Pretreatment process of the front surface of polycrystalline silicon carbide slabs

Abstract

The present invention relates to a process for polishing the front surface of a polycrystalline silicon carbide slab having a surface area at least partially damaged by grinding effects, the process comprising: moving the rotary-driven grinding wheel and the polycrystalline silicon carbide slab relative to one another until a layer having an at least partially damaged surface area and exhibiting a thickness of 3 μm or less is removed from the polycrystalline silicon carbide slab with the rotating grinding wheel in contact with the front surface of the slab; ceasing the relative motion and maintaining contact of the rotating grinding wheel with the front surface of the polycrystalline silicon carbide slab for a period of more than 15 seconds; Includes:

Description

[Technical Field]

[0001] The present invention relates to a pretreatment process for the front surface of a polycrystalline silicon carbide slab, a manufacturing process for a polycrystalline silicon carbide slab using the pretreatment process for the front surface of the slab, and a pretreatment process for a multi-layer structure using the manufacturing process for a polycrystalline silicon carbide slab. [Background technology]

[0002] Silicon carbide (SiC) is being used increasingly widely in power electronics applications, particularly to meet the needs of an expanding range of electronic devices, such as electric vehicles. Power devices and integrated power systems based on single-crystal SiC can effectively manage much higher power densities than their conventional silicon counterparts, and can do so with smaller active area sizes.

[0003] Nevertheless, substrates made of single-crystal SiC for the microelectronics industry remain expensive and difficult to source in large sizes, so it is typically advantageous to use layer transfer solutions to produce composite structures comprising a thin layer of single-crystal SiC (m-SiC) on a lower-cost polycrystalline SiC (p-SiC) support substrate.

[0004] One well-known thin layer transfer solution is the Smart Cut™ process, which makes it possible to produce composite structures with thin layers made of, for example, monocrystalline SiC, which are taken from a donor substrate made of monocrystalline SiC and placed in direct contact with a support substrate made of polycrystalline SiC.

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

[0006] The thinning may include, for example, in order, very coarse thinning (by electro-discharge machining or grinding), for example, to remove a thickness of about 150 μm or more, coarse grinding, for example, to remove a thickness of about 20 μm, and fine grinding, for example, to remove a thickness of about 3 μm. Such thinning is preferably performed on both the front and back surfaces of the polycrystalline silicon carbide slab. The very coarse grinding may be omitted if the polycrystalline silicon carbide slab is thinner, for example, 400 μm thick.

[0007] The thinning makes it possible to obtain wafers that are self-supporting, i.e., have a thickness such that they do not break or undergo plastic deformation under the influence of their own weight, such a thickness being, for example, greater than or equal to 325 μm, preferably around 350 μm.

[0008] The thinning of the slab can be followed by a step of wafer surface finishing, which is aimed in particular at making the wafer smoother.

[0009] Nevertheless, it remains difficult to produce high-quality bonds between two single-crystal SiC and polycrystalline SiC substrates because of the complexities of controlling the surface condition and roughness of the substrates.

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

[0011] Although the use of grinding wheels with very small abrasive grains (less than 1.5 μm in size, i.e., more than 15,000 mesh) makes it possible to obtain monocrystalline materials with very low roughness, the performance quality achieved with this same type of grinding wheel on polycrystalline materials such as polycrystalline silicon carbide is not reproducible or sufficiently satisfactory to allow effective bonding of monocrystalline SiC substrates to polycrystalline SiC substrates, because the material removal by the teeth of such types of grinding wheels is non-uniform at the abrasive grains and grain boundaries, which is why a certain roughness occurs.

[0012] Furthermore, chemical mechanical polishing, a smoothing technique known to those skilled in the art as CMP (chemical mechanical polishing), is an effective process for single crystal substrates, but it also results in the appearance of grain boundaries in polycrystalline SiC substrates, and therefore too much roughness to allow good bonding of the single crystal SiC substrate to the polycrystalline SiC substrate.

[0013] The object of the present invention is to design a front surface pre-treatment process to enable high quality bonding of a thin layer of monocrystalline material to the front surface of a p-SiC substrate.

[0014] To this end, the present invention provides a polishing process for the front surface of a polycrystalline silicon carbide slab having a surface area at least partially damaged by grinding effects, the process comprising the following steps: moving the rotary-driven grinding wheel relative to the polycrystalline silicon carbide slab until a layer having an at least partially damaged surface region and exhibiting a thickness of 3 μm or less is removed from the polycrystalline silicon carbide slab with the rotating grinding wheel in contact with the front surface of the slab. Ceasing the relative motion and maintaining contact of the rotating grinding wheel with the front surface of the polycrystalline silicon carbide slab for a period of more than 15 seconds.

