Silicon carbide crystal laser slicing method and system

Abstract

The invention discloses a silicon carbide crystal laser slicing method and system, and relates to the technical field of silicon carbide crystal slicing. The method includes rotating a silicon carbide crystal about a central axis; according to the preset laser processing depth, the laser spot linearly moves to the center axis in the radial direction along the edge of the silicon carbide crystal at the preset linear speed v till the laser spot moves to the center axis, and laser processing is completed; during laser processing, the distance between the surface of the silicon carbide crystal and the laser objective lens is adjusted according to the current processing radius and a specific compensation formula; and finally separating to obtain the silicon carbide wafer. The system provided by the invention comprises a laser processing module, a lifting module, a rotating module, a translation module and a control module. Compared with linear motion, due to the fact that acceleration and deceleration are not involved, the machining speed can be remarkably increased; good processing depth compensation can be realized for the middle convex / concave crystal, so that the processing is at the same depth, and the smoothness of the separated wafer is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of silicon carbide crystal slicing, and particularly to a laser slicing method and system for silicon carbide crystals. Background Art

[0002] As a third-generation semiconductor material, silicon carbide (SiC) has a wide range of application prospects in the fields of power electronics, new energy vehicles, etc. due to its high hardness (Mohs hardness 9.2), high thermal conductivity (4.9 W / m·K), and excellent chemical stability. The traditional mechanical cutting method of silicon carbide crystals is gradually being replaced by laser slicing due to large losses.

[0003] Laser slicing technology separates the crystal body from the wafer by inputting laser energy into the silicon carbide crystal to form a damaged layer. Although laser slicing technology can reduce material loss, there are problems such as slow processing speed of the damaged layer and poor consistency of processing depth. For example, the wafer manufacturing method provided by the Chinese authorized patent CN113714649B, the low-loss silicon carbide ingot laser slicing method provided by the Chinese patent application CN117139865A, and the SiC wafer generation method provided by the Chinese authorized patent CN108372434B all use a coordinated manner of linear motion in the x-axis and y-axis directions for laser processing of silicon carbide crystals. However, since each linear motion in the x-axis or y-axis direction involves acceleration and deceleration processes, nearly 2 / 3 of the time is consumed in the acceleration and deceleration processes, resulting in limited processing efficiency and restricting the processing speed.

[0004] Before laser processing of silicon carbide crystals, the crystal surface needs to be ground to make the surface smooth so that the laser can pass through the surface and be focused inside the crystal. Currently, diamond grinding wheels are generally used to achieve grinding of the entire surface by the rotation of the grinding wheel and the crystal. However, it is difficult to ensure strict parallelism between the grinding wheel and the crystal, resulting in a microscopic morphology of the processed surface that is convex or concave in the middle, with a height difference of 10 μm between the middle and the edge.

[0005] To alleviate / solve the problem of height difference, strict calibration work needs to be carried out before the grinding of silicon carbide crystals. However, if strict calibration is carried out before each crystal grinding, it will not only significantly increase the loss of the grinding wheel and the crystal, but also prolong the time required for the grinding process. Even through strict calibration, it is still difficult for the height difference between the middle region and the edge region to be less than 4 μm. Generally speaking, the laser refractive index of silicon carbide crystals is very large (about 2.6). When laser enters the silicon carbide crystal from the air environment, obvious refraction will occur, causing the laser to focus on a deeper depth (i.e., the target depth), which can generally be evaluated by multiplying the focal depth of the objective lens by a value slightly larger than the refractive index (about 2.9). Even if there is only a 4-μm height difference between the middle region and the edge region of the silicon carbide crystal, the actual processing deviation after refraction will still reach more than 10 μm. During the high-speed linear motion of laser processing, it is very difficult to achieve real-time non-linear height compensation. As a result, the damage layers formed by laser processing in the middle region and the edge region of the silicon carbide wafer are not at the same depth. This will not only lead to an increase in loss, but also greatly improve the difficulty of subsequent separation. Summary of the Invention

[0006] The present invention provides a method and system for laser slicing of silicon carbide crystals. By controlling the movement trajectory of the laser spot inside the silicon carbide crystal and simultaneously dynamically adjusting the distance between the surface of the silicon carbide crystal and the laser objective lens in real time, the constancy of the processing depth of the laser inside the silicon carbide crystal is ensured, and the compensation accuracy for silicon carbide crystals / wafers with a surface topography deviation of convex or concave in the middle is ≤2 μm. The technical solution of the present invention is specifically realized through the following technologies.

