JP6047424B2 - Method for producing silicon ingot - Google Patents

Description

  The present invention relates to a method for manufacturing a silicon ingot.

  A solar cell element is an element that converts light energy into electrical energy. A silicon substrate is widely used as a semiconductor substrate in solar cell elements. This silicon substrate is obtained by slicing a single crystal or polycrystalline silicon ingot to a desired thickness by a wire saw device or the like.

  By the way, as a method for producing a single crystal silicon ingot, for example, the Czochralski (CZ) method and the floating zone (FZ) method are known. As a method for producing a polycrystalline silicon ingot, for example, a casting method or the like is known. It is generally known that a polycrystalline silicon ingot can be more easily produced than a single crystal silicon ingot. However, the photoelectric conversion efficiency in a solar cell element in which a polycrystalline silicon substrate is used as a semiconductor substrate is a solar cell in which a single crystal silicon substrate is used due to the presence of crystal defects such as crystal grain boundaries. It is generally known that the photoelectric conversion efficiency of the device is lower.

  In view of this, there has been proposed a method of manufacturing a silicon ingot with few defects such as crystal grain boundaries and having a uniform crystal orientation by using a casting method (see, for example, Patent Documents 1 and 2 below). In this manufacturing method, in a state where single crystal silicon as a seed crystal is arranged on the bottom surface in the mold, a silicon melt is injected into the mold, so that the silicon melt is unidirectional from the seed crystal as a starting point. A solidifying method (also referred to as a seed casting method) is employed.

Japanese Patent Laid-Open No. 10-194718 JP-T 2009-523893

  However, in the seed cast method, when the silicon melt is injected into the mold, the high temperature silicon melt suddenly comes into contact with the seed crystal disposed on the bottom surface in the mold, thereby causing local thermal expansion or the like. Therefore, the seed crystal is easily broken. Further, the silicon melt is kept in intensive contact with a part of the seed crystal disposed on the bottom surface in the mold, so that the seed crystal is likely to be locally melted. In this case, when the cracking and melting of the seed crystal reach the bottom surface of the mold, crystal growth not starting from the seed crystal can be started in a portion corresponding to a part of the bottom portion of the silicon ingot. Further, in the initial stage where the silicon melt is injected into the mold, the silicon melt is likely to be scattered near the outer peripheral portion on the upper surface of the seed crystal. At this time, the silicon melt scattered in the vicinity of the outer peripheral portion is easily solidified in a form having a random crystal orientation without being affected by the crystal orientation of the seed crystal by rapid cooling. As a result, crystal growth that does not start from the seed crystal may further occur. That is, in the seed cast method, the crystallinity of the silicon ingot tends to be lowered.

  Therefore, a manufacturing method that can easily manufacture a high-quality silicon ingot having a uniform crystal orientation is desired.

In order to solve the above-described problem, a method for manufacturing a silicon ingot according to one aspect includes a plate-like single crystal silicon seed crystal disposed on a bottom surface in a mold, and a solid silicon mass is formed on the seed crystal. A first step of disposing, a second step of melting the silicon mass and covering an upper surface of the seed crystal with a first silicon melt, and an upper surface of the seed crystal being covered with the first silicon melt. A third step of cooling the first silicon melt from the bottom surface side of the mold and injecting the second silicon melt into the mold; and the first silicon melt and the the third silicon melt second silicon melt is mixed, have a fourth step for causing the unidirectional solidification directed upwardly from the bottom portion of the mold, in the third step, the Injection of the second silicon melt into the mold Before starting, cooling of the first silicon melt in the mold starts, so that the carbon contained in the first silicon melt is precipitated in the form of silicon carbide, and the mold An injection step of reducing the carbon concentration in the first silicon melt in the mold is sequentially performed by injecting the second silicon melt into the mold .

  According to the above-described method for manufacturing a silicon ingot, the high-temperature second silicon melt does not intensively contact a part of the seed crystal disposed in the mold, and the second silicon melt has a peripheral edge on the upper surface of the seed crystal. Therefore, crystal growth that does not start from the seed crystal is difficult to occur. Therefore, a high-quality silicon ingot having a uniform crystal orientation can be easily produced.

It is sectional drawing which shows typically the example of 1 structure of the manufacturing apparatus of a silicon ingot. It is sectional drawing which shows an example of operation | movement of a cooling plate typically. It is a flowchart which shows the manufacturing process of a silicon ingot. It is sectional drawing which shows typically the state by which the mold release material was apply | coated to the inner wall of a casting_mold | template. It is sectional drawing which shows typically the state by which the circular seed crystal was distribute | arranged to the bottom part of the casting_mold | template. It is a top view which shows typically the state by which the circular seed crystal was distribute | arranged to the bottom part of the casting_mold | template. It is sectional drawing which shows typically the state by which the several seed crystal was arranged in the bottom part of the casting_mold | template. It is a top view which shows typically the state by which the several seed crystal was arranged in the bottom part of a casting_mold | template. It is sectional drawing which shows typically the state with which the silicon lump was filled on the seed crystal. It is sectional drawing which shows typically the state with which the silicon lump was filled in the crucible. It is sectional drawing which shows typically the state by which the silicon lump in a casting_mold | template was fuse | melted. It is sectional drawing which shows typically the state by which the cooling plate was earth | grounded by the mold holding part. It is sectional drawing which shows typically the state by which the silicon lump in a crucible was fuse | melted. It is sectional drawing which shows typically a mode that silicon melt is poured into a casting_mold | template from a crucible. It is sectional drawing which shows typically a mode that a silicon melt solidifies to one direction. It is sectional drawing which shows the cross section of a silicon ingot typically. It is a figure which illustrates distribution of the normalized specific resistance ((rho) b value) according to the density | concentration distribution of boron. It is a figure which illustrates distribution of the normalized specific resistance ((rho) b value) according to the density | concentration distribution of boron. It is a figure which shows typically an example of the solidification form of a silicon melt. It is a figure which shows the relationship between the dimension of a seed crystal, the residual condition of a seed crystal, and the evaluation result concerning the formation state of a single crystal region. It is a top view which shows typically the external appearance of the light-receiving surface of a photoelectric conversion element. It is a top view which shows typically the external appearance of the non-light-receiving surface of a photoelectric conversion element. It is a figure which shows the XZ cross section in the position shown with the dashed-dotted line XXIII-XXIII in FIG. 21 and FIG. It is a figure which shows typically the position where silicon carbide precipitates. It is a figure which shows typically the area | region where a strain relaxation layer arises.

  Hereinafter, an embodiment and a modification of the present invention will be described with reference to the drawings. In the drawings, parts having the same configuration and function are denoted by the same reference numerals, and redundant description is omitted in the following description. Further, the drawings are schematically shown, and the sizes and positional relationships of various structures in each drawing can be appropriately changed. 1, 2, 4 to 16, 19, 24, and 25, a right-handed system in which the upward direction of the silicon ingot manufacturing apparatus (the upward direction in the drawing of FIG. 1) is the + Z direction is shown. An XYZ coordinate system is attached. In FIGS. 2 and 11 to 15, the movement of the cooling plate 123 is indicated by black arrows, the movement of heat is indicated by white arrows, and the application of heat by heating is shaded. This is indicated by the hatched arrows. Further, in FIGS. 15 and 19, a state in which the unidirectional solidification of the third silicon melt MS <b> 3 proceeds is indicated by a thick dashed arrow.

<(1) One Embodiment>
<(1-1) Silicon Ingot Manufacturing Equipment>
As shown in FIG. 1, the manufacturing apparatus 100 for manufacturing a silicon ingot includes an upper unit 110, a lower unit 120, and a control unit 130.

  The upper unit 110 includes a crucible 111, an upper heater H1u, and a side heater H1s. The lower unit 120 includes a mold 121, a mold holding part 122, a cooling plate 123, a rotating shaft 124, an upper heater H2u, a lower heater H2l, and temperature measuring parts CHA and CHB. The material of the crucible 111 and the mold 121 may be any material as long as it does not easily melt, deform and decompose at a temperature equal to or higher than the melting point of silicon, and does not easily react with silicon and has reduced impurities.

  The crucible 111 includes a main body 111b, an upper opening 111uo, an internal space 111i, and a lower opening 111bo. The main body 111b has a generally cylindrical shape with a bottom. The material of the crucible 111 may be, for example, quartz glass. The upper heater H1u is arranged in an annular shape in plan view just above the upper opening 111uo. The side heater H1s is arranged in an annular shape in plan view so as to surround the main body 111b from the side.

  Here, at the time of manufacturing the silicon ingot, the internal space 111i of the crucible 111 is filled with, for example, a plurality of silicon lumps (also referred to as silicon lump) in a solid state as a raw material of the silicon ingot from the upper opening 111uo. In addition, this silicon lump may contain the thing of a powder state. The silicon lump filled in the internal space 111i is melted by heating by the upper heater H1u and the side heater H1s. For example, the silicon lump provided on the lower opening 111bo is heated and melted, so that the molten silicon (also referred to as silicon melt) in the internal space 111i passes through the lower opening 111bo to the lower unit. It is poured toward 120 molds 121. In addition, the lower opening 111bo is not provided in the crucible 111, and the crucible 111 may be inclined so that the silicon melt is poured from the crucible 111 toward the mold 121.

