JP2013199417A - Manufacturing apparatus and manufacturing method of cryatal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a crystal manufacturing apparatus and a manufacturing method capable of solving such a problem that a part of a crystal obtained by unidirectional solidification of a metal and a semiconductor is refined and the quality of the crystal may be deteriorated.SOLUTION: In a manufacturing apparatus and a manufacturing method of a crystal, the crystal manufacturing apparatus is a metal or semiconductor crystal manufacturing apparatus for advancing unidirectional solidification from a cooling end face to a heating end face by heating a single-side end face of a metal or semiconductor material melted in a mold, cooling the opposed end face and simultaneously rigidly insulating side faces and includes a heat quantity detection mechanism capable of detecting the amount of heat flow received from a solidified object by a cooling mechanism on the side of a solidification start face at accuracy of 1/10 or less of the amount of heat flow corresponding to solidification latent heat generation quantity per averaged unit time within time in which crystal solidification is advanced.

Description

  The present invention relates to an apparatus and a manufacturing method for manufacturing a semiconductor such as silicon or a crystal of a metal used for manufacturing a solar cell or the like.

  Many of the solar cells on the market today have a so-called semiconductor PN junction formed on a thin substrate obtained by slicing a semiconductor crystal such as silicon. The semiconductor crystal used for the substrate is required to be a high-quality product with less metal impurities such as iron, precipitates, crystal grain boundaries, dislocations, etc. so that the characteristics of the solar cell can be maximized. For this reason, in the manufacture of crystals for solar cells, a unidirectional solidification technique that facilitates the formation of larger crystal grains and less contamination of impurities is employed. Unidirectional solidification is also used in a method for purifying low-purity silicon, which is a raw material for silicon crystals. Here, a metal element component having a small segregation coefficient is removed by solidification segregation, and simultaneously melted. Foreign substances such as precipitates floating in the liquid are also removed. In addition to silicon for solar cells, such unidirectional solidification technology is also applied to the manufacture of alloys used in aircraft jet engines and generator gas turbines.

  In order to perform this unidirectional solidification, it is necessary to slowly solidify the melt raw material held in the mold in one direction (for example, upward from the bottom) while keeping the solidification interface flat. Here, FIG. 1 shows a structure of a general unidirectional solidification apparatus. In this unidirectional solidification apparatus, a mold 3 for injecting a raw material 2 such as silicon into a path body container 1 and dissolving and holding the same is shown. A heater 6 for setting the upper part to a high temperature is installed, and a cooling water channel 10 connected to a cooling water introduction pipe 11 and a discharge pipe 12 is provided below the mold 3. A cooling source such as a water cooling board 9 is installed, and solidification progresses upward from the bottom surface of the mold. In order to ensure this, the side surface of the mold 3 parallel to the progressing direction of solidification The upper side of the heater 6 is strongly insulated by the thick heat insulating materials 7 and 8. FIG. 1 shows a unidirectional solidification apparatus when the raw material 2 is silicon. In the crystal manufacturing process, a silicon crystal 2a is present on the water-cooling board 9 side, and a silicon melt 2b is present on the silicon crystal 2a on the heater 6 side. To do. Reference numeral 5 in the figure denotes a support leg.

  Specifically, for example, unidirectional solidification furnaces having various structures as disclosed in Patent Documents 1 to 4 and Non-Patent Documents 1 to 4 have been proposed. As for the solidification method for obtaining high-quality crystals, various methods such as a method for controlling the amount of heat generated by the heater and a method for controlling the amount of heat removed from the mold as in Patent Document 1 have been proposed. .

Japanese Patent Laid-Open No. 10-182,137 Japanese Patent Laid-Open No. 11-092,284 Japanese Patent Laid-Open No. 2002-293,527 Special table 2008-534,414 gazette

D. Helmreich & E. Sirtll, Journal of Crystal Growth, Vol. 79 (1986), p.562-571 C. P. Khatacl et al, Proc. 25th IEEE PVSC (1986), p.597-600 I. Steinbach et al, Solar Energy & Solar Cells, Vol.72 (2002), p.59-68 M. C. Flemings, "Solidification Processing", (1974), McGraw-Hill, p.134 Fujiwara et al., Journal of Crystal Growth, 243, (2002), p.275

  Under such circumstances, it has been clarified that there is a phenomenon that the size of crystal grains in the crystal is locally fine in a silicon crystal manufactured by unidirectional solidification in order to collect a substrate of a solar cell. It is widely known that when a solar cell is manufactured from such crystals with small crystal grains, the power generation efficiency of the solar cell is not good. Further, it is well known that in the purification of silicon raw material, when crystal grains are refined, the effect of eliminating impurities by segregation is significantly hindered. Furthermore, when crystal refinement occurs during the solidification of unidirectional solidification, the portion that solidifies afterwards tends to be covered with fine crystal grains, and its recovery is difficult. Further, the occurrence of crystal grain refinement often involves precipitates such as SiC, and such precipitates are known to cause processing defects such as cracks and scratches when slicing a crystal. ing.

  Also, in alloys used for aircraft jet engines and generator gas turbines, it is important that the grain boundaries of the material be arranged parallel to the stress during use because of the severe thermal stress environment. It is not possible to use crystals that are small and equiaxed.

  For these reasons, it is an important condition for obtaining a high-quality and high-yield crystal to surely carry out unidirectional solidification so that crystal grains are not refined. Conventionally, however, the crystal grains are refined or it is not possible to detect the refinement during the production of the crystals. This refinement of the crystal grains is performed after the unidirectional solidification is finished. This was first understood when the cross section was taken out and the cross section was observed.

  However, if it is possible to know the occurrence of grain refinement during solidification and sort out the refined crystals, the problematic solidification process can be interrupted immediately, and as a result, the process It also becomes possible to shorten the cycle time, and further, it becomes easy to remove defective parts from the product. In addition, knowing the occurrence of crystal grain refinement during solidification makes it possible to clarify the operating conditions in which the refinement occurs and to improve the operating conditions efficiently.