[0007] The present invention provides a method for laser slicing of silicon carbide crystals, including the following steps:

[0008] Install and fix the silicon carbide crystal so that the silicon carbide crystal rotates around the central axis;

[0009] According to the preset laser processing depth, the laser spot moves linearly in the radial direction from the initial position of the edge of the silicon carbide crystal to the central axis at a preset translation speed u until the laser spot moves to the central axis to complete the laser treatment;

[0010] Separate to obtain silicon carbide wafers;

[0011] When performing laser treatment, according to the compensation formula , adjust the distance between the surface of the silicon carbide crystal and the laser objective lens;

[0012] According to the formula and the formula , calculate the k value;

[0013] Among them, Z(r) is the height difference between the current position of the laser spot and the initial position of the edge, k is the topography coefficient, r is the linear distance from the laser spot to the center of the surface of the silicon carbide crystal, R0 is the crystal radius, Z0 is the height difference between the center of the silicon carbide crystal and the initial position of the edge, n is the refractive coefficient, n1 is the refractive index of air, n2 is the refractive index of the silicon carbide crystal, and NA is the numerical aperture of the objective lens used for laser processing.

[0014] In the above method provided by the present invention, while the silicon carbide crystal rotates around its own central axis, the laser spot moves radially relative to the surface of the silicon carbide crystal from the edge to the center. The radial movement of the laser spot can be achieved by keeping the laser processing module stationary and controlling the movement of the silicon carbide crystal itself, or by keeping the silicon carbide crystal rotating in place and controlling the movement of the laser processing module. Finally, the laser spot leaves a corresponding movement trajectory inside the silicon carbide crystal.

[0015] In the above method provided by the present invention, three-dimensional scanning is performed on the surface topography before processing to obtain high-precision height data of the crystal surface. By dynamically adjusting the distance between the surface of the silicon carbide crystal and the laser objective lens, the laser processing accuracy is improved, the stability of the laser processing depth is ensured, and the degree of middle convex or middle concave on the surface of the silicon carbide crystal / wafers after slicing is minimized as much as possible.

[0016] The method of dynamic adjustment is specifically implemented according to the compensation formula to achieve. Before processing, the surface height of the silicon carbide crystal is measured, and the topography coefficient k value is obtained according to the compensation formula. The compensation height can be determined by the relative value of the initial position of the edge of the silicon carbide crystal. Denote the compensation height at the processing starting point (crystal edge) as 0, measure the height difference between the center and the edge of the crystal, and then determine the topography coefficient k. During processing, the distance between the surface of the silicon carbide crystal and the laser objective lens is dynamically compensated according to the current processing radius r without real-time monitoring. The compensation accuracy achieved by this method is ≤1 μm, which is applicable to various crystals with surface topography deviations of middle convex or middle concave.

[0017] Furthermore, the movement trajectory of the laser spot inside the silicon carbide crystal is a spiral line that shrinks from the periphery to the central axis.

[0018] Even further, the movement arc length of each laser pulse of the laser spot inside the silicon carbide crystal is the same, and the spiral intervals of the spiral line are evenly distributed.

[0019] To ensure the consistency of the processing effect, it is necessary to make: (1) the path length of the laser spot moving inside the silicon carbide crystal within each laser pulse time is the same, that is, the arc length swept by the laser spot inside the silicon carbide crystal within each laser pulse time is equal; on the premise that the laser repetition frequency is constant (i.e., maintaining the initial set frequency unchanged), it is achieved by ensuring a constant linear velocity; (2) the spiral intervals of the spiral are evenly distributed.

