Processing method for modifying and thinning silicon carbide wafer by using laser and silicon carbide wafer thinning sheet

Abstract

The invention provides a processing method for modifying and thinning a silicon carbide wafer by using laser and a silicon carbide wafer thinning sheet, and relates to the technical field of crystal laser processing. The method comprises the following steps: S1, carrying out first laser scanning on a to-be-thinned surface of a silicon carbide wafer to form a modified layer in the silicon carbide wafer; s2, performing second laser scanning on the circumferential edge part of the to-be-thinned surface after the first laser scanning to enable a modified layer formed on the circumferential edge part to extend towards the to-be-thinned surface; s3, sequentially carrying out separation and grinding wheel thinning on the silicon carbide wafer subjected to the second laser scanning to obtain a silicon carbide wafer thinned sheet; wherein the thickness of the modified layer after the second laser scanning is greater than that of the modified layer after the first laser scanning. Through two times of laser scanning, the modification method is optimized, the separation difficulty and the warping problem of the thinned wafer can be greatly reduced, and the abrasion of the grinding wheel and the cracking risk of the wafer are reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of crystal laser processing, in particular to a processing method for reducing the thickness of a silicon carbide wafer by laser modification and a silicon carbide wafer thinning sheet. BACKGROUND

[0002] Silicon carbide (SiC) is a semiconductor material with excellent performance, widely used in high-power, high-frequency and high-temperature electronic device fields. With the development of SiC technology, its application in power electronics, optoelectronic devices and high-temperature devices is continuously expanding, especially in the application of MOSFET devices in power management applications. MOSFET is a vertical device, the thinner the total thickness of the device, the lower the on-resistance, and the power consumption will also be smaller, and the drain is generally on the back of the wafer, so after the front surface process of the wafer is completed, the thickness of the SiC wafer needs to be reduced to reduce the on-resistance of the device. The current SiC wafer thinning method has many shortcomings, low production efficiency and high production cost restrict the application and promotion of the process.

[0003] The existing SiC wafer thinning processing method is mainly based on the "IN-Feed principle" of the grinding wheel thinning process. The wafer is adsorbed on the vacuum table, and the rotation of the table and the longitudinal feed motion of the grinding wheel are used to realize the thinning of the wafer. This process, on the one hand, because of the high hardness of silicon carbide, the grinding wheel consumption is high, and the efficiency is low. In the thinning process, because of the "hard collision" grinding method, the direct contact between the grinding wheel abrasive and the wafer will introduce mechanical stress at the contact point, which is easy to cause cracks and edge collapse defects, affecting the wafer yield. On the other hand, the thinner the wafer thickness, the greater the warping, the greater the TTV of the substrate after thinning, and the more difficult it is to control the thickness accurately, increasing the processing difficulty.

[0004] In view of the above problems, on the basis of the traditional grinding wheel thinning, some people have proposed a "Taiko process" method, that is, when thinning the back surface of the wafer, the edge part (about 3mm) of the wafer periphery is retained, and only the inner part of the wafer is thinned and removed. By introducing this technology, the handling risk of thin wafers can be reduced and the warping problem can be reduced; on this basis, some people have developed a thinning processing method for the inner part of the wafer, which separates the inner part of the wafer by laser, and then processes the edge of the wafer by ring cutting process, which can further reduce the processing cost on the basis of ensuring the warping of the wafer.

[0005] The existing silicon carbide wafer thinning method is mainly carried out by the way of grinding wheel thinning. In the process of mutual movement between the grinding wheel and the wafer, the mechanical friction is formed between the diamond particles adhered to the grinding wheel and the silicon carbide, and the silicon carbide wafer is removed by the hardness difference between the diamond and the silicon carbide. Although diamond is the hardest material in nature, because the Mohs hardness of silicon carbide material is 9.2-9.5, which is the second highest hardness material in nature only next to diamond (10), which also determines that the grinding wheel thinning process is realized by "hard grinding". This process is very low in efficiency, and the diamond grinding wheel is consumed greatly, and mechanical stress is easily introduced in the process, which affects the wafer yield. In summary, this process has the disadvantages of slow processing efficiency, high processing cost, easy introduction of mechanical stress damage and the like; another scheme adopts a ring cutting process scheme of first processing the inside of the wafer by laser or grinding wheel and then cutting the outside edge, which is high in technical complexity, and has the risk of cutting and cracking due to the introduction of the ring cutting step, thereby reducing the wafer yield and increasing the production cost. SUMMARY

[0006] In order to solve the above problems, the first aspect of the present application provides a processing method for thinning silicon carbide wafer by using laser modification, which comprises the following steps:

[0007] S1: performing first laser scanning on the surface of the silicon carbide wafer to be thinned to form a modified layer in the inside of the silicon carbide wafer.

