Method for producing a sapphire crystal

Abstract

The invention relates to a method for producing a sapphire crystal by growing from a melt, wherein a disc-shaped seed crystal is arranged on a base of a crucible and a starting material formed from aluminium oxide is filled into the crucible on top of the seed crystal. This starting material is heated until the melt forms and then a recrystallisation of the starting material is performed on the seed crystal by cooling of the melt. A plurality of tiles are joined together here in a mosaic-like manner and the seed crystal is formed by these joined-together tiles.

Description

[0001] METHOD FOR PRODUCING A SAPPHIRE CRYSTAL

[0002] The invention relates to a method for producing a sapphire crystal by growing it from a melt.

[0003] Synthetically produced single-crystal materials have a wide range of technical applications. Depending on the type of material, different methods are suitable for producing the corresponding single crystals. For this purpose, the usually polycrystalline starting or raw material (powder or granules) must be recrystallized. The production processes can be differentiated according to the phase transitions that lead to the single crystal. These can be growth from the melt, from the solution, or from the gas phase. A so-called seed crystal or nucleus crystal forms the basis for the deposition of further layers of lattice planes formed by atoms of the crystal lattice. When growing a single crystal from a melt, the starting material must be heated above its melting temperature in a crucible of a suitable furnace and thus liquefied.For the melt to crystallize at the seed crystal, the temperature must drop slowly enough below the melting point. To achieve this, the seed crystal is kept slightly below the melting temperature by cooling.

[0004] Sapphire single crystals are particularly important for technical and industrial applications. These can be produced synthetically from molten aluminum oxide (Al2O3). Sapphire is acid-resistant and, due to its high scratch resistance, is used, for example, as sapphire glass for watch lenses or as scratch-resistant smartphone displays.

[0005] The object of the present invention was to create a process for producing crystals with improved properties.

[0006] This object is achieved by a method according to the claims.

[0007] In the inventive method for producing a sapphire crystal by growing it from a melt, a disk-shaped seed crystal is arranged at the bottom of a crucible, and a starting material made of aluminum oxide is poured into the crucible above it. The starting material comprises aluminum oxide in lumpy, granular, or powdered form. The starting material is heated until the melt forms, and then, by cooling the melt, the starting material is recrystallized on the seed crystal. A plurality of tiles are assembled in a mosaic-like manner, with these assembled tiles forming the seed crystal. This achieves the advantage of forming a crystal with greater fracture strength. Furthermore, crystals with larger diameters can be produced.

[0008] Another advantage is the refinement of the process, in which the tiles are produced with a uniform external shape, and the arrangement of the plurality of tiles forms a two-dimensional, macroscopic crystal structure. This enables the targeted creation of an axially symmetric distribution of crystal dislocations in the crystal produced by the process.

[0009] It is also advantageous if the outer shape of the tiles is in the form of symmetrical hexagons.

[0010] In an alternative procedure, it can also be provided that an outer shape of the tiles has the shape of equilateral triangles.

[0011] In another alternative, the outer shape of the tiles is provided in the form of squares that are equal to one another.

[0012] It is also advantageous if the single crystals of the individual tiles are spatially aligned in the same way.

[0013] The procedure in which the crystallographic c-axis of the crystal lattice of the seed crystal is aligned parallel with respect to a surface normal of a top surface of the seed crystal has the advantage that preferred optical properties can be more easily achieved in the subsequent production of wafers from the synthetic crystal.

[0014] For a better understanding of the invention, it is explained in more detail using the following figures.

[0015] They show in a highly simplified, schematic representation:

[0016] Fig. 1 shows an apparatus for producing a single crystal by growth from the melt; Fig. 2 shows a cross-section of the apparatus or crucible according to Fig. 1;

[0017] Fig. 3 shows a cross section of the crucible according to Fig. 1, according to a second embodiment;

[0018] Fig. 4 shows a cross section of the crucible according to Fig. 1, according to a third embodiment.

[0019] By way of introduction, it should be noted that in the variously described embodiments, identical parts are provided with identical reference symbols or component designations. The disclosures contained throughout the description can be applied analogously to identical parts with identical reference symbols or component designations. Furthermore, the positional information chosen in the description, such as top, bottom, side, etc., refers to the directly described and illustrated figure, and these positional information must be applied analogously to the new position in the event of a change in position.

[0020] Figure 1 shows a first embodiment of an apparatus 1 for producing a single crystal by growth from a melt, shown in section. The apparatus 1 comprises a crucible 2, the outer circumference of which is surrounded by a heating device 3—illustrated only schematically here. The crucible 2 is approximately pot-shaped with a base 4.

[0021] To carry out a single crystal manufacturing process, the crucible 2, filled with a corresponding starting material 5, is placed in a furnace (not shown here). This furnace is specially equipped for use at particularly high temperatures. It features thermal insulation to prevent heat or energy loss, as well as shielding against atmospheric influences such as the ingress of atmospheric oxygen. Alternatively, portions of the furnace's interior volume can be filled with protective gas or evacuated by vacuum pumps. The starting material 5 can have a lumpy, granular, or even powdery structure. Larger pieces can also be used to achieve a better filling density in the crucible 2.

