Method for Pulling a Cylindrical Crystal From a Melt

Abstract

A method for pulling a cylindrical crystal from a melt by a crystal pulling unit includes measuring an actual value of a diameter of the crystal at a surface of the melt, comparing the actual value with a setpoint value for the diameter of the crystal, and setting a height of the annular gap as a function of a deviation between the actual value and the setpoint value using a first controller which has a first readjustment time.

Claims

1.-9. (canceled)

10. A method for pulling a cylindrical crystal (5) from a melt (2) by a crystal pulling unit, the crystal pulling unit comprising: a crucible (3) in which the melt (2) is disposed; a crucible heater (4) which surrounds the crucible (3) annularly; a crucible lifting device (7) for lifting the crucible (3); a crystal lifting device (6) for pulling the crystal (5) from the melt (2); and a heat shield (10) surrounding the crystal (5) annularly and having a lower edge which ends above the melt (2) and forms an annular gap (12); and comprising the steps of: measuring an actual value of a diameter of the crystal at a surface of the melt; comparing the actual value with a setpoint value for the diameter of the crystal; and setting a height of the annular gap (12) as a function of a deviation between the actual value and the setpoint value using a first controller which has a first readjustment time.

11. The method according to claim 10, wherein the first readjustment time is selected so as to compensate influences of melting convection at a melt-contacting end face of the crystal and of the melt.

12. The method according to claim 10, wherein when the actual value is smaller than the setpoint value the height is reduced and wherein when the actual value is larger than the setpoint value the height is increased.

13. The method according to claim 10, wherein for changing the height, a lifting rate (14) of the crucible (3) and in equal measure a lifting rate (13) of the crystal (5) are varied so that a crystal pulling rate remains constant.

14. The method according to claim 13, wherein for the lifting rate (14) of the crucible (3) a first base value is set which compensates a volume consumption of the melt (2), wherein a second base value is set for the lifting rate (13) of crystal (5), and wherein for regulating the height a same offset value is added to both the first base value and the second base value.

15. The method according to claim 10, wherein a control value for the height is supplied as an actual value for regulating a heating power of the crucible heater (4) by a second controller, which has a second readjustment time, with a medial annular gap height being prespecified as a setpoint value.

16. The method according to claim 15, wherein the second readjustment time is selected so as to compensate influences of changes in a position of the surface of the melt in the crucible that come about during the pulling.

17. The method according to claim 16, wherein the first readjustment time is smaller than the second readjustment time.

18. The method according to claim 16, wherein the control value for the height is supplied via a PT1 filter (40) as an actual value to a heating power regulator (28).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] FIG. 1 shows a schematic representation of a Czochralski-process crystal pulling unit;

[0031] FIG. 2 shows a closed-loop control scheme according to the state of the art;

[0032] FIG. 3 shows a closed-loop control scheme in accordance with the invention; and

[0033] FIG. 4 shows a record of a crystal pulling procedure in diagram form.

DETAILED DESCRIPTION OF THE DRAWINGS

[0034] Reference is made first to FIG. 1. According to FIG. 1, the crystal pulling unit consists of a unit housing 1, in which a crucible 3 filled with a melt 2 is located. Located around the crucible is a crucible heater 4, which is used to melt the material in the crucible 3 and then maintain it at temperature.

[0035] A cylindrical crystal 5 is pulled slowly from the melt 2 by means of a crystal lifting device 6, with the atoms of the melt 2 taking their place at the lower end face of the crystal 5 and forming a crystal lattice as the crystal is pulled out of the melt 2. In the course of this process, material in the crucible 3 is consumed, and so crystal pulling units typically provide a crucible lifting device 7, with which the crucible 3 is repositioned, so that the melt surface is always located at a constant level in relation to the unit housing 1.

[0036] The crystal diameter is captured by means of a camera 8, which is directed at the margin of the crystal at the transition to the melt. The crystal diameter can be derived from the images obtained by the camera 8. The image analysis processes employed for this purpose are known to the skilled person and need not, therefore, be elucidated in more detail here.

[0037] The crystal pulling procedure is regulated by means of a control device 9. This device contains information from the camera 8 and transmits control variables to the crystal lifting device 6 and to the crucible lifting device 7.

[0038] So that the crystal pulled from the melt can gradually cool, a heat shield 10 surrounding the crystal is necessary, which keeps the radiation from the crucible heater 4 and from the hotmelt away from the upper region of the lateral surface of the crystal 5.

[0039] The lower edge of the heat shield 10 ends above the surface of the melt 2, and so there is an annular gap 12 between the edge of the heat shield 10 and the melt. By virtue of this annular gap 12, heat is irradiated into the lower region of the lateral surface of the crystal 5.

[0040] The parameters relevant to the control of the pulling procedure are as follows: the decisive parameter ultimately is the crystal pulling rate, being the rate of the crystal relative to the melt surface. It is determined by the absolute lifting rate 13 of the crystal, which is the rate at which the crystal is moved relative to the unit, and by the rate of the melt surface relative to the unit. If this is set at zero, and the melt surface is therefore held at a constant level in relation to the unit, the crystal pulling rate corresponds to the absolute lifting rate 13 of the crystal.

