Method for preparing front surface of polycrystalline silicon carbide plate

Abstract

The invention relates to a method for polishing the front face of a polycrystalline silicon carbide plate comprising a surface area which is at least partially damaged by grinding, said method comprising:-a relative movement of a rotationally driven grinding wheel and the polycrystalline silicon carbide plate, the relative movement is carried out with a rotating grinding wheel in contact with the front surface of the plate until a layer comprising at least part of the surface area of the machining damage and having a thickness of less than or equal to 3 [mu] m is removed from the polycrystalline silicon carbide plate, stopping the relative movement and keeping the rotating grinding wheel in contact with the front surface of the polycrystalline silicon carbide plate for more than 15 seconds.

Description

Technical Field

[0001] The invention relates to a method for preparing the front side of a polycrystalline silicon carbide plate, a method for manufacturing a polycrystalline silicon carbide plate using the method for preparing the front side of said plate, and a method for manufacturing a multilayer structure using the method for manufacturing a polycrystalline silicon carbide plate. Background Art

[0002] Silicon carbide (SiC) is increasingly being used in power electronics applications, especially to meet the demands of a growing range of electronic devices, such as electric vehicles. Single-crystal SiC-based power devices and integrated power systems can actually manage much higher power densities than their conventional silicon counterparts, and do so with smaller active area sizes.

[0003] However, substrates made of single-crystal SiC intended for use in the microelectronics industry are still expensive and difficult to supply in large sizes. Therefore, it is advantageous to adopt a layer transfer scheme to produce a composite structure, which generally includes a thin layer made of single-crystal SiC (m-SiC) on a lower-cost polycrystalline SiC (p-SiC) support substrate.

[0004] A well-known thin-layer transfer solution is Smart Cut TM This method makes it possible, for example, to manufacture a composite structure comprising a thin layer made of single-crystalline SiC, taken from a donor substrate made of single-crystalline SiC, in direct contact with a supporting substrate made of polycrystalline SiC.

[0005] Such a support substrate made of polycrystalline SiC can be formed, for example, by vapor depositing p-SiC on a growth substrate (e.g., a graphite substrate) to form a relatively thick p-SiC plate (e.g., 0.4 to 3 mm thick), and then removing the growth substrate and thinning the p-SiC plate to obtain one or more p-SiC wafers having a desired shape (in particular, a chamfered outer edge) and a desired thickness.

[0006] The thinning comprises, for example, sequentially very coarse thinning (by electrical discharge machining or grinding), which will remove, for example, a thickness of more than about 150 μm; coarse grinding, which will remove, for example, a thickness of about 20 μm; and fine grinding, which will remove, for example, a thickness of about 3 μm. Preferably, this thinning is performed on the front and back sides of the polycrystalline silicon carbide plate. When the polycrystalline silicon carbide plate is thinner, for example, with a thickness of 400 μm, the very coarse grinding can be omitted.

[0007] The thinning makes it possible to obtain a self-supporting wafer, in other words a thickness such that it does not break or deform plastically under its own weight. Such a thickness is, for example, greater than or equal to 325 μm, preferably about 350 μm.

[0008] The thinning of the plate may be followed by a surface finishing phase of the wafer, in particular aimed at making it smoother.

[0009] However, it remains difficult to produce high-quality bonds between two single-crystal SiC and multicrystalline SiC substrates because managing the surface condition and roughness of the substrates is complex.

[0010] For example, atomic diffusion bonding (ADB) involving a 10nm silicon layer requires less than RMS roughness. This bonding involves depositing silicon under vacuum and then bringing the two substrates into contact. The structure is then heated to diffuse the silicon and thus enable bonding of the substrates.

[0011] Although the use of grinding wheels with very small grains (less than 1.5 μm in size, i.e. greater than 15000 mesh) makes it possible to obtain very low roughnesses on single-crystalline materials, the quality of performance achieved with this same type of grinding wheel on multicrystalline materials (e.g. polycrystalline silicon carbide) is neither reproducible nor sufficiently satisfactory for effectively bonding a single-crystalline SiC substrate to a polycrystalline SiC substrate. This is because the material removed by the teeth of this type of grinding wheel is not uniform at the grains and at the grain boundaries, thus producing a rough form.

[0012] Furthermore, while chemical mechanical polishing (a smoothing technique known to those skilled in the art as CMP) is an effective method for single crystal substrates, this also results in the display of grain boundaries on polycrystalline SiC substrates and, therefore, in a roughness that is too great to allow good bonding of single crystal SiC substrates to polycrystalline SiC substrates. Summary of the invention

[0013] The object of the invention is to devise a method for preparing the front side of a p-SiC substrate in such a way that a thin layer of single-crystalline material can be bonded to said front side with high quality.

