APPARATUS FOR MANUFACTURING A SINGLE CRYSTAL

Abstract: In manufacturing a single crystal by means of the apparatus according to the invention, the crystal (20) is first pulled up at a relatively low speed V₁. When the diameter D of the crystal reaches D o - ΔD, where D o is the target diameter and ΔD is a deviation allowance, the pull-up speed is changed to a second speed V₂ which is much higher than V₁. When the diameter D reaches a peak value D , the pull-up speed is changed from V₂ to another speed V o which is lower than V₂. At the same time, PID control is performed based on Di and D, thereby controlling D so that it may become equal to Di. At the start of the PID control, D i is set to be the peak value D , and is then reduced by [K(D i - D o )], where 0 < K < 1. When D i becomes substantially equal to D o , the target value D o is used to control the diameter D of the crystal.

Description: The present invention relates to an apparatus for manufacturing a single crystal by the Czochralski method. More particularly, it relates to an apparatus which can automatically shape a single crystal, while being pulled up from a crucible. Most of the single crystals of silicon are made by the Czochralsky method in which a seed crystal is dipped in molten silicon in a crucible, a Dash's neck is formed of the growing single crystal, and the crystal is pulled up at a controlled speed while the temperature of the molten silicon is being controlled and while the crystal is growing further. In this method, the crystal is made to grow until it has a desired diameter and then is pulled up while maintaining this diameter. Hithereto, whether or not the crystal may have a desired diameter largely depends on the skill of the workers. That is, until its diameter increases to a desired value, the crystal is first pulled at a speed lower than that when a trunk of the crystal is formed. When a skilled worker determines, by using or not using a scale, that the diameter of the crystal has become close to the desired value, he begins to pull it faster to prevent it from growing thicker. Thereafter, he lowers the pull-up speed to a predetermined value, thus forming the trunk of the crystal. The diameter of the trunk may be adjusted by PID control. With the conventional method which much depends on the skills of a worker, it is difficult to produce single crystals of the same shape; single crystals of different diameters or single crystals with lowered shoulders may be produced. Further, each crystal made by this method may have thick and thin portions. If the crystals are subjected to an excessive PID control test they will probably have varying diameters, they may also have striations or an uneven impurity concentration distribution. Even if they have the same diameter, they must be ground when they are too thick or they must be discarded when they are too thin. Both grinding and discarding reduces the yield. A process of controlling the diameter of single crystals being made by the Czochralski method is disclosed in Japanese Patent Disclosure No. 55-130895. This process uses a ITV camera to monitor how the diameter of a single crystal being pulled up from the crucible changes with time. The rate of diameter change is used to control the speed of pulling up the crystal and to adjust the electric power supply for heating the crucible, so as to control the diameter of the final product. This method has a drawback, however. Since the diameter of the product is controlled in accordance with the rate that its diameter increases (i.e., the speed of the crystal growth), it is difficult to desirably shape the shoulder of the crystal since the shoulder is formed while the speed is rapidly changing. Further, the larger the diameter, the more slowly the growth catches up with the changes in manufacturing conditions. Documents "IBM Technical Disclosure Bulletin" Vol. 19 No. 3, August 1976 at pages 869 and 870 and "Instruments and Control Systems" Vol. 47 No. 7 July 1974 at pages 26, 28 and 30 both describe automated apparatus for manufacturing single crystals by a Czochralski method. In both cases the automation is effected by feedback processes. The present invention provides an apparatus as defined in claim 1 of the appended claims, which enables the disadvantages of the prior apparatus to be mitigated. The apparatus of the invention can automatically manufacture a single crystal with a shoulder of the desired shape and with a trunk of a predetermined diameter. The crystal made by the apparatus has no striation and is of a high quality. The apparatus needs no skilled labor. Further, it can make single crystals in a short time with a high reproducibility. It helps to reduce the material loss as compared