Process for fabricating a composite structure including a grading step

Abstract

The invention relates to a process for fabricating a composite structure comprising a thin layer made of single-crystal silicon carbide placed on a carrier substrate made of polycrystalline silicon carbide, the process comprising the following steps: 1) providing at least one donor substrate made of single-crystal silicon carbide, having a front side and a rear side, the front side potentially having defects, called primary defects; 2) controlling the quality of the - at least one - donor substrate by means of a photoluminescence-based imaging technique, so as to extract a map of the front side, called the first map, cataloguing the primary defects identified as being of micro-hole type, of the complex type consisting of star stacking faults or of point-defect type; 3) transferring a thin layer, obtained from a surface layer of the - at least one - donor substrate, onto a carrier substrate made of polycrystalline silicon carbide, in order to obtain a composite structure and a residual donor substrate; 4) inspecting a free surface of the thin layer of the composite structure by means of a technique for inspecting for defects by means of scattering of an ultraviolet laser beam, so as to extract a map of the free surface, called the second map, cataloguing defects called secondary defects; 5) grading the composite structure, this including a comparison of the first map and of the second map.

Description

Method of manufacturing a composite structure including a gradation step FIELD OF THE INVENTION The present invention relates to the field of semiconductor materials, in particular composite structures comprising a thin layer (from a monocrystalline silicon carbide donor substrate) transferred onto a polycrystalline silicon carbide support substrate. It relates in particular to a method for manufacturing such a composite structure, including a gradation step based on the control mapping of the donor substrate and on the control mapping of the thin layer of the composite structure. TECHNOLOGICAL BACKGROUND OF THE INVENTION Silicon carbide is a particularly interesting material for the manufacture of power devices, radio frequencies or even devices operating at very high temperatures. The quality of monocrystalline silicon carbide (c-SiC) substrates has improved significantly over the last ten years, accompanied by increasingly precise knowledge and detection of the different types of crystalline defects likely to be present in this material. The JEITA ("Japan Electronics and Information Technology Industries Association") standard also includes four documents relating to the zoology of defects on / in a layer grown by homoepitaxy on a 4H-SiC substrate (EDR 4712 / 100) and to non-destructive procedures for optical inspection of these defects (EDR 4712 / 200, / 300, / 400). In particular, a procedure is indicated for evaluating and referencing defects by combining optical inspection and photoluminescence imaging. Commercially available equipment, such as the SICA88 from Lasertec, allows the combination of visible light and photoluminescence confocal Nomarski prism microscopy techniques (as described by [1] T.Kimoto et al., “Fundamentals of Silicon Carbide Technology Growth Characterization Devices and Applications”, p.126, or [2] D. Baierhofer et al., Materials Science in Semiconductor processing 140 (2022) 106414) and is commonly used to inspect c-SiC substrates before or after epitaxy. As an example, [3] Das et al (“Statistical analysis of killer and non-killer defects in SiC and the impact of device performance”, Material Science Forum, ISSN 1662-9752, Vol.1004, pp 458-463 (2020) Trans Tech Publications Ltd) proposes a statistical analysis of “killer” defects for devices grown on a homoepitaxial c-SiC layer, and shows optical and photoluminescence images of typical c-SiC defects. Although rapidly developing, high-quality c-SiC substrates remain expensive and difficult to source in large sizes. It is therefore advantageous to use layer transfer solutions to develop composite structures comprising a thin monocrystalline SiC layer (derived from the high-quality c-SiC donor substrate) on a lower-cost support substrate, for example polycrystalline SiC (p-SiC), which can also offer advantages in terms of electrical conductivity. A well-known thin-film transfer solution is the Smart Cut process. TM, based on light ion implantation and direct bonding of a c-SiC donor substrate to a support substrate at a bonding interface. The implantation creates a buried fragile plane along which separation occurs, leading to the transfer of a thin c-SiC layer onto the support substrate to form the composite structure, and allowing the recovery and recycling of the remainder of the donor substrate to potentially perform one or more other layer transfers. Epitaxy can then be performed on the thin layer of the composite structure, followed by the development of electronic devices. For the implementation of a layer transfer process to be economically viable, it is important to know how to control the quality of the donor substrates, so as to avoid carrying out the transfer of a layer which would inevitably lead to a downgrading of the composite structure, or which would give rise to an epitaxial layer which is out of specification, in terms of "killer" defects. The zoology of c-SiC defects is relatively well known, and numerous studies (some of which are referenced above) tend to define the type and size of defects, present in an epitaxial layer of c-SiC, which would be killer for the components; nevertheless, the applicant has observed that the criteria for classifying defects in a donor substrate are not necessarily the same when epitaxy is carried out on said substrate and when a thin film transfer is carried out from said substrate. It is therefore important to detect, but above all to precisely classify the crystalline defects (called primary defects) present on the donor substrates, in order to downgrade those, among these donor substrates, which will not allow the production of a thin layer of the required quality. Furthermore, the detection and recognition of defects (so-called secondary defects) on and / or in the thin layer (from a donor substrate) of a composite structure whose support substrate is made of polycrystalline SiC (p-SiC) are complex because the p-SiC grains are visible under said thin layer, in particular with equipment combining confocal microscopy with Nomarski prism in visible light and photoluminescence imaging. However, reliable and efficient control (not requiring too much inspection time per structure) of the quality of the thin layer is required, to avoid continuing the epitaxy steps if secondary defects in the thin layer are likely to generate an out-of-specification density of killer defects in the homoepitaxial