Process for manufacturing polycrystalline silicon carbide support substrates

Abstract

The present invention relates to a process for producing a polycrystalline silicon carbide support substrate comprising the steps of: a) growing an initial polycrystalline silicon carbide substrate on a graphite or silicon carbide seed; b) forming a stiffening carbon film on a front surface of the initial substrate, the initial substrate having, at the plane of the front surface, a first average silicon carbide grain size immediately prior to the formation of the stiffening film; c) removing the seed to free a rear surface of the initial substrate, the initial substrate having, at the plane of the rear surface, immediately following the removal of the seed, a second average silicon carbide grain size smaller than the first average size; d) thinning the rear surface of the initial substrate to a thickness where the initial substrate has, at the plane of the rear surface, a third average grain size equal to within ±30% of the first average grain size, the thinned initial substrate forming a support substrate.

Description

[Technical field]

[0001] The present invention relates to the field of semiconductor materials for microelectronic components. The present invention is particularly concerned with a process for manufacturing a polycrystalline silicon carbide support substrate, which is particularly suitable for producing a composite structure comprising a thin layer of monocrystalline silicon carbide arranged on said support substrate. [Background technology]

[0002] SiC is being increasingly used to manufacture innovative power devices to meet the needs of emerging areas of electronics, such as electric vehicles, among others. In particular, power devices and integrated power supply systems based on single crystal silicon carbide can handle much higher power densities than their traditional silicon counterparts, and can do so due to the smaller size of the active area.

[0003] Nevertheless, high-quality single-crystal SiC substrates (c-SiC) intended for the microelectronics industry remain expensive and difficult to supply in large sizes. It is thus advantageous to resort to layer transfer solutions to fabricate composite structures that typically include thin layers of single-crystal SiC (obtained from high-quality c-SiC substrates) on lower-cost support substrates, for example made from polycrystalline SiC (p-SiC). One well-known thin-layer transfer solution is the Smart Cut® process, based on implanting light ions and bonding by direct bonding at the bonding interface.

[0004] US Patent Application Publication No. 2019153616 provides a process for producing a p-SiC support substrate onto which a thin c-SiC layer can be transferred, the support substrate comprising grains of an average size of approximately 10 μm and having a degree of variation in grain size between the front and rear surfaces of the support substrate relative to the thickness of the support substrate of 0.43% or less, the latter feature making it possible to limit residual stresses in the support substrate and thus curvature of the support substrate.

[0005] The manufacturing process involves a first carbon-based substrate, on which a thick layer of p-SiC (typically 2 mm) is produced by chemical vapor deposition. A second p-SiC substrate approximately 350 μm thick is extracted from the thick layer of p-SiC by removing the first carbon-based substrate and mechanically thinning both sides of the thick layer. The second base substrate has a degree of variation in grain size between the front and rear sides of the two base substrates relative to the thickness of the second base substrate of 0.43% or less. A new p-SiC layer (typically around 400 μm) is then formed on the second base substrate by chemical vapor deposition, and is separated from the second base substrate, for example by laser irradiation, forming a p-SiC support substrate intended for use in a composite structure. The second base substrate can then be reused.

[0006] In practice, the steps consisting in forming the second base substrate may prove to be complicated, since the removal of the first carbon base substrate generally induces a very large curvature in the thick p-SiC layer, which may cause said thick layer to break or at least complicate or prevent the thinning steps required to achieve the thickness of the second base substrate. In addition, this thinning is very substantial (approximately 1.5 mm) and costly in terms of p-SiC material and deposition and thinning steps. Summary of the Invention

[0007] The present invention proposes a manufacturing process that addresses the above mentioned problems. The present invention relates to an economical and simplified process for manufacturing a polycrystalline SiC support substrate, said support substrate being even more particularly suitable for the manufacture of composite structures comprising a thin c-SiC layer arranged on said p-SiC support substrate.