  The mold 121 has a bottomed cylindrical shape as a whole. Specifically, the mold 121 includes a bottom part 121b, a side wall part 121s, an internal space 121i, and an upper opening part 121o arranged on the upper part of the mold 121. For example, the bottom 121b and the upper opening 121o may have a substantially square shape. And one side of bottom 121b and upper opening 121o should just be 300 mm or more and about 800 mm or less, for example. Upper opening 121o accepts injection of silicon melt from crucible 111 into internal space 121i. Here, as a material of the side wall part 121s and the bottom part 121b, for example, silica or the like may be employed. The side wall 121s may be formed by being combined with, for example, a carbon fiber reinforced carbon composite material or the like, and a felt or the like as a heat insulating material.

  Further, as shown in FIG. 1, the upper heater H <b> 2 u is arranged in an annular shape directly above the upper opening 121 o of the mold 121. The lower heater H2l is arranged in an annular shape so as to surround a portion from the lower part to the upper part in the + Z direction of the side wall part 121s of the mold 121 from the side. The lower heater H2l may be divided into a plurality of regions, and each region may be configured to be temperature controlled independently.

  The mold holding unit 122 holds the mold 121 from below and is in close contact with the lower surface of the mold 121. The material of the mold holding part 122 may be a material having high heat conductivity such as graphite, and may further include a heat insulating part (not shown) between the side wall part 121s of the mold 121. Thereby, heat can be preferentially transmitted from the bottom 121b to the cooling plate 123 rather than the side wall 121s. The material of the heat insulating portion may be, for example, felt.

  As shown in FIG. 2, the cooling plate 123 rises by the rotation of the rotating shaft 124 and contacts the lower surface of the mold holding unit 122. The contact of the cooling plate 123 with the lower surface of the mold holding unit 122 is also referred to as grounding. For example, the cooling plate 123 may have a structure in which water or gas circulates inside a hollow metal plate or the like. When the silicon ingot is manufactured, the cooling plate 123 comes into contact with the lower surface of the mold holding part 122 in a state where the internal space 121i is filled with the silicon melt MS, thereby cooling the silicon ingot from the silicon melt MS via the mold holding part 122. Heat removal is performed so that heat is transmitted to the plate 123. That is, the silicon melt MS is cooled by the cooling plate 123 from the bottom 121b side.

  The temperature measuring parts CHA and CHB are parts for measuring the temperature. The temperature measuring units CHA and CHB can measure the temperature with a thermocouple or the like covered with a thin tube made of alumina, for example. In the temperature detection unit (not shown), the voltage generated in each temperature measuring unit CHA and CHB. A temperature corresponding to is detected.

  Here, the temperature measuring part CHA is arranged in the vicinity of the lower heater H2l. The temperature measuring part CHB is arranged near the lower surface of the central part of the bottom 121b of the mold 121.

  The control unit 130 is a part that controls the entire manufacturing apparatus 100. The control unit 130 includes, for example, a processor, a memory, a storage unit, and the like, and may be any unit that can perform various controls by executing a program stored in the storage unit by the processor. For example, the controller 130 controls the outputs of the upper heaters H1u and H2u, the side heater H1s, and the lower heater H2l. In addition, in the control part 130, the output of each heater H1s, H1u, H2l, H2u is controlled according to at least one of the temperature obtained by each temperature measuring part CHA, CHB, and the passage of time, for example.

<(1-2) Manufacturing method of silicon ingot>
Next, a method for manufacturing a silicon ingot using the manufacturing apparatus 100 will be described. As shown in FIG. 3, in the method for manufacturing a silicon ingot according to this embodiment, the first to fourth steps of Steps Sp1 to Sp4 are performed in order, so that a high-quality silicon ingot with a uniform crystal orientation can be easily obtained. Manufactured. In the present embodiment, in the first process of step Sp1, four processes of steps Sp11 to Sp14 are performed in order, and in the third process of step Sp3, three processes of steps Sp31 to Sp33 are performed in order. 4 to 15 schematically show at least one state of the mold 121 and the crucible 111 in each step.

<(1-2-1) First step>
In the first step of Step Sp1, a plate-like single crystal silicon seed crystal 200s is disposed on the bottom surface in the mold 121, and a silicon block PS1 that is solid silicon is disposed on the seed crystal 200s. . Here, four steps of Steps Sp11 to Sp14 performed in order in the first step will be described.

  In Step Sp11, as shown in FIG. 4, a release material layer Mr1 is formed by applying a release material to the inner wall surface of the mold 121. Due to the presence of the release material layer Mr1, fusion of the silicon ingot to the inner wall of the mold 121 in the process of solidifying the first silicon melt MS1 is reduced. As the material of the release material layer Mr1, for example, silicon nitride, silicon carbide, silicon oxide, or the like can be employed. The release material layer Mr1 can be formed, for example, by coating a slurry containing one or more of silicon nitride, silicon carbide, and silicon oxide on the inner wall of the mold 121 by coating or spraying. Here, the slurry is, for example, in a solution in which powder of any one or a mixture of two or more of silicon nitride, silicon carbide, and silicon oxide mainly includes an organic binder such as PVA (polyvinyl alcohol) and a solvent. It can be formed by stirring the mixture.

  In step Sp12, as shown in FIGS. 5 and 6, seed crystal 200s is arranged on the inner wall surface of bottom 121b as the bottom of mold 121. Here, for example, when the release material layer Mr1 formed on the inner wall surface of the mold 121 in Step Sp11 is dried, the seed crystal 200s may be attached to the release material layer Mr1. In the present embodiment, plate-like single crystal silicon having a first main surface and a second main surface located on the opposite side of the first main surface is employed as the seed crystal 200s.

  Here, the plane orientation on the upper surface of the seed crystal 200s may be, for example, (100) in the Miller index. In this case, the seed crystal 200s can be easily manufactured, and the speed of crystal growth can be improved when unidirectional solidification of the third silicon melt MS3 described later is performed. The shape of the seed crystal 200s may be a circular shape when viewed in plan, as shown in FIG. 6, for example. In this case, the diameter of the seed crystal 200s may be about 180 mm or more and about 300 mm or less, for example. Further, the thickness of the seed crystal 200s is set to a thickness that does not melt to the bottom when the second silicon melt MS2 is injected into the mold 121. The thickness of the seed crystal 200s may be, for example, 20 mm or more and about 40 mm or less.

  The shape of the surface of the seed crystal 200s may be, for example, a substantially square shape. In this case, each corner may be chamfered. Further, in consideration of improvement of casting efficiency due to enlargement of the bottom area of the silicon ingot and difficulty of enlargement of the seed crystal 200s, as shown in FIGS. A plurality of seed crystals 200 s may be disposed on the inner wall surface of the bottom 121 b of the base plate. In this case, one side of the various crystals 200s may be, for example, about 150 mm or more and about 200 mm or less. For example, as shown in FIG. 7 and FIG. 8, a mode in which four seed crystals 200 s having a substantially square board surface are spread on the bottom 121 b of the mold 121 so as to form a substantially square shape can be employed. In this case, for example, a mode in which four seed crystals 200 s are arranged so that the gap GA <b> 2 is formed substantially at the center on the bottom 121 b of the mold 121 can be adopted. If such a gap GA2 is provided, a region that relaxes strain in a region located directly above the gap GA2 (also referred to as a strain relaxation region) when unidirectional solidification of a third silicon melt MS3 described later is performed. This contributes to improving the crystallinity of the silicon ingot.

  By the way, the seed crystal 200s may be arranged so that, for example, the outer peripheral portion of the seed crystal 200s and the side surface portion of the inner wall as the inner peripheral side surface portion of the mold 121 are separated from each other with the gap GA1. As a result, when a third silicon melt MS3, which will be described later, is solidified, the boundary Bd2 between the third silicon melt MS3 and the solid silicon can have a convex shape that protrudes upward. Due to the solidification mode of the third silicon melt MS3, impurities in the third silicon melt MS3 are discharged in the vicinity of the inner peripheral side surface portion of the mold 121, and in a wide range centering on the central portion of the silicon ingot. The occurrence of dislocations and defects due to the precipitation of the compound can be reduced. As a result, the crystallinity of the silicon ingot can be improved.

Specifically, for example, the bottom area Sa (unit: mm ² ) in the mold 121, the area of the upper surface of the seed crystal 200s (also referred to as the upper area) Sb (unit: mm ² ), and the thickness of the seed crystal 200s. d (unit: mm) may satisfy the relationship of the formulas [I] and [II]. The bottom area Sa is the area of the inner wall surface of the bottom 121b in the mold 121. When a plurality of seed crystals 200s are spread on the bottom 121b of the mold 121, the upper area Sb of the seed crystals 200s may be the total area Sb of the upper surfaces of the plurality of seed crystals 200s. That is, the seed crystal 200s satisfying the formula [II] may be disposed in the mold 121 satisfying the formula [I].

0.21 ≦ Sb / Sa <1 [I]
Sb ^1/2 /d≦9.8 (II).

