WO2014141473A1

WO2014141473A1 - Method for producing and device for producing polycrystalline silicon ingot

Info

Publication number: WO2014141473A1
Application number: PCT/JP2013/057470
Authority: WO
Inventors: 章一日和佐
Original assignee: Hiwasa Shoichi
Priority date: 2013-03-15
Filing date: 2013-03-15
Publication date: 2014-09-18
Also published as: TW201435160A

Abstract

This device that is for producing a polycrystalline silicon ingot and that contains a crucible, which can house a silicon starting material, a heater, which is disposed at the periphery of the crucible, a thermal transmitter, which is disposed below the crucible, and a cooling means, which cools the thermal transmitter, is characterized by the thermal transmitter forming a bottomed cylinder that opens downwards, the outer surface of the bottom of the thermal transmitter being disposed opposing the outer surface of the bottom of the crucible, and the vicinity of the open end of the thermal transmitter being cooled by the cooling means.

Description

Polycrystalline silicon ingot manufacturing apparatus and manufacturing method thereof

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

Multi-crystalline silicon is mainly used as a substrate material for solar cells. Polycrystalline silicon used for solar cells and the like is usually manufactured by cutting a polycrystalline silicon ingot into an arbitrary size and shape. An ingot of polycrystalline silicon is usually contained in a crucible, and is manufactured by heating the crucible to obtain molten silicon, and then cooling the crucible from its bottom to solidify the silicon (see Patent Documents 1 and 2). ). In recent years, in order to obtain a large-diameter polycrystalline silicon wafer, there is a demand for manufacturing a larger polycrystalline silicon ingot. In order to manufacture a large polycrystalline silicon ingot, it is required to appropriately control the solidification rate of silicon by controlling the amount of heat removed from the bottom of the crucible. However, it is extremely difficult to control the solidification rate of silicon because the amount of heat removal needs to be delicately and precisely controlled (Patent Document 3). For example, during the solidification of molten silicon, the amount of heat removed from the bottom of the crucible tended to be large in the conventional heat removal method, so the temperature of the molten silicon (melt temperature) was set so that solidification did not proceed rapidly from the initial solidification to the middle. It had to be kept at a temperature well above the solidification temperature of silicon (1410 ° C.) (eg 1440-1460 ° C.). The silicon crystal obtained under such conditions has many lattice defects and has a quality problem such as low energy conversion efficiency when processed into a solar cell substrate. In addition, in order to appropriately control the solidification rate of silicon, a method for controlling the amount of heat removal by controlling the position of the crucible in the apparatus (Patent Document 3) and a method for controlling the temperature distribution in the crucible ( Patent Document 4) is known. However, with these methods, it has been difficult to completely control the amount of heat removed from the crucible.

JP 60-103017 A Japanese Unexamined Patent Publication No. Sho 63-166711 JP 2002-308616 A

An object of this invention is to provide the manufacturing apparatus for manufacturing a polycrystalline-silicon ingot which can control appropriately the amount of heat removal from a crucible.
Another object of the present invention is to provide a method for producing a good polycrystalline silicon ingot by appropriately controlling the amount of heat removed from the crucible.
In particular, the present invention can produce polycrystalline silicon ingots of various sizes by employing the above production apparatus and production method, and has very few crystal defects in the entire height direction of the polycrystalline silicon ingot. An object is to obtain a polycrystalline silicon ingot having excellent crystal quality.
The present invention uses a heat conductor for cooling a crucible disposed below a crucible in manufacturing a polycrystalline silicon ingot, the heat conductor has a specific shape, and a method for cooling the heat conductor This is based on the knowledge that the heat removal amount of the crucible can be controlled appropriately by optimizing the above.

Specifically, the present invention can include the following contents.
[1] Polycrystalline silicon including a crucible capable of containing a silicon raw material, a heater disposed around the crucible, a heat conductor disposed below the crucible, and a cooling means for cooling the heat conductor An ingot manufacturing apparatus, wherein the heat conductor is formed in a bottomed cylindrical body that opens downward, and a bottom outer surface of the heat conductor is disposed to face a bottom outer surface of the crucible, The polycrystalline silicon ingot manufacturing apparatus is cooled by the cooling means while arbitrarily changing an opening area in the vicinity of the opening end over the entire solidification period.
[2] The polycrystalline silicon ingot manufacturing apparatus according to [1], wherein the thermal conductor has a thermal conductivity of 30 to 150 W / m · K measured at 25 ° C. based on a flash method of JIS R1611.
[3] The polycrystalline silicon ingot manufacturing apparatus according to [1] or [2], wherein the thermal conductor is made of graphite.
[4] The polycrystalline silicon ingot manufacturing apparatus according to any one of [1] to [3], further including an induction heating coil for heating the heat conductor.
[5] The polycrystalline silicon ingot manufacturing apparatus according to any one of [1] to [4], further including a heat insulating material covering at least part of the crucible, the heater, and the heat conductor.
[6] The polycrystalline silicon ingot according to any one of [1] to [5], further comprising a lid that covers an open end of the crucible and a nozzle for blowing an inert gas into the crucible. Manufacturing equipment.