layer. In view of the growing use of layer transfer techniques in the manufacturing chain of electronic devices on c-SiC, there is therefore a strong need to define control steps allowing reliable and rapid inspection, both of the donor substrates, at the start of the chain, and of the composite structures, in the middle of the chain, so as to assign a grade, as soon as possible in the overall manufacturing chain, to the donor substrates or composite structures and allow the achievement of the specifications and yields expected for the devices. SUBJECT OF THE INVENTION The present invention addresses the stated problem. The invention relates to a method for manufacturing a composite structure comprising a thin layer of monocrystalline silicon carbide transferred onto a support substrate of polycrystalline silicon carbide. The manufacturing method comprises an original, reliable and rapid gradation step of the composite structure, combining an initial control of the donor substrate and a final control of said structure. BRIEF DESCRIPTION OF THE INVENTION The invention relates to a method for manufacturing a composite structure comprising a thin layer of monocrystalline silicon carbide arranged on a support substrate of polycrystalline silicon carbide, the method comprising the following steps: 1) the provision of at least one donor substrate made of monocrystalline silicon carbide, having a front face and a back face, the front face potentially having defects, called primary defects; 2) quality control of the – at least one – donor substrate using a photoluminescence imaging technique so as to extract a map of the front face, called the first map, listing the primary defects identified as being of the micro-hole type, of the star stacking fault complex type or of the point defect type; 3) the transfer of a thin layer, originating from a surface layer of the – at least one – donor substrate, onto a polycrystalline silicon carbide support substrate, to obtain a composite structure and a residual donor substrate; 4) inspection of a free surface of the thin layer of the composite structure using a defect inspection technique involving diffusion of an ultraviolet laser beam, so as to extract a map of the free surface, called a second map, listing defects, called secondary defects; 5) the gradation of the composite structure, including a comparison of the first mapping and the second mapping. According to other advantageous and non-limiting characteristics of the invention, taken alone or in any technically feasible combination: step 2) comprises the following sub-steps: i) inspecting a front face of the donor substrate to detect primary defects, due to local variations in the intensity of a photoluminescence signal emitted by the front face following excitation by an incident beam, and to form, for each primary defect, a photoluminescence image, ii) assigning to each detected primary defect a labeled defect type with a certain level of similarity, using an image recognition algorithm, trained on different types of defects likely to be present on the front face of a monocrystalline silicon carbide donor substrate, such as micro-holes, star stacking fault complexes and point defects, in particular related to inclusions,a contrast level being associated with the photoluminescence image of each detected primary defect,iii) the classification of each primary defect by applying the following conditions:- if the labeled defect type is micro-hole and if the similarity level is higher than a first level, the primary defect is classified as a critical defect of micro-hole type,- if the labeled defect type is complex star stacking faults and if the similarity level is higher than a second level, the primary defect is classified as a critical defect of complex star stacking fault type,- if the labeled defect type is point defect, if the similarity level is higher than a third level, and if the contrast level is higher than a predetermined threshold, the primary defect is classified as a critical defect of point defect type,- in other cases,the primary defect is classified as a non-critical defect.the inspection step i) is carried out with an incident beam of wavelength 313nm and the emitted photoluminescence signal is collected in a wavelength range from 700nm to 1000nm;the predetermined threshold is established empirically for a type of donor substrate, on the basis of a correlation study between the contrast value of primary defects detected on a front face of a test donor substrate and the presence of secondary defects on a thin layer, originating from the test donor substrate, transferred to a support substrate;a donor substrate having a density of primary defects, classified as critical defects regardless of the type, greater than 1 defect / cm, 2 , or even preferably greater than 0.25 defect / cm 2, is downgraded at the end of step 2) and is not used for step 3);step 5) includes a reconciliation between the primary defects classified as critical defects and the secondary defects, each primary defect classified as critical and not associated with a secondary defect being added to the defects of the second mapping to be taken into account in a grading decision;the grading decision corresponds to a downgrading of the composite structure if the latter comprises:- a density of secondary defects associated with critical primary defects and primary defects classified as critical defects added to the second mapping, greater than 0.25 defects / cm 2 , and / or- a density of secondary defects only detected on the second mapping, not associated with critical primary defects, greater than 0.2 defects / cm 2 ;step 5) comprises a reconciliation between the primary defects classified as critical defects and the secondary defects, and step 5) comprises a complementary control by a technique coupling visible light optical microscopy and photoluminescence imaging, making it possible to verify whether defects are present in the thin layer, at the locations of said primary defects classified as critical defects;step 3) comprises the following sub-steps:3a) the implantation of light species in a donor substrate, to form a buried fragile plane delimiting, with a front face of the donor substrate, the surface layer to be transferred;3b) the assembly of a polycrystalline silicon carbide support substrate with the donor substrate implanted in step 3a);3c) separation along the buried fragile plane to form an intermediate composite structure comprising the surface layer after transfer and the support substrate, on the one hand, and the remainder of the donor substrate, called the residual donor substrate, on the other hand; 3d) application of thermal, mechanical and / or chemical treatments to a free surface of the surface layer, to form the composite structure provided with the thin layer of monocrystalline silicon carbide; in the composite