[0008] Optionally, a protective film is first attached to the surface of the silicon carbide wafer which is not irradiated by laser, as shown in Figure 2 The protective film can be a pyrolytic blue film or a UV film for ordinary thinning. The film can be attached manually or by equipment. After the film is attached, the excess film on the surface of the wafer needs to be accurately cut along the circumferential direction of the wafer. In the working process, the silicon carbide wafer with the film attached to the surface not irradiated by laser is conveyed to the wafer stage of the laser scanning equipment, and the surface of the silicon carbide wafer to be thinned (the surface irradiated by laser) faces upward, as shown in Figure 3 The laser generator is moved continuously within a certain interval to scan the surface of the wafer to complete the modification treatment of the target thickness.

[0009] Optionally, the silicon carbide wafer can be obtained by laser peeling of a silicon carbide ingot.

[0010] S2: performing second laser scanning on the circumferential edge part of the surface to be thinned after the first laser scanning, so that the modified layer formed on the circumferential edge part extends to the surface to be thinned; wherein the thickness of the modified layer after the second laser scanning is greater than the thickness of the modified layer after the first laser scanning.

[0011] The first laser scanning is performed on the whole surface of the to-be-thinned surface of the silicon carbide wafer, after the laser modification on the whole surface is completed, the focal length position of the laser acting on the silicon carbide wafer is increased upward (toward the laser irradiation surface) on the basis of the focal length position of the first laser scanning, only the circumferential edge portion of the to-be-thinned surface of the silicon carbide wafer is subjected to the second laser scanning, the modified layer formed by the circumferential edge portion extends in the direction of the to-be-thinned surface, the overall thickness of the modified layer formed by the circumferential edge portion increases on the basis of the first laser modification, the thickness of the modified layer after the second laser scanning is greater than the thickness of the modified layer after the first laser scanning, that is, the total thickness of the modified layer after the second laser scanning is greater than the thickness of the modified layer after the first laser scanning.

[0012] S3: sequentially performing separation and grinding wheel thinning on the silicon carbide wafer subjected to the second laser scanning to obtain a silicon carbide wafer thinning sheet. The flowchart of the laser modification thinning method for processing a silicon carbide wafer is shown in Figure 1 .

[0013] Optionally, the thickness of the modified layer subjected to the second laser scanning is increased by 3-7 μm than the thickness of the modified layer subjected to the first laser scanning. Optionally, the thickness of the modified layer subjected to the second laser scanning is increased by 3-5 μm than the thickness of the modified layer subjected to the first laser scanning. For example, the thickness of the modified layer subjected to the second laser scanning can be increased by 3 μm, 3.2 μm, 3.5 μm, 3.8 μm, 4 μm, 4.2 μm, 4.5 μm, 4.8 μm, 5 μm or any value and range therebetween than the thickness of the modified layer subjected to the first laser scanning.

[0014] Optionally, the inner diameter of the circumferential edge portion accounts for 0.6-0.8 times the radius of the whole wafer with the center of the silicon carbide wafer as the center. Further, reference is made to Figure 4 . Figure 4 An exemplary embodiment of a position diagram of a silicon carbide wafer is shown. The silicon carbide wafer is divided into a circumferential edge portion and a wafer inner portion with the center of the silicon carbide wafer as the center, R2 is the radius of the silicon carbide wafer, R1 is the inner diameter of the circumferential edge portion (or the radius of the wafer inner portion), and the second laser scanning region is a region with a radius greater than R1.

[0015] Optionally, the laser modification thinning thickness is >80 μm. Compared with the traditional direct grinding wheel thinning process, the laser modification method of the present application can directly process according to the target thickness, the thicker the target thickness, the higher the time efficiency, generally when the thinning thickness is >80 μm, the laser modification time is much lower than the traditional direct grinding wheel thinning time, and the overall processing time can be greatly reduced.