[0041] The rate of the melt surface in relation to the unit is determined by the absolute lifting rate 14 of the crucible, taking account of the lowering of the melt surface within the crucible caused by the consumption of material. The lifting rate of the crucible may be determined, for example, by ascertaining the increase in material of the crystal 5 using the crystal pulling rate, and so the lowering of the melt surface in relation to the crucible 3 is compensated by a corresponding lifting of the crucible 3, so that the melt surface remains at a constant level in relation to the unit.

[0042] FIG. 2 depicts a closed-loop control scheme which is used to regulate a crystal pulling procedure, according to the state of the art, in order to achieve a very largely constant diameter of the crystal. This scheme consists of a diameter controller 20, which receives an actual diameter value at a first input 21 and a setpoint diameter value at a second input 22. A control value for the crystal lifting device 6 is output at the output 23 of the diameter controller 20.

[0043] In order to prevent excessive drift of the crystal pulling rate, the control value is also supplied to a PT1 filter 24, whose output 25 is supplied as actual value to a controller 26 for a medial crystal pulling rate. For this purpose there is a setpoint value for the medial crystal pulling rate at an input 27. Through a comparison of actual value and setpoint value, a control value for the heating power of the crucible heater 4 is generated, and is supplied to a heating power regulator 28.

[0044] In contrast to this, according to FIG. 3, the closed-loop control scheme of the invention consists of a diameter controller 30, which is supplied at a first input 31 with an actual value for the crystal diameter and at a second input 32 with a setpoint value for the crystal diameter. At the output 33, the diameter controller 30 generates a control value for the annular gap height, which is made available to an annular gap height regulator 34.

[0045] This regulator generates an offset value at its output 35, the value being supplied both to a crystal lifting regulator 36 and to a crucible lifting regulator 37. In the regulators 36 and 37, the offset value is added to a base value for the crucible lifting rate 14 and to a base value for crystal lifting rate 13, respectively, and so the crystal lifting rate 13 and the crucible lifting rate 14 change in equal measure, without any change in the crystal pulling rate. The crucible lifting base value is the value with which the melt surface, taking account of the consumption, is maintained at a constant level. The crystal lifting base value is the value of the desired crystal pulling rate.

[0046] Critical to the invention, therefore, is that while there is a change in the crystal lifting rate 13, there is no change in the crystal pulling rate at which the crystal 5 lifts from the melt 2. The crucible 3 changes its position here in such a way that the melt surface is displaced in relation to the bottom edge of the heat shield 10, causing a change in the height of the annular gap 12. The inward radiation of heat from the crucible heater 4 into the lower lateral surface of the crystal can therefore be kept at a value at which, at least in the lower region of the crystal, there is no irradiation of heat, and so, for a constant crystal pulling rate, the diameter of the crystal 5 is constant and the ratio of temperature gradient to crystal pulling rate is optimal.

[0047] Here as well, a slow external closed-loop control circuit is overlaid on the inner closed-loop control circuit. The control value for the gap height regulator is supplied via a PT1 filter 40 to a controller 41 for the medial annular gap height. This value is compared with a setpoint value at the input 42 for the medial annular gap height, and, as a result, a corresponding control value is made available for the heating power regulator 28. Because the PT1 filter 40 changes the actual value, relevant for the heating power regulator 28, in proportion to the temporal change in the control value for the annular gap height regulator 34, the control procedure for the external closed-loop control circuit is slowed.

[0048] FIG. 4 shows the temporal profile, over several hours, of the actual crystal diameter value (curve 50), measured with a camera, the crystal pulling rate (curve 51), the height of the annular gap (curve 52) and the temperature of the crucible heater (curve 53).

[0049] In the first phase of the growing experiment—shown up to the first timeline 54—the crystal diameter is regulated with a conventional controller cascade by way of the pulling rate in the internal closed-loop control circuit, and via the temperature of the main heater in the external closed-loop control circuit. Clearly apparent are the fluctuations in the pulling rate (indicator arrow 55) on the timescale (A), which accompany a significant change in the crystal diameter (indicator arrow 56).

[0050] Subsequently (from the first timeline 54 on), the control of the crystal pulling unit was switched over to regulation in accordance with the invention, using the annular gap height as control variable in the internal closed-loop control circuit, and using, still, the temperature of the main heater as control variable in the external closed-loop control circuit. It is apparent that after the switch, the pulling rate was kept entirely constant (indicator arrow 57), whereas now there is fluctuation of the annular gap height on the timescale (A) (indicator arrow 58). With this regulation, a substantially consistent crystal diameter is achieved (indicator arrow 59). The experiment was discontinued at a second timeline 60.

LIST OF REFERENCE CHARACTERS

[0051]

TABLE-US-00001 1 Unit housing 2 Melt 3 Crucible 4 Crucible heater 5 Crystal 6 Crystal lifting device 7 Crucible lifting device 8 Camera 9 Control Device 10 Heat shield 12 Annular gap 13 Crystal lifting rate 14 Crucible lifting rate 20 Diameter controller 21 Input 22 Input 23 Output 24 PT1 filter 25 Output 26 Controller 27 Input 28 Heating power regulator 30 Diameter controller 31 Input 32 Input 33 Output 34 Annular gap height regulator 35 Output 36 Crystal lift regulator 37 Crucible lift regulator 40 PT1 filter 41 Controller 42 Input 50 Curve 51 Curve 52 Curve 53 Curve 54 Timeline 55 Indicator arrow 56 Indicator arrow 57 Indicator arrow 58 Indicator arrow 59 Indicator arrow 60 Timeline