METHOD FOR REDUCING STRUCTURAL DAMAGE TO THE SURFACE OF MONOCRYSTALLINE ALUMINIUM-NITRIDE SUBSTRATES, AND MONOCRYSTALLINE ALUMINIUM-NITRIDE SUBSTRATES THAT CAN BE PRODUCED BY A METHOD OF THIS TYPE

Abstract

The present invention relates to a method for reducing structural damage to the surface of monocrystalline aluminium-nitride substrates, according to which the substrate undergoes thermal treatment in a crucible in an autoclave, during which treatment the aluminum-nitride substrate is sublimated in the damaged regions of the surface of the substrate and is removed. The method is used to prepare the surface of monocrystalline aluminium-nitride (AlN), in particular the aim of the invention is to eliminate, or at least significantly reduce near-surface structural damage to the monocrystalline material caused by mechanical processing. The invention also relates to aluminium-nitride substrates that are treated in this way.

Claims

1-12. (canceled)

13. A method for preparing the surface of a monocrystalline aluminium-nitride substrate, the method comprising heating the substrate undergoes in a crucible in an autoclave, which treatment results in a sublimation and removal of the aluminium nitride in damaged regions on the surface, wherein the heat treatment is carried out at a temperature of at least 2000° C. and in an atmosphere at an oxygen partial pressure of at most 10.sup.−4 mbar.

14. The method according to claim 13, wherein the heat treatment is carried out at a temperature of from 2000 to 2350° C.

15. The method according to claim 13, wherein the heat treatment is carried out under vacuum at 1 mbar to 10.sup.−4 mbar or in a protective gas atmosphere at 1 mbar to 1.5×10.sup.3 mbar, wherein nitrogen, argon, helium or a combination thereof is selected as the protective gas.

16. The method according to claim 13, wherein, during the heat treatment, the temperature gradient is at least 5° C./cm and the temperature gradient is perpendicular to the substrate surface.

17. The method according to claim 13, wherein, during the heat treatment, the temperature gradient is at most 1° C./cm and the temperature gradient is parallel to the substrate surface in order to allow for lateral homogeneous material removal.

18. The method according to claim 13, wherein, during the heat treatment, the temperature gradient is at least 1° C./cm and is parallel to the substrate surface, in order to set laterally inhomogeneous material removal for producing tilting of the surface normal relative to the <0001> crystal axis.

19. The method according to claim 13, wherein a nitrogen-polar surface of aluminium nitride is removed from the substrate in a specific manner at an orientation of +/−5° relative to the <0001> crystal axis.

20. The method according to claim 13, wherein the atmosphere comprises carbon.

21. The method according to claim 13, wherein the crucible comprises tantalum carbide, from which carbon is released during the heat treatment.

22. The method according to claim 13, wherein damage caused by pretreatment of the substrate is removed by a sawing process, a grinding process, a polishing process, or a combination thereof.

23. The method according to claim 13, which includes removing the damage below the surface of the substrate.

24. The method according to claim 13, which includes removing the damage in the rim region and/or to the edges of the substrate.

25. A method of producing monocrystalline aluminium-nitride substrate, the method comprising heating an aluminium-nitride substrate in a crucible in an autoclave, which treatment results in a sublimation and removal of the aluminium nitride in damaged regions on the surface, wherein the heat treatment is carried out at a temperature of at least 2000° C. and in an atmosphere at an oxygen partial pressure of at most 10.sup.−4 mbar.

Description

[0027] The subject matter according to the invention will be explained in greater detail with reference to the following figures and examples, without restricting it to the specific embodiments set out here.

[0028] FIG. 1 is a schematic view of what happens with a rounded-edge wafer.

[0029] FIG. 2 shows a wafer in the growth process with a shielded wafer edge.

[0030] FIG. 3 shows the structure of a crucible used according to the invention.

[0031] FIG. 4 shows X-ray topography images of mechanically polished and CMP-polished AlN wafers once without thermal treatment (a) and once with thermal treatment (b).

[0032] FIG. 5 shows an AFM image of an AlN wafer processed according to the invention.

[0033] FIG. 1 shows a wafer 1 that has undergone mechanical edge rounding. In this mechanical processing, structural damage 2 to the rounded wafer edge occurs. According to the prior art, wafers that have been previously damaged in this way generally undergo a growth process, with an attempt being made to shield the edge regions by means of covers 3. This is shown in FIG. 2.

[0034] FIG. 3 shows a crucible 11 used according to the invention, in which the aluminium-nitrite wafer is arranged on a holder 12. In this case, partial sublimation occurs on the surface of the aluminium-nitrite wafer 13 by means of a suitable temperature profile.

[0035] FIG. 4 shows X-ray topography images of a wafer mechanically processed according to the prior art (FIG. 4a) and a wafer processed according to the invention (FIG. 4b). It can be seen here that, in FIG. 4a, the wafer has a dark contrast on the wafer rim. This is the damage to the rim region resulting from the mechanical processing. The view in FIG. 4b then shows a wafer thermally treated according to the invention, which does not have any mechanical damage at all on the wafer rim, with no dark contrast being visible here.

[0036] FIG. 5 shows an AFM image of an AlN wafer according to the invention, with the surface having a stepped structure here. This wafer surface can then be used directly as a seed plate for a new PVT growth process for producing aluminium-nitrite crystals.

EXAMPLE

[0037] The wafer is placed onto a TaC ceramic plate on the crucible base with the surface to be treated upwards within the tungsten crucible (typical height: 3 cm). Here, the diameter of the crucible is determined by the size of the wafer diameter and typically exceeds this by >1 cm. At the rim of the TaC ceramic described, additional AlN polymaterial (mass of the size of the mass of the wafer to be treated) having the fewest possible impurities of oxygen (<200 ppm) is added in order to produce an additional partial pressure on AlN species in the gas phase during the thermal treatment of the wafer surface to better control the removal of AlN material from the wafer surface by sublimation. For the process control, as is common, the temperature at the crucible lid (control temperature) is pyrometrically determined and is set for the various process steps in a targeted manner. Care should be taken here to ensure that, for the described crucible configuration, the control temperature is 50-70° C. below the temperature of the wafer surface to be treated.

[0038] For this configuration, it is then inserted into an autoclave and exposed to the following process conditions (the stated temperature corresponds to the control temperature here): [0039] 1. Before the heating step and during a first heating step up to approx. 500-700° C., oxygen residues in the autoclave should be reduced as far as possible by flushing multiple times with >5N nitrogen (720 mbar) and repeated evacuation to a vacuum of <1E-2 mbar. As a last step, the autoclave is filled with >5N nitrogen (720 mbar). [0040] 2. By heating (RF heating or resistance heating), the control temperature is increased to 2100° C. at a rate of 12-15° C./min. [0041] 3. By heating, the control temperature is increased from 2100° C. to 2200° C. at a rate of 2.5-3.0° C./min. [0042] 4. The temperature of 2200° C. is maintained for 5-25 min in order to achieve typical surface removal of 20-50 μm. To do this, an axial temperature gradient of approx. 20° C./cm is aimed for on the wafer surface in the direction of the crucible lid. This gradient is set by the crucible geometry and a suitable selection of the geometry and material for the thermal insulation material surrounding the crucible within the autoclave. [0043] 5. The control temperature is then reduced by reducing the heating until room temperature is reached. Typical cooling rates are 4-5° C./min.