  Here, as shown by the formula [I], if the value Sb / Sa obtained by dividing the top area Sb of the seed crystal 200s by the bottom area Sa in the mold 121 is less than 1, the outer circumference of the seed crystal 200s The part and the side part of the inner wall of the mold 121 may be separated from each other. In addition, if Sb / Sa is 0.21 or more, a single crystal region (also referred to as a single crystal region) having a sufficiently large area in the XY section is obtained by solidification of a third silicon melt MS3 in a mold 121 described later. A silicon ingot can be produced. Note that if the upper area Sb of the seed crystal 200s is small, the manufacturing cost of the seed crystal 200s can be reduced. On the other hand, if the upper area Sb of the seed crystal 200s is large, the single crystal region in the silicon ingot can be expanded. That is, the crystallinity of the silicon ingot can be improved.

As shown in [II] above, if the value Sb ^1/2 / d obtained by dividing the square root of the upper area Sb of the seed crystal 200s by the thickness d of the seed crystal 200s is 9.8 or less, the seed crystal 200s is not an excessively thin shape. As a result, when the silicon block PS1 disposed in the mold 121, which will be described later, is melted and the second silicon melt MS2 is injected into the mold 121, melting variation in the seed crystal 200s hardly occurs. As a result, the single crystal region in the silicon ingot can be expanded. That is, the crystallinity of the silicon ingot can be improved.

  In step Sp13, as shown in FIG. 9, the solid silicon block PS1 is arranged on the seed crystal 200s arranged in step Sp12. Here, it suffices if the lower region in the mold 121 is filled with the silicon block PS1. Here, the silicon lump PS1 may be, for example, a lump of polysilicon as a part of the raw material of the silicon ingot. The polysilicon lump may be a relatively fine block-like lump of silicon, for example. In addition, the total weight of the seed crystal 200s and the silicon lump PS1 may be about 3 kg or more and about 20 kg or less, for example.

  In Step Sp14, as shown in FIG. 10, the silicon block PS2 is introduced into the internal space 111i of the crucible 111. Here, it suffices if silicon block PS2 is filled from the lower region in crucible 111 toward the upper region. At this time, for example, an element serving as a dopant in the silicon ingot may be mixed with the silicon lump PS2. Here, the silicon lump PS2 may be, for example, a lump of polysilicon as a raw material of a silicon ingot. The polysilicon lump may be a relatively fine block-like lump of silicon, for example. When a p-type silicon ingot is manufactured, the dopant element may be, for example, boron and gallium. When an n-type silicon ingot is manufactured, the element serving as a dopant may be, for example, phosphorus. Further, here, the silicon lump PS3 is filled so as to cover the lower opening 111bo of the crucible 111. As a result, the path from the internal space 111i to the lower opening 111bo is closed.

  In addition, before the 2nd process is started, it sets to the state by which the cooling plate 123 is not earth | grounded on the lower surface of the casting_mold | template holding | maintenance part 122. FIG.

<(1-2-2) Second step>
In the second step of step Sp2, as shown in FIG. 11, the silicon lump PS1 disposed in the mold 121 is melted, and the upper surface of the seed crystal 200s is covered with the molten first silicon melt MS1. Set to state. Here, since the silicon lump PS1 on the seed crystal 200s is melted earlier than the seed crystal 200s, the entire upper surface of the seed crystal 200s is likely to be heated substantially uniformly, and the surface layer portion on the upper surface side of the seed crystal 200s is approximately. It can melt uniformly.

  Specifically, here, for example, by the upper heater H2u and the lower heater H2l disposed above and to the side of the mold 121, the temperature range of 1414 ° C. or higher and 1500 ° C. or lower where the silicon mass PS1 is higher than the melting point. It may be heated up to. At this time, for example, the output of the upper heater H2u is higher than the output of the lower heater H2l, so that the silicon block PS1 may be preferentially melted over the seed crystal 200s. Since the seed crystal 200s is in close contact with the bottom 121b of the mold 121, it remains without being dissolved by heat transfer from the seed crystal 200s to the bottom 121b. At this time, a part of the silicon lump PS3 covering the lower opening 111bo of the crucible 111 may be melted by the heating by the upper heater H2u and dropped onto the mold 121.

<(1-2-3) Third step>
In the third step of step Sp3, in the state where the upper surface of the seed crystal 200s is covered with the first silicon melt MS1, the first silicon melt MS1 is cooled from the bottom surface side of the mold 121 and into the mold 121. The second silicon melt MS2 is injected. Here, three steps of Steps Sp31 to Sp33 performed in order in the third step will be described.

  In step Sp31, as shown in FIG. 12, the cooling plate 123 is grounded to the lower surface of the mold holder 122. Thereby, heat removal from the first silicon melt MS1 to the cooling plate 123 via the mold holding unit 122 is started. That is, in the state where the upper surface of the seed crystal 200s is covered with the first silicon melt MS1, the cooling of the first silicon melt MS1 from the bottom 121b side by the cooling plate 123 is started. Just before the cooling plate 123 is grounded to the lower surface of the mold holding part 122, for example, the outputs of the upper heater H2u and the lower heater H2l may be reduced. By the grounding of the cooling plate 123 to the lower surface of the mold holding part 122, the first silicon melt MS1 may start to be slightly solidified in the vicinity of the upper surface of the seed crystal 200s.

  Here, the timing at which the cooling plate 123 is grounded on the lower surface of the mold holding unit 122 (also referred to as a grounding timing) is, for example, the timing at which a first predetermined time set in advance has elapsed since the start of heating in the second step. I just need it. Note that the contact timing may be, for example, a timing at which a preset second predetermined time has elapsed from the start of melting of the silicon block PS1. The start of melting of the silicon lump PS1 can be visually recognized through a peephole provided in the manufacturing apparatus 100, for example. Further, the grounding timing may be controlled according to the temperature detected by the temperature measuring unit in the manufacturing apparatus 100 such as the temperature measuring units CHA and CHB, for example.

  In step Sp32, as shown in FIG. 13, the silicon lump PS2 disposed in the crucible 111 is melted by heating, and the second silicon melt MS2 is stored in the crucible 111. Here, for example, by the upper heater H1u and the side heater H1s arranged above and to the side of the crucible 111, the silicon mass PS2 is heated to a temperature range of 1414 ° C. or higher exceeding the melting point and 1500 ° C. or lower, The second silicon melt MS2 may be used. The timing at which the silicon mass PS2 in the crucible 111 starts to melt may be immediately after the grounding timing, for example.

  In step Sp33, as shown in FIG. 14, the silicon lump PS3 covering the lower opening 111bo of the crucible 111 is heated to melt the silicon lump PS3. A heater or the like may be used as the heating means. Thereby, the path | route from the internal space 111i in the crucible 111 to the lower opening part 111bo will be in the state opened. As a result, the second silicon melt MS2 in the crucible 111 is poured into the mold 121 through the lower opening 111bo. Therefore, the second silicon melt MS2 is injected into the mold 121 in a state where the upper surface of the seed crystal 200s is covered with the first silicon melt MS1. Thereby, in the mold 121, the first silicon melt MS1 and the second silicon melt MS2 are mixed to become the third silicon melt MS3.

  At this time, the second silicon melt MS2 does not intensively contact with a part of the seed crystal 200s arranged on the bottom surface in the mold 121. For this reason, the generation | occurrence | production of the malfunction that the crack of the seed crystal 200s resulting from local thermal expansion etc. and the fusion | melting of the seed crystal 200s arrives on the bottom part 121b of the casting_mold | template 121 can be reduced. In addition, at the initial stage when the second silicon melt MS2 is injected into the mold 121, the second silicon melt MS2 hardly scatters in the vicinity of the outer peripheral portion on the upper surface of the seed crystal 200s. This makes it difficult for silicon to grow without starting from the seed crystal 200s. That is, the crystallinity of the silicon ingot is easily improved.

  Here, the timing at which the heating of the silicon block PS3 is started may be, for example, a timing at which a preset third predetermined time has elapsed since the heating start time in the second step.

<(1-2-4) Fourth step>
In the fourth step of Step Sp4, as shown in FIG. 15, the third silicon melt MS3 in which the first silicon melt MS1 and the second silicon melt MS2 in the mold 121 are mixed is cooled from the bottom surface side. Is done. As a result, unidirectional solidification (also referred to as unidirectional solidification) is performed upward from the bottom surface side of the third silicon melt MS3. That is, unidirectional solidification of the third silicon melt MS3 in which the first silicon melt MS1 and the second silicon melt MS2 are mixed is performed.

  At this time, for example, the outputs of the upper heater H2u and the lower heater H2l arranged above and on the side of the mold 121 are controlled according to the temperature detected by the temperature measuring units CHA, CHB, etc. in the manufacturing apparatus 100. . Here, the temperature in the vicinity of the upper heater H2u and the lower heater H2l may be maintained, for example, in the vicinity of the melting point of silicon. Thus, silicon crystal growth from the side of the mold 121 is difficult to occur, and single crystal silicon crystal growth in the + Z direction as the upper side is likely to occur. At this time, for example, using the divided lower heater H2l, the third silicon melt MS3 is heated by the upper heater H2u and a part of the divided lower heater H2l, and the divided lower heater H2l In the other part, the third silicon melt MS3 may not be heated.

