manufacturing process for a composite structure including a grading step

Abstract

The invention relates to a method for manufacturing a composite structure comprising a thin layer of monocrystalline silicon carbide disposed on a support substrate of polycrystalline silicon carbide, the method comprising the following steps: 1) the supply of at least one donor substrate of monocrystalline silicon carbide, having a front face and a back face, the front face potentially having defects, called primary defects; 2) the quality control of the – at least one – donor substrate by 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 complex type of star stacking faults or of the point defect type;3) the transfer of a thin film, originating from a surface layer of at least one donor substrate, onto a polycrystalline silicon carbide support substrate, to obtain a composite structure and a residual donor substrate; 4) the inspection of a free surface of the thin film of the composite structure using an ultraviolet laser beam scattering defect inspection technique, in order to extract a map of the free surface, referred to as the second map, listing defects, referred to as secondary defects; 5) the grading of the composite structure, including a comparison of the first and second maps. Figure to be published with the abstract: No figure;

Description

Title of the invention: Method for manufacturing a composite structure including a grading step FIELD OF INVENTION

[0001] The present invention relates to the field of semiconductor materials, particularly composite structures comprising a thin film (derived from a single-crystal 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 grading step based on the control mapping of the donor substrate and the control mapping of the thin film of the composite structure.

[0002] TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0003] Silicon carbide is a material of particular interest for the manufacture of power devices, radio frequencies or devices operating at very high temperatures.

[0004] The quality of single-crystal silicon carbide (c-SiC) substrates has improved significantly over the past ten years, accompanied by increasingly precise knowledge and detection of the various types of crystalline defects that may be present in this material. The JEITA (Japan Electronics and Information Technology Industries Association) standard includes four documents relating to the zoology of defects on / in a layer produced by ho-moepitaxy on a 4H-SiC substrate (EDR 4712 / 100) and to non-destructive procedures for the optical inspection of these defects (EDR 4712 / 200, / 300, / 400). In particular, it specifies a procedure for evaluating and referencing defects by combining optical inspection and photoluminescence imaging.

[0005] Commercially available equipment, such as in particular the SICA88 from Lasertec, allows the combination of Nomarski prism confocal microscopy techniques in visible light and by photoluminescence (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.

[0006] By way of 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 fabricated on a layer homoepitaxial in c-SiC, and shows optical and photoluminescence images of typical c-SiC defects.

[0007] Although rapidly developing, high-quality c-SiC substrates remain expensive and difficult to source in large sizes. Therefore, it is advantageous to use layer transfer solutions to create composite structures comprising a thin film of monocrystalline SiC (derived from the high-quality c-SiC donor substrate) on a lower-cost support substrate, for example, polycrystalline SiC (p-SiC), which may also offer advantages in terms of electrical conductivity. A well-known thin-film transfer solution is the Smart Cut™ process, based on light ion implantation and direct bonding between a c-SiC donor substrate and a support substrate at a bonding interface.The implantation creates a fragile, buried plane along which a separation occurs, leading to the transfer of a thin c-SiC layer onto the supporting substrate to form the composite structure. This allows for the recovery and recycling of the remaining donor substrate, potentially enabling one or more further layer transfers. Epitaxy can then be performed on the thin layer of the composite structure, followed by the fabrication of the electronic devices.

[0008] For the implementation of a layer transfer process to be economically viable, it is important to be able to control the quality of the donor substrates in order to avoid transferring a layer that would inevitably lead to a downgrade of the composite structure, or that would result in an epitaxial layer that is out of specification, in terms of "killer" defects. The defect zoology of c-SiC 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 epitaxially grown c-SiC layer that would be fatal to the components; however, the applicant has observed that the criteria for classifying defects in a donor substrate are not necessarily the same when epitaxy is performed on said substrate and when a thin film transfer is performed from said substrate.

[0009] It is therefore important to detect, but above all to accurately 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 fabrication of a thin film of required quality.

[0010] Furthermore, the detection and recognition of defects (so-called secondary defects) on and / or in the thin film (derived from a donor substrate) of a composite structure whose support substrate is polycrystalline SiC (p-SiC) is complex because the p-SiC grains are visible under said thin film, particularly with equipment combining Nomarski prism confocal microscopy in visible light and photoluminescence imaging. However, reliable and efficient control (not requiring too much inspection time per structure) of the thin film quality is required, to avoid continuing the epitaxial steps if secondary defects in the thin film are likely to generate an out-of-specification killer defect density in the homoepitaxial layer.

