Regeneration method and application of silicon oxide polishing waste liquid

Abstract

The invention discloses a regeneration method of silicon oxide polishing waste liquid and application thereof. The regeneration method comprises the following steps: S1, collecting crystallized or agglomerated silicon oxide polishing waste liquid; s2, adding a hydrofluoric acid aqueous solution into the waste liquid, uniformly mixing for reaction, eroding bonding points on the surfaces of the agglomerated silicon oxide particles by utilizing hydrofluoric acid, converting part of silicon oxide into movable fluosilicate ions, and combining the fluosilicate ions with metal cations in the waste liquid to generate fluosilicate precipitates; s3, performing primary filtration on the mixed solution treated in the step S2 through a first filter medium to remove particles with the particle size greater than a first threshold value so as to obtain a primary filtrate; s4, the primary filtrate is subjected to secondary filtration through a second filter medium, particles with the particle size larger than a second threshold value are removed, fine filtrate is obtained, and the pore diameter of the second filter medium is smaller than that of the first filter medium; and S5, collecting the fine filtrate as a regenerated silicon oxide polishing solution. The invention aims to reduce the chemical mechanical polishing processing cost of the silicon carbide substrate.

Description

A method for regenerating silicon oxide polishing waste liquid and its application Technical Field

[0001] This application relates to the field of chemical mechanical polishing slurry recycling technology, and mainly to a method for regenerating silicon oxide polishing waste slurry and its application. Background Technology

[0002] Silicon carbide (SiC), a key material for third-generation semiconductors, faces severe challenges in precision surface processing due to its extremely high hardness and chemical inertness. Currently, the only mature technology capable of economically and efficiently achieving globally ultra-smooth, ultra-flat, and ultra-low-damage surface processing of SiC substrates is chemical mechanical polishing (CMP). The silicon oxide (SiO2) polishing slurry plays an almost irreplaceable role in this process. Specific chemical components in the SiO2 slurry first react with the SiC surface, transforming it into a softer, easily removable silica or silicate reaction layer. Simultaneously, a soft silica gel coating layer forms on the surface of the suspended nano-SiO2 abrasive particles. Under the pressure and relative motion of the polishing pad, the abrasive particles coated with the gel layer preferentially remove the soft reaction layer from the SiC surface through mechanical friction, exposing a new SiC surface for further chemical reactions. This cyclical process of "surface chemical transformation—soft layer formation—mechanical removal" makes it possible to achieve efficient, controllable, and low-damage polishing of SiC materials under relatively low mechanical pressure.

[0003] However, silica polishing slurries are thermodynamically unstable systems, and their colloidal stability is highly susceptible to environmental factors. During actual storage and use, the double-layer structure on the surface of silica particles can be disrupted due to factors such as excessively high local concentration caused by moisture evaporation, decreased pH value caused by absorption of CO2 from the air, drastic temperature fluctuations, or the introduction of metal ion impurities. This weakens the repulsive force between particles, leading to uncontrolled agglomeration, flocculation, and even the precipitation of hard crystals.

[0004] Once silicon oxide polishing slurry crystallizes, it will have serious consequences: (1) Crystallized silicon oxide particles or hard agglomerates act like coarse abrasive grains during polishing, which can easily cause severe micro-scratches or deep pits on the surface of SiC wafers. For semiconductor substrates with extremely high surface quality requirements, these defects are often fatal, directly leading to wafer scrapping or a significant drop in device yield; (2) Crystallization will reduce the concentration of effective active particles in the polishing slurry and change the particle size distribution, thereby causing a decrease in material removal rate and a decrease in uniformity within and between wafers. In addition, crystallized particles can easily clog the delivery pipes and polishing pad nozzles, leading to interruption of liquid supply and affecting production efficiency; (3) At present, the industry usually lacks effective repair methods for silicon oxide polishing slurries that have crystallized or agglomerated. In order to avoid the risk of scratches, once the polishing slurry shows signs of crystallization or reaches a certain service life, it is often directly disposed of as hazardous waste. Considering that SiC polishing slurry itself is expensive and the CMP process consumes a lot of it, this "disposable" or "short-life" usage method greatly increases the manufacturing cost of SiC substrates and also increases the environmental pressure of waste liquid treatment.

[0005] Therefore, there is an urgent need to develop a recycling method that can effectively remove crystalline substances from silicon oxide polishing slurries, restore their dispersion stability, and allow for safe recycling, in order to reduce production costs and ensure processing yield. Summary of the Invention

[0006] One objective of this application is to provide a method for regenerating silicon oxide polishing waste liquid to reduce the cost of chemical mechanical polishing of silicon carbide substrates.

