WO2013031923A1

WO2013031923A1 - Method for producing semiconductor ingot

Info

Publication number: WO2013031923A1
Application number: PCT/JP2012/072067
Authority: WO
Inventors: 跡部　淳一
Original assignee: 京セラ株式会社
Priority date: 2011-08-30
Filing date: 2012-08-30
Publication date: 2013-03-07
Also published as: JP5312713B1; JPWO2013031923A1

Abstract

This method for producing a semiconductor ingot comprises: a seed crystal preparation step for preparing a seed crystal that has a first main surface on which a semiconductor melt is poured and a second main surface which is on the reverse side of the first main surface, with the central part being protruded toward the second main surface side; a mold preparation step for preparing a mold that has an opening through which the semiconductor melt is poured, a bottom surface, and an inner lateral surface that is positioned between the opening and the bottom surface; a seed crystal arrangement step for arranging the seed crystal on the bottom surface of the mold with the second main surface down; a pouring step for pouring the semiconductor melt toward the central part of the first main surface of the seed crystal; and a solidification step for solidifying the semiconductor melt in the mold.

Description

Manufacturing method of semiconductor ingot

The present invention relates to a method for manufacturing a semiconductor ingot.

Solar cell elements are elements that convert light energy into electrical energy, and those using a silicon substrate as a semiconductor substrate for solar cell elements are widely used. The silicon substrate is obtained by slicing a monocrystalline or polycrystalline silicon ingot to a predetermined thickness using a wire saw device or the like.

For example, a Czochralski (CZ) method or a floating zone (FZ) method is known as a method for producing a single crystal silicon ingot. A casting method or the like is known as a method for producing a polycrystalline silicon ingot.

In general, a polycrystalline silicon ingot can be easily manufactured as compared with a single crystal silicon ingot. However, the photoelectric conversion efficiency of a solar cell element using a polycrystalline silicon substrate as a semiconductor substrate is reduced due to crystal defects such as grain boundaries. This is generally inferior to a solar cell element using a single crystal silicon substrate.

Therefore, as a method of manufacturing a high-quality crystal silicon ingot with few crystal defects such as crystal grain boundaries and impurities by using a casting method, after placing a seed crystal with few impurities and defects on the bottom surface of the mold, There has been proposed a method (seed casting method) in which a melt is injected into a mold and solidified in one direction starting from a seed crystal (for example, Japanese Patent Laid-Open No. 10-194718 and Japanese Translation of PCT International Publication No. 2009-523893). See).

However, in the seed casting method, when the silicon melt is injected into the mold, the silicon melt keeps in contact with a part of the seed crystal arranged on the bottom surface of the mold, so the seed crystal melts locally. It's easy to do. When the seed crystal melts to the bottom surface of the mold, a part of the bottom surface of the silicon ingot starts crystal growth without starting from the seed crystal. For this reason, a silicon ingot with low crystal quality may be formed. This problem is particularly noticeable in the production of large ingots using a large amount of melt.

Also, in order to avoid the above problem, it is not desirable to prepare a seed crystal that is entirely thick so that the seed crystal does not dissolve to the bottom of the mold because this leads to an increase in the manufacturing costs of the seed crystal and ingot.

Furthermore, in a seed crystal of a certain thickness used in the conventional seed casting method, the heat removal from the melt is performed from the bottom surface and the inner peripheral surface of the mold, so the solid-liquid interface during crystal growth has a convex shape. Prone. Therefore, the crystal growth is in the direction from the outer periphery to the center of the ingot, so that the high-quality crystal region becomes smaller as the ingot grows, and crystal defects and impurity defects caused by the mold or the release material are reduced. It will increase.

Accordingly, one object of the present invention is to manufacture a high-quality and large-sized semiconductor ingot such as a silicon ingot, and using a substrate cut out from the semiconductor ingot, particularly a solar cell element excellent in characteristics such as photoelectric conversion efficiency It is an object of the present invention to provide a method for manufacturing a semiconductor ingot capable of obtaining the above.

A method for manufacturing a semiconductor ingot according to the present invention has a first main surface into which a semiconductor melt is poured and a second main surface located on the opposite side of the first main surface, and the second main surface. A seed crystal preparation step for preparing a seed crystal in which a central portion on the surface side protrudes; an opening portion into which the semiconductor melt is poured; a bottom surface portion; and a position between the opening portion and the bottom surface portion. A mold preparing step of preparing a mold having an inner peripheral side surface portion, and a seed crystal arranging step of arranging the seed crystal on the bottom surface portion of the mold with the second main surface of the seed crystal facing down And an injection step of injecting the semiconductor melt toward the center of the first main surface of the seed crystal, and a solidification step of solidifying the semiconductor melt in the mold.

