US20090044926A1

US20090044926A1 - Silicon casting apparatus

Info

Publication number: US20090044926A1
Application number: US12/222,743
Authority: US
Inventors: Michio Kida; Kenichi Sasatani; Mitsuo Yoshihara; Tomohiro Onizuka
Original assignee: Sumco Solar Corp
Current assignee: Sumco Solar Corp
Priority date: 2007-08-17
Filing date: 2008-08-15
Publication date: 2009-02-19
Also published as: JP5040521B2; JP2009046339A

Abstract

In a silicon casting apparatus according to the present invention, silicon melted by electromagnetic induction heating is continuously solidified using an electrically-conductive bottomless cold crucible and an induction coil surrounding the cold crucible. The cold crucible is made of copper alloy containing beryllium (desirably containing beryllium of 0.1 to 5 mass %), whereby the generation of electric-discharge flaw can be effectively prevented in performing electromagnetic casting. The use of the silicon casting apparatus according to the present invention can greatly extend a crucible life to reduce facility costs. Additionally, a solar-cell silicon ingot can be produced with high quality.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a silicon casting apparatus suitable for production of a solar-cell silicon ingot and the like. The silicon casting apparatus includes an electrically-conductive copper-alloy cold crucible divided into a plurality of elements in a circumferential direction, and is used when silicon as raw material is melted by electromagnetic induction heating and solidified to perform continuous casting of a polycrystalline silicon.
2. Description of the Related Art
A silicon crystal is used as a substrate material in almost all currently-produced solar cells. The silicon crystal is classified into a single crystal and a polycrystal. Usually, solar cells having higher efficiency in converting incident light energy into electric energy can be obtained when the single crystal is used as the substrate.
The single-crystal silicon is produced by the Czochralski method, and a high-quality dislocation-free crystal can be obtained. However, when compared with the polycrystalline silicon, solar-cell production costs are increased since the silicon production costs are increased. On the other hand, the polycrystalline silicon is usually produced by a casting method (hereinafter also referred to as “cast method”) in which a molten silicon is solidified by a mold, or a continuous casting method (hereinafter also referred to as “electromagnetic casting method”) in which electromagnetic induction is utilized. The substrate material can be produced at low costs in comparison with the single-crystal silicon substrate produced by the Czochralski method.
However, since the cast method is an ingot making method of solidifying the molten silicon with a quartz crucible or a graphite mold, impurity contamination is generated due to contact of the molten silicon with the wall of a container such as the crucible. There is also a problem that a mold lubricant used to prevent fusion between the ingot and the mold is mixed into the molten silicon. Additionally, since continuous casting can be hardly performed by the cast method, production efficiency is inevitably lowered.
The electromagnetic casting method is a method which has been developed in order to solve the problems of the cast method. In the electromagnetic casting method, the silicon crystal can be cast while the molten silicon scarcely contacts the crucible or the mold.
