Methods for reducing structural damage on the surface of single-crystal aluminium nitride substrates and single-crystal aluminium nitride substrates producible in this way

Abstract

Method for surface preparation of single-crystal aluminium nitride substrates, wherein the substrate is subjected to a temperature treatment in a crucible in an autoclave, which leads to sublimation and removal of the aluminium nitride in the damaged areas on the surface, wherein the temperature treatment is carried out at a temperature of 2000 to 2350°C and in an atmosphere with an oxygen partial pressure of at most 10 -4 mbar, whereby the temperature treatment is carried out under vacuum at 1 mbar to 10 -4 mbar or in a protective gas atmosphere at 1 mbar up to 1.5×10 3 mbar, wherein the temperature gradient perpendicular to the substrate surface is at least 5°C / cm during the temperature treatment, and wherein the nitrogen-polar surface of aluminium nitride with an orientation of + / - 5° relative to the substrate is specifically selected. <0001> -Crystal axis is eroded.

Description

[0001] The present invention relates to a method for reducing structural damage on the surface of single-crystal aluminum nitride substrates, in which the substrate is subjected to a heat treatment in a crucible within an autoclave, during which the aluminum nitride in the damaged areas on the substrate's surface is sublimated and removed. The method serves for the surface preparation of single-crystal aluminum nitride (AlN), in particular to eliminate or at least significantly reduce near-surface structural damage in the single-crystal material caused by mechanical processing. The invention also relates to aluminum nitride substrates treated in this way.

[0002] The production of substrate discs, e.g., wafers, or seed plates from grown single crystals routinely involves several, mostly mechanical, processing steps. These include cylindrical or surface grinding, sawing, edge rounding, lapping, and polishing. If this mechanical processing concludes with a polishing step, surface defects in the single-crystal material are often detectable.

[0003] A distinction is made between various near-surface areas that may exhibit defects. These include, firstly, the polishing layer directly at the surface with a thickness of 0.1 to 1 µm, followed by a substrate layer in the range of 1 to 100 µm from the surface, a deformation layer in the range of 1 to 200 µm from the surface, and finally the effect-free substrate.

[0004] Surface treatments for removing such surface damage, which can also affect regions below the visible surface ("subsurface damage"), are known in the prior art. These include chemical etching (Müller S., Sumakeris J., Brady M., Glass R., Hobgood H., Jenny J., Carter C. (2004). Defects in SiC substrates and epitaxial layers affecting semiconductor device performance. The European Physical Journal Applied Physics, 27(1-3), 29-35. doi:10.1051 / epjap:2004085) and / or so-called "chemical-mechanical polishing steps" (CMP) (Kevin Moeggenborg, Methods for polishing aluminum nitride, Patent Pub. No.: US 2010 / 0062601 A1, Pub. Date: Mar. 11, 2010). However, damage to a wafer edge after a standard edge rounding cannot be addressed, especially with a standard CMP step.

[0005] US 2013-0 152 852 A1 describes a process for producing aluminum nitride single crystals by sublimation.

[0006] For single crystals such as the semiconductors aluminum nitride (AlN) and silicon carbide (SiC), which are produced using a PVT process, it has been shown that the quality of surface preparation of a seed surface can have a direct impact on the dislocation density of material deposited on it during growth. Removing or reducing near-surface damage directly improves the dislocation density at the phase boundary between the seed and the newly grown crystal material, thus also reducing the dislocation density within the bulk of the new single crystal.

[0007] Existing near-surface damage, particularly at the wafer edge, is often deliberately accepted when using these wafers as seed plates for a growth process (e.g., in bulk crystal growth, but also in epitaxial processes like MOVPE for layer deposition). This results in the deposition of new material of inferior crystalline quality in this edge region during the growth process. Therefore, attempts are made to geometrically shield these edge regions from the growth flow using other materials and, if necessary, to adjust the thermal boundary conditions in this area to prevent or at least reduce material deposition there.

