METHOD FOR PRODUCING A POLYCRYSTALLINE SILICON CARBIDE SUPPORT SUBSTRATE

Description

DOMAINE DE L'INVENTION

[0001] The present invention relates to the field of semiconductor materials for microelectronic components. It relates in particular to a method for manufacturing a support substrate made of polycrystalline silicon carbide, particularly suitable for producing a composite structure comprising a thin layer of monocrystalline silicon carbide arranged on said support substrate. ARRIERE PLAN TECHNOLOGIQUE DE L'INVENTION

[0002] SiC is increasingly used to manufacture innovative power devices to meet the needs of emerging electronics fields, such as electric vehicles. Indeed, power devices and integrated power systems based on monocrystalline silicon carbide can handle much higher power density compared to their traditional silicon counterparts, and with smaller active area dimensions.

[0003] High-quality monocrystalline SiC (c-SiC) substrates for the microelectronics industry remain expensive and difficult to source in large quantities. It is therefore advantageous to use layer transfer solutions to develop composite structures typically comprising a thin monocrystalline SiC layer (from the high-quality c-SiC substrate) on a lower-cost support substrate, for example polycrystalline SiC (p-SiC). A well-known thin-layer transfer solution is the Smart Cut ® process, based on light ion implantation and direct bonding at a bonding interface.

[0004] Document US2019153616 proposes a method for manufacturing a p-SiC support substrate onto which a thin c-SiC layer can be transferred. The support substrate comprises grains of average size of the order of 10 µm and has a rate of variation of the grain size between its front and rear faces, reduced to its thickness, less than or equal to 0.43%; this latter characteristic makes it possible to limit the residual stress in the support substrate and therefore its curvature.

[0005] The manufacturing process involves a first carbon base substrate on which a thick layer (typically 2mm) of p-SiC is produced by chemical vapor deposition. A second p-SiC base substrate, approximately 350µm thick, is extracted from the thick p-SiC layer, by removing the first carbon base substrate and mechanically thinning both faces of the thick layer. The second base substrate has a grain size variation rate between its front and back faces, reduced to its thickness, of less than or equal to 0.43%. A new p-SiC layer (typically of the order of 400µm) is then formed by chemical vapor deposition on the second base substrate: separated from the second base substrate, for example by laser irradiation, this new p-SiC layer constitutes the p-SiC support substrate intended to be used in a composite structure. The second base substrate can then be reused.

[0006] In practice, the step of forming the second base substrate can be complex because the removal of the first carbon base substrate generally induces a very significant curvature of the thick p-SiC layer, which can cause the breakage of said thick layer, or at least complicates or even prevents the performance of the thinning step required to reach the thickness of the second base substrate. In addition, this thinning is very substantial (of the order of 1.5 mm) and expensive in p-SiC material and in deposition and thinning steps. OBJET DE L'INVENTION

[0007] The present invention provides a manufacturing method addressing the aforementioned problem. It relates to a method for manufacturing a polycrystalline SiC support substrate, which is economical and simplified. Said support substrate is furthermore particularly suitable for manufacturing a composite structure comprising a thin c-SiC layer arranged on said p-SiC support substrate. BREVE DESCRIPTION DE L'INVENTION

[0008] The invention relates to a method for manufacturing a polycrystalline silicon carbide support substrate comprising the following steps: a) the growth of an initial polycrystalline silicon carbide substrate on a graphite or silicon carbide seed;at the end of step a), the initial substrate having a free front face and a rear face in contact with the seed, b) the formation of a stiffening film of carbon, on the front face of the initial substrate, the initial substrate having, in the plane of its front face and just before the formation of the stiffening film, a first average size of silicon carbide grains, c) the removal of the seed, so as to release the rear face of the initial substrate, the latter having, in the plane of its rear face and just after the removal of the seed, a second average size of silicon carbide grains, smaller than the first average size, d) the thinning of the rear face of the initial substrate, to a thickness for which the initial substrate has, in the plane of its thinned rear face, a third average size of grains equal to the first average size of grains to within + / - 30%, the thinned initial substrate forming the support substrate. ;

