JP2006273669A - Method for manufacturing semiconductor ingot - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To propose a method for manufacturing a semiconductor ingot which is small in variation of specific resistance, uniform in quality and excellent in characteristics. <P>SOLUTION: In the method for manufacturing the semiconductor ingot by solidifying a semiconductor melt 9 containing a first dopant material regulating a first conduction type in the prescribed direction within a casting mold, the liquid surface of the semiconductor melt 9 in the solidification process is measured and the semiconductor raw material having the first dopant concentration lower than that of the semiconductor melt is supplied into the semiconductor melt according to the measured value. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for manufacturing a semiconductor ingot, and more particularly to a technique for adjusting the specific resistance of a semiconductor ingot.

  At present, silicon ingots are often used as semiconductor substrates for solar cells, and further improvements in the quality of the solar cells are demanded for higher performance of solar cells.

  Among them, in order to adjust the specific resistance of the silicon ingot within a predetermined range and improve the quality, a technique of adding a dopant material, which is a III-valent or V-valent impurity element, to the silicon melt is used. This dopant material is usually supplied in the form of granular silicon crystals containing an impurity element at a high concentration, and this is melted simultaneously with the silicon material to obtain a silicon melt having a desired dopant concentration.

  When such a silicon melt is solidified, there is a problem that the specific resistance value fluctuates in the crystal growth direction.

  That is, the dopant element in the silicon melt is taken into the silicon solid crystal at a certain ratio by the segregation phenomenon during the solidification growth. At this time, the dopant concentration (CS) in the crystal is expressed by CS = kCL (k: segregation coefficient, CL: dopant concentration in the melt). The dopant concentration that lowers the temperature increases from the bottom of the mold to the top. Accordingly, the specific resistance is highest at the bottom of the mold and decreases in the height direction. Therefore, there has been a problem that it deviates from a desired specific resistance value range, and the portion cannot be used as a product.

  In order to solve this problem, the following method has been proposed as a specific resistance adjustment technique when single crystal silicon is grown by the CZ method.

For example, in the middle of pulling a single crystal from a silicon melt containing a first dopant material, a method of supplying a second dopant material exhibiting a conductivity type opposite to that of the first dopant material (for example, see Patent Document 1), or The specific resistance of the entire silicon ingot by the method of adding phosphorus of the second dopant material at a concentration of 25 to 35% with respect to the boron concentration of the first dopant material (for example, see Patent Document 2) to the initial silicon melt There are those that make the values uniform.
Japanese Patent Laid-Open No. 7-309963 JP 2003-137687 A

  However, in the method of supplying the second dopant material exhibiting the opposite conductivity type while pulling up the single crystal as in Patent Document 1, the tail portion of the pulled silicon single crystal is controlled depending on the timing of supplying the second dopant material. There is a problem that the specific resistance varies greatly. In particular, unlike the CZ method, in which the silicon solidification rate can be easily estimated from the pulling height, when the silicon melt is solidified from the bottom of the mold by the casting method, it is difficult to observe the solid-liquid interface from the outside. It was extremely difficult to control the supply timing.

  Further, in the method of adding the second dopant material in advance to the initial melt as in Patent Document 2, various conditions for controlling various quality characteristics of the silicon ingot (temperature profile, silicon pulling speed, and rotation of the seed crystal) The uniformity of the specific resistance needs to be adjusted depending on the speed). In addition, in this case, the dopant concentration is increased as compared with the conventional method, and it is not a preferable method from the viewpoint of homogenizing the ingot quality.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to propose a method for producing a semiconductor ingot having a small specific resistance variation, uniform quality and excellent characteristics in a method of manufacturing a semiconductor ingot by a casting method. To do.

  The method for producing a semiconductor ingot according to the present invention comprises solidifying a semiconductor melt containing a first dopant material defining a first conductivity type in a predetermined direction in a mold, wherein the semiconductor melt in the solidification process The liquid level is measured, and a semiconductor raw material having a first dopant concentration lower than the first dopant concentration of the semiconductor melt is supplied into the semiconductor melt according to the measured value. Here, the “semiconductor raw material” is a concept including both a solid semiconductor material and a liquid semiconductor melt.

