Silicon carbide wafer thinning method

Abstract

The invention belongs to the technical field of silicon carbide processing, and particularly discloses a silicon carbide wafer thinning method, which comprises the following steps that S1, the focus of a first laser beam is aligned with a peeling plane of a preset depth of a silicon carbide wafer, and the silicon carbide wafer of the peeling plane absorbs laser energy to form modified points, the modified points are communicated in a laser irradiation scanning mode to form a modified layer; s2, the modification points are connected in series through microcracks through a crack extension technology, the silicon carbide wafer is peeled off along the modification layer, and a peeled surface is formed; s3, the silicon carbide wafer is placed in a cavity of a high-pressure cavity, laser cladding is conducted on the stripping face through a second laser beam, and a fused surface is formed; and S4, grinding the molten surface by using a diamond grinding wheel. The peeled silicon carbide wafer peeling surface is subjected to laser melting in a high-pressure environment, and the roughness of the molten surface is improved, so that the loss of the peeling surface to a grinding wheel is effectively reduced.

Description

Technical Field

[0001] This invention belongs to the field of silicon carbide processing technology, and specifically relates to a method for thinning silicon carbide wafers. Background Technology

[0002] Silicon carbide (SiC), as a third-generation semiconductor material, is an ideal choice for fabricating high-performance electronic devices due to its excellent properties such as wide bandgap, high electron mobility, high breakdown field strength, and high thermal conductivity. To improve current handling capability, SiC power devices typically employ a vertical structure design, where the on-resistance decreases as the thickness decreases, contributing to lower power consumption. However, during manufacturing, to ensure sufficient mechanical strength of the wafer, the initial thickness is usually maintained at around 350 μm. After completing the complex front-side structure fabrication, the wafer needs to be thinned to 150 μm or even 100 μm from the back side to optimize device performance.

[0003] Because SiC material has extremely high hardness (Mohs hardness of 9.5), using only traditional diamond grinding wheels for thinning from 350μm to 100μm results in high wheel wear and high costs. To reduce costs, existing technologies employ a composite process of "laser radiation ablation + grinding wheel polishing": First, a modified layer is introduced into the wafer at a thickness close to the target using a laser. Local ablation is then achieved by inducing crack propagation along the modified layer, removing excess material. Next, grinding wheel polishing removes the laser-damaged layer and planarizes the surface to meet the required roughness for the device. This method can reduce the amount of grinding wheel work by approximately 200μm, significantly reducing grinding wheel wear.

[0004] However, in practical applications, such as Figure 3 As shown, due to the approximately 4° tilt angle between the SiC wafer surface and its crystal plane, periodic steps formed by natural cleavage occur on the surface after laser lift-off, with a height difference of up to approximately 20 μm. Practice has shown that this type of step structure significantly increases the wear of the grinding wheel during subsequent damage removal polishing, reaching 3 to 4 times that under smooth surface polishing conditions. This weakens the original cost advantage of the laser lift-off process, and its benefits are not significant compared to traditional purely mechanical polishing methods. Summary of the Invention

[0005] The purpose of this invention is to provide a method for thinning silicon carbide wafers, which significantly improves the surface roughness by laser melting and peeling, thereby effectively reducing the wear of the peeling surface on the grinding wheel.

[0006] The objective of this invention is achieved through the following technical solution: a method for thinning silicon carbide wafers, comprising the following steps:

[0007] S1. The focus of the first laser beam is aligned with the peeling plane of the silicon carbide wafer at a preset depth. The silicon carbide wafer on the peeling plane absorbs the laser energy to form modification points, and the modification points are connected to form a modification layer by laser irradiation scanning.

[0008] S2. By using crack propagation technology, the modified points are connected through microcracks, and the silicon carbide wafer is peeled off along the modified layer to form a peeling surface;

[0009] S3. Place the silicon carbide wafer inside the high-pressure chamber and use a second laser beam to laser clad the peeling surface to form a molten surface;

[0010] S4. Grind the molten surface using a diamond grinding wheel.

