(19) Japan Patent Office (JP)
`
`(12) Gazette of Unexamined
`Patent Applications (A)
`
`(11) Publication Number
`H07-106611
`
`(43) Publication Date April 21, 1995
`
`(51) Int.Cl.6
`H01L 31/04
`
`ID Codes
`
`Int. Ref. Nos.
`
`FI
`
`Theme Codes (Ref.)
`
`7376-4M
`
`H01L 31/04
`
`A
`
`Examination Request Not Yet Received No. of Claims 5 OL (Total of 10 Pages)
`
`(21) Application No. H05-243040
`(22) Filing Date September 29, 1993
`
`(71) Applicant
`390022998
`Tonen Corporation
`1-1-1, Hitotsubashi, Chiyoda-ku, Tokyo
`(72) Inventor
`Yasunori SAGAWA
`Tonen Corporation, Research &
`Development Center, 1-3-1,
`Nishitsurugaoka, Oi-machi, Irima-gun,
`Saitama-ken
`(72) Inventor
`Yoshinori OKAYASU
`Tonen Corporation, Research &
`Development Center, 1-3-1,
`Nishitsurugaoka, Oi-machi, Irima-gun,
`Saitama-ken
`(74) Agent
`Attorney Kohei KUBOTA (and 1 other)
`
`(54) [Title of the Invention]
`BSF-Type Solar Cell Production Method
`(57) [Abstract]
`[Problem]
`To provide a method that enables the stable
`production of BSF-type solar cells with high
`photoelectric conversion efficiency.
`[Solution]
`Provided is a method for producing a BSF-type
`solar cell comprising an n- single crystal silicon
`(Si) substrate 1, a p-type poly-Si layer 2 on the
`light-entering side of the substrate, and an n+
`poly-Si layer 5 on the opposite side forming an HL
`junction with the Si substrate, the method
`characterized by forming the n+ poly-Si layer by
`thermal CVD using B2H6/SiH4/SiF4 as a feedstock
`gas. Because the production of suspended
`particles in the film-forming chamber can be
`prevented despite the use of halogen atom-
`emitting SiF4 and
`the substrate heating
`temperature can be reduced despite the use of
`thermal CVD, this method improves the film
`quality of the n+ poly-Si layer and enables the
`stable production of BSF-type solar cells with high
`photoelectric conversion efficiency.
`
`HANWHA 1031
`
`

`

`
`
`[Claims]
`[Claim 1] A method for producing a BSF-type
`solar cell provided with an n-type or p-type
`silicon substrate, a p-type or n-type silicon layer
`provided on the light-entering side of the silicon
`substrate and forming a p-n junction with the
`silicon substrate, and an n+ or p+ silicon layer
`provided on the opposite side of the silicon
`substrate and forming an HL junction with the
`silicon substrate,
`the method comprising
`supplying a film-forming gas containing silicon
`atoms, SiF4 gas, and an n-type or p-type dopant
`gas to a film-forming chamber in which an n-
`type or p-type silicon substrate has been
`arranged, and thermally decomposing the
`supplied gas in the vicinity of the heated silicon
`substrate while allowing the fluorine radicals
`generated by the reaction between the radical
`component of the thermally decomposed film-
`forming gas and the SiF4 gas to act as etching
`components to form an n+ or p+ silicon layer on
`the n-type or p-type silicon substrate surface.
`[Claim 2] A method for producing a BSF-type
`solar cell provided with a n-type or p-type silicon
`substrate, a p-type or n-type silicon layer
`provided on the light-entering side of the silicon
`substrate and forming a p-n junction with the
`silicon substrate, and an n+ or p+ silicon layer
`provided on the opposite side of the silicon
`substrate and forming an HL junction with the
`silicon substrate,
`the method comprising
`supplying a
`film-forming gas
`consisting
`primarily of silicon hydride and fluorinated
`silane gas represented by SiHmF4-m (where m is
`1 to 3) and an n-type or p-type dopant gas to a
`film-forming chamber in which an n-type or p-
`type silicon substrate has been arranged, and
`thermally decomposing the supplied gas in the
`vicinity of the heated silicon substrate while
`allowing the fluorinated radicals generated by
`the thermal decomposition of the fluorinated
`silane gas and fluorinated radicals generated by
`the reaction of radical components of the
`thermally decomposed film-forming gas with
`the fluorinated silane gas to act as etching
`components to form an n+ or p+ silicon layer on
`the n-type or p-type silicon substrate surface.
