal direction of the antenna element 6 is exponentially attenuated in the z-axis direction from the feeding point and does not propagate as a wave. Therefore, resonance cannot occur even if the length is (2n + 1) / 4 times the wavelength, and as a result, energy cannot be efficiently introduced into the plasma. This relationship is shown in FIG.

On the other hand, if the antenna element 6 has a dielectric coating, the second term of the equation (6) can be made a positive value.
At this time, the propagation constant γ becomes an imaginary number, and the radio wave propagates along the antenna element 6. This value (critical value bc
) If a larger outer diameter b of the dielectric 8 is selected, it becomes possible to propagate a high frequency current in the z-axis direction as shown in FIG. 10, and it becomes possible to radiate a radio wave from the antenna element 6. In this case, the antenna 2 operates as a resonator by setting the length of the antenna element 6 to be (2n + 1) / 4 times the wavelength.

Next, the relationship between the coating thickness of the dielectric 8 and the current flowing through the antenna element 6 will be described. The current strength in the longitudinal direction of the antenna element 6 is highest at the supply end and zero at the tip. Therefore, high-frequency currents are supplied to the antenna elements 6 having opposite polarities in mutually opposite directions, radio waves emitted from the antenna elements 6 are combined to form a uniform plasma, and a vapor deposition film having a uniform thickness can be obtained. Become.

By the way, when the radius of the dielectric 8 is set to a thickness exceeding the critical b _c and a high frequency current can be propagated in the antenna element 6, the intensity of the current Ia, Ib flowing through each of the antenna elements 6 of opposite polarities is reduced. Since the attenuation curve is a cosine curve, and the energy is proportional to the square of the energy, the sum of the energy due to the current intensities Ia and Ib is proportional to the sum 1 of the squares of the sine and the cosine. That is, as shown in the graph of FIG. 11B, the antenna element 6 is always flat in the axial direction, and the spatial density of the generated plasma in the chamber 3 can be made uniform.

From the critical value bc, the thickness tc of the dielectric 8 (=
bc-a) is obtained, and the relationship with the plasma frequency fp is obtained, so that FIG. 12 is obtained. Plasma electron density is 10
^{If it is 16} (1 / m ³ ), the plasma frequency fp will be about 1 GHz. When this value is put in FIG. 12, the critical value tc of the dielectric thickness is about 2 mm.

As is clear from the above description, according to the present invention, it has become possible to efficiently carry out film formation, etching, etc. of large thin film solar cells of 1 m × 1 m, which could not be realized conventionally, and liquid crystals. The plasma generation antenna of the present invention can be appropriately used for purposes other than the CVD vapor deposition, the liquid crystal film formation, and the etching described above.

[0047]

As described above, in the antenna for large-area plasma generation of the present invention, the surface of the rod-shaped antenna element forming the array antenna is covered with the dielectric, so that the radiated radio wave energy can be efficiently supplied to the gas around the electrode. It has become possible to generate a spatially homogeneous plasma with a density as high as possible. Therefore, it can be applied to the production of large-sized thin film solar cells that can be used for power supply, the production of liquid crystal thin films, the etching of semiconductors, and various other industrial applications.

[Brief description of drawings]

FIG. 1A shows a basic shape of an antenna of the present invention, and shows a pair of a case where a free end side of an antenna element is a wall surface W _R side and a case where a free end side of an antenna element is W _L. FIG. 7B is a diagram showing that there is a certain thing, and FIG. .

FIG. 2 is a plan view for explaining the outline of the internal configuration of the plasma CVD apparatus provided with the plasma generation antenna according to the embodiment of the present invention.

3 is a sectional view taken along line III-III in FIG.

4 is an enlarged partial sectional view showing a state of a portion where the rod-shaped antenna element shown in FIG. 1 is attached to a chamber wall.

5 is a partial enlarged cross-sectional view for explaining how plasma is generated around the antenna element of FIG.

6 is a cross-sectional view in the direction perpendicular to FIG.

FIG. 7 is a graph showing the relationship between electron density n _e and plasma frequency fp.

8A and 8B are graphs showing a relationship between x and a function F (x) in the equation (7), (A) is a graph when x is 0.01 to 1, and (B) is x. Shows the value of F (x) when is 1 to 20.

FIG. 9 is a graph illustrating a shielding effect when plasma is generated using a rod-shaped antenna element made of a normal conductor.

FIG. 10: z when the thickness t of the dielectric layer is greater than t _c
It is a figure showing that an electric wave propagates infinitely in the axial direction without being attenuated.

FIG. 11 is an explanatory diagram of a method for optimizing the current distribution in the longitudinal direction of the 1 / 4λ-long antenna element of the present invention, (A)
[Fig. 3] is a cross-sectional view of the antenna element, and (B) is a graph showing the optimized distribution.

FIG. 12 is a graph showing a limit dielectric coating thickness that allows a traveling wave to be fed to an antenna element when the dielectric constant of the dielectric is 5.0.

[Explanation of symbols]

2 array antenna 6 antenna elements 8 Dielectric

   ─────────────────────────────────────────────────── ─── Continued front page    F-term (reference) 4K030 FA03 JA01 KA15 KA30 LA16                 5F004 AA01 BA20 BB11 BB16 BC08                       BD04 DA00 DB02                 5F045 AC01 AE15 AE17 AE19 BB01                       BB08 CA13 CA15 DP11 EH02                       EH17

Claims (3)

[Claims] 1. An array antenna in which a plurality of antenna elements made of columnar conductors are alternately arranged in parallel and planarly with the feeding directions reversed, and the antenna element comprises:
Both are large-area plasma generation antennas whose surface is covered with a dielectric. 2. The length of the antenna element is substantially equal to (2n +) of a high frequency wavelength supplied to the array antenna.
The large area plasma generating antenna according to claim 1, wherein the length is 1) / 4 times (where n in the formula is zero or a positive integer). 3. The large-area plasma generating antenna according to claim 1, wherein the planar arrangement is a planar arrangement.