Silicon carbide-based composite structure exhibiting good vertical electrical conductivity

Abstract

The invention relates to a composite structure having a front face and a rear face extending parallel to a principal plane, and comprising: - a useful layer of monocrystalline silicon carbide, of which a free face constitutes the front face, and having a first concentration of type N dopants, - a support substrate of polycrystalline silicon carbide, of which a free face constitutes the rear face, and having a second concentration of type N dopants, the second concentration being greater than the first concentration, - an intermediate region extending along the principal plane and including an interface zone between an assembled face of the useful layer and an assembled face of the support substrate;The composite structure is remarkable in that the intermediate region comprises single-crystal silicon carbide inclusions in direct contact with the useful layer and extending, in a direction normal to the principal plane, between grains of the supporting substrate, said inclusions having a third concentration of N-type dopants situated between the first and second concentrations. The invention also relates to a method for manufacturing such a composite structure. Figure N / A;

Description

Title of the invention: Silicon carbide-based composite structure exhibiting good vertical electrical conductivity. Scope of the invention

[0001] The present invention relates to the field of microelectronics and semiconductors. In particular, the invention relates to a composite structure comprising a useful layer of monocrystalline silicon carbide, a support substrate of polycrystalline silicon carbide, and an intermediate region, between the useful layer and the support substrate, promoting vertical electrical conduction. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] Silicon carbide (SiC) is increasingly used in the manufacture of high-performance power devices. However, high-quality single-crystal SiC (c-SiC) substrates for the microelectronics industry remain expensive and difficult to source in large quantities. Therefore, it is advantageous to use layer transfer solutions to create composite structures typically comprising a thin c-SiC layer (derived from a high-quality c-SiC substrate and intended to host the sensitive functional parts of the devices) on a lower-cost support substrate, for example, polycrystalline SiC (p-SiC). It should also be noted that a composite structure can provide additional functionality and performance, particularly by providing a support substrate with advantageous mechanical, electrical, and / or thermal properties.

[0003] A well-known thin film transfer solution is the Smart Cut® process, based on the implantation of light ions in a donor substrate (c-SiC) and on assembly, by direct bonding, at the level of a bonding interface between the donor substrate and a support substrate (for example in p-SiC).

[0004] In the field of power electronics, vertical components require good electrical conductivity between the thin film (also called the useful layer) and the substrate supporting the composite structure. In particular, the bonding interface between a useful layer and a substrate supporting the material must have a resistivity that is as low as possible, preferably less than 1 / 1000 ohms.cm², or even less than 0.1 / 1000 ohms.cm².

[0005] Document WO2022 / 008809 proposes forming an interface zone from a very thin layer of a metallic material, for example tungsten, in a SiC / SiC semiconductor structure. Considering a useful layer and a support substrate of single-crystal SiC with a resistivity of 20 mΩ·cm, and a 1 nm to 3 nm layer of tungsten

[0006] (W) sandwiched between the useful layer and the supporting substrate, it is possible to obtain an interface zone resistivity less than or equal to 0.1 mohm.cm2, after application of an appropriate heat treatment and formation of very fine nodules largely made of metal (W), responsible for the good vertical conduction of the

[0007] interface zone.

[0008] Document WO2022 / 129726 proposes forming an interface zone in a SiC / SiC semiconductor structure, comprising regions of direct contact between the active layer and the substrate, and agglomerates of a semiconductor material different from that of the active layer and the substrate, and having a thickness of 250 nm or less. The semiconductor material is chosen for its particular affinity for oxygen. Thus, the agglomerates are able to efficiently trap any oxygen that may be present at the bonding interface; the regions of direct contact between the active layer and the substrate, free of native oxide residues in particular, allow for efficient and high-quality electrical conduction and / or vertical semiconductor-to-semiconductor contact. Considering a active layer and a substrate made of c-SiC with a resistivity of 20 mΩ.cm, and a silicon semiconductor material, it is possible to obtain an interface zone resistivity less than or equal to 0.1 mohm.cm2, for example 0.032 mohm.cm2 (after annealing the structure at 1370°C) or 0.0076 mohm.cm2 (after annealing at a very high temperature, namely 1900°C). . SUBJECT OF THE INVENTION

[0009] The present invention proposes an alternative solution particularly favorable for obtaining very low interface resistivities in composite structures whose useful layer is made of c-SiC and whose support substrate is made of p-SiC, and whose doping levels are not necessarily extremely high. The invention relates to a composite structure comprising an intermediate region including an interface zone between an assembled face of the useful layer and an assembled face of the support substrate, and comprising single-crystal silicon carbide inclusions in direct contact with the useful layer and extending between grains of the support substrate. These inclusions are particularly favorable for vertical electrical conduction. The invention also relates to a method for manufacturing said composite structure. BRIEF DESCRIPTION OF THE INVENTION

[0010] The present invention relates to a composite structure having a front face and a rear face extending parallel to a principal plane, and comprising:

[0011] - a useful layer of single-crystal silicon carbide, one free face of which constitutes the front face, and having a first concentration of N-type dopants,

[0012] - a polycrystalline silicon carbide support substrate, of which a free face constitutes the rear face, and presents a second concentration of dopants of the type N, the second concentration being greater than the first concentration,

[0013] - an intermediate region extending along the principal plane and including an area interface between an assembled face of the useful layer and an assembled face of the supporting substrate.

