Preparation method of 4H-SiC micron-sized modified layer plastically deformed to be decomposed

Abstract

A preparation method of a 4H-SiC micron-sized modified layer plastically deformed to be decomposed comprises the following steps: firstly, ultrasonically cleaning a 4H-SiC wafer sample by using an acetone solution, then establishing a light path, and adjusting the wavelength, repetition frequency and pulse width of laser output by a femtosecond laser by using a control system; a 4H-SiC wafer sample is fixed to a machining station of a six-axis displacement table, a focus position is found by scribing on the sample, the machining position in the sample is adjusted through the six-axis displacement table, the focus position is moved to achieve preparation of modified layers with different depths, and the six-axis displacement table is controlled by a control system to move in the horizontal direction; the method comprises the following steps: focusing femtosecond laser in a 4H-SiC sample material, adjusting pulse density and laser energy flux to prepare modified layers with different depths and thicknesses in a 4H-SiC wafer sample, and finally forming a thin and uniform modified layer decomposed into carbon and silicon in 4H-SiC; parameters of the modified layer prepared by the method can reach that the thickness of the modified layer is less than or equal to 30 microns, the depth of a stress region is less than or equal to 1 micron, the method is low in energy and controllable, and the processing efficiency is greatly improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of laser stealth dicing of silicon carbide (SiC) ingots, and particularly relates to a method for preparing a 4H-SiC micron-level modified layer with plastic deformation to decomposition. Background Art

[0002] Due to its excellent physical properties, SiC is widely used in fields such as radio frequency devices, power devices, and new energy vehicles. There are many problems with traditional SiC wire saw cutting methods, such as large saw kerf losses, rapid wire saw wear, low cutting efficiency, and it is difficult to achieve large-size cutting. At the same time, the thickness of the sliced wafers is large and the comprehensive cost is extremely high, which seriously hinders the mass production and market promotion of SiC wafers. Laser stealth dicing technology is expected to replace wire saw cutting as the mainstream technology for SiC ingot slicing. First, femtosecond lasers are used to generate a modified layer inside SiC, and the material is separated at the modified layer by means of stress to achieve stealth dicing. Among them, precise modification by ultrafast lasers is the primary problem in internal stealth dicing of SiC ingots.

[0003] Combined with the research status at home and abroad, the main problems in realizing internal modification of SiC by ultrafast lasers are as follows:

[0004] Internal modification inside SiC by ultrafast lasers often adopts the processing method of tightly focusing with an objective lens ([1] Y.L. Zhang, X.Z. Xie, Y.A. Huang, W. Hu, J.Y. Long, Internal modified structure of silicon carbide prepared by ultrafast laser for wafer slicing, Ceramics International 49(3) (2023) 5249-5260. https: / / doi.org / 10.1016 / j.ceramint.2022.10.043; [2] M. Yamamoto, M. Deki, T. Takahashi, T. Tomita, T. Okada, S. Matsuo, S. Hashimoto, M. Yamaguchi, K. Nakagawa, N. Uehara, M. Kamano, Raman Spectroscopic Stress Evaluation of Femtosecond-Laser-Modified Region Inside 4H-SiC, Applied Physics Express 3(1) (2010). https: / / doi.org / 10.1143 / apex.3.016603.). Modification is achieved through a single pulse, making it difficult to precisely control the modification. Uneven stress distribution leads to poor consistency in the degree of modification in each modified region. Insufficient modification results in discontinuous modified regions, making it difficult to generate large-area cracks for slicing subsequently. Excessive modification makes the modified layer too thick, resulting in large losses.

[0005] Current objective lens processing ([3] L.F. Wang, C. Zhang, F. Liu, H. Zheng, G.J. Cheng, Ultrafast pulsed laser stealth dicing of 4H-SiC wafer: Structure evolution and defect generation, Journal of Manufacturing Processes 81 (2022) 562-570. https: / / doi.org / 10.1016 / j.jmapro.2022.06.064.) produces a self-focusing effect due to the relatively high peak power density, resulting in a narrow range and low precision for controlling the modified shape. The main reasons include the narrow and long modified region, poor position accuracy, and uncontrollable shape, all of which lead to a too thick modified layer and large losses.