[0022] Before filling the crucible 2 with the starting material 5, a seed crystal or seed crystal 6 is arranged on the bottom 4 of the crucible 2. A monocrystalline, thin slice of the crystalline material to be produced is used as the seed crystal 6. The seed crystal 6 preferably extends over almost the entire inner diameter 7 of the crucible 2. Following the attachment of the seed crystal 6 to the bottom 4 of the crucible 2, the remaining volume of the crucible 2 is filled with the starting material 5. The crucible 2 can finally be closed with a crucible lid 8. Suitable materials for the production of the crucible 2 and the crucible lid 8 include materials from the group of iridium, tungsten, or molybdenum.

[0023] Once inserted into the furnace, the crucible 2 containing the starting material 5 is heated, transforming the initially solid starting material 5 into a melt 9. In the boundary region between the seed crystal 6 and the melt 9, partial melting of the surface of the seed crystal 6 may occur. By keeping the bottom 4 at a slightly lower temperature than the melting temperature of the starting material 5, a temperature gradient forms in the region of the bottom 4 of the crucible 2, which leads to heat being removed from the melt 9. This simultaneously causes atoms from the melt 9 to attach to the seed crystal 6, thus leading to progressive crystal growth. This continues until the entire amount of the starting material 5 or the entire volume of the melt 9 has been transformed or recrystallized into a single crystal of the material.

[0024] In the case of the production of sapphire single crystals, aluminum oxide of varying consistencies is used as starting material 5. This can be polycrystalline lumps, granular material, or powder of varying grain sizes. Aluminum oxide (Al2O3) requires heating to over 2050°C to liquefy it into the melt 9. Previously artificially produced sapphire crystals, specifically in the form of a so-called "waver" (thin disc), are preferably used as the seed crystal 6. The seed crystal 6 for the production of sapphire single crystals preferably has a thickness 10 with a value within a range of 0.3 mm to 1 mm. Furthermore, the seed crystal 6 is preferably manufactured such that its crystal lattice is aligned in a preferred relative position with respect to the macroscopic surfaces of the seed crystal 6.The crystallographic c-axis of the crystal lattice of the seed crystal 6 is preferably aligned parallel with respect to a surface normal 11 of an upper side 12 of the seed crystal 6. Fig. 2 shows a cross-section of the device 1 or of the crucible 2 according to Fig. 1. The illustration in Fig. 2 corresponds to a plan view from above of the crucible 2 in the state not yet filled with the starting material 5, wherein the seed crystal 6 is already arranged on the bottom 4 of the crucible 2. According to this exemplary embodiment, the seed crystal 6 comprises a mosaic-like arrangement of tiles 13. The tiles 13 of the seed crystal 6 have, in particular, an identical external shape in the form of symmetrical hexagons. Side edges 14 of the tiles 13 are joined closely together, so that the arrangement of the tiles 13 forms a hexagonal pattern.An arrangement of tiles 13 is to be understood as a flat arrangement of thin slices with the same thickness as the thickness 10 of the seed crystal 6. The seed crystal 6 consisting of the tiles 13 is manufactured in such a way that further tiles 13 are joined together to form the pattern around a first tile 13 arranged centrally around the central axis 15 in a radial direction, progressing from the inside to the outside. This is done in particular in such a way that the mutually facing side edges 14 of immediately adjacent tiles 13 touch one another, i.e., that no joints remain between them. The upper sides of the tiles 13 therefore together form a seamless, connected surface, i.e., the surface of the upper side 12 of the seed crystal 6.

[0025] When producing the seed crystal 6 from an arrangement of multiple tiles 13, it is preferably provided that the single crystals of the individual tiles 13 are spatially aligned in the same way. This means that lattice planes of the same type in a first tile 13 and an adjacent second tile 13 are spatially aligned in the same way.

[0026] The production of a sapphire single crystal using such a seed crystal 6 formed from a plurality of tiles 13 enables a targeted influencing of crystal growth. The tiles 13, formed as symmetrical hexagons, induce a hexagonal pattern of crystal dislocations in the forming single crystal. Overall, this results in an axially symmetric distribution of crystal dislocations with a hexagonal pattern with respect to a central axis 15 of the seed crystal 6.

[0027] The sapphire crystal ultimately manufactured in this way exhibits a higher fracture strength than the fracture strength of sapphire single crystals with fewer crystal dislocations. Figure 3 shows a further, possibly independent embodiment of the device 1, wherein the same reference numerals or component designations are used for identical parts as in the preceding Figures 1 and 2. To avoid unnecessary repetition, reference is made to the detailed description in the preceding Figures 1 and 2.