  The present invention has been made in view of the above-described problems of the prior art, and an object thereof is to provide a crystal manufacturing apparatus and a manufacturing method capable of detecting crystal grain refinement during crystal manufacturing.

  In view of the above, the present inventors have conducted many tests on the unidirectional solidification of silicon, observed the crystal grain structure and precipitates of the obtained silicon, and analyzed the corresponding heat transfer numerical simulation model. went. As a result of these many tests and analyses, the present inventors have found that most of the starting points of grain refinement are aligned along the isothermal line expected from the heat transfer analysis. The refinement of crystal grains is considered to occur all at once at the time of solidification. Furthermore, from the detailed observation of the crystal structure, the refinement of crystal grains often involves fine precipitates such as SiC. I found it.

  As a result of the above tests and analyses, the present inventors have found that the precipitation nuclei suddenly occur all at once in the supercooled melt, and the solidification interface rapidly moves forward while causing shape instability. The conclusion was reached that the so-called “recareless phenomenon (re-brightening phenomenon)” is the cause of crystal grain refinement. Also, if the grain refinement is due to such a phenomenon of recurrence, a considerable latent heat of solidification should be generated due to rapid solidification when this phenomenon occurs. If sudden generation of latent heat of solidification could be detected, we thought that crystal refinement could be detected in real time.

The present invention has been invented from such a viewpoint, and the crystal manufacturing apparatus and manufacturing method of the present invention are configured as follows.
(1) In the mold, one end face of the melt of metal or semiconductor is heated, the other end face opposite to the end face is cooled, and the side face of the melt is insulated to thereby cool the end face. A device for producing a metal or semiconductor crystal by solidifying in one direction from the heating side to the heating end surface side, wherein the amount of heat per unit time that the heat removal mechanism on the cooling end surface side receives from the solidified body An apparatus for producing a crystal, comprising: a heat quantity detection mechanism capable of detecting with an accuracy of 1/10 or less of a heat quantity corresponding to a solidification latent heat generation amount per unit time as solidification progresses.

(2) The heat removal mechanism uses heat exchange by a cooling fluid, and the heat quantity detection mechanism supplies a cooling fluid flow rate per unit time used by the heat removal mechanism and the heat removal mechanism. It has a mechanism for continuously measuring the amount of heat per unit time that the heat removal mechanism receives from the solidified body from the temperature difference between the temperature of the cooling fluid that is input and the temperature of the cooling fluid that is discharged. (1) The crystal production apparatus according to (1).

(3) The heat quantity detection mechanism detects a discontinuous increase in the amount of heat per unit time received by the heat removal mechanism from the solidified body, and the increase is generated by solidification latent heat per unit time as the solidification of the melt progresses. The crystal manufacturing apparatus according to (1) or (2), further comprising an alarm mechanism for detecting an alarm exceeding 1/10 of the amount and issuing an alarm.

(4) Using the crystal manufacturing apparatus according to any one of (1) to (3), the amount of heat per unit time received by the heat removal mechanism on the cooling end face side from the solidified body during solidification of the melt is detected. A method for producing a crystal, which is detected as a heat flow rate by a mechanism and which controls the progress of solidification of the melt so as not to cause a sudden increase in heat flow rate based on the detection result.

(5) The method for producing a crystal according to (4), wherein the progress of solidification of the melt is controlled by temperature control of a heater.

  With the crystal manufacturing apparatus of the present invention, it is possible to detect in real time the refinement of the crystal grains that cause the characteristic defect of the unidirectionally solidified crystal, thereby making it easy to improve the crystal quality and improve the yield. In actual operation, quality abnormalities can be identified in real time, and the cycle time can be shortened by measures such as immediately interrupting the solidification process in which the problem occurred. Since the defective part can be known at least, it is very easy to select a good product. In this way, the present invention contributes to producing high-purity crystals at low cost and in large quantities.

FIG. 1 is an explanatory diagram for explaining an outline of a general unidirectional solidification apparatus. FIG. 2 is an explanatory diagram for explaining the outline of the unidirectional solidification device according to an example of the embodiment of the present invention.

FIG. 3 is an explanatory diagram for explaining an outline of an electrical instrumentation system mounted on the unidirectional solidification apparatus of the present invention. FIG. 4 is an explanatory diagram for explaining an outline in the case where the present invention is applied to a general silicon unidirectional solidification apparatus that does not use a water-cooled disc at the bottom of the mold.

FIG. 5 is an explanatory diagram for explaining an outline of a unidirectional solidification apparatus that performs unidirectional solidification in the horizontal direction. FIG. 6 is an explanatory diagram for explaining an outline of the in-furnace structure common to the unidirectional solidification apparatuses of Examples 1 to 3 and Comparative Example 1 of the present invention.

FIG. 7 is an explanatory diagram for explaining the outline of the electric instrumentation system of the unidirectional solidification apparatus according to Comparative Example 1 of the present invention. FIG. 8 is a graph showing process data during solidification in Comparative Example 1 of the present invention.

FIG. 9 is an explanatory diagram for explaining the outline of the vertical cross section of the crystal obtained in Comparative Example 1 of the present invention. FIG. 10 is an explanatory diagram for explaining an outline of the electric instrumentation system of the unidirectional solidification apparatus according to the first embodiment of the present invention.

FIG. 11 is a graph showing process data during solidification in Example 1 of the present invention. FIG. 12 is a graph showing changes in the value of the heat flow rate Q observed corresponding to before and after the time when crystal refinement occurred during solidification in Example 1 of the present invention.

FIG. 13 is an explanatory diagram for explaining the outline of the vertical cross section of the crystal obtained in Example 1 of the present invention. FIG. 14 is a graph showing process data during solidification in Example 2 of the present invention.

FIG. 15 is an explanatory diagram for explaining the outline of the vertical cross section of the crystal obtained in Example 2 of the present invention. FIG. 16 is a graph showing process data during solidification in Example 3 of the present invention.

FIG. 17 is an explanatory diagram for explaining an outline of an electrical instrumentation system of a unidirectional solidification apparatus according to Embodiment 5 of the present invention.

