Method for improving SiC crystal ingot laser slicing surface roughness through laser annealing

Abstract

The invention relates to a method for improving SiC crystal ingot laser slicing surface roughness through laser annealing. The method comprises the following steps: firstly stripping a SiC wafer from a SiC crystal ingot in a laser stripping manner, then forming an annealing layer on the surface of the stripped SiC wafer by using laser, and finally pasting a UV film on the smooth surface of the annealed SiC wafer, and putting the annealed SiC wafer into a thinning machine for thinning, thereby completing the laser slicing of the SiC crystal ingot. According to the method for improving the surface roughness of the SiC crystal ingot laser slicing, the laser with the specific wavelength is utilized, the annealing layer is formed on the surface of the SiC wafer after stripping, the surface roughness of the SiC wafer is reduced, the strength of the surface area of the SiC wafer is reduced, disintegrating slag hidden in tool mark gaps of the SiC wafer after the SiC wafer is stripped can be reduced, and the service life of the SiC wafer is prolonged. The cost of the grinding wheel in subsequent thinning of the SiC wafer is reduced, and the cracking rate of the SiC wafer is also reduced.

Description

Technical Field

[0001] The invention relates to a method for improving the surface roughness of SiC crystal ingot laser slicing through laser annealing, belonging to the technical field of laser annealing. Background Art

[0002] Silicon carbide (SiC), as a third-generation semiconductor material, has attracted much attention due to its unique physical properties. This material has remarkable characteristics such as wide bandgap, high breakdown electric field, high thermal conductivity, high electron saturation rate and high radiation resistance. These characteristics enable SiC to work stably under extreme environments such as high temperature, high voltage, high frequency and other conditions, making it very suitable for the manufacture of high-temperature, high-frequency, radiation-resistant and high-power devices. SiC has a wide range of applications, including high-voltage professional applications such as automotive electronics, motor drives, and solar energy. Especially in the field of new energy vehicles, the application of SiC power devices can significantly improve the operating efficiency and endurance of motors.

[0003] Silicon carbide (SiC) ingot slicing is to process large-sized ingots into wafers of a specific thickness for subsequent semiconductor device manufacturing. Slicing methods vary according to technology and needs, and mainly include the following methods: First, multi-wire slicing, which works by using high-speed diamond wires to drive the synergistic effect of abrasives and cutting fluids to slice the ingots. Its processing characteristics are: 1. High cutting efficiency, suitable for mass production; 2. Uniform slicing, relatively less material waste; 3. High requirements for equipment and operating technology, currently the mainstream method of SiC ingot slicing in the current industry. Second, laser slicing, which works by focusing a high-energy laser beam at a certain depth of the ingot to form a modified layer to cut the ingot layer by layer, and by laser-induced microcracks in the crystal to assist in the subsequent peeling of the wafer. Its processing characteristics are: 1. High slicing accuracy, suitable for ultra-thin wafer manufacturing; 2. Non-contact processing to avoid problems such as chip edge collapse; 3. Suitable for low-damage cutting of large-sized ingots. Currently, laser slicing is mainly used in high-precision cutting scenarios, such as the manufacture of ultra-thin wafers or large-sized wafers.

[0004] At present, in the silicon carbide ingot slicing process, a laser slicing method is used to meet certain high-precision cutting requirements. For example, Chinese patent document CN105855732A discloses a method for generating a wafer, the main steps of which include: 1) using a first laser beam of a specific wavelength to form a modified layer and cracks inside the ingot to create a separation starting point; 2) using a second laser beam with a higher repetition frequency to further reduce the bonding strength between the modified layer and the cracks; 3) using ultrasonic waves to act on the modified layer in the ingot in water, and then peeling out the wafer. Although this method improves production efficiency, reduces material waste, and is suitable for slicing silicon carbide ingots with high hardness, it does not mention the thinning process of the wafer after peeling. The surface roughness of the wafer after peeling may be poor, and crystal debris will appear when the wafer is separated, resulting in the occurrence of cracks in the subsequent thinning process.

[0005] Chinese patent document CN115635183A discloses a method for laser stripping of workpieces, wafers, and semiconductor devices, the main steps of which include: 1) using a short pulse width pulse laser to form a modified point on a preset stripping surface inside the workpiece; 2) using a long pulse width pulse laser to focus on the modified point to form a modified area and a crack extending radially along the preset stripping surface; 3) dividing the workpiece into two units along the preset stripping surface. Similarly, this method does not study the thinning of the wafer, the surface roughness of the wafer after stripping cannot be guaranteed, and the problem of thinning and cracking is not solved.