[0028] A further exemplary embodiment of the device 1 and the method for producing a single crystal by growing it from a melt is described with reference to Fig. 3. Fig. 3 shows a cross-section of the crucible 2 of the device 1 according to Fig. 1, not yet filled with the starting material 5. The seed crystal 6 is already arranged on the bottom 4 of the crucible 2. According to this exemplary embodiment, the seed crystal 6 is composed of triangular tiles 16. The tiles 16 of this seed crystal 6 have an external shape in the form of isosceles triangles. For this purpose, further tiles 16 are added radially around a first tile 16 arranged centrally around the central axis 15.

[0029] The triangular tiles 16 are cut from previously artificially produced single crystals of the corresponding material. The tiles 16 are manufactured in a size such that the side edges 17 have a length 18 ranging from 15 mm to 35 mm.

[0030] For the production of the seed crystal 6, tiles 13, 16 with the same external shape and dimensions are preferably used. By arranging the plurality of tiles 13, 16, the formation of a two-dimensional crystal structure can be achieved. This means that the tiles 13, 16 are combined to form the seed crystal 6 with a periodicity corresponding to a macroscopic crystal structure.

[0031] Fig. 4 shows a cross-section of the crucible 2 of the device 1 according to Fig. 1 with an alternative embodiment of the seed crystal 6. The representation in Fig. 4 again corresponds to a plan view from above of the crucible 2 in the state not yet filled with the starting material 5. The seed crystal 6 at the bottom 4 of the crucible 2 in this embodiment is composed of tiles 19 with an outer contour in the shape of a square. Further square tiles 19 are arranged around a first tile 19 arranged axially symmetrically about the central axis 15. The arrangement of the tiles 19 of the seed crystal 6 forms a two-dimensional macroscopic crystal system. Using the described method, single crystals of the desired material can be produced into which a distribution of crystal dislocations is imprinted to a predetermined extent.In addition to the resulting changes in the physical and chemical properties of the crystals produced in this way, the process according to the invention also has the advantage that crystals with larger diameters 7 can be produced.

[0032] The embodiments show possible embodiments, whereby it should be noted at this point that the invention is not limited to the specifically illustrated embodiments thereof, but rather various combinations of the individual embodiments with each other are also possible and this possibility of variation lies within the skill of the person skilled in the art in this technical field due to the teaching of technical action by means of the objective invention.

[0033] The scope of protection is determined by the claims. However, the description and drawings must be used to interpret the claims. Individual features or combinations of features from the various embodiments shown and described may represent independent inventive solutions. The problem underlying these independent inventive solutions can be derived from the description.

[0034] All information on value ranges in this description is to be understood as including any and all sub-ranges thereof, e.g. the information 1 to 10 is to be understood as including all sub-ranges starting from the lower limit of 1 and the upper limit of 10, ie all sub-ranges begin with a lower limit of 1 or greater and end with an upper limit of 10 or less, e.g. 1 to 1.7, or 3.2 to 8.1, or 5.5 to 10.

[0035] For the sake of clarity, it should be noted that some elements have been shown not to scale and / or enlarged and / or reduced in size to improve understanding of the structure.

[0036] device

[0037] crucible

[0038] Heating device

[0039] Floor

[0040] Output material

[0041] Seed crystal

[0042] diameter

[0043] Crucible lid

[0044] melt

[0045] thickness

[0046] Surface normal

[0047] Top

[0048] tile

[0049] side edge

[0050] central axis

[0051] tile

[0052] side edge

[0053] length

[0054] tile

Claims

Patent claims 1. A method for producing a sapphire crystal by growing it from a melt (9), wherein a disk-shaped seed crystal (6) is arranged on a bottom (4) of a crucible (2) and a starting material (5) made of aluminum oxide is filled into the crucible (2) above it, wherein the starting material (5) comprises aluminum oxide in lumpy, granular or powdery form, and the starting material (5) is heated until the melt (9) is formed and then a recrystallization of the starting material (5) on the seed crystal (6) is carried out by cooling the melt (9), characterized in that a plurality of tiles (13) are joined together in a mosaic manner, the seed crystal (6) being formed by the joined tiles (13).

2. Method according to one of the preceding claims, characterized in that the tiles (13) are produced with a same external shape and the arrangement of the plurality of tiles (13) forms a two-dimensional, macroscopic crystal structure.

3. Method according to one of the preceding claims, characterized in that an outer shape of the tiles (13) has the shape of symmetrical hexagons.

4. Method according to one of the preceding claims, characterized in that an outer shape of the tiles (13) has the shape of equilateral triangles.

5. Method according to one of the preceding claims, characterized in that an outer shape of the tiles (13) has a square shape.

6. Method according to one of the preceding claims, characterized in that the single crystals of the individual tiles (13) are spatially aligned in the same way.

7. Method according to one of the preceding claims, characterized in that the crystallographic c-axis of the crystal lattice of the seed crystal (6) is aligned parallel with respect to a surface normal (11) of an upper side (12) of the seed crystal (6).

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