For example, as disclosed in Japanese Patent Application Publication No. 61-52962, a bottomless cold crucible is used in the electromagnetic casting method. The cold crucible is made of material (usually, copper) having good electric conductivity and thermal conductivity, being comprised of a plurality of strip-like elements inside a high-frequency induction coil, the strip-like elements are electrically insulated from one other in a circumferential direction, and the inside of the element is water cooled. A sectional shape of the coil and a sectional shape of a portion surrounded by the strip-like elements constituting a crucible may be formed into either a cylindrical body or a rectangular cylindrical body.
Silicon as raw material is loaded in the copper cold crucible which is formed as a melting vessel, and an alternating current is passed through the high-frequency induction coil. Since the strip-like elements constituting the cold crucible are electrically insulated from each other, the current forms a loop in each element, and the current on the inner wall side of the cold crucible forms a magnetic field in the cold crucible to heat and thermally melt the silicon in the crucible. An inwardly force in a direction normal to the surface of the molten silicon is exerted to the molten silicon in the crucible by interaction of the magnetic field formed by the current of the cold crucible inner wall and the skin current of the molten silicon, whereby the silicon is melted without contacting the crucible.
When a support pedestal which retains the molten silicon and an ingot beneath the cold crucible is moved downward while the silicon is melted in the crucible, the induction magnetic field becomes weaker as the support pedestal is moved away from a lower end of the high-frequency induction coil. Therefore, the generated current is lowered to decrease an amount of heat generation, and solidification progresses upward in a bottom portion of the molten silicon. In accordance with the downward movement of the support pedestal, the raw material is continuously loaded from above the crucible to continue the melting and solidification, whereby the silicon polycrystal is continuously cast while solidified unidirectionally.
In the electromagnetic casting method, the impurity contamination can be prevented since the molten silicon scarcely contacts the crucible wall. Since the contamination from the crucible is eliminated, advantageously, it is not necessary to use a high-purity material as a crucible material. Additionally, the production costs can be substantially reduced due to the continuous casting.
However, in the actual operation, warping or damaging takes place in slits between the strip-like elements constituting the cold crucible. Therefore, a frequent repair is required, and a crucible life is short, which increases facility costs. The slit warping or damaging is caused by electric discharging which is generated by the contact of the molten silicon with the electrically-charged crucible surface (that is, surface of the each strip-like element). The slit warping or damaging is generally called “electric-discharge flaw”. When the slit is degraded due to the increase in electric-discharge flaw thus rendering the slit warping or damaging sorely, an eddy current generated in the molten silicon is weakened to decrease heat quantity for melting the raw material. The casting becomes unstable to thereby lower the obtained ingot quality.
FIGS. 3A to 3C are photographs illustrating progression of deterioration of the slit portion in the copper cold crucible which is used for the purpose of comparison in the embodiments described below. FIG. 3A shows the normal slit portion before the casting is performed, and FIGS. 3B and 3C show the slit portion after the casting is performed. FIG. 3B shows the state of the slit portion after being used 6 times in the casting, and FIG. 3C shows the state of the slit portion after being used 8 times in the casting. As can be seen from FIGS. 3A to 3C, the deterioration of the slit portion becomes much distinctive as the frequency of use thereof in the casting is increased

