Method for preparing a carrier substrate made of polycrystalline material and method for manufacturing a composite structure from the carrier substrate

Abstract

The invention relates to a method for preparing a carrier substrate from a disc made of polycrystalline material having a front face and a rear face, based on a sequence for mechanical grinding of the front face, the sequence comprising: a) arranging the rear face of the disc on a plate which is capable of being rotated and which is arranged parallel to a grinding wheel, which is itself capable of being rotated and of moving vertically until it comes into contact with the front face in order to mechanically remove material; b) a first material-removal step, involving a first descent speed of the grinding wheel; c) a second material-removal step, involving a second descent speed of the grinding wheel lower than the first descent speed; d) a finishing step, involving a zero descent speed of the grinding wheel, for a duration greater than or equal to 2 minutes, at the end of which step the carrier substrate is obtained.

Description

Method for preparing a support substrate made of polycrystalline material and method for manufacturing a composite structure from said support substrate FIELD OF INVENTION

[0001] The present invention relates to the field of semiconductor materials for microelectronic components. It relates in particular to a method for preparing a support substrate made of polycrystalline material, especially SiC, which can be used for the fabrication of a composite structure including a single-crystal thin film transferred onto said support substrate. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] Silicon carbide (SiC) is increasingly used in the fabrication of innovative devices (power components, radio frequencies, etc.) to meet the needs of emerging areas of electronics, such as electric vehicles. Indeed, power devices and integrated power systems based on monocrystalline silicon carbide can handle significantly higher power densities compared to their traditional silicon counterparts, and with smaller active area dimensions.

[0003] High-quality single-crystal SiC (c-SiC) substrates for the microelectronics industry remain expensive and difficult to source in large quantities. Therefore, layer transfer solutions offer an advantage for creating composite structures typically comprising a thin single-crystal SiC layer (derived from the high-quality c-SiC substrate) on a lower-cost support substrate, such as polycrystalline SiC (p-SiC). Electronic components can then be fabricated on and / or within the thin film. It is also worth noting that a composite structure can provide additional functionality and performance benefits, particularly by offering a support substrate with advantageous mechanical, electrical, and / or thermal properties.

[0004] A well-known thin-film transfer solution is the Smart Cut process. TM, based on the implantation of light ions and on an assembly, by direct bonding, between a single-crystal donor substrate and a support substrate, at the level of a bonding interface.

[0005] To be compatible with a thin-film transfer process and ensure high-quality transferred layers, the substrate must have a very good surface finish; in particular, the roughness level must be sufficiently low (typically less than 1 nm RMS per AFM measurement on a 5x5 μm scan). 2 or more) to ensure proper bonding and low defects in the resulting bonding interface.

[0006] While the desired surface finish is obtained relatively easily with conventional processes in the case of single-crystal materials, it is much more complex to achieve a satisfactory surface finish in the case of polycrystalline materials.

[0007] A polycrystalline SiC support substrate can, for example, be formed by vapor deposition of p-SiC onto a growth substrate (such as graphite) to create a relatively thick (e.g., 0.4 to 3 mm) p-SiC blank. This blank is then separated from the growth substrate and thinned to obtain the support substrate with the desired shape (including a beveled edge) and thickness. The thinning process generally involves a series of mechanical grinding steps, progressing from coarse to fine, applied to both faces of the blank.

[0008] Mechanical or chemical-mechanical polishing steps can then be performed to further improve the surface roughness of the substrate. However, the polycrystalline surface of the substrate complicates the application of such steps because the differential chemical attack between grains and grain boundaries creates surface texturing. Furthermore, polishing techniques tend to degrade the edge relief of the wafers.

[0009] Efforts are therefore focused on fine grinding techniques, using very small grain wheels (typically with an average grain size of less than 3 μm), to obtain the lowest possible roughness on the front face of the substrate intended for assembly. This is, for example, the case of the process described in document WO2024047305.

[0010] However, performance in terms of surface roughness still needs improvement to promote the quality of layer transfer and the final composite structure.

[0011] SUBJECT OF THE INVENTION

[0012] The present invention proposes a method for preparing a support substrate in polycrystalline material, in particular in p-SiC, which allows the achievement of roughness levels compatible with molecular adhesion bonding and favorable to high-quality thin film transfers.

