Large-size thin wafer electric spark discharge-etching auxiliary grinding method and device

Abstract

The invention provides a large-size thin wafer electric spark discharge-etching auxiliary grinding method and device.The specific machining method comprises the steps that a high-voltage input end is connected with a column electrode, a low-voltage input end is connected with a ring electrode, water vapor, N2 and Ar are mixed and then conveyed to the position between the ring electrode and the column electrode of an electric spark generating device through a gas path, and the high-voltage input end and the low-voltage input end are connected; high-concentration hydroxyl radicals are generated in a non-uniform electric field between ring-column electrodes, the hydroxyl radicals oxidize SiC to form SiO2 with lower hardness, and N2 < + > reacts with SiO2 to obtain conductive Si2N2O. The conductive Si2N2O is electrified, under the electrochemical auxiliary etching effect, hydroxyl ions in the Si2N2O and SiO2 areas are gathered, the local concentration is remarkably increased, and the reaction rate with the Si2N2O and SiO2 is increased. And removing the silicate surface with lower hardness by using an ultrasonic vibration device. Therefore, processing of low / stress-free large-size silicon carbide is realized, and the advantages of high precision, low load and high efficiency in grinding of large-size wafers are achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of semiconductor material processing, and in particular to an ultra-precision polishing technology for large-size silicon carbide wafers. background

[0002] As a key semiconductor material, SiC is in huge demand in the fields of new energy, 5G communications, etc. However, its traditional mechanical processing methods such as mechanical grinding are inefficient and prone to cracks and defects. Chemical mechanical polishing is also difficult to achieve both efficiency and precision. Although energy field assisted chemical mechanical polishing can improve efficiency, for large-sized thin SiC wafers, ultrasonic vibration is easy to break the material. Laser assisted processing has problems such as energy control and thermal stress. Ion beam polishing will produce excessive thermal stress, resulting in a decrease in the optical life of the wafer. Traditional CMP processing has only Si atoms and hydroxide ions (OH - ) undergoes a silicon-water oxidation reaction, while other surfaces (such as the C-plane) lack dangling Si bonds and lack the conditions for this chemical reaction. Large-sized SiC wafers have a large surface area, and achieving uniform polishing across the entire wafer surface requires extremely precise control of polishing pressure. As wafer size increases, the pressure distribution across the wafer becomes more complex, making the polishing process more susceptible to stress and making it difficult to control yield.

[0003] Chinese invention patent application CN 117464549 A, filed on January 30, 2024, describes a silicon carbide polishing device and process. The device includes a polishing disc, a carrier block, an LED ultraviolet light source, a graphite electrode, and a power supply. The positive electrode of the power supply forms a current loop with the carrier block, polishing fluid, graphite electrode, and negative electrode of the power supply. Photocatalytic-assisted polishing utilizes nano-photocatalysts under ultraviolet light to generate hydroxyl radicals (·OH) to oxidize silicon carbide. However, this process has a material removal rate of 1.18 μm / h, which is insufficient for polishing efficiency.

[0004] Chinese invention patent application CN 114619296 A, published on June 14, 2022, describes a silicon carbide atmospheric plasma polishing apparatus and method. The apparatus includes a workbench, an ion torch electrode, a radio frequency power supply, an Ar carrier gas source, a liquid ethanol source, a fluorine-containing active gas source, an auxiliary gas source, and a gas-liquid mixing evaporator. The apparatus chemically modifies the surface of the SiC workpiece to produce a gaseous reactant, thereby achieving high-precision polishing. However, the reaction products and reaction gases are toxic, and the polishing method is difficult to adapt to large workpieces.

[0005] EDM (Electrodischarge Plasma) is a non-contact machining method. Its powerful chemical removal capabilities negligibly affect the physical properties of the sample, preventing mechanical damage while achieving efficient, residual stress-free polishing. Furthermore, the use of spark discharge to generate plasma allows machining at atmospheric pressure, overcoming the high manufacturing and maintenance costs associated with vacuum equipment. EDM plasma, with its high energy and activity, can rapidly induce chemical reactions on the material surface, forming easily removable compounds or gaseous products. Summary of the Invention