structure, the thin layer comprises a buried face arranged opposite the support substrate, corresponding to the front face of the donor substrate, and another face, opposite the front face, corresponding to a free surface of said layer; and a mirror flip is applied to the first map or to the second map to compare them; step 3) comprises the following sub-steps:; 3i) the implantation of light species in a donor substrate, to form a buried fragile plane delimiting, with a front face of the donor substrate, the surface layer to be transferred; 3ii) assembling a temporary substrate with the donor substrate implanted in step 3i), via a first bonding interface, 3iii) separation along the buried fragile plane to form an intermediate assembly comprising the surface layer after transfer and the temporary substrate, on the one hand, and the remainder of the donor substrate, called residual donor substrate, on the other hand, 3iv) the application of thermal, mechanical and / or chemical treatments to a free surface of the surface layer, to form a temporary structure provided with the thin layer of monocrystalline silicon carbide arranged on the temporary substrate, 3v) the assembly of a polycrystalline silicon carbide support substrate with the thin layer of the temporary structure, via a second bonding interface; 3vi) disassembly along the first bonding interface to separate the temporary substrate from the composite structure provided with the thin layer of monocrystalline silicon carbide; the manufacturing method comprises a step 6) of recycling the residual donor substrate to make it a recycled donor substrate, and in which: - the recycled donor substrate is introduced directly in step 3) as a donor substrate, and - in the following step 4), the gradation of the resulting composite structure uses the first mapping established for the initial donor substrate from which the recycled donor substrate originates. The invention also relates to a method comprising the following steps: 1) the provision of at least one donor substrate made of monocrystalline silicon carbide, having a front face and a rear face, the front face potentially having defects, called primary defects, which are through and also exist on the rear face; 2) quality control of the – at least one – donor substrate using a photoluminescence imaging technique so as to extract a map of the rear face, called the first map, listing the primary defects identified as being of the micro-hole type, of the star stacking fault complex type or of the point defect type; 3) the transfer of a thin layer, originating from a surface layer of the – at least one – donor substrate on the side of its front face, onto a polycrystalline silicon carbide support substrate, to obtain a composite structure and a residual donor substrate; 4) inspection of a free surface of the thin layer of the composite structure using a defect inspection technique involving diffusion of an ultraviolet laser beam, so as to extract a map of the free surface, called a second map, listing defects, called secondary defects; 5) the gradation of the composite structure, including a comparison of the first mapping and the second mapping. BRIEF DESCRIPTION OF THE FIGURES 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: It presents a donor substrate, a composite structure without and with epitaxial layer; La, la, laand lapresent examples of a correlation study, carried out on SICA88 type equipment, between primary defects present on a front face of a donor substrate and associated secondary defects, present on the free face of a thin layer in a composite structure; the primary defects are imaged by photoluminescence microscopy; they give rise to secondary defects when they have a contrast value greater than a predetermined threshold; the secondary defects are imaged by confocal optical microscopy with a Nomarski prism; Figures 3a, 3b, 3c, 3d, 3e and 3f show steps of a method of manufacturing a composite structure according to the present invention; The present map (first map) lists the primary defects of a donor substrate, in step 2) of the method according to the invention; Laet present two examples of mapping (second mapping) listing the secondary defects of a composite structure (the thin layer being derived from the donor substrate, the first mapping of which is also illustrated in the figures), in step 4) of the method according to the invention; Figures 3a, 3b, 3c, 3d, 3e, 3e' and 3e'' show steps of an alternative embodiment of the method of manufacturing a composite structure according to the present invention. DETAILED DESCRIPTION OF THE INVENTION The invention relates to a method for manufacturing a composite structure 100 comprising a thin layer 10 of monocrystalline silicon carbide arranged on a support substrate 20 of polycrystalline silicon carbide. It will of course be understood that, even if the description only mentions the manufacturing of one composite structure 100 for reasons of readability, the method applies to the manufacturing of a plurality of composite structures 100. In the description to follow, we will conventionally call “primary defects” the defects present on and / or in a donor substrate 1 from which the thin layer 10 is derived, “secondary defects” the defects present on and / or in the thin layer of a composite structure 100, and “tertiary defects” the defects present on and / or in the raw epitaxial layer on the thin layer 10. In a main plane (x,y), the donor substrate 1 and the composite structure 100 are preferably in the form of circular wafers with a diameter of 100mm, 150mm, 200mm, or even more. They could nevertheless be in any other form allowing their subsequent processing for the manufacture of components. The thickness of the substrates and structures extends along the z axis on the. It is the front faces 1a, 10a, 150a of the substrate 1 and the structure 100 which are inspected and likely to contain the aforementioned defects. The manufacturing method comprises a first step 1) of providing a donor substrate 1 made of monocrystalline silicon carbide. The monocrystalline SiC can be of polytype 4H, 6H or 3C. The donor substrate 1 is preferably in the form of a wafer with a diameter identical to or very close to that of the support substrate 20 with which it will subsequently be assembled, and with a thickness typically between 300 μm and 800 μm. It has a front face 1a and a rear face 1b (). The surface roughness of the front face 1a is advantageously chosen to be less than 1 nm RMS, or even less than 0.5 nm RMS, measured by atomic force microscopy (AFM) on a scan of 20 μm x 20 μm. The type of doping and the resistivity of the donor substrate 1 are defined according to the application and the devices targeted.Preferably, the front face 1a is a “carbon” face [000-1], to provide, after the transfer, a thin layer 10 with a front face 10a of “silicon” type.