[0009] According to other advantageous and non-limiting characteristics of the invention, taken alone or in any technically feasible combination: the stiffening film has a thickness of between 100nm and several millimeters, for example 10mm; the stiffening film has a thickness of between 100nm and 10µm; the carbon stiffening film has a crystallographic structure of the diamond type or of the glassy carbon type; step b) is carried out by spreading a polymer resin comprising carbon-carbon bonds preformed in three dimensions, in the form of a viscous layer, on the front face of the initial substrate, and by annealing at a temperature of between 500°C and 2000°C, to form the carbon stiffening film; the polymer resin is based on coal tar, phenol formaldehyde, polyfurfuryl alcohol, polyvinyl alcohol, polyacrylonitrile, polyvinylidene chloride, and / or polystyrene; step b) is carried out by plasma deposition, ion bombardment deposition or evaporation deposition;the manufacturing method comprises a step a'), between step a) and step b), of grinding the front face and / or a periphery of the initial substrate, to reduce a surface roughness of said face and / or a variation in thickness of said substrate, and / or to regularize its periphery; step a') comprises mechanical or chemical-mechanical thinning; the manufacturing method comprises: a step e), after step d), of removing the stiffening film, and / or a step, after step d) or after step e), of heat treatment at a temperature greater than or equal to 1500°C.;

[0010] The invention also relates to a method for manufacturing a composite structure, implementing the above method and further comprising a step f) of transferring a thin layer of monocrystalline silicon carbide onto a first or second face of the support substrate, directly or via an intermediate layer, to form the composite structure.

[0011] According to other advantageous and non-limiting characteristics of the invention, taken alone or in any technically feasible combination: the intermediate layer is formed by the carbon stiffening film retained on the first face of the support substrate; the transfer of the thin layer is carried out on one of the faces of the support substrate and an additional carbon film is placed on the other free face of the support substrate prior to the transfer; the additional film is removed, preferably after the composite structure has undergone all heat treatments at temperatures above 1400°C required for its manufacture or that of components on and / or in said structure. BREVE DESCRIPTION DES FIGURES

[0012] 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: [ Fig. 1a ] [ Fig. 1b ] [ Fig. 1c ] [ Fig. 1d ] [ Fig. 1e ] [ Fig. 1f ] THE figures 1a à 1f present steps of a manufacturing method in accordance with the invention; [ Fig. 2a ] [ Fig. 2b ] [ Fig. 2c ] [ Fig. 2d ] THE figures 2a à 2d present other steps of the manufacturing process according to the invention; [ Fig. 3a ] [ Fig. 3b ] [ Fig. 3c ] [ Fig. 3d ] [ Fig. 3e ] THE figures 3a à 3e present variants of steps of the manufacturing method according to the invention.

[0013] The same references in the figures may be used for elements of the same type. The 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 are not necessarily respected in the figures. DESCRIPTION DETAILLEE DE L'INVENTION

[0014] The present invention relates to a method of manufacturing a support substrate 10 made of polycrystalline silicon carbide (p-SiC).

[0015] The method firstly comprises a step a) of growing an initial substrate 1 made of polycrystalline silicon carbide on a seed 2 made of graphite or low-quality monocrystalline or polycrystalline silicon carbide ( figure 1a ). The seed 2 is preferably in the form of a plate whose diameter is substantially that which is targeted for the support substrate 10, for example 100mm, 150mm, 200mm or even 300mm.

[0016] The growth of the initial substrate 1 in p-SiC is carried out by a known chemical vapor deposition (CVD) technique, at a temperature typically between 1100°C and 1500°C. The precursors can be chosen from methylsilane, dimethyldichlorosilane or even dichlorosilane and i-butane, preferably with a C / Si ratio close to or greater than 1. Optionally, doping species (such as nitrogen or phosphorus, for example) may be introduced during the CVD deposition, so as to adjust the resistivity of the initial substrate 1 (from which the support substrate 10 will be derived) to the specifications of the final product, in particular of the targeted composite structure. The doping levels usually targeted are greater than 1E18 / cm 3< , or even greater than 1E20 / cm 3< .