  And the silicon ingot is manufactured in the mold 121 by the unidirectional solidification of the third silicon melt MS3 proceeding slowly.

<Specific example of (1-3) silicon ingot>
FIG. 16 illustrates a cross-sectional view schematically showing a cross-sectional photograph of a silicon ingot manufactured by the manufacturing method according to this embodiment described above.

  Here, a seed crystal 200s having a substantially circular disk surface with a diameter of 222 mm and a thickness of 30 mm was used, and a p-type silicon ingot was manufactured using boron as an element serving as a dopant. Then, a silicon plate was formed by cutting a substantially central portion in the Y direction of the silicon ingot along the XZ plane with a band saw. Thereafter, this silicon plate was subjected to mirror finishing etching (also called mirror etching). Further, etching (also referred to as selective etching) for revealing crystal defects was performed on the silicon plate subjected to mirror etching. Thereafter, the board surface of the silicon plate was photographed to obtain a cross-sectional photograph as shown in FIG.

  In the mirror etching, the silicon plate was sequentially immersed in a hydrofluoric acid solution for 180 seconds, washed with water, immersed in a hydrofluoric acid aqueous solution for 30 seconds, washed with water, and dried. Here, the hydrofluoric acid solution was generated by mixing 70% nitric acid and 50% hydrofluoric acid in a ratio of 7: 2. The hydrofluoric acid aqueous solution was produced by mixing pure water and 50% hydrofluoric acid in a ratio of 20: 1. In the selective etching, a selective etching solution described in JIS standard H0609, that is, 70% nitric acid, 99% acetic acid, 50% hydrofluoric acid, and water is applied to the silicon plate at 1: 12.7: 3: 3. 5 minutes immersion in a solution mixed at a ratio of .7, washing with water and drying were sequentially performed.

  As shown in FIG. 16, a single crystal region (also referred to as a single crystal region) A1sc having a width Wd1 was formed on a region (also referred to as a seed crystal region) A1sd where the seed crystal 200s remained. The width Wd1 was 217 mm. In addition, a polycrystalline region (also referred to as a polycrystalline region) A1p having relatively high dislocations and defects was formed on the sides of the seed crystal region A1sd and the single crystal region A1sc. Here, the boundary Bd1 between the seed crystal region A1sd and the single crystal region A1sc was confirmed by an apparent change in the density (EPD) of etch pits obtained by visual identification and observation with a microscope. Note that the apparent change in EPD was presumed to be due to some dislocations generated in the seed crystal 200s by heating. In FIG. 16, a broken line is attached along the boundary Bd1. The single crystal region A1sc was confirmed by analyzing the crystal orientation by EBSD (electron backscattering diffraction method).

  Further, the thickness HT1 in the + Z direction of the seed crystal region A1sd is the largest at about 26 mm at the substantially central portion of the silicon ingot, and it becomes smaller as it approaches the end of the seed crystal region A1sd in the ± X direction. In the vicinity of the end portion in the ± X direction of seed crystal region A1sd, thickness HT1 was about 8 mm. It was confirmed that there was no problem that the seed crystal 200 s cracked and the seed crystal 200 s melted to reach the bottom 121 b of the mold 121. It was estimated that such cracks and defects were not caused by the manufacturing method according to this embodiment in which the second silicon melt MS2 does not intensively contact the seed crystal 200s disposed on the bottom surface in the mold 121. In addition, at the initial stage when the second silicon melt MS2 is injected into the mold 121, the second silicon melt MS2 is scattered near the outer peripheral portion on the upper surface of the seed crystal 200s, so that silicon that does not originate from the seed crystal 200s is used. It was confirmed that there was no defect that caused crystal growth. That is, it was found that the single crystal region A1sc having excellent crystallinity was formed over a wide range excluding the vicinity of the outer peripheral portion of the silicon ingot. In other words, it was found that a high-quality silicon ingot having a uniform crystal orientation in a wide range of silicon ingots can be produced.

  Further, by measuring the electrical resistance between the front surface and the back surface of the silicon plate subjected to the selective etching described above, the distribution of specific resistance (ρb value) corresponding to the concentration distribution of boron as a dopant is obtained. Obtained. FIG. 17 shows the distribution of the normalized ρb value in each part where the distance from the lower surface of the silicon ingot is 30, 60, and 90, where the distance from the lower surface to the upper surface of the silicon ingot is 100. ing. In FIG. 17, the vertical axis represents the normalized ρb value, and the horizontal axis represents the position in the horizontal direction (± X direction) in the silicon ingot. And the distribution of the normalized ρb value in the portion where the distance from the lower surface of the silicon ingot is 30 is indicated by a one-dot chain line. The distribution of the normalized ρb value in the portion where the distance from the lower surface of the silicon ingot is 60 is indicated by a broken line. The distribution of the normalized ρb value in the portion where the distance from the lower surface of the silicon ingot is 90 is shown by a solid line.

  As shown in FIG. 17, it was confirmed that the ρb value decreased according to the distance from the lower surface of the silicon ingot. Thereby, it was estimated that the ρb value was lowered by increasing the boron concentration according to the distance from the lower surface of the silicon ingot. Such a tendency of the ρb value indicates that when the third silicon melt MS3 solidifies, the concentration of boron as an impurity in the third silicon melt MS3 increases from the initial stage of solidification to the end of solidification. It was thought to be consistent with this.

  In addition, when the distance from the lower surface of the silicon ingot was the same, the ρb value tended to decrease as it approached the lateral end (± X direction) of the silicon ingot. For this reason, if the distance from the bottom 121b of the mold 121 is the same, it is estimated that the third silicon melt MS3 solidified faster as the distance from the side wall 121s of the mold 121 increases. Instead of the seed crystal 200s having a substantially circular disk surface, as shown in FIGS. 7 and 8, four seed crystals 200s having a substantially square disk surface with a side of 150 mm and a thickness of 40 mm are arranged. In this case, as shown in FIG. 18, the result of the same tendency was obtained. However, in this case, the region along the XZ plane including the substantially central portion of the two seed crystals 200s adjacent to each other among the four seed crystals 200s is sliced along the XZ plane by the band saw, A silicon plate was formed.

  Therefore, from the results shown in FIGS. 17 and 18, as shown in FIG. 19, when the third silicon melt MS3 is solidified, the boundary Bd2 between the third silicon melt MS3 and the solid silicon is obtained. However, it was estimated that it had a convex shape projecting upward. According to such solidification mode of the third silicon melt MS3, it is presumed that impurities contained in the third silicon melt MS3 are likely to segregate on the inner peripheral side surface portion on the side wall 121s side of the mold 121. It was. As a result, as shown in FIG. 16, it was estimated that precipitates and defects of impurities were formed in the polycrystalline region A1p in the vicinity of the side surface in the mold 121 of the silicon ingot. Then, it was estimated that a single crystal region A1sc excellent in crystallinity was formed in a wide range centering on the central portion of the silicon ingot.

  Table 1 and FIG. 20 show the remaining state and single crystal region of the seed crystal 200s for nine silicon ingots manufactured under nine conditions in which the dimensions of the seed crystal 200s are made different according to the manufacturing method according to this embodiment described above. The evaluation result which concerns on the formation state of is illustrated.

Here, a mold 121 was used in which the shape of the inner wall surface of the bottom 121b was a substantially square with a side of about 350 mm. For this reason, as shown in Table 1, the area (also referred to as the bottom area) Sa of the inner wall surface of the bottom 121b was about 122500 mm ² . Under conditions 1 and 2, four seed crystals 200 s each having a substantially square board surface with a side of about 313 mm were spread on the bottom 121 b of the mold 121. Under conditions 3 to 9, one seed crystal 200s having a substantially circular disk surface with a diameter of approximately 222, 205, 180, or 150 mm was placed on the bottom 121b of the mold 121. In other words, the area Sb on the seed crystal 200s is about 97969Mm ² In condition 1, about 38708Mm ² In condition 3-6, about 33006Mm ² In condition 7, about 25447Mm ² In condition 8, about the condition 9 17671mm ² and It was done. As a result, the value obtained by dividing the top area Sb by the bottom area Sa (Sb / Sa) is about 0.80 for the conditions 1 and 2, about 0.32 for the conditions 3 to 6, and about 0.27 for the condition 7. 8 was about 0.21, and condition 9 was about 0.14. The thickness d of the seed crystal 200s is about 40 mm under conditions 1 and 3, about 30 mm under conditions 2 and 4, about 24 mm under condition 8, about 20 mm under conditions 5, 7, and 9, and about 18 mm under condition 6. It was. As a result, the value obtained by dividing the square root of the upper area Sb by the thickness d (Sb ^1/2 / d) is about 7.8 in the condition 1, about 10.4 in the condition 2, and about 4.9 in the condition 3. The conditions 4, 8, and 9 were about 6.6, the condition 5 was about 9.8, the condition 6 was about 10.9, and the condition 7 was about 9.1.

