Method for treating a silicon carbide substrate

Abstract

The invention relates to a method for treating a substrate (10) comprising at least one superficial layer of monocrystalline silicon carbide, said method comprising: · placing said substrate (10) in a furnace, · lowering the pressure in the furnace to a pressure (pF) of between 0.01 and 0.5 Pa, · heating the substrate to a temperature (TF) above a sublimation temperature for silicon at said pressure (pF), so as to form a layer of carbon on the layer of monocrystalline silicon carbide by sublimation of the silicon of the surface of said layer of monocrystalline silicon carbide, and · after formation of the carbon layer (30), heat-treating the substrate at a heat treatment temperature (TT) strictly above 1300°C and under a pressure (pT) above the pressure (pF) of formation of the carbon layer.

Description

[0001] METHOD FOR PROCESSING A SILICON CARBIDE SUBSTRATE

[0002] FIELD OF THE INVENTION

[0003] The present invention relates generally to the field of manufacturing silicon carbide substrates for forming electronic components. More particularly, it provides a method of treating a surface of a monocrystalline silicon carbide substrate.

[0004] STATE OF THE ART

[0005] Monocrystalline silicon carbide (m-SiC) is used for the formation of electronic components, for example for power electronics or radio frequency applications. In some applications, substrates of bulk monocrystalline SiC are used, in other applications substrates of, for example, polycrystalline silicon carbide, with a surface layer of monocrystalline SiC.

[0006] Microelectronics manufacturing processes often involve thermal treatments of substrates, for example to activate dopants, to heal defects in the crystal lattice caused by implantation of ionic species, or to modify the crystal structure of the substrate. Such treatments are often carried out at high temperatures of up to 2000°C or more.

[0007] However, the surface of monocrystalline SiC is susceptible to destabilization under the effect of high temperature. This destabilization can result in degradation of the surface, in particular by a modification of its morphology and / or composition.

[0008] In particular, as illustrated in Figure 1, smooth SiC surfaces are sensitive to high temperature treatments, typically from about 1200°C. In the absence of adequate protection of said surface, the morphology reorganizes to minimize the surface energy, forming terraces 22. This effect is known as "step bunching". The reorganization of the surface 222 is due to a diffusion of atoms on the surface when it is exposed to high temperatures. It also depends on the atmosphere and pressure to which the surface is exposed.

[0009] To obtain smooth surfaces, we therefore seek to avoid the formation of terraces or to remove them in a later step. For example, mechanical or chemical-mechanical polishing (CMP) can be carried out after heat treatment. However, if the height of the terraces formed is too great, for example greater than 80 nm between two consecutive plates, the thickness to be removed is too great, which implies a long and laborious process. In addition, the remaining thickness of m-SiC is often too thin for the intended applications.

[0010] Another method is to use very rapid annealing ramps to limit the appearance of terraces. This method requires temperature increases of approximately 400°C per minute and is therefore not compatible with industrial furnaces.

[0011] Alternatively, silane overpressure can be applied to limit silicon sublimation into SiC, thus preventing terrace formation. This method is described in the article by R. Zhang et al. However, it requires the introduction of SiH4 gas and equipment to create the overpressure. In addition, silicon droplets can appear on the SiC surface at temperatures above 1600°C.

[0012] Another possibility to avoid terrace formation is to use a protective layer. The article by J. Bao et al. proposes a carbon protective layer obtained by a process of centrifugation of a resin on a SiC surface. The resin is annealed at 750°C - 900°C to form a carbon film of approximately 1 μm thickness. This technique involves the addition of additional steps in the formation process, the use of additional equipment and carries a risk of contamination of the substrate by the resin.

[0013] In some cases, the protective layer can also be formed from aluminum nitride (AIN). However, this material is difficult to remove, especially after heat treatment at high temperatures.

[0014] STATEMENT OF THE INVENTION

[0015] An aim of the invention is to provide a method for treating a surface of a monocrystalline silicon carbide substrate making it possible to reduce the formation of terraces by a step that is easy to implement. Such a treatment must be easy to integrate into the method for manufacturing a substrate comprising a monocrystalline silicon carbide surface, and not present any risk of contamination of such a substrate.