`[Claim 3] The method for producing a BSF-type
`solar cell according to claims 1 or 2, wherein a
`mesh-like heat-generating member is arranged
`in the space near the n-type or p-type silicon
`substrate placed
`inside
`the
`film-forming
`chamber so that the substrate is covered.
`[Claim 4] A method for producing a BSF-type
`solar cell provided with a n-type or p-type silicon
`substrate, a p-type or n-type silicon layer
`provided on the light-entering side of the silicon
`substrate and forming a p-n junction with the
`silicon substrate, and an n+ or p+ silicon layer
`
`(2) JP H07-106611 A
`
`provided on the opposite side of the silicon
`substrate and forming an HL junction with the
`silicon substrate,
`the method comprising
`forming the n+ or p+ silicon layer on the heated
`silicon substrate surface by optical CVD.
`[Claim 5] A method for producing a BSF-type
`solar cell provided with a n-type or p-type silicon
`substrate, a p-type or n-type silicon layer
`provided on the light-entering side of the silicon
`substrate and forming a p-n junction with the
`silicon substrate, and an n+ or p+ silicon layer
`provided on the opposite side of the silicon
`substrate and forming an HL junction with the
`silicon substrate,
`the method comprising
`supplying fluorinated silane gas represented by
`SiHmF4-m (where m is 1 to 3) or SiF4 gas to a
`film-forming chamber in which an n-type or p-
`type silicon substrate has been arranged, and
`forming an n+ or p+ silicon layer on the heated
`silicon substrate surface by ion plating in the
`presence of the fluorinated silane gas or SiF4 gas.
`[Detailed Description of the Invention]
`[0001]
`[Field of Industrial Applicability] The present
`invention relates to a method for producing a
`back surface field (BSF) type solar cell, and in
`particular, to a method for producing a BSF type
`solar cell that enables stable production of a BSF
`type solar cell with high photoelectric
`conversion efficiency.
`[0002]
`[Prior Art] As shown, for example, in Fig. 7 and
`Fig. 8, a BSF type solar cell has, as its main
`components, a p-type single crystal silicon
`substrate (a), an n+ silicon layer (b) provided on
`the light-entering side of the silicon substrate
`(a) and forming a p-n junction with the silicon
`substrate (a), an indium tin oxide (ITO)
`antireflection layer (c) uniformly formed on the
`n+ silicon layer (b), a comb-shaped electrode
`(d) provided on the antireflection layer (c), a p+
`silicon layer (e) provided on the opposite side of
`the silicon substrate (a) and forming a high-low
`(HL) junction with the silicon substrate (a), and
`a back side electrode (f) uniformly provided on
`the back side of the p+ silicon layer (e), and a
`structure
`in which electrons and holes
`generated by incidence light are taken out as
`electric current from electrodes (d) and (f) is
`known.
`[0003] Fig. 9 is a conceptual diagram used to
`describe the structure of this BSF type solar cell
`in modeling terms, and Fig. 10 is an energy
`band diagram of a BSF type solar cell with this
`structure.
`[0004] Such a BSF type solar cell has the
`advantage
`of
`improving
`photoelectric
`conversion efficiency compared to solar cells
`with a structure not equipped with a p+ silicon
`
`

`

`
`
`layer (e) because the built-in electric field
`between the silicon substrate (a) and the p+
`silicon layer (e) forming the HL junction (g) acts
`as a barrier to the diffusion of minority carriers
`(in this case, electrons) to the back electrode f,
`increasing the apparent diffusion length of
`electrons, and the series resistance is reduced
`because the p+ silicon layer (e) becomes an
`ohmic electrode with low resistance to holes.
