A process method for laser stress relief to achieve ultra-low warpage of thinned SiC

Abstract

The application relates to the field of SiC material processing, in particular to a laser stress release process method for realizing ultra-low warping of thinned SiC. The method comprises the following steps: array laser drilling pretreatment is carried out on the C / Si surface of the SiC wafer before thinning; stress buffer belts are formed through accurate drilling by ultraviolet laser; slag is removed in combination with ultrasonic cleaning; grinding parameters of a grinding wheel and double-path cooling water design are optimized; and mechanical stress in the thinning process is dynamically offset. The application can significantly reduce residual stress and warping degree, meet the requirements of high-performance SiC devices on processing quality and reliability, and is suitable for mass production.

Description

Technical Field

[0001] This invention belongs to the field of SiC material processing. Background Technology

[0002] With the widespread application of silicon carbide (SiC) materials in semiconductor devices, stress control in its processing, especially after thinning, has gradually become a research hotspot. SiC is renowned for its high hardness, high thermal conductivity, and excellent electrical properties, but its brittleness makes it prone to warping due to residual stress during thinning, thus affecting subsequent processing and device performance. Therefore, achieving ultra-low warping after SiC thinning through processes such as laser stress release has become a key technical issue for improving the reliability and yield of SiC devices. While some stress release solutions exist in the prior art, significant shortcomings remain. For example, patent CN118023727B proposes a technique to reduce thermal stress by lowering the internal temperature gradient of the material through preheating. This method is mainly applicable to stress release during laser cutting and does not address the optimization of the overall stress distribution after SiC thinning. Furthermore, its preheating method may not meet the stringent requirements for ultra-low warping of SiC after thinning. In addition, the complex structure of this equipment and the need for coordinated control of the preheating and cutting processes may increase process difficulty and cost. Another patent, CN102285624B, improves the mechanical properties of bonded wafers by designing a thermal stress relief structure and combining it with laser scribing to suppress thermally induced cracks. However, this technology mainly targets thermal stress relief in bonded wafers and does not consider the unique stress distribution caused by the crystal structure and processing characteristics of SiC material after thinning. Furthermore, its thermal stress relief structure design is difficult to directly apply to the thinning process of SiC material, and its adaptability to complex stress fields is limited. These issues indicate that existing laser stress relief technology still has certain shortcomings in the application of ultra-low warpage after SiC material thinning, especially in the optimization of stress distribution, process simplification, and warpage control accuracy tailored to SiC material characteristics. This invention aims to significantly reduce the residual stress and warpage of SiC material after thinning by optimizing the laser action path, controlling the stress relief region, and adjusting process parameters, thereby meeting the requirements of high-performance SiC devices for processing quality and reliability. Summary of the Invention

[0003] This invention proposes a novel solution to the problem of warpage deterioration caused by residual stress in existing SiC thinning processes. To this end, this invention employs the following technical solution:

[0004] This invention provides a laser stress relief process for achieving ultra-low warpage after SiC thinning. The method involves pre-processing the C- and Si-planes of the SiC wafer with arrayed laser drilling before thinning, forming a stress buffer zone on the wafer surface to dynamically offset the mechanical stress generated during rapid material removal via grinding. Furthermore, by combining optimized thinning process parameters and a cooling system design, this method significantly reduces residual stress and warpage after thinning, thereby meeting the requirements of high-performance SiC devices for processing quality and reliability.

[0005] The specific implementation method of laser drilling pretreatment includes the following steps: S1, array-type laser drilling is performed on the C-side and Si-side of the SiC wafer using ultraviolet light with a wavelength of 355nm; S2, the laser parameters are adjusted according to the different physicochemical properties of the C-side and Si-side, wherein the pulse frequency is set to 10kHz to 13kHz, the spot size is controlled within the range of 10μm to 20μm, and the laser power is set to 10W to 20W; S3, the diameter of the laser-drilled hole is 10μm to 20μm, the hole depth is 25μm on the C-side and 15μm on the Si-side, the hole spacing is 15mm on the C-side and 20μm on the Si-side, and the hole shape is cylindrical to ensure that the hole is hollow and free of slag and other particulate impurities. In particular, for the characteristics of the C-side, the laser pulse frequency is increased by 10% to 20% compared to the Si-side, and the laser focus is moved down 20μm from the wafer surface to enhance the bottom energy density, thereby adapting to the higher intensity characteristics and chemical activity differences of the C-side. The laser power and hole depth energy density calculation formulas are as follows:

[0006] P: power, f: frequency, d: beam diameter.