[0020] Furthermore, the equation of the spiral is ω = v / r, where v is the linear velocity of the rotation of the silicon carbide crystal at the current position of the laser spot, and ω is the angular velocity of the rotation of the silicon carbide crystal matching v.

[0021] As the laser spot continuously moves at a translation speed u, the straight-line distance r from the laser spot to the center of the silicon carbide crystal gradually decreases. To ensure that the movement arc length of each laser pulse inside the silicon carbide crystal is the same, the linear velocity of the rotation of the silicon carbide crystal at the current position of the laser spot needs to be kept constant, and the angular velocity ω of the rotation of the silicon carbide crystal is dynamically adjusted (i.e., continuously increased) with the radius r according to the above formula.

[0022] The translation speed u changes with the change of the radius to ensure a constant spiral interval of the spiral.

[0023] Optionally, the change formula of the translation speed u is ; where d is the spiral interval and v is the linear velocity of the rotation of the silicon carbide crystal at the current position of the laser spot.

[0024] Through the combined control of the rotation of the silicon carbide crystal and the movement of the laser spot, the final movement trajectory of the laser spot inside the silicon carbide crystal is an Archimedean spiral that shrinks from the edge to the center.

[0025] Although the angular velocity ω of the rotation of the silicon carbide crystal is continuously increasing, the inherent angular velocity of the device has an upper limit and cannot be increased infinitely. Therefore, when the angular velocity that the device can reach reaches the upper limit, there is a corresponding critical radius. When r is less than this critical radius value, it is impossible to achieve a constant linear velocity by changing the angular velocity. At this time, to ensure that the movement arc length of each laser pulse of the laser spot inside the silicon carbide crystal is the same, it is necessary to reduce the laser repetition frequency to match the current linear velocity.

[0026] Furthermore, when r is lower than the critical radius, keep ω at the maximum value unchanged and keep the translation speed u unchanged, and adjust the laser repetition frequency according to the formula ;

[0027] where f is the laser repetition frequency to be adjusted and f0 is the initial set frequency of the laser.

[0028] Further, v is 10 - 1000 mm / s.

[0029] Further, the laser processing depth is 200 - 600 μm.

[0030] Further, during the Z(r) compensation, the compensation resolution ≤ 1 μm. The compensation resolution refers to the accuracy that can be achieved for height compensation. At present, it is very difficult to achieve real-time compensation in the linear processing method in the industry. In order to reflect the compensation effect, since this method reduces the compensation difficulty, the compensation resolution can be improved. The present invention also provides a silicon carbide crystal laser slicing system for performing the above method, which comprises:

[0031] A laser processing module, including a laser light source and an objective lens, for generating a laser and focusing the laser inside the silicon carbide crystal;

[0032] A lifting module, for adjusting the distance between the silicon carbide crystal and the laser objective lens;

[0033] A rotating module, for fixing and rotating the silicon carbide crystal;

[0034] A translation module, for controlling the movement of the laser processing module or the silicon carbide crystal, so that the laser spot moves linearly along the edge towards the central axis relative to the surface of the silicon carbide crystal;

[0035] A control module, for controlling the operation of the laser processing module, the lifting module, the rotating module and the translation module.

[0036] The above-mentioned lifting module, rotating module and translation module provided by the present invention can be realized by selecting a mature mechanical structure on the current market based on their purposes. The function of the control module can also be realized by selecting a common and mature control chip on the market.

[0037] Compared with the prior art, the beneficial effect of the present invention is that: by regulating the movement trajectory of the laser spot while rotating, and at the same time regulating the distance between the surface of the silicon carbide crystal and the laser objective lens, the present invention compensates for the surface topography deviation (convex in the middle or concave in the middle) of the silicon carbide crystal, thereby reducing the notch loss caused by the uneven processing surface. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 It is the overall system schematic diagram of the embodiment of the present invention.

[0039] Figure 2 It is the movement schematic diagram of the silicon carbide crystal in the embodiment of the present invention.

[0040] Figure 3 It is the processing trajectory schematic diagram in the embodiment of the present invention.

[0041] Figure 4 Schematic diagrams of the concave (left figure) and convex (right figure) surfaces in the silicon carbide crystal in the embodiment of the present invention.

