JPH0696443B2 - Casting method for polycrystalline objects such as silicon - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention provides a melt of a polycrystalline raw material such as polycrystalline silicon for solar cells, in particular, for a solar cell, by supplying the melt to a mold to cast a desired object. The present invention relates to a method for casting a polycrystalline object in which a good quality cast product can be obtained by aligning the crystal growth direction during solidification in one direction.

[0002]

2. Description of the Related Art As is known, in order to carry out this type of casting method, a crystalline raw material such as silicon is superheated and melted in an inert atmosphere 1 as illustrated in FIG. Since the melt 2 thus obtained is also made to flow down to the mold 3 in the inert atmosphere 1 and supplied all at once until the melt 2 reaches a predetermined high liquid level H in the mold 3. is there.

In this way, when the melt 2 is stored in the mold 3, the polycrystalline body is cast by cooling and solidifying the melt 2. The crystal growth caused by the cooling of the melt 2 is performed. A good quality cast product cannot be obtained unless the directions are aligned in a certain direction. For this reason, as shown in the drawing, the temperature gradient heater 4 is provided above the mold 3 so that the temperature gradually increases from the lower side to the upper side of the mold 3 as indicated by the arrow T. It gives a gradient.

[0004]

By doing so, it is sure that the melt 2 in the mold 3 grows in a certain direction from the bottom to the top, and the crystal C1 thereof grows as shown in FIG. Becomes However, at this time, it is not possible to completely eliminate the crystal growth nuclei on the side wall surface 3a of the mold 3,
Inevitably, there is a crystal C2 that grows laterally inward from the side wall surface 3a. Therefore, not only the crystal growth directions are generated in both vertical and horizontal directions and are not stabilized, but also the crystal C1 growing from the bottom and the crystal C2 collide with each other. In addition, a large number of strains and crystal defects are generated, and as a result, it is impossible to cast a polycrystalline object of satisfactory quality.

Therefore, when the conventional casting method is used, as described above, in order to prevent the crystal C2 from growing laterally from the side wall surface 3a of the mold 3, the temperature of the side wall surface 3a is set to the temperature of the polycrystalline material. The temperature is kept above the melting point of the object, and although some improvement in quality can be expected, of course, this is not sufficient, and the mold 3 is heated to a high temperature at this time. The deterioration of the side wall surface 3a of the mold 3 progresses, and as a result, defects such as long-term use of the mold 3 cannot be avoided.

Further, in the same casting method as above, the melt 2 is entirely poured into the mold 3 from the beginning, and this is solidified over a long period of time. Impurities such as oxygen, silicon nitride, and carbon easily dissolve into the crystal of the melt 2 from the mold 3 that is left to stand, and as a result, the adverse effect on the crystal becomes large.

The present invention has been made in view of the above-mentioned defects of the conventional method.
According to the first aspect of the present invention, instead of injecting the melt into the mold 3 all at once as described above, the melt is gradually added in synchronism with the rate at which the crystals of the melt grow. By continuing to supply into the mold, it is possible to prevent the generation of strains and crystal defects due to the collision of crystals as in the conventional method, and to eliminate the need to heat the mold at a high temperature to further increase its durability. In order to obtain high quality products, it is possible to prevent impurities from melting into the crystal from the mold.
That is the purpose.

In the second aspect, in order to synchronize the crystal growth rate in the first aspect with the melt injection rate, the liquid level of the melt is detected by using a liquid level measuring thermocouple or an infrared sensor. Then, based on this, the set temperature of the heater for melting the crystalline raw material in the crucible is controlled to ensure the synchronization.

In the third aspect of the present invention, the synchronization in the first aspect is performed by controlling the pressure applied to the crucible containing the melt, rather than adjusting the set temperature of the heater. In addition, it is possible to perform a highly responsive synchronization, and according to claim 4, by further measuring the weight of the whole including the mold by a load sensor or the like, a more highly accurate synchronization is guaranteed. Trying to.