[0007] To achieve the above objectives, the technical solution adopted in this application is as follows: a method for regenerating silicon oxide polishing waste liquid, comprising the following steps: S1. Collecting silicon oxide polishing waste liquid that has crystallized or agglomerated, wherein the waste liquid contains metal cations; S2. Adding hydrofluoric acid aqueous solution to the waste liquid and mixing it evenly to react, wherein the mass concentration of the hydrofluoric acid aqueous solution is 0.5%~5%, and in step S2, the volume ratio of the hydrofluoric acid aqueous solution to the waste liquid is 1:8~1:12, using hydrofluoric acid to erode the bonding points on the surface of the agglomerated silicon oxide particles and converting part of the silicon oxide into mobile fluorosilicate ions, wherein the fluorosilicate ions combine with the metal cations in the waste liquid to form fluorosilicate precipitates; S3. Performing primary filtration on the mixture after step S2 through a first filter medium to remove particles with a particle size larger than a first threshold, obtaining a pre-filtrate; S4. Performing secondary filtration on the pre-filtrate through a second filter medium to remove particles with a particle size larger than a second threshold, obtaining a fine filtrate, wherein the pore size of the second filter medium is smaller than the pore size of the first filter medium; S5. The fine filtrate is collected as a regenerated silica polishing slurry.

[0008] As a preferred embodiment, in step S2, the mass concentration of the hydrofluoric acid aqueous solution is 0.5% to 3%.

[0009] As a preferred embodiment, in step S2, the volume ratio of the hydrofluoric acid aqueous solution to the waste liquid is 1:10.

[0010] As a preferred method, in step S2, the mixing is carried out by stirring at a speed of 100-300 rpm for a reaction time of 0.5-3 hours.

[0011] As a preferred embodiment, in step S3, the mesh size of the first filter medium is 2000-6000 mesh, or the pore size of the first filter medium is 2.5μm-9μm; the mesh size of the second filter medium is 8000-15000 mesh, or the pore size of the second filter medium is 0.8μm-2μm.

[0012] As a preferred embodiment, in step S3, the first filter medium has a mesh size of 4000 mesh, and the second filter medium has a mesh size of 10000 mesh.

[0013] As a preferred embodiment, in step S1, the metal cations in the waste liquid include Na. + K + The content of the metal cation is 1000ppm~3000ppm.

[0014] As a preferred embodiment, step S6 is included after step S5: detecting the pH value and solid content of the fine filtrate, and adding deionized water or pH adjuster to adjust the fine filtrate to restore it to the preset polishing solution process parameters.

[0015] As a preferred embodiment, a chemical mechanical polishing method for wafers uses a regenerated polishing slurry prepared as described in any of the above methods when polishing the wafer.

[0016] As a preferred embodiment, the wafer is a silicon carbide single crystal.

[0017] Compared with the prior art, the beneficial effects of this application are as follows: (1) Eliminating scratches and improving yield: This invention innovatively introduces a dual removal mechanism of "chemical complexation + physical sieving", which transforms hard particles that are difficult to be physically intercepted into large particles that are easy to filter or directly dissolves them. Experimental data show that when SiC wafers are processed with polishing fluid treated by this invention, the total length of scratches is significantly reduced from 24.9 mm in the unfiltered crystal liquid to 3.4 mm, which significantly improves the surface quality of the wafer.

[0018] (2) Significantly reduce costs and achieve green recycling: This invention breaks the industry's convention of directly discarding crystallizing polishing liquid to avoid scratches. The polishing liquid treated by this method does not contain agglomerated particles and the particle size distribution is restored to normal. It can be recycled more than 5 times. Under the premise of ensuring that the polishing removal rate (0.6-0.8um / h) and surface roughness (Ra<0.1nm) are not reduced, the cost of using the polishing liquid is reduced by about 80%, which has extremely high economic benefits and environmental value.

[0019] (3) High process stability: Through dual-stage filtration, the problem of easy clogging or incomplete interception of single-stage filtration is effectively prevented, ensuring the consistency and stability of the regenerated polishing fluid.