  FIG. 2 shows a unidirectional solidification apparatus for silicon according to an example of the present invention. In this apparatus, a pedestal 4 is installed on the inner bottom surface of a furnace container 1 that can be sealed with an inert gas, and a mold 3 for holding the silicon raw material 2 is installed on the pedestal 4. A heater 6 is set for transitioning the silicon raw material 2 from room temperature to a solidified state through a molten state. Further, thick carbon fiber heat insulating materials 7 and 8 are respectively installed on the side surface of the mold 3 and the upper surface of the heater 6 so as to surround the mold 3 and the heater 6. The heat flow from 3 to the furnace body container 1 is greatly suppressed. A water cooling board 9 for cooling the lower part of the silicon raw material 2 is installed at the lower part of the mold 3. The pedestal 4 on which the mold 3 is installed is supported by four support legs 5, and the cross-sectional area of these support legs 5 is so small that it can be ignored with respect to the cooling area of the water cooling board 9. The portion that contacts the bottom surface of the fuser 1 is also strongly insulated. Due to the structure of the lower part of the mold, most of the heat extracted from the lower surface of the crystal is absorbed by the water cooling board 9.

  A cooling water channel 10 is formed inside the water cooling board 9 and is connected to a cooling water introduction pipe 11 and a discharge pipe 12. It is kept at a temperature close to room temperature. The water cooling board 9 can be moved in the vertical direction by a bellows 13 and a drive mechanism, and its upper surface can be brought into close contact with the lower surface of the base 4 on which the mold 3 is placed. The cooling water introduction pipe 11 is provided with a thermometer contact 15 for measuring the water temperature, and the cooling water discharge pipe 12 is provided with a thermometer contact 16 for measuring the water temperature and a flow rate thereof. And a flow meter 14 for measuring. Each thermometer contact 15, 16 is fixed so as to be positioned at the center of each pipe cross section by a sheath pipe (not shown) in order to accurately measure the temperature of the cooling water. In FIG. 2, reference numeral 17 denotes a temperature control thermocouple, and reference numeral 18 denotes a service port.

FIG. 3 is a schematic diagram of an electrical instrumentation system including a flow meter 14 and thermometer contacts 15 and 16 arranged in the cooling water piping system of the water cooling board 9 of the unidirectional solidification furnace. Here, as will be described later, it is desirable to use a thermometer having the highest possible sensitivity as the thermometer contacts 15 and 16. The outputs from the two thermometer contacts 15 and 16 are output as a signal of the temperature difference ΔT (° C.) between the cooling water introduction side and the discharge side through the signal converter 19 and input to the furnace I / O panel 20. Is done. The output signal F (L / sec) of the flow meter 14 is also input to the I / O board 20 of the furnace in parallel. Based on these two signals, the furnace control device 21 receives the amount of heat received by the cooling water per unit time (in other words, the amount of heat removed by the cooling water per unit time, hereinafter referred to as “heat flow rate”). Q is calculated in real time, and these time-sequential values of Q, ΔT, and F are displayed on the operation panel 22, and at the same time, stored in the recording device 23 as time passes for unidirectional solidification. Here, the heat flow rate Q (L / sec) can be expressed by the following relational expression (1), where the specific heat of the cooling medium is α (J / Kg / ° C.). When the cooling medium is water, α is 4200 (J / Kg / ° C.).
Q = αΔTF (1)

  In the present invention, since it is necessary to detect the generation of latent heat of solidification due to abnormal fluctuations in the solidification speed, the resolution Qres of the thermal flow rate Q detected by the instrumentation system of the control device 21 from the flow meter 14 shown in FIG. It must be possible to detect a smaller amount than the amount Qv of latent heat generated per unit time calculated from the normal solidification rate V. According to the results of many experiments conducted by the present inventors, it has been found that if Qres is 1/10 or less of Qv, the occurrence of crystal grain refinement can be detected sufficiently.

Here, Qv is calculated by the following equation (2), where L is the solidification latent heat, ρ is the density at the melting point of the crystal, and A is the area of the bottom of the mold.
Qv = ρLVA (2)
Therefore, the required detection resolution Qres is expressed by the following equation (3).
Qres <ρLVA × 0.1 (3)

Here, assuming that the relative accuracy of the flow meter 14 is δ _F , from the relational expressions (1) and (3), the resolution ΔTres of the temperature difference ΔT detected by the flow meter 14 and the thermometer contact 15 is It can be expressed by equation (4).
ΔTres = ρLVαF1 + δF × 0.1 (4)

However, in a commercially available flow meter widely used in industry, the relative accuracy δ _F of the output value _F is 0.5% or less, so the influence of δ _F in equation (4) can be ignored in practice. The required accuracy of ΔT can be determined by the following equation (5).
ΔTres〜ρLVαF × 0.1 (5)

In the unidirectional solidification of silicon for solar cells, the value of V is about 2 × 10 ⁻⁶ (m / sec), the cross-sectional area of the crystal is 0.64 (m ² ), and the cooling water flow rate is 2.5. If (Kg / sec) (= 150 L / min), ρ = 2.35 × 10 ³ (Kg / m ³ ) and L = 1.80 × 10 ⁶ (J / Kg). Is substituted into the equation (4), the required resolution (ΔTres) of the cooling water temperature becomes the following relational expression.
ΔTres <0.042 (℃) (6)

  However, since the resolution of a general-purpose general temperature measuring device is about 0.1 ° C., the following device is necessary to obtain this accuracy. For example, it is effective to select a 64.4 (μV / ° C.) E-type thermocouple having a large temperature conversion count as an electromotive force per unit temperature as a thermometer. In this case, the noise performance of the signal converter 19 is required to be 1.2 μV or less from the value of the equation (6) and the temperature conversion count. For this purpose, it is used for the thermometer contacts 15 and 16. At the same time as the E-type thermocouple differential wiring, a high-accuracy voltage amplifier used for strain measurement or the like can be used as the signal converter. In addition, since rapid solidification with crystal refinement is not a change in seconds, in the internal circuit of the signal converter 19, the signal of the temperature difference ΔT from the thermocouple is smoothed for several tens of seconds. To improve the noise characteristics.