[0010]

In order to achieve the above object, the present invention provides a crucible with an outlet in an inert atmosphere which can be heated to a desired temperature by a heater. , A crystalline raw material such as silicon is stored, and a temperature gradient is applied to the mold placed below it so that the temperature rises from the lower side to the upper side in order to determine the predetermined crystal growth direction. That is, the crystalline raw material in the crucible, is heated and melted by the heater, from the outlet of the crucible, when the melt is supplied downward into the mold, when the melt flowed into the mold Melt, from the bottom to the top by the temperature gradient, characterized in that the injection rate of the same melt melt down flow supplied from the outlet is synchronized with the growth rate of the crystal. A casting method for polycrystalline objects such as silicon is proposed. I am trying to.

Next, in the case of claim 2, claim 1
In the melt, from the bottom to the top by the temperature gradient, the rate at which the crystal grows, by the thermocouple for liquid level measurement previously applied to the mold, the temperature at the time of solidification of the melt By measuring the change, by detecting the liquid surface of the melt by this, or by measuring the change in infrared radiation during solidification of the melt by an infrared sensor, the same as above to detect the liquid surface of the melt Grasping the same, the injection speed of the melt that is supplied downward from the outlet is adjusted by controlling the set temperature of the heater that melts the crystalline raw material in the crucible based on the above detection result. The content is what you did.

Further, according to the third aspect of the invention, the rate at which the crystals of the melt in the first aspect grow from the bottom to the top due to the temperature gradient is determined by an infrared sensor when the melt is solidified. By measuring the change in infrared radiation in, the liquid level of the melt is detected and grasped, and the injection speed of the melt that is supplied downward from the outlet is based on the above detection result, and the pressure controller The pressure applied to the crucible is controlled to be a negative pressure or a positive pressure by the above, and the same as above is adjusted by turning the melt on and off.

Further, in the case of claim 4, claim 1
In the same as above, the melt is heated from the bottom to the top by a temperature gradient, the rate at which the crystals grow, by measuring the change in infrared radiation during solidification of the melt by an infrared sensor, the liquid of the melt. As well as detecting the surface, grasp by measuring the weight of the melt in the mold at the time of detection,
The injection speed of the melt which is supplied downward from the outlet is the same as above.
Based on the above detection result, or to control the set temperature of the heater to melt the crystalline raw material in the crucible, the pressure applied to the crucible by the pressure controller, negative pressure, to control the positive pressure, The content is that adjustment is made by turning the melt on and off.

[0014]

According to the present invention, the melt contained in the mold grows from the bottom to the top of the melt without causing the melt of a large amount of crystalline raw material to flow down into the mold all at once. In synchronism with the growth rate to be performed, since the melt will be injected into the mold, of course, the melt is not accumulated in the upper part of the mold, therefore, the crystal It does not grow laterally from the side wall surface of the mold, the crystal growth direction is constant, strain and crystal defects do not occur, and the mold is kept at a temperature higher than the melting point of silicon etc. Not only is it unnecessary, but the time of staying in the mold as a melt is also reduced, so that the dissolution of impurities such as silicon nitride and carbon from the mold into the crystals of the melt is also reduced.

As a specific example of synchronizing the supply rate of the melt of the polycrystalline raw material flowing down with the crystal growth rate as described above, first, as disclosed in claim 2, an inert atmosphere is first prepared. When the polycrystalline raw material is housed in the crucible inside and heated by a heater, the heating temperature is adjustable. On the other hand, by measuring the liquid level of the melt, the growth rate can be grasped as a result, so that the heating temperature can be controlled based on the grasped result.

In the third aspect of the present invention, regarding the crystal growth rate of the melt, the change in infrared radiation when the melt is solidified is detected by the infrared sensor to detect the liquid level of the melt. On the other hand, the amount of the melt supplied to the mold is adjusted by the pressure level detected by the pressure controller for the crucible to be positive or negative depending on the detected liquid level, thereby turning the supply of the melt ON-OFF. It is controlled so that the supply amount and the crystal growth rate are synchronized with each other.

