Method for preparing the front face of a polycrystalline silicon carbide slab

Abstract

The invention relates to a method for polishing the front face of a polycrystalline silicon carbide slab comprising a surface region at least partially work-hardened under the effect of grinding, comprising: - relative movement of a rotating grinding wheel and the polycrystalline silicon carbide slab until, with the rotating grinding wheel in contact with the front face of the slab, a layer of the polycrystalline silicon carbide slab has been removed, said layer comprising the at least partially work-hardened surface region and having a thickness of less than or equal to 3 µm; - stopping the relative movement and maintaining the rotating grinding wheel in contact with the front face of the polycrystalline silicon carbide slab for a period of more than 15 seconds.

Description

[0001] DESCRIPTION

[0002] Process for preparing the front face of a polycrystalline silicon carbide plate

[0003] TECHNICAL FIELD

[0004] The invention relates to a method for preparing the front face of a polycrystalline silicon carbide wafer, a method for manufacturing a polycrystalline silicon carbide wafer using the method for preparing the front face of said wafer, and a method for preparing a multilayer structure using the method for manufacturing a polycrystalline silicon carbide wafer.

[0005] STATE OF THE ART

[0006] Silicon carbide (SiC) is increasingly widely used in power electronics applications, particularly to meet the needs of emerging electronics fields such as electric vehicles. Power devices and integrated power systems based on single-crystal SiC can effectively handle much higher power density than their traditional silicon counterparts, even with smaller active area dimensions.

[0007] Monocrystalline SiC substrates for the microelectronics industry remain expensive and difficult to source in large quantities. It is therefore advantageous to use layer transfer solutions to develop composite structures typically comprising a thin monocrystalline SiC (m-SiC) layer on a lower-cost polycrystalline SiC (p-SiC) support substrate.

[0008] A well-known thin-film transfer solution is the Smart Cut™ process. Such a process makes it possible, for example, to manufacture a composite structure comprising a thin monocrystalline SiC layer, taken from a monocrystalline SiC donor substrate, in direct contact with a polycrystalline SiC support substrate.

[0009] Such a polycrystalline SiC support substrate may for example be formed by vapor deposition of p-SiC on a growth substrate (for example a graphite substrate), so as to form a relatively thick p-SiC plate (generally referred to by the English term slab) (for example 0.4 to 3 mm thick) followed by removal of the growth substrate and thinning of the p-SiC plate, so as to obtain one or more p-SiC wafers having a desired shape (in particular a beveled edge) and a desired thickness. The thinning comprises for example successively very coarse thinning (by electroerosion or grinding) for example removing a thickness of the order of 150 μm or more, coarse grinding for example removing a thickness of the order of 20 μm and fine grinding for example removing a thickness of the order of 3 μm.Preferably, such thinning is carried out on the front and back faces of the polycrystalline silicon carbide plate. When the polycrystalline silicon carbide plate is thinner, for example with a thickness of 400 μm, very rough grinding can be omitted.

[0010] Thinning makes it possible to obtain a self-supporting wafer, i.e. one whose thickness is such that it does not break or deform plastically under the effect of its own weight. Such a thickness is, for example, greater than or equal to 325 μm, preferably of the order of 350 μm.

[0011] Thinning the plate can be followed by surface finishing steps aimed in particular at making it smoother.

[0012] However, it remains difficult to achieve good quality bonding between two monocrystalline SiC and polycrystalline SiC substrates because managing the roughness and surface condition of said substrates is complex.

[0013] For example, an atomic diffusion bonding (ADB) involving a 10 nm silicon layer requires a roughness of less than 10 Å RMS. Such a bonding involves the deposition of silicon in a vacuum, then bringing the two substrates into contact. The structure is then heated to allow the diffusion of the silicon and thus the bonding of the said substrates.

[0014] If the use of a grinding wheel with very small grains (size less than 1.5 pm, i.e. a mesh greater than 15,000) makes it possible to obtain a very low roughness on a monocrystalline material, the performances achieved with this same type of grinding wheel on a polycrystalline material such as polycrystalline silicon carbide are neither reproducible nor sufficiently satisfactory to allow effective bonding of a monocrystalline SiC substrate with a polycrystalline SiC substrate. Indeed, the removal of material by the teeth of such a type of grinding wheel is non-uniform at the level of the grains and at the level of the grain boundaries, thus generating a form of roughness.

[0015] Furthermore, although chemical mechanical polishing, a smoothing technique known to those skilled in the art as CMP (from its English acronym Chemical Mechanical Polishing), is an effective process for monocrystalline substrates, it also leads to the highlighting of grain boundaries on a polycrystalline SiC substrate and therefore to a roughness that is too great to allow good bonding of a monocrystalline SiC substrate with a polycrystalline SiC substrate.