[0006] Chinese patent document CN115410979A discloses a wafer peeling method and a laser slicing method, including loading, clamping, fixing and peeling steps. First, the wafer and the hot melt adhesive sheet are installed between the temperature control fixture; then, the wafer and the adhesive sheet are clamped by the linear drive module; then, the wafer is heated, kept warm, pressurized and cooled to fix the adhesive sheet; finally, the fixture is moved in the opposite direction to separate the modified layer of the wafer to achieve peeling. The laser slicing method involves using a laser to form a modified layer inside the wafer, and then using the above-mentioned peeling method to obtain the wafer and the remaining wafer, and the remaining wafer is processed by cyclic grinding, laser and peeling to continuously and automatically produce multiple wafers. However, there are two problems with this method. On the one hand, the problem of wafer cracking occurs during the mechanical peeling process, and on the other hand, the surface roughness of the wafer still does not meet the requirements for direct thinning, which may cause the problem of wafer cracking in subsequent thinning.

[0007] At the same time, some of the above-mentioned prior arts do not mention the grinding of the wafer, and some use a thinning machine for grinding. However, after the wafer is peeled off, the surface roughness of the wafer is poor, and there are serrated micro grooves on the surface. These serrated structures will increase the loss of the thinning grinding wheel and thus increase the cost; and there are fine SiC debris inside the grooves. These debris may cause cracks in the wafer during the grinding process, thereby increasing the chipping rate of the wafer grinding. Summary of the invention

[0008] In view of the shortcomings of the prior art, the present invention provides a method for improving the surface roughness of SiC ingot laser slicing by laser annealing. The method can solve the problems of low grinding efficiency, high production cost and high chipping rate of the existing grinding scheme, and can also adjust the polarization direction and power of the laser through the polarization beam splitter PBS and the half-wave plate. The method can be applied to the thinning of silicon carbide peeling wafers of different sizes and doping elements, and can also anneal peeling wafers with different surface roughness.

[0009] A method for improving the surface roughness of SiC ingot laser slicing by laser annealing is to first peel the SiC wafer from the SiC ingot by laser peeling, then use laser to form an annealing layer on the surface of the peeled SiC wafer, and finally stick a UV film on the smooth surface of the annealed SiC wafer and put it into a thinning machine for thinning to complete the SiC ingot laser slicing.

[0010] The above method for improving the surface roughness of SiC ingot laser slicing by laser annealing has the following specific steps:

[0011] (1) Wafer peeling: After pre-treatment, the SiC ingot is fixed on a two-dimensional motion platform and processed once by laser to form a modified layer and cracks inside the SiC ingot; then the laser parameters are adjusted and a secondary processing is performed along the path of the primary processing to reduce the strength of the modified layer and cracks formed inside the SiC ingot; finally, the SiC ingot after the two processing is removed and immersed in water, and the wafer is peeled by ultrasonic vibration to obtain a SiC wafer;

[0012] (2) fixing the SiC wafer obtained in step (1) on a two-dimensional motion platform, and then placing the two-dimensional motion platform in a closed cavity with a transparent top, and forming a uniform annealing layer on the upper surface of the SiC wafer by laser scanning in an oxygen atmosphere to obtain an annealed SiC wafer;

[0013] (3) The annealed SiC wafer obtained in step (2) is placed in a film laminating machine with the smooth surface facing upward, a UV film is affixed on the smooth surface, and the film-laminating SiC wafer is fixed in a thinning machine for grinding to complete the laser slicing of the SiC ingot and obtain a SiC wafer of a specified thickness.

[0014] Preferably, according to the present invention, in step (1), the diameter of the SiC ingot is greater than 4 inches, and the model is a semi-insulating or conductive 4H-SiC ingot; the pretreatment is: double-sided fine grinding of the SiC ingot until it is smooth.

[0015] Preferably, in step (1) of the present invention, the parameters of the one-time processing are: laser wavelength of 1000-1200nm, pulse width of 5-30ns, repetition frequency of 20-200kHz, light output power of 2-12W, two-dimensional platform moving speed of 50-500mm / s, and the depth of the modified layer after one-time processing is 400-600μm, so that a modified layer and cracks are formed inside the SiC ingot, and a separation starting point is created. In this process, real-time height measurement compensation is used to ensure that the laser beam can be processed at a predetermined depth.

[0016] Further preferably, the parameters of the one-time processing are: laser wavelength is 1030nm or 1064nm, pulse width is 20ns, repetition frequency is 180kHz, light output power is 8W, two-dimensional platform moving speed is 400mm / s, and the depth of the modified layer after one-time processing is 470μm.

[0017] Preferably, according to the present invention, in step (1), the secondary processing is to adjust the repetition frequency in the primary processing to 300-400 kHz, so that the strength of the modified layer and cracks formed previously is further reduced, thereby reducing the bonding strength between the SiC wafer and the SiC ingot.