According to the above method for manufacturing a semiconductor ingot, when the melt is injected into the mold, the melt continues to contact a part of the seed crystal disposed on the bottom surface of the mold so that the seed crystal is locally Even if it is melted, the seed crystal does not reach the bottom surface due to the presence of the protrusions, so that an ingot with few crystal defects is formed.

Also, since the contact area between the seed crystal and the bottom of the mold is narrower than that of the conventional seed crystal, the diffusion of impurities from the mold and the release material can be reduced.

Thereby, since a high-quality semiconductor ingot can be manufactured, a silicon ingot capable of producing a solar cell element having excellent photoelectric conversion efficiency can be provided.

FIG. 1 is a cross-sectional view schematically showing one embodiment of a method for manufacturing a semiconductor ingot according to the present invention. Fig.2 (a) is a top view which shows typically an example of the seed crystal used for one Embodiment of the manufacturing method of the semiconductor ingot based on this invention, FIG.2 (b) is the side view. Fig.3 (a) is a top view which shows typically an example of the seed crystal used for one Embodiment of the manufacturing method of the semiconductor ingot based on this invention, FIG.3 (b) is the side view. FIG. 4A is a plan view schematically showing an example of a seed crystal used in one embodiment of a method for producing a semiconductor ingot according to the present invention, and FIG. 4B is a side view thereof. Fig.5 (a) is a top view which shows typically an example of the seed crystal used for one Embodiment of the manufacturing method of the semiconductor ingot based on this invention, FIG.5 (b) is the side view. FIG. 6 is a cross-sectional view schematically showing an example of a manufacturing apparatus used in an embodiment of a method for manufacturing a semiconductor ingot according to the present invention. FIG. 7 is a cross-sectional view schematically showing an example of a manufacturing apparatus used in an embodiment of a method for manufacturing a semiconductor ingot according to the present invention. FIG. 8 is a cross-sectional view schematically showing an example of a manufacturing apparatus used in an embodiment of a method for manufacturing a semiconductor ingot according to the present invention. FIG. 9 is a cross-sectional view schematically showing an example of a manufacturing apparatus used in an embodiment of a method for manufacturing a semiconductor ingot according to the present invention. FIG. 10 is a diagram schematically showing an example of the solar cell element, and is a plan view seen from the light receiving surface side. FIG. 11 is a diagram schematically showing an example of the solar cell element, and is a plan view seen from the non-light-receiving surface side. FIG. 12 is a diagram schematically showing an example of the solar cell element, and is a cross-sectional view showing a state cut along the line KK in FIG.

Hereinafter, an embodiment according to the present invention will be described with reference to the drawings. In addition, drawing is shown typically, and the same code | symbol is attached | subjected about the part which has the same structure and function in drawing. In addition, the sizes and positional relationships of the various components in each drawing are not accurately illustrated.

<Manufacturing equipment>
First, a manufacturing apparatus for manufacturing a semiconductor ingot will be described. In FIG. 1, a seed crystal 4 made of single crystal or polycrystalline silicon or germanium for manufacturing a semiconductor ingot is arranged in a mold 2, and a semiconductor made of the same material as the seed crystal 4 in an opening 2 a of the mold 2. It shows how the melt is poured.

The mold 2 has a cylindrical shape with a bottom as a whole, and is located, for example, between an opening 2a into which a semiconductor melt is poured from the crucible 1, a bottom surface 2b, and the opening 2a and the bottom surface 2b. And an inner peripheral side surface portion 2c. The outer shape of the opening 2a is a square shape or a circular shape. In addition, as for the method of pouring the semiconductor melt into the mold 2, the semiconductor melt may be poured from an opening provided above the crucible 1 as shown in the figure. For example, a nozzle is provided on the lower surface of the crucible 1. The semiconductor melt may be poured from the nozzle into the mold 2.

Both the crucible 1 and the mold 2 are not easily melted, deformed and decomposed at a temperature equal to or higher than the melting point of the semiconductor material such as silicon, and are less likely to react with the semiconductor material. It is made of a material in which impurities that reduce the characteristics of the solar cell element using the material are reduced as much as possible. For example, when producing a silicon ingot, both the crucible 1 and the mold 2 can use quartz or graphite.

As shown in FIG. 1, a holder 5 that supports the mold 2 is disposed on the lower surface of the mold 2. The holder 5 is made of, for example, quartz, graphite, or a carbon fiber reinforced carbon composite material, and has a function and a shape that removes heat from the bottom surface portion 2b while keeping it in close contact with the bottom surface portion 2b of the mold 2. ing.

