Method for polishing a substrate

Abstract

The present invention relates to a method for polishing a substrate (100) having a monocrystalline silicon carbide portion (20) and a carbon layer (30) on a front face (301), the method comprising: bringing a polishing pad (70) into contact with the front face (301); a first step of polishing for a first duration (t1) in which the pad and / or the substrate (100) is rotating at a first speed and the pad exerts a first mechanical pressure (p1) on the front face so as to at least partially remove the carbon layer (30); a second step of polishing for a second duration (t2) greater than the first duration (t1), in which the pad (70) and / or the substrate (100) is rotating at a second speed lower than the first speed and / or the pad exerts a second mechanical pressure (p2) lower than the first mechanical pressure (p1) on the front face.

Description

[0001] METHOD FOR POLISHING A SUBSTRATE

[0002] FIELD OF INVENTION

[0003] The present invention relates to a method for polishing a substrate having a front face of single-crystal silicon carbide and a layer of carbon covering said front face.

[0004] STATE OF THE ART

[0005] Semiconductor materials, particularly silicon carbide (SiC), are widely used in the manufacture of power electronics and radio frequency components. In some applications, bulk single-crystal SiC substrates or substrates with a single-crystal SiC surface layer are used, for example, transferred by a process such as SmartCut™.

[0006] Substrates for electronic applications often require heat treatments during their fabrication, such as annealing to heal defects in the crystal lattice caused during ion implantation or to modify the substrate's crystal structure. These treatments can lead to degradation of the free face of single-crystal SiC, for example, the formation of terraces, also known as "step bunching." Referring to Figure 1, it is known to form a carbon layer 30 on the free face of the single-crystal SiC portion 20 to protect the substrate against degradation during high-temperature treatments. Under certain conditions, the spontaneous formation of such a layer can be observed. For illustrative purposes, but not as a limitation, the single-crystal SiC portion 20 can be placed on a support substrate 10, for example, polycrystalline SiC.

[0007] Following the heat treatments, as shown in Figure 2, the carbon layer 30 is removed, and then the single-crystal SiC face 201 is smoothed. After smoothing, a single-crystal SiC layer 40 is typically epitaxially deposited, as illustrated in Figure 3. This epitaxially deposited layer 40 makes the single-crystal SiC portion 10, 40 sufficiently thick for the fabrication of electronic components.

[0008] Carbon layer removal and smoothing are performed together in a chemical-mechanical polishing (CMP) process using a polishing fluid. Preferably, polishing fluids free of abrasive particles are used to avoid contamination of the silicon carbide and the formation of scratches on the free face of the substrate. Such fluids typically contain potassium permanganate or another chemical agent that oxidizes the silicon carbide. The oxide produced during the chemical reaction is removed by the mechanical action of polishing.

[0009] However, the carbon layer is chemically inert to polishing fluids. Therefore, the polishing action on the carbon layer is purely mechanical, and consequently, the carbon layer removal time is long, significantly increasing the polishing process time.

[0010] During polishing, the temperature of the substrate's free face increases due to the chemical reaction of the silicon carbide. Therefore, the temperature of the substrate's top face can be used as an indicator of polishing progress. Figure 4 shows the temperature profile during the polishing of a single-crystal SiC substrate with a protective carbon layer (curve A) and a single-crystal SiC substrate without such a layer (curve B). The substrate without the carbon layer reaches a temperature plateau after approximately 30 seconds, indicating that the polishing process is effective across the entire substrate. The substrate protected by the carbon layer exhibits a considerably more gradual temperature rise due to the carbon layer's chemical inertness. A plateau is reached after approximately 150 seconds, indicating that the chemical reaction polishing is effective after this time.Therefore, a prolonged polishing time is required to prepare a substrate protected by a carbon layer before carrying out the next step, namely a SiC deposition by epitaxy.

[0011] Furthermore, carbon layers can vary in thickness across the substrate and between different substrates within a batch. This means that the polishing time required to remove the carbon layer varies considerably from one substrate to another.

[0012] In an automated polishing process, the maximum polishing time must be considered to ensure complete polishing of all substrates in each batch. Therefore, varying polishing times extend the overall duration of the automated process.