[0018] Preferably, according to the present invention, in step (1), the frequency of the ultrasonic vibration is controlled to be 20 to 45 kHz, and a specific ultrasonic vibrator is used at a certain distance from the crystal ingot to act on the modified layer in the crystal ingot, thereby further reducing the bonding strength of the modified layer until the SiC wafer is peeled off from the SiC crystal ingot.

[0019] Preferably, according to the present invention, in step (2), the wavelength of the laser is 200-800nm, the pulse width is 10ps-300ns, the frequency is 10-500kHz, the energy of a single focal point is 10-100μJ, and the laser beam is focused onto the surface of the SiC wafer through the transparent upper surface of the cavity, and an annealing layer is uniformly formed on its surface by cooperating with a two-dimensional motion platform.

[0020] Further preferably, the wavelength of the laser is 532 nm, the pulse width is 20 ps, ​​the frequency is 50 kHz, and the energy of a single focal point is 80 μJ.

[0021] Preferably, in step (2), the laser scanning is specifically as follows: the laser beam is first adjusted to have a power and polarization direction by a polarization beam splitter PBS and a half-wave plate, and then the laser is precisely focused to the upper surface of the SiC wafer by a high reflector and a high-speed galvanometer, and the polarization direction of the laser is the same as the scanning direction; after one laser scan, an annealing layer is formed in an area of ​​1×1cm or 2×2cm, and the laser scanning operation is repeated to form a uniform annealing layer on the upper surface of the SiC wafer. By controlling the size of the annealing area, the efficiency of the annealing process can be adjusted on the one hand, and the stability of the annealing effect can be adjusted on the other hand.

[0022] Preferably, according to the present invention, in step (3), the parameters of the thinning machine are: the spindle speed is 1400-2200 rpm, the worktable speed is 100-300 rpm, and the spindle feed speed is 0.1-0.5 μm / s.

[0023] Further preferably, the parameters of the thinning machine are: the spindle speed is 1800 rpm, the worktable speed is 260 rpm, and the spindle feed speed is 0.4 m / s.

[0024] Beneficial effects of the present invention:

[0025] 1. The method for improving the surface roughness of SiC ingot laser slicing provided by the present invention utilizes a laser of a specific wavelength to form an annealing layer on the surface of the SiC wafer after peeling, thereby reducing the roughness of the SiC wafer surface, reducing the strength of the SiC wafer surface area, and being able to reduce the debris hidden in the knife marks and gaps of the SiC wafer after the SiC wafer is peeled off, reducing the grinding wheel cost of the SiC wafer in the subsequent thinning, and also reducing the cracking rate of the SiC wafer.

[0026] 2. The surface roughness of the SiC wafer obtained by the method of the present invention is between 2 and 3 μm, wherein the depth of the sawtooth grooves is below 5 μm, thereby improving the surface quality of the SiC wafer.

[0027] 3. The method provided by the present invention for improving the surface roughness of SiC ingot laser slicing by laser annealing significantly reduces the loss ratio of SiC wafer and grinding wheel, reducing the loss ratio of SiC wafer and grinding wheel to 1: (1.2-1.3), thereby greatly reducing production costs.

[0028] 4. The method provided by the present invention for improving the surface roughness of SiC ingot laser slices by laser annealing can be applied to the annealing and thinning of silicon carbide peeling wafers of different sizes (above 4 inches) and different doping elements.

[0029] 5. The method provided by the present invention for improving the surface roughness of SiC crystal ingot laser slices by laser annealing has wide applicability. The output power of the laser and the polarization direction of the output light are adjusted by the half-wave plate angle and the polarization beam splitter prism PBS, and the focusing position of the laser is accurately controlled by the high-speed galvanometer, so as to achieve precise control of the annealing layer on the surface of the wafer, and the laser power can be adjusted by adjusting the half-wave plate according to the different doped resistivity crystal ingots to form a good annealing layer. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 The figure is a flowchart of a method for improving the surface roughness of a SiC wafer by laser annealing in an embodiment of the present invention.

[0031] Figure 2 Schematic diagram of the surface microstructure of a SiC wafer during peeling process in an embodiment of the present invention;

[0032] Among them, A is the laser processing line mark, and B is the SiC wafer structure after peeling.

[0033] Figure 3 Schematic diagram of using laser to form an annealing layer on the peeled surface of a SiC wafer in an embodiment of the present invention.

[0034] Figure 4 This is the processing path of the laser when forming the surface annealing layer in the embodiment of the present invention.

[0035] Figure 5 It is a schematic diagram of thinning and polishing a wafer after annealing in an embodiment of the present invention.

[0036] Figure 6 It is the white light interference test pattern of the wafer after annealing and the sawtooth morphology of the cross section.

