METHOD FOR PREPARING A SUPPORT SUBSTRATE MADE OF POLYCRYSTALLINE MATERIAL AND METHOD FOR MANUFACTURING A COMPOSITE STRUCTURE INCLUDING SAID SUPPORT SUBSTRATE

Abstract

The invention relates to a method for preparing a support substrate made of polycrystalline material, the preparation method comprising the following steps: a) providing a raw disc made of polycrystalline material, having two faces; b) rough grinding at least one of the faces of the raw disc to obtain a ground disc having at least one ground face, the rough grinding inducing, at said ground face, the formation of a surface damaged region; the surface damaged region comprising a surface work-hardened layer and an underlying stressed layer; c) applying a heat treatment to the ground disc, the temperature and duration of the heat treatment being defined so as to relax stresses present in the stressed layer;step c) leading to obtaining an annealed disc comprising, at the level of the -at least one- ground face, a first superficial annealed layer, in place of the work-hardened layer, and a second underlying annealed layer, in place of the constrained layer; d) thinning the annealed disc, at the level of the -at least one- ground face, to remove the first annealed layer; step d) leading to obtaining the support substrate in which all or part of the second annealed layer is retained. Figure to be published with the abstract: No figure;

Description

Title of the invention: METHOD FOR PREPARING A SUPPORT SUBSTRATE MADE OF POLYCRYSTALLINE MATERIAL AND METHOD FOR MANUFACTURING A COMPOSITE STRUCTURE INCLUDING SAID SUPPORT SUBSTRATE FIELD OF THE INVENTION

[0001] The present invention relates to the field of semiconductor materials for microelectronic components. It relates in particular to a method for preparing a support substrate made of polycrystalline material, in particular SiC, capable of being used for the manufacture of a composite structure including a thin monocrystalline layer transferred onto said support substrate. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] SiC is increasingly widely used for the manufacture of innovative devices (power components, radio frequencies, etc.), to meet the needs of rising electronics fields, such as electric vehicles. Indeed, power devices and integrated power systems based on monocrystalline silicon carbide can handle a much higher power density compared to their traditional silicon counterparts, and this with smaller active area dimensions.

[0003] High-quality monocrystalline SiC (c-SiC) substrates intended for the microelectronics industry nevertheless remain expensive and difficult to supply in large sizes. It is therefore advantageous to use layer transfer solutions to develop composite structures typically comprising a thin monocrystalline SiC layer (derived from the high-quality c-SiC substrate) on a lower-cost support substrate, for example polycrystalline SiC (p-SiC). The electronic components can then be developed on and / or in the thin layer. It should also be noted that a composite structure can provide additional functionalities and performances, in particular by providing a support substrate with advantageous mechanical, electrical and / or thermal properties.

[0004] A well-known thin-layer transfer solution is the Smart Cut™ process, based on light ion implantation and direct bonding between a monocrystalline donor substrate and a support substrate at a bonding interface.

[0005] To be compatible with a thin film transfer process, the support substrate must have as little curvature or deformation as possible. Low curvature is also desired to ensure the performance of the steps of fabrication (e.g., photolithography) of the components on / in the thin layer of the composite structure.

[0006] Document FR3132381 proposes a method for manufacturing a non-deformable p-SiC wafer.

[0007] Starting from a p-SiC plate separated from its growth substrate, this method comprises a heat treatment step at a temperature between 1650°C and 2000°C, followed by a thinning step including in particular very rough grinding, rough grinding and fine grinding. The heat treatment is likely to cause deformation of the plate. The thinning is then adapted to include a correction of said deformation, by locally adjusted material removal.

[0008] SUBJECT OF THE INVENTION

[0009] The present invention provides a method for preparing a support substrate made of polycrystalline material, in particular p-SiC, as an alternative to the methods of the prior art, which promotes low curvature of the support substrate and which minimizes manufacturing costs. The invention also relates to a method for manufacturing a composite structure including said support substrate.

