Pressurized skull crucible for crystal growth using the Czochralski technique

• US 5,863,326 A
• Filed: 03/14/1997
• Issued: 01/26/1999
• Est. Priority Date: 07/03/1996
• Status: Expired due to Fees

• Current Assignee: Cermet, Inc.
• Original Assignee: Cermet, Inc.
• Inventors: Nause, Jeffrey E., Hill, D. Norman, Pope, Stephen G.
• Primary Examiner(s): KUNEMUND, ROBERT M
• Application Number:
  US08/818,393
• Time in Patent Office: 683 Days
• Field of Search: 117/77, 117/30, 117/223, 117/35, 117/81, 117/49, 117/217
• US Class Current: 117/217
• CPC Class Codes: C30B 11/002 Crucibles or containers for supporting the melt
• CPC Class Codes: C30B 11/003 Heating or cooling of the melt or the crystallised material
• CPC Class Codes: C30B 15/00 Single-crystal growth by pulling from a melt, e.g. Czochralski method under a protective fluid C30B27/00
• CPC Class Codes: C30B 15/10 Crucibles or containers for supporting the melt
• CPC Class Codes: C30B 15/14 Heating of the melt or the crystallised materials
• CPC Class Codes: Y10T 117/1068 including heating or cooling details [e.g., shield configuration]
• CPC Class Codes: Y10T 117/1092 Shape defined by a solid member other than seed or product [e.g., Bridgman-Stockbarger]

Abstract

The invention is directed to an apparatus, system and methods for growing high-purity crystals of substances that are peritectic at atmospheric pressure using the Czochralski technique. The apparatus includes a pressure vessel that contains a pressurized gas. The apparatus also includes a cooling unit that is situated in the pressure vessel. The cooling unit receives a coolant flow from outside of the vessel, and has cooled surfaces that define an enclosure that receives the charge material. The apparatus further includes an inductive heating element situated in the vessel, that is coupled to receive electric power externally to the vessel. The element heats the interior portion of the charge material to form a molten interior portion contained by a relatively cool, exterior solid-phase portion of the charge material that is closer relative to the molten interior, to the cooled surfaces of the cooling unit. Because the exterior portion is the same material as the contained molten interior portion, few impurities are introduced to the molten interior portion of the charge material. Due to the pressure exerted by the gas contained in the vessel, the liquid interior of the charge material becomes congruently melting to prevent its peritectic decomposition. Therefore, crystals of substances that are peritectic at atmospheric pressure, can be produced from the liquid phase with the apparatus of this invention using the Czochralski technique. In addition to electric power, the heating element receives a coolant flow from a feedthrough that extends through a wall of the pressure vessel. In proximity to the vessel wall, the feedthrough has two coaxial conductors to improve the electric power transfer to the heating element and to reduce heating of the external surfaces of the vessel. The two conductors of the feedthrough are cylindrical in shape, and define two channels for channeling a coolant flow to and from, respectively, the heating element. A shield formed of a cylindrical sheet of metal, for example, is positioned in the vessel to surround the heating element to focus energy emitted by the element to the charge material.

Claims

1. An apparatus for growing a crystal from a charge material with a seed crystal, the apparatus comprising:

a pressure vessel for containing a pressurized gas;

a cooling unit for situation in the pressure vessel, having cooled surfaces defining an enclosure to receive the charge material, the enclosure being partially opened to expose the charge material to pressure exerted by the gas contained in the pressure vessel;

an induction heating element for situation in the pressure vessel, for generating radio-frequency (rf) radiation to heat an interior portion of the charge material to form a melt zone that is contained by a relatively cool, exterior portion of the charge material that is closer relative to the melt zone, to the cooled surfaces of the cooling unit;

a shield situated in the pressure vessel in operation of the apparatus, and arranged between the induction heating element and a wall of the pressure vessel, the shield preventing the rf radiation generated by the induction heating element from significantly heating the pressure vessel'"'"'s wall; and

a pull rod situated in the pressure vessel, for holding the seed crystal in proximity to the charge material contained in the cooling unit.

