WO2011024910A1 - Silicon wafer for solar cells and production method therefor - Google Patents

Silicon wafer for solar cells and production method therefor Download PDF

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Publication number
WO2011024910A1
WO2011024910A1 PCT/JP2010/064510 JP2010064510W WO2011024910A1 WO 2011024910 A1 WO2011024910 A1 WO 2011024910A1 JP 2010064510 W JP2010064510 W JP 2010064510W WO 2011024910 A1 WO2011024910 A1 WO 2011024910A1
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wire
silicon wafer
silicon
solar cells
wafer
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PCT/JP2010/064510
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French (fr)
Japanese (ja)
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亮 中島
之信 貝賀
有樹 村田
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株式会社Sumco
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B28—WORKING CEMENT, CLAY, OR STONE
    • B28D—WORKING STONE OR STONE-LIKE MATERIALS
    • B28D5/00—Fine working of gems, jewels, crystals, e.g. of semiconductor material; apparatus or devices therefor
    • B28D5/04—Fine working of gems, jewels, crystals, e.g. of semiconductor material; apparatus or devices therefor by tools other than rotary type, e.g. reciprocating tools
    • B28D5/045—Fine working of gems, jewels, crystals, e.g. of semiconductor material; apparatus or devices therefor by tools other than rotary type, e.g. reciprocating tools by cutting with wires or closed-loop blades
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B28—WORKING CEMENT, CLAY, OR STONE
    • B28D—WORKING STONE OR STONE-LIKE MATERIALS
    • B28D5/00—Fine working of gems, jewels, crystals, e.g. of semiconductor material; apparatus or devices therefor
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/02—Details
    • H01L31/0236—Special surface textures
    • H01L31/02363—Special surface textures of the semiconductor body itself, e.g. textured active layers
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy

Definitions

  • the present invention relates to a silicon wafer for a solar cell and a method for manufacturing the same, and more particularly to a silicon wafer for a solar cell that is obtained by slicing a silicon ingot for a solar cell as a base material for a silicon-based solar cell.
  • a silicon-based solar cell is manufactured by forming a PN junction on a solar cell silicon wafer obtained by slicing a single crystal silicon ingot or a polycrystalline silicon ingot, and forming electrodes on the front and back surfaces of the wafer.
  • wire saws have been used for general slices. In the slicing method using a wire saw, first, one wire is bridged between two to four groove rollers at a predetermined pitch to form a wire row. Thereafter, each groove roller is rotated to run the wire, and the solar cell silicon ingot is pressed against the wire row while supplying slurry containing loose abrasive grains onto the wire row on both sides of the ingot. Thereby, many silicon wafers for solar cells are obtained simultaneously.
  • the thickness of a general silicon wafer for solar cells is as thin as about 200 ⁇ m. Therefore, in the bi-directional feed of the wire, the silicon wafer for solar cells is cracked or chipped at the time of slicing, and the wire is easily broken. Therefore, in order to solve this problem, unidirectional feed that provides high smoothness of the slice surface has been adopted as one of the wire operating conditions (wire feed direction). As a result, the front and back surfaces of the solar cell silicon wafer were so smooth (Rmax 5 to 10 ⁇ m) that the saw marks on the grinding marks could not be confirmed.
  • silicon-based solar cells have innumerable fine irregularities formed on the surface (light-receiving surface) of a solar cell silicon wafer, and the light reflectance on this surface is reduced.
  • a method of forming irregularities on the surface of a silicon wafer for solar cells conventionally, utilizing the difference in etching rate depending on the silicon surface orientation, an alkaline aqueous solution is brought into contact with the silicon wafer for solar cells, and irregularities are formed on the wafer surface by etching.
  • a method has been developed (for example, Patent Document 1).
  • the surface roughness of Rmax of 20 ⁇ m or more could not be realized with respect to the surface of the solar cell silicon wafer due to the characteristics of crystal anisotropy.
  • an etching process is employed for forming the irregularities, an etching process different from slicing is required when manufacturing a silicon-based solar cell from a silicon wafer for solar cells. Thereby, the silicon wafer for solar cells was expensive. This also applies to the case where irregularities are formed on the surface of the solar cell silicon wafer by a method different from etching (for example, grinding, laser irradiation, etc.).
  • the invention provides a silicon wafer for a solar cell that can increase the light receiving area, obtain high photoelectric conversion efficiency, simplify the unevenness forming process different from slicing, and reduce the manufacturing cost of silicon-based solar cells. And it aims at providing the manufacturing method.
  • the present invention relates to a silicon wafer for a solar cell in which a PN junction and an electrode are formed to be processed into a silicon-based solar cell, and a large number of linear concave grooves directed in the same direction, which are saw marks that appear at the time of slicing. It is the silicon wafer for solar cells formed in the front and back.
  • the silicon wafer for solar cells has a large number of linear concave grooves in the same direction that appear on the front and back surfaces of the silicon wafer by traveling in both directions. That is, the silicon wafer for solar cells as a product is obtained without performing flattening processing such as etching, grinding, polishing, etc., which is generally performed on the silicon wafer after slicing. Thereby, the light receiving area of the silicon wafer for solar cells is expanded (wafer surface roughness of 20 ⁇ m or more is possible at Rmax), and high photoelectric conversion efficiency is obtained.
  • silicon solar cells include single crystal silicon solar cells and polycrystalline silicon solar cells.
  • a material of the silicon wafer for solar cells single crystal silicon, polycrystalline silicon, or the like can be employed.
  • a shape of the silicon wafer for solar cell a circle, a rectangle with chamfered corners (made of single crystal silicon), a rectangle (made of polycrystalline silicon), or the like can be adopted.
  • “Straight groove” means a saw mark at the time of slicing consisting of a large number of grooves that are linearly and parallelly formed in the same direction at almost equal intervals over the entire surface of the silicon wafer for solar cells. Grinding marks). This saw mark is formed by slicing a silicon ingot for solar cells while a fixed abrasive wire is traveling in both directions. For this reason, not only the surface of the silicon wafer for solar cells but also the back surface has the same linear groove on the back surface in the same direction as the groove on the wafer surface (for example, the groove on the front and back surfaces of the wafer in the X direction). Further, there may be a curved portion (sag) in one direction at both end portions of the linear groove.
  • the groove is a saw mark, there is a finer surface roughness of about 2 to 3 ⁇ m in Rmax between the bottom surface of the groove and the top surface of the wafer. Further, since the concave grooves are saw marks, the pitch and depth of the concave grooves on the wafer surface and the pitch and depth of the concave grooves on the back surface of the wafer are substantially the same. Furthermore, in the pitch direction of the silicon wafer, the groove formation position on the wafer surface and the groove formation position on the wafer back surface are substantially the same.
  • the thickness of the silicon wafer for solar cells is 160 to 220 ⁇ m. If it is less than 160 micrometers, the silicon wafer for solar cells is too thin, and the crack of a wafer increases remarkably on wafer handling. Therefore, it is effective to increase the thickness of the silicon wafer for solar cells as a countermeasure against this crack. However, if it exceeds 220 ⁇ m, the amount of silicon used is increased, and the production cost of the silicon wafer for solar cells is increased.
  • On the front and back surfaces of the silicon wafer for solar cells there is a processing damage of 1 to 5 ⁇ m (on one side of the wafer) that appears by slicing the fixed abrasive wire in both directions. Incidentally, when a wire is cut using loose abrasive grains (wires are moved in both directions), a processing damage of 5 to 15 ⁇ m generally appears on one side of the wafer.
  • the concave grooves have a pitch of 0.1 to 5 mm and a depth of 1 to 50 ⁇ m.
  • the pitch of the concave grooves formed on the front and back surfaces of the wafer is set to 0.1 to 5 mm, and the depth of the concave grooves is set to 1 to 50 ⁇ m, thereby preventing a reduction in strength of the silicon wafer for solar cells due to the formation of the concave grooves.
  • expansion of the light receiving area of the silicon wafer for solar cells and high photoelectric conversion efficiency can be satisfied at the same time.
  • the pitch of the concave grooves is less than 0.1 mm, the strength of the silicon wafer for solar cells due to the formation of the concave grooves tends to decrease. Moreover, if the pitch of a ditch
  • the present invention relates to a silicon wafer for solar cells obtained by slicing a plurality of silicon wafers for solar cells by pressing a silicon ingot for solar cells while supplying slurry to a wire row traveling between a plurality of groove rollers of a wire saw.
  • a fixed abrasive wire having abrasive grains fixed on an outer peripheral surface is used as a wire constituting the wire row, and the silicon ingot for solar cells is sliced while the wire row is traveling in both directions.
  • This is a method for manufacturing a silicon wafer for solar cells, in which a large number of linear concave grooves directed in the same direction as saw marks are formed on the front and back surfaces of each silicon wafer for solar cells.
  • a solar cell silicon ingot is pressed against a wire row composed of fixed abrasive wires traveling in both directions (forward and backward directions).
  • a large number of rough grooves (saw marks) extending in the same direction appearing when the wire row is run in both directions for example, Rmax of 20 ⁇ m or more
  • Rmax for example, 20 ⁇ m or more
  • linear grooves (saw marks) extending in the same direction with a finer Rmax of about 2 to 3 ⁇ m are formed by the fixed abrasive grains on the wire surface.
  • the use of the fixed abrasive wire increases the cutting efficiency compared with the case of slicing with a wire saw using loose abrasive grains, and the slicing speed of the silicon ingot for solar cells is increased. Even when etching for forming the groove is performed after that, the etching time can be shortened.
  • the number of wire rollers used in the wire saw is, for example, 2, 3, or 4.
  • the fixed abrasive wire is a diamond abrasive grain having a particle size of 7 to 25 ⁇ m, heat-cured or UV-irradiated with a binder on a piano wire with a concentration of 20 to 55, preferably 50 and a diameter of 100 to 300 ⁇ m, etc. Can be adopted.
  • a binder an epoxy resin, a phenol resin, an acrylic urethane resin, or the like can be used.
  • a diamond abrasive grain may be electrodeposited together with nickel on the outer peripheral surface of the wire.
  • the feed rate of the fixed abrasive wire is 500 to 1500 m / min.
  • the slurry (wrapping oil) that does not contain loose abrasive grains is not brought into the cutting portion of the ingot by the wire, and the cutting efficiency decreases. If it exceeds 1500 m / min, the slurry adhering to the wire is blown off, and the cutting efficiency is lowered.
  • the advance amount of the fixed abrasive wire is 250 to 450 m, and the retract amount of the fixed abrasive wire is 248 to 499 m.
  • the cycle time for forward and reverse is 47.7-88.9 seconds.
