EP4271857A1

EP4271857A1 - Method for producing an artificial sapphire single crystal

Info

Publication number: EP4271857A1
Application number: EP21840775.7A
Authority: EP
Inventors: Ghassan Barbar; Robert Ebner; Jong Kwan Park; Gourav SEN
Original assignee: Fametec GmbH
Current assignee: Fametec GmbH
Priority date: 2020-12-29
Filing date: 2021-12-28
Publication date: 2023-11-08
Also published as: AT524603A1; AT524603B1; WO2022140808A1

Abstract

The invention relates to a method for producing an artificial sapphire single crystal. Starting with the step of crystal growth, a temperature difference ΔΤ, selected from a range of 1 °C to 60 °C, is set between an Al₂O₃ melt surface and a boundary surface of the growing sapphire single crystal to the Al₂O₃ melt, and this temperature difference ΔΤ is kept constant at least over a greater part of the duration of the crystal growth.

Description

PROCESS FOR PRODUCTION OF AN ARTIFICIAL SAPPHIRE SINGLE CRYSTAL

The invention relates to methods for producing an artificial single crystal of sapphire.

Methods for producing or growing single crystals are very energy-consuming and time-consuming processes, in which crystal growth of a single crystal can take several days or even weeks. To grow sapphire single crystals, methods are used, among other things, in which a sapphire single crystal is grown by slow crystallite growth or solidification from an Al ₂ O ₃ melt, usually on a seed or seed crystal provided is made in a crucible. Generic methods are, for example, the vertical temperature gradient method, also known as the Tammann-Stöber method, or the Bridgeman method, although some modifications of these methods have also become known in the past. As a rule, in the known methods, vertical crystal growth takes place in the crucible, although at least partial growth can also take place in the horizontal direction.

In order to form sapphire monocrystals that are as perfect as possible, very slow crystal growth is necessary in order to prevent the formation of defects such as multicrystallites or gas inclusions. In particular, this requires precise zonal temperature control in the crucible at high temperatures for a long period of time.

Some devices and methods for temperature control in crucibles during crystal growth have already been proposed in the past. For example, JP 2016033102 A proposes a specific temperature gradient in both directions along the liquid-solid phase boundary in the crucible during crystal growth in connection with a specific growth rate. Due to the dynamics of temperature regulation, many factors affect the growth process. In particular, the growth speed varies from 01 mm/h to 5 mm/h, which translates into several g/h to 450 g/h. Both the stability of the power grid and the consistency of the water cooling also influence the temperature gradient. The design of the heater and insulator is also of great influence. However, setting such a dedicated temperature gradient around the growing and therefore non-stationary liquid-solid phase boundary in the crucible is very complex in terms of control technology and also very error-prone, sometimes even hardly realizable.

There is still a need for improvement with regard to temperature management when growing a sapphire single crystal from an Al ₂ O ₃ melt in a crucible.

The object of the present invention was to overcome the disadvantages of the prior art that still exist and to provide a method by means of which temperature management in a crucible that is relatively easy to implement in terms of control technology is accomplished during the growth of a sapphire single crystal in a crucible and by means of which a sapphire monocrystal of high quality can be produced at the same time.

This object is solved by a method according to the claims.

The method for producing or for growing an artificial sapphire single crystal includes the steps

- Filling a crucible with a sapphire seed crystal or sapphire seed crystal and Al ₂ O ₃ - raw material,

- Melting of the Al ₂ O ₃ raw material to form an Al ₂ O ₃ melt and at least superficial melting of the sapphire seed crystal,

- Crystal growth at a boundary surface of the growing sapphire single crystal to the Al ₂ O ₃ - melt by generating a temperature gradient in the Al ₂ O ₃ melt in the crucible and crystallization of the melt at the boundary surface of the growing sapphire single crystal up to complete solidification of the Al ₂ O ₃ melt or the sapphire monocrystal in the crucible, - cooling and removal of the sapphire monocrystal from the crucible.