[0016] Optionally, the first laser scanning and the second laser scanning both use picosecond laser equipment; further, the laser wavelength is 532-1064 nm, and the pulse width is 45-500 ps. Optionally, the laser power of the first laser scanning and the second laser scanning is 2-22 W, and the scanning rate is 300-800 mm / s. Optionally, the scanning path of the first laser scanning and the second laser scanning is moved laterally within a certain interval along the <11-20> crystal plane to form a plurality of single scanning paths, and the interval between adjacent single scanning paths is 120-350 μm. Further, the present application is based on a silicon carbide modification thinning method using picosecond laser equipment. The wavelength determines the penetration depth of the laser, and the longer the wavelength, the deeper the depth. The pulse width determines the number of local laser actions, and the shorter the pulse width, the better the local processing. By introducing picosecond laser equipment, the laser of a specific wavelength and pulse width emitted by the laser is focused near the silicon carbide crystal to form a high-power density processing focal point area. The crystal molecules in this area absorb photon energy and emit free electrons, and the micro-explosion caused by the Coulomb repulsion makes the molecular bonds of silicon carbide broken, and the crystal structure changes. The single crystal structure on both sides of the laser focal point is cracked due to the nonlinear stress. A modification layer is formed at the target thickness inside the silicon carbide wafer, and subsequent application of reasonable external force in different directions can achieve complete separation of the wafer without affecting the wafer warping. However, the implementation of the laser of the present application is not limited to this, and those skilled in the art can achieve the same effect by using different types of lasers such as femtosecond lasers or different scanning methods.

[0017] After the two laser scans of the present application, the total thickness of the modification layer of the second laser scan is increased compared to the thickness of the modification layer of the first laser scan. By applying reasonable external force in different directions, complete separation of the wafer can be achieved without affecting the wafer warping. The separated wafer has basically reached the desired target thickness, and there is still a crack damage layer on the surface caused by laser modification, which needs to be thinned again by a grinding wheel. At this time, the crystal phase structure on the surface of the silicon carbide wafer changes due to the laser modification method, and the resistance is smaller during the grinding process due to the existence of local micro-cracks. Thinning with a grinding wheel of appropriate hardness and mesh can reduce the loss, reduce the production cost, and obtain a wafer thickness and surface state consistent with traditional thinning. Because the edge thickness is higher than the internal thickness, the stress in the crystal is basically balanced at this time, and the crack rate and surface warping can be reduced during the grinding process.

[0018] Optionally, step S3 further comprises the following steps:

[0019] S31: The surface to be thinned of the silicon carbide wafer subjected to the second laser scanning is first laminated, fixed on a lamination ring, adsorbed on the lamination ring by a ring-mounted adsorption clamp, and separated along the modification layer to obtain a silicon carbide wafer by applying tension.

[0020] Further, the surface to be thinned (laser irradiation surface) of the silicon carbide wafer after the second laser scanning is first attached with a protective film, as shown in Figure 5 The silicon carbide wafer is attached with a film, as shown in the schematic diagram of the silicon carbide wafer attached with a film, Figure 5 The left side is a perspective view, and the right side is a side view. For example, a blue film or a UV film can be attached, which provides an adsorption plane for the subsequent ring-shaped adsorption clamp, so as to avoid the influence of breakage or bending on the silicon carbide wafer after separation. The film (blue film or UV film) is fixed on the attached ring, the ring-shaped adsorption clamp is used to adsorb the attached ring, and the non-laser irradiation surface of the silicon carbide wafer can be fixed by a vacuum chuck during the separation process. For example, the ring-shaped adsorption clamp includes suction nozzles and connecting rods arranged in a ring shape at a certain interval. The upper end of the suction nozzle is connected to a vacuum generator, one end of the connecting rod is connected to the suction nozzle, and a pressure sensor is connected to the connecting rod. Optionally, the number of suction nozzles is not less than 6. For example, the number of suction nozzles is 8. The suction nozzles are uniformly distributed at a certain interval on the adsorption clamp. During the separation process, each suction nozzle is adsorbed on the attached ring by the vacuum generator. The wafer separation schematic diagram is shown in Figure 6 The same pulling force is gradually applied from the Notch to the opposite direction of the Notch. The pressure sensor detects the pulling force value in each direction at any time and records it (ensures that the pulling force and time are linearly related). At this time, the surface wafer will gradually separate until the pulling force is 0. If the separation is not successful at one time, the initial pulling force can be increased to try again. The pulling force is 8-16 N, and the maximum pulling force is generally not more than 16 N. In addition, the silicon carbide wafer after the two treatments in the laser modification process has a uniform separation direction (the Notch of the wafer needs to be ensured to be in a fixed direction during each feeding), so that the force conduction during the separation is along the <11-20> direction, which greatly reduces the separation difficulty and the cracking situation.