SUMMARY OF THE INVENTION

An object of the present invention is to provide a silicon casting apparatus having a cold crucible which can extend the crucible life while heat quantity for melting the raw material is ensured by preventing the generation of the electric-discharge flaw in the surface of each strip-like element constituting the cold crucible, when the polycrystalline silicon is continuously cast by solidifying silicon as raw material after the silicon as raw material is melted by electromagnetic induction heating using the electrically-conductive bottomless cold crucible which is divided into a plurality of elements in a circumferential direction.
The inventors studied adoption of a crucible made of material having an excellent resistance to electrically discharging generated by the contact of the molten silicon with the crucible surface (surface of the each strip-like element), in other words, material in which the warping or damaging is hardly generated in the slits, as one of the means for solving the problems. The reason why they studied adoption of the crucible is that even if the electric discharging is generated, when the electric-discharge flaw is hardly generated due to the great resistance to the electric discharging, the slit warping or damaging is reduced to extend the cold crucible life, and the increase in facility costs can be suppressed.
In addition to the excellent electric conductivity and thermal conductivity, the conditions which should be possessed by the cold crucible include excellent hardness, strength, and wear-resistant property at high temperatures in order to prevent or suppress the generation of the electric-discharge flaw.
From such standpoints, the inventors focused attention on a copper alloy containing beryllium. For example, beryllium copper (a copper alloy in which beryllium of about 2 mass % and a small amount of cobalt are contained in copper) defined by JIS H 3270 is utilized as a high-conductivity spring, a spot welding electrode, and the like, so that the beryllium copper is considered to be excellent in terms of hardness, strength, and wear-resistant property at high temperatures when compared with copper.
The heat treatment and characteristics of the casting beryllium copper are described in Standard Metal Engineering Course 4 “Non-Ferrous Metal Material”, Masataka Sugiyama, Corona Publishing Co., Ltd., First edition in 30 Jun. 1963 (hereinafter referred to as “Non-Patent Document”).
FIG. 1 shows tensile strength and Brinell hardness (either one indicating a value after a solution heat treatment and aging treatment), excerpts from Table 1-56 (heat treatment and characteristics of casting beryllium copper) of page 105 in Non-Patent Document, while a horizontal axis indicates a beryllium content. In Table 1-56, the beryllium content, the tensile strength, and the Brinell hardness each are grouped in certain range. However, for the purpose of convenience, a median in each numerical range is adopted in FIG. 1. The tensile strength is expressed while a unit of (kg/mm²) is converted into a unit of (N/mm²).
As can be seen from FIG. 1, both the tensile strength and the Brinell hardness are substantially linearly increased with the beryllium content increasing. Although FIG. 1 shows the test data measured at room temperature, it is considered that the beryllium copper is excellent in hardness and strength at high temperatures since the beryllium copper is used in a welding electrode. Therefore, it is assumed that the tendency shown in FIG. 1 is sustained at high temperatures.
Accordingly, the material of the cold crucible is changed from pure copper to beryllium copper, the cold crucible is prepared, and the electromagnetic casting is actually performed. As a result, it is confirmed that the generation of the electric-discharge flaw is prevented to thereby substantially extend the cold crucible life.
The present invention is made based on the above-described study result, and the gist thereof pertains to the silicon casting apparatus below.
In accordance with an aspect of the present invention, a silicon casting apparatus in which it includes: (a) an electrically-conductive bottomless cold crucible in which a portion of the cold crucible along its axial length is divided into a plurality of elements in a circumferential direction; and (b) an induction coil which surrounds the cold crucible, thereby enabling silicon to be melted by electromagnetic induction heating using the induction coil and to be withdrawn downward and solidified, is characterized in that said cold crucible is made of copper alloy containing beryllium.
In the silicon casting apparatus according to the present invention, it is preferable that said copper alloy contains beryllium of 0.1 to 5 mass %, since a cold crucible life extension effect is clearly recognized without accompanying any difficulty in preparing the cold crucible.
The silicon casting apparatus according to the present invention, as a vessel for melting the raw material, includes a bottomless cold crucible made of copper alloy containing beryllium, in which the crucible is divided into a plurality of elements in a circumferential direction. In performing the electromagnetic casting, the silicon casting apparatus can prevent effectively the generation of the electric-discharge flaw in comparison with the use of the conventional copper bottomless cold crucible.
Therefore, the crucible life can be remarkably extended to contribute to the facility cost reduction, and the heat quantity for melting the raw material can be ensured to stably perform the casting. Additionally, the impurity contamination can be prevented since the molten silicon scarcely contacts the crucible wall. Therefore, the silicon casting apparatus according to the present invention is suitable for the production of the solar-cell silicon ingot requiring high quality.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view based on Table 1-56 of Non-Patent Document showing a relationship between a beryllium content in a Cu—Be alloy and tensile strength and Brinell hardness;

FIG. 2 schematically shows a configuration example of a silicon casting apparatus according to the present invention;

FIGS. 3A to 3C are photographs illustrating progression of deterioration of a slit portion in a copper cold crucible which is used for the purpose of comparison, FIG. 3A shows the normal slit portion before casting is performed, and FIGS. 3B and 3C show the slit portions after used 6 and 8 times in the casting, respectively; and