[0015] Holding a rotating grinding wheel in contact with the front surface of a polycrystalline silicon carbide slab for a period of time to polish the front surface of the p-SiC slab without removing material allows the grinding wheel to be used in a "rubbing" mode, which avoids the appearance of grain boundaries in the p-SiC substrate observed in more conventional polishing processes such as chemical mechanical polishing, thereby allowing for a level of front surface roughness compatible with bonding layers of single-crystalline material.

[0016] Some preferred, non-limiting aspects of this process are as follows: The grinding wheel exhibits a hardness higher than the maximum hardness that allows removal of an undamaged layer made of polycrystalline silicon carbide at least partially equal in thickness to the damaged surface area. The grinding wheel comprises diamond abrasive grains coated with a bond, the size of the diamond abrasive grains being larger than 2 μm (12000 mesh). During the relative movement between the grinding wheel and the polycrystalline silicon carbide slab, the relative speed between the grinding wheel and the front surface of the polycrystalline silicon carbide slab is 0.05 μm / s to 0.5 μm / s, preferentially 0.1 μm / s to 0.45 μm / s, and even more preferentially 0.15 μm / s to 0.45 μm / s until a thickness of 3 μm or less has been removed from the polycrystalline silicon carbide slab. The step of holding the rotating grinding wheel in contact with the front surface of the polycrystalline silicon carbide slab is carried out for a period of time greater than or equal to 30 seconds, and preferentially greater than or equal to 40 seconds. The step of holding the rotating grinding wheel in contact with the front surface of the polycrystalline silicon carbide slab for a period of time of at least 30 seconds, preferentially at least 40 seconds, is divided into multiple sequences. The process further includes, after the step of stopping the relative motion and holding the rotating grinding wheel in contact with the front surface of the polycrystalline silicon carbide slab, the additional step of relatively moving the grinding wheel and the front surface of the polycrystalline silicon carbide slab with the rotating grinding wheel in contact with the front surface of the slab until an additional thickness of 1 μm or less has been removed from the polycrystalline silicon carbide slab. The rotation speed of the grinding wheel during grinding is between 10 m / s and 45 m / s, preferentially between 10 m / s and 25 m / s. The front surface of the polycrystalline silicon carbide slab is driven to rotate at a speed of 200 rpm to 600 rpm during polishing.

[0017] The present invention is applicable to a process for manufacturing polycrystalline silicon carbide slabs, which includes the steps of thinning a thick polycrystalline silicon carbide slab by grinding at least its front surface to obtain a thinned polycrystalline silicon carbide slab, and polishing the front surface of the thinned polycrystalline silicon carbide slab. The step of thinning at least its front surface can be performed using a grinding wheel with an abrasive grain size of 10 μm (2000 mesh) to 30 μm (600 mesh).

[0018] The present invention also applies to a pretreatment process for a multilayer structure, which process includes the steps of manufacturing a polycrystalline silicon carbide slab according to the present invention, providing a substrate that is a donor for the layer to be transferred, and transferring the layer to be transferred from the donor substrate to the front surface of the polycrystalline silicon carbide slab. The donor substrate may be a monocrystalline silicon carbide substrate. The transfer step may include the steps of forming a weakened zone by implanting atomic entities into the donor substrate to define the boundary of the layer to be transferred, bonding the implanted surface of the donor substrate to the front surface of the polycrystalline silicon carbide slab, and peeling it from the donor substrate along the weakened zone. The bonding may be direct bonding, for example atomic diffusion bonding. [Brief explanation of the drawings]

[0019] Other features and advantages of the present invention will become apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which: [Figure 1] FIG. 1 shows abrasive grains in a grinding wheel bond before (left panel) and after (right panel) blunting. [Figure 2] 1A-1C are diagrams showing various states of wear of abrasive grains on the periphery of a grinding wheel. [Figure 3] FIG. 1 compares the bonding efficiency and haze (an arbitrary unit that is an indication of roughness) of polycrystalline silicon carbide slabs where the front side of the polycrystalline silicon carbide slabs have been pretreated according to a prior art process with the bonding efficiency and haze of slabs where the front side of the slabs have been pretreated according to two different embodiments of the process of the present invention.

