A method for improving surface roughness of a SiC crystal ingot laser slice by laser annealing

Abstract

The application relates to a method for improving the surface roughness of a SiC crystal ingot laser slice through laser annealing. The SiC wafer is first peeled from the SiC crystal ingot through laser peeling, then an annealing layer is formed on the surface of the peeled SiC wafer through laser, finally, the smooth surface of the annealed SiC wafer is pasted with a UV film and put into a thinning machine for thinning to complete the SiC crystal ingot laser slicing. The method for improving the surface roughness of the SiC crystal ingot laser slice provided by the application utilizes laser of a specific wavelength to form an annealing layer on the surface of the peeled SiC wafer, reduces the roughness of the surface of the SiC wafer, reduces the intensity of the surface area of the SiC wafer, and can reduce the slag hidden in the SiC wafer scratch gap after the SiC wafer is peeled, the cost of the grinding wheel in the subsequent thinning of the SiC wafer is reduced, and the wafer cracking rate of the SiC wafer is also reduced.

Description

TECHNICAL FIELD

[0001] The application relates to a method for improving the surface roughness of a SiC crystal ingot laser slice through laser annealing and belongs to the technical field of laser annealing. BACKGROUND

[0002] Silicon carbide (SiC) is a third-generation semiconductor material that has attracted much attention due to its unique physical properties. This material has a wide bandgap, high breakdown field, high thermal conductivity, high electron saturation velocity, and high radiation resistance, among other notable features. These characteristics enable SiC to operate stably in extreme environments, such as high temperatures, high voltages, and high frequencies, making it ideal for use in high-temperature, high-frequency, radiation-resistant, and high-power devices. SiC has a wide range of applications, including automotive electronics, motor drives, and solar energy, among other high-voltage professional applications. In particular, in the field of new energy vehicles, the application of SiC power devices can significantly improve the operating efficiency and endurance of the motor.

[0003] SiC crystal ingot slicing is the process of machining a large-sized crystal ingot into a specific thickness wafer for subsequent semiconductor device manufacturing. Depending on the technology and requirements, there are mainly two methods for slicing: one is multi-wire slicing, which works by using high-speed moving diamond fine wires to drive abrasive and cutting fluid to cooperatively slice the crystal ingot. 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 operation technology, which is currently the mainstream method for SiC crystal ingot slicing in the industry. The other is laser slicing, which works by focusing a high-energy laser beam at a certain depth of the crystal ingot to form a modified layer for layer-by-layer cutting of the crystal ingot. The micro-cracks generated by laser-induced crystal assist in subsequent peeling of the wafer. Its processing characteristics are: 1. High slicing precision, suitable for ultra-thin wafer manufacturing; 2. Non-contact processing, avoiding problems such as wafer edge collapse; 3. Suitable for low-damage cutting of large-size crystal ingots, and currently laser slicing is mainly applied in high-precision cutting demand scenarios, such as the manufacturing of ultra-thin wafers or large-size wafers.

[0004] At present, in the process of slicing silicon carbide ingot, laser slicing method is used for some high-precision cutting requirements. For example, Chinese patent document CN105855732A discloses a wafer generation method, the main steps of which include: 1) using a first laser beam of a specific wavelength to form a modified layer and a crack inside the ingot to create a separation starting point; 2) using a second laser beam with a larger repetition frequency to further reduce the bonding strength of the modified layer and the crack; 3) using ultrasonic waves in water to act on the modified layer in the ingot, and then peeling off the wafer. This method improves production efficiency and reduces material waste, and is suitable for slicing silicon carbide ingots with high hardness. However, it does not mention the thinning process of the wafer after peeling, and the surface roughness of the wafer after peeling may be poor, and the wafer may be broken during separation, resulting in cracks in the subsequent thinning process.