[0011] 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 for reliable and rapid inspection of both donor substrates, at the beginning of the chain, and composite structures, in the middle of the chain, in order to assign a grade, as soon as possible in the overall manufacturing chain, to the donor substrates or composite structures and to allow the achievement of the expected specifications and yields for the devices.

[0012] SUBJECT OF THE INVENTION

[0013] 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 polycrystalline silicon carbide support substrate. The manufacturing method includes an original, reliable, and rapid grading step of the composite structure, combining an initial inspection of the donor substrate and a final inspection of said structure.

[0014] BRIEF DESCRIPTION OF THE INVENTION

[0015] The invention relates to a method for manufacturing a composite structure comprising a thin layer of monocrystalline silicon carbide deposited on a polycrystalline silicon carbide support substrate, the method comprising the following steps:

[0016] 1) the supply of at least one mono-silicon carbide donor substrate crystalline, having a front face and a back face, the front face potentially presenting defects, called primary defects;

[0017] 2) quality control of the - at least one - donor substrate by a technique photoluminescence imaging in order 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 complex type of star stacking faults or of the point defect type;

[0018] 3) the transfer of a thin layer, originating from a surface layer of the - at least one - donor substrate, on a polycrystalline silicon carbide support substrate, to obtain a composite structure and a residual donor substrate;

[0019] 4) the inspection of a free surface of the thin layer of the composite structure by a technique for inspecting defects by scattering an ultraviolet laser beam, in order to extract a map of the free surface, called a second map, listing defects, called secondary defects;

[0020] 5) the gradation of the composite structure, including a comparison of the first mapping and the second mapping.

[0021] According to other advantageous and non-limiting features of the invention, taken alone or in any technically feasible combination: • Step 2) includes the following sub-steps: (i) the inspection of 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) the assignment to each detected primary defect of 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 single-crystal silicon carbide donor substrate, such as micro-holes, star stacking fault complexes and point defects, particularly those related to inclusions, with 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 greater than a first level, the primary defect is classified as a critical micro-hole defect, - if the labeled type of fault is complex star stacking faults and if the similarity level is greater than a second level, the primary fault is classified as a critical fault of complex star stacking fault type, - if the labeled type of defect is a point defect, if the similarity level is greater than a third level, and if the contrast level is greater than a predetermined threshold, the primary defect is classified as a critical point defect. - 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, based on 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, taken from the test donor substrate, transferred onto a support substrate; a donor substrate with a density of primary defects, classified as critical defects of any type, greater than 1 defect / cm2, or preferably greater than 0.25 defects / cm2, is downgraded at the end of step 2) and is not used for step 3); step 5) includes a reconciliation between primary defects classified as critical defects and secondary defects, each primary defect classified as critical and not associated with a secondary defect being added to the defects in 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 includes: - 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 / cm2, and / or - a density of secondary defects detected only on the second mapping, not associated with critical primary defects, greater than 0.2 defects / cm2; step 5) includes a reconciliation between the primary defects classified as critical defects and the secondary defects, and step 5) includes a complementary check by a technique coupling optical microscopy in visible light and photoluminescence imaging, allowing to check if defects are present in the thin film, at the locations of said primary defects classified as critical defects; Step 3) includes the following sub-steps: 3a) the implantation of light species in a donor substrate, to form a fragile buried 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) the separation along the buried fragile plane to form an intermediate composite structure comprising the surface layer after transfer and the supporting substrate, on the one hand, and the remainder of the donor substrate, called residual donor substrate, on the other hand; 3d) the application of thermal, mechanical and / or chemical treatment(s) to a free surface of the surface layer, to form the composite structure equipped with the thin layer of single-crystal silicon carbide; The manufacturing process includes a step 6) of recycling the residual donor substrate to make a recycled donor substrate, and in which: - the recycled donor substrate is introduced directly in step 3) as a donor substrate, and - in the next 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 is derived. BRIEF DESCRIPTION OF THE FIGURES

[0022] Other features and advantages of the invention will become apparent from the detailed description of the invention which follows with reference to the accompanying figures in which:

[0023] [Fig.1] The [Fig.1] presents a donor substrate, a composite structure without and with epitaxial layer;