[0020] The drawings are not necessarily drawn to scale for ease of reading. DETAILED DESCRIPTION OF THE INVENTION

[0021] Detailed Description of the Embodiments The present invention relates to polishing the front surface of a p-SiC substrate to enable high-quality bonding of a thin layer of monocrystalline material to said front surface. This surface preparation is carried out using a grinding tool, such as a grinding wheel, instead of a conventionally used polishing tool, such as a chemical-mechanical polishing tool. This is because chemical-mechanical polishing, or CMP, allows for the realization of monocrystalline substrates with very low roughness. However, this process exposes the grain boundaries of polycrystalline silicon carbide substrates, thus creating new forms of roughness.

[0022] More particularly, the present invention relates to polishing the front surface of a polycrystalline silicon carbide slab, which has at least a partially processed and damaged surface area caused by grinding, at least on the front side of the polycrystalline silicon carbide slab. Such polishing can be carried out, for example, using a grinding wheel having coarse abrasive grains (typically abrasive grains with a size of 10 μm to 18 μm, i.e., 1000 mesh to 2000 mesh) to pre-thin at least the front surface of the polycrystalline silicon carbide slab, and preferentially the front and back surfaces.

[0023] Any grinding process leaves at least a partially damaged surface area, also known to those skilled in the art as a "damage layer." In other words, after grinding, the slab exhibits a surface area at the front of the slab where the crystals are broken down by scratches and fracture zones. The thickness of the damaged surface area varies depending on the grinding process performed; the coarser the grinding, the thicker the damaged surface area. The thickness of the damaged surface area is typically between 500 nm and 1 μm.

[0024] 1 and 2, the term "grinding wheel" refers to a tool having rotational symmetry and comprising abrasive grains made primarily of an abrasive material called abrasive grains 1, embedded in a bond 2 exhibiting pores 3, all deposited on a support perpendicular to the axis of rotation of the grinding wheel. The bond 2 may, by way of example, comprise a resin, ceramic, or metal. The abrasive grains 1 are, for example, diamond abrasive grains. The bond 2 generally defines a surface opposite the support from which the abrasive grains 1 protrude, making said surface abrasive.

[0025] Such grinding wheels are conventionally used to remove material in order to thin the substrate in a "grinding" step.

[0026] The substrate 4 to be thinned typically contacts the abrasive surface of a rotating grinding wheel with its front surface (see FIG. 1). Relative axial movement of the grinding wheel and the substrate to be thinned along the grinding wheel's rotational axis allows pressure to be applied to the front surface of the substrate 4. Under the influence of pressure, each abrasive grain on the grinding wheel's peripheral edge acts as a separate cutting tool, scattering small pieces of the substrate to be thinned from the front surface of the substrate. As the abrasive grains become dull (see FIG. 1, moving from left to right), the pressure and heat generated by the machining process cause them to break and then split (see FIG. 2), revealing new abrasive grains with fresh, sharp edges on their peripheries and thus restoring the abrasiveness of the grinding wheel's surface. As long as the grinding wheel and substrate are in relative translational movement, new abrasive grains continue to remove material from the substrate to be thinned.

[0027] The present invention uses such a grinding wheel not just to remove material from a polycrystalline silicon carbide slab, but also to grind the front surface of the slab by contact between the front surface of the slab and the abrasive surface of the rotating grinding wheel.

[0028] To this end, the process according to the present invention for polishing the front surface of a polycrystalline silicon carbide slab with an at least partially processed damaged surface region as described above comprises the following steps: moving the rotary-driven grinding wheel relative to the polycrystalline silicon carbide slab until a layer having an at least partially damaged surface region and exhibiting a thickness of 3 μm or less is removed from the polycrystalline silicon carbide slab with the rotating grinding wheel contacting the front surface of the slab. Ceasing the relative motion and maintaining contact of the rotating grinding wheel with the front surface of the polycrystalline silicon carbide slab for a period of more than 15 seconds.

[0029] The relative movement of the grinding wheel and the polycrystalline silicon carbide slab until at least the damaged surface area is removed dulls the abrasive grains of the grinding wheel without promoting grain splitting. Thus, when the rotating grinding wheel maintains contact with the front surface of the slab after the relative movement between the slab and the grinding wheel has stopped, it is the dulled abrasive grains, rather than new, sharp abrasive grains, that rub against the front surface of the slab. This rubbing action ensures a smooth grinding of the front surface without exposing the boundaries between the silicon carbide abrasive grains.

[0030] To protect the grinding tool, for example to avoid large fluctuations in the motor's supply current, it is conventional to hold the rotating grinding wheel in contact with the front surface of the polycrystalline silicon carbide slab for less than 5 seconds without any relative axial movement between the slab and the grinding wheel. In accordance with the present invention, holding the rotating grinding wheel in contact with the front surface of the polycrystalline silicon carbide slab for more than 15 seconds allows a reduction in surface roughness and a significant polishing effect to be observed.