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

[0006] Chinese patent document CN115410979A discloses a wafer peeling method and a laser slicing method, which includes the steps of feeding, clamping, fixing and peeling. First, the wafer and the hot-melt adhesive sheet are installed between the temperature control clamps; then the wafer and the adhesive sheet are clamped by the linear drive module; then the adhesive sheet is heated, temperature-controlled, pressure-controlled and cooled to fix the wafer; finally, the clamps are moved in reverse to separate the modified layer of the wafer, achieving peeling. The laser slicing method involves forming a modified layer inside the wafer using a laser, and then using the above peeling method to obtain a wafer and a remaining wafer, and continuously and automatically producing multiple wafers by repeatedly grinding, laser and peeling the remaining wafer. However, this method has two problems, on the one hand, it causes wafer cracking during mechanical peeling, on the other hand, the surface roughness of the wafer still does not meet the requirements of direct thinning, which may cause wafer cracking during subsequent thinning.

[0007] Meanwhile, some of the above-mentioned prior art does not mention grinding, 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 jagged microgrooves on the surface. These jagged structures will increase the wear of the thinning grinding wheel and thus increase the cost. In addition, there are small SiC chips inside the grooves, which may cause cracks in the wafer during grinding and increase the wafer cracking rate. SUMMARY

[0008] In view of the deficiencies of the prior art, the present application provides a method for improving the surface roughness of a SiC ingot laser slice by laser annealing. This method can solve the problems of low grinding efficiency, high production cost and high cracking rate of the existing grinding scheme. In addition, the polarization direction and power of the laser can be adjusted by a polarizing beam splitter PBS and a half-wave plate. This method can be applied to the thinning of exfoliated SiC wafers of different sizes and different doping elements, and can also be used for annealing exfoliated wafers with different surface roughness.

[0009] A method for improving the surface roughness of a SiC ingot laser slice by laser annealing, which comprises the following steps: first, exfoliating a SiC wafer from a SiC ingot by laser exfoliation; then, forming an annealing layer on the surface of the exfoliated SiC wafer by laser; finally, placing the smooth surface of the annealed SiC wafer on a UV film and putting it into a thinning machine for thinning to complete the laser slicing of the SiC ingot.

[0010] The method for improving the surface roughness of a SiC ingot laser slice by laser annealing comprises the following specific steps:

[0011] (1) Wafer exfoliation: After pretreatment, the SiC ingot is fixed on a two-dimensional motion platform, and laser processing is performed once to form a modified layer and a crack inside the SiC ingot; then, the laser parameters are adjusted, and the second processing is performed along the path of the first processing to reduce the strength of the modified layer and the crack formed inside the SiC ingot; finally, the SiC ingot after the two processes is taken out and immersed in water, and wafer exfoliation is performed by ultrasonic vibration to obtain a SiC wafer;

[0012] (2) The SiC wafer obtained in step (1) is fixed on a two-dimensional motion platform, and the two-dimensional motion platform is placed in a closed and transparent cavity. In an oxygen atmosphere, a uniform annealing layer is formed on the upper surface of the SiC wafer by laser scanning 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 up, and a UV film is attached to the smooth surface. Then, the SiC wafer after film lamination is fixed in a thinning machine for grinding to complete the laser slicing of the SiC ingot and obtain a SiC wafer with a specified thickness.

[0014] According to the preferred embodiment of the present application, in step (1), the diameter of the SiC ingot is greater than 4 inches, and the type of the SiC ingot is semi-insulating or conductive 4H-SiC ingot; the pretreatment is to grind both sides of the SiC ingot to smoothness.

[0015] According to the application, preferably, in step (1), the parameters of the primary processing are as follows: the laser wavelength is 1000-1200 nm, the pulse width is 5-30 ns, the repetition frequency is 20-200 kHz, the light output power is 2-12 W, the two-dimensional platform moving speed is 50-500 mm / s, and the modified layer depth after the primary processing is 400-600 μm, so that the modified layer and the crack are formed inside the SiC crystal ingot, the separation starting point is created, and the laser beam is ensured to be processed at the predetermined depth through the real-time height compensation method in the process.