[0017] At the end of step a), the initial substrate 1 has a free front face 1a and a rear face 1b in contact with the seed 2. The thickness of the initial substrate 1 is less than 1mm, preferably less than 550µm. Note that the range of thicknesses usually desired, for a support substrate 10 intended for the production of a composite structure, is 100µm - 500µm.

[0018] The initial substrate 1 may comprise grains of silicon carbide of type 4H, 6H and / or 3C, depending on the CVD deposition conditions.

[0019] The average grain size at the rear face 1b of the initial substrate 1 is relatively small, typically less than or equal to 1µm, or even less than or equal to 100nm; said grains correspond to the p-SiC material produced at the start of CVD deposition (nucleation phase) on the graphite seed 2.

[0020] Let us recall that the size of a grain, delimited by the grain boundaries, corresponds to the largest dimension of said grain, in the plane of the considered face of the substrate. The average grain size is defined by the average of the sizes of the different grains in said plane. To measure the dimensions of the grains or distances between grain boundaries, it is possible to use images obtained by conventional scanning electron microscopy (SEM) or by using electron diffraction (EBSD "Electron Back Scattered Diffraction"). It is also possible to use X-ray crystallography. When the considered face mainly comprises grains of micrometric size (typically from a few microns to a few tens of microns), very small grains, typically less than 50nm, are preferentially excluded from the measurement, to limit measurement uncertainties.

[0021] As CVD deposition progresses, the p-SiC grains increase in size, until they reach a relatively stable average size, for a deposit thickness that can vary between a few micrometers and a few tens of micrometers, depending on the deposition conditions.

[0022] Thus, depending on the thickness of p-SiC deposited to grow the initial substrate 1, the average grain size at the front face of said substrate 1 may typically vary between 1 and 10 µm.

[0023] Subsequently, the average size of the p-SiC grains at the front face 1a of the initial substrate 1 will be called the first average size, and the average size of the p-SiC grains at the back face 1b of the initial substrate 1 will be called the second average size.

[0024] The first average p-SiC grain size (front face side 1a) is larger than the second average grain size (back face side 1b), the latter corresponding to the nucleation phase.

[0025] The manufacturing method then comprises a step b) of forming a stiffening film 3 made of carbon, on the front face 1a of the initial substrate 1 ( figure 1b ). The stiffening film 3 may have a thickness ranging from 100nm to several millimeters, for example 10mm. Preferably, its thickness is between 100nm and 10µm.

[0026] Advantageously, the stiffening film 3 made of carbon has a crystallographic structure of the diamond type, that is to say comprising sp3 carbon-carbon atomic bonds, or of the glassy carbon type, comprising sp2 carbon-carbon atomic bonds.

[0027] The stiffening film 3 can be formed by various conventional deposition techniques, such as plasma deposition, ion bombardment deposition or evaporation deposition.

[0028] Alternatively, step b) can be carried out by spreading a polymer resin comprising carbon-carbon bonds preformed in three dimensions, in the form of a viscous layer, on the front face 1a of the initial substrate 1. This spreading can be carried out by centrifugation. Then, annealing is applied at a temperature between 500°C and 2000°C, typically between 600°C and 1100°C, under nitrogen, to form the stiffening film 3 made of carbon, by chemical decomposition of the resin (pyrolysis). The temperature ramps are typically chosen to be of the order of 10°C / min and the duration of the annealing is of the order of an hour. The temperature rise is controlled so that the effective temperature remains below the resin / carbon glass transition temperature.

[0029] The polymer resin may be formed from coal tar, phenol formaldehyde, polyfurfuryl alcohol, polyvinyl alcohol, polyacrylonitrile, polyvinylidene chloride, and / or polystyrene, etc.

[0030] For example, known photosensitive resins may be used, such as the commercial products AZ-4330, AZ-P4620 (registered trademarks) (based on 1-methoxy-2-propanol acetate, diazonaphthoquinonesulfonic esters, 2-methoxy-1-propanol acetate, Cresol novolak resin), OCG-825 (based on ethyl-3-ethoxypropionate), SU-8 2000 (based on cyclopentanone, triarylsulfonium / hexafluoroantimonate salts, propylene carbonate, epoxy resin), usually used for photolithography steps in the field of microelectronics.