  • the ingot slice average speed is 400 to 1200 ⁇ m / min. In order to further improve the wafer quality, a lower speed condition is preferable, but if it is less than 400 ⁇ m / min, the productivity is lowered, and the amount of wire used may increase, resulting in an increase in cost. On the other hand, if it exceeds 1200 ⁇ m / min, an excessive load acts on the wire, the fixed abrasive grains are worn out or fall off, making ingot cutting impossible, and wire breakage is likely to occur.
  • the preferred slicing speed of the ingot is 500 to 1000 ⁇ m / min. Within this range, the flatness of the wafer can be further increased.
  • the silicon ingot for solar cells a single crystal silicon ingot or a polycrystalline silicon ingot can be adopted.
  • the single crystal silicon ingot is grown by, for example, the Czochralski (CZ) method or the floating zone melting (FZ) method.
  • the polycrystalline silicon ingot is manufactured by, for example, the Siemens method.
  • the silicon ingot for solar cells is made of polycrystalline silicon, there is a crystal grain size distribution, so unevenness with uniform size cannot be formed by conventional etching, resulting in variations in power generation efficiency within the wafer surface. There is a risk.
  • the linear concave grooves are mechanically formed by the fixed abrasive wire, a uniform power generation efficiency can be obtained in the wafer plane without depending on the crystal grain size distribution.
  • a large number of linear concave grooves are formed on the front and back surfaces of the sliced solar cell silicon wafer. Since a linear concave groove is also formed on the back surface of the silicon wafer for solar cells, when a thin film such as aluminum is formed on the back surface of the wafer in the solar cell module manufacturing process, the contact area with the thin film increases, Adhesive strength can be increased.
  • the concave groove on the back surface side may be removed by a flattening process such as polishing.
  • the operating condition of the wire at the time of slicing traveling in both directions of the wire row is adopted. For this reason, a large number of linear fine grooves in the same direction as saw marks are formed on the front and back surfaces of the silicon wafer for solar cell. Thereby, the light receiving area of the silicon wafer for solar cells is expanded, and high photoelectric conversion efficiency is obtained. In addition, it is possible to simplify the unevenness forming process different from slicing, and to reduce the manufacturing cost of the silicon-based solar cell. Moreover, by adopting a fixed abrasive wire as a wire, at the time of slicing, linear striation is performed on the front and back surfaces of the solar cell silicon wafer by the fixed abrasive.
  • linear grooves having a finer Rmax of about 2 to 3 ⁇ m are formed by the fixed abrasive grains on the wire surface.
  • the use of the fixed abrasive wire increases the cutting efficiency as compared with slicing using loose abrasive grains, and increases the slicing speed of the silicon ingot for solar cells.
  • the pitch of the groove is 0.1 to 5 mm and the depth of the groove is 1 to 50 ⁇ m.
  • FIG. 1 It is a top view of the silicon wafer for solar cells which concerns on Example 1 of this invention. It is a principal part expanded sectional view of the silicon wafer for solar cells which concerns on Example 1 of this invention.
  • A is a perspective view of the use condition of the wire saw used with the manufacturing method of the silicon wafer for solar cells which concerns on Example 1 of this invention.
  • B is a principal part enlarged view of Fig.3 (a). It is a principal part enlarged plan view which shows the slice process of the silicon ingot for solar cells which uses the fixed abrasive wire in the manufacturing method of the silicon wafer for solar cells which concerns on Example 1 of this invention.
  • W is a silicon wafer for solar cells according to Example 1 of the present invention.
  • This silicon wafer W for solar cells is formed by slicing a silicon ingot for solar cells with a wire saw to form a PN junction and an electrode. And processed into a silicon-based solar cell.
  • a large number of linear concave grooves Wa appearing in the slicing process and extending in the same direction are formed on the entire front and back surfaces of the silicon wafer W for solar cells.
  • the pitch a of the concave grooves Wa is about 1000 ⁇ m, and the depth b of the concave grooves Wa is about 5 ⁇ m.
  • the degree of surface roughness (fine groove group) c of the silicon wafer W for solar cells is about 1 ⁇ m.
  • groove Wa on the wafer back surface are the same.
  • the pitch and depth of the concave grooves Wa on the front surface of the silicon wafer W and the pitch and depth of the concave grooves Wa on the rear surface of the silicon wafer W are substantially the same.
  • the pitch direction of the concave groove Wa of the silicon wafer W the formation position of the concave groove Wa on the wafer surface and the formation position of the concave groove Wa on the back surface of the wafer are substantially the same.
  • the polycrystalline silicon ingot is crushed into a lump of a predetermined size to obtain a molten raw material for casting a polycrystalline silicon ingot for a solar cell.
  • the obtained polycrystal silicon lump is put into a crucible, and a 160 mm square polycrystal silicon ingot is manufactured by an electromagnetic melting continuous casting method.
  • a conductive bottomless crucible having an induction coil arranged on the outer periphery is used.
  • the raw material silicon inserted into the bottomless crucible is heated and melted above the melting point of silicon in a non-contact state on the inner wall of the crucible by electromagnetic induction (15 kHz, 300 kW) of the induction coil.
  • the melt in the bottomless crucible is gradually lowered downward by a drawing device and solidified by a slow cooling device disposed immediately below the bottomless crucible.
  • a polycrystalline silicon ingot is continuously manufactured.
  • the continuously cast polycrystalline silicon ingot is cut every 400 mm in length and finished to a prism having a side of 156 mm by grinding.
  • 10 is a wire saw, and this wire saw 10 is an apparatus for slicing a polycrystalline silicon ingot I cast by an electromagnetic melting continuous casting method into a silicon wafer W for solar cells made of a large number of polycrystalline silicon. It is.
  • the wire saw 10 has two groove rollers 12A and 12B arranged in a rectangular shape when viewed from the front.
  • the groove roller 12A is a driving roller connected so that the rotational force of the driving motor can be transmitted
  • the groove roller 12B is a driven roller.
  • a single fixed abrasive wire 11a is wound around the groove rollers 12A and 12B in parallel with each other at a pitch of 370 ⁇ m. Thereby, the wire row 11 appears between the groove rollers 12A and 12B.
  • the wire row 11 is reciprocated between the two groove rollers 12A and 12B by a drive motor.
  • the middle of the groove rollers 12A and 12B is the ingot cutting position a1 of the wire row 11 that cuts the polycrystalline silicon ingot I.
  • the polycrystalline silicon ingot I is fixed to the lower surface of the lifting platform 19 for raising and lowering the polycrystalline silicon ingot I through the carbon bed 19a.
  • a pair of slurry nozzles 30 for continuously supplying the slurry S onto the wire row 11 are disposed above both sides of the ingot cutting position a1.
  • wrapping oil 100 liters / min
  • the groove rollers 12A and 12B have a cylindrical shape, and their outer peripheral surfaces are covered with a lining material having a predetermined thickness made of urethane rubber.
  • a wire groove 12d is formed on the outer peripheral surface of each lining material (FIG. 3B).
  • a fixed abrasive wire 11a having a large number of abrasive grains 11b fixed to the outer peripheral surface is used as the wire.
  • a fixed abrasive wire 11a in which abrasive grains 11b made of diamond having a particle diameter of 10 to 25 ⁇ m are fixed to a wire having a diameter of 0.12 mm by Ni plating by an electrodeposition method is used.
  • the wire 11a is led out from the bobbin 20 of the feeding device 13, and is bridged over the groove rollers 12A and 12B via the supply-side guide roller. After that, it is wound around the bobbin 21 of the winding device 15 via the guide roller on the outlet side.
  • the rotating shafts of the bobbins 20 and 21 are connected to corresponding output shafts of the drive motors 16 and 17, respectively.
  • the bobbins 20 and 21 pivotally supported by the pair of bearings 18 are rotated clockwise or counterclockwise in FIG.
  • the wire 11a travels in both directions.
  • the bobbin 20 of the feeding device 13 is driven by the drive motor 16 while supplying the slurry S from the slurry nozzle 30 to the wire row 11 at 100 liters / minute. Rotate. Thereby, the wire 11a is supplied to the groove rollers 12A and 12B. At the same time, the bobbin 21 of the winding device 15 is rotated by the drive motor 17 to wind the wire 11a via the groove rollers 12A and 12B. At that time, the rotation direction of each of the bobbins 20 and 21 is changed at a constant cycle, and the wire 11a is caused to travel in both directions.
  • the advance amount of the fixed abrasive wire 11a is 250 m
  • the retreat amount of the fixed abrasive wire 11a is 248 m
  • the cycle time for changing between advance and retreat is 47.7 seconds.
  • the feed rate of the fixed abrasive wire 11a is 900 m / min.
  • the slice speed of the polycrystalline silicon ingot I is 700 ⁇ m / min.
  • the polycrystalline silicon ingot I is pressed against the wire row 11 from above.
  • the polycrystalline silicon ingot I has a rectangular shape with a length of 156 mm and a width of 120 mm, a boron concentration of 1.4 ⁇ 10 16 atoms / cm 3 , and a specific resistance of 1.0 m ⁇ ⁇ cm (P-type). It slices into the silicon wafer W for solar cells. That is, during the reciprocating traveling of the wire row 11, a large number of abrasive grains 11b are rubbed against the bottom of the cutting groove by the fixed abrasive wire 11a of the wire row 11, and the bottom is gradually scraped off by a grinding action (FIG. 4).
  • a PN junction is formed on the silicon wafer W for solar cells, and electrodes are formed on the front and back surfaces of the wafer. Specifically, phosphorus (P) is thermally diffused on the wafer surface to form an N-type diffusion layer, and then a back electrode made of aluminum is formed on the back surface of the silicon wafer W for solar cells, and silicon for solar cells. A surface electrode made of silver is formed on the surface of the wafer W.
  • each concave groove Wa has a pitch a of about 1000 ⁇ m, a depth b of about 5 ⁇ m, and a surface roughness c of about 1 ⁇ m.
  • an unevenness forming step for example, an etching step
  • the manufacturing cost of the silicon-based solar cell can be reduced.
  • the solar cell silicon wafer is compared with the case of slicing using slurry containing loose abrasive grains.
  • a rough linear groove Wa can be formed on the front and back surfaces of W. That is, the pitch a of the concave grooves Wa is about 1000 ⁇ m, and the depth b of the concave grooves Wa is about 5 ⁇ m. Further, the degree of surface roughness c of the solar cell silicon wafer W is about 1 ⁇ m.
  • the fixed abrasive wire 11a since the fixed abrasive wire 11a is used, the cutting efficiency of the polycrystalline silicon ingot I is higher than when slicing using a slurry containing loose abrasive grains. Therefore, the slice speed of the polycrystalline silicon ingot I is increased.
  • the abrasive grains 11b and the fixed abrasive wire 11a are integrated, and the moving speed of the fixed abrasive wire 11a and the moving speed of the abrasive grains 11b in the slice are the same. Because it becomes.
