JP2015214473A - Method for manufacturing ingot of polycrystal silicon - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing without using a seed crystal an ingot of polycrystal silicon suitable for producing a solar battery element capable of improving conversion efficiency.SOLUTION: A method for manufacturing an ingot of polycrystal silicon includes a step for preparing a mold 3 having a release layer 32 on each of an inner side wall 3e and an inner bottom wall 3d, a step for disposing a buffer silicon 4 which is not a seed crystal on the inner bottom wall 3d of the mold 3, a step for pouring silicon melt 2 into the mold 3 so that the melt collides with the buffer silicon 4 to supply a predetermined amount of the silicon melt 2 to the mold 3, and a step for subjecting the silicon melt 2 to unidirectional solidification from the inner bottom wall 3d of the mold 3 upward to obtain an ingot of polycrystal silicon.

Description

  The present invention relates to a method for producing a polycrystalline silicon ingot.

  A polycrystalline silicon substrate used for a crystalline silicon solar cell is manufactured by, for example, slicing a polycrystalline silicon ingot to a desired thickness using a wire saw device or the like. This ingot is produced by a casting method or the like. In the casting method, for example, a silicon melt supplied to a mold having a bottom surface and a side surface is solidified unidirectionally upward from the bottom surface of the mold to produce a polycrystalline silicon ingot.

  As a means for supplying the silicon melt to the mold, there is known a system (hereinafter referred to as a pouring method) in which a silicon melt melted in a crucible separate from the mold is poured into the mold from the crucible. (For example, refer to Patent Document 1 below). In this pouring method, the raw material is melted and solidified in separate crucibles and molds. For this reason, since the melt and the mold do not come into contact with each other during the melting of the silicon raw material, the amount of impurities caused by the mold contained in the melt can be kept low. In addition, since the preparation of the mold and the melting of the silicon raw material can be performed in parallel, the manufacturing tact of the ingot can be shortened. In addition, by making the crucible and the mold suitable, it is possible to improve the productivity of the ingot and the characteristics of the solar cell.

  However, in the casting method by the pouring method, crystal defects may occur in the ingot. The occurrence of this crystal defect is due to the fact that when the silicon melt is poured into a mold having a release material layer on the inner wall, a part of the release material layer is peeled off by the impact of the silicon melt colliding with the release material layer. it is conceivable that. Crystal defects generated in the ingot can cause deterioration of device characteristics such as conversion efficiency of solar cells.

  As a method of producing an ingot with few crystal defects, a method of crystallizing a silicon melt from a seed crystal arranged at the bottom of a mold (hereinafter referred to as seed casting method) has been proposed (for example, the following patents) References 2 and 3).

JP 2001-19591 A JP 2010-534179 gazette JP 2007-63049 A

However, the seed casting method has the following problems (1) to (2).
(1) It is necessary to prepare a seed crystal having a high quality and a large area.
(2) Since the crystal is grown with the seed crystal not melted up to the bottom surface of the mold in the state where the entire surface of the seed crystal is melted, the silicon melt for growing the crystal while leaving the seed crystal and It is difficult to control the temperature of the mold.

  Therefore, the present invention has been made in view of the above problems, and it is possible to provide a polycrystalline silicon ingot suitable for the production of a solar cell capable of improving the conversion efficiency without using a seed crystal. Objective.

  The method for producing a polycrystalline silicon ingot according to the present invention comprises a step of preparing a mold having a release material layer on each of a side inner wall and a bottom inner wall, and non-seed buffer silicon on the bottom inner wall of the mold. Placing a silicon melt into the mold so that the silicon melt collides with the buffer silicon, supplying a predetermined amount of the silicon melt into the mold, and adding the silicon melt to the mold. And unidirectionally solidifying upward from the bottom to obtain a polycrystalline silicon ingot.

  According to the above manufacturing method, it is possible to simply provide a polycrystalline silicon ingot with few crystal defects such as dislocations, which is suitable for the production of a solar cell that can improve the conversion efficiency without using a seed crystal. it can.

