JP5464429B2 - Method for growing single crystal silicon having a square cross section - Google Patents

Description

  The present invention relates to a method for growing single crystal silicon having a square cross section using a floating zone method (hereinafter referred to as “FZ method”), and a square silicon wafer obtained from single crystal silicon by the FZ method. -Regarding c.

A single crystal silicon solar cell has a conversion efficiency of 10% or more higher than that of a polycrystalline silicon solar cell, and has the best characteristics for general use. Until now, single crystal silicon solar cells have been manufactured using circular silicon wafers obtained by slicing cylindrical single crystal silicon grown by the Czochralski (CZ) method.
However, since the space of the solar cell panel cannot be effectively filled with the circular silicon wafer, the efficiency of the panel may be lower than that of the polycrystal even if the element efficiency is high.

  On the other hand, single crystal silicon used for high-power semiconductor elements and the like may be manufactured using the FZ method. In the FZ method, single crystal silicon is grown by heating and melting while lowering a silicon raw material rod by an induction heating coil provided in a growth furnace. Patent Literatures 1 to 3).

  In Patent Document 1, a ring-shaped member is provided on the upper surface side and / or the lower surface side of the induction heating coil, and the inner peripheral edge of the ring-shaped member is inserted into a molten zone in which the silicon raw material rod is melted. Although it is described that a silicon single crystal is grown in a process "(Claim 1), the ring-shaped member floats on the surface of the molten zone among the polycrystalline grains separated from the solid-liquid interface on the raw material rod side. It has only been shown to be used to prevent the downward movement of the polycrystalline grains (paragraph [0017]), suggesting that the ring-shaped member controls the melt from which the silicon raw material rod is melted. It has not been.

  Patent Document 2 discloses that in order to prevent discharge between any one or more of the raw material polycrystalline rod, the grown single crystal rod, and the floating zone and the induction heating coil, the raw material polycrystalline rod, the grown single crystal rod, the floating zone An insulating member is disposed between any one or more of the above and the induction heating coil ”(Claim 1),“ the insulating member is at least the upper surface, the lower surface, and the inner periphery of the induction heating coil. It should be arranged on one or more of the surfaces ”(Claim 2) and“ The insulating member is ring-shaped ”(Claim 3). The insulating member is for preventing discharge, and is not for controlling the melt in which the raw material polycrystalline rod is melted.

  Patent Document 3 states that “a liquid silicon melting cap is generated on a seed crystal using an annular induction heating coil, the seed crystal is lowered at a constant tensile speed, and the silicon in the melting cap is solidified on the seed crystal. A method for producing a large-diameter ingot-type silicon single crystal that forms a single crystal, wherein the solid silicon is melted in a melting apparatus and supplied in a liquid state to a melting cap. The invention of item 1) is described, and “then the diameter of the grown single crystal is continuously increased and the so-called initial cone is pulled. A ring, preferably a quartz ring, is interposed between the induction heating coil and the melting cap. However, by this time, the diameter of the grown single crystal should be at least 20 mm larger than the diameter of the ring. "Wave formation on the surface of the cup is suppressed, and the temperature of the melt cap is made uniform, and non-molten silicon particles are prevented from reaching the phase boundary" (paragraph [0009]). However, in the present invention, what is used to "suppress wave formation on the surface of the melt cap and make the temperature of the melt cap uniform" is limited to "ring", Since a special method of “melting in a melting apparatus and supplying it to the melt cap in a liquid state” is adopted, it is not suggested to be applied to a method for growing single crystal silicon having a square cross section by the FZ method. .

In order to improve the disadvantage that the circular silicon wafer obtained by slicing cylindrical single crystal silicon as described above cannot effectively fill the space of the solar cell panel, in recent years, the vertical gradient solidification method (VGF method) has been used for solar cells. And the invention of a method for producing single crystal silicon having a square cross section using the CZ method has been proposed (see Patent Documents 4 and 5).
However, the VGF method of Patent Document 4 has a problem that it is difficult to obtain single crystal silicon having a uniform square cross section, and the CZ method of Patent Document 5 has a problem that the growth rate of single crystal silicon having a square section is slow. was there.

In addition, a method for producing single crystal silicon having a square cross section using the FZ method has been reported (see Non-Patent Document 1). In this manufacturing method, a square temperature distribution is created by a square-shaped high-frequency coil having four slits, and single-crystal silicon having a square cross section is produced using facet growth. In a high frequency coil having a coil and four slits, there is a problem that it is difficult to melt a round raw material rod.
Furthermore, in order to produce a quadrilateral crystal using facet growth, the temperature distribution of the melt needs to be a quadrilateral distribution, which is difficult with only a high-frequency coil. In addition, since the melt temperature distribution is made to be a square shape using only a high-frequency coil, it is difficult to control the convection of the melt and the interface shape, and the in-plane distribution of resistivity is poor.