[0010] BRIEF DESCRIPTION OF THE INVENTION

[0011] The invention relates to a method for preparing a support substrate made of material polycrystalline, the preparation process comprising the following steps:

[0012] a) providing a raw disc made of polycrystalline material, having two faces;

[0013] b) rough grinding of at least one of the faces of the raw disc to obtain a ground disc having at least one ground face, the rough grinding inducing, at the level of said ground face, the formation of a surface damaged region; the surface damaged region comprising a surface work-hardened layer and an underlying stressed layer;

[0014] c) applying a heat treatment to the ground disc, the temperature and duration of the heat treatment being defined so as to relax stresses present in the stressed layer; step c) leading to obtaining an annealed disc comprising, at the level of the -at least one- ground face, a first surface annealed layer, in place of the work-hardened layer, and a second underlying annealed layer, in place of the stressed layer;

[0015] d) thinning the annealed disc, at the level of the -at least one- ground face, to remove the first annealed layer; step d) leading to obtaining the support substrate in which all or part of the second annealed layer is preserved.

[0016] According to other advantageous and non-limiting characteristics of the invention, taken alone or in any technically feasible combination: • step a) comprises the following sub-steps:

[0017]

[0018]

[0019]

[0020]

[0021] (al) chemical vapor deposition of a layer formed from the polycrystalline material, on a growth substrate, a2) removal of the growth substrate to obtain the raw disc.; • the polycrystalline material is silicon carbide or aluminum nitride; • step b) is applied to both sides of the raw disc, and / or step d) is applied to both sides of the raw disc; • coarse grinding is carried out with a grinding wheel whose abrasive grains have an average size greater than or equal to 10 qm, or even greater than or equal to 18 qm; • rough grinding, applied to one and / or the other of the faces of the raw disc, removes a first thickness greater than or equal to 150qm; • the heat treatment of step c) is carried out at a temperature greater than or equal to 1600°C, preferably greater than or equal to 1700°C, still preferably greater than or equal to 1800°C, still preferably greater than or equal to 1850°C, or even still preferably greater than or equal to 2000°C; • the thinning of step d) includes fine grinding carried out with a grinding wheel whose abrasive grains have an average size less than or equal to 5qm, less than or equal to 4qm, or even less than or equal to 3qm; • fine grinding, applied to one and / or the other of the faces of the annealed disc, removes a second thickness less than or equal to 50qm; • the second thickness is less than or equal to 20qm, preferably between 5qm and 20qm; • step d) comprises, after fine grinding, a surface preparation comprising polishing, cleaning, annealing, etching and / or deposition of a film, so as to obtain a surface condition of the support substrate compatible with direct assembly on another substrate. The invention also relates to a method of manufacturing a composite structure comprising the following steps: - the preparation of a support substrate made of a polycrystalline material as above, and - the transfer of a thin layer of monocrystalline material, from a donor substrate, onto said support substrate. Advantageously, the polycrystalline material is chosen from silicon carbide and aluminum nitride. Advantageously, the monocrystalline material is chosen from silicon carbide, gallium nitride, gallium oxide, diamond, silicon, germanium, indium phosphide.

[0022] The manufacturing method may further comprise forming electronic components in or on the thin layer. BRIEF DESCRIPTION OF THE FIGURES

[0023] Other characteristics and advantages of the invention will emerge from the detailed description of the invention which follows with reference to the appended figures in which:

[0024] [Fig. 1a]

[0025] [Fig. 1a]

[0026] [Fig.lb]

[0027] [Fig. the]

[0028] [Fig.ld] [Fig.la], [Fig.la], [Fig.lb], [Fig.le] and [Fig.ld] show a method for preparing a support substrate according to the present invention;

[0029] [Fig.2a]

[0030] [Fig.2b] [Fig.2a] and [Fig.2b] show composite structures from a manufacturing method in accordance with the invention;

[0031] [Fig.3a]

[0032] [Fig.3b] [Fig.3a] and [Fig.3b] present cross-sectional images, obtained by mi scanning electron microscopy of discs ground after step b) of a method according to the present invention; in particular, the surface work-hardened layer can be observed.

