JP2009038317A - Method of forming film of thin film solar battery, and film-forming device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To form a film of microcrystalline silicon having a large area and excellent film quality (high modulus of crystalization and low modulus of failure) in generating microcrystalline silicon using a high frequency plasma CVD method. <P>SOLUTION: The control of the film quality (modulus of crystallization and modulus of failure) of microcrystalline silicon is made easy by eliminating a restriction on the introduction of material gas by performing radical film formation using argon gas in forming the film of microcrystalline silicon for an optical absorption layer (an i layer) of a solar battery by using surface wave excitation plasma. High frequency power for use in surface wave excitation plasma treatment is set low by performing radical film formation using argon gas. Film formation with a large area is made possible by setting the radical of argon uniform. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a solar cell, a solar cell film forming method, and a film forming apparatus, and more particularly to a film forming method in which a light absorption layer (I layer) of a thin film solar cell is formed of a microcrystalline silicon film.

  A solar cell using amorphous silicon (a-Si) is known. Although this amorphous silicon solar cell is less efficient than a solar cell, it can easily be made large in area and can be manufactured at a low manufacturing cost. However, when it is irradiated with light for a long time, its characteristics gradually increase. While there is a problem of light degradation that decreases, solar cells using microcrystalline silicon (μc-Si) do not exhibit light degradation, and the same process as a-Si solar cells can be applied, resulting in lower costs. It is supposed to be possible (Patent Document 1).

  Conventionally, it is known to manufacture a solar cell with a plasma CVD apparatus. FIG. 3 is a diagram showing a configuration example of this plasma CVD apparatus.

An anode 208 and a cathode 203 are arranged opposite to each other in the reaction vessel 202, and the inside is evacuated by an exhaust device 207. After forming the transparent electrode on the substrate 210, SiH ₄ is supplied from the introduction pipe 206 as the main gas, H ₂ as the rare gas, and B ₂ H ₆ as the doping gas, and from the power source 204 through the impedance matching unit 205. A high frequency is applied to the cathode 203 to generate discharge plasma 209 between the anode 208 and the cathode 203. The plasma 209 ionizes each source gas to form reactive ions, and the reactive ions react with the surface of the substrate 210 to deposit a p-type crystalline silicon layer on the substrate. Next, SiH ₄ is used as a main gas and H ₂ is used as a rare gas to supply the reaction vessel 202 to form an i-type microcrystalline silicon layer on the microcrystalline p layer. Further, an n-type microcrystalline silicon layer is formed by supplying SiH ₄ as a main gas, H ₂ as a rare gas, and PH ₃ as a doping gas. Further, a back electrode made of a metal such as aluminum is formed on the n-type microcrystalline silicon. Thereby, a solar cell is formed in a pin structure including positive and negative electrodes (Patent Document 2).

  As described above, the microcrystalline silicon film is generally generated by a parallel plate plasma CVD apparatus. When a film having a large area and a high crystal ratio and few crystal defects is formed at high speed using this parallel plate plasma CVD apparatus, a complicated electrode structure is required to prevent the occurrence of abnormal discharge. In addition, it is necessary to apply a high frequency to generate a high-density crystal, but it is difficult to apply a high frequency from a complicated electrode structure.

  Therefore, in order to form a microcrystalline silicon film having a high crystallinity at a high film formation rate using a parallel plate plasma CVD apparatus, it is necessary to set the frequency range to be lower than that of RF or VHF and to a high temperature. When film formation is performed in a low frequency range, there is a problem that crystal defects are likely to occur. Moreover, when it is set as a high temperature process, the product adhering to the inside of a plasma CVD apparatus will increase, and the problem that maintenance frequency increases will generate | occur | produce.

  Patent Document 3 proposes that a light absorption layer (i layer) of a solar cell is generated from a microcrystalline silicon film by a high-frequency plasma CVD method. In order to generate a light absorption layer (i-layer) of a solar cell with microcrystalline silicon, it is necessary to input high-frequency power, but when high-frequency power is continuously applied, the material gas is uniformly diffused on the substrate. Therefore, there is a problem that the film thickness and crystallinity of the crystalline thin film formed on the substrate are likely to be non-uniform. In Patent Document 3, in order to solve this problem, by performing pulse modulation on the high-frequency power, the material gas is diffused during the time when the high-frequency power is not applied, and the film thickness and crystallinity of the crystalline thin film are made uniform. .