  In particular, the first dopant concentration of the semiconductor raw material is lower than the first dopant concentration before the start of solidification of the semiconductor melt.

  The method for producing a semiconductor ingot according to the present invention comprises solidifying a semiconductor melt containing a first dopant material defining a first conductivity type in a predetermined direction in a mold, wherein the semiconductor melt in the solidification process Since a semiconductor material having a first dopant concentration lower than the first dopant concentration of the semiconductor melt is supplied into the semiconductor melt according to the measured value, the semiconductor melt in the solidification process is measured. Estimating the solidification rate of the semiconductor melt using the liquid surface height as an index, and controlling the dopant concentration of the semiconductor melt, the desired specific resistance can be obtained even on the top of the semiconductor ingot, thereby suppressing quality variations. Is possible.

  In particular, since the impurity concentration in the semiconductor melt can be controlled at an arbitrary timing, the degree of freedom in setting the heating and cooling conditions for controlling the solidification rate and temperature gradient can be secured, and as a substrate for solar cells It is possible to simultaneously satisfy various conditions for improving the characteristics.

  In addition, since the dopant material which prescribes | regulates the conductivity type contrary to a 1st conductivity type is not included at all, the semiconductor ingot by which the quality variation was suppressed simultaneously with the specific resistance variation can be manufactured.

  Next, since the first dopant concentration of the semiconductor material is configured to be lower than the first dopant concentration before the start of solidification of the semiconductor melt, it is possible to effectively vary the resistivity using a relatively small amount of semiconductor material. Can be suppressed.

  Hereinafter, the manufacturing method of the semiconductor ingot of this invention is demonstrated in detail using drawing.

  FIG. 1 is a schematic cross-sectional view showing a configuration of a general semiconductor (silicon) casting apparatus, and FIG. 2 is a schematic cross-sectional view of a solidified portion in the semiconductor (silicon) casting apparatus. In the figure, 1 is a melting crucible, 2 is a holding crucible, 3 is a heating means, 3a is an upper heating means, 3b is a side heating means, 4 is a silicon material, 5 is a mold, 6 is a mold holding table, and 7 is a cooling plate. , 8 is a mold heating means, and 9 is a silicon melt.

  First, a semiconductor (silicon) casting apparatus used in the method for producing a semiconductor ingot according to the present invention will be described.

  As the melting crucible 1, high-purity quartz or the like is used so that impurities are not mixed into the molten silicon melt. In addition, the melting crucible 1 is held by a holding crucible 2 made of graphite or the like in order to prevent softening and deformation when the temperature becomes higher than the melting point of silicon. The size of the melting crucible 1 and the holding crucible 2 needs to be a size that can contain a silicon material according to the melting amount to be melted at one time. The melting amount of the silicon material 4 is, for example, in the range of 1 kg to 250 kg.

  A heating means 3 is installed around the melting crucible 1 and the holding crucible 2. For example, an upper heating means 3 a and a side heating means 3 b are provided on the upper and side portions of the melting crucible 1. A silicon melt is obtained by heating and melting the silicon material 4. As the heating means 3, for example, a resistance heating type heater or an induction heating type coil can be used.

  Further, the silicon melt heated and melted inside the melting crucible 1 is poured into a mold 5 having an opening upward by tilting the melting crucible 1. Then, the silicon melt 9 poured into the mold 5 is removed from the bottom by the cooling plate 7 provided at the lower part of the mold 5 and simultaneously heated from the top by the mold heating means 8, and the temperature is increased in the height direction. By providing the distribution, solidification progresses from the bottom toward the top.