[0024]

[0025]

[0026] [Fig.2a] [Fig.2b] [Fig.2c]

[0027] [Fig.2d] Fig.2a, Fig.2b, Fig.2c and Fig.2d present examples of a correlation study, carried out on a 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 film 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 Nomarski prism confocal optical microscopy;

[0028]

[0029]

[0030]

[0031]

[0032] [Fig.3a] [Fig.3b] [Fig.3c] [Fig.3d] [Fig.3e]

[0033] [Fig.3f] Figures 3a, 3b, 3c, 3d, 3e and 3f show steps of a manufacturing process for a composite structure according to the present invention;

[0034] [Fig.4] Fig.4 presents a map (first map) listing the primary defects of a donor substrate, in step 2) of the process according to the invention;

[0035] [Fig.5a]

[0036] [Fig. 5b] Figures [Fig. 5a] and [Fig. 5b] present two examples of mapping (second mapping) listing the secondary defects of a composite structure (the thin layer being derived from the donor substrate whose first mapping is (also illustrated in the figures), in step 4) of the process according to the invention. DETAILED DESCRIPTION OF THE INVENTION

[0037] The invention relates to a method for manufacturing a composite structure 100 comprising a thin layer 10 of monocrystalline silicon carbide deposited on a support substrate 20 of polycrystalline silicon carbide. It will be understood that, although the description only mentions the manufacture of one composite structure 100 for reasons of readability, the method is applicable to the manufacture of a plurality of composite structures 100.

[0038] In the following description, 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.

[0039] In a principal plane (x,y), the donor substrate 1 and the composite structure 100 are preferably in the form of circular wafers with diameters of 100 mm, 150 mm, 200 mm, or even larger. They could, however, be in any other form allowing for their subsequent processing in the manufacture of components. The thickness of the substrate and structures extends along the z-axis in [Fig. 1]. It is the front faces 1a, 10a, 150a of the substrate 1 and the structure 100 that are inspected and are likely to contain the aforementioned defects.

[0040] The manufacturing process comprises a first step 1) of supplying a donor substrate 1 made of single-crystal silicon carbide. The single-crystal 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 be subsequently assembled, and a thickness typically between 300 µm and 800 µm. It has a front face 1a and a back face 1b ([Fig. 3a]). The surface roughness of the front face 1a is advantageously chosen to be less than 1 / 8 m RMS, or even less than 0.5 nm RMS, measured by atomic force microscopy (AFM) on a 20 µm x 20 µm scan. The type of doping and the resistivity of the donor substrate 1 are defined according to the application and the devices intended.Preferably, the front face is a "carbon" face [000-1], to provide, after the transfer, a thin layer 10 with a front face 10a of the "silicon" type

[0001] in the composite structure 100.

[0041] Even though 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 setting.

[0042] The second step 2) of the process comprises quality control of at least one donor substrate 1 by a photoluminescence imaging technique in order to extract a map of the front face, referred to as the first map, listing the primary defects identified as being of the micro-hole type, complex star stacking fault type, and point defect type (for example, a defect related to a species inclusion). Indeed, the applicant has identified that these types of defects can constitute critical defects that generate problematic secondary defects in the thin layer 10 of the composite structure 100.

[0043] 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. These local variations in the intensity of the PL signal reflect modified properties of the material (stress, roughness, flatness, etc.), corresponding to the primary defect. The latter can appear on the generated PL image as white or black spot(s), depending on its characteristics.

[0044] This inspection step can be performed on known equipment, for example, the SICA88 (Lasertec), the "Photoluminescence scanner" (Intego), or the "MiPlato SiC" (EtaMax). These devices typically combine an optical microscopy image in visible light with a photoluminescence image of the same defect to assign dimensional (size, area) and intensity (contrast) criteria based on the measured optical and photoluminescence signals. They can also assign a typology criterion (predefined defect classes) to each of the primary defects, based, for example, on a machine learning algorithm. The predefined classes typically correspond to micro-hole ("micro-pipe") defects, star stacking fault complexes, point defects (particularly those related to inclusions), scratches, particles, etc.