JP 2004-31518 A JP 2003-158276 A JP 2005-50905 A

  As described above, in order to generate a light absorption layer (i layer) of a solar cell from a microcrystalline silicon film by a high frequency plasma CVD method, it is necessary to input a high frequency power. There is a problem that a device for applying high frequency power becomes complicated because pulse modulation is required.

  Further, it is conceivable to use a surface wave excitation plasma processing apparatus as the high frequency plasma CVD method. In order to form a film with a large area, a high crystallinity, and a small number of crystal defects at high speed using a surface wave excitation plasma processing apparatus, the introduction method of the material gas is complicated and the separation of the material gas is promoted. There is a problem that a large amount of power is required to secure the discharge.

  In addition, since the introduction of the material gas is restricted, there is a problem that it is difficult to control the film quality (crystal ratio and defect rate) of the generated microcrystalline silicon.

  Therefore, the present invention solves the above-described problems and forms microcrystalline silicon having a large area and good film quality (high crystal ratio, low defect rate) in the production of microcrystalline silicon using the high-frequency plasma CVD method. For the purpose.

  Another object of the present invention is to reduce the high-frequency power used for the surface wave excitation plasma processing when the light absorption layer (i layer) of the solar cell is generated by generating the microcrystalline silicon film by the surface wave excitation plasma processing. .

  The present invention eliminates restrictions on the introduction of material gas by performing radical film formation using argon gas in the formation of microcrystalline silicon for a light absorption layer (i layer) of a solar cell by surface wave excitation plasma. The film quality (crystal ratio, defect rate) of microcrystalline silicon can be easily controlled. Further, by performing radical film formation using argon gas, the high-frequency power used for the surface wave excitation plasma treatment is reduced. In addition, by making the argon radial uniform, a large-area film can be formed.

  The present invention can be applied to various aspects of an aspect of a film formation method, an aspect of a film formation apparatus, and an aspect of a solar cell formed by the film formation method.

An aspect of the film forming method of the present invention is a film forming method in which a light absorption layer (i layer) of a thin film solar cell is formed of a microcrystalline silicon film, and the microcrystalline silicon film is formed by surface wave excitation plasma treatment. In this surface wave excitation plasma treatment, radicals are generated using argon gas as a process gas. This argon radical promotes the separation of SiH ₄ and H ₂ which are material gases. According to surface wave excitation plasma, radicals can be easily obtained by separating argon. In addition, since radicals can be easily obtained, large-capacity high-frequency power is not necessary, and power can be reduced.

  In addition, since argon is not a reactive species, there is an advantage that it does not become a hindrance to the formation of the microcrystalline silicon film.

In a more detailed aspect of the film formation method of the present invention, in the film formation method in which the light absorption layer (I layer) of the thin film solar cell is formed of a microcrystalline silicon film, the region in which surface waves are formed by the supply of microwaves is provided. Argon gas (Ar) gas is introduced, and the argon gas is excited by the surface wave to generate radicals to form a plasma state. The material gas is introduced into the plasma, and the material gas SiH ₄ is separated by the plasma. Then, a microcrystalline silicon film is formed on the semiconductor surface by the separated silicon.

  A film forming apparatus according to the present invention includes a film forming apparatus for forming a light absorption layer (I layer) of a thin film solar cell with a microcrystalline silicon film, a film forming chamber for performing surface wave excitation plasma treatment on a semiconductor surface, A process gas introduction unit that introduces a process gas into the film chamber and a material gas introduction unit that introduces a material gas into the film formation chamber are provided.

The process gas introduction unit introduces an argon gas (Ar) gas into a region where a surface wave is formed by the supply of microwaves, and the argon gas is excited by surface waves to generate radicals to form a plasma state. Further, the material gas introduction unit introduces a material gas into the plasma by surface wave excitation, causes the material gas SiH ₄ to be separated by the plasma, and forms a microcrystalline silicon film on the semiconductor surface by the separated silicon.