[0008] The present invention provides a process for producing a polycrystalline silicon carbide support substrate, comprising: a) growing an initial polycrystalline silicon carbide substrate on a graphite or silicon carbide seed, at the end of step a) the initial substrate having a free front surface and a rear surface in contact with the seed, b) forming a stiffening carbon film on a front surface of an initial substrate, the initial substrate having a first average silicon carbide grain size at a plane of the front surface of the initial substrate and immediately prior to the formation of the stiffening film; c) removing the seeds to free a rear surface of the initial substrate, the initial substrate having, in a plane of the rear surface of the initial substrate and immediately after removal of the seeds, a second average silicon carbide grain size smaller than the first average size; d) thinning the rear surface of the initial substrate to a thickness where the initial substrate has a third average grain size equal to within ±30% of the first average grain size in the plane of the thinned rear surface of the initial substrate, the thinned initial substrate forming a support substrate. It relates to the process, including

[0009] According to other advantageous and non-limiting features of the present invention, taken alone or in any technically feasible combination, The stiffening membrane has a thickness between 100 nm and several millimeters, for example 10 mm; The stiffening film has a thickness between 100 nm and 10 μm; the stiffening carbon film has a diamond-like or glassy carbon-like crystallographic structure; Step b) is carried out by applying a polymeric resin having preformed carbon-carbon bonds in three dimensions as a viscous layer on the front surface of the initial substrate and annealing at a temperature between 500° C. and 2000° C. to form a stiffening carbon film; the polymeric resin is based on coal tar, phenol formaldehyde, polyfurfuryl alcohol, polyvinyl alcohol, polyacrylonitrile, polyvinylidene chloride, and / or polystyrene; Step b) is carried out by plasma deposition, ion bombardment deposition or evaporation deposition, the manufacturing process comprises, between steps a) and b), a step a') of grinding the front surface and / or the periphery of the initial substrate to reduce the surface roughness of said surface and / or the variation in the thickness of said substrate and / or to uniformize the periphery of the initial substrate, Step a') comprises mechanical or mechanochemical thinning, The manufacturing process is After step d), a step e) of removing the stiffening membrane; and / or After step d) or after step e), a step of heat treatment at a temperature of 1500° C. or higher. Includes.

[0010] The present invention further relates to a process for manufacturing a composite structure, which involves the process described above and also comprises a step f) of transferring a thin layer of monocrystalline silicon carbide onto the first or second side of a support substrate, either directly or via an intermediate layer, to form a composite structure.

[0011] According to other advantageous and non-limiting features of the present invention, taken alone or in any technically feasible combination, the intermediate layer is formed by a stiffening carbon film supported on a first surface of a supporting substrate; The transfer of the thin layer is performed on one of the first and second sides of the support substrate, and an additional carbon film is disposed on the other free side of the support substrate prior to the transfer, The additional film is preferentially removed after the composite structure has undergone any heat treatment at temperatures above 1400° C. required for the manufacture of the composite structure or for the manufacture of components on and / or within said structure.

[0012] Other features and advantages of the present invention will become apparent from the following detailed description of the invention, which is given with reference to the accompanying drawings. [Brief description of the drawings]

[0013] [Figure 1a] 1 illustrates steps of a manufacturing process according to the present invention. [Figure 1b] 1 illustrates steps of a manufacturing process according to the present invention. [Figure 1c] 1 illustrates steps of a manufacturing process according to the present invention. [Figure 1d] 1 illustrates steps of a manufacturing process according to the present invention. [Figure 1e] 1 illustrates steps of a manufacturing process according to the present invention. [Figure 1f] 1 illustrates steps of a manufacturing process according to the present invention. [Figure 2a] FIG. 4 illustrates another step of the manufacturing process according to the present invention. [Figure 2b] FIG. 4 illustrates another step of the manufacturing process according to the present invention. [Figure 2c] FIG. 4 illustrates another step of the manufacturing process according to the present invention. [Figure 2d] FIG. 4 illustrates another step of the manufacturing process according to the present invention. [Figure 3a] 5A-5C illustrate variations of the steps of the manufacturing process according to the present invention. [Figure 3b] 5A-5C illustrate variations of the steps of the manufacturing process according to the present invention. [Figure 3c] 5A-5C illustrate variations of the steps of the manufacturing process according to the present invention. [Figure 3d] 5A-5C illustrate variations of the steps of the manufacturing process according to the present invention. [Figure 3e] 5A-5C illustrate variations of the steps of the manufacturing process according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] The same reference numbers in the figures may be used for the same types of elements. The figures are schematic representations that are not to scale for clarity. In particular, the thicknesses of layers along the z-axis are not to scale with respect to lateral dimensions along the x and y-axes, and the relative thicknesses of layers with respect to one another are not necessarily emphasized in the figures.

[0015] The present invention relates to a process for fabricating a polycrystalline silicon carbide (p-SiC) support substrate 10.