  The mold 5 is made of silicon oxide, silicon nitride, graphite or the like. Moreover, it is desirable to provide a release material layer (not shown) on the inner surface of the mold 5 in order to prevent the inner wall of the mold 5 and the silicon ingot from fusing after the silicon melt 9 is solidified. As a material for the release material layer, for example, silicon nitride, silicon carbide, silicon oxide or the like can be used.

  Further, a heat insulating material layer (not shown) made of a carbon material or the like can be provided on the outer periphery of the mold 5. By selecting the material, thickness and the like of the heat insulating material layer, the solidification rate and solidification time of the silicon melt poured into the mold 5 can be controlled.

  Further, the cooling plate 7 is in contact with the bottom of the mold 5 or the lower surface of the mold holding table 6 to cool the silicon melt 9 in the mold 5 from the bottom. The thing of the structure of circulating gas etc. can be used.

  Further, a mold heating means 8 composed of a resistance heating type heater, an induction heating type coil or the like that can heat the mold 5 from above may be disposed, whereby the silicon melt 9 poured into the mold 5 can be disposed. The surface can be heated moderately, and the temperature gradient from the bottom to the top can be controlled more accurately.

  In addition, these casting apparatuses are desirable in that they are placed in a vacuum vessel (not shown) and performed in a reducing atmosphere such as an inert gas to prevent impurities from being mixed and oxidized.

  Next, a method for manufacturing a semiconductor ingot using the casting apparatus will be described.

  First, after a predetermined amount of silicon material 4 and a first dopant material containing P-type impurities such as boron and gallium in such a manner that the specific resistance at the bottom of the ingot is adjusted to a desired value are put into the melting crucible 1. Then, the silicon material 4 in the melting crucible is melted by the heating means 3 to obtain a silicon melt at a desired temperature. At this time, the dopant material may be the impurity element itself, or may be a silicon material in which the impurity element is previously doped at a high concentration.

  Next, the melting crucible 1 is tilted and the silicon melt 9 heated and melted in the mold 5 is poured. The mold 5 filled with a predetermined amount of the silicon melt 9 moves to the solidification step, and the cooling plate 7 is brought into contact with the lower surface of the mold holding table 6 and at the same time, the silicon melt 9 is heated from above by the mold heating means 8. The output of the mold heating means 8 is adjusted so that the silicon melt 9 gradually solidifies from the bottom to the top of the mold 5.

  Here, the solidification rate of the silicon melt is estimated using the height of the surface of the silicon melt as an index, and when the liquid surface height of the silicon melt 9 reaches a height corresponding to the desired solidification rate. Supplying an additional silicon material or silicon melt containing a first dopant material lower than the dopant concentration in the silicon melt in the mold.

  By using such a method, the solidification rate of the silicon melt is estimated from the height of the silicon melt, and the same conductivity type is used in the silicon melt in which the dopant concentration increases as the solidification progresses. By supplying an additional silicon material or silicon melt containing a first dopant material whose concentration is lower than the dopant concentration in the silicon melt, the dopant concentration in the silicon melt is reduced and the decrease in resistivity is suppressed. It is possible to manufacture a silicon ingot having a uniform quality and a small variation in specific resistance.

  Further, since the silicon melt 9 is solidified from the bottom of the mold 5 and the additional silicon material or silicon melt is supplied to the surface of the silicon melt heated by the mold heating means 8, the silicon melt near the solid-liquid interface is used. The dopant concentration of the liquid does not change abruptly, and additional silicon material or silicon melt is diffused by thermal convection generated in the silicon melt, so that the silicon melt in the mold has a uniform dopant concentration.

  Furthermore, the degree of freedom of manufacturing conditions is not restricted, and various temperature conditions can be freely changed to control the solidification rate, so that various conditions for improving solar cell characteristics can be satisfied at the same time. It becomes possible.