[0001] in the composite structure 100. Even if it is stated in step 1) the provision of a donor substrate 1, it can of course be provided at this step a plurality of donor substrates 1, in an industrial framework. The second step 2) of the method comprises controlling the quality of the – at least one – donor substrate 1 by a photoluminescence imaging technique so as to extract a map of the front face 1a, called the first map, listing the primary defects identified as being of the micro-hole type, of the complex star stacking fault type and of the point defect type (for example, a defect linked to a species inclusion). Indeed, the applicant has identified that these types of defects could constitute critical defects which generate problematic secondary defects in the thin layer 10 of the composite structure 100. The primary defects of a donor substrate 1 are made visible in photoluminescence imaging due to local variations in the intensity of a photoluminescence (PL) signal emitted by the front face 1a, following excitation by an incident beam. The local variations in the intensity of the PL signal reflect modified properties of the material (stresses, roughness, flatness, etc.), corresponding to the primary defect. The latter can appear on the generated PL image, in the form of white or black spot(s) depending on its characteristics. This control step can be carried out on known equipment, for example SICA88 (Lasertec company) or “Photoluminescence scanner” (Intego company) or “MiPlato SiC” (EtaMax company). These equipments traditionally use the combination of a visible light optical microscopy image and the photoluminescence image of the same defect to assign dimensional (size, surface) and intensity (contrast) criteria from the optical microscope and measured photoluminescence signals; they can also assign a typology criterion (predefined classes of defects) to each of the primary defects, based, for example, on a learning algorithm. The predefined classes typically correspond to defects of the micro-hole type (“micro-pipe”), star stacking fault complex, point defect (particularly linked to inclusions), scratch, particle, etc. Advantageously, step 2) comprises the three sub-steps described below: Inspection of the front face 1a of the donor substrate 1: This step aims to detect primary defects, due to local variations in the intensity of a photoluminescence signal emitted by the front face 1a following excitation by an incident beam, and to form, for each primary defect, a photoluminescence image. In SICA88 equipment, which will be favored in the remainder of this description, the incident excitation beam has a wavelength of 313nm and the emitted photoluminescence signal is collected in a wavelength range from 700nm to 1000nm. The assignment to each detected primary defect of a labeled defect type: An image recognition algorithm, based for example on a SSD (Single Shot Detector) type model for object detection, is fed and trained on different types of defects likely to be present on the front face 1a of a donor substrate 1 made of monocrystalline silicon carbide, such as micro-holes, star stacking fault complexes and point defects in particular related to inclusions (labeled defect types). The equipment can thus assign to each detected primary defect a labeled defect type, with a certain level of similarity. A contrast level is further associated with the photoluminescence image of each detected primary defect.The classification of each primary defect by applying the following conditions: if the labeled defect type is micro-hole and if the similarity level is higher than a first level, the primary defect is classified as a critical defect of micro-hole type, if the labeled defect type is complex star stacking fault and if the similarity level is higher than a second level, the primary defect is classified as a critical defect of complex star stacking fault type, if the labeled defect type is point defect, if the similarity level is higher than a third level, and if the contrast level is higher than a predetermined threshold, the primary defect is classified as a critical defect of point defect type, in other cases, the primary defect is classified as a non-critical defect. The first, second and third levels of similarity can be different from each other, as they depend on the typology of the defect and the number of defects of each type used to train the image recognition algorithm. The predetermined contrast threshold can be established empirically for a given type of donor substrate 1 (supplier, crystal structure, roughness of the front face, manufacturing technique, etc.), on the basis of a correlation study between the contrast value of primary defects detected on a front face 1a of one (or preferably several) test donor substrate(s) and the presence of secondary defects on one (or preferably a plurality of) thin layer(s) 10, originating from the test donor substrate(s), transferred to a support substrate 20. The table shows the correlation that can be observed between a critical primary defect ((a)) on a donor substrate 1 and a secondary defect ((b)) induced in the thin layer 10 from this substrate 1. The tables in Figures 2b, 2c and 2d show that not all primary defects detected on a donor substrate 1 give rise to secondary defects in the reported thin layer 10, and that it is therefore required to define criteria to correctly classify these primary defects into critical or non-critical defects. In addition to the similarity level applied to each primary defect associated with a labeled defect type, the contrast level of the photoluminescence image is an important criterion, in particular to judge the criticality of point defects, because these defects can present a wide range of contrasts, from low to high. This is the reason why a predetermined contrast threshold is used, to separate critical defects from non-critical defects. In the example of 1a, the predetermined contrast threshold is set at 600 auThis threshold obviously depends on the recipe parameters, the characteristics of the support substrate 1 (roughness, doping, residual stress, production method, etc.) and a universal value cannot in any case be proposed. For microhole or star-shaped stacking fault complexes, the similarity level is often sufficient because these defects have a very specific signature and associated high contrast. The photoluminescence image can also be combined with the optical microscopy image of these defects to further improve their classification. Recall that the contrast on a photoluminescence image can be defined as the normalized difference between a photoluminescence signal intensity at the primary defect and a surrounding intensity in a field less than or equal to the microscope field size. Other calculations of the contrast value would give different numerical results and a predetermined contrast threshold, but with a similar meaning. The example of a first mapping is given on the: it lists the primary defects detected on a donor substrate 1 and classified as critical defects (in particular of the complex type of star stacking faults, micro-hole or point defect of the species inclusion type); another defect, classified as non-critical is illustrated at the top left of the. According to an advantageous embodiment, in step 2) of the method according to the invention, a donor substrate 1 having a density of primary defects, classified as critical defects regardless of the type, greater than 1 defect / cm 2 , or even greater than 0.25 defects / cm 2 , is downgraded at the end of step 