FIGS. 4A and 4B are photographs illustrating progression of deterioration of a slit portion in a cold crucible made of a copper alloy containing beryllium, which is used in the silicon casting apparatus of the present invention, and FIGS. 4A and 4B show the slit portions after being used 8 and 16 times in the casting, respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As described above, a silicon casting apparatus according to the present invention is characterized in that a cold crucible is made of copper alloy containing beryllium in an apparatus for producing a silicon ingot by an electromagnetic casting method using an electrically-conductive bottomless cold crucible in which a portion thereof along an axial length is divided into a plurality of elements in a circumferential direction.
FIG. 2 schematically shows a configuration example of a silicon casting apparatus according to the present invention. The silicon casting apparatus of the present invention is similar to the conventional silicon casting apparatus in the basic configuration.
As shown in FIG. 2, a chamber 1 is formed by a water-cooled vessel having a double walled structure, an upper portion thereof is coupled to a raw material loading device partitioned by a blocking means 2, and a withdrawing port 4 for extracting an ingot 3 is provided in a bottom portion thereof. An inert-gas introducing port 5 is provided at a sidewall in the upper portion of the chamber 1, and a vacuum suction port 6 is provided at the side wall in a lower portion of the chamber 1.
A cold crucible 7, an induction coil 8 and an afterheater 9 constituting an electromagnetic casting means are provided in a central portion of the chamber 1. The cold crucible 7 is a bottomless crucible, which is formed by a rectangular cylindrical body cooled body made of a copper alloy containing beryllium and is divided into a plurality of elements in a circumferential portion while an upper portion of the body is left without being divided, the elements each being in a strip-like form. The induction coil 8 is coaxially provided around the outside circumference of the cold crucible 7 and is connected to a power supply through a coaxial cable (not shown). Likewise, side by side, the afterheater 9 is coaxially provided at the lower portion of the cold crucible 7, and heats the ingot 3 which is withdrawn down from the cold crucible 7 to give a predetermined temperature gradient in an axial direction thereof.
A raw material introducing pipe 10 is provided below a blocking means 2 disposed at the upper portion of the chamber 1, and a granular and aggregated silicon raw material 11 loaded into the raw material introducing pipe 10 is supplied to a molten silicon 12 in the cold crucible 7. An auxiliary heater 13 made of graphite or the like is provided in an upward/downward movable fashion, immediately above the cold crucible 7, such that the auxiliary heater 13 be inserted into the cold crucible 7 in the lowered state.
A gas sealing portion 14 is provided below the afterheater 9, and a withdrawing device 15 is attached to withdraw the ingot 3 downward while supporting the ingot 3. A diamond cutter 16 as a mechanical cutting means is provided below the gas sealing portion 14 outside the chamber 1. The diamond cutter 16 is configured so that the cutter is lowered in synchronization with a withdrawing speed of the ingot 3 and can dynamically cut the ingot 3 being withdrawn outside the chamber 1 through the withdrawing port 4 while following the movement of the ingot 3.
One of the features of the silicon casting apparatus of the present invention is that the cold crucible which is a component of the electromagnetic casting means is made of copper alloy containing beryllium.
The reason why the cold crucible is made of copper alloy containing beryllium is that, as described above, the cold crucible life is markedly extended while the generation of the electric-discharge flaw (slit warping or damaging) caused by the contact of the molten silicon with the crucible surface (surface of the each strip-like element) is prevented to ensure the heat quantity for melting the silicon as raw material.
There is no particular limitation to a beryllium content. As shown in FIG. 1, by analogy with the fact that hardness and strength substantially linearly increase as the beryllium content increases, when even a small amount of beryllium is contained, a resistance to the generation of the electric-discharge flaw should be exhibited to an extent corresponding to the beryllium content, and the resistance is increased as the beryllium content is increased. An upper limit of the beryllium content is restricted from the viewpoints of melting, production, and processing of the copper alloy.
However, as far as beryllium of 0.1 mass % or more is contained, the electric-discharge flaw generation preventing effect is clearly recognized in the cold crucible. When the beryllium content exceeds 5 mass %, uneven precipitation of the beryllium is likely generated in welding repair, which causes an undesirable local rupture or cracking of the beryllium copper. Accordingly, the desirable beryllium content ranges from 0.1 to 5 mass %.