JP-A-8-104590 JP 2006-169059 A Japanese Patent Laid-Open No. 9-142988 JP 2007-284343 A JP 2010-37142 A

International Scientific Colloquium, Modeling for Material Processing, Riga, June 8-9, 2006

  The present invention is intended to solve the above-described problems, has a high growth rate, can easily control a rectangular temperature distribution without relying only on a high-frequency coil, suppresses the formation of crystal defects, and has a resistance. It is an object of the present invention to provide a method for growing single crystal silicon having a quadrangular cross section with a uniform in-plane distribution of the rate, and a quadrangular silicon wafer.

The present invention employs the following means in order to solve the above problems.
(1) In the method for growing single crystal silicon having a quadrangular cross section by the FZ method in which a silicon raw material rod is heated and melted by a high frequency coil provided in a growth furnace to grow single crystal silicon, a square is formed on the lower surface side of the high frequency coil. A method for growing single crystal silicon having a quadrangular cross section, wherein the quadrangular mold is contacted with a melt obtained by melting the silicon raw material rod.
(2) The method for growing single crystal silicon having a quadrangular cross section according to (1), wherein the convection and / or interface shape of the melt is controlled by the quadrangular formwork.
(3) A single crystal having a quadrangular cross section according to (1) or (2), wherein a seeded crystal is used to form an enlarged pyramid portion obtained by gradually increasing the diameter of one side to form a single crystal. This is a method for growing silicon.
(4) The method for growing single crystal silicon having a quadrangular cross section according to (3), wherein the size of the quadrangular formwork is changed as the diameter of the enlarged pyramid portion increases.
(5) The method for growing single crystal silicon having a quadrangular section according to any one of (1) to (4), wherein the high-frequency coil is quadrangular.

In the method for growing single crystal silicon according to the present invention, the convection and interface shape of the melt are controlled by bringing a square formwork into contact with the melt produced by the high frequency coil, and the square is relatively easily formed in the melt. A temperature distribution can be created and the in-plane resistivity distribution can be improved.
In addition, by bringing a square formwork into contact with the melt, the melt can be easily restrained, and single crystal silicon having a square cross section can be grown at high speed.
Furthermore, a rectangular single crystal that does not rely solely on the facet growth mechanism can be produced by a rectangular mold that is in contact with the melt. Facet growth has the disadvantage that the growth rate cannot be increased because it requires supercooling of the temperature. However, in the present invention, the growth rate can be increased, and swirl defects can be formed during slow growth. Can be suppressed.
Since the single crystal silicon having a square cross section is obtained by the method for growing single crystal silicon according to the present invention, this can be sliced to obtain a square single crystal silicon wafer used for a solar cell panel as it is.

It is a schematic sectional drawing of the FZ apparatus using the formwork of this invention. It is a figure which shows the high frequency coil and formwork (round high frequency coil without a slit + formwork A) used in Example 1. FIG. It is a figure which shows the high frequency coil and mold which were used in Example 2 (round high frequency coil without slit + mold B). It is a figure which shows the high frequency coil and formwork (square high frequency coil without a slit + formwork A) used in Example 3. It is a figure which shows the high frequency coil and formwork (square high frequency coil without a slit + formwork B) used in Example 4. It is a figure which shows the square high frequency coil containing the slit of 4 places used by the comparative example. It is a figure which shows the in-plane distribution of the resistivity of the single crystal silicon which has a square cross section. It is a figure which shows the in-plane distribution of the resistivity of the square wafer which sliced the single crystal silicon which has the square cross section of Example 4. FIG. It is a figure which shows the in-plane distribution of the resistivity of the square wafer produced from the conventional cylindrical silicon single crystal.

FIG. 1 shows a schematic cross-sectional view of an FZ apparatus (growing furnace) used in the method for growing single crystal silicon according to the present invention.
In the growth furnace, a silicon raw material rod (silicon polycrystalline raw material rod) 1 held by the raw material rod holding portion 11 of the upper shaft 7 and a seed crystal 5 held by the seed crystal holding portion 10 of the lower shaft 6 are accommodated. ing. An annular high frequency coil (high frequency induction heating coil) 8 for partially heating and melting the lower end of the silicon raw material rod 1 is arranged coaxially with the silicon raw material rod 1 so as to surround the silicon raw material rod 1. Further, an inert gas is supplied from 12 and discharged from 13.

  When single crystal silicon is grown using this FZ apparatus, first, the tip of the silicon raw material rod 1 formed in a tapered shape is heated and melted by the high frequency coil 8 to form a melting zone, and the seed crystal 5 is fused. . In the present invention, it is preferable from the viewpoint of the production cost of the seed to use a <001> -oriented single crystal rod having a square cross section (a quadratic prism having a {110} plane as a side surface) as the seed crystal 5. However, since the crystal structure does not depend on the shape of the seed crystal, a single crystal having a square cross section can be grown even from a cylindrical or conical seed crystal. Therefore, a cylindrical or conical seed crystal 5 may be used. Next, a drawing allowance for performing seed drawing is formed on the silicon raw material rod 1 side, and thereafter, a seed drawing portion 4 for making dislocation-free having a side diameter of, for example, about 3 mm is formed.