[0014] To this end, the present invention provides a method for polishing the front side of a polycrystalline silicon carbide plate, the polycrystalline silicon carbide plate comprising a surface area at least partially damaged by grinding, the method comprising:

[0015] - moving the rotationally driven grinding wheel and the polycrystalline silicon carbide plate relative to each other until a layer having a surface area including at least part of the machining damage and having a thickness of less than or equal to 3 μm is removed from the polycrystalline silicon carbide plate with the rotating grinding wheel in contact with the front side of the plate,

[0016] - Stop the relative motion and maintain the rotating grinding wheel in contact with the front face of the polycrystalline silicon carbide plate for a period greater than 15 seconds.

[0017] Keeping the rotating grinding wheel in contact with the front side of the polycrystalline silicon carbide plate for a period of time makes it possible to use the grinding wheel in a "rubbing" mode so as to polish the front side of the p-SiC plate without removing material. The manifestation 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 front side roughness level compatible with the bonding of a layer of single-crystalline material.

[0018] Some preferred but non-limiting aspects of the method are as follows:

[0019] - the hardness of the grinding wheel is greater than a maximum hardness, making it possible to remove a non-machine-damaged layer of the same thickness as the at least partially machine-damaged surface area made of polycrystalline silicon carbide;

[0020] - The grinding wheel comprises diamond particles coated in a binder, the size of the diamond particles being greater than 2 μm (12000 mesh);

[0021] - during the relative movement of the grinding wheel and the polycrystalline silicon carbide plate, the relative speed of the grinding wheel and the front side of the polycrystalline silicon carbide plate is 0.05 μm / s to 0.5 μm / s, preferably 0.1 μm / s to 0.45 μm / s, more preferably 0.15 μm / s to 0.45 μm / s, until a thickness of less than or equal to 3 μm is removed from the polycrystalline silicon carbide plate;

[0022] - Maintaining the rotating grinding wheel in contact with the front face of the polycrystalline silicon carbide plate for a time greater than or equal to 30 seconds, preferably greater than or equal to 40 seconds;

[0023] - maintaining the rotating grinding wheel in contact with the front surface of the polycrystalline silicon carbide plate for a time greater than or equal to 30 seconds, preferably greater than or equal to 40 seconds, divided into multiple steps;

[0024] - the method further comprises: after stopping the relative movement and maintaining the rotating grinding wheel in contact with the front face of the polycrystalline silicon carbide plate, performing an additional phase of relative movement of the grinding wheel and the front face of the polycrystalline silicon carbide plate until an additional thickness of less than or equal to 1 μm is removed from the polycrystalline silicon carbide plate with the rotating grinding wheel in contact with the front face of the plate;

[0025] - during polishing, the rotation speed of the grinding wheel is 10 m / s to 45 m / s, preferably 10 m / s to 25 m / s;

[0026] During polishing, the front side of the polycrystalline silicon carbide plate is driven to rotate at a speed of 200 to 600 rpm.

[0027] The present invention is applicable to a method for manufacturing a polycrystalline silicon carbide plate, the method comprising: thinning a thick polycrystalline silicon carbide plate by grinding at least on the front side of the thick polycrystalline silicon carbide plate to obtain a thinned polycrystalline silicon carbide plate, and polishing the front side of the thinned polycrystalline silicon carbide plate according to the present invention. The thinning at least on the front side of the thick polycrystalline silicon carbide plate can be performed by a grinding wheel having a particle size of 10 μm (2000 mesh) to 30 μm (600 mesh).

[0028] The present invention is also applicable to a method for preparing a multilayer structure, the method comprising manufacturing a polycrystalline silicon carbide plate according to the present invention, providing a substrate as a donor of the layer to be transferred, and transferring the layer to be transferred from the donor substrate to the front side of the polycrystalline silicon carbide plate. The donor substrate may be a single crystal silicon carbide substrate. The transfer stage may include: forming a weakened zone by implanting atomic entities in the donor substrate to define the layer to be transferred; bonding the implanted face of the donor substrate to the front side of the polycrystalline silicon carbide plate; and separating from the donor substrate along the weakened zone. The bonding may be direct bonding, such as atomic diffusion bonding. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Other features and advantages of the present invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:

[0030] - Figure 1 Represents the abrasive grains in the binder of the grinding wheel before passivation (left side) and after passivation (right side);

[0031] - Figure 2 Indicates the different wear states of the abrasive grains on the outer edge of the grinding wheel;

[0032] - Figure 3 The bonding efficiency and haze values ​​(arbitrary units, representing roughness) of polycrystalline silicon carbide panels whose front sides were prepared according to the prior art method were compared with the bonding efficiency and haze values ​​of panels whose front sides were prepared according to two different embodiments of the method of the present invention.