with the known apparatus, and to enhance the production yield. In short, it increases productivity since it automatically controls the shape of the crystal being produced. Embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which : Fig. 1 is a block diagram of an apparatus according to the invention; Fig. 2 is a block diagram illustrating how the device of Fig. 1 operates: Fig. 3 is a flow chart showing how the device automatically operates; and Figs. 4 and 5 are graphs showing the control mode of the device. Fig. 1 schematically shows an apparatus of the invention for manufacturing a single crystal. The apparatus comprises a chamber 10, a crucible 12 provided in the chamber 10 and filled with molten silicon 14, and an annular heater 16 placed in the chamber 10 around the crucible 12. The chamber 10 has an opening 11 made in its top portion. A rod 22 vertically extends in the chamber 10 through this opening 11. It is slidably supported by a support (not shown and provided outside the chamber 10). It may be moved up and down by a motor 24. A seed crystal is attached to the lower end of the rod 22. The rod 22 is lowered while being rotated, thereby dipping the rotating seed crystal in the molten silicon 14. It is then lifted, thereby pulling up the crystal 20 grown on the seed crystal. A window 18 is made in the upper portion of the chamber 10. An industrial TV (ITV) camera 30 is so positioned as to monitor the crystal 20 through the window 18. The output signal of the camera 30 is input to a diameter measuring device 32, which measures the diameter D of the crystal 20. The output signal of the diameter measuring device 32 is converted to a digital signal by an A/D converter 34. The digital signal is supplied to, and latched by, an input port 36. The output data from the input port 36 is input to a microcomputer 38. The microcomputer 38 has a CPU (central processing unit) 42, a RAM (random-access memory) 40 connected to CPU 42, and a ROM (read-only memory) 44 also connected to CPU 42. A timer 46 is connected to determine the one-cycle period of the CPU 42. ROM 44 stores a program for controlling the CPU 42. The CPU 42 therefore functions according to this program during its one-cycle period determined by the timer 46. More specifically, it receives the data showing the diameter D, exchanges data with the RAM 40 and calculates the most suitable speed at which to pull up the single crystal 20. The data representing this speed is supplied via an output port 48 to a D/A converter 50 and is converted to an analog signal. The analog signal is input to a rotational speed controller 52, which controls the rotational speed of the motor 24 so as to lift the rod 22 at the speed calculated by the microcomputer 38. It will now be explained how the microcomputer 38 functions with reference to Fig. 2 and Fig. 3 (flow chart). The CPU 42 operates as follows in accordance with the program stored in ROM 44, while exchanging data with RAM 40. The output from the diameter measuring device 32 is supplied to first and second detecting means 62 and 70 through the A/D converter 34 and input port 36. The first detecting means 62 has been given data showing a target diameter D O for the crystal 20 by a target diameter setting means 66. When the measured diameter D exceeds D O - ΔD (where ΔD is the deviation allowance), the first detecting means 62 generates a first detection signal and supplies it to a second speed setting means 64, which outputs data representing a second pull-up speed V₂. This speed data is supplied to a switching means 76. The output data of a first speed setting means 60, which represents a first pull-up speed V₁, is also supplied to the switching means 76. The second detection means 70 supplies a second detection signal to a third speed setting means 72 when it detects that the measured diameter reaches the peak value D . Upon receipt of the second detection signal, the third speed setting means 72 produces data showing a third pull-up speed V₃. The diameter previously measured by the diameter measuring device 32 is used as the peak value D when it is greater than the measured diameter D. The third speed setting means 72 controls the speed V₃ so that the diameter D of the crystal 20 may equal the control target value D i . The initial value V O for the speed V₃ has been given to the third speed setting means 72. When the measured diameter D is different from the diameter D i , the third speed setting means 72 calculates the deviation allowance ΔV and then the output speed data V₃ (= V O + ΔV). ΔV is calculated by the PID (proportional integral derivative) control method. D i is input by operating a target value correction means 68 and is input to the third speed setting