  Needless to say, the signal sent from the signal converter 19 and the flow meter 14 to the furnace I / O board 20 and the control device 21 pays sufficient attention to the accuracy when digitized so as not to impair the measurement accuracy. Must-have. It is difficult to directly measure the normal solidification rate V, which is a reference for temperature measurement accuracy, but the average solidification rate obtained by dividing the required time from the start of solidification to the completion of solidification by the ingot height. May be substituted. For example, in FIG. 2, a service port 18 is installed in the center of the furnace ceiling. By using this port 18, a graphite or quartz probe rod 28 can be inserted to measure the position of the solidification interface. It is possible, and the coagulation velocity V can be obtained sequentially from the position change per time.

  If the flow rate F of the cooling water introduced into the solidification apparatus is extremely stable and is regarded as a constant, the heat flow rate Q is simply proportional to the temperature difference ΔT as seen from equation (1). Therefore, it is not necessary to use the flow meter 14, and the occurrence of crystal grain refinement can be detected by measuring only the ΔT signal.

  Furthermore, in this apparatus, since the heat flow rate (heat extraction amount) Q from the crystal is measured in real time, when it is detected that Q has suddenly increased, a function to immediately issue an alarm may be added. Is possible. Specifically, the control device of this device is provided with an input 30 for the value of coagulation velocity V and an indicator lamp 29 to add the following functions to the control device 22.

(A) A function of calculating the latent heat generation amount Qv at the solidification rate V from the solidification rate V according to the equation (2).
Here, V may be a constant obtained by obtaining past operation results, or may be a value calculated from the indicated position of the search bar 28.

(B) A function of calculating an increment ΔQ per unit time by comparing a value Q at a predetermined unit time, for example, two minutes before the heat flow rate Q recorded continuously, with a value Q at that time.
(C) A function of comparing the increment ΔQ obtained in (B) with Qv obtained in (A) and calculating the ratio (ΔQ / Qv) of ΔQ and Qv.

(D) A function of determining that ΔQ / Qv exceeds a predetermined value, for example, 0.1.
(E) Based on the determination result of (D), a function for issuing an alarm and turning on the indicator lamp.
With this function, it is possible to immediately notify the operator that crystal grains may have been refined during operation by turning on the indicator lamp 29.

  Moreover, since the occurrence of crystal grain refinement accompanied by recurrence is generally due to supercooling of the crystal growth interface, this is more likely to occur as the melt temperature is lower. Therefore, keeping the heater control temperature as high as possible is an effective method for preventing the refinement of crystal grains. Even with a general unidirectional solidification device, even if the produced crystal is divided and the location of the refinement of the crystal grains can be specified, the exact time at which the refinement of the crystal has occurred is accurately determined. Although not required, if the apparatus of the present invention is used, it can be surely grasped at which time the crystal is refined. Because of this advantage, if the refinement of crystal grains frequently occurs in the operation of the apparatus of the present invention, the control temperature of the heater is set to a high value in the time zone where the refinement of crystal grains frequently occurs and the cooling is performed. By slowing down the speed, it is possible to operate without crystal grain refinement. Here, the increase in the control temperature of the heater necessary to prevent recurrence is sufficient if the cooling rate is slightly slower than the generation limit because in principle, recurrence is a critical phenomenon. It is difficult to know exactly. On the other hand, it is known that the supercooling required for the progress of solidification is about 10 ° C. at most in solidification of silicon (Non-patent Document 5). For this reason, the control temperature increment of the heater for preventing recurrence is about 10 ° C., but this value can be reduced according to the results.

  Further, as a unidirectional solidification apparatus for producing a polycrystalline silicon ingot for collecting silicon wafers for solar cells in recent years, as shown in FIG. A device that cools the lower part of a crystal by using a cooling block 25 made of a mold as a base for a mold and directly receiving the radiant heat emitted from the lower side surface of the graphite block 25 on a water cooling wall 27 installed at the lower part of the furnace body 1 However, even in such an apparatus, the same cooling water thermometer as shown in FIG. 3 is provided in the cooling water introduction pipe 11 and the discharge pipe 12 to the water cooling wall 27 at the bottom of the furnace body. A heat flow instrumentation system comprising the contacts 15, 16 and the flow meter 14 may be installed. In FIG. 4, reference numeral 24 denotes a mold bottom plate.

  Further, as shown in FIG. 5, the heater 6 is disposed on the left side of the mold 3, the water cooling board 9 is disposed on the right side, and the left side of the heater 6 and the upper and lower surfaces of the mold 3 are thickened. Even in the device structure in which the crystal is unidirectionally solidified in the horizontal direction from right to left by the heat insulating materials 7, 8, 26, the instrumentation system shown in FIG. Applicable.

  Recurrence, that is, the phenomenon of crystal grain refinement accompanying supercooling, which is the principle of the present invention, is not a phenomenon peculiar to the solidification of silicon, but a single element metal such as Ni, Al, Ag, Co, or Ge. It has been observed not only in semiconductors but also in alloy systems such as Ni-Fe, Ni-Cu, Cu-Sn, etc., and its mechanism has been shown to be a universal phenomenon in the solidification of metals and semiconductors (non-patent) Reference 4). Therefore, the present invention can be similarly applied not only to silicon but also to unidirectional solidification of other semiconductor materials and metal materials.

  Hereinafter, the crystal production apparatus and production method of the present invention will be described in detail with reference to examples and comparative examples, using a unidirectional solidification apparatus and a solidification method for purifying silicon raw materials for solar cells as examples. The present invention is not limited to the following examples and comparative examples.

  FIG. 6 shows a common basic structure in the unidirectional solidification apparatuses of Examples 1 to 4 and Comparative Example 1 taken up here. This is in accordance with the unidirectional solidification apparatus described with reference to FIG. Here, the furnace body container 1 has a box-shaped water-cooled jacket structure, and the mold 3 to be used is a rectangular quartz mold having an inner size of 830 mm × 830 mm and a height of 450 mm that is commercially available for silicon for solar cells. Yes, 450 kg of low-purity silicon raw material 2 is charged into the mold 3, the heater 6 is energized by resistance heating, and the energization power of the heater 6 is indicated by the temperature control thermocouple 17 installed near the heater 6. While the PID control is performed, the charged silicon raw material 2 is dissolved and the entire melt is kept at a temperature higher than the melting point, and then the water cooling plate 9 at the bottom of the mold is brought into contact with the lower surface of the pedestal 4, The silicon in the mold is unidirectionally solidified while lowering the temperature, thereby obtaining high-purity silicon crystals using solidification segregation.