[0020] (4) Unlike existing technologies that recover solid precipitates by mixing strong acids or etching waste liquids, this application strictly controls the concentration (0.5%~3%) and addition ratio of hydrofluoric acid. This specific low concentration range creates a "micro-etching" environment, which is limited to destroying the bonding points of high surface energy crystalline aggregates and precipitating harmful metal ions, without excessively dissolving normal nano-silica abrasive spheres. At the same time, with the specific two-stage filtration pore size (especially the second stage 0.8μm~2μm), this application achieves "removing coarse and retaining fine", using the filtrate directly as a regenerated product instead of recycling the filter cake, thereby maximizing the retention of high-value components of the polishing slurry. Attached Figure Description

[0021] Figure 1 is a schematic diagram of the process for recycling silicon oxide polishing waste liquid provided in this application. Detailed Implementation

[0022] The present application will be further described below with reference to specific embodiments. It should be noted that, without conflict, the various embodiments or technical features described below can be arbitrarily combined to form new embodiments.

[0023] In the description of this application, it should be noted that the terms "center", "lateral", "longitudinal", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", etc., which indicate the orientation and positional relationship based on the orientation or positional relationship shown in the accompanying drawings, are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and should not be construed as limiting the specific protection scope of this application.

[0024] It should be noted that the terms "first," "second," etc., in the specification and claims of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence.

[0025] The terms “comprising” and “having”, and any variations thereof, in the specification and claims of this application are intended to cover non-exclusive inclusion, for example, a process, method, system, product, or device that includes a series of steps or units is not necessarily limited to those steps or units that are explicitly listed, but may include other steps or units that are not explicitly listed or that are inherent to such process, method, product, or device.

[0026] It is understandable that used silica polishing slurry or silica polishing slurry that has been left to expire for a long time will experience uncontrollable aggregation, precipitation, agglomeration, and even the formation of hard crystals from the originally uniformly dispersed silica particles. The crystallization of the polishing slurry will have the following adverse consequences: (1) Blockage of the delivery system and nozzles on the polishing pad, resulting in interruption of the liquid supply. (2) Scratching of the wafer surface. Large aggregates or hard lumps will scratch the wafer surface during the polishing process, causing fatal defects. (3) Decreased polishing effect: Changes in particle size distribution and reduction of active particles lead to a decrease in material removal rate, an increase in surface roughness, and a deterioration in uniformity. (4) Waste polishing slurry: Severely crystallized polishing slurry is usually irreversible and can only be scrapped, resulting in economic losses.

[0027] Based on this, this application provides a method for regenerating silicon carbide polishing waste liquid, comprising the following steps: S1. Collecting silicon oxide polishing waste liquid that has crystallized or agglomerated, wherein the waste liquid contains metal cations; S2. Adding hydrofluoric acid aqueous solution to the waste liquid and mixing it evenly to react, wherein the mass concentration of the hydrofluoric acid aqueous solution is 0.5%~5%, and in step S2, the volume ratio of the hydrofluoric acid aqueous solution to the waste liquid is 1:8~1:12, using hydrofluoric acid to erode the bonding points on the surface of the agglomerated silicon oxide particles and converting part of the silicon oxide into mobile fluorosilicate ions, wherein the fluorosilicate ions combine with the metal cations in the waste liquid to form fluorosilicate precipitates; S3. Performing primary filtration on the mixture after step S2 through a first filter medium to remove particles with a particle size larger than a first threshold, obtaining a pre-filtrate; S4. Performing secondary filtration on the pre-filtrate through a second filter medium to remove particles with a particle size larger than a second threshold, obtaining a fine filtrate, wherein the pore size of the second filter medium is smaller than the pore size of the first filter medium; S5. The fine filtrate is collected as a regenerated silica polishing slurry.

[0028] It is worth mentioning that during the entire polishing process and waste liquid recycling, more metal ions may be introduced or dissolved from equipment, consumables, and process water. For example, the raw materials for producing silica particles (such as sodium silicate) may contain metal impurities such as sodium, potassium, calcium, and iron. The pH adjusters used (such as KOH and NaOH), polishing pads, equipment pipes, and water may also generate metal impurities during polishing. This application addresses this by dissolving the bonding points on the surface of the crystals with hydrofluoric acid aqueous solution, disrupting their structure, and converting some SiO2 into mobile fluorosilicate ions (SiF6). 2- ), SiF6 2- It combines with metal cations (such as sodium, potassium, calcium, iron, etc.) in the waste liquid to form insoluble fluorosilicate precipitates, for example: H₂SiF₆(aq) + 2Na + (aq) →Na2SiF6(aq)↓ + 2H + (aq), and then effectively removes fluorosilicate precipitates through filtration. This not only effectively removes large-sized agglomerates that cause crystal surface scratches, but also simultaneously purifies metal ion impurities in the polishing waste liquid. This restores the particle size distribution stability and chemical cleanliness of the polishing liquid, significantly reducing the risk of wafer scratches caused by crystal particles, improving surface processing quality, and enabling efficient recycling of the polishing liquid. This greatly saves production costs and reduces the environmental burden caused by waste liquid discharge.