[0013] BRIEF DESCRIPTION OF THE INVENTION

[0014] The invention relates to a method for preparing a support substrate from a polycrystalline material disc having a front face and a back face, based on a mechanical grinding sequence of the front face, said sequence comprising:

[0015] a) the arrangement of the rear face of the disc on a plate capable of being rotated and arranged parallel to a grinding wheel, itself capable of being rotated and moving vertically until it comes into contact with the front face to achieve a mechanical removal of material;

[0016] b) a first stage of material removal, involving a first descent speed of the grinding wheel;

[0017] c) a second stage of material removal, involving a second descent speed of the grinding wheel lower than the first descent speed;

[0018] d) a finishing step, involving a zero descent speed of the grinding wheel, for a duration greater than or equal to 2 minutes, at the end of which the substrate support is obtained.

[0019] According to other advantageous and non-limiting features of the invention, taken alone or in any technically feasible combination: the polycrystalline material is silicon carbide or aluminum nitride or silicon; the first descent speed is between 0.15μm / s and 0.25μm / s; the second descent speed is between 0.05μm / s and 0.15μm / s; the second rotation speed of the grinding wheel is less than the first rotation speed of the grinding wheel, and the second rotation speed of the plate is less than the first rotation speed of the plate; step b) induces a material removal of less than 1μm, preferably between 0.2μm and 0.8 μm; step c) induces a material removal of less than 5 μm, preferably between 3 μm and 4 μm; in step a), at least the front face of the disc has a work-hardened surface region, induced by prior coarse grinding; the grinding wheel comprises abrasive grains having an average size less than or equal to 4 μm, less than or equal to 2 μm, or even less than or equal to 1 μm; the duration of step d) is greater than or equal to 3 min, preferably between 3 min and 5 min; the preparation process includes a third material removal step c2), after step c) and before step d), involving a third grinding wheel descent speed greater than the second grinding wheel descent speed; the third descent speed is between 0.15 μm / s and 0.25 μm / s; step c2) induces a material removal of less than 0.5μm, preferably between 0.2μm and 0.3μm.

[0020] The invention also relates to a method for manufacturing a composite structure comprising:

[0021] - the preparation of a support substrate in a polycrystalline material according to the aforementioned process, and

[0022] - the transfer of a thin layer in a single-crystal material, from a donor substrate, onto said supporting substrate.

[0023] The single-crystal material can be chosen from silicon carbide (SiC), gallium nitride (GaN), gallium oxide (Ga2O3), diamond, silicon, germanium, indium phosphide (InP). BRIEF DESCRIPTION OF THE FIGURES

[0024] 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:

[0025] Presents simplified views of a mechanical grinding equipment, in perspective, side view and top view;

[0026]

[0027]

[0028]

[0029] La, la, la and la respectively represent a polycrystalline material disc intended to have the preparation process according to the invention applied to it (), a support substrate obtained in accordance with the process of the invention (), composite structures resulting from a manufacturing process according to the invention (,);

[0030] Present two graphs representing the local value of "haze" as a function of the diameter of the support substrate prepared according to the prior art (A) or according to a preparation process according to the invention (B);

[0031] Presents average "haze" values ​​for support substrates obtained from the preparation process according to the invention, and for support substrates manufactured according to the prior art;

[0032] This presents a graph illustrating the load (resistance opposing the rotation of the grinding wheel due to contact friction with the polycrystalline material disc to be thinned) and the thickness of the disc, as a function of time, during a preparation process according to the present invention.

[0033] The same references in the figures can be used for elements of the same type. Some figures are schematic representations which, for the sake of clarity, are not drawn 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; and the relative thicknesses of the layers are not necessarily to scale in the figures. DETAILED DESCRIPTION OF THE INVENTION

[0034] The present invention relates to a method for preparing a support substrate 20 made of polycrystalline material. Preferably, said material is silicon carbide (SiC) or aluminum nitride (AlN) or even silicon.

[0035] Several techniques exist for the fabrication or growth of polycrystalline materials in the form of discs, intended to form substrates or wafers (known as "wafers" in English), particularly for the semiconductor industry. These techniques include, among others, sintering, PVT (Physical Vapor Transport), CVD (Chemical Vapor Deposition), and HTCVD (High Temperature Chemical Vapor Deposition).