[0033] The same references in the figures may be used for elements of the same type. Some figures are schematic representations which, for the sake of readability, are not to scale. In particular, the thicknesses of the layers along the z axis are not to scale with respect to the lateral dimensions along the x and y axes; and the relative thicknesses of the layers to each other are not necessarily respected in the figures. DETAILED DESCRIPTION OF THE INVENTION

[0034] The present invention relates to a method for preparing a support substrate 200 made of polycrystalline material. Preferably, said material is silicon carbide (SiC) or aluminum nitride (AIN).

[0035] There are several techniques for producing or growing polycrystalline materials in the form of discs, intended to form substrates or wafers, in particular for the semiconductor industry. These techniques include manufacturing by sintering, PVT (Physical Vapor Transport), CVD (Chemical Vapor Deposition) or HTCVD (High Temperature Chemical Vapor Deposition), etc.

[0036] The first step a) of the preparation process according to the invention corresponds to the supply of a raw disc 2 made of polycrystalline material, produced by one of the aforementioned known techniques. The raw disc 2 has two faces which extend substantially parallel to a main plane (x,y).

[0037] Preferably, step a) involves a first sub-step a1) of chemical vapor deposition (CVD) of a layer 2' of polycrystalline material, on a so-called growth substrate 2” ([Fig.la]).

[0038] In the case where the polycrystalline material is SiC, this technique involves a gas mixture comprising at least one silicon precursor gas (such as a silane or a chlorosilane) and / or at least one carbon precursor gas (such as an alkane or an alkene), and / or at least one silicon and carbon precursor gas (such as methyltrichlorosilane, abbreviated MTCS), and, if necessary, at least one doping gas. These gases may be diluted in a carrier gas, which may be a reducing gas such as hydrogen and / or an inert gas such as argon. From this gas mixture, the polycrystalline SiC layer 2' is formed on the growth substrate 2”. The latter is preferably made of fine-grained, purified isostatic graphite, to form a 3C-SiC polytype. The reactor temperature during CVD deposition of SiC must be between approximately 1000°C and approximately 1600°C.For example, the deposition can be carried out on a graphite disc, with a diameter approaching a nominal 150 mm and a thickness greater than 2 mm to ensure sufficient flatness of the growth substrate / p-SiC layers assembly. The diameter and thickness of the 2” graphite disc can of course vary, if one wishes to prepare a support substrate with a diameter of 100 mm, 200 mm or other.

[0039] The following sub-step a2) corresponds to the removal of the growth substrate 2” to obtain a raw disc 2 made of polycrystalline material having two faces 2a, 2b ([Fig.la]').

[0040] Returning to the previous example relating to SiC, the graphite growth substrate 2', coated with the p-SiC deposition layer 2', is machined, then oxidized in air typically at 900°C to remove any graphite residue. Note that the removal of the graphite 2” could also be carried out by purely mechanical machining techniques or even essentially by burning / oxidation. In practice, a raw p-SiC disc 2 is often recovered at each face of the growth substrate 2”.

[0041] For a diameter of 150mm or 200mm, the thickness of the raw disc 2 of polycrystalline material provided in step a) is typically between 700qm and 3000qm.

[0042] The method then comprises a step b) consisting of a rough grinding of at least one of the faces 2a, 2b of the raw disc 2, with a grinding wheel adapted to a high removal of material.

[0043] As is known per se, a grinding wheel is a tool with symmetry of re volution composed of abrasive grains embedded in a binding matrix. The abrasive grains are, for example, diamond grains. To grind one face of the raw disc 2, the grinding wheel is set in rotation and lowers gradually to abrade the polycrystalline material and thus remove material to a certain thickness.

[0044] The grinding of step b) is described as “coarse” because it uses a grinding wheel with a large grain size.

[0045] In particular, the abrasive grains have an average size (or average diameter) greater than or equal to 10 μm, typically between 10 μm and 160 μm. By average size is meant the average of the sizes (diameters or equivalent diameters) of the abrasive grains forming the grinding wheel.

[0046] In other words, the mesh (or grit according to English terminology) of the grinding wheel, the value of which varies inversely with the size of the abrasive grains, is less than or equal to 2000, typically between 100 and 2000.