[0013] DESCRIPTION OF THE INVENTION

[0014] One object of the invention is to provide a faster polishing process for a silicon carbide substrate coated with a carbon layer, enabling efficient and reliable removal of the carbon layer followed by polishing of the silicon carbide for a subsequent epitaxy step.

[0015] To this end, the invention proposes a method for polishing a substrate having a portion of single-crystal silicon carbide extending along a principal plane of the substrate and a layer of carbon disposed on the portion of single-crystal silicon carbide and extending over a front face of the substrate,

[0016] said process comprising:

[0017] • bringing a polishing pad into contact with the front face of the substrate and applying a polishing fluid between the pad and the substrate;

[0018] • a first polishing step during a first duration in which the pad is rotating relative to the substrate at a first speed and / or the substrate is rotating relative to the pad at a first rotation speed of the substrate and the pad exerts a first mechanical pressure on the front face of the substrate, so as to remove at least partially the carbon layer;

[0019] • a second polishing step for a second duration greater than the first duration, in which the pad is rotating relative to the substrate at a second speed less than the first speed and / or the substrate is rotating relative to the pad at a second rotation speed of the substrate less than the first rotation speed of the substrate and / or the pad exerts a second mechanical pressure less than the first mechanical pressure on the front face of the substrate.

[0020] Preferably, the first speed is between 140% and 170% of the second speed.

[0021] In some embodiments, the first pressure is between 105% and 120% of the second pressure.

[0022] Advantageously, the first duration is between 10 and 100 s.

[0023] The first duration can be between 10% and 75% of the second duration. Advantageously, the polishing fluid contains potassium permanganate or sodium permanganate.

[0024] Preferably, the polishing fluid is free of abrasive particles.

[0025] Advantageously, the first polishing step is carried out under a first flow rate of polishing fluid and the second step is carried out under a second flow rate of polishing fluid lower than the first flow rate.

[0026] The invention also relates to a method for treating a substrate having a portion of single-crystal silicon carbide extending over a front face of the substrate, said method comprising successively

[0027] • the formation of a carbon layer on the portion made of single-crystal silicon carbide,

[0028] • heat treatment at a temperature above 1400°C, and

[0029] • a mechano-chemical polishing by a process as described above. The invention also relates to a method for manufacturing a substrate, comprising: • the implantation of atomic or ionic species in a single-crystal silicon carbide donor substrate to form a weakening zone delimiting a first layer of single-crystal silicon carbide,

[0030] • the bonding of the donor substrate to a recipient substrate,

[0031] • the detachment of the donor substrate along the embrittlement zone so as to transfer the first layer of single-crystal SiC onto the receiving substrate, • the treatment of the first layer of single-crystal silicon carbide transferred onto the receiving substrate by the process as described above.

[0032] Preferably, the receiving substrate is made of polycrystalline silicon carbide.

[0033] Advantageously, the process for manufacturing a substrate further includes, after the treatment of the first layer of single-crystal silicon carbide, the formation by epitaxial growth of a second layer of single-crystal silicon carbide on the first layer of silicon carbide.

[0034] BRIEF DESCRIPTION OF THE FIGURES

[0035] Other features and advantages of the invention will become apparent from the detailed description that follows, with reference to the attached drawings, in which:

[0036] Figure 1 illustrates a substrate comprising a portion of single-crystal silicon carbide and a protective layer of carbon.

[0037] Figure 2 illustrates the substrate of Figure 1 after chemical polishing. Figure 3 illustrates the substrate of Figure 2 after a single-crystal silicon carbide epitaxy step.

[0038] Figure 4 shows the temperature of a substrate with a carbon protective layer as a function of time, compared with a substrate having a top face of single-crystal SiC, during a mechano-chemical polishing by a known process.

[0039] Figures 5A and 5B illustrate a mechano-chemical polishing device. Figure 6 shows a rotational speed profile of a process according to the invention. Figure 7 shows a mechanical pressure profile of a process according to the invention. Figure 8 shows the temperature of a substrate protected by a carbon layer as a function of time during a known process and a process according to the invention.