[0031] Next, we consider the selection of a grinding wheel in an embodiment of the process according to the invention.

[0032] Various types of grinding wheels are distinguished, for example, according to the size or hardness of the abrasive grains of the grinding wheel.

[0033] It should be noted that the hardness of a grinding wheel is defined as a measure of the force holding the abrasive grains within the wheel. The harder the wheel, the more difficult it is to remove the dulled grains from the wheel and regenerate the abrasive surface of the wheel. Conversely, the softer the wheel, the easier it is for the rows of abrasive grains to break apart and form a continuous chain. The hardness of a grinding wheel varies depending on the nature of the bond, the porosity, and the size of the abrasive grains.

[0034] Those skilled in the art traditionally select a grinding wheel that is not too hard so that the cutting grains are continually regenerated to remove material, and in addition, a grinding wheel that is too hard is less effective at removing material and creates the risk of overheating and therefore breaking the grinding wheel or slab, as well as the risk of jamming and therefore scratching the slab.

[0035] To carry out the polishing process according to the present invention, the inventors recommend using a grinding wheel that is harder than the grinding wheels traditionally used to remove material. This is because the partially damaged nature of the surface area to be removed advantageously allows the abrasive grains of the harder grinding wheel to be dulled without causing the wheel to break or jam. In addition, because the grinding wheel is harder, it is easier to dull the abrasive grains without splitting them.

[0036] Therefore, a grinding wheel having a hardness higher than the maximum hardness that allows the removal of an undamaged layer of the same material and thickness as the at least partially damaged surface area to be removed is preferentially selected. In other words, the preferentially used grinding wheel is too hard to remove an undamaged layer of the same material and thickness as the at least partially damaged surface area to be removed, in particular without causing breakage or clogging.

[0037] The grinding wheels used are, by way of example, those from the company Accretech, referenced alphabetically before the letter I, for example the Accretech HW8000VB-I144 grinding wheel.

[0038] Regarding the selection of the size of the abrasive grains of the grinding wheel, it is known that in the case of a single crystal substrate, it is desirable to obtain a surface that exhibits low roughness after grinding, so that grinding wheels with smaller abrasive grains are used. Thus, a grinding wheel with very small abrasive grains (typically less than 1.5 μm in size, i.e., more than 15,000 mesh) makes it possible to obtain a free surface of a single crystal silicon carbide substrate with a very low roughness, for example, less than 1 nm RMS.

[0039] However, the results obtained with this type of grinding wheel with very small abrasive grains on polycrystalline silicon carbide slabs are still insufficient to allow high-quality bonding of a monocrystalline substrate to said slab, such as the bonding carried out in connection with the SmartCut™ process.

[0040] The inventors have noticed that grinding wheels with very small abrasive grains tend to expose the grain boundaries of polycrystalline materials, resulting in the observed high roughness. The inventors believe that this result is due to the fact that with these grain sizes, the grinding wheels used are not necessarily hard enough, so that during the rotation of the grinding wheel, dulled grains are more easily broken off and new grains with fresh, sharp edges that tend to "scrape away" the grain boundaries are more easily placed on the surface.

[0041] In the process according to the present invention, therefore, a grinding wheel having a smaller mesh (i.e., a coarser abrasive grain) than the grinding wheels with very small abrasive grains conventionally used to obtain very small surface roughness, but exhibiting a hardness that cannot be achieved by grinding wheels with such very small abrasive grains, is preferentially used. Therefore, a grinding wheel having an abrasive grain size of 2 μm to 3 μm (8000 mesh to 12000 mesh) is preferentially used. In addition, the hardness of the grinding wheel is preferentially higher than the hardness that can be achieved by a grinding wheel having an abrasive grain size of 1.5 μm or less (15000 mesh or more).

[0042] This type of grinding wheel is advantageous because it promotes dulling of the abrasive grains while removing at least partially damaged surface areas, without breaking the grains and replacing them with new grains.

[0043] The relative movement between the grinding wheel and the silicon carbide slab is preferably carried out so that the total thickness of the removed material does not exceed the thickness of the at least partially damaged surface area by more than 2.5 μm. Thus, if the at least partially damaged surface area has a thickness of 500 nm, the total thickness of the removed material is preferably 3 μm or less. Not removing a total thickness of material that exceeds the thickness of the damaged surface area by more than 2.5 μm has the advantage of making it possible to avoid exposing new rows of new abrasive grains with sharp edges on the surface.