  According to the present invention, in the production of microcrystalline silicon using the high-frequency plasma CVD method, it is possible to form microcrystalline silicon having a large area, a high crystal ratio, and a low defect rate, and a good film quality.

  Further, when the light absorption layer (i layer) of the solar cell is generated by generating the microcrystalline silicon film by the surface wave excitation plasma treatment, the high frequency power used for the surface wave excitation plasma treatment can be reduced.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  First, a solar cell according to an embodiment of the present invention will be described with reference to FIG. FIG. 1 is a cross-sectional view of a solar cell according to an embodiment of the present invention. The solar cell 100 includes a transparent insulating substrate 101, a first transparent electrode 102, a p-type microcrystalline Si layer (p layer) 103, an i-type microcrystalline Si layer (i layer) 104, and an n-type microcrystalline Si. The layer (n layer) 105, the second transparent electrode 106, and the back electrode 107 are formed in order.

  The surface wave excitation plasma CVD apparatus 13 forms the light absorption layer (I layer) of a thin film solar cell with a microcrystalline silicon film. In addition, a transfer device is provided, and the substrate is carried into the surface wave excitation plasma CVD apparatus 13 in the chamber 2, and the substrate on which microcrystalline silicon is formed by the surface wave excitation plasma CVD apparatus 13 is taken out of the chamber. Can do.

  A surface wave excitation plasma CVD apparatus (hereinafter referred to as SWP-CVD apparatus) can easily generate a high-density plasma with a large area using surface waves, and this plasma is a surface wave excitation plasma (SWP: It is called “Surface Wave Plasma”. In addition, the above-mentioned code | symbol is based on FIG.

  Hereinafter, the configuration of the film forming apparatus will be described with reference to the schematic diagram shown in FIG. In FIG. 2, the film forming apparatus 1 includes a chamber 2, a dielectric plate 3, a microwave waveguide 4, a process gas introduction pipe 5, a material gas introduction pipe 6, a vacuum exhaust pipe 7, a stage 8, and a heater 9.

  The dielectric plate 3 is supplied with microwaves from the microwave generation source 14 through the microwave waveguide 4 and constitutes a part of a surface wave excitation plasma apparatus 13 that excites plasma in the vicinity of the surface of the dielectric plate 3. . A microwave waveguide 4 is installed in contact with the upper surface of the dielectric plate 3, and a plurality of slot antennas formed by openings are provided on the bottom plate of the microwave waveguide 4 in contact with the dielectric plate 3. Microwave is introduced through the antenna. The dielectric plate 3 can be made of, for example, quartz, alumina, zirconia, or the like. The dielectric plate 3 may be composed of a plurality of portions.

  Further, the microwave waveguide 4 can be provided with an end matching device, a side reflector or the like so that the standing wave can be stably formed by matching the microwave wave ends.

  The chamber 2 is a hermetically sealed container that forms a film on the surface of the substrate placed on the stage 8 using surface wave excitation plasma generated in the internal space, and is evacuated to evacuate the inside. A tube 7 is provided and evacuated by a vacuum pump (not shown). The stage 8 can move and rotate in the vertical direction, and can heat a substrate that is a film formation target by a heater 9 provided therein. Moreover, it is good also as a structure which cools and a structure which applies an electric field as needed.

  The dielectric plate 3 is provided with a process gas introduction pipe 5, and a process gas such as Ar gas is introduced from the mass flow controller (MFC) 15 through the adjustment valve 12.

  The dielectric plate 3 is provided with a plurality of microwave waveguides 4, and each microwave waveguide 4 is supplied with microwave power from a plurality of microwave generation sources 14. The dielectric plate 3 is provided with a plurality of process gas introduction pipes 5, and process gas is introduced into each process gas introduction pipe 5.

  By introducing microwaves from the microwave waveguide 4 into the dielectric plate 3, plasma is excited in the chamber 2. Thus, the dielectric plate 3 has a plurality of microwave waveguides 4 and a plurality of processes. By providing the gas introduction tube 5, the discharge region can be uniformly excited in the chamber 2. In FIG. 2, two sets of the microwave waveguide 4 and the plurality of process gas introduction pipes 5 are shown, but the number provided is not limited to two sets, and is matched to the size and shape of the dielectric plate 3. Can be determined.