[0008] Both methods have significant disadvantages for the growth process. With the permitted deposition of material of lower crystalline quality, the achievable crystal diameter with material of higher crystalline quality is limited by this marginal region. The diameter of the region of higher crystalline quality can even be further reduced by parasitic intrusion of the lower crystalline quality material into this region. Using a geometric covering over a marginal region with surface damage results in similar disadvantages regarding the diameter reduction of the region of higher crystalline quality. In particular, care must be taken to ensure that polycrystalline deposition of material on the covering itself can be avoided, if necessary, through a suitable combination of the choice of covering material and appropriate thermal boundary conditions during growth.Depending on the material being bred and the necessary breeding conditions, this is often not possible or only achievable with a great deal of effort.

[0009] On the other hand, attempts to remove near-surface damage by chemical etching, e.g., using KOH solutions or melts, can be associated with other specific problems. These include potentially difficult-to-remove surface contamination from the chemicals used and the formation of a surface morphology that is unfavorable for further use in growth processes. This is often due to the selective etching attack on dislocations or other lattice defects.

[0010] Specifically for optimizing nucleation in PVT growth of the semiconductor SiC, it has been proposed to remove surface damage or surface contamination of the polished nucleus at the beginning of the growth process by reversing the temperature gradient (T(nucleus) > T(source)) as an in-situ step, in order to simultaneously avoid material deposition during the heating process at low temperatures (MM Anikin et al., Proc. ICSCRM-95, Kyoto, Japan, 1996, p.33 and Temperature gradient controlled SiC crystal growth, M. Anikin, R. Madar, Materials Science and Engineering B46 (1997) 278-286).However, in practice the method is difficult to implement for SiC growth and does not represent a replacement for mechanical polishing steps after a sawing process for SiC, since the non-stoichiometric sublimation of SiC and the necessary, significant material removal can easily lead to graphitization of the surface and thus render the surface of SiC unusable as a nucleation surface.

[0011] It was therefore an object of the present invention to provide a method for treating surfaces of single-crystal aluminium nitride substrates with the aim of enabling a surface that is as free of damage as possible or of minimizing damage in near-surface areas of the aluminium nitride substrate, with the aim of achieving the most complete treatment possible in all surface areas.

[0012] This problem is solved by the method for surface preparation of single-crystal aluminum nitride substrates with the features of claim 1 and corresponding single-crystal aluminum nitride substrates with the features of claim 10. The further dependent claims describe advantageous embodiments.

[0013] According to the invention, a method for the surface preparation of single-crystal aluminum nitride substrates is provided, in which the substrate is subjected to a heat treatment in a crucible within an autoclave, leading to sublimation and removal of the aluminum nitride in the damaged areas on the surface. The heat treatment is carried out at a temperature of 2000 to 2350°C and in an atmosphere with an oxygen partial pressure of at most 10 -4 mbar.

[0014] Since, unlike silicon carbide, the sublimation of aluminum nitride occurs essentially stoichiometrically, the thermal treatment of the semiconductor aluminum nitride according to the invention can be carried out without the disadvantages described for silicon carbide. This means, in particular, that even with longer treatment times or higher material removal rates, the formation of an undesirable surface layer due to a non-stoichiometric decomposition reaction does not occur.

[0015] The temperature during the heat treatment is chosen so that a sufficiently high aluminum partial pressure is generated at the substrate interface to enable material removal of the damaged material by sublimation from the substrate surface.

[0016] A sufficiently large and simultaneously controlled removal rate can be set by the absolute temperature in the autoclave, the temperature gradient at the substrate surface, and the ambient pressure in the autoclave.

[0017] To prevent unwanted oxidation of the aluminum nitride substrate surface, the oxygen concentration inside the autoclave must be kept as low as possible. Therefore, a maximum oxygen partial pressure of 10⁻⁵ g / cm² should be maintained. -4 mbar can be selected.

[0018] It is preferred that the heat treatment be carried out at a temperature of 2150 °C to 2250 °C.