[0016] BRIEF DESCRIPTION OF THE INVENTION

[0017] An aim of the invention is to design a method for preparing the front face of a p-SiC substrate to enable good quality bonding of a thin layer of a monocrystalline material on said front face.

[0018] To this end, the invention proposes a method for polishing the front face of a polycrystalline silicon carbide plate comprising a surface region at least partially work-hardened under the effect of grinding, comprising:

[0019] - the relative movement of a rotating grinding wheel and the polycrystalline silicon carbide plate until, with the rotating grinding wheel in contact with the front face of the plate, a layer comprising the at least partially work-hardened surface region and having a thickness less than or equal to 3 μm of the polycrystalline silicon carbide plate is removed,

[0020] - stopping the relative movement and keeping the grinding wheel rotating in contact with the front face of the polycrystalline silicon carbide plate for a period of more than 15 seconds.

[0021] Maintaining the rotating grinding wheel in contact with the front face of the polycrystalline silicon carbide plate for a period of time allows the grinding wheel to be used in "friction" mode to polish the front face of the p-SiC plate, without removing material. The exposure of the grain boundaries of the p-SiC substrate observed in more conventional polishing processes, such as chemical-mechanical polishing, is then avoided, which therefore makes it possible to obtain a level of roughness of the front face compatible with the bonding of a layer of a monocrystalline material.

[0022] Some preferred but non-limiting aspects of this method are as follows: the grinding wheel has a hardness greater than the maximum hardness allowing the removal of an unworked layer of the same thickness as the at least partially work-hardened surface region and made of polycrystalline silicon carbide; the grinding wheel comprises diamond grains coated in a binder, the size of the diamond grains being greater than 2 pm (mesh 12000); during the relative movement of the grinding wheel and the polycrystalline silicon carbide plate, the relative speed of the grinding wheel and the front face of the polycrystalline silicon carbide plate is between 0.05 pm / s and 0.5 pm / s, preferably between 0.1 pm / s and 0.45 pm / s, more preferably between 0.15 pm / s and 0.45 pm / s, until the thickness of the polycrystalline silicon carbide plate is removed to a thickness of less than or equal to 3 pm;maintaining the grinding wheel in rotation in contact with the front face of the polycrystalline silicon carbide plate is implemented for a duration greater than or equal to 30 seconds, preferably a duration greater than or equal to 40 seconds, maintaining the grinding wheel in rotation in contact with the front face of the polycrystalline silicon carbide plate for a duration greater than or equal to 30 seconds, preferably a duration greater than or equal to 40 seconds, is split into several sequences;it further comprises, after stopping the relative movement and maintaining the grinding wheel in rotation in contact with the front face of the polycrystalline silicon carbide plate, an additional step of relative movement of the grinding wheel and the front face of the polycrystalline silicon carbide plate until removing, with the grinding wheel rotating in contact with the front face of the plate, an additional thickness of the polycrystalline silicon carbide plate less than or equal to 1 μm; the rotation speed of the grinding wheel during polishing is between 10 m / s and 45 m / s, preferably between 10 m / s and 25 m / s; during polishing, the front face of the polycrystalline silicon carbide plate is rotated at a speed between 200 rpm and 600 rpm.;

[0023] The invention extends to a method for manufacturing a polycrystalline silicon carbide plate comprising thinning by grinding a thick polycrystalline silicon carbide plate on at least its front face, so as to obtain a thinned polycrystalline silicon carbide plate and polishing the front face of the thinned polycrystalline silicon carbide plate in accordance with the invention. The thinning of the thick polycrystalline silicon carbide plate on at least its front face can be carried out by means of a grinding wheel whose grain size is between 10 μm (2000 mesh) and 30 μm (600 mesh).

[0024] The invention further extends to a method for preparing a multilayer structure comprising manufacturing a polycrystalline silicon carbide wafer in accordance with the invention, providing a donor substrate for a layer to be transferred, and transferring the layer to be transferred from the donor substrate to the front face of the polycrystalline silicon carbide wafer. The donor substrate may be a monocrystalline silicon carbide substrate. The transfer step may comprise forming a weakening zone by implanting atomic species in the donor substrate so as to delimit the layer to be transferred, bonding the implanted face of the donor substrate to the front face of the polycrystalline silicon carbide wafer, and detaching the donor substrate along the weakening zone. The bonding may be direct bonding, for example atomic diffusion bonding. BRIEF DESCRIPTION OF THE FIGURES

[0025] Other characteristics and advantages of the invention will emerge from the detailed description which follows, with reference to the appended drawings, in which:

[0026] - Figure 1 shows an abrasive grain in the binder of a grinding wheel, before (left representation) and after (right representation) blunting.