[0021] S32: The separation surface of the silicon carbide wafer is thinned by a grinding wheel to obtain a silicon carbide wafer thinning piece.

[0022] Optionally, the separation surface of the silicon carbide wafer is thinned by a grinding wheel. During the thinning process, as shown in Figure 7 The stage and the grinding wheel rotate relative to each other, and the grinding wheel gradually feeds from top to bottom. During this process, the influence area of the modified layer gradually weakens from the surface to the inside. By controlling the feed amount and the feed speed, different grinding wheel abrasions can be obtained.

[0023] Optionally, the first feeding amount is 15-25 μm, the feeding speed is 0.5-0.6 μm / s; the second feeding amount is 8-12 μm, the feeding speed is 0.3-0.4 μm / s; the third feeding amount is 8-12 μm, the feeding speed is 0.2-0.25 μm / s. By gradually feeding the feeding amount and gradually reducing the feeding speed, the feeding speed is reduced to half of the initial value when the feeding amount is 80% of the target thickness, and the minimum grinding wheel loss can be obtained.

[0024] Further, the separated silicon carbide wafer has an edge thickness higher than an internal thickness, and the stress in the crystal is basically balanced at this time, so that the single grinding wheel (an 8000# grinding wheel can be used for the SIC substrate) can be used for thinning processing. Compared with the double-grinding wheel use scheme of "rough + fine", the single-grinding wheel processing can avoid the additional introduction of local stress of the grinding wheel and the breaking of the balance of the stress in the wafer, so that the wafer is not easy to have the phenomenon of abnormal warping of the center ("one-word type" surface). Under the condition that the TTV level is the same, the center area of the image is better in flatness, which is more conducive to the processing of the subsequent process. At the same time, the use of the single grinding wheel can reduce the grinding wheel use cost by at least 30% (related to the thinning thickness).

[0025] The second aspect of the present application provides a silicon carbide wafer thinning sheet, wherein the TTV of the silicon carbide wafer thinning sheet is not higher than 1.2 μm, and the wafer breakage rate is not higher than 4%.

[0026] Optionally, the TTV of the silicon carbide wafer thinning sheet is not higher than 0.8 μm, and the wafer breakage rate is not higher than 3%.

[0027] Optionally, the silicon carbide wafer thinning sheet can be any one of 4-12 inches.

[0028] Optionally, the silicon carbide wafer thinning sheet can be N-type or H-type.

[0029] Compared with the prior art, the present application at least has one of the following beneficial effects:

[0030] (1) The present application uses a laser modification thinning method for silicon carbide wafers, which can greatly reduce the separation difficulty and the warping problem after wafer thinning by optimizing the modification method through two laser scans.

[0031] (2) the present application uses laser modification thinning silicon carbide wafer processing method, in the final thinning process due to the presence of wafer surface modification layer, reduce the grinding wear and wafer cracking risk, the current measured with the same thinning thickness wear can be reduced by more than 20%, the integration of grinding wheel after the overall cost is reduced by about 30%, the crack rate is less than 4%, at the same time, the final need for grinding wheel thinning thickness (generally <80um) after laser modification is much lower than the target thickness (about 250um) of traditional grinding wheel thinning, which can further reduce the processing cost on the basis of ensuring wafer warping. BRIEF DESCRIPTION OF DRAWINGS

[0032] The accompanying drawings, which are included to provide a further understanding of the application and are incorporated in and constitute a part of this application, illustrate embodiments of the application and together with the description serve to explain the application. In the drawings:

[0033] Figure 1 An exemplary embodiment of the process flow diagram of the present application for thinning silicon carbide wafer using laser modification is shown;

[0034] Figure 2 An exemplary embodiment of the schematic diagram of the protective film attached to the non-laser irradiated surface of the silicon carbide wafer is shown;

[0035] Figure 3 An exemplary embodiment of the schematic diagram of the laser modification of the silicon carbide wafer is shown;

[0036] Figure 4 An exemplary embodiment of the schematic diagram of the second laser scanning area position of the silicon carbide wafer is shown;

[0037] Figure 5 An exemplary embodiment of the schematic diagram of the protective film attached to the laser irradiated surface of the silicon carbide wafer after the second laser scanning is shown;

[0038] Figure 6 An exemplary embodiment of the schematic diagram of the separation operation of the silicon carbide wafer is shown;

[0039] Figure 7 An exemplary embodiment of the schematic diagram of the thinning of the silicon carbide wafer is shown;

[0040] Figure 8 An exemplary embodiment of the surface profile diagram of the silicon carbide wafer thinning sheet of Example 1 is shown;

[0041] Figure 9 An exemplary embodiment of the surface profile diagram of the silicon carbide wafer thinning sheet of Comparative Example 2 is shown;

[0042] Figure 10 An exemplary embodiment of the side view of the internal modification layer of the silicon carbide wafer after the second laser scanning of Example 1 is shown;

[0043] Figure 11 FIG. 1 shows an exemplary embodiment of a side view of a modified layer inside a silicon carbide wafer after a first laser scan of Comparative Example 1;

[0044] Figure 12 FIG. 2 shows an exemplary embodiment of a schematic diagram of a silicon carbide wafer thickness test position in Table 3.

[0045] BRIEF DESCRIPTION OF DRAWINGS

[0046] 1 - silicon carbide wafer; 2 - protective film; 3 - laser generator; 4 - slide stage; 5 - patch ring; 6 - suction nozzle; 7 - connecting rod; 8 - opening; 9 - ring-shaped suction clamp; 10 - vacuum chuck; 11 - grinding wheel. DETAILED DESCRIPTION

[0047] In order to more clearly illustrate the overall concept of the present application, the following detailed description is made with reference to the accompanying drawings.

[0048] In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present application. However, it will be apparent to one skilled in the art that the present application can be practiced without the specific details presented in this disclosure. Therefore, the scope of the present application is not limited to the specific embodiments disclosed in this description.

[0049] Example 1

[0050] In an exemplary embodiment of the present application, a method for processing a silicon carbide wafer using laser modification and thinning includes the following steps:

[0051] S1 : Blue film is attached to the non-laser irradiated surface of the silicon carbide wafer, the excess blue film on the surface of the wafer is precisely cut along the circumferential direction of the wafer, and the silicon carbide wafer with the film attached to the non-laser irradiated surface is transferred to the slide stage of the laser scanning equipment with the surface to be thinned facing upwards. The picosecond laser emitted by the laser generator has a wavelength of 532 nm and a pulse width of 500 ps. The laser focal length X is controlled to a certain depth (target wafer thickness) inside the wafer. By controlling the local laser output power to be 10 W and the movement speed to be 400 m / s, a very high power density processing focal point area can be formed locally. The crystal molecules in this area absorb photon energy and emit free electrons, which produce micro-explosions through Coulomb repulsion, causing the molecular bonds of silicon carbide to be broken and the crystal structure to change. The single crystal structure on both sides of the laser focal point produces cracks due to nonlinear stress. The laser scanning path moves horizontally along the <11-20> crystal plane. After a single path scan is completed, a second path scan is performed after moving a certain interval. The interval between two adjacent moving tracks is 120 μm. After the entire surface is scanned, a modified layer is formed along the 0001 plane in a certain depth area inside the ingot, accompanied by the generation of local micro-cracks.

[0052] S2: With the center of the silicon carbide wafer as the center, the inner diameter of the circumferential edge portion is 0.7 times the radius of the entire wafer. A second laser scan is performed on the area outside the inner diameter of the circumferential edge portion of the surface to be thinned after the first laser scan. The conditions for the second laser scan are the same as those for the first laser scan. The focal length of the laser acting on the silicon carbide wafer is reduced, so that the modified layer formed in the area outside the inner diameter of the circumferential edge portion extends towards the laser irradiation surface. The total thickness of the modified layer in the second laser scan is 5 μm thicker than that in the first laser scan.