[0016] The process first comprises a step a) of growing an initial polycrystalline silicon carbide substrate 1 on a seed 2 of graphite or low-quality monocrystalline or polycrystalline silicon carbide (FIG. 1a). The seed 2 is preferentially in the form of a wafer, the diameter of which is substantially the diameter targeted for the support substrate 10, for example 100 mm, 150 mm, 200 mm or even 300 mm.

[0017] The growth of the initial p-SiC substrate 1 is carried out by known chemical vapor deposition (CVD) techniques, typically at temperatures between 1100° C. and 1500° C. The precursors may preferentially be chosen from methylsilane, dimethyldichlorosilane, or dichlorosilane and i-butane, with a C / Si ratio close to or greater than 1.

[0018] Optionally, doping species (for example nitrogen or phosphorus) may be introduced during the CVD deposition to adjust the resistivity of the initial substrate 1 (from which the support substrate 10 will be obtained) to the specifications of the final product, in particular the targeted composite structure. Typically targeted doping levels are 1E18 / cm 3 or greater than 1E20 / cm 3 It is even greater than that.

[0019] At the conclusion of step a), the initial substrate 1 has a free front face 1a and a rear face 1b in contact with the seed 2. The thickness of the initial substrate 1 is less than 1 mm, preferentially less than 550 μm. It should be noted that the usual desired thickness range for the support substrate 10 intended for the production of composite structures is between 100 μm and 500 μm.

[0020] The initial substrate 1 may comprise 4H, 6H, and / or 3C type silicon carbide particles, depending on the CVD deposition conditions.

[0021] The average size of the grains at the rear face 1b of the initial substrate 1 is relatively small, typically below 1 μm or even below 100 nm, said grains corresponding to the p-SiC material produced at the start of the CVD deposition on the graphite seeds 2 (nucleation stage).

[0022] It should be recalled that the size of a grain delimited by a grain boundary corresponds to the largest dimension of said grain in the plane of the considered surface of the substrate. The average grain size is defined as the average of the sizes of the individual grains in said surface. The grain size or grain boundary distance can be measured on the basis of images obtained by conventional scanning electron microscopy (SEM) or with electron backscatter diffraction (EBSD). It may even be envisaged to use X-ray crystallography. When the surface under consideration mainly comprises grains of micrometer size (typically a few microns to tens of microns), very small grains, typically less than 50 nm, are preferentially excluded from the measurement so as to limit the measurement uncertainty.

[0023] As the CVD deposition progresses, the p-SiC grains grow in size until they reach a relatively stable average size for a deposit thickness that can vary between a few micrometers to tens of micrometers depending on the deposition conditions.

[0024] Thus, depending on the thickness of p-SiC deposited to grow the initial substrate 1, the average grain size at the front surface of said substrate 1 may typically vary between 1 and 10 μm.

[0025] Hereinafter, the average size of the p-SiC grains at the front surface 1a of the initial substrate 1 will be referred to as the first average size, and the average size of the p-SiC grains at the rear surface 1b of the initial substrate 1 will be referred to as the second average size.

[0026] The first average p-SiC grain size (front side 1a) is larger than the second average grain size (back side 1b), the latter corresponding to the nucleation stage.

[0027] The manufacturing process then comprises a step b) of forming a stiffening carbon film 3 on the front surface 1a of the initial substrate 1 (FIG. 1b). The stiffening film 3 has a thickness ranging from 100 nm to a few millimeters, for example 10 mm. Preferentially, the thickness of the stiffening film is between 100 nm and 10 μm.

[0028] The stiffening carbon film 3 is, namely, sp 3 Diamond-type crystallographic structure containing carbon-carbon bonds, or sp 2 It is advantageous to have a glassy carbon type structure, which includes carbon-carbon atom bonds.

[0029] The stiffening film 3 may be formed by various conventional deposition techniques, such as plasma deposition, ion bombardment deposition, or evaporation deposition, among others.

[0030] Alternatively, step b) may be performed by applying a polymeric resin containing preformed carbon-carbon bonds in three dimensions as a viscous layer on the front surface 1a of the initial substrate 1. This application may be performed by centrifugation. Annealing is then applied under nitrogen at a temperature between 500°C and 2000°C, typically between 600°C and 1100°C, in order to form a stiffening carbon film 3 by chemical decomposition (pyrolysis) of the resin. The chosen temperature ramp is typically around 10°C / min and the annealing time is around 1 hour. The temperature increase is controlled so that the effective temperature remains below the resin / carbon glass transition temperature.