[0037] Figure 7 It is the white light interference test pattern of the wafer before annealing and the sawtooth morphology of the cross section.

[0038] In the above figure, 1 is SiC ingot, 2 is laser processing line mark, 3 is serrated structure, 4 is SiC wafer, 5 is No. 2 laser, 6 is half-wave plate, 7 is polarization beam splitter prism PBS, 8 is high reflection mirror, 9 is high-speed galvanometer, 10 is glass cover, 11 is cavity, 12 is air inlet pipe, 13 is air outlet pipe, 14 is two-dimensional motion platform, 15 is annealing zone, 16 is laser scanning path, 17 is rotating spindle, 18 is grinding wheel, 19 is grinding wheel teeth, and 20 is rotatable worktable. DETAILED DESCRIPTION

[0039] The specific implementation modes of the present invention are described below so that those skilled in the art can understand the present invention. However, it should be clear that the present invention is not limited to the scope of the specific implementation modes. For those of ordinary skill in the art, as long as various changes are within the spirit and scope of the present invention as defined and determined by the attached claims, these changes are obvious, and all inventions and creations utilizing the concept of the present invention are protected.

[0040] like Figure 1 As shown, the method of improving the surface roughness of SiC crystal ingot laser slicing by laser annealing of the present invention generally comprises the following steps: firstly, the SiC wafer is peeled off from the SiC crystal ingot by laser peeling, then the annealing layer is formed on the surface of the peeled SiC wafer by laser, and finally, the smooth surface of the annealed SiC wafer is pasted with UV film and placed in a thinning machine for thinning, thereby completing the SiC crystal ingot laser slicing. The specific implementation process is shown in the following embodiment.

[0041] like Figure 2 As shown, laser light of a certain wavelength is focused at a certain thickness inside the SiC ingot 1, and laser processing lines 2 are formed along a specified processing path.

[0042] Example 1

[0043] A method for improving the surface roughness of SiC ingot laser slicing by laser annealing, the specific steps are as follows:

[0044] (1) Wafer peeling: The high resistivity conductive 4H-SiC ingot after double-sided fine grinding is fixed on a two-dimensional motion platform by vacuum adsorption, and is processed once by a No. 1 laser to form a modified layer and cracks inside the SiC ingot; then the parameters of the No. 1 laser are adjusted, and a secondary processing is performed along the path of the primary processing, so that the strength of the modified layer and cracks formed inside the SiC ingot is reduced, and a separation starting point is created to facilitate the subsequent wafer peeling step. During this process, real-time height measurement compensation can be used to ensure that the laser beam can be processed at a predetermined depth; finally, the SiC ingot with a good modified layer formed after the two processings is removed and placed in a water tank, and water that can submerge the SiC ingot is poured into the water tank, and an ultrasonic vibrator is used to act on the modified layer of the SiC ingot. The frequency of the ultrasonic vibration is 40kHz, thereby further reducing the bonding strength of the modified layer until the SiC wafer is peeled off from the ingot;

[0045] The parameters of the primary processing are: laser wavelength of 1030nm, pulse width of 20ns, repetition frequency of 180kHz, light output power of 8W, two-dimensional platform moving speed of 400mm / s, and the depth of the modified layer after the primary processing is 470μm; the secondary processing is to adjust the repetition frequency of the primary processing to 360kHz;

[0046] The traces of primary and secondary processing are as follows: Figure 2 As shown in A, the laser forms laser processing line marks 2 on the SiC crystal ingot 1 according to the specified processing path;

[0047] The microstructure of the SiC wafer after peeling is as follows Figure 2 As shown in B, the SiC wafer 4 includes a sawtooth structure 3, and SiC crystal fragments may remain in the sawtooth structure 3, which is the main reason for the wafer to split during the thinning process;

[0048] (2) Figure 3 As shown, the SiC wafer obtained in step (1) is cleaned and fixed on a two-dimensional motion platform 14, and then the two-dimensional motion platform 14 is placed in a cavity 11 with an open upper cover, a glass cover plate 10 is placed above the cavity 11, an air inlet pipe 12 is arranged on the side of the cavity 11, and oxygen is introduced into the cavity 11, which helps to form SI-O compounds on the surface of the wafer, and an air outlet pipe 13 is arranged on the other side of the cavity 11 to discharge the introduced oxygen;

[0049] In an oxygen atmosphere, a laser beam is emitted by a second laser 5. The laser beam first passes through a half-wave plate 6 and a polarization beam splitter PBS7 to adjust the output power and polarization direction of the laser beam, and then is guided into a high-speed galvanometer 9 through a high-reflection mirror 8. The laser beam is focused to a distance from the surface of the SiC wafer through the high-speed galvanometer 9. After one laser scan, an annealing area with an area of ​​1×1 cm is formed on the surface of the SiC wafer. The laser scanning operation is repeated to form a uniform annealing layer on the upper surface of the SiC wafer.