[0033] For reasons of readability, the drawings are not necessarily drawn to scale. DETAILED DESCRIPTION

[0034] The present invention relates to the polishing of the front side of a p-SiC substrate in order to enable a thin layer of single-crystalline material to be bonded to the front side with high quality. The surface preparation is carried out by replacing conventional polishing tools (e.g. chemical mechanical polishing tools) with grinding tools (e.g. grinding wheels). This is because chemical mechanical polishing or CMP can achieve very low roughness on single-crystalline substrates. However, this process reveals the grain boundaries of the polycrystalline silicon carbide substrate and thus generates new roughness forms.

[0035] The invention relates more particularly to the polishing of the front side of a polycrystalline silicon carbide plate, said polycrystalline silicon carbide plate comprising, at least on its front side, a surface region at least partially damaged by machining produced by grinding. Such grinding can be carried out, for example, using a grinding wheel with coarse grains (usually the size of the abrasive grains is 10 μm to 18 μm, i.e. 1000 to 2000 mesh) to thin the polycrystalline silicon carbide plate, which has previously been carried out on at least its front side, preferably on its front side and its back side.

[0036] Any grinding leaves behind an at least partially machine-damaged surface area, also referred to as "damage layer" by those skilled in the art. In other words, after grinding, the plate has a surface area on its front side in which the crystals are disordered, with areas of scratches and fractures. The thickness of the machine-damaged surface area depends on the grinding performed: the coarser the grinding, the thicker the machine-damaged surface area. The thickness of the machine-damaged surface area is typically between 500 nm and 1 μm.

[0037] refer to Figure 1 and 2 The term "grinding wheel" refers to a tool with rotational symmetry, which mainly comprises particles made of abrasive (called abrasive grains 1), which are embedded in a binder 2 with holes 3, and they are all deposited on a support perpendicular to the rotation axis of the grinding wheel. For example, the binder 2 can include resin, ceramic or metal. The abrasive grains 1 are, for example, diamond grains. The binder 2 generally defines a surface opposite to the support, on which the abrasive grains 1 protrude, making the surface abrasive.

[0038] This type of wheel is typically used to remove material in order to thin the substrate during the "grinding" stage.

[0039] Typically, the substrate 4 to be thinned is in contact with the grinding surface of a rotating grinding wheel on its front side (see Figure 1 ). The relative axial movement of the grinding wheel and the substrate to be thinned along the axis of rotation of the grinding wheel makes it possible to apply pressure to the front side of the substrate 4. Under the effect of the pressure, each grain of the abrasive material on the outer edge of the grinding wheel then acts as an individual cutting tool, which causes tiny fragments of the substrate to be thinned to fly out of its front side. When the abrasive grains become dull (see Figure 1 , moving from left to right), the pressure and heat generated by the process cause the dulled abrasive grains to fracture and then split (see Figure 2 ), so that new abrasive grains with new sharp edges appear at the outer edge, thus regenerating the abrasiveness of the grinding wheel surface. As long as the grinding wheel and substrate are in relative translational motion, material continues to be removed from the substrate to be thinned by the new abrasive grains.

[0040] The present invention includes the use of such a grinding wheel not only for removing material from a polycrystalline silicon carbide plate, but also for polishing the front side of the plate by contact between the front side of the plate and the abrasive surface of the rotating grinding wheel.

[0041] To this end, a method according to the invention for polishing the front side of a polycrystalline silicon carbide plate comprising an at least partially machine-damaged surface region as described above comprises:

[0042] - moving a grinding wheel driven in rotation and a polycrystalline silicon carbide plate relative to each other until a layer having a surface area including at least part of the machining damage and having a thickness less than or equal to 3 μm is removed from the polycrystalline silicon carbide plate with the rotating grinding wheel in contact with the front side of the plate,

[0043] - stopping the relative movement and maintaining the rotating grinding wheel in contact with the front side of the polycrystalline silicon carbide plate for a time greater than 15 seconds.

[0044] The relative movement of the grinding wheel and the polycrystalline silicon carbide plate until at least part of the surface area damaged by machining is removed makes it possible to passivate the grains of the grinding wheel without promoting their splitting. Thus, when the rotating grinding wheel remains in contact with the front face of the plate after the relative movement of the plate and the grinding wheel has ceased, it is the passivated grains that rub against the front face of the plate, rather than new very sharp grains. This rubbing action ensures a gentle polishing of the front face, without revealing the boundaries between the silicon carbide grains.