[0045] Advantageously, step 2) comprises the three substeps described below: i. Inspection of the front face of the donor substrate 1:

[0046] This step aims 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. In a SICA88 device, which will be preferred in the remainder of this description, the incident excitation beam has a wavelength of 313 nm and the emitted photoluminescence signal is collected in a wavelength range from 700 nm to 1000 nm. i. Assigning a labeled defect type to each detected primary defect:

[0047] An image recognition algorithm, based for example on an SSD (Single Shot Detector) type model, in French: The single-port detector (SPC) for object detection is fed and driven onto various types of defects that may be present on the front face of a single-crystal silicon carbide donor substrate, such as microholes, star stacking fault complexes, and point defects, particularly those related to inclusions (labeled defect types). The equipment can thus assign each detected primary defect a labeled defect type, with a certain level of similarity. A contrast level is also associated with the photoluminescence image of each detected primary defect. i. 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 greater than a first level, the primary defect is classified as a critical micro-hole defect, - if the labeled type of fault is complex star stacking faults and if the similarity level is greater than a second level, the primary fault is classified as a critical fault of complex star stacking fault type, - if the labeled type of defect is a point defect, if the similarity level is greater than a third level, and if the contrast level is greater than a predetermined threshold, the primary defect is classified as a critical point defect. - In other cases, the primary defect is classified as a non-critical defect.

[0048] The first, second, and third levels of similarity may differ 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.

[0049] The predetermined contrast threshold can be established empirically for a given type of donor substrate 1 (supplier, crystalline structure, front face roughness, manufacturing technique, etc.), based on a correlation study between the contrast value of primary defects detected on a front face 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, from the test donor substrate(s), transferred onto a support substrate 20.

[0050] The table in [Fig.2a] shows the correlation that can be observed between a critical primary defect ([Fig.2a] (a)) on a donor substrate 1 and a secondary defect ( [Fig.2a] (b)) induced in the thin film 10 from this substrate 1.

[0051] 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 transferred thin film 10, and that it is therefore necessary to define criteria for classifying cor These primary defects are classified as critical or non-critical. In addition to the similarity level applied to each primary defect associated with a labeled defect type, the contrast level of the photoluminescent image is an important criterion, particularly for judging the criticality of point defects, as these defects can exhibit a wide range of contrasts, from low to high. This is why a predetermined contrast threshold is used to separate critical from non-critical defects. In the example in [Fig. 2b], the predetermined contrast threshold is set at 600 ua. This threshold obviously depends on the recipe parameters, the characteristics of the substrate (roughness, doping, residual stress, manufacturing method, etc.), and a universal value cannot be proposed under any circumstances.

[0052] For microhole defects or star stacking fault complexes, the level of similarity is often sufficient because these defects have a very specific signature and high associated contrast. The photoluminescence image can also be combined with the optical microscopy image of these defects to further improve their classification.

[0053] Recall that the contrast in a photoluminescent image can be defined as the normalized difference between the intensity of the photoluminescence signal at the primary defect and the surrounding intensity in a field less than or equal to the field size of the microscope. Other calculations of the contrast value would give different numerical results and a predetermined contrast threshold, but with a similar meaning.

[0054] An example of a first mapping is given in [Fig.4]: it lists the primary defects detected on a donor substrate 1 and classified as critical defects (in particular complex types of star stacking faults, micro-holes or point defects of species inclusion type); another defect, classified as non-critical, is illustrated in the top left of [Fig.4].

[0055] According to an advantageous embodiment, in step 2) of the process according to the invention, a donor substrate 1 having a density of primary defects, classified as critical defects of any type, greater than 1 defect / cm2, or even greater than 0.25 defect / cm2, is downgraded at the end of step 2) and is not used for the subsequent step 3).

[0056] The manufacturing process then includes a third step 3) of transferring a thin layer 10, 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'.

[0057] Advantageously, the surface layer 10' is transferred by a technique of Thin-film transfer such as Smart Cut. A first substep 3a) involves implanting light species into a donor substrate 1 to form a buried fragile plane 11. This plane, along with a front face of the donor substrate 1, delimits the surface layer to be transferred 10' ([Fig. 3b]). The light species are preferentially hydrogen and / or helium and are implanted into the donor substrate 1 at a depth consistent with the thickness of the target thin film 10. These light species will form microcavities around the determined depth, distributed within a thin layer parallel to the free surface of the donor substrate 1, i.e., parallel to the (x,y) plane in the figures. For simplicity, this thin layer is called the buried fragile plane 11. The implantation energy of the light species is chosen 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 5E16 / cm² and 1E17 / cm², to delimit a thin layer 10 with a thickness on the order of 100 nm at 1500 nm. Note that a protective layer may be deposited on the front face 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 substep 3b). This substep corresponds to the assembly of a support substrate 20 made of polycrystalline silicon carbide, on its front face 20a, with the implanted donor substrate 1, also on its front face ([Fig. 3c]). The support substrate 20 corresponds to the mechanical support of the future composite structure 100.The lateral dimensions in the principal 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 about 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.