FIG. 1 is a cross-sectional view illustrating the main part of an ingot manufacturing apparatus used in a manufacturing method according to an embodiment of the present invention. FIG. 2 is a view for explaining a state in a mold used in a manufacturing method according to an embodiment of the present invention, and FIG. 2 (a) is a cross section showing a state in which a plurality of massive buffer silicons are arranged in the mold. FIG. 2 and FIG. 2B are cross-sectional views showing a state in which a plurality of plate-shaped buffer silicons are arranged in a mold. FIG. 3 is a view for explaining a state in the mold used in the manufacturing method according to one embodiment of the present invention, and FIG. 3A is a cross section showing a state in which a plurality of massive buffer silicons are arranged in the mold. FIG. 3B is a cross-sectional view showing a state in which a plurality of massive buffer silicons having different average particle diameters are arranged in the upper and lower sides in the mold. FIG. 4 is a view for explaining a state in the mold used in the manufacturing method according to the embodiment of the present invention, and FIG. 4A is a cross section showing a state in which a plurality of massive buffer silicons are arranged in the mold. FIG. 4 and FIG. 4B are cross-sectional views showing a state in which disc-shaped buffer silicon is disposed in the mold. 5A to 5C are cross-sectional views showing the steps of the manufacturing method according to the embodiment of the present invention. 6 (a) to 6 (c) are cross-sectional views showing the steps of the manufacturing method according to the embodiment of the present invention. FIG. 7 is a graph showing the relationship between the measurement position and the etch pit density.

  Embodiments of a method for producing a polycrystalline silicon ingot according to the present invention will be described below in detail with reference to the drawings. In addition, all the drawings are schematically shown, and the size and positional relationship of constituent members in each drawing can be appropriately changed. Further, in the following description, “ingot” simply means “ingot of polycrystalline silicon”.

<Ingot production equipment>
The main part of the ingot manufacturing apparatus S is shown in FIG. As shown in FIG. 1, a crucible 1 that is a constituent element of an ingot manufacturing apparatus S is for melting, for example, solid raw material silicon into a silicon melt 2, and the silicon melt 2 enters inside thereof. . Further, a heating means (not shown) such as a heater is disposed around the outside of the crucible 1. By this heating means, for example, solid raw material silicon provided in the crucible 1 is heated to a melting point or higher and melted.

Similar to the crucible 1, the mold 3 that is a component of the ingot manufacturing apparatus S has a main body 31 and a release material layer 32. Moreover, the casting_mold | template 3 has the opening part 3a into which the silicon melt 2 can be poured from the crucible 1 in the upper part. Moreover, the casting_mold | template 3 has the side part 3b and the bottom part 3c for hold | maintaining the poured silicon melt 2. FIG. Moreover, the main body 31 of the casting_mold | template 3 is good to be comprised from the some side plate 31a and the bottom plate 31b which can be assembled and disassembled.

  A release material layer 32 is disposed on each inner wall (side inner wall 3e and bottom inner wall 3d) of the side 3b and the bottom 3c of the mold 3 so that the solidified ingot does not directly adhere to the main body 31 of the mold 3. ing. That is, the release material layer 32 is applied to the inner surface of the main body 31 of the mold 3, for example. In addition, in order to form a desired temperature gradient in the mold 3, a heating means (not shown), a heat insulating member, a cooling mechanism, and the like are arranged around the outside of the mold 3.

  In the mold 3, the buffer silicon 4 is disposed on the release material layer 32. Here, the part 5 of the bottom 3c of the mold 3 where the buffer silicon 4 is disposed may be provided with a parting material layer 32 that is partially or entirely thicker than the part where the buffer silicon 4 is not disposed. Thereby, since the buffer silicon 4 is easily held by the release material layer 32, the buffer silicon 4 is not easily displaced.