[0006] In view of the shortcomings of existing SiC processing technology, the present invention aims to achieve high-efficiency, ultra-smooth, low / no damage ultra-precision processing of large-sized thin SiC wafers. The device includes a workbench, a polishing pad, a high-frequency pulse power supply, a workpiece to be processed, an electric spark generating device (ring electrode, column electrode, insulator, insulating nozzle, high-voltage input, low-voltage input, gas path), KOH etching solution, a power amplifier, a signal generator, an ultrasonic vibration device (piezoelectric ceramic sheet, brush ring, polishing head, rotating shaft), a machine tool spindle, and an inert electrode. Its main feature is that the plasma is generated by the uneven electric field between the ring and column electrodes of the device, and then sprayed onto the surface of the workpiece through the insulating nozzle, so that the sprayed area of ​​the surface is chemically modified and injected with conductive nitrogen ions. By utilizing the conductivity of Si2N2O, hydroxide ions (OH - ) are aggregated, and electrochemically assisted etching is used to react oxidized SiO2 and Si2N2O into silicates with lower hardness, increasing the reaction rate. The workpiece surface, where the silicate reaction has occurred, is then rotated by a polishing table to the ultrasonic vibration polishing area. The polishing pad then polishes the workpiece surface under ultrasonic vibration, achieving a high-precision, wear-free, and sub-damage-free machining method that couples the effects of plasma, electrochemically assisted etching, and ultrasonic vibration polishing.

[0007] In order to achieve the above-mentioned purpose, the technical solution adopted by the present invention will be divided into the following steps: Step 1, adjusting the position of the tool and the workpiece: fix the workpiece to be polished on the polishing pad, adjust the relative position of the ring electrode and the columnar electrode to the workpiece, so that the discharge gap is in an appropriate range, adjust the position of the ring-column electrode insulating nozzle and the ultrasonic vibration device polishing head, and ensure that the two are located in concentric circle positions on the workbench. Step 2, plasma modification: turn on the high-frequency pulse power supply to make the ring-column electrode generate an uneven electric field. The electric field strength exceeds the breakdown threshold of the gas, and electrons are released from the electrode surface or gas molecules to form free electrons. The free electrons are accelerated in the electric field and collide with neutral gas molecules, resulting in collision ionization, generating more electrons and positive ions, and forming an ionization channel. The reaction gas in the gas path obtains hydroxyl radicals (·OH) through the ionization channel, which are sprayed onto the surface of the workpiece through the insulating nozzle, chemically modifying the surface of the workpiece to obtain SiO2 with lower hardness, and injecting N2 + SiO2 is reacted to produce conductive Si2N2O. The etching effect of the electric spark plasma on the workpiece surface is controlled by adjusting the parameters. Step 3: Electrochemical etching: The Si2N2O area is connected to electricity and serves as the electrolysis cathode, and the inert electrode serves as the electrolysis anode. Under the action of electrochemically assisted etching, the concentration of hydroxyl radicals (·OH) in the Si2N2O and SiO2 areas will increase, increasing the removal rate of Si2N2O and SiO2, ultimately modifying the hard SiC surface into a lower hardness silicate and removing the ionic impurities introduced by the plasma modification. Step 4: Ultrasonic vibration polishing: The polishing table rotates the reaction area to the ultrasonic vibration polishing area, a conductive layer is coated on both sides of the piezoelectric ceramic, and a high-frequency alternating voltage is applied. The piezoelectric ceramic can then generate high-frequency vibrations through the inverse piezoelectric effect. The ultrasonic cavitation effect is used to assist in the removal of etching products and micro-protrusions on the workpiece surface, promote the flow and renewal of the etching solution, and improve the uniformity of the polishing effect. The ultrasonic vibration polishing device can achieve removal and polishing of the workpiece surface. Step 5. Repeat steps 1 to 4: Use the machine tool spindle to synchronize the movement of the electric spark plasma device and the ultrasonic vibration polishing device to ensure that the action areas of the two are on the concentric circles of the workpiece surface, and achieve precise polishing processing on the workpiece surface. Continue the above-mentioned collaborative polishing process. According to the requirements of the workpiece surface roughness, after polishing is completed, turn off all power supplies and systems, remove the workpiece, and clean and dry it.

[0008] The electric spark generating device is arranged above the workbench, the high voltage input of the power supply is connected to the column electrode through an impedance matcher, the low voltage input of the power supply is connected to the ring electrode, and the workbench is grounded.

[0009] The workpiece is immersed in an etching solution, causing the workpiece surface to react with the etching solution during operation. The ultrasonic vibration polishing device is located on the other side of the EDM generator. The polishing table rotates the work surface area oxidized by the EDM device to the ultrasonic vibration polishing area. The power amplifier is bolted to the reaction chamber, with its bottom communicating with the interior of the reaction chamber, effectively transmitting ultrasonic vibrations into the reaction chamber. A piezoelectric ceramic disc is attached to the polishing head with a special glue to generate vibrations. The polishing pad is glued to a specific position at the bottom of the reaction chamber, ensuring its stable position and sufficient contact with the SiC wafer surface, polishing the wafer under the action of ultrasonic vibrations.