[0040] Figure 9 illustrates the temperature of a plurality of substrates as a function of time, during a known process and during a process according to the invention.

[0041] Figure 10 is a comparison of the shrinkage rate between a known CMP process and a process according to the invention. Figures 11, 12 and 13 illustrate the transfer steps of a single-crystal SiC layer onto a support substrate.

[0042] For reasons of readability of the figures, the illustrated elements are not necessarily represented to scale.

[0043] DETAILED DESCRIPTION OF IMPLEMENTATION METHODS

[0044] The invention relates to a polishing method for a substrate comprising a portion of single-crystal silicon carbide. Typically, said portion is disposed on a support substrate, for example, a polycrystalline silicon carbide support substrate. Alternatively, the substrate may be made of solid single-crystal silicon carbide. A carbon layer extends over the single-crystal silicon carbide portion on the front face of the substrate.

[0045] In the following description, the single-crystal silicon carbide face, protected by a carbon layer before the polishing process and exposed after the polishing process, is referred to as the "front face" of the substrate.

[0046] During the polishing process, the substrate is held in a chemical-mechanical polishing (CMP) device as illustrated in Figures 5A and 5B. The device typically includes a support to hold the substrate in front of a polishing pad 70. The polishing pad 70 may be coated with a specific material 71 to optimize contact with the substrate and a polishing fluid 60.

[0047] The substrate and the polishing pad can be actuated in a direction perpendicular to the front face of the substrate (i.e. a vertical direction if the front face of the substrate is positioned in a horizontal plane) relative to each other in order to bring the pad closer to the substrate to be polished and to apply a mechanical pressure p between the pad and the substrate.

[0048] Advantageously, the polishing pad 70 has a diameter D at least twice the diameter d of the substrate. This polishing pad 70 can be rotated relative to the substrate about an axis of rotation z, typically oriented perpendicular to the front face of the substrate, which extends in a plane (x,y). The substrate is preferably offset from the pad's axis of rotation z. This offset prevents repetitive circular patterns on the substrate's front face and ensures a homogeneous distribution of abrasive forces.

[0049] During the polishing process, a polishing fluid 60 is applied between the pad 70 and the substrate to generate a chemical etch and ensure controlled lubrication. This lubrication prevents excessive adhesion of the substrate to the polishing pad 70 and overheating of the substrate. The polishing fluid 60 typically contains a reactive agent such as potassium permanganate or sodium permanganate to promote the oxidation of the silicon carbide. In some cases, permanganate-free polishing fluids may be used. Preferably, the polishing fluid 60 does not contain abrasive particles to avoid contamination of the silicon carbide and the formation of scratches on the front face of the substrate.

[0050] A CT temperature sensor is arranged on the polishing pad 70. At each rotation, the CT temperature sensor comes into contact with the front face of the substrate and can measure the temperature T of said front face in order to control the chemical reaction between the polishing fluid and the substrate.

[0051] Polishing process

[0052] At the beginning of the polishing process, the substrate is brought into contact with the polishing pad 70 by vertical actuation of the substrate held in a holder, and / or the polishing pad 70. The polishing fluid 60 is applied between the pad 70 and the front face 301 of the substrate. A mechanical pressure p is applied to press the front face 301 of the substrate against the pad 70. Simultaneously, the polishing pad 70 is rotated relative to the substrate.

[0053] During a ramp-up phase, the rotational speed v of the buffer and the mechanical pressure p are gradually increased to reach an initial speed vi and pressure pi. Simultaneously, the substrate can be rotated relative to the buffer around a substrate rotation axis z. s , for example a central axis of the substrate.

[0054] In this case, it is advantageous to maintain the ratio between the rotation speed of the buffer and the rotation speed of the substrate constant during the duration of the process, in order to limit the stress on the machine motors.

[0055] The initial rotational speed of the pad is typically higher than a default speed used for polishing single-crystal silicon carbide surfaces without a carbon layer in the same setup and with the same polishing fluid. The initial mechanical pressure is preferably greater than or equal to the default pressure used for polishing single-crystal silicon carbide surfaces without a carbon layer under similar polishing conditions.