[0036] The preparation process according to the invention is applied to a disc 2 made of polycrystalline material, produced by a known technique such as those mentioned above. The disc 2 has two faces extending substantially parallel to a principal plane (x,y); one will be called the front face 2a and is intended to form the front face 20a of the support substrate 20, which will be assembled with a thin monocrystalline layer in a composite structure 100. The other face will be called the back face 2b and is intended to form the back face 20b of the support substrate 20 and of the composite structure 100.

[0037] The preparation process is based on a particular sequence of mechanical rectification of the front face 2a of the disc 2. Of course, such a sequence can also be applied to the rear face 2b if necessary.

[0038] Step a) corresponds to the arrangement of the rear face 2b of the disc 2 on a plate E2 suitable for rotation and arranged parallel to a grinding wheel E1, itself suitable for rotation and for moving vertically until it comes into contact with the front face 2a to achieve a mechanical removal of material ().

[0039] As a reminder, a grinding wheel is a rotationally symmetrical tool consisting primarily of abrasive grains embedded in a bonding agent, all deposited on a support perpendicular to the wheel's axis of revolution. For example, the bonding agent can be resins, ceramics, or metals. The abrasive grains are, for instance, diamond grains. The bonding agent essentially defines a surface opposite the support, onto which the abrasive grains are exposed, making the wheel's surface abrasive and capable of mechanically attacking the polycrystalline material of the front face 2a of the disc 2.

[0040] In a mechanical grinding machine, the disc 2, fixed to the plate E2, is rotated around an axis of rotation called the plate's axis of rotation z2, parallel to the (z) axis of the orthonormal reference frame. The grinding wheel E1 is also rotated around another axis of rotation called the wheel's axis of rotation z1, also parallel to the (z) axis but offset from the plate's axis of rotation z2 in the principal (x,y) plane. The grinding wheel E1 is then moved downwards along the (z) axis until it contacts the face of the disc 2 to be thinned. A continuous flow of liquid (usually water) is sprayed into the contact area between the wheel E1 and the disc 2 to cool and remove dust from the material being ground on the disc 2 and from the wear on the wheel E1.

[0041] The preparation process thus includes a step b), called the first material removal step, involving an initial descent speed of the grinding wheel E1. Preferably, the first descent speed is between 0.15 μm / s and 0.25 μm / s. It is relatively high to ensure the initial attack efficiency of the disc 2 surface. The duration of step b) is relatively short, preferably between 5 s and 40 s.

[0042] Typically, step b) involves a first rotational speed of the grinding wheel between 1200 rpm and 3000 rpm, and a first rotational speed of the platter between 100 rpm and 500 rpm. The coolant flow rate can typically vary between 3 and 4 L / min.

[0043] During this step b), the relative axial movement of the grinding wheel E1 and the disc 2 along the (z) axis allows pressure to be applied to the front face 2a. Under the effect of the pressure, the abrasive grains of the wheel act as cutting tools, removing tiny chips of material from the front face 2a. As the abrasive grains become dull, the pressure and heat generated by the machining cause the dull abrasive grains to break and then cleave, exposing new abrasive grains with sharp edges and thus regenerating the abrasiveness of the grinding wheel surface.

[0044] The range chosen for the first descent speed allows for good regeneration of the grinding wheel E1 surface; thus, during step b), the mechanical resistance due to contact friction (resistance opposing the rotation of the grinding wheel, usually called "load," and generally measured by grinding equipment) tends to decrease. Conversely, the strong regeneration of the wheel leads to more pronounced marking (in the form of striations or grooves, synonymous with roughness) on the thinned face due to the presence of new and very sharp abrasive grains.

[0045] This is why the preparation process includes a step c), called the second material removal step, involving a second descent speed of the grinding wheel E1, lower than the first descent speed. Preferably, the second descent speed is between 0.05 μm / s and 0.15 μm / s. Reducing the wheel's descent speed allows for a less aggressive cutting regime than in step b).

[0046] The range chosen for the second descent speed allows for more limited regeneration of the grinding wheel surface E1; thus, during step c), the abrasive grains of the wheel remain in a dull state for longer and produce a gentler attack (fewer induced marks) on the front face 2a. The mechanical resistance due to contact friction (resistance opposing the rotation of the grinding wheel, "load") tends to increase over time (). It has been observed that this increase in mechanical resistance, at least in the final moments of step c), improves the overall surface finish of the front face 2a.This can be observed, in particular, on haze maps (diffuse background noise signal) obtained using full-plate inspection systems, such as the KLA-Tencor Surfscan SP1 or the Lasertec SICA88, which employ laser scanning techniques to scan the surface and recover the scattered optical signal. Haze maps or average haze values ​​are characteristic of the roughness of the inspected surface: generally, the greater the roughness, the greater the diffusion of that surface and the higher the measured haze.

