Precision machining method of thin aluminum nitride wafer

Abstract

The invention belongs to the technical field of semiconductor processing, and particularly relates to a precision processing method of a thin aluminum nitride wafer. The method comprises the following steps: firstly, enabling the aluminum surface of thin aluminum nitride to face upwards, carrying out thinning, coarse grinding, accurate grinding and polishing, and then enabling the nitrogen surface to face upwards, and carrying out thinning, coarse grinding, accurate grinding and polishing. The method can be used for precision machining of the thin aluminum nitride wafer, and the problems that in the polishing process of the thin wafer, due to long machining time and large pressure, the wafer is prone to being broken, and the surface quality of the wafer is poor are solved; the rapid processing of the aluminum nitride wafer can be realized, and the processing efficiency of the wafer is obviously improved; and the processed wafer is high in integrity, and the fracturing phenomenon is greatly reduced.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor processing technology, and specifically relates to a precision processing method for thin aluminum nitride wafers. Background Technology

[0002] Aluminum nitride (AlN), a key material for third-generation semiconductors, has become an ideal substrate for high-end fields such as deep ultraviolet optoelectronic devices, high-frequency high-power devices, and high-precision surface acoustic wave filters due to its ultra-wide bandgap (~6.2 eV), high thermal conductivity (~340 W / mK), and excellent insulation and piezoelectric properties. However, AlN single crystals are hard and brittle, making them extremely difficult to process with precision. Only by obtaining wafers with non-destructive and ultra-smooth surfaces can the material's potential be fully realized.

[0003] Currently, the traditional processing of AlN wafers follows a process of "cutting—grinding (rough grinding and fine grinding)—polishing". This process is still applicable to wafers of conventional thickness (usually ≥500μm) because they possess sufficient mechanical strength to withstand processing stress. However, with the development of devices towards vertical integration and miniaturization, the market demand for thin AlN wafers (usually ≤300μm) is becoming increasingly urgent. Such wafers have significant advantages in reducing thermal resistance and achieving heterogeneous integration.

[0004] The fundamental contradiction facing existing technologies lies in the irreconcilable conflict between the processing requirements of thin wafers and the capabilities of traditional processes. This conflict is specifically manifested in the significant difference in polarity between AlN crystals. The (0001) facet (aluminum facet) has high chemical inertness and a slow material removal rate; while the (000-1) facet (nitrogen facet) has high chemical reactivity and a fast removal rate. For wafers of conventional thickness, there is considerable room for choice in the processing sequence (which facet to process first), often prioritizing final surface quality or efficiency. However, if this conventional sequence (especially processing the nitrogen facet first for efficiency) is directly applied to thin wafers, the difficult-to-remove aluminum facet will undergo prolonged, high-stress polishing after the wafer has been thinned to a relatively thin thickness, directly leading to lengthy processing times, soaring breakage rates, and uncontrolled yield.

[0005] To address the breakage problem during wafer fabrication, one industry solution is to introduce specialized clamping systems for temporary bonding / debonding. While this solution provides mechanical support, it introduces complex process steps, high costs for specialized materials and equipment, and may lead to colloidal contamination and additional thermal stress. If the goal is to avoid these costs and complexities by directly machining wafers on traditional fixtures, the inability to overcome the inherent contradiction between "fragility" and "differences in polarity removal rates" results in an extremely narrow processing window and very low yield.

[0006] Therefore, there is a significant gap in existing technology: the lack of a method for stable and efficient ultra-precision machining of thin AlN wafers using low-cost, readily available traditional machining fixtures. Either economic efficiency and process simplicity are sacrificed, or machining yield and efficiency are compromised. This invention addresses this gap by proposing an innovative integrated process solution based on traditional fixtures. Its core inventive point lies not in introducing new fixtures or equipment, but in the creative design of an optimized machining sequence: aluminum surface first, then nitrogen surface. At the stage where the wafer is thickest and has the highest mechanical strength, the difficult-to-machine aluminum surface is tackled; in the final stage where the wafer is thinned to its most vulnerable state, the high removal rate of the nitrogen surface is utilized to quickly complete the polishing process in a shorter time and with gentler parameters. This optimized sequence makes it possible to achieve high-yield, high-efficiency, and ultra-precision machining of thin AlN wafers using traditional fixtures, effectively overcoming a key bottleneck in industrialization. Summary of the Invention

