Method of automatic control of growing neck portion of a single crystal by the CZ method

Abstract: A method for automatically controlling growing a single crystal neck portion by the CZ method, comprising the steps of pulling up a seed crystal (30) at 2 mm/min. for five minutes so as to grow a single crystal 32; next measuring a diameter of the lower end of the crystal; modifying an electric power for heating a melt based on the difference between the measured diameter and a reference value; waiting for five minutes; keeping the electric power constant for 10 minutes with controlling the pulling up speed so as to approach the diameter of the crystal to the reference value and with measuring the pulling up speed repeatedly; and next modifying the power based on the difference between the mean speed and a reference value. The last two steps are repeated alternatively.

Description: BACKGROUND OF THE INVENTION The present invention relates to a method of automatically controlling the growth of a neck portion between a seed crystal and a cone portion of a single crystal, which is applicable to an apparatus for growing a single crystal by the Czochralski method. In apparatus for growing a single crystal by the CZ method, there is an established method of automatic control which is applicable only to those cylindrical sections of the grown single crystal that are grown after a cone section. The growth of the neck portion of the single crystal, however, which is grown since the initiation of the growth of the crystal until the growth of a cone section, is controlled manually by a skilled operator. This is because there are elaborate control requirements to expel dislocations from the crystal surface in the neck portion. For example, in the neck portion the diameter of the grown crystal should be reduced to a value of about 2 - 5 mm; the crystal should be pulled up at a relatively high speed of not lower than 2 mm/min.; the absolute value of the diameter deviation should be controlled to be not larger than about 0.5 mm and the reduced portion should be grown to a length which must be more than 10 times as large as that diameter. The process of growing the crystal into a desired configuration of the reduced portion, that has to get free of dislocations which is to be followed by unusual increase in the crystal diameter, is so hard a process that even a skilled operator will be able to succeed in such process at the rate of as low as 10 %. If the crystal diameter is too reduced, then the lower end of the crystal can be often disconnected from the melt surface with the result that the growth can no longer be continued, or if not disconnected the reduced diameter portion cannot be strong enough to support a cylindrical portion which will normally be grown. If, on the contrary, the crystal diameter is too thick, then dislocations cannot be expelled out sufficiently with the result that the growing process cannot proceed to the step of growing the cone section. Patent abstracts of Japan, Vol. 10, No. 23 (C-325) [2080], 29.01.1986; JP-A-60-176989 disclose a method for growing a single crystal neck portion of a single crystal by the CZ method, wherein the heater power is continually controlled while growing the whole neck portion of the crystal. During this process the crystal is pulled up at a constant speed. US-A-3958129 discloses a method for growing the neck portion of a single crystal by the CZ method, wherein the seed crystal is withdrawn at a predetermined speed to form the neck portion. No reference is taken to the diameter of the crystal for controlling the pull-up speed. US-A-3621213 shows a according method, in which during the growth of the neck portion the heater power is only changed according to precalculated and computer-stored data, which are parcelled out in terms of incremental power changes at predetermined process time increments. This means that no real control of the crystal growing in the so-called neck-in phase takes place, as all other controlled process variables and especially the pulling-up speed of the crystal are maintained constant. To summarize the prior art does not show any method of automatically controlling the growth of a single crystal neck portion by the CZ method. SUMMARY OF THE INVENTION In view of the above-mentioned problems, an object of the present invention is therefore to provide a method of automatically controlling the growth of a single crystal neck portion by the CZ method. The neck portion can be grown in the same manner as the cone portion or cylindrical portion when the crystal diameter is controlled by regulating the speed at which the crystal is pulled up, but it cannot be so grown when the crystal diameter is controlled by regulating the temperature of a melt. This is because the response in the crystal diameter is significantly lower to the melt temperature than to the speed at which the crystal is pulled up and, hence, the diameter deviation would exceed an allowable value in the reduced-diameter portion if the crystal diameter is controlled by regulating the melt temperature during the growth of the reduced-diameter portion in the same manner as that in which the cone or cylindrical portion is grown. To improve the controllability of the neck portion diameter, however, not only the crystal pull-up speed but also the melt temperature should be involved in the control of the diameter in some manner, since the crystal diameter is largely dependent on both of the melt temperature and the crystal pull-up speed. In accordance with the present invention, the above object is achieved by involving