[0058] The assembly is performed by direct bonding, by molecular adhesion, along a bonding interface 40. Optionally, an intermediate layer may be formed on the front face 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 may 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 may also be deposited on the face 20a to be assembled of the support substrate 20, prior to assembly; it may be of the same or different nature as the intermediate layer mentioned for the donor substrate 1. An intermediate layer may 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. (particularly 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 bonded; another advantage of this / these intermediate layer(s) may be to improve the vertical electrical conductivity of the bonding interface 40. The intermediate layer(s) is / are intended to be embedded in the bonded assembly 50 after assembly, and ultimately, in the composite structure 100.

[0059] Direct bonding by molecular adhesion does not require an adhesive material, as bonds are established at the atomic scale between the bonded surfaces. Several types of molecular adhesion bonding exist, which differ in particular in their temperature, pressure, atmospheric conditions, or pretreatments prior to contacting the surfaces.Examples include room temperature bonding with or without prior plasma activation of the surfaces to be joined, atomic diffusion bonding ("Atomic diffusion bonding" or ADB according to Anglo-Saxon terminology), surface-activated bonding ("SAB"), etc.

[0060] It should be noted that the assembly step 3b) may include, prior to bringing the faces la,20a to be assembled into contact, conventional sequences of chemical cleaning (for example, RCA cleaning), surface activation (for example, by oxygen or nitrogen plasma) or other surface preparations (such as scrubbing), which may promote the quality of the bonding interface 40 (low defect, high adhesion energy).

[0061] The following substep 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 supporting substrate 20, on the one hand, and the remainder of the donor substrate 1', on the other hand ([Fig. 3d]). The separation along the buried fragile plane 11 is usually achieved 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 a fracture propagates along said fragile plane 11. Alternatively or concurrently, mechanical stress can be applied to the bonded assembly 50 and in particular to the buried fragile plane 11, so as to propagate or help to propagate mechanically the fracture leading to the separation.Following this separation, we obtain on the one hand the intermediate composite structure 100' and on the other hand 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 between 5nm and 100nm RMS.

[0062] The next sub-step 3d) includes the application of thermal, mechanical and / or chemical treatment(s) to a free surface 10'a of the surface layer 10', to form the composite structure 100 equipped with the single-crystal silicon carbide thin layer 10 having a surface roughness less than or equal to 0.5 nm RMS, or even less than or equal to 0.1 nm RMS (AFM scan 10 x 100 pm² or 20 x 20 pm²) ([Fig. 3e]). In particular, this substep 3d) may include a mechano-chemical smoothing treatment of the free surface 10'a of the surface layer 10'. A removal of between 50 nm and 300 nm effectively restores the surface condition of said layer. It may also include at least one heat treatment at a temperature between 1200°C and 1800°C. Such a heat treatment is applied to remove 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 strengthens the bonding interface 40.

[0063] At this stage of the process, the thin film 10 of the composite structure 100 typically has a thickness between a few tens of nm and a few hundred nm, for example, between 50 nm and 800 nm. The types and levels of doping of the thin film 10 and the support substrate 20 are defined according to the intended applications and devices.

[0064] Returning to the general description of the manufacturing process according to the invention, step 3) of transferring the thin film 10 onto the support substrate 20 is followed by a fourth step 4) of inspecting the free surface 10a of said thin film 10 by a defect inspection technique based on dark-field and bright-field imaging combined with scanning the surface 10a with a DUV laser beam (for "deep UV", typically with a wavelength of approximately 200-280 nm). 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 dimensions. By way of example, equipment such as the KLA SPA2 can be used. The DUV laser beam penetrates very weakly into the thickness of the thin film 10, and is therefore not contaminated 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 film 10 and to associate them with known defects in the equipment database such as holes, bubbles, scratches, particles, other point or agglomerated defects, etc.