  And the remaining condition of the seed crystal 200s was confirmed by observing the lower surface of each silicon ingot. Each silicon ingot is sliced along the XZ plane by a band saw to form a silicon plate. After the silicon plate is mirror-etched, the size of the single crystal region is visually confirmed. It was.

  As shown in Table 1, for the silicon ingots according to the conditions 1, 3, 4, and 7, the shape of the portion (also referred to as the bottom surface portion) on the −Z side of the seed crystal 200s before being heated is almost as it is. It was confirmed that it was maintained. It was confirmed that the single crystal region A1sc was formed in a wide range on the seed crystal region A1sd where the seed crystal 200s remained. That is, according to the conditions 1, 3, 4, and 7, it was found that a very good silicon ingot in which the single crystal region A1sc excellent in crystallinity was formed in a wide range was obtained. Therefore, very good evaluation results were obtained for the conditions 1, 3, 4, and 7. Moreover, about the silicon ingot which concerns on the conditions 5 and 8, although a part of outer edge part of the bottom face part of the seed crystal 200s before heating is melt | dissolved, the shape of the bottom face part of the seed crystal 200s is maintained to some extent. It has been confirmed. Then, it was confirmed that the single crystal region A1sc was formed in a somewhat wide range on the seed crystal region A1sd where the seed crystal 200s remained. That is, according to the conditions 5 and 8, it was found that a good silicon ingot in which the single crystal region A1sc excellent in crystallinity was formed in a certain range was obtained. Therefore, good evaluation results were obtained for conditions 5 and 8.

  On the other hand, with respect to the silicon ingots according to the conditions 2 and 6, it was confirmed that the portion where the seed crystal 200s was melted to the bottom surface portion was wider than the conditions 5 and 8. And it was confirmed that the formation range of the single crystal region A1sc is in a small state to some extent. Further, with respect to the silicon ingot according to condition 9, it was confirmed that the diameter of the seed crystal 200s was too small before heating, and the formation range of the single crystal region A1sc was limited to a narrow range to some extent.

Here, as shown in FIG. 20, when the values of Sb / Sa and Sb ^1/2 / d according to the conditions 1 to 9 are plotted, the silicon ingot is satisfied when the above-described equations [I] and [II] are satisfied. It was confirmed that a single crystal region A1sc having excellent crystallinity was formed in a wide range. In FIG. 20, the horizontal axis indicates the value of Sb / Sa, and the vertical axis indicates the value of Sb ^1/2 / d. In FIG. 20, the plots of conditions 1, 3, 4 and 7 where the evaluation results were very good are double circles, and the plots of conditions 5 and 8 where the evaluation results were good are normal circles. The plots of conditions 2, 6, and 9 in which the evaluation results were not good compared to the other cases are indicated by triangles, respectively. In FIG. 20, sandy areas are hatched in a numerical range satisfying the above-described equations [I] and [II].

<(1-4) Solar cell element>
The silicon substrate cut out from the silicon ingot manufactured by the manufacturing method according to this embodiment described above can be used as a semiconductor substrate of the solar cell element 10.

  Here, the basic configuration of the solar cell element 10 will be described. As shown in FIGS. 21 to 23, the solar cell element 10 includes a light receiving surface (upper surface in FIG. 23) on which light is incident and a non-light receiving surface (in FIG. 23) that is a surface opposite to the light receiving surface 10a. Lower surface) 10b. This solar cell element 10 has a semiconductor substrate 1. The semiconductor substrate 1 includes a first semiconductor layer 1p which is a one-conductivity-type semiconductor layer, and a second semiconductor layer 1n which is a reverse-conductivity-type semiconductor layer provided on the light-receiving surface 10a side of the first semiconductor layer 1p. Have. An antireflection layer 2 is provided on the first main surface 1 a on the light receiving surface 10 a side of the semiconductor substrate 1. Further, the solar cell element 10 includes a first electrode 4 provided on the first main surface 1 a on the light receiving surface 10 a side of the semiconductor substrate 1 and a second main surface 1 b on the non-light receiving surface 10 b side of the semiconductor substrate 1. And a second electrode 5 provided.

  Next, a more specific configuration example of the solar cell element 10 will be described. First, a silicon substrate having one conductivity type (for example, p-type) is prepared as the semiconductor substrate 1. As a silicon substrate, after the silicon ingot produced by the method for producing a silicon ingot according to the present invention is cut into blocks having a desired shape, the substrate shape is obtained by using a multi-wire saw device or the like to make a thin slice. Can be used. The thickness of the semiconductor substrate 1 may be, for example, 300 μm or less, and if it is 200 μm or less, the manufacturing cost of the solar cell element 10 can be reduced due to effective use of resources.

For example, boron is used as an element serving as a dopant for making the conductivity type of the silicon ingot p-type. If the boron concentration in the silicon ingot is about 1 × 10 ¹⁶ to 1 × 10 ¹⁷ [atoms / cm ³ ], the specific resistance of the silicon substrate is about 0.2 to 2 Ω · cm. As a method for doping boron into the silicon substrate, for example, a method in which an appropriate amount of boron element alone or an appropriate amount of silicon block having a known boron concentration is mixed at the time of manufacturing the silicon ingot may be employed.

  When the semiconductor substrate 1 is a silicon substrate exhibiting p-type conductivity, impurities such as phosphorus are diffused into the surface layer portion on the first main surface 1a side of the semiconductor substrate 1 to form the second semiconductor layer 1n. obtain. The first semiconductor layer 1p and the second semiconductor layer 1n form a pn junction region.

  The antireflection layer 2 serves to reduce the reflectance of the light receiving surface 10a with respect to light in a desired wavelength region, and facilitate the absorption of light in the desired wavelength region in the semiconductor substrate 1. Thereby, the amount of carriers generated by photoelectric conversion in the semiconductor substrate 1 can be increased. As a material of the antireflection layer 2, for example, silicon nitride, titanium oxide, silicon oxide, or the like can be employed. The thickness of the antireflection layer 2 may be set as appropriate depending on the material of the antireflection layer 2 so that a condition in which appropriate incident light hardly reflects (also referred to as a non-reflection condition) may be realized. For example, when the semiconductor substrate 1 is a silicon substrate, the refractive index of the antireflection layer 2 may be about 1.8 to 2.3, and the thickness of the antireflection layer 2 is about 500 to 1200 mm. good.

A BSF (Back-Surface-Field) region 1Hp is provided on the second main surface 1b side of the semiconductor substrate 1. The BSF region 1Hp has a role of forming an internal electric field on the second main surface 1b side of the semiconductor substrate 1 and reducing carrier recombination in the vicinity of the second main surface 1b. Thereby, the fall of the photoelectric conversion efficiency in the solar cell element 10 can be reduced. The BSF region 1Hp has the same conductivity type as the first semiconductor layer 1p, and the concentration of majority carriers contained in the BSF region 1Hp is higher than the concentration of majority carriers contained in the first semiconductor layer 1p. When the semiconductor substrate 1 is p-type, for example, an element serving as a dopant such as boron or aluminum is diffused in the surface layer portion on the second main surface 1b side of the semiconductor substrate 1 to form the BSF region 1Hp. obtain. At this time, the concentration of the dopant in the BSF region 1Hp may be about 1 × 10 ^{18 to} 5 × 10 ²¹ atoms / cm ³ , for example.

  As shown in FIG. 21, the first electrode 4 has a first output extraction electrode 4a and a plurality of linear first current collecting electrodes 4b. At least a part of the first output extraction electrode 4a intersects the first collector electrode 4b. The line width of the first output extraction electrode 4a may be, for example, about 1.3 to 2.5 mm. On the other hand, the line width of the 1st current collection electrode 4b is narrower than the line width of the 1st output extraction electrode 4a, for example, should just be about 50-200 micrometers. In addition, the plurality of first current collecting electrodes 4b are arranged with an interval. This interval may be about 1.5 to 3 mm. Moreover, the thickness of the 1st electrode 4 should just be about 10-40 micrometers. The first electrode 4 may include a linear auxiliary electrode 4c that connects one end portions and the other end portions of the plurality of first current collecting electrodes 4b. For example, the line width of the auxiliary electrode 4c may be equal to the line width of the first current collecting electrode 4b. The 1st electrode 4 which has the said structure can be formed by baking, after apply | coating the silver paste to the area | region of the desired pattern on the 1st main surface 1a of the semiconductor substrate 1, for example. The silver paste may be generated by mixing, for example, silver powder, glass frit, organic vehicle, and the like. The silver paste application method may be, for example, a screen printing method.

  As shown in FIG. 22, the second electrode 5 includes a second output extraction electrode 5a and a second current collecting electrode 5b. The thickness of the second output extraction electrode 5a may be about 10 to 30 μm, for example. The line width of the second output extraction electrode 5a may be about 1.3 to 7 mm. The second output extraction electrode 5a can be formed of the same material and manufacturing method as the first electrode 4 described above. For example, the silver paste can be formed by being applied to a desired pattern region on the second main surface 1b of the semiconductor substrate 1 and then firing. Moreover, the thickness of the 2nd current collection electrode 5b should just be about 15-50 micrometers. The second current collecting electrode 5b is formed on substantially the entire surface of the second main surface 1b of the semiconductor substrate 1 except for most of the region where the second output extraction electrode 5a is formed. The second current collecting electrode 5b can be formed, for example, by applying an aluminum paste to a region of a desired pattern on the second main surface 1b of the semiconductor substrate 1 and then baking it. The aluminum paste may be generated, for example, by mixing aluminum powder, glass frit, organic vehicle, and the like. The method for applying the aluminum paste may be, for example, a screen printing method.