[0007] The purpose of this invention is to provide a rapid and precise machining method for aluminum nitride thin wafers, which solves the problems of wafer breakage and poor wafer surface quality caused by long processing time and high pressure during the polishing process, and can effectively shorten the processing time.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: A precision machining method for thin aluminum nitride wafers includes the following steps: Step S1: Place the aluminum nitride wafer to be processed and the quartz plate on the heating table for preheating, and then use paraffin wax to stick the aluminum side of the aluminum nitride wafer to be processed onto the quartz plate with the aluminum side facing up; if there is only one wafer, stick the aluminum nitride wafer to be processed to the center of the quartz plate; if there are two or more wafers, stick the wafers to be processed at equal intervals to the edge of the quartz plate. Step S2: Place the quartz plate from step S1 on the workpiece table of the thinning machine, evacuate the vacuum, set the tool, and then thin the wafer. Step S3: Place the thinned wafer on a copper disk, drip diamond solution onto it, and perform rough grinding; Step S4: Place the coarsely ground wafer on a rubber disc, drip in molten diamond, and perform fine grinding; Step S5: Place the finely ground wafer on a polyurethane polishing pad, add polishing liquid, and polish. Step S6: After polishing the aluminum surface, flip the wafer to be processed over so that the nitrogen side is facing up, keep the pasting position unchanged, and re-paste it onto the quartz plate; Step S7: Repeat steps S2-S5.

[0009] Preferably, the preheating temperature in step S1 is 90℃~120℃, and the total thickness deviation of the quartz plate is 0~3μm (inclusive).

[0010] Preferably, in step S1, after the paraffin wax is heated and melted on a quartz plate, the wafer to be processed is attached to the quartz plate, and then pressure is applied with a heavy object and cooled to room temperature.

[0011] Preferably, in step S2, the grinding wheel of the thinning machine has a mesh size of 300~2000, the workpiece rotation speed is 100~300rpm, the grinding wheel rotation speed is 1000~3000rpm, and the feed speed is 0.1~0.5μm / s.

[0012] Preferably, the diamond abrasive particles in step S3 have a particle size of 3~8μm, and the polishing pressure is 300g / cm. 2 ~700g / cm 2 The main speed range is 20 pm to 60 rpm.

[0013] Preferably, the diamond slurry abrasive grains in step S4 have a particle size of 1~3μm, and the polishing pressure is 300g / cm. 2 ~700g / cm 2 The main disc rotation speed is 20rpm~60rpm.

[0014] Preferably, the polishing slurry in step S5 is a silica sol polishing slurry, and the polishing pressure is 10 g / cm. 2 ~500g / cm 2 The main disc rotates at 30 rpm to 60 rpm.

[0015] Preferably, the wafer thickness before N-side polishing in step S7 is 250~400μm, and the wafer thickness after N-side polishing is 50~300μm.

[0016] Preferably, the polishing slurry in step S7 is a silica sol polishing slurry, and the polishing pressure is 10 g / cm. 2 ~500g / cm 2 The main disc rotates at 15 rpm to 50 rpm.

[0017] The thinning method is self-rotating grinding, which achieves rapid wafer leveling by bonding abrasive and high-speed wafer rotation. Grinding the wafer directly on the copper disk will result in low processing efficiency due to the excessive wafer TTV. Step S2 can significantly improve the wafer removal rate, thereby improving processing efficiency.

[0018] The polishing method is chemical mechanical polishing, which has strong chemical and mechanical effects. The removal rate of the aluminum surface is much lower than that of the nitrogen surface. If the aluminum surface of the thin aluminum nitride wafer is polished in the last step, there will be problems of long processing time and high cracking rate. Using the method in step S8 to polish the nitrogen surface in the last step can achieve precision processing of thin aluminum nitride while ensuring the quality of the crystal surface.