[7] (1) The process of accommodating the silicon raw material in a temperature-controllable crucible,
(2) a step of melting the silicon raw material by heating the silicon raw material in the crucible to a temperature equal to or higher than the melting point of silicon;
(3) The step of cooling the molten silicon in the crucible through a heat conductor disposed below the crucible to obtain a polycrystalline silicon ingot;
A method for producing a polycrystalline silicon ingot, comprising: a bottomed tubular body in which the heat conductor is opened downward, and a bottom outer surface of the heat conductor is disposed to face a bottom outer surface of the crucible, A method for producing a polycrystalline silicon ingot, wherein the vicinity of the open end of the heat conductor is cooled by the cooling means.
[8] The cooling causes the temperature (Tg) (° C.) of the bottom outer surface of the heat conductor to be expressed by the following linear approximation Tg = a−bt
(Wherein, a is 1,250 to 1,400, b is 10 to 35, and t is the elapsed time (hours) from the start of cooling)
The method for producing a polycrystalline silicon ingot according to [7], which is performed under the conditions:
[9] The method for producing a polycrystalline silicon ingot according to [7] or [8], wherein the heating is performed by a heater disposed around the crucible and the thermal conductor heated by an induction heating coil. .
[10] The method for producing a polycrystalline silicon ingot according to any one of [7] to [9], wherein the heating and cooling steps are performed in an inert gas atmosphere.

According to the present invention, it is possible to provide a manufacturing apparatus for manufacturing a polycrystalline silicon ingot capable of appropriately controlling the amount of heat removed from the crucible. In particular, heat can be appropriately removed from the crucible by the heat conductor having a special shape arranged below the crucible. In addition, according to the present invention, it is possible to provide a method for producing a polycrystalline silicon ingot having crystals of good quality by appropriately controlling the amount of heat removed from the crucible. In particular, the crucible is supercooled by controlling the cooling of the crucible by focusing on the temperature of the heat conductor close to the crucible without relying on the conventional crucible top temperature or the cooling conditions of the crucible itself. It can be cooled at an appropriate temperature without being done. In particular, by maintaining the superheat degree of molten silicon (molten silicon melt temperature−silicon solidification temperature) at 40 ° C. or less, preferably 10 ° C. or less, more preferably 2 to 5 ° C. High-quality polycrystalline silicon with very few crystal defects can be obtained.

It is the schematic of the heat conductor of this invention. It is a longitudinal cross-sectional view of the heat conductor of this invention. It is sectional drawing of the polycrystalline-silicon ingot manufacturing apparatus of this invention in the process of heating a crucible. It is sectional drawing of the polycrystalline-silicon ingot manufacturing apparatus of this invention in the process of cooling a crucible. FIG. 6 shows time changes from the start to the end of solidification of the temperature Ts (° C.) of molten silicon, the temperature Tx (° C.) inside the bottom of the crucible, and the temperature Tg (° C.) of the outer surface of the bottom of the heat conductor in Example 1. FIG. 4 shows time changes from the start to the end of solidification of molten silicon temperature Ts (° C.), crucible bottom inner temperature Tx (° C.), and heat conductor bottom outer surface temperature Tg (° C.) in Example 2. FIG. 5 is a time change from the start to the end of solidification of a temperature Ts (° C.) of molten silicon and a temperature Tg (° C.) of a bottom outer surface of a water-cooled copper cooling chiller (a graphite support plate in contact with a crucible bottom) in Comparative Example 1; It is a time change from the solidification start to completion | finish of solidification speed | rate (mm / min) in Example 1, 2 and Comparative Example 1. FIG.

[A] Polycrystalline silicon manufacturing apparatus Hereinafter, the polycrystalline silicon ingot manufacturing apparatus of the present invention will be described in detail.
(1) The polycrystalline silicon ingot manufacturing apparatus of the present invention includes a crucible that can contain a silicon raw material, a heater disposed around the crucible, a heat conductor disposed below the crucible, and the heat conductor. Cooling means for cooling. Moreover, the polycrystalline silicon ingot manufacturing apparatus of the present invention optionally includes an induction heating coil, a heat insulating material, a lid, a nozzle for blowing an inert gas, and the like. Hereinafter, each member will be described.