[0047] Typically, step c) involves a second rotational speed of the grinding wheel E1 between 1200 rpm and 3000 rpm, and a second rotational speed of the platen E2 between 100 rpm and 500 rpm. The coolant flow rate can typically vary between 3 and 4 L / min.

[0048] Advantageously, the second rotational speed of the grinding wheel E1 is lower than the first rotational speed, and the second rotational speed of the plate E2 is lower than the first rotational speed. This allows for the smoothest possible cutting action.

[0049] It is in step c) that the majority of the material removal in the preparation process according to the invention takes place, down to the final target thickness. Its duration is therefore adjusted to the thickness of material to be removed. Typically, it can vary between 30 and 200 seconds.

[0050] Step b) preferentially induces a material shrinkage of less than or equal to 1 μm, preferably between 0.4 μm and 0.8 μm. Step c) preferentially induces a material shrinkage of less than 5 μm, preferably between 3 μm and 4 μm.

[0051] According to an advantageous variant, the preparation process includes a third material removal step (c2), after step (c) and before step (d), involving a third descent speed of the grinding wheel E1 that is higher than the second descent speed of the grinding wheel E1. The third descent speed can be between 0.15μm / s and 0.25μm / s and its duration is typically between 2s and 25s.

[0052] Step c2) is relatively short; its objective is to increase mechanical resistance (load), as illustrated on the, without returning to an aggressive cutting regime such as in step b) by regeneration (re-sharpening) of the wheel E1, because the latter, in this regime, tends to mark the surface.

[0053] Typically, step c2) involves a third rotational speed of the grinding wheel E1 between 1200 rpm and 3000 rpm, and a third rotational speed of the platen E2 between 100 rpm and 500 rpm. The coolant flow rate can vary between 3 and 4 L / min.

[0054] As stated in reference to step c), the claimant observed a benefit from an increase in load (which reflects an increase in friction between wheel E1 and front face 2a), a few seconds to a few tens of seconds before switching to the next step d).

[0055] Preferably, step c2) induces a material removal of less than 0.5μm, preferably between 0.2μm and 0.3μm.

[0056] Finally, the preparation process according to the invention includes a step d) called the finishing step, which involves a descent speed of zero and a duration greater than or equal to 2 minutes ( ). The grinding wheel E1 is therefore in a stationary vertical position opposite the front face 2a of the disc 2.

[0057] Holding the grinding wheel in rotation without axial movement for a period ranging from a few seconds to several tens of seconds is typically performed at the end of the grinding process ("spark out"), before raising the grinding wheel. This helps prevent significant fluctuations in the motor's power supply current. Current practices suggest holding the wheel in rotation without axial movement for longer than 15 seconds, or even 30 or 40 seconds, to reduce surface roughness.

[0058] Step d) of the process goes well beyond the durations proposed by the prior art, by proposing a very significant extension of the duration of holding the grinding wheel E1 in a stationary vertical position, at the end of step c): this brings an unexpected effect of exacerbated smoothing in a peripheral annular region of the front face 2a.

[0059] This presents two curves illustrating the local haze value along the diameter of two 200 mm p-SiC support substrates. Curve (A) corresponds to a support substrate obtained by a prior art process, with a spark-out time of less than 1 min (typically 15 s, 30 s, or 40 s). Curve (B) corresponds to a support substrate obtained by a preparation process according to the present invention (in particular, a process according to the detailed embodiment described later). A significant reduction in haze can be observed on the front face 20a, in a peripheral annular region extending over ½ to ¾ of the radius of the support substrate 20. The central part of the front face also shows a reduced haze compared to the prior art substrate.