[0014] The composite structure is remarkable in that the intermediate region comprises single-crystal silicon carbide inclusions in direct contact with the useful layer and extending, in a direction normal to the principal plane, between grains of the supporting substrate, said inclusions having a third concentration of N-type dopants between the first and second concentrations.

[0015] According to advantageous features of the invention, taken alone or in any feasible combination: • the substrate support, at least on the side of its assembled face, comprises silicon carbide grains having a size less than or equal to 500 nm, said grains covering in the principal plane, a surface greater than or equal to 20% of the assembled face; • silicon carbide grains with a size less than 500 nm cover, in the principal plane (x,y), a surface area greater than or equal to 50%, or even 80%, of the assembled face of the support substrate; • at least 20% of the silicon carbide grains, on the side of the assembled face of the support substrate, have a misorientation, between themselves or with the

[111] orientation, greater than or equal to 30°; • the intermediate region comprises silicon nodules, arranged in the interface zone or under the inclusions between grains of the supporting substrate; • the first concentration is between 1.1018 / cm3 and 5.1018 / cm3; • the second concentration is between 1.1020 / cm3 and 5.1020 / cm3, preferably between 1.1020 / cm3 and 2.1020 / cm3; • the inclusions have a thickness, along an axis normal to the principal plane, of between 20 nm and 70 nm, and a width, in the principal plane, of between 100 nm and 700 nm; • the intermediate region has an inclusion density, in the principal plane, of between 0.1 / pm2 and 1 / pm2.

[0016] The invention also relates to a method for manufacturing a composite structure such as the above, comprising the following steps:

[0017] a) the supply of a single-crystal silicon carbide donor substrate having a first concentration of N-type dopants, and comprising a buried brittle plane induced by an ionic implantation of light species;

[0018] b) the supply of the polycrystalline support substrate;

[0019] c) the deposition of a silicon film on a face to be assembled of the donor substrate and / or on the side of the substrate to be assembled;

[0020] d) the assembly of the donor substrate and the support substrate at the level of their faces to be assembled, so as to obtain a bonded assembly comprising an intercalated silicon layer, from the film(s) deposited in step c), between the donor substrate and the support substrate, said intercalated layer having a thickness between 8 nm and 30 nm;

[0021] e) the separation along the buried fragile plane to form a transferred structure comprising the useful layer, transferred from the donor substrate, disposed on the intercalated layer, itself disposed on the supporting substrate;

[0022] f) applying a heat treatment to the transferred structure, at a holding temperature between 1500°C and 2000°C, to form the composite structure.

[0023] According to advantageous features of the invention, taken alone or in any feasible combination: • the support substrate comprises, at least on the side of one face to be assembled, silicon carbide grains having a size less than or equal to 500 nm, said grains covering, in the principal plane, a surface greater than or equal to 20% of the face to be assembled; • at least 20% of the silicon carbide grains, on the side of the assembled face of the support substrate, have a misorientation, between themselves or with the

[111] orientation, greater than or equal to 30°; • the holding temperature is maintained for a period of between 30 minutes and 4 hours; • the treatment of step f) has a temperature ramp of less than or equal to 20°C / min, or even less than or equal to 10°C / min; • the second concentration is on the order of 1.1020 / cm3, the first concentration is on the order of 3.1018 / cm3, and the silicon carbide grains having a size less than or equal to 500 nm cover, in the principal plane, a surface greater than or equal to 50%, or even 60%, of the face to be assembled of the support substrate; • the second concentration is on the order of 1.1020 / cm3, the first concentration is on the order of 1.1018 / cm3, and the silicon carbide grains having a size less than or equal to 500 nm cover, in the principal plane, a surface greater than or equal to 80% of the face to be assembled of the support substrate. Brief description of the drawings

[0024] Other features and advantages of the invention will become apparent from the detailed description that follows, with reference to the accompanying figures in which:

[0025] [Fig.1] The [Fig.1] has a composite structure according to the invention;

[0026] [Fig.2] The [Fig.2] illustrates the intermediate region of a composite structure according to the invention, on the left of the figure schematically, on the right with a transmission electron microscopy image;

[0027] [Fig.3a]

[0028] [Fig.3b]

[0029] [Fig.3c]

[0030] [Fig.3d]

[0031] [Fig.3e]

[0032] [Fig.3f] Figures 3a to 3f show steps of a manufacturing process for a composite structure according to the present invention;

[0033] [Fig.4] Fig.4 shows the intermediate region of a composite structure according to the invention, on the left of the figure schematically, on the right with a transmission electron microscopy image;

[0034] [Fig.5] Fig.5 presents the intermediate region of a composite structure according to the invention, with a transmission electron microscopy image;

[0035] [Fig.6] Fig.6 presents theoretical curves (numerical simulations) relating the resistivity of the intermediate region of a composite structure as a function of the percentage of surface area of ​​the support substrate in which the p-SiC grains have a size less than or equal to 500 nm, for different doping concentrations of the useful layer (NmonoSic) and the support substrate (NpoiySic)-

[0036] Some figures are schematic representations which, for the sake of readability, are not to scale. In particular, the layer thicknesses along the z-axis are not to scale with respect to the lateral dimensions along the x and y axes.