Here, the heater 6 is made of graphite, the outer heat insulating materials 7 and 8 are made of carbon fiber, and the base 4 and the support leg 5 are made of carbon fiber composite. The water cooling plate 9 at the lower part of the mold is made of copper and has a cooling water channel 10 formed therein, and its surface area is 0.64 m ^{2. In} the cooling water channel 10 inside, 80 L (== 1.33 Kg / sec) of room temperature cooling water is introduced. Further, in order to investigate the progress of solidification, a furnace is provided on the upper surface of the furnace, and a graphite search rod 28 for measuring the solidification interface position is inserted from this service port 18, and the solidification interface height is determined from the insertion length. The position can be measured sequentially. In addition, the silicon ingot manufactured by these apparatuses is a rectangular parallelepiped having a width of 830 mm × 830 mm and a height of 275 mm.

[Comparative Example 1]
The unidirectional solidification apparatus of the comparative example 1 is the unidirectional solidification apparatus having the structure described above, and has an electrical instrumentation system as shown in FIG. Here, no thermometer is set for the thermometer contact 15 on the cooling water introduction side, and the platinum resistor sensor 16a having a temperature measurement resolution of 0.1 ° C. only on the thermometer contact 16 on the discharge side. Is set. The output signal from the platinum resistor sensor 16a is input to a signal converter 19a for the platinum resistor, and is input as temperature data to the control device 21 via the furnace I / O panel 20 and displayed on the operation panel 22. At the same time, it is stored in the recording device 23 as time passes for unidirectional solidification. Further, a commercially available float type is installed as the cooling water flow meter 14a, and the flow rate is confirmed by visually reading the position of the float.

  Using the unidirectional solidification apparatus of Comparative Example 1 equipped with the instrumentation system shown in FIG. 7, 450 kg of silicon raw material having an iron concentration of 200 ppmw is melted, and the temperature of the thermocouple 17 for temperature control becomes 1495 ° C. Thus, the electric power applied to the heater 6 was adjusted, and the melt temperature was stabilized by maintaining the temperature, and then the temperature was controlled according to the time steps shown in Table 1 to perform unidirectional solidification of silicon.

  FIG. 8 shows the change with time of each operation parameter from the start to the end in the solidification process of Comparative Example 1. The solid line a) shows the indicated temperature T of the thermocouple 17 for temperature control above the liquid level. Further, the broken line b) shows the distance between the crystal growth interface and the inner surface of the mold bottom obtained from the position of the solidification interface measured by the solidification interface position search rod 28, that is, the solidification height h, and c). Indicates the temperature Tw on the drain side of the cooling water. As can be seen from the time-dependent change b) of the solidification height h, in Comparative Example 1, solidification started at time 120 minutes, and at time 1920 minutes, the solidification height h reached 275 mm, and unidirectional solidification was completed. .

  While this solidification was in progress, there was no rapid change in the indicated temperature of the temperature control thermocouple 17 on the liquid surface. Further, although the cooling water drain side temperature Tw has a long-period pulsation and noise of about 0.2 ° C. in amplitude, as far as the recorded data is seen, solidification is in progress. There was no sudden change in the cooling water temperature. Further, the cooling water flow rate was monitored by visually reading the indicator scale of the float every hour, but no fluctuations in the flow rate were observed.

  The silicon ingot obtained by completing the unidirectional solidification in this way was cut into two equal parts by cutting in the solidification (vertical) direction in order to inspect the inside. FIG. 9 is an explanatory diagram showing an outline of the crystal structure of the divided cross section, and hatched portions where crystal grains are miniaturized. In a good ingot, grain refinement usually occurs only in the part where compositional supercooling due to concentration of impurity elements occurs in the final stage of unidirectional solidification. In addition to the area A of 25 mm from the top surface corresponding to the end of solidification, the refined portion has four horizontal positions (indicated by B, C, D and E in the figure) from the position of 45 mm to the position of 80 mm from the bottom surface. It was caused to spread long and narrow. Further, when the concentration of iron in the regions A, B, C, D, and E was measured with a simple fluorescent X-ray analyzer, they were 2120 ppmw, 29 ppmw, 30 ppmw, 27 ppmw, and 24 ppmw, respectively. When the iron concentration in the thick columnar crystal portion other than the regions A, B, C, D, and E was measured, it was found that the concentration was 1 ppmw or less of the detection lower limit value.

  Usually, the product yield is 87% in order to remove 25 mm from the top surface and 10 mm from the bottom surface of the region A out of 275 mm, so that the product yield is 87%. However, in this comparative example 1, the regions B, C, D, The part E deviated from the standard for raw materials for solar cells (SEMI-PV170611), and the actual yield rate was only 72.3%.

[Example 1]
The silicon unidirectional solidification apparatus of Example 1 has the same furnace structure as that of Comparative Example 1, but a cooling water electric instrumentation system having a configuration as shown in FIG. 10 was used. Here, E-type thermocouples 15b and 16b of JIS-1 class are attached to the thermometer contact 15 on the cooling water introduction side and the thermometer contact 16 on the discharge side, respectively. These terminals are connected to a high-precision signal converter 19b for strain gauges by differential wiring, and signals from the thermocouples 15b and 16b are input to the signal converter 19b. In this instrumentation system, since the thermocouples 15b and 16b are differentially connected, the output from the signal converter 19b is only the temperature difference ΔT of the cooling water.