[0011] A modified layer is formed at a predetermined depth by irradiation and scanning with a first laser beam. Then, the modified layer is peeled off using crack propagation technology. The peeled silicon carbide wafer is then placed in the chamber of a high-pressure cavity. A second laser beam irradiates the silicon carbide wafer, melting the peeled surface. This causes the periodic steps formed during the peeling process to melt and then solidify, restoring the wafer to a near-planar state. Finally, a grinding wheel is used to remove the damage caused by the laser modification, further reducing the surface roughness.

[0012] Preferably, in step S3, the chamber of the high-pressure cavity contains an inert gas.

[0013] Preferably, in step S3, the chamber of the high-pressure cavity contains Si gas.

[0014] Preferably, the wavelength of the first laser beam in step S1 is >380nm.

[0015] Preferably, the wavelength of the second laser beam in step S3 is <380nm.

[0016] Preferably, in step S1, when forming the modified layer on the peeling plane by laser irradiation scanning, the edge portion of the silicon carbide wafer is not modified, forming an unmodified area, and a trench is provided between the modified area and the unmodified area.

[0017] Preferably, the crack propagation technique in step S2 includes one of ultrasonic wave, laser heating, and microwave heating.

[0018] Preferably, in step S4, the grinding of the laser-melted surface with a diamond grinding wheel includes the following steps:

[0019] S41. Roughly grind the molten surface with a coarse diamond grinding wheel;

[0020] S42. Use a precision diamond grinding wheel to finely grind the molten surface.

[0021] Because of the adoption of the above technical solution, the present invention has the following advantages:

[0022] After being stripped, the silicon carbide wafer is melted by laser under high pressure, which improves the roughness of the molten surface and effectively reduces the wear of the stripping surface on the grinding wheel. Attached Figure Description

[0023] To more clearly illustrate the specific embodiments of the present invention, the accompanying drawings used in the specific embodiments will be briefly introduced below. In all the drawings, the elements or parts are not necessarily drawn to scale.

[0024] Figure 1 This is a schematic diagram illustrating the steps of a silicon carbide wafer thinning method according to the present invention;

[0025] Figure 2 This is a schematic diagram of the high-pressure cavity.

[0026] Figure 3 This is a schematic diagram showing the periodic steps on the surface after laser ablation.

[0027] Figure 4 A schematic diagram of a silicon carbide wafer;

[0028] Figure 5 Schematic diagram of Si and C planes;

[0029] Figure 6 This is a schematic diagram of the modified layer in Example 1;

[0030] Figure 7 This is a schematic diagram of the modified layer in Example 2;

[0031] Figure 8 This is a schematic diagram of a diamond grinding wheel grinding a molten surface.

[0032] Figure label:

[0033] 1-Box body, 11-Cavity, 12-Irradiation port, 13-Air inlet, 14-Exhaust port;

[0034] 2-Bearing platform; 3-Box cover; 4-First pressure regulating valve; 5-Second pressure regulating valve; 6-Pressure gauge; 7-Drive source;

[0035] 8-Silicon carbide wafer / SiC wafer, 81-Si surface, 82-C surface, 83-Modification point, 84-Modified layer, 85-Unmodified area, 86-Trench, 87-Melted surface; 9-Diamond grinding wheel. Detailed Implementation

[0036] The embodiments of the technical solution of the present invention will now be described in detail with reference to the accompanying drawings.

[0037] Please see Figure 1 , Figure 2 , Figure 4 and Figure 5 A method for thinning silicon carbide wafers specifically includes the following steps:

[0038] S1. The focus of the first laser beam is aligned with the peeling plane of the silicon carbide wafer 1 at a preset depth. The silicon carbide wafer 1 on the peeling plane absorbs the laser energy to form modification points 83. The modification points 83 are connected by laser irradiation scanning to form a modification layer 84.