[0050] The specific path of laser scanning is as follows Figure 4 As shown, a single laser can only form an annealing zone 15 on the SiC wafer 4. After a single processing, the two-dimensional motion platform 14 below the SiC wafer 4 is moved to process the wafer according to a specified laser scanning path 16, so that a uniform annealing layer is formed on the upper surface of the SiC wafer to obtain an annealed SiC wafer;

[0051] The wavelength of the laser is 532nm, the pulse width is 20ps, the frequency is 50kHz, and the energy of a single focal point is 80μJ;

[0052] (3) Clean the smooth surface of the annealed SiC wafer obtained in step (2) with a dust-free cloth, place it in a film laminating machine with the smooth surface facing upward, attach a UV film to the smooth surface, and then fix the SiC wafer after laminating on a ceramic suction cup on a thinning machine workbench by vacuum adsorption for grinding, remove the SiC wafer by a specified thickness, and complete the laser slicing of the SiC ingot;

[0053] Among them, the structure of the thinning machine is as follows Figure 5As shown, it includes a high-speed rotating module and a low-speed high-speed rotating module. The high-speed rotating module is composed of a rotating spindle 17 and a grinding wheel 18 connected to the rotating spindle. The edge of the grinding wheel 18 is composed of a plurality of grinding wheel teeth 19. The low-speed rotating module is composed of a rotatable workbench 20 and a SiC wafer 4 with a UV film attached.

[0054] The parameters of the thinning machine are: the spindle speed is 1800 rpm, the table speed is 260 rpm, and the spindle feed speed is 0.4 m / s.

[0055] The surface of the SiC wafer after annealing obtained in step (2) of this embodiment is as follows Figure 6 As shown, it can be seen that the jagged upper end of the wafer is ablated by the laser, the depth of the jagged grooves of the wafer is greatly reduced, and the SiC crystal debris on the surface of the wafer is also blown away by the oxygen introduced during the laser annealing process. Finally, the surface roughness of the wafer is reduced and the overall quality is better.

[0056] In this embodiment, 20 SiC wafers were annealed repeatedly according to step (2). After annealing, the average surface roughness of the SiC wafers was measured by white light interferometer to be 2-3 μm, and the depth of the surface sawtooth grooves was less than 5 μm. Subsequently, the SiC wafers were ground to a thickness of 370 μm according to step (3), and the grinding wheel loss ratio was 1:1.2. After grinding 20 annealed SiC wafers, no cracking occurred.

[0057] Example 2

[0058] A method for improving the surface roughness of SiC ingot laser slicing by laser annealing, the specific steps are as follows:

[0059] (1) Wafer stripping: The low-resistivity conductive 8-inch 4H-SiC ingot after double-sided fine grinding is fixed on a two-dimensional motion platform by vacuum adsorption, and processed once by laser No. 1 to form a modified layer and cracks inside the SiC ingot; then the parameters of laser No. 1 are adjusted, and secondary processing is performed along the path of the primary processing, so that the strength of the modified layer and cracks formed inside the SiC ingot is reduced, and a separation starting point is created to facilitate the subsequent wafer stripping step. During this process, real-time height measurement compensation can be used to ensure that the laser beam can be processed at a predetermined depth; finally, the SiC ingot with a good modified layer after the two processes is removed and placed in a water tank, and water that can submerge the SiC ingot is poured into the water tank, and the modified layer of the SiC ingot is acted on by an ultrasonic vibrator, and the frequency of the ultrasonic vibration is 40kHz, thereby further reducing the bonding strength of the modified layer until the SiC wafer is stripped from the ingot;

[0060] The parameters of the primary processing are: laser wavelength of 1030nm, pulse width of 20ns, repetition frequency of 200kHz, light output power of 12W, two-dimensional platform moving speed of 400mm / s, and the depth of the modified layer after the primary processing is 470μm; the secondary processing is to adjust the repetition frequency of the primary processing to 400kHz;

[0061] (2) The SiC wafer obtained in step (1) is cleaned and fixed on a two-dimensional motion platform 14, and then the two-dimensional motion platform 14 is placed in a cavity 11 with an open upper cover, a glass cover plate 10 is placed above the cavity 11, an air inlet pipe 12 is passed through the side of the cavity 11, and oxygen is passed into the air pipe to help form SI-O compounds on the surface of the wafer, and at the same time, an air outlet pipe 13 is passed through the other side of the cavity 11 to release the oxygen that has been passed;

[0062] In an oxygen atmosphere, a laser beam is emitted by a second laser 5. The laser beam first passes through a half-wave plate 6 and a polarization beam splitter PBS7 to adjust the output power and polarization direction of the laser beam, and then is guided into a high-speed galvanometer 9 through a high-reflection mirror 8. The laser beam is focused to a distance from the surface of the SiC wafer through the high-speed galvanometer 9. After one laser scan, an annealing area with an area of ​​1×1 cm is formed on the surface of the SiC wafer. The laser scanning operation is repeated to form a uniform annealing layer on the upper surface of the SiC wafer.