[0006] Currently, this research is often achieved with an objective lens of 20 times or even higher magnification. Through a large focal length lens (focal length 30 mm), it is possible to achieve extremely small material removal and affected areas under the action of low energy density. In theory, an amorphous modified layer with an infinitely small thickness can be obtained, but the above method has not been publicly disclosed yet. Summary of the Invention

[0007] In order to overcome the above-mentioned disadvantages of the prior art, the purpose of the present invention is to provide a method for preparing a 4H-SiC micron-level modified layer by plastic deformation to decomposition. The parameters of the prepared modified layer can reach a modified layer thickness ≤ 30 μm and a stress zone depth ≤ μm. This method has low energy controllability and greatly improves the processing efficiency.

[0008] In order to achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0009] A method for preparing a 4H-SiC micron-level modified layer by plastic deformation to decomposition, comprising the following steps:

[0010] 1) Using a 4H-SiC wafer sample as the processing object, ultrasonically cleaning the 4H-SiC wafer sample with acetone solution and drying it.

[0011] 2) Setting up the optical path. The optical path includes a femtosecond laser 1. The output light of the femtosecond laser 1 sequentially passes through a shutter 4, a half-wave plate 5, a polarization beam splitter prism 6, and a cemented lens 9 and perpendicularly irradiates the 4H-SiC wafer sample 10 clamped on a six-axis displacement stage 11. The femtosecond laser 1, the shutter 4, the six-axis displacement stage 11, and a control system 2 are connected. Among them, the half-wave plate 5 and the polarization beam splitter prism 6 achieve stepless adjustment of the laser energy. The control system 2 controls the shutter 4 to further control the on / off of the optical path. The focal length of the cemented lens 9 is less than or equal to 30 mm.

[0012] 3) Using the control system 2 to adjust the output laser wavelength of the femtosecond laser 1 to 1030 ± 10 nm, the repetition frequency to 0 - 1000 kHz, and the pulse width to 246 fs.

[0013] 4) Fixing the 4H-SiC wafer sample 10 on the processing station of the six-axis displacement stage 11, finding the focal position by scribing on the sample, adjusting the processing position inside the sample through the six-axis displacement stage 11, and moving the focal position to achieve the preparation of modified layers with different depths. Using the control system 2 to control the six-axis displacement stage 11 to move horizontally.

[0014] 5) Focus the femtosecond laser inside the 4H-SiC sample material. Set the pulse frequency of the laser to 1 - 5 kHz respectively. Calculate the actual pulse density of 1 - 5 pulses / μm through the moving speed and pulse frequency. Adjust the pulse density and laser energy flux to prepare modified layers with different depths and thicknesses inside the 4H-SiC wafer sample 10. Finally, a thin and uniform modified layer decomposed into carbon and silicon is formed inside the 4H-SiC.

[0015] The preparation process is to induce nanoscale plastic deformation in the far field, and achieve decomposition through near-field enhancement under subsequent pulses, completing the preparation of the modified layer inside the 4H-SiC wafer sample 10.

[0016] The laser energy flux mentioned above is 0.09 J / cm 2 - 0.33 J / cm 2 .

[0017] The moving speed of the six-axis displacement stage 11 in the horizontal direction is 0 - 10 mm / s.

[0018] Compared with the existing technology, the beneficial effects of the present invention are:

[0019] During the preparation of the modified layer inside the 4H-SiC sample wafer, the laser energy flux uses a much lower flux than the phase explosion ablation threshold of 22.2 J / cm 2 ([4] H.Y. Shi, Q. Song, Y. Hou, S. Yue, Y. Li, Z. Zhang, M. Li, K.P. Zhang, Z.C. Zhang, Investigation of structural transformation and residual stress under single femtosecond laser pulse irradiation of 4H-SiC, Ceramics International 48(17)(2022)24276 - 24282. https: / / doi.org / 10.1016 / j.ceramint.2022.03.063.). The initial laser pulse induces nanoscale plastic deformation and other products in the far field, and achieves decomposition through near-field enhancement under subsequent pulses. The modified layer prepared under the optical near-field effect can break through the optical processing limit. The thickness of the modified layer is thinner, and the thickness of the modified layer ≤ 30 μm. The low flux makes the depth of the stress area around the prepared modified structure smaller, and the depth of the stress area ≤ 1 μm. On the one hand, the present invention has low-energy controllability, solving the randomness of blasting processing; on the other hand, it can be applied to high-speed scanning galvanometers and even cylindrical mirror processing schemes, greatly improving the processing efficiency. Description of the Drawings

[0020] Figure 1 Schematic diagram of the method principle of the embodiment of the present invention.

[0021] Figure 2 Schematic diagram of the optical path of the embodiment of the present invention.

[0022] Figure 3 Result diagram of Embodiment 1 (depth 100 μm, pulse density 1 pulse / μm) of the present invention.