[0010] As a further preferred technical solution of the present invention, it is necessary to control the duration and power of the spark discharge plasma oxidation to reduce the generation of Si3N4.

[0011] As a further preferred technical solution of the present invention, diamond abrasive particles may be appropriately added to increase the polishing rate.

[0012] As a further preferred technical solution of the present invention, a mass flow controller is used to control the flow rates of Ar, N2 and H2O within a suitable range according to process requirements during the processing.

[0013] As a further preferred technical solution of the present invention, when the KOH etching solution etches SiO2, the temperature can be appropriately increased to increase the reaction rate.

[0014] As a further preferred technical solution of the present invention, a collection device should be provided to collect the post-reaction gas and the residual input gas to prevent them from interfering with the subsequent ultrasonic vibration device.

[0015] As a further preferred technical solution of the present invention, the ultrasonic vibration polishing device can be installed with an adjustable bracket so that the device can change the working area to achieve precise positioning polishing.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] ① Continuously and stably supply high concentrations of hydroxyl radicals (·OH) to increase the oxidation rate of SiC. The design of an electrospark discharge plasma jet device with a needle-ring structure can generate a more stable electric field, thereby uniformly generating plasma. The reaction gas is non-toxic and the generated plasma concentration is high. The hydroxyl radicals (·OH) generated by the electrospark plasma device are injected into the surface of the SiC workpiece through the nozzle, oxidizing it to SiO2. By controlling the voltage of the working power supply and moving the nozzle position, the surface oxidation rate can be precisely controlled. The ring electrode can continuously provide initial electrons for the discharge, which is conducive to the formation of a stable and continuous plasma jet, ensuring a uniformly distributed reaction area.

[0018] ② Generate conductive Si2N2O products, locally gather hydroxide ions (OH - ), which improves the removal rate of Si2N2O and SiO2. The Si2N2O area is connected to electricity and used as the electrolysis cathode, and the inert electrode is used as the electrolysis anode. Under the action of electrochemical assisted etching, the hydroxide ions (OH - ) concentration increases, increasing the removal rate of Si2N2O and SiO2, ultimately modifying the hard SiC surface into a lower-hardness silicate and removing ionic impurities introduced during plasma modification. This significantly improves the removal efficiency of SiC, a hard and brittle material that is difficult to machine.

[0019] ③ Multiple energy field coupling effects take into account both grinding rate and precision. The three processes of plasma generated by electric spark discharge, chemical modification of etching solution, and ultrasonic vibration polishing are coupled. Simple mechanical polishing is less efficient for SiC because of its high hardness and it is difficult for abrasives to remove the material quickly. This device combines plasma generated by electric spark discharge, chemical modification of etching solution, and ultrasonic vibration polishing. First, plasma is generated by an electric spark discharge plasma jet device to oxidize the SiC surface to SiO2. During the etching process, by selecting a suitable etching solution, SiO2 can be reacted to obtain silicate with lower hardness. At the same time, ultrasonic vibration polishing can use the powerful impact force generated by the tiny bubbles generated by the ultrasonic cavitation effect when they burst to assist in removing materials on the SiC surface. In short, the three-coupled polishing method achieves high-precision and non-sub-damage processing of SiC.

[0020] ④ The use of an electrochemically assisted wet etching method, which is more compatible with large-scale SiC and is safe and non-toxic. Currently, dry etching is more commonly used, but this may not be ideal for high-precision polishing of large-scale SiC. This is because dry etching relies on the reaction of reactant gases with the SiC surface to produce gases, thereby achieving surface polishing. However, due to the toxicity of both the generated and reactant gases, the process must be carried out under closed conditions. Furthermore, when polishing large-scale SiC, the generated and reactant gases are mixed under closed conditions, making it more difficult to ensure high-precision polishing of the SiC surface by the reactant gases. By combining wet etching with electrochemical assistance, the etch rate is further increased. While KOH replaces the most commonly used HF etchant, the etch rate may be slightly reduced, but the etching process does not produce toxic gases or liquids. Compared to high-precision wet etching of SiC, HF / HNO3 etching reduces surface quality and increases roughness. In contrast, the surface roughness after KOH etching remains unchanged or slightly reduced. This process achieves high-speed and high-precision polishing for large-scale SiC. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a schematic diagram of the processing principle of the large-size thin wafer electrospark discharge-etching assisted grinding technology of the present invention.

