JPH0912390A - Production of planar material - Google Patents

Abstract

PURPOSE: To reduce cost by forming a melting zone in a planar raw material by high-frequency induction heating and moving this melting zone in the planar raw material and crystallizing or purifying the planar raw material, thereby producing the planar material. CONSTITUTION: The planar raw material 12, such as polycrystal Si, is held perpendicularly in its longitudinal direction in an inert gaseous atmosphere of N2 , etc., contg. <=1000ppm impurity and highfrequency current is passed to a coil 11 for hating having a slitlike groove 14 held horizontally so as to enclose the planar raw material 12. The limited part of the planar raw material 12 enclosed by the coil 11 is melted to a band form by high-frequency induction heating and the planar crystal 13' which is a seed is fused to this molten part; thereafter, the position of the coil 11 is moved from the seed crystal toward the planar raw material 12 to move the melting zone, by which the planar material, such as planar single crystal Si, is produced. An element for imparting electrical conductivity of n-type or p-type is incorporated into the planar raw material 12 or the atmosphere gas, by which the electrical conductivity and electric conductivity of the Si to be produced are adjusted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a novel method for producing a plate-like material, particularly a plate-like silicon crystal useful as a substrate material for solar cells.

[0002]

2. Description of the Related Art Solar cells, especially solar cells using a semiconductor material such as silicon, are attracting attention as a clean alternative energy source from the viewpoint of environmental measures such as global warming in recent years. As a silicon solar cell, single crystal, polycrystal,
A number of proposals have been made for the three types of amorphous, and also for the configuration and structure of the solar cell, and some of them have already been put into practical use, but there are many problems in view of the characteristics of solar cells and the cost of alternative energy. I have it. As a conventional silicon solar cell, as shown in FIG.
On the surface of the silicon semiconductor substrate 1 of the mold, an n layer 2 in which P or the like is diffused, a surface electrode 4 is formed thereon via an antireflection film 3, and a layer formed by printing an Al paste or the like is formed on the back surface. In general, the p ⁺ layer 5 is fired and the back electrode 6 is formed on the p ⁺ layer 5 by printing an Al paste. When this solar cell is irradiated with sunlight 7, light electrons incident on silicon generate free electron and hole pairs, and a current can be taken out to an external circuit between the back electrode 6 and the front electrode 4. Solar energy can be directly converted into electrical energy.

However, in order to reduce the cost of the crystal substrate in the cost of the solar cell, a method for producing thin ribbon-shaped or plate-shaped silicon has been researched and proposed as a typical method. EF for growing a plate crystal by raising the silicon melt by capillarity using a jig having a slit-shaped opening and crystallizing at the upper end thereof
G (Edge-defined Film fed Growth) method, Gompertz-Stepanov method of directly pulling out and crystallizing a silicon melt from the slit-shaped opening, or a modification of these methods, a method of pulling out a silicon melt downward, or a silicon melt. Research and proposals have been made on methods such as a dendrite growing method for growing plate crystals without using a jig while keeping the liquid surface in a supercooled state, or a Web method. However, in any of these methods, it is difficult to keep the temperature of the solid-liquid interface where the plate-like silicon grows stable over the entire area, so that plate-like silicon with a constant thickness is continuously stable. It has a drawback that it is extremely difficult to grow.

That is, in the plate-like silicon manufacturing method according to the prior art, the growth rate is determined by the heat of solidification generated and the heat from the solid-liquid interface determined by the temperature distribution near the solid-liquid interface between the plate-like silicon and the melt. Determined by the amount of movement.
Therefore, in order to grow plate-like silicon stably at a constant rate, it is necessary to precisely control the temperature at the solid-liquid interface and the temperature distribution in the vicinity of it, and measure the width and thickness of the plate-like silicon during growth. It is desirable to control the temperature and its distribution by feedback.However, considering the structure of the growth furnace and the heat capacity of the members, the time constant of the temperature control system is generally large and the response is inferior. However, the growth rate must be kept low as a result. Further, for example, in the case of the EFG method, since the supply of the melt to the solid-liquid interface depends on the capillary phenomenon at the opening of the jig, the growth rate also depends on the properties of the interface between the jig and the melt. Therefore, stable manufacturing becomes more difficult. Furthermore, in the method using the jig, the quality and shape of the jig surface layer may change due to the chemical reaction between the silicon melt and the jig material, and the purity of the silicon may decrease due to contamination from the jig material. Inevitably, when growing a single crystal in particular, it is difficult to obtain a stable single crystal plate-like silicon which becomes crystalline and has many crystal defects due to these causes.