  Further, a general-purpose electromagnetic velocimeter 14b is used as a flow meter for cooling water, and the measurement accuracy is 0.5% of full scale 200 (L / min), which is 1 (L / min). The output of the temperature difference ΔT and the flow rate F from the signal converter 19b and the anemometer 14b is converted into a digital signal and input to the furnace control device 21, and the value of the heat flow rate Q is obtained by the above equation (1). This heat flow rate Q is displayed on the operation panel 22 and simultaneously recorded in the recording device 23 as time elapses in the unidirectional solidification.

  Using this unidirectional solidification apparatus of silicon having such an instrumentation system, 450 kg of silicon raw material having an iron concentration of 200 ppmw is melted and the temperature of the control thermocouple 17 is maintained at 1495 ° C. as in Comparative Example 1. Then, after the melt temperature was stabilized, the silicon was unidirectionally solidified while controlling the temperature of the thermocouple 17 for temperature control according to the time step shown in Table 1 exactly as in the condition of Comparative Example 1. FIG. 11 shows the change with time of each operation parameter from the beginning to the end in the solidification process of Example 1, and the solid line a) shows the indicated temperature T of the thermocouple 17 for temperature control above the liquid level. The broken line b) indicates the solidification height h obtained from the position of the solidification interface measured by the search rod 28, c) indicates the flow rate F of the cooling water, d) indicates the temperature difference ΔT of the cooling water, and e ) Indicates the heat quantity Q per unit time received by the cooling water.

  As in Comparative Example 1, in Example 1, as can be seen from the time-dependent change b) of the solidification height h, solidification started at time 120 minutes, and at time 1920 minutes, the solidification height h reached 275 mm. Unidirectional solidification is complete.

  During the solidification, as in Comparative Example 1, the temperature T on the liquid surface and the solidification height h did not change suddenly. However, when the cooling water temperature difference ΔT is observed, the cooling water flow rate Although it is difficult to understand due to the influence of pulsation, a slight spike-like disturbance can be confirmed at 350 minutes. On the other hand, in the data of the heat flow rate Q, the influence of the pulsation of the flow rate F and the long-term fluctuation of the cooling water temperature has been removed, and a spike-like change was detected in the vicinity of the time 352 minutes while it smoothly decreased monotonously. It was.

  FIG. 12 is an enlarged graph showing the change in Q for 50 minutes before and after the time when the spike-like fluctuation of the heat flow Q occurs. Looking at this, Q has increased rapidly from 30.0 KW to 31.6 KW from 352 minutes to 354 minutes, and the incremental heat quantity ΔQ is 1.6 KW, which is calculated from equation (2). This corresponds to the amount of latent heat generated at a speed of 0.035 mm / min. The solidification rate determined by the search rod 28 at this time is 0.20 mm / min, and the solidification latent heat generation amount Qv at this rate is calculated as 9.0 KW from the equation (2). Therefore, although the detection resolution Qres of the amount of heat required for this apparatus is 0.9 KW or less, the instrumentation system of this apparatus has sufficient detection resolution from the graph of FIG. 12, and sufficiently detects the actual ΔQ. You can see that it is made. Further, the solidification height h measured by the search rod 28 when a spike-like change was detected in the vicinity of this time 352 minutes was 45 mm.

  The silicon ingot obtained by completing the unidirectional solidification in Example 1 was divided into two equal parts in the solidification (vertical) direction in the same manner as in Comparative Example 1, and the cross section was examined. FIG. 13 is an explanatory diagram for explaining the outline of the crystal structure of the divided cross section, and a portion where crystal grains are miniaturized is indicated by hatching. In a good ingot, grain refinement usually occurs in the final stage of unidirectional solidification, where the compositional supercooling occurs due to the concentration of impurity elements, but in this ingot, the grain size is fine. In addition to the area A 25 mm from the top surface corresponding to the end of coagulation, three parts (indicated by B, C, and D in the figure) extend in the horizontal direction from the bottom surface 45 mm to the 85 mm position. Had occurred. Further, when the iron concentrations in the regions A, B, C, and D were measured with a simple fluorescent X-ray analyzer, the values were 2250 ppmw, 29 ppmw, 35 ppmw, and 24 ppmw, respectively. When the concentration of iron in columnar crystal portions other than regions A, B, C, and D was measured, it was found that iron was not detected and that the concentration was 1 ppmw or less of the lower limit of detection. Thus, the refinement | solidification of the solidification structure | tissue appeared in the coagulation position which detected the spike-like change of the heat flow rate Q. In other words, it was possible to confirm the occurrence position of the solidification purification failure due to the actual refinement of the crystal by looking at the solidification process data.

[Example 2]
In the same unidirectional solidification apparatus of silicon as in Example 1, 450 kg of silicon raw material having an iron concentration of 200 ppmw was dissolved, and the temperature of the control thermocouple 17 was maintained at 1495 ° C., and the melt temperature was stabilized. In accordance with the time steps shown in Table 2 in which the temperature was lowered more gently than the conditions of Example 1 and Example 1, unidirectional solidification of silicon was performed while controlling the heater power.

  The heater temperature control pattern in this Example 2 is that the heater control temperature is always compared between 320 minutes and 620 minutes across the time 352 minutes when crystal refinement occurred in Comparative Example 1 and Example 1. By setting the temperature at 320 minutes to 1465 ° C. so as to be higher by about 10 ° C. than in the case of Example 1 and Example 1, the temperature gradient from 60 minutes to 320 minutes is reduced.

  FIG. 14 shows the change over time of each operating parameter from the beginning to the end in the solidification process of Example 2, a) shows the temperature T of the control thermocouple 17 above the liquid level, b) Represents the height h of the crystal growth interface measured with the search rod 28, c) the flow rate F of the cooling water, d) the temperature difference ΔT of the cooling water, and e) the unit received by the cooling water. The amount of heat Q per hour is shown.

  During this solidification process, as in the case of Comparative Example 1, no rapid changes were observed in the temperature and solidification height of the control thermocouple 17. Further, the spike-like increase in the cooling water temperature difference ΔT is not confirmed as in the first embodiment, and the spike-like data is also obtained in the heat flow rate Q data from which the influence of the cooling water flow rate and the temperature on the water supply side is removed. No increase was detected.