[0060] The combination of step c), providing a gentle cutting regime and an increase in mechanical strength at the end of the step (optionally accentuated by step c2), and step d), maintaining the substrate at a stationary height for an extended period, allows for a significant improvement in the surface finish of the resulting support substrate 20. This presents statistics relating to an average haze value measured on a plurality of p-SiC support substrates 20 obtained according to the invention and on a plurality of p-SiC support substrates obtained by a prior art process. The average haze value for each substrate is calculated by taking the average of the haze values ​​obtained at all points on the front face 2a, using SURFSCAN SP1 type inspection equipment (KLA company), and adding to this average three times the standard deviation of the distribution of said values. Each point on the haze graph represents a support substrate.We note the drastic improvement in surface condition provided by the process according to the invention, in addition to excellent reproducibility resulting in a very tight distribution of "haze" values.

[0061] The average value of "haze", as reported on the, is targeted to be less than or equal to 100 (ua), or even 80, or even preferentially 50, and thus agrees with the expected roughness target (less than or equal to 1nm RMS).

[0062] Advantageously, the duration of step d) is greater than or equal to 3 min, preferably between 3 min and 5 min.

[0063] Step d) may involve a grinding wheel rotation speed between 1200 rpm and 3000 rpm, and a disc rotation speed between 100 rpm and 500 rpm. The coolant flow rate may be between 3 and 4 L / min.

[0064] The mechanical grinding sequence according to the process can be applied regardless of the characteristics of the grinding wheel E1 (more or less coarse or fine). It is, however, particularly suited to fine grinding, at the end of which a surface roughness on the front face 20a of the support substrate 20 is targeted, compatible with a molecular adhesion assembly step. As mentioned in the introduction, a roughness of less than or equal to 1 nm RMS is targeted by AFM measurement (5x5 μm scan). 2 or more) or by optical profilometry (for example on WYKO-type equipment, at x10 magnification). It is even advantageous to achieve a roughness less than or equal to 0.8nm RMS.

[0065] The mechanical grinding sequence according to the invention therefore preferentially provides a low material removal rate (preferably less than 10 μm, less than 8 μm, or even less than 5 μm) and is carried out with a fine-grain grinding wheel. The grinding wheel may, in particular, comprise abrasive grains having an average size less than or equal to 4 μm, less than or equal to 2 μm, or even less than or equal to 1 μm, which corresponds respectively to a wheel mesh size greater than 8000, greater than 10000, or even greater than 20000.

[0066] For example, wheel references GFSC-30D-VH08A (Disco company) [mesh 20k, average grain size 1µm] or W8000VG-I348-C3W5X-320 or HW8000VB-I144 (Accretech company) [mesh 8000, average grain size 2.5µm] can be used.

[0067] In a preferred embodiment, at least the front face 2a of the disc 2 has a work-hardened surface region, induced by coarse grinding prior to the application of the preparation process according to the invention. A work-hardened surface region is characterized by the presence of significant surface scratches (resulting in, in particular, in high surface roughness) and the presence of cracks and stresses in the material. It has a typical thickness of between 0.5 μm and 5 μm, or even greater than 5 μm for the very large average abrasive grain sizes used during very coarse grinding. The work-hardened surface region can be characterized, in particular, by transmission electron microscopy (TEM or TEM) or by scanning electron microscopy (SEM or SEM).

[0068] Coarse grinding may for example have been carried out with one (or more) grinding wheel(s) comprising abrasive grains with an average size greater than or equal to 5μm, greater than or equal to 18μm, or even greater than or equal to 50μm, which corresponds respectively to a wheel mesh of less than 4000, less than 1000, or even less than 350.

[0069] The roughness of the front face 2a of the disk 2, when it reaches step a), can then be on the order of 10 to 50 nm RMS (AFM or optical profilometer), respectively after coarse grinding with a wheel mesh of 2000 and 1000.

[0070] In this preferred embodiment, the mechanical grinding sequence of the preparation process aims to remove all or part of the work-hardened surface region.

[0071] Note that heat treatments and / or chemical cleaning can be applied to the disc 2 or the support substrate 20 after its preparation by the process of the invention. The support substrate 20 typically has a thickness of between 300 and 500 μm, for diameters of 150 mm or 200 mm.

[0072] The invention also relates to a method of manufacturing a composite structure 100 comprising the preparation of a support substrate 20 in a polycrystalline material in accordance with the previously described method, and the transfer of a thin layer 10 in monocrystalline material, from a donor substrate 1 onto said support substrate 20.