[0019] Wafers with high flatness and low surface roughness can be quickly obtained through thinning, grinding, and polyurethane polishing.

[0020] Compared with the prior art, the present invention has the following advantages: (1) The method of the present invention can be used for precision processing of thin aluminum nitride wafers, which solves the problem that thin wafers are prone to breakage and poor surface quality due to long processing time and high pressure during polishing. (2) The method of the present invention can realize the rapid processing of aluminum nitride wafers and significantly improve the processing efficiency of wafers.

[0021] (3) The wafers processed by the method of the present invention have high integrity and greatly reduce the occurrence of cracking. Attached Figure Description

[0022] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 This is a flowchart illustrating the rapid and precise machining process of the aluminum nitride wafer described in this invention. Figure 2 This is an image of the thin aluminum nitride wafer in Example 2; Figure 3 AFM image of the aluminum surface of the thin aluminum nitride wafer in Example 2; Figure 4 This is an optical microscope image of an aluminum nitride wafer with scratches caused by edge chipping in Comparative Example 1. Figure 5 This is an image of the broken thin aluminum nitride wafer in Comparative Example 2. Detailed Implementation

[0023] The present invention will be further described below with reference to specific embodiments, and the advantages and features of the present invention will become clearer with the description. However, the embodiments are merely exemplary and do not constitute any limitation on the scope of the present invention. Those skilled in the art should understand that modifications or substitutions can be made to the details and form of the technical solutions of the present invention without departing from the spirit and scope of the present invention, but all such modifications and substitutions fall within the protection scope of the present invention.

[0024] Example 1 A precision machining method for thin aluminum nitride wafers includes the following steps: S1. Take three 5.0 mm × 5.0 mm c-plane aluminum nitride single crystals (thickness 700~900 μm) as the wafers to be processed. Place the wafers and a quartz plate with a TTV of less than 1 μm on a heating stage and preheat at 100°C for 5 minutes. Melt the paraffin wax and attach the wafers with their aluminum side facing up, at equal intervals, to the edge of the quartz plate. Then apply pressure with a flat weight and cool to room temperature.

[0025] S2. Mount the quartz plate with the attached wafer onto the workpiece stage of the thinning machine and fix it under vacuum. Select a 320-mesh diamond grinding wheel, set the workpiece stage speed to 200 rpm and the grinding wheel speed to 2000 rpm, and perform self-rotating grinding and thinning at a feed rate of 0.1~0.2μm / s until the wafer thickness is 500μm.

[0026] S3. Remove the thinned wafer and mount it on a copper disk. Use a diamond solution with a particle size of 3μm at a concentration of 300~500g / cm³. 2 Rough grinding is performed under pressure and a large disc speed of 40~50 rpm.

[0027] S4. Transfer the coarsely ground wafer to a rubber disk. Use a diamond solution with a particle size of 1 μm, at a concentration of 300~500 g / cm³. 2 Fine grinding is performed under pressure and a large disc speed of 40~50 rpm.

[0028] S5. Transfer the finely ground wafer to a polyurethane polishing pad. Use a silica sol polishing slurry at a concentration of 300~500 g / cm³. 2 Chemical mechanical polishing was performed under pressure and a large disk rotation speed of 40~50 rpm to obtain a smooth aluminum surface; the wafer thickness was 450μm.

[0029] S6. After removing the wafer from the quartz plate, repeat the preheating and bonding process of step S1, but this time flip the wafer so that its nitrogen side is facing up.

[0030] S7. Repeat steps S2-S5 to thin and polish the nitrogen-coated surface. Transfer the finely polished wafer (approximately 300 μm thick) onto a polyurethane polishing pad. Use a silica sol polishing slurry at 50–200 g / cm³. 2 The nitrogen surface is then polished under relatively low pressure and at a large disc speed of 20-40 rpm until the target thickness of 100 μm is achieved.