[0037] The same references in the figures or in the description may be used for elements of the same nature. DETAILED DESCRIPTION OF THE INVENTION

[0038] The invention relates to a composite structure 100 and an associated manufacturing process. The composite structure 100 has a front face 100a and a back face 100b extending parallel to a principal plane (x,y). It comprises a useful layer 10 of monocrystalline silicon carbide, preferably of polytype 4H, a free face of which constitutes the front face 100a, and a support substrate 20 of polycrystalline silicon carbide 3C, a free face of which constitutes the back face 100b ([Fig. 1]). The composite structure 100 also comprises an intermediate region 30 extending along the principal plane (x,y) and including an interface zone between an assembled face of the useful layer 10 and an assembled face of the support substrate 20. The intermediate region 30 comprises inclusions 31 of monocrystalline silicon carbide in direct contact with the useful layer 10 and extending, in a direction (z) normal to the main plane (x,y), between grains 21i,21j of the support substrate 20 ([Fig.2]).

[0039] To facilitate understanding of the particular properties of the composite structure 100, related in particular to this intermediate region 30, we will first describe the manufacturing process of said structure 100.

[0040] The process according to the invention includes a step a) of supplying a donor substrate 1 of monocrystalline silicon carbide (for example, of polytype 4H) having a given concentration of type N dopants (called the first concentration). Typically, the first concentration is between 1 x 10¹⁸ / cm³ and 5 x 10¹⁸ / cm³, corresponding to a resistivity range between 20 and 30 mΩ·cm.

[0041] The donor substrate 1 has a front face 1a intended for assembly, and a rear face 1b ([Fig. 3a]). It includes a buried fragile plane 11 induced by an ionic implantation of light species (referring to the Smart Cut process). These species are, for example, hydrogen, helium, or a combination of these two species. The buried fragile plane, together with the front face 1a of the donor substrate 1, delimits the useful layer 10 to be transferred.

[0042] The process includes a step b) of supplying a support substrate 20 made of polycrystalline silicon carbide (of polytype 3C). The support substrate 20 has a front face 20a for assembly and a rear face 20b ([Fig. 3b]). It has a higher concentration of type N dopants (referred to as the second concentration) than the first concentration. Typically, the second concentration is between 1 x 10²⁰ / cm³ and 5 x 10²⁰ / cm³. To simplify the supply of the support substrate 20, the second dopant concentration can preferably be limited to the range of 1 x 10²⁰ / cm³ and 2 x 10²⁰ / cm³.

[0043] The N-type dopant, in the donor substrate 1 or in the support substrate 20, can in particular be nitrogen.

[0044] Typically, in the field of microelectronics, the support substrate 20 and the donor substrate 1 are in the form of circular wafers with diameters typically ranging from 100 mm to 200 mm (or even more). The front (circular) faces 1a,20a and rear faces 1b,20b of the substrates 1,20 extend parallel to the principal plane (x,y). The thicknesses of the donor substrate 1 and support substrate 20 are typically between 200 µm and 800 µm.

[0045] Advantageously, the support substrate 20 has the particularity of comprising, at least on the side of its face to be assembled 20a, silicon carbide grains 21i, 21j having a size less than or equal to 2 µm, less than or equal to 1 µm, or even less than or equal to 500 nm, said grains covering, in the principal plane (x,y), a surface area greater than or equal to 20% of the face to be assembled 20a. The size of a grain 21i,21j, delimited by the grain boundaries 22, corresponds to the largest dimension of said grain 21i,21j, in the principal (x,y) plane. To measure the dimensions of grains 21i,21j or the distances between grain boundaries 22, it is possible to rely on images obtained by conventional scanning electron microscopy (SEM) or by electron back-scattered diffraction (EBSD). X-ray crystallography can also be used.

[0046] Advantageously, the p-SiC grains 21i,21j having a size less than or equal to 2 pm, less than or equal to 1 pm, or even less than or equal to 500 nm, cover, in the principal plane (x,y), a surface greater than or equal to 30%, 40%, 50%, 60%, 70%, or even greater than or equal to 80% of the front face to be assembled 20a of the support substrate 20. It is understood that the remainder of the surface is occupied by larger grains.