[0007] Furthermore, after laser drilling, an ultrasonic cleaning step is used to remove slag and other particulate impurities from the hole. The ultrasonic cleaning process includes the following steps: S4, using ultrasonic waves at a frequency of 28 kHz to clean the C-surface to remove large particles of slag such as stubborn carbon deposits; S5, using ultrasonic waves at a frequency of 120 kHz to clean the Si-surface to remove fine particles such as silicon oxide slag; S6, controlling the cleaning water temperature within the range of 40°C to 60°C and maintaining the cleaning time for 1 hour to ensure the cleaning effect.

[0008] After laser drilling pretreatment and ultrasonic cleaning, the SiC wafer is thinned using a grinding wheel. The specific implementation of the thinning process includes the following steps: S7, selecting a grinding wheel with a tooth height of 5mm, setting its rotation speed to 1200rpm / min in the rough thinning stage and 900rpm / min in the fine thinning stage; S8, controlling the dynamic balance value to ≤0.2g to reduce the impact of vibration on processing accuracy; S9, setting the feed rate according to the differences in physicochemical properties between the C-side and Si-side, with the C-side feed rate at 0.3μm / s and the Si-side feed rate at 0.5μm / s in the rough thinning stage, and the feed rate at 0.2μm / s in the fine thinning stage; S10, setting the cooling water flow rate to 20L / min to 22L / min to effectively reduce the heat impact during processing; S11, setting the thickness of the thinned wafer to 370μm±1μm, with 20μm removed from the Si-side and 30μm removed from the C-side.

[0009] Furthermore, to optimize the cooling effect during the thinning process, this invention designs a dual-channel cooling water supply system. The cooling system design includes the following specific implementation methods: S12, the first channel of cooling water is sprayed from top to bottom onto the wafer surface through the internal pipe of the grinding wheel spindle, with the water column evenly dispersed into 12 streams to avoid direct contact with the grinding wheel teeth; S13, the second channel of cooling water is rinsed on the wafer edge through a universal water pipe, with the water column forming an acute angle of less than 45° with the wafer surface to ensure that the cooling water can cover the wafer surface and remove heat; S14, through the above cooling water design, the inside and outside of the grinding wheel are cooled simultaneously, which, combined with the laser pretreatment method, achieves the goal of zero warpage after thinning.

[0010] Specifically, the present invention also includes a specific implementation method for process flow control. The process flow control includes the following steps: S15, fixing the wafer using vacuum adsorption, with the vacuum adsorption value set to -0.8MPa to ensure the stability of the wafer during processing; S16, setting the internal air pressure of the thinning equipment to 0.6MPa to 0.8MPa to maintain the stability of the equipment operating environment; S17, executing the process steps in the following order: first, performing laser drilling pretreatment on the wafer, followed by ultrasonic cleaning to remove slag, then using a grinding wheel to thin the Si and C surfaces sequentially, and finally cleaning and drying the wafer after completing the thinning of both sides before proceeding to the next wafer processing step.

[0011] Compared with existing technologies, the advantages of this invention are as follows: By performing array-type laser drilling pretreatment on the C-side and Si-side of the SiC wafer and pre-setting a stress buffer zone, mechanical stress can be dynamically offset during the thinning process, thereby significantly reducing residual stress. Furthermore, by optimizing the grinding wheel, stress release cycle process, and dual-channel cooling water design, the wafer warpage is ≤0.05mm / m (test method: SEMI MF657), fundamentally solving the technical problem of excessive warpage after thinning. In addition, this invention employs an automated laser drilling and cooling system design, improving process efficiency and making it suitable for mass commercial production. By strictly controlling laser parameters, ultrasonic cleaning conditions, and thinning process parameters, the processed SiC wafer is ensured to have excellent surface quality and stability, thereby meeting the requirements of high-performance SiC devices for processing quality and reliability.

[0012] In summary, this invention provides an innovative SiC thinning process method. Through laser stress release pretreatment and optimized thinning process parameters, it achieves the technical goal of ultra-low warpage after thinning, providing reliable technical support for the fabrication of high-performance SiC devices. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of laser drilling on the surface of a SiC wafer.