As described above, the material to be used for making the cold crucible included in the silicon casting apparatus of the present invention is a copper alloy containing beryllium. However, the copper alloy may contain alloy elements other than beryllium, as long as it has the excellent electric conductivity and thermal conductivity and the electric-discharge flaw generation preventing effect is not impaired. For example, the copper alloy may contain a small amount of cobalt and nickel in addition to beryllium. When the small amount of cobalt is contained, a crystal grain growth preventing effect and the like is recognized in the solution heat treatment.
For the copper alloy containing beryllium, “beryllium copper” is defined in JIS H 3270, and “beryllium copper for spring” is defined in JIS H 3130. A chemical composition of the beryllium copper includes Be: 1.8 to 2.00% (“%” means “mass %”, hereinafter), Ni+Co: 0.20% or more, Ni+Co+Fe: 0.6% or less, and Cu+Be+Ni+Co+Fe: 99.5% or more. A chemical composition of the beryllium copper for spring includes Be: 1.60 to 1.79%, and other elements are identical to those of the beryllium copper. In the silicon casting apparatus of the present invention, an equivalent to this beryllium copper or to that beryllium copper for spring may be used as material for the cold crucible.
In order to produce the silicon ingot by the electromagnetic casting method using the silicon casting apparatus of the present invention shown in FIG. 2, silicon as raw material 11 is loaded in the cold crucible 7 made of copper alloy containing beryllium, and an alternating current is passed through the induction coil 8. Therefore, as described above, the magnetic field is formed in the cold crucible 7 to heat and melt silicon 11 as raw material. At this point, the inward force in a direction normal to the surface of the molten silicon 12 is applied to the molten silicon 12, and the molten silicon 12 is melted without contacting the cold crucible 7, thereby preventing the contamination of the ingot 3 incurred by the contact with the cold crucible 7.
Then, as described above, while silicon as raw material 11 is melted in the cold crucible 7, the withdrawing device 15 which retains the molten silicon 12 and the ingot 3 in the lower portion of the chamber 1 is moved downward to go on with the solidification of the molten silicon 12, and the silicon as raw material 11 is continuously loaded from above the cold crucible 7 to continue the melting and solidification, which allows the continuous casting of the silicon polycrystal.
Thus, the use of the silicon casting apparatus of the present invention including the cold crucible whose material is changed from the pure copper to the copper alloy containing beryllium can prevent the generation of the electric-discharge flaw to markedly extend the cold crucible life. Additionally, the heat quantity for melting the raw material is ensured to stably perform the casting, and the impurity contamination can be prevented since the molten silicon scarcely contacts the crucible wall. Therefore, the high-quality silicon ingot suitable for the solar-cell substrate material can be produced.
Furthermore, the following additional effect can be also obtained since mechanical strength of the crucible is remarkably increased.
The copper alloy containing beryllium is an alloy having a large precipitation hardening property, and the mechanical strength (tensile strength) is remarkably improved by performing a proper heat treatment. The pure copper (Cu: not less than 99.9%) has the tensile strength of about 275 N/mm², which is designated as an alloy number C1100 in JIS H 3100. On the other hand, the beryllium copper after an aging hardening treatment has the tensile strength of about 1400 N/mm², which is designated as an alloy number C1720H therein, and the tensile strength of beryllium copper is usually 4 times that of the pure copper or more.
Accordingly, in the case where a 345 mm-square ingot is cast using the pure-copper cold crucible (hereinafter also referred to as “mold” since the crucible has the rectangular cylindrical body), a 505 mm-square ingot can be cast without changing a thickness of the mold by using the mold made of beryllium copper. This can be realized by the remarkable improvement of the mechanical strength of the mold material, and the cost saving effect by virtue of material change is prominent in preparing the expensive mold.
Since the thickness of the cooled mold can be thinned, a magnetic-field loss incurred by the thickness of the mold is decreased, and the transfer of the magnetic field toward the inside of the mold from the high-frequency induction coil disposed around the outer circumference of the crucible is increased, which can advantageously enhance the heat generation efficiency.