  • the solar cell silicon ingot was sliced in accordance with the method for producing the solar cell silicon wafer of Example 1.
  • the data of the surface roughness of the silicon wafer for solar cells at that time are shown in FIG.
  • a contact roughness meter (Surfcom 130A) manufactured by Tokyo Seimitsu Co., Ltd. was used.
  • the measurement length was 5 mm
  • the measurement speed was 0.3 mm / s
  • CutOFF was 0.8 mm.
  • the wafer surface roughness profile in the graph of FIG. 6 compared with the conventional method (surface roughness Rmax of about 5 ⁇ m) using a slurry containing loose abrasive grains and running the wire in one direction, it is for solar cells.
  • surface roughness Rmax of about 5 ⁇ m
  • FIG. 7a shows a magnification of 200 times
  • FIG. 7a shows a magnification of 200 times
  • FIG. 8 shows the results of measuring the surface roughness of the silicon wafer for solar cells using Keyence VK8500.
  • a slurry in which 110 kg of free abrasive grains (GC abrasive grains) having an average particle size of 7 to 8 ⁇ m are mixed with 100 liters of wrapping oil is used instead of the non-abrasive slurry.
  • the magnification of the site on the wafer surface is 200 times in FIG. 8a and 1000 times in FIG. 8b.
  • no linear grooves (irregularities) were observed on the surface of the solar cell silicon wafer at both low magnification and high magnification.
  • This invention is useful, for example, for silicon wafers for solar cells for power generation.

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  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Physics & Mathematics (AREA)
  • Condensed Matter Physics & Semiconductors (AREA)
  • Electromagnetism (AREA)
  • General Physics & Mathematics (AREA)
  • Computer Hardware Design (AREA)
  • Microelectronics & Electronic Packaging (AREA)
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  • Finish Polishing, Edge Sharpening, And Grinding By Specific Grinding Devices (AREA)
  • Photovoltaic Devices (AREA)
  • Mechanical Treatment Of Semiconductor (AREA)

Abstract

Disclosed is a silicon wafer for solar cells that has a plurality of similarly-oriented, minute, linear depressions that are saw marks formed upon the front and back surfaces of the wafer, whereby the light receiving area of the wafer is expanded and high photoelectric conversion efficiency can be obtained. Furthermore, projection/depression formation processes other than slicing are simplified, and the production cost for silicon solar cells is reduced.

Description

太陽電池用シリコンウェーハおよびその製造方法Silicon wafer for solar cell and manufacturing method thereof
　この発明は、太陽電池用シリコンウェーハおよびその製造方法、詳しくはシリコン系太陽電池の基材となり、太陽電池用シリコンインゴットをスライスして得られる太陽電池用シリコンウェーハおよびその製造方法に関する。 The present invention relates to a silicon wafer for a solar cell and a method for manufacturing the same, and more particularly to a silicon wafer for a solar cell that is obtained by slicing a silicon ingot for a solar cell as a base material for a silicon-based solar cell.
　シリコン系太陽電池は、単結晶シリコンインゴットまたは多結晶シリコンインゴットをスライスして得られた太陽電池用シリコンウェーハにＰＮ接合を形成し、ウェーハ表裏面に電極を形成して製造される。近年、一般的なスライスではワイヤソーが利用される。
　ワイヤソーによるスライス方法は、まず１本のワイヤを２～４本のグルーブローラ間に所定ピッチで架け渡し、ワイヤ列を形成する。その後、各グルーブローラを回転させてワイヤを走行させ、遊離砥粒を含むスラリーをインゴット両側方のワイヤ列上に供給しながら、太陽電池用シリコンインゴットをワイヤ列に押し付ける。これにより、多数枚の太陽電池用シリコンウェーハが同時に得られる。 A silicon-based solar cell is manufactured by forming a PN junction on a solar cell silicon wafer obtained by slicing a single crystal silicon ingot or a polycrystalline silicon ingot, and forming electrodes on the front and back surfaces of the wafer. In recent years, wire saws have been used for general slices.
In the slicing method using a wire saw, first, one wire is bridged between two to four groove rollers at a predetermined pitch to form a wire row. Thereafter, each groove roller is rotated to run the wire, and the solar cell silicon ingot is pressed against the wire row while supplying slurry containing loose abrasive grains onto the wire row on both sides of the ingot. Thereby, many silicon wafers for solar cells are obtained simultaneously.
　ところで、一般的な太陽電池用シリコンウェーハの厚さは、２００μｍ前後と薄肉である。そのため、ワイヤの双方向送りでは、スライス時に太陽電池用シリコンウェーハの割れやチッピングが生じ、ワイヤの断線が発生し易かった。そこで、この課題を解消するため、ワイヤの動作条件（ワイヤ列の送り方向）の１つとして、スライス面の高い平滑性が得られる一方向送りが採用されていた。これにより、太陽電池用シリコンウェーハの表裏面は、研削跡のソーマークが確認できないほど高平滑化（Ｒｍａｘ５～１０μｍ）していた。 Incidentally, the thickness of a general silicon wafer for solar cells is as thin as about 200 μm. Therefore, in the bi-directional feed of the wire, the silicon wafer for solar cells is cracked or chipped at the time of slicing, and the wire is easily broken. Therefore, in order to solve this problem, unidirectional feed that provides high smoothness of the slice surface has been adopted as one of the wire operating conditions (wire feed direction). As a result, the front and back surfaces of the solar cell silicon wafer were so smooth (Rmax 5 to 10 μm) that the saw marks on the grinding marks could not be confirmed.
　また、シリコン系太陽電池は、光電変換効率を高めるため、太陽電池用シリコンウェーハの表面（受光面）に無数の微細な凹凸を形成し、この表面での光反射率の低減を図っている。太陽電池用シリコンウェーハの表面に凹凸を形成する方法として、従来、シリコンの面方位によるエッチング速度の違いを利用し、アルカリ性水溶液を太陽電池用シリコンウェーハに接触させ、エッチングによりウェーハ表面に凹凸を形成する方法が開発されている（例えば特許文献１）。 In addition, in order to increase photoelectric conversion efficiency, silicon-based solar cells have innumerable fine irregularities formed on the surface (light-receiving surface) of a solar cell silicon wafer, and the light reflectance on this surface is reduced. As a method of forming irregularities on the surface of a silicon wafer for solar cells, conventionally, utilizing the difference in etching rate depending on the silicon surface orientation, an alkaline aqueous solution is brought into contact with the silicon wafer for solar cells, and irregularities are formed on the wafer surface by etching. A method has been developed (for example, Patent Document 1).
日本国特開２００６－２０２８３１号公報Japanese Unexamined Patent Publication No. 2006-202831
　しかしながら、このようなエッチング法によれば、結晶異方性の特性上、太陽電池用シリコンウェーハの表面に対して、Ｒｍａｘで２０μｍ以上の表面粗さを実現させることができなかった。その結果、高い光電変換効率を得る際に必要な太陽電池用シリコンウェーハの受光面積（表面積）の拡大を十分に行うことができなかった。
　また、前記凹凸の形成にエッチング処理を採用した場合には、太陽電池用シリコンウェーハからシリコン系太陽電池を製造する際、スライスとは別のエッチング工程が必要であった。これにより、太陽電池用シリコンウェーハがコスト高となっていた。このことは、エッチングとは異なる手法（例えば研削、レーザ照射など）によって、太陽電池用シリコンウェーハの表面に凹凸を形成させる場合も同じである。 However, according to such an etching method, the surface roughness of Rmax of 20 μm or more could not be realized with respect to the surface of the solar cell silicon wafer due to the characteristics of crystal anisotropy. As a result, it was not possible to sufficiently expand the light receiving area (surface area) of the silicon wafer for solar cells necessary for obtaining high photoelectric conversion efficiency.
In addition, when an etching process is employed for forming the irregularities, an etching process different from slicing is required when manufacturing a silicon-based solar cell from a silicon wafer for solar cells. Thereby, the silicon wafer for solar cells was expensive. This also applies to the case where irregularities are formed on the surface of the solar cell silicon wafer by a method different from etching (for example, grinding, laser irradiation, etc.).
　そこで、発明者は鋭意研究の結果、これまではウェーハ割れなどを誘発するとして危惧されていたワイヤの双方向走行によるシリコンインゴットのスライスに着目した。すなわち、太陽電池用シリコンウェーハの表裏面に、ワイヤ列を双方向へ走行させた際に形成されるソーマークと呼ばれる同一方向に向かう多数の直線状の凹溝が存在するものとすれば、上述した全ての問題が解消されることを知見し、この発明を完成させた。 Therefore, as a result of diligent research, the inventor focused attention on a slice of a silicon ingot by two-way traveling of a wire, which has been feared to induce wafer cracking. That is, if there are a large number of linear concave grooves in the same direction called saw marks formed on the front and back surfaces of the silicon wafer for solar cells when the wire row is run in both directions, It was found that all problems could be solved, and the present invention was completed.
　この発明は、受光面積を拡大させて高い光電変換効率が得られるとともに、スライスとは別の凹凸形成工程が簡略化でき、シリコン系太陽電池の製造コストも低減させることができる太陽電池用シリコンウェーハおよびその製造方法を提供することを目的としている。 The invention provides a silicon wafer for a solar cell that can increase the light receiving area, obtain high photoelectric conversion efficiency, simplify the unevenness forming process different from slicing, and reduce the manufacturing cost of silicon-based solar cells. And it aims at providing the manufacturing method.
　本発明は、ＰＮ接合および電極が形成されてシリコン系太陽電池に加工される太陽電池用シリコンウェーハにおいて、スライス加工の際に現出したソーマークである多数の同一方向に向かう直線状の凹溝が、表裏面に形成された太陽電池用シリコンウェーハである。 The present invention relates to a silicon wafer for a solar cell in which a PN junction and an electrode are formed to be processed into a silicon-based solar cell, and a large number of linear concave grooves directed in the same direction, which are saw marks that appear at the time of slicing. It is the silicon wafer for solar cells formed in the front and back.