the melt temperature in a moderate manner in the following diameter control methods: (1) According to claim 1 a method of automatic control of growing a single crystal neck portion between a seed crystal and a cone portion of the single crystal by the CZ method, comprises serial repeated steps of: maintaining constant, for a period of time, a heater power supplied to a heater for heating a melt contained in a crucible; controlling a pulling up speed of the single crystal growing from said melt such that a diameter of the lower end of the single crystal will approach a reference value during said period; measuring said speed repeatedly during said period (118 - 124); and modifying said heater power based on the difference between the mean value of said speed and a target value of said speed after said first period (126 - 130). A preferred embodiment of this method is given by claim 2 and comprises the steps of: (110, 112) pulling up the seed crystal (30) at a constant speed for a second period of time so as to grow the single crystal (32) with maintaining said heater power constant (113) measuring a diameter of the lower end of said single crystal after said second period has elapsed; and (114) modifying said heater power based on the difference between the measured diameter and said target value of said diameter. It is possible to improve the success rate of the automatic neck-section control by carrying out the method according to claim 1 after the method steps of claim 2. In such a case, the method steps of claim 1 can be started immediately after the method steps of claim 2 have been completed since, at the moment the latter steps are completed, a throttled section of reduced diameter is not yet reached. With a preset third time period of e.g. 5 minutes being interposed according to claim 3 between the method steps of claim 2 and 1, the melt temperature can be stabilized by the modification made to the power supply to the heater so that the method according to claim 1 can be carried out more effectively. BRIEF DESCRIPTION OF THE DRAWINGS An embodiment of the present invention will now be described with reference to the accompanying drawings, in which: Fig. 1 is a block diagram showing the arrangement of the principal parts of an automatic apparatus for growing a single crystal neck section; Figs. 2A - 2C are flow charts showing the sequence of control for growing the neck section using a microcomputer 58; Fig. 3 is a graph showing an example of an input-to-output relation of a diameter setting unit 42; and Fig. 4 is a graph of data taken by implementing the embodiment, showing the change of the pull up speed V, crystal diameter D and melt surface temperature T, versus the elapsed time t since the moment the pulling up of the single crystal was started. DESCRIPTION OF THE PREFERRED EMBODIMENT Fig. 1 shows the principal parts of an automatic apparatus for growing a single crystal neck section to which the method according to the present invention is applied. On a table 12 which is fixed to the top of a shaft 10, there is mounted a graphite crucible 14 in which a quartz crucible 16 is fitted. The graphite crucible 14 is surrounded by a heater 18 which in turn is surrounded by a heat insulator of graphite 20. A piece of polycrystalline silicon is put in the quartz crucible 16, and electric power is supplied to the heater so as to melt the polycrystalline silicon into a melt 22. A seed crystal 30 is attached via a seed holder 28 to the lower end of a wire 26 which can be moved up and down by a motor 24 which is located above the melt 22. The seed crystal 30 is positioned such that its lower end comes into contact with the molten surface 22S of the melt 22 and is then pulled up, whereby a silicon single crystal 32 is grown on the free end of the seed crystal 30. The growth of the silicon single crystal 32 is caused to occur within a chamber 34 which is purged of air by argon gas. To take a measurement of the diameter D of the lower end of the silicon single crystal 32, a CCD camera 38, with its optical axis directed toward the center of the melt surface 22S, is placed above a window 36 which is provided at a shoulder of the chamber 34. A video signal output from the CCD camera 38 is provided to a diameter measuring unit 40 which determines, by picture processing, the diameter D of a brilliant annulus formed at the interface between the silicon single crystal 32 and the melt surface 22S, i.e. the lower end diameter D of the silicon single crystal 32. Since the diameter of the silicon single crystal 32 is reduced at its throttled section, the CCD camera 38 has a high magnification so as to have a high precision of measurement, such that one scanning line width corresponds to an actual dimension of e.g. 0.05 mm. With respect to the crystal diameter D, a target value D₀ is stored as a function of the length L of the silicon single crystal 32 in a diameter setting unit 42 which is constituted by a memory. The relation between the crystal length L and the target diameter D₀ is e.g. one which is shown in Fig. 3. The crystal length L is derived from an up-down counter 46 which counts the number of pulses output from a rotary encoder 44 whose axis of rotation is connected to the drive shaft of the motor 24. The count of the up-down counter 46 is cleared when the seed crystal 30 is at its upper limit position or has come into contact with the melt surface 22S. The contact may be sensed e.g. by an arrangement in which a voltage is applied between the wire 26 and the shaft 10, an electric current flowing through them being detected as an indication