[0045] Maintaining the rotating grinding wheel in frontal contact with the polycrystalline silicon carbide plate for less than 5 seconds without relative axial movement of the plate and the grinding wheel can be conventionally performed, thereby protecting the grinding tool, for example, avoiding large changes in the supply current of the motor. According to the present invention, maintaining the rotating grinding wheel in frontal contact with the polycrystalline silicon carbide plate for more than 15 seconds makes it possible to observe a reduction in surface roughness and a significant polishing effect.

[0046] Next, the selection of a grinding wheel for carrying out the method of the present invention is considered.

[0047] Different types of grinding wheels are distinguished, for example, according to the size of their abrasive grains or their hardness.

[0048] It should be noted that the hardness of a grinding wheel is defined as a measure of the force used to hold the particles in the grinding wheel. The harder the grinding wheel, the less likely it is that the dulled particles will break away from the grinding wheel, thereby regenerating the grinding surface of the grinding wheel. Conversely, the less hard the grinding wheel, the easier it is for multiple rows of abrasive grains to break apart and connect to each other. The hardness of a grinding wheel depends on the nature of the binder, the porosity and the size of the abrasive grains.

[0049] To remove matter, the skilled person generally chooses a grinding wheel that is not too hard, so that the cutting particles can be continuously regenerated. In addition, a grinding wheel that is too hard is less effective in removing matter and creates a risk of heating and therefore a risk of cracking the grinding wheel or the plate, as well as a risk of seizing and therefore a risk of scratching the plate.

[0050] In order to carry out the polishing process according to the invention, the inventors propose to use a grinding wheel that is harder than the grinding wheels conventionally used for removing substances. This is because the nature of the partial machining damage of the surface area to be removed advantageously enables the abrasive grains of the harder grinding wheel to be passivated without causing breakage or clogging of said grinding wheel. In addition, since the grinding wheel is harder, the passivation of the abrasive grains is facilitated without causing them to split.

[0051] Therefore, it is preferred to select a grinding wheel having a hardness greater than a maximum hardness that enables removal of a non-machine-damaged layer having the same thickness as the surface area of ​​at least a portion of the machine-damaged surface to be removed and made of the same material as said surface area. In other words, it is preferred to use a grinding wheel that is too hard to remove a non-machine-damaged layer made of the same material and the same thickness as the surface area of ​​the portion of the machine-damaged surface to be removed, in particular without causing breakage or seizure.

[0052] For example, the grinding wheels used are grinding wheels from Accretech, which are alphabetically labeled with the letter I or higher, such as Accretech HW8000VB-I144 grinding wheel.

[0053] As regards the choice of the grain size of the grinding wheel, for single-crystal substrates it is known to use grinding wheels with smaller abrasive grains, since it is desirable to obtain a surface with low roughness after grinding. Thus, a grinding wheel comprising very small grains (typically with a size of less than 1.5 μm, i.e. greater than 15000 mesh) makes it possible, for example, to obtain a free surface of a single-crystal silicon carbide substrate with very low roughness (less than 1 nm RMS).

[0054] However, the results obtained with this type of grinding wheel with very small grains on polycrystalline silicon carbide plates are still not sufficient to enable high-quality bonding of single-crystalline substrates to said plates, such as in Smart Cut TM Bonding performed in the context of the process.

[0055] The inventors have noticed that grinding wheels with very small grains tend to expose the grain boundaries of polycrystalline materials, which leads to the high roughness observed. The inventors attribute this effect to the fact that at these grain sizes, the grinding wheels used are not necessarily hard enough, so that during the rotation of the grinding wheel, dull grains are more easily broken and new grains with brand new sharp edges (which tend to "scrape" the grain boundaries) are more easily placed on the surface.

[0056] Therefore, in the method of the present invention, it is preferred to use a grinding wheel with a lower mesh number (and therefore coarser abrasive grain) than the grinding wheels with very small particles conventionally used to obtain very low surface roughness, but which exhibits a hardness that cannot be achieved for these grinding wheels with very small particles. Therefore, it is preferred to use a grinding wheel with a particle size of 2 μm to 3 μm (8000 to 12000 mesh). In addition, the hardness of the grinding wheel is preferably greater than the hardness that can be achieved by a grinding wheel whose particle size is less than or equal to 1.5 μm (greater than or equal to 15000 mesh).

[0057] Grinding wheels of this type advantageously promote the dulling of the abrasive grains during the removal of at least a portion of the machine-damaged surface region without the abrasive grains breaking off and being replaced by new abrasive grains.