[0042] Figure 5 Photo of the damaged layer of the silicon carbide crystal after processing according to the present invention.

[0043] In the figure: 100, objective lens; 400, lifting module; 300, rotating module; 200, suction cup; 500, translation module; 600, silicon carbide crystal. Detailed implementation manners

[0044] The technical solutions of the present invention will be described clearly and completely below. Apparently, the described embodiments are only a part of the embodiments of the present invention, rather than all of them. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts belong to the scope of protection of the present invention.

[0045] In some embodiments of the present invention, a method for laser slicing of silicon carbide crystals is provided, including the following steps:

[0046] Install and fix the silicon carbide crystal to rotate the silicon carbide crystal around the central axis;

[0047] According to the preset laser processing depth, the laser spot moves linearly in the radial direction from the initial position of the edge of the silicon carbide crystal to the central axis at a preset linear velocity v until the laser spot moves to the central axis to complete the laser treatment;

[0048] Separate to obtain silicon carbide wafers;

[0049] When performing the laser treatment, according to the current processing radius r, according to the compensation formula Adjust the distance between the surface of the silicon carbide crystal and the laser objective lens;

[0050] According to the formula and the formula , calculate the k value;

[0051] Wherein, Z(r) is the height difference between the current position of the laser spot and the initial position of the edge, k is the topography coefficient, r is the linear distance from the laser spot to the center of the surface of the silicon carbide crystal, R0 is the crystal radius, Z0 is the height difference between the center of the silicon carbide crystal and the initial position of the edge, n is the refractive coefficient, n1 is the refractive index of air, n2 is the refractive index of the silicon carbide crystal, and NA is the numerical aperture of the objective lens used for laser processing.

[0052] Optionally, the movement trajectory of the laser spot inside the silicon carbide crystal is a spiral line that shrinks from the periphery to the central axis.

[0053] Optionally, the movement arc length of each laser pulse of the laser spot inside the silicon carbide crystal is the same, and the spiral intervals of the spiral are evenly distributed. For example, according to the formula Adjust the translation speeds u, where v is the linear speed of the rotation of the silicon carbide crystal at the current position of the laser spot.

[0054] Further optionally, the equation of the spiral is ω = v / r, where v is the linear speed of the rotation of the silicon carbide crystal at the current position of the laser spot, and ω is the angular speed of the rotation of the silicon carbide crystal matching v.

[0055] Optionally, when r is lower than the critical radius, keep ω unchanged at the maximum value and keep the translation speed u unchanged. According to the formula , adjust the laser repetition frequency;

[0056] where f is the laser repetition frequency to be adjusted, and f0 is the initial set frequency of the laser.

[0057] Optionally, v is 10 - 1000 mm / s.

[0058] Optionally, the laser processing depth is 200 - 600 μm.

[0059] Optionally, the compensation resolution during Z(r) compensation is ≤1 μm.

[0060] Embodiment

[0061] A laser slicing system for silicon carbide crystals provided in this embodiment is as shown in Figure 1 and Figure 2 . The system includes:

[0062] A laser processing module, including a laser light source and an objective lens 100, for generating a laser and focusing the laser inside the silicon carbide crystal;

[0063] A lifting module 400, for adjusting the distance between the silicon carbide crystal and the laser objective lens, as indicated by the bidirectional arrow of 310 in Figure 2 ;

[0064] A rotation module 300, on which a suction cup 200 is further provided, for fixing and rotating the silicon carbide crystal, as indicated by the rotation direction of the arrow of 410 in Figure 2 ;

[0065] A translation module 500, for controlling the movement of the silicon carbide crystal, so that the laser spot moves linearly along the edge towards the central axis relative to the surface of the silicon carbide crystal, as indicated by the direction of the arrow of 510 in Figure 2 ;

[0066] A control module for controlling the operation of a laser processing module, a lifting module, a rotating module, and a translation module.