[0016] To this end, the invention proposes a method for treating a substrate comprising at least one surface layer of monocrystalline silicon carbide, said method comprising

[0017] • placing said substrate in an oven,

[0018] • lowering the pressure in the furnace to a pressure between 0.01 and 0.5 Pa,

[0019] • heating the substrate to a temperature above a sublimation temperature of silicon at said pressure, so as to form a carbon layer on the monocrystalline silicon carbide layer by sublimation of silicon from the surface of said monocrystalline silicon carbide layer, and,

[0020] • after the formation of the carbon layer, a heat treatment of the substrate at a heat treatment temperature (TT) strictly greater than 1300°C and under a pressure (pr) greater than the pressure (pp) for forming the carbon layer, the carbon layer protecting the surface of the monocrystalline silicon carbide layer against the formation of steps.

[0021] The carbon layer serves to protect the silicon carbide during heat treatment at a higher temperature, and thus minimize the formation of steps on the surface of the silicon carbide.

[0022] Pumping at a pressure below 0.01 to 0.5 Pa (0.1 to 5 pbar) allows the rapid removal of evaporating silicon atoms from the substrate surface. In addition, in the case of a plurality of substrates in the same furnace, silicon atoms are removed from the space between the silicon carbide substrates. Such removal provides a relatively homogeneous atmosphere on the substrate surface, thereby forming a protective layer with a small variation in thickness between the edge and the center of each substrate.

[0023] Advantageously, the formation temperature of the carbon layer is between 1200°C and 1500°C.

[0024] Preferably, the carbon layer is a graphene layer.

[0025] Advantageously, the carbon layer has a thickness of between 0.2 nm and 1.2 nm.

[0026] Advantageously, a plurality of substrates are processed simultaneously in the same oven.

[0027] The substrates can be vertically stacked in the oven so that a vertical space is provided between two adjacent substrates.

[0028] Preferably, said vertical space between two adjacent substrates has a height of between 4 and 10 mm.

[0029] In some embodiments the method comprises, after forming the carbon layer, heat treating the substrate at a temperature above the carbon layer formation temperature and under a pressure above the carbon layer formation pressure.

[0030] Advantageously, the temperature of the heat treatment is greater than or equal to 1500°C, preferably greater than or equal to 1700°C. The temperature (TT) of the heat treatment may be less than or equal to 2000°C.

[0031] The heat treatment pressure is preferably greater than or equal to 2 Pa.

[0032] Advantageously, the formation of the carbon layer and the heat treatment are carried out in the same furnace.

[0033] Preferably, the method further comprises a step of removing the carbon layer after the heat treatment.

[0034] Advantageously, the method further comprises a step of chemical-mechanical polishing after the heat treatment to remove steps formed on the surface of the silicon carbide layer during said heat treatment.

[0035] Said chemical-mechanical polishing can lead to the removal of a silicon carbide thickness of less than 200 nm.

[0036] The invention also relates to a method for manufacturing a structure comprising a layer of monocrystalline silicon carbide on a support substrate, successively comprising:

[0037] - the formation of a weakening zone in a monocrystalline silicon carbide donor substrate by implantation of ionic species, so as to delimit a layer of monocrystalline silicon carbide to be transferred,

[0038] - bonding of the donor substrate to the support substrate,

[0039] - detachment of the donor substrate along the weakening zone so as to transfer the layer of monocrystalline silicon carbide onto the support substrate,

[0040] - implementing the treatment as described above on the transferred monocrystalline silicon carbide layer.

[0041] Preferably, the monocrystalline silicon carbide layer comprises dopants and in that the heat treatment is carried out at a temperature suitable for activating said dopants.

[0042] Advantageously, the silicon carbide layer comprises defects due to the implantation of ionic species and in that the heat treatment is carried out at a temperature suitable for curing at least part of said defects.

[0043] Advantageously, the method further comprises, after the chemical-mechanical polishing, a step of resuming epitaxy of monocrystalline silicon carbide on the surface of the monocrystalline silicon carbide layer.

[0044] BRIEF DESCRIPTION OF THE FIGURES

[0045] Other characteristics and advantages of the invention will emerge from the detailed description which follows, with reference to the appended drawings, in which: Figure 1 illustrates the formation of terraces on a front face made of a monocrystalline semiconductor material.

[0046] Figure 2 illustrates a substrate having a monocrystalline SiC portion on its front face.

[0047] Figure 3A illustrates the substrate of Figure 2 after the formation of a carbon layer.

[0048] Figure 3B illustrates the substrate of Figure 2 during the formation of a carbon layer.

[0049] Figure 3C illustrates the substrate of Figure 2 after the formation of a particular embodiment of the carbon layer.