  In order to inspect the inside of the silicon ingot obtained by completing the unidirectional solidification in Example 2, as in Comparative Example 1 and Example 1, the silicon ingot was cut in the solidification (vertical) direction and divided into two equal parts. did. FIG. 15 is a schematic diagram of the crystal structure of the divided cross section, and hatched portions where crystal grains are made finer. In the final stage of unidirectional solidification, there was no portion where the crystal grains were refined other than the region A of 25 mm from the top surface where compositional supercooling occurred due to the concentration of impurity elements. Further, the concentration of iron in the region A measured by a simple fluorescent X-ray analyzer was 2290 ppmw, iron was not detected in the columnar crystal portion other than the region A, and the concentration was 1 ppmw or less of the lower limit of detection. .

Example 3
In the same unidirectional solidification apparatus of silicon as in Examples 1 and 2, after 450 kg of silicon raw material having an iron concentration of 200 ppmw was dissolved and the temperature of the control thermocouple 17 was maintained at 1495 ° C., the melt temperature was stabilized. In accordance with the time steps shown in Table 3, unidirectional solidification of silicon was performed while controlling the temperature of the heater power.

  The control pattern of this heater is exactly the same as the conditions of Comparative Example 1 and Example 1 until time 320, but the time between the time 352 minutes when the crystal grain refinement occurred in Comparative Example 1 and Example 1 is sandwiched. For the 80 minutes from 320 minutes to 400 minutes, by keeping the heater control temperature at 1455 ° C., the cooling rate of the silicon melt is lowered, and thereafter the same conditions as in Comparative Example 1 and Example 1 The temperature decreases with the gradient. The time required for solidification with this control pattern is about 80 minutes longer than that of Comparative Example 1 and Example 1, and the control temperature of the heater when fine grain refinement occurs in Example 1 is 1453 ° C. However, in Example 3, the temperature was maintained at 1455 ° C., which was 2 ° C. higher than that.

  FIG. 16 shows the change with time of each operation parameter from the beginning to the end in the solidification process of Example 3, a) shows the temperature T of the control thermocouple 17 above the liquid level, b). Represents the height h of the crystal growth interface measured with the search rod 28, c) the flow rate F of the cooling water, d) the temperature difference ΔT of the cooling water, and e) the unit received by the cooling water. The amount of heat Q per hour is shown.

  During this coagulation process, a decrease in coagulation rate was observed between 320 minutes and 400 minutes when the temperature was maintained at 1455 ° C. As in Example 2, the temperature of the thermocouple 17 for control, There was no rapid change in the solidification height. Further, the temperature difference ΔT of the cooling water does not increase in a spike shape as in the first embodiment, and the data of the heat flow Q from which the influence of the flow rate of the cooling water and the temperature on the water supply side is removed is also spiked. No increase was detected.

  In order to inspect the inside of the silicon ingot obtained by completing the unidirectional solidification in Example 3, as in Comparative Example 1 and Examples 1 and 2, it was cut in the solidification (vertical) direction to 2 Divided equally. The outline of the crystal structure of the divided cross section is the same as that in the case of Example 2 shown in FIG. Except for the region A, no crystal grain refined portion was observed. Further, the concentration of iron in region A measured by a simple fluorescent X-ray analyzer was 2045 ppmw, iron was not detected in columnar crystal parts other than region A, and the concentration was 1 ppmw or less of the lower limit of detection. .

Example 4
In the unidirectional solidification using the apparatus of Example 1, the unidirectional solidification was repeated 150 times for 450 kg of silicon raw materials having various iron concentrations (100 ppmw to 500 ppmw). Of the 150 ingots obtained, refinement of crystal grains was observed in the region other than the final solidification part in 10 of the obtained ingots, and the refinement of crystal grains other than the final part of solidification was observed in the remaining 140 ingots. I was not able to admit.

  Reviewing the 10 operation data where the crystal refinement occurred, in any case, per unit time received by the cooling water at the time corresponding to the position where the crystal grain refinement occurred in the ingot. A spike-like increase was detected in the heat quantity Q data. Table 4 shows that in these 10 cases, in addition to the solidification rate V immediately before the increase in the heat flow rate Q was detected, the solidification latent heat generation amount Qv corresponding to this solidification rate V, and the increase ΔQ in Q, ΔQ This shows the relationship of the ratio of Qv (ΔQ / Qv).

  As can be seen from Table 4, the increase ΔQ in the heat flow rate Q generated when crystal refinement occurs is 1/10 or more of the latent heat generation Qv corresponding to the solidification rate in any case. It has been found that it is sufficient that the detection accuracy Qres of the necessary heat flow rate Q is 1/10 or less of Qv.

Example 5
The silicon unidirectional solidification apparatus of Example 5 has a structure as shown in FIG. 4 and is a unidirectional solidification apparatus for producing a silicon ingot for collecting a polycrystalline wafer for a solar cell. A commercially available quartz mold having an inner dimension of 830 mm × 830 mm and a height of 450 mm was used. In this apparatus, the heat from the bottom surface 0.69 m ² of silicon in the mold is transmitted to the graphite block 25 arranged immediately below, and most of the heat is generated by the radiation from the side surface of the block 25. In the lower part of the wall, it is structured to be absorbed by the water-cooled wall 27 arranged to face the block 25.

  In addition, an electrical instrumentation system shown in FIG. 17 is incorporated in the cooling water introduction pipe 11 and the discharge pipe 12 connected to the water cooling wall 27. In this instrumentation system, JIS-1 class E-type thermocouples 15b, 16b are connected to the thermometer contact 15 on the introduction side of the cooling water and the thermometer contact 16 on the discharge side, respectively, and these thermocouples 15b, Each terminal of 16b is connected to a high-accuracy signal converter 19b, and signals from thermocouples 15b and 16b are input to the signal converter 19b. The signal converter 19b used is exactly the same as that of the apparatus of the second embodiment. Further, a general-purpose electromagnetic velocimeter 14b is used as a flow meter for the cooling water, and the measurement accuracy is 1 (L / min).