[0031] The above experiment was repeated 20 times, and the total processing time in this embodiment averaged 5.5 hours. The final thin aluminum nitride wafers had a breakage rate of less than 15% and a surface roughness of less than 0.5 nm.

[0032] Example 2 A precision machining method for thin aluminum nitride wafers includes the following steps: S1. Take a 10.0 mm × 10.0 mm c-plane aluminum nitride single crystal (thickness 800~1000μm) as the wafer to be processed. Place the wafer and a quartz plate with a TTV of 1~3μm on a heating stage and preheat at 120℃ for 5 minutes. Melt the paraffin wax and attach the wafer with its aluminum side facing up to the center of the quartz plate. Then apply pressure with a flat weight and cool to room temperature.

[0033] S2. Mount the quartz plate with the attached wafer onto the workpiece stage of the thinning machine and fix it under vacuum. Select a 320-mesh diamond grinding wheel, set the workpiece stage speed to 200 rpm and the grinding wheel speed to 1000 rpm, and perform self-rotating grinding and thinning at a feed rate of 0.5 μm / s until the wafer thickness is 550 μm.

[0034] S3. Remove the thinned wafer and mount it on a copper disk. Use a diamond solution with a particle size of 3 μm at a concentration of 300~500 g / cm³. 2 Rough grinding is performed under pressure and a large disc speed of 40~50 rpm.

[0035] S4. Transfer the coarsely ground wafer to a rubber disk. Use a diamond solution with a particle size of 1 μm, at a concentration of 300~500 g / cm³. 2 Fine grinding is performed under pressure and a large disc speed of 40~50 rpm.

[0036] S5. Transfer the finely ground wafer to a polyurethane polishing pad. Use a silica sol polishing slurry at a concentration of 300~500 g / cm³. 2 Chemical mechanical polishing was performed under pressure and a large disk rotation speed of 40~50 rpm to obtain a smooth aluminum surface; the wafer thickness was 500μm.

[0037] S6. After removing the wafer from the quartz plate, repeat the preheating and bonding process of step S1, but this time flip the wafer so that its nitrogen side is facing up.

[0038] S7. Repeat steps S2-S5 to thin and polish the nitrogen-coated surface. Transfer the finely polished wafer (approximately 350 μm thick) onto a polyurethane polishing pad. Use a silica sol polishing slurry at 50–200 g / cm³. 2 The nitrogen surface was finally polished under relatively low pressure and a large disk rotation speed of 20 rpm. The finished wafer image is shown below. Figure 2 As shown.

[0039] The above experiment was repeated 20 times, with a total processing time of an average of 5.5 hours in this embodiment. The final thin aluminum nitride wafer had a breakage rate of less than 10%, an average thickness of 150 μm, and a surface roughness of less than 0.6 nm. A schematic diagram of the surface after polishing using atomic force microscopy (AFM) is shown below. Figure 3 As shown.

[0040] Example 3 A precision machining method for thin aluminum nitride wafers includes the following steps: S1. Take one 8.0 mm × 8.0 mm c-plane aluminum nitride single crystal (original thickness 800~1000μm) as the wafer to be processed. Place the wafer and a quartz plate with a TTV of 1~3μm on a heating stage and preheat at 110℃ for 5 minutes. Melt the paraffin wax and attach the wafer with its aluminum side facing up to the center of the quartz plate. Then apply pressure with a flat weight and cool to room temperature.

[0041] S2. Mount the quartz plate with the attached wafer onto the workpiece stage of the thinning machine and fix it under vacuum. Select a 2000-grit diamond grinding wheel, set the workpiece stage speed to 200 rpm and the grinding wheel speed to 2000 rpm, and perform self-rotating grinding and thinning at a feed rate of 0.1 μm / s until the wafer thickness is 550 μm.

[0042] S3. Remove the thinned wafer and mount it on a copper disk. Use a diamond solution with a particle size of 3 μm at a concentration of 300~500 g / cm³. 2 Rough grinding is performed under pressure and a large disc speed of 40~50 rpm.