`[0005] A p+ silicon layer (e) that increases the
`open circuit voltage has been formed in the past
`by using the "thermal diffusion method”. Here,
`a single-crystal silicon substrate
`(a)
`is
`introduced into a reaction chamber filled with a
`p-type dopant gas containing, for example, B
`(boron), the dopant gas is thermally diffused
`into the silicon substrate a under high-
`temperature conditions of around 1,000°C, a p-
`type dopant layer such as aluminum (Al) is
`laminated on the surface of the single-crystal
`silicon substrate (a), and firing is conducted
`under high-temperature conditions of around
`1,000°C to form a p+ silicon layer (e).
`[0006] However, there are disadvantages when
`the p+ silicon layer (e) is formed using the
`"thermal diffusion method." The thickness of the
`p+ silicon layer (e) is difficult to control and
`tends to be thicker, ranging from several
`hundred angstroms to several micrometers, and
`the diffusion distance of dopant gases such as B,
`A1, etc. tends to easily change due to slight
`variations in thermal conditions, making the
`characteristics
`of
`silicon
`substrate
`(a)
`susceptible to variation.
`[0007] There are additional disadvantages with
`the "thermal diffusion method," such as the
`dopant gas readily becoming trapped and the
`dopant gas thermally diffusing into the n+ silicon
`layer (b) to form a p-n junction with the silicon
`substrate. The silicon substrate
`is also
`disadvantageously
`exposed
`to
`high
`temperatures of around 1,000°C, which causes
`thermal decomposition of the substrate.
`[0008] Against this technical background, the
`present applicant has already proposed a
`production method for BSF solar cells in which
`an n+ or p+ silicon layer forming an HL junction
`with such an n-type or p-type silicon substrate
`is formed by plasma CVD. This production
`method provides advantages in that it has more
`relaxed thermal conditions during formation of
`the n+ or p+ silicon layer than the conventional
`thermal diffusion method, making degradation
`of the characteristics of the silicon substrate less
`likely, and in that the thickness of the n+ or p+
`silicon layer can be reduced to several dozen to
`several hundreds of angstroms, which improves
`the photoelectric conversion efficiency.
`[0009]
`
`(3) JP H07-106611 A
`
`[Problem to be Solved by the Invention] This
`production method using plasma CVD provides
`these advantages because it allows for lower
`temperature film formation of n+ and p+ silicon
`layers and can be used to produce thinner n+
`and p+ silicon layers. However, there is also a
`disadvantage in that during the film formation
`process of the n+ or p+ silicon layer, a large
`amount of powdery suspended foreign matter is
`readily generated in the film formation chamber,
`and these suspended particles can enter the n+
`or p+ silicon layer during film formation and
`degrade the film properties.
`[0010] In the plasma CVD method, the plasma-
`forming
`feedstock gases are uniformly
`distributed in the plasma-forming region inside
`the film-forming chamber, and the plasma-
`forming feedstock gases are in an excited state.
`As a result, the feedstock gases react with each
`other in any space inside the film-forming
`chamber to produce silicon powder. Silicon is
`deposited by the reaction of the plasma-forming
`feedstock gases on the inner walls of the film-
`forming chamber, that is, somewhere other than
`on the substrate, and a large amount of
`powdery suspended foreign matter is readily
`generated which, for example, delaminate as a
`powdery substance.
`[0011] In the plasma CVD method, halogen gas
`may be mixed with, for example, SiH4 gas to
`serve as a film-forming gas, but the etching
`action of the halogen atoms easily corrodes the
`inner walls of the film-forming chamber and the
`surfaces of the internal tools, causing these
`corroded surfaces to delaminate and join the
`floating particles mentioned above.
`[0012] Therefore, while the production method
`using plasma CVD has the advantages described
`above, it has difficulty forming good quality n+
`or p+ silicon layers, and as a result, the
`photoelectric conversion efficiency of
`the
`resulting BSF solar cells has certain limitations.