[0027] - figure 2 represents different states of wear of an abrasive grain at the periphery of a grinding wheel,

[0028] - Figure 3 compares the bonding efficiency and Haze value of polycrystalline silicon carbide plates (arbitrary unit indicative of roughness) whose front face was prepared according to a state-of-the-art method with the bonding efficiency and Haze values ​​of plates whose front face was prepared according to two different embodiments of the method of the invention.

[0029] For readability reasons, the drawings are not necessarily drawn to scale.

[0030] DETAILED DESCRIPTION OF EMBODIMENTS

[0031] The invention relates to the polishing of the front face of a p-SiC substrate to enable good quality bonding of a thin layer of a monocrystalline material on said front face. This surface preparation is carried out using a grinding tool, such as a grinding wheel, instead of the tools conventionally used for polishing, such as chemical-mechanical polishing tools. Indeed, chemical-mechanical polishing or CMP, makes it possible to achieve very low roughness on monocrystalline substrates. However, this process reveals the grain boundaries of a polycrystalline silicon carbide substrate, thus creating a new form of roughness.

[0032] The invention relates more particularly to the polishing of the front face of a polycrystalline silicon carbide plate which comprises, at least on its front face side, an at least partially work-hardened surface region generated by grinding. Such grinding could be implemented to previously thin the polycrystalline silicon carbide plate on at least its front face, preferably on its front face and on its rear face, for example using a coarse-grained grinding wheel (typically abrasive grains of size between 10 μm and 18 μm, i.e. a mesh of between 1000 and 2000).

[0033] Any grinding leaves an at least partially work-hardened surface region, also known to those skilled in the art as a “damaged layer.” In other words, following grinding, the plate has a surface region on its front face in which the crystal is disorganized with scratches and fractured areas. The thickness of the work-hardened surface region depends on the grinding used: the coarser the grinding, the thicker the work-hardened surface region. The thickness of the work-hardened surface region is typically between 500 nm and 1 pm.

[0034] With reference to figures 1 and 2, a grinding wheel is a tool with rotational symmetry mainly comprising grains of an abrasive material - called abrasive grains 1 - embedded in a binder 2 having pores 3, the whole being deposited on a support perpendicular to the axis of revolution of the grinding wheel. For example, the binder 2 may comprise resins, ceramics, metals. The abrasive grains 1 are for example diamond grains. The binder 2 generally defines a surface opposite the support on which abrasive grains 1 are exposed, making said surface abrasive.

[0035] Such grinding wheels are conventionally used to remove material in order to thin substrates in so-called grinding steps.

[0036] Typically, the substrate to be thinned 4 is brought into contact on its front face with the abrasive surface of a rotating grinding wheel (see figure 1). A relative axial displacement of the grinding wheel and the substrate to be thinned along the axis of revolution of the grinding wheel makes it possible to apply pressure to the front face of said substrate 4. Under the effect of the pressure, each grain of abrasive material on the periphery of the grinding wheel then acts as a separate cutting tool which blows off a tiny chip of the substrate to be thinned on its front face. As the abrasive grains become blunt (see figure 1, passage from left to right), the pressure and heat generated by machining cause the blunt abrasive grains to break and then split (see figure 2), so as to cause new abrasive grains with brand new sharp edges to appear on the periphery and thus regenerate the abrasiveness of the surface of the grinding wheel.The removal of material from the substrate to be thinned continues with new abrasive grains as long as the grinding wheel and the substrate are in relative translational movement.

[0037] The invention consists in using such a grinding wheel, not simply to remove material from a polycrystalline silicon carbide plate, but also to polish the front face of said plate, by contact between the front face of the plate and the abrasive surface of the rotating grinding wheel.

[0038] To this end, the method, according to the invention, for polishing the front face of a polycrystalline silicon carbide plate comprising an at least partially work-hardened surface region as previously described comprises:

[0039] - the relative movement of the rotating grinding wheel and the polycrystalline silicon carbide plate until, with the rotating grinding wheel in contact with the front face of the plate, a layer comprising the at least partially work-hardened surface region and having a thickness less than or equal to 3 μm of the polycrystalline silicon carbide plate is removed,

[0040] - stopping the relative movement and keeping the grinding wheel rotating in contact with the front face of the polycrystalline silicon carbide plate for a period of more than 15 seconds.

[0041] The relative movement of the grinding wheel and the polycrystalline silicon carbide plate until at least the at least partially work-hardened surface region is removed makes it possible to blunt the grains of the grinding wheel without promoting their cleavage. Thus, when the grinding wheel is kept rotating in contact with the front face of the plate following the cessation of the relative movement of the plate and the grinding wheel, it is the blunt grains that rub against the front face of the plate and not very sharp new grains. This friction ensures gentle polishing of said front face without revealing the joints between the silicon carbide grains.