[0053] S31: A blue film is first applied to the thinned surface (laser irradiation surface) of the silicon carbide wafer after the second laser scan. The blue film is fixed onto the mounting ring, and a ring-shaped adsorption fixture is used to adsorb the mounting ring. The ring-shaped adsorption fixture includes eight suction nozzles arranged in a ring at certain intervals and eight connecting rods. The upper end of each suction nozzle is opened and connected to a vacuum generator. One end of each connecting rod is connected to a suction nozzle, and the other end is connected to a pressure sensor. During the separation process, each suction nozzle is adsorbed onto the mounting ring by the action of the vacuum generator. (See the wafer separation diagram for reference.) Figure 6 As shown. A uniform pulling force is gradually applied from the notch port towards the opposite side of the notch. Pressure sensors continuously monitor and record the pulling force values ​​in each direction. During this process, the surface wafer will gradually separate until the pulling force reaches zero. If separation fails on the first attempt, the initial pulling force can be increased and the attempt repeated. The pulling force should be 8-16N, with a maximum generally not exceeding 16N.

[0054] S32: The separation surface of the silicon carbide sub-wafer is thinned using a grinding wheel. The stage and the grinding wheel rotate relative to each other, and the grinding wheel is gradually fed from top to bottom. After the grinding wheel thinning is completed, the blue film on the un-laser-irradiated surface of the silicon carbide wafer is removed to obtain a thinned silicon carbide wafer sheet. The grinding wheel thinning processing parameters are shown in Table 1.

[0055] Table 1 Grinding wheel thinning parameters

[0056]

[0057] Example 2

[0058] In an exemplary embodiment of the present invention, the processing method for thinning silicon carbide wafers using laser modification includes the following steps;

[0059] S1: Blue film is pasted on the non-laser irradiation surface of the silicon carbide wafer, the excess blue film on the wafer surface is accurately cut along the wafer circumferential direction, and the silicon carbide wafer with the film pasted on the non-laser irradiation surface is conveyed to the sample stage of the laser scanning equipment with the thinning surface facing up. The picosecond laser is generated by the laser generator, the wavelength of the laser is 1064 nm, the pulse width is 400 ps, the focal length X of the laser is controlled to a certain depth (target wafer thickness) inside the wafer, by controlling the local laser light output power to be 20 W and the movement speed to be 800 mm / s, a very high power density processing focal point area can be formed locally, the crystal molecules in this area absorb photon energy and emit free electrons, the micro-explosion caused by the Coulomb repulsion makes the molecular bonds of silicon carbide broken, and the crystal structure changes. The laser scanning path moves transversely along the <11-20> crystal surface, after a single path scanning is completed, a second path scanning is performed after moving a certain interval, the interval between two adjacent moving tracks is 200 μm, and after the whole surface scanning is completed, a modified layer will be formed along the 0001 surface in the certain depth area inside the wafer, accompanied by the generation of local micro-cracks.

[0060] S2: Taking the center of the silicon carbide wafer as the center, the inner diameter of the circumferential edge part is 0.7 times the radius of the whole wafer, the area with an inner diameter larger than the circumferential edge part of the wafer after the first laser scanning is scanned by the second laser, the conditions of the second laser scanning are the same as those of the first laser scanning, the focal length position of the laser acting on the silicon carbide wafer is lowered, the modified layer formed in the area outside the inner diameter of the circumferential edge part extends to the laser irradiation surface, and the total thickness of the modified layer in the second laser scanning is increased by 5 μm than that of the modified layer in the first laser scanning.

[0061] S31: The thinning surface (laser irradiation surface) of the silicon carbide wafer after the second laser scanning is pasted with a blue film, the blue film is fixed on the pasting ring, and the ring-mounted adsorption clamp is used to adsorb the pasting ring. The ring-mounted adsorption clamp includes 8 suction nozzles and 8 connecting rods arranged in a certain interval in a ring shape, the upper end of the suction nozzle is connected to a vacuum generator, one end of the connecting rod is connected to the suction nozzle, and the other end of the connecting rod is connected to a pressure sensor. During the separation process, each suction nozzle is adsorbed on the pasting ring by the vacuum generator, and the wafer separation schematic diagram is shown in Figure 6 The same pulling force is gradually applied from the Notch to the opposite direction of the Notch, the pressure sensor detects the pulling force value in each direction at any time and records it, at this time, the surface wafer will gradually separate until the pulling force is 0. If the first separation is unsuccessful, the initial pulling force can be increased to try again, the pulling force is 8-16 N, and the maximum pulling force is generally not more than 16 N.