2) and is not used for the subsequent step 3). The manufacturing method then comprises a third step 3) of transferring a thin layer 10, originating from a surface layer 10' of the donor substrate 1, onto a support substrate 20 made of polycrystalline silicon carbide, to obtain the composite structure 100 and a residual donor substrate 1' devoid of the surface layer 10'. In the composite structure 100, the front face 1a of the donor substrate 1 is thus arranged opposite a front face 20a of the support substrate 20, and it corresponds to a buried face of the thin layer 10, which is not accessible for surface inspection; the other face 10a of the thin layer 10, opposite the buried face (front face 1a) is free and will be inspected in the following step. Advantageously, the surface layer 10' is transferred by a thin layer transfer technique such as Smart Cut. A first sub-step 3a) comprises the implantation of light species in a donor substrate 1, to form a buried fragile plane 11 delimiting, with a front face 1a of the donor substrate 1, the surface layer to be transferred 10' (). The light species are preferably hydrogen and / or helium, and are implanted in the donor substrate 1, at a depth consistent with the thickness of the targeted thin layer 10. These light species will form, around the determined depth, microcavities distributed in a thin layer parallel to the free surface 1a of the donor substrate 1, i.e. parallel to the plane (x,y) in the figures. This thin layer is called the buried fragile plane 11, for the sake of simplification. 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 thin layer 10 having a thickness of the order of 100nm to 1500nm. Note that a protective layer may be deposited on the front face 1a of the donor substrate 1, 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 may be removed prior to the following sub-step 3b). The latter corresponds to the assembly of a support substrate 20 made of polycrystalline silicon carbide, on the side of its front face 20a, with the implanted donor substrate 1, also on the side of its front face 1a (). The support substrate 20 corresponds to the mechanical support of the future composite structure 100. The lateral dimensions in the main plane (x,y) of the support substrate 20 (its diameter in particular) are the same as those of the composite structure 100.The support substrate 20 has a thickness typically between approximately 50 μm and several hundred micrometers, for example between 50 μm and 650 μm, or between 100 μm and 450 μm, or between 200 μm and 350 μm. The assembly is made by direct bonding, by molecular adhesion, along a bonding interface 40. Optionally, an intermediate layer can be formed on the front face 1a of the donor substrate 1, before or after the introduction of the light species, and in any case, before the assembly phase. This intermediate layer can be made of a dielectric, semiconductor or metallic material (such as for example silicon oxide, silicon, silicon carbide, tungsten, titanium, etc.). Optionally, an intermediate layer can also be deposited on the face 20a to be assembled of the support substrate 20, prior to the assembly; it can be chosen to be of the same nature or of a different nature from the intermediate layer mentioned for the donor substrate 1. An intermediate layer can optionally be deposited on either of the two substrates 1, 20 to be assembled.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 assembled directly; another advantage of this (these) intermediate layer(s) may be to improve the vertical electrical conduction of the bonding interface 40. The intermediate layer(s) is (are) intended to be buried in the bonded assembly 50 after assembly, and ultimately, in the composite structure 100. Direct bonding by molecular adhesion does not require an adhesive material, because bonds are established at the atomic level between the assembled surfaces. Several types of molecular adhesion bonding exist, which differ in particular in their temperature, pressure, atmosphere or treatment conditions prior to bringing the surfaces into contact. Examples include room temperature bonding with or without prior plasma activation of the surfaces to be joined, atomic diffusion bonding (ADB), surface-activated bonding (SAB), etc. Let us recall that the assembly step 3b) may comprise, prior to bringing the faces 1a, 20a 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 40 (low defectivity, high adhesion energy). The following sub-step 3c) corresponds to a separation along the buried fragile plane 11 to form an intermediate composite structure 100' comprising the surface layer 10' after transfer and the support substrate 20, on the one hand, and the remainder of the donor substrate 1', on the other hand (). The separation along the buried fragile plane 11 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 11, and their pressurization by the light species present in gaseous form, until the propagation of a fracture along said fragile plane 11. Alternatively or jointly, a mechanical stress can be applied to the bonded assembly 50 and in particular at the buried fragile plane 11, so as to propagate or help to mechanically propagate the fracture leading to the separation.At the end of this separation, we obtain on the one hand the intermediate composite structure 100' and the remainder 1' of the donor substrate. The free surface 10'a of the surface layer 10' is usually rough after separation: for example, it has a roughness of between 5nm and 100nm RMS. The following sub-step 3d) comprises the application of thermal, mechanical and / or chemical treatments to a free surface 10'a of the surface layer 10', to form the composite structure 100 provided with the thin layer 10 of monocrystalline silicon carbide having a surface roughness less than or equal to 0.5nm RMS, or even less than or equal to 0.1nm RMS (AFM scan 10x10μm 2 or 20x20μm 2) (). In particular, this sub-step 3d) may comprise a mechanical-chemical smoothing treatment of the free surface 10'a of the surface layer 10'. A removal of between 50nm and 300nm makes it possible to effectively restore the surface state of said layer. It may also comprise at least one heat treatment at a temperature of between 1200°C and 1800°C. Such a heat treatment is applied to remove the residual light species from the surface layer 10' and to promote the rearrangement of its crystal lattice, thus forming the thin layer 10. It also makes it possible to reinforce the bonding interface 40. At this stage of the process, the thin layer 10 of the composite structure 100 has a thickness typically between a few tens of nm and a few hundreds of nm, for example, between 50 nm and 800 nm. The types and doping levels of the thin layer 10 and the support substrate 20 are defined according to the intended applications and devices. Returning to the general description of the manufacturing method according to the invention, step 3) of transferring the thin layer 10 onto the support substrate 20 is followed by a fourth step 4) of inspecting the free surface 10a of said thin layer 10 by a defect inspection technique based on dark field and bright field imaging associated with scanning the surface 10a with a DUV laser beam (for "deep UV", typically with a wavelength of the order of 200-280nm); the laser beam is scattered by the defects proportionally to their size, which allows the localization of said defects on the surface 10a and the estimation of their dimension. For example, equipment such as KLA SPA2 can be used. The DUV laser beam penetrates very weakly into the