[0031] Epoxy resins, such as for example the product Epoxy Novolac EPON (registered trademark), proposed for covering and protecting different surfaces in various fields (aeronautics, marine, automotive, construction, etc.), can also be used in step b) of the process according to the invention.

[0032] When using resins, it is important to take into account the contraction that the viscous layer of resin will undergo during annealing, to define its initial thickness sufficient to obtain the desired stiffening film thickness 3. The contraction in thickness can typically be between 70% and 95%. The carbon ratio, i.e. the ratio between the mass of the polymer resin layer after pyrolysis (corresponding to the stiffening film 3) and the initial mass of the spread polymer resin layer, must be at least 5%, preferably greater than 50%.

[0033] Optionally, the manufacturing method may comprise a step a'), between step a) and step b), of grinding the front face 1a and / or a periphery 1c of the initial substrate 1, to reduce a surface roughness of said face 1a and / or reduce a variation in thickness of said substrate 1, and / or to regularize the periphery 1c.

[0034] Step a') may include mechanical or chemical-mechanical thinning (polishing), with material removal of the order of a few microns to a few tens of microns.

[0035] The manufacturing method according to the invention then comprises a step c) of removing the seed 2, so as to release the rear face 1b of the initial substrate 1 ( figure 1c ).

[0036] When the seed 2 is made of graphite, the removal can be carried out by burning the graphite, by applying a heat treatment, in an oxygen-rich atmosphere (air, for example), at a temperature above 400°C, preferably above 550°C.

[0037] It is also possible to mechanically detach the seed 2, whether it is made of graphite or silicon carbide, for example by localized application of mechanical stress, at or near the interface between the seed 3 and the initial substrate 1.

[0038] If any residues remain on the rear face 1b of the initial substrate 1 after removal of the seed 2, they can be burned (when they are made of graphite) or removed mechanically or chemically by polishing or etching (when they are made of graphite or SiC).

[0039] This shrinkage typically generates a strong curvature of the initial substrate 1, which can be up to 500µm for a diameter of 150mm. This curvature is mainly due to the constraints associated with the difference in grain sizes between the second face 1b (nucleation grains, of small average size) and the first face 1a.

[0040] In the context of the invention, the carbon stiffening film 3 makes it possible to greatly limit the increase in curvature during the removal of the seed 2, by mechanically maintaining the initial substrate 1 by its front face 1a. The curvature of the initial substrate 1, provided with the carbon stiffening film 3, does not exceed 200 µm for a substrate diameter of 150 mm; or even, the curvature is kept below 100 µm. In these curvature ranges, the initial substrate 1 can be processed without problem in standard lines and equipment, without risk of breakage or equipment failure; these problems are essentially encountered for curvatures greater than 300 µm (diameter 150 mm).

[0041] The manufacturing method finally comprises a step d) of thinning the rear face 1b of the initial substrate 1. The thinned initial substrate 1 forms the support substrate 10 ( figure 1d ).

[0042] The thinning of step d) is carried out by mechanical grinding, mechanical polishing and / or chemical-mechanical polishing of the rear face 1b. The material removal is typically between a few tens of microns and 200 µm, depending on the thickness of the initial substrate 1 at the entry of step d), and of course depending on the thickness targeted for the support substrate 10.

[0043] The thinning is carried out to a thickness for which the initial substrate 1 has, in the plane of its thinned rear face 1b', a third average grain size equal to the first average grain size to within + / - 30%. In other words, if the first average size is for example 5µm, the third average size is expected to be between 4µm and 6µm.

[0044] It may happen that the grain sizes in the plane of the front face 1a or in the plane of the rear face 1b are distributed according to a double population, each peak following a substantially Gaussian distribution. According to a first option, the average grain size is calculated by taking the overall average including the two populations, and the first and third average sizes must not differ by more than 30%. According to a second option, the first two average sizes (corresponding to the double population on the side of the front face 1a) and the third two average sizes (corresponding to the double population on the side of the rear face 1b) will be taken into consideration, which must not differ from each other by more than 30% respectively.