[0044] During the relative axial movement between the grinding wheel and the polycrystalline silicon carbide slab, the relative axial speed between the grinding wheel and the front surface of the polycrystalline silicon carbide slab is 0.05 μm / s to 0.5 μm / s, preferentially 0.1 μm / s to 0.45 μm / s, and even more preferentially 0.15 μm / s to 0.45 μm / s until a thickness of 3 μm or less has been removed from the polycrystalline silicon carbide slab.

[0045] Speeds above 0.5 μm / s cause the previously dulled line of abrasive grains to split, necessitating replacement with a new line of abrasive grains, or risking breakage of the grinding wheel, which is desirable to avoid. Speeds below 0.005 μm / s are too slow to be economically profitable.

[0046] The rotational speed of the grinding wheel during grinding is, for example, 10 m / s to 45 m / s, preferentially 10 m / s to 25 m / s. Thus, for a grinding wheel with a diameter of 300 mm, the rotational speed of the grinding wheel is 500 rpm to 1500 rpm. A grinding wheel rotational speed of less than 10 m / s unnecessarily lengthens the duration of this stage and is not economically advantageous. A speed of more than 45 m / s would risk splitting the previously dulled abrasive grains of the grinding wheel and / or causing breakage of the grinding wheel and / or the polycrystalline silicon carbide slab.

[0047] FIG. 3 presents a comparison of the results obtained with an embodiment of this process for pretreating the front surface of a polycrystalline silicon carbide slab (histograms (c) and (d)) with the results obtained with an embodiment of a prior art pretreatment process (histograms (a) and (b)).

[0048] Histograms (a) and (b) correspond to fine grinding, using a grinding wheel with a lower hardness than the grinding wheel preferentially used in the present invention, removing a thickness of the order of 10 μm. Histograms (c) and (d) correspond to polishing, in accordance with the present invention, using a harder grinding wheel to remove a thickness of 10 μm, followed by polishing, performed by holding the rotating grinding wheel in contact with the front surface of the polycrystalline silicon carbide slab, here for a time period of less than 5 seconds.

[0049] Histograms (a) and (c) correspond to the percentage of polycrystalline silicon carbide slabs whose front surface condition allows for high-quality bonding of the front surface to a monocrystalline substrate. In other words, histograms (a) and (c) correspond to the percentage of monocrystalline substrates bonded to polycrystalline silicon carbide slabs without defects. The term "defect" should be understood to mean, for example, partial bonding of the monocrystalline substrate, where a layer of the monocrystalline substrate has not been transferred to a specific portion of the polycrystalline silicon carbide slab. Histograms (b) and (d) correspond to the haze value measured by light scattering of the polycrystalline silicon carbide slab for portions of histograms (b) and (d), for example, using a KLA-Tencor Surfscan SP1 inspection system or a Lasertec SICA88 inspection system. More precisely, it is an average value measured at multiple points on the polycrystalline silicon carbide slab. The haze value is an indirect tool for measuring the roughness of the front surface of polycrystalline silicon carbide slabs. This haze value is obtained by a method using the light reflection properties of the surface to be characterized and corresponds to the light signal scattered by the surface due to the micro-roughness of the surface; the rougher the surface, the more scattering occurs at the surface and the higher the measured haze value.

[0050] It can therefore be observed that an embodiment of the process according to the invention makes it possible to obtain a bonding efficiency of the order of 30%, which is higher than the efficiency of a process removing a thickness of the order of 10 μm, which is itself of the order of 10%. Furthermore, the haze value of the front surface of the silicon carbide slab pretreated with the process according to the invention is lower than the haze value of the front surface pretreated with a process according to the prior art.

[0051] To maintain frictional contact between the grinding wheel and the slab for as long as possible (while maintaining the same row of previously dulled abrasive grains), it is advantageous to hold the rotating grinding wheel in contact with the front surface of the polycrystalline silicon carbide slab for a period of time greater than 30 seconds, and more preferably even greater than 40 seconds.

[0052] Preferably, the holding of the rotating grinding wheel in contact with the front surface of the polycrystalline silicon carbide slab for a period of more than 30 seconds, preferentially more than 40 seconds, is divided into multiple sequences, whereby contact between the grinding wheel and the front surface of the polycrystalline silicon carbide slab is released by relative movement between the grinding wheel and the polycrystalline silicon carbide slab between each sequence. In this case, the sum of the durations of the sequences in which the rotating grinding wheel is held in contact with the front surface of the polycrystalline silicon carbide slab still equals the selected period of more than 40 seconds. For example, the holding of the grinding wheel can include two sequences, the first of which has a duration of 5 to 10 seconds, and the second of which lasts such that the sum of the duration of the first sequence and the duration of the second sequence is at least 30 seconds, preferentially more than 40 seconds. Dividing the rubbing time between the rotating grinding wheel and the front surface of the polycrystalline silicon carbide slab makes it possible to avoid the development of overheating and thus the splitting of the previously dulled lines of abrasive grains.