[0047] The manufacturing process then comprises a step c) of deposition of a silicon film 3' (amorphous or polycrystalline) onto the face to be assembled la of the donor substrate 1, onto the face to be assembled 20a of the support substrate 20, or onto both faces la,20a ([Fig. 3c]). Advantageously, this deposition is carried out by low-temperature sputtering, and involves a high vacuum, typically between 3 x 10⁶ Pa and 8 x 10⁶ Pa, before the injection of the sputtering gas (Ar), then an atmosphere of around 10¹ Pa and 10² Pa during the deposition. The thickness of the deposited film 3' can vary from a few nm to a few tens of nm.

[0048] The next step d) corresponds to the assembly, by direct bonding, of the donor substrate 1 and the support substrate 20 at their faces to be joined, 1a, 20a, so as to obtain a bonded assembly 50 comprising an interlayer 3 of silicon, derived from the film(s) 3' deposited in step c), between the donor substrate 1 and the support substrate 20 ([Fig. 3d]). This interlayer 3 advantageously has a thickness of between 8 nm and 30 nm.

[0049] The assembly is carried out by molecular adhesion bonding, consisting of bringing the faces to be joined into contact, 20a, without the addition of any adhesive material. This may involve direct bonding between the useful layer 10 and the silicon film 3', when the latter has been deposited only on the support substrate 20, or direct bonding between the support substrate 20 and the film 3', when the latter has been deposited only on the donor substrate 1, or even direct bonding between two silicon films 3', when they have been deposited on the donor substrate 1 and on the support substrate 20. Direct assembly is preferably carried out under a high vacuum, on the order of 10⁶ Pa or less.

[0050] Advantageously, the deposition of step c) and the direct assembly of step d) are linked without breaking the vacuum, in-situ or in multi-chamber equipment. For example, atomic diffusion bonding equipment (“Atomic Diffusion Bonding”) BV7000 from Canon, in which it is possible to successively deposit the 3' film and perform direct bonding, while maintaining a controlled atmosphere.

[0051] The process then includes a step e) involving a separation along the buried fragile plane 11 to form a transferred structure 100' comprising the useful layer 10, transferred from the donor substrate 1, disposed on the intercalated layer 3, itself disposed on the support substrate 20. Such a separation can be carried out during a heat treatment, typically around 900°C-1000°C, capable of growing cavities and microcracks, induced by the implanted species, in the buried fragile plane 11. The separation can also be carried out by the application of a mechanical stress, or by the combination of thermal and mechanical stresses, as is well known with reference to the Smart Cut process.

[0052] Cleaning, smoothing, polishing or engraving sequences of the separated face 10a of the useful layer 10 and / or of the separated face l'a from the remainder 1' of the donor substrate may be carried out in such a way as to restore a good surface quality, in particular in terms of roughness, defects and other contaminations.

[0053] Finally, the process includes a step f) corresponding to the application of a heat treatment to the transferred structure 100', at a holding temperature between 1500°C and 2000°C, advantageously between 1750°C and 2000°C, to form the composite structure 100 ([Fig.3f], left). This temperature range induces the melting of the silicon in the interlayer 3. The interlayer 3 will segment into droplets to optimize its surface energy, and the liquid silicon 3” from at least some of these droplets will diffuse into the grain boundaries 22. It will then be able to interact with the SiC grains 21i, 21j of the substrate and dissolve a certain amount of them. The dissolved carbon, whose diffusion coefficient in liquid silicon is 1.7 x 10⁸ m² / s, is then transported from the dissolution zone to the interface between the liquid silicon and the single-crystal useful layer 10.By recombining with silicon, the carbon enables the growth of single-crystal SiC from the useful layer 10, the latter acting as a seed ([Fig. 3f], right). This is similar to growth in a TSSG (top-seeded solution growth) process, which consists of placing liquid silicon in a graphite crucible and then positioning a high-quality single-crystal SiC seed on the surface of the liquid silicon; the carbon present on the crucible walls dissolves upon contact with the liquid silicon and diffuses into it via a temperature gradient to the SiC seed. Single-crystal SiC then grows from this seed. In the present invention, this growth process occurs on a very local scale and at the interface zone between the useful layer 10 and the support substrate 20: the graphite crucible is replaced by the 2l1,2l1j grains of the [material]. substrate support in p-SiC, the seed by the useful layer 10 in c-SiC and the silicon source comes from the intercalated layer 3.

[0054] The composite structure 100 resulting from this step f) therefore has an intermediate region 30, which extends along the principal plane (x,y) and comprises inclusions 31 of single-crystal silicon carbide in direct contact with the useful layer 10 and extending, in a direction (z) normal to the principal plane (x,y), between grains 21i, 21j of the support substrate 20. The growth mode of these inclusions 31 promotes the diffusion of dopants, present in the grains 21i, 21j, into said inclusions 31. Thus, the latter exhibit a third concentration of N-type dopants between the first and second concentrations, which is very favorable to establishing good electrical conduction between the useful layer 10 and the support substrate 20. For example, for a first dopant concentration of 5 x 10¹⁸ / cm³ and a second dopant concentration of 5.1020 / cm3, the third concentration of dopants is expected to be between 1.1020 and 1.5 x 1020 / cm3. .