2. An apparatus as claimed in claim 1, further comprising:

a pressurized gas unit coupled to communicate with a space enclosed by the pressure vessel, for supplying the pressurized gas to the pressure vessel.

3. An apparatus as claimed in claim 2, further comprising:

a conduit extending through a wall of the pressure vessel, having a first end communicating with the space enclosed by the inside of the pressure vessel and a second end situated outside of the pressure vessel;

a fitting coupled to form a pressure-tight seal between the wall and the conduit; and

a valve coupled between the second end of the conduit and the pressurized gas unit, the valve controlling an amount of pressure of the gas to a preselected value.

4. An apparatus as claimed in claim 1, wherein the cooling unit includes

a manifold defining input and output passages coupled to the first and second conduits, respectively, to receive and expel the coolant flow from the cooling unit;

and

a plurality of elongated cooling members that are circularly arranged to define the inner side cooling surfaces of the enclosure, the elongated cooling members having input and output conduits defined by coaxial tubes, the cooling members being mounted to the manifold so that the input and output conduits of the cooling members communicate with the input and output passages, respectively, of the manifold,

the cooled surfaces of the cooling members that face inwardly toward a center axis of the circular arrangement of the cooling members and a surface of the manifold together defining the enclosure.

5. An apparatus as claimed in claim 4, further comprising:

a protective cylinder fitted in sleeve-like fashion outside of the elongated cooling members, for containing the charge material.

6. An apparatus as claimed in claim 1, further comprising:

a feedthrough extending through a wall of the pressure vessel, and having a first end situated inside of the pressure vessel, coupled to the heating element, and a second end opposite the first end, situated outside of the pressure vessel; and

a coolant supply unit coupled to the feedthrough to circulate the coolant through the inductive heating element.

7. An apparatus as claimed in claim 6, further comprising:

a fitting coupled to form a pressure-tight seal between the feedthrough and the wall of the pressure vessel.

8. An apparatus as claimed in claim 6, further comprising:

an electric power generator situated outside of the pressure vessel and coupled to the feedthrough, generating an alternating current that is supplied to the inductive heating element to heat the interior portion of the charge material.

9. An apparatus as claimed in claim 1, wherein the apparatus receives a coolant flow and electric power, the apparatus further comprising:

a feedthrough extending through a wall of the pressure vessel, the feedthrough including

a first elongated electric conductor having an inner wall defining a conduit for channeling the coolant flow, and

a second elongated electric conductor enclosing a portion of the first conductor in a region in which the feedthrough extends through the wall, the second conductor having an inner wall that is spaced from an outer wall of the first conductor to define a conduit for channeling the coolant flow between the inner wall of the second conductor and the outer wall of the first conductor,

the first and second electric conductors having respective first ends coupled inside of the pressure vessel to respective terminals of the heating element, to supply the electric power to the heating element, and coupled to supply the coolant flow to the heating element through one of the first and second conduits, and to receive the return coolant flow from the heating element through the other of the first and second conduits, and

the first and second electric conductors having second ends coupled outside of the pressure vessel to receive the electric power, and coupled so that the one of the first and second conduits receives the coolant flow for supply to the heating element, and the other of the first and second conduits outputs the coolant flow from the heating element.

10. An apparatus as claimed in claim 9, further comprising:

an electrical insulator surrounding and contacting the first conductor, to provide electrical insulation between the first and second conductors, an outer wall of the electrical insulator and the inner wall of the second conductor defining the channel between the outer wall of the first conductor and the inner wall of the second conductor.

11. An apparatus as claimed in claim 1, wherein the heating element includes a coil encircling a longitudinal axis of the cooling unit.

12. An apparatus as claimed in claim 1, wherein the shield is formed of metal.

13. An apparatus as claimed in claim 1, further comprising:

a support member situated in the pressure vessel and supporting the shield in a position to surround the heating element.

14. An apparatus as claimed in claim 1, further comprising:

at least one bracket mounted to an inside wall of the pressure vessel, the bracket attached to the shield to support the shield.