[0073] Although any known thin-film transfer process can be used, the Smart Cut process is particularly noteworthy. TM, based on the formation of a fragile plane buried in a donor substrate of monocrystalline material, by ionic implantation of light species (for example, H, He or a combination of these two species), and on a direct assembly (by molecular adhesion) between said donor substrate and the support substrate 20. A separation in the buried fragile plane then allows the transfer of a thin monocrystalline layer 10, from the donor substrate, onto the support substrate 20 ( ), while preserving the rest of the donor substrate for future reuse.

[0074] The single-crystal thin film 10 can notably be made of silicon carbide, diamond, silicon, II-VI or III-V semiconductor compounds (e.g. AlN, GaN,...), gallium oxide (Ga2O3), or any wide bandgap semiconductor material.

[0075] As an example, the thin film 10 of the composite structure 100 has a thickness ranging from a few tens of nm to a few hundred nm, for example, between 50 nm and 800 nm. Epitaxial steps can be performed on this thin film 10 to thicken it (homoepitaxy) or to grow other materials (heteroepitaxy), depending on the requirements of the electronic components to be manufactured. The thin film 10 has an electrical resistivity suitable for the application and the intended components.

[0076] The composite structure 100 may include a continuous or discontinuous intermediate layer 30, disposed between the thin film 10 and the support substrate 20 and composed of at least one metallic or semiconductor material. As is known per se, with reference to a manufacturing process for the composite structure 100, the intermediate layer 30 may be formed on the side of the thin film 10, on the side of the support substrate 20, or on both sides, prior to assembly along a bonding interface between the donor substrate and the support substrate 20. The intermediate layer 30 may, for example, be composed of silicon, silicon carbide, tungsten, and / or titanium. Its thickness is typically between a few nm and a few hundred nm, preferably between 2 nm and 50 nm.The assembly is advantageously carried out by a known molecular adhesion bonding technique, for example by ADB (atomic diffusion bonding) or direct plasma activation bonding.

[0077] The very low surface roughness of the front face 20a of the support substrate 20 gives excellent quality to the bonding interface: low defect, high bonding force, good bonding at the edges (low delamination), etc.

[0078] The resulting composite structure 100 is extremely robust to very high temperature heat treatments that may be applied to improve the quality of the thin film 10 or to manufacture components on and / or in said layer 10. The composite structure 100 is particularly suitable for the development of one (or more) electronic component(s), especially high voltage, such as for example Schottky diodes, MOSFET or HEMT transistors, and / or high frequency (RF), in and / or on the thin film. Example of a completed project

[0079] Chemical vapor deposition (CVD) is performed to grow a polycrystalline SiC layer on a 200 mm diameter graphite growth substrate at a temperature typically between 1200°C and 1400°C. At the end of the deposition, the growth substrate is removed to obtain a raw p-SiC disk with two faces. The thickness of the raw disk is typically between 700 μm and 3000 μm.

[0080] A rectification of the edges of the raw disc and a thinning of the two faces of the raw disc by coarse grinding are then applied, to obtain a regular chamfered edge on all the perimeter, a diameter of 200mm and a thickness of 355μm, and form disc 2. The surface roughness of the front face 2a of disc 2 is on the order of 50 nm RMS.

[0081] The preparation process according to the invention is then applied. Loaded onto the chuck E2 of a grinding machine (step a), the disc 2 undergoes successive steps b), c) and d) of the process:

[0082] b) first stage of material removal: grinding wheel with 10000 mesh, first wheel descent speed 0.15 μm / s, first wheel rotation speed 2200 rpm, first plate rotation speed 400 rpm, duration 10 s => i.e. a material removal of 0.5 μm;

[0083] c) second material removal stage: 10000 mesh grinding wheel, second wheel descent speed 0.1 μm / s, second wheel rotation speed 2200 rpm, second plate rotation speed 400 rpm, duration 150 s => i.e. a material removal of 3 μm;

[0084] d) Finishing stage: 10000 mesh grinding wheel, wheel descent speed 0 μm / s, wheel rotation speed 2200 rpm, plate rotation speed 400 rpm, duration 3 min.