EXAMPLES

The electromagnetic casting was performed using the silicon casting apparatus of the present invention having the schematic configuration of FIG. 2, and the state of the slit deterioration associated with the repetitive use of the cooled mold and the mold life were investigated. Given that the use of the mold from the start to the end in the continuous casting is deemed to be one (1) time in the frequency of use, the mold life was evaluated in terms of the frequency of use in the casting.
The crucible made of beryllium copper equivalent defined in JIS H 3270 was used as the cold crucible made of copper alloy containing beryllium.
FIGS. 3A to 3C illustrate the progression of the deterioration of the slit portion in the copper cold crucible which is used for the purpose of comparison. FIG. 3A shows the normal slit portion before casting is performed, and FIGS. 3B and 3C show the slit portions after being used 6 and 8 times in the casting. The deterioration aggravates considerably in the slit portion shown in FIG. 3C, and it is determined that the FIG. 3C slit portion should indicate a service-life limit of the crucible. That is, the 8-time use is regarded as the copper cold crucible life when evaluated in terms of the frequency of use in the casting.
FIGS. 4A and 4B illustrate progression of deterioration of the slit portion in the cold crucible made of copper alloy containing beryllium (the beryllium-copper crucible), which is used in the silicon casting apparatus of the present invention. FIG. 4A shows the slit portion after being used 8 times in the casting and FIG. 4B shows the slit portion after being used 16 times in the casting.
The photographs of FIGS. 4A and 3C differ remarkably from each other in the progression of the deterioration of the slit portion although the slit portions are used 8 times. In the cold crucible made of copper alloy containing beryllium, it is found that the generation of the electric-discharge flaw is effectively prevented. However, as shown in FIG. 4B, when the frequency of use reaches 16 times, the partial slit warping is remarkably observed even in the copper alloy containing beryllium cold crucible. In this case, it is evaluated that the mold life is 16 times.
As can be seen from the results shown in FIGS. 3A to 3C, 4A, and 4B, in performing the electromagnetic casting, the use of the silicon casting apparatus of the present invention including the bottomless cold crucible made of the copper alloy containing beryllium can prevent the generation of the electric-discharge flaw to greatly extend the crucible life. The same holds true for the cylindrical mold, not only the cold crucible.
Thus, the silicon casting apparatus according to the present invention includes the bottomless cold crucible made of the copper alloy containing beryllium, and the generation of the electric-discharge flaw can be effectively prevented in performing the electromagnetic casting. The use of the silicon casting apparatus of the present invention can greatly extend the crucible life to reduce the facility costs.
The heat quantity for melting silicon as raw material is ensured to stably perform the casting, and the impurity contamination from the crucible wall is prevented, so that the high-quality silicon ingot can be produced. Accordingly, the silicon casting apparatus of the present invention can be suitably utilized in the production of the polycrystalline silicon used for the solar-cell substrate material in which high quality is demanded.

Claims

1. A silicon casting apparatus in which it includes: (a)an electrically-conductive bottomless cold crucible in which a portion of the cold crucible along its axial direction is divided into a plurality of elements in a circumferential direction; and (b)an induction coil which surrounds the cold crucible, thereby enabling silicon to be melted by electromagnetic induction heating using the induction coil and to be withdrawn downward and solidified,

wherein the cold crucible is made of copper alloy containing beryllium.

2. The silicon casting apparatus according to claim 1, wherein the copper alloy contains beryllium of 0.1 to 5 mass %

3. The silicon casting apparatus according to claim 1, wherein the silicon casting apparatus is used to produce a solar-cell silicon ingot.

4. The silicon casting apparatus according to claim 2, wherein the silicon casting apparatus is used to produce a solar-cell silicon ingot.