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

[0017] According to the application, preferably, 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 the crack formed before is further reduced, and the bonding strength of the SiC wafer and the SiC crystal ingot is reduced.

[0018] According to the application, preferably, in step (1), the frequency of the ultrasonic vibration is controlled to be 20-45 kHz, the modified layer in the crystal ingot is acted on by a specific ultrasonic vibration head at a certain distance from the crystal ingot, and the bonding strength of the modified layer is further reduced until the SiC wafer is peeled off from the SiC crystal ingot.

[0019] According to the application, preferably, in step (2), the wavelength of the laser is 200-800 nm, the pulse width is 10 ps-300 ns, the frequency is 10-500 kHz, and the energy of a single light spot is 10-100 μJ, the laser beam is focused on the surface of the SiC wafer through the transparent upper surface of the cavity, and a layer of annealing layer is uniformly formed on the surface of the SiC wafer by cooperating with the 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 light spot is 80 μJ.

[0021] According to the application, preferably, in step (2), the laser scanning is specifically as follows: the laser beam passes through a polarization beam splitter prism PBS and a half-wave plate to adjust the laser beam power and polarization direction, and then passes through a high-reflectivity mirror and a high-speed galvanometer to precisely control the laser to focus on the upper surface of the SiC wafer, and the polarization direction of the laser is the same as the scanning direction; after one laser scanning, an annealing layer is formed in an area with a size 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. By controlling the size of the annealing area, on the one hand, the efficiency of the annealing process can be adjusted, and on the other hand, the stability of the annealing effect can be adjusted.

[0022] According to the application, preferably, in step (3), the parameters of the thinning machine are as follows: the spindle speed is 1400-2200 rpm, the workbench speed is 100-300 rpm, and the feed speed of the spindle is 0.1-0.5 μm / s.

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

[0024] The application has the following beneficial effects:

[0025] 1. The method for improving the surface roughness of the SiC crystal ingot laser slice provided by the application utilizes a specific wavelength of laser to form an annealing layer on the surface of the SiC wafer after peeling, reduces the surface roughness of the SiC wafer, reduces the intensity of the surface area of the SiC wafer, and can reduce the slag hidden in the SiC wafer cut mark gap after peeling, thereby reducing the cost of the grinding wheel in the subsequent thinning of the SiC wafer and reducing the wafer cracking rate of the SiC wafer.

[0026] 2. The SiC wafer obtained by the method has a surface roughness of 2-3 μm, and the depth of the jagged gully is less than 5 μm, thereby improving the quality of the SiC wafer surface.

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

[0028] 4. The method for improving the surface roughness of the SiC crystal ingot laser slice provided by the application can be applied to the annealing and thinning of SiC peeled wafers with different sizes (more than 4 inches) and different doping elements.

[0029] 5、The method for improving the surface roughness of a SiC ingot laser slice by laser annealing provided by the application 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 precisely controlled by a high-speed galvanometer, so that the annealing layer on the wafer surface is precisely controlled, and the laser power can also be adjusted by adjusting the half-wave plate to form a good annealing layer according to wafers of different resistivities. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 A flowchart of the method for improving the surface roughness of a SiC wafer by laser annealing of a SiC wafer in an embodiment of the application.

[0031] Figure 2 A schematic diagram of the surface microstructure of a SiC wafer in a peeling process in an embodiment of the application.

[0032] Wherein, A is a laser processing line mark, and B is the structure of a SiC wafer after peeling.

[0033] Figure 3 A schematic diagram of forming an annealing layer on the peeling surface of a SiC wafer by using a laser in an embodiment of the application.

[0034] Figure 4 A processing path of a laser when forming a surface annealing layer in an embodiment of the application.

[0035] Figure 5 A schematic diagram of thinning and polishing a wafer after annealing in an embodiment of the application.