[0042] Maintaining the grinding wheel in rotation in contact with the front face of the polycrystalline silicon carbide plate without relative axial displacement of the plate and the grinding wheel for a period of less than 5 seconds can be conventionally carried out to preserve the grinding tool, for example to avoid large variations in the motor supply current. Maintaining the grinding wheel in rotation in contact with the front face of the polycrystalline silicon carbide plate for a period of more than 15 seconds in accordance with the invention makes it possible to observe a reduction in surface roughness and a significant polishing effect.

[0043] In the following, we will focus on the choice of the grinding wheel for implementing the method according to the invention.

[0044] There are different types of grinding wheels, depending for example on the size of the abrasive grains of the wheels or their hardness.

[0045] It should be noted that the hardness of the grinding wheel is defined as a measure of the holding force of the grains in the grinding wheel. The harder a grinding wheel is, the less easily the dull grains detach from the grinding wheel, so as to regenerate the abrasive surface of said grinding wheel. Conversely, the less hard the grinding wheel is, the more easily the rows of abrasive grains cleave and succeed each other. The hardness of the grinding wheel depends on the nature of the bond, the porosity and the size of the abrasive grains. To remove material, the skilled person traditionally chooses a grinding wheel that is not too hard, so as to constantly regenerate cutting grains. In addition, a grinding wheel that is too hard is less effective at removing material and creates a risk of heating - and therefore breakage of the grinding wheel or plate - and seizure - therefore scratches of the plate.

[0046] To implement the polishing method according to the invention, the inventors recommend the use of a harder grinding wheel than the grinding wheels conventionally used for removing material. Indeed, the partially work-hardened nature of the surface region to be removed advantageously allows the abrasive grains of the harder grinding wheel to be blunted without causing breakage or seizure of said grinding wheel. Furthermore, because the grinding wheel is harder, blunting of the grains without causing them to split is facilitated.

[0047] Thus, a grinding wheel with a hardness greater than the maximum hardness allowing the removal of an unworked layer of the same thickness as the at least partially work-hardened surface region to be removed and made of the same material as said surface region is preferably chosen. In other words, the grinding wheel preferably used is too hard to remove an unworked layer made of the same material and of the same thickness as the partially work-hardened surface region to be removed, in particular without causing breakage or seizure.

[0048] For example, the grinding wheel used is a grinding wheel referenced at ACCRETECH with the letter I or higher in alphabetical order, for example the ACCRETECH HW8000VB-I144 grinding wheel.

[0049] Concerning the choice of the size of the grinding wheel grains, for a monocrystalline substrate, it is known to use grinding wheels having abrasive grains that are all the smaller as one wishes to obtain, following grinding, a surface with low roughness. Thus, a grinding wheel comprising very small grains (typically a size less than 1.5 μm, i.e. a mesh greater than 15,000) makes it possible, for example, to obtain free surfaces of monocrystalline silicon carbide substrates with very low roughness, less than 1 nm RMS.

[0050] The result obtained with this type of very small grain grinding wheel on a polycrystalline silicon carbide plate, however, remains insufficient to allow quality bonding of a monocrystalline substrate on said plate, for example bonding carried out using the Smart Cut ™ process.

[0051] The inventors noticed that grinding wheels with very small grains tend to reveal the grain boundaries of the polycrystalline material, which leads to the high roughness observed. The inventors attribute this effect to the fact that at these grain sizes, the grinding wheel used is necessarily not hard enough, and therefore when the grinding wheel rotates, there is more easily a breakage of the blunt grains and the surfacing of new grains with brand new sharp edges which tend to "scrape" the grain boundaries.

[0052] In the method according to the invention, a grinding wheel with a smaller mesh (therefore larger abrasive grains) is therefore preferably used than the very small grain grinding wheels conventionally used to obtain very low surface roughness, but which has a hardness which is not achievable for these very small grain grinding wheels. Thus, a grinding wheel with a grain size of between 2 μm and 3 μm (mesh between 8000 and 12000) is preferably used. Furthermore, the hardness of said grinding wheel is preferably greater than the hardness achievable by a grinding wheel with a grain size of less than or equal to 1.5 μm (mesh is greater than or equal to 15000).

[0053] This type of grinding wheel advantageously promotes the blunting of the abrasive grains during the removal of the at least partially work-hardened surface region without causing them to break and be replaced by new grains.