Here, starting with the step of crystal growth between an Al ₂ O ₃ melt surface and the boundary surface of the growing sapphire single crystal to the Al ₂ O ₃ melt, a temperature difference ΔT is selected from a range of 1°C to 60°C, esp - set between 1°-11°, preferably between 1°-9°, particularly preferably between 1-5°, and this temperature difference ΔT is kept constant at least over a predominant period of crystal growth. The boundary surface between the melt and the growing sapphire single crystal is in an equilibrium state of liquid and solid. A temperature gradient of z. B. 1°C in this interface creates a driving force so that the crystal solidifies in a certain direction (here upwards). Here, a desired temperature gradient is created in the melt and crystallization is initiated from a hotter side of the melt to a cooler, growing single crystal. A range of between 1 and 60 degrees is set as the temperature gradient in the melt. (Temperature gradients usually create convection in the melt, which transports the heat further to the interface. It has been found that a higher temperature gradient makes the melt unstable due to the creation of rapid convection, thereby disrupting crystal growth.

The sapphire single crystal naturally grows in the crucible in the crystal growth step, and it may preferably be vertical growth. Due to the specified process measures, due to the local displacement of the liquid-solid phase boundary, spatially or, for example, vertically across the crucible, variable temperature gradients can therefore also occur across the liquid-solid phase boundary or the interface .

By means of the specified measures, a temperature control that is relatively easy to implement in terms of control technology can be achieved in or via the crucible. At the same time, however, it has also been shown that sapphire monocrystals of very high quality can be grown by means of the appropriate temperature control. Preferably, starting with the crystal growth step between the Al ₂ O ₃ melt surface and the boundary surface of the growing sapphire single crystal for the Al ₂ O ₃ melt, a temperature difference ΔT selected from a range of 1° to 60° C., in particular between 1° to 15° C., preferably between 1° and 9°, particularly preferably between 1° and 5° C., and this temperature difference ΔT is kept constant at least over the majority of the crystal growth period. A temperature gradient of between 1° and 5° is preferably also set in the crystallization region between the melt and the single crystal and kept constant. The temperature control in the crucible can of course be automated by means of a corresponding control device. Such a control device can be designed, among other things, to regulate the output of heating and/or cooling elements of a furnace for accommodating the crucible. In addition or as an alternative, the control device can also be designed to adjust, in particular to adjust the height of, heating and/or cooling elements of the furnace, but also, for example, to adjust the crucible itself in the furnace or from a furnace chamber. Some exemplary embodiments for the temperature management or the temperature control are explained in more detail below using a figure. In principle, all known and suitable measures and devices can be used to control the temperature in the crucible, and reference is accordingly made to the relevant literature.

Due to the very slow crystallization, the temperature at the boundary surface of the growing sapphire single crystal can essentially correspond to the melting temperature of sapphire, and the same can therefore be assumed to be given. In principle, the position or the height of the Al ₂ O ₃ melt surface in the crucible and due to the known crucible geometry and filling quantity of the crucible with Al ₂ O ₃ raw material can also be used as known for control. Slow growth of the sapphire monocrystal can be carried out by cooling the Al ₂ O ₃ melt in zones in the crucible, for example vertically from bottom to top, as is usually the case.

In a preferred development of the method, however, it can also be provided that a distance ΔT between the Al ₂ O ₃ melt surface and the boundary surface of the growing sapphire single crystal is determined at least at one point in the crucible during the crystal growth.

This measure of distance measurement enables improved or more precise temperature control in the crucible during the growth of the sapphire single crystal. The position or height of the Al ₂ O ₃ melt surface in the crucible can be recorded, for example, mechanically or by means of optical or image-recording methods. The position or height of the boundary surface of the growing sapphire single crystal can, for example, be recorded mechanically at any point in the crucible using a measuring rod that can be lowered into the Al ₂ O ₃ melt, which measuring rod rests against the boundary surface of the growing sapphire single crystal . In a further embodiment variant, however, a position of the boundary surface of the growing sapphire monocrystal can also be detected by sensors at least at one point in the crucible by irradiating a beam of electromagnetic radiation or mechanical vibration into the crucible.

Contactless detection of the position of the boundary surface of the growing sapphire monocrystal can be advantageous here. The distance ΔT can be determined or calculated from the known and/or also measured position of the Al ₂ O ₃ melt surface.

For example, the beam can be introduced into the crucible from above. Depending on the type of beam, however, it can also be introduced, for example, through a crucible wall or the crucible floor if at least partial transparency is provided here. Furthermore, the beam can be coupled into the crucible, for example, by means of a line, in the case of light, for example, via an optical waveguide.

Subsequently, a change in a property of the beam after contact with the boundary surface of the growing sapphire monocrystal can be detected by means of suitable sensors, for example a weakening or deflection of the beam at a specific angle. Ultimately, the position or height of the boundary surface of the growing sapphire single crystal can be deduced from the measured change.