Furthermore, the holder 5 is preferably supported by a cooling mechanism 6 as shown. The cooling mechanism 6 is made of, for example, a metal such as stainless steel, and is a cooling plate or a water cooling jacket into which a cooling liquid such as water is introduced.

For example, as shown in FIG. 2, the seed crystal 4 has a first main surface 4a into which the semiconductor melt 3 is poured and a second main surface 4b located on the opposite side of the first main surface 4a. It is a plate-like body. The seed crystal 4 has, for example, a square shape or a circular shape when viewed in plan, and each corner may be chamfered in an arc shape in the case of the square shape.

The seed crystal 4 has at least one protrusion 4c on the second main surface 4b side. The protrusion 4c is disposed at least at the center of the second main surface 4b. Further, the projecting portion 4 c is further arranged, for example, in a ring shape at a portion along the peripheral edge of the second main surface 4 b, so that the seed crystal 4 can be stabilized in the mold 2. Further, as will be described later, the seed crystal 4 may be constituted by combining a plurality of small seed crystals.

The seed crystal 4 is a semiconductor crystal with few crystal defects such as impurities and crystal grain boundaries, and is preferably a single crystal, but may be a polycrystal. When using a polycrystal, it is preferable to use the one containing a single crystal region having a large grain size. The size and shape of the projecting portion 4 c of the seed crystal 4 and the thickness of the plate-like body are set so as not to melt to the bottom when the semiconductor melt 3 is injected. For example, the protrusion 4c may be a columnar shape with a diameter of about 50 to 200 mm or a prismatic shape with a cross-sectional area of the same, the thickness is about 5 to 50 mm, and the thickness of the plate portion is about 5 to 50 mm. Preferably there is.

In addition, the seed crystal 4 may be integrated including the protruding portion 4c, or a plurality of small seed crystals may be combined. When a small seed crystal is combined, it can be easily applied to the mold 2 having various shapes, and the manufacturing costs of the seed crystal and the ingot can be reduced.

An example in which the seed crystal 4 is composed of a plurality of small seed crystals is shown in FIGS. 2 to 5, the region divided by the broken line 9 in the drawing is a small seed crystal. For example, as shown in FIGS. 2 (a) and 2 (b), a total of five small seeds in which seed crystals smaller than these small seed crystals are arranged under four small seed crystals equally divided into four. It may be composed of crystals.

Further, for example, as shown in FIGS. 3A and 3B, a total of 10 small seeds in which seed crystals larger than these small seed crystals are arranged under the small seed crystals equally divided into nine. It may be composed of crystals.

Further, as shown in FIGS. 4A and 4B, the seed crystal 4 may be a small seed crystal that is equally divided into nine parts on a plane, and only the small seed crystal disposed in the lower center portion may be thickened. In this case, in order to stably dispose the seed crystal 4 in the mold 2, the seed crystal 4 is also interposed between the small seed crystal located around the seed crystal 4 and the inner peripheral side surface of the mold 2 via the release material 8. The crystal 4 is preferably fixed.

Further, as shown in FIG. 5 (a), the seed crystal 4 is located under the center part of these small seed crystals, as shown in FIG. 5 (b). One small seed crystal corresponding to the first protrusion 4c1 and two small seed crystals having the same length as one side of the seed crystal 4 at the left end and the right end as shown in FIG. 5B, A total of 12 small seed crystals may be included. Further, instead of the two long small seed crystals, an annular small seed crystal may be arranged.

As the seed crystal 4 shown in FIG. 5, if the seed crystal 4 having a projecting portion along the peripheral edge on the second main surface 4 b side is prepared, the seed crystal 4 can be further stabilized in the mold 2. It can be arranged.

Further, the seed crystal 4 can be reliably fixed to the mold 2 by drying the slurry-like release material 8 applied to the bottom surface portion 2b and the inner peripheral side surface portion 2c in the mold 2. At this time, only the projecting portion 4 c of the seed crystal 4 comes into contact with the bottom surface portion 2 b of the mold 2. Therefore, the diffusion of impurities contained in the mold 2 and the release material 8 into the semiconductor ingot is reduced, and a high-quality semiconductor ingot with few impurities can be manufactured.

In addition, after a dry release material is thinly provided in the mold 2, a slurry-like release material is applied onto a pre-formed release material, and a seed crystal 4 is disposed thereon and dried. The mold release property can be improved while fixing the seed crystal 4 in the mold 2.

The mold release material 8 applied in the mold 2 is originally used for the purpose of reducing the adhesion of the ingot to the mold 2 and the diffusion of impurities from the mold 2 to the semiconductor ingot, but when the semiconductor melt 3 is injected. In addition, it also has an effect of suppressing that the seed crystal 4 moves and normal seed casting is not performed.