  The buffer silicon 4 is a non-seed crystal. That is, the buffer silicon 4 is a non-seed crystal in which crystal growth with the buffer silicon as a nucleus is not performed when the silicon melt 2 in the mold 3 is cooled and crystal is grown. The buffer silicon 4 may be polycrystalline or single crystal silicon, but may not be a high-quality single crystal like a seed crystal, and it is not necessary to prepare a large area like a seed crystal. Further, since the buffer silicon 4 does not grow a crystal using it as a nucleus, it is not necessary to control the temperature of the silicon melt 2 and the mold 3 with high accuracy as compared with the seed casting method using a seed crystal. For this reason, heating means, such as a heater, or the ingot manufacturing apparatus S can be simplified. Moreover, since the buffer silicon 4 does not require preheating like the seed crystal, the thermal load on the mold and the ingot manufacturing apparatus S is small. For this reason, mixing of foreign matters and impurities into the ingot is suppressed, and the durability (life) of the mold is also improved.

  The buffer silicon 4 softens the impact when the silicon melt 2 first collides with the mold 3 when the silicon melt 2 is supplied into the mold 3. The buffer silicon 4 prevents the first droplet 2 a of the dropped silicon melt 2 from directly colliding with the release material layer 32. In this way, the part where the silicon melt 2 first collides becomes the surface 4 a of the buffer silicon 4. The buffer silicon 4 only needs to relieve the impact of the silicon melt 2 on the mold 3, and the entire buffer silicon 4 or a part thereof may be melted to a lower part in contact with the mold 3. Thereby, the buffer silicon 4 can be made thin with a small amount as compared with the case where the seed crystal is used.

  The buffer silicon 4 is, for example, a single plate as shown in FIG. In addition, the buffer silicon 4 may be a plurality of, for example, a lump as shown in FIGS. 2 (a), 3 (a), 3 (b), and 4 (a). Here, the plurality of lumps may include a collection of a large number of irregular lumps, or may include a collection of a large number of spherical bodies having a substantially constant particle size. The buffer silicon 4 may be in the form of particles having an average particle diameter of about 3 to 40 mm, for example, or may be in a finer powder form.

  Moreover, the buffer silicon | silicone 4 may arrange | position the several plate-shaped body side by side, for example, as shown in FIG.2 (b). In FIG. 2 (b), 4b indicates the boundaries between a plurality of plate-like bodies. Further, as shown in FIG. 4B, the buffer silicon 4 may be a single plate-like plate-like body that does not contact the inner wall of the mold 3.

In order to supply a predetermined amount of the silicon melt 2 into the mold 3, for example, as shown in FIG. 1, the silicon melt 2 in the crucible 1 is supplied into the mold 3 by tilting the crucible 1. As described above, in this embodiment, the raw material is melted and solidified by the separate crucible 1 and the mold 3, respectively. For this reason, the preparation of the mold 3 and the melting of the silicon raw material can be performed in parallel, and the manufacturing tact of the ingot can be shortened. Moreover, since it can be set as the structure suitable for each function of the crucible 1 and the casting_mold | template 3, the productivity and characteristic of a solar cell can be improved.

  Both the crucible 1 and the mold 3 are made of a material that hardly melts, deforms, decomposes or the like and does not easily react with silicon at a temperature equal to or higher than the melting point of silicon. Furthermore, this material uses what reduced the impurity which reduces the characteristic of the solar cell manufactured from an ingot as much as possible. For example, quartz or carbon can be used as the material for the crucible 1 and the mold 3. Moreover, as a material of the release material layer 32, for example, a material such as silicon oxide, silicon nitride, or silicon carbide, or a mixture of a plurality of types of these materials is used.

<Ingot production method>
The production of the ingot includes at least the following steps 1 to 4. The main reference drawings are FIG. 5 and FIG.

  Step 1 is a step of preparing the mold 3. In step 1, a mold 3 having a release material layer 32 on each inner wall (side inner wall 3e and bottom inner wall 3d) of the side portion 3b and the bottom portion 3c is prepared.

  Step 2 is a step of disposing buffer silicon 4. In step 2, non-seed buffer silicon 4 is disposed on the bottom inner wall 3 d in the mold 3.

  Step 3 is a step of supplying a predetermined amount of the silicon melt 2. In step 3, the silicon melt is poured into the mold 3 so as to collide with the buffer silicon 4, and a predetermined amount of the silicon melt 2 is supplied into the mold 3. In step 3, the region from the portion where the silicon melt 2 collides with the buffer silicon 4 to the bottom inner wall 3d of the mold 3 located below may be melted.