<Specific example of (1-5) solar cell element>
Below, the Example which further actualized the solar cell element 10 which concerns on this embodiment mentioned above is demonstrated.

  Here, the silicon ingot was manufactured by using the silicon ingot manufacturing apparatus shown in FIG. 1 and the silicon ingot manufacturing method shown in FIGS.

  In the silicon ingot manufacturing apparatus, quartz glass is adopted as the material of the crucible 111. Further, the mold 121 has a first layer including a portion on the side of the internal space 121i of the side wall 121s and a bottom 121b. Further, in the side wall 121s, the second layer and the third layer are sequentially arranged around the first layer. The ones arranged are adopted. Here, silica was employed as the first layer material, carbon fiber reinforced carbon composite material was employed as the second layer material, and felt as a heat insulating material was employed as the third layer material.

  As the seed crystal 200s, single crystal silicon having a surface orientation of (100) in the Miller index was prepared. Specifically, a disc-shaped single crystal silicon having a diameter of 222 mm and a thickness of 40 mm was prepared as the seed crystal 200 s according to Example 1. Further, four seed crystals 200s having a configuration in which four corners of single crystal silicon having a square disk surface with a side of 150 mm and a thickness of 30 mm are chamfered are prepared as seed crystal 200s according to specific example 2. It was done.

First, as shown in FIG. 4, the release material is applied to the inner wall surface of the mold 121 so that the application weight per unit area is about 0.1 g / cm ² , so that the release material layer Mr1 is formed. Been formed. Here, a release material formed into a slurry by mixing a silicon nitride powder having an average particle size of about 0.5 μm, a silicon oxide powder having an average particle size of about 20 μm, and a PVA aqueous solution as a binder solution. Used.

  Next, seed crystal 200 s was placed on the inner wall surface of bottom 121 b as the bottom of mold 121. Here, for example, when the release material layer Mr1 formed on the inner wall surface of the mold 121 is dried, the seed crystal 200s is attached to the release material layer Mr1. Here, in the first specific example, as shown in FIGS. 5 and 6, the disc-shaped single crystal silicon as the seed crystal 200 s was arranged at the approximate center on the bottom 121 b of the mold 121. At this time, the outer peripheral portion of the seed crystal 200s and the inner peripheral side surface portion of the side wall portion 121s in the mold 121 were separated from each other so as to have a gap GA1. Moreover, in the specific example 2, as shown in FIGS. 7 and 8, the single crystal silicon having a substantially square disk surface as the four seed crystals 200s is formed on the bottom 121b of the mold 121 so as to form a substantially square shape. Spread all over. More specifically, the four seed crystals 200 s are arranged so that the gap GA <b> 2 is formed at the approximate center on the bottom 121 b of the mold 121. At this time, the outer peripheral portion of the four seed crystals 200s and the inner peripheral side surface portion of the side wall portion 121s in the mold 121 were separated from each other so as to have a gap GA1.

  Then, as shown in FIG. 9, in the lower region in the mold 121, the seed crystal 200s was filled with the relatively fine silicon lump PS1. At this time, in specific example 1, the total weight of one seed crystal 200s and silicon lump PS1 was 10 kg. In the specific example 2, the total weight of the four seed crystals 200s and the silicon lump PS1 was 10 kg. Further, as shown in FIG. 10, a large number of silicon chunks PS2 having a total amount of 90 kg were put into the crucible 111.

  Next, the silicon mass PS1 in the mold 121 was melted by heating by the upper heater H2u and the lower heater H2l arranged around the mold 121. As a result, as shown in FIG. 11, the upper surface of the seed crystal 200s was set to be covered with the first silicon melt MS1. At this time, the temperature of the first silicon melt MS1 was raised to about 1420 ° C.

  As shown in FIG. 12, the cooling plate 123 was grounded to the lower surface of the mold holding part 122 at the timing when almost all of the silicon block PS1 in the mold 121 was dissolved. Thus, in the state where the upper surface of the seed crystal 200s is covered with the first silicon melt MS1, the cooling of the first silicon melt MS1 from the bottom 121b side by the cooling plate 123 is started. Immediately after that, heating of the silicon lump PS2 arranged in the crucible 111 was started by the upper heater H1u and the side heater H1s arranged around the crucible 111. As a result, as shown in FIG. 13, the silicon mass PS <b> 2 disposed in the crucible 111 was dissolved, and the second silicon melt MS <b> 2 was stored in the crucible 111. At this time, the temperature of the second silicon melt MS2 was raised to about 1420 ° C.

  At the timing when almost all of the silicon lump PS2 disposed in the crucible 111 was melted, the silicon lump PS3 covering the lower opening 111bo of the crucible 111 was heated by heating means such as a heater (not shown). Thereby, as shown in FIG. 14, the silicon lump PS3 was melted, and the second silicon melt MS2 in the crucible 111 was poured into the mold 121 through the lower opening 111bo. That is, the second silicon melt MS2 was injected into the mold 121 in a state where the upper surface of the seed crystal 200s was covered with the first silicon melt MS1. As a result, a third silicon melt MS3 in which the first silicon melt MS1 and the second silicon melt MS2 are mixed is generated in the mold 121.

  Thereafter, as shown in FIG. 15, the third silicon melt MS3 in the mold 121 is cooled from the bottom 121b side, so that unidirectional solidification occurs upward from the bottom 121b side of the third silicon melt MS3. It was. At this time, the temperature in the vicinity of the upper heater H2u and the lower heater H2l was maintained at about 1413 to 1414 ° C., which is in the vicinity of the melting point of silicon. At this time, the third silicon melt MS3 was heated by the upper heater H2u and the lower heater H2l. As a result, the unidirectional solidification of the third silicon melt MS3 was performed, and then the silicon ingot was produced in the mold 121 by air cooling.

  As a reference example, the second silicon melt MS2 in the crucible 111 is poured into the mold 121 through the lower opening 111bo without the silicon lump PS1 being disposed in the lower area in the mold 121. As a result, a silicon ingot was produced. At this time, the arrangement of the seed crystal 200s is performed under the same conditions as in the first specific example, and the processes other than the melting of the silicon block PS1 in the mold 121 are performed under the same conditions as in the first and second specific examples. It was.

  Here, when the cross section of the silicon ingot according to the specific example 1 and the specific example 2 is visually observed, there is a problem that the seed crystal 200 s cracks and the seed crystal 200 s melting reaches the bottom 121 b of the mold 121. Not confirmed. Furthermore, the cross section of the silicon ingot according to the specific example 1, the specific example 2 and the reference example is visually observed, so that the seed crystal 200 s is formed in a wide range except for the vicinity of the outer peripheral portion of the silicon ingot according to the specific example 1 and the specific example 2. It was confirmed that the single crystal grew from the starting point. On the other hand, the cross section of the silicon ingot according to the reference example is visually observed, so that substantially the entire area of the silicon ingot according to the reference example is composed of polycrystalline silicon, and a single crystal grows starting from the seed crystal 200s. It was confirmed that they did not.

  Next, the silicon ingot according to the specific example 1, the specific example 2 and the reference example was sliced along a plane parallel to the bottom surface of the silicon ingot, so that a silicon substrate corresponding to the semiconductor substrate 1 was manufactured. And the solar cell element 10 (refer FIGS. 21-23) which uses the silicon substrate obtained here as the semiconductor substrate 1 was produced according to the following processes.

  Here, first, a silicon substrate was fabricated by slicing each silicon ingot. At this time, a silicon substrate having a square board surface with a thickness of 200 μm and a side of 150 mm was produced by a wire saw device. At this time, the damaged layer generated when the silicon ingot was cut in the surface layer of the silicon substrate was removed by etching with a sodium hydroxide solution.

Next, a texture structure with fine irregularities was formed on the first main surface 1a of the semiconductor substrate 1 by dry etching. Then, the second semiconductor layer 1n and the phosphor glass on the second semiconductor layer 1n were formed by vapor phase thermal diffusion using POCl ₃ as a diffusion source. At this time, the sheet resistance of the second semiconductor layer 1n was 70Ω / □. Further, after removing phosphorous glass by etching using a hydrofluoric acid solution and performing pn separation by laser beam, a silicon nitride film as an antireflection layer 2 was formed on the first main surface 1a by PECVD. .

  Thereafter, an aluminum paste was applied to the second main surface 1b of the semiconductor substrate 1 over substantially the entire surface, and this aluminum paste was baked to form the BSF region 1Hp and the second current collecting electrode 5b. Further, a silver paste is applied on the first main surface 1a and the second main surface 1b of the semiconductor substrate 1, and the silver paste is baked to form the first electrode 4 and the second output extraction electrode 5a. It was done. Thereby, the solar cell element 10 was produced.