thickness of the thin layer 10, it is therefore not polluted by the presence of the p-SiC grains of the support substrate 20.This technique makes it possible to detect secondary defects present on the surface of the thin layer 10 and to associate them with known defects in the equipment database such as holes, bubbles, scratches, particles, other point or agglomerated defects, etc. This inspection step is widely used in the semiconductor industry (especially silicon), making it compatible with industrial speed standards and requirements. Secondary defects may be induced by primary defects (crystalline defects present on the donor substrate 1 having given rise to the thin layer 10), by particles or other surface contaminations which have not been completely eliminated, before the assembly of the donor substrate 1 and the support substrate 20, or even by a specific problem at a stage of the manufacturing process (scratches, deposited particles, flaking, etc.). These may therefore be defects of crystalline origin, or defects linked to the transfer such as holes (local absence of thin layer 10), bubbles (defect at the bonding interface 40, at which the thin layer 10 is not bonded and forms a blister), particles, scratches, etc. At the end of step 4), a map of the free surface 10a, called the second map, can be extracted, listing the identified secondary defects. An example of a second map listing the secondary defects detected on a composite structure 100 whose thin layer 10 comes from the donor substrate 1 (first map) is given on the. Note that to compare the first and second maps, it is required to apply a “mirror” flip of the first map, since the front face 1a donor substrate 1 is assembled on the support substrate 20, and that it therefore undergoes a flip. In this example, several primary defects classified as critical (visible on the first map) are also visible on the second map. Another example is given on the : here, several primary defects classified as critical and visible on the first map, do not stand out on the second map. The advantage of this inspection step 4) is that it is fast and therefore allows 100% control of composite structures in a production line. However, it is likely not to detect defects of crystalline origin present in the thin layer, because some of these defects do not induce diffusion of the UV laser beam and remain invisible to this inspection. Conversely, a control technique combining visible light optical microscopy and photoluminescence imaging can make it possible to detect these defects of crystalline origin, but inspection may be made difficult by the presence of underlying p-SiC grains and, above all, requires a much longer processing time per structure, which is impactful in a high-volume production line. Thus, the applicant has defined a fifth step 5) of grading the composite structure 100 which is reliable and efficient, based on the use of the first mapping of the donor substrate 1 (step 2) from which the thin layer 10 of the composite structure 100 in question is derived, and of the second mapping of said structure 100 (step 4). This grading aims to separate the structures falling within the targeted specifications and the downgraded structures. The second mapping is compared to the first mapping, having of course taken the precaution of applying a mirror reversal to the first mapping since the donor substrate 1, bonded to the support substrate 20, has its front face 1a (buried face of the thin layer 10) reversed. It is on the basis of this comparison that the gradation of the composite structure 100 is decided. Advantageously, step 5) comprises a reconciliation between the primary defects classified as critical defects of the first mapping and the secondary defects of the second mapping. Such a reconciliation is possible because the first and second mappings are established in the same orthonormal reference frame, and each point of one or other of the mappings is identified by a Cartesian or polar coordinate system for example. When a primary defect classified as a critical defect cannot be associated with a secondary defect, because no defect is detected on the second mapping, at the location of said primary defect, an additional defect is added to the defects of the second mapping, to be taken into consideration in the grading decision.The primary defects identified as critical (i.e. generating defects in the transferred thin layer) are thus considered for the evaluation of the quality of the composite structures 100, without requiring a final inspection, which is complex and time-consuming, on the composite structures 100. According to a variant, step 5) comprises a reconciliation between the primary defects classified as critical defects and the secondary defects, and if a large number of primary defects classified as critical defects are not each associated with a secondary defect, step 5) comprises a complementary control of the composite structure 100 in question, by a technique coupling confocal microscopy in visible light and photoluminescence imaging, making it possible to verify whether defects are actually present in the thin layer 10 at the locations of said critical primary defects. The grading step 5) of the manufacturing method according to the invention makes it possible to decide reliably and efficiently the grade to be assigned to the composite structures 100, by benefiting from precise control of the donor substrates 1 adapted to the field of layer transfer, by carrying out a rapid and standardized quality control in the end on all the composite structures 100, and by combining the teaching of these two controls. In particular, a composite structure 100 may be downgraded if it has: a density of secondary defects associated with critical primary defects and primary defects classified as critical defects added to the second mapping, greater than 0.25 defects / cm 2 , and / or a density of secondary defects only detected on the second mapping (i.e. not associated with critical primary defects) greater than 0.2 defects / cm 2 . In a subsequent step, SiC epitaxial growth can be carried out on the thin layer 10 of the composite structures 100 having positively passed the gradation step 5), in order to increase the thickness of the latter and form the epitaxial layer 150 on and in which the devices will be produced (). The epitaxial growth step can be carried out according to the techniques known in the state of the art. The manufacturing method may also comprise a sixth step 6) of recycling the residual donor substrate 1' to make it a recycled donor substrate. Step 6) may comprise mechanical and / or chemical treatments, similar to those applied to the composite structure 100, implemented at the front face 1'a of the residual donor substrate 1'. The recycled donor substrate is then directly introduced in step 3) as a new donor substrate 1. In step 5), the gradation of the resulting composite structure 100 uses the first mapping (step 2) established for the initial donor substrate 1 from which the recycled donor substrate originates. The first mapping reflecting the quality of the initial donor substrate 1 is therefore used to grade each composite structure 100 produced from this substrate 1, in its initial state and after a plurality of recyclings. Although this control is time-consuming, it is industrially viable because it is amortized over a very large number of composite structures 100 manufactured from the donor substrate 1. The manufacturing method has been described with a step 3) of transferring the thin layer 10 onto