[0022] Figure 2 This is a diagram of the equipment required for the large-size thin wafer electrospark discharge-etching assisted grinding technology of the present invention.

[0023] Figure 3 This is a cross-sectional view of the main processing device for large-size thin wafer electrospark discharge-etching assisted grinding of the present invention.

[0024] Figure numerals: 1. Workbench; 2. Polishing pad; 3. High-frequency pulse power supply; 4. Workpiece to be processed; 5. Electric spark generating device; 5-1. Ring electrode; 5-2. Column electrode; 5-3. Insulator; 5-4. Insulating nozzle; 5-5. High-voltage input; 5-6. Low-voltage input; 5-7. Gas path; 6. Etching liquid; 7. Power amplifier; 8. Signal generator; 9. Ultrasonic vibration device; 9-1. Piezoelectric ceramic sheet; 9-2. Brush ring; 9-3. Polishing head; 9-4. Rotating shaft; 10. Machine tool spindle; 11. Inert electrode. Specific implementation methods

[0025] Step 1. Adjust the position of the tool and the workpiece: Fix the workpiece to be polished on the polishing pad, adjust the relative position of the ring electrode and the cylindrical electrode to the workpiece so that the discharge gap is in the appropriate range, and adjust the position of the ring-cylindrical electrode insulating nozzle and the ultrasonic vibration device polishing head to ensure that they are located in the concentric circle position of the workbench.

[0026] Step 2: Plasma modification: Turn on the high-frequency pulse power supply to generate an uneven electric field at the ring-column electrode. The electric field strength exceeds the breakdown threshold of the gas, and electrons are released from the electrode surface or gas molecules to form free electrons. The free electrons are accelerated in the electric field and collide with neutral gas molecules, resulting in collision ionization, generating more electrons and positive ions, forming an ionization channel. The reactive gas in the gas path obtains hydroxyl radicals (·OH) through the ionization channel, which are sprayed onto the workpiece surface through an insulating nozzle, chemically modifying the workpiece surface to obtain SiO2 with lower hardness, and injecting N2 + SiO2 is reacted to obtain conductive Si2N2O, and the etching effect of the electric spark plasma on the workpiece surface is controlled by adjusting the parameters.

[0027] Step 3: Electrochemical etching: The Si2N2O region is connected to electricity and used as the electrolysis cathode, and the inert electrode is used as the electrolysis anode. Under the action of electrochemical assisted etching, the hydroxide ions (OH - ) concentration will increase, which will increase the removal rate of Si2N2O and SiO2, and ultimately modify the SiC surface with high hardness into silicate with lower hardness, and remove the ionic impurities introduced during plasma modification.

[0028] Step 4: Ultrasonic Vibration Polishing: The polishing table rotates the reaction area to the ultrasonic vibration polishing area. A conductive layer is applied to both sides of the piezoelectric ceramic. A high-frequency alternating voltage is applied, causing the piezoelectric ceramic to generate high-frequency vibrations through the inverse piezoelectric effect. The ultrasonic cavitation effect assists in removing etched products and microscopic protrusions from the workpiece surface, promoting the flow and renewal of the etching solution, and improving the uniformity of the polishing effect. The ultrasonic vibration polishing device achieves both surface removal and polishing.

[0029] The specific reaction formulas for steps 2 and 3 are as follows: e+H2O→e+·H+·OH SiC+8·OH→SiO2+CO2+4H2O; SiO2+2KOH→+H2O; Si2N2O+4KOH+H2O→2K2SiO3+2NH3

[0030] Step 5. Repeat steps 1 to 4: Use the machine tool spindle to synchronize the movement of the electric spark plasma device and the ultrasonic vibration polishing device to ensure that the action areas of the two are on the concentric circles of the workpiece surface, and achieve precise polishing processing on the workpiece surface. Continue the above-mentioned collaborative polishing process. According to the requirements of the workpiece surface roughness, the polishing time can be adjusted between 10-600 minutes. After polishing is completed, turn off all power supplies and systems, remove the workpiece, and clean and dry it.