[0065] This inspection step is widely used in the semiconductor industry (especially silicon), making it a step compatible with industrial rate standards and requirements.

[0066] Secondary defects can be induced by primary defects (crystalline defects present on the donor substrate 1 that gave rise to the thin film 10), by particles or other surface contaminants that were not completely removed before the assembly of the donor substrate 1 and the support substrate 20, or by a one-off problem at a stage of the manufacturing process (scratches, deposited particles, flaking...). It can therefore be defects of crystalline origin, or defects related to transfer such as holes (local absence of thin layer 10), bubbles (defect at the bonding interface 40, at the level of which the thin layer 10 is not bonded and forms a blister), particles, scratches, etc.

[0067] Following 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 is derived from the donor substrate 1 (first map) is given in [Fig. 5a]. Note that to compare the first and second maps, it is necessary to apply a "mirror" flip of the first map, since the front face of the donor substrate 1 is assembled onto the support substrate 20, and 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 in [Fig.5b]: Here, several primary defects classified as critical and visible on the first map do not appear on the second map.

[0068] The advantage of this inspection step 4) is that it is quick and therefore allows for 100% inspection of composite structures in a production line. However, it may not detect crystalline defects present in the thin film, because some of these defects do not cause scattering of the UV laser beam and remain invisible to this inspection.

[0069] Conversely, a control technique combining optical microscopy in visible light and photoluminescence imaging can detect these defects of crystalline origin, but the 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.

[0070] Thus, the applicant has defined a fifth step 5) for grading the composite structure 100 reliably and efficiently, 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 originates, and the second mapping of said structure 100 (step 4). This grading aims to separate the structures that meet the specified requirements from the downgraded structures.

[0071] The second map is compared to the first map, having of course taken the precaution of applying a mirror image to the first map since the donor substrate 1, bonded to the support substrate 20, has its front face (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.

[0072] Advantageously, step 5) includes a reconciliation between the primary defects Defects classified as critical in the first mapping and secondary in the second mapping are identified. When a primary defect classified as critical cannot be associated with a secondary defect because no defect is detected in the second mapping at the location of the primary defect, an additional defect is added to the defects in the second mapping for consideration in the grading decision. Primary defects identified as critical (i.e., those that generate defects in the transferred thin layer) are thus considered for the quality assessment of composite structures, without requiring a complex and time-consuming final inspection of the composite structures.

[0073] According to one variant, step 5) includes a reconciliation between primary defects classified as critical defects and secondary defects, and if a large number of primary defects classified as critical defects are not, each, associated with a secondary defect, step 5) includes a further check of the composite structure 100 in question, by a technique combining confocal microscopy in visible light and photoluminescence imaging, making it possible to verify whether defects are indeed present in the thin layer 10 at the locations of said critical primary defects.

[0074] The grading step 5) of the manufacturing process according to the invention makes it possible to reliably and efficiently decide the grade to be assigned to the composite structures 100, by benefiting from a 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.

[0075] In particular, a composite structure 100 may be downgraded if it exhibits: • 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 / cm2, and / or • a density of secondary defects detected only on the second mapping (i.e. not associated with critical primary defects) greater than 0.2 defects / cm2.

[0076] In a subsequent step, SiC epitaxial growth can be performed on the thin layer 10 of the composite structures 100 that have successfully passed step 5) of grading, in order to increase its thickness and form the epitaxial layer 150 on and in which the devices will be fabricated ([Fig. 3f]). The epitaxial growth step can be carried out according to known prior art techniques.

[0077] The manufacturing process may also include a sixth step 6) of recycling the residual donor substrate 1' to make a recycled donor substrate. Step 6) may include mechanical and / or chemical treatments, similar to those applied to the composite structure 100, implemented at the front face l'a of the residual donor substrate 1'.

[0078] The recycled donor substrate is then directly introduced in step 3) as the 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 is derived.

[0079] The initial mapping of the quality of the original donor substrate 1 is therefore used to grade each composite structure 100 produced from this substrate 1, both in its initial state and after multiple recycling cycles. Although this control is time-consuming, it is industrially viable because the cost is amortized over a very large number of composite structures 100 manufactured from the donor substrate 1.