[0023] Figure 4 Result diagram of Embodiment 2 (depth 100 μm, pulse density 2 pulses / μm) of the present invention.

[0024] Figure 5 Result diagram of Embodiment 3 (depth 200 μm, pulse density 5 pulses / μm) of the present invention.

[0025] Figure 6 TEM detection of the modified layer prepared in Embodiment 3 of the present invention, where (a) is the TEM image of the modified area and (b) is the TEM image of the unmodified area. Detailed implementation manners

[0026] The present invention will be further described below in conjunction with embodiments and drawings.

[0027] Embodiment 1, referring to Figure 1 , a preparation method for a 4H-SiC micron-level modified layer with plastic deformation to decomposition, comprising the following steps:

[0028] 1) Taking a 4H-SiC wafer sample with a single-piece size of 5×5×0.35 mm as the processing object, performing ultrasonic cleaning of the 4H-SiC wafer sample with acetone solution, and drying it;

[0029] 2) Building an optical path, referring to Figure 2 , the optical path includes a femtosecond laser 1, a control system 2, a first reflector 3, a shutter 4, a half-wave plate 5, a polarization beam splitter prism 6, a second reflector 7, a third reflector 8, a cemented lens 9, a 4H-SiC wafer sample 10, and a six-axis displacement stage 11; the light output by the femtosecond laser 1 passes through the first reflector 3 to turn the optical path by 90°, and the reflected light sequentially passes through the shutter 4, the half-wave plate 5, the polarization beam splitter prism 6, the second reflector 7, the third reflector 8, and the cemented lens 9 and vertically irradiates the 4H-SiC wafer sample 10 clamped on the six-axis displacement stage 11. The femtosecond laser 1, the shutter 4, the six-axis displacement stage 11, and the control system 2 are connected. Among them, the half-wave plate 5 and the polarization beam splitter prism 6 realize stepless adjustment of the laser energy. At the same time, the control system 2 controls the shutter 4 to further control the on / off of the optical path. A cemented lens 9 with a focal length of 30 mm is used for focusing the laser output by the femtosecond laser 1. The smaller the focal length, the easier it is to prepare a sample with no surface damage but internal modification;

[0030] 3) Using the control system 2, the femtosecond laser 1 is adjusted to output a laser wavelength of 1030 nm, a repetition rate of 0-1000 kHz, and a pulse width of 246 fs;

[0031] 4) Fixing the 4H-SiC wafer sample 10 on the processing station of the six-axis translation stage 11, the focus position is found by drawing a line on the sample, and the processing position inside the sample is adjusted by the six-axis translation stage 11. The focus position is moved to achieve the preparation of modified layers of different depths. The six-axis translation stage 11 is controlled by the control system 2 to move horizontally at a speed of 1 mm / s;

[0032] The femtosecond laser is focused inside the 4H-SiC sample material, and the laser pulse frequency is set to 1kHz. The actual pulse density can be calculated as 1 pulse / μm through the movement speed and pulse frequency. By adjusting the pulse density and laser energy flux, the preparation of modified layers of different depths and thicknesses inside the 4H-SiC wafer sample 10 can be achieved. Ultimately, a thin and uniform modified layer decomposed into carbon and silicon can be formed inside the 4H-SiC.

[0033] The preparation process of this embodiment is to induce nanoscale plastic deformation and other products in the far field, and to achieve decomposition through near-field enhancement under subsequent pulses to complete the preparation of the internal modified layer of the 4H-SiC wafer sample 10.

[0034] Reference Figure 3 In this embodiment, the modified layer was prepared at a depth of 100 μm, and the laser energy flux was 0.33 J / cm 2 ; A sample with no surface damage but internal modification that meets the application conditions was obtained with a pulse density of 1 pulse / μm; it was observed that the surface of the 4H-SiC sample was undamaged, and the modified layer could be observed by focusing inside the 4H-SiC; a cross-section was obtained by breaking the sample, and the thickness of the modified layer was observed to be 15.74μm under a laser confocal microscope (LSCM) and 11.70μm under a field emission scanning electron microscope (SEM); because SEM lacks the ability to detect optical modifications, the modification was tested by Raman spectroscopy, and the main peak hardly moved on the TO(2 / 4) mode, indicating that the stress-affected area was very small.

[0035] Example 2 (controllable thickness of modified layer), refer to Figure 4 , which is different from Example 1: at the same modification depth of 100 μm, a lower laser energy flux (F = 0.17 J / cm 2, a modified layer was prepared with a pulse density of 2 pulses / μm; the thickness of the modified layer was observed to be 5.48 μm under LSCM and 4.58 μm under SEM; through the new method of preparing the modified layer, the thickness of the modified layer was further reduced. Obviously, a thinner modified layer can be obtained by continuously reducing the pulsed laser fluence and increasing the pulse density, but it is more difficult to observe.