[0031] The polymeric resin may be formed from coal tar, phenol formaldehyde, polyfurfuryl alcohol, polyvinyl alcohol, polyacrylonitrile, polyvinylidene chloride, and / or polystyrene, among others.

[0032] By way of example, known photosensitive resins may be used, such as the commercial products AZ-4330, AZ-P4620® (based on 1-methoxy-2-propanol acetate, diazonaphthoquinone sulfonic acid ester, 2-methoxy-1-propanol acetate, cresol novolac resin), OCG-825 (based on 3-ethoxyethyl propionate), SU-8 2000 (based on cyclopentanone, triarylsulfonium / hexafluoroantimonate, propylene carbonate, epoxy resin), which are usually used for photolithography steps in the field of microelectronics.

[0033] Epoxy resins, illustratively Epoxy Novolac EPON products, proposed for coating and protecting various surfaces in various sectors (aviation, maritime, automotive, construction, etc.), may furthermore be used in step b) of the process according to the invention.

[0034] When using resin, it is important to take into account the shrinkage that the viscous resin layer will undergo during annealing in order to define an initial thickness of the viscous resin layer sufficient to obtain the targeted thickness of the stiffening membrane 3. The thickness shrinkage may typically be between 70% and 95%. The carbon fraction, i.e. the ratio between the mass of the polymeric resin layer after pyrolysis (corresponding to the stiffening membrane 3) and the initial mass of the applied polymeric resin layer, must be at least 5%, preferentially greater than 50%.

[0035] Optionally, the manufacturing process may comprise, between steps a) and b), a step a') of grinding the front surface 1a and / or the peripheral portion 1c of the initial substrate 1 to reduce the surface roughness of said surface 1a and / or to reduce thickness variations of said substrate 1 and / or to uniformize the peripheral portion 1c.

[0036] Step a') may comprise a mechanical or mechanochemical thinning (polishing) involving the removal of approximately a few microns to a few tens of microns of material.

[0037] The manufacturing process according to the invention then comprises a step c) of removing the seed 2 to free the rear face 1b of the initial substrate 1 (FIG. 1c).

[0038] When the seed 2 is made of graphite, the removal can be carried out by burning the graphite by applying a thermal treatment in an oxygen-rich atmosphere (e.g. air) at a temperature above 400°C, preferentially above 550°C.

[0039] It is further possible to mechanically detach the seeds 2, whether made from graphite or from silicon carbide, for example by localized application of mechanical stress at or near the interface between the seeds 2 and the initial substrate 1.

[0040] If any residues remain on the rear face 1b of the initial substrate 1 after removal of the seeds 2, they can be burned off (if they are made of graphite) or removed mechanically or chemically by polishing or etching (if they are made of graphite or SiC).

[0041] This removal typically results in a strong curvature of the initial substrate 1, which can be up to 500 μm for a diameter of 150 mm. This curvature is mainly due to stresses associated with the difference in grain size between the second face 1b (nucleation grains, small average size) and the first face 1a.

[0042] In the context of the present invention, the stiffening carbon film 3 makes it possible to significantly limit the increase in curvature during removal of the seed 2 by mechanically holding the initial substrate 1 by its front surface 1a. The curvature of the initial substrate 1 provided with the stiffening carbon film 3 does not exceed 200 μm for a substrate diameter of 150 mm, and even more so, the curvature is kept below 100 μm. In these curvature ranges, the initial substrate 1 can be processed without any problems in standard lines and equipment without any risk of breakage or of equipment failure, these problems being mainly encountered for curvatures greater than 300 μm (diameter 150 mm).

[0043] Finally, the manufacturing process comprises a step d) of thinning the rear face 1b of the initial substrate 1. The thinned initial substrate 1 forms a support substrate 10 (FIG. 1d).

[0044] The thinning in step d) is carried out by mechanical grinding, mechanical polishing and / or mechanical-chemical polishing of the rear face 1b. The material removal is typically between a few tens of microns and 200 μm, depending on the thickness of the initial substrate 1 at the start of step d) and, of course, on the thickness targeted for the support substrate 10.