[0080] Of course, the invention is not limited to the embodiments and examples described, and alternative embodiments may be made without departing from the scope of the invention as defined by the claims

Claims

Demands

1. A method for manufacturing a composite structure (100) comprising a thin layer (10) of monocrystalline silicon carbide deposited on a support substrate (20) of polycrystalline silicon carbide, the method comprising the following steps: 1) the supply of at least one donor substrate (1) in single-crystal silicon carbide, having a front face (la) and a back face (1b), the front face (la) potentially having defects, called primary defects; 2) quality control of - at least one - donor substrate (1) by a photoluminescence imaging technique in order to extract a map of the front face (the), called the first map, listing the primary defects identified as being of microhole type, of complex star stacking fault type or of point defect type; 3) the transfer of a thin layer (10), from a surface layer (10') of - at least one - donor substrate (1), onto a support substrate (20) 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 defect inspection technique by diffusion of an ultraviolet laser beam, so as to extract a map of the free surface (10a), called 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.

2. A method for manufacturing a composite structure (100) according to claim 1, wherein step 2) comprises the following substeps: i) inspecting a front face (la) 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 (la) following excitation by an incident beam, and to form, for each primary defect, a photoluminescence image, (ii) the assignment to each detected primary defect of 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 (la) of a donor substrate (1) in single-crystal silicon carbide, such as microholes, star stacking fault complexes, and point defects, particularly those related to inclusions, with a contrast level 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 microhole and the similarity level is greater than a first level, the primary defect is classified as a critical microhole defect; - if the labeled defect type is a star stacking fault complex and the similarity level is greater than a second level, the primary defect is classified as a critical star stacking fault complex; - if the labeled defect type is a point defect, the similarity level is greater than a third level, and the contrast level is greater than a predetermined threshold,The primary fault is classified as a critical fault of the point fault type; in other cases, the primary fault is classified as a non-critical fault.

3. 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.

4. Method of manufacturing a composite structure (100) according to any one of claims 2 and 3, wherein the predetermined threshold is established empirically for a type of donor substrate (1), based on a correlation study between the contrast value of primary defects detected on a front face (la) of a test donor substrate (1) and the presence of secondary defects on a thin layer (10), from the test donor substrate, transferred onto a support substrate (20).

5. A method for manufacturing a composite structure (100) according to any one of claims 2 to 4, wherein a donor substrate (1) having a density of primary defects, classified as critical defects of any type, greater than 1 defect / cm2, or preferably greater than 0.25 defects / cm2, is downgraded at the end of step 2) and is not used for step 3).

6. Method of manufacturing a composite structure (100) according to one of the claims 2 to 5, wherein step 5) includes a reconciliation between primary defects classified as critical defects and secondary defects, each primary defect classified as critical and not associated with a secondary defect being added to the defects in the second mapping to be taken into account in a grading decision.

7. A method for manufacturing a composite structure (100) according to claim 6, wherein the grading decision corresponds to a downgrading of the composite structure if the latter has: - 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 / cm2, and / or - a density of secondary defects only detected on the second mapping, not associated with critical primary defects, greater than 0.2 defects / cm2.

8. A method for manufacturing a composite structure (100) according to any one of claims 2 to 5, wherein step 5) includes a reconciliation between the primary defects classified as critical defects and the secondary defects, and step 5) includes a supplementary check by a technique coupling optical microscopy in visible light and photoluminescence imaging, allowing verification of whether defects are present in the thin film (10), at the locations of said primary defects classified as critical defects.

9. A method for manufacturing a composite structure (100) according to any one of the preceding claims, wherein step 3) comprises the following substeps: 3a) the implantation of light species in a donor substrate (1), to form a buried brittle plane (11) delimiting, with a front face (la) of the donor substrate (1), the surface layer (10') to be transferred; 3b) the assembly of a support substrate (20) of polycrystalline silicon carbide with the donor substrate (1) implanted in step 3a); 3c) the separation along the buried brittle 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, referred to as the residual donor substrate (1'), on the other hand;3d) the application of thermal, mechanical and / or chemical treatment(s) to a free surface (10'a) of the surface layer (10'), to form the composite structure (100) equipped with the thin layer (10); made of single-crystal silicon carbide.

10. A method for manufacturing a composite structure according to any one of the preceding claims, comprising a step 6) of recycling the residual donor substrate (1') to make a recycled donor substrate (1”), and wherein: - the recycled donor substrate (1”) is introduced directly in step 3) as donor substrate (1), and - in the next 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”) is derived.

Images