A crucible is used to contain a silicon raw material, heat the silicon raw material to obtain molten silicon, and further cool it to produce a polycrystalline silicon ingot. The shape of the crucible is preferably a bottomed cylindrical body having an open upper end, and more preferably the bottom is flat. The bottom may be a circle or a polygon that is greater than or equal to a triangle, but is preferably a circle or a rectangle. Although the size of the crucible depends on the size of the polycrystalline silicon to be produced, for example, when the crucible has a bottomed and square bottom, the outer size of the bottom is, for example, 300 to 1500 mm square, preferably Is 500 to 1200 mm square, more preferably 670 to 1000 mm square, and the thickness of the bottom is, for example, 1 to 50 mm, preferably 5 to 30 mm, more preferably 10 to 25 mm. The height of the crucible is, for example, 100 to 1000 mm, preferably 200 to 800 mm, more preferably 300 to 700 mm. For example, when the crucible is a bottomed cylinder, the bottom is, for example, a circle having a radius of 150 to 750 mm, preferably a radius of 250 to 600 mm, more preferably a radius of 335 to 500 mm, and the thickness of the bottom is, for example, 1 mm to It is appropriate that the thickness is 100 mm, preferably 5 mm to 70 mm, more preferably 10 mm to 50 mm. The height of the crucible is, for example, 100 to 1000 mm, preferably 200 to 800 mm, more preferably 300 to 500 mm. The thickness of the crucible wall is suitably 1 to 50 mm, preferably 5 to 30 mm, more preferably 10 to 25 mm, for example. The thermal conductivity of the crucible is, for example, 1.2 to 11.6 W / m · K, preferably 1.7 to 5.8 W / m · K, more preferably 2.3 to 4.7 W when measured at 25 ° C. based on the flash method of JIS R1611. / m · K is appropriate.
A lid can be provided on the top of the crucible.
The crucible and lid are not particularly limited, and known crucibles and lids used in ordinary polycrystalline silicon ingot production apparatuses can be used. For example, the crucible is preferably opaque quartz and the lid is preferably graphite. The wall surface of the crucible is preferably coated with silicon nitride containing, for example, 1 to 10% by mass, preferably 3 to 7% by mass, more preferably 4 to 6% by mass silica.

-Nozzle During the manufacture of the polycrystalline silicon ingot, particularly when heating and cooling the silicon raw material, a nozzle for blowing an inert gas into the crucible may be provided, and the inert gas may be blown into the crucible from the nozzle. As the inert gas, a rare gas is preferable, and for example, helium gas and argon gas are preferable. In addition to the inert gas, a gas such as carbon monoxide gas that can lower and control the oxygen concentration may be added. When carbon monoxide gas is added to the argon gas, the concentration of carbon monoxide gas is, for example, 600 to 1400 ppmm, preferably 800 to 1200 ppm, more preferably 1000 to 1100 ppm.
-Silicon raw material As the silicon raw material contained in the crucible, for example, polysilicon having a purity of 6-N or higher, preferably 7-N or higher, more preferably 9-N or higher (that is, a purity of 99.9999999%) is appropriate. It is.

Heater The heater is arranged around the crucible, that is, above and / or at least one side, preferably above. As the heater, if it is a temperature adjustable heater, a known heater such as a resistance heater or an induction heater using a graphite such as isotropic graphite or a carbon fiber composite (CCM) as a heating element is used. can do.
-Heat conductor A heat conductor is arrange | positioned under the crucible. One or more crucibles may be arranged on one heat conductor. Further, the heat conductor and the crucible may be in close contact with each other or may be separated from each other, and a support plate for supporting the crucible may be provided between the heat conductor and the crucible. The heat conductor is disposed so that the bottom outer surface of the heat conductor faces the bottom outer surface of the crucible. With this arrangement, the crucible is efficiently cooled through the heat conductor and sometimes heated. An example of the outer shape of the heat conductor is shown in FIG. FIG. 1 (a) shows a cylindrical body having a rectangular or square bottom. FIG. 1B shows a cylindrical body having a shape in which the bottom portion is rectangular or square and the bottom portion protrudes outward from the wall portion of the side wall by a certain length. FIG.1 (c) is a cylindrical body with a circular bottom part. The length of the bottom of the cylindrical body in FIG. 1B protruding from the side wall is, for example, 70 to 220 mm, preferably 60 to 120 mm, more preferably 50 to 100 mm. It may be protruding up to 30%, preferably 5-20%. The shape of the heat conductor is preferably a cylindrical body having a bottom with a bottom that is open at one end. The shape of the longitudinal section of the heat conductor is preferably concave (see FIG. 1 (a) or (c)), π-type (or saddle-shaped, see FIG. 1 (b)), and particularly concave. It is preferable. A longitudinal sectional view of the concave heat conductor is shown in FIG. The heat conductor has a bottomed cylindrical body that is open on the side opposite to the side where the crucible is arranged, that is, the bottom, and the inside of the heat conductor is hollowed out. The shape of the longitudinal section of the inner part of the heat conductor, that is, the hollowed-out part, is a polygon such as a triangle (see FIGS. 2B and 2C), a quadrangle (see FIG. 2D), or a half It may be a curved shape such as a circle (see FIG. 2E).
Further, as shown in FIG. 2 (a), the longitudinal sectional shape of the opening end portion (FIG. 2 (a) 9) may be a rectangle in which the thickness of the wall portion does not change, but as shown in FIG. 2 (f). Further, it may be thinner by one step or two steps than the thickness of the wall portion. For example, when the height of the wall (FIG. 2 (a) 7) is 500 to 1000 mm and the wall thickness is 60 to 180 mm, the height of the opening end (FIG. 2 (f) 9) is 50 to Appropriately, the thickness is 200 mm (5 to 20% of the height of the wall) and the thickness of the opening end is about 30 to 100 mm (30 to 60% of the thickness of the wall).