[0014] Figure 2 This is a schematic diagram of the cross-section of a laser-drilled hole.

[0015] Figure 3 This is a schematic diagram of the thinning process after surface treatment;

[0016] Figure 4 This is a schematic diagram of the cooling water pipes inside the grinding wheel spindle.

[0017] In the image: 1. Chip serial number marking area; 2. Laser drilling area; 3. Chip C-side; 4. Drilling spacing 15mm;

[0018] 5. Si surface of the wafer; 6. Drilling spacing 20mm; 7. Crystal growth core; 8. C-face cross section; 9. Si-face cross section; 10. Hole bottom morphology; 11. Thinning grinding wheel; 12. SiC wafer; 13. Worktable; 14. Axial feed; 15. Cooling water universal pipe; 16. Thinning grinding wheel; 17. Spindle cooling water; 18. Cooling water outlet. Detailed Implementation

[0019] This invention provides a laser stress relief process for achieving ultra-low warpage after SiC thinning. The core of this method lies in pre-treating the C- and Si-faces of the SiC wafer with arrayed laser drilling before thinning, forming a stress buffer zone on the wafer surface to dynamically offset the mechanical stress generated during the rapid material removal process via grinding. Combined with optimized thinning process parameters and a cooling system design, this significantly reduces residual stress and warpage after thinning, thereby meeting the requirements of high-performance SiC devices for processing quality and reliability. The specific embodiments of this invention are described in detail below with reference to the accompanying drawings.

[0020] like Figure 1 As shown, the schematic diagram of laser drilling on the SiC wafer surface includes the wafer number marking area 1, the laser drilling area 2, the wafer C-side 3, the 15mm drilling spacing mark 4, the wafer Si-side 5, the 20mm drilling spacing mark 6, and the crystal growth core 7. In the implementation process, the wafer first needs to undergo laser drilling pretreatment. Array-type laser drilling is performed on the C-side 3 and Si-side 5 of the SiC wafer using 355nm ultraviolet light. The pulse frequency is set to 10kHz to 13kHz, the spot size is controlled within the range of 10μm to 20μm, and the laser power is set to 10W to 20W. The formula for calculating laser power and hole depth energy density is as follows:

[0021] P: power, f: frequency, d: beam diameter;

[0022] Laser parameters were adjusted based on the different physicochemical properties of C-side 3 and Si-side 5. For the characteristics of C-side 3, the laser pulse frequency was increased by 10% to 20% compared to Si-side 5, and the laser focal length was extended 20 μm into the material to enhance the bottom energy density and accommodate the higher strength characteristics and chemical reactivity differences of the C-side. The laser-drilled holes had diameters of 10 μm to 20 μm, a depth of 25 μm on the C-side and 15 μm on the Si-side, and a spacing of 15 μm on the C-side and 20 μm on the Si-side, respectively. The holes were cylindrical to ensure they were hollow and free of slag and other particulate impurities. After single-sided drilling of all wafers was completed, a wafer flipper was used to flip the wafers in the chuck before laser drilling on the other side. Table 1 shows the residual stress release effect under different laser parameters.

[0023]

[0024] Table 1

[0025] After laser drilling, an ultrasonic cleaning process is used to remove slag and other particulate impurities from the hole. For example... Figure 2As shown in the schematic diagram of the laser drilling cross-section, the C-side section 8, the Si-side section 9, and the bottom morphology 10 are illustrated. The ultrasonic cleaning process includes the following steps: ultrasonic cleaning of the C-side 3 at a frequency of 28 kHz to remove large particles of slag such as stubborn carbon deposits; and ultrasonic cleaning of the Si-side 5 at a frequency of 120 kHz to remove fine particles such as silicon oxide slag. The cleaning water temperature is controlled within the range of 40℃ to 60℃, and the cleaning time is maintained for 1 hour to ensure the cleaning effect. After cleaning, an Olympus optical microscope is used for inspection to confirm that the hole diameter, hole depth, and hole shape meet the requirements. The aperture was confirmed by an Olympus optical microscope, and the aperture depth was confirmed by adjusting the microscope's focal length and recording the difference in the position of the clear focus. The specific operation is to first place the focus directly above the surface of the wafer where the hole is to be drilled, observe the page until it is blurry and there is no clear field of view, then adjust the stage to gradually raise it. The first thing observed is the wafer surface (aperture), and the position of the focus knob at this time is recorded. Continue to raise the stage until a bright spot appears in the field of view. This is the diffuse reflection light source at the bottom of the hole, indicating that the focus has reached the bottom of the hole. Record the position of the focus knob at this time. The difference between the two is the laser drilling depth.