[0039] S2. By using crack propagation technology, the modified points 83 are connected through microcracks, and the silicon carbide wafer 1 is peeled off along the modified layer 84 to form a peeling surface;

[0040] S3. Place the silicon carbide wafer 1 in the high-pressure cavity and use the second laser beam to laser clad the peeling surface to form a molten surface 87.

[0041] S4. Grind the molten surface 87 using a diamond grinding wheel 9.

[0042] Specifically, in this application, the silicon carbide (SiC) wafer 1 has a diameter of 6 inches and a thickness of 350 μm. Since SiC is a polar material, the silicon carbide wafer 1 comprises two faces with different characteristics, named Si face 81 and C face 82 based on the distribution of Si and C atoms, respectively. To facilitate epitaxial growth and defect control, the surface of the silicon carbide wafer 1 forms a 4° angle with the 0001 crystal plane of the SiC lattice. The preset depth of the peeling plane is set at a position 150 μm away from Si face 81 of the silicon carbide wafer 1. It should be noted that the necessary epitaxial growth and device fabrication processes on Si face 81 have been completed before the silicon carbide wafer 1 is thinned; the specific preset depth depends on actual requirements.

[0043] The high-pressure chamber includes a housing 1, a support platform 2, a housing cover 3 for sealing the housing, a first pressure regulating valve 4, a second pressure regulating valve 5, and a pressure gauge 6. The housing 1 has a chamber 11 inside. The surface material of the housing 1 is quartz, which is transparent. An irradiation port 12 is opened on the surface of the housing 1. An air inlet 13 and an exhaust port 14 are provided on the side of the housing 1. The irradiation port 12, the air inlet 13, and the exhaust port 14 are respectively connected to the chamber 11. A second laser beam passes through the irradiation port 12 to irradiate the silicon carbide wafer 1. Gas is introduced into the chamber 11 through the air inlet 13 to increase the air pressure in the chamber 11. The gas in the chamber 11 is discharged from the housing 1 through the exhaust port 14 to reduce the air pressure in the chamber 11. The first pressure regulating valve 4 is fixedly installed at the air inlet 13 of the housing 1 and is used to regulate the gas flow through the air inlet 13. It is connected to an external gas device, which provides the required gas. The second pressure regulating valve 5 is fixedly installed at the exhaust port 14 of the housing 1 and is used to regulate the gas flow through the exhaust port 14. Using existing technology, the support platform 2 is slidably installed on the bottom surface of the housing 1 to support the stripped silicon carbide wafer. The silicon carbide wafer is fixed to the support platform 2 by an electrostatic chuck, as is done in existing technology. The bottom surface of the housing 1 is provided with two slide rails, the area of ​​which is below the irradiation port 12. Under the action of external force, the support platform 2 slides along the slide rails. The housing cover 3 is provided on one side of the housing 1 using existing technology. The housing cover 3 is rotatably connected to the housing 1 and seals the housing 1. It is used to remove / place the silicon carbide wafer 1. The pressure gauge 6 is fixedly connected to the surface of the housing 1. The monitoring end of the pressure gauge 6 is connected to the chamber 11 to monitor the pressure in the chamber 11. The high-pressure chamber is also equipped with a drive source 7, which drives the support platform 2 to move along the slide rail. Preferably, the drive source 7 is an existing electric push rod. In use, the box cover 3 is opened, the silicon carbide wafer is fixed on the support platform 2, the box cover 3 is closed, the second laser beam and the drive source 7 are started, the second laser beam irradiates the silicon carbide wafer, and the drive source 7 drives the support platform 2 to move along the slide rail. After melting is completed, the second laser beam and the drive source 7 are turned off, and the silicon carbide wafer is removed.