[0067] A method for laser slicing of silicon carbide crystals using the above system is as follows:

[0068] (1) The selected silicon carbide crystal has a specification of a diameter of 200 mm (radius R0 = 100 mm). Through measurement with a rangefinder, it is determined that the surface topography is convex in the middle, and the center of the surface is 4 μm higher than the edge. Install and fix the silicon carbide crystal so that the silicon carbide crystal rotates around the central axis;

[0069] (2) The preset target parameters for laser processing are: target linear velocity v = 100 mm / s, and spiral pitch 200 μm.

[0070] The initial set frequency of the laser f0 = 100 kHz, and the pulse interval is 1 μm. The rotational speed limit of the rotating module is 500 rpm (i.e., the angular velocity ω is approximately 52.36 rad / s).

[0071] Control the simultaneous operation of the laser processing module, the lifting module, the rotating module, and the translation module. The laser spot moves linearly along the edge of the silicon carbide crystal towards the central axis at the preset translation speed v until the laser spot moves to the central axis to complete the laser treatment. As Figure 3 shown, the overall movement trajectory of the laser spot inside the silicon carbide crystal is a spiral line that contracts from the periphery to the central axis with a constant spiral pitch.

[0072] At the initial position of the edge of the laser spot on the surface of the silicon carbide crystal, r = 100 mm, and the angular velocity ω = 1 rad / s (approximately 9.55 rpm).

[0073] In this embodiment, the compensation height at the initial position of the edge is recorded as 0, and the rangefinder measures that the center of the surface of the silicon carbide crystal is 4 μm higher than the edge. That is, the silicon carbide wafer used in this embodiment is a crystal convex in the middle ( Figure 4 right figure).

[0074] When performing Z(r) compensation, according to the radius r corresponding to the current position of the laser spot, adjust the distance between the surface of the silicon carbide crystal and the laser objective lens according to the compensation formula

[0075] According to the formula and the formula , calculate the k value.

[0076] ​Among them, n is the refractive coefficient, n1 is the refractive index of air, which is taken as 1 in this embodiment; n2 is the refractive index of the silicon carbide crystal, which is taken as 2.6 in this embodiment; NA is the numerical aperture of the objective lens used for laser processing, which is an inherent parameter of the laser objective lens and is taken as 0.65 in this embodiment. It is calculated that k = 1.4×10 -7 .

[0077] Therefore, the height compensation formula is Z(r) = 1.4×10 -7 ×(r 2 - 10000) mm.

[0078] Based on this method, the slice flatness can be reduced from greater than 10 μm to within 2 μm.

[0079] The translation speed u at the initial position of the edge is approximately 0.0318 mm / s (the corresponding angular velocity ω is 1 rad / s). As the silicon carbide crystal continues to translate, r gradually decreases.

[0080] ① When r does not reach the critical radius, the translation speed u increases as the processing radius r decreases.

[0081] Specifically, the translation speed u is adjusted according to the formula .

[0082] ② When r reaches the critical radius (the radius when the rotation speed reaches the limit) of 1.91 mm, the angular velocity ω of the rotation module is 52.36 rad / s (i.e., the rotation speed limit of 500 rpm) is reached. At this time, the laser repetition frequency = 100 kHz, and the translation speed u is approximately 1.67 mm / s.

[0083] ③ When r is less than the critical radius, ω is kept at the maximum value unchanged, and the translation speed u also remains unchanged according to the formula .

[0084] At this time, it is necessary to reduce the laser repetition frequency according to the formula to ensure the consistency of the processing effect.

[0085] Finally, the termination point position of the laser treatment is r = 0.2 mm. At this time, the laser repetition frequency f drops to approximately 10.47 kHz. The total processing time is approximately 30 min.

[0086] The translation speed u is the feed speed of the translation module to control the silicon carbide wafer along its radial direction, which is different from the constant linear speed (100 mm / s) v in the tangential direction. Due to the use of spiral motion, the translation speed is much lower than the high-speed lateral movement in traditional linear processing (usually ≥200 mm / s), allowing sufficient time to perform height compensation (such as the compensated curve fitted by preprocessing or real-time fine-tuning) to ensure the consistency of the processing depth. Figure 5It is a photo of the damaged layer after actual processing.