  And each solar cell element 10 using the semiconductor substrate 1 obtained from the silicon ingot which concerns on the specific example 1, the specific example 2, and a reference example was made into object, and the photoelectric conversion efficiency was measured. The measurement of the photoelectric conversion efficiency was performed in accordance with JIS C 8913 (1998). The measurement results are shown in Table 2. Table 2 shows that each sun in which the semiconductor substrate 1 of each part in which the distance from the lower surface of the silicon ingot is 16, 50, and 82 when the distance from the lower surface to the upper surface of the silicon ingot is 100 is used. The photoelectric conversion efficiency for the battery element 10 is shown. However, the photoelectric conversion efficiencies shown in Table 2 are standardized with the photoelectric conversion efficiency of the solar cell element 10 according to the reference example when the distance from the bottom surface of the silicon ingot is 82 as 100.

  As shown in Table 2, the photoelectric conversion efficiency of the solar cell element 10 according to the reference example was 94.7 to 100, whereas the photoelectric conversion efficiency of the solar cell element 10 according to Specific Example 1 and Specific Example 2 was The conversion efficiency was 102.1 to 103.5. From these results, it was confirmed that the photoelectric conversion efficiency was improved for the solar cell element 10 using the semiconductor substrate 1 obtained from a high-quality silicon ingot having a uniform crystal orientation.

<(1-6) Summary>
As described above, in the method for manufacturing a silicon ingot according to an embodiment, the silicon block PS1 disposed on the seed crystal 200s in the mold 121 is melted, and the upper surface of the seed crystal 200s is the first silicon melt MS1. It is covered. In this state, the second silicon melt MS2 is injected from the crucible 111 into the mold 121. Thereby, the high-temperature second silicon melt MS2 does not intensively contact the seed crystal 200s arranged in the mold 121. For this reason, it is difficult to cause a problem that the seed crystal 200 s cracks and the seed crystal 200 s melts to reach the bottom 121 b of the mold 121. Further, in the initial stage when the second silicon melt MS2 is injected into the mold 121, the second silicon melt MS2 is scattered near the outer periphery on the upper surface of the seed crystal 200s, so that the crystal growth does not start from the seed crystal 200s. It is difficult to cause defects. As a result, a silicon single crystal having excellent crystallinity can be formed over a wide range excluding the vicinity of the outer peripheral portion of the silicon ingot. That is, a high quality silicon ingot having a uniform crystal orientation can be manufactured.

  Further, since the silicon block PS1 on the seed crystal 200s is melted earlier than the seed crystal 200s, the entire upper surface of the seed crystal 200s is easily heated substantially uniformly, and the surface layer portion on the upper surface side of the seed crystal 200s is substantially uniform. Can melt. That is, it is difficult to cause a problem that unevenness is generated on the upper surface of the seed crystal 200s and the unidirectional solidification of the third silicon melt MS3 is inhibited. Furthermore, since the weight of the silicon lump PS1 disposed on the seed crystal 200s in the mold 121 can be adjusted here, the first silicon melt MS1 covering the upper surface of the seed crystal 200s in the mold 121 is provided. The height of the can be adjusted. Therefore, when the second silicon melt MS2 is poured into the mold 121, the amount of the first silicon melt MS1 is within a range in which the seed silicon 200s is not cracked and unevenly melted by the second silicon melt MS2. If it is reduced, the temperature of the first first silicon melt MS1 can be easily controlled. For example, the temperature of the first silicon melt MS1 can be easily controlled by heating with the upper heater H2u and the lower heater H2l and grounding the cooling plate 123 to the lower surface of the mold holding unit 122.

<(2) Other>
Note that the present invention is not limited to the above-described embodiment, and various modifications and improvements can be made without departing from the gist of the present invention.

  For example, in the manufacturing method according to the above-described embodiment, in the first step, after the silicon block PS1 is filled in the mold 121, the silicon block PS2 is filled in the crucible 111. However, the present invention is not limited to this. For example, after filling the crucible 111 with the silicon lump PS2, the mold 121 may be filled with the silicon lump PS1.

  In the manufacturing method according to the above embodiment, in the third step, the cooling plate 123 is grounded to the lower surface of the mold holding part 122, the silicon mass PS2 in the crucible 111 is melted, and the second silicon melt in the mold 121 is melted. The injection of the liquid MS2 was performed in this order, but is not limited to this. For example, after the melting of the silicon lump PS2 in the crucible 111 is started, the cooling plate 123 may be grounded to the lower surface of the mold holding unit 122. Further, for example, the cooling plate 123 may be grounded to the lower surface of the mold holding part 122 after the injection of the second silicon melt MS2 into the mold 121 is started. Further, for example, the grounding of the cooling plate 123 to the lower surface of the mold holding unit 122 and the start of the injection of the second silicon melt MS2 into the mold 121 may be performed substantially simultaneously.

  However, in the second step, the CO gas generated by the reaction of the carbon contained in the second and third layers of the upper heater H2u, the lower heater H2l and the mold 121 with the SiO vapor evaporated from the first silicon melt MS1 is generated. And dissolved in the first silicon melt MS1. Here, if a structure such as a lid for reducing the dissolution of the CO gas into the first silicon melt MS1 is provided, evaporation of impurities from the first silicon melt MS1 is inhibited and the mold 121 is used. Heat can easily accumulate inside, and the rate of unidirectional solidification of the third silicon melt MS3 can be reduced. Further, if an exhaust path or the like for promoting the evaporation of impurities is added to the manufacturing apparatus 100, various problems such as an increase in size, complexity, and an increase in manufacturing cost of the manufacturing apparatus 100 occur.

  Therefore, in the third step, if the cooling of the first silicon melt MS1 in the mold 121 is started before the injection of the second silicon melt MS2 into the mold 121 is started, the seed crystal 200s is melted. Thus, a deposition step in which carbon contained in the first silicon melt MS1 is deposited in the form of silicon carbide can be performed. In addition, this precipitation process may be performed in step Sp31 of the one embodiment. Thus, silicon carbide is positively precipitated in first silicon melt MS1 that solidifies in the vicinity of seed crystal 200s, and the generation of dislocations starting from this silicon carbide precipitate is promoted. As a result, in the vicinity of the seed crystal 200s, a region (also referred to as a strain relaxation region) that relaxes strain generated in the solidified silicon when the unidirectional solidification of the third silicon melt MS3 is performed can be formed. This strain relaxation region contributes to improvement of crystallinity in the initial stage of unidirectional solidification of the third silicon melt MS3. In addition, the strain relaxation region can be a region where impurities in the first to third silicon melts MS1 to MS3 are captured by defects or the like. As a result, single crystal silicon excellent in crystallinity can be generated in a wide range excluding the outer peripheral portion of the silicon ingot.

  Then, after the deposition step, the second silicon melt MS2 is injected from the crucible 111 into the mold 121, so that the first silicon melt MS1 in the mold 121 has a reduced carbon concentration. The melt is MS3. That is, an injection process in which the carbon concentration in the first silicon melt MS1 in the mold 121 is reduced can be performed. Thereby, when the unidirectional solidification of the third silicon melt MS3 is performed, the carbon concentration in the third silicon melt MS3 is hardly increased until the end of the unidirectional solidification. As a result, silicon carbide is deposited in the vicinity of the upper end portion of the silicon ingot, and single crystal silicon having excellent crystallinity can be formed in a wide range excluding the outer peripheral portion of the silicon ingot. That is, the region of single crystal silicon excellent in crystallinity occupying the silicon ingot can be increased.

  Of the silicon ingot produced as described above, the region near the outer peripheral portion of the silicon ingot including the strain relaxation layer containing a relatively large amount of silicon carbide precipitates, dislocations, defects, and impurities is excised. Relatively large single crystal silicon can be obtained. That is, the yield in the production of single crystal silicon can be improved.

  Further, in the precipitation step, if the first silicon melt MS1 is cooled by the cooling plate 123 from the bottom 121b side, silicon carbide is likely to precipitate in a region closer to the seed crystal 200s. For this reason, in the precipitation process, the first silicon melt MS1 may be cooled so that the cooling rate and the temperature decrease amount are larger on the lower surface side of the mold 121 than on the side surface side of the mold 121. That is, the cooling rate (first cooling rate) and the amount of decrease in temperature (first decrease amount) on the lower surface side of the mold 121 are the same as the cooling rate (second cooling rate) and the temperature decrease amount (first amount) on the side surface side of the mold 121. The first silicon melt MS1 may be cooled so as to be larger than (2 decrease amount).

  Here, the silicon ingot IG2 manufactured by the manufacturing method in which the arrangement of the seed crystal 200s in the manufacturing method according to the embodiment is omitted is sliced along the XZ plane with a band saw, and the obtained silicon plate is obtained. The board surface was observed with a scanning electron microscope (SEM). In addition, before the observation by SEM, mirror etching was performed on the silicon plate. As a result of this observation, as shown in FIG. 24, about 10 μm along a region AR1 (a linear region indicated by a broken line in the figure) separated by a distance (also referred to as a height) H1 from the bottom surface of the silicon ingot IG2. It was confirmed that precipitates of silicon carbide having a diameter of 5 mm were concentrated.