[0063] The wavelength of the laser is 532nm, the pulse width is 20ps, the frequency is 30kHz, and the energy of a single focal point is 90μJ;

[0064] (3) Clean the smooth surface of the annealed SiC wafer obtained in step (2) with a dust-free cloth, place it in a film laminating machine with the smooth surface facing upward, attach a UV film to the smooth surface, and then fix the film-attached SiC wafer on a ceramic suction cup on a thinning machine workbench by vacuum adsorption for grinding, remove a specified thickness from the SiC wafer, and complete the laser slicing of the SiC ingot;

[0065] The parameters of the thinning machine are: the spindle speed is 1800 rpm, the table speed is 260 rpm, and the spindle feed speed is 0.4 m / s.

[0066] The surface of the SiC wafer after annealing obtained in step (2) of this embodiment is as follows Figure 6 As shown, it can be seen that the jagged upper end of the wafer is ablated by the laser, the depth of the jagged grooves of the wafer is greatly reduced, and the SiC crystal debris on the surface of the wafer is also blown away by the oxygen introduced during the laser annealing process. Finally, the surface roughness of the wafer is reduced and the overall quality is better.

[0067] In this embodiment, 20 SiC wafers were annealed repeatedly according to step (2). After annealing, the average surface roughness of the SiC wafers was measured by white light interferometer to be 2-3 μm, and the depth of the surface sawtooth grooves was less than 5 μm. Subsequently, according to step (3), the SiC wafers were ground to a thickness of 370 μm, and the grinding wheel loss ratio was 1:1.25. After grinding 20 annealed SiC wafers, no cracking occurred.

[0068] Example 3

[0069] A method for improving the surface roughness of SiC ingot laser slicing by laser annealing, the specific steps are as follows:

[0070] (1) Wafer peeling: The semi-insulating 4H-SiC ingot after double-sided fine grinding is fixed on a two-dimensional motion platform by vacuum adsorption, and is processed once by laser No. 1 to form a modified layer and cracks inside the SiC ingot; then the parameters of laser No. 1 are adjusted, and secondary processing is performed along the path of the primary processing, so that the strength of the modified layer and cracks formed inside the SiC ingot is reduced, and a separation starting point is created to facilitate the subsequent wafer peeling step. During this process, real-time height measurement compensation can be used to ensure that the laser beam can be processed at a predetermined depth; finally, the SiC ingot with a good modified layer formed after the two processes is removed and placed in a water tank, and water that can submerge the SiC ingot is poured into the water tank, and the modified layer of the SiC ingot is acted on by an ultrasonic vibrator, and the frequency of the ultrasonic vibration is 40kHz, thereby further reducing the bonding strength of the modified layer until the SiC wafer is peeled off from the ingot;

[0071] The parameters of the primary processing are: laser wavelength of 1065nm, pulse width of 20ns, repetition frequency of 180kHz, light output power of 2W, two-dimensional platform moving speed of 400mm / s, and the depth of the modified layer after the primary processing is 470μm; the secondary processing is to adjust the repetition frequency of the primary processing to 360kHz;

[0072] (2) The SiC wafer obtained in step (1) is cleaned and fixed on a two-dimensional motion platform 14, and then the two-dimensional motion platform 14 is placed in a cavity 11 with an open upper cover, a glass cover plate 10 is placed above the cavity 11, an air inlet pipe 12 is passed through the side of the cavity 11, and oxygen is passed into the air pipe to help form SI-O compounds on the surface of the wafer, and at the same time, an air outlet pipe 13 is passed through the other side of the cavity 11 to release the oxygen that has been passed;

[0073] In an oxygen atmosphere, a laser beam is emitted by a second laser 5. The laser beam first passes through a half-wave plate 6 and a polarization beam splitter PBS7 to adjust the output power and polarization direction of the laser beam, and then is guided into a high-speed galvanometer 9 through a high-reflection mirror 8. The laser beam is focused to a distance from the surface of the SiC wafer through the high-speed galvanometer 9. After one laser scan, an annealing area with an area of ​​1×1 cm is formed on the surface of the SiC wafer. The laser scanning operation is repeated to form a uniform annealing layer on the upper surface of the SiC wafer.