[0036] Figure 6 A white light interference test pattern and a sawtooth-shaped morphology of a cross section of a wafer after annealing.

[0037] Figure 7 A white light interference test pattern and a sawtooth-shaped morphology of a cross section of a wafer before annealing.

[0038] In the above figures, 1 is a SiC ingot, 2 is a laser processing line mark, 3 is a sawtooth structure, 4 is a SiC wafer, 5 is a second laser, 6 is a half-wave plate, 7 is a polarization beam splitter prism PBS, 8 is a high-reflectivity mirror, 9 is a high-speed galvanometer, 10 is a glass cover plate, 11 is a cavity, 12 is an air inlet pipe, 13 is an air outlet pipe, 14 is a two-dimensional motion platform, 15 is an annealing area, 16 is a laser scanning path, 17 is a rotating main shaft, 18 is a grinding wheel, 19 is a grinding wheel tooth, and 20 is a rotatable workbench. DETAILED DESCRIPTION

[0039] The specific embodiments of the present application are described below to facilitate the understanding of the present application for those skilled in the art, but it should be clear that the present application is not limited to the scope of the specific embodiments, and for those skilled in the art, it is obvious that various changes are within the spirit and scope of the present application defined and determined by the appended claims, and all the inventions utilizing the concept of the present application are within the scope of protection.

[0040] As shown in Figure 1 The method for improving the surface roughness of a SiC crystal ingot laser slice by laser annealing according to the present application generally comprises the following steps: first, the SiC wafer is peeled off from the SiC crystal ingot by laser peeling; then, an annealing layer is formed on the surface of the peeled SiC wafer by laser; finally, the smooth surface of the annealed SiC wafer is pasted with a UV film and put into a thinning machine for thinning to complete the SiC crystal ingot laser slice. The specific implementation process is shown in the following examples.

[0041] As shown in Figure 2 A certain wavelength of laser is focused at a certain thickness inside the SiC crystal ingot 1, and a row of laser processing lines 2 are formed according to the specified processing path.

[0042] Example 1

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

[0044] (1) Wafer peeling: the high resistivity conductive type 4H-SiC crystal ingot after double side fine grinding is fixed on a two-dimensional motion platform by vacuum adsorption, and a first processing is performed by a first laser to form a modified layer and a crack inside the SiC crystal ingot; then, the parameters of the first laser are adjusted, and a second processing is performed along the path of the first processing to reduce the strength of the modified layer and the crack formed inside the SiC crystal ingot, to create a separation starting point, facilitating the subsequent wafer peeling step. In this process, the laser beam can be ensured to process at the predetermined depth by real-time height compensation; finally, the SiC crystal ingot with a good modified layer after the two processes is taken out and put into a water tank, water is poured into the water tank to immerse the SiC crystal ingot, and an ultrasonic vibration head is used to act on the modified layer of the SiC crystal ingot, the frequency of the ultrasonic vibration is 40 kHz, and then the bonding strength of the modified layer is further reduced until the SiC wafer is peeled off from the crystal ingot;

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

[0046] the traces of primary and secondary processing such as Figure 2 As shown in A, the laser forms a series of laser processing lines 2 on the SiC crystal ingot 1 according to the specified processing path;

[0047] The microstructure of the peeled SiC wafer is shown in Figure 2 As shown in B, the SiC wafer 4 contains jagged structures 3, which may contain crystal debris of SiC, which is the main cause of wafer cracking during thinning;

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

[0049] Under the oxygen atmosphere, a laser beam is emitted by the second laser 5. The laser beam first passes through a half-wave plate 6 and a polarization beam splitter prism PBS 7 to adjust the output power and polarization direction of the laser beam, and then passes through a high-reflectivity mirror 8 to guide the laser beam into a high-speed galvanometer 9. The laser is focused to the surface of the SiC wafer through the high-speed galvanometer 9. After a single 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 shown in Figure 4 A single laser can only form an annealing area 15 on the SiC wafer 4. After single processing, the two-dimensional motion platform 14 under the SiC wafer 4 is moved to make the wafer process according to the specified laser scanning path 16 to form a uniform annealing layer on the upper surface of the SiC wafer, and the annealed SiC wafer is obtained;