  Step 4 is a step of solidifying the silicon melt 2 in one direction. In step 4, the bottom 3c of the mold 3 is cooled, and the silicon melt 2 in the mold 3 is solidified in one direction upward from the bottom 3c of the mold 3 to obtain the ingot 6.

  Here, in step 3, all of the buffer silicon 4 may be melted. Further, in step 2, the buffer silicon 4 may be arranged so that the outer peripheral portion of the buffer silicon 4 does not contact the side inner wall 3e of the mold 3. In step 2, the buffer silicon 4 may be fixed to the bottom inner wall 3 d of the mold 3. Further, in step 2, the buffer silicon 4 may include a plurality of lump bodies or one or more plate-like buffer silicon bodies. Further, in step 2, the buffer silicon 4 may be arranged so as to have a circular shape in plan view. Further, in step 2, the buffer silicon 4 may include a plurality of lumps, and a plurality of types of lumps having different average particle diameters may be disposed. Further, in step 2, the buffer silicon 4 may include a large number of lumps, and the lumps having a smaller average particle diameter may be disposed below the upper part. Further, in step 2, the buffer silicon 4 may include a combination of a plurality of plate-like bodies.

  Further, in step 2, the buffer silicon 4 is arranged so that a part of the buffer silicon 4 has a thick protrusion, and in step 3, the first contact portion of the silicon melt 2 is the protrusion of the buffer silicon 4. May be.

  Next, according to FIGS. 5A to 5C and FIGS. 6A to 6C, an example of the process flow of this embodiment will be specifically described.

As shown in FIG. 5A, a release material layer 32 is provided on the inner surface of the mold 3. The release material layer 32 is formed by first preparing a slurry, applying the slurry into the mold 3, and then drying the applied slurry. Thereby, it can be set as the casting_mold | template 3 which has the mold release material layer 32 in the side part inner wall 3e and the bottom part inner wall 3d (process 1). The slurry is prepared by, for example, mixing and stirring ceramic particles such as silicon nitride in a solution composed of an organic binder such as polyvinyl alcohol, polyvinyl butyral, or methyl cellulose and a solvent.

  Further, as shown in FIG. 5A, the buffer silicon 4 is disposed on the bottom 3c in the mold 3 (step 2). At this time, the buffer silicon 4 may be disposed on the release material layer 32 after the release material layer 32 is formed. However, as will be described later, after the slurry that becomes the release material layer 32 is applied in the mold 3, Silicon 4 may be placed, after which the slurry may be dried. Alternatively, a slurry that becomes the release material layer 32 may be applied to one surface of the buffer silicon 4, placed on the bottom of the mold 3 with the application surface down, and dried.

  Next, as shown in FIGS. 5 (b) to 6 (a), the silicon melt 2 is poured into the buffer silicon 4, and a predetermined amount of the silicon melt 2 is poured into the mold 3 while melting the buffer silicon 4. Supply. In addition, 2b in the figure has shown the flow of the silicon melt 2, and has shown a mode that the silicon melt 2 is continuously inject | poured from the crucible 1. FIG.

  A predetermined amount of silicon melt 2 formed in the crucible 1 is applied to the buffer silicon 4 disposed in the mold 3 to melt the buffer silicon 4. At this time, at least in the buffer silicon 4, the region from the portion where the silicon melt 2 first contacts to the portion located immediately below the portion on the bottom inner wall 3 d side in the mold 3 is melted, and the inside of the mold 3 is melted. A predetermined amount of the silicon melt 2 is supplied to (step 3).