15. An apparatus as claimed in claim 1, the apparatus further comprising:

an actuator situated in the pressure vessel and coupled between the cooling unit and an inner wall of the enclosure, for moving the cooling unit relative to an end of the pull rod at which the seed crystal is held, to form the crystal of the charge material.

16. An apparatus as claimed in claim 15, wherein the actuator includes

a hydraulic fluid;

a hydraulic device situated inside of the pressure vessel, mechanically coupled between the cooling unit and the inner wall of the pressure vessel, for moving the cooling unit relative to the end of the pull rod at which the seed crystal is held, based on a flow of the hydraulic fluid, and

a hydraulic pump situated outside of the pressure vessel, and coupled to communicate with the hydraulic device, the hydraulic pump actuatable to move the cooling unit relative to the pull rod to form an elongated crystal from the charge material.

17. An apparatus as claimed in claim 15, further comprising:

a motor coupled to the pull rod, for rotating the pull rod.

18. An apparatus as claimed in claim 17, further comprising:

a motor controller coupled to the motor and situated externally to the pressure vessel, for controlling the rotation speed of the motor that is imparted to the pull rod.

19. An apparatus as claimed in claim 18, further comprising:

a worm gear box situated in the pressure vessel and coupled to the pull rod, for rotating the pull rod and moving the pull rod relative to the cooling unit; and

a motor coupled to the worm gear box, for driving the worm gear box to rotate and move the pull rod.

20. An apparatus as claimed in claim 19, further comprising:

a motor controller coupled to the motor and situated externally to the pressure vessel, for controlling the rotation speed imparted to the pull rod by the motor.

21. An apparatus as claimed in claim 1, further comprising:

a worm gear box situated in the pressure vessel and coupled to the pull rod, for rotating the pull rod and moving the pull rod relative to the cooling unit; and

a motor coupled to the worm gear box for driving the worm gear box to rotate and move the pull rod.

22. An apparatus as claimed in claim 21, further comprising:

a motor controller coupled to the motor and situated externally to the pressure vessel, for controlling the rotation speed imparted to the pull rod by the motor.

23. An apparatus as claimed in claim 1, further comprising:

at least one heat shield attached to an end of the pull rod, and sized to fit into an open top of the enclosure defined by the cooling unit.

24. An apparatus as claimed in claim 1, further comprising:

a window fixed to the pressure vessel, allowing light generated by the charge material to escape the pressure vessel.

25. An apparatus as claimed in claim 24, further comprising:

a reflector positioned in the pressure vessel, to direct light to the window.

26. An apparatus as claimed in claim 24, wherein the light is indicative of the temperature of the charge material.

34. An apparatus as claimed in claim 1, wherein the shield reflects at least some of the rf radiation away from the pressure vessel'"'"'s wall.

35. An apparatus as claimed in claim 1, wherein the shield reflects at least some of the rf radiation toward the charge material contained in the cooling unit.

36. An apparatus as claimed in claim 1, wherein the shield is electrically conductive.

37. An apparatus as claimed in claim 36, wherein the shield is electrically coupled to an end of the heating element.

38. An apparatus as claimed in claim 37, wherein one end of the heating element is electrically grounded.

39. An apparatus as claimed in claim 1, wherein the shield has a cylindrical wall that surrounds the heating element.

40. An apparatus as claimed in claim 1, wherein the shield is formed of metal.

41. An apparatus as claimed in claim 40, wherein the metal includes copper.

27. A system receiving a pressurized gas, the system comprising:

a pressure vessel for containing the pressurized gas;

a cooling unit situated in the pressure vessel, having cooled surfaces defining an enclosure with at least one opening;

a charge material positioned inside of the enclosure of the cooling unit and exposed through the opening of the enclosure to pressure exerted by the gas contained in the pressure vessel;

an induction heating element situated in the pressure vessel, for generating radio-frequency (rf) radiation to heat an interior portion of the charge material to form a melt zone that is contained by a relatively cool, exterior portion of the charge material that is closer relative to the melt zone, to the cooled surfaces of the cooling unit;

a shield situated in the pressure vessel in operation of the apparatus, and arranged between the induction heating element and a wall of the pressure vessel, the shield preventing the rf radiation generated by the induction heating element from significantly heating the pressure vessel'"'"'s wall;

a seed crystal; and

a pull rod situated in the pressure vessel and having an end that holds the seed crystal, for moving the seed crystal into contact with the melt zone of the charge material and for moving the seed crystal away from the charge material to grow a crystal from the melt zone of the charge material.