[0051] 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 condensing point is 80 μJ;

[0052] (3) Use a dust-free cloth to clean the smooth surface of the annealed SiC wafer obtained in step (2). Place the SiC wafer with the smooth surface facing up into a film sticking machine, attach a UV film to the smooth surface, and then fix the film-stuck SiC wafer on the ceramic suction cup of the thinning machine workbench by vacuum suction for grinding. The specified thickness of the SiC wafer is removed, and the laser slicing of the SiC ingot is completed;

[0053] The structure of the thinning machine is shown in Figure 5As shown, including high-speed rotation module and low-speed high-speed rotation module, high-speed rotation module is composed of rotating main shaft 17, the sand wheel 18 connected below the rotating main shaft, the edge on the sand wheel 18 is composed of several sand wheel teeth 19;Low-speed rotation module is composed of rotatable workbench 20 and SiC wafer 4 attached to UV film;

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

[0055] The surface of the annealed SiC wafer obtained in step (2) of the embodiment is shown in Figure 6 As shown, the sawtooth-shaped upper end of the wafer is ablated by laser, the depth of the sawtooth-shaped groove of the wafer is greatly reduced, the SiC crystal debris on the surface of the wafer is blown away by the oxygen introduced in the process of laser annealing, and finally the surface roughness of the wafer is reduced and the overall quality is good.

[0056] In this embodiment, the annealing of 20 SiC wafers is repeated according to step (2), and the average surface roughness of the annealed SiC wafer is measured by white light interferometer to be 2-3 μm, and the depth of the sawtooth-shaped groove on the surface is less than 5 μm. Subsequently, the SiC wafer is ground to a thickness of 370 μm according to step (3), the grinding ratio of the grinding wheel is 1:1.2, and no cracking occurs during the grinding of the 20 annealed SiC wafers.

[0057] Example 2

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

[0059] (1) Wafer peeling: the double-side fine-ground low-resistivity conductive type 8-inch 4H-SiC ingot is fixed on a two-dimensional motion platform by vacuum suction, a first processing is performed by a first laser to form a modified layer and a crack in the SiC ingot; then the parameters of the first laser are adjusted, and a second processing is performed along the path of the first processing to reduce the strength of the modified layer and the crack formed in the SiC ingot, to create a separation starting point and facilitate the subsequent wafer peeling step. In this process, real-time height compensation can be used to ensure that the laser beam can process at the predetermined depth;Finally, the SiC ingot with a good modified layer after the two processes is taken out and placed in a water tank, water is poured into the water tank to submerge the SiC ingot, and an ultrasonic horn is used to act on the modified layer of the SiC ingot, the frequency of the ultrasonic vibration is 40 kHz, and then the bonding strength of the modified layer is further reduced until the SiC wafer is peeled off from the ingot;

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

[0061] (2) The SiC wafer obtained in step (1) is cleaned and fixed on the two-dimensional motion platform 14, and the two-dimensional motion platform 14 is placed in the cavity 11 with the upper cover opened. The cavity 11 is provided with a glass cover plate 10 above it, an air inlet pipe 12 is connected to the side of the cavity 11, and oxygen is introduced into the air inlet pipe 12, which is helpful for the formation of SI-O compounds on the surface of the wafer. At the same time, an air outlet pipe 13 is connected to the other side of the cavity 11 to discharge the introduced oxygen.

[0062] Under the oxygen atmosphere, the laser beam is emitted by the second laser 5. The laser beam first passes through the half-wave plate 6 and the polarization beam splitter prism PBS 7 to adjust the output power and polarization direction of the laser beam, and then passes through the high-reflectivity mirror 8 to guide the laser beam into the high-speed galvanometer 9. The laser is focused to the surface of the SiC wafer through the high-speed galvanometer 9. After one laser scanning, 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 532 nm, the pulse width is 20 ps, the frequency is 30 kHz, and the energy of a single condensing point is 90 μJ.