In one embodiment, it can be provided in particular that a laser beam is used as the beam from the electromagnetic radiation.

In particular, it can be provided that the temperature difference ΔT between the Al ₂ O ₃ melt surface and the boundary surface of the growing sapphire single crystal is set and kept constant on the basis of the determined distance ΔT.

As a result, a high-precision method can be provided or carried out with very precise, zone-wise control of the temperature in the crucible. This is because the position, in particular the height of the two surfaces, is not assumed or estimated, but rather the control can be carried out according to corresponding measured values.

Keeping the temperature difference between the Al ₂ O ₃ melt surface and the boundary surface of the growing sapphire single crystal constant over the majority of the time of the crystal growth step has proven to be advantageous. Specifically can the temperature difference ΔT between the Al ₂ O ₃ melt surface and the boundary surface of the growing sapphire single crystal is kept constant for 95% to 99.9% of the entire crystal growth period, beginning with the crystal growth process step. The temperature difference ΔT between the Al ₂ O ₃ melt surface and the boundary surface of the growing sapphire monocrystal can preferably be 98% to 99.9%, in particular 99% to 99.9%, of the entire period, starting with the process step of crystal growth of crystal growth are kept constant. Of course, towards the end of the crystal growth, the temperature can be lowered to or below the crystallization temperature in the entire crucible for complete crystallization of the Al ₂ O ₃ melt.

In one possible method implementation, it can additionally be provided that a temperature of the Al ₂ O ₃ melt surface is detected by sensors during the crystal growth.

The temperature control in the crucible can also be further improved or made more precise by this further measure. The temperature measurement at the Al ₂ O ₃ melt surface can be done, for example, using a pyrometer. However, it is also conceivable to measure the temperature using a high-temperature sensor, such as a suitable thermocouple, by bringing it up to or also slightly immersing it in the Al ₂ O ₃ melt.

As already mentioned above, a temperature at the boundary surface of the growing sapphire single crystal can inevitably correspond essentially or very precisely to the crystallization temperature of sapphire, since the liquid and solid phases are in equilibrium here. Due to the very slow crystallization, the crystallization temperature is largely the same as the melting point of sapphire. Therefore, the temperature at the interface surface of the growing sapphire single crystal can be taken as given. However, it is also possible to measure the temperature at the boundary surface during crystal growth and to use such measured values as a basis for the temperature control. A measurement can be carried out, for example, by bringing insulated or coated high-temperature sensors to the boundary surface or contacting the boundary surface with such sensors. For example, a hollow measuring rod can be used to detect the position or height of the boundary surface of the growing sapphire single crystal. Finally, it can be specifically provided in the method that, starting with the crystal growth step, a temperature of the Al ₂ O ₃ melt surface is set to a value selected from a range of 2040 °C to 2100 °C and over a predominant duration of the Crystal growth is kept constant. Starting with the crystal growth step, a temperature of the Al ₂ O ₃ -Schinclzc surface can preferably be adjusted to a value selected from a range from 2041° C. to 2056° C., in particular from 2041° C. to 2046° C., and over a predominant period of crystal growth are kept constant. Of course, for the growth of the sapphire single crystal, the temperature at each position and height of the boundary surface of the growing single crystal in the crucible must be controlled to about the crystallization temperature of sapphire or slightly lower, and Accordingly, a temperature control takes place, which takes into account the migration or the change in position of this boundary surface during crystal growth.

For a better understanding of the invention, it is explained in more detail with reference to the following figures.

They each show in a greatly simplified, schematic representation:

Fig. 1 a crucible filled with a sapphire seed crystal and molten Al ₂ O ₃ -

Raw material at the beginning of a crystal growth, in sectional view;

2 shows a crucible with growing sapphire monocrystal and Al ₂ O ₃ melt during crystal growth, in a sectional view;

3 shows a crucible with a completely hardened sapphire single crystal at the end of the crystal growth, in a sectional view;

4 shows an exemplary embodiment of a furnace chamber with a filled crucible, in a sectional view.

As an introduction, it should be noted that in the differently described embodiments, the same parts are provided with the same reference numbers or the same component designations, with the disclosures contained throughout the description corresponding to the same Common parts with the same reference numbers or the same component designations can be transferred. The position information selected in the description, such as top, bottom, side, etc., is related to the directly described and illustrated figure and these position information are to be transferred to the new position in the event of a change of position.