  The solid surface ratio of the silicon melt 9 can be estimated by, for example, observing the liquid level of the silicon melt 9 from a viewing window or the like provided in the casting apparatus as needed. In addition, it is possible to estimate the solidification rate more accurately by providing a mark such as a scale on the surface of the release material layer previously applied to the side surface of the mold 5. It is also possible to control the supply of additional silicon material or silicon melt by monitoring the liquid level using a video camera or the like.

  Further, by supplying an additional silicon material or a silicon melt having a lower concentration than the dopant concentration of the silicon melt in the mold a plurality of times, a silicon ingot having a smaller specific resistance variation and a uniform quality can be manufactured. Therefore, the solidification rate of the silicon melt when supplying the silicon material or silicon melt, the dopant concentration and supply amount of the additional silicon material or silicon melt, and the number of times of supply are appropriately determined so that variation in the specific resistance of the ingot can be suppressed. Adjust it.

  As described above, the specific resistance value of the silicon ingot produced by the casting method described above is such that the difference in specific resistance between the bottom and top of the silicon ingot is small in the specific resistance variation of 0.2Ω · cm to 0.5Ω · cm. It is.

  The present invention is not limited to the above-described embodiment, and various changes and improvements can be made without departing from the gist of the present invention.

  For example, in the above-described embodiment, the description has been given using the silicon casting apparatus additionally provided with the melting crucible on the upper side. However, the present invention is not limited thereto, and the mold for melting the silicon material inside the mold to form the silicon melt An inner melting type silicon casting apparatus may be used. In the in-mold melting type silicon casting apparatus, the bulk specific gravity of the silicon material in the mold is usually 1.0 to 1.5, which is about half of the specific gravity of silicon 2.33. The height of the silicon ingot formed in this way is only about half that of the mold. Therefore, it is preferable to supply an additional silicon material in order to effectively use the mold, and at this time, it is particularly effective to use the manufacturing method according to the embodiment of the present invention.

  Further, instead of the above-described embodiment, a silicon melt containing the first dopant material is prepared in the melting crucible 1, and the first dopant material is separately disposed inside the mold 5, so that the silicon melt in the melting crucible 1 is disposed. A certain amount of liquid is supplied into the mold to melt the silicon material in the mold. Then, the silicon melt in the mold is solidified, the solidification rate of the silicon melt is estimated using the height of the silicon melt as an index, and the liquid surface height of the silicon melt 9 is the desired solidification rate. When the height corresponding to the above is reached, the silicon melt remaining in the melting crucible 1 is supplied to solidify the silicon melt. Further, the silicon melt to be added may be supplied in a plurality of times.

  In the above embodiment, the dopant concentration of the silicon melt in the melting crucible 1 is set low, and the dopant concentration is adjusted in the mold when a certain amount of the silicon melt is poured into the mold. By disposing the first dopant material, the specific resistance value at the bottom of the silicon ingot may be set to a desired value. And since the silicon melt in the melting crucible is only poured into a plurality of times, the manufacturing apparatus is not complicated, and the dopant concentration and specific resistance can be adjusted by a simple method.

  Furthermore, it can replace with the said embodiment and can satisfy | fill the effect of this invention even if a 1st dopant material is an N-type dopant. As the N-type dopant, a donor element such as P (phosphorus), As (arsenic), or Sb (antimony) is used. As the P-type dopant, B (boron), Ga (gallium), In (indium), Al ( An acceptor element such as aluminum is used.

  Hereinafter, the Example which implemented the manufacturing method of the semiconductor ingot mentioned above and the comparative example implemented by the manufacturing method of the prior art for the comparison are described using FIG.