The thermal conductor has a thermal conductivity measured at room temperature (25 ° C.) (see JIS R1611 flash method), 30 to 150 W / m · K, preferably 40 to 120 W / m · K, more preferably 50 to Any material can be used as long as it is a solid of 100 W / m · K, but it is preferably made of graphite. Particularly preferable graphite that can be used as the heat conductor includes isotropic graphites G535, G330, G320, and G347 manufactured by Tokai Carbon Co., Ltd. The support plate is also preferably a solid having a thermal conductivity similar to that of the heat conductor, and more preferably made of graphite similar to the heat conductor.
In order to maintain the control accuracy of solidification, the size of the heat conductor is such that the heat capacity of the heat conductor is equal to or less than the sum of the heat capacity of the crucible at the start of solidification and the heat capacity of the raw silicon and the solidification latent heat of the raw silicon. It is important that it is 90% or less, preferably 1 to 80%, more preferably 10 to 60%. For example, when the heat conductor has a bottom and the bottom (that is, the inner bottom of the cylindrical body (FIG. 2 (a) 2)) is a square cylinder, the outer dimension of the bottom (FIG. 2 (a) 4 ) Is, for example, ± 400 mm of the outer dimensions of the crucible (the sum of the outer dimensions of each crucible when a plurality of crucibles are arranged), preferably ± 200 mm of the outer dimensions of the bottom of the crucible, more preferably ± 50 mm of the outer dimensions of the crucible. A size equal to is appropriate. Specifically, the outer dimension of the bottom of the heat conductor is, for example, 100 to 4000 mm square, preferably 200 to 2000 mm square, more preferably 300 to 1500 mm square. The thickness of the bottom (FIG. 2 (a) 6) is, for example, 5 mm to 200 mm, preferably 20 mm to 100 mm, more preferably 30 mm to 60 mm. The height of the cylindrical wall portion (FIG. 2 (a) 7) is suitably, for example, 250 mm to 2000 mm, preferably 400 mm to 1500 mm, more preferably 500 mm to 1000 mm. The thickness of the cylindrical wall (FIG. 2 (a) 8) is, for example, 20 mm to 300 mm, preferably 40 mm to 250 mm, and more preferably 60 to 180 mm.
For example, when the heat conductor has a bottomed cylindrical shape, the radius of the bottom is suitably, for example, ± 200 mm, preferably ± 100 mm, more preferably ± 50 mm of the radius of the crucible. Specifically, the radius of the bottom is, for example, 100 to 1000 mm, preferably 150 to 800 mm, more preferably 200 to 500 mm. The thickness of the bottom is, for example, 5 mm to 200 mm, preferably 20 mm to 100 mm, more preferably 30 mm to 60 mm. The thickness of the cylindrical wall is, for example, 20 mm to 100 mm, preferably 30 mm to 70 mm, more preferably 40 mm to 50 mm, and the height of the cylindrical wall is, for example, 250 mm to 700 mm, preferably 350 mm to 550 mm. The thickness is preferably 400 mm to 500 mm. The thickness of the support plate is, for example, 5 to 100 mm, preferably 10 to 60 mm, more preferably 20 to 50 mm.