  Here, the temperature of the silicon melt 2 at the time of pouring is preferably about 10 to 100 ° C. higher than the melting point of silicon, for example. In addition, as shown in FIG. 1, the injection | pouring of the silicon melt 2 from the crucible 1 to the casting_mold | template 3 may be performed from the opening part located in the upper part of the crucible 1 by inclining the crucible 1, or the crucible 1 A supply port may be provided in advance at the bottom, and the silicon melt 2 may be injected from the supply port. Further, as shown in FIG. 1, the initial injection of the silicon melt 2 may be performed intermittently or may be performed continuously as shown in FIG. However, it is desirable to intermittently perform the initial injection of the silicon melt 2 in order to reduce the impact. At this time, it is desirable from the viewpoint of manufacturing efficiency that the silicon melt 2 in an amount sufficient to cover the bottom 3c and the buffer silicon 4 in the mold 3 is intermittently supplied and then continuously supplied.

  In this way, a predetermined amount of the silicon melt 2 is supplied to the inside of the mold 3 as shown in FIG.

  Next, as shown in FIG. 6B, the bottom 3c of the mold 3 is cooled using a cooling mechanism described later. By controlling the temperature in the mold 3 during cooling, the silicon melt 2 is solidified in one direction upward from the bottom 3c of the mold 3 to form the ingot 6 (step 4). Then, as shown in FIG. 6C, an ingot 6 in which all the silicon melt 2 is solidified is obtained. In order to solidify the silicon melt 2 in one direction, for example, the following configuration is adopted. A heating means such as a heater or a heat insulating member such as felt-like carbon fiber is disposed above and to the side of the mold 3. In addition, below the mold 3, for example, a cooling mechanism in which a coolant such as water is circulated inside a plate of a good heat conductive material such as metal is disposed. In this way, a temperature gradient in the vertical direction is formed between the silicon melt 2 in the mold 3 and the solidified ingot 6 so that the silicon melt 2 can be easily moved in one direction upward from the bottom of the mold 3. It can be solidified.

As described above, in the present embodiment, by placing the non-seed buffer silicon 4 in the mold 3, the impact on the release material layer 32 due to the first injection of the silicon melt 2 is reduced. Further, the buffer silicon 4 is smoothly melted by the silicon melt 2 supplied into the mold 3. As a result, crystal defects in the ingot 6 are reduced, so that a high-quality silicon substrate suitable for manufacturing a solar cell element with high conversion efficiency can be obtained without using a seed crystal.

<Type of buffer silicon>
Below, the form of the buffer silicon | silicone 4 is demonstrated in detail.

  If one or more plate-like silicons are used as the buffer silicon 4, it is possible to use an end material when the ingot is cut or a silicon substrate that is no longer needed. Further, the buffer silicon 4 may be easily fixed to the release material layer 32.

  On the other hand, if a plurality of bulk silicon is used as the buffer silicon 4, a part of the raw material silicon put into the crucible 1 can be used as the buffer silicon 4. For this reason, a plurality of lumps are desirable because the surface area per volume is larger than that of the plate-like body, so that the entire buffer silicon 4 can be melted more uniformly in a shorter time.

  Further, it is desirable that the buffer silicon 4 is melted when the silicon melt 2 is injected over the entire region. This is because if a penetration region is formed by melting only a partial region of the buffer silicon 4, the boundary between the penetration region and the remaining region can be the starting point of a crack when the ingot 6 is cut. It is. In particular, if the shape of the buffer silicon 4 when viewed from above (plan view) inside the mold 3 in which the buffer silicon 4 is arranged is substantially circular, the buffer silicon 4 is compared with other shapes such as a rectangle. It is sufficient that the undissolved residue at the end of the arrangement region is less likely to occur.

  On the other hand, the buffer silicon 4 may be arranged so that the height thereof is uniform, and includes a portion where the silicon melt 2 first collides as shown in FIG. 3A or 4A. You may arrange | position so that only an area | region may become high. In particular, when the silicon melt 2 is supplied into the mold 3, damage to the release material layer 32 can be reduced if the concentrated and colliding portions have protrusions such as thick cones. It is possible that crystal defects are less likely to occur. The maximum thickness of the buffer silicon 4 may be about 5 mm to 50 mm.

  The buffer silicon 4 may be fixed to the mold 3 by drying the release material layer 32 applied to the inside of the mold 3. Thereby, it is possible to reduce an increase in the damage of the release material layer 32 due to the silicon melt 2 due to the buffer silicon 4 being displaced due to the impact when the silicon melt 2 is supplied. In addition, if a slurry to be the release material layer 32 is applied to one surface of the buffer silicon 4 and placed on the bottom of the mold 3 with the application surface down and dried, the fixing strength may be increased.