Further, in the case of claim 4, the change in infrared radiation when the melt is solidified is detected by the infrared sensor as in claim 3, and the liquid level height of the melt is detected. In addition to measuring the total weight including the mold at this time, the injection speed of the melt is controlled by the weight value, and by doing so, only the height of the liquid surface is measured. In this case, the volume shrinkage that occurs when the melt solidifies causes an error in the measured value of the crystal growth rate, but it is necessary to detect it by weight without grasping the change in height. Therefore, the injection rate of the melt, that is, the dropping amount can be determined by the more accurate detection result of the crystal growth rate, and the desired synchronization can be achieved.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A method for casting a polycrystalline object according to the present invention will be described in detail with reference to FIGS. 1 and 2. First, a casting furnace 10 that can be used for carrying out the method will be described. As is the case with the conventional example, a crucible 12 capable of containing a polycrystalline raw material 11 made of silicon or the like in an inert atmosphere 10a in a furnace, and a heater 13 for heating the crucible 12 are used.
Further, in the lower part of the crucible 12, a mold 14 constituted by coating the surface of carbon with silicon nitride is provided. Reference numeral 15 denotes a temperature gradient heater for applying a predetermined temperature gradient to the mold 14 such that the temperature gradually increases upward as indicated by an arrow T.
Reference characters a in the crucible 12 indicate outlets formed by pinholes or the like formed in the bottom of the crucible 12, respectively.

On the other hand, the casting furnace 1 shown in FIG.
In No. 0, the following components are added to the casting furnace 10 of FIG. That is, the side wall 14a of the mold 14
, Liquid level measuring thermocouples 16 are attached with a required plurality of height differences, and the set temperature of the heater 13 is controlled by an output signal from these liquid level measuring thermocouples 16 due to a temperature change. It has become.

On the other hand, the infrared sensor 17 outputs the infrared radiation 18 detected by the infrared sensor 17 according to the output generated by the infrared sensor 17, and the heater 13 or the pressure controller 19 is operated.
Is controlled so that the pressure of the inert gas from the pressure controller 19 can be applied to the crucible 12 in the closed state. Of the melt 11a.

Therefore, in order to carry out the casting method according to the present invention using the above casting furnace 10, the mold 14 is placed under the temperature distribution as shown by the arrow T as in the conventional method, and the heating by the heater 13 is performed. Polycrystalline raw material 11 stored in crucible 12 is melted. The melt 11a thus obtained
Is dropped from the outlet 12a of the crucible 12 and dropped into the mold 14. At this time, the heating temperature of the heater 13 is variable, and, for example, about 10
1500-1600 ° C for melting kg of silicon
The temperature can be controlled within the range of, and the time required for the silicon to melt is about 30.
Be prepared to adjust within the range of 1 minute to 1 hour.

On the other hand, in the present invention, the melt 11a flowing down into the mold 14 as described above is synchronized with the rate at which the crystal C1 grows from the bottom to the top by the temperature gradient shown by the arrow T. Therefore, the moving speed of the solid-liquid interface F1 shown in the figure, that is, the moving speed V1 of the crystal C1 and the moving speed V2 of the liquid surface F2 by the melt 11a supplied downward are allowed. Make them as equal as possible.

As a concrete example for supplying the melt 11a from the crucible 12 so that V2 is synchronized with V1 as described above, in the casting furnace 10 shown in FIG. The temperature change when the melt 11a of the polycrystalline raw material 11 such as silicon is solidified is previously measured by the thermocouple 16 for use, and the temperature change is measured by the temperature change. It is possible to grasp the change and control the heating set temperature by the heater 13 based on the result obtained from the output of the liquid level measuring thermocouple 16, thereby supplying the melt 11a by the polycrystalline raw material 11. If the amount is adjusted, it is possible to supply the melt 11a downward in synchronism with the growth rate V1 of the crystal of the melt 11a as described above.