[0047] Preferably, the abrasive grains have an average size greater than or equal to 18qm, typically between 18qm and 50qm (i.e. a wheel mesh between 350 and 1000), or even between 50qm and 160qm (i.e. a wheel mesh between 100 and 350).

[0048] This rough grinding causes the formation of a superficial damaged region, which can extend over several micrometers, several tens of micrometers, or even a few hundred micrometers (for example, between 15qm and 600qm).

[0049] The damaged region comprises two layers: a surface work-hardened layer 21 and an underlying stressed layer 22. The work-hardened layer 21 is characterized by the presence of significant scratches on the surface (in particular causing significant surface roughness) and the presence of cracks and stresses in the material. It has a typical thickness of between 0.5qm and 50qm, or even between 5qm and 50qm for the largest average abrasive grain sizes.

[0050] The constrained layer 22, as its name indicates, is characterized by constraints in the material, but without cracks. It is located under the work-hardened layer 21 and has a typical thickness of between 10 μm and 500 μm.

[0051] The thicknesses of the work-hardened layer 21 and of the constrained layer 22 depend in particular on the size of the abrasive grains of the grinding wheel.

[0052] The damaged region with its two layers 21, 22 can be characterized in particular by transmission electron microscopy (TEM or TEM). The work-hardened layer 21 can also be identified by scanning electron microscopy (SEM or SEM): two examples are illustrated in Figures 3a and 3b, with a work-hardened layer 21, in which cracks can be seen extending, and which respectively has a thickness of the order of 4um and 1 Oum.

[0053] Advantageously, the rough grinding is carried out on the two faces 2a, 2b of the raw disc 2. Indeed, the raw disc 2 usually has a large thickness variation (TTV) and requires grinding on the side of the face where the growth took place. In addition, the face initially in contact with the growth substrate 2'' has a nucleation zone with very small grains, a zone likely to create a stress differential with the other face, and which it is therefore preferable to also grind.

[0054] Preferably, the rough grinding removes a thickness (called first thickness) on one or each face 2a, 2b of the raw disc 2, greater than or equal to 10 μm, greater than or equal to 50 μm, greater than or equal to 150 μm, greater than or equal to 250 μm, greater than or equal to 500 μm, greater than or equal to 1 mm. The first thickness is advantageously between 100 μm and 300 μm.

[0055] At the end of this step, a so-called rectified disc 20 is obtained which has two faces 20a, 20b ([Fig.lb]).

[0056] After step c), a ground disc 20 with a diameter of 150 mm has a typical thickness of between 400 μm and 800 μm. For a ground disc 20 with a diameter of 200 mm, the typical thickness is between 550 μm and 900 μm.

[0057] The following step c) provides for the application of a heat treatment to the ground disc 20. The temperature and duration of the heat treatment are defined so as to relax all or part of the stresses present in the stressed layer 22. Here we are only talking about one stressed layer 22, but as appears in [Fig.lb], the ground disc 20 may have two damaged superficial regions (therefore two stressed layers 22) at the level of one and the other of its faces 20a, 20b, if they have both been ground in step b).

[0058] The heat treatment is carried out at a temperature higher than the growth / formation temperature of the raw disc 2 and lower than the melting temperature of the polycrystalline material.

[0059] In particular, the temperature applied in step c) is greater than or equal to 1500°C (for example in the case of p-AIN), or even greater than or equal to 1600°C (for example in the case of p-SiC) for a duration greater than or equal to 10 min.

[0060] Preferably, the heat treatment of step c) is even carried out at a temperature greater than or equal to 1700°C, 1800°C, 1850°C, or even 2000°C. Also preferably, the duration of the heat treatment is greater than or equal to 30 min, greater than or equal to 1 h, or even greater than or equal to 2 h. The annealing atmosphere is advantageously neutral, for example based on argon. The heat treatment can advantageously be carried out under a pressure greater than atmospheric pressure.

[0061] At the end of this step, we obtain a disc called annealed 20' ([Fig.1e]).