[0058] Preferably, the relative movement of the grinding wheel and the silicon carbide plate is performed such that the total thickness of the material removed is not more than 2.5 μm greater than the thickness of the at least partially machine-damaged surface area. Thus, if the at least partially machine-damaged surface area has a thickness of 500 nm, the total thickness of the material removed is preferably less than or equal to 3 μm. The total thickness of the material removed is not more than 2.5 μm greater than the thickness of the machine-damaged surface area, which advantageously makes it possible to avoid the formation of a new row of sharp-edged particles on the surface.

[0059] 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 surface of the polycrystalline silicon carbide plate is 0.05 μm / s to 0.5 μm / s, preferably 0.1 μm / s to 0.45 μm / s, more preferably 0.15 μm / s to 0.45 μm / s, until a thickness of less than or equal to 3 μm is removed from the polycrystalline silicon carbide plate.

[0060] Speeds greater than 0.5 μm / s risk causing a series of previously dulled abrasive grains to break apart, thereby creating a series of new abrasive grains (which needs to be avoided), or causing the grinding wheel to break. Speeds less than 0.005 μm / s are too slow and not economically viable.

[0061] The rotation speed of the grinding wheel during polishing is, for example, 10 m / s to 45 m / s, preferably 10 m / s to 25 m / s. Thus, for a grinding wheel with a diameter of 300 mm, the rotation speed of the grinding wheel is 500 rev / min to 1500 rev / min. A rotation speed of less than 10 m / s of the grinding wheel unnecessarily increases the duration of this phase and is economically disadvantageous. A speed greater than 45 m / s will result in the splitting of previously passivated abrasive grains of the grinding wheel and / or the risk of causing a crack in the grinding wheel and / or the polycrystalline silicon carbide plate.

[0062] Figure 3A comparison is given of the results obtained by implementing this method for producing the front side of a polycrystalline silicon carbide plate (bar graphs (c) and (d)) with the results obtained by implementing a production method according to the prior art (bar graphs (a) and (b)).

[0063] The histograms (a) and (b) correspond to fine grinding to remove a thickness of about 10 μm using a grinding wheel having a hardness less than that of the grinding wheel preferably used in accordance with the present invention. The histograms (c) and (d) correspond to polishing performed in accordance with the present invention, wherein a thickness of 10 μm is removed by using a hard grinding wheel and then keeping the rotating grinding wheel in contact with the front side of the polycrystalline silicon carbide plate for a period of time (the time used here is less than 5 seconds).

[0064] The histograms (a) and (c) correspond to the percentage of polycrystalline silicon carbide plates whose surface conditions on the front side allow the front side to be bonded to a single crystal substrate with high quality. In other words, the histograms (a) and (c) correspond to the percentage of single crystal substrates bonded to the polycrystalline silicon carbide plates flawlessly. The term "defect" should be understood to mean, for example, partial bonding of a single crystal substrate, where the single crystal substrate layer is not transferred to certain parts of the polycrystalline silicon carbide plate. The histograms (b) and (d) themselves correspond to the haze values ​​measured on the polycrystalline silicon carbide plates by light scattering, for example using a Surfscan SP1 detection system from KLA-Tencor or by a SICA88 detection system from Lasertec. More precisely, it is an average value measured at several points on the polycrystalline silicon carbide plate. The haze value is an indirect tool for measuring the roughness of the front side of the polycrystalline silicon carbide plate. This haze value results from a method using the optical reflection properties of the surface to be characterized and corresponds to the optical signal scattered by the surface due to its micro-roughness: the greater the roughness of the surface, the greater the scattering on said surface and the greater the haze value measured.

[0065] Thus, it can be observed that implementation of the method of the invention makes it possible to obtain a bonding efficiency of about 30%, which is greater than the efficiency of the method with a thickness of about 10 μm removed (which itself is about 10%). Furthermore, the haze value of the front side of the silicon carbide plate prepared by the method according to the invention is lower than the haze value of the front side prepared by the method according to the prior art.

[0066] Maintaining the rotating grinding wheel in frontal contact with the polycrystalline silicon carbide plate may advantageously be performed for a time greater than 30 seconds, more preferably greater than 40 seconds, in order to maintain frictional contact between the grinding wheel and the plate (while maintaining the same row of previously passivated abrasive grains) for as long as possible.

