LASER-BASED AFTERHEATING FOR CRYSTAL GROWTH

Abstract

A crystal-growth apparatus (10, 10’,10”) and a crystal-growth method for growing a crystal (21) from a molten feed material (23) are presented, where in addition to a molten-zone heater, at least one afterheater laser (5) is arranged to heat an extended afterheater zone (50), the afterheater zone (50) at least partly overlapping a solidification zone (210) adjacent to the molten zone (230). The crystal-growth apparatus (10, 10’,10”) and the crystal-growth method may be used for thermal treatment to reduce crack formation or thermal stress in grown crystals (21).

Claims

1. A crystal-growth apparatus (10, 10’,10”) for growing a crystal (21) from a molten feed material (23), comprising a molten-zone heater to melt the feed material in a molten zone (230); at least one afterheater laser (5) arranged to emit an afterheater laser beam (51) to heat an extended afterheater zone (50), the afterheater zone (50) at least partly overlapping a solidification zone (210) adjacent to the molten zone (230).

2. The crystal-growth apparatus (10, 10’,10”) according to claim 1, further comprising irradiation-area adjustment means to adjust the irradiation area of the afterheater laser beam (51).

3. The crystal-growth apparatus (10, 10’,10”) according to claim 2, wherein the irradiation-area adjustment means comprise at least one adjustable defocusing means.

4. The crystal-growth apparatus (10, 10’,10”) according to claim 2, wherein the irradiation-area adjustment means comprise at least one movable lens.

5. The crystal-growth apparatus (10, 10’,10”) according to claim 1, wherein the at least one afterheater laser is a diode laser (5) with or without adjustable output power.

6. The crystal-growth apparatus (10″) according to claim 1, wherein the crystal-growth apparatus (10″) comprises an odd number N of afterheater lasers (5) with N > 1, the afterheater lasers (5) circumferentially surrounding the afterheater zone (50).

7. The crystal-growth apparatus (10′) according to claim 1, wherein the crystal-growth apparatus (10′) comprises several afterheater lasers (5) arranged to have variable and/or superimposable irradiation areas and/or arranged to regulate the temperature profile (4′) of the afterheater zone (50).

8. The crystal-growth apparatus (10, 10′, 10″) according to claim 1, wherein the at least one afterheater laser (5) is arranged to heat an afterheater zone (50) which is directly adjacent to the molten zone (230) and/or at least partly overlaps the molten zone (230).

9. The crystal-growth apparatus (10, 10,10”) according to claim 1, wherein the afterheater laser (5) is arranged to heat an afterheater zone (50) overlapping at least partly with the solidification zone (210) and the zone of the feed material (220) which is adjacent to the molten zone (230).

10. A crystal-growth method for growing a crystal (21) from a molten feed material (23), wherein in addition to heating the molten zone (230), an extended afterheater zone (50) which partly overlaps a solidification zone (210) adjacent to the molten zone (230), is heated by at least one afterheater laser beam (51) emitted by at least one afterheater laser (5).

11. The crystal-growth method of claim 10, wherein the irradiation area of the afterheater laser beam (51) emitted by the afterheater laser (5) is adjustable by irradiation-area adjustment means.

12. The crystal-growth method according to claim 10, wherein the temperature profile (4′) of the afterheater zone (50) is adjustable.

13. The crystal-growth method according to claim 10, wherein the afterheater zone (50) is directly adjacent to the molten zone (230) or at least partly overlaps the molten zone (230).

14. The crystal-growth method according to claim 10, wherein the afterheater zone (50) at least partly overlaps the solidification zone (210) and the zone of the feed material (220) which is adjacent to the molten zone (230).

15. A zone-melting-type or a Czochralski-type or a Bridgman-type apparatus comprising the crystal-growth apparatus according to claim 1.

16. A method for thermal treatment to reduce crack formation or thermal stress in crystals (21) grown from a molten feed material (23) and/or for facilitating the melting process of the feed material (23), said method comprising performing the method according to claim 10.

17. The crystal-growth apparatus (10, 10’,10”) according to claim 3, wherein the irradiation-area adjustment means comprise at least one movable lens.

18. The crystal-growth apparatus (10, 10’,10”) according to claim 17, wherein: the at least one afterheater laser is a diode laser (5) with or without adjustable output power.

19. The crystal-growth apparatus (10″) according to claim 18, wherein the crystal-growth apparatus (10″) comprises an odd number N of afterheater lasers (5) with N > 1, the afterheater lasers (5) circumferentially surrounding the afterheater zone (50).

20. The crystal-growth apparatus (10′) according to claim 19, wherein the crystal-growth apparatus (10′) comprises several afterheater lasers (5) arranged to have variable and/or superimposable irradiation areas and/or arranged to regulate the temperature profile (4′) of the afterheater zone (50).

Description

[0052] Implementations of the invention will be described, by way of example only, with reference to accompanying drawings in which:

[0053] FIG. 1 schematically shows a crystal-growth apparatus according to prior art and the associated temperature profile across the specimen along the direction of motion of the molten zone;

[0054] FIG. 2 schematically shows a crystal-growth apparatus according to the invention and the associated temperature profile across the specimen along the direction of motion of the molten zone;

[0055] FIG. 3 schematically shows a crystal-growth apparatus according to the invention where the afterheater zone partly overlaps the zone of the feed material and the solidification zone as well as the molten zone;

[0056] FIG. 4 schematically shows a crystal-growth apparatus according to the invention where several afterheater lasers are successively arranged in the direction of motion of the molten zone;

[0057] FIG. 5 schematically shows a crystal-growth apparatus according to the invention where several afterheater lasers are arranged circumferentially surrounding the specimen.