`[0013] It is an object of the present invention
`to address these problems by providing a
`method that allows for stable production BSF
`solar
`cells with
`improved photoelectric
`conversion efficiency by selecting thermal CVD,
`optical CVD, and ion plating, which are able to
`form high-quality n+ or p+ silicon layers under
`low-temperature film formation conditions.
`[0014]
`[Means for Solving the Problem] The invention
`according to claim 1 is premised on a method
`for producing a BSF-type solar cell provided with
`a n-type or p-type silicon substrate, a p-type or
`n-type silicon layer provided on the light-
`entering side of the silicon substrate and
`forming a p-n junction with the silicon substrate,
`and an n+ or p+ silicon layer provided on the
`
`

`

`
`
`opposite side of the silicon substrate and
`forming an HL junction with the silicon substrate,
`and is characterized by supplying a film-forming
`gas containing silicon atoms, SiF4 gas, and an
`n-type or p-type dopant gas to a film-forming
`chamber in which an n-type or p-type silicon
`substrate has been arranged, and thermally
`decomposing the supplied gas in the vicinity of
`the heated silicon substrate while allowing the
`fluorine radicals generated by the reaction
`between the radical component of the thermally
`decomposed film-forming gas and the SiF4 gas
`to act as etching components to form an n+ or
`p+ silicon layer on the n-type or p-type silicon
`substrate surface. The invention according to
`claim 2 is premised on a method for producing
`a BSF-type solar cell provided with a n-type or
`p-type silicon substrate, a p-type or n-type
`silicon layer provided on the light-entering side
`of the silicon substrate and forming a p-n
`junction with the silicon substrate, and an n+ or
`p+ silicon layer provided on the opposite side of
`the silicon substrate and forming an HL junction
`with the silicon substrate, and is characterized
`by supplying a film-forming gas consisting
`primarily of silicon hydride and fluorinated
`silane gas represented by SiHmF4-m (where m is
`1 to 3) and an n-type or p-type dopant gas to a
`film-forming chamber in which an n-type or p-
`type silicon substrate has been arranged, and
`thermally decomposing the supplied gas in the
`vicinity of the heated silicon substrate while
`allowing the fluorinated radicals generated by
`the thermal decomposition of the fluorinated
`silane gas and fluorinated radicals generated by
`the reaction of radical components of the
`thermally decomposed film-forming gas with
`the fluorinated silane gas to act as etching
`components to form an n+ or p+ silicon layer on
`the n-type or p-type silicon substrate surface.
`[0015] Because the invention according to claim
`1 uses SiF4 gas, which is not very susceptible to
`thermal decomposition, as a halogen atom-
`releasing gas, and the invention according to
`claim 2 uses fluorinated silane gas, which has
`an even higher
`thermal decomposition
`temperature, as a halogen atom-releasing gas,
`thermal decomposition of SiF4 and fluorinated
`silane gas is much less likely to occur in the low-
`temperature distribution regions away from the
`substrate in the film-forming chamber, and the
`concentration of fluorinated radicals in these
`regions is thus low.
`[0016] Therefore, compared to the conventional
`production method using plasma CVD, the film-
`forming gas is less likely to react with fluorine
`radicals in other spaces inside the film-forming
`chamber to generate silicon powder, and
`corrosion of the inner wall surfaces of the film-
`
`(4) JP H07-106611 A
`
`forming chamber and internal tool surfaces due
`to the action of these fluorinated radicals is less
`likely to occur. As a result, despite using SiF4 or
`fluorinated silane gas, which releases halogen
`atoms, the generation of suspended particles in
`the film-forming chamber can be prevented.