[0050] Figure 3D illustrates the distribution of different thicknesses of a carbon layer on the surface of a substrate.

[0051] Figure 4 shows the thermal profile used for a process according to the invention.

[0052] Figures 5A to 5C illustrate the fabrication of a substrate having a surface portion of m-SiC on its front face.

[0053] Figure 6 illustrates an annealing furnace suitable for forming a carbon layer according to a preferred embodiment of the invention.

[0054] Figure 7 shows the roughness of a single-crystal SiC surface as a function of radial position on the substrate.

[0055] Figure 8 shows the step height between terraces formed on a monocrystalline SiC surface as a function of radial position on the substrate.

[0056] Figures 9A and 9B are electron beam scanning microscopy images of a substrate after heat treatment according to a known method.

[0057] Figures 10A and 10B are electron beam scanning microscopy images of a substrate after heat treatment according to a method according to the invention.

[0058] DETAILED DESCRIPTION OF EMBODIMENTS

[0059] Generally, a protective carbon layer is formed on a substrate having a surface portion of monocrystalline silicon carbide. The carbon layer temporarily protects the surface of the substrate against the formation of terraces during a heat treatment. The protective layer is formed by transforming the surface into m-SiC under temperature and pressure conditions suitable for causing sublimation of the silicon atoms present on said surface while minimizing the formation of terraces. After the heat treatment, the protective layer is removed from the surface of the substrate. Figure 2 illustrates a substrate having a surface layer 20 made of monocrystalline silicon carbide. Said layer 20 is arranged on a support 11 formed of one or more materials different from monocrystalline silicon carbide.The support may comprise a monocrystalline or polycrystalline semiconductor material, for example polycrystalline silicon carbide. Alternatively, the substrate may be a solid monocrystalline silicon carbide substrate, the layer 20 forming a surface portion of said substrate.

[0060] Formation of the carbon layer

[0061] Referring to Figure 3A, a carbon layer 30 is formed on the surface 200 of the m-SiC layer 20. The carbon layer 30 protects the surface of the layer 20 against the formation of terraces during heating to a high temperature. It thus stabilizes the crystalline structure of the surface.

[0062] The carbon layer 30 is formed by sublimation of silicon onto the surface of the m-SiC layer. Advantageously, the formation of the carbon layer is carried out in a furnace which will be described later. In order to obtain such sublimation, the pressure in the furnace is lowered to a pressure pp. The pressure pp is preferably between 0.01 and 0.5 Pa (0.1 and 5 pbar).

[0063] No additional gas is injected into the furnace, which keeps pressure low and avoids any risk of contamination by impurities that may be present in such gas.

[0064] During pressure reduction to pressure pp, the substrate is heated to a temperature Tp. The pressure reduction and heating processes are adjusted so that pressure pp is reached before or simultaneously with reaching temperature Tp.

[0065] Alternatively, the temperature is increased after the pressure has been lowered to pressure pp. The temperature Tp is higher than the sublimation temperature of silicon at said pressure pp in the furnace. Typically, the temperature Tp is between 1200°C and 1500°C.

[0066] A temperature below 1200°C does not allow silicon sublimation in the pp pressure range that can be achieved in a substrate processing furnace. A temperature above 1500°C causes terrace formation that competes with silicon sublimation, leading to the formation of excessively high steps between the terraces.

[0067] The aim is to limit the formation of steps between terraces to a height of less than 25 nm in order to minimize the polishing required to remove said steps. The formation of higher terraces would require long and laborious polishing. In addition, such polishing often leads to the creation of defects such as scratches in the m-SiC, which increases the number of defective substrates. Lowering the pressure in the furnace allows silicon to sublimate at a temperature TF lower than the temperature at which terrace formation becomes significant. The pressure pp is thus adjusted to avoid or reduce terrace formation during carbon layer formation.

[0068] The pressure pp and the temperature TF are maintained until a carbon layer 30 covering the entire surface of the monocrystalline silicon carbide is obtained. In order to maintain the pressure pp, vacuum pumping of the furnace is maintained throughout the carbon layer formation stage. Continuous pumping causes the evacuation of silicon atoms from the furnace. Such evacuation is necessary to lower the partial pressure of silicon and thus allow the sublimation of other silicon atoms from the substrate surface.

[0069] When the carbon layer is finalized, the carbon layer prevents the silicon from sublimating. At this stage, the terraces and the height of the steps are fixed. As a result, it is possible to continue the heat treatment at a higher temperature without degrading the surface finish. Typically, the pressure is increased by injecting an inert gas, such as argon, into the furnace.