[0055] The inclusions 31 typically have a thickness, along an axis (z) normal to the principal plane (x,y), of between 20 nm and 70 nm, and a width, in the principal plane (x,y), of between 100 nm and 700 nm. The size of the inclusions 31 is notably a function of the thickness of the intercalated silicon layer 3.

[0056] The intermediate region 30 has an inclusion density 31, in the principal plane (x,y), typically between 0.1 / pm2 and 1 / pm2. This density is all the greater as small grains 21i,21j (less than or equal to 2 pm, less than or equal to 1 pm, or even less than or equal to 500 nm) cover a significant surface of the assembled face 20a of the support substrate 20. According to the invention, these small grains cover more than 20% of the surface of said assembled face 20a, more than 30%, more than 40%, more than 50%, more than 60%, more than 70%, or even 80% and more.

[0057] The crystallographic orientations of the grains 21i,21j of the assembled face 20a of the support substrate 20 can also influence the formation, size and density of the inclusions 31. The density of inclusions 31 is also related to the thickness of the intercalated silicon layer 3, a greater quantity of silicon being able to increase said density in the intermediate region 30.

[0058] It should be noted that a significant misorientation of the grains relative to each other or to the orientation

[111] is favorable to the interaction between the silicon droplets and the grain boundaries 22, because this misorientation is accompanied by unstable grain boundaries 22, and it is then energetically favorable for silicon to dissolve carbon atoms at said grain boundaries 22. A misorientation greater than 10°, 20°, or even 30° appears particularly advantageous. The grain orientation can be observed by transmission electron microscopy (TEM) or by electron back-scattered diffraction (EBSD).

[0059] Such disorientation is expected between the grains 21i,21j covering at least 20%, 30%, 40% or even more than 50% of the surface of the assembled face 20a of the support substrate 20; this objective can be evaluated by statistical measurements on said face 20a.

[0060] The c-SiC inclusions 31 between the p-SiC (3C) grains have a particular geometry. For example, in the case of a useful c-SiC layer 10 of polytype 4H, it has been observed that the basis of inclusions 31 is perpendicular to the

[0004] direction of the c-SiC of the useful layer 10, parallel to the {0004} planes. Thus, the basis of at least some inclusions 31 is not parallel to the principal (x,y) plane, but is misoriented by 4° with respect to it. This is due to the fact that the {0004} planes of the SiC-4H single crystal are misoriented by 4° with respect to the assembled face la of the useful layer 10 (and therefore with respect to the principal plane) before step e). Since the growth of SiC in the form of inclusions 31 occurs by epitaxy from the useful layer 10, the crystal planes maintain the same orientation. Growth therefore terminates on {0004} planes, at 4° to the principal (x,y) plane.

[0061] The orientation of the flanks of the inclusion 31 c-SiC appears to depend on the stability of the facets between the hexagonal single crystal and the cubic polycrystal. The inclination of the flanks also depends on the respective orientations of the grains 21i,21j in contact with said inclusion 31. In general, the flanks of the inclusions 31 have an inclination between 110° and 135° with respect to the base.

[0062] The heat treatment in step f) plays an important role in the formation of these c-SiC inclusions 31. Thus, beyond a plateau temperature between 1500°C and 2000°C, the duration of this plateau and the temperature ramps can also play a role.

[0063] The holding temperature is preferably 1600°C, 1700°C, 1750°C, 1800°C, 1850°C, 1900°C, or 1950°C. The holding time is defined to allow the diffusion of carbon in the liquid silicon from the droplets at the grain boundaries 22, and the growth of inclusions 31 from the useful layer 10. These inclusions 31 promote vertical electrical conduction; therefore, their size and density should be optimized. Preferably, the holding temperature is maintained for a period of 30 min, 1 h, 2 h, 3 h, 4 h, or even longer.

[0064] The temperature ramp rate (up to the plateau) is advantageously chosen to be less than or equal to 20°C / min, less than or equal to 10°C / min, or even less than or equal to 5°C / min, as this promotes the fining of the silicon in the interlayer layer 3 towards the grain boundaries 22. The same ramp rates can be applied to the temperature ramp rate. Furthermore, these ramp rates reduce the risk of formation of "bubble" type defects at the assembly interface.

[0065] The intermediate region 30 may also include silicon nodules 33, 32, arranged in the interface zone, or under the inclusions 31 between grains 21i,2Ij of the support substrate 20 ([Fig.4]). Indeed, some silicon droplets, resulting from the segmentation of the intercalated layer 3 during the heat treatment of step f), may be located opposite a grain 2li,2Ij and not a grain boundary 22: in this case, following their solidification during the temperature ramp of the heat treatment, they form silicon nodules 33 arranged between the useful layer 10 and the support substrate 20, in the composite structure 100. Such a nodule 33 can have a thickness, along an axis (z) normal to the principal plane (x,y), of between 50 nm and 200 nm, and a width, in the principal plane (x,y), of between 200 nm and 400 nm.