In each of these methods, the raw material silicon melt is housed in a heat-resistant container, and plate crystals are grown from this, so that the silicon is contaminated from the container material as in the case of the above-mentioned jig, and the growth of the crystal is promoted. As the amount of silicon in the container decreases with the progress, the thermal condition near the solid-liquid interface of crystal growth changes, and it becomes extremely difficult to form a plate-shaped crystal having a constant shape. An RTR (Ribon to Ribon) method for growing a plate-shaped crystal from a plate-shaped raw material has been proposed in order to overcome the drawbacks of the conventional plate-shaped crystal growing method. In this method, a laser is incident on the side surface of the plate-shaped material from a direction perpendicular to the lengthwise direction of the plate-shaped material to melt a part of the plate-shaped material into an extremely thin band, and this molten band is cut from one end of the material plate. A plate crystal is created by moving it toward the other end,
In this method, a plate-shaped crystal to be grown is uniformly heated by a laser beam over the entire width, so a high-power laser light source and a complicated and precise optical system are required. In addition, special equipment is required to secure the stability of the laser beam for a long time, which makes the device extremely expensive, and the efficiency of the laser output energy with respect to the input energy is extremely low. There are many problems in the economical production of plate crystals.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and provides a plate-shaped material, particularly a plate-shaped silicon crystal useful as a substrate material for a solar cell, with high quality, high accuracy and low cost. The purpose is to obtain at high cost and high productivity.

[0007]

The present inventors have succeeded in stably producing a plate-shaped raw material into a high-quality plate-shaped material by using a high-frequency induction heating method. This is a method of producing a plate-shaped material by crystallizing or purifying the plate-shaped raw material by forming a melting zone in the plate-shaped raw material by heating and moving the melting zone in the plate-shaped raw material. At this time, hold one end of the plate-shaped raw material so that the lengthwise direction is vertical,
After melt-liquefying the other end by high-frequency induction heating, a plate-like crystal serving as a seed is fused to the melt, and the melted portion is moved toward one end where the plate-like raw material is held while keeping the band-like shape,
It is preferable to crystallize or purify the plate-shaped raw material. And these methods are preferably carried out in an atmosphere of an inert gas, for example, at least one of argon, helium, and nitrogen, or under reduced pressure. Impurities of the atmosphere gas used are 1,000 ppm or less,
And the moisture content in the atmosphere is the vapor pressure at 25 ℃
It should be 10 ^-3 atm or less. Furthermore, in these methods, the shape of the plate-shaped material produced can be controlled by high-frequency heating power, the solidification rate of the plate-shaped material, or both. Further, it is useful for producing a plate-shaped silicon crystal by using the plate-shaped raw material as silicon. In this case, the conductivity and the conductivity of the plate-like silicon to be produced should be adjusted by adding an element that imparts n-type or p-type conductivity to silicon in the plate-like raw material or the atmosphere gas or both. Is possible. Since the plate-shaped silicon crystal obtained by this method has high quality and low cost, it can be used as a substrate for solar cells, and a high-efficiency, low-cost solar cell is provided.