　本発明によれば、太陽電池用シリコンウェーハを、その表裏面に、ワイヤ列を双方向へ走行させることで現出する、同一方向に向かう多数の直線状の凹溝を有するものとした。すなわち、一般的にスライス加工後のシリコンウェーハに施されるエッチング、研削、研磨などの平坦化処理がなされず、製品としての太陽電池用シリコンウェーハとなる。これにより、太陽電池用シリコンウェーハの受光面積が拡大（Ｒｍａｘで２０μｍ以上のウェーハ表面粗さも可能）し、高い光電変換効率が得られる。また、シリコンウェーハの裏面にも、ウェーハ表面の凹溝と同じ方向に向かう多数の直線状の凹溝が形成されることで、太陽電池モジュール製造工程でウェーハ裏面にアルミニウムなどの薄膜を形成した際、薄膜との接触面積が増大して両者の接着強度を高めることができる。しかも、スライスとは別の凹凸形成工程が簡略化でき、シリコン系太陽電池の製造コストを低減させることができる。本発明において、「スライスとは別の凹凸形成工程が簡略化できる」とは、（１）スライスとは別の凹凸形成工程が不要になるか、（２）スライスとは別に凹凸を形成する場合でもこの凹凸形成工程の時間が短縮できることをいう。 According to the present invention, the silicon wafer for solar cells has a large number of linear concave grooves in the same direction that appear on the front and back surfaces of the silicon wafer by traveling in both directions. That is, the silicon wafer for solar cells as a product is obtained without performing flattening processing such as etching, grinding, polishing, etc., which is generally performed on the silicon wafer after slicing. Thereby, the light receiving area of the silicon wafer for solar cells is expanded (wafer surface roughness of 20 μm or more is possible at Rmax), and high photoelectric conversion efficiency is obtained. In addition, when a number of linear grooves are formed on the back surface of the silicon wafer in the same direction as the grooves on the wafer surface, when a thin film such as aluminum is formed on the wafer back surface in the solar cell module manufacturing process. In addition, the contact area with the thin film can be increased, and the adhesive strength between them can be increased. In addition, it is possible to simplify the unevenness forming process different from the slicing, and to reduce the manufacturing cost of the silicon-based solar cell. In the present invention, “an unevenness forming process different from a slice can be simplified” means that (1) an unevenness forming process different from a slice is not required, or (2) an unevenness is formed separately from a slice. However, it means that the time for the unevenness forming process can be shortened.
　シリコン系太陽電池としては、単結晶シリコン系太陽電池、多結晶シリコン系太陽電池などが挙げられる。
　太陽電池用シリコンウェーハの素材としては、単結晶シリコン、多結晶シリコンなどを採用することができる。
　太陽電池用シリコンウェーハの形状としては、円形、角部が面取りされた四角形（単結晶シリコン製）、四角形（多結晶シリコン製）などを採用することができる。 Examples of silicon solar cells include single crystal silicon solar cells and polycrystalline silicon solar cells.
As a material of the silicon wafer for solar cells, single crystal silicon, polycrystalline silicon, or the like can be employed.
As the shape of the silicon wafer for solar cell, a circle, a rectangle with chamfered corners (made of single crystal silicon), a rectangle (made of polycrystalline silicon), or the like can be adopted.
　「直線状の凹溝」とは、太陽電池用シリコンウェーハの表裏面の全域にわたって、ほぼ等間隔で同一方向に向かう直線的かつ平行に形成された多数の凹溝からなるスライス加工時のソーマーク（研削痕）である。このソーマークは、固定砥粒ワイヤを双方向へ走行させながら、太陽電池用シリコンインゴットをスライスすることで形成される。そのため、太陽電池用シリコンウェーハの表面だけでなく、裏面にもウェーハ表面の凹溝と同じ方向（例えば、ウェーハ表裏面の凹溝ともＸ方向）に向かう同様の直線状の凹溝が存在する。また、直線状の凹溝の両端部分には、一方向への湾曲部（ダレ）が存在する場合もある。凹溝はソーマークであるので、凹溝の底面とウェーハ上面とには、Ｒｍａｘで２～３μｍ程度のさらに微細な面荒れも存在する。さらに、凹溝はソーマークであることから、ウェーハ表面の凹溝のピッチおよび深さと、ウェーハ裏面の凹溝のピッチおよび深さとは略同一となる。さらにまた、シリコンウェーハの前記ピッチ方向において、ウェーハ表面の凹溝の形成位置とウェーハ裏面の凹溝の形成位置とは略同一となる。 “Straight groove” means a saw mark at the time of slicing consisting of a large number of grooves that are linearly and parallelly formed in the same direction at almost equal intervals over the entire surface of the silicon wafer for solar cells. Grinding marks). This saw mark is formed by slicing a silicon ingot for solar cells while a fixed abrasive wire is traveling in both directions. For this reason, not only the surface of the silicon wafer for solar cells but also the back surface has the same linear groove on the back surface in the same direction as the groove on the wafer surface (for example, the groove on the front and back surfaces of the wafer in the X direction). Further, there may be a curved portion (sag) in one direction at both end portions of the linear groove. Since the groove is a saw mark, there is a finer surface roughness of about 2 to 3 μm in Rmax between the bottom surface of the groove and the top surface of the wafer. Further, since the concave grooves are saw marks, the pitch and depth of the concave grooves on the wafer surface and the pitch and depth of the concave grooves on the back surface of the wafer are substantially the same. Furthermore, in the pitch direction of the silicon wafer, the groove formation position on the wafer surface and the groove formation position on the wafer back surface are substantially the same.
　太陽電池用シリコンウェーハの厚さは、１６０～２２０μｍである。１６０μｍ未満では、太陽電池用シリコンウェーハが薄すぎて、ウェーハ取り扱い上でウェーハの割れが著しく増加する。そのため、この割れ対策には太陽電池用シリコンウェーハの厚肉化が有効である。しかしながら、２２０μｍを超えれば、シリコンの使用量が増加し、太陽電池用シリコンウェーハの製造コストが高まるので好ましくない。
　太陽電池用シリコンウェーハの表裏面には、固定砥粒ワイヤを双方向へ走行させるスライスにより現出した１～５μｍ（ウェーハ片面）の加工ダメージが存在する。ちなみに、遊離砥粒を利用してワイヤ切断（ワイヤを双方向へ走行）した場合には、一般的にウェーハ片面に５～１５μｍの加工ダメージが現出する。 The thickness of the silicon wafer for solar cells is 160 to 220 μm. If it is less than 160 micrometers, the silicon wafer for solar cells is too thin, and the crack of a wafer increases remarkably on wafer handling. Therefore, it is effective to increase the thickness of the silicon wafer for solar cells as a countermeasure against this crack. However, if it exceeds 220 μm, the amount of silicon used is increased, and the production cost of the silicon wafer for solar cells is increased.
On the front and back surfaces of the silicon wafer for solar cells, there is a processing damage of 1 to 5 μm (on one side of the wafer) that appears by slicing the fixed abrasive wire in both directions. Incidentally, when a wire is cut using loose abrasive grains (wires are moved in both directions), a processing damage of 5 to 15 μm generally appears on one side of the wafer.
　本発明において、前記凹溝は、ピッチが０．１～５ｍｍで、深さが１～５０μｍとした方が望ましい。 In the present invention, it is desirable that the concave grooves have a pitch of 0.1 to 5 mm and a depth of 1 to 50 μm.
　これにより、ウェーハ表裏面に形成された凹溝のピッチを０．１～５ｍｍとし、凹溝の深さを１～５０μｍとしたので、凹溝の形成による太陽電池用シリコンウェーハの強度低下を防止しながら、太陽電池用シリコンウェーハの受光面積の拡大と、高い光電変換効率とを同時に満足させることができる。 As a result, the pitch of the concave grooves formed on the front and back surfaces of the wafer is set to 0.1 to 5 mm, and the depth of the concave grooves is set to 1 to 50 μm, thereby preventing a reduction in strength of the silicon wafer for solar cells due to the formation of the concave grooves. However, expansion of the light receiving area of the silicon wafer for solar cells and high photoelectric conversion efficiency can be satisfied at the same time.
　凹溝のピッチが０．１ｍｍ未満であれば、凹溝の形成による太陽電池用シリコンウェーハの強度が低下し易い。また、凹溝のピッチが５ｍｍを超えれば、太陽電池用シリコンウェーハの受光面積の拡大が不十分で、高い光電変換効率が得られない。
　凹溝の深さが１μｍ未満では、太陽電池用シリコンウェーハの受光面積の拡大が不十分で、高い光電変換効率が得られない。また、凹溝の深さが５０μｍを超えれば、凹溝の形成による太陽電池用シリコンウェーハの強度が低下するおそれがある。特に、凹溝がウェーハ表裏面で重なった部分では１００μｍを超える減厚となり、ウェーハの割れが発生し易くなる。 If the pitch of the concave grooves is less than 0.1 mm, the strength of the silicon wafer for solar cells due to the formation of the concave grooves tends to decrease. Moreover, if the pitch of a ditch | groove exceeds 5 mm, expansion of the light-receiving area of the silicon wafer for solar cells is inadequate, and high photoelectric conversion efficiency cannot be obtained.
When the depth of the groove is less than 1 μm, the light receiving area of the solar cell silicon wafer is not sufficiently enlarged, and high photoelectric conversion efficiency cannot be obtained. Moreover, if the depth of the ditch | groove exceeds 50 micrometers, there exists a possibility that the intensity | strength of the silicon wafer for solar cells by formation of a ditch | groove may fall. In particular, the thickness where the concave grooves overlap on the front and back surfaces of the wafer is reduced to more than 100 μm, and the wafer is likely to be cracked.
　本発明は、ワイヤソーの複数本のグルーブローラ間で走行中のワイヤ列に、スラリーを供給しながら太陽電池用シリコンインゴットを押し付け、多数枚の太陽電池用シリコンウェーハをスライスにより得る太陽電池用シリコンウェーハの製造方法において、前記ワイヤ列を構成するワイヤとして、外周面に砥粒が固定された固定砥粒ワイヤを使用し、前記ワイヤ列を双方向に走行させながら前記太陽電池用シリコンインゴットをスライスすることで、前記各太陽電池用シリコンウェーハの表裏面に、ソーマークである多数の同一方向に向かう直線状の凹溝を形成する太陽電池用シリコンウェーハの製造方法である。 The present invention relates to a silicon wafer for solar cells obtained by slicing a plurality of silicon wafers for solar cells by pressing a silicon ingot for solar cells while supplying slurry to a wire row traveling between a plurality of groove rollers of a wire saw. In this manufacturing method, a fixed abrasive wire having abrasive grains fixed on an outer peripheral surface is used as a wire constituting the wire row, and the silicon ingot for solar cells is sliced while the wire row is traveling in both directions. This is a method for manufacturing a silicon wafer for solar cells, in which a large number of linear concave grooves directed in the same direction as saw marks are formed on the front and back surfaces of each silicon wafer for solar cells.