[0066] It is also observed that liquid silicon 3” having diffused between grains 21i,21j, along grain boundaries 22, can form a nodule 32 if it is not completely consumed in the growth of an inclusion 31: in this case, a silicon nodule 32 is found disposed under an inclusion 31 and between grains 21i,21j of the support substrate 20, in the composite structure 100 at the end of step e). This type of nodule 32 can have dimensions, in the principal plane (x,y), on the order of 200 nm to 400 nm, and a typical thickness, along the (z) axis, of a few nm to a few pm. The interface between the c-SiC inclusion 31 and the underlying silicon nodule 32 is misoriented by 4° with respect to the principal plane (x,y).

[0067] The intermediate region 30 further includes an interface zone between an assembled face 1a of the useful layer 10 and an assembled face 20a of the support substrate 20, said interface zone being due to the segmentation of the interlayer 3 which causes the appearance of direct contact zones between the c-SiC useful layer 10 and the p-SiC support substrate 20. These direct contact zones can form steps 34 as can be seen in [Fig. 5]. It is assumed that the solubilization of the p-SiC (3C) in the liquid silicon 3” during step f) allows the c-SiC (4H) of the useful layer 10 to be reconstructed in the form of steps 34 (“step bunching”) in at least part of the direct contact zones between the thin layer 10 and the support substrate 20.It appears that the steps 34 are located at the level of direct contact zones where the grains 21i,21j of the supporting substrate 20 exhibit small misorientations with respect to each other, whereas the c-SiC inclusions 31 described previously are located at the level of grains 21i,21j which are very misoriented and / or not very thermodynamically stable.

[0068] The total thickness of the intermediate region 30, along an axis (z) normal to the principal plane (x,y), is greater than or equal to the maximum thickness of the inclusions 31. The presence of silicon nodules 32 under all or part of the inclusions 31 can of course increase this total thickness.

[0069] To achieve good vertical electrical conduction, in other words, a resistivity of the intermediate region 30 as low as possible, preferably less than 1 mohm.cm2, or even less than 0.1 mohm.cm2, certain embodiments Preferential options are proposed, depending on the doping levels of the useful layer 10 and the supporting substrate 20.

[0070] According to a first embodiment, the first concentration (useful layer 10) of N-type dopants (nitrogen) is on the order of 5 x 10¹⁸ / cm³, and the second concentration (support substrate 20) of N-type dopants (nitrogen) is on the order of 1 x 10²⁰ / cm³. To obtain the desired low resistivity, the support substrate 20 is chosen such that the silicon carbide grains 21i, 21j, having a size less than or equal to 500 nm, cover, in the principal plane (x,y), a surface area greater than or equal to 20% of its face to be assembled. This contributes to obtaining a density of c-SiC inclusions 31, favorable to vertical electrical conduction, compatible with the desired objective. The third concentration of dopants (inclusions) is between the first and second concentrations.

[0071] According to a second embodiment, the first concentration of N-type dopants (useful layer) is on the order of 3 x 10¹⁸ / cm³, and the second concentration of N-type dopants (supporting substrate) is on the order of 1 x 10²⁰ / cm³. To obtain the desired low resistivity, the supporting substrate is chosen such that silicon carbide grains with a size less than or equal to 500 nm cover, in the principal plane, a surface area greater than or equal to 50%, or even 60%, of its face to be assembled. This contributes to obtaining a density of c-SiC inclusions with a third dopant concentration between the first and second concentrations, inclusions favorable to vertical electrical conduction, consistent with the desired objective.

[0072] According to a third embodiment, the first concentration of N-type dopants (useful layer) is on the order of 1 x 10¹⁸ / cm³, and the second concentration of N-type dopants (supporting substrate) is on the order of 1 x 10²⁰ / cm³. To obtain the desired low resistivity, the supporting substrate is chosen such that silicon carbide grains with a size less than or equal to 500 nm cover, in the principal plane, a surface area greater than or equal to 80% of its face to be assembled. Again, this contributes to obtaining a density of c-SiC inclusions with a third dopant concentration between the first and second concentrations, inclusions that are favorable to vertical electrical conduction, consistent with the desired objective.