(Example)
The melting crucible 1 made of high-purity quartz was held by a holding crucible 2 made of graphite, and the calculation was performed in advance so that the specific resistance of the 80 kg silicon material and the bottom of the cast ingot was 1.5 Ω · cm in the melting crucible. A first dopant material composed of a predetermined amount of boron was added. The raw material in the crucible was heated and melted by the upper heating means 3a and the side heating means 3b to form a silicon melt, which was poured into the quartz mold 5. In the solidification process, the mold 5 poured with the silicon melt 9 was removed from the bottom by the cooling plate 7 and heated from the top by the mold heating means 8 to start unidirectional solidification from the bottom to the top. Further, 20 kg of silicon material and 70% of the first dopant material with respect to the dopant concentration were supplied into the melting crucible 1 and heated and melted by the heating means 3 to obtain a silicon melt. Then, the level of the silicon melt is monitored by using a video camera to estimate the solidification rate of the silicon melt. When the solidification rate reaches 80%, the silicon melt in the melting crucible is put into the mold 5. Additional supply was performed to solidify the silicon melt to obtain a silicon ingot.

(Comparative example)
The molten crucible 1 made of high-purity quartz was held by a holding crucible 2 made of graphite, and 100 kg of silicon material and the specific resistance at the bottom of the cast ingot were calculated in advance so that the specific resistance was 1.5 Ω · cm. A predetermined amount of the first dopant raw material was charged. The raw material in the crucible was heated and melted by the upper heating means 3a and the side heating means 3b to form a silicon melt, which was poured into the quartz mold 5. In the solidification process, the mold 5 poured with the silicon melt 9 is removed from the bottom by the cooling plate 7 and heated from the top by the mold heating means 8 to perform unidirectional solidification from the bottom to the top, and the silicon ingot Got.

  The center part of the silicon ingot cast by the methods of the above examples and comparative examples was cut in the vertical direction, and the specific resistance value of the cross section was measured with a four-terminal measuring instrument. The result is shown in FIG. Here, the vertical axis represents the specific resistance value of the silicon ingot, the horizontal axis represents the solidification rate of the obtained silicon ingot, and the top of the silicon ingot is 1.

  The specific resistance difference between the bottom and top of the ingot of the example according to the present invention was 0.2 to 0.5 Ω · cm, whereas the specific resistance difference between the bottom and top of the ingot of the comparative example was 0.7Ω. -Cm. Moreover, the silicon ingot obtained by the above-mentioned method was each sliced to a predetermined thickness to produce a general bulk type solar cell element, and the conversion efficiency of the solar cell element was evaluated as its characteristics. As a result, the average value of the conversion efficiencies of the elements using the substrates produced in the examples was 15.8%, while those using the substrates produced in the comparative examples were 15.4%. It was. As described above, the solar cell element formed using the semiconductor substrate according to the present invention of the example obtained better characteristics than those of the comparative example according to the conventional method. The silicon ingot formed using the manufacturing method is presumed to have a quality with a uniform impurity concentration and a small variation in specific resistance. As described above, the effects of the present invention could be confirmed by the examples.

It is sectional drawing which shows an example of the casting apparatus of the supply process of the semiconductor raw material in connection with the manufacturing method of the semiconductor ingot of this invention. It is sectional drawing which shows an example of the casting apparatus of the solidification process in connection with the manufacturing method of the semiconductor ingot of this invention. It is a graph which shows the specific resistance of the silicon ingot produced by the manufacturing method of the semiconductor ingot of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Melting crucible 2 ... Holding crucible 3 ... Heating means 3a ... Upper heating means 3b ... Side part heating means 4 ... Silicon material (semiconductor material)
5 ... Mold 6 ... Mold holder 7 ... Cooling plate 8 ... Mold heating means 9 ... Silicon melt (semiconductor melt)

Claims (2)

In a method for producing a semiconductor ingot obtained by solidifying a semiconductor melt containing a first dopant material defining a first conductivity type in a predetermined direction in a mold,
The liquid level of the semiconductor melt in the solidification process is measured, and a semiconductor raw material having a first dopant concentration lower than the first dopant concentration of the semiconductor melt is supplied into the semiconductor melt according to the measured value. A method for manufacturing a semiconductor ingot. 2. The method of manufacturing a semiconductor ingot according to claim 1, wherein the first dopant concentration of the semiconductor raw material is lower than the first dopant concentration before the solidification of the semiconductor melt is started.

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