of the occurrence of the contact. The apparatus for growing a single crystal is arranged to cascade control the pull up speed V of the wire 26 in such a manner that the crystal diameter D will come nearer to the target value D₀. Specifically, the crystal diameter D and the target diameter D₀ are provided to a PID controller 48. An output voltage EV₀ from the PID controller 48 is provided to a variable speed motor controller 50 as the target value to be reached by the speed of rotation of the motor 24. The output from the rotary encoder 44 is converted by a frequency-to-voltage converter 52 into a voltage EV which is proportional to the frequency. The voltage EV is provided as a feedback quantity to the variable speed motor controller 50. The variable speed motor controller 50 is normally a PID controller. The controller 50 will control the speed of rotation of the motor 24, or the pull up speed V of the wire 26, by means of an analog changeover switch 54 and a driver 56 such that EV will come nearer to EV₀. This analog changeover switch 54 is arranged to selectively provide to the driver 56 either the output from the variable speed motor controller 50 or the output from a microcomputer 58. When closed-loop control is to be effected to maintain constant the speed of rotation of the motor 24, the output from the microcomputer 58 is used. The crystal diameter D is largely dependent on the temperature of the melt 22 but will still more quickly respond to the single crystal pulling up speed. To control the melt temperature so as to adjust the crystal diameter D of the neck section having a nallow allowable range of diameter control deviation of about ±0.5 mm, an especial manner of control is required which is different from those applicable to the corn or straight body section of the grown single crystal. Thus, the single crystal growing apparatus has the following arrangement for controlling the power supply to the heater 18 in such a manner that the crystal diameter D will come nearer to the target value D₀. Specifically, a two-color pyrometer 62 is provided over a window 60 provided at a shoulder of the chamber 34, with the optical axis of the pyrometer being directed to the melt surface 22S at a point which is somewhat spaced away from the center of the melt surface to the left in Fig. 1. An output from the two-color pyrometer 62 is provided to the microcomputer 58 after being converted into a digital form by an analog-to-digital (A/D) converter 64. The microcomputer 58 calculates a quantity of adjustment to the electric power supply to the heater 18 for adjusting the power supplied to the heater 18 by means of a driver 66, using the crystal diameter D from the diameter measuring unit 40, the pull up speed V from the rotary encoder 44 (since "V" is inversely proportional to the period of the output pulse from the rotary encoder 44, this period is determined by software to calculate the speed V) and the melt surface temperature T from the A/D converter 64, as will be described later in more detail. Next, the sequence of control effected by the microcomputer 58 for controlling the growth of a single crystal neck section, indicated in Figs. 2A - 2C, will be described. Individual steps of this sequence are roughly classified into the following phases A) - C) which correspond to Figs. 2A - 2C, respectively. A) CONTROL UNTIL THE START OF PULL UP STEP 100: The seed crystal 30 is initially at its upper limit position. Then, the melt surface temperature T is read out from the A/D converter 64 and electric power is supplied to the heater 18 via the driver 66 such that the value of the melt surface temperature T will come nearer to a preset temperature. This preset temperature is the temperature selected to cause the diameter of the silicon single crystal 32 to become equal to the diameter of the seed crystal 30, e.g. 10 mm, when the seed crystal 30 is pulled up at a constant speed of e.g. 2 mm/min., and such temperature is empirically determined. STEP 102: The analog changeover switch 54 is changed over to the position associated with the microcomputer 58 to turn on the motor 24 to lower the wire 26 at a constant speed. The motor 24 is turned off just before the lower end of the seed crystal 30 comes into contact with the melt surface 22S. The stop position of the seed crystal 30 is determined when the count of the up-down counter 46 has reached a preset value. STEP 104: For preheating the seed crystal 30, a predetermined period of time, 7 minutes, for example, is allowed to elapse. STEP 106: The motor 24 is operated to lower the seed crystal 30 until the lower end of the seed crystal 30 comes into contact with the melt 22. STEP 108: For causing the lower end of the seed crystal 30 to be in intimate contact with the melt surface 22S, a predetermined period of time, 5 minutes, for example, is allowed to elapse. The foregoing steps 100 - 108 are already known in the art. B) PULL UP AT CONSTANT SPEED STEP 110: Subsequently, the motor 24 is again turned on so as to pull up the seed crystal 30 at a constant speed, 2 mm/min., for example. STEP 112: A predetermined time period of 5 minutes, for example, is allowed to elapse for seeing whether the temperature of the melt 22 is suitable or not. STEP 113: The motor 24 is turned off and the crystal diameter D is read out from the diameter measuring unit 40. STEP 114: A quantity of adjustment ΔP to be applied to the electric power supply to the heater 18 is calculated dependent on the difference between the actual value D and the expected value of the crystal diameter. Such expected value is, in this