the support substrate 20, without intermediate manipulation, which can be called simple transfer. According to an alternative embodiment, step 3) may consist of a double transfer, illustrated in FIGS. 6a to 6f, during which the thin layer 10 is first transferred from the donor substrate 1 onto a temporary substrate 30, via a first bonding interface 60 (FIGS. 6a, 6b, 6c, 6d, 6e), then transferred a second time from the temporary substrate 30 onto the support substrate 20 (FIGS. 6e', 6e''), via a second bonding interface 40. Step 3) can thus include the following sub-steps: 3i) the implantation of light species in a donor substrate 1, to form a buried fragile plane 11 delimiting, with a front face 1a of the donor substrate 1, the surface layer 10' to be transferred; 3ii) the assembly of a temporary substrate 30 with the donor substrate 1 implanted in step 3i), via a first bonding interface 60, 3iii) separation along the buried fragile plane 11 to form an intermediate assembly comprising the surface layer 10' after transfer and the temporary substrate 30, on the one hand, and the remainder 1' of the donor substrate, called residual donor substrate 1', on the other hand, 3iv) the application of thermal, mechanical and / or chemical treatments to a free surface 10'a of the surface layer 10', to form a temporary structure provided with the thin layer 10 of monocrystalline silicon carbide arranged on the temporary substrate 30, 3v) the assembly of a support substrate 20 made of polycrystalline silicon carbide with the thin layer 10 of the temporary structure, via a second bonding interface 40; 3vi) disassembly along the first bonding interface 60 to separate the temporary substrate 30 from the composite structure 100 provided with the thin layer 10 of monocrystalline silicon carbide. The first bonding interface 60 may be based on molecular adhesion bonding, as may the second interface 40. The temporary substrate 30 may be chosen from any material compatible with direct bonding, in particular silicon, silicon carbide, etc. The temporary substrate 30 may be removed by mechanical disassembly, for example. The composite structure 100, obtained in this alternative embodiment, comprises a thin layer 10 whose free surface 10b corresponds to the front face 1a of the donor substrate 1, taking into account the double transfer. Thus, in step 5) of grading the composite structure 100, the comparison of the first mapping and the second mapping can be carried out without mirror reversal. According to another particular embodiment, involving a simple transfer in step 3), the method may comprise the following steps: 1) the provision of at least one donor substrate 1 made of monocrystalline silicon carbide, having a front face 1a and a rear face 1b, the front face 1a potentially having defects, called primary defects, which are through and also exist on the rear face 1b; 2) controlling the quality of the – at least one – donor substrate 1 by a photoluminescence imaging technique so as to extract a map of the rear face 1b, called the first map, listing the primary defects identified as being of the micro-hole type, of the complex star stacking fault type or of the point defect type; 3) the transfer of a thin layer 10, originating from a surface layer 10' of the – at least one – donor substrate 1 on the side of its front face 1a, onto a support substrate 20 made of polycrystalline silicon carbide, to obtain a composite structure 100 and a residual donor substrate 1'; 4) the inspection of a free surface 10a of the thin layer 10 of the composite structure 100 by a technique of inspecting defects by scattering an ultraviolet laser beam, so as to extract a map of the free surface 10a, called the second map, listing defects, called secondary defects; 5) the gradation of the composite structure 100, including a comparison of the first mapping and the second mapping. According to this particular embodiment, the quality control of the donor substrate 1 is carried out on its rear face 1b, considering that the critical primary defects are mainly through and will therefore be detectable on the rear face 1b, as on the front face 1a. In step 5), the comparison of the first mapping and the second mapping can be carried out without mirror reversal. Note that the generic details of steps 1) to 5) set forth in this description in the case of a single transfer may apply to this particular embodiment, as well as to the alternative embodiment based on a double transfer. 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 as defined by the claims.

Claims

A method of manufacturing a composite structure (100) comprising a thin layer (10) of monocrystalline silicon carbide arranged on a support substrate (20) of polycrystalline silicon carbide, the method comprising the following steps: 1) providing at least one donor substrate (1) of monocrystalline silicon carbide, having a front face (1a) and a rear face (1b), the front face (1a) potentially having defects, called primary defects; 2) controlling the quality of the – at least one – donor substrate (1) by a photoluminescence imaging technique so as to extract a map of the front face (1a), called the first map, listing the primary defects identified as being of the micro-hole type, of the complex star stacking fault type or of the point defect type;3) transferring a thin layer (10), originating from a surface layer (10') of the – at least one – donor substrate (1), onto a support substrate (20) made of polycrystalline silicon carbide, to obtain a composite structure (100) and a residual donor substrate (1');4) inspecting a free surface (10a, 10b) of the thin layer (10) of the composite structure (100) using a defect inspection technique by scattering an ultraviolet laser beam, so as to extract a map of the free surface (10a, 10b), called the second map, listing defects, called secondary defects;5) grading the composite structure (100), including a comparison of the first map and the second map.; A method of manufacturing a composite structure (100) according to claim 1, wherein step 2) comprises the following sub-steps:i) inspecting a front face (1a) of the donor substrate (1) to detect primary defects, due to local variations in the intensity of a photoluminescence signal emitted by the front face (1a) following excitation by an incident beam, and to form, for each primary defect, a photoluminescence image,ii) assigning to each detected primary defect a labeled defect type with a certain level of similarity, using an image recognition algorithm, trained on different types of defects likely to be present on the front face (1a) of a donor substrate (1) made of monocrystalline silicon carbide, such as micro-holes, star stacking fault complexes and point defects in particular linked to inclusions,a contrast level being associated with the photoluminescence image of each detected primary defect,iii) the classification of each primary defect by applying the following conditions:- if the labeled defect type is micro-hole and if the similarity level is higher than a first level, the primary defect is classified as a critical defect of micro-hole type,- if the labeled defect type is complex of star stacking faults and if the similarity level is higher than a second level, the primary defect is classified as a critical defect of complex of star stacking faults type,- if the labeled defect type is point defect, if the similarity level is higher than a third level, and if the contrast level is higher than a predetermined threshold, the primary defect is classified as a critical defect of