  In the chamber 2, a plurality of material gas introduction pipes 6 for introducing a material gas are provided. FIG. 2 shows a configuration in which the first material gas introduction pipe 6 a is arranged in the center portion of the chamber 2, and the second material gas introduction pipe 6 b and the third material gas introduction pipe 6 c are arranged on the side surface side of the chamber 2. . The first material gas introduction tube 6a introduces a material gas into the region 10a in the central portion of the chamber 2, and the second material gas introduction tube 6b and the third material gas introduction tube 6c are formed in the region 10b on the side portion of the chamber 2, The material gas is introduced into 10c. The material gas is supplied from the mass flow controllers (MFCs) 16a to 16c to the first material gas introduction pipe 6a to the third material gas introduction pipe 6c.

  In FIG. 2, three sets of the material gas introduction pipes 6 are shown, but the number provided is not limited to three sets, and can be determined according to the size and shape of the chamber 2 and the stage 8.

  Further, the heaters 9 provided on the stage 8 can also be provided at a plurality of locations according to the respective regions 10 described above. FIG. 2 shows an example in which the heater 9a is provided at a position corresponding to the area 10a, the heater 9b is provided at a position corresponding to the area 10b, and the heater 9c is provided at a position corresponding to the area 10c.

Of the deposition gas, the process gas introduced from the process gas introduction pipe 5 into the chamber 2 is reactive activity such as N ₂ gas, O ₂ gas, H ₂ gas, NO ₂ gas, NO gas, NH ₃ gas, etc. In addition to the gas used as the seed material, it is a rare gas such as Ar gas, He gas, Ne gas, Kr gas, or Xe gas. Of the film forming gas, the material gas introduced into the chamber 2 from the material gas introduction pipe 6 is a gas containing Si element which is a component of a silicon thin film or silicon compound thin film such as SiH ₄ gas and Si ₂ H ₆ gas. .

  A vacuum exhaust pipe 7 connected to a vacuum exhaust pump (not shown) is disposed on the bottom plate of the chamber 2. By exhausting while introducing a predetermined gas into the chamber 2 at a predetermined flow rate through the process gas introduction pipe 5 and the material gas introduction pipe 6, the inside of the chamber 2 can be maintained at a predetermined pressure.

  In the surface wave excitation plasma apparatus 13 configured as described above, a microwave having a frequency of 2.45 GHz is propagated from the microwave generation source 14 into the microwave waveguide 5, and TE10 is transmitted by a termination matching unit (not shown). A standing wave T of a mode electromagnetic wave is generated. The standing wave T is radiated to the dielectric plate 3 from a slot antenna (not shown) installed at intervals of the wavelength of the standing wave T. The electromagnetic wave radiated from the slot antenna (not shown) propagates inside the dielectric plate 3 and generates a unique standing wave in a range surrounded by the side reflectors (not shown). A surface wave (SW) is generated on the surface of the plate 3. This surface wave (SW) ionizes and dissociates the deposition gas immediately below the dielectric plate 3 to generate surface wave excited plasma P.

  Surface-wave-excited plasma has an electron density higher than the cut-off density of the microwave, totally reflects the electromagnetic wave at the plasma interface, and does not absorb the electromagnetic wave into the plasma. Energy maintains a low temperature of 10 eV or less. In the surface wave excitation plasma, energy is transferred between the surface of the dielectric plate 3 and the plasma boundary surface, and high energy plasma is distributed only in the vicinity of the surface of the dielectric plate 3, and as the distance from the dielectric surface increases. The energy level decreases exponentially. At a distance of about 200 mm from the dielectric plate 3, the ion energy is 20 ev or less. In this way, the surface wave-excited plasma has a high energy region and a low energy region. By generating radicals in the high energy region and introducing a material gas into the low energy region, high efficiency radical generation and low damage are achieved. High-speed film formation is possible.

  Since the surface wave (SW) spreads over the entire inner surface of the dielectric plate 3, the surface wave excited plasma also spreads in the corresponding region in the chamber 2. Therefore, it is possible to cope with a large area by expanding the dielectric plate 3. Using this surface wave excitation plasma, a microcrystalline silicon film is formed on the substrate.

