JP2014024716A - Method for manufacturing silicon ingot - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a silicon ingot by which it is possible to prevent an area of a crystal grain nucleated on a crucible wall from increasing with progression of crystal growth and it is possible to manufacture the silicon ingot including, at a high rate, a single crystal part which inherits an orientation of a seed crystal.SOLUTION: A method for manufacturing a silicon ingot, in which a silicon seed crystal placed at a crucible bottom is grown in one direction from a lower part to an upper part so that an orientation of the seed crystal is inherited, using a silicon melt in the crucible, includes: preparing a seed crystal composed of plural single crystal silicon blocks having different orientations; and setting boundary positions between the single crystal silicon blocks at distances of 5 mm or more and 50 mm or less from the crucible wall in order to form crystal grain boundaries in the grown silicon ingot in such a manner that the boundaries in the seed crystal are inherited.

Description

  The present invention relates to a method for manufacturing a silicon ingot, and more particularly to a method for manufacturing a polycrystalline silicon ingot suitable as a solar cell.

  About 90% of solar cells as of 2011 are produced using crystalline silicon as a material. Solar cells using crystalline silicon are expected to continue to be a major solar cell material in the future due to their relatively high efficiency, mass production, rich resources and safety. Crystalline silicon for solar cells includes single crystals and polycrystals, but the production rate as of 2011 is reported to be approximately 25% for the former and 75% for the latter. For the production of polycrystalline silicon, a cast growth method utilizing unidirectional growth is generally used. The cast growth method using unidirectional growth is a method for producing a silicon ingot by solidifying a silicon melt placed in a crucible in one direction from the bottom of the crucible upward. An advantage of the cast growth method is that a large-capacity polycrystalline ingot can be manufactured relatively easily at a low cost.

  In recent years, in the production of silicon ingots for solar cells, monocast growth methods (also called seed cast methods and pseudo single crystal methods) have been actively researched and developed, and ingots produced by monocast growth methods have begun to be supplied to the market. Yes. In the monocast method, first, a seed crystal of single crystal silicon is arranged on the bottom of the crucible, and a silicon raw material is arranged on the seed crystal. Thereafter, all of the silicon raw material and a part of the seed crystal are melted so that at least a part of the seed crystal remains, and then the silicon melt is unidirectionally solidified in the same manner as in the cast growth method. In the monocast method, since the crystal is grown by taking over the crystal orientation of the seed crystal of the single crystal, it is possible to produce an ingot of a quality close to that of a single crystal. Therefore, when a solar cell is produced, it is produced from a conventional polycrystalline ingot. It is said that it is highly efficient.

  Further, since the growth apparatus and growth method are similar to the cast growth method, it is said that low cost and mass production close to the cast growth method are possible. The monocast method has been actively researched and developed in Japan and overseas in recent years, and the details of the monocast method and the characteristics of the ingot produced by the same method are known in various forms (for example, Non-Patent Document 1 and Patent Document 1).

  In addition to the monocast method, as a method for producing a silicon ingot using a seed crystal, a method of controlling a grain boundary in a polycrystalline ingot using a plurality of seed crystals (for example, Non-Patent Document 2), A method of using crystalline silicon as a seed crystal (for example, Patent Document 2) has been reported.

JP-A-10-007493 WO2007 / 004631 Publication

Nathan Stoddard, Bei Wu, Lan Witting, Magnus Wagener, Yondkook Park, George Rozgonyi and Roger Clark, “Casting Single Crystal Silicon: Novel Defect Profiles from BP Solar ’s Mono2 Wafer”, Solid State Phenomena 131-133 (2008) pp1-8. K. Kutsukake, N. Usami, K. Fujiwara, Y. Nose, and K. Nakajima, “Influence of structural imperfection of sigma 5 grain boundaries in bulk multicrystalline Si on their electrical activities”, Journal of Applied Physics. 101 (2007) 063509.

  In the above method including the monocast growth method, unlike the Czochralski method of producing single crystal silicon, the crystal grows by touching the crucible wall in the crucible. Different crystal grains nucleate from the crucible wall. These crystal grains having different crystal orientations expand in area as the crystal grows, and the area of the portion of the single crystal that inherits the orientation of the seed crystal is reduced, that is, polycrystallization is a problem.