[0044] The silicon carbide wafer 1 has a melting point as high as 2700℃. However, when the silicon carbide wafer 1 is directly heated under normal pressure or vacuum, the C-Si bonds in the silicon carbide wafer 1 will break when heated to 2300℃, and Si atoms will volatilize first. Therefore, it is impossible to directly obtain molten silicon carbide wafer 1. However, when placed in a high-pressure chamber, the silicon carbide wafer 1 has an increased saturated vapor pressure under high pressure during laser melting and peeling, thereby suppressing the volatilization of Si atoms. Its surface can be briefly in a liquid phase state under laser irradiation, and a smooth surface is formed under the action of liquid surface tension. After the surface is clad with the modified layer 84, the unevenness is reduced to below 5μm.

[0045] This invention discloses a method for thinning silicon carbide wafers. A modified layer 84 is formed on silicon carbide by radiation and scanning with a first laser beam. Then, a crack propagation technique is used to peel off the modified layer 84, forming a peeling surface. The peeled silicon carbide wafer 1 is then placed in a high-pressure chamber, and a second laser beam passes through an irradiation port 12 to irradiate the silicon carbide wafer 1, melting the peeling surface. This causes the periodic steps formed during the peeling process to melt and then solidify, restoring the wafer to a near-planar state. Finally, a grinding wheel is used to remove the damage caused by the laser modification, further reducing surface roughness. Through laser melting under high pressure, the roughness after peeling is improved, thereby effectively reducing the wear on the grinding wheel on the peeling surface.

[0046] Further, in step S3, the chamber pressure of the high-pressure chamber is 2-1000 times atmospheric pressure. Specifically, the gas device and the first pressure regulating valve 4 are activated, allowing gas from the gas device to be injected into the chamber 11 through the air inlet 13. When the pressure displayed on the pressure gauge 6 reaches a predetermined value, the first pressure regulating valve 4 is closed to stop the gas from entering the chamber 11. During the laser melting process, the state of the first pressure regulating valve 4 or the second pressure regulating valve 5 can be adjusted to ensure that the pressure in the chamber 11 reaches a specified amount. The specific pressure is determined by the actual site conditions and process requirements, which are not specifically limited in this embodiment. The housing 1 of this application withstands a pressure of 1200 times atmospheric pressure, therefore the pressure in the high-pressure chamber cannot exceed 1000 times atmospheric pressure.

[0047] Furthermore, in step S3, the chamber of the high-pressure cavity contains an inert gas. The inert gas includes, but is not limited to, argon, helium, or mixtures thereof. Using an inert gas prevents silicon from reacting with the gas at high temperatures to produce substances that could affect the quality of the molten surface.

[0048] Furthermore, in step S3, the high-pressure chamber contains a Si gas. The Si gas includes, but is not limited to, SiH4 or a mixture of an inert gas and a Si-based gas. At high temperatures, silicon (Si) atoms in SiC evaporate more readily than carbon (C) atoms (because the saturated vapor pressure of silicon is much higher than that of carbon). When a laser heats a localized area to extremely high temperatures, silicon atoms on the surface preferentially escape, leaving a carbon-rich surface. If molten in a Si-containing gas, the high concentration of silicon in the environment inhibits the decomposition of SiC and promotes the recombination of silicon atoms into the crystal lattice. This effectively prevents silicon evaporation from the wafer body and improves the quality of the molten surface.

[0049] Furthermore, the wavelength of the first laser beam in step S1 is >380nm. A laser beam with a wavelength >380nm will not be directly absorbed by the SiC material, allowing it to penetrate into the silicon carbide wafer 1. At the focal point, the laser energy density is high, and the silicon carbide wafer absorbs the laser energy through a multiphoton absorption mechanism, thereby forming a modified region. Preferably, the wavelength of the first laser beam is 1064nm. According to the formula for calculating the energy of a laser photon, E = hc / λ, its energy E is approximately 1.17eV, far less than 3.26eV (the bandgap of SiC). A 1064nm photon does not have enough energy to directly excite electrons in the SiC valence band to the conduction band. Therefore, the 1064nm laser can penetrate the SiC wafer without being absorbed in large quantities, reaching deep into the wafer's interior, or even completely penetrating it.