[0062] S32: The separation surface of the silicon carbide wafer is ground by a grinding wheel, the stage and the grinding wheel rotate relative to each other, the grinding wheel gradually advances from top to bottom, after the grinding wheel thinning is completed, the blue film on the non-laser irradiation surface of the silicon carbide wafer is removed, and a silicon carbide wafer thinning piece is obtained. The grinding wheel thinning processing parameters are shown in Table 1.

[0063] Example 3

[0064] Based on Example 1, the main difference is that the total thickness of the modified layer of the second laser scanning is increased by 3 μm than the thickness of the modified layer of the first laser scanning. The other steps are the same as Example 1.

[0065] Example 4

[0066] Based on Example 1, the main difference is that the inner diameter of the circumferential edge part is 0.8 times the radius of the entire wafer with the center of the silicon carbide wafer as the center. The other steps are the same as Example 1.

[0067] Example 5

[0068] Based on Example 1, the main difference is that the total thickness of the modified layer of the second laser scanning is increased by 7 μm than the thickness of the modified layer of the first laser scanning. The other steps are the same as Example 1.

[0069] Comparative Example 1

[0070] Based on Example 1, the main difference is that the step S2 of the second laser scanning on the area outside the inner diameter of the circumferential edge part is not involved, and the silicon carbide wafer after the first laser scanning is directly separated and subjected to subsequent thinning treatment.

[0071] Comparative Example 2

[0072] The silicon carbide wafer is directly thinned by a grinding wheel.

[0073] Test Example

[0074] The 8-inch, 200-μm-thick silicon carbide wafer thinning pieces prepared in the above examples and comparative examples were tested, and the results are shown in Table 2.

[0075] Table 2

[0076]

[0077] According to Table 2, the TTV of the silicon carbide wafer thinning piece prepared by the laser modification and thinning method of the present application is not higher than 1.2 μm, and the crack rate is not higher than 4%. It can be seen that the processing method of the present application greatly reduces the problem of cutting cracks, improves the yield of the wafer, and reduces the production cost.

[0078] Table 3

[0079]

[0080] Referring to Table 3, the thickness variation of the silicon carbide wafer obtained by the second laser modification separation of Example 1 and the thickness variation of the silicon carbide wafer obtained by the first laser modification separation of Comparative Example 1 are tested respectively. The testing method is shown in Figure 12 Referring to Table 3, it can be seen that the silicon carbide wafer obtained by the first laser processing of Comparative Example 1 is relatively uniform in thickness; the silicon carbide wafer obtained by the second laser processing of the circumferential edge portion on the basis of the first laser processing of Example 1 is higher in thickness at the edge than in the interior, indicating that the modification layer formed at the circumferential edge portion extends in the direction of the surface to be thinned (or the direction of laser irradiation) during the second laser processing of the circumferential edge portion, and the thickness of the modification layer increases.

[0081] Referring to Figure 8 , Figure 8 An exemplary embodiment of the surface profile of the silicon carbide wafer thinned sheet of Example 1 is shown, and the wafer after separation is partially thinned. On the one hand, the edge of the wafer is subjected to two layers of laser processing, which reduces the difficulty of separation because the stress of the edge is smaller than that of the interior; on the other hand, the thickness of the target wafer edge is higher than that of the center after separation, at which time the stress in the crystal is basically in a balanced state (the sum of the tensile and compressive stresses of the wafer edge and center is zero), and at this time, a single grinding wheel (for example, 8000#) can be used for thinning processing. Because the direction of the grinding force is edge-center-edge, the additional introduction of local stress of the grinding wheel can be avoided when the thinning is completed once, the balance of the stress in the wafer is not broken, and the wafer is not prone to abnormal warping in the center (in the form of a "one-character" surface), which is better reflected in the image in the flatness of the center region.