28. A system as claimed in claim 27, wherein the charge material and the seed crystal include at least one of GaN, AlN, SiC, YBa₂ Cu₃ O_x and AlGaN_x.

29. A system as claimed in claim 27, wherein the charge material and the seed crystal are composed of a substance that undergoes a peritectic reaction at atmospheric pressure.

30. A system as claimed in claim 27, wherein the charge material includes susceptor material to facilitate melting of the interior portion of the charge material.

31. A method comprising the steps of:

a) heating an interior portion of a charge material body with radio-frequency (rf) radiation to melt the interior portion;

b) during performance of the step (a), cooling an exterior portion of the charge material body so that the exterior portion is in a solid phase and contains the molten interior portion;

c) pressurizing the charge material body in a pressure vessel with a gas during performance of the steps (a) and (b);

d) shielding the pressure vessel from the rf radiation used to heat the interior portion in the step (a);

e) moving a seed crystal into contact with the molten interior portion of the charge material; and

f) moving the seed crystal away from the charge material body to pull an elongated crystal from the molten interior portion of the charge material body.

32. A method as claimed in claim 31, wherein the step (e) is performed at a rate of at least one millimeter per hour.

d) shielding the inductive heating element to focus the heating in the step (a) in the interior portion of the charge material body.

33. A method as claimed in claim 31, wherein the shielding in the step (d) is performed so as to focus the heating in the step (a) in the interior portion of the charge material body.

42. A method as claimed in claim 31, wherein said step (c) is performed above one-hundred atmospheres of pressure.

43. A method as claimed in claim 31, wherein said step (c) is performed above one-hundred-and-fifty atmospheres.

44. A method as claimed in claim 31, wherein the shielding of said step (d) is performed so as to direct at least some radiation toward the charge material body.

45. An apparatus for growing a crystal from a charge material with a seed crystal, the apparatus coupled to receive electric power and coolant, the apparatus comprising:

a pressure vessel for containing a pressurized gas;

a cooling unit for situation in the pressure vessel, having cooled surfaces defining an enclosure to receive the charge material;

an induction heating element for situation in the pressure vessel, for heating an interior portion of the charge material to form a melt zone that is contained by a relatively cool, exterior portion of the charge material that is closer relative to the melt zone, to the cooled surfaces of the cooling unit, the heating element defining a conduit having two opposite terminal ends through which the coolant passes;

a feedthrough extending through a wall of the pressure vessel, and having a first end situated inside of the pressure vessel, that is coupled to the heating element, and a second end opposite the first end, situated outside of the pressure vessel, the feedthrough having first and second conductors that are coaxial at least in a portion of the feedthrough that extends through the pressure vessel wall, the first and second conductors of the feedthrough coupled to respective terminal ends of the heating element to supply the electric power to the heating element and to circulate the coolant through the heating element; and

a pull rod situated in the pressure vessel, for holding the seed crystal in proximity to the charge material contained in the cooling unit.

46. An apparatus as claimed in claim 45, further comprising:

a fitting coupled to form a pressure-tight seal between the feedthrough and the wall of the pressure vessel.

47. An apparatus as claimed in claim 46, wherein the seal formed by the fitting can withstand a pressure above one-hundred atmospheres.

48. An apparatus as claimed in claim 46, wherein the seal formed by the fitting can withstand a pressure above one-hundred-and-fifty atmospheres.

49. An apparatus as claimed in claim 45, further comprising:

a coolant supply unit coupled to circulate the coolant through the inductive heating element.

50. An apparatus as claimed in claim 45, further comprising:

an electrical insulator surrounding and contacting the first conductor, to provide electrical insulation between the first and second conductors, an outer wall of the electrical insulator and the inner wall of the second conductor defining the channel between the outer wall of the first conductor and the inner wall of the second conductor.