`[0017] In the invention according to claim 1,
`while the SiF4 gas is less susceptible to thermal
`decomposition,
`thermal decomposition
`is
`accelerated by
`the reaction with radical
`components of the film-forming gas such as
`thermally decomposed silane compounds. As a
`result, regions with a high concentration of
`these radical components, that is, the regions
`near the n-type or p-type silicon substrate, are
`susceptible to decomposition, resulting in a high
`concentration of unevenly distributed fluorine
`radicals toward these regions. Also, in the
`invention according to claim 2, fluorinated silane
`gases such as SiH2F2 are susceptible to thermal
`decomposition near the n-type or p-type silicon
`substrate
`at
`high
`temperatures,
`and
`decomposition is accelerated by the reaction
`with radical components (SiHx radicals, H-based
`radicals, etc.) of the thermally decomposed
`film-forming gas,
`resulting
`in a high
`concentration of unevenly distributed fluorine
`radicals toward the region near the substrate.
`[0018] Therefore, because only impurities
`mixed into the n+ or p+ silicon layer on the n-
`type or p-type silicon substrate or the growing
`n+ or p+ silicon layer are selectively etched away
`due to the action of the fluorine radicals, the
`invention according to claim 1 or 2 allows for a
`reduction in the substrate heating temperature
`despite use of thermal CVD, which requires
`high-temperature heat treatment to remove
`these impurities.
`[0019] If the invention according to claims 1 or
`2 is provided with a heating means for heating
`the substrate in the space near the substrate
`inside the film-forming chamber, the substrate
`heating temperature can be further reduced
`because the thermal decomposition of the film-
`forming gas is accelerated. The invention
`according to claim 3 is based on this technical
`rationale.
`[0020] The invention according to claim 3 is
`premised on the method for producing a BSF-
`type solar cell according to claim 1 or 2, and is
`characterized by a mesh-like heat-generating
`member arranged in the space near the n-type
`or p-type silicon substrate placed inside the
`film-forming chamber so that the substrate is
`covered.
`[0021] This heat-generating member, which is
`placed in the space near the n-type or p-type
`silicon substrate, has a shape that does not
`interfere with film formation of the n+ or p+
`
`

`

`
`
`silicon layer even if arranged so as to cover the
`entire substrate surface, and
`the heat
`generating member is composed of a material
`that generates heat to the extent that the film-
`forming gas, for example, silicon hydride, is
`thermally decomposed in the region near the
`substrate. Examples include reticulated thermal
`filaments made of tungsten, thoriated tungsten,
`etc.
`[0022] In the thermal CVD method according to
`claims 1 to 3, the vacuum in the chamber is
`continuously evacuated downstream of the
`chamber during the film-forming process to
`maintain a constant vacuum level, and to
`remove fluorine radicals used in the etching
`process or excess fluorine radicals from the film-
`forming chamber in the exhaust.
`[0023] In the invention according to claims 1 to
`3, the n+ or p+ silicon layer forming a HL junction
`with the n-type or p-type silicon substrate can
`be amorphous silicon layers, polysilicon layers,
`or single-crystal silicon layers with introduced n-
`type or p-type dopant gases. Here, from the
`standpoint of reducing internal resistance, a
`single-crystal silicon layer with low resistance is
`preferable to a polysilicon
`layer, and a
`polysilicon
`layer with
`low
`resistance
`is
`preferable to an amorphous silicon layer.
`[0024] Compounds containing antimony (Sb),
`arsenic (As), and phosphorus (P) from Group III
`of the periodic table of elements can be used as
`n-type dopant gases. Specific examples include
`SbH3, SbCl3, AsH3, As2H4, and PH3. Compounds
`containing gallium (Ga), boron (B), indium (In),
`and aluminum (Al) from Group V of the periodic
`table of elements can be used as p-type dopant
`gases. Specific examples include B2H6, Ga(CH3)3,
`In(CH3)3, and Al(CH3)3.
`[0025] The film-forming gas containing silicon
`atoms used in the invention according to claims
`1 or 3 can be easily thermally decomposed
`silicon hydrides such as SiH4, Si2H6, and Si3H8,
`or silane halide represented by SiHmX4-m (where
`m is 1 to 3, preferably 2 to 3, and X is a Cl or F
`atom, preferably a F atom). Hydrogen gas may
`be added to the film-forming gas. Hydrogen
`radicals act on SiF4 gas to accelerate its
`decomposition. When a silane halide gas is used
`as the film-forming gas, a mixing ratio for the
`silane halide gas that is too high may cause
`floating foreign matter described above to be
`produced
`in
`the
`film-forming
`chamber.