[0070] Structure of the carbon layer

[0071] The free face 200 of the m-SiC layer 20 is a single-crystal silicon carbide surface of the 4H or 6H polytype with one face (0001). This crystal orientation favors the growth of carbon in the form of two distinct layers on the surface.

[0072] Optionally, the free face 200 forms an angle of between 0 and 8 degrees, for example between 3 and 5 degrees, with the (0001) face of the SiC single crystal. The free face 200 is, in particular, formed by a set of (0001) facets separated from each other by atomic steps. Such a disorientation limits, during epitaxy on this free face, the risks of generating 3C polytype zones.

[0073] By convention, the (0001) face of m-SiC is generally called the “Si face”, and the (000-1) face is called the “carbon face”.

[0074] At the start of the sublimation of the silicon of the surface of the layer 20, with reference to FIG. 3B, a first layer 31 of carbon is formed on the surface. The layer 31 extends over the entire surface 200 of the m-SiC layer 20 and takes on the function of a buffer layer arranged between the surface 200 of the substrate and any carbon layers formed subsequently. The first layer 31 is made of hexagonal carbon with a low proportion of sp3 hybridization bonded to the silicon atoms of the layer 20. The proportion of sp3 bonds decreases from the face in contact with the layer 20 towards the free face of the layer 31.

[0075] As silicon sublimation continues, further carbon layers are formed on the buffer layer 31. The carbon layers on the buffer layer 31 are graphene monolayers, i.e., these layers comprise only sp2 bonds between the carbon atoms. The stacked graphene monolayers together form a surface layer 32.

[0076] The formation and characterization of such a carbon layer on an m-SiC surface are for example described by Strupinski et al. and by Razado-Colambo et al.

[0077] Thus, with reference to Figure 3C, the carbon layer 30 is typically composed of a buffer layer 31 in contact with the m-SiC surface of the substrate, and a surface layer 32 arranged on the surface of the buffer layer 31. Preferably, the surface layer 32 comprises between one and three monolayers of graphene. The presence of a carbon layer composed of a buffer layer 31 and a surface layer 32 greatly limits the formation of terraces during a high temperature heat treatment.

[0078] The m-SiC surfaces of the 4H and 6H polytype promote graphene growth in a plane parallel to the surface of the buffer layer 31. Such parallel growth limits the evaporation of silicon from the surface during subsequent heat treatment and thus prevents thermal decomposition of the silicon carbide layer. The growth in the form of graphene also limits the mobility of silicon and carbon atoms on the substrate surface. In addition, m-SiC surfaces of such a crystal structure are particularly affected by terrace formation.

[0079] In the case of a substrate with a large surface area, it is possible that the thickness of the carbon layer may have inhomogeneities. By forming a buffer layer and a surface layer, it is ensured that the entire surface of the substrate is protected by a carbon layer and the risk of forming areas without a carbon layer on the substrate surface is prevented.

[0080] In some embodiments, only a buffer layer 31 is formed. In this case, the carbon layer formation process is stopped when the buffer layer is finalized, and no surface layer 32 is formed. Such a buffer layer alone can also limit the formation of terraces on the surface of the m-SiC layer 20.

[0081] Generally, the buffer layer 31 extends over the entire surface of the substrate. On the other hand, the surface layer of graphene 32 may be arranged over the entirety or only over certain areas of the buffer layer 31, and / or have different thicknesses depending on the position on the substrate.

[0082] Typically, with increasing distance from the center of the substrate, the thickness of the carbon layer increases due to the presence of the graphene monolayers. Such a distribution of the graphene monolayers typically occurs when simultaneously processing several superimposed substrates, due to the more efficient evacuation of silicon atoms in the areas close to the edge B of the substrate. Illustratively and not limitingly, with reference to FIG. 3D, the carbon layer may consist of a bare buffer layer 31 in a first area close to the center C of the substrate. The carbon layer may comprise a second area 321 in which the carbon layer comprises a single graphene monolayer arranged on the buffer layer 31, and a third area 322 in which the carbon layer comprises two graphene monolayers arranged on the buffer layer 31.In a fourth area extending from edge B, the carbon layer comprises three graphene monolayers arranged on the buffer layer 31.

[0083] The buffer layer 31 typically has a thickness of approximately 0.2 nm. The surface layer 32 typically has a thickness of between 0 and 1 nm. Thus, the thickness of the carbon layer 30 is between 0.2 and 1.2 nm.