As already described at the outset, the method for producing or for growing an artificial sapphire monocrystal quite generally comprises the steps

- Filling a crucible with a sapphire seed crystal and Al ₂ O ₃ raw material,

- Crystal growth at a boundary surface of the growing sapphire single crystal to the Al ₂ O ₃ - melt by generating a temperature gradient in the Al ₂ O ₃ melt in the crucible and crystallization of the melt at the boundary surface of the growing sapphire single crystal up to complete solidification of the Al ₂ O ₃ melt in the crucible,

- Cooling and removal of the sapphire single crystal from the crucible.

An interface between melt and crystal is in a state of equilibrium of liquid and solid. A temperature gradient of z. B. 1°C in this interface creates a driving force so that the crystal solidifies in a certain direction (here upwards). Here, a desired temperature gradient is created in the melt and crystallization is initiated from a hotter side to a cooler one. A range between 1 and 60 degrees is set as the temperature gradient in the melt. (Temperature gradients usually create convection in the melt, which transports the heat further to the interface. It has been found that a higher temperature gradient makes the melt unstable due to the creation of rapid convection, and crystal growth is thereby disturbed.

To illustrate the general course of the process step of crystal growth, each stage of this step of crystal growth is shown in FIGS. 1 to 3, or the process step of crystal growth is best illustrated by looking at FIGS 3 can be seen.

Of course, the solution according to the invention can also be used to grow a number of crystals simultaneously in a furnace by arranging a number of crucibles in the furnace at the same time. 1 shows a crucible 1 with a sapphire seed crystal 2 or a sapphire seed crystal placed at the bottom, which serves as a nucleus or, so to speak, as a crystallite template for the sapphire single crystal. In FIG. 1, an Al ₂ O ₃ melt 3 covering the sapphire seed crystal 2 is also shown in the crucible 1, which Al ₂ O ₃ melt 3 was obtained by melting Al ₂ O ₃ raw material. Accordingly, an Al ₂ O ₃ melt surface 4 is formed on the upper side of the Al ₂ O ₃ melt 3 after the Al ₂ O ₃ raw material has been melted. Usually, when the Al ₂ O ₃ raw material is melted to form the Al ₂ O ₃ melt 3 , at least superficial melting of the Al ₂ O ₃ seed crystal 2 in the crucible 1 can also take place.

As is known per se, the crucible 1 can consist of a high-temperature-resistant and high-purity material, such as tungsten, molybdenum, alloys thereof or other high-temperature-resistant materials that are able to withstand the high temperatures in the course of the process. Al ₂ O ₃ powder, Al ₂ O ₃ chips or, for example, sapphire crystallite fragments or also mixtures of different Al ₂ O ₃ materials can be provided as Al ₂ O ₃ raw material. Depending on the exact requirement, a high degree of purity of the Al ₂ O ₃ raw material provided is of course indicated for growing a sapphire single crystal.

At this point it should be pointed out that several crystals can also be grown in one furnace at the same time by arranging several crucibles 3 in the furnace. If several crystals are grown in a furnace at the same time, the procedure described here is carried out for each crystal. The simultaneous cultivation of several crystals in one furnace is particularly advantageous in terms of energy consumption. 2 shows the crucible 1 with a growing sapphire single crystal 5 and the Al ₂ O ₃ melt 3 covering it during or in the course of the crystal growth of the sapphire single crystal 5 . As shown in FIG. 2, in the course of the crystal growth of the sapphire single crystal 5, a boundary surface 6 of the growing sapphire single crystal 5 with the Al ₂ O ₃ melt 3 in the crucible 1 is formed.

In the embodiment shown in FIGS. 1 to 3, as illustrated and best seen in FIG standard method mentioned at the outset for single-crystal growth in a crucible 1 is customary. In principle, however, at least partial crystal growth can also take place in the horizontal direction, or horizontal growth components can be included.

In order to achieve crystal growth, the Al ₂ O ₃ melt 3 can be cooled in zones in the crucible 1, in which in FIGS. A corresponding temperature control in the crucible 1 can, of course, take place automatically by means of a corresponding control device, not shown in detail, which is used to control various control elements suitable and designed for temperature control, such as heating and/or cooling devices or adjustment devices - Devices, isolating devices, and so on can be designed. Some examples of controllable temperature control elements and devices are briefly described below with reference to an exemplary embodiment of a furnace chamber shown in FIG. In principle, any known and suitable control elements for controlling the temperature in the crucible 1 can be used, and reference is also made to the relevant literature on this at this point.