[0074] The wavelength of the laser is 532nm, the pulse width is 20ps, the frequency is 30kHz, and the energy of a single focal point is 48μJ;

[0075] (3) Clean the smooth surface of the annealed SiC wafer obtained in step (2) with a dust-free cloth, place it in a film laminating machine with the smooth surface facing upward, attach a UV film to the smooth surface, and then fix the film-attached SiC wafer on a ceramic suction cup on a thinning machine workbench by vacuum adsorption for grinding, remove a specified thickness from the SiC wafer, and complete the laser slicing of the SiC ingot;

[0076] The parameters of the thinning machine are: the spindle speed is 1800 rpm, the table speed is 260 rpm, and the spindle feed speed is 0.4 m / s.

[0077] The surface of the SiC wafer after annealing obtained in step (2) of this embodiment is as follows Figure 6 As shown, it can be seen that the jagged upper end of the wafer is ablated by the laser, the depth of the jagged grooves of the wafer is greatly reduced, and the SiC crystal debris on the surface of the wafer is also blown away by the oxygen introduced during the laser annealing process. Finally, the surface roughness of the wafer is reduced and the overall quality is better.

[0078] In this embodiment, 20 SiC wafers were annealed repeatedly according to step (2). After annealing, the average surface roughness of the SiC wafers was measured by white light interferometer to be 2-3 μm, and the depth of the surface sawtooth grooves was less than 5 μm. Subsequently, according to step (3), the SiC wafers were ground to a thickness of 370 μm, and the grinding wheel loss ratio was 1:1.3. After grinding 20 annealed SiC wafers, no cracking occurred.

[0079] Comparative Example 1

[0080] A wafer peeling and laser slicing method, the specific steps are as follows:

[0081] (1) Wafer peeling: The conductive 8-inch 4H-SiC ingot after double-sided fine grinding is fixed on a two-dimensional motion platform by vacuum adsorption, and a first processing is performed by a No. 1 laser to form a modified layer and cracks inside the SiC ingot; then the parameters of the No. 1 laser are adjusted, and a second processing is performed along the path of the first processing; finally, the SiC ingot after the second processing is removed and placed in a water tank, and water that can submerge the SiC ingot is poured into the water tank, and an ultrasonic vibrator is used to act on the modified layer of the SiC ingot. The frequency of the ultrasonic vibration is 40kHz, until the SiC wafer is peeled off from the ingot;

[0082] The parameters of the primary processing are: laser wavelength of 1030nm, pulse width of 20ns, repetition frequency of 200kHz, light output power of 12W, two-dimensional platform moving speed of 400mm / s, and the depth of the modified layer after the primary processing is 470μm; the secondary processing is to adjust the repetition frequency of the primary processing to 400kHz;

[0083] (2) using a dust-free cloth to clean the smooth surface of the SiC wafer after laser peeling obtained in step (1), placing it into a film laminating machine with the smooth surface facing upward, attaching a UV film to the smooth surface, and then fixing the SiC wafer after film lamination on a ceramic suction cup on a thinning machine workbench by vacuum adsorption for grinding, removing a specified thickness from the SiC wafer, and completing the laser slicing of the SiC ingot;

[0084] The parameters of the thinning machine are: the spindle speed is 1800 rpm, the table speed is 260 rpm, and the spindle feed speed is 0.4 m / s.

[0085] The surface of the SiC wafer obtained in this comparative example is as follows Figure 7 As shown, a series of sawtooth grooves can be seen on the surface of the wafer. There are some SiC crystal fragments produced by peeling in these grooves, which will cause the wafer to crack during the subsequent grinding process.

[0086] The peeling of 20 SiC wafers was repeated, and the average surface roughness was measured by white light interferometer to be 5-9μm, and the depth of the surface jagged grooves was more than 15μm. The SiC wafers were subsequently ground to a thickness of 370μm, with a grinding wheel loss ratio of 1:1.9 and a cracking rate of 30% after grinding 20 wafers.

[0087] By comparing Examples 1 to 3 with Comparative Example 1, it can be seen that the method for improving the surface roughness of SiC ingot laser slicing provided by the present invention utilizes a laser of a specific wavelength to form an annealing layer on the surface of the SiC wafer after peeling, thereby reducing the roughness of the SiC wafer surface, reducing the strength of the SiC wafer surface area, and reducing the debris hidden in the knife marks and gaps of the SiC wafer after the SiC wafer is peeled off, reducing the grinding wheel cost of the SiC wafer in the subsequent thinning, and also reducing the cracking rate of the SiC wafer. The surface roughness of the SiC wafer is between 2 and 3 μm, wherein the depth of the sawtooth groove is below 5 μm, and the loss ratio of the SiC wafer and the grinding wheel is reduced to 1: (1.2 to 1.3), thereby improving the surface quality of the SiC wafer and greatly reducing the production cost.