  Countermeasures against this polycrystallization are taken by devising the temperature distribution in the growth apparatus, but the polycrystallization has not been completely suppressed. In addition, there has been almost no effort to devise the seed crystal side for polycrystallization. For example, in Patent Document 1 and Non-Patent Document 2, ingot production is performed using a plurality of seed crystals whose crystal orientations are controlled, but crystal grain boundaries formed between the seed crystals suppress polycrystallization. The effect is not sung. In addition, regarding the arrangement of the boundaries between the seed crystals, the effect of suppressing polycrystallization and improving the ingot quality yield is not considered.

  The present invention has been made in view of the above-described situation, and prevents the crystal grains nucleated by the crucible wall from expanding the area with the progress of crystal growth and inherits the crystal orientation of the seed crystal. The present invention provides a method for producing a silicon ingot capable of producing a high-quality silicon ingot having a high proportion of portions.

  According to the present invention, there is provided a method for producing a silicon ingot in which a silicon melt in a crucible inherits the crystal orientation of a silicon seed crystal disposed at the bottom of the crucible and grows unidirectionally from below to the seed crystal. It is composed of a plurality of single crystal silicon blocks with different orientations, and the boundary position between the single crystal silicon blocks is arranged at a distance of 5 mm or more and 50 mm or less from the crucible wall, taking over the boundary position of the seed crystal, and in the grown silicon ingot An ingot manufacturing method characterized in that a crystal grain boundary is formed is obtained.

  Furthermore, according to the present invention, there is obtained an ingot manufacturing method characterized in that the crystal orientation in the direction perpendicular to the bottom surface of the crucible is aligned within ± 10 ° in all the single crystal silicon blocks of the seed crystal.

  Furthermore, according to the present invention, the crystal orientation of the plane serving as the boundary between the blocks of the single crystal silicon block is a plane within ± 10 ° from the plane of mirror symmetry with respect to the crystal orientation of the single crystal silicon blocks on both sides. The ingot manufacturing method characterized by this can be obtained.

  According to the present invention, the crystal grains nucleated on the crucible wall are prevented from expanding the area as the crystal growth proceeds, and a high-quality silicon ingot having a large proportion of the single crystal portion that inherits the orientation of the seed crystal is produced. The effect that the manufacturing method of a silicon ingot which can perform and a silicon ingot can be provided is acquired.

Schematic diagram of seed crystal arrangement in crucible A schematic diagram in which a silicon ingot grows in a crucible and polycrystallization is suppressed by a grain boundary. The schematic diagram of the crystal orientation of the single crystal silicon block of a seed crystal. Illustration of mirror symmetry plane The schematic diagram of the seed crystal used for the Example. The cross-sectional structure of the silicon ingot produced in the Example. Distance from seed crystal [mm], A: 0, B: 4, C: 8, D: 12, E: 16, F: 20, G: 24, H: 28, I: 32. The dashes are those in which the crystal grains nucleated from the crucible wall are painted in gray for easy viewing.

  The present invention relates to a method for producing a silicon ingot, wherein a silicon melt in a crucible is unidirectionally grown from the bottom to the top while inheriting the crystal orientation of a silicon seed crystal disposed at the bottom of the crucible. The seed crystal is composed of a plurality of single crystal silicon blocks having different crystal orientations, and the boundary position between the single crystal silicon blocks is arranged at a distance of 5 mm to 50 mm from the crucible wall, and the seed crystal boundary position is taken over and grown. It is characterized in that crystal grain boundaries are formed in the silicon ingot.