For example, in the case of producing a silicon ingot, the release material 8 is composed of a powder of silicon oxide, silicon nitride, silicon carbide, or the like, or a mixture thereof including a binder such as PVA (polyvinyl alcohol) and a solvent such as water or alcohol. The resulting mixture is stirred to form a slurry, which is coated on the inner wall of the mold by means such as coating or spraying. In addition, you may mix suitably the additive material etc. for improving fluidity | liquidity with the slurry-like mold release material 8, for example.

The mold release material 8 can be used repeatedly by suppressing the fusion of the inner wall of the mold 2 and the silicon ingot after the silicon melt is solidified. Moreover, since the joint part in the bottom face part and inner peripheral side face part of the mold 2 is reliably sealed by the applied release material 8, leakage of the silicon melt is reduced.

The peripheral edge 4d of the seed crystal 4 and the inner peripheral side face 2c of the mold 2 may be sealed with a release material 8 or the like, or a gap may be left. When sealed with the release material 8 or the like, the semiconductor melt 3 cannot reach the lower part of the seed crystal 4, so that the diffusion of impurities from the bottom surface 2 b of the mold 2 and the release material 8 to the semiconductor ingot. In addition, it is easy to form a solid-liquid interface having an upwardly convex shape during crystal growth, so that a semiconductor ingot with good quality can be manufactured.

On the other hand, when the gap between the peripheral edge portion 4d of the seed crystal 4 and the inner peripheral side surface portion 2c of the mold 2 is vacant, the semiconductor melt 3 reaches the bottom surface portion 2b of the mold 2 and solidifies. Since a gap is formed between the seed crystal 4 and the seed crystal 4, the diffusion of impurities contained in the mold 2 and the release material 8 into the semiconductor ingot is reduced, and a high-quality semiconductor ingot with few impurities can be manufactured.

By disposing the protrusion 4c of the seed crystal 4 at the center of the seed crystal on the second main surface 4b side, the semiconductor ingot is cooled from the center of the bottom surface. Furthermore, since the crystal growth direction is from the central part of the ingot to the peripheral part, crystal defects such as crystal grain boundaries and dislocations can be reduced.

By installing a plate-like body 7 made of a material having a different thermal conductivity from that of the seed crystal 4 between the projecting portion 4c of the seed crystal 4 and the bottom surface portion 2b of the mold 2, heat conduction from the bottom surface of the mold 2 can be achieved. Control becomes possible. As a result, the crystal growth rate and the solid-liquid interface during solidification can be controlled, and crystal defects in the semiconductor ingot can be reduced. Furthermore, the diffusion of impurities from the mold 2 and the release material 8 to the semiconductor ingot can be reduced.

As a material of the plate-like body 7 disposed between the projecting portion 4c of the seed crystal 4 and the bottom surface portion 2b of the mold 2, there is no melting, deformation, or decomposition at a temperature higher than the melting point of a semiconductor material such as silicon. It is possible to use ceramics or metal, etc., which are less likely to react with a semiconductor material such as silicon, and reduce impurities as much as possible to reduce the characteristics of the finished solar cell element in the semiconductor ingot. Silicon dioxide, silicon carbide Silicon nitride or the like is preferably used.

<Manufacturing method>
Hereinafter, an example of a method for manufacturing a semiconductor ingot will be described.

The basic manufacturing method will be described. First, it has the 1st main surface 4a into which a semiconductor melt is poured, and the 2nd main surface 4b located in the other side of the 1st main surface 4a, and the center part by the side of the 2nd main surface 4b protrudes. A seed crystal preparation step for preparing the seed crystal 4 is performed.

Next, a mold 2 having an opening 2a into which the semiconductor melt is poured, a bottom surface 2b, and an inner peripheral side surface 2c located between the opening 2a and the bottom 2b is prepared. A mold preparation process is performed.

Next, a seed crystal disposing step is performed in which the seed crystal 4 is disposed on the bottom surface 2b of the mold 2 with the second main surface 4a of the seed crystal 4 facing down.

Next, an injection step of injecting the semiconductor melt 3 toward the central portion 4g of the first main surface 4a of the seed crystal 4 is performed. As shown in FIG. 1, the semiconductor melt 3 is injected from the crucible 1 into the mold 2 by tilting the crucible 1 containing the semiconductor melt into the mold 2 from the opening at the top of the crucible 1. Alternatively, a liquid injection port may be provided at the bottom of the crucible 1 and the semiconductor melt 3 may be injected into the mold 2 from the liquid injection port.