[0045] The thinning is performed to a thickness such that the initial substrate 1 has a third average grain size equal to within ±30% of the first average grain size in the plane of the thinned rear surface 1b' of the initial substrate 1. In other words, if the first average size is for example 5 μm, the third average size is expected to be between 4 μm and 6 μm.

[0046] It may happen that the particle sizes in the plane of the front side 1a or in the plane of the rear side 1b are distributed in a double population, with each peak following a substantially Gaussian distribution. According to a first option, the average particle size is calculated by taking the overall average including both populations, and the first and third average sizes should not differ by more than 30%. According to a second option, the first two average sizes (corresponding to the double population on the front side 1a) and the third two average sizes (corresponding to the double population on the rear side 1b) are taken into account, and they should not differ from each other by more than 30%, respectively.

[0047] After thinning in step d), the thinned rear surface 1b' of the initial substrate 1 has an average p-SiC grain size that differs from the average grain size of the front surface 1a by less than 30%. The residual stresses in the thinned initial substrate 1 (forming the support substrate 10) are therefore at least manageable in a manufacturing line, which is adapted to small curvatures.

[0048] The manufacturing process may then comprise a step e) of removing the stiffening membrane 3, for example by dry or wet chemical etching (FIG. 1e). After this removal, the support substrate 10 has a curvature of less than 200 μm, or even less than 100 μm (for a diameter of 150 mm), due to reduced residual stresses in the volume of the support substrate 10.

[0049] In this aspect, the support substrate 10 has a first surface 10a, a second surface 10b and an edge 10c which respectively correspond to the front surface 1a, the thinned rear surface 1b' and the edge 1c of the initial substrate 1 after step d).

[0050] As an example, to form a support substrate 350 μm thick and 150 mm in diameter, a 500 μm initial substrate 1 may be produced on the seed 2, the initial substrate 1 having a first average p-SiC grain size on the front surface 1a of the initial substrate 1 of about 4 μm. A step to correct the thickness uniformity of the initial substrate 1 may be performed, for example by removal of 50 μm. A 4 μm stiffening carbon film 3 is formed on this front surface 1a. After removal of the graphite seed 2, the second average grain size on the rear surface 1b of the initial substrate 1 is less than 100 nm, but the curvature of the initial substrate 1 remains below 150 μm due to the presence of the stiffening film 3. A removal of 100 μm on the rear surface 1b of the initial substrate 1 is performed, and a third average grain size of p-SiC on the thinned rear surface 1b' of approximately 3 μm satisfies the condition of equality to within 30% of the first average grain size. Thus, after removal of the stiffening membrane 3 , the curvature of the supporting substrate 10 is less than 200 μm, making it compatible with the subsequent steps for manufacturing the composite structure 100 .

[0051] Optionally, after step e), a surface treatment may be applied to the first side 10a of the support substrate 10, in particular if this side 10a is intended to receive the thin layer 20 of the composite structure 100 in a subsequent step f) of the process. This surface treatment may comprise mechanical grinding, mechanochemical polishing or other chemical cleaning operations, depending on the surface roughness of the first side 10a.

[0052] If the second side 10b of the support substrate 10 is intended to receive the thin layer 20 and step d) did not achieve a sufficiently low level of roughness (typically <1 nm RMS, measured by atomic force microscopy on a 20 μm x 20 μm scan), an additional surface treatment may further be applied to the second side 10b.

[0053] The surface of the support substrate 10 intended to form the rear surface of the composite structure 100 may have a higher surface roughness, for example of the order of 10 nm RMS.

[0054] The manufacturing process may also include a heat treatment after step d) or after step e) at a temperature above 1500° C., typically between 1500° C. and 1900° C., so as to stabilise the polycrystalline structure of the support substrate 10. Moreover, these temperature ranges are likely to be applied later in the process, especially for the manufacture of composite structures.

[0055] Thanks to the manufacturing process according to the invention, a support substrate 10 having mechanical properties meeting the specifications of composite structures for microelectronic applications may be obtained in a simple manner, without the need to deposit a very thick initial p-SiC substrate, which is removed by more than 80% in order to select a very small useful portion of p-SiC, as is performed in the prior art processes. In the manufacturing process according to the invention, the thickness of the initial substrate 1 formed is less than 1 mm, and the material removal at the front side 1a of the initial substrate 1 and / or at the rear side 1b of the initial substrate 1 is less than 70% or even less than 50% of the initial thickness, which results in savings in material and technological steps.