[0053] In one possible embodiment, grinding further includes an additional step of relative movement between the grinding wheel and the front surface of the polycrystalline silicon carbide slab after stopping the relative movement and maintaining the rotating grinding wheel in contact with the front surface of the polycrystalline silicon carbide slab. The additional step of relative movement between the grinding wheel and the front surface can include, in a first step, releasing contact between the grinding wheel and the front surface, and then, in a second step, re-establishing said contact. The additional step of relative movement between the grinding wheel and the front surface of the polycrystalline silicon carbide slab is stopped, for example by an operator, as soon as contact between the grinding wheel and the front surface of the polycrystalline silicon carbide slab is re-established, in this case, so as not to disrupt previously dulled abrasive grains of the grinding wheel.

[0054] The additional step of relative movement is carried out in such a way that it results in the removal of a very small additional thickness from the polycrystalline silicon carbide slab, preferentially an additional thickness of 1 μm or less from the polycrystalline silicon carbide slab, more preferably a thickness of less than 200 nm.

[0055] This additional stage of relative movement between the grinding wheel and the front surface of the polycrystalline silicon carbide slab allows for further reduction in the roughness of the front surface of the polycrystalline silicon carbide slab, thus improving the surface quality of the front surface.

[0056] In Figure 3, histograms (e) and (f) respectively represent the efficiency and haze values ​​obtained after an embodiment of the procedure followed for histograms (c) and (d), supplemented with this additional step of 1 μm thick removal. Figure 3 therefore demonstrates the effectiveness of this step, as the efficiency is further increased, reaching values ​​of around 80%, and the haze values ​​are further reduced compared to histogram (d).

[0057] In a particular embodiment of the invention, the polycrystalline silicon carbide slab is rotated about an axis X that is perpendicular to the front surface of the polycrystalline silicon carbide slab, and the grinding wheel itself rotates about an axis Y that is perpendicular to the abrasive surface of the grinding wheel and is parallel to axis X.

[0058] The polycrystalline silicon carbide slab is, for example, a slab with a diameter of 150 mm, and the grinding wheel is, for example, a grinding wheel with a diameter of 300 mm.

[0059] The front surface of the polycrystalline silicon carbide slab can be driven to rotate at a speed of, for example, 200 rpm to 600 rpm.

[0060] In this embodiment, the relative movement between the grinding wheel and the polycrystalline silicon carbide slab comprises, for example, axial movement of the rotating grinding wheel along axis Y to bring the abrasive surface of the grinding wheel into contact with the front surface of the polycrystalline silicon carbide slab and then remove a thickness of 3 μm or less from said slab under the influence of pressure exerted by the axially moving grinding wheel.

[0061] The movement of the grinding wheel along axis Y is then stopped, and the grinding wheel and slab are still rotating about axes X and Y, respectively, and contact between the grinding wheel and the front surface of the polycrystalline silicon carbide slab is maintained for a period of more than 15 seconds, preferentially more than 30 seconds, and even more preferentially more than 40 seconds.

[0062] According to this embodiment, when the holding of the rotating grinding wheel in contact with the front surface of the polycrystalline silicon carbide slab for a duration of more than 30 seconds, preferentially more than 40 seconds, is divided into multiple sequences as described above, the release of the grinding wheel from contact with the front surface can be performed by translating the grinding wheel being held in rotation along axis Y so as to move said grinding wheel away from the front surface of the slab. The holding of the grinding wheel can, by way of example, include a first sequence of 10 seconds duration, a 1 minute separation period during which the grinding wheel is released from contact with the front surface of the slab, and a second sequence of 30 seconds.

[0063] The present invention is applied to a pretreatment process for polycrystalline silicon carbide slabs, which process comprises thinning the front and optionally the back surface of a thick polycrystalline silicon carbide slab using a first type of grinding wheel to obtain a thinned polycrystalline silicon carbide slab.Preferably, the polycrystalline silicon carbide slab thinned using the first type of grinding wheel exhibits a thickness of 325 μm or more, more preferably 350 μm.