[0084] A carbon layer thickness of between 0.2 and 1.2 nm is sufficient to limit the formation of terraces during heat treatment to a height below 25 nm. At pressure pp and temperature TF in the furnace, it is possible to cover the entire surface of the substrate with a carbon layer of such thickness. A greater thickness is difficult to obtain because the sublimation of silicon atoms from the upper surface of the substrate is blocked by the presence of the already formed carbon layer. The formation of additional graphene monolayers is therefore longer than the formation of the first monolayers and would unnecessarily prolong the process.

[0085] High temperature steps requiring a protective layer

[0086] After the formation of the carbon layer, further steps can be carried out on the substrate at a heat treatment temperature TT, typically higher than the carbon layer formation temperature TF. Such steps are, for example, annealing for dopant activation or annealing to heal defects in the crystal lattice caused during ionic species implantation. Such implantation can be used for the transfer of the m-SiC layer onto a base substrate as described below. Ionic species implantation can also be carried out for doping the m-SiC.

[0087] During the annealing step(s), the surface of the monocrystalline semiconductor material is protected by the carbon layer and the formation of terraces on said surface is thus avoided. An annealing step is typically carried out at a heat treatment temperature TT strictly greater than 1300°C, preferably greater than 1500° and more preferably greater than 1700°. Advantageously, the heat treatment temperature is less than or equal to 2000°C. Typically, the heat treatment is carried out under an atmosphere of a protective gas, for example argon. During this step, the heat treatment pressure PT in the furnace is between the pressure PF and atmospheric pressure, for example greater than or equal to 2 Pa. Advantageously, such heat treatments are carried out directly after the formation of the carbon layer in the same furnace.The temperature rise to the heat treatment temperature can be achieved from the TF temperature without intermediate cooling. The pressure rise can be achieved from the pp pressure under a protective gas atmosphere, without exposing the substrate to ambient air. Subsequent treatment in the same furnace avoids intermediate cooling steps, exposure to ambient air and removal and loading into different enclosures. This reduces the necessary labor, cooling and heating time, and the risk of substrate contamination.

[0088] Temperature profile

[0089] As an illustration and not a limitation, Figure 4 shows a temperature profile of such a process. The substrate is placed in the furnace at time to. The pressure is lowered to reach pressure pp. Subsequently, the temperature is increased to temperature TF at time ti. This temperature is, for example, approximately 1450°C for a pressure pp of 2.10 -1Pa. The carbon layer is formed by sublimation until time t2. The duration of the carbon layer formation is typically between 10 and 120 minutes. The pressure in the furnace is then increased to stop the sublimation of silicon and the formation of carbon on the substrate surface. Typically, the pressure is increased to about 2 Pa under an argon atmosphere. The substrate is heated until time ta where the temperature TT required for heat treatment is reached, for example 1700°C. The heat treatment temperature TT is maintained depending on the type of substrate and the objective of the heat treatment. After the heat treatment, at time t4, the furnace temperature is lowered. After the substrate has cooled, at time ts, the substrate can be removed from the furnace for further treatments such as polishing.

[0090] Removal of terraces and carbon layer

[0091] At the end of the carbon layer formation and heat treatment stages, the temperature is lowered in order to remove the substrate from the oven.

[0092] The terraces formed during heat treatment can then be removed, for example by mechanical or chemical-mechanical polishing. The carbon layer is removed in the same step. The low thickness of the carbon layer avoids additional steps of carbon layer removal.

[0093] The low height of the steps that can appear during heat treatment and the low thickness of the carbon layer make it possible to limit the shrinkage of the silicon carbide to a thickness less than or equal to 200 nm. This makes it possible to considerably limit the polishing time and material consumption compared to the polishing steps of a substrate treated using a process without a protective layer.

[0094] At the end of this step, the substrate is finalized. The free m-SiC surface can then be used for the epitaxial growth of a layer of a monocrystalline semiconductor material, for example another layer of m-SiC or a layer of GaN, and / or for the manufacture of a power or radiofrequency electronic component.

[0095] Method of manufacturing a semiconductor structure

[0096] We will now describe a method of manufacturing a semiconductor structure for electronic applications, having a surface layer 20 of monocrystalline silicon carbide.