The method specifically provides that, beginning with the crystal growth step, a temperature difference ΔT is selected between the Al ₂ O ₃ melt surface 4 and the boundary surface 6 of the growing sapphire single crystal 5 for the Al ₂ O ₃ melt 3 from a range of 1 °C to 60 °C and this temperature difference ΔT is kept constant at least over the majority of the crystal growth period. A temperature _difference _.DELTA.T can _preferably be selected from a range of _1.degree to 15° C., particularly preferably from 1° C. to 5° C., and this temperature difference .DELTA.T is kept constant at least over a predominant period of crystal growth. As a result, due to the local displacement of the liquid-solid phase boundary or the boundary surface 6 of the growing sapphire monocrystal 5 to the Al ₂ O ₃ melt 3 , spatially, for example, vertically over the crucible 1 , and thus variable temperature gradients also occur across the interface 6 in the Al ₂ O ₃ melt 3 and the growing sapphire single crystal 5 .

Towards the end of the crystal growth step, the remaining Al ₂ O ₃ melt 3 in the crucible 1 can of course also be crystallized out by appropriate temperature control, so that the temperature difference ΔT here may vary from the specified range deviate, in particular be smaller or even approach 0. However, it can preferably be provided that the temperature difference ΔT between the Al ₂ O ₃ melt surface 4 and the boundary surface 6 of the growing sapphire single crystal 5 starts with the process step of crystal growth for 95% to 99.9% of the entire period of time Crystal growth is kept constant. Particularly preferably, the temperature difference ΔT between the Al ₂ O ₃ Schinclzc surface 4 and the boundary surface 6 of the growing sapphire single crystal 5 can be 98% to 99.9%, in particular 99 to 99.9%, starting with the process step of crystal growth % of the total crystal growth period are kept constant.

In the specific case of growing or producing a sapphire single crystal 5, starting with the crystal growth step, a temperature of the Al ₂ O ₃ melt surface 4 can be set to a value selected from a range of 2040° C. to 2100° C., preferably 2041° C to 2056° C., in particular from 2041° C. to 2046° C., and kept constant over a predominant period of crystal growth. During the crystal growth, the temperature in the crucible 1 can be lowered gradually, for example from bottom to top, so that the boundary surface 6 of the growing sapphire single crystal 5 to the Al ₂ O ₃ melt 3 in the crucible 1 slowly migrates upwards. In the respective area of the boundary surface 6, a temperature can naturally be in the range of the crystallization temperature or the melting point of sapphire, ie about 2050.degree.

The height of the Al ₂ O ₃ melt surface in the crucible can in principle be used as known per se for temperature control due to the known geometry of the crucible and the filling quantity of the crucible with Al ₂ O ₃ raw material. In principle, the position or height of the boundary surface 6 of the growing sapphire monocrystal 5 can at least be estimated at any time during the crystal growth on the basis of the temperature set in the crucible 1 or the corresponding technical control specifications.

In a preferred embodiment of the method, a distance ΔT between the Al ₂ O ₃ melt surface 4 and the boundary surface 6 of the growing sapphire single crystal 5 can also be determined at least at one point in the crucible 1 during the crystal growth is indicated in FIG. The position or height of the Al ₂ O ₃ melt surface 4 in the crucible 1 can, for example, be recorded mechanically or also by means of optical or image-recording methods. The position or height of the boundary surface 6 of the growing Sapphire monocrystal 5 can, for example, be measured mechanically at any point in the crucible 1 by means of a measuring rod 7 that can be lowered into the Al ₂ O ₃ melt, which measuring rod 7 is positioned on the boundary surface 6 of the growing sapphire as shown in FIG. Single crystal 5 is present. Such a measuring rod 7 can, of course, like the crucible 1, also consist of a high-temperature-resistant material, for example the same material as the crucible 1 itself.