[0062] This heat treatment, carried out after the rough grinding and prior to thinning by fine grinding (next step d)) allows healing of all or part of the stressed layer 22 and possibly partial healing of the work-hardened layer 21 of the damaged region (of one or both faces 20a, 20b) of the ground disc 20. This healing results in a relaxation (therefore a reduction) of the stresses present in the damaged region.

[0063] The annealed disc 20' then comprises a first annealed layer 21', formerly the work-hardened layer 21 of the ground disc 20. The first annealed layer 21' extends over the same thickness as the work-hardened layer 21, but can benefit from a lower level of defectivity and stress due to the heat treatment.

[0064] The annealed disc 20' also comprises a second annealed layer 22', located under the first annealed layer 21', and formerly the stressed layer 22 of the ground disc 20. The second annealed layer 22' is wholly or partly relaxed, that is to say it is no longer likely (or less likely) to generate a deformation of the disc during subsequent thinning or heat treatment steps. The second annealed layer 22' extends over the same thickness as the stressed layer 22.

[0065] The method then comprises a step d) of thinning the annealed disc 20', at the level of the -at least one- face ground during step b). This step aims to remove the first annealed layer 21', while the second annealed layer 22' is retained at least in part, preferably, it is retained in full or in major part (more than 80%, or even more than 90%).

[0066] Step d) includes fine grinding with a grinding wheel whose abrasive grains have an average size (or average diameter) less than or equal to 10 μm (i.e. a wheel mesh greater than or equal to 2000). Preferably, the average size of the abrasive grains is less than or equal to 8 μm, 7 μm, 6 μm, 5 μm, 4 μm, or even 3 μm (corresponding respectively to a wheel mesh greater than or equal to approximately 2400, 2800, 3300, 4000, 5000, 7000).

[0067] At the end of step e), the support substrate 200 is obtained ([Fig. 1 d]).

[0068] Due to the application of the heat treatment in step c), it is possible to remove less material by fine grinding in this step d). According to the invention, the fine grinding aims to remove the entire first annealed layer 21' or in any case a major part of it (typically more than 90%, or even more than 95%); on the other hand, it does not remove the underlying second annealed layer 22' or only a minor part of it (typically less than 20%, less than 10%, or even less than 5%), because the potential damage that the second annealed layer 22' could cause on the curvature is deactivated by step c).

[0069] Advantageously, the fine grinding removes a thickness (called second thickness) on one or each face 20a, 20b of the annealed disc 20', less than or equal to 50 pm, less than or equal to 20 pm, preferably between 5 pm and 20 pm.

[0070] For a diameter of 150 mm, the support substrate 200, after fine grinding, typically has a thickness of between 1 lOpm and 400 pm; for a diameter of 200 mm, its typical thickness is between 1 lOpm and 600 pm.

[0071] Very fine grain grinding wheels undergo significant and rapid wear with the thickness removed, so it is not economically advantageous to remove too much material with these wheels. Their advantage, however, is that they provide very favorable preliminary surface conditions (low roughness and low thickness of induced damaged region).

[0072] Because the thickness to be removed is small in step d), due to the application of the heat treatment directly after the rough grinding, it is economically viable to carry out the fine grinding with a grinding wheel whose abrasive grains are fine, or even very fine, such as having an average size less than or equal to 5 pm, less than or equal to 4 pm, or even less than or equal to 3 pm.

[0073] For example, step d) may be carried out on a 20' annealed p-SiC disc with a 4000 mesh grinding wheel, or even an 8000 mesh grinding wheel, by applying a rotation of the head supporting the wheel ("spindle") between 1000 and 2000 revolutions per minute (rpm) and a rotation of the plate supporting the 20' annealed disc of the order of 200 to 400 rpm, with a removal speed of between 0.1 pm / s and 0.5 pm / s, for example 0.15 pm / s.

[0074] After the fine grinding, step d) advantageously comprises a surface treatment comprising chemical-mechanical polishing, cleaning, annealing, chemical etching (wet or dry) and / or film deposition, so as to obtain a surface condition of the support substrate 200 compatible with direct assembly on another substrate. Such a surface condition typically corresponds to a roughness less than or equal to Inm RMS (measured by atomic force microscopy on scans of 20 pm x 20 pm).