[0067] The rotating grinding wheel is kept in contact with the front face of the polycrystalline silicon carbide plate for a time greater than 30 seconds, preferably greater than 40 seconds, which is preferably divided into a plurality of steps so that the contact between the grinding wheel and the front face of the polycrystalline silicon carbide plate is broken between each step by the relative movement of the grinding wheel and the polycrystalline silicon carbide plate. In this case, the sum of the duration of each step of keeping the rotating grinding wheel in contact with the front face of the polycrystalline silicon carbide plate is still equal to a selected time of more than 40 seconds. For example, the holding of the grinding wheel can include two steps: a first step, the duration of which is 5 to 10 seconds; followed by a second step, so that the sum of the duration of the first step and the second step is greater than or equal to 30 seconds, preferably greater than 40 seconds. Dividing the friction time between the rotating grinding wheel and the front face of the polycrystalline silicon carbide plate makes it possible to avoid overheating events, thereby avoiding the splitting of a series of previously passivated abrasive grains.

[0068] In one possible embodiment, polishing further comprises: after stopping the relative movement and maintaining the rotating grinding wheel in contact with the front side of the polycrystalline silicon carbide plate, performing an additional stage of relative movement of the grinding wheel and the front side. The additional stage of relative movement of the grinding wheel and the front side may comprise breaking the contact between the grinding wheel and the front side in a first step and then reestablishing the contact in a second step. In this case, once the contact between the grinding wheel and the front side of the polycrystalline silicon carbide plate is reestablished, the additional stage of relative movement of the grinding wheel and the front side of the polycrystalline silicon carbide plate is stopped (e.g., by an operator) to avoid causing the previously dulled abrasive grains of the grinding wheel to break apart.

[0069] An additional relative movement phase is performed in order to remove a very small additional thickness from the polycrystalline silicon carbide plate, preferably an additional thickness less than or equal to 1 μm, more preferably less than 200 nm, from the polycrystalline silicon carbide plate.

[0070] This additional stage of relative movement of the grinding wheel and the front side of the polycrystalline silicon carbide plate makes it possible to further reduce the roughness of the front side of the polycrystalline silicon carbide plate, thereby improving its surface quality.

[0071] exist Figure 3 In the graphs (e) and (f), the efficiency and haze values ​​respectively obtained by removing 1 μm of thickness in this additional stage after implementing the scheme followed by the graphs (c) and (d). Figure 3 The effectiveness of this stage is shown, since the efficiency increases further to reach a value of about 80% and the haze values ​​decrease further compared to histogram (d).

[0072] In a specific embodiment of the invention, the polycrystalline silicon carbide plate rotates around an X-axis perpendicular to the front side of the polycrystalline silicon carbide plate. The grinding wheel itself rotates around a Y-axis perpendicular to the grinding surface of the grinding wheel, and the Y-axis is parallel to the X axis.

[0073] The polycrystalline silicon carbide plate is, for example, a plate with a diameter of 150 mm, and the grinding wheel is, for example, a grinding wheel with a diameter of 300 mm.

[0074] For example, the front side of the polycrystalline silicon carbide plate may be driven to rotate at a speed of 200 to 600 rpm.

[0075] In this embodiment, the relative movement of the grinding wheel and the polycrystalline silicon carbide plate includes, for example, axial movement of the rotating grinding wheel along the Y-axis so that the abrasive surface of the grinding wheel contacts the front side of the polycrystalline silicon carbide plate, and then a thickness of less than or equal to 3 μm is removed from the plate under the action of the pressure applied by the grinding wheel during the axial movement.

[0076] The grinding wheel is then stopped from moving along the Y axis, and the grinding wheel and the plate are kept rotating about the X axis and the Y axis respectively so as to maintain contact between the grinding wheel and the front side of the polycrystalline silicon carbide plate for a time greater than 15 seconds, preferably greater than 30 seconds, and more preferably greater than 40 seconds.

[0077] According to this embodiment, if the duration of the contact between the rotating grinding wheel and the front face of the polycrystalline silicon carbide plate is greater than 30 seconds, preferably greater than 40 seconds, divided into a plurality of steps as described above, the breaking of the contact between the grinding wheel and the front face can be performed by translating along the Y axis of the grinding wheel that is kept rotating so as to move the grinding wheel away from the front face of the plate. For example, the holding of the grinding wheel can include a first step of duration of 10 seconds, a separation time of 1 minute (during which the contact between the grinding wheel and the front face of the plate is broken), and a second step of 30 seconds.

[0078] The present invention is applicable to a method for preparing a polycrystalline silicon carbide plate. The method for preparing a polycrystalline silicon carbide plate comprises thinning a thick polycrystalline silicon carbide plate on the front side and optionally on the back side of the thick polycrystalline silicon carbide plate by a first type of grinding wheel to obtain a thinned polycrystalline silicon carbide plate. Preferably, the polycrystalline silicon carbide plate thinned by the first type of grinding wheel has a thickness greater than or equal to 325 μm, more preferably a thickness of 350 μm.