[0045] After the thinning of step d), the thinned rear face 1b' of the initial substrate 1 has an average p-SiC grain size differing by less than 30% from the average grain size of the front face 1a. The residual stress in the thinned initial substrate 1 (which forms the support substrate 10) is then compatible with a low curvature, and at least manageable in the manufacturing lines.

[0046] The manufacturing method may then comprise a step e) of removing the stiffening film 3, for example by dry or wet chemical etching ( figure 1e ). After this removal, the support substrate 10 has a curvature of less than 200 µm, or even less than 100 µm (for a diameter of 150 mm), due to the reduced residual stress in its volume.

[0047] At this stage, the support substrate 10 has a first face 10a, a second face 10b and an edge 10c, corresponding respectively to the front face 1a, to the thinned rear face 1b' and to the edge 1c of the initial substrate 1 after step d).

[0048] For example, to form a support substrate 350 µm thick and 150 mm in diameter, an initial substrate 1 of 500 µm can be produced on the seed 2, the initial substrate 1 having a first average p-SiC grain size at its front face 1a of the order of 4 µm. A step of correcting the uniformity of the thickness of the initial substrate 1 can be carried out, for example by removing 50 µm. A stiffening film 3 of carbon of 4 µm is formed on this front face 1a. After the removal of the graphite seed 2, the second average grain size at the rear face 1b of the initial substrate 1 is less than 100 nm; but the curvature of the initial substrate 1, due to the presence of the stiffening film 3, is kept less than 150 µm.A removal of 100 µm at its rear face 1b is carried out, and the third average size of p-SiC grains at the thinned rear face 1b', of the order of 3 µm, respects the condition of equality to within 30% with respect to the first average size of grains. Thus, after removal of the stiffening film 3, the curvature of the support substrate 10 is less than 200 µm and compatible with subsequent steps of manufacturing a composite structure 100.

[0049] Optionally, after step e), a surface treatment may be applied to the first face 10a of the support substrate 10, in particular if this face 10a is intended to receive the thin layer 20 of the composite structure 100, at a subsequent step f) of the method. This surface treatment may comprise mechanical grinding, chemical-mechanical polishing or other chemical cleaning, depending on the surface roughness of the first face 10a.

[0050] If the second face 10b of the support substrate 10 is intended to receive the thin layer 20, and step d) has not made it possible to achieve a sufficiently low level of roughness (typically < 1nm RMS, measured by atomic force microscopy on scans of 20µmx20µm), an additional surface treatment may also be applied to it.

[0051] The face of the support substrate 10 intended to form the rear face of the composite structure 100 may have a greater surface roughness, for example of the order of 10nm RMS.

[0052] The manufacturing method may also comprise a heat treatment, after step d) or after step e), at a temperature greater than or equal to 1500°C, typically between 1500°C and 1900°C, so as to stabilize the polycrystalline structure of the support substrate 10. Indeed, these temperature ranges are likely to be applied later in the method, in particular for the manufacture of a composite structure.

[0053] Thanks to the manufacturing method according to the present invention, a support substrate 10, having mechanical characteristics compatible with the specifications of a composite structure for microelectronic applications, can be obtained in a simple manner, without requiring the deposition of an initial p-SiC substrate of very high thickness, which is more than 80% removed, to select a tiny useful portion of p-SiC, as is practiced in the methods of the state of the art. In the manufacturing method according to the invention, the thickness of the initial substrate 1 formed is less than or equal to 1 mm and the removal of material at its front face 1a and / or its rear face 1b is less than 70%, or even less than 50% of the initial thickness, which provides a saving of material and technological steps.

[0054] In the context of the development of a composite structure 100, the manufacturing method according to the invention can continue with a step f) of transferring a useful layer 20 of monocrystalline silicon carbide onto the support substrate 10, based on bonding by molecular adhesion ( figure 1f ).

[0055] There are various options known from the state of the art for performing a layer transfer, which will not be described exhaustively here.

[0056] According to a preferred embodiment, step f) of the method involves implantation of light species according to the principle of the Smart Cut ® method.