[0056] In the context of the development of the composite structure 100, the manufacturing process according to the invention may be continued by step f) of transferring a working layer 20 made of monocrystalline silicon carbide onto the support substrate 10 based on molecular adhesive bonding (FIG. 1f).

[0057] There are various options known in the art for performing layer transfer that will not be described in detail here.

[0058] According to a preferred mode, step f) of the process involves the injection of light species according to the principle of the Smart-Cut® process.

[0059] In a first step f1), a monocrystalline silicon carbide donor substrate 21 is prepared, from which the working layer 20 is to be obtained (FIG. 2a). The donor substrate 1 is preferentially in the form of a wafer with a diameter of 100 mm, 150 mm, 200 mm or even 300 mm (identical or very similar to the diameter of the support substrate 10) and with a thickness typically between 300 μm and 800 μm. The donor substrate has a front face 21a and a rear face 21b. The surface roughness chosen for the front face 1a is advantageously less than 1 nm RMS, or even less than 0.5 nm RMS, measured by atomic force microscopy (AFM) in a 20 μm×20 μm scan. The donor substrate 21 may be of polytype 4H or 6H and may have n- or p-type doping, depending on the requirements of the components to be expressed on and / or in the working layer 20 of the composite structure 100.

[0060] A second step f2) corresponds to the introduction of light chemical species into the donor substrate 21 in order to form a buried fragile plane 22 which, together with the front surface 21a of the donor substrate 21, defines the boundary of the processing layer 20 to be transferred (FIG. 2b).

[0061] Light species, preferentially hydrogen, helium or a co-implantation of these two species, are implanted into the donor substrate 21 to a given depth, which corresponds to the targeted thickness of the working layer 20. These light species will form, around the given depth, microcavities distributed as a thin layer parallel to the free surface 21a of the donor substrate 21, i.e. parallel to the plane (x,y) in the figure. This thin layer is called, for simplicity, the buried weak plane 22.

[0062] The energy of the implantation of the light species is selected to reach a given depth. For example, hydrogen ions are implanted at energies between 10 keV and 250 keV and at 50 keV to define a working layer 20 with a thickness of approximately 100 nm to 1500 nm. E 16 / cm 2 ~1 E 17 / cm2 It should be noted that a protective layer may be deposited on the front side 21a of the donor substrate 21 prior to the ion implantation step. This protective layer may consist of a material such as silicon oxide or silicon nitride. The protective layer is removed prior to the next stage.

[0063] Optionally, an intermediate layer 4 may be formed on the front surface 21a of the donor substrate 21 before or after the second stage f2) of introduction of light chemical species (Figures 3b, 3c, 3d, 3e). This intermediate layer 4 may be made of a semiconductor material, for example silicon or silicon carbide, or of a metallic material such as tungsten, titanium, etc. The thickness of the intermediate layer 4 is advantageously limited, typically between a few nanometers and a few tens of nanometers.

[0064] When the intermediate layer 4 is formed before step f2), the implantation energy (and potentially the dose) of the light species will be adjusted for the traverse of this additional layer. When the intermediate layer 4 is formed after step f2), care will be taken to form this layer by applying a thermal budget lower than the bubbling thermal budget, which corresponds to the appearance of blisters at the surface of the donor substrate 21 due to excessive growth and pressurization of microcavities in the buried weak plane 22.

[0065] The transfer step f) then comprises a third stage f3) of assembling the donor substrate 21 on the front side 21a of the donor substrate 21 with the support substrate 10 along a bonding interface 30 by molecular adhesion bonding, either on the first side 10a of the support substrate 10 or on the second side 10b of the support substrate 10 (Figure 2c).

[0066] Optionally, an intermediate layer 4' may further be deposited on the side to be brought together of the support substrate 10 prior to the bringing together step f3) (FIGS. 3d, 3e), the intermediate layer 4' being chosen to be of the same nature as the intermediate layer 4 mentioned for the donor substrate 21 or of a different nature from the intermediate layer 4. The intermediate layer 4, 4' may optionally be deposited only on one or the other of the two substrates 21, 10 to be brought together.

[0067] The purpose of the intermediate layer(s) is essentially to increase the bonding energy (especially in the temperature range below 1100°C) due to the formation of covalent bonds at lower temperatures than in the case of two directly joined SiC surfaces; another advantage of this(these) intermediate layer(s) may be to improve the vertical electrical conduction of the bonding interface 30.