[0079] Thick polycrystalline silicon carbide plates (eg 0.4 to 3 mm thick) can be produced, for example, by depositing p-SiC on a growth substrate (eg a graphite substrate), typically by chemical vapor deposition at a temperature of 1200 to 1400°C.

[0080] Prior to thinning by means of a grinding wheel of the first type, one or more very coarse grinding operations may optionally be carried out on the front side, preferably on the front side and on the back side, of the thick silicon carbide plate.

[0081] The first type of grinding wheel is a grinding wheel comprising abrasive grains coated in a binder, the grain size of the first type of grinding wheel being 10 μm to 30 μm (600 mesh to 2000 mesh). Given the relatively large grain size of the first type of grinding wheel, the thinning of the polycrystalline silicon carbide plate therefore corresponds to what a person skilled in the art would describe as a rough grinding operation.

[0082] A thick polycrystalline silicon carbide plate is thinned on its front side by a first type of grinding wheel, on which an at least partially machine-damaged surface region is generated, i.e. a region of crystal disorder in which scratches and / or fracture zones are present. The smaller the mesh number of the first type of grinding wheel (and therefore the larger the size of the abrasive grains of the grinding wheel), the thicker the at least partially machine-damaged surface region. Given a range of mesh numbers of the first type of grinding wheel, the thickness of the machine-damaged surface region is 500 nm to 1 μm.

[0083] The method of producing a polycrystalline silicon carbide plate of the present invention further comprises polishing the front side of the polycrystalline silicon carbide plate thinned by the first type of grinding wheel, the polishing being performed by the second type of grinding wheel according to any one of the embodiments of the above-mentioned polishing method.

[0084] The second type of grinding wheel preferably has the characteristics of grain size and hardness as described above.

[0085] Therefore, the second type of grinding wheel preferably has a hardness greater than the maximum hardness that enables removal of a non-machine-damaged layer having the same thickness and made of the same material as the surface area of ​​at least partial machine damage produced by the first type of grinding wheel.

[0086] The second type of grinding wheel preferably has a grain size of 2 to 3 μm (8000 to 12000 mesh). In addition, the hardness of the second type of grinding wheel is preferably greater than the hardness achievable by a grinding wheel having a grain size of less than or equal to 1.5 μm (greater than or equal to 15000 mesh).

[0087] In this method for manufacturing a polycrystalline silicon carbide plate, the front side of the plate previously thinned by grinding with a first type of grinding wheel is polished, which comprises relative movement of a second type of grinding wheel and the polycrystalline silicon carbide plate until a thickness of less than or equal to 3 μm is removed, and then maintaining the rotating grinding wheel in contact with the front side of the polycrystalline silicon carbide plate for a time greater than 15 seconds, preferably greater than 30 seconds, and more preferably greater than 40 seconds. Removing a thickness of less than or equal to 3 μm includes removing at least part of a surface area of ​​machining damage previously produced by thinning with the aid of the first type of grinding wheel.

[0088] The method of the invention thus makes it possible to polish the front side of a polycrystalline silicon carbide plate directly after thinning by rough grinding at least on the front side of said plate, without requiring an additional finer grinding step.

[0089] The invention also applies to a method of manufacturing a multilayer structure comprising polishing of a p-SiC plate as described above and transferring a thin layer of single-crystalline material from a substrate of said single-crystalline material to a polycrystalline silicon carbide plate.

[0090] The single crystal substrate is, for example, a single crystal silicon carbide (m-SiC) substrate. Alternatively, the single crystal substrate is a gallium nitride (GaN), indium phosphide (InP), diamond or silicon substrate. Optionally, an intermediate silicon dioxide (SiO2) layer is provided between the polycrystalline silicon carbide plate and the single crystal substrate.

[0091] The transfer of thin layers of single crystal materials can be done according to Smart Cut TM The process may include implanting ions into a single crystal material substrate so as to form therein a weakened plane defining the thin layer to be transferred, bonding the single crystal material substrate to a polycrystalline silicon carbide plate (via one or more bonding layers where appropriate), and then separating the single crystal material substrate along the weakened plane (by heat treatment, mechanical action or a combination of these means), thereby transferring the thin active layer to the polycrystalline silicon carbide plate. The single crystal material substrate may be subjected to mechanical or chemical mechanical polishing before bonding. The method for manufacturing a composite structure may further include forming electronic components, in particular power or radio frequency components, in the transferred thin layer.