[0056] During the first polishing step, the initial pad rotational speed vi and mechanical pressure pi are maintained. During this step, the carbon front face 301 is in contact with the polishing pad 70 and is retracted mechanically. When the substrate is rotating, the speed v si The rotation of the substrate is also kept constant during the first stage.

[0057] The duration of the first stage depends on the type of polishing fluid, the size of the substrate, the values ​​of the first velocity vi and the first mechanical pressure pi, the initial thickness of the carbon layer 30 and, where applicable, the velocity v si of substrate rotation. Typically, the duration of the first polishing step is between 10 and 100 s.

[0058] At the end of this step, the carbon layer 30 is removed, and the front face of the substrate is at least partially a single-crystal silicon carbide 201 face. Preferably, the carbon layer 30 is completely removed.

[0059] Typically, during the first stage, a centrifugal effect is exerted on the polishing fluid due to the high rotational speed vi. Therefore, a high flow rate di of polishing fluid is required. For example, in the case of a substrate with a diameter of 150 mm, the flow rate di during the first polishing stage is approximately 125 mL / min.

[0060] After the first step, the rotational speed v and / or the mechanical pressure p and / or, where applicable, the speed v siThe rotational speed of the substrate is modified. In some cases, only one of the parameters, rotational speed v and mechanical pressure p, is modified. Preferably, the rotational speed v is modified due to the good repeatability of carbon layer removal. Furthermore, in some polishing devices, the speed can be adjusted more precisely and repeatably than the pressure. In other configurations, it may be appropriate to modify only the mechanical pressure p or a combination of the parameters: rotational speed v, mechanical pressure p, and rotational speed of the substrate.

[0061] Depending on the parameters chosen, the speed v can be lowered to a second speed V2, which is lower than the first speed vi. Typically, the first speed vi is between 140% and 170% of the second speed V2. The second speed may correspond to the default speed for polishing silicon carbide surfaces.

[0062] When the substrate is rotating around a substrate rotation axis z s , the speed v si the substrate rotation speed can be kept constant or reduced to a second substrate rotation speed v S 2 after the first step.

[0063] The mechanical pressure can be reduced to a second mechanical pressure p2. This second mechanical pressure can correspond to the default mechanical pressure for polishing silicon carbide surfaces. The first mechanical pressure pi can be between 100% and 120% of the second mechanical pressure p2, preferably between 105% and 120%. During a second polishing step, immediately following the first step, the second rotational speed V2 of the pad and the second mechanical pressure p2 are maintained. The second polishing step is intended for smoothing the front face 201 made of monocrystalline silicon carbide. During this step, the front face 201 made of monocrystalline silicon carbide is in contact with the polishing pad 70.The front face 201 is smoothed mainly by the action of the chemical agent present in the polishing fluid, for example by the formation of an oxide which is easily removed by the mechanical action of the polishing fluid 60 and the pad 70.

[0064] During the second stage, particularly when the second pad rotation speed (v2) is lower than the first pad rotation speed (vi), the polishing fluid flow rate is typically lower than that of the first stage. For example, with a substrate having a diameter of 150 mm, the flow rate might be approximately 100 mL / min. Generally, the polishing fluid flow rate (di) during the first stage can be between 120% and 200% of the polishing fluid flow rate (d2) of the second stage.

[0065] The duration t2 of the second step is greater than the duration ti of the first step. Typically, the duration of the first step is between 10% and 75% of the duration of the second step. For illustrative purposes, and without limitation, for a substrate with a diameter of approximately 150 mm (6 inches), the duration of the first step might be 55% of the duration of the second step. For a substrate with a diameter of approximately 200 mm (8 inches), the duration of the first step might be 13% of the duration of the second step.

[0066] At the end of the second step, the rotational speed v of the pad and the mechanical pressure p are progressively reduced until rotation stops and the polishing pad 70 detaches from the front face 201 of the substrate. When the substrate is rotating around a substrate rotation axis z sThe rotation speed of the substrate is also reduced until it stops. This slowdown phase is known by the English term "ramp-down".