[0064] Thick polycrystalline silicon carbide slabs (e.g., 0.4 mm to 3 mm thick) can be produced, for example, by depositing p-SiC on a growth substrate (e.g., a graphite substrate), typically by chemical vapor deposition at temperatures of 1200°C to 1400°C.

[0065] Prior to thinning with the first type of grinding wheel, optionally one or more very coarse grinding steps can be carried out on the front surface, preferentially the front and back surfaces of the thick silicon carbide slab.

[0066] The first type of grinding wheel is a grinding wheel with abrasive grains coated with a bond, and the size of the abrasive grains of the first type of grinding wheel is 10 μm to 30 μm (600 mesh to 2000 mesh). Given the relatively large size of the abrasive grains of the first type of grinding wheel, thinning of the polycrystalline silicon carbide slab corresponds to a grinding process that would be described as coarse by those skilled in the art.

[0067] Thinning the front surface of a thick polycrystalline silicon carbide slab using a first type of grinding wheel produces an at least partially damaged surface region on the front surface, i.e., a region where the crystals are broken down and exhibits flaws and / or fractured zones. The smaller the mesh of the first type of grinding wheel (and therefore the larger the size of the abrasive grains in the grinding wheel), the thicker the at least partially damaged surface region. Given the mesh range of the first type of grinding wheel, the thickness of the damaged surface region is between 500 nm and 1 μm.

[0068] The pretreatment process for polycrystalline silicon carbide slabs according to the present invention further comprises polishing the front surface of the thinned polycrystalline silicon carbide slab using a first type of grinding wheel, wherein the polishing is carried out according to any one of the embodiments of the polishing process described above using a second type of grinding wheel.

[0069] The second type of grinding wheel preferentially exhibits the grain size and hardness characteristics described above.

[0070] The second type of grinding wheel therefore preferentially exhibits a hardness higher than the maximum hardness produced by the first type of grinding wheel that allows the removal of an undamaged layer of the same material and of the same thickness as the at least partially damaged surface area.

[0071] The second type of grinding wheel preferably exhibits an abrasive grain size of 2 μm to 3 μm (8000 mesh to 12000 mesh), and the hardness of said second type of grinding wheel preferably is higher than the hardness achievable by a grinding wheel having an abrasive grain size of 1.5 μm or less (15000 mesh or more).

[0072] In this process for producing polycrystalline silicon carbide slabs, the step of polishing the front surface of the slab previously thinned by grinding with a first type of grinding wheel comprises moving the second type of grinding wheel and the polycrystalline silicon carbide slab relative to each other until a thickness of 3 μm or less is removed, the removal of which comprises at least partially removing the damaged surface area previously produced by the thinning performed with the first type of grinding wheel, before maintaining the rotating grinding wheel in contact with the front surface of the polycrystalline silicon carbide slab for a period of more than 15 seconds, preferentially more than 30 seconds, and even more preferentially more than 40 seconds.

[0073] The process according to the invention therefore allows polishing the front surface of a polycrystalline silicon carbide slab immediately after thinning by coarse grinding of at least the front surface of said slab, eliminating the need for an additional fine grinding step.

[0074] The present invention also applies to a process for manufacturing a multi-layer structure, comprising the steps of polishing a p-SiC slab as described above and transferring a thin layer of monocrystalline material from said substrate of monocrystalline material to a polycrystalline silicon carbide slab.

[0075] 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 disposed between the polycrystalline silicon carbide slab and the single crystal substrate.

[0076] The transfer of the thin layer of single-crystal material can be carried out according to the Smart-Cut™ technology and can therefore include the injection of ionic entities into the single-crystal material substrate to form weakened planes in the single-crystal material substrate that define the boundaries of the thin layer to be transferred, bonding the single-crystal material substrate to a polycrystalline silicon carbide slab (if appropriate via one or more bonding layers), and then peeling the single-crystal material substrate along the weakened planes (caused by heat treatment, mechanical action, or a combination of these means) to transfer the thin active layer to the polycrystalline silicon carbide slab. The single-crystal material substrate can be subjected to mechanical or chemical-mechanical polishing before bonding. The process for producing a composite structure can further include the step of forming electronic components, in particular power or radio-frequency components, in the transferred thin layer.

[0077] The monocrystalline material substrate on the polycrystalline silicon carbide slab is bonded by bonding, such as ADB bonding. ADB bonding involves, for example, the deposition of a silicon layer, preferably 4 to 20 nm thick, more preferably 10 nm thick, on each of the surfaces to be bonded. This is because a silicon layer less than 4 nm thick poses the risk of bubble formation. Alternatively, the monocrystalline material substrate on the polycrystalline silicon carbide slab is bonded by surface activation bonding (SAB). The polishing process according to the present invention is advantageous because it makes it possible to achieve the very low roughness required for these bonding methods.