[0054] Preferably, the relative movement of the grinding wheel and the silicon carbide plate is implemented so that the total thickness of the material removal does not exceed the thickness of the at least partially work-hardened surface region by more than 2.5 μm. Thus, if the at least partially work-hardened surface region has a thickness of 500 nm, the total thickness of the material removal is preferably less than or equal to 3 μm. Not removing a total thickness of material that exceeds the thickness of the work-hardened surface region by more than 2.5 μm advantageously makes it possible not to expose a new row of new grains with sharp edges on the surface.

[0055] During the relative axial movement of the grinding wheel and the polycrystalline silicon carbide plate, the relative axial speed of the grinding wheel and the front face of the polycrystalline silicon carbide plate is between 0.05 pm / s and 0.5 pm / s, preferably between 0.1 pm / s and 0.45 pm / s, more preferably between 0.15 pm / s and 0.45 pm / s, until the thickness of the polycrystalline silicon carbide plate is removed to a thickness of less than or equal to 3 pm.

[0056] A speed higher than 0.5 pm / s risks causing the cleavage of the line of previously blunted abrasive grains, and therefore the exposure of a new line of abrasive grains that we are trying to avoid, or the breakage of the grinding wheel. A speed lower than 0.005 pm / s is too slow to be economically viable. The rotation speed of the grinding wheel during polishing is for example between 10 m / s and 45 m / s, preferably between 10 m / s and 25 m / s. Thus, for a grinding wheel with a diameter of 300 mm, the rotation speed of the grinding wheel is between 500 rpm and 1500 rpm. A grinding wheel rotation speed lower than 10 m / s unnecessarily increases the duration of the step and is not economically interesting. A speed greater than 45 m / s could cause the previously blunt abrasive grains of the grinding wheel to split and / or break the grinding wheel and / or the polycrystalline silicon carbide plate.

[0057] Figure 3 compares the results obtained by implementing such a method for preparing the front face of a polycrystalline silicon carbide plate (histograms c and d) with the results obtained by implementing a preparation method according to the state of the art (histograms a and b).

[0058] Histograms (a) and (b) correspond to fine grinding removing a thickness of the order of 10 μm using a less hard grinding wheel than that preferably used by the invention. Histograms (c) and (d) correspond to polishing carried out according to the invention by removing a thickness of 10 μm using a hard grinding wheel and then maintaining the grinding wheel in rotation in contact with the front face of the polycrystalline silicon carbide plate for a duration taken here to be less than 5 seconds.

[0059] Histograms (a) and (c) correspond to the percentage of polycrystalline silicon carbide wafers whose front surface condition allows good bonding quality of said front surface with a monocrystalline substrate. In other words, histograms (a) and (c) correspond to the percentage of monocrystalline substrates bonded to a polycrystalline silicon carbide wafer without defects. By defect, for example, is meant a partial bonding of the monocrystalline substrate, the monocrystalline substrate layer not being transferred to certain parts of the polycrystalline silicon carbide wafer. Histograms (b) and (d) correspond to the Haze value measured on a polycrystalline silicon carbide wafer by light scattering, for example using the Surfscan SP1 inspection system from KLA-Tencor or the SICA88 inspection system from Lasertec.More precisely, it is an average value measured at several points on a polycrystalline silicon carbide plate. The Haze value is an indirect tool for measuring the roughness of the front face of the polycrystalline silicon carbide plate. This haze value is derived from a method using the optical reflectivity properties of the surface to be characterized and corresponds to an optical signal scattered by the surface, due to its microroughness: the greater the roughness of the surface, the greater the scattering on said surface and the greater the measured Haze value. Thus, it can be seen that the implementation of the method according to the invention makes it possible to obtain a bonding efficiency of the order of 30%, higher than the efficiency of the method according to which a thickness of the order of 10 μm is removed, which is itself of the order of 10%.Furthermore, the Haze value of a front face of a silicon carbide plate prepared by the method according to the invention is lower than the Haze value of a front face prepared by the method according to the state of the art.

[0060] Maintaining the rotating grinding wheel in contact with the front face of the polycrystalline silicon carbide plate can advantageously be carried out for a period greater than 30 seconds and even more preferably greater than 40 seconds, in order to maintain frictional contact between the grinding wheel and the plate for as long as possible (while retaining the same row of previously blunted abrasive grains).