Hereinafter, the present invention will be described in detail. The present invention provides a new method for producing a plate-like material, particularly a plate-like silicon crystal, which has economically stable quality and overcomes the above-mentioned drawbacks and problems of the prior art. Hereinafter, the present invention will be described focusing on the case of producing a plate crystal of silicon, but the present invention is not limited to silicon. The method of the present invention is characterized mainly in that a melting zone perpendicular to the longitudinal direction of the plate-shaped raw material, particularly plate-shaped silicon, is formed by high-frequency heating in order to solve the problems of the laser heating RTR method. That is, the present inventors have already developed a method for producing fine linear silicon having a thickness of 1 mm or less by high frequency heating and applied for a patent (Japanese Patent Application No. 6-326012). According to the method, it is possible to effectively form a melting zone with a small amount of energy compared to laser heating, and it is possible to keep the melting zone stable by precisely controlling the high frequency current flowing in the coil. By arranging auxiliary rings above and below the working coil to form a desirable temperature distribution in the silicon above and below the melting zone and precisely control the moving speed of the melting zone, in other words, the crystal growth rate to a considerably high value. It was discovered that it was possible to apply it to the method for producing plate-shaped silicon crystals, and it achieved low energy consumption similar to the production of thin-line silicon. Therefore, it became clear that a plate-shaped silicon crystal can be produced stably and at high speed.

When a plate-shaped silicon crystal is produced, for example, as shown in FIG. 1, the plate-shaped silicon 12 as a raw material is held vertically in the longitudinal direction and horizontally held to surround the plate-shaped raw material silicon. Similarly, a current from a high frequency power source is applied to the heating coil 11 having the slit-shaped groove 14, and the extremely limited portion in the length direction of the plate-shaped raw material silicon surrounded by the coil is melted in a strip shape to change the coil position. By relatively moving the plate-shaped raw material silicon in the lengthwise direction (vertical direction), the melting zone is moved in one direction upward or downward in the plate-shaped raw material silicon to remove the silicon on the side relatively separated from the coil. It can be crystallized.

In the case of producing plate-shaped or ribbon-shaped single crystal silicon, one end of the plate-shaped raw material silicon is held,
The other end is melted directly by a high-frequency heating coil having a slit-shaped groove as described above, or after being melted by using an auxiliary heater together, and then a plate-shaped or ribbon-shaped single crystal silicon seed is fused to the melted portion, or a plate One end of the plate-shaped or ribbon-shaped single crystal silicon is brought into contact with one end of the raw material silicon which is not held so as not to create voids as much as possible, and the contact portion is melted by the high frequency coil, and then the coil position is changed to a single position. It can be created by moving the molten zone relatively from the crystalline silicon toward the plate-shaped raw material silicon and moving the melting zone. The method of making this plate-shaped or ribbon-shaped single crystal silicon is, for example, when making a thin silicon plate from a thick raw material silicon plate, the thickness of the plate-shaped raw material silicon is larger than the slit width of the coil. It can also be applied when the value is large.

The plate-shaped raw material silicon may be either polycrystalline or single crystal, and its quality is closely related to the characteristics of the produced plate-shaped silicon crystal. Since the generated plate-like silicon crystal has a significantly large surface area to volume ratio, volatile impurities are easily volatilized and removed before and after the plate-like silicon is made, and point defects and dislocations are also diffused to the outside. ,
It is easy to obtain high-quality silicon. However, if the impurities contained in the plate-shaped raw material silicon exceed 0.1%, the obtained plate-shaped silicon crystals do not have high purity even if such an impurity removing effect is obtained. Is preferred. Further, contamination from the growing atmosphere is also a problem, and in order to obtain an extremely high-purity plate-like silicon crystal, the pressure is reduced, for example, 1 Torr or less, preferably 10 ⁻³ Torr or less, or a rare gas such as Ar or He or N ₂
Using an inert gas atmosphere such as, 1000p of impurities such as oxygen, hydrogen or hydrocarbons contained in it
It is preferable to control to pm or less. The moisture in the atmosphere is
The vapor pressure at 25 ° C is preferably 10 ^-3 atmospheres or less. Furthermore, the electrical characteristics of the plate-shaped silicon, that is, its conductivity type and resistance value, are the same as those of the plate-shaped raw material silicon.
-Type or p-type conductivity-imparting elements or by doping these elements into the compound form (for example, B ₂ H ₆ , PH ₃ ) in the atmosphere when growing plate-like silicon.
It is possible to control it arbitrarily by including in.