  Further, here, by manufacturing the silicon ingot IG2 under manufacturing conditions in which the first cooling rate, the second cooling rate, the first temperature decrease amount, and the second temperature decrease amount in the precipitation process are made different, the region AR1 The difference in height H1 was confirmed.

  Here, when the silicon ingot IG2 is manufactured under the three manufacturing conditions, the relationship between the time elapsed from the time when the heating of the silicon lump PS1 is started and the temperature detected by the temperature measuring units CHA and CHB (temperature (History) will be described.

  Under the first manufacturing condition, the first cooling rate and the first decrease amount of the temperature obtained from the detection result by the temperature measuring unit CHB are the second cooling rate and the second temperature obtained from the detection result by the temperature measuring unit CHA. The first silicon melt MS1 was cooled so as to be smaller than the decrease amount.

  On the other hand, under the second manufacturing condition, the first silicon melt MS1 was cooled such that the first cooling rate and the first decrease amount of the temperature were slightly larger than the second cooling rate and the second decrease amount of the temperature.

  Further, under the third manufacturing condition, the first silicon melt MS1 is cooled such that the first cooling rate and the first decrease amount of the temperature are several times larger than the second cooling rate and the second decrease amount of the temperature. It was.

  Thus, as a result of making the first cooling rate, the second cooling rate, the first decrease amount of the temperature, and the second decrease amount of the temperature different, the height H1 of the area AR1 becomes 35 mm according to the first manufacturing condition. According to the second manufacturing condition, it was 25 mm, and according to the third manufacturing condition, it was 12 mm. That is, if the first silicon melt MS1 is cooled such that the first cooling rate and the first decrease amount of the temperature are larger than the second cooling rate and the second decrease amount of the temperature, the height H1 of the area AR1. Was confirmed to be low. In addition, it was confirmed that the height H1 of the area AR1 is decreased as the first cooling rate and the first decrease amount of the temperature are larger than the second cooling rate and the second decrease amount of the temperature. Thereby, it turned out that many good quality areas of silicon ingot IG2 can be secured.

  Further, in the above-described embodiment, the mode in which the four seed crystals 200s having a substantially square disk surface are spread on the bottom 121b of the mold 121 so as to form a substantially square in the first step has been described. I can't. For example, two or more seed crystals 200 s such as 9, 25 may be spread on the bottom 121 b of the mold 121. In this case, if two or more seed crystals 200s are arranged on the bottom 121b of the mold 121 with a gap between them, as shown in FIG. 25, a specific extension extending in the + Z direction immediately above the gap in the silicon ingot. It is possible to concentrate dislocations and defects in the region A2. Note that the shape of the surface of the two or more seed crystals 200s may be, for example, a substantially square shape.

  This specific region A2 can be generated starting from a gap portion between two or more seed crystals 200s. Specifically, when unidirectional solidification of the third silicon melt MS3 is performed, crystal defects (dislocation clusters) are generated from a gap portion between two or more seed crystals 200s. At this time, if crystal growth occurs from the gap portion, a region where polycrystalline silicon grows may be formed, but it is easy to form so that crystal defects also extend in the + Z direction, which is the direction of crystal growth in unidirectional solidification. For this reason, dislocations and defects may be concentrated in a specific region A2 extending in the + Z direction at a position directly above the gap.

  And this specific area | region A2 functions as an area | region (it also calls a distortion relaxation area | region) which relieve | moderates the distortion which arises in the silicon | silicone solidified when the unidirectional solidification of 3rd silicon melt MS3 is performed. Further, the strain relaxation region A2 can be a region where impurities in the first to third silicon melts MS1 to MS3 are captured by defects or the like. As a result, single crystal silicon having excellent crystallinity can be generated in a wide range excluding the outer peripheral portion of the silicon ingot and the strain relaxation region A2. Specifically, the single crystal region A1sc can be formed immediately above the various crystals 200s. Then, by removing the strain relaxation region A2 and the region in the vicinity of the outer periphery of the silicon ingot from the silicon ingot manufactured in this way, single crystal silicon having relatively large crystallinity can be obtained.

  The gap between the two or more seed crystals 200s causing the strain relaxation region A2 may be, for example, a slit-like gap or a gap surrounded by the two or more seed crystals 200s. As the gap surrounded by two or more seed crystals 200s, for example, a gap GA2 surrounded by four or more seed crystals 200s as shown in FIGS. 7 and 8 may be employed. In such an embodiment in which the gap GA2 surrounded by the two or more seed crystals 200s is formed, the region where the strain relaxation region A2 is formed can be limited to a relatively small region. Thereby, when single crystal silicon is cut out from the silicon ingot, the strain relaxation region A2 to be excised is reduced, so that the yield in manufacturing the single crystal silicon can be improved. The gap surrounded by the two or more seed crystals 200s is appropriately adjusted in the shape of the front and back surfaces of the seed crystal 200s, so that any number of two or more seed crystals 200s arranged on the bottom 121b of the mold 121 is obtained. Can be formed. That is, in the first step, two or more seed crystals 200s can be arranged on the bottom 121b of the mold 121 so as to surround the gap GA2.

  By the way, in the technique of the said patent document 2, the aspect by which many seed crystals are spread | laid without gaps on the bottom part of a casting_mold | template is employ | adopted. In this aspect, if there is a deviation in crystal orientation among a large number of seed crystals, the growth of unidirectional solidification of silicon and the solidification of silicon near the interface where single crystal regions formed on various crystals collide with each other. Defects and cracks due to the resulting strain can occur. On the other hand, in an embodiment in which two or more seed crystals 200s are arranged so as to sandwich the gap on the bottom 121b of the mold 121 or surround the gap GA2, defects concentrate on the strain relaxation area A2 as a specific area. Can do. As a result, the strain caused by the solidification of silicon can be relaxed in the strain relaxation region A2. For this reason, in such an aspect, when two or more seed crystals 200s are disposed on the bottom 121b of the mold 121 in the first step, the two or more seed crystals 200s may not be precisely disposed. Therefore, the first step can be easily performed.

  Needless to say, all or a part of each of the above-described embodiment and other various aspects can be appropriately combined within a consistent range.

DESCRIPTION OF SYMBOLS 100 Manufacturing apparatus 111 Crucible 121 Mold 121b Bottom part 122 Mold holding part 123 Cooling plate 130 Control part 200s Seed crystal A1p Polycrystalline region A1sc Single crystal region A1sd Seed crystal region A2 Strain relaxation region Bd1, Bd2 Boundary GA1, GA2 Gap IG2 MS1 Ingot 1st silicon melt MS2 2nd silicon melt MS3 3rd silicon melt PS1-PS3 Silicon lump

Claims (6)

A first step of disposing a seed crystal of plate-like single crystal silicon on the bottom surface in the mold and disposing a solid silicon mass on the seed crystal;
A second step of melting the silicon mass and covering an upper surface of the seed crystal with a first silicon melt;
In a state where the upper surface of the seed crystal is covered with the first silicon melt, the first silicon melt is cooled from the bottom surface side of the mold, and the second silicon melt is poured into the mold. A third step of injecting
The third silicon melt said first silicon melt and the second silicon melt is mixed, have a fourth step for causing the unidirectional solidification directed upwardly from the bottom portion of the mold ,
In the third step, before the injection of the second silicon melt into the mold is started, the cooling of the first silicon melt in the mold is started, whereby the first silicon melt is injected into the mold. Implantation for reducing the carbon concentration in the first silicon melt in the mold by depositing the carbon contained in the form of silicon carbide and injecting the second silicon melt into the mold The manufacturing method of the silicon ingot which performs a process in order .   2. The method for producing a silicon ingot according to claim 1, wherein in the first step, the seed crystal is disposed on the bottom surface portion of the mold such that an inner peripheral side surface portion of the mold is separated from an outer peripheral portion of the seed crystal. . 3. The method for producing a silicon ingot according to claim 2, wherein in the first step, the seed crystal satisfying the following formula [II] is arranged in the mold satisfying the following formula [I].
[I] 0.21 ≦ Sb / Sa <1
[II] Sb ^1/2 /d≦9.8
(However, Sa [mm ² ]: Area of the bottom surface of the mold, Sb [mm ² ]: Area of the upper surface of the seed crystal, d [mm]: Thickness of the seed crystal) In the precipitation step, the first cooling rate on the lower surface side of the mold is larger than the second cooling rate on the side surface side of the mold, and the first decrease in temperature on the lower surface side of the mold is the temperature on the side surface side of the mold. The method for manufacturing a silicon ingot according to any one of claims 1 to 3 , wherein the first silicon melt is cooled so as to be larger than a second decrease amount of the first silicon melt. In the first step, on the bottom of the mold, a method of manufacturing a silicon ingot according to claim 1, any one of claims 4 to place two or more of the seed crystal across the gap . The method for manufacturing a silicon ingot according to claim 5 , wherein in the first step, two or more seed crystals are arranged on the bottom surface of the mold so as to surround the gap.

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