[0075] Taking into account the successive application of steps b), c) and d), without any other intervening step, the curvature of the p-SiC support substrate 200 remains within a reasonable range, being derived from a raw disc 2 of economically viable thickness and after implementation of a simple process, minimizing manufacturing costs.

[0076] The curvature can be measured using a white light confocal sensor which scans a surface of the substrate, the latter being placed on a support plane of the measuring tool. For a support substrate 200 with a diameter of 150mm or 200mm, the curvature is typically less than or equal to 150pm, 100pm, or even 50pm.

[0077] The sequence of steps b), c) and d) also presents an economic interest because the rough grinding is rapid and the associated grinding wheel supports thick removals without prohibitive wear. In addition, since step d) requires low removal, a high grinding wheel mesh can be used, which facilitates the subsequent surface treatment aimed at obtaining a surface condition of the support substrate 200 compatible with direct assembly.

[0078] The support substrate 200 thus prepared is intended to be used for the manufacture of a composite structure 100. The method for manufacturing this structure 100 comprises the transfer of a thin layer 10 made of a monocrystalline material onto said support substrate 200. Although any known method for transferring a thin layer can be used, reference can in particular be made to the Smart Cut™ method, based on the formation of a fragile plane buried in a donor substrate made of monocrystalline material, by ion implantation of light species (for example, H, He or a combination of these two species), and on a direct assembly (by molecular adhesion) between said donor substrate and the support substrate. A separation in the buried fragile plane then makes it possible to transfer a monocrystalline thin layer 10, originating from the donor substrate, onto the support substrate 200 ([Fig.2a]), while retaining the remainder of the donor substrate for subsequent reuse.

[0079] The monocrystalline thin layer 10 may in particular be made of silicon carbide, diamond, silicon, ILVI or IILV semiconductor compounds (for example AIN, GaN, etc.), gallium oxide (Ga2O3), or any wide bandgap semiconductor material.

[0080] By way of example, the thin layer 10 of the composite structure 100 has a thickness of between a few tens of nm and a few hundreds of nm, for example, between 50 nm and 800 nm. We will see later that epitaxy steps can be implemented on said thin layer 10, so as to thicken it (homoepitaxy) or to grow other materials (heteroepitaxy), for the needs of the electronic components to be manufactured. The thin layer 10 has an electrical resistivity adapted to the application and the components targeted.

[0081] In the composite structure 100, the support substrate 200 has a thickness and a curvature as previously stated. The range of radius of curvature of the support substrate 200 makes the latter perfectly compatible with the specifications of a composite structure 100 provided with a thin monocrystalline layer 10, with the method of manufacturing such a structure 100 and with the subsequent development of microelectronic components on and / or in the thin layer 10. Note that the curvature of the composite structure 100 remains close to the curvature of the support substrate 200.

[0082] The composite structure 100 may comprise a continuous intermediate layer 30 or discontinuous, arranged between the thin layer 10 and the support substrate 200 and composed of at least one metallic or semiconductor material ([Fig.2b]). As is known per se, with reference to a method for manufacturing the composite structure 100, the intermediate layer 30 may be formed on the side of the thin layer 10, on the side of the support substrate 200 or on both sides, prior to assembly along a bonding interface of the donor substrate and the support substrate 200. The intermediate layer 30 may for example be composed of silicon, silicon carbide, tungsten and / or titanium. Its thickness is typically between a few nm and a few hundred nm, preferably between 2nm and 50nm.

[0083] The composite structure 100 obtained is extremely robust to very high temperature heat treatments that may be applied to improve the quality of the thin layer 10 or to manufacture components on and / or in said layer 10. The support substrate 200 in the composite structure 100 is stable and does not see its curvature increase prohibitively during the high temperature heat treatments applied to the composite structure 100 for its manufacture and subsequently.

[0084] The composite structure 100 according to the invention is particularly suitable for the production of one (or more) electronic component(s), in particular high voltage, such as for example Schottky diodes, MOSFET or HEMT transistors, and / or high frequencies (RF), in and / or on the thin layer.