[0067] Figure 6 shows a temperature profile measured by a CT sensor mounted on the polishing pad during a process according to the invention (solid curve 6A), and during a known polishing process (dashed curve 6B). For the process according to the invention, the start-up step begins at t=0 and lasts until time ti2, at which the pad rotation speed vi is reached. In the example in Figure 6, the initial pad rotation speed is approximately vi = 300 rpm, and the substrate rotation speed is v si = 250 rpm. The rotational speed of the buffer vi and the rotational speed of the substrate v siare maintained until time ti2, at which point the buffer rotation speed is reduced to the second rotation speed V2 and the second stage begins. In the example in Figure 6, the second buffer rotation speed is approximately V2 = 185 rpm and the substrate rotation speed is v S 2 = 158 rpm. At time t23, the device begins the deceleration stage until the device comes to a stop. In the known process, with reference to curve 6B, at time t12 the rotational speed V2 is reached and maintained until the end of the process at time 4. In the known process, it is necessary to maintain the rotational speed of the buffer V2 for a duration greater than the sum of the first and second stages of the process according to the invention.

[0068] Figure 7 shows a profile of mechanical pressures p during a process according to the invention (solid curve 7A), and during a known polishing process (dashed curve 7B). For the process according to the invention, the start-up step begins at t=0 and lasts until time ti2, at which point the mechanical pressure pi is reached. In the example in Figure 6, the first pressure is approximately 48.3 kPa, corresponding to 7 psi (pounds per square inch). The mechanical pressure pi is maintained until time ti2, at which point the mechanical pressure is reduced to the second mechanical pressure p2, and the second step begins. In the example in Figure 6, the first mechanical pressure p2 is approximately p2 = 43.4 kPa (6.3 psi). Mechanical pressure is maintained until time t23. From time t23, the pressure is reduced and the device begins the deceleration phase until the device comes to a stop at time tfj. n .

[0069] In the known process, with reference to curve 6B, after start-up, at time ti2, the mechanical pressure p c is reached and maintained until the start of the deceleration step at time t4. Subsequently, the pressure is reduced during the deceleration phase until the end of the process at time tfj n The time required to maintain mechanical pressure p c during the known process is therefore greater than the sum of the steps of application of the mechanical pressures pi and p2 of the process according to the invention.

[0070] Figure 8 shows the temperature profile measured by the CT sensor for the processes described in Figures 6 and 7. Temperature is an indicator of the chemical reaction between the polishing fluid and the substrate's front face. Since a chemical reaction is expected only when a portion of single-crystal silicon carbide is exposed, the temperature also reflects the progression of carbon layer shrinkage.

[0071] Curve 8A shows the temperature of a substrate treated by a process according to the invention. The temperature reaches a maximum value at time ti2 and remains approximately constant until the beginning of the ramp-down phase of the process. The ripples around the maximum temperature occur due to the positioning of the sensor on the polishing pad. During the pad's rotation, the sensor is not in continuous contact with the substrate's front face. The substrate temperature corresponds to the maximum value of the ripples and is approximately 60°C. For the known process, with reference to curve 8B, the carbon layer removal time is considerably longer than for the process according to the invention. The temperature indicating a chemical reaction of the substrate's front face is reached only at time t2.From the end of the start-up step at time ti2, the time until the removal of the carbon layer is therefore about twice as long as in a process according to the invention.

[0072] It can be deduced that, to obtain a comparable quality of polishing, the duration of the known process is longer than the duration of the process according to the invention.

[0073] Figure 9 shows the temperature indicating the start of the chemical reaction between silicon carbide and the chemical agent in the polishing fluid for a batch of substrates. The solid curves 11A, 12A, 13A, and 14A correspond to substrates treated by a process according to the invention. The dashed curves 11B, 12B, 13B, and 14B correspond to substrates treated by a known process, in which the rotational speeds of the pad and the substrate and the mechanical pressure correspond to the rotational speed V2 and the mechanical pressure P2 in the second step of the process according to the invention. The values ​​of the process parameters in Figure 9 are given in Table 1.