  The peripheral edge of the buffer silicon 4 may be in contact with the side inner wall 3 e of the mold 3. Since the peripheral edge of the buffer silicon 4 is in contact with the side inner wall 3e of the mold, the release material layer 32 can be protected over a wide area of the bottom 3c of the mold 3. Further, if the buffer silicon 4 is provided at a position where the silicon melt 2 is poured, the peripheral edge of the buffer silicon 4 may be separated from the side inner wall 3 e of the mold 3. Since the peripheral edge portion of the buffer silicon 4 is separated from the side inner wall 3e of the mold 3, the amount (region) in which the buffer silicon 4 is disposed can be reduced.

  When the shape of the buffer silicon 4 is granular, the stress generation locations on the bottom surface of the ingot 6 are dispersed as compared with the case where the shape is a plate shape. As a result, the stress in the ingot 6 and defects generated by this stress are reduced. Further, if grains having an average particle size of about 1 mm to 40 mm are used as the buffer silicon 4, the poured silicon melt 2 is not directly in contact with the mold release material 32 (the mold release material in plan view of the pouring region). It becomes easy to fill and arrange in the mold 3 at an appropriate height (so that the layer 32 is not visible).

  When the buffer silicon 4 is arranged, a plurality of granular (or powdered) buffer silicon 4 having different average particle diameters are laminated, or a plurality of types of granular and powdery aggregates having different average particle diameters are mixed together. May be arranged. At this time, as shown in FIG. 3B, buffer silicon 41 having a large average particle diameter may be disposed in the upper portion, and buffer silicon 42 having a small average particle diameter may be disposed in the lower portion. That is, the buffer silicon 4 may include a plurality of lumps, and the lumps having a smaller average particle diameter may be disposed below the upper part. This is because the entire buffer silicon 4 is easily melted more uniformly while protecting the release material layer 32 from the poured silicon melt 2. When the buffer silicon 4 has a large particle size as a whole, a gap is formed between the buffer silicon 4 and the release material layer 32 is easily damaged by the silicon melt 2. Further, when the buffer silicon 4 has a small particle size as a whole, the bulk buffer silicon 4 is moved by an impact during pouring of the silicon melt 2 and the release material layer 32 is easily damaged. On the other hand, as shown in FIG. 3 (b) or FIG. 4 (a), by arranging a plurality of types of buffer silicon 41 and 42 having different average particle diameters, the buffer silicon 4 is disposed without a gap, Each buffer silicon 4 may be difficult to move. Further, by arranging a plurality of types of buffer silicon 41 and 42 having different average particle diameters (or sizes) in a mixed manner, the heat conduction between the buffer silicons 4 may be improved and easily melted.

  Further, when supplying the silicon melt 2 into the mold 3, the melt supply amount is decreased immediately after the start of supply, and after the silicon melt 2 supplies a sufficient amount to cover the bottom surface of the mold 3, If the melt supply amount is increased, both the protection of the release material layer 32 and the shortening of the pouring time can be achieved.

  It is desirable that the buffer silicon 4 is completely dissolved by the silicon melt 2. Even if there is a region where the buffer silicon 4 is not completely dissolved when the silicon melt 2 is poured, the region does not grow with the buffer silicon 4 as a nucleus (the growth start site of the ingot 6 and the buffer silicon 4 are If it is an area | region which has cut | disconnected in crystal | crystallization, there can exist the effect of this embodiment mentioned above.

  When the buffer silicon 4 remains after the silicon melt 2 is solidified, the obtained ingot 6 includes the first crystal formed on the buffer silicon 4 remaining unmelted on the bottom surface and the bottom surface of the inner wall of the mold 3. In some cases, a second crystal formed directly on the substrate exists. According to such an ingot 6, in addition to the operational effects of the above embodiment, a difference in cooling rate from the bottom of the mold 3 during solidification can be provided between the first crystal and the second crystal. Thereby, it is possible to expect the effect that the temperature distribution in the mold 3 can be easily controlled and the high-quality ingot 6 can be easily obtained.