　本発明によれば、スライスに際して、双方向（往方向および復方向）へ走行中の固定砥粒ワイヤから構成されるワイヤ列に、太陽電池用シリコンインゴットを押し付ける。これにより、太陽電池用シリコンウェーハの表裏面に、ワイヤ列を双方向へ走行させた際に現出する多数の同一方向に向かう粗い（例えばＲｍａｘで２０μｍ以上）直線状の凹溝（ソーマーク）が形成される。それと同時に、ワイヤ表面の固定砥粒によって、さらに細かいＲｍａｘで２～３μｍ程度の同一方向に向かう直線状の溝（ソーマーク）が形成される。その結果、太陽電池用シリコンウェーハの受光面積が拡大し、高い光電変換効率が得られる。しかも、スライスとは別の凹凸形成工程が簡略化でき、シリコン系太陽電池の製造コストを低減させることができる。さらに、固定砥粒ワイヤの使用により、遊離砥粒を使用したワイヤソーによるスライス時に比べて高切削効率となり、太陽電池用シリコンインゴットのスライス速度が高まる。なお、その後に凹溝形成用のエッチングを実施する場合でも、エッチング時間の短縮が図れる。 According to the present invention, when slicing, a solar cell silicon ingot is pressed against a wire row composed of fixed abrasive wires traveling in both directions (forward and backward directions). As a result, a large number of rough grooves (saw marks) extending in the same direction appearing when the wire row is run in both directions (for example, Rmax of 20 μm or more) are formed on the front and back surfaces of the solar cell silicon wafer. It is formed. At the same time, linear grooves (saw marks) extending in the same direction with a finer Rmax of about 2 to 3 μm are formed by the fixed abrasive grains on the wire surface. As a result, the light receiving area of the solar cell silicon wafer is expanded, and high photoelectric conversion efficiency is obtained. In addition, it is possible to simplify the unevenness forming process different from the slicing, and to reduce the manufacturing cost of the silicon-based solar cell. Further, the use of the fixed abrasive wire increases the cutting efficiency compared with the case of slicing with a wire saw using loose abrasive grains, and the slicing speed of the silicon ingot for solar cells is increased. Even when etching for forming the groove is performed after that, the etching time can be shortened.
　ワイヤソーのグルーブローラの使用本数は、例えば２本、３本または４本である。
　固定砥粒ワイヤとしては、粒度が７～２５μｍのダイヤモンド砥粒を、集中度２０～５５、好ましくは５０で、直径１００～３００μｍのピアノ線に、バインダにより加熱硬化または紫外線照射硬化させたものなどを採用することができる。バインダとしては、エポキシ樹脂、フェノール樹脂、アクリルウレタン樹脂などを採用することができる。また、ワイヤの外周面にニッケルとともに、ダイヤモンド砥粒を電着させたものでもよい。
　固定砥粒ワイヤの送り速度は、５００～１５００ｍ／分である。５００ｍ／分未満では、遊離砥粒を含まないスラリー（ラッピングオイル）が、ワイヤによってインゴットの切断箇所へ持ち込まれず、切断効率が低下する。１５００ｍ／分を超えれば、ワイヤに付着したスラリーが吹き飛ばされて切断効率が低下する。 The number of wire rollers used in the wire saw is, for example, 2, 3, or 4.
The fixed abrasive wire is a diamond abrasive grain having a particle size of 7 to 25 μm, heat-cured or UV-irradiated with a binder on a piano wire with a concentration of 20 to 55, preferably 50 and a diameter of 100 to 300 μm, etc. Can be adopted. As the binder, an epoxy resin, a phenol resin, an acrylic urethane resin, or the like can be used. Further, a diamond abrasive grain may be electrodeposited together with nickel on the outer peripheral surface of the wire.
The feed rate of the fixed abrasive wire is 500 to 1500 m / min. If it is less than 500 m / min, the slurry (wrapping oil) that does not contain loose abrasive grains is not brought into the cutting portion of the ingot by the wire, and the cutting efficiency decreases. If it exceeds 1500 m / min, the slurry adhering to the wire is blown off, and the cutting efficiency is lowered.
　固定砥粒ワイヤの前進量は２５０～４５０ｍ、固定砥粒ワイヤの後退量は２４８～４９９ｍである。前進と後退とのサイクル時間は、４７．７～８８．９秒である。
　インゴットのスライス平均速度は、４００～１２００μｍ／分である。ウェーハ品質をさらに高めるためには、より低速な条件が好ましいが、４００μｍ／分未満では生産性が低下し、かつワイヤ使用量が増加してコスト高を招くおそれがある。また、１２００μｍ／分を超えれば、ワイヤに過剰な負荷が作用し、固定砥粒の磨滅や脱落が生じてインゴット切断が不可能となり、ワイヤ断線が発生し易くなる。インゴットの好ましいスライス速度は、５００～１０００μｍ／分である。この範囲であれば、ウェーハの平坦度をより高めることができる。 The advance amount of the fixed abrasive wire is 250 to 450 m, and the retract amount of the fixed abrasive wire is 248 to 499 m. The cycle time for forward and reverse is 47.7-88.9 seconds.
The ingot slice average speed is 400 to 1200 μm / min. In order to further improve the wafer quality, a lower speed condition is preferable, but if it is less than 400 μm / min, the productivity is lowered, and the amount of wire used may increase, resulting in an increase in cost. On the other hand, if it exceeds 1200 μm / min, an excessive load acts on the wire, the fixed abrasive grains are worn out or fall off, making ingot cutting impossible, and wire breakage is likely to occur. The preferred slicing speed of the ingot is 500 to 1000 μm / min. Within this range, the flatness of the wafer can be further increased.
　太陽電池用シリコンインゴットとしては、単結晶シリコンインゴット、多結晶シリコンインゴットを採用することができる。単結晶シリコンインゴットは、例えばチョクラルスキー（ＣＺ）法または浮遊帯域融解（ＦＺ）法により育成される。多結晶シリコンインゴットは、例えばシーメンス法により作製される。
　特に、太陽電池用シリコンインゴットが多結晶シリコン製の場合には、結晶粒度分布が存在するため、従来法のエッチングではサイズが均一な凹凸は形成できず、ウェーハ面内で発電効率にバラツキが発生するおそれがある。しかしながら、この発明では、固定砥粒ワイヤによって機械的に直線状の凹溝を形成するので、結晶粒度分布に依存せず、ウェーハ面内で均一な発電効率が得られる。
　スライス後の太陽電池用シリコンウェーハには、その表裏面に多数の直線状の凹溝が形成される。太陽電池用シリコンウェーハの裏面にも直線状の凹溝が形成されるので、太陽電池モジュール製造工程において、ウェーハ裏面にアルミニウムなどの薄膜を形成した際、薄膜との接触面積が増大し、両者の接着強度を高めることができる。なお、太陽電池モジュール製造工程における工程フローによっては、裏面側の凹溝は研磨などの平坦化処理により除去してもよい。 As the silicon ingot for solar cells, a single crystal silicon ingot or a polycrystalline silicon ingot can be adopted. The single crystal silicon ingot is grown by, for example, the Czochralski (CZ) method or the floating zone melting (FZ) method. The polycrystalline silicon ingot is manufactured by, for example, the Siemens method.
In particular, when the silicon ingot for solar cells is made of polycrystalline silicon, there is a crystal grain size distribution, so unevenness with uniform size cannot be formed by conventional etching, resulting in variations in power generation efficiency within the wafer surface. There is a risk. However, in the present invention, since the linear concave grooves are mechanically formed by the fixed abrasive wire, a uniform power generation efficiency can be obtained in the wafer plane without depending on the crystal grain size distribution.
A large number of linear concave grooves are formed on the front and back surfaces of the sliced solar cell silicon wafer. Since a linear concave groove is also formed on the back surface of the silicon wafer for solar cells, when a thin film such as aluminum is formed on the back surface of the wafer in the solar cell module manufacturing process, the contact area with the thin film increases, Adhesive strength can be increased. Depending on the process flow in the solar cell module manufacturing process, the concave groove on the back surface side may be removed by a flattening process such as polishing.
　本発明によれば、スライス時のワイヤの動作条件として、ワイヤ列の双方向への走行を採用している。そのため、太陽電池用シリコンウェーハの表裏面に、ソーマークである多数の同一方向に向かう直線状の微細な凹溝が形成される。これにより、太陽電池用シリコンウェーハの受光面積が拡大し、高い光電変換効率が得られる。しかも、スライスとは別の凹凸形成工程が簡略化でき、シリコン系太陽電池の製造コストの低減が図れる。
　また、ワイヤとして固定砥粒ワイヤを採用することで、スライス時、太陽電池用シリコンウェーハの表裏面に、固定砥粒によって直線的な筋引きが行われる。これにより、ウェーハ表裏面に多数本の粗い直線状の凹溝が形成される。それと同時に、ワイヤ表面の固定砥粒によって、さらに細かいＲｍａｘで２～３μｍ程度の直線状の溝が形成される。しかも、固定砥粒ワイヤの使用により遊離砥粒を使用したスライス時に比べて高切削効率となり、太陽電池用シリコンインゴットのスライス速度が高まる。 According to the present invention, as the operating condition of the wire at the time of slicing, traveling in both directions of the wire row is adopted. For this reason, a large number of linear fine grooves in the same direction as saw marks are formed on the front and back surfaces of the silicon wafer for solar cell. Thereby, the light receiving area of the silicon wafer for solar cells is expanded, and high photoelectric conversion efficiency is obtained. In addition, it is possible to simplify the unevenness forming process different from slicing, and to reduce the manufacturing cost of the silicon-based solar cell.
Moreover, by adopting a fixed abrasive wire as a wire, at the time of slicing, linear striation is performed on the front and back surfaces of the solar cell silicon wafer by the fixed abrasive. Thereby, a large number of rough linear grooves are formed on the front and back surfaces of the wafer. At the same time, linear grooves having a finer Rmax of about 2 to 3 μm are formed by the fixed abrasive grains on the wire surface. In addition, the use of the fixed abrasive wire increases the cutting efficiency as compared with slicing using loose abrasive grains, and increases the slicing speed of the silicon ingot for solar cells.
　本発明において、凹溝のピッチを０．１～５ｍｍとし、凹溝の深さを１～５０μｍとした方が望ましい。これにより、凹溝の形成による太陽電池用シリコンウェーハの強度低下を防止しながら、太陽電池用シリコンウェーハの受光面積の拡大と、高い光電変換効率とを同時に満足させることができる。 In the present invention, it is desirable that the pitch of the groove is 0.1 to 5 mm and the depth of the groove is 1 to 50 μm. Thereby, expansion of the light receiving area of the silicon wafer for solar cells and high photoelectric conversion efficiency can be satisfied at the same time while preventing the strength reduction of the silicon wafer for solar cells due to the formation of the concave grooves.