means 72. The initial input data from the correction means 68 is the peak value D . The means 68 varies gradually the control target value D i until it has a value nearly equal to the target diameter D O , when the former is different from the latter. For instance, it calculates the control target value Di(n) by correcting the previous control target value Di(n-1) as follows: Di(n) = Di(n-1) - K{Di(n-1) - D O } (1) where K is a constant of proportion (0 < K < 1). Di is corrected at the end of every control cycle preset by the timer 46 and gradually approaches the target diameter D O . The pull-up speed V₃ set by the third speed setting means 72 is supplied to the switching means 76. The detection signals generated by the first and second detecting means 62, 70 are also input to a selection means 74. At first the means 76 inputs the speed V₁ set by the first speed setting means 60 to a rotational speed controller 52. Upon receipt of the first detection signal, the selection means 74 actuates the switching means 76, thereby supplying speed V₂ from the second speed setting means 64 to the controller 52. Upon receipt of the second detection signal, it supplies speed V₃ from the third speed setting means 72 to the controller 52. It will now be described how the apparatus of Figs. 1 and 2 is operated to manufacture a single crystal, with reference to Figs. 3 - 5. Fig. 3 is a flow chart; Fig. 4 shows how the diameter of the crystal changes with time; and Fig. 5 illustrates how the pull-up speed is changed with time. First, the rod 22 with a seed crystal at its lower end is pulled from the crucible 14 at speed V₁. When the program stored in the ROM 44 starts running, the CPU 42 starts operating. Its operation cycle (several seconds to one minute) is determined by the timer 46. In step P1, mode "F = 1" is input. In step P2, the diameter of the crystal measured by the diameter measuring device 32 is read in CPU 42. It is checked in step P3 what flag is set. Since F = 1, the operation branches to step P4. In step P4 it is determined whether or not the measured diameter D is equal to or greater than D O - ΔD. If D < D O - ΔD, the operation returns to step P2, whereby the speed at which the rod 22 is being pulled up is still V₁ as shown in Fig. 5. Step P3 is repeated in every operation cycle of CPU 42. When the diameter D becomes equal to or greater than D O - ΔD upon lapse of time t₁, the answer imposed in step P4 is "YES". The operation proceeds to step P5, in which speed V₂ is written in CPU 42 as shown in Fig. 5. In step P6, the mode is changed to "F = 2". In the next operation cycle of CPU 42, D is written in RAM 40 in step P₂. In step P₃ it is checked what flag is set. Since F = 2, the operation proceeds to step P7, in which it is determined whether or not diameter D is the peak value D . If D has not reached D , the operation returns to step P2. Since speed V₂ is higher than speed V₁, the speed of the growth of the crystal is lowered. Thereafter, the diameter D reaches the peak value D , whereby the answer asked in step P7 is "YES". The operation then proceeds to step P8. In step P8, the third pull-up speed V₃ is input to CPU 42. This speed V₃ is of the initial value V O . Further, the control target diameter D i is set to be peak value D . In step P9, the mode is changed to "F = 3". The operation returns to step P2. In the next operation cycle of CPU 42, D is written in CPU 42. In step P3, it is checked what flag is set. Since F = 3, the operation goes to step P10, in which it is decided whether or not D i is nearly equal to D O . Since D i equals to D and greater than D O , the answer is "NO". Therefore, step P11 is carried out, thereby correcting the target value D i according to the equation (1). Thereafter, in step P12, the deviation allowance ΔV is calculated by PID method. V₃ is therefore set to V O + ΔV so that D may equal D i . In step P13, it is determined whether or not the single crystal has been pulled out of the crucible 12. If the answer is "NO", the operation returns to step P2. Then, the diameter of the trunk of the single crystal is controlled again. The flag indicating that the crystal has been pulled out of the crucible 14 is set when the length of the crystal reaches a predetermined value. The length of the cyrstal is measured by a measuring device (not shown). Until this flag is set, step P12, i.e., the PID process, is repeated. The control target value D i gradually decreases as step P11 is repeated. When D i becomes nearly equal to D O , the answer imposed in step P10 is "YES". Then, the operation jumps to step P12 over step P11. Step P12 is repeated until the flag is set in step P13. The program in CPU 42 then stops running. Ten single crystals of