[0026] After laser drilling pretreatment and ultrasonic cleaning, the SiC wafer is thinned using a grinding wheel. For example... Figure 3 As shown in the schematic diagram of the surface treatment thinning process, the thinning wheel 11, SiC wafer 12, worktable 13, axial feed 14, and cooling water universal pipe 15 are illustrated. A grinding wheel with a tooth height of 5mm is selected, and its rotational speed is set to 1200 rpm / min in the rough thinning stage and 900 rpm / min in the fine thinning stage. The dynamic balance value is controlled to ≤0.2g to reduce the impact of vibration on machining accuracy. Based on the differences in physicochemical properties between C-surface 3 and Si-surface 5, the feed rates are set separately: 0.3 μm / s for C-surface and 0.5 μm / s for Si-surface in the rough thinning stage, and 0.2 μm / s for the fine thinning stage. The cooling water flow rate is set to 20 L / min to 22 L / min to effectively reduce the thermal impact during the machining process. The initial wafer thickness was 420μm ± 5μm, and the thinned wafer thickness was set to 370μm ± 1μm, with 20μm removed from the Si surface and 30μm removed from the C surface. Two cooling water supplies were used during the thinning process. One supply sprayed water from top to bottom onto the wafer surface through internal pipes of the Spindle spindle, with the water column evenly branching into 12 streams from a single point. The other supply rinsed the edges of the SiC wafer through a universal water pipe. The outlet of the universal cooling water pipe was positioned horizontally above the wafer surface, ensuring the water column impacted the wafer surface without directly contacting the thinning grinding wheel teeth, and maintaining an acute angle of less than 45° between the water column and the wafer surface.

[0027] To optimize the cooling effect during the thinning process, this invention designs a dual-path cooling water supply system. For example... Figure 4As shown in the schematic diagram of the internal cooling water pipes of the grinding wheel spindle, the thinning grinding wheel 16, the spindle cooling water 17, and the cooling water outlet 18 are illustrated. The grinding wheel spindle is equipped with a 12-hole distributor (material: 316L stainless steel), with orifices of 0.5mm ± 0.01mm, evenly spaced in a ring. The distributor forms a 15° divergence angle with the spindle centerline. The first stream of cooling water sprays from top to bottom onto the wafer surface through the internal pipes of the grinding wheel spindle, the water column evenly distributed into 12 streams to avoid direct contact with the grinding wheel teeth. The second stream of cooling water rinses the wafer edge through a universal water pipe, with the water column forming an acute angle of less than 45° with the wafer surface to ensure that the cooling water covers the wafer surface and removes heat. This cooling water design allows for simultaneous cooling of the grinding wheel's internal and external surfaces, achieving zero warpage after thinning in conjunction with laser pretreatment.

[0028] The specific implementation of process flow control includes the following operations: Vacuum adsorption is used to fix the wafer, with the vacuum adsorption value set at -0.8 MPa to ensure wafer stability during processing. The internal air pressure of the thinning equipment is set between 0.6 MPa and 0.8 MPa to maintain the stability of the equipment's operating environment. The process steps are executed in the following order: First, the wafer undergoes laser drilling pretreatment, followed by ultrasonic cleaning to remove slag. Then, a grinding wheel is used to thin the Si surface (5) and C surface (3) sequentially. Finally, after thinning both sides, the wafer is cleaned, dried, and then proceeds to the next wafer.