Claims

1. A large-scale thin wafer plasma etching combined with ultrasonic vibration atomic scale processing device, the method requiring devices including a workbench (1), a polishing pad (2), a high-frequency pulse power supply (3), a workpiece to be processed (4), an electric spark generating device (5) (ring electrode (5-1), column electrode (5-2), insulator (5-3), insulating nozzle (5-4), high-voltage input (5-5), low-voltage input (5-6), gas path (5-7)), KOH etching solution (6), a power amplifier (7), a signal generator (8), an ultrasonic vibration device (9) (piezoelectric ceramic piece (9-1), brush ring (9-2), polishing head (9-3), rotating shaft (9-4), machine tool spindle (10), and an inert electrode (11). The electric spark generating device is arranged above the workbench, the high voltage input of the power supply is connected to the column electrode through an impedance matcher, the low voltage input of the power supply is connected to the ring electrode, and the workbench is grounded. The workpiece is immersed in the etching liquid, and the workpiece surface can react with the etching liquid during operation; the ultrasonic vibration polishing device is arranged on the other side of the electric spark generating device, and the working surface area oxidized by the electric spark device is rotated to the polishing area through the polishing table. The polishing pad is attached to a specific position at the bottom of the reaction chamber via a suction cup to ensure that the polishing pad is in a stable position and can fully contact the surface of the SiC wafer, thereby polishing the wafer under the action of ultrasonic vibration.

2. The SiC atmospheric plasma polishing equipment according to claim 1, characterized in that: The power supply frequency range is between 18kHz and 24kHz, and the high-frequency power supply power also needs to match the ultrasonic vibration parameters.

3. A large-scale thin wafer electrospark discharge-etching assisted grinding process, characterized by: A method and device for electrospark discharge-etching assisted grinding of large-size thin wafers according to claim 1 includes the following steps, which are performed in sequence: step 1, adjusting the position of the tool and the workpiece: fixing the workpiece to be polished on the polishing pad (2), immersing the workpiece (4) in the KOH etching solution (6), adjusting the relative positions of the ring electrode (5-1) and the columnar electrode (5-2) and the workpiece (4), and adjusting the distance between the ring-column electrode insulating nozzle (5-4) and the polishing head (9-4) of the ultrasonic vibration device to ensure that the two are located in concentric circle positions on the workbench. Step 2: Plasma modification: Turn on the high-frequency pulse power supply (3) to generate an uneven electric field in the ring-column electrode. The reaction gas in the gas path passes through the ionization channel to obtain hydroxyl radicals (·OH), which are sprayed onto the workpiece surface (10) through the insulating nozzle (5-4) to chemically modify the workpiece surface to obtain SiO2 with lower hardness, and inject N2 + React SiO2 into Si2N2O which has conductivity. Step 3: Electrochemical etching: The Si2N2O region is connected to electricity and used as the electrolysis cathode, and the inert electrode is used as the electrolysis anode. Under the action of electrochemical assisted etching, the hydroxide ions (OH - ) concentration will increase, which will increase the removal rate of Si2N2O and SiO2, and ultimately modify the SiC surface with high hardness into silicate with lower hardness, and remove the ionic impurities introduced during plasma modification. Step 4: Ultrasonic vibration polishing: Coat the conductive layer on both sides of the piezoelectric ceramic sheet (8-1) and apply high-frequency alternating voltage. The piezoelectric ceramic sheet can generate high-frequency vibration through the inverse piezoelectric effect, and use the ultrasonic cavitation effect to assist in removing the erosion products and micro-convexities on the surface of the workpiece. rise,. Step 5: Repeat steps 1 to 4: synchronize the movement of the electric spark plasma device (5) and the ultrasonic vibration polishing device (9) through the machine tool spindle (10), ensure that the action areas of the two are on the concentric circles of the workpiece surface, achieve precise polishing processing on the workpiece surface, and continue the above-mentioned coordinated polishing process.

4. The large-size thin wafer electrospark discharge-etching assisted grinding process according to claim 1 is characterized in that: The concentration of the etching solution is between 5-20 wt%.

5. The large-size thin wafer electrospark discharge-etching assisted grinding process according to claim 1 is characterized in that: The pulse width of the high-frequency pulse power supply is between 1-100 μs, the pulse interval is between 10-500 μs, and the discharge energy is between 0.1-10J.

6. The method according to claim 1, wherein the rotation speed of the polishing table spindle (10) is 0-100 r / min, and the rotation speed of the polishing shaft (9-5) of the ultrasonic vibration device is 10-100 r / min, and the two rotate in the same direction.

7. The method according to claim 1, wherein the nozzle (5-4) of the electric spark generating device needs to be insulated and immersed in the etching solution (6), characterized in that: The nozzle is between 1-5mm from the workpiece surface.

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