[0097] A donor substrate 220 made of monocrystalline silicon carbide is provided from which the surface layer of monocrystalline silicon carbide will be formed. Typically, the donor substrate also comprises dopants incorporated in the crystalline matrix. With reference to FIG. 5A, as shown schematically by the arrows, an implantation of ionic species, such as hydrogen and / or helium, is carried out so as to form a weakening zone 21 in the donor substrate 220. Said weakening zone 21 defines a layer 20 of monocrystalline silicon carbide to be transferred.

[0098] With reference to Figure 5B, the donor substrate 220 thus implanted is bonded to a base substrate 11. The donor substrate can be bonded directly to the base substrate as illustrated in Figure 5B, or via one or more intermediate layers which can be arranged on the donor substrate and / or on the base substrate before bonding (not shown).

[0099] Referring to Figure 5C, the donor substrate 220 is then detached along the weakening zone 21, which leads to the transfer of the monocrystalline silicon carbide layer onto the base substrate 11, thus forming the substrate 10 as shown in Figure 2. The detached portion 221 of the donor substrate can be reused for the transfer of other monocrystalline silicon carbide layers onto other base substrates.

[0100] A treatment method is then carried out, successively comprising a step of stabilizing the surface by forming a protective carbon layer 30, high-temperature annealing intended to activate the dopants and / or cure the defects created in the monocrystalline silicon carbide layer during the implantation of the ionic species, and the subsequent removal of the protective carbon layer.

[0101] Presentation of the oven

[0102] In order to make a process for treating a plurality of substrates efficient and rapid, an annealing furnace is preferably used that can simultaneously treat a batch of substrates under identical conditions. Such a furnace is conventionally called a "batch anneal" furnace in the field of microelectronics.

[0103] Figure 6 illustrates a furnace 70 adapted to receive a quantity of between 120 and 150 substrates in an annealing chamber. The substrates 10 are positioned horizontally in the furnace and are superimposed vertically in order to treat a maximum number of substrates. A free space is maintained between each substrate 10 and the adjacent substrates 10 to optimize the gas flow and to obtain a homogeneous pressure when the pressure is lowered. A homogeneous pressure makes it possible to facilitate the formation of the carbon layer and to obtain a more homogeneous thickness of the carbon layer over the extent of each substrate. The optimal vertical distance depends on the diameter of the substrates. Typically, the vertical distance between two superimposed substrates is between 4 and 10 mm for substrates with a diameter of between 150 and 200 mm.The distance between substrates can be increased for large substrate diameters, and decreased for smaller diameters to allow gas flow between two adjacent substrates, while optimizing the space in the furnace to allow the simultaneous processing of a large number of substrates.

[0104] Such a furnace uses a “top flow” configuration, i.e. a gas flow can be fed into the furnace from the top 71 and / or discharged through an outlet 79 at the bottom of the furnace. During the formation of the carbon layer, no gas is injected into the furnace. The pressure is lowered by pumping at the outlet 79 of the furnace. During the pressure build-up, a gas flow can be fed into the furnace from the top 71.

[0105] All elements of the annealing chamber inside the furnace, i.e. the inner walls 75, the substrate holders for holding the substrates in a horizontal and vertically superimposed position, and the gas included in the annealing chamber are at the same temperature during use of the furnace, with the exception of the outer elements such as the outer wall 76 and the ends of the gas inlet pipes. By the same temperature, it is meant that the temperature difference between the top end 74 of the furnace and the bottom end 78 of the furnace is minimal, typically a few degrees °C, for example less than 3 degrees for a temperature in the furnace between 800 and 1200°C.

[0106] The substrates are heated mainly by conduction from the furnace walls. The gases are injected at the same temperature as the temperature inside the furnace.

[0107] An annealing furnace is not intended for the formation of layers but for the application of a heat treatment to the substrates placed in the furnace. These furnaces are therefore optimized for high temperature homogeneity of the substrates received there, but not to ensure homogeneity of gas evacuation in the space between the substrates. The superimposed positioning of the substrates is particularly detrimental to this homogeneity since, when the pressure is lowered, it is more difficult to lower the pressure in the space between the substrates near the center of the substrates than at their edges. It is also more difficult to eliminate silicon atoms following sublimation during the creation of the carbon layer.

[0108] In order to obtain a homogeneous carbon layer, it is necessary to control the thickness so as to obtain a sufficient thickness over the entire surface area of ​​all the substrates contained in the furnace, particularly in the central area of ​​the substrates.