[0082] Referring to Figure 9 , Figure 9 An exemplary embodiment of the surface profile of the silicon carbide wafer thinned sheet of Comparative Example 2 is shown, and the surface profile is in the form of M, and the wafer center is abnormally warped.

[0083] Referring to Figure 10 , Figure 10 An exemplary embodiment of the side view of the internal modification layer of the silicon carbide wafer after the second laser scanning of Example 1 is shown in Figure 11 An exemplary embodiment of the side view of the internal modification layer of the silicon carbide wafer after the first laser scanning of Comparative Example 1 is shown. The applicant found that Figure 10 , 11The modified layer is indicated by a circle. It can be seen that the overall modified layer is thickened after the second laser scanning.

[0084] The thickness of the laser thinning is greater than 80 μm, but the thickness of the final grinding thinning after the laser modification is less than 80 μm. Compared with Comparative Example 2, the grinding wear can be reduced by more than 20% under the same thinning thickness. When the traditional grinding thinning is used in Comparative Example 2, the thickness of the grinding thinning is greater than 200 μm. It is further proved that the processing method of the application further reduces the processing cost and shortens the thinning time on the basis of ensuring the wafer warping.

[0085] The above only describes the embodiments of the application and is not intended to limit the application. The application can have various modifications and changes for those skilled in the art. Any modification, equivalent replacement, improvement, etc. within the spirit and principles of the application shall be included in the scope of the claims of the application.

Claims

1. A method of processing a silicon carbide wafer using laser modification thinning, the method comprising: The processing method comprises the following steps: ​ S1: performing a first laser scanning on a surface to be thinned of a silicon carbide wafer to perform surface finishing, and forming a modified layer inside the silicon carbide wafer; S2: performing a second laser scanning on a circumferential edge portion of the surface to be thinned after the first laser scanning, so that the modified layer formed by the circumferential edge portion extends to the surface to be thinned; S3: sequentially performing separation and grinding thinning on the silicon carbide wafer after the second laser scanning to obtain a silicon carbide wafer thinning sheet; The thickness of the modified layer after the second laser scanning is greater than the thickness of the modified layer after the first laser scanning.

2. The method of claim 1, wherein, The thickness of the modified layer after the second laser scanning is increased by 3-5 μm compared with the thickness of the modified layer after the first laser scanning.

3. The method of claim 1 wherein, The inner diameter of the circumferential edge portion accounts for 0.6-0.8 times of the radius of the entire wafer with the center of the silicon carbide wafer as the center.

4. The method of claim 1 wherein, The laser modification and thinning thickness is greater than 80 μm.

5. The method of claim 1 wherein, The first laser scanning and the second laser scanning both use a picosecond laser device; further, the laser wavelength is 532-1064 nm, and the pulse width is 45-500 ps.

6. The method of claim 1 wherein, The laser power of the first laser scanning and the second laser scanning is 2-22 W, and the scanning rate is 300-800 mm / s.

7. The method of claim 1 wherein, The scanning path of the first laser scanning and the second laser scanning moves laterally within a certain interval along the <11-20> crystal surface to form a plurality of single scanning paths, and the interval between adjacent single scanning paths is 120-350 um.

8. The method of claim 5, wherein, Step S3 further comprises the following steps: S31: the surface to be thinned of the silicon carbide wafer after the second laser scanning is first laminated, fixed on a lamination ring, adsorbed on the lamination ring by using a ring-mounted adsorption clamp, and separated along the modified layer to obtain a silicon carbide wafer by applying tension; S32: performing grinding thinning on the separation surface of the silicon carbide wafer to obtain a silicon carbide wafer thinning sheet.

9. The method of claim 8, wherein, The ring-mounted adsorption clamp comprises suction nozzles and connecting rods arranged in a certain interval in a ring shape, the upper end of the suction nozzle is connected to a vacuum generator through a hole, one end of the connecting rod is connected to the suction nozzle, and the other end of the connecting rod is connected to a pressure sensor.

10. A silicon carbide wafer thinned sheet obtained by the processing method according to any one of claims 1 to 9, characterized by The TTV of the silicon carbide wafer thinning sheet is not higher than 1.2 μm, and the wafer breakage rate is not higher than 4%.

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