[0043] S4. Transfer the coarsely ground wafer to a rubber disk. Use a diamond solution with a particle size of 1 μm, at a concentration of 300~500 g / cm³. 2 Fine grinding is performed under pressure and a large disc speed of 40~50 rpm.

[0044] S5. Transfer the finely ground wafer to a polyurethane polishing pad. Use a silica sol polishing slurry at 300 g / cm². 2 Chemical mechanical polishing was performed under pressure and a large disk rotation speed of 40~50 rpm to obtain a smooth aluminum surface; the wafer thickness was 545μm.

[0045] S6. After removing the wafer from the quartz plate, repeat the preheating and bonding process of step S1, but this time flip the wafer so that its nitrogen side is facing up.

[0046] S7. Repeat steps S2-S5 to thin and polish the nitrogen-coated surface. Transfer the finely polished wafer onto a polyurethane polishing pad. Use a silica sol polishing slurry at 30 g / cm³. 2The nitrogen surface is then polished under low pressure and at a large disc speed of 20 rpm.

[0047] The above experiment was repeated 20 times, and the total processing time in this embodiment averaged 5 hours. The final thin aluminum nitride wafer had a breakage rate of less than 10%, an average thickness of 200 μm, and a surface roughness of less than 0.6 nm.

[0048] Comparative Example 1 This comparative example uses the traditional method, which involves processing the nitrogen-coated surface first and then the aluminum surface.

[0049] S1. Take three 5.0 mm × 5.0 mm c-plane aluminum nitride single crystals (thickness 800~1000 μm) as the wafers to be processed. Place the wafers and a quartz plate with a TTV of less than 1 μm on a heating stage and preheat at 100°C for 5 minutes. Melt the paraffin wax and attach the wafers with their nitrogen-side up, at equal intervals, to the edge of the quartz plate. Then apply pressure with a flat weight and cool to room temperature.

[0050] S2. Mount the quartz plate with the attached wafer onto the workpiece stage of the thinning machine and fix it under vacuum. Select a 320-mesh diamond grinding wheel, set the workpiece stage speed to 200 rpm and the grinding wheel speed to 2000 rpm, and perform self-rotating grinding and thinning at a feed rate of 0.1~0.2μm / s until the wafer thickness is 350μm.

[0051] S3. Remove the thinned wafer and mount it on a copper disk. Use a diamond solution with a particle size of 3 μm at a concentration of 300~500 g / cm³. 2 Rough grinding is performed under pressure and a large disc speed of 40~50 rpm.

[0052] S4. Transfer the coarsely ground wafer to a rubber disk. Use a diamond solution with a particle size of 1 μm, at a concentration of 300~500 g / cm³. 2 Fine grinding is performed under pressure and a large disc speed of 40~50 rpm.

[0053] S5. Transfer the finely ground wafer to a polyurethane polishing pad. Use a silica sol polishing slurry at a concentration of 300~500 g / cm³. 2 Chemical mechanical polishing was performed under pressure and a large disk rotation speed of 40-50 rpm to obtain a smooth nitrogen-coated surface. The wafer thickness was 300 μm.

[0054] S6. After removing the wafer from the quartz plate, repeat the preheating and bonding process of step S1, but this time flip the wafer so that its aluminum side is facing up.

[0055] S7. Repeat steps S2-S5 to thin and polish the aluminum surface. Transfer the finely polished wafer (approximately 300 μm thick) onto a polyurethane polishing pad. Use a silica sol polishing slurry at 50–200 g / cm³. 2 The aluminum surface is then polished under relatively low pressure and at a large disc speed of 20-40 rpm.

[0056] In this comparative example, during the grinding of the aluminum surface in step S7, the wafer was too thin, resulting in noticeable edge chipping. Subsequent polishing of the aluminum surface was also affected by this chipping, easily introducing scratches during the polishing process. Figure 4 As shown, the surface roughness of the wafers is greater than 1 nm. The total processing time exceeds 10 hours, and the processing efficiency is significantly lower than that of Example 1.