silicon were manufactured by means of the apparatus according to the invention. The crucible 12 was made of quartz and had an inner diameter of 300 mm. About 5 Kg of silicon was put into the crucible 12 and heated by the heater 16 until it was melted. Speed V₁, speed V₂, and speed V O (initial value) input to the microcomputer 38 were 25 mm/hour, 200 mm/hour and 60 mm/hour, respectively. The target diameter D O of 80 mm and the deviation allowance ΔD of 10 mm had been input to the microcomputer 38. Each single crystal was made in the following manner. The pull rod 22 with a seed crystal was lowered until the seed crystal was fully dipped in the molten silicon 14. Thereafter, a Dash's neck was formed. The rod 22 was pulled up at 25 mm/hour. At the same time, the temperature of the molten silicon was reduced to lower the output voltage of the pyrometer, which was used to detect said temperature, by approximately 100 microvolts. Consequently, the crystal grew, forming its shoulder. Thirty minutes (t₁) after the start of the pulling of the rod 22, the crystal had a diameter of 70 mm and the pull-up speed had increased to 200 mm/hour. Upon a lapse of 3 minutes, the peak value D was reached and the pull-up speed was reduced to 60 mm/hour. Thereafter, the diameter of the crystal was controlled by the PID method, and a single crystal of silicon, about 4 Kg, was manufactured. The ten cyrstals had no concaves or projections in their shoulders. Their trunks had a diameter of 80 ± 0.4 mm. It was found that the diameter of each crystal and the shape of the shoulder thereof were automatically controlled to become substantially identical with the designed shape and designed diameter. Hence, the yield was good.

Claims: An apparatus for manufacturing a single crystal having a diameter DO by a Czochralski method, comprising: drive means (24, 52) for pulling up the crystal from a crucible; measuring means (30, 32) for measuring an actual diameter D of the crystal (20) being pulled up from the crucible (12); timer (46); control means (38) connected to said drive means, said measuring means and said timer, for controlling the pull-up speed of the crystal (20), wherein (i) said timer (46) defines one control cycle period; (ii) said control means (38) stores therein the target diameter DO, for performing a pull-up control processing in synchronism with every one control cycle period in such a manner that, when the measured diameter D of the crystal being pulled up deviates from said target diameter DO, said drive means (24, 52) is controlled to change the crystal pull-up speed thereof by gradually correcting a control target value Di until the value Di reaches the target value DO; and (iii) said control means (38) comprises, first speed setting means (60) for setting a speed V1 at which the drive means is to pull up the crystal, target diameter setting means (66) for setting the target diameter DO, second speed setting means (64) for setting a speed V2 at which said drive means is to pull up the crystal, first detecting means (62) for detecting that the measured diameter D exceeds a value DO - ΔD, where ΔD is a predetermined value, and for outputting a first detection signal indicating that D exceeds said value, third speed setting means (72) for setting a speed V3 at which said drive means is to pull up the crystal, and for controlling the speed V3 so that the diameter D may equal to a control target value Di, second detecting means (70) for detecting that the measured diameter D reaches a peak value D and outputting a second detection signal indicating that D= D , correction means (68) for setting the peak value D as the initial value for the control target value Di and for gradually correcting the control target value Di until the value Di reaches the target diameter DO, and switching means (76) for supplying said drive means with a signal representing the speed V2 in response to the first detection signal and a signal representing the speed V3 in response to the second detection signal. The apparatus according to claim 1, characterized in that said control means comprises a digitial microcomputer. The apparatus according to claim 1, characterized in that said peak value D , serving as the initial value of the control target value Di, is greater than the target diameter DO. The apparatus according to claim 3, characterized in that said correction means (68), in use, gradually decreases the control target value Di, once at the end of every control cycle period, until it reaches the target diameter DO.

Inventors: WASHIZUKA, SYOICHI C/O PATENT DIVISION, WATANABE, MASAYUKI C/O PATENT DIVISION
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1984
Main classification: C30B15
IPC codes: C30B15