-Cooling means The heat conductor is cooled by the cooling means in the vicinity of the open end (FIG. 2 (a) 9) of the heat conductor. In this way, by cooling only the vicinity of the opening end, heat is directly transferred through the inside of the heat conductor at the bottom of the heat conductor close to the opening end, and the bottom is cooled (heat conduction). Further, at the bottom of the thermal conductor far from the opening end, heat is transmitted to the vicinity of the opening end by radiation through the opening of the thermal conductor, and the bottom is cooled (thermal radiation). As described above, the heat conductor of the present invention has the opening and the vicinity of the opening end of the heat conductor is cooled by the cooling means, whereby the bottom of the heat conductor can be uniformly cooled.
The cooling means includes a cooling portion such as an induction heating coil cooled by a refrigerant for cooling the wall of the lower chamber and the coil portion that are cooled on the inside. Since these cooling parts are arranged on the outer side facing the opening end of the heat conductor, the surface from the opening end of the heat conductor to the cooling part is not contacted with the opening end of the heat conductor. Cooling is performed using radiant heat transfer. For cooling the cooling part, a liquid medium method such as a water cooling method or a gas medium method such as an inert gas or air cooling method can be used. The amount of radiant heat transfer (Q) is proportional to the difference between the fourth power of the surface temperature (T1) of the open end of the heat conductor and the fourth power of the surface temperature (T2) of the cooling part (Q = A × B × k × (T1 ⁴ -T2 ⁴ ), where A is the surface area of the open end of the heat conductor, B is the shape factor, k is the Boltzmann coefficient), and the surface temperature of the cooling part is measured with a temperature sensor, The amount of heat removal can be controlled while arbitrarily adjusting the surface area (A) of the open end of the conductor.
The portion near the opening end of the heat conductor cooled by the cooling means is not particularly limited, but when the height of the heat conductor cylindrical wall is 100%, it is 1 to 70%, preferably 1 from the opening end. An area with a height of ˜50%, more preferably 1-40%. The opening end itself may be included in the vicinity of the opening end. Specifically, the gap with the cooling means provided around the opening end of the heat conductor is, for example, larger than 0 mm and not more than 400 mm, preferably 1 mm to 300 mm, more preferably 10 mm to 200 mm.

-Heat insulating material When the silicon raw material in the crucible is heated by a heater, it is preferable from the viewpoint of thermal efficiency that the entire polycrystalline silicon manufacturing apparatus of the present invention, in particular, the entire crucible and the entire heat conductor is insulated from the outside. Therefore, it is preferable that the heat conductor and the cooling means are insulated during heating. As a means to insulate, for example, a mounting table for a heat conductor and a heat insulating material covering the crucible and the heat conductor may be closely attached. On the other hand, during cooling, for example, the lower heat insulating material in the vicinity of the opening end portion of the heat conductor is lowered by the lifting cylinder of the heat insulating material to create a gap in the vicinity of the opening end portion of the heat conductor. Through this gap, cooling is performed by radiant heat transfer to a cooling portion disposed on the outer peripheral portion without contacting a heat conductor such as a wall of the lower chamber or an induction heating coil. The amount of radiant heat transfer can be arbitrarily controlled by changing the area of the opening at the opening end of the heat conductor according to the temperature at the opening end surface of the heat conductor and the surface temperature of the cooling portion. Here, the heat insulating material can cover at least a part of the crucible, the heater back surface, and the heat conductor. As the heat insulating material, a heat insulating material having heat resistance of at least 2000 ° C. or more, preferably 2500 ° C. or more is preferable. As the heat insulating material, a carbon fiber molded heat insulating material or the like is desirable. Examples of preferable heat insulating materials include DONACARBO (DON-1000, DON-2000, DON-3000, DON-4000) manufactured by Osaka Gas Chemical Co., Ltd.

-Induction heating coil A heat conductor may be heated by an induction heating coil, and may be used as a heater which can be heated from the lower part of a crucible. Since the induction heating coil can be arranged separately from the heat conductor, the heat conductor can be heated from the outside of the heat insulating material even when the heat conductor is covered with the heat insulating material. In addition, since the induction heating coil can heat not only the surface of the heat conductor but also the inside of the heat conductor, there is an advantage that the bottom of the heat conductor can be uniformly heated. The induction heating coil is preferably provided with a temperature sensor so that the temperature can be adjusted. The induction coil is controlled to a frequency of 1 to 500 Hz, preferably 10 to 300 Hz, more preferably 30 to 100 Hz, for example.

[B] Polycrystalline silicon ingot manufacturing method The polycrystalline silicon manufacturing method of the present invention is performed using the above-described polycrystalline silicon ingot manufacturing apparatus, and includes the following steps:
(1) A process of containing silicon raw material in a crucible whose temperature can be adjusted,
(2) a step of melting the silicon raw material by heating the silicon raw material in the crucible to a temperature equal to or higher than the melting point of silicon;
(3) a step of cooling the molten silicon in the crucible through a heat conductor disposed below the crucible to obtain polycrystalline silicon;
including.