[0057] Comparative Example 2 This comparative example shows the results of using excessive pressure in the final polishing step.

[0058] All steps are exactly the same as in Example 2, until the nitrogen surface is finally polished in step S7.

[0059] In this step, the polishing pressure is changed to 600 g / cm. 2 Other parameters remain unchanged.

[0060] Under high pressure, the wafer shatters, such as Figure 5 As shown, this indicates that the lower pressure range employed in this invention is crucial for ensuring yield during the final polishing of thin wafers.

Claims

1. A precision machining method for thin aluminum nitride wafers, characterized in that, Includes the following steps: Step S1: Place the aluminum nitride wafer to be processed and the quartz plate on the heating table for preheating, and then use paraffin wax to stick the aluminum side of the aluminum nitride wafer to be processed onto the quartz plate with the aluminum side facing up; if there is only one wafer, stick the aluminum nitride wafer to be processed to the center of the quartz plate; if there are two or more wafers, stick the wafers to be processed at equal intervals to the edge of the quartz plate. Step S2: Place the quartz plate from step S1 on the workpiece table of the thinning machine, evacuate the vacuum, set the tool, and then thin the wafer. Step S3: Place the thinned wafer on a copper disk, drip diamond solution onto it, and perform rough grinding; Step S4: Place the coarsely ground wafer on a rubber disc, drip in molten diamond, and perform fine grinding; Step S5: Place the finely ground wafer on a polyurethane polishing pad, add polishing liquid, and polish. Step S6: After polishing the aluminum surface, flip the wafer to be processed over so that the nitrogen side is facing up, keep the pasting position unchanged, and re-paste it onto the quartz plate; Step S7: Repeat steps S2-S5.

2. The precision machining method for thin aluminum nitride wafers according to claim 1, characterized in that, The preheating temperature in step S1 is 90℃~120℃, and the total thickness deviation of the quartz plate is 0~3μm, including 0.

3. The precision machining method for thin aluminum nitride wafers according to claim 1, characterized in that, In step S1, after the paraffin wax is heated and melted on a quartz plate, the wafer to be processed is attached to the quartz plate, and then pressure is applied with a heavy object and cooled to room temperature.

4. The precision machining method for thin aluminum nitride wafers according to claim 1, characterized in that, In step S2, the grinding wheel of the thinning machine has a mesh size of 300~2000, the workpiece rotation speed is 100~300rpm, the grinding wheel rotation speed is 1000~3000rpm, and the feed speed is 0.1~0.5μm / s.

5. The precision machining method for thin aluminum nitride wafers according to claim 1, characterized in that, The diamond slurry abrasive particles in step S3 have a particle size of 3~8μm, and the polishing pressure is 300g / cm. 2 ~700g / cm 2 The main speed range is 20 pm to 60 rpm.

6. The precision machining method for thin aluminum nitride wafers according to claim 1, characterized in that, The diamond slurry abrasive particles in step S4 have a particle size of 1~3μm, and the polishing pressure is 300g / cm. 2 ~700g / cm 2 The main disc rotation speed is 20rpm~60rpm.

7. The precision machining method for thin aluminum nitride wafers according to claim 1, characterized in that, The polishing slurry mentioned in step S5 is a silica sol polishing slurry, and the polishing pressure is 10 g / cm. 2 ~500g / cm 2 The main disc rotates at 30 rpm to 60 rpm.

8. The precision machining method for thin aluminum nitride wafers according to claim 1, characterized in that, The wafer thickness before N-side polishing in step S7 is 250~400μm, and the wafer thickness after N-side polishing is 50~300μm.

9. The precision machining method for thin aluminum nitride wafers according to claim 1, characterized in that, The polishing slurry mentioned in step S7 is a silica sol polishing slurry, and the polishing pressure is 10 g / cm. 2 ~500g / cm 2 The main disc rotates at 15 rpm to 50 rpm.

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