[0064] (3) The smooth surface of the annealed SiC wafer obtained in step (2) is cleaned with a dust-free cloth. The SiC wafer is placed in a film sticking machine with the smooth surface facing up, and a UV film is attached to the smooth surface. The SiC wafer after film sticking is fixed on a ceramic suction cup of a thinning machine workbench by vacuum suction for grinding. The specified thickness of the SiC wafer is removed, and the SiC ingot laser slicing is completed.

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

[0066] The surface of the annealed SiC wafer obtained in step (2) of the embodiment is shown in Figure 6 As can be seen from the figure, the serrated upper end of the wafer is ablated by the laser, the depth of the serrated groove of the wafer is greatly reduced, the SiC crystal debris on the surface of the wafer is blown away by the introduced oxygen during the laser annealing process, and finally the surface roughness of the wafer is reduced and the overall quality is good.

[0067] The annealing of 20 SiC wafers is repeated according to step (2), and the average surface roughness of the SiC wafers after annealing is measured by a white light interferometer to be 2-3 μm, and the surface jagged groove depth is below 5 μm. Subsequently, the SiC wafers are ground to a thickness of 370 μm according to step (3), and the grinding wheel loss ratio is 1:1.25. No cracks occur in the 20 annealed SiC wafers after grinding.

[0068] Example 3

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

[0070] (1) Wafer peeling: the double-sided fine-ground semi-insulating 4H-SiC crystal ingot is fixed on a two-dimensional motion platform by vacuum adsorption, and a first processing is performed by a first laser to form a modified layer and a crack inside the SiC crystal ingot; then the parameters of the first laser are adjusted, and a second processing is performed along the path of the first processing to reduce the strength of the modified layer and the crack formed inside the SiC crystal ingot, create a separation starting point, and facilitate the subsequent wafer peeling step. In this process, the real-time height compensation method can be used to ensure that the laser beam can process at the predetermined depth; finally, the SiC crystal ingot with a good modified layer after the two processes is taken out and placed in a water tank, water is poured into the water tank to submerge the SiC crystal ingot, and an ultrasonic horn is used to act on the modified layer of the SiC crystal ingot, the frequency of the ultrasonic vibration is 40 kHz, and then the bonding strength of the modified layer is further reduced until the SiC wafer is peeled off from the crystal ingot;

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

[0072] (2) The SiC wafer obtained in step (1) is cleaned and fixed on a two-dimensional motion platform 14, and the two-dimensional motion platform 14 is placed in a cavity 11 with the upper cover opened. The cavity 11 is provided with a glass cover plate 10 above the cavity 11, and an air inlet pipe 12 is arranged on the side of the cavity 11 to introduce oxygen into the air pipe, which is helpful for the formation of SI-O compound on the surface of the wafer. At the same time, an air outlet pipe 13 is arranged on the other side of the cavity 11 to exhaust the introduced oxygen;

[0073] In the oxygen atmosphere, the laser beam is emitted by the second laser 5, and the laser beam is first adjusted in output power and polarization direction by the half-wave plate 6 and the polarization beam splitter prism PBS 7, and then introduced into the high-speed galvanometer 9 by the high-reflection mirror 8, and the laser is focused to the surface of the SiC wafer by the high-speed galvanometer 9, and after one laser scanning, an annealing area with an area of 1*1 cm is formed on the surface of the SiC wafer; and 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 532 nm, the pulse width is 20 ps, the frequency is 30 kHz, and the energy of a single focusing point is 48 muJ.