[0050] Furthermore, the wavelength of the second laser beam in step S3 is <380nm. Preferably, the wavelength of the second laser beam is 355nm. The photon energy of 355nm ultraviolet light is about 3.49eV, which can be directly absorbed by the surface SiC, thereby generating heat and melting the surface material.

[0051] Furthermore, in step S1, when the modified layer 84 is formed on the peeling plane by laser irradiation scanning, the edge portion of the silicon carbide wafer 1 is not modified, forming an unmodified region 85, and a trench 86 is provided between the modified region and the unmodified region 85.

[0052] Example 1

[0053] Please see Figure 4 and Figure 6 The SiC wafer, with a diameter of 6 inches and an initial thickness of 350 μm, has undergone the necessary epitaxial growth and device fabrication processes on the Si surface 81 before thinning.

[0054] A first laser beam with a wavelength of 1064 nm is used to penetrate the silicon carbide wafer 1 from the C-plane 82, and the focal point is set at a position 150 μm away from the Si-plane 81 of the silicon carbide wafer 1, forming modification points 83, as shown. Figure 6 As shown. By translating the silicon carbide wafer 1, the laser beam performs a serpentine scan across the entire surface of the silicon carbide wafer 1, but leaves a 3-5 mm width from the edge unscanned (i.e., the unmodified area). Thousands of modification points 83 form a modification surface parallel to the Si surface 81 in the middle of the wafer. The area of ​​the modification surface is the modification area.

[0055] A groove 86 is cut on the outer ring of the modification zone using a cutting wheel, with the bottom of the groove 86 at the same height as the modification point 83. For example... Figure 5 As shown. Using crack propagation techniques, such as ultrasound, cracks are caused to originate from modification point 83 and extend along the 0001 crystal plane, thus connecting the modification points 83 to each other.

[0056] The silicon carbide wafer 1 is peeled in half from the modified layer 84 formed by the modification points 83, retaining the half containing the device structure, forming a concave wafer structure that is thick at the edges and thin in the middle. Because there is a 4° angle between the modified surface and the crystal plane, the peeled surface is stepped, causing significant wear on the grinding wheel. Therefore, further flattening is required before grinding. The retained half of the wafer is placed in a high-pressure chamber 11, which is filled with SiH4 at a pressure of 50 atmospheres. By translating the silicon carbide wafer 1, a laser beam scans within the peeled surface. A second laser beam, with a wavelength of 355 nm and an energy density of 9 joules per square centimeter, is used to melt the peeled surface, allowing it to be directly absorbed by SiC, causing the peeled surface to melt and then solidify, eliminating the surface steps.

[0057] Example 2

[0058] The only difference from Example 1 is that the scan starts at the edge of silicon carbide wafer 1, and there is no unmodified region. Please refer to [link / reference]. Figure 7 .

[0059] Furthermore, the crack propagation technology in step S2 includes one of ultrasonic waves, laser heating, and microwave heating. The crack propagation technology in step S2 employs existing technologies, such as patent CN115971642B, entitled "A Laser-Induced Cracking Silicon Carbide Stripping Sheet and Processing Method," which uses laser-induced cracking to strip silicon carbide. Another example is patent CN120503329A, entitled "Silicon Carbide Stripping Method and Silicon Carbide Stripping Device," which applies ultrasonic waves to the silicon carbide semi-finished product to improve the separation efficiency of silicon carbide.

[0060] Further, in step S4, the grinding of the laser-melted surface 87 by the diamond grinding wheel 9 includes the following steps:

[0061] S41. Roughly grind the molten surface with a coarse diamond grinding wheel;

[0062] S42. Use a precision diamond grinding wheel to finely grind the molten surface.