Claims

1. A process for polishing the front surface of a polycrystalline silicon carbide slab having a surface area that has suffered at least partial processing damage due to the effects of grinding, wherein the process comprises: The steps include moving the rotary-driven grinding wheel and the polycrystalline silicon carbide slab relative to each other until the layer having at least partially machined damaged surface region and having a thickness of 3 μm or less is removed from the polycrystalline silicon carbide slab using a rotating grinding wheel in contact with the front surface of the slab, The steps include stopping the relative movement and maintaining contact between the rotating grinding wheel and the front surface of the polycrystalline silicon carbide slab for a period of time exceeding 15 seconds. A process that includes this.

2. The process according to claim 1, wherein the grinding wheel exhibits a hardness higher than the maximum hardness that enables the removal of an undamaged layer made of polycrystalline silicon carbide, having the same thickness as the at least partially damaged surface region.

3. The process according to claim 1 or 2, wherein the grinding wheel comprises diamond abrasive grains coated with a binder, and the size of the diamond abrasive grains is greater than 2 μm (12,000 mesh).

4. The process according to claim 1 or 2, wherein, during the relative movement between the grinding wheel and the polycrystalline silicon carbide slab, the relative speed between the grinding wheel and the front surface of the polycrystalline silicon carbide slab is 0.05 μm / sec to 0.5 μm / sec, preferably 0.1 μm / sec to 0.45 μm / sec, and even more preferably 0.15 μm / sec to 0.45 μm / sec, until a thickness of 3 μm or less is removed from the polycrystalline silicon carbide slab.

5. The process according to claim 1 or 2, wherein the step of holding the rotating grinding wheel in contact with the front surface of the polycrystalline silicon carbide slab is performed for a period of 30 seconds or more, preferably 40 seconds or more.

6. The process according to claim 5, wherein the step of keeping the rotating grinding wheel in contact with the front surface of the polycrystalline silicon carbide slab for 30 seconds or more, preferably 40 seconds or more, is divided into a plurality of sequences.

7. The process according to claim 1 or 2, further comprising the step of stopping the relative movement and holding the rotating grinding wheel in contact with the front surface of the polycrystalline silicon carbide slab, and then moving the grinding wheel and the front surface of the polycrystalline silicon carbide slab in relative motion until an additional thickness of 1 μm or less is removed from the polycrystalline silicon carbide slab, while the rotating grinding wheel is in contact with the front surface of the slab.

8. The process according to claim 1 or 2, wherein the rotational speed of the grinding wheel during polishing is 10 m / sec to 45 m / sec, preferably 10 m / sec to 25 m / sec.

9. The process according to claim 1 or 2, wherein the front surface of the polycrystalline silicon carbide slab is driven to rotate at a speed of 200 revolutions per minute to 600 revolutions per minute during polishing.

10. A process for manufacturing polycrystalline silicon carbide slabs, wherein the process comprises: The steps include thinning a thick polycrystalline silicon carbide slab by grinding at least its front surface to obtain a thinned polycrystalline silicon carbide slab, The steps include polishing the front surface of the thinned polycrystalline silicon carbide slab and A process comprising the step of polishing, wherein the polishing step is performed by the process according to claim 1 or 2.

11. The process according to claim 10, wherein the step of thinning at least the front surface of the thick polycrystalline silicon carbide slab is performed using a grinding wheel having abrasive grain sizes of 10 μm (2000 mesh) to 30 μm (600 mesh).

12. A preprocessing process for a multilayer structure, wherein the process is: A step of manufacturing a polycrystalline silicon carbide slab by the process described in claim 10, The steps include: preparing a substrate which will be the donor for the layer to be transferred, The steps include transferring the layer to be transferred from the donor substrate to the front surface of the polycrystalline silicon carbide slab. A process that includes this.

13. The process according to claim 12, wherein the donor substrate is a single-crystal silicon carbide substrate.

14. The transcription step is The steps include forming a weakened zone by injecting atomic material into the donor substrate in order to define the boundary of the layer to be transferred, The steps include: bonding the injection surface of the donor substrate to the front surface of the polycrystalline silicon carbide slab; The steps of peeling the substrate off along the weakened zone and The process according to claim 12, including the process described in claim 12.

15. The process according to claim 14, wherein the bonding is a direct bonding, preferably an atomic diffusion bonding.

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