[0075] (3) The smooth surface of the annealed SiC wafer obtained in step (2) is cleaned by using a dust-free cloth, the smooth surface is placed upward in a film sticking machine, a UV film is attached on the smooth surface, and then the SiC wafer after film sticking is fixed on a ceramic suction cup of a thinning machine workbench by vacuum suction for grinding, so that the SiC wafer is removed to a specified thickness, and the SiC ingot laser slicing is completed.

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

[0077] The surface of the annealed SiC wafer obtained in step (2) of the embodiment is shown in Figure 6 It can be seen that the serrated upper end of the wafer is ablated by the laser, the depth of the serrated groove of the wafer is greatly reduced, the SiC crystal debris on the surface of the wafer is blown away by the oxygen introduced in the laser annealing process, and finally the surface roughness of the wafer is reduced and the overall quality is good.

[0078] According to step (2), the annealing of 20 SiC wafers is repeated, and the average surface roughness of the annealed SiC wafer is measured by a white light interferometer to be 2-3 mu m, and the depth of the serrated groove on the surface is less than 5 mu m. According to step (3), the SiC wafer is ground to a thickness of 370 mu m, the grinding wheel loss ratio is 1:1.3, and no wafer cracking occurs during the grinding of 20 annealed SiC wafers.

[0079] Comparative Example 1

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

[0081] (1) wafer stripping: the conductive type 8-inch 4H-SiC crystal ingot after double-sided fine grinding is fixed on a two-dimensional motion platform by vacuum adsorption, a first processing is performed by a first laser to form a modified layer and a crack in the SiC crystal ingot; then the parameters of the first laser are adjusted, and a second processing is performed along the path of the first processing; finally, the SiC crystal ingot after the two processes is taken out and placed in a water tank, water is poured into the water tank to submerge the SiC crystal ingot, and the modified layer of the SiC crystal ingot is acted on by an ultrasonic vibration head, the frequency of the ultrasonic vibration is 40 kHz, until the SiC wafer is stripped from the crystal ingot;

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

[0083] (2) the smooth surface of the SiC wafer obtained after laser stripping in step (1) is cleaned with a dust-free cloth, the SiC wafer is placed in a film sticking machine with the smooth surface facing up, a UV film is attached to the smooth surface, and then the SiC wafer after film sticking is fixed on a ceramic suction cup of a thinning machine workbench by vacuum adsorption for grinding, the specified thickness of the SiC wafer is removed, and the laser slicing of the SiC crystal ingot is completed;

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

[0085] The surface of the SiC wafer obtained in the comparative example is shown in Figure 7 It can be seen that a series of sawtooth-shaped grooves appear on the surface of the wafer, and there are some SiC crystal debris generated by stripping in the grooves, which will cause the wafer cracking problem in subsequent grinding processing.

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

[0087] By comparing the examples 1-3 and the comparative example 1, it can be seen that the method for improving the surface roughness of the SiC crystal ingot laser cutting provided by the present application utilizes the laser of a specific wavelength to form an annealing layer on the surface of the SiC wafer after peeling, which reduces the surface roughness of the SiC wafer, reduces the intensity of the surface area of the SiC wafer, and can reduce the slag hidden in the SiC wafer cutting mark gap after the SiC wafer is peeled, the cost of the grinding wheel in the subsequent thinning of the SiC wafer is reduced, and the SiC wafer cracking rate is also reduced. The surface roughness of the SiC wafer is between 2-3 μm, the depth of the jagged gully is less than 5 μm, the SiC wafer and the grinding wheel loss ratio is reduced to 1:(1.2-1.3), the quality of the SiC wafer surface is improved, and the production cost is greatly reduced.

[0088] The above is the preferred embodiment of the present application, it should be noted that for those skilled in the art, without departing from the principles of the present application, can also make a number of improvements and refinements, these improvements and refinements should also be considered as the protection scope of the present application.