[0074] Method according to the invention Known method V1 300 rpm —

[0075] V S 1250 rpm

[0076] V2185 rpm 185 rpm

[0077] Vs2 158 rpm 158 rpm

[0078] P1 48.3 kPa —

[0079] P2 43.4 kPa —

[0080] Pc — 43.4 kPa

[0081] you 9s 9s

[0082] tl2 46s —

[0083] t23 125s —

[0084] t4— 180s

[0085] tfin 139s 193s

[0086]

[0087] The curves for substrates treated by a process according to the invention are very close to each other, and the temperature plateau indicating a chemical reaction between the polishing fluid and the silicon carbide of the substrate is reached at approximately 50 s for all substrates subjected to the process according to the invention. The curves for the known process indicate a later onset of the chemical reaction. Furthermore, a significant temporal dispersion is observed for the onset of said chemical reaction, which begins between approximately t2 = 65 s for curve 11B and approximately t2 = 110 s for curve 14B.

[0088] Therefore, in order to obtain a sufficient chemical reaction to ensure the polishing of each substrate, the duration of the process up to time t4 is longer than the duration of the process according to the invention up to time t23. In this example, the process according to the invention can be stopped after a duration t23 of approximately 130 s, and the known process is maintained up to t4 for approximately 180 s in order to obtain a polish of comparable quality.

[0089] Figure 10 shows a comparison of the shrinkage rate for substrates treated by a process according to the invention and substrates treated by a known process using the same polishing fluid free of abrasive particles. For the process according to the invention, an average increase in the shrinkage rate of 12% is observed compared to the known process. Furthermore, the standard deviation of the rate between the different substrates in a batch is reduced by 13.5% compared to the known process; therefore, the process according to the invention provides more homogeneous and repeatable results.

[0090] Table 2 shows the statistical parameters of the analysis of the shrinkage rate RR of Figure 10. It should be noted that the dispersion for the process according to the invention is considerably lower between the different substrates treated than for the known process, despite the greater number of substrates treated by a process according to the invention.

[0091] Average number of samples, deviation - error, 95% of samples, standard, lower, upper, Process, 16, 1.01, 0.12, 0.03, 0.94, 1.07, according to

[0092] the invention

[0093] Process 12 1.09 0.10 0.03 1.03 1.16 known

[0094]

[0095] Substrate fabrication

[0096] A substrate comprising a portion of single-crystal silicon carbide is preferably made from a single-crystal silicon carbide donor substrate.

[0097] With reference to Figure 11, as shown schematically by the arrows, an implantation of ionic species, such as hydrogen and / or helium, is implemented so as to form a weakening zone 21 in the donor substrate 200. Said weakening zone 21 defines a layer 20 of single-crystal SiC to be transferred.

[0098] With reference to figure 12, the weakened donor substrate 200 is glued onto a support substrate 10. Typically, the support substrate is made of polycrystalline silicon carbide.

[0099] Referring to Figure 13, the donor substrate 200 is then detached along the embrittlement zone 21, resulting in the transfer of the single-crystal silicon carbide layer 20 onto the support substrate 10, thus forming a substrate for the fabrication of electronic components. The remaining portion 201 of the donor substrate can be reused for the transfer of one or more additional single-crystal SiC layers onto other support substrates.

[0100] The carbon layer is formed by transforming the surface into m-SiC under temperature and pressure conditions adapted to induce sublimation of the silicon atoms present on said surface while minimizing terrace formation. To achieve such sublimation, the pressure in the furnace is lowered to a pressure pp. The pressure pp is preferably between 0.01 and 0.5 Pa (0.1 and 5 pbar). During the pressure reduction to the pressure pp, the substrate is started to be heated to a temperature Tp. The pressure reduction and heating processes are adjusted so that the pressure pp is reached before or simultaneously with the reaching of the temperature Tp. Typically, the temperature Tp is between 1200°C and 1500°C. Alternatively, the temperature ramp-up is carried out after the pressure has been lowered to the pressure pp.The temperature Tp is higher than the sublimation temperature of silicon at the pressure pp in the furnace. Thus, the carbon layer forms before excessive terraces, for example greater than 25 nm, form on the front face of the substrate.