`Therefore, when halogenated silane gas
`represented by SiHmX4-m is used, the mixing
`ratio should be set so as to be less than that of
`SiF4.
`[0026] In the invention according to claim 2 or
`3, SiH4, Si2H6, and Si3H8, etc. can be used as the
`silicon hydride
`constituting
`the primary
`
`(5) JP H07-106611 A
`
`component of the film-forming gas. As in the
`invention according to claim 1 or 3, hydrogen
`gas may be added to the film-forming gas. This
`is because hydrogen radicals also react with
`fluorinated silane gases such as SiH2F2 to
`promote their thermal decomposition.
`[0027] Note that an inert gas such as helium,
`neon, or argon may be added as a dilution gas
`to the film-forming gas in the invention
`according to claims 1 to 3.
`[0028] In the method for producing a BSF solar
`cell according to the invention described in
`claims 1 to 3, the vacuum in the film-forming
`chamber before the start of film formation is set
`to a level at which some impurities remain in the
`chamber (about 10-5 Torr) and the temperature
`of the n-type or p-type silicon substrate placed
`in the chamber is set to about 500°C to 700°C.
`Then, either at least the film-forming gas
`described above, SiF4 gas, and n-type or p-type
`dopant gas are supplied, or at least the film-
`forming gas described above, fluorinated silane
`gas represented by SiHmF4-m (where m is 1 to 3)
`and n-type or p-type dopant gas are supplied to
`form an n+ or p+ silicon layer on a n-type or p-
`type silicon substrate. At this time, impurities
`are selectively removed by the etching action of
`the fluorinated radicals even when the substrate
`heating temperature is set to low temperature
`conditions of 500°C to 700°C. In addition,
`because hardly any floating foreign matter is
`produced that is likely to become mixed into the
`n+ or p+ silicon layer, there is little possibility of
`it adversely affecting the film properties of the
`n+ or p+ silicon layer.
`[0029] This has the advantage of enabling the
`stable production of BSF solar cells with
`improved photoelectric conversion efficiency.
`[0030] The invention according to claims 4 to 5
`relates to a method for producing a BSF solar
`cell in which optical CVD or ion plating are used
`instead of thermal CVD as in the invention
`according to claims 1 to 3 to form an n+ or p+
`silicon layer.
`[0031] The invention according to claim 4 is
`premised on a method for producing a BSF-type
`solar cell provided with an n-type or p-type
`silicon substrate, a p-type or n-type silicon layer
`provided on the light-entering side of the silicon
`substrate and forming a p-n junction with the
`silicon substrate, and an n+ or p+ silicon layer
`provided on the opposite side of the silicon
`substrate and forming an HL junction with the
`silicon substrate, and is characterized by
`forming the n+ or p+ silicon layer on the heated
`silicon substrate surface by optical CVD.
`[0032] As shown in Fig. 5, the optical CD device
`has, as its main components, a film-forming
`chamber 11, a UV lamp 31 provided in the film-
`
`

`

`
`
`forming chamber 11 via a quartz window 30, a
`substrate holder 13 provided on the opposite
`side of the UV lamp 31 and incorporating a
`heater 15, gas supply sources 16, 17 that
`supply film-forming gases such as SiH4, a
`dopant gas if appropriate, and hydrogen gas,
`etc. to the film-forming chamber 11, and a
`vacuum pump 18 connected via a pressure
`regulating valve 19. In this method, an n-type
`or p-type silicon substrate 14 is placed in the
`film-forming chamber 11, the silicon substrate
`14 is heated to about 500 to 600°C, film-
`forming gases, dopant gases, etc. are supplied
`from the gas supply sources 16, 17, UV light
`irradiation is performed, and an n+ or p+ silicon
`layer is formed on the n-type or p-type silicon
`substrate 14.