  The temperature difference ΔT from the signal converter 19b, the anemometer 14b, and the output of the flow rate F are converted into digital signals and input to the furnace control device 21, and the unit received by the water cooling wall according to the above equation (1) The amount of heat Q per time is obtained, and the obtained heat flow rate Q is displayed on the operation panel 22 and simultaneously recorded on the recording device 23 as time elapses in one-way solidification. Further, this apparatus includes a logic circuit for calculating Qv from the solidification rate V based on the above-described equation (2) and a heat flow rate Q when the heat flow rate Q is discontinuously increased. And a theoretical circuit for calculating the increase ΔQ based on the time series data of the above, and when this ΔQ becomes a value of 1/10 or more of Qv, a lamp indicating coagulation abnormality is immediately displayed on the indicator lamp 29. An alarm mechanism that turns on and displays an alarm on the screen of the control device 22 is also provided.

In this Example 5, the temperature of the thermocouple 17 for temperature control becomes 1480 ° C. after 450 kg of silicon raw material having a solar cell grade iron concentration of 0.1 ppmw and a carbon concentration of 40 ppmw is dissolved by the unidirectional solidification device. In this way, the electric power supplied to the heater was adjusted to maintain the temperature and stabilize the melt temperature, and then the silicon was unidirectionally solidified by temperature control according to the time steps shown in Table 5. In the case of unidirectional solidification at the time step shown in Table 5, solidification starts when the heater temperature is 1465 ° C. at 120 minutes, and solidification is completed when the heater temperature is 1415 ° C. at 1700 minutes. The average solidification rate in this unidirectional solidification is 0.17 mm / min, that is, 0.29 × 10 ⁻⁶ m / sec, and this solidification rate is a constant, and Qv in this apparatus is calculated. Is input to the logic circuit.

  At a time of 1050 minutes from the start of the solidification process, the indication of solidification abnormality was lit on the indicator lamp 29, but the heater temperature was controlled as shown in Table 5 of the initial plan, and the unidirectional solidification was completed. The silicon ingot taken out of the furnace was divided into 25 blocks of 156 mm × 156 mm × height 275 mm in order to collect a 6-inch square polycrystalline silicon wafer as usual. However, in this coagulation process, since abnormal coagulation was detected, an inspection by infrared fluoroscopy was performed specially. As a result of the inspection, in 8 out of 28 blocks, dark bands extending in the horizontal direction having a width of about 15 to 30 mm where fine SiC precipitates were densely located above the position of 150 mm in height from the bottom surface were recognized. Since such a dark band of precipitates has a high risk of causing damage or scratching of the wafer during slicing, this portion was cut and removed to obtain a product.

  Thus, in Example 5, the defective part of the ingot could be efficiently removed by the apparatus of this example having an alarm mechanism for alarming the occurrence of unidirectional solidification.

DESCRIPTION OF SYMBOLS 1 ... Furnace container, 2 ... Silicon raw material, 2a ... Silicon crystal, 2b ... Silicon melt, 3 ... Mold, 4 ... Base, 5 ... Support leg, 6 ... Heater, 7 ... Heat insulation material (side surface), 8 ... Heat insulation Material (top surface), 9 ... Water cooling board, 10 ... Cooling water channel, 11 ... Cooling water introduction pipe, 12 ... Cooling water discharge pipe, 13 ... Bellows, 14 ... Flow meter, 14a ... Float type flow meter, 14b ... Electromagnetic flow meter, 15 ... Thermometer contact on cooling water introduction side, 15b ... Thermocouple on cooling water introduction side, 16 ... Thermometer contact on cooling water discharge side, 16a ... Resistance temperature detector on cooling water discharge side, 16b ... Thermocouple on the cooling water discharge side, 17 ... Thermocouple for temperature control, 18 ... Service port, 19 ... Signal converter, 19a ... Signal converter for RTD, 19b ... Signal converter for thermocouple, 20 ... I / O board, 21 ... control device, 22 ... control panel, 23 ... recording device, 24 ... mold bottom plate, 25 ... cooling block, 26 ... heat insulating material (lower surface), 27 ... water cooling wall, 28 ... solidification field Search bar for position measurement, 29 ... indicator lights, 30 ... input of the solidification rate.

Claims (5)

  Heating from the cooling end face side by heating one end face of the melt of metal or semiconductor in the mold, cooling the other end face opposite to the end face, and insulating the side face of the melt. This is a device that produces metal or semiconductor crystals by solidifying in one direction toward the end face, and the amount of heat per unit time received from the solidified body by the heat removal mechanism on the cooling end face is used for the progress of solidification of the melt. A crystal production apparatus comprising a heat quantity detection mechanism capable of detecting with an accuracy of 1/10 or less of the heat quantity corresponding to the amount of generated solidification latent heat per unit time.   The heat removal mechanism on the cooling end face side of the unidirectional solidification uses heat exchange by a cooling fluid, and the heat quantity detection mechanism is configured to detect a flow rate of the cooling fluid per unit time used by the heat removal mechanism and the heat removal mechanism. A mechanism for continuously measuring the amount of heat per unit time that the heat removal mechanism receives from the solidified body from the temperature difference between the temperature of the cooling fluid supplied to the heat mechanism and the temperature of the discharged cooling fluid. The crystal manufacturing apparatus according to claim 1, wherein the crystal manufacturing apparatus is provided.   The heat amount detection mechanism detects a discontinuous increase in the amount of heat per unit time that the heat removal mechanism receives from the solidified body, and the increase is 1 of the amount of latent heat of solidification generated per unit time as the solidification of the melt progresses. 3. The crystal manufacturing apparatus according to claim 1, further comprising an alarm mechanism that issues an alarm upon detecting that the ratio exceeds / 10.   The amount of heat per unit time received by the heat removal mechanism on the cooling end face side from the solidified body during solidification of the melt as the heat flow rate by the heat quantity detection mechanism using the crystal production apparatus according to any one of claims 1 to 3. Detecting and controlling the progress of solidification of the melt so as not to cause a sudden increase in heat flow based on the detection result.   The method for producing a crystal according to claim 4, wherein the progress of solidification of the melt is controlled by temperature control of a heater.

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