  FIG. 1 shows an example of the seed crystal arrangement in the crucible. According to this silicon ingot manufacturing method, the crystal grain boundary controlled and controlled by the seed crystal extends almost in the growth direction of the ingot. Therefore, when the crystal grains nucleated on the crucible wall expand inward with the growth of the ingot, Since it collides with the crystal grain boundary, further area expansion is suppressed. FIG. 2 is a schematic view in which a silicon ingot grows in a crucible and polycrystallization is suppressed by a crystal grain boundary. When the boundary position of the single crystal silicon block of the seed crystal is closer than 5 mm from the crucible wall, the effect of suppressing polycrystallization cannot be sufficiently obtained, and when it is separated from the crucible wall by 50 mm or more, the portion not used as a solar cell near the crucible wall Therefore, the range of polycrystallization expands beyond this range, leading to a decrease in ingot quality yield. Therefore, by arranging the boundary of the single crystal silicon block of the seed crystal at a distance of 5 mm or more and 50 mm or less from the crucible wall, it is possible to obtain effective suppression of polycrystallization for improving the ingot quality yield.

  FIG. 3 shows a schematic diagram of the orientation of the single crystal silicon block. According to this invention, the grain boundary formed by growing from the boundary between the single crystal silicon blocks aligned in the direction perpendicular to the bottom surface of the crucible grows from the boundary of the single crystal silicon block of random crystal orientation. Since the structure is more stable than the grain boundary formed in this manner, a flat grain interface perpendicular to the bottom of the crucible is obtained, and a high inhibitory effect on the progress of polycrystallization is obtained. However, if the crystal orientation in the direction perpendicular to the bottom of the crucible of the single crystal silicon block differs by 10 ° or more, the structure of the grain boundary formed by growing from the boundary between the blocks is a single crystal silicon block having a random crystal orientation. Therefore, in order to obtain the above effect, it is necessary to align the crystal orientation to 10 ° or less.

  FIG. 4 is an explanatory diagram of a mirror-symmetric surface. According to the present invention, when the crystal grain interface is a mirror symmetry plane with respect to the crystal orientation of the crystal grains on both sides of the crystal grain boundary, the structure has a stable structure, so that a high suppression effect on the progress of polycrystallization Is obtained. However, if the plane of the crystal grain boundary deviates by 10 ° or more from the mirror symmetry plane, the structure approaches a random structure. Therefore, the crystal grain boundary plane can be controlled to a plane within 10 ° from the mirror symmetry plane. is necessary.

  The crystal orientation and interface orientation of the single crystal silicon block can take any orientation, but in order to increase the effect of suppressing polycrystallization, it is better to satisfy a specific orientation relationship. Specifically, the crystal orientation in the direction perpendicular to the bottom surface of the crucible of all the blocks is desirably aligned with any of low index orientations such as <100>, <110>, and <111> with an accuracy of ± 10 ° or less. .

  The single crystal silicon block used for the seed crystal of the present invention is not only a single crystal silicon ingot produced by the Czochralski method or the floating zone method, but also reuses the seed crystal used in this production method. It is also possible to use a silicon block cut out from a silicon ingot produced by the method of the present invention.

  The crucible used in the present invention is made of silica having a square or round horizontal cross section, and is used by applying a release agent such as silicon nitride powder on the inner surface. Further, the unidirectional solidification method of the present invention includes a method of moving the crucible position downward, a method of moving the heater upward, and a heater output gradually when the temperature gradient is spatially low at the bottom and high at the top. Any method can be used as long as unidirectional solidification is possible, such as a method, a method of removing heat from the lower part by opening the shutter, or a combination of these methods.

  Examples carried out to confirm the effects of the present invention are shown below. A silicon seed crystal having an outer diameter of 30 mm and a height of 50 mm was set in a silica crucible having an inner diameter of 31 mm and a depth of 70 mm coated with silicon nitride powder on the inner surface. FIG. 5 shows a schematic diagram of the seed crystal used in the examples. It is composed of a central 20 mm square silicon block and four pieces of silicon blocks surrounding it. In these silicon blocks, the crystal orientation in the direction perpendicular to the bottom surface of the crucible is set to <100> orientation, and the boundary surface between the central 20 mm square portion and the surrounding portion is a mirror surface with respect to the crystal orientation of the crystal on both sides of the boundary. It became a symmetrical plane, and the crystal orientation difference (angle in FIG. 4-34) of the crystals on both sides was 37 °. The crystal orientation control accuracy for the direction perpendicular to the bottom surface of the crucible and the mirror symmetry of the boundary surface of this seed crystal was 2 ° or less. In this example, the seed crystal itself was used as a silicon raw material, and a part of the seed crystal was melted to form a silicon melt and used for ingot growth.