embodiment, equivalent to the output D₀ from the diameter setting unit 42. The electric power supplied to the heater 18 is changed from the present value by the calculated ΔP. The ΔP is calculated from e.g. the following equation: ΔP = K( D - D₀ ) (1) STEP 116: A predetermined time period, 5 minutes, for example, is allowed to elapse so that the temperature of the melt 22 can become stable. C) CONTROL OF THE DIAMETER OF THROTTLED SECTION STEP 118: The analog changeover switch 54 is changed over to the position associated with the variable speed motor controller 50 and the motor 24 is turned on so as to PID control the pull up speed of the wire 26. STEP 120: To obtain the mean value of the pull up speed V over a predetermined time period t₀, 10 minutes, for example, the present value of the pull up speed V is added to the sum S to obtain a new value of the sum S. The initial value of the sum S is 0. STEP 122: A comparison is made between the values of the crystal length L and L₀. This "L₀" is for determining the time point the growth of the neck section is to be terminated, and has a value of 150 mm, for example. If L < L₀, then control is transferred to the next step 124. STEP 124: A comparison is made between the values of t₀ and the elapsed time t since the time the step 118 was made for first time. If t < t₀, then control is returned to step 118. If t ≧ t₀, then control is transferred to the next step 126. STEP 126: The mean value of the pull up speed V , i.e. V = S/N , is derived. This "N" represents the number of times the series of steps 118 - 124 has been repeated. STEP 128: A comparison is made between the values of | V - V s | and ΔV. This "V s " represents the target value of the pull up speed of the throttled section and, in the case of Fig. 4, has a value of 3.5 mm/min. ΔV has a value of 0.5 mm/min., for example. If | V - V s | ≧ ΔV , then control is transferred to the next step 130. Otherwise, step 130 is omitted and control is directly transferred to step 132 so as to render moderate the temperature change to reduce the diameter control deviation. STEP 130: A quantity of adjustment ΔP to be applied to the electric power supply to the heater 18 is calculated dependent on the the value of | V - V s |. The electric power supplied to the heater 18 is changed from the present value by the calculated ΔP. The equation for this calculation may be expressed e.g. as follows: Δ P = K( V - V s ) (2) STEP 132: A value of t₁, 10 minutes, for example, is added to the value of t₀ so as to obtain a new value of t₀ and the sum S is changed into 0. Control is then returned to the above step 120. When L ≧ L₀ is established in step 122, then the control of the neck section growth is terminated and control of the growth of a cone section is initiated. In accordance with the foregoing steps of control, a neck section can automatically be grown with a high success rate. Fig. 4 shows the change, occurring as a result of the foregoing control sequence, of the crystal diameter D, pull up speed V, and melt surface temperature T, versus the elapsed time t since the moment the pulling up of the single crystal was started. In this example, said predetermined time period in steps 112 and 116 is set to 5 minutes; the target value of the diameter D₀ versus the crystal length L is identical to that of Fig. 3; V s in step 128 has a value of 3.5 mm/min.; ΔV has a value of 0.5 mm/min. Control of the growth of a neck section was effected 170 times under such conditions, with a success rate of about 90 %. This is approximately the same as the success rate of manual control by a skilled operator.

Claims: A method of automatic control of growing a single crystal neck portion between a seed crystal and a cone portion of the single crystal by the CZ method, comprising serial repeated steps of: maintaining constant, for a first period of time, a heater power supplied to a heater for heating a melt contained in a crucible; controlling a pulling up speed of the single crystal growing from said melt such that a diameter of the lower end of the single crystal will approach a target value of said diameter during said first period; measuring said speed repeatedly during said first period (118 - 124); and modifying said heater power based on the difference between a mean value of said speed and a target value of said speed after said first period (126 - 130). A method according to claim 1, wherein said serial repeated steps are carried out after the steps of: (110, 112) pulling up said seed crystal (30) at a constant speed for a second period of time so as to grow said single crystal (32) with maintaining said heater power constant; (113) measuring a diameter of the lower end of said single crystal after said second period has elapsed; and (114) modifying said heater power based on the difference between the measured diameter and said target value of said diameter. A method according to claim 2, wherein between said additional steps defined in claim 2 and said serial repeated steps defined in claim 1, a step of waiting a third period of time with stopping pulling up said crystal (116) is carried out. A method according to claim 1, wherein said first period of time is about 10 minutes. A method according to claim 2, wherein said second period of time is about 5 minutes. A method according to claim 3, wherein said third period of time is about 5 minutes.

Inventors: BABA MASAHIKO, OHTSUNA HIROSHI, BABA, MASAHIKO, OHTSUNA, HIROSHI
Applicants: SHINETSU HANDOTAI KK, SHIN-ETSU HANDOTAI COMPANY LIMITED
Application year: 1991
Main classification: C30B15
IPC codes: C30B15, C30B29