[0057] In a first phase f1), a donor substrate 21 made of monocrystalline silicon carbide, from which the useful layer 20 will be produced, is provided ( figure 2a ). The donor substrate 1 is preferably in the form of a wafer with a diameter of 100mm, 150mm, 200mm or even 300mm (identical to or very close to that of the support substrate 10) and a thickness typically between 300µm and 800µm. It has a front face 21a and a rear face 21b. The surface roughness of the front face 1a is advantageously chosen to be less than 1nm RMS, or even less than 0.5nm RMS, measured by atomic force microscopy (AFM) on a 20µm x 20µm scan. The donor substrate 21 may be of 4H or 6H polytype, and have n or p type doping, depending on the requirements of the components that will be produced on and / or in the useful layer 20 of the composite structure 100.

[0058] A second phase f2) corresponds to the introduction of light species into the donor substrate 21 to form a buried fragile plane 22 delimiting, with a front face 21a of the donor substrate 21, the useful layer 20 to be transferred ( figure 2b ).

[0059] The light species are preferably hydrogen, helium or a co-implantation of these two species, and are implanted at a determined depth in the donor substrate 21, consistent with the thickness of the targeted useful layer 20. These light species will form, around the determined depth, microcavities distributed in a thin layer parallel to the free surface 21a of the donor substrate 21, i.e. parallel to the plane (x,y) in the figures. This thin layer is called the buried fragile plane 22, for the sake of simplification.

[0060] The implantation energy of the light species is chosen so as to reach the determined depth. For example, hydrogen ions will be implanted at an energy between 10 keV and 250 keV, and at a dose between 5 E< 16 / cm 2< and 1 E< 17 / cm 2< , to delimit a useful layer 20 having a thickness of the order of 100 nm to 1500 nm. Note that a protective layer may be deposited on the front face 21a of the donor substrate 21, prior to the ion implantation step. This protective layer may be composed of a material such as silicon oxide or silicon nitride for example. It is removed prior to the following phase.

[0061] Optionally, an intermediate layer 4 can be formed on the front face 21a of the donor substrate 21, before or after the second phase f2) of introduction of the light species ( figures 3b, 3c , 3d, 3e ). This intermediate layer 4 may be made of a semiconductor material, for example silicon or silicon carbide, or of a metallic material such as tungsten, titanium, etc. The thickness of the intermediate layer 4 is advantageously limited, typically between a few nanometers and a few tens of nanometers.

[0062] In the case where the intermediate layer 4 is formed before phase f2), the implantation energy (and potentially the dose) of the light species will be adjusted to the crossing of this additional layer. In the case where the intermediate layer 4 is formed after phase f2), care will be taken to form this layer by applying a thermal budget lower than the bubbling thermal budget, said bubbling thermal budget corresponding to the appearance of blisters on the surface of the donor substrate 21 due to growth and excessive pressurization of the microcavities in the buried fragile plane 22.

[0063] The transfer step f) then comprises a third phase f3) of assembling the donor substrate 21, on the side of its front face 21a, on the support substrate 10, on the side of its first face 10a or its second face 10b, by bonding by molecular adhesion, along a bonding interface 30 ( figure 2c ).

[0064] Optionally, an intermediate layer 4' can also be deposited on the face to be assembled of the support substrate 10, prior to the assembly phase f3) ( figures 3d, 3e ); it can be chosen to be of the same nature or of a different nature from the intermediate layer 4 mentioned for the donor substrate 21. An intermediate layer 4,4' can optionally only be deposited on one or the other of the two substrates 21,10 to be assembled.

[0065] The objective of the intermediate layer(s) is essentially to promote the bonding energy (in particular in the temperature range below 1100°C), due to the formation of covalent bonds at lower temperatures than in the case of two SiC surfaces directly assembled; another advantage of this(these) intermediate layer(s) may be to improve the vertical electrical conduction of the bonding interface 30.

[0066] According to a possible variant, the intermediate layer can be formed by the stiffening film 3 made of carbon, kept on the first face 10a of the support substrate 10 ( figures 3a , 3c). In this case, step e) of the manufacturing method according to the invention is not carried out and the face to be assembled of the support substrate 10 is its first face 10a provided with the film 3. To promote vertical electrical conduction through the stiffening film 3, in the final composite structure 100, a carbon film with a diamond-type crystallographic structure will be preferred.