[0036] Example 3 (the position of the modified layer is controllable), referring to Figure 5 , different from Example 1: a modified layer was prepared at a depth of 200 μm, the laser energy fluence was 0.09 J / cm 2 , the pulse density was 5 pulses / μm; the thickness of the modified layer was observed to be 7.34 μm under LSCM and 6.70 μm under SEM, and the stress-affected zone was only 4.97 nm observed in the transmission electron microscope (TEM), referring to Figure 6 , it can be seen that the modified structure prepared by this method has a thinner stress zone, which is beneficial to reducing material loss.

[0037] The beneficial effects of this example: the thickness and position of the modified layer are controllable. At a depth of 200 μm, the laser energy fluence F = 0.09 J / cm 2 , the pulse density was 5 pulses / μm, and the frequency was 1 kHz. The thickness of the modified layer was observed to be 7.34 μm under the optical microscope and 6.70 μm under the electron microscope. Li et al. prepared a modified layer with a length of about 28 μm and a thickness of only 13.67 μm under the electron microscope through tight focusing treatment ([5] Y.H. Li, Z.Zhang, Q.Song, H.Y. Shi, Y.H. Hou, S.Yue, R.Wang, S.S.Cai, Z.C.Zhang, Surface micromorphology and nanostructures evolution in hybrid laser processes of slicing and polishing single crystal 4H-SiC, Journal of Materials Science & Technology 184 (2024) 235-244. https: / / doi.org / 10.1016 / j.jmst.2023.09.046.). In comparison, a thinner modified layer with a thickness of 6.70 μm was prepared on a length of 90 μm, and the thickness was reduced by 50.99%. At the same time, the stress-affected zone was only 4.97 nm, which is beneficial to reducing loss.

Claims

1. A preparation method for a micron-level modified layer of 4H-SiC with plastic deformation to decomposition, characterized in that, The steps include the following: 1) Using a 4H-SiC wafer sample as the processing object, ultrasonically cleaning the 4H-SiC wafer sample with acetone solution and drying it; 2) Setting up an optical path, which includes a femtosecond laser (1). The output light of the femtosecond laser (1) sequentially passes through a shutter (4), a half-wave plate (5), a polarization beam splitter prism (6), and a cemented lens (9) and vertically irradiates the 4H-SiC wafer sample (10) clamped on a six-axis displacement stage (11). The femtosecond laser (1), the shutter (4), the six-axis displacement stage (11), and a control system (2) are connected. Among them, the half-wave plate (5) and the polarization beam splitter prism (6) achieve stepless adjustment of the laser energy. The control system (2) controls the shutter (4) to further control the on / off of the optical path. The focal length of the cemented lens (9) is less than or equal to 30 mm; 3) Using the control system (2) to adjust the output laser wavelength of the femtosecond laser (1) to 1030 nm, the repetition rate to 0 - 1000 kHz, and the pulse width to 246 fs; 4) Fixing the 4H-SiC wafer sample (10) on the processing station of the six-axis displacement stage (11), finding the focal position by scribing on the sample. The processing position inside the sample is adjusted by the six-axis displacement stage (11), and different depth modified layers are prepared by moving the focal position. The control system (2) is used to control the six-axis displacement stage (11) to move horizontally; 5) Focusing the femtosecond laser inside the 4H-SiC sample material, setting the pulse frequency of the laser to 1 - 5 kHz respectively, calculating the actual pulse density of 1 - 5 pulses / μm by the moving speed and the pulse frequency, and adjusting the pulse density and the laser energy flux to prepare modified layers with different depths and thicknesses inside the 4H-SiC wafer sample (10), and finally forming a thin and uniform modified layer decomposed into carbon and silicon inside the 4H-SiC; 2. The preparation method according to claim 1, characterized in that: The preparation process is to induce nanoscale plastic deformation in the far field and achieve decomposition through near-field enhancement under subsequent pulses to complete the preparation of the modified layer inside the 4H-SiC wafer sample (10).

3. The preparation method according to claim 1, wherein: The laser energy flux described in step 4) is 0.09 J / cm 2 -0.33 J / cm 2 .

4. The preparation method according to claim 1, characterized in that: The moving speed of the six-axis displacement stage (11) horizontally is 0 - 10 mm / s.

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