(1) Step of accommodating silicon raw material First, a silicon raw material is accommodated in a crucible. The details of the silicon raw material and the crucible are as described above.
(2) Step of heating crucible Next, the silicon raw material in the crucible is heated to a temperature equal to or higher than the melting point of silicon to melt the silicon raw material. Heating is performed by a heater arranged around the crucible, preferably above. Details of the heater are as described above.
Since the melting point of silicon is about 1410 ° C., the temperature of the molten silicon raw material in the crucible is sufficiently higher than the melting point, for example, 1450 ° C. or more, preferably 1450 to 1600 ° C., more preferably The temperature is about 1500 to 1550 ° C.
Heating may be performed by a combination of the heater and a heat conductor heated by an induction heating coil. Heating with a heater and a heat conductor is efficient because it can be heated from above and below the crucible. The details of the induction heating coil are as described above.

(3) Step of cooling the crucible The molten silicon in the crucible is cooled via a heat conductor disposed below the crucible. In order to produce homogeneous polycrystalline silicon, it is particularly preferable that the cooling be performed while keeping the temperature of the bottom outer surface of the heat conductor (the side close to the bottom outer surface of the crucible) uniform. When the temperature of the bottom outer surface of the heat conductor is Tg (° C.), Tg is the following general formula (primary approximation formula):
Tg = a−bt
It is preferable to lower the temperature under conditions that satisfy In the formula, a is 1,250 to 1,400 (° C.), preferably 1,290 to 1,380 (° C.), more preferably 1,300 to 1,360 (° C.), and b is 10 to 35, preferably 15 to 33, more preferably 17 to 30 and t is the elapsed time (hours) from the start of cooling. In other words, it is preferable to lower Tg almost linearly at a rate of 10 to 35 ° C./hour, preferably 15 to 33 ° C./hour, more preferably 17 to 30 ° C./hour.
The temperature Tx at the portion where the molten silicon and the bottom of the crucible are in contact is suitably 40 to 200 ° C., preferably 50 to 150 ° C., more preferably 60 to 120 ° C. higher than Tg. Here, the solidification rate of the molten silicon is, for example, 0.1 to 1.5 mm / min, preferably 0.2 to 1.0 mm / min, and more preferably 0.3 to 0.5 mm / min. Is appropriate. The superheat degree of molten silicon (the temperature of molten silicon−the solidification temperature of silicon) is suitably maintained at, for example, 40 ° C. or less, preferably 10 ° C. or less, more preferably 2 to 5 ° C. over the entire solidification time. .

[C] Description of Polycrystalline Silicon Manufacturing Apparatus with Drawings Hereinafter, a polycrystalline silicon manufacturing apparatus according to the present invention will be described with reference to the drawings.
FIG. 3A is a cross-sectional view of the polycrystalline silicon manufacturing apparatus of the present invention in the process of heating the crucible. FIG.3 (b) is sectional drawing of the polycrystalline-silicon manufacturing apparatus of this invention in the process of cooling a crucible.
As shown in FIGS. 3 (a) and 3 (b), the polycrystalline silicon manufacturing apparatus (101) of the present invention is arranged above the crucible (103) capable of accommodating the silicon raw material (102) and the crucible (103). An upper chamber (105) including a heater (104), a heat conductor (106) disposed below the crucible (103), a cooling means (107) for cooling the heat conductor (106), and cooling A lower chamber (108) including portions (108, 113). The inside of the wall of the lower chamber (108) used as a cooling part and the inside of the induction heating coil (113) are cooled by a liquid medium method such as a water cooling method or a gas medium method such as an inert gas or air cooling method. A lid (109) may be provided on the upper part of the crucible (103), and a support plate (110) for supporting the crucible (103) may be provided on the lower part of the crucible. A nozzle (111) may be provided in the crucible (103), and an inert gas or the like is blown from the nozzle (111). The crucible (103), the heater (104), and the heat conductor (106) are covered with heat insulating materials (112) and (114). In FIG. 3A, since the lower heat insulating material (114) constituting the cooling means (107) is in contact with the upper heat insulating material (112), the gap (116) which is also a part of the cooling means (107). (See FIG. 3B) is closed. The heat conductor (106) is heated by an induction heating coil (113) arranged outside the heat insulating material (112). Therefore, the cooling means (107) does not function in FIG. On the other hand, as shown in FIG. 3B, in the process of cooling the crucible (103), the lower heat insulating material (114) is lowered by the cylinder (115), and a gap (116) is provided. As a result, the vicinity of the open end of the heat conductor (106) in contact with the gap (116) is cooled. In addition, the bottom temperature of the heater (104), the heat conductor (106), the opening end surface temperature, the front side temperature of the induction heating coil (113), and the furnace inner surface of the wall of the lower chamber are respectively temperature sensors (117). Can be measured.