`[0033] Because an n+ or p+ silicon layer with
`good film quality can be formed on a p-type
`silicon substrate 14 under low-temperature film
`formation conditions, the production method
`according to the invention described in claim 4
`provides the same advantage as the invention
`according to claims 1 to 3, namely, stable
`production of BSF solar cells with improved
`photoelectric conversion efficiency.
`[0034] The invention according to claim 5 is
`premised on a method for producing a BSF-type
`solar cell provided with an n-type or p-type
`silicon substrate, a p-type or n-type silicon layer
`provided on the light-entering side of the silicon
`substrate and forming a p-n junction with the
`silicon substrate, and an n+ or p+ silicon layer
`provided on the opposite side of the silicon
`substrate and forming an HL junction with the
`silicon substrate, and is characterized by
`supplying fluorinated silane gas represented by
`SiHmF4-m (where m is 1 to 3) or SiF4 gas to a
`film-forming chamber in which an n-type or p-
`type silicon substrate has been arranged, and
`forming an n+ or p+ silicon layer on the heated
`silicon substrate surface by ion plating in the
`presence of the fluorinated silane gas or SiF4 gas.
`[0035] As shown in Fig. 6, the ion plating device
`has, as its main components, a film-forming
`chamber 11, a resistance heating vaporization
`source 40 provided in the lower end of the film-
`forming chamber 11 and containing a Si target
`mixed with dopants as the film-forming material,
`an RF coil 41 provided in the upper space of the
`resistance heating vaporization source 40, a
`shutter 42 that adjusts the amount supplied
`from the resistance heating vaporization source
`40, and a substrate holder 43 provided in the
`upper end of the film-forming chamber 11 to
`heat the silicon substrate 14 to about 600 to
`700°C. Here, a high-frequency power supply 44
`is connected to the RF coil 41, and a positive
`high-voltage power supply (not shown) is
`
`(6) JP H07-106611 A
`
`connected to the substrate holder 43. In this
`method, an n-type or p-type silicon substrate 14
`is placed in the film-forming chamber 11,
`fluorinated silane gas or SiF4 gas is supplied to
`the
`film-forming chamber 11, the silicon
`substrate 14 is heated to about 600 to 700°C,
`dopants and silicon are vaporized from the
`resistance heating vaporization source 40, and
`an n+ or p+ silicon layer is formed on an n-type
`or p-type silicon substrate 14.
`[0036] In the invention according to claim 5, as
`in the invention according to claims 1 to 3,
`because fluorinated silane gas or SiF4 gas, which
`is
`less
`likely
`to
`experience
`thermal
`decomposition, is used as a halogen atom-
`releasing gas, a high concentration of fluorine
`radicals is unevenly distributed toward the
`region near the n-type or p-type silicon
`substrate. In other words, because the radical
`component of thermally decomposed silicon
`molecules is unevenly distributed toward the
`region near the n-type or p-type silicon
`substrate,
`the
`reaction of
`the
`radical
`components with the fluorinated silane gas or
`SiF4 gas results in a high concentration of
`unevenly distributed fluorinated radicals. Also,
`because the fluorine radicals selectively etch
`and remove only impurities in the n+ or p+
`silicon layer on the n-type or p-type silicon
`substrate or in the growing n+ or p+ silicon layer,
`an n+ or p+ silicon layer with good film quality
`can be formed on an n-type or p-type silicon
`substrate under low-temperature film formation
`conditions and, as in the invention according to
`claims 1 to 4, this provides the advantage of
`stable production of BSF solar cells with
`improved photoelectric conversion efficiency.