この発明の実施例１に係る太陽電池用シリコンウェーハの平面図である。It is a top view of the silicon wafer for solar cells which concerns on Example 1 of this invention. この発明の実施例１に係る太陽電池用シリコンウェーハの要部拡大断面図である。It is a principal part expanded sectional view of the silicon wafer for solar cells which concerns on Example 1 of this invention. （ａ）は、この発明の実施例１に係る太陽電池用シリコンウェーハの製造方法で使用されるワイヤソーの使用状態の斜視図である。（ｂ）は、図３（ａ）の要部拡大図である。(A) is a perspective view of the use condition of the wire saw used with the manufacturing method of the silicon wafer for solar cells which concerns on Example 1 of this invention. (B) is a principal part enlarged view of Fig.3 (a). この発明の実施例１に係る太陽電池用シリコンウェーハの製造方法における固定砥粒ワイヤを使用した太陽電池用シリコンインゴットのスライス工程を示す要部拡大平面図である。It is a principal part enlarged plan view which shows the slice process of the silicon ingot for solar cells which uses the fixed abrasive wire in the manufacturing method of the silicon wafer for solar cells which concerns on Example 1 of this invention. 従来手段に係る太陽電池用シリコンウェーハの製造方法における遊離砥粒を含むスラリーを使用した太陽電池用シリコンインゴットのスライス工程を示す要部拡大平面図である。It is a principal part enlarged plan view which shows the slice process of the silicon ingot for solar cells using the slurry containing the loose abrasive grain in the manufacturing method of the silicon wafer for solar cells which concerns on the conventional means. この発明の実施例１に係る固定砥粒ワイヤを使用するワイヤソーでの双方向走行によってスライスされた太陽電池用シリコンウェーハの表面粗さプロファイルを示すグラフである。It is a graph which shows the surface roughness profile of the silicon wafer for solar cells sliced by bidirectional | two-way driving | running | working with the wire saw using the fixed abrasive wire which concerns on Example 1 of this invention. 固定砥粒ワイヤを使用し、ワイヤの送りを双方向としてスライスした太陽電池用シリコンウェーハの表面のサイト的な表面粗さの分布図（倍率：２００倍）である。It is a distribution map (magnification: 200 times) of the site-like surface roughness of the surface of the silicon wafer for solar cells sliced by using a fixed abrasive wire and feeding the wire in both directions. 固定砥粒ワイヤを使用し、ワイヤの送りを双方向としてスライスした太陽電池用シリコンウェーハの表面のサイト的な表面粗さの分布図（倍率：１０００倍）である。It is a distribution map (magnification: 1000 times) of the site-like surface roughness of the surface of the silicon wafer for solar cells sliced by using a fixed abrasive wire and feeding the wire in both directions. 遊離砥粒を含むスラリーを使用し、ワイヤの送りを一方向としてスライスした太陽電池用シリコンウェーハの表面のサイト的な表面粗さの分布図（倍率：２００倍）である。It is a distribution map (magnification: 200 times) of the site-like surface roughness of the surface of the silicon wafer for solar cells which used the slurry containing loose abrasive grains and sliced it by making wire feeding into one direction. 遊離砥粒を含むスラリーを使用し、ワイヤの送りを一方向としてスライスした太陽電池用シリコンウェーハの表面のサイト的な表面粗さの分布図（倍率：１０００倍）である。It is a distribution map (magnification: 1000 times) of the site-like surface roughness of the surface of the silicon wafer for solar cells sliced using a slurry containing loose abrasive grains with the wire feeding as one direction.
１０　ワイヤソー、
１１　ワイヤ列、
１１ａ　固定砥粒ワイヤ、
１１ｂ　砥粒、
１２Ａ，１２Ｂ　グルーブローラ、
Ｉ　多結晶シリコンインゴット（太陽電池用シリコンインゴット）、
Ｓ　スラリー、
Ｗ　太陽電池用シリコンウェーハ、
Ｗａ　凹溝、
ａ　ピッチ、
ｂ　深さ。 10 Wire saw,
11 Wire row,
11a Fixed abrasive wire,
11b abrasive grains,
12A, 12B Groove roller,
I polycrystalline silicon ingot (silicon ingot for solar cell),
S slurry,
W Silicon wafer for solar cells,
Wa groove,
a pitch,
b Depth.
　以下、この発明の実施例を具体的に説明する。 Hereinafter, embodiments of the present invention will be described in detail.
　図１および図２において、Ｗはこの発明の実施例１に係る太陽電池用シリコンウェーハで、この太陽電池用シリコンウェーハＷは、太陽電池用シリコンインゴットをワイヤソーによりスライスし、ＰＮ接合および電極が形成されてシリコン系太陽電池に加工されるものである。
　太陽電池用シリコンウェーハＷの表裏面の全域には、スライス加工の際に現出した多数の同一方向に向かう直線状の凹溝Ｗａが形成されている。凹溝Ｗａのピッチａは１０００μｍ程度、凹溝Ｗａの深さｂは５μｍ程度である。また、太陽電池用シリコンウェーハＷの面荒れ（微細な溝群）ｃの程度は、１μｍ程度である。なお、ウェーハ表面の各凹溝Ｗａの向き（例えばＸ方向）と、ウェーハ裏面の各凹溝Ｗａの向き（Ｘ方向）とは同じである。このことは、微細な溝群である面荒れｃの向きについても同様である。さらに、ソーマークであることから、シリコンウェーハＷの表面の凹溝Ｗａのピッチおよび深さと、シリコンウェーハＷの裏面の凹溝Ｗａのピッチおよび深さとは略同一となる。さらにまた、シリコンウェーハＷの凹溝Ｗａのピッチ方向において、ウェーハ表面の凹溝Ｗａの形成位置とウェーハ裏面の凹溝Ｗａの形成位置とは略同一となる。 1 and 2, W is a silicon wafer for solar cells according to Example 1 of the present invention. This silicon wafer W for solar cells is formed by slicing a silicon ingot for solar cells with a wire saw to form a PN junction and an electrode. And processed into a silicon-based solar cell.
A large number of linear concave grooves Wa appearing in the slicing process and extending in the same direction are formed on the entire front and back surfaces of the silicon wafer W for solar cells. The pitch a of the concave grooves Wa is about 1000 μm, and the depth b of the concave grooves Wa is about 5 μm. The degree of surface roughness (fine groove group) c of the silicon wafer W for solar cells is about 1 μm. In addition, the direction (for example, X direction) of each ditch | groove Wa on the wafer surface and the direction (X direction) of each ditch | groove Wa on the wafer back surface are the same. The same applies to the direction of surface roughness c, which is a fine groove group. Further, since it is a saw mark, the pitch and depth of the concave grooves Wa on the front surface of the silicon wafer W and the pitch and depth of the concave grooves Wa on the rear surface of the silicon wafer W are substantially the same. Furthermore, in the pitch direction of the concave groove Wa of the silicon wafer W, the formation position of the concave groove Wa on the wafer surface and the formation position of the concave groove Wa on the back surface of the wafer are substantially the same.
　このように、太陽電池用シリコンウェーハＷの表裏面全域、特に受光面となる表面全域に、ピッチが１０００μｍ、深さが５μｍ程度の多数の直線状の凹溝Ｗａを形成したので、太陽電池用シリコンウェーハＷの受光面積が拡大し、高い光電変換効率が得られる。しかも、従来のようにスライスとは別の凹凸形成工程が簡略化でき、シリコン系太陽電池の製造コストの低減が図れる。 As described above, since a large number of linear concave grooves Wa having a pitch of about 1000 μm and a depth of about 5 μm are formed on the entire front and back surfaces of the silicon wafer W for solar cells, particularly on the entire surface serving as the light receiving surface. The light receiving area of the silicon wafer W is expanded, and high photoelectric conversion efficiency is obtained. In addition, it is possible to simplify the unevenness forming process different from the slicing as in the prior art, and to reduce the manufacturing cost of the silicon solar cell.
　次に、この発明の実施例１の太陽電池用シリコンウェーハＷの製造方法を説明する。
　まず、中間化合物であるトリクロロシラン（ＳｉＨＣｌ３）を水素により還元することで、多結晶シリコンを得るシーメンス法（Ｓｉｅｍｅｎｓ　Ｍｅｔｈｏｄ）を利用した太陽電池用シリコンインゴットの製造方法を説明する。
　これは、水冷したベルジャー型の反応器の中にシリコンの種棒を設置し、種棒に所定の電圧を印加して種棒を１１００℃に加熱し、反応器内にトリクロロシラン（ＳｉＨＣｌ３）、還元剤の水素およびドーパントとしてのボロンガスを下方から導入する。これにより、シリコン塩化物を還元し、生成したシリコンが選択的に種棒の表面に付着することで、棒状の多結晶シリコンが気相成長する。
　次に、多結晶シリコンインゴットを所定サイズの塊に破砕して、太陽電池用の多結晶シリコンインゴットを鋳造する溶融原料とする。 Next, the manufacturing method of the silicon wafer W for solar cells of Example 1 of this invention is demonstrated.
First, by reducing trichlorosilane which is an intermediate compound (SiHCl 3) with hydrogen, Siemens method to obtain a polycrystalline silicon manufacturing method of a silicon ingot for a solar cell using (Siemens Method) will be described.
This is done by installing a silicon seed rod in a water-cooled bell jar type reactor, applying a predetermined voltage to the seed rod and heating the seed rod to 1100 ° C., and trichlorosilane (SiHCl 3 ) in the reactor. Then, hydrogen as a reducing agent and boron gas as a dopant are introduced from below. Thereby, silicon chloride is reduced, and the generated silicon selectively adheres to the surface of the seed rod, so that rod-shaped polycrystalline silicon is vapor-phase grown.
Next, the polycrystalline silicon ingot is crushed into a lump of a predetermined size to obtain a molten raw material for casting a polycrystalline silicon ingot for a solar cell.
　得られた多結晶シリコンの塊をルツボに投入し、電磁溶解連続鋳造方法により１辺の長さ１６０ｍｍ角の多結晶シリコンインゴットを製造する。この方法では、外周に誘導コイルが配置された導電性の無底ルツボを使用する。無底ルツボに挿入された原料シリコンは、誘導コイルの電磁誘導（１５ｋＨｚ、３００ｋＷ）により、ルツボ内壁に非接触状態でシリコンの融点以上に加熱されて溶解する。その後、無底ルツボに原料シリコンを供給しながら、引き抜き装置により無底ルツボ内の融液を下方へ徐々に引き下げ、無底ルツボの直下に配置された徐冷装置により凝固させる。これにより、多結晶シリコンインゴットが連続的に製造される。連続鋳造された多結晶シリコンインゴットは、長さ４００ｍｍ毎に切断され、研削により１辺が１５６ｍｍの角柱に仕上げられる。 The obtained polycrystal silicon lump is put into a crucible, and a 160 mm square polycrystal silicon ingot is manufactured by an electromagnetic melting continuous casting method. In this method, a conductive bottomless crucible having an induction coil arranged on the outer periphery is used. The raw material silicon inserted into the bottomless crucible is heated and melted above the melting point of silicon in a non-contact state on the inner wall of the crucible by electromagnetic induction (15 kHz, 300 kW) of the induction coil. After that, while supplying raw material silicon to the bottomless crucible, the melt in the bottomless crucible is gradually lowered downward by a drawing device and solidified by a slow cooling device disposed immediately below the bottomless crucible. Thereby, a polycrystalline silicon ingot is continuously manufactured. The continuously cast polycrystalline silicon ingot is cut every 400 mm in length and finished to a prism having a side of 156 mm by grinding.