  Hereinafter, examples in which the above embodiment is more concretely described. An ingot was manufactured by the manufacturing method shown in FIGS. 5 and 6 using the ingot manufacturing apparatus S shown in FIGS.

  First, a crucible 1 made of quartz and a mold 3 made of CCM (carbon fiber reinforced carbon composite material) were prepared. Furthermore, granular and plate-like buffer silicon 4 was prepared.

  The release material layer 32 provided in the mold 3 was formed as follows. First, granular silicon nitride having an average particle diameter of 0.8 μm and granular silicon oxide having an average particle diameter of 30 μm were mixed, and stirred and mixed with an aqueous polyvinyl alcohol solution to form a slurry. Next, this slurry was applied to the inner surface of the main body 31 of the mold 3 and further dried to form a release material layer 32 having a thickness of about 2 mm on the inner surface of the mold 3.

  Thereafter, the buffer silicon 4 was disposed in the mold 3 on the inner surface 3d of the bottom 3c of the mold 3 with the release material layer 32 interposed therebetween. Here, as the buffer silicon 4, in Example 1, granular silicon having a particle size of about 1 to 15 mm was disposed at a height of about 15 mm over the entire bottom surface of the mold 3. Further, in Example 2, the buffer silicon 4 made of a single plate-like polycrystalline silicon having a planar shape of a square having a side of about 150 mm and a height of about 15 mm was arranged at the center of the bottom surface of the mold 3.

  Further, about 100 kg of raw material silicon lump was put into the crucible 1, and the silicon in the crucible 1 was heated and melted by the heating means arranged around the outside of the crucible 1.

  Thereafter, a silicon melt 2 at about 1450 ° C. was poured from the crucible 1 into the mold 3 (on the buffer silicon 4 in Examples 1 and 2). Then, using the cooling mechanism provided below the mold 3, a temperature gradient was formed upward from the bottom 3 c of the mold 3 to produce the ingot 6 while cooling the silicon melt 2.

  On the other hand, as a comparative example, the ingot 6 was produced under the same conditions as in Examples 1 and 2 without using the buffer silicon 4.

  Next, the block obtained by cutting the end face and the like of the obtained ingot was sliced almost horizontally with respect to the bottom surface thereof, and a silicon substrate having a thickness of about 200 μm and a plane of 156 mm × 156 mm was rectangular. . In any ingot including the comparative example, cracks starting from the bottom surface did not occur during the cutting process.

  The surface of the obtained silicon substrate was etched to form etch pits, and the density of the etch pits (hereinafter referred to as EPD) was measured. The EPD measurement method is as follows.

  First, mirror etching for mirror finishing and selective etching for revealing crystal defects were performed on the surface of the silicon substrate. In this mirror etching, the silicon substrate was sequentially immersed in a hydrofluoric acid solution for 180 seconds and washed with water, then immersed in a hydrofluoric acid aqueous solution for 30 seconds, washed with water, and dried. Here, the hydrofluoric acid solution was prepared by mixing 70 mass% nitric acid and 50 mass% hydrofluoric acid in a ratio of 7: 2. The hydrofluoric acid aqueous solution was prepared by mixing pure water and 50% by mass hydrofluoric acid at a ratio of 20: 1. In the selective etching, a selective etching solution (70% by mass nitric acid, 99% by mass acetic acid, 50% by mass hydrofluoric acid, and pure water is 1: 12.7 with respect to the silicon substrate in accordance with JIS standard H0609. : 3: 3.7 mixed solution) for 5 minutes, followed by water washing and drying.

  Thereafter, the etched surface of the silicon substrate was photographed to measure EPD. EPD measurement is performed on each silicon substrate by observing a rectangular region (each side is 250 μm) at 3 × 3 = 9 positions at approximately equal intervals in the XY direction of the silicon substrate by using an optical microscope. Was measured. Then, the EPD was calculated by dividing the number of etch pits by the area of the observation region. In addition, it is clear from the fact that a large number of crystal grain boundaries were observed by visual observation and observation with an optical microscope that the silicon substrates are all polycrystalline.