[0067] Optionally, an additional carbon film 5 is placed on the face opposite the face to be assembled of the support substrate 10, always prior to the assembly phase f3) ( figure 3e ). Its characteristics can for example be chosen from those proposed for the stiffening film 3 previously in this description.

[0068] Even though the presence of this additional film 5 was illustrated on the figure 3e in combination of intermediate layers 4,4' on the faces to be assembled respectively of the donor substrate 21 and of the support substrate 10, this additional film 5 can be implemented in any of the possible configurations mentioned, in particular those illustrated on the figures 3a à 3c The additional film 5 may be removed subsequently, preferably after the composite structure 100 has undergone all heat treatments at temperatures above 1400°C required for its manufacture or that of components on and / or in said structure 100.

[0069] Returning to the description of the assembly phase f3), and as is well known in itself, direct bonding by molecular adhesion does not require an adhesive material, because bonds are established at the atomic scale between the assembled surfaces. Several types of bonding by molecular adhesion exist, which differ in particular by their conditions of temperature, pressure, atmosphere or treatments prior to bringing the surfaces into contact. Examples include bonding at room temperature with or without prior plasma activation of the surfaces to be assembled, bonding by atomic diffusion ("Atomic diffusion bonding" or ADB according to English terminology), bonding with surface activation ("Surface-activated bonding" or SAB), etc.

[0070] The assembly phase f3) may comprise, prior to bringing the faces 21a, 10a to be assembled into contact, conventional sequences of cleaning by chemical means (for example, RCA cleaning), surface activation (for example, by oxygen or nitrogen plasma) or other surface preparations (such as cleaning by brushing (“scrubbing”)), capable of promoting the quality of the bonding interface 30 (low defectivity, high adhesion energy).

[0071] Finally, a fourth phase f4) comprises the separation along the buried fragile plane 22, which leads to the transfer of the useful layer 20 onto the support substrate 10 ( figure 2d ).

[0072] The separation along the buried fragile plane 22 is usually carried out by applying a heat treatment at a temperature between 800°C and 1200°C. Such a heat treatment induces the development of cavities and microcracks in the buried fragile plane 22, and their pressurization by the light species present in gaseous form, until the propagation of a fracture along said fragile plane 22. Alternatively or jointly, a mechanical stress can be applied to the bonded assembly and in particular at the buried fragile plane 22, so as to propagate or help to mechanically propagate the fracture leading to the separation. At the end of this separation, on the one hand the semiconductor structure 100 comprising the support substrate 10 and the useful layer 20 transferred in monocrystalline SiC is obtained, and on the other hand, the remainder 21' of the donor substrate.The level and type of doping of the useful layer 20 are defined by the choice of the properties of the donor substrate 21 or can be adjusted subsequently via known techniques for doping semiconductor layers.

[0073] The free surface 20a of the useful layer 20 is usually rough after separation: for example, it has a roughness of between 5nm and 100nm RMS (AFM, 20µm x 20µm scan). Cleaning and / or smoothing phases may be applied to restore a good surface condition (typically, a roughness of less than a few angstroms RMS on a 20µm x 20µm scan by AFM). In particular, these phases may comprise a mechanical-chemical smoothing treatment of the free surface of the useful layer 20. A removal of between 50nm and 300nm makes it possible to effectively restore the surface condition of said layer 20. They may also comprise at least one heat treatment at a temperature of between 1300°C and 1800°C. Such a heat treatment is applied to remove the residual light species from the useful layer 20 and to promote the rearrangement of the crystal lattice of the useful layer 20. It also makes it possible to strengthen the bonding interface 30.

[0074] The heat treatment may also include or correspond to an epitaxy of silicon carbide on the thin layer 20.

[0075] Finally, note that the transfer step f) may comprise a step of reconditioning the remainder 21' of the donor substrate with a view to reuse as a donor substrate 21 for a new composite structure 100. Mechanical and / or chemical treatments, similar to those applied to the composite structure 100, may be implemented at the front face 21'a of the remaining substrate 21'.

[0076] The composite structure 100 obtained is extremely robust to very high temperature heat treatments likely to be applied to improve the quality of the useful layer 20 or to manufacture components on and / or in said layer 20.