Then, a solidification process for solidifying the semiconductor melt in the mold 2 is performed. At this time, since the projecting portion 4c exists in the central portion of the seed crystal 4, the initial crystal growth direction is directed from the central portion of the ingot to the peripheral portion, so that crystal defects such as crystal grain boundaries and dislocations can be reduced.

As described above, in the seed crystal preparation step, it is preferable to prepare a seed crystal 4 in which a portion along the peripheral edge on the second main surface 4a side also protrudes.

Also, as described above, in the seed crystal preparation step, a seed crystal 4 in which a plurality of small seed crystals are combined may be prepared.

Further, in the seed crystal arranging step, the seed crystal 4 may be arranged on the bottom surface portion 2b of the mold 2 via the release material 8.

Further, as shown in FIG. 6, in the seed crystal arranging step, a plate-like body 7 having a thermal conductivity different from that of the seed crystal 4 is arranged on the bottom surface portion 2 b of the mold 2, and on the plate-like body 7. A seed crystal 4 may be disposed. Such an arrangement makes it easy to form a solid-liquid interface having an upwardly convex shape during crystal growth in the solidification step, so that a semiconductor ingot of good quality can be manufactured. Furthermore, crystal defects due to segregation of crystal grain boundaries and impurities can be reduced in the grown crystal.

Moreover, as shown in FIG. 7, in the said crystal | crystallization arrangement | positioning process, as the plate-shaped body 7, the 1st plate-shaped body 7a and several 2nd plate-shaped body with smaller heat conductivity than this 1st plate-shaped body 7a. 7b and before placing the seed crystal 4, the central portion of the bottom surface portion 2b of the mold 2 so that the first plate 7a is in contact with the central portion 4g of the second main surface 4b of the seed crystal 4 The plurality of second plate-like bodies 7b may be arranged around the first plate-like body 7a so as to come into contact with the peripheral edge of the seed crystal 4 on the second main surface 4b side.

The material of the first plate-like body 7a and the second plate-like body 7b may be the same or different from each other as long as the thermal conductivity is different. For example, two types of silicon dioxide plates 7a and 7b having different thermal conductivities may be used, or a combination of a first plate 7a made of silicon carbide and a second plate 7b made of silicon nitride. May be used.

In the solidification step, the center portion of the bottom surface portion 2b of the mold 2 may be cooled so that the temperature is lower than the peripheral portion of the center portion. Specifically, the center part of the holder 5 or the cooling mechanism 6 may be cooled so that the temperature is lower than the peripheral part.

Further, in the solidification step, with the holder 5 supporting the mold 2 from the bottom surface 2b side of the mold 2, the outer surface portion corresponding to the center portion of the bottom surface portion 2b of the mold 2 is defined as the outer surface portion. It is good to cool so that temperature may become lower than the circumference | surroundings. Specifically, for example, as shown in FIG. 8, the cooling mechanism 6 is divided into a central portion 6 a and a peripheral edge portion 6 b, and a separate coolant whose temperature is controlled is supplied to each of the cooling mechanism 6 and the cooling mechanism 6. What is necessary is just to set the temperature of the cooling liquid supplied to the center part 6a of this to be lower than the temperature of the cooling liquid supplied to the peripheral part 6b of the cooling mechanism 6.

In the solidification step, the holder 5 may be made of a material whose portion facing the outer surface portion of the mold 2 is made of a material having a higher thermal conductivity than the periphery of the portion. Specifically, for example, as shown in FIG. 9, the holder 5 is divided into a central portion 5a and a peripheral portion 5b, and the central portion 5a is made of carbon graphite, and the peripheral portion 5b is heated more than that. What is necessary is just to make it the holder 5 by producing with a carbon fiber reinforced carbon composite material with small conductivity, combining both, and integrating.

According to the semiconductor ingot manufacturing method described above, when the semiconductor melt is poured into the mold 2, the semiconductor melt continues to contact a part of the seed crystal 4 disposed on the bottom surface of the mold 2, Even if the seed crystal 4 is locally melted, the presence of the protrusion 4c prevents the seed crystal 4 from melting to the bottom surface, so that an ingot with few crystal defects is formed.

Further, since the contact area with the bottom surface of the mold 2 is narrower than that of the conventional seed crystal, the diffusion of impurities from the mold 2 and the release material 8 can be reduced. Thereby, since a high quality semiconductor ingot can be manufactured, the silicon ingot which can produce the solar cell element excellent in photoelectric conversion efficiency can be provided.

<Solar cell element>
Next, a semiconductor substrate can be cut out from the semiconductor ingot produced as described above and used as a semiconductor substrate of a solar cell element.

