CRUCIBLE FOR GROWING CRYSTALS

Abstract

Crucible for growing single crystals, formed from W, Mo, Re, an alloy or a base alloy of these metals, and a process for producing a crucible (2), wherein at least part of an outwardly facing outer face (4) of the crucible (2) has, at least in certain regions, a profile with a mean profile depth (a) of between 5 and 500 m.

Claims

1. Crucible for growing crystals, in particular for growing single crystals, formed from W, Mo, Re, an alloy or a base alloy of these metals, wherein an outer face of the crucible has, at least in certain regions, a profile with a mean profile depth between 5 and 500 m.

2. Crucible according to claim 1, wherein the profile has a mean profile depth (a) of between 10 and 300 m.

3. Crucible according to claim 1, wherein the profile has a recess or a multiplicity of recesses, which are arranged spaced uniformly apart at least in certain regions over the outer face of the crucible.

4. Crucible according to claim 1, wherein the profile is in the form of a recess circulating around the crucible (2), in particular a score or groove, or a multiplicity of recesses circulating around the crucible (2).

5. Crucible according to claim 1, wherein the profile has a recess or a multiplicity of recesses with a part-circular, trapezoidal, wedge-shaped, conical and/or rectangular cross section.

6. Crucible according to claim 1, wherein the profile, at least in certain regions, has a recess or a multiplicity of recesses with a part-circular cross section with a radius of between 0.2 and 10 mm.

7. Crucible according to claim 1, wherein the profile has a recess or a multiplicity of recesses with, at least in certain regions, a part-circular cross section with a radius of between 0.8 and 6 mm.

8. Crucible according to claim 1, wherein the profile has a multiplicity of recesses and the mean spacing between adjacent recesses in the axial direction of the crucible is, at least in certain regions, between 0.2 and 10 mm.

9. Crucible according to claim 1, wherein the profile has a multiplicity of recesses and the spacing between adjacent recesses in the axial direction of the crucible is, at least in certain regions, between 0.8 and 6 mm.

10. Crucible according to claim 1, wherein the outer faces of the crucible which are exposed during the production of a single crystal have the profile, in particular the side walls of the crucible.

11. Crucible according to claim 1, wherein an inner face of the crucible facing toward an internal volume has, at least in certain regions, a mean roughness value Ra of between 0.1 and 1.6 m, in particular is ground axially and/or polished axially.

12. Process for producing a crucible for growing crystals, in particular a crucible according to claim 1, said process comprising the following steps: providing a pressed main crucible body, a pressed and sintered main crucible body, a pressed, sintered and deformed main crucible body or a main crucible body produced by way of a coating process, including machining or coating an outer face of the main crucible body, such that at least part of the outer face has a profile with, at least in certain regions, a mean profile depth of between 5 and 500 m.

13. Process according to claim 12, further comprising the following step: machining an inner face of the crucible facing toward an internal volume, such that the inner face has, at least in certain regions, a mean roughness value Ra of between 0.1 and 1.6 m.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0020] Embodiments of the invention will be explained in more detail with reference to the figures.

[0021] FIG. 1 shows a schematic sectional view, not true to scale, of a crucible during the production of a single crystal.

[0022] FIGS. 2a-2b show schematic illustrations, not true to scale, of an outer face and inner face of the crucible shown in FIG. 1.

[0023] FIG. 3 shows the result of a contour measurement.

DETAILED DESCRIPTION

[0024] FIG. 1 shows a schematic sectional view, not true to scale, of a crucible 2 during the production of a single crystal. The crucible 2 is produced from W, Mo, Re or an alloy of these materials, in order to withstand the high temperatures during the production of a single crystal, such as e.g. a sapphire single crystal.

[0025] The schematically illustrated crucible 2 is designed so as to be rotationally symmetrical about its axis A, e.g. cylindrical or substantially cylindrical. The crucible 2 can have a conical form, in order to facilitate the removal of a single crystal 8 produced therein. The outer dimensions of the crucible 2 can be adapted to the desired size of the single crystal to be produced. By way of example, 2 sapphire single crystals with a weight of 30 kg, 60 kg, 90 kg, 120 kg or more can be produced with an appropriate crucible. By way of example, a crucible 2 can have a diameter of 500 mm and a height of approximately 600 mm.

[0026] The schematically illustrated side wall heating systems 10, 10 and bottom heating system 10 are intended to illustrate the heating of the crucible 2 by means of thermal radiation. A seed crystal 12, on the basis of which the single crystal growth is effected, is illustrated in sketched form above the crucible 2. The seed crystal 12 is held in a seed crystal holder 14 and, for producing the single crystal, is pulled slowly from a molten mass (Al.sub.2O.sub.3 in the case of sapphire single crystals) in the crucible 2. Shown adjoining the seed crystal 12 is a single crystal 8, which has already been pulled from the molten mass in the lower region of the crucible 2. As is shown schematically here, it is possible to use, for example, the Nacken-Kyropoulus process or the Czochralski process, in which a seed crystal 12 is dipped from above into the molten mass. Alternatively (not shown), a seed crystal can be placed in the bottom region of the crucible 2 and countercooled in a controlled manner, in order to achieve slow solidification from the molten mass.