[0029] Table 2 shows a comparison of subsurface damage (SSD) and surface roughness (Ra) between traditional thinning methods and this process:

[0030]

[0031] Table 2

[0032] By performing array-type laser drilling pretreatment on the C-plane 3 and Si-plane 5 of the SiC wafer, a pre-set stress buffer band is created, which dynamically offsets mechanical stress during the thinning process, thereby significantly reducing residual stress. Furthermore, as shown in Table 3, by optimizing the grinding wheel, stress release cycle process, and dual-channel cooling water design, the wafer warpage is reduced to ≤0.05 mm / m (test method: SEMI MF657), fundamentally solving the technical problem of excessive warpage after thinning. In addition, this invention employs an automated laser drilling and cooling system design, improving process efficiency and making it suitable for mass commercial production. By strictly controlling laser parameters, ultrasonic cleaning conditions, and thinning process parameters, the processed SiC wafers are ensured to have excellent surface quality and stability, thus meeting the processing quality and reliability requirements of high-performance SiC devices.

[0033] Existing technology none Single-channel cooling 0.32 This invention have Dual-channel cooling 0.04

[0034] Table 3

[0035] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A laser stress relief process for achieving ultra-low warpage after SiC thinning, characterized in that... Includes the following steps: S1, using ultraviolet light with a wavelength of 355nm to perform array-type laser drilling on the C-side (3) and Si-side (5) of the SiC wafer respectively; S2, adjust the laser parameters according to the different physicochemical properties of the C surface (3) and the Si surface (5), wherein the pulse frequency is set to 10kHz to 13kHz, the spot size is controlled within the range of 10μm to 20μm, and the laser power is set to 10w to 20w. Formulas for calculating laser power and aperture depth energy density: P: power, f: frequency, d: spot diameter; E: laser energy density; S3, the diameter of the hole formed by laser drilling is 10μm to 20μm, the hole depth is 25μm on the C surface (3) and 15μm on the Si surface (5), the hole spacing is 15mm on the C surface (3) and 20mm on the Si surface (5), and the hole shape is cylindrical; S4, use ultrasonic cleaning at a frequency of 28kHz to clean surface C (3) to remove slag; S5, use ultrasonic cleaning at a frequency of 120kHz to clean the Si surface (5) to remove slag; S6, control the cleaning water temperature within the range of 40℃ to 60℃, and maintain the cleaning time for 1 hour; S7, select a grinding wheel with a tooth height of 5mm to perform thinning processing on SiC wafers. The rotation speed is set to 1200rpm / min in the rough thinning stage and 900rpm / min in the fine thinning stage. S8, dynamic balance value controlled within ≤0.2g; S9, cooling water flow rate set to 20L / min to 22L / min; S10 has an initial wafer thickness of 420μm±5μm and a set wafer thickness of 370μm±1μm after thinning.

2. The laser stress relief process method as described in claim 1, characterized in that... In the laser drilling process, taking into account the characteristics of the C-side (3), the laser pulse frequency is increased by 10% to 20% compared to the Si-side (5), and the laser focus is moved down 20μm from the wafer surface.

3. The laser stress relief process method as described in claim 2, characterized in that... Further, the holes formed by laser drilling are required to be hollow and free of slag and other particulate impurities.

4. The laser stress relief process method as described in claim 1, characterized in that... During the ultrasonic cleaning process, the pore size and depth were checked using an Olympus optical microscope to ensure that the pore size was 10 μm to 20 μm and the pore depth was 25 μm on the C surface (3) and 15 μm on the Si surface (5).

5. The laser stress relief process method as described in claim 1, characterized in that... During the thinning process, the feed rate of the C surface (3) in the rough thinning stage is 0.3 μm / s, the feed rate of the Si surface (5) is 0.5 μm / s, and the feed rate of the fine thinning stage is 0.2 μm / s.

6. The laser stress relief process method as described in claim 5, characterized in that... The amount of Si (5) removed during the thinning process is further limited to 20 μm, and the amount of C (3) removed is 30 μm.

7. The laser stress relief process method as described in claim 1, characterized in that... In step S9, the first cooling water is sprayed from the inside pipe of the grinding wheel spindle from top to bottom onto the wafer surface, and the water column is evenly dispersed into 12 streams.

8. The laser stress relief process method as described in claim 7, characterized in that... The second cooling water is further limited to rinsing the wafer edge through a universal water pipe, with the angle between the water column and the wafer surface being less than 45° acute angle.

9. The laser stress relief process method as described in claim 1, characterized in that... In the process flow control, the wafer is fixed by vacuum adsorption, and the vacuum adsorption value is set to -0.8MPa.

10. The laser stress relief process method as described in claim 9, characterized in that... The internal air pressure of the thinning equipment is further limited to 0.6MPa to 0.8MPa.

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