First, the basic configuration of the solar cell element will be described. As shown in FIGS. 10 to 12, the solar cell element 30 includes a light receiving surface (upper surface in FIG. 12) 29a on which light is incident and a non-light receiving surface (lower surface in FIG. 12) that is the surface opposite to the light receiving surface 29a. 29b.

The solar cell element 30 includes a semiconductor substrate 21, and the semiconductor substrate 21 is provided on the first semiconductor layer 22, which is a semiconductor layer of one conductivity type, and on the light receiving surface 29 a side in the first semiconductor layer 22. And a second semiconductor layer 23 which is a semiconductor layer of a reverse conductivity type. An antireflection layer 24 is provided on the light receiving surface 29 a of the semiconductor substrate 21. The solar cell element 30 includes a first electrode 25 provided on the light receiving surface 29 a of the semiconductor substrate 21 and a second electrode 26 provided on the non-light receiving surface 29 b of the semiconductor substrate 21.

Next, a more specific configuration example of the solar cell element 30 will be described. As the semiconductor substrate 21 including the first semiconductor layer 22 having one conductivity type (for example, p-type), a silicon substrate is preferably used.

First, a p-type silicon substrate is prepared as a semiconductor substrate. As the silicon substrate, a polycrystalline silicon ingot produced by the method for producing an ingot according to the present invention is cut into a block having a desired shape, and then sliced using a multi-wire saw device or the like to form a substrate. be able to.

Boron (B) is preferably used as the p-type doping element, and the concentration is about 1 × 10 ¹⁶ to 1 × 10 ¹⁷ [atoms / cm ³ ]. At this time, the specific resistance value of the silicon substrate is 0.2. It is about 2Ω · cm. As a method for doping boron into the silicon substrate, an appropriate amount of boron element alone may be included at the time of manufacturing the silicon ingot, or an appropriate amount of boron-containing silicon lump whose doping concentration is already known may be included.

The thickness of the semiconductor substrate 21 is, for example, preferably 300 μm or less, and more preferably 200 μm or less.

The second semiconductor layer 23 that forms a pn junction with the first semiconductor layer 22 is a layer having a conductivity type opposite to that of the first semiconductor layer 22, and is provided on the light receiving surface 29 a side of the semiconductor substrate 21. In a silicon substrate in which the semiconductor substrate 21 exhibits p-type conductivity, the second semiconductor layer 23 can be formed by diffusing impurities such as phosphorus (P) on the light receiving surface 29 a side of the semiconductor substrate 21.

The antireflection layer 24 plays a role of reducing the reflectance of light in a desired wavelength region and increasing the amount of photogenerated carriers. The antireflection layer 24 is made of, for example, a silicon nitride film, a titanium oxide film, or a silicon oxide film. The thickness of the antireflective layer 24 is appropriately selected depending on the constituent material, and is set so as to realize a nonreflective condition with respect to appropriate incident light. For example, the semiconductor substrate 21 made of silicon preferably has a refractive index of about 1.8 to 2.3 and a thickness of about 500 to 1200 mm.

A BSF (Back-Surface-Field) region 7 provided on the non-light-receiving surface 29b side of the semiconductor substrate 21 has a role of reducing a decrease in efficiency due to carrier recombination in the vicinity of the non-light-receiving surface 29b. An internal electric field is formed on the light receiving surface 29b side. The BSF region 27 has the same conductivity type as that of the first semiconductor layer 22, but has a majority carrier having a concentration higher than that of the majority carrier contained in the first semiconductor layer 22. If the semiconductor substrate 21 is p-type, the BSF region 6 has a concentration of these dopant elements of 1 × 10 ¹⁸ to 5 by diffusing a dopant element such as boron or aluminum on the non-light-receiving surface 29b side. It is preferably formed so as to be about × 10 ²¹ atoms / cm ³ .

As shown in FIG. 10, the first electrode 25 has a first output extraction electrode 25a and a plurality of linear first current collecting electrodes 25b. At least a part of the first output extraction electrode 25a intersects the first current collection electrode 25b. The first output extraction electrode 25a has a width of about 1.3 to 2.5 mm, for example.

On the other hand, the first collector electrode 25b has a line width of about 50 to 200 μm and is thinner than the first output extraction electrode 25a. A plurality of first current collecting electrodes 25b are provided with an interval of about 1.5 to 3 mm.

The thickness of the first electrode 25 is about 10 to 40 μm. The first electrode 25 can be formed by applying a paste for forming an electrode made of, for example, silver powder, glass frit, an organic vehicle or the like into a desired shape by screen printing or the like, and then baking the paste.