point defect type,- in other cases, the primary defect is classified as a non-critical defect. A method of manufacturing a composite structure (100) according to claim 2, wherein the inspection step i) is carried out with an incident beam of wavelength 313nm and the emitted photoluminescence signal is collected in a wavelength range from 700nm to 1000nm. Method for manufacturing a composite structure (100) according to one of claims 2 and 3, in which the predetermined threshold is established empirically for a type of donor substrate (1), on the basis of a correlation study between the contrast value of primary defects detected on a front face (1a) of a test donor substrate (1) and the presence of secondary defects on a thin layer (10), originating from the test donor substrate, transferred to a support substrate (20). A method of manufacturing a composite structure (100) according to one of claims 2 to 4, wherein a donor substrate (1) having a density of primary defects, classified as critical defects regardless of the type, greater than 1 defect / cm 2 , or even preferably greater than 0.25 defect / cm 2 , is downgraded at the end of step 2) and is not used for step 3). Method for manufacturing a composite structure (100) according to one of claims 2 to 5, in which step 5) comprises a reconciliation between the primary defects classified as critical defects and the secondary defects, each primary defect classified as critical defect and not associated with a secondary defect being added to the defects of the second mapping to be taken into account in a grading decision. A method of manufacturing a composite structure (100) according to claim 6, wherein the grading decision corresponds to a downgrading of the composite structure if the latter comprises:- a density of secondary defects associated with critical primary defects and of primary defects classified as critical defects added to the second mapping, greater than 0.25 defects / cm 2 , and / or- a density of secondary defects only detected on the second mapping, not associated with critical primary defects, greater than 0.2 defects / cm 2 . Method for manufacturing a composite structure (100) according to one of claims 2 to 5, in which step 5) comprises a reconciliation between the primary defects classified as critical defects and the secondary defects, and step 5) comprises a complementary control by a technique coupling optical microscopy in visible light and photoluminescence imaging, making it possible to verify whether defects are present in the thin layer (10), at the locations of said primary defects classified as critical defects. A method of manufacturing a composite structure (100) according to one of the preceding claims, wherein step 3) comprises the following sub-steps:3a) implanting light species in a donor substrate (1), to form a buried fragile plane (11) delimiting, with a front face (1a) of the donor substrate (1), the surface layer (10') to be transferred;3b) assembling a support substrate (20) made of polycrystalline silicon carbide with the donor substrate (1) implanted in step 3a);3c) separating along the buried fragile plane (11) to form an intermediate composite structure (100') comprising the surface layer (10') after transfer and the support substrate (20), on the one hand, and the remainder (1') of the donor substrate, called the residual donor substrate (1'), on the other hand;3d) the application of thermal, mechanical and / or chemical treatments to a free surface (10'a) of the surface layer (10'), to form the composite structure (100) provided with the thin layer (10) of monocrystalline silicon carbide.; Method for manufacturing a composite structure (100) according to one of the preceding claims, in which, in the composite structure (100): - the thin layer (10) comprises a buried face arranged opposite the support substrate (20) corresponding to the front face (1a) of the donor substrate (1), and another face, opposite the front face (1a), corresponding to a free surface (10a) of said layer (10), - in step 5), a mirror reversal being applied to the first or second mapping to compare them. Method for manufacturing a composite structure (100) according to one of claims 1 to 8, in which step 3) comprises the following sub-steps: 3i) the implantation of light species in a donor substrate (1), to form a buried fragile plane (11) delimiting, with a front face (1a) of the donor substrate (1), the surface layer (10') to be transferred;3ii) assembling a temporary substrate (30) with the donor substrate (1) implanted in step 3i), via a first bonding interface (60),3iii) separating along the buried fragile plane (11) to form an intermediate assembly comprising the surface layer (10') after transfer and the temporary substrate (30), on the one hand, and the remainder (1') of the donor substrate, called residual donor substrate (1'), on the other hand,3iv) applying thermal, mechanical and / or chemical treatments to a free surface (10'a) of the surface layer (10'), to form a temporary structure provided with the thin layer (10) of monocrystalline silicon carbide arranged on the temporary substrate (30),3v) assembling a support substrate (20) of polycrystalline silicon carbide with the thin layer (10) of the temporary structure, via a second bonding interface (40);3vi) disassembly along the first bonding interface (60) to separate the temporary substrate (30) from the composite structure (100) provided with the thin layer (10) of monocrystalline silicon carbide.; A method of manufacturing a composite structure according to one of the preceding claims, comprising a step 6) of recycling the residual donor substrate (1') to make it a recycled donor substrate (1''), and in which:- the recycled donor substrate (1'') is introduced directly in step 3) as a donor substrate (1), and- in the following step 4), the gradation of the resulting composite structure (100) uses the first mapping established for the initial donor substrate (10) from which the recycled donor substrate (1'') originates. A method of manufacturing a composite structure (100) comprising a thin layer (10) of monocrystalline silicon carbide arranged on a support substrate (20) of polycrystalline silicon carbide, the method comprising the following steps: 1) providing at least one donor substrate (1) of monocrystalline silicon carbide, having a front face (1a) and a rear face (1b), the front face (1a) potentially having defects, called primary defects, which are through-face and also exist on the rear face (1b); 2) controlling the quality of the – at least one – donor substrate (1) by a photoluminescence imaging technique so as to extract a map of the rear face (1b), called the first map, listing the primary defects identified as being of the micro-hole type, of the complex star stacking fault type or of the point defect type;3) transferring a thin layer (10), originating from a surface layer (10') of the – at least one – donor substrate (1) on the side of its front face (1a), onto a support substrate (20) made of polycrystalline silicon carbide, to obtain a composite structure (100) and a residual donor substrate (1');4) inspecting a free surface (10a) of the thin layer (10) of the composite structure (100) by a technique for inspecting defects by scattering an ultraviolet laser beam, so as to extract a map of the free surface (10a), called the second map, listing defects, called secondary defects;5) grading the composite structure (100), including a comparison of the first map and the second map.

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