[0061] Maintaining the rotating grinding wheel in contact with the front face of the polycrystalline silicon carbide plate for a duration greater than 30 seconds, preferably a duration greater than 40 seconds, is preferably split into several sequences, so that the contact between the grinding wheel and the front face of the polycrystalline silicon carbide plate is broken between each sequence by relative movement of the grinding wheel and the polycrystalline silicon carbide plate. In this case, the sum of the durations of each sequence during which the rotating grinding wheel is maintained in contact with the front face of the polycrystalline silicon carbide plate remains equal to the duration greater than 40 seconds chosen.For example, the holding of the grinding wheel may comprise two sequences: a first sequence whose duration is between 5 seconds and 10 seconds, then a second sequence such that the sum of the durations of the first sequence and the second sequence is greater than or equal to 30 seconds, preferably greater than 40 seconds. The splitting of the friction period between the rotating grinding wheel and the front face of the polycrystalline silicon carbide plate makes it possible to avoid heating and therefore the splitting of the line of previously blunted abrasive grains.

[0062] In one possible embodiment, the polishing further comprises, after stopping the relative movement and maintaining the grinding wheel in rotation in contact with the front face of the polycrystalline silicon carbide plate, an additional step of relative movement of the grinding wheel and the front face of the polycrystalline silicon carbide plate. The additional step of relative movement of the grinding wheel and the front face may comprise, initially, breaking the contact between the grinding wheel and the front face and then, in a second step, reestablishing said contact. The additional step of relative movement of the grinding wheel and the front face of the polycrystalline silicon carbide plate is in this case stopped, for example by the operator, as soon as the contact of the grinding wheel and the front face of the polycrystalline silicon carbide plate is reestablished, so as not to split the previously blunt abrasive grains of the grinding wheel.

[0063] The additional step of relative displacement is carried out so as to lead to the removal of a very small additional thickness of the polycrystalline silicon carbide plate, preferably an additional thickness of the polycrystalline silicon carbide plate less than or equal to 1 μm, more preferably a thickness less than 200 nm.

[0064] This additional step of relative movement of the grinding wheel and the front face of the polycrystalline silicon carbide plate makes it possible to further reduce the roughness of the front face of the polycrystalline silicon carbide plate and therefore to improve its surface quality.

[0065] In Figure 3, histograms (e) and (f) represent respectively the yield and the Haze value obtained following the implementation of the protocol followed for histograms (c) and (d) supplemented by this additional step removing a thickness of 1 pm. Thus, Figure 3 highlights the effectiveness of this step, since the yield increases further to reach a value of the order of 80% and the Haze value decreases further compared to histogram (d).

[0066] In a particular embodiment of the invention, the polycrystalline silicon carbide plate is rotated about an X axis perpendicular to the front face of the polycrystalline silicon carbide plate. The grinding wheel is itself rotating about a Y axis perpendicular to the abrasive surface of said grinding wheel, the Y axis being parallel to the X axis.

[0067] The polycrystalline silicon carbide plate is for example a 150 mm diameter plate and the grinding wheel is for example a 300 mm diameter grinding wheel.

[0068] For example, the front face of the polycrystalline silicon carbide plate can be rotated at a speed between 200 rpm and 600 rpm.

[0069] In this embodiment, the relative movement of the grinding wheel and the polycrystalline silicon carbide plate comprises, for example, the axial movement of the grinding wheel in rotation along the Y axis so as to bring the abrasive surface of the grinding wheel into contact with the front face of the polycrystalline silicon carbide plate and then to remove the thickness less than or equal to 3 μm from said plate under the effect of the pressure exerted by the grinding wheel in axial movement.

[0070] Then, the movement of the grinding wheel along the Y axis is blocked, the grinding wheel and the plate remaining in rotation respectively around the X and Y axes, so as to maintain contact between the grinding wheel and the front face of the polycrystalline silicon carbide plate for a period greater than 15 seconds, preferably a period greater than 30 seconds, still preferably a period greater than 40 seconds.

[0071] According to this embodiment, if maintaining the grinding wheel in rotation in contact with the front face of the polycrystalline silicon carbide plate for a period of more than 30 seconds, preferably for a period of more than 40 seconds, is split into several sequences as previously described, breaking the contact between the grinding wheel and the front face can be implemented by translation along the Y axis of the grinding wheel maintained in rotation, so as to move said grinding wheel away from the front face of the plate. For example, maintaining the grinding wheel can comprise a first sequence of 10 seconds, a separation period of one minute during which the contact is broken between the grinding wheel and the front face of the plate and a second sequence of 30 seconds.

[0072] The invention extends to a method for preparing a polycrystalline silicon carbide plate. Said method for preparing a polycrystalline silicon carbide plate comprises thinning, by means of a grinding wheel of a first type, a thick polycrystalline silicon carbide plate on its front face and optionally on its rear face, so as to obtain a thinned polycrystalline silicon carbide plate. Preferably, the polycrystalline silicon carbide plate thinned by means of the grinding wheel of the first type has a thickness greater than or equal to 325 μm, more preferably a thickness of 350 μm.