[0058] FIG. 1 shows a crystal-growth apparatus, w.l.o.g. a floating-zone (FZ) apparatus 1, acting on a specimen 2, according to prior art without the use of an afterheating system. The specimen 2 comprises a crystalline part 21 in the solidification zone 210. The crystalline material 21 is grown by melting a polycrystalline-material feed rod 22 located in the feed-material zone 220 and moving the melt pool 23 in the narrow molten zone 230 along the rod 22 by moving the specimen 2 in the direction of motion depicted by arrows 24. The molten zone 230 is heated by thermal radiation to a temperature slightly above the melting temperature of the feed material T.sub.melt. The thermal radiation may be focused laser light 3 emitted by e.g. at least one CO.sub.2 or YAG laser, or focused light of at least one polychromatic arc lamp or halogen lamp (not shown for the sake of clarity). The temperature profile 4 that develops over the length of the specimen 2, the length being the dimension of the specimen 2 in the direction of motion 24 of the specimen 2, exhibits a narrow peak 41 in the molten zone 230 with large temperature gradients in the feed-material zone 220 and the solidification zone 210. Especially this steep decrease of temperature in the solidification zone 210 may lead to pronounced thermal stress in the single-crystalline material 21.

[0059] FIG. 2 shows a crystal-growth apparatus, w.l.o.g. a floating-zone (FZ) apparatus 10, according to the invention. In addition to the molten-zone heater (not shown), the FZ apparatus 10 comprises at least one diode laser 5 heating an afterheater zone 50, which partly overlaps the solidification zone 210. Adjustable defocusing means (not shown) act on the diode laser 5, which thus emits defocused laser light 51, in contrast to the focused laser light 3 emitted by the molten-zone heater. The temperature profile 4′ over the length of the specimen 2 exhibits a narrow peak 41′ in the molten zone 230, similar to that shown in FIG. 1. In contrast to the temperature profile 4 of FIG. 1, the temperature profile 4′ is characterised by a shoulder 42 in the feed-material zone 220 and a pronounced shoulder 43 in the solidification zone 210, which occur because of the additional heat input by the diode laser 5 into the afterheater zone 50. While the afterheater zone 50, comprising at least a part of the solidification zone 210, is directly irradiated by the diode laser 5, the shoulder 42 in the feed-material zone 220 occurs mainly due to conduction of heat from the solidification zone 210 to the feed-material zone 220. The shoulder 43 results in a significantly reduced temperature gradient in the solidification zone 210, thus reducing thermal stress and the proneness to the formation of cracks in the single-crystalline material 21.

[0060] FIG. 3 shows an FZ apparatus 10 similar to that of FIG. 2, wherein the defocusing means (not shown) acting on the diode laser 5 are adjusted such that the irradiation area of the defocused laser light 51, and thus the afterheater zone 50, is larger than in FIG. 2. The afterheater zone 50 not only overlaps with at least a part of the solidification zone 210, but also the molten zone 230 and at least a part of the feed-material zone 220.

[0061] The focused laser light acting on the melt pool 23 has been omitted for the sake of clarity.

[0062] FIG. 4 shows an FZ apparatus 10′ similar to that of FIG. 2 with three diode lasers 5. The diode lasers 5 are positioned and/or their respective defocusing means (not shown) are adjusted such that their irradiation areas overlap, which ensures that the temperature gradient remains small across the entire length of the afterheater zone 50. The afterheater zone 50 may extend over the whole solidification zone 210, the molten zone 230 and the whole feed-material zone 220. The apparatus 10′ allows for controlled cooling of the crystalline material 21 in the solidification zone 210, i.e. the temperature profile across the length of the specimen 2 can be adjusted to largely reduce thermal stress.

[0063] The focused laser light acting on the melt pool 23 has been omitted for the sake of clarity.

[0064] FIG. 5 shows the bottom view of an FZ apparatus 10″ with five diode lasers 5 arranged around the circumference of the specimen 2, which ensures a uniform temperature distribution in relation to the cross-section of the specimen 2. The diode lasers 5 are positioned outside a process chamber 6, which may provide an environment as needed for growing certain types of crystals, e.g. a high partial pressure of oxygen. The arrangement of an odd number of diode lasers 5 enables the use of beam traps 52 to absorb the defocused laser light 51 transmitted through the specimen 2, wherein one beam trap 52 is assigned to each diode laser 5.

TABLE-US-00001 List of reference signs 1 FZ apparatus according to prior art 10, 10’,10” FZ apparatus according to the invention 2 specimen 21 Crystalline material 210 Solidification zone 22 Polycrystalline material feed rod 220 Feed-material zone 23 Melt pool 230 Molten zone 24 Direction of motion of the specimen 3 Focused laser light 4, 4′ Temperature profile 41, 41′ Temperature peak 42 Shoulder in temperature profile 43 Shoulder in temperature profile 5 Diode laser 50 Afterheater zone 51 Defocused laser light 52 Beam trap 6 Process chamber