Subsequently, while the silicon raw material rod 1 is gradually moved downward, it is heated and melted by the high-frequency coil 8 to gradually enlarge the diameter of one side to form a single-crystallized enlarged pyramid portion 3.
The present invention is characterized in that the square mold 9 is brought into contact with a melt obtained by melting the silicon raw material rod 1. In a four-fold symmetrical temperature distribution (heat generation distribution heated by induction current generated by the high-frequency coil 8), the {110} side face of the seed crystal is opposed to the high temperature part of the temperature distribution, and the {111} facet surface is { 110} Growing on the side surface portion increases the size of the grown single crystal while maintaining the rectangular cross section, and forms the increased-diameter pyramid portion 3.
In the formation of the increased-diameter pyramid portion 3, the increased-diameter pyramid portion 3 can be easily formed by using a mold. That is, by using the square variable mold 9 and enlarging the size of the mold in accordance with the increase in diameter, it is possible to easily form the increased-diameter pyramid portion 3.

Then, the diameter of the enlarged pyramid portion 3 is further expanded, and when a side of the square cross section has an appropriate length, growth of a portion that becomes the single crystal silicon (product single crystal rod) 2 is started. By shifting the position of the melting zone formed by the high-frequency coil 8 or the position of the grown single crystal, the single crystal is grown from one end of the silicon raw material rod 1 toward the other end.
In the present invention, the square fixed mold 9 can be used in the process of growing the product single crystal rod 2 having a square cross section.

  The high frequency coil 8 used in the present invention may be round or square, and the shape is not particularly limited. However, in order to make the in-plane distribution of resistivity of the single crystal silicon 2 more uniform, a rectangular high frequency coil is preferable.

The mold 9 used in the present invention is basically a quadrangle in order to produce a single crystal silicon 2 having a quadrangular cross section, but it is sufficient that the convection and / or interface shape of the melt can be controlled. Therefore, it does not have to be a true square, and includes a case where the corner is rounded and a case where the whole is curved.
As the material of the mold 9, high-purity quartz, silicon nitride, silicon or the like can be used so as not to lower the purity of the single crystal. High purity silicon is preferable when mixing with oxygen and nitrogen is disliked.

  The single crystal silicon 2 grown in the present invention has a quadrangular cross section, and it is preferable that the corners are not rounded as much as possible, but those having slightly rounded corners are acceptable. The same can be said for a rectangular single crystal silicon wafer obtained by slicing single crystal silicon having a rectangular cross section. Further, the single crystal silicon 2 is not necessarily free of dislocations, but by controlling the shape of the solid-liquid interface, the accumulation and growth of dislocations are eliminated, and polycrystallization is prevented. For this reason, it is preferable to optimize the thermal design of the growth furnace.

  Since the FZ method employed in the present invention melts a part of the raw material rod and continuously performs crystal growth, segregation of doping impurities can be made constant. For this reason, impurities such as phosphorus having a small segregation coefficient can be distributed at a uniform concentration from the beginning to the end of crystal growth.

In the FZ method, since the raw material rod is directly heated at a high frequency, the crystal grown by the heat radiation from the heater as in the CZ method is not irradiated. That is, generally, the FZ method can increase the growth rate (3 mm / min) compared to the CZ method (1 mm / min) and the cast method (0.1 mm / min).
In the present invention, the growth rate (growth rate) is set to 2 to 3 mm / min.
In addition, since the FZ apparatus has less heat radiation, the distance from the growth interface to the diameter and temperature measuring device can be shortened, so that automatic operation control is easy.

  Examples will be described below with reference to the drawings. However, these drawings are merely examples, and the size of the mold is not limited.

Example 1
By using a silicon polycrystalline raw material rod having a diameter of 120 mm, a slit-shaped round high-frequency coil 8 shown in FIG. 2 and a rectangular frame (located above the single-crystal silicon 2; the same applies to the following embodiments). Five single crystal silicons (FZ crystals) 2 each having a side of 105 mm and a length of 800 mm and having a square cross section were manufactured.
The crystal growth rate was 2.0 mm / min, there was no spillage of the melt, and a dislocation-free single crystal could be grown.

(Example 2)
Using a silicon polycrystalline raw material rod having a diameter of 120 mm, three FZ crystals 2 each having a square section of 105 mm and a length of 800 mm are manufactured by a round high-frequency coil 8 without slits and a rectangular frame shown in FIG. However, the result was the same as the arrangement of the square form in FIG.