[0068] According to a possible variant, the intermediate layer may be formed by a stiffening carbon film 3 held on the first side 10a of the support substrate 10 (FIGS. 3a, 3c). In this case, step e) of the manufacturing process according to the invention is not carried out and the side of the support substrate 10 that will be brought together is the first side 10a of the support substrate 10, to which the film 3 is applied. In the final composite structure 100, a carbon film with a diamond-type crystallographic structure will be preferred to promote vertical electrical conduction through the stiffening film 3.

[0069] Optionally, an additional carbon film 5 is arranged, still prior to the assembly step f3), on the face of the support substrate 10 opposite to the face to be assembled (FIG. 3e). The properties of the additional carbon film 5 may for example be chosen from the properties proposed for the stiffening film 3 earlier in the description.

[0070] Although the presence of this additional film 5 is illustrated in Figure 3e in combination with intermediate layers 4, 4' on the to-be-assembled faces of the donor substrate 21 and the support substrate 10, respectively, this additional film 5 may be realised in any of the possible configurations mentioned, in particular in the configurations illustrated in Figures 3a to 3c.

[0071] The additional film 5 may be removed at a later time, preferentially after the composite structure 100 has undergone any thermal treatment at temperatures above 1400°C required for the manufacture of the composite structure 100 or for the manufacture of components on and / or within said structure 100.

[0072] Returning to the description of the combining step f3), and as is well known per se, direct molecular adhesive bonding does not require adhesive materials, since the bond is established at the atomic level between the surfaces to be combined. Several types of molecular adhesive bonds exist, which differ, inter alia, in their temperature, pressure or atmospheric conditions, or in the treatment prior to bringing the surfaces into contact. One can mention bonding at room temperature, with or without prior plasma activation of the surfaces to be combined, atomic diffusion bonding (ADB), surface activated bonding (SAB), etc.

[0073] The alignment step f3) may include, prior to bringing the faces 21a, 10a to be aligned in contact, a conventional sequence of chemical cleaning (e.g., RCA cleaning) and of surface activation (e.g., by means of oxygen or nitrogen plasma) or other surface preparation (scrubbing, etc.) likely to enhance the quality of the bonding interface 30 (low defect density, high adhesion energy).

[0074] Finally, a fourth step f4) involves detachment along the embedded plane of weakness 22 leading to the transfer of the engineered layer 20 onto the support substrate (FIG. 2d).

[0075] The separation along the embedded plane of weakness 22 is usually carried out by applying a heat treatment at a temperature between 800 ° C. and 1200 ° C. Such a heat treatment causes cavities and microcracks to appear in the embedded plane of weakness 22, which cavities and microcracks are pressurized by light species present in gaseous form, until a fracture propagates along said plane of weakness 22. Alternatively, or jointly, a mechanical stress can be applied to the bonded assembly, and in particular to the embedded plane of weakness 22, so as to propagate or to help mechanically propagate the fracture leading to the separation. At the end of this separation, a semiconductor structure 100 is obtained, which comprises, on the one hand, the support substrate 10 and the transferred working layer 20 made of monocrystalline SiC, and, on the other hand, the remainder 21 ′ of the donor substrate. The level and type of doping of the working layer 20 is defined by the choice of the properties of the donor substrate 21 or can be adjusted later by known techniques for doping semiconductor layers.

[0076] The free surface 20a of the working layer 20 is usually rough after separation, for example the free surface 20a has a roughness between 5 nm and 100 nm RMS (AFM, 20 μm×20 μm scan). Cleaning and / or smoothing steps can be applied to restore a good surface finish (typically less than a few Angstroms RMS roughness in a 20 μm×20 μm AFM scan). In particular, these steps may include mechanochemical smoothing treatments of the free surface of the working layer 20. A removal of between 50 nm and 300 nm makes it possible to effectively restore the surface finish of said layer 20. Said steps may also include at least a thermal treatment at a temperature between 1300 ° C. and 1800 ° C. Such a thermal treatment is applied to remove residual light species from the working layer 20 and to promote a rearrangement of the crystal lattice of the working layer 20. The thermal treatment further makes it possible to strengthen the bonding interface 30. The thermal treatment may also include or correspond to the epitaxy of silicon carbide in the thin layer 20 .