  Crystal growth was performed according to the procedure described below. After placing the crucible in which the seed crystal is set at a predetermined position in the manufacturing apparatus, the temperature is raised in an Ar gas atmosphere, and the temperature of the seed crystal is about 10 ° C./cm in which the lower temperature is low and the upper temperature is high. The seed crystal was melted leaving the lower 10 mm. Next, the crucible was moved 100 mm downward at a speed of 0.2 mm / min to solidify the silicon melt in the crucible unidirectionally upward. After the movement of the crucible was completed, the temperature in the manufacturing apparatus was lowered to room temperature to complete the growth.

  The obtained silicon ingot was cut along a cross section perpendicular to the ingot growth direction, and from observation of the crystal grain distribution in the cross section, the area expansion accompanying crystal growth of the crystal grains generated from the crucible wall was examined. FIG. 6 shows a cross-sectional structure of the silicon ingot produced in the example. It can be confirmed that a plurality of crystal grains are generated from the crucible wall in the cross section at a position of 8 mm from the seed crystal (FIG. 6C). Furthermore, it can be seen that the area of the crystal grains is increased as the crystal growth proceeds, that is, as the distance from the seed crystal increases. However, at a position of 16 mm from the seed crystal (FIG. 6E), these crystal grains collide with the grain boundary formed by controlling the boundary position between the silicon blocks of the seed crystal, and thereafter the crystal inside the ingot. It can be seen that the grain area expansion was suppressed. Finally, in the cross section at a position of 32 mm from the seed crystal (FIG. 6I), the area of the central 20 mm square region grown by taking over the crystal orientation of the seed crystal is almost preserved. As described above, according to the present invention, the crystal grains nucleated from the crucible wall can be prevented from expanding the area with the progress of crystal growth, and the ratio of the area of the portion inheriting the crystal orientation of the seed crystal is high. It was confirmed that high-quality silicon ingots can be provided.

  According to the present invention, the crystal grains nucleated on the crucible wall are prevented from expanding the area with the progress of crystal growth, and a high-quality silicon ingot having a high proportion of the single crystal portion that inherits the crystal orientation of the seed crystal is produced. Therefore, it is possible to manufacture a silicon ingot for a solar cell with high performance and low cost.

1: crucible, 2: seed crystal, 3: crystal grown by inheriting crystal orientation of seed crystal, 4: silicon melt, 5: boundary of silicon block of seed crystal, 6: crystal formed by controlling with seed crystal Grain boundary, 7: crystal grains having a crystal orientation different from the crystal orientation of the seed crystal nucleated by the crucible wall.
21: Crystal orientation in a direction perpendicular to the bottom surface of the crucible in the left block, 22: Crystal orientation in a direction perpendicular to the bottom surface of the crucible in the right block, 23: Crystal orientation in a boundary surface between the blocks (left block), 24: Crystal orientation of the interface between blocks (right block).
31: Boundary surface between blocks of seed crystal, 32: Crystal lattice surface of left and right blocks, 33: Permissible range of angle error of boundary surface, 34: Difference in crystal orientation between left and right blocks.

Claims (3)

  A method for producing a silicon ingot, in which a silicon melt in a crucible inherits the crystal orientation of a silicon seed crystal disposed at the bottom of the crucible and grows in one direction from the bottom to the top. It is composed of crystalline silicon blocks, and the boundary position between the single crystal silicon blocks is arranged at a distance of 5 mm to 50 mm from the crucible wall, and the grain boundary is formed in the grown silicon ingot by taking over the boundary position of the seed crystal. An ingot manufacturing method characterized by the above.   2. The ingot manufacturing method according to claim 1, wherein the crystal orientation in a direction perpendicular to the bottom surface of the crucible is aligned within ± 10 ° in all the single crystal silicon blocks of the seed crystal.   3. The ingot manufacturing method according to claim 1, wherein the crystal orientation of a plane serving as a boundary between the blocks of the single crystal silicon block is ±± from a plane that is mirror-symmetric with respect to the crystal orientation of the single crystal silicon blocks on both sides. An ingot manufacturing method characterized by being a surface within 10 °.