Instead of a mechanical measurement of the height of the boundary surface 6 of the growing sapphire single crystal 5 in the crucible, as also illustrated in FIG Beam 8 from an electromagnetic radiation or a mechanical vibration in the crucible 1 are detected by sensors. Depending on the type of beam 8, the same can be radiated into the crucible 1 from above, for example. However, it is also possible, for example, for a beam 8, for example in the case of a light beam, to be coupled into the crucible 1 by means of a line 9, for example an optical fiber, as can also be seen in FIG. In principle, depending on its type, a beam 8 can also be introduced, for example, through a crucible wall or the crucible bottom of the crucible 1 if the crucible 1 is at least partially transparent for a beam 8 or has been made transparent at least in sections. A laser beam can particularly preferably be used as the beam 8 from the electromagnetic radiation.

Subsequently, a change in a property of the beam 8 after contact with the boundary surface 6 of the growing sapphire single crystal 5 can be detected by means of suitable sensors, not shown in detail in FIG. 2, for example a sensor-detected weakening or deflection of the Beam 8 at a certain angle. Ultimately, the position or height of the boundary surface 6 of the growing sapphire single crystal 5 in the crucible 1 can be deduced from the measured change. By measuring the position or height of the boundary surface 6 of the growing sapphire single crystal 5 in the crucible and the known or also measured height of the Al ₂ O ₃ melt surface 4, at any point in time during the crystal growth step, the figure shown in Fig. 2 illustrated distance ΔT between the Al ₂ O ₃ melt surface 4 and the boundary surface 6 of the growing sapphire single crystal 5 can be determined.

As a further consequence, it can preferably be provided in the method that the temperature difference ΔT between the Al ₂ O ₃ melt surface 4 and the boundary surface 6 of the growing sapphire monocrystal 5 is set and kept constant at least during the predominant period of crystal growth, starting with the crystal growth process step, on the basis of the determined distance ΔT.

In addition, it can also be advantageous in the method, especially from a control point of view, if a temperature of the Al ₂ O ₃ melt surface 4 is detected or monitored by sensors during the crystal growth. This can be done, for example, by means of a pyrometer, not shown in detail. However, it is also conceivable to measure the temperature using a high-temperature sensor, for example using a suitable thermocouple by bringing it up to or also slightly immersing it in the Al ₂ O ₃ melt 3 .

In principle, it is also possible to measure the temperature during the crystal growth at the boundary surface 6 of the growing sapphire monocrystal 5 and to use this as a basis for the temperature control. A measurement can be carried out, for example, by bringing insulated or sheathed high-temperature sensors to the boundary surface or contacting the boundary surface with such sensors. For example, a hollow measuring rod 7 can be used to detect the position or height of the boundary surface 6 of the growing sapphire single crystal 5 .

For the sake of completeness, FIG. 3 shows another state in the crucible 1 when the crystal growth is complete or the Al ₂ O ₃ melt has cooled completely. The sapphire monocrystal 5 which has then grown completely can be removed from the crucible 1 after cooling down if necessary.

FIG. 4 shows another exemplary embodiment of a device 10 for carrying out the method in a roughly schematic manner. The device 1 shown by way of example comprises a furnace chamber 11 with thermal insulation 12, within which the crucible 1 is arranged. Also shown are heating elements 13 arranged circumferentially around the crucible 1 and above and below the crucible 1, by means of which a crucible filling can be temperature-controlled.

In the exemplary embodiment shown, a plurality of heating elements 13 are arranged at different heights around the circumference of the crucible 1, so that different temperatures can be set in the crucible at different heights or in different zones.

For this purpose, the heating elements 13 shown can, for example, be controlled independently of one another, for example be designed to be heat output-controlled. A controller of the heating output can take place, for example, by variable charging of the heating elements 13 with electrical energy. In addition, however, other measures can also be used to control the temperature in the crucible 1, for example through the targeted supply of a coolant. As illustrated by way of example in FIG. 1 , a cooling gas can be supplied to an underside or the bottom of the crucible 1 , for example via a cooling channel 14 .

In addition, a temperature control in the crucible 1, in particular a targeted, zone-wise temperature reduction from bottom to top for progressive crystal growth, can be accomplished by adjusting the crucible 1 itself, in particular a successive adjustment vertically downwards out of the area of the heating elements 13, such as this is illustrated in FIG. 4 by the adjustment direction 15 indicated by the arrow.

The means for temperature control shown in FIG. 4 and briefly described above are only to be regarded as examples. A large number of measures and devices are known in principle for zone-specific, in particular height-specific, temperature control in crucibles for crystal growth, and reference is again made here to the corresponding, relevant literature.