[0027] The outer face 4 or side faces of the crucible 2 have a profile, which is shown on an enlarged scale and by way of example in FIG. 2a. The profile or the surface structure has a mean profile depth a of between 5 and 500 m, 10 and 300 m, 15 and 150 m, 20 and 100 m or 30 and 80 m. The profile depth is measured here using a contour measuring appliance, for example a Mitutoyo Formtracer SV-C3200. The reference points for a recess are formed here by two elevations and a recess enclosed thereby. To determine the mean profile depth a, an average is formed at least over 5 measurement results. As is shown in FIG. 2a, the profile can be formed from a multiplicity of recesses which are arranged alongside one another and have the aforementioned profile depth. If at least 5 recesses are arranged alongside one another, 5 recesses which follow one another directly are used for determining the mean profile depth a. The result of an exemplary contour measurement is shown in FIG. 3. Here, the mean value of at least 5 elevations which follow one another directly is calculated, and the mean profile depth is thus determined.

[0028] The profile or the structure of the outer face 4 can be produced easily by means of turning or milling, for example. A profile with a thread-like progression can be produced easily and quickly in a turning operation using an appropriately shaped tool, with an appropriately set cutting depth and an appropriately set advance (millimetres per revolution). By way of example, the recesses have a conical, wedge-shaped, trapezoidal, part-circular or rectangular cross section, it being possible for the cross-sectional shape to be established easily, for example, via the selection of the appropriate tool or the cutting edge shape of the tool. According to one example, the thread-like profile has a recess with a part-circular cross section with a mean radius of between 0.2 and 10 mm, 0.6 and 5 mm or 0.8 and 2 mm. To determine the mean radius, an average is again formed over at least 5 measurement results.

[0029] The mean spacing between adjacent recesses in the axial direction of the crucible 2 can be between 0.2 and 10 mm, 0.5 and 8 mm, 0.6 and 5 mm, 0.7 and 2 mm or 0.8 and 1.5 mm. During the production, an advance of 0.2 to 10 mm per revolution, 0.5 to 8 mm per revolution, 0.6 to 5 mm per revolution or 0.7 to 2 mm per revolution is set. To determine the mean spacing, too, at least 5 measurement results are used in turn.

[0030] Suitable materials for machining the extremely hard and brittle crucible material are, for example, tools with cutting edges made from polycrystalline diamond (PCD) or cubic crystalline boron nitride (CBN).

[0031] The inner face 6 of the crucible 2, in contrast to the outer face 4, has a very smooth form, such that the inner face 6 has, at least in certain regions, a (radial and axial) mean roughness value Ra of between 0.1 and 1.6 m, 0.1 and 1 m or 0.2 and 0.3 m. By way of example, the inner face 6 is ground axially. In addition, the inner face 6 can be polished in the axial direction of the crucible 2 in order to produce a particularly smooth surface.

[0032] Owing to the profile, the outer face of the crucible 2 hascompared to a smooth facea high emissivity and degree of absorption. The emitted or absorbed thermal radiation of the rough outer face 4 compared to the smooth inner face 6 of the crucible is shown in qualitative terms by means of arrows in FIGS. 2a-b.

[0033] Owing to the high emissivity/degree of absorption of the outer face 4, if the heating power is reduced, heat is emitted quicker by the crucible 2, and if the heating power is increased, the heat generated is absorbed quicker by the crucible 2. Owing to the profiled or rough surface, the crucible 2 therefore reacts quicker to changes in temperature or changes in power of the heating system 10, 10, such that the temperature and the temperature gradient of the single crystal 8 in the crucible 2 can be precisely regulated. In this way, it is possible to achieve stable, repeatable growth results or a constantly good quality of the single crystals 8 produced using the crucible 2.

[0034] Compared to the rough outer face 4, the very smooth inner face 6 has only a low emissivity and degree of absorption. Therefore, only little heat is irradiated onto the single crystal 8 via the inner face 6 in the upper region of the crucible 2, where a single crystal 8 has already been grown and which is not in contact with the inner face 4 of the crucible 2. In the lower region of the crucible 2, where the molten mass is in contact with the inner face 6 or the crucible wall, heat is efficiently transmitted from the crucible 2 onto the molten mass by means of heat conduction. As a result, it is possible to efficiently control the temperature gradient in the single crystal 8 produced. This is particularly advantageous for the production of a single crystal by means of the Nacken-Kyropoulus process, in which the temperature or the temperature gradient of the single crystal and of the molten mass has to be precisely controlled.

LIST OF REFERENCE SIGNS

[0035] 2 Crucible [0036] 4 Outer face [0037] 6 Inner face [0038] 8 Single crystal/ingot [0039] 10, 10, 10 Heating system [0040] 12 Seed crystal [0041] 14 Seed crystal holder [0042] A Crucible axis [0043] a Profile depth [0044] b Spacing/advance