[0063] Specifically, please refer to Figure 8The specific mesh count, roughing thickness, and finishing thickness of the coarse and fine diamond grinding wheels are determined by the actual site conditions and process requirements, and this embodiment does not impose specific limitations on these. In this application, taking a 6-inch SiC wafer with an initial thickness of 350μm as an example, the coarse diamond grinding wheel has a mesh count of 2000, and the roughing thickness is 40μm, effectively removing the cut marks and damaged layer on the molten surface; the fine diamond grinding wheel has a mesh count of 8000, and the finishing thickness is 10μm, removing the damaged layer left by the roughing grinding. After the above process, the roughness of the molten surface 87 is reduced to below 10nm, enhancing the cost advantage originally possessed by the laser ablation process, and showing significant benefits compared to traditional pure mechanical grinding methods.

[0064] This invention discloses a method for thinning a silicon carbide wafer. A modified layer 84 is formed on a silicon carbide wafer 1 by irradiation and scanning with a first laser beam with a wavelength greater than 380 nm. Then, a crack propagation technique is used to peel off the modified layer 84. The peeled silicon carbide wafer 1 is then placed in a high-pressure chamber filled with a corresponding gas. A second laser beam with a wavelength less than 380 nm irradiates the silicon carbide wafer 1, melting the peeled surface. This melts and then solidifies the periodic steps formed during the peeling process, reducing the roughness of the molten surface 87 to below 10 nm, restoring it to a near-planar state. Finally, coarse and fine diamond grinding wheels are used to grind and remove the damage caused by the laser modification, further reducing the surface roughness. Through laser melting under high pressure and a corresponding gas environment, the roughness after peeling is improved, thereby effectively reducing the wear on the grinding wheels on the peeled surface.

[0065] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the scope of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for thinning silicon carbide wafers, characterized in that, Includes the following steps: S1. The focus of the first laser beam is aligned with the peeling plane of the silicon carbide wafer (1) at a preset depth. The silicon carbide wafer (1) on the peeling plane absorbs laser energy to form modification points (83), and the modification points (83) are connected by laser irradiation scanning to form a modification layer (84). S2. By using crack propagation technology, the modified points (83) are connected through microcracks, and the silicon carbide wafer (1) is peeled off along the modified layer (84) to form a peeling surface; S3. Place the silicon carbide wafer (1) in the high-pressure cavity and laser clad the peeling surface with the second laser beam to form a molten surface (87); S4. Grind the laser-melted surface (87) using a diamond grinding wheel (9).

2. The silicon carbide wafer thinning method according to claim 1, characterized in that, In step S3, the chamber pressure of the high-pressure cavity is 2-1000 times atmospheric pressure.

3. The silicon carbide wafer thinning method according to claim 1 or 2, characterized in that, In step S3, the chamber of the high-pressure cavity contains an inert gas.

4. The silicon carbide wafer thinning method according to claim 1 or 2, characterized in that, In step S3, the chamber of the high-pressure cavity contains Si gas.

5. The silicon carbide wafer thinning method according to claim 1 or 2, characterized in that, The wavelength of the first laser beam in step S1 is >380nm.

6. The silicon carbide wafer thinning method according to claim 4, characterized in that, The wavelength of the first laser beam in step S1 is >380nm.

7. The silicon carbide wafer thinning method according to claim 1, 2, or 6, characterized in that, The wavelength of the second laser beam in step S4 is <380nm.

8. The silicon carbide wafer thinning method according to claim 1, 2, or 6, characterized in that, In step S1, when the modified layer (84) is formed on the peeling plane by laser irradiation scanning, the edge part of the silicon carbide wafer (1) is not modified, forming an unmodified area (85), and a trench (86) is provided between the modified area and the unmodified area (85).

9. The silicon carbide wafer thinning method according to claim 1, 2, or 6, characterized in that, Furthermore, the crack propagation technique in step S2 includes one of ultrasonic heating, laser heating, and microwave heating.

10. The silicon carbide wafer thinning method according to claim 1, 2, or 6, characterized in that, In step S4, the diamond grinding wheel (9) grinds the molten surface (87) including the following steps: S41. Roughly grind the molten surface with a coarse diamond grinding wheel; S42. Use a precision diamond grinding wheel to finely grind the molten surface.

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