[0019] According to the invention, the temperature treatment is carried out under vacuum at a pressure of 1 mbar to 10 -4 mbar or in a protective gas atmosphere at 1 mbar up to 1.5 × 10 3The procedure is carried out at mbar. Nitrogen, argon, helium, or combinations thereof are preferably used as the protective gas.

[0020] According to the invention, during temperature treatment the temperature gradient perpendicular to the substrate surface is at least 5 °C / cm.

[0021] Another preferred embodiment provides that during heat treatment, the temperature gradient parallel to the substrate surface is a maximum of 1°C / cm. If this gradient is maintained, a laterally homogeneous material removal is obtained.

[0022] In a further preferred embodiment, the temperature gradient during the heat treatment is selected to be at least 1°C / cm parallel to the substrate surface. This allows for laterally inhomogeneous material removal, generating a tilting of the surface normal relative to the substrate. <0001> -Crystal axis is reached.

[0023] According to the invention, the nitrogen-polar surface of aluminum nitride with an orientation of + / - 5° relative to the substrate is specifically adapted. <0001> -Crystal axis eroded.

[0024] It is further preferred that carbon in the form of carbon-containing species is present in this autoclave during atmospheric temperature treatment. This prevents the formation of aluminum droplets on the treated aluminum nitride surface during cooling. This is preferably achieved by including or consisting of tantalum carbide in a component of the inner surface of the crucible containing the aluminum nitride substrate, so that a carbon partial pressure develops during the temperature treatment.

[0025] The method according to the invention is particularly well suited for removing damage resulting from pretreatment of the substrate, in particular by a sawing process, a grinding process, a polishing process or combinations of these processes, and thus providing surfaces that are essentially free of damage.

[0026] It is further advantageous that the method according to the invention can also remove damage below the surface of the substrate. Likewise, it is possible to remove damage in the marginal area and / or at the edges of the substrate.

[0027] According to the invention, a single-crystal aluminium nitride substrate is also provided which is essentially free of damage on the surface or in near-surface areas.

[0028] The following figures and examples are intended to explain the subject matter of the invention in more detail, without limiting it to the specific embodiments shown here. Fig. Figure 1 shows a schematic representation of the effects that occur in a wafer with rounded edges. Fig. Figure 2 shows a wafer in the growing process with a shielded wafer edge. Fig. Figure 3 shows the construction of a crucible used according to the invention. Fig. Figure 4 shows X-ray topography images of mechanically and CMP-polished AlN wafers, once without thermal treatment (a) and once with thermal treatment (b). Fig. Figure 5 shows an AFM image of an AIN wafer processed according to the invention.

[0029] In Fig. Figure 1 shows a wafer 1 that has undergone mechanical edge rounding. This mechanical processing results in structural damage 2 to the rounded wafer edge. According to the prior art, wafers damaged in this way are generally subjected to a growth process in which attempts are made to shield the edge areas using apertures 3. This is in Fig. 2 shown.

[0030] In Fig. Figure 3 shows a crucible 11 used according to the invention, in which the aluminum nitride wafer is arranged on a holder 12. Partial sublimation occurs on the surface of the aluminum nitride wafer 13 with the aid of a suitable temperature profile.

[0031] In Fig. 4 are X-ray topography images of a wafer mechanically processed according to the state of the art ( Fig. 4a) and a wafer processed according to the invention ( Fig. 4b) shown. Here it can be seen that in Fig. 4a The wafer exhibits a dark contrast at the wafer edge. This is due to damage in the edge area caused by mechanical processing. A view of Fig. Figure 4b then shows a wafer thermally treated according to the invention, which has no mechanical damage at the wafer edge, and no dark contrast is visible.