As shown in FIG. 11, the second electrode 26 has a second output extraction electrode 26a and a second current collecting electrode 26b. In the present embodiment, the second output extraction electrode 26a has a thickness of about 10 to 30 μm and a width of about 1.3 to 7 mm. The second output extraction electrode 26a can be formed of the same material and manufacturing method as the first electrode 25 described above. For example, you may form by apply | coating a silver paste to a desired shape, and baking. The second collector electrode 26b has a thickness of about 15 to 50 μm, and is formed on substantially the entire surface of the non-light-receiving surface 29b of the semiconductor substrate 21 excluding a part of the second output extraction electrode 26a. The second current collecting electrode 26b can be formed, for example, by applying an aluminum paste in a desired shape and baking it.

Hereinafter, examples that further embody the above-described embodiment will be described.

A silicon ingot was produced using a seed crystal and a mold for producing a semiconductor ingot shown in FIGS.

First, a quartz crucible 1 shown in FIG. 1, a graphite mold 2 and a seed crystal 4 composed of five small seed crystals as shown in FIG. 2 were prepared.

In Example 1, as the seed crystal 4, five pieces of single crystal silicon having a Miller index having a (100) plane crystal orientation and a 150 mm square and a height of 10 mm were used.

Further, in Example 2, the seed crystal 4 used was a seed crystal 4 of Example 1 provided with a small seed crystal of 20 mm × 150 mm and a height of 10 mm at the periphery.

Further, in Example 3, in the case of Example 2, a carbon fiber having a thermal conductivity of 100 W / m · K or more at the center portion of the holder and a thermal conductivity smaller than this at the holder peripheral portion. Reinforced carbon composite was used.

Further, in the fourth embodiment, in the case of the second embodiment, the cooling mechanism 6 is divided into the central portion 6a and the peripheral portion 6b, and the cooling water is allowed to flow through two systems of the central portion and the peripheral portion. The ingot was manufactured by setting the set temperature of the cooling water supplied to the central portion 6a to 15 ° C and the set temperature of the cooling water supplied to the peripheral portion 6b to 20 ° C.

Then, a powder made of silicon nitride having an average particle size of about 0.5 μm, a powder made of silicon oxide having an average particle size of about 20 μm, and a PVA aqueous solution as a binder solution are mixed on the inner peripheral side surface portion 2 c of the mold 2. The release material 8 in the form of a slurry was applied so that the coating weight per unit area was about 0.1 g / cm ² . Thereafter, the projecting portion 4 c of the seed crystal 4 was fixed in the mold 2 via a release material 8. Further, a location located in the vicinity of the inner peripheral side surface portion 2 c of the small-crystal crystal mold 2 was also fixed through the release material 8.

A large number of silicon chunks of a total amount of 100 kg are put into the crucible 1, and the silicon in the crucible 1 is heated and melted by heating means (not shown) arranged around the crucible 1 to obtain a silicon melt 3 of about 1420 ° C. Was injected toward the central portion 4 g of the seed crystal 4 to produce a silicon ingot in the mold 2.

Further, as a comparative example, a silicon ingot was produced under the same conditions as in the example using a seed crystal having no protrusion.

As a result, in the comparative example, in the central part of the seed crystal 4 where the silicon melt 3 is concentrated and in contact, the seed crystal 4 melts until reaching the bottom surface part 2b of the mold 2, and a fine polycrystal grows from the central part of the silicon ingot. This was observed by visual observation of the cross section of the ingot and by observation of etch pits using a mixed acid composed of a mixture of hydrofluoric acid, nitric acid and acetic acid.

On the other hand, in each of Examples 1 to 4, it was confirmed by visual observation of the ingot cross section and etch pit observation by the mixed acid that the central part of the silicon ingot was grown from the seed crystal 4 as a starting point. .

Next, a silicon substrate was sliced from the center of the obtained silicon ingot, and then a solar cell element using this as a semiconductor substrate was produced as follows (refer to FIGS. 10 to 12 for the solar cell element). .

First, the polycrystalline silicon ingot produced in this example was sliced into a polycrystalline silicon substrate (semiconductor substrate 21) having a thickness of 200 μm, an outer shape of 150 mm × 150 mm, and a specific resistance of 1 to 1.2 Ω · cm. The damaged layer on the surface was cleaned by etching with a NaOH solution.

Next, a texture was formed on the light receiving surface 29a by dry etching. Then, the second semiconductor layer 23 was formed by a vapor phase thermal diffusion method using POCl ₃ as a diffusion source. At this time, the sheet resistance of the second semiconductor layer 23 was 70Ω / □. Further, after removing the phosphor glass by etching with a hydrofluoric acid solution and performing pn separation using a laser beam, a silicon nitride film to be the antireflection layer 24 was formed on the light receiving surface 29a by PECVD.