[0073] The thick polycrystalline silicon carbide plate (e.g. 0.4 mm to 3 mm thick) can for example be made by deposition of p-SiC on a growth substrate (e.g. graphite substrate), typically chemical vapor deposition at a temperature between 1200°C and 1400°C.

[0074] Prior to thinning using the grinding wheel of the first type, one or more very coarse grinding operations may optionally be carried out on the front face, preferably the front face and the rear face, of the thick silicon carbide plate. The grinding wheel of the first type is a grinding wheel comprising abrasive grains coated in a binder, the grain size of the grinding wheel of the first type being between 10 μm and 30 μm (mesh between 600 and 2000). Given the relatively large abrasive grain size of the grinding wheel of the first type, the thinning of the polycrystalline silicon carbide plate therefore corresponds to grinding described as coarse by a person skilled in the art.

[0075] The thinning of the thick polycrystalline silicon carbide plate on its front face by means of the grinding wheel of the first type generates an at least partially work-hardened surface region on said front face, i.e. a region in which the crystal is disorganized having scratches and / or fractured zones. The smaller the mesh of the grinding wheel of the first type (and therefore the size of the abrasive grains of said grinding wheels), the thicker the at least partially work-hardened surface region. Given the mesh range of the grinding wheel of the first type, the thickness of the work-hardened surface region is between 500 nm and 1 pm].

[0076] The method for preparing a polycrystalline silicon carbide plate according to the invention further comprises polishing the front face of the thinned polycrystalline silicon carbide plate using the grinding wheel of the first type, said polishing being carried out according to any embodiment of the polishing method previously described using a grinding wheel of a second type.

[0077] The grinding wheel of the second type preferably has the characteristics of grain sizes and hardness as previously stated.

[0078] Thus, the grinding wheel of the second type preferably has a hardness greater than the maximum hardness allowing the removal of an unworked layer of the same thickness as the at least partially work-hardened surface region generated by the grinding wheel of the first type and made of the same material as said surface region.

[0079] The grinding wheel of the second type preferably has a grain size of between 2 pm and 3 pm (mesh between 8000 and 12000). In addition, the hardness of said grinding wheel of the second type is preferably greater than the hardness achievable by a grinding wheel whose grain size is less than or equal to 1.5 pm (mesh greater than or equal to 15000).

[0080] In this method for manufacturing a polycrystalline silicon carbide plate, the polishing of the front face of said plate previously thinned by grinding using the grinding wheel of the first type comprises the relative movement of the grinding wheel of the second type and the polycrystalline silicon carbide plate until a thickness less than or equal to 3 μm is removed before maintaining the grinding wheel in rotation in contact with the front face of the polycrystalline silicon carbide plate for a period of time greater than 15 seconds, preferably a period of time greater than 30 seconds, more preferably a period of time greater than 40 seconds. The removal of the thickness less than or equal to 3 μm comprises the removal of the at least partially work-hardened surface region previously generated by the thinning carried out using the grinding wheel of the first type.

[0081] Thus, the method according to the invention makes it possible to polish the front face of the polycrystalline silicon carbide plate directly after thinning by rough grinding on at least the front face of said plate, avoiding additional steps of finer grinding.

[0082] The invention further extends to a method of manufacturing a multilayer structure, comprising the polishing of a p-SiC plate as previously described and the transfer of a thin layer of a monocrystalline material from a substrate of said monocrystalline material to the polycrystalline silicon carbide plate.

[0083] The monocrystalline substrate is, for example, a monocrystalline silicon carbide (m-SiC) substrate. Alternatively, the monocrystalline substrate is a gallium nitride (GaN), indium phosphide (InP), diamond, or silicon substrate. Optionally, an intermediate layer of silicon dioxide (SiO2) is arranged between the polycrystalline silicon carbide plate and the monocrystalline substrate.

[0084] The transfer of the thin layer of the monocrystalline material can be carried out using Smart Cut™ technology and thus comprise an implantation of ionic species in the substrate of the monocrystalline material so as to form therein a weakening plane delimiting the thin layer to be transferred, the bonding of the substrate of the monocrystalline material with the polycrystalline silicon carbide plate (where appropriate by means of one or more bonding layers) then the detachment (caused by a heat treatment, a mechanical action, or a combination of these means) of the substrate of the monocrystalline material along the weakening plane so as to transfer the thin active layer to the polycrystalline silicon carbide plate. The substrate of monocrystalline material can be subjected to mechanical or chemical-mechanical polishing before bonding.The method of manufacturing the composite structure may further comprise the formation of electronic components, in particular power or radio frequency components in the transferred thin layer.