[0092] The bonding of the single crystal material substrate on the polycrystalline silicon carbide plate is carried out by bonding such as ADB bonding. ADB bonding includes, for example, depositing a silicon layer on each surface to be bonded, the thickness of the silicon layer preferably being 4nm to 20nm, more preferably 10nm. This is because a silicon layer with a thickness of less than 4nm will bring the risk of blistering. Alternatively, the single crystal material substrate on the polycrystalline silicon carbide plate is bonded by surface activated bonding (SAB). The polishing method according to the present invention advantageously makes it possible to achieve the very low roughness required by these bonding methods.

Claims

1. A method for polishing the front side of a polycrystalline silicon carbide plate, the polycrystalline silicon carbide plate including a surface area at least partially damaged by grinding, the method comprising: - moving a rotationally driven grinding wheel and a polycrystalline silicon carbide plate relative to each other until a layer having a thickness less than or equal to 3 μm and including the surface area of ​​the at least partial machining damage is removed from the polycrystalline silicon carbide plate with the rotating grinding wheel in contact with the front side of the plate, - stopping the relative movement and maintaining the rotating grinding wheel in contact with the front side of the polycrystalline silicon carbide plate for a time greater than 15 seconds.

2. The method of claim 1, wherein the hardness of the grinding wheel is greater than a maximum hardness so that a non-machine-damaged layer made of polycrystalline silicon carbide having the same thickness as the at least partially machine-damaged surface area can be removed.

3. The method of claim 1 or 2, wherein the grinding wheel comprises diamond particles coated in a binder, the diamond particles having a size greater than 2 μm (12000 mesh).

4. The method according to any one of claims 1 to 3, wherein during the relative movement of the grinding wheel and the polycrystalline silicon carbide plate, the relative speed of the grinding wheel and the front surface of the polycrystalline silicon carbide plate is 0.05 μm / s to 0.5 μm / s, preferably 0.1 μm / s to 0.45 μm / s, more preferably 0.15 μm / s to 0.45 μm / s, until a thickness of less than or equal to 3 μm is removed from the polycrystalline silicon carbide plate.

5. The method according to any one of claims 1 to 4, wherein the rotating grinding wheel is kept in contact with the front side of the polycrystalline silicon carbide plate for a time greater than or equal to 30 seconds, preferably greater than or equal to 40 seconds.

6. The method of claim 5, wherein the method is divided into a plurality of steps to keep the rotating grinding wheel in contact with the front surface of the polycrystalline silicon carbide plate for a time greater than or equal to 30 seconds, preferably greater than or equal to 40 seconds.

7. The method according to any one of claims 1 to 6, further comprising: After stopping the relative movement and maintaining the rotating grinding wheel in contact with the front side of the polycrystalline silicon carbide plate, additional stages of relative movement of the grinding wheel and the front side of the polycrystalline silicon carbide plate are performed until an additional thickness of less than or equal to 1 μm is removed from the polycrystalline silicon carbide plate with the rotating grinding wheel in contact with the front side of the plate.

8. The method according to any one of claims 1 to 7, wherein during polishing, the rotation speed of the grinding wheel is 10 to 45 m / s, preferably 10 to 25 m / s.

9. The method according to any one of claims 1 to 8, wherein during polishing, the front side of the polycrystalline silicon carbide plate is driven to rotate at a speed of 200 to 600 rpm.

10. A method for manufacturing a polycrystalline silicon carbide plate, comprising: - thinning the thick polycrystalline silicon carbide plate by grinding said thick polycrystalline silicon carbide plate at least on its front side to obtain a thinned polycrystalline silicon carbide plate, - polishing the front side of the thinned polycrystalline silicon carbide plate, the polishing being performed by the method according to any one of claims 1 to 9.

11. The method of claim 10, wherein the thinning at least on the front side of the thick polycrystalline silicon carbide plate is performed by a grinding wheel having a grain size of 10 μm (2000 mesh) to 30 μm (600 mesh).

12. A method for preparing a multilayer structure, comprising: - manufacturing a polycrystalline silicon carbide plate by the method according to claim 10 or 11, - providing a substrate, which serves as a donor for the layer to be transferred, - Transferring the layer to be transferred from a donor substrate onto the front side of the polycrystalline silicon carbide plate.

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

14. The method of claim 12 or 13, wherein the transfer phase comprises: - forming a weakened zone by implanting atomic entities in the donor substrate to define the layer to be transferred, - bonding the implantation face of the donor substrate to the front side of the polycrystalline silicon carbide plate, - separating from the donor substrate along the weakened area.

15. The method of claim 14, wherein the bonding is direct bonding, preferably atomic diffusion bonding.

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