[0073] Figure 6 shows theoretical curves obtained by numerical simulations of vertical electron transport in the composite structure 100. They illustrate the evolution of the resistivity of the intermediate region 30 as a function of the percentage of the surface area of ​​the support substrate 20 (on the side of its assembled face 20a) in which the grains 21i, 21j have a size less than or equal to 500 nm. When the second dopant concentration of the support substrate 20 (denoted NpoiySic in Figure 6) is very high (typically 5.1020 / cm3), it is not critical that the support substrate 20 has a certain percentage of small grains (< 500 nm), a resistivity of less than 105 ohm.cm2 can be achieved regardless of the level of doping of the useful layer 10 (between 1.1018 / cm3 and 5.1018 / cm3). On the other hand, when the second concentration of dopants is on the order of 1.1020 / cm3, obtaining a resistivity of less than 105 ohm.cm2 will be conditioned on the presence of small grains favorable to the formation of c-SiC inclusions 31, which benefit from a third concentration of dopants higher than the first concentration of the useful layer 10. The lower the first concentration, the higher the percentage of surface of the assembled face 20a of the support substrate 20 comprising grains 21i,21j of size less than or equal to 500 nm must be.

[0074] The invention also relates to a composite structure 100 having a front face 100a and a rear face 100b extending parallel to the principal plane (x,y) ([Fig. 1]). It comprises a useful layer 10 having a first concentration of N-type dopants. Typically, the first concentration is between 1 x 10¹⁸ / cm³ and 5 x 10¹⁸ / cm³.

[0075] It also includes a support substrate 20 having a second concentration of N-type dopants, higher than the first concentration. Typically, the second concentration is between 1.1020 / cm3 and 5.1020 / cm3, or even preferably limited to the range of 1.1020 / cm3 and 2.1020 / cm3.

[0076] Advantageously, the support substrate 20, at least on the side of its assembled face 20a, comprises silicon carbide grains 21i, 21j having a size less than or equal to 2 pm, less than or equal to 1 pm, or even less than or equal to 500 nm, said grains covering, in the principal plane (x,y), a surface area greater than or equal to 20% of the assembled face 20a. In particular embodiments, the silicon carbide grains 21i, 21j having a size less than or equal to 2 pm, less than or equal to 1 pm, or even less than 500 nm cover, in the principal plane (x,y), a surface area greater than or equal to 30%, 40%, 50%, 60%, 70%, or even 80% of the assembled face 20a of the support substrate 20.

[0077] Advantageously, the grains of the support substrate 20, on the side of its assembled face 20a, are significantly misoriented with respect to each other (i.e., between two adjacent grains) or with respect to the orientation

[111] : as mentioned previously in the process according to the invention, this is favorable to the interaction between the silicon droplets and the grain boundaries 22 (in step f), because the low stability of the grain boundaries 22, linked to this misorientation, makes the dissolution of carbon atoms by silicon at said grain boundaries 22 energetically favorable. A misorientation greater than 10°, 20°, or even 30° appears particularly favorable. Such a misorientation is expected between the grains 21i, 21j covering at least 20%, 30%, 40% or even more than 50% of the surface of the assembled face 20a of the support substrate 20.

[0078] The composite structure 100 also includes an intermediate region 30 extending along the principal plane (x,y) and including an interface zone between an assembled face la of the useful layer 10 and an assembled face 20a of the support substrate 20.

[0079] The intermediate region 30 comprises single-crystal silicon carbide inclusions 31 in direct contact with the useful layer 10 and extending, in a direction (z) normal to the principal plane (x,y), between grains 21i, 21j of the supporting substrate 20. These inclusions 31 have a third concentration of N-type dopants located between the first and second concentrations. The presence of these inclusions 31 provides a significant advantage in terms of electrical conductivity of the intermediate region 30 of the composite structure 100.

[0080] The intermediate region 30 may have an inclusion density 31, in the principal plane (x,y), between 0.1 / pm2 and 1 / pm2.

[0081] Typically, the inclusions 31 have a thickness, along an axis (z) normal to the principal plane (x,y), of between 20 nm and 70 nm, and a width, in the principal plane (x,y), of between 100 nm and 700 nm.

[0082] As mentioned previously in the description of the manufacturing process of the composite structure 100, the intermediate region 30 can also include silicon nodules 32,33 and steps 34 at the interface zone between the assembled face of the useful layer 10 and that of the support substrate 20.

[0083] Of course, the invention is not limited to the embodiments described and alternative embodiments can be made without departing from the scope of the invention as defined by the claims.

Claims

Demands

1. Composite structure (100) having a front face (100a) and a rear face (100b) extending parallel to a principal plane (x,y), and comprising: - a useful layer (10) of monocrystalline silicon carbide, of which a free face constitutes the front face (100a), and having a first concentration of type N dopants, - a support substrate (20) of polycrystalline silicon carbide, of which a free face constitutes the rear face (100b), and having a second concentration of type N dopants, the second concentration being greater than the first concentration, - an intermediate region (30) extending along the principal plane (x,y) and including an interface zone between an assembled face (la) of the useful layer (10) and an assembled face (20a) of the support substrate (20);the composite structure (100) being characterized in that the intermediate region (30) comprises inclusions (31) of single-crystal silicon carbide in direct contact with the useful layer (10) and extending, in a direction (z) normal to the principal plane (x,y), between grains (21i,21j) of the supporting substrate (20), said inclusions (31) having a third concentration of type N dopants between the first and second concentrations.;

2. Composite structure (100) according to claim 1, wherein the support substrate (20), at least on the side of its assembled face (20a), comprises silicon carbide grains (2li,2Ij) having a size less than or equal to 500 nm, said grains (2li,2Ij) covering in the principal plane (x,y), a surface greater than or equal to 20% of the assembled face (20a).