(Example 3)
Using a silicon polycrystalline raw material rod having a diameter of 120 mm, five FZ crystals 2 each having a square section of 105 mm and a length of 800 mm were manufactured by using the rectangular high-frequency coil 8 without slits and a rectangular frame shown in FIG. .
The crystal growth rate was 2.5 mm / min, there was no spillage of the melt, and a dislocation-free single crystal could be grown.

Example 4
Using a silicon polycrystalline raw material rod having a diameter of 120 mm, three FZ crystals 2 having a side of 105 mm and a length of 800 mm and having a square cross section were produced by the rectangular high-frequency coil 8 and the rectangular mold shown in FIG. However, the result was the same as the arrangement of the square form in FIG.

(Comparative example)
Using a silicon polycrystalline raw material rod having a diameter of 120 mm, five FZ crystals 2 each having a square section of 105 mm and a length of 800 mm were manufactured by the rectangular high-frequency coil 8 having four slits shown in FIG. .
The growth rate of the crystal was 1.0 mm / min, and melt spilling occurred twice, resulting in dislocations, and the three crystals were dislocation-free single crystals with no melt spilling.

The grown single crystal silicon having a square cross section with a side of 105 mm was shaped by machining to have a side diameter of 100 mm, and this was sliced to obtain a square single crystal silicon wafer with a side of 100 mm.
Table 1 and FIG. 7 show the in-plane distribution of resistivity in a wafer sampled from a 400 mm long portion of Comparative Example, Example 2 and Example 4 (on a line parallel to one side through the center of the wafer). Resistivity distribution).
In the comparative example, the in-plane distribution of resistivity was as bad as 30%, but it was improved to 22% in Example 2 and 12% in Example 4.

FIG. 8 shows a two-dimensional in-plane distribution of wafer resistivity (one side is 100 mm) of the portion of the crystal manufactured in Example 4 having a length of 400 mm. It can be seen from FIG. 8 that the rectangular single crystal silicon wafer of the present invention has an in-plane distribution with a four-fold resistivity. Further, in this wafer, the resistivity is higher in the central portion and the outer portion than in the intermediate portion, but it can be seen that the in-plane distribution of the resistivity approaches flat.
FIG. 9 shows a two-dimensional in-plane distribution of resistivity of a rectangular single crystal silicon wafer manufactured from a conventional cylindrical silicon single crystal. It is an axisymmetric (concentric) resistivity distribution, which is different from the in-plane resistivity distribution of the rectangular single crystal silicon wafer of the present invention.

(Explanation of symbols)
1: Silicon raw material rod 2: Single crystal silicon 3: Increased diameter pyramid part 4: Seed restricting part 5: Seed crystal 6: Lower shaft 7: Upper shaft 8: High-frequency coil 9: Square mold 10: Seed crystal holding part 11 : Raw material rod holding unit 12: Gas supply path 13: Gas discharge path

Single crystal silicon grown by the FZ method of the present invention has a rectangular cross section, and a rectangular silicon wafer can be obtained, so that it can be used for solar cells.
In addition, since the FZ method of the present invention does not use a crucible, oxygen is not mixed into the grown single crystal silicon from the quartz crucible, so that the oxygen impurity concentration is kept low, and carrier annihilation due to oxygen compounds and oxygen precipitates is eliminated. The impact can be ignored. This improves the diffusion length of the photogenerated carrier and contributes to an increase in the efficiency of the solar cell. The same applies to the mixing of other transition metal impurities, which enables the growth of high-purity crystals without reducing the purity of the raw material silicon, improves the diffusion length of photogenerated carriers, and contributes to the increase in solar cell efficiency. . Therefore, it is optimal as single crystal silicon for solar cells.

Claims (5)

  In the method for growing single crystal silicon having a square cross section by the FZ method in which a silicon raw material rod is heated and melted by a high frequency coil provided in a growth furnace to grow single crystal silicon, a rectangular mold is formed on the lower surface side of the high frequency coil. A method for growing single crystal silicon having a quadrangular cross section, wherein the quadrangular mold is brought into contact with a melt obtained by melting the silicon raw material rod.   2. The method for growing single crystal silicon having a quadrangular cross section according to claim 1, wherein the convection and / or interface shape of the melt is controlled by the quadrangular formwork.   3. The method for growing single crystal silicon having a quadrangular cross section according to claim 1, wherein a diameter-increased pyramid portion is formed by gradually increasing the diameter of one side to form a single crystal by using a seed crystal. .   4. The method for growing single crystal silicon having a quadrangular cross section according to claim 3, wherein the size of the quadrangular formwork is changed as the diameter of the enlarged pyramid portion increases.   The method for growing single crystal silicon having a quadrangular cross section according to any one of claims 1 to 4, wherein the high-frequency coil is quadrangular.