[0085] In an advantageous variant of this embodiment, step c2) is applied between step c) and step d):

[0086] c2) third stage of material removal: grinding wheel with 10000 mesh, third wheel descent speed 0.15 μm / s, third wheel rotation speed 2200 rpm, third plate rotation speed 400 rpm, duration 5 s => i.e. a material removal of 0.25 μm;

[0087] The haze value obtained on the front face 20a of the support substrate 20 is approximately 50 ppm (measured on Surfscan SP1). The roughness value measured on the front face 20a is approximately 0.8 nm by AFM 5x5 µm, which is perfectly compatible with a layer transfer process.

[0088] A single-crystal SiC donor substrate is implanted with hydrogen ions at a depth of 850 nm (120 keV - 6 e 16 H / cm 2 ) to generate a fragile, buried plane. The donor substrate and the support substrate 20 are introduced into an ADB bonding device. A 4nm silicon film is deposited on their respective front faces, then direct bonding is performed between these two faces, sandwiching the two silicon films and forming an intermediate layer buried in a bonded assembly.

[0089] A separation anneal is applied to the bonded assembly at approximately 1000°C to induce the development of microcavities / microcracks in the embedded weak plane and to create a fracture within that plane. This allows for the separation of the composite structure on one side and the remaining donor substrate on the other. The composite structure is then processed (polishing, heat treatments, cleaning, etc.) to restore perfect crystalline quality to the transferred thin layer and to strengthen the bond interface.

[0090] Of course, the invention is not limited to the embodiments and examples described, and alternative embodiments can be made without departing from the scope of the invention.

Claims

Method for preparing a support substrate (20) from a disc (2) made of polycrystalline material having a front face (2a) and a rear face (2b), based on a mechanical grinding sequence of the front face (2a), said sequence comprising: a) the arrangement of the rear face (2b) of the disc (2) on a plate suitable for rotation and arranged parallel to a grinding wheel, itself suitable for rotation and for moving vertically until it comes into contact with the front face (2a) to effect a mechanical removal of material; b) a first step of material removal, involving a first descent speed of the grinding wheel; c) a second step of material removal, involving a second descent speed of the grinding wheel lower than the first descent speed;d) a finishing step, involving a zero descent speed of the grinding wheel, for a duration greater than or equal to 2 minutes, at the end of which the substrate support (20) is obtained.; A preparation method according to claim 1, wherein the polycrystalline material is silicon carbide or aluminum nitride or silicon. A preparation method according to any one of the preceding claims, wherein the first descent velocity is between 0.15μm / s and 0.25μm / s. A preparation method according to any one of the preceding claims, wherein the second descent velocity is between 0.05μm / s and 0.15μm / s. A preparation method according to any one of the preceding claims, wherein the second rotational speed of the grinding wheel is less than the first rotational speed of the grinding wheel, and the second rotational speed of the platter is less than the first rotational speed of the platter. A preparation method according to any one of the preceding claims, wherein step b) induces a material shrinkage of less than 1μm, preferably between 0.2μm and 0.8μm. A preparation method according to any one of the preceding claims, wherein step c) induces a material shrinkage of less than 5μm, preferably between 3μm and 4μm. Preparation method according to any one of the preceding claims, wherein, in step a), at least the front face (2a) of the disc (2) has a work-hardened surface region, induced by a rough grinding carried out beforehand. Preparation method according to any one of the preceding claims, wherein the grinding wheel comprises abrasive grains having an average size less than or equal to 4μm, less than or equal to 2μm, or even less than or equal to 1μm. A preparation method according to any one of the preceding claims, wherein the duration of step d) is greater than or equal to 3 min, preferably between 3 min and 5 min. Preparation process according to any one of the preceding claims, comprising a third material removal step c2), after step c) and before step d), involving a third grinding wheel descent speed greater than the second grinding wheel descent speed. Preparation method according to the preceding claim, wherein the third descent velocity is between 0.15μm / s and 0.25μm / s. Preparation process according to one of the two preceding claims, wherein step c2) induces a material shrinkage of less than 0.5μm, preferably between 0.2μm and 0.3μm. Method of manufacturing a composite structure (100) comprising: - the preparation of a support substrate (20) in a polycrystalline material according to the method according to any one of claims 1 to 13, and - the transfer of a thin layer (10) in a monocrystalline material, from a donor substrate, onto said support substrate (20). Method of manufacturing a composite structure (100) according to the preceding claim, wherein the single-crystal material is selected from silicon carbide (SiC), gallium nitride (GaN), gallium oxide (Ga2O3), diamond, silicon, germanium, indium phosphide (InP).

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