  The EPD measurement results are shown in FIG. Here, the “measurement position” on the horizontal axis in FIG. 7 indicates the solidification rate (the bottom of the ingot 6 is 0% and the top is 100%). The EPD on the vertical axis indicates the numerical value obtained by normalizing the EPD (average value of nine measured values) of each substrate with the EPD at the side fixed position 15% of the comparative example being 1.

As is clear from FIG. 7, in Example 1 and Example 2, EPD was smaller than that in the comparative example, and it was found that crystal defects such as dislocations in the ingot 6 were reduced. Furthermore, when the solar cell element was produced using the silicon substrate obtained from the respective positions of the solidification rates of 15%, 35% and 50% of each ingot, in Example 1 and Example 2, in any position The conversion efficiency of the solar cell element was higher than that of the comparative example.

  From the above, it was confirmed that an ingot was obtained by this example, from which a silicon substrate suitable for the production of a solar cell element capable of improving the conversion efficiency was obtained without using a seed crystal.

1: Crucible 2: Silicon melt 3: Mold 3a: Opening 3b: Side 3c: Bottom 3d: Bottom inner wall 3e: Side inner wall 31: Main body 32: Release material layer 4: Buffer silicon 6: Ingot S: Ingot manufacture apparatus

Claims (11)

Preparing a mold having a release material layer on each of the side inner wall and the bottom inner wall;
Disposing non-seed buffer silicon on the bottom inner wall of the mold;
Pouring the silicon melt into the mold so as to collide with the buffer silicon, and supplying a predetermined amount of the silicon melt into the mold;
Solidifying the silicon melt upward from the bottom inner wall of the mold to obtain a polycrystalline silicon ingot;
A method for producing an ingot of polycrystalline silicon having:   2. The polycrystal according to claim 1, wherein in the step of supplying a predetermined amount of the silicon melt, a region from the portion where the silicon melt collides with the buffer silicon to the bottom inner wall of the mold located below is melted. Silicon ingot manufacturing method.   The method for producing an ingot of polycrystalline silicon according to claim 1 or 2, wherein in the step of supplying a predetermined amount of the silicon melt, all of the buffer silicon is melted.   4. The polycrystalline silicon ingot according to claim 1, wherein in the step of disposing buffer silicon, the buffer silicon is disposed such that an outer peripheral portion of the buffer silicon does not contact the inner wall of the side of the mold. Production method.   5. The method for producing a polycrystalline silicon ingot according to claim 1, wherein, in the step of arranging the buffer silicon, the buffer silicon is fixed to the inner wall of the bottom of the mold. In the step of disposing the buffer silicon, the buffer silicon has a part with a thick protrusion,
6. The polycrystalline silicon ingot according to claim 1, wherein in the step of supplying a predetermined amount of the silicon melt, a portion where the silicon melt first contacts is the protruding portion of the buffer silicon. Manufacturing method.   The method for producing a polycrystalline silicon ingot according to any one of claims 1 to 6, wherein in the step of arranging the buffer silicon, the buffer silicon includes a plurality of lump bodies or one or more plate-shaped buffer silicon.   8. The method of manufacturing a polycrystalline silicon ingot according to claim 7, wherein, in the step of arranging the buffer silicon, the buffer silicon is arranged so as to have a circular shape in plan view.   The method for producing an ingot of polycrystalline silicon according to claim 7 or 8, wherein, in the step of arranging the buffer silicon, the buffer silicon includes a large number of aggregates, and a plurality of types of aggregates having different average particle diameters are disposed.   The polycrystalline silicon according to any one of claims 7 to 9, wherein in the step of disposing the buffer silicon, the buffer silicon includes a large number of lumps, and the lumps having a smaller average particle diameter are disposed below the upper part. Ingot manufacturing method.   The method for producing a polycrystalline silicon ingot according to claim 7, wherein in the step of arranging the buffer silicon, the buffer silicon includes a combination of a plurality of plate-like bodies.

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