[0087] (3) Separate to obtain a silicon carbide wafer.

[0088] The method of this embodiment is also equally applicable to a crystal with a middle concave as shown in the left figure. When performing laser processing (that is, while the rotation module and the translation module are working), a similar method is used for adjustment to ensure that the damaged layer formed by laser processing is on the same depth plane. Figure 4

[0089] The above specific embodiments have described the implementation of the present invention in detail. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the claims and technical concept of the present invention, various simple modifications and changes can be made to the technical solution of the present invention, and these simple variations all belong to the protection scope of the present invention.​

Claims

1. A method for laser slicing of silicon carbide crystals, characterized in that, It includes the following steps: Install and fix the silicon carbide crystal to make the silicon carbide crystal rotate around the central axis; According to the preset laser processing depth, the laser spot moves linearly in the radial direction from the initial position of the edge of the silicon carbide crystal to the central axis at a preset translation speed u until the laser spot moves to the central axis to complete the laser treatment; Separate to obtain silicon carbide wafers; When performing laser processing, adjust the distance between the surface of the silicon carbide crystal and the laser objective lens according to the compensation formula ; According to the formula and the formula , calculate the value of k; Wherein, Z(r) is the height difference between the current position of the laser spot and the initial position of the edge, k is the topography coefficient, r is the linear distance from the laser spot to the center of the surface of the silicon carbide crystal, R0 is the crystal radius, Z0 is the height difference between the center of the silicon carbide crystal and the initial position of the edge, n is the refractive coefficient, n1 is the refractive index of air, n2 is the refractive index of the silicon carbide crystal, and NA is the numerical aperture of the objective lens used for laser processing.

2. The silicon carbide crystal laser slicing method according to claim 1, wherein The movement trajectory of the laser spot inside the silicon carbide crystal is a spiral line that shrinks from the periphery to the central axis.

3. The silicon carbide crystal laser slicing method according to claim 2, wherein The movement arc length of each laser pulse of the laser spot inside the silicon carbide crystal is the same, and the spiral intervals of the spiral line are evenly distributed.

4. The silicon carbide crystal laser slicing method according to claim 3, wherein The method for the spiral intervals of the spiral line to be evenly distributed is as follows: the translation speed u is adjusted according to the formula where d is the spiral interval and v is the linear velocity of the rotation of the silicon carbide crystal at the current position of the laser spot.

5. The silicon carbide crystal laser slicing method according to claim 2, wherein The equation of the spiral line is ω = v / r, where v is the linear velocity of the rotation of the silicon carbide crystal at the current position of the laser spot, and ω is the angular velocity of the rotation of the silicon carbide crystal matching v.

6. The silicon carbide crystal laser slicing method according to claim 5, characterized in that, When r is lower than the critical radius, keep ω at its maximum value and keep the translation speed u unchanged, and adjust the laser repetition frequency according to the formula ; Wherein, f is the laser repetition frequency to be adjusted, and f0 is the initial set frequency of the laser.

7. The silicon carbide crystal laser slicing method according to claim 1, wherein v is 10 - 1000 mm / s.

8. The silicon carbide crystal laser slicing method according to claim 1, wherein The laser processing depth is 200 - 600 μm.

9. The silicon carbide crystal laser slicing method according to claim 1, wherein, The compensation resolution during Z(r) compensation ≤ 1 μm.

10. A silicon carbide crystal laser slicing system, characterized in that, A method for laser slicing of silicon carbide crystals for any one of claims 1 - 9; includes: A laser processing module, including a laser light source and an objective lens, for generating laser light and focusing the laser light inside the silicon carbide crystal; A lifting module for adjusting the distance between the silicon carbide crystal and the laser objective lens; A rotation module for fixing and rotating the silicon carbide crystal; A translation module for controlling the movement of the laser processing module or the silicon carbide crystal, so that the laser spot moves linearly in the radial direction from the edge to the central axis relative to the surface of the silicon carbide crystal; A control module for controlling the operation of the laser processing module, the lifting module, the rotation module, and the translation module.

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