`[0037]
`[Operation] In the invention according to claim
`1, SiF4 gas, which is resistant to thermal
`decomposition, is used as a halogen atom-
`releasing gas, and the concentration of fluorine
`radicals in the film-forming chamber is low with
`respect to the portion of this gas that is less
`likely to experience thermal decomposition. As
`a result, the film-forming gas reacting with
`fluorine radicals to form silicon powder in any
`space inside the film-forming chamber and the
`inner wall surfaces of the chamber or internal
`tool surfaces becoming corroded by fluorine
`radicals is much less likely to occur compared to
`the conventional production methods using
`plasma CVD, and the production of suspended
`particles in the film-forming room can be
`reliably prevented despite using halogen atom-
`releasing SiF4 gas.
`thermal
`because
`[0038]
`However,
`decomposition of SiF4 gas is accelerated by the
`reaction with radical components (SiHx radicals,
`
`

`

`(7) JP H07-106611 A
`
`film-forming gas in the region near the
`substrate above. As a result, the substrate
`heating temperature can be reduced further due
`to the synergy with the etching action of the
`fluorinated radicals.
`[0042] In the invention according to claim 4,
`because an n+ or p+ silicon layer is formed on
`the heated silicon substrate surface by optical
`CVD, an n+ or p+ silicon layer with good film
`quality can be formed on the n-type or p-type
`silicon substrate under low-temperature film
`formation conditions.
`[0043] In the invention according to claim 5,
`because an n+ or p+ silicon layer is formed on
`the heated silicon substrate surface by ion
`plating under the conditions in which fluorinated
`silane gas or SiF4 gas is present, an n+ or p+
`silicon layer with good film quality can be
`formed on the n-type or p-type silicon substrate
`under
`low-temperature
`film
`formation
`conditions.
`[0044]
`[Examples] The present invention will now be
`described in greater detail with reference to
`examples. In these examples, as in the prior art
`and as shown in Fig. 1, the BSF type solar cell
`in these examples has, as its main components,
`an n- single crystal silicon substrate (100
`orientation, specific resistance 1 to 10 Ω·cm) 1,
`a p-type amorphous silicon or polysilicon layer
`2 on the light-entering side of the silicon
`substrate 1 and forming a p-n junction with the
`silicon substrate 1, an antireflection layer 3
`made of indium tin oxide (ITO) uniformly
`deposited on the p-type amorphous silicon or
`polysilicon layer 2, a comb-shaped electrode 4
`formed with silver paste on the antireflection
`layer 3, an n+ amorphous silicon or polysilicon
`layer 5 provided on the opposite side of the
`silicon substrate 1 and forming an HL junction
`with the silicon substrate 1, and a backside
`electrode 6 uniformly provided on the back side
`of the n+ amorphous silicon or polysilicon layer
`5 and formed of aluminum.
`[0045] This BSF solar cell is produced using the
`following process.
`[0046] [Example 1] Fig. 3 is a diagram used to
`explain the thermal CVD device used in this
`example. In this figure, 11 is a film-forming
`chamber, 13 is a substrate holder containing a
`heater 15, 14 is an n- single crystal silicon
`substrate attached to the substrate holders 13,
`16, and 17 are feedstock gas sources, and 18 is
`a vacuum pump connected to the film-forming
`chamber 11 via a pressure regulating valve 19.
`[0047] First, the film-forming chamber 11 was
`evacuated to a vacuum of 1 × 10-5 Torr. Then,
`the feedstock gas was introduced to the film-
`forming chamber 11, and a p-type polysilicon
`
` H
`
` radicals, etc.) in the film-forming gas, a high
`concentration of fluorine radicals is more likely
`to be unevenly distributed toward the vicinity of
`the n-type or p-type silicon substrate, where the
`radical components are high, and because only
`impurities in the n+ or p+ silicon layer on the
`substrate or in the growing n+ or p+ silicon layer
`are selectively etched and removed by the
`action of the fluorine radicals, the substrate
`heating temperature can be reduced despite use
`of
`thermal CVD, which
`requires high-
`temperature heat treatment to remove these
`impurities.
`[0039] In the invention according to claim 2,
`fluorinated silane gas, which has a higher
`thermal decomposition temperature than the
`film-forming gas above, is used as a halogen
`atom-releasing gas, ther