In order to precisely control the shape of the plate-like silicon crystal to be produced, the shape and size of the silicon melting zone are enlarged and observed or measured by, for example, a CCD camera or the like, and the variation is measured by high frequency power or It is fed back to the relative movement speed (solidification speed of the plate-shaped material) between the plate-shaped raw material silicon and the generated plate-shaped silicon crystal,
Alternatively, it is possible to control the shape of the produced plate-like silicon crystal and the growth rate extremely stably by performing feedback control on both of them.

As a matter of course, the method of the present invention is based on the same principle as the floating zone method (FZ method for short) which is one of the methods for producing a silicon single crystal ingot. Can be single-crystallized and the plate-shaped silicon as a raw material can be highly purified. Therefore, in the method of the present invention, it is desirable that a material other than silicon, for example, a metal material such as Fe, Cu, Al, etc. is required to have a high degree of purification, or single crystallization or special crystal grains are grown. It can also be applied to cases.

[0014]

【Example】

(Embodiment 1) Using a device similar to the silicon single crystal growth device by the FZ method as shown in FIG.
A plate-shaped raw material silicon polycrystal having a width of 10 mm, a length of 100 mm and an n-type of 500 Ωcm was set to have a vertical direction in its length direction, and its upper end was held by an upper drive shaft. This was held horizontally in the growth chamber and was concentrically penetrated through the slit of a copper high-frequency coil having a rectangular slit-shaped groove with a width of 3 mm and a length of 20 mm, which was connected to an external high-frequency power source, and was placed concentrically. The lower end of the silicon polycrystal was fixed to the lower drive shaft. The position of the coil was 10 mm above the lower end of the plate-like silicon polycrystal. An argon atmosphere (1 atm) was applied in the growth chamber, and a high frequency (2 MHz) current from a high frequency power source was controlled to flow through the coil to form a molten zone in a portion surrounded by the plate-shaped raw material silicon coil. While observing the shape of the melting zone with a CCD camera, the lower drive shaft is moved downward at a speed of 2 mm / min, while the upper drive shaft is moved downward at an average speed of 1 mm / min, thereby A plate-like silicon crystal having a width of 10 mm and a thickness of 1 mm was prepared in the area of. The obtained plate-like silicon crystal was also polycrystalline, but the width was 10 mm and the thickness was 1 mm, and the resistance was about 600 Ωcm.

(Embodiment 2) Using a device similar to that of Embodiment 1, a plate-shaped raw material silicon is formed with a width of 10 mm, a thickness of 2 mm, a length of 100 mm, and n.
Type 500 Ωcm polycrystal was used, and its upper end was fixed to the upper drive shaft so that its length direction was aligned with the vertical direction. The same coil as in Example 1 was used, and the plate-shaped raw material silicon was arranged concentrically with the slit of the coil. A plate-shaped raw material in which a single-crystal silicon seed plate with a width of 10 mm, a thickness of 0.8 mm, and a length of 20 mm was fixed to the lower drive shaft so that the length direction was aligned with the vertical direction, and then the upper end was fixed to the upper drive shaft. Keep the plate contacted with the lower end of the silicon polycrystal so that they are in contact with each other, and move them using the upper and lower drive shafts so that the contact part between the raw material and the seed crystal plate is located in the slit part of the coil. set. After forming a melting zone at the contact area with a high frequency current (frequency 2MHz), while observing the melting zone with a CCD camera, the lower drive shaft moves downward at an average speed of 2mm / min and the upper drive shaft moves downwards at an average speed of 0.8mm / min. It was moved to grow a silicon single crystal plate above the seed crystal plate. Atmosphere is B _{2 at} 50 ppm
Argon containing H ₆ (flow rate 2 L / min) at 1 atm, B ₂
Impurities other than H ₆ were 50 ppm, and the dew point was -50 ° C. As a result, a plate-like silicon single crystal having a width of 10 ± 0.5 mm, a thickness of 0.8 ± 0.1 mm and a length of 200 mm was grown, and this was a single crystal having the same orientation as the seed crystal plate and having a p-type of 10 Ωcm.