[D] Evaluation of polycrystalline silicon The characteristics of the polycrystalline silicon ingot produced by the above method can be evaluated by measuring the energy conversion efficiency as a solar cell. The energy conversion efficiency is usually evaluated based on STC (Standard Test Cell conditions) using a solar simulator or the like manufactured by Solar Light. Note that STC is evaluated based on the maximum output power of a solar cell measured at a solar cell temperature of 25 ° C. using a solar simulator with solar radiation intensity of 1.0 kW / m ² and air mass AM = 1.5 as a light source. As a sample, a solar cell prepared by cutting a silicon wafer from the obtained polycrystalline silicon ingot can be used.
EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention further in detail, an Example is an example of this invention and does not limit the scope of the present invention.

Examples 1 and 2
Polycrystalline silicon was manufactured using a polycrystalline silicon manufacturing apparatus as shown in FIGS. 3 (a) and 3 (b). However, the support plate (110) was not used in the examples and comparative examples.
As the crucible, a bottomed cylindrical crucible having a square bottom of 1000 mm square and a height of 650 mm was used. The thermal conductivity of the crucible was 4.5 W / m · K (measurement temperature 950 ° C.). The thickness of the crucible is 25 mm. The crucible is made of opaque quartz, and a coating of silicon nitride added with 5% by mass of silica is applied to the inner surface of the crucible. As the heat conductor, an isotropic graphite heat conductor G535 made by Tokai Carbon Co., Ltd. having a concave longitudinal section was used. The heat conductor has a bottomed outer dimension of a square of 1100 mm square, and has a bottomed cylindrical shape with a wall height (Fig. 2 (a) 7) of 500 mm (Examples 1 and 2) (Fig. 1). (See (a), FIG. 2 (a)). The thickness of the bottom and wall of the heat conductor (FIGS. 2 (a) 6 and 8) is 150 mm (Examples 1 and 2). The thermal conductivity of the thermal conductor was 81 W / m · K (measured at room temperature (see JIS R1611 flash method)). The air gap of the cooling means provided around the open end of the heat conductor was adjusted in the range of greater than 0 mm and not greater than 200 mm.
Polysilicon (purchased by Wacker) with a purity of 99.9999999% or more was used as the silicon raw material contained in the crucible.
As a heater, isotropic graphite G535 manufactured by Tokai Carbon Co., Ltd. was used, and as a dielectric heating coil, a low frequency induction heating device (output 50 kW) manufactured by FUJI Electric Furnace was used. Moreover, Osaka Gas Chemical Co., Ltd. DONACARBO (DON-1000) was used for the heat insulating material in an apparatus.
First, the crucible was heated by a heater above the crucible and a heat conductor heated by a dielectric heating coil (50 Hz) so that the temperature of the molten silicon was 1550 ° C.
After all of the silicon raw material has become molten silicon, stop heating the heat conductor, lower the lower heat insulating material for the heat conductor, provide a gap near the opening end of the heat conductor, and cool the heat conductor. Started. The temperature profile of the temperature Tg (° C.) of the bottom outer surface of the heat conductor is the following first-order approximation:
Tg = a−bt
Can be expressed as In the formula, a and b are as shown in Table 1 below, and t is the elapsed time (hours) from the start of cooling. In Examples 1 and 2, the height of silicon added to the crucible is 300 mm, and the temperature Ts of the molten silicon surface during solidification is 2 ° C. higher than the melting point 1410 ° C. (superheat = 2 ° C., Ts = 1412 ° C). The solidification speed Vs is a target of 0.3 mm / min in Example 1, and a target of 0.5 mm / min in Example 2. The graphs showing changes in Ts (° C.), Tx (° C.), Tg (° C.) and coagulation rate in Examples 1 and 2 are shown in FIGS. 4, 5, and 7, respectively. In the final solidification stage, the heater output level above the crucible was kept constant in order to solidify the molten silicon.

Comparative Example 1
Instead of the heat conductor of the present invention, a water-cooled copper chiller (commercially deoxidized copper, 1,000 mm square, box-type cooling structure, cooling plate thickness: 12 mm, cooling water: A polycrystalline silicon ingot was produced in the same manner as in Example 1 except that a silicon raw material having a height of 250 mm was used and the silicon ingot was solidified when placed in the crucible. . Changes in the molten silicon surface temperature Ts (° C.), the temperature Tg (° C.) of the bottom outer surface of the water-cooled copper cooling chiller (the side in contact with the crucible bottom), and the solidification rate are shown in FIGS.