Further, as the synchronizing means for V1 and V2, as shown in FIG.
By rotating 7 in the vertical direction as shown by arrow 20, a change in infrared radiation 18 when the melt 11a of silicon or the like is solidified in the mold 14 is detected, whereby the melt 11a is detected. When the liquid level F2 of the heater 13 is detected, the output from the infrared sensor 17 based on the detection causes the set temperature of the heater 13 or the pressure controller 19 to be turned on as described above.
N-OFF control is performed.

By doing so, in the case of the pressure controller 19, the inert gas from now on causes the crucible 1
The pressure for 2 is adjusted to a negative pressure or a positive pressure, and as a result, the injection of the melt 11a flowing out from the outlet 12a of the crucible 12 is repeated ON-OFF, and the moving speed of the solid-liquid interface F1 is increased. Thus, the moving speed V2 of the liquid surface F2 can be made to follow.

Here, when the present invention was actually carried out using a 20 cm square mold, when the above-mentioned 10 kg of silicon was solidified by injecting all the melt into the mold from the beginning as in the conventional method, it was already described. Although the crystal growth from the side wall surface of the mold was considerably observed as shown in the figure, almost no crystal growth from the side wall surface was observed because the melt was gradually and synchronously injected continuously over 3 hours. It was

Further, in the above implementation, the temperature of the side wall surface of the side wall 14a of the mold 14 is set to be equal to or lower than the melting point of silicon or the like. Even in this case, the crystal growth from the side wall surface is suppressed. I was able to confirm.

Further, another embodiment for synchronizing the above V1 and V2 will be shown here, but the apparatus as shown in FIG. 3 will be used for this. This device is different from the above-mentioned device in that the mold 14 is placed on the support 21, and the support column 21a of the support 21 is connected to the outside via a gas seal 22 provided at the bottom of the casting furnace 10. That is, the load sensor 24 is provided between the support column 21a and the base portion 23 fixed at a predetermined position. And
The output signal from the load sensor 24 is input to the controller 25, and the controller 25
The output signal from the infrared sensor 17 described above is also input, and the heater 13 or the pressure controller 19 is controlled by the output of the controller 25.

By using the above apparatus, the infrared sensor 17 detects the height of the liquid surface F2 of the melt 11a as described above, and the detection result does not control the heater 13 or the pressure controller 19, but Liquid level F2
The total weight including the mold 14 is measured by the load sensor 24 corresponding to the above detection, and the controller 25 outputs an output based on the weight change.
3 and the pressure controller 19 are controlled.

Therefore, in the case of control by the height of the liquid surface F2, when the melt 11a is changed from liquid to solid, its volume is contracted (the liquid 1 in the case of silicon is 1).
The weight of cc is 2.1 g, while the weight of 1 cc of solid is 2.3 g. Therefore, in the end, an error occurs in measuring the crystal growth rate. However, as in this case, the crystal growth rate should be grasped by the measured value of the weight of the melt 11a by the load sensor 24. If
The above error is eliminated, and as a result, a more desirable V
1 and V2 will be synchronized.

[0031]

Since the present invention can be carried out as described above, according to claim 1, in synchronization with the rate at which the crystal of the melt grows from below to above due to the temperature gradient,
Since the melt is supplied to the mold, crystal growth in the lateral direction from the side wall of the mold is suppressed, and it is dominated by only upward crystal growth, so that it is possible to obtain a good quality cast product with no distortion or crystal defects. it can.

Further, according to the second and third aspects, the crystal growth of the melt is obtained by grasping the liquid level change in the melt in the mold using a thermocouple for measuring the liquid level and an infrared sensor. Knowing the speed, and based on the results of this grasp, the set temperature of the heater that melts the polycrystalline raw material and the pressure applied in the crucible were controlled, so the crystal growth rate and the melt to the mold were controlled. The supply amount is accurately synchronized, and the desired result can be obtained in the production of high quality cast products.