The exemplary embodiments show possible variants, it being noted at this point that the invention is not limited to the specifically illustrated variants of the same, rather various combinations of the individual variants are also possible with one another and these possible variations are based on the teaching of technical action through a concrete invention lies within the ability of the person skilled in the art working in this technical field.

The scope of protection is determined by the claims. However, the description and drawings should be used to interpret the claims. Individual features or combinations of features from the different exemplary embodiments shown and described can represent independent inventive solutions. The task on which the independent inventive solutions are based can be found in the description.

All information on value ranges in the present description is to be understood in such a way that it includes any and all sub-ranges, eg the information 1 to 10 is to be understood in such a way that all sub-ranges, starting from the lower limit 1 and the upper limit 10 are also included, ie all partial ranges start with a lower limit of 1 or greater and end with an upper limit of 10 or less, eg 1 to 1.7, or 3.2 to 8.1, or 5.5 until 10.

Finally, for the sake of clarity, it should be pointed out that, in order to better understand the structure, some elements are shown not to scale and/or enlarged and/or reduced.

List of reference symbols Crucible Sapphire seed crystal Al ₂ O ₃ melt Al ₂ O ₃ melt surface Sapphire single crystal Boundary surface Measuring rod Beam line Device Furnace chamber Heat insulation Heating element Cooling channel Adjustment direction

Claims

P atentClaims

Claims 1. A method for producing an artificial sapphire single crystal (5), comprising the steps of

- Filling a crucible (1) with a sapphire seed crystal (2) and Al ₂ O ₃ raw material,

- Melting of the Al ₂ O ₃ raw material to form an Al ₂ O ₃ melt (3) and at least superficial melting of the sapphire seed crystal (2),

- Crystal growth at a boundary surface (6) of the growing sapphire single crystal (5) to the Al ₂ O ₃ melt (3) by generating a temperature gradient in the Al ₂ O ₃ melt

(3) in the crucible (1) and crystallization of the melt at the boundary surface of the growing sapphire single crystal (5) until the Al ₂ O ₃ melt (3) has completely solidified in the crucible,

- Cooling and removal of the sapphire single crystal (5) from the crucible (1), characterized in that beginning with the crystal growth step between an Al ₂ O ₃ melt surface

(4) and the boundary surface (6) of the growing sapphire single crystal (5) to the Al ₂ O ₃ melt (3) a temperature difference ΔT selected from a range of 1 °C to 60 °C, in particular between 1 °C - 15 ° C, preferably between 1 ° C - 9 ° C, particularly preferably between

1 °C-5 °C is set and this temperature difference ΔT is kept constant at least over a predominant period of crystal growth.

2. The method according to claim 1, characterized in that during the crystal growth at least at one point in the crucible (1) a distance .DELTA.T between the Al ₂ O ₃ - melt surface (4) and the boundary surface (6) of the growing sapphire Single crystal (5) is determined.

3. The method according to claim 2, characterized in that at least at one point in the crucible (1) a position of the boundary surface (6) of the growing sapphire single crystal (5) by irradiation of a beam (8) of electromagnetic radiation or a mechanical vibration in the crucible (1) is detected by sensors.

4. The method according to claim 3, characterized in that a laser beam is used as the beam (8) from the electromagnetic radiation.

5. The method according to any one of claims 2 to 4, characterized in that the temperature difference ΔT between the Al ₂ O ₃ melt surface (4) and the Grenzoberflä- surface (6) of the growing sapphire single crystal (5) based on the determined distance ΔT is set and kept constant.

6. The method according to any one of the preceding claims, characterized in that the temperature difference .DELTA.T between the Al ₂ O ₃ -Schinclzc surface (4) and the boundary surface (6) of the growing sapphire single crystal (5) starting with the process step of crystal growth for 95% to 99.9% of the total crystal growth period is kept constant.

7. The method according to any one of the preceding claims, characterized in that a temperature of the Al ₂ O ₃ melt surface (4) is detected by sensors during crystal growth.

8. The method according to any one of the preceding claims, characterized in that starting with the step of crystal growth, a temperature of the Al ₂ O ₃ -Schmelze- surface (4) to a value selected from a range of 2040 ° C to 2100 ° C, esp - special from 2041 ° C to 2056 ° C, particularly preferably from 2041 ° C to 2046 ° C is set and is kept constant over a predominant period of crystal growth.