Example 3 The silicon single crystal plate obtained in Example 2 was cut to a length of 10 mm, phosphorus was diffused at 850 ° C. to form a pn junction, and then the back surface diffusion layer was removed. A TiO ₂ antireflection film is formed on the n-type surface, an Al-Si surface electrode is formed by photolithography, and an Al-Si back surface electrode is similarly formed on the back surface to form a solar cell as shown in FIG. It was made. The conversion efficiency of this solar cell was measured under the conditions of AM 1.5, 100 mW / cm ² , and 28 ° C., and a value of 14.5% was obtained.

[0017]

According to the present invention, a highly pure and highly accurate plate-shaped material, particularly a plate-shaped silicon crystal, can be stably obtained at low cost. Therefore, it contributes to the cost reduction of the solar cell made from such a plate-like silicon crystal, which can contribute to the spread of the solar cell.

[Brief description of the drawings]

FIG. 1 is a schematic view of a method for producing a plate-shaped material of the present invention. (A) Sectional view (b) Perspective view of enlarged fusion zone

FIG. 2 is a cross-sectional view of a conventional solar cell.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... P-type silicon semiconductor substrate 11 ... High frequency heating coil 2 ... N layer 12 ... Plate-shaped raw material silicon 3 ... Antireflection film 13 ... Plate-shaped silicon crystal 4 ... Surface electrode 14 ... Slit-shaped groove 5 ... P ⁺ layer 6 ... Back electrode 7 ... sunlight

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Masatsugu Ueoka 3-2-1 Sakado, Takatsu-ku, Kawasaki City, Kanagawa Prefecture Corporate Research Center, Shin-Etsu Chemical Co., Ltd. (72) Teruhiko Hirasawa, Sakado, Takatsu-ku, Kawasaki City, Kanagawa Prefecture 3-2-1, Shin-Etsu Chemical Co., Ltd. Corporate Research Center

Claims (12)

[Claims] 1. A melting zone is formed in a plate-shaped raw material by high-frequency induction heating, and the melting zone is moved in the plate-shaped raw material,
A method for producing a plate-shaped material by crystallizing or purifying a plate-shaped raw material. 2. A plate-shaped raw material is held at one end so that the lengthwise direction is vertical, and the other end is melted and liquefied by high frequency induction heating, and then a plate-shaped crystal serving as a seed is fused to the melt. A method for producing a plate-shaped material for crystallizing or purifying the plate-shaped raw material by moving the molten portion toward one end where the plate-shaped raw material is held while keeping the melted portion in a band shape. 3. The method according to claim 1, which is carried out in an inert gas atmosphere. 4. The method according to claim 1, 2 or 3, wherein the inert gas is at least one of argon, helium and nitrogen. 5. The method according to claim 1 or claim 2 to claim 4, which is carried out under reduced pressure. 6. The method according to claim 1 or claim 2 or claim 5, wherein the impurities in the atmospheric gas are 1,000 ppm or less. 7. The method according to claim 6, wherein the water content in the atmosphere is 10 ⁻³ atm or less in vapor pressure at 25 ° C. 8. The method according to claim 1, wherein the shape of the plate-shaped material to be manufactured is controlled by high-frequency heating power, the solidification rate of the plate-shaped material, or both of them. 9. The method according to claim 1, wherein the plate-shaped raw material is silicon. 10. A plate-shaped raw material, an atmosphere gas, or both of them are mixed with an element that imparts n-type or p-type conductivity to silicon, thereby adjusting the conductivity and conductivity of the plate-shaped silicon produced. The method of claim 9, wherein 11. A plate-like silicon crystal produced by the method according to claim 9. 12. A solar cell produced by using the plate-like silicon crystal according to claim 11.