3. Composite structure (100) according to any one of the preceding claims, wherein the silicon carbide grains (21i,21j) having a size less than 500 nm cover, in the principal plane (x,y), a surface greater than or equal to 50%, or even 80%, of the assembled face (20a) of the support substrate (20).

4. Composite structure (100) according to any one of the preceding claims, wherein at least 20% of the silicon carbide grains (2li,2Ij), on the side of the assembled face (20a) of the support substrate (20), have a misorientation, with each other or with the orientation [111], greater than or equal to 30°.

5. Composite structure (100) according to any one of the preceding claims, wherein the intermediate region (30) comprises silicon nodules (32,33), arranged in the interface zone or under the inclusions (31) between grains (21i,21j) of the support substrate (20).

6. Composite structure (100) according to any one of the preceding claims, wherein the first concentration is between 1.1018 / cm3 and 5.1018 / cm3.

7. Composite structure (100) according to any one of the preceding claims, wherein the second concentration is between 1.1020 / cm3 and 5.1020 / cm3, preferably between 1.1020 / cm3 and 2.1020 / cm3.

8. Composite structure (100) according to any one of the preceding claims, wherein the inclusions (31) have a thickness, along an axis (z) normal to the principal plane (x,y), of between 20 nm and 70 nm, and a width, in the principal plane (x,y), of between 100 nm and 700 nm

9. llili. Composite structure (100) according to any one of the preceding claims, wherein the intermediate region (30) has an inclusion density (31), in the principal plane (x,y), between 0.1 / pm2 and 1 / pm2.

10. A method for manufacturing a composite structure (100) according to any one of claims 1 to 9, comprising the following steps: a) supplying a donor substrate (1) of monocrystalline silicon carbide having a first concentration of N-type dopants, and comprising a buried brittle plane (11) induced by an ionic implantation of light species; b) supplying the polycrystalline support substrate (20); c) deposition of a silicon film (3') on a face to be assembled (la) of the donor substrate (1) and / or on the face to be assembled (20a) of the support substrate (20);d) the assembly of the donor substrate (1) and the support substrate (20) at the level of their faces to be joined (la,20a), so as to obtain a bonded assembly (50) comprising an interlayer layer (3) of silicon, from the film(s) (3') deposited in step c), between the donor substrate (1) and the support substrate (20), said interlayer layer (3) having a thickness between 8 nm and 30 nm; e) the separation along the buried fragile plane (11) to form a transferred structure (100') comprising the useful layer (10), transferred from the donor substrate (1), disposed on the interlayer layer (3), itself; arranged on the support substrate (20); f) the application of a heat treatment to the transferred structure (100'), at a holding temperature between 1500°C and 2000°C, to form the composite structure (100).

11. A method for manufacturing a composite structure (100) according to claim 10, wherein the support substrate (20) comprises, at least on the side of a face to be assembled (20a), silicon carbide grains (21i,21j) having a size less than or equal to 500 nm, said grains (21i,21j) covering, in the principal plane (x,y), a surface greater than or equal to 20% of the face to be assembled (20a).

12. A method for manufacturing a composite structure (100) according to any one of claims 10 and 11, wherein at least 20% of the silicon carbide grains (21i,21j) on the side of the assembled face (20a) of the support substrate (20) have a misorientation, with each other or with the orientation [111], greater than or equal to 30°.

13. Method of manufacturing a composite structure (100) according to any one of claims 10 to 12, wherein the holding temperature is maintained for a period of between 30 min and 4 h.

14. A method for manufacturing a composite structure (100) according to any one of claims 10 and 13, wherein the treatment in step f) has a temperature ramp of less than or equal to 20°C / min, or even less than or equal to 10°C / min.

15. A method for manufacturing a composite structure (100) according to any one of claims 10 to 14, wherein: - the second concentration is on the order of 1.1020 / cm3, - the first concentration is on the order of 3.1018 / cm3, and - the silicon carbide grains (2li,2Ij) having a size less than or equal to 500 nm cover, in the principal plane (x,y), a surface greater than or equal to 50%, or even 60%, of the face to be assembled (20a) of the support substrate (20).

16. A method for manufacturing a composite structure (100) according to any one of claims 10 to 14, wherein: - the second concentration is on the order of 1.1020 / cm3, - the first concentration is on the order of 1.1018 / cm3, and - the silicon carbide grains (2li,2Ij) having a size less than or equal to 500 nm cover, in the principal plane (x,y), a surface greater than or equal to 80% of the face to be assembled (20a) of the support substrate (20).

Images