[0101] It is also possible to deposit a layer of carbon on the front face of single-crystal silicon carbide, for example by a reaction with carbon present in the furnace.

[0102] After the carbon layer has formed, the substrate is treated by one or more high-temperature steps, typically above 1400°C. During these steps, the carbon layer protects the front face against terrace formation.

[0103] After the completion of the steps at temperatures above 1400°C which can cause the formation of terraces, the process according to the invention can be implemented in order to remove the carbon layer and smooth the front face of single-crystal silicon carbide.

[0104] The substrate and free surface of the single-crystal silicon carbide portion can now be used for the epitaxial growth of a layer of single-crystal semiconductor material, as illustrated in Figure 3, and / or the fabrication of a power electronic or radio frequency component. The single-crystal semiconductor material deposited in this step may be the same as, or different from, the single-crystal silicon carbide of the substrate.

Claims

DEMANDS 1. A method for polishing a substrate (100) having a portion of single-crystal silicon carbide (20) extending along a principal plane (x,y) of the substrate and a layer of carbon (30) disposed on the portion of single-crystal silicon carbide (20) and extending over a front face (301) of the substrate, said process comprising: • bringing a polishing pad (70) into contact with the front face (301) of the substrate and applying a polishing fluid (60) between the pad (70) and the substrate (100); • a first polishing step for a first duration (h) in which the pad is rotating relative to the substrate at a first speed (vi) and / or the substrate (100) is rotating relative to the pad (70) at a first speed (vis) of rotation of the substrate and the pad exerts a first mechanical pressure (pi) on the front face of the substrate, so as to remove at least partially the carbon layer (30); • a second polishing step lasting for a second duration (t2) longer than the first duration (h), in which the pad (70) is rotating relative to the substrate (100) at a second speed (V2) lower than the first speed (vi) and / or the substrate (100) is rotating relative to the pad (70) at a second speed (V2 S) rotation of the substrate less than the first speed (screw) of rotation of the substrate and / or the buffer exerts a second mechanical pressure (P2) less than the first mechanical pressure (pi) on the front face (301, 201) of the substrate.

2. Polishing method according to claim 1, wherein the first speed (vi) is between 140% and 170% of the second speed (V2).

3. Method according to claim 1 or claim 2, wherein the first pressure (pi) is between 105% and 120% of the second pressure (P2).

4. A method according to any one of the preceding claims, wherein the first duration is between 10 and 100 s.

5. A method according to any one of the preceding claims, wherein the first duration (h) is between 10% and 75% of the second duration.

6. A method according to any one of the preceding claims, wherein the polishing fluid (60) comprises potassium permanganate or sodium permanganate.

7. A method according to any one of the preceding claims, wherein the polishing fluid (60) is free of abrasive particles.

8. A method according to any one of the preceding claims, wherein the first polishing step is carried out under a first flow rate of polishing fluid and the second step is carried out under a second flow rate of polishing fluid lower than the first flow rate.

9. A process for treating a substrate having a portion of single-crystal silicon carbide (20) extending over a front face (201) of the substrate, said process successively comprising • the formation of a carbon layer (30) on the portion made of single-crystal silicon carbide (20), • heat treatment at a temperature above 1400°C, and • a mechano-chemical polishing by the process according to any one of claims 1 to 8.

10. A method for manufacturing a substrate, comprising: • the implantation of atomic or ionic species in a donor substrate (200) of monocrystalline silicon carbide to form a weakening zone (21) delimiting a first layer of monocrystalline silicon carbide (20), • the bonding of the donor substrate (200) onto a recipient substrate, • the detachment of the donor substrate along the embrittlement zone so as to transfer the first layer of single-crystal SiC onto the receiving substrate (10), • the treatment of the first layer of single-crystal silicon carbide (20) transferred onto the receiving substrate (10) by the process according to claim 9.

11. A method according to claim 10, wherein the receiving substrate (10) is made of polycrystalline silicon carbide.

12. A method according to claim 10 or claim 11, further comprising, after the treatment of the first layer of monocrystalline silicon carbide (20), the formation by epitaxial growth of a second layer of monocrystalline silicon carbide (40) on the first layer of silicon carbide (20).

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