Then, the BSF region 27 and the second current collecting electrode 26b were formed by applying and baking an aluminum paste on substantially the entire surface of the non-light-receiving surface 29b of the semiconductor substrate 21. In addition, a silver paste was applied and fired on the light receiving surface 29a and the non-light receiving surface 29b to form the first electrode 25 and the second output extraction electrode 26a to obtain the solar cell element 30.

Furthermore, when the photoelectric conversion efficiency was measured in accordance with JIS C 8913 (1998) for the solar cell element using the semiconductor substrate obtained in each example, the solar cell using the semiconductor substrate of the comparative example was measured. The battery element was 16.2%, whereas the solar cell element using the semiconductor substrate of Example 1 was 16.8%, and the solar cell element using the semiconductor substrate of Example 2 was 16.5%. %, 16.6% for the solar cell element using the semiconductor substrate of Example 3, and 16.7% for the solar cell element using the semiconductor substrate of Example 4. From these results, it was also confirmed that the characteristics could be improved as the solar cell element using a semiconductor substrate with good crystallinity.

1: crucible 2: mold 2a: opening 2b: bottom surface 2c: inner peripheral side surface 3: semiconductor melt 4: seed crystal 4a: first main surface 4b: second main surface 4c: protrusion 5: holder 6: Cooling mechanism 7: plate-like body 7a: first plate-like body 7b: second plate-like body 8: release material 21: semiconductor substrate (silicon substrate)
22: 1st semiconductor layer 23: 2nd semiconductor layer 24: Antireflection layer 25: 1st electrode 25a: 1st output extraction electrode 25b: 1st current collection electrode 25c: Auxiliary electrode 26: 2nd electrode 6a: 2nd output Extraction electrode 6b: second current collecting electrode 27: BSF region 29a: light receiving surface 29b: non-light receiving surface 30: solar cell element

Claims

A seed crystal having a first main surface into which a semiconductor melt is poured and a second main surface located on the opposite side of the first main surface, and a central portion protruding from the second main surface side A seed crystal preparation process for preparing
A mold preparation step of preparing a mold having an opening into which the semiconductor melt is poured, a bottom surface, and an inner peripheral side surface positioned between the opening and the bottom;
A seed crystal disposing step of disposing the seed crystal on the bottom surface of the template with the second main surface of the seed crystal facing down;
An injection step of injecting the semiconductor melt toward a central portion of the first main surface of the seed crystal;
A solidification step of solidifying the semiconductor melt in the mold.
Manufacturing method of semiconductor ingot.
The method for manufacturing a semiconductor ingot according to claim 1, wherein, in the seed crystal preparation step, a seed crystal that also projects from a peripheral portion on the second main surface side is prepared as the seed crystal.
The method for manufacturing a semiconductor ingot according to claim 1 or 2, wherein, in the seed crystal preparation step, a seed crystal in which a plurality of small seed crystals are combined is prepared.
The method for manufacturing a semiconductor ingot according to any one of claims 1 to 3, wherein, in the seed crystal arranging step, the seed crystal is arranged on the bottom surface of the mold via a release material.
The said seed crystal arrangement | positioning process WHEREIN: The plate-like body from which the said seed crystal differs in thermal conductivity is arrange | positioned in the said bottom face part of the said casting_mold | template, and the said seed crystal is arrange | positioned on this plate-like body. The manufacturing method of the semiconductor ingot in any one of.
In the crystal arranging step, a first plate and a plurality of second plates having a lower thermal conductivity than the first plate are prepared as the plate, and the seed crystal is arranged. Before the first plate-like body is disposed at the center portion of the bottom surface portion of the mold so as to contact the center portion of the second main surface of the seed crystal, a plurality of the second plate-like bodies are arranged. 6. The method for manufacturing a semiconductor ingot according to claim 5, wherein the first ingot is disposed around the first plate-like body so as to be in contact with a peripheral portion of the seed crystal on the second main surface side.
The method for manufacturing a semiconductor ingot according to any one of claims 1 to 6, wherein, in the solidification step, a central portion of the bottom surface portion of the mold is cooled so that a temperature is lower than a peripheral portion of the central portion.
In the solidification step, the outer surface portion corresponding to the central portion of the bottom surface portion of the mold is positioned more than the periphery of the outer surface portion in a state where the holder for supporting the mold is disposed from the bottom surface portion side of the mold. The method of manufacturing a semiconductor ingot according to claim 1, wherein the cooling is performed so that the temperature is lowered.
9. The semiconductor ingot according to claim 8, wherein in the solidification step, the holder is made of a material that has a portion of the mold facing the outer surface portion that has a higher thermal conductivity than the periphery of the portion. Production method.