[0088] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for improving the surface roughness of SiC ingot laser slices by laser annealing, characterized in that: The SiC wafer is first peeled off from the SiC ingot by laser peeling, and then an annealing layer is formed on the surface of the peeled SiC wafer using laser. Finally, the smooth surface of the annealed SiC wafer is covered with UV film and placed in a thinning machine for thinning to complete the laser slicing of the SiC ingot.

2. The method for improving the surface roughness of SiC ingot laser slicing by laser annealing according to claim 1, characterized in that: The specific steps are as follows: (1) Wafer peeling: After pre-treatment, the SiC ingot is fixed on a two-dimensional motion platform and processed by laser to form a modified layer and cracks inside the SiC ingot. Then, the laser parameters are adjusted to perform secondary processing along the path of the primary processing. Finally, the SiC ingot after the two processing is removed and immersed in water, and the wafer is peeled by ultrasonic vibration to obtain a SiC wafer. (2) fixing the SiC wafer obtained in step (1) on a two-dimensional motion platform, and then placing the two-dimensional motion platform in a closed cavity with a transparent top, and forming a uniform annealing layer on the upper surface of the SiC wafer by laser scanning in an oxygen atmosphere to obtain an annealed SiC wafer; (3) The annealed SiC wafer obtained in step (2) is placed in a film laminating machine with the smooth surface facing upward, a UV film is affixed on the smooth surface, and the film-laminating SiC wafer is fixed in a thinning machine for grinding to complete the laser slicing of the SiC ingot and obtain a SiC wafer of a specified thickness.

3. The method for improving the surface roughness of SiC ingot laser slicing by laser annealing according to claim 2, characterized in that: In step (1), the diameter of the SiC ingot is greater than 4 inches, and the type is a semi-insulating or conductive 4H-SiC ingot; the pretreatment is: double-sided grinding of the SiC ingot to make it smooth.

4. The method for improving the surface roughness of SiC ingot laser slicing by laser annealing according to claim 2, characterized in that: The parameters of the one-time processing are: laser wavelength of 1000-1200nm, pulse width of 5-30ns, repetition frequency of 20-200kHz, light output power of 2-12W, two-dimensional platform moving speed of 50-500mm / s, and the depth of the modified layer after one-time processing is 400-600μm; Further preferably, the parameters of the one-time processing are: laser wavelength is 1030nm or 1064nm, pulse width is 20ns, repetition frequency is 180kHz, light output power is 8W, two-dimensional platform moving speed is 400mm / s, and the depth of the modified layer after one-time processing is 470μm.

5. The method for improving the surface roughness of SiC ingot laser slicing by laser annealing according to claim 2, characterized in that: In step (1), the secondary processing is to adjust the repetition frequency in the primary processing to 300-400 kHz.

6. The method for improving the surface roughness of SiC ingot laser slicing by laser annealing according to claim 2, characterized in that: In step (1), the frequency of the ultrasonic vibration is controlled to be 20 to 45 kHz.

7. The method for improving the surface roughness of SiC ingot laser slicing by laser annealing according to claim 2, characterized in that: In step (2), the wavelength of the laser is 200 to 800 nm, the pulse width is 10 ps to 300 ns, the frequency is 10 to 500 kHz, and the energy of a single focal point is 10 to 100 μJ.

8. The method for improving the surface roughness of SiC ingot laser slicing by laser annealing according to claim 7, characterized in that: The wavelength of the laser is 532 nm, the pulse width is 20 ps, ​​the frequency is 50 kHz, and the energy of a single focal point is 80 μJ.

9. The method for improving the surface roughness of SiC ingot laser slicing by laser annealing according to claim 2, characterized in that: In step (2), the laser scanning is specifically as follows: the laser beam is first adjusted in power and polarization direction by a polarization beam splitter prism PBS and a half-wave plate, and then the laser is precisely focused on the upper surface of the SiC wafer by a high-reflection mirror and a high-speed galvanometer, wherein the polarization direction of the laser is the same as the scanning direction; after one laser scan, an annealing layer is formed in an area of ​​1×1 cm or 2×2 cm; and the laser scanning operation is repeated to form a uniform annealing layer on the upper surface of the SiC wafer.

10. The method for improving the surface roughness of SiC ingot laser slices by laser annealing according to claim 2, characterized in that: In step (3), the parameters of the thinning machine are: the spindle speed is 1400-2200 rpm, the table speed is 100-300 rpm, and the spindle feed speed is 0.1-0.5 μm / s; Further preferably, the parameters of the thinning machine are: the spindle speed is 1800 rpm, the worktable speed is 260 rpm, and the spindle feed speed is 0.4 m / s.

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