[0077] Finally, it should be noted that the transfer step f) may also include a step of modifying the remaining part 21' of the donor substrate for reuse as a donor substrate 21 for the new composite structure 100. Mechanical and / or chemical treatments similar to those applied to the composite structure 100 may be applied to the front side 21'a of the remaining substrate 21'.

[0078] The resulting composite structure 100 is extremely robust with respect to very high temperature thermal treatments that may be applied to improve the quality of the processing layer 20 or to manufacture components on and / or within said layer 20.

[0079] The composite structure 100 according to the invention is particularly suitable for producing one or more high voltage microelectronic component(s), illustratively Schottky diodes, MOSFET transistors, etc. More generally, the composite structure 100 is suitable for power microelectronic applications, allowing excellent vertical electrical conduction, good thermal conductivity, and resulting in high quality c-SiC processed layers.

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

Claims

Claim 1 A process for manufacturing a polycrystalline silicon carbide support substrate (10), comprising: a) growing an initial polycrystalline silicon carbide substrate (1) on a graphite or silicon carbide seed (2), wherein at the end of step a), the initial substrate (1) has a free front surface (1a) and a rear surface (1b) in contact with the seed (2); b) forming a stiffening carbon film (3) on the front surface (1a) of the initial substrate (1), wherein the initial substrate (1) has a first average silicon carbide particle size in the plane of the front surface (1a) of the initial substrate and immediately before the formation of the stiffening film (3); c) removing the seed (2) to free the rear surface (1b) of the initial substrate (1), wherein the initial substrate has a second average silicon carbide particle size smaller than the first average size in the plane of the rear surface (1b) of the initial substrate and immediately after the removal of the seed (2); d) thinning the rear surface (1b) of the initial substrate (1) to a thickness having a third average particle size equal to the first average particle size within ±30% in the plane of the thinned rear surface (1b') of the initial substrate (1), wherein the thinned initial substrate (1) forms the support substrate (10). A process for manufacturing, comprising the above steps. Claim 2 The process according to claim 1, wherein the stiffening film (3) has a thickness between 100 nm and several millimeters, for example 10 mm. Claim 3 The process according to claim 2, wherein the stiffening film (3) has a thickness between 100 nm and 10 µm. Claim 4 The process according to claim 1, wherein the stiffening carbon film (3) has a diamond-like or glassy carbon-like crystallographic structure. Claim 5 The process according to claim 1, wherein step b) is carried out by applying a polymer resin having pre-formed carbon-carbon bonds three-dimensionally as a viscous layer on the front surface of the initial substrate (1) and annealing at a temperature between 500 °C and 2000 °C to form the stiffening carbon film (3). Claim 6 The process according to claim 5, wherein the polymer resin is based on coal tar, phenol formaldehyde, polyfurfuryl alcohol, polyvinyl alcohol, polyacrylonitrile, polyvinylidene chloride, and / or polystyrene.

7. The process according to claim 1, wherein step b) is carried out by plasma deposition, ion impact deposition, or vapor deposition.

8. The process according to claim 1, including step a') between step a) and step b), of grinding the front face (1a) and / or the periphery of the initial substrate (1) to reduce the surface roughness of the face (1a), and / or to reduce the thickness variation of the substrate (1), and / or to uniformize the periphery of the initial substrate.

9. The process according to claim 8, wherein step a') includes mechanical or mechanochemical thinning.

10. After step d), step e) of removing the stiffening film, and / or After step d), or after step e), a heat treatment step at a temperature of 1500 °C or higher The process according to claim 1.

11. A process for manufacturing a composite structure (100), involving the process according to any one of claims 1 to 10, and including step f) of transferring a thin layer (20) of single-crystalline silicon carbide directly or via an intermediate layer onto the first face (10a) or the second face (10b) of the support substrate (10) to form the composite structure (100).

12. The process according to claim 11, wherein the intermediate layer is formed by the stiffening carbon film (3) retained on the first face (10a) of the support substrate (10).

13. The transfer of the thin layer (20) is carried out on one of the first face and the second face (10a, 10b) of the support substrate (10), and an additional carbon film (5) is disposed on the other free face (10b, 10a) of the support substrate (10) prior to the transfer. The process according to claim 11.

14. The process according to claim 13, wherein the additional film (5) is preferably removed after any heat treatment at a temperature above 1400 °C that the composite structure (100) is subjected to for the manufacture of the composite structure or for the manufacture of components on and / or within the structure (100).

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