[0085] The substrate of the monocrystalline material on the polycrystalline silicon carbide plate is bonded, for example, by a bonding such as an ADB bonding. The ADB bonding comprises, for example, the deposition of a silicon layer with a thickness preferably between 4 nm and 20 nm, more preferably a thickness of 10 nm on each of the surfaces to be bonded. Indeed, a silicon layer with a thickness of less than 4 nm leads to risks of bubbling. Alternatively, the substrate of the monocrystalline material on the polycrystalline silicon carbide plate is bonded by means of a bonding by surface activation SAB (acronym for

[0086] Surface Activated Bonding). The polishing process according to the invention advantageously makes it possible to achieve the very low roughness required by these bonding methods.

Claims

CLAIMS 1. Method for polishing the front face of a polycrystalline silicon carbide plate comprising a surface region at least partially work-hardened under the effect of grinding, said method comprising: - the relative movement of a rotating grinding wheel and the polycrystalline silicon carbide plate until, with the rotating grinding wheel in contact with the front face of the plate, a layer comprising the at least partially work-hardened surface region and having a thickness less than or equal to 3 μm of the polycrystalline silicon carbide plate is removed, - stopping the relative movement and keeping the grinding wheel rotating in contact with the front face of the polycrystalline silicon carbide plate for a period of more than 15 seconds.

2. Method according to claim 1, in which the grinding wheel has a hardness greater than the maximum hardness allowing the removal of an unworked layer of the same thickness as the at least partially work-hardened surface region and made of polycrystalline silicon carbide.

3. Method according to one of claims 1 to 2, in which the grinding wheel comprises diamond grains coated in a binder, the size of the diamond grains being greater than 2 pm (mesh 12000).

4. Method according to one of claims 1 to 3, in which, during the relative movement of the grinding wheel and the polycrystalline silicon carbide plate, the relative speed of the grinding wheel and the front face of the polycrystalline silicon carbide plate is between 0.05 pm / s and 0.5 pm / s, preferably between 0.1 pm / s and 0.45 pm / s, more preferably between 0.15 pm / s and 0.45 pm / s, until the thickness of the polycrystalline silicon carbide plate is less than or equal to 3 pm.

5. Method according to one of claims 1 to 4, in which the grinding wheel is kept rotating in contact with the front face of the polycrystalline silicon carbide plate for a duration greater than or equal to 30 seconds, preferably a duration greater than or equal to 40 seconds.

6. Method according to claim 5, in which maintaining the rotating grinding wheel in contact with the front face of the polycrystalline silicon carbide plate for a duration greater than or equal to 30 seconds, preferably a duration greater than or equal to 40 seconds, is divided into several sequences.

7. Method according to one of claims 1 to 6, further comprising, after stopping the relative movement and maintaining the grinding wheel in rotation in contact with the front face of the polycrystalline silicon carbide plate, an additional step of relative movement of the grinding wheel and the front face of the polycrystalline silicon carbide plate until removing, with the grinding wheel rotating in contact with the front face of the plate, an additional thickness of the polycrystalline silicon carbide plate less than or equal to 1 μm.

8. Method according to one of claims 1 to 7, in which the rotation speed of the grinding wheel during polishing is between 10 m / s and 45 m / s, preferably between 10 m / s and 25 m / s.

9. Method according to one of claims 1 to 8, in which, during polishing, the front face of the polycrystalline silicon carbide plate is rotated at a speed of between 200 rpm and 600 rpm.

10. Method of manufacturing a polycrystalline silicon carbide plate comprising: - thinning by grinding a thick polycrystalline silicon carbide plate on at least its front face, so as to obtain a thinned polycrystalline silicon carbide plate, - polishing the front face of the thinned polycrystalline silicon carbide plate, said polishing being carried out by a method according to one of claims 1 to 9.

11. Method according to claim 10, in which the thinning of the thick polycrystalline silicon carbide plate on at least its front face is carried out by means of a grinding wheel whose grain size is between 10 pm (mesh 2000) and 30 pm (mesh 600).

12. Method for preparing a multilayer structure comprising: - the manufacture of a polycrystalline silicon carbide plate by a method according to one of claims 10 to 11, - the supply of a donor substrate for a layer to be transferred, - the transfer of the layer to be transferred from the donor substrate onto the front face of the polycrystalline silicon carbide plate.

13. The method of claim 12, wherein the donor substrate is a single crystal silicon carbide substrate.

14. Method according to one of claims 12 or 13, in which the transfer step comprises: - the formation of a weakening zone by implantation of atomic species in the donor substrate so as to delimit the layer to be transferred, - the bonding of the implanted face of the donor substrate on the front face of the polycrystalline silicon carbide plate, - detachment of the donor substrate along the weakening zone.

15. Method according to claim 14, in which the bonding is a direct bonding, preferably a bonding by atomic diffusion.

Images