[0085] Of course, the invention is not limited to the embodiments and examples described, and variant embodiments can be made without departing from the scope of the invention.

Claims

Claims

1. A method for preparing a support substrate (200) made of polycrystalline material, the preparation method comprising the following steps: a) providing a raw disc (2) made of polycrystalline material, having two faces (2a, 2b); b) rough grinding at least one of the faces (2a, 2b) of the raw disc (2) to obtain a ground disc (20) having at least one ground face (20a, 20b), the rough grinding inducing, at said ground face (20a, 20b), the formation of a surface damaged region; the surface damaged region comprising a surface work-hardened layer (21) and an underlying stressed layer (22); c) applying a heat treatment to the ground disc (20), the temperature and duration of the heat treatment being defined so as to relax stresses present in the stressed layer (22);step c) leading to obtaining an annealed disc (20') comprising, at the level of the -at least one- ground face (20a, 20b), a first superficial annealed layer (21'), in place of the work-hardened layer (21), and a second underlying annealed layer (22'), in place of the constrained layer (22); d) thinning the annealed disc (20'), at the level of the -at least one- ground face (20a, 20b), to remove the first annealed layer (21'); step d) leading to obtaining the support substrate (200) in which all or part of the second annealed layer (22') is preserved.;

2. Preparation method according to claim 1, wherein step a) comprises the following sub-steps: a1) chemical vapor deposition of a layer formed from the polycrystalline material (2'), on a growth substrate (2”), a2) removal of the growth substrate (2”) to obtain the raw disc (2).

3. Preparation method according to one of the preceding claims, wherein the polycrystalline material is silicon carbide (SiC) or aluminum nitride (AIN).

4. Preparation method according to one of the preceding claims, wherein step b) is applied to both faces of the raw disc (2), and / or step d) is applied to both faces of the raw disc (2).

5. Preparation process according to one of the preceding claims in in which the rough grinding is carried out with a grinding wheel whose abrasive grains have an average size greater than or equal to 10 qm, or even greater than or equal to 18 qm.

6. Preparation method according to one of the preceding claims in which the rough grinding, applied to one and / or the other of the faces of the raw disc (2), removes a first thickness greater than or equal to 150qm.

7. Preparation process according to one of the preceding claims, in which the heat treatment of step c) is carried out at a temperature greater than or equal to 1600°C, preferably greater than or equal to 1700°C, still preferably greater than or equal to 1800°C, still preferably greater than or equal to 1850°C, or even still preferably greater than or equal to 2000°C.

8. Preparation method according to one of the preceding claims, in which the thinning of step d) includes fine grinding carried out with a grinding wheel whose abrasive grains have an average size less than or equal to 5qm, less than or equal to 4qm, or even less than or equal to 3qm.

9. Preparation method according to claim 8, in which the fine grinding, applied to one and / or the other of the faces of the annealed disc (20'), removes a second thickness less than or equal to 50qm.

10. Preparation process according to claim 9, in which the second thickness is less than or equal to 20qm, preferably between 5qm and 20qm.

11. Preparation method according to one of claims 8 to 10, in which step d) comprises, after fine grinding, a surface preparation comprising polishing, cleaning, annealing, etching and / or deposition of a film, so as to obtain a surface condition of the support substrate (200) compatible with direct assembly on another substrate.

12. Method for manufacturing a composite structure (100) comprising: - preparing a support substrate (200) made of a polycrystalline material in accordance with the method according to one of the preceding claims, and - transferring a thin layer (10) made of a monocrystalline material, from a donor substrate (1), onto said support substrate (200).

13. A method of manufacturing a composite structure (100) according to claim 12, wherein the polycrystalline material is silicon carbide (SiC) or aluminum nitride (AIN), and wherein the monocrystalline material is selected from silicon carbide (SiC), gallium nitride (GaN), gallium oxide (Ga2O3), diamond, silicon, germanium, indium phosphide (InP).

14. A manufacturing method according to one of claims 12 and 13, further comprising forming electronic components in or on the thin layer (10).

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