[0109] The pressure pp between 0.01 and 0.5 Pa is particularly suitable for processing a batch of substrates in such a furnace. On the one hand, a pressure higher than 0.5 Pa causes insufficient evacuation of silicon atoms between two adjacent substrates, especially near the center of the substrate. Insufficient evacuation results in the presence of a significant number of silicon atoms close to the substrate surface and thus prevents the sublimation of other silicon atoms from the m-SiC of the lower substrate surface. Such blocking of silicon sublimation therefore prevents the formation of the carbon layer.

[0110] On the other hand, a pressure below 0.01 Pa is difficult to achieve in an industrial furnace and requires significant pumping effort and time. Such pumping requires additional equipment and slows down the process considerably. In addition, it is difficult to ensure sufficient sealing of the furnace to reliably achieve lower pressures.

[0111] The formation of a buffer layer and one or more thicknesses of graphene on the surface of each substrate allows to obtain a sufficient and sufficiently homogeneous thickness to limit the formation of terraces on each substrate. When the substrates are superimposed in a furnace, a greater thickness of the carbon layer is typically obtained near the edge of each substrate, and a thinner thickness near the center of the substrate. Using a carbon layer formation pressure pp between 0.01 and 0.5 Pa, the carbon layer typically has a thickness between 0.2 and 1.2 nm over the entire extent of the substrate.

[0112] Unlike a deposition chamber, such an oven allows a large number of substrates to be treated simultaneously thanks to the vertical stacking arrangement.

[0113] Homogeneity of the protective layer and the effect on the formation of terraces

[0114] Figure 7 shows the root mean square (RMS) value of the surface roughness caused by terrace formation on a single-crystal SiC surface, measured by AFM. The roughness is measured as a function of the radial position on the substrate.

[0115] Curve n corresponds to the roughness measured on substrates that have undergone heat treatment at 1700°C for 30 minutes without the formation of a protective carbon layer. Due to the limited gas flow between the substrates, significant inhomogeneity is observed between the measurement points close to the center C of the substrate, and the measurement points close to the edge B of the substrate. The average roughness is greater than 15 nm near the center C, and between about 7 and 9 nm near the edge B.

[0116] Curve r2 corresponds to the roughness of substrates after a treatment comprising the formation of a carbon layer and a heat treatment at 1700°C for 30 minutes. The temperature and duration of the heat treatment are therefore identical to the temperature and duration of the treatment of the substrate in curve n. During the heat treatment, each substrate was therefore protected by a respective carbon layer. The roughness is approximately 5 nm near the center C and between 3 and 4 nm near the edge B of each substrate. The roughness of a substrate with a carbon layer formed by a process as described above (curve r2) is therefore clearly lower than the roughness of a substrate treated without a carbon layer (curve n).

[0117] In both processes (curves n and r2), the substrates were arranged within a batch of substrates in a furnace as shown above. The vertical distance between two adjacent substrates was 4 mm. The difference in roughness at the center and near the edge is due to the limited gas flow and the difference in partial pressures of different gases in the central area between two substrates superimposed in the furnace.

[0118] Figure 8 shows the step height between two terraces formed on the monocrystalline SiC surfaces of the same substrates as in Figure 7. The step height is evaluated as the average height in a 30x30 pm area 2 . The abscissa axis corresponds to the radial position on the substrate.

[0119] The step height of the substrates that have undergone heat treatment without formation of a carbon layer is illustrated by the curve Hi. The average step height Hi is about 80 nm near the center C and about 40 nm near the edge B of the substrate. The curve H2 shows the average step height for substrates after a process comprising the formation of a carbon layer as described above, followed by a heat treatment identical to the heat treatment of the substrates in curve H1. The step height H2 on the substrate protected by a carbon layer is between 30 and 40 nm near the center C of the substrate, and between 25 nm and 30 nm near the edge B of the substrate. The step height formed during the heat treatment is therefore considerably reduced by the presence of a carbon layer for the two respective radial positions.

[0120] Figures 9A and 9B are electron beam scanning microscopy images of the m-SiC surface of a substrate after heat treatment by a known method without carbon layer formation. Each image represents a 5x5 pm area 2 Figure 9A is an image of an area near the center of the substrate. Figure 9B is an image of an area near the edge of the same substrate. The terraces have a greater extent and height in the area near the center of the substrate.