In the present invention, in the film formation of microcrystalline silicon, Ar gas is introduced from the process gas introduction tube 4 into the high energy region of the surface wave excited plasma to generate radicals, and the material is used in the low energy region of the surface wave excited plasma. By introducing the material gas from the gas introduction pipe 6, high-speed film formation with low damage is performed. Argon gas radicals promote the separation of the material gases SiH ₄ and H ₂ .

Here, the conditions for depositing microcrystalline silicon with a crystal ratio Ic / Ia of 3 to 9 by surface wave excitation plasma using argon gas are, for example, an argon gas amount of 144 sccm and a microwave power of 2. The pressure in the chamber is 7.0 to 14 Pa, the film forming temperature is 150 ° C., the amount of SiH ₄ gas is 18 sccm, and the amount of H ₂ gas is 0 to 80 sccm.

  In this embodiment, since the reaction gas is completely dissociated by the high-density plasma CVD apparatus, the amount of products attached to the inside of the high-density plasma CVD apparatus is small. For this reason, the frequency of cleaning of the high-density plasma CVD apparatus is reduced. Further, since the temperature inside the high-density plasma CVD apparatus can be lowered, the time required for the temperature to be lowered to a temperature at which the cleaning operation can be performed is short. For this reason, the time for stopping the high-density plasma CVD apparatus for cleaning can be shortened.

  The present invention is not limited to the thin film for solar cells, and can be applied to the deposition of a thin film on a substrate having similar deposition requirements.

It is sectional drawing of the solar cell by embodiment of this invention. It is the schematic for demonstrating the structure of the film-forming apparatus of this invention. It is the schematic for demonstrating the structure of the conventional parallel plate plasma CVD apparatus.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Film-forming apparatus, 2 ... Chamber, 3 ... Derivative plate, 4 ... Microwave introduction pipe, 5, 5a, 5b ... Process gas introduction pipe, 6, 6a, 6b, 6c ... Material gas introduction pipe, 7 ... Vacuum exhaust Tube: 8 ... Stage, 9a, 9b, 9c ... Heater, 10, 10a, 10b, 10c ... Region, 11 ... Discharge region, 12 ... Adjustment valve, 13 ... Surface wave excitation plasma device, 14 ... Microwave generation source, 15 ... mass flow controller, 16, 16a, 16b, 16c ... mass flow controller, 100 ... solar cell.

Claims (4)

In a film forming method for forming a light absorption layer (i layer) of a thin film solar cell with a microcrystalline silicon film,
A method for forming a thin-film solar cell, wherein the microcrystalline silicon film is formed by surface wave excitation plasma treatment, and the surface wave excitation plasma treatment generates radicals using argon gas as a process gas. In a film forming method for forming a light absorption layer (i layer) of a thin film solar cell with a microcrystalline silicon film,
Argon gas (Ar) gas is introduced into a region where surface waves are formed by the supply of microwaves, and the argon gas is excited by the surface waves to generate radicals into a plasma state,
A method of forming a thin film solar cell, comprising introducing a material gas into the plasma, causing the plasma to dissociate SiH ₄ of the material gas, and forming a microcrystalline silicon film on the semiconductor surface with the dissociated silicon .   A thin film solar cell formed by the film forming method according to claim 1. In a film forming apparatus for forming a light absorption layer (i layer) of a thin film solar cell with a microcrystalline silicon film,
A film forming chamber for performing surface wave excitation plasma treatment on the semiconductor surface;
A process gas introduction section for introducing a process gas into the film forming chamber;
A material gas introduction unit for introducing a material gas into the film formation chamber;
The process gas introduction unit introduces an argon gas (Ar) gas into a region where a surface wave is formed by supplying microwaves, generates a radical by exciting the argon gas with a surface wave, and enters a plasma state.
The material gas introduction unit introduces a material gas into the plasma by the surface wave excitation, dissociates the material gas SiH ₄ by the plasma, and forms a microcrystalline silicon film on the semiconductor surface by the dissociated silicon. A thin film solar cell film forming apparatus.

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