　次に、図３を参照して、ワイヤソーを具体的に説明する。
　図３（ａ）において、１０はワイヤソーで、このワイヤソー１０は、電磁溶解連続鋳造方法により鋳造された多結晶シリコンインゴットＩを多数枚の多結晶シリコンからなる太陽電池用シリコンウェーハＷにスライスする装置である。 Next, the wire saw will be specifically described with reference to FIG.
In FIG. 3A, 10 is a wire saw, and this wire saw 10 is an apparatus for slicing a polycrystalline silicon ingot I cast by an electromagnetic melting continuous casting method into a silicon wafer W for solar cells made of a large number of polycrystalline silicon. It is.
　ワイヤソー１０は、正面視して矩形状に配置された２本のグルーブローラ１２Ａ、１２Ｂを有している。このうち、グルーブローラ１２Ａが駆動モータの回転力を伝達可能に連結された駆動側のローラ、グルーブローラ１２Ｂが従動側のローラである。グルーブローラ１２Ａ、１２Ｂ間には、１本の固定砥粒ワイヤ１１ａが、互いに平行かつ３７０μｍピッチで巻き架けられている。これにより、グルーブローラ１２Ａ、１２Ｂ間にワイヤ列１１が現出される。
　ワイヤ列１１は、２本のグルーブローラ１２Ａ、１２Ｂ間で駆動モータにより往復走行される。グルーブローラ１２Ａ、１２Ｂの中間が、多結晶シリコンインゴットＩを切断するワイヤ列１１のインゴット切断位置ａ１である。 The wire saw 10 has two groove rollers 12A and 12B arranged in a rectangular shape when viewed from the front. Among these, the groove roller 12A is a driving roller connected so that the rotational force of the driving motor can be transmitted, and the groove roller 12B is a driven roller. A single fixed abrasive wire 11a is wound around the groove rollers 12A and 12B in parallel with each other at a pitch of 370 μm. Thereby, the wire row 11 appears between the groove rollers 12A and 12B.
The wire row 11 is reciprocated between the two groove rollers 12A and 12B by a drive motor. The middle of the groove rollers 12A and 12B is the ingot cutting position a1 of the wire row 11 that cuts the polycrystalline silicon ingot I.
　多結晶シリコンインゴットＩは、カーボンベッド１９ａを介して、多結晶シリコンインゴットＩを昇降させる昇降台１９の下面に固定されている。インゴット切断位置ａ１の両側の上方には、スラリーＳをワイヤ列１１上に連続供給するスラリーノズル３０が、一対配設されている。スラリーＳとしては、遊離砥粒を含まないラッピングオイル（１００リットル／分）を採用している。
　グルーブローラ１２Ａ、１２Ｂは円筒形状で、これらの外周面は、ウレタンゴムからなる所定厚さのライニング材により被覆されている。各ライニング材の外周面には、ワイヤ溝１２ｄが刻設されている（図３（ｂ））。 The polycrystalline silicon ingot I is fixed to the lower surface of the lifting platform 19 for raising and lowering the polycrystalline silicon ingot I through the carbon bed 19a. A pair of slurry nozzles 30 for continuously supplying the slurry S onto the wire row 11 are disposed above both sides of the ingot cutting position a1. As the slurry S, wrapping oil (100 liters / min) that does not contain loose abrasive grains is employed.
The groove rollers 12A and 12B have a cylindrical shape, and their outer peripheral surfaces are covered with a lining material having a predetermined thickness made of urethane rubber. A wire groove 12d is formed on the outer peripheral surface of each lining material (FIG. 3B).
　ワイヤとして外周面に多数の砥粒１１ｂが固着された固定砥粒ワイヤ１１ａを使用する。具体的には、直径０．１２ｍｍのワイヤに粒径１０～２５μｍのダイヤモンドからなる砥粒１１ｂを電着方式によるＮｉメッキにより固着させた固定砥粒ワイヤ１１ａを用いる。ワイヤ１１ａは、繰出し装置１３のボビン２０から導出され、供給側のガイドローラを介して、グルーブローラ１２Ａ、１２Ｂに架け渡される。その後、導出側のガイドローラを介して、巻取り装置１５のボビン２１に巻き取られる。ボビン２０、２１の各回転軸は、駆動モータ１６、１７の対応する出力軸にそれぞれ連結されている。
　各駆動モータ１６、１７を同期して駆動することで、一対の軸受１８に軸支された各ボビン２０、２１が、その軸線を中心として図３（ａ）における時計回り方向または反時計回り方向に回転して、ワイヤ１１ａが双方向へ走行する。 A fixed abrasive wire 11a having a large number of abrasive grains 11b fixed to the outer peripheral surface is used as the wire. Specifically, a fixed abrasive wire 11a in which abrasive grains 11b made of diamond having a particle diameter of 10 to 25 μm are fixed to a wire having a diameter of 0.12 mm by Ni plating by an electrodeposition method is used. The wire 11a is led out from the bobbin 20 of the feeding device 13, and is bridged over the groove rollers 12A and 12B via the supply-side guide roller. After that, it is wound around the bobbin 21 of the winding device 15 via the guide roller on the outlet side. The rotating shafts of the bobbins 20 and 21 are connected to corresponding output shafts of the drive motors 16 and 17, respectively.
By driving the drive motors 16 and 17 synchronously, the bobbins 20 and 21 pivotally supported by the pair of bearings 18 are rotated clockwise or counterclockwise in FIG. The wire 11a travels in both directions.
　図３（ａ）に示すように、多結晶シリコンインゴットＩのスライス時には、スラリーＳを１００リットル／分でスラリーノズル３０よりワイヤ列１１に供給しながら、駆動モータ１６により繰出し装置１３のボビン２０を回転させる。これにより、ワイヤ１１ａをグルーブローラ１２Ａ、１２Ｂに供給する。これと同時に、駆動モータ１７により巻取り装置１５のボビン２１を回転し、グルーブローラ１２Ａ、１２Ｂを介して、ワイヤ１１ａを巻き取る。
　その際、一定の周期で各ボビン２０、２１の回転方向を変更し、ワイヤ１１ａを双方向へ走行させる。具体的には、固定砥粒ワイヤ１１ａの前進量を２５０ｍ、固定砥粒ワイヤ１１ａの後退量を２４８ｍ、前進と後退とを変更するサイクル時間を４７．７秒とする。固定砥粒ワイヤ１１ａの送り速度は９００ｍ／分である。多結晶シリコンインゴットＩのスライス速度は７００μｍ／分とする。 As shown in FIG. 3A, when the polycrystalline silicon ingot I is sliced, the bobbin 20 of the feeding device 13 is driven by the drive motor 16 while supplying the slurry S from the slurry nozzle 30 to the wire row 11 at 100 liters / minute. Rotate. Thereby, the wire 11a is supplied to the groove rollers 12A and 12B. At the same time, the bobbin 21 of the winding device 15 is rotated by the drive motor 17 to wind the wire 11a via the groove rollers 12A and 12B.
At that time, the rotation direction of each of the bobbins 20 and 21 is changed at a constant cycle, and the wire 11a is caused to travel in both directions. Specifically, the advance amount of the fixed abrasive wire 11a is 250 m, the retreat amount of the fixed abrasive wire 11a is 248 m, and the cycle time for changing between advance and retreat is 47.7 seconds. The feed rate of the fixed abrasive wire 11a is 900 m / min. The slice speed of the polycrystalline silicon ingot I is 700 μm / min.
　ワイヤ列１１の往復走行中、上方から多結晶シリコンインゴットＩをワイヤ列１１へ押し付ける。これにより、多結晶シリコンインゴットＩが縦１５６ｍｍ、横１２０ｍｍの矩形状を有し、かつボロン濃度が１．４×１０１６ａｔｏｍｓ／ｃｍ３、比抵抗が１．０ｍΩ・ｃｍ（Ｐ形）の多数枚の太陽電池用シリコンウェーハＷにスライスされる。すなわち、ワイヤ列１１の往復走行時に、多数の砥粒１１ｂがワイヤ列１１の固定砥粒ワイヤ１１ａにより切断溝の底部に擦り付けられ、その底部が研削作用により徐々に削り取られる（図４）。
　その後、太陽電池用シリコンウェーハＷにＰＮ接合を形成し、ウェーハ表裏面に電極を形成して製造される。具体的には、ウェーハ表面にリン（Ｐ）を熱拡散させてＮ型拡散層を形成し、その後、太陽電池用シリコンウェーハＷの裏面にアルミニウムからなる裏面電極を形成するとともに、太陽電池用シリコンウェーハＷの表面に銀からなる表面電極を形成する。 During the reciprocating traveling of the wire row 11, the polycrystalline silicon ingot I is pressed against the wire row 11 from above. As a result, the polycrystalline silicon ingot I has a rectangular shape with a length of 156 mm and a width of 120 mm, a boron concentration of 1.4 × 10 16 atoms / cm 3 , and a specific resistance of 1.0 mΩ · cm (P-type). It slices into the silicon wafer W for solar cells. That is, during the reciprocating traveling of the wire row 11, a large number of abrasive grains 11b are rubbed against the bottom of the cutting groove by the fixed abrasive wire 11a of the wire row 11, and the bottom is gradually scraped off by a grinding action (FIG. 4).
Thereafter, a PN junction is formed on the silicon wafer W for solar cells, and electrodes are formed on the front and back surfaces of the wafer. Specifically, phosphorus (P) is thermally diffused on the wafer surface to form an N-type diffusion layer, and then a back electrode made of aluminum is formed on the back surface of the silicon wafer W for solar cells, and silicon for solar cells. A surface electrode made of silver is formed on the surface of the wafer W.
　このように、スライス時に、ワイヤ列１１を双方向へ往復走行させて多結晶シリコンインゴットＩをスライスするので、太陽電池用シリコンウェーハＷの表裏面の全域に、ワイヤ列１１を双方向へ走行させた際に現出する多数の同一方向（例えばＸ方向）に向かう直線状の微細な凹溝Ｗａが形成される。各凹溝Ｗａは、ピッチａが１０００μｍ、深さｂが５μｍ程度、面荒れｃが１μｍ程度のものである。その結果、太陽電池用シリコンウェーハＷの受光面積が拡大し、高い光電変換効率が得られる。しかも、従来法のようにスライスとは別の凹凸形成工程（例えばエッチング工程）が簡略化でき、シリコン系太陽電池の製造コストを低減させることができる。 As described above, since the polycrystalline silicon ingot I is sliced by reciprocating the wire row 11 in both directions at the time of slicing, the wire row 11 is caused to travel in both directions over the entire front and back surfaces of the silicon wafer W for solar cells. In this case, a large number of linear fine grooves Wa extending in the same direction (for example, the X direction) appear. Each concave groove Wa has a pitch a of about 1000 μm, a depth b of about 5 μm, and a surface roughness c of about 1 μm. As a result, the light receiving area of the solar cell silicon wafer W is expanded, and high photoelectric conversion efficiency is obtained. In addition, an unevenness forming step (for example, an etching step) different from slicing as in the conventional method can be simplified, and the manufacturing cost of the silicon-based solar cell can be reduced.