[0121] Figures 10A and 10B are electron beam scanning microscopy images of the m-SiC surface of a substrate after heat treatment according to a method as described above, including the formation of a carbon layer by sublimation of silicon. Each image represents a 5x5 μm area 2. Figure 10A is an image of an area near the center of a substrate. Figure 10B is an image of an area near the edge of the same substrate. As with the substrate in Figures 9A and 9B, the terraces have a larger width and height in the area near the center of the substrate. However, the terraces have a considerably smaller extent and size than all the terraces obtained on the substrate after the heat treatment without the formation of a carbon layer.

[0122] The extent of terraces near the center of the substrate that is protected by a carbon layer (Figure 10A) is smaller than the size of terraces at the edge and center of the substrate treated without the formation of such a protective layer (Figure 9A). Moreover, in the area near the edge of the substrate (Figures 9B and 10B), terrace formation was almost suppressed by the presence of the carbon layer.

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[0127] I. Razado-Colambo et al. « Structural determination of bilayer graphene on SiC(0001) using synchrotron radiation photoelectron diffraction”, Scientific Reports 2018, 8:10190

Claims

CLAIMS 1. A method of treating a substrate (10) comprising at least one surface layer of monocrystalline silicon carbide, said method comprising: o placing said substrate (10) in a furnace, o lowering the pressure in the furnace to a pressure (pp) of between 0.01 and 0.5 Pa, o heating the substrate to a temperature (TF) above a sublimation temperature of silicon at said pressure (pp), so as to form a carbon layer on the monocrystalline silicon carbide layer by sublimation of silicon from the surface of said monocrystalline silicon carbide layer, and, o after the formation of the carbon layer (30), heat treating the substrate at a heat treatment temperature (TT) strictly above 1300°C and under a pressure (pr) above the pressure (pp) for forming the carbon layer,the carbon layer protecting the surface of the monocrystalline silicon carbide layer against step formation., 2. Method according to claim 1, in which the temperature (TF) for forming the carbon layer (30) is between 1200°C and 1500°C.

3. Method according to any one of the preceding claims, in which the carbon layer (30) is a graphene layer.

4. Method according to any one of the preceding claims, in which the carbon layer (30) has a thickness of between 0.2 nm and 1.2 nm.

5. Method according to one of the preceding claims, in which a plurality of substrates (10) are treated simultaneously in the same oven (70).

6. Method according to claim 5, in which the substrates (10) are vertically superimposed in the oven (70) so that a vertical space is provided between two adjacent substrates.

7. Method according to claim 6, wherein said vertical space between two adjacent substrates (10) has a height of between 4 and 10 mm.

8. Method according to any one of the preceding claims, in which the temperature (TT) of the heat treatment is greater than or equal to 1500°C, preferably greater than or equal to 1700°C.

9. Method according to any one of the preceding claims, in which the temperature (TT) of the heat treatment is less than or equal to 2000°C.

10. Method according to one of the preceding claims, in which the pressure (PT) of the heat treatment is greater than or equal to 2 Pa.

11. Method according to one of the preceding claims, in which the formation of the carbon layer and the heat treatment are carried out in the same furnace.

12. Method according to one of the preceding claims, further comprising a step of removing the carbon layer (30) after the heat treatment.

13. Method according to one of the preceding claims, further comprising a step of chemical-mechanical polishing after the heat treatment to remove steps formed on the surface of the silicon carbide layer during said heat treatment.

14. The method of claim 13, wherein said chemical-mechanical polishing results in the removal of a silicon carbide thickness of less than 200 nm.

15. Method for manufacturing a structure comprising a layer of monocrystalline silicon carbide on a support substrate, successively comprising: - the formation of a weakening zone in a monocrystalline silicon carbide donor substrate by implantation of ionic species, so as to delimit a layer of monocrystalline silicon carbide to be transferred, - bonding of the donor substrate to the support substrate, - detachment of the donor substrate along the weakening zone so as to transfer the layer of monocrystalline silicon carbide onto the support substrate, - implementing the treatment according to one of the preceding claims on the transferred monocrystalline silicon carbide layer.

16. Method according to claim 15, in which the layer of monocrystalline silicon carbide comprises dopants and in that the heat treatment is carried out at a temperature suitable for activating said dopants.

17. Method according to one of claims 15 or 16, in which the silicon carbide layer comprises defects due to the implantation of ionic species and in that the heat treatment is carried out at a temperature suitable for curing at least part of said defects.

18. Method according to one of claims 15 to 17 in their dependent relationship with claims 13 or 14, further comprising, after the chemical-mechanical polishing, a step of resuming epitaxy of monocrystalline silicon carbide on the surface of the monocrystalline silicon carbide layer.

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