[0077] The composite structure 100 according to the invention is particularly suitable for the production of one (or more) high-voltage microelectronic component(s), such as for example Schottky diodes, MOSFET transistors, etc. It more generally meets power microelectronic applications, by allowing excellent vertical electrical conduction, good thermal conductivity and by providing a useful layer of high-quality c-SiC.

[0078] Of course, the invention is not limited to the embodiments and examples described, and variant embodiments may be made without departing from the scope of the invention as defined by the claims.

Claims

1. Method for producing a polycrystalline silicon carbide support substrate (10) comprising the following steps: a) growing an initial substrate (1) consisting of polycrystalline silicon carbide on a seed (2) consisting of graphite or silicon carbide; at the end of step a), the initial substrate (1) having a free front face (1a) and a rear face (1b) in contact with the seed (2), b) forming a stiffening film (3) made of carbon on the front face (1a) of the initial substrate (1), the initial substrate (1) having, in the plane of its front face (1a) and just prior to the formation of the stiffening film (3), a first average silicon carbide grain size, c) removing the seed (2) so as to free the rear face (1b) of the initial substrate (1), the initial substrate having, in the plane of its rear face (1b) and just after removal of the seed (2), a second average silicon carbide grain size, smaller than the first average size, d) thinning the rear face (1b) of the initial substrate (1) to a thickness at which the initial substrate (1) has, in the plane of its thinned rear face (1b'), a third average grain size equal to the first average grain size to within + / - 30%, the thinned initial substrate (1) forming the support substrate (10).

2. Production method according to the preceding claim, wherein the stiffening film (3) has a thickness of between 100 nm and several millimeters, for example 10 mm.

3. Production method according to the preceding claim, wherein the stiffening film (3) has a thickness of between 100 nm and 10 µm.

4. Production method according to any of the preceding claims, wherein the carbon stiffening film (3) has a diamond-type or glassy carbon-type crystallographic structure.

5. Production method according to any of the preceding claims, wherein step b) is carried out by spreading a polymer resin including three-dimensionally preformed carbon-carbon bonds, in the form of a viscous layer, on the front face of the initial substrate (1), and by annealing at a temperature of between 500°C and 2000°C, to form the carbon stiffening film (3).

6. Production method according to the preceding claim, wherein the polymer resin is made from coal tar, phenol formaldehyde, polyfurfuryl alcohol, polyvinyl alcohol, polyacrylonitrile, polyvinylidene chloride and / or polystyrene.

7. Production method according to any of claims 1 to 4, wherein step b) is carried out by plasma deposition, ion bombardment deposition or evaporation deposition.

8. Production method according to any of the preceding claims, comprising a step a'), between step a) and step b), of grinding the front face (1a) and / or a periphery of the initial substrate (1) to reduce a surface roughness of said face (1a) and / or a thickness variation of said substrate (1), and / or to regularize its periphery.

9. Production method according to the preceding claim, wherein step a') comprises mechanical or mechanochemical thinning.

10. Production method according to any of the preceding claims, comprising: - a step e), after step d), of removing the stiffening film, and / or - a step, after step d) or after step e), of heat treatment at a temperature greater than or equal to 1500°C.

11. Method for producing a composite structure (100), using the method according to any of the preceding claims and further comprising a step f) of transferring a thin layer (20) of monocrystalline silicon carbide onto a first (10a) or second (10b) face of the support substrate (10), directly or via an intermediate layer, to form the composite structure (100).

12. Production method according to the preceding claim, wherein the intermediate layer is formed by the carbon stiffening film (3) retained on the first face (10a) of the support substrate (10).

13. Production method according to claim 11, wherein the thin layer (20) is transferred onto one of the faces (10a; 10b) of the support substrate (10) and an additional carbon film (5) is arranged on the other free face (10b; 10a) of the support substrate (10) prior to the transfer.

14. Production method according to the preceding claim, wherein the additional film (5) is removed, preferably after the composite structure (100) has undergone all heat treatments at temperatures greater than 1400°C which are required for its production or that of components on and / or in said structure (100).