Further, according to claim 4, the change in the melt surface level is known by the infrared sensor in claim 3 and the crystal growth rate is ascertained by this, whereas the weight of the melt in the mold is determined. By knowing the change by the load sensor, the crystal growth rate is measured as above. Therefore, the melt supply amount can be synchronized with the rate with higher accuracy.

[Brief description of drawings]

FIG. 1 is a front vertical cross-sectional explanatory view showing an example of a casting furnace that can be used for carrying out the method for casting a polycrystalline object according to the present invention.

FIG. 2 is a front vertical cross-sectional explanatory view showing a different example of a casting furnace that can be used to carry out the above-described casting method.

FIG. 3 is a front vertical cross-sectional explanatory view showing another example of a casting furnace that can be used to carry out the above-described casting method.

FIG. 4 is a vertical sectional front view showing a casting furnace used for carrying out a conventional method for casting a polycrystalline object.

[Explanation of symbols]

 10a Inert Atmosphere 11 Polycrystalline Raw Material 11a Melt 12 Crucible 12a Outlet 13 Heater 14 Mold 16 Thermocouple for Liquid Level Measurement 17 Infrared Sensor 18 Infrared Radiation 19 Pressure Controller

Claims (4)

[Claims] 1. A crystalline raw material such as silicon is housed in a crucible with an outlet that can be heated to a desired temperature by a heater in an inert atmosphere, and a mold disposed below the crystalline raw material is In order to determine the direction of a predetermined crystal growth, a temperature gradient is applied so that the temperature becomes higher from the lower side to the upper side, and the crystalline raw material in the crucible is heated and melted by the heater, From the outlet of the crucible the melt, during the down-flow supply into the mold, the melt flowed down in the mold, from the bottom to the top due to the temperature gradient,
A method for casting a polycrystalline object such as silicon, characterized in that the speed at which the crystal grows is synchronized with the injection speed of the melt that is supplied downward from the outlet. 2. The crystalline raw material in the crucible is heated and melted by a heater, and when the melt is supplied from the outlet of the crucible into the mold, the melt flowed down into the mold is From the bottom to the top by the gradient, the rate at which the crystal grows, the temperature change at the time of solidification of the melt is measured by a thermocouple for measuring the liquid level applied to the mold in advance. By detecting the liquid level of the melt, or by measuring the change in infrared radiation during solidification of the melt with an infrared sensor, the same as above, the liquid level of the melt is detected and grasped, and is supplied downward from the outlet. The same as above, the injection speed of the melt is adjusted by controlling the set temperature of the heater for melting the crystalline raw material in the crucible based on the above detection result. A method for casting a crystalline object. 3. The crystalline raw material in the crucible is heated and melted by a heater, and when the melt is supplied from the outlet of the crucible into the mold, the melt flowed down into the mold is The rate at which the crystal grows from the bottom to the top according to the gradient is detected by grasping the liquid level of the melt by measuring the change in infrared radiation during solidification of the melt with an infrared sensor. Then, the injection speed of the same melt that is supplied downward from the outlet, based on the above detection result, the pressure applied to the crucible by the pressure controller is controlled to a negative pressure, a positive pressure, 2. The method for casting a polycrystalline object such as silicon according to claim 1, wherein the adjustment is made by turning the melt on and off. 4. The crystalline raw material in the crucible is heated and melted by a heater, and when the melt is supplied from the crucible outlet into the mold, the melt flowed into the mold is From bottom to top by the gradient, the rate at which the crystal grows, by measuring the change in infrared radiation during solidification of the melt by an infrared sensor, while detecting the liquid surface of the melt, Grasping by measuring the weight of the melt in the mold at the time of the detection, the injection speed of the melt melted down supplied from the outlet, based on the above detection results, the crystalline raw material in the crucible Or the pressure applied to the crucible by the pressure controller is controlled to be a negative pressure or a positive pressure, and the same as above. Siri according to claim 1. Casting method for polycrystalline objects such as concrete.