Evaluation The lower end 20 mm, the upper end 10 mm, and the side 20 mm of the silicon ingots of Examples and Comparative Examples manufactured as described above were cut off, and 36 150 mm square blocks were cut out from the remaining ingots. From 36 blocks, a silicon wafer having a cross-sectional size of 150 mm and a thickness of 200 μm was cut out to produce solar cells. The energy conversion efficiency of the solar cell was measured under the above-mentioned STC conditions using a solar simulator (16S-300-002) manufactured by Solarlight. Table 2 summarizes the distribution of energy conversion efficiency for all wafers corresponding to the silicon ingots obtained in Example 1, Example 2, and Comparative Example.

Table 1

Table 2

* The numerical values in Examples 1 and 2 and Comparative Example 1 are the ratio (%) of the number of wafers showing the corresponding conversion efficiency in all wafers.

As shown in Table 2 above, in both Examples 1 and 2, the number of silicon wafers having an energy conversion efficiency of 17% or more, which is said to be relatively high, was obtained by 70% or more of the total, whereas 20% in the conventional method The yield was less than one third.
As a result, the polycrystalline silicon ingot producing apparatus of the present invention was able to maintain the molten silicon temperature at a temperature slightly higher than the melting point over the entire solidification region, and also maintain the solidification rate at a low and constant level throughout the solidification region. Polycrystalline silicon produced from the obtained polycrystalline silicon ingot had energy conversion efficiency superior to that of the comparative example.

DESCRIPTION OF SYMBOLS 1 Bottom part outer surface 2 Bottom part inner surface 3 Wall part 4 Bottom part outside dimension 5 Bottom part inside dimension 6 Bottom part thickness 7 Wall part height 8 Wall part thickness 9 Opening end part 101 Polycrystalline silicon manufacturing apparatus 102 Silicon raw material 103 Crucible 104 Heater 105 Upper chamber 106 Thermal conductor 107 Cooling means 108 Lower chamber 109 Lid 110 Support plate 111 Nozzle 112 Upper heat insulating material 113 Induction heating coil 114 Lower heat insulating material 115 Cylinder 116 Air gap 117 Temperature sensor

Claims

A polycrystalline silicon ingot manufacturing apparatus comprising a crucible capable of containing a silicon raw material, a heater disposed around the crucible, a heat conductor disposed below the crucible, and a cooling means for cooling the heat conductor The heat conductor is a bottomed cylindrical body that opens downward, the bottom outer surface of the heat conductor is disposed to face the bottom outer surface of the crucible, and the open end of the heat conductor An apparatus for producing a polycrystalline silicon ingot, wherein the vicinity is cooled by the cooling means.
2. The polycrystalline silicon ingot producing apparatus according to claim 1, wherein the thermal conductor has a thermal conductivity of 30 to 150 W / m · K measured at 25 ° C. based on a flash method of JIS R1611.
The polycrystalline silicon ingot manufacturing apparatus according to claim 1 or 2, wherein the thermal conductor is made of graphite.
The polycrystalline silicon ingot manufacturing apparatus according to any one of claims 1 to 3, further comprising an induction heating coil for heating the heat conductor.
The polycrystalline silicon ingot manufacturing apparatus according to any one of claims 1 to 4, further comprising a heat insulating material covering at least part of the crucible, the heater, and the heat conductor.
The polycrystalline silicon ingot manufacturing apparatus according to any one of claims 1 to 5, further comprising a lid that covers an open end of the crucible and a nozzle for blowing an inert gas into the crucible.
(1) A process of containing silicon raw material in a crucible whose temperature can be adjusted,
(2) a step of melting the silicon raw material by heating the silicon raw material in the crucible to a temperature equal to or higher than the melting point of silicon;
(3) The step of cooling the molten silicon in the crucible through a heat conductor disposed below the crucible to obtain a polycrystalline silicon ingot;
A method for producing a polycrystalline silicon ingot, comprising: a bottomed tubular body in which the heat conductor is opened downward, and a bottom outer surface of the heat conductor is disposed to face a bottom outer surface of the crucible, A method for producing a polycrystalline silicon ingot, wherein the vicinity of the open end of the heat conductor is cooled by the cooling means.
The cooling reduces the temperature (Tg) (° C.) of the bottom outer surface of the heat conductor to the following first-order approximation Tg = a−bt
(Wherein, a is 1,250 to 1,400, b is 10 to 35, and t is the elapsed time (hours) from the start of cooling)
The method for producing a polycrystalline silicon ingot according to claim 7, wherein the method is performed under the following conditions.
The method for producing a polycrystalline silicon ingot according to claim 7 or 8, wherein the heating is performed by a heater disposed around the crucible and the thermal conductor heated by an induction heating coil.
The method for producing a polycrystalline silicon ingot according to any one of claims 7 to 9, wherein the heating and cooling steps are performed in an inert gas atmosphere.