Claims

1. A method for improving surface roughness of a laser-sliced surface of a SiC ingot by laser annealing, the method comprising: The specific steps are as follows: ​ (1) The SiC crystal ingot is pretreated and fixed on a two-dimensional motion platform, and a modified layer and a crack are formed in the SiC crystal ingot by laser processing once; then the laser parameters are adjusted, and secondary processing is performed along the path of the first processing; finally, the SiC crystal ingot after two processes is taken out and immersed in water, and wafer peeling is performed by ultrasonic vibration, to obtain a SiC wafer; The parameters of the first processing are: the laser wavelength is 1000-1200 nm, the pulse width is 5-30 ns, the repetition frequency is 20-200 kHz, the light output power is 2-12 W, the two-dimensional platform moving speed is 50-500 mm / s, and the modified layer depth after the first processing is 400-600 pm; (2) The SiC wafer obtained in step (1) is fixed on a two-dimensional motion platform, and the two-dimensional motion platform is placed in a closed and transparent cavity, and a uniform annealing layer is formed on the upper surface of the SiC wafer by laser scanning under an oxygen atmosphere, to obtain an annealed SiC wafer; The laser scanning is as follows: the laser beam first passes through a polarization beam splitter PBS and a half-wave plate to adjust the laser beam power and polarization direction, and then passes through a high-reflectivity mirror and a high-speed galvanometer to accurately control the laser focus on the upper surface of the SiC wafer, and the polarization direction of the laser is the same as the scanning direction; after one laser scanning, an annealing layer is formed in an area of 1x1 cm or 2x2 cm; and the laser scanning operation is repeated to form a uniform annealing layer on the upper surface of the SiC wafer; (3) The annealed SiC wafer obtained in step (2) is placed in a film sticking machine with the smooth surface facing up, and a UV film is attached to the smooth surface, and then the SiC wafer after film sticking is fixed in a thinning machine for grinding, to complete the laser slicing of the SiC crystal ingot, and obtain a SiC wafer with a specified thickness; The parameters of the thinning machine are: the spindle speed is 1400-2200 rpm, the workbench speed is 100-300 rpm, and the feed speed of the spindle is 0.1-0.5 pm / s.

2. The method of improving surface roughness of a laser sliced SiC wafer from a SiC ingot by laser annealing as claimed in claim 1, wherein, In step (1), the diameter of the SiC crystal ingot is more than 4 inches, and the type is semi-insulating or conductive 4H-SiC crystal ingot; the pretreatment is to polish the SiC crystal ingot to smoothness on both sides.

3. The method for improving surface roughness of a laser sliced SiC wafer from a SiC ingot by laser annealing as claimed in claim 1, wherein, In step (1), the parameters of the first processing are: the laser wavelength is 1030 nm or 1064 nm, the pulse width is 20 ns, the repetition frequency is 180 kHz, the light output power is 8 W, the two-dimensional platform moving speed is 400 mm / s, and the modified layer depth after the first processing is 470 pm.

4. The method for improving surface roughness of a laser sliced SiC wafer from a SiC ingot by laser annealing as claimed in claim 1, wherein, In step (1), the secondary processing is to adjust the repetition frequency in the first processing to 300-400 kHz.

5. The method for improving surface roughness of a laser sliced SiC wafer from a SiC ingot by laser annealing as claimed in claim 1, wherein, In step (1), the frequency of the ultrasonic vibration is controlled to be 20-45 kHz.

6. The method for improving surface roughness of a laser sliced SiC wafer from a SiC ingot by laser annealing as claimed in claim 1, wherein, In step (2), the wavelength of the laser is 200-800 nm, the pulse width is 10 ps-300 ns, the frequency is 10-500 kHz, and the energy of a single focusing point is 10-100 pJ.

7. The method of improving surface roughness of a laser sliced SiC wafer from a SiC ingot by laser annealing as claimed in claim 6, wherein, In step (2), 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 focusing point is 80 pJ.

8. The method of improving surface roughness of a laser sliced SiC wafer from a SiC ingot by laser annealing as claimed in claim 1, wherein, In step (3), 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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