　また、固定砥粒ワイヤ１１ａを双方向へ走行させて太陽電池用シリコンインゴットＩをスライスするように構成したので、遊離砥粒を含むスラリーを用いてスライスする場合に比べて、太陽電池用シリコンウェーハＷの表裏面に、粗い直線状の凹溝Ｗａを形成させることができる。すなわち、凹溝Ｗａのピッチａが１０００μｍ程度、凹溝Ｗａの深さｂが５μｍ程度である。また、太陽電池用シリコンウェーハＷの面荒れｃの程度は、１μｍ程度である。 In addition, since the fixed abrasive wire 11a is moved in both directions to slice the solar cell silicon ingot I, the solar cell silicon wafer is compared with the case of slicing using slurry containing loose abrasive grains. A rough linear groove Wa can be formed on the front and back surfaces of W. That is, the pitch a of the concave grooves Wa is about 1000 μm, and the depth b of the concave grooves Wa is about 5 μm. Further, the degree of surface roughness c of the solar cell silicon wafer W is about 1 μm.
　さらに、固定砥粒ワイヤ１１ａを使用するので、遊離砥粒を含むスラリーを使用したスライス時に比べて、多結晶シリコンインゴットＩの切削効率が高い。そのため、多結晶シリコンインゴットＩのスライス速度が速まる。これは、固定砥粒ワイヤ１１ａを利用したスライスの場合、砥粒１１ｂと固定砥粒ワイヤ１１ａとが一体化し、スライス中の固定砥粒ワイヤ１１ａの移動速度と砥粒１１ｂの移動速度とが同一になるためである。その結果、多結晶シリコンインゴットＩの切断溝の全域（インゴットＩの一端から他端）にわたり、均一なスライス速度が得られ、多結晶シリコンインゴットＩの切削効率が高まる。これに対して、遊離砥粒１１ｄを利用したスライスの場合、遊離砥粒１１ｄとワイヤ１１ｃとが一体化していないので、スライス中のワイヤ１１ｃの移動速度（送り速度）より遊離砥粒１１ｄの移動速度が遅くなる。その結果、遊離砥粒１１ｄを利用したスライスの場合において、多結晶シリコンインゴットＩは、その切断開始端から切断終了端に向かって徐々に切断効率が低下していた（図５）。 Furthermore, since the fixed abrasive wire 11a is used, the cutting efficiency of the polycrystalline silicon ingot I is higher than when slicing using a slurry containing loose abrasive grains. Therefore, the slice speed of the polycrystalline silicon ingot I is increased. In the case of slicing using the fixed abrasive wire 11a, the abrasive grains 11b and the fixed abrasive wire 11a are integrated, and the moving speed of the fixed abrasive wire 11a and the moving speed of the abrasive grains 11b in the slice are the same. Because it becomes. As a result, a uniform slicing speed is obtained over the entire cutting groove of the polycrystalline silicon ingot I (from one end to the other end of the ingot I), and the cutting efficiency of the polycrystalline silicon ingot I is increased. On the other hand, in the case of the slice using the free abrasive grains 11d, the free abrasive grains 11d and the wire 11c are not integrated, and therefore the movement of the free abrasive grains 11d from the moving speed (feed speed) of the wire 11c in the slice. The speed is slow. As a result, in the case of slicing using the free abrasive grains 11d, the cutting efficiency of the polycrystalline silicon ingot I gradually decreased from the cutting start end toward the cutting end end (FIG. 5).
　実際に、実施例１の太陽電池用シリコンウェーハの製造方法に則り、太陽電池用シリコンインゴットをスライスした。そのときの太陽電池用シリコンウェーハの表面粗さのデータを図６に示す。
　測定には、株式会社東京精密製の接触式粗さ計（Ｓｕｒｆｃｏｍ　１３０Ａ）を使用した。測定条件としては、測定長が５ｍｍ、測定速度が０．３ｍｍ／ｓ、ＣｕｔＯＦＦが０．８ｍｍとした。
　図６のグラフ中のウェーハ表面粗さプロファイルから明らかなように、遊離砥粒を含むスラリーを使用し、ワイヤを一方向へ走行させる従来法（表面粗さＲｍａｘ５μｍ程度）に比べて、太陽電池用シリコンウェーハＷの表裏面に、粗い直線状の凹溝Ｗａが現出した。 Actually, the solar cell silicon ingot was sliced in accordance with the method for producing the solar cell silicon wafer of Example 1. The data of the surface roughness of the silicon wafer for solar cells at that time are shown in FIG.
For the measurement, a contact roughness meter (Surfcom 130A) manufactured by Tokyo Seimitsu Co., Ltd. was used. As measurement conditions, the measurement length was 5 mm, the measurement speed was 0.3 mm / s, and CutOFF was 0.8 mm.
As apparent from the wafer surface roughness profile in the graph of FIG. 6, compared with the conventional method (surface roughness Rmax of about 5 μm) using a slurry containing loose abrasive grains and running the wire in one direction, it is for solar cells. On the front and back surfaces of the silicon wafer W, rough linear grooves Wa appeared.
　また、同一の太陽電池用シリコンインゴットをスライスして得られた別の太陽電池用シリコンウェーハについて、Ｋｅｙｅｎｃｅ　ＶＫ８５００を用いて、ウェーハ表面のサイト的な粗さ分布を測定した結果を図７に示す。図７ａは倍率が２００倍、図７ｂは１０００倍である。これらの立体的な分布図から明らかなように、低倍率（２００倍）での観察では、ワイヤ双方向の大きな凹溝（ワイヤの走行跡）を確認することができた。一方、高倍率（１０００倍）では、直線的な凹溝（固定砥粒による研削跡）を確認することができた。
　比較のため、固定砥粒ワイヤに代えて一般的なワイヤ（直径１６０μｍ、高張力鋼鉄線製）を使用し、同様に太陽電池用シリコンインゴットを切断して得られた太陽電池用シリコンウェーハについて、Ｋｅｙｅｎｃｅ　ＶＫ８５００を使用し、その太陽電池用シリコンウェーハの表面粗さを測定した結果を図８に示す。なお、ここでは無砥粒のスラリーに代えて、ラッピングオイル１００リットルに対して、平均粒度７～８μｍの遊離砥粒（ＧＣ砥粒）が１１０ｋｇ混入されたスラリーを使用する。ウェーハ表面のサイトの倍率は、図８ａが２００倍、図８ｂが１０００倍である。これらの立体的な分布図から明らかなように、低倍率、高倍率の何れにおいても、太陽電池用シリコンウェーハの表面には直線的な凹溝（凹凸）は確認できなかった。 Moreover, about the silicon wafer for solar cells obtained by slicing the same silicon ingot for solar cells, the result of having measured the site-like roughness distribution of the wafer surface using Keyence VK8500 is shown in FIG. FIG. 7a shows a magnification of 200 times and FIG. As is clear from these three-dimensional distribution diagrams, a large concave groove (wire running trace) in both directions of the wire could be confirmed in the observation at a low magnification (200 times). On the other hand, at a high magnification (1000 times), it was possible to confirm a linear concave groove (a grinding mark by fixed abrasive grains).
For comparison, using a general wire (diameter 160 μm, made of high-tensile steel wire) instead of a fixed abrasive wire, and similarly for a solar cell silicon wafer obtained by cutting a solar cell silicon ingot, FIG. 8 shows the results of measuring the surface roughness of the silicon wafer for solar cells using Keyence VK8500. Here, instead of the non-abrasive slurry, a slurry in which 110 kg of free abrasive grains (GC abrasive grains) having an average particle size of 7 to 8 μm are mixed with 100 liters of wrapping oil is used. The magnification of the site on the wafer surface is 200 times in FIG. 8a and 1000 times in FIG. 8b. As is apparent from these three-dimensional distribution diagrams, no linear grooves (irregularities) were observed on the surface of the solar cell silicon wafer at both low magnification and high magnification.
　この発明は、例えば発電用の太陽電池用シリコンウェーハに有用である。 This invention is useful, for example, for silicon wafers for solar cells for power generation.

Claims (3)

  1. 　ＰＮ接合および電極が形成されてシリコン系太陽電池に加工される太陽電池用シリコンウェーハにおいて、
    　スライス加工の際に現出したソーマークである多数の同一方向に向かう直線状の凹溝が、表裏面に形成された太陽電池用シリコンウェーハ。     In a silicon wafer for solar cells in which a PN junction and an electrode are formed and processed into a silicon-based solar cell,
    A silicon wafer for solar cells in which a large number of linear concave grooves directed in the same direction, which are saw marks appearing during slicing, are formed on the front and back surfaces.
  2. 　前記凹溝は、ピッチが０．１～５ｍｍで、深さが１～５０μｍである請求項１に記載の太陽電池用シリコンウェーハ。 2. The silicon wafer for a solar cell according to claim 1, wherein the groove has a pitch of 0.1 to 5 mm and a depth of 1 to 50 μm.
  3. 　ワイヤソーの複数本のグルーブローラ間で走行中のワイヤ列に、スラリーを供給しながら太陽電池用シリコンインゴットを押し付け、多数枚の太陽電池用シリコンウェーハをスライスにより得る太陽電池用シリコンウェーハの製造方法において、
    　前記ワイヤ列を構成するワイヤとして、外周面に砥粒が固定された固定砥粒ワイヤを使用し、
    　前記ワイヤ列を双方向に走行させながら前記太陽電池用シリコンインゴットをスライスすることで、前記各太陽電池用シリコンウェーハの表裏面に、ソーマークである多数の同一方向に向かう直線状の凹溝を形成する太陽電池用シリコンウェーハの製造方法。     In a method for producing a silicon wafer for a solar cell, a silicon cell ingot for a solar cell is pressed against a wire row traveling between a plurality of groove rollers of a wire saw while supplying slurry, and a plurality of silicon wafers for a solar cell are obtained by slicing. ,
    As a wire constituting the wire row, a fixed abrasive wire in which abrasive grains are fixed to the outer peripheral surface is used,
    By slicing the solar cell silicon ingot while running the wire row in both directions, a large number of linear concave grooves directed in the same direction as saw marks are formed on the front and back surfaces of each solar cell silicon wafer. Manufacturing method of silicon wafer for solar cell.
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