[0032] In Fig. Figure 5 shows an AFM image of an AIN wafer according to the invention, the surface of which has a stepped structure. This wafer surface can then be used directly as a seed plate for a new PVT growth process for the production of aluminum nitride starter crystals. Example

[0033] The wafer, with the surface to be treated facing upwards, is placed inside a tungsten crucible (typically 3 cm high) onto a TaC ceramic plate located at the bottom of the crucible. The crucible diameter is determined by the size of the wafer and typically exceeds it by more than 1 cm. Additional AlN polymer material (mass on the order of the mass of the wafer to be treated) with minimal oxygen impurities (<200 ppm) is added to the edge of the TaC ceramic plate. This creates an additional partial pressure of AlN species in the gas phase during the thermal treatment of the wafer surface, thus better controlling the removal of AlN material from the wafer surface via sublimation. For process control, the temperature at the crucible lid (control temperature) is typically determined pyrometrically and precisely adjusted for the various process steps.It should be noted that for the described crucible configuration, the control temperature is 50-70 °C below the temperature of the wafer surface to be treated.

[0034] This configuration is now placed in an autoclave and exposed to the following process conditions (the specified temperature corresponds to the control temperature): 1. Before the heating step and during the first heating step up to approximately 500-700 °C, residual oxygen in the autoclave should be reduced as much as possible by repeated purging with >5N nitrogen (720 mbar) and repeated pumping down to a vacuum <1E-2 mbar. As a final step, the autoclave is filled with >5N nitrogen (720 mbar). 2. The control temperature is increased to 2100 °C by heating (RF or resistance heating) at a rate of 12-15 °C / min. 3. The control temperature is increased from 2100 °C to 2200 °C by heating at a rate of 2.5-3.0 °C / min. 4. The temperature of 2200 °C is maintained for 5–25 minutes to achieve a typical surface removal of 20–50 µm. For this purpose, an axial temperature gradient of approximately 20 °C / cm is targeted at the wafer surface in the direction of the crucible lid. This gradient is set by the crucible geometry and a suitable choice of geometry and material for the thermal insulation material surrounding the crucible within the autoclave. 5. The control temperature is now reduced by lowering the heating until room temperature is reached. Typical cooling rates are 4-5 °C / min.

Claims

[1] Method for surface preparation of single-crystal aluminium nitride substrates, wherein the substrate is subjected in a crucible in an autoclave to a heat treatment which leads to sublimation and removal of the aluminium nitride in the damaged areas on the surface, wherein the heat treatment is carried out at a temperature of 2000 to 2350°C and in an atmosphere at an oxygen partial pressure of at most 10 -4 mbar, whereby the temperature treatment is carried out under vacuum at 1 mbar to 10 -4 mbar or in a protective gas atmosphere at 1 mbar up to 1.5×10 3 mbar, wherein the temperature gradient perpendicular to the substrate surface is at least 5°C / cm during the temperature treatment, and wherein the nitrogen-polar surface of aluminium nitride with an orientation of + / - 5° relative to the substrate is specifically selected. <0001> -Crystal axis is eroded. [2] Method according to claim 1, characterized bythat the heat treatment takes place at a temperature of 2150 to 2250°C. [3] Method according to one of claims 1 or 2, characterized by that nitrogen, argon, helium or combinations thereof are selected as the protective gas. [4] Method according to any one of claims 1 to 3, characterized by , that during heat treatment the temperature gradient parallel to the substrate surface is a maximum of 1°C / cm in order to enable laterally homogeneous material removal. [5] Method according to any one of claims 1 to 3, characterized by , that during heat treatment the temperature gradient parallel to the substrate surface is at least 1°C / cm in order to create a laterally inhomogeneous material removal and thus a tilting of the surface normal relative to the <0001> -To adjust the crystal axis. [6] Method according to any one of claims 1 to 5, characterized bythat carbon is contained in the atmosphere, wherein the crucible preferably contains tantalum carbide, from which the carbon is released during the temperature treatment. [7] Method according to any one of claims 1 to 6, characterized by , that damage caused by pretreatment of the substrate, in particular by a sawing process, grinding process, polishing process or combinations thereof, is removed. [8] Method according to any one of claims 1 to 7, characterized by , that damage below the surface of the substrate is removed. [9] Method according to any one of claims 1 to 8, characterized by , that damage in the marginal area and / or on the edges of the substrate is removed. [10] Single-crystal aluminium nitride substrate producible by the method according to any one of claims 1 to 9.

Images