[0029] In some embodiments, in step S1, the metal cations in the waste liquid include Na. + K + The content of metal cations ranges from 1000 ppm to 3000 ppm. It is understandable that metal ions compress the electric double layer on the surface of nano-SiO2 particles, neutralizing their surface charge and severely weakening the electrostatic repulsion between particles. This is one of the core chemical reasons for the irreversible agglomeration, flocculation, and even crystallization of silica particles in the polishing slurry.

[0030] In step S1, crystalline or agglomerated SiO2 particles typically have high surface energy and a strong tendency to aggregate, forming hard or large-sized agglomerates in the polishing slurry. These agglomerates are difficult to redisperse by conventional physical methods (such as stirring or ultrasonication). If such waste liquid is directly reused in the polishing process, the large-sized agglomerated particles will act as a hard abrasion source, generating macroscopic scratches and subsurface damage layers on the wafer surface, resulting in a significant decrease in product yield, an increase in rework rate, and even wafer scrapping.

[0031] In step S2, this application introduces a low-concentration hydrofluoric acid aqueous solution to trigger the following series of reactions: the hydrofluoric acid aqueous solution erodes the surface bonding sites of agglomerated particles in the silicon oxide polishing waste liquid, destroying their macroscopic structure; some SiO2 is converted into mobile fluorosilicate ions (SiF6). 2-), and with Na in the waste liquid + K + Ca 2+ When metal ions combine, insoluble fluorosilicate precipitates are formed. Compared to crystalline or agglomerated SiO2 particles that are difficult to filter out, fluorosilicate crystals have higher density and a regular, rigid structure, allowing them to be filtered out more quickly. This effectively removes crystals from polishing wastewater while maintaining process efficiency and economy, providing a feasible and scalable technical basis for the recycling of polishing fluids.

[0032] In step S3, filtration with a larger pore size effectively traps and removes the large fluorosilicate complex precipitates generated during the reaction, preventing them from clogging subsequent fine filtration stages. Then, in step S4, filtration with a smaller pore size further separates fine or incompletely reacted precipitate particles, thereby further purifying the regenerated polishing solution and restoring its particle size to near that of new polishing solution, thus improving its quality. It is worth noting that the multi-stage filtration mode consisting of steps S3 and S4 can not only remove fluorosilicate complex precipitates but also other solid substances exceeding the size limit in the waste liquid, achieving deep purification of the polishing waste liquid.

[0033] In some embodiments, in step S2, the mass concentration of the hydrofluoric acid aqueous solution is 0.5% to 5%, preferably 0.5% to 3%, and more preferably 1%. Specifically, a low concentration range (especially around 1%) ensures that the hydrofluoric acid effectively erodes the bonding points on the surface of the silica agglomerates and destroys their structure, while avoiding excessive dissolution of silica particles or collapse of the dispersion system due to overly vigorous reaction, thereby achieving an ideal balance between destroying agglomerates and maintaining the integrity of polishing functional particles. Within this concentration range, the generated fluorosilicate ions (SiF6) 2- A moderate concentration is beneficial for reacting with the inherent metal cations (Na+) in the waste liquid. + K + Ca 2+(etc.) are orderly combined to form a fluorosilicate crystal precipitate with uniform particle size and dense structure. This type of precipitate, due to its high density and good crystallinity, not only settles quickly but is also easier to separate through subsequent filtration. Furthermore, low-concentration hydrofluoric acid significantly reduces operational risks, has less corrosiveness to equipment, and its moderate dosage helps control regeneration costs. A preferred concentration of 1% ensures reaction efficiency while minimizing the consumption of chemical reagents and the burden of subsequent waste acid treatment. In step S2, the selection of hydrofluoric acid concentration is crucial. If the concentration is too high (e.g., using conventional etching waste liquid or a concentration >5%), it will cause a large amount of silica particles to dissolve, disrupting the solid content of the colloidal system and even leading to complete instability of the colloid; if the concentration is too low, it cannot effectively break the hard bonds of the aggregates. The concentration range of 0.5% to 3% specified in this application is precisely at the equilibrium point of disrupting the aggregate structure without destroying the monodisperse particles, which is key to achieving liquid regeneration.

[0034] In some embodiments, in step S2, the volume ratio of the hydrofluoric acid aqueous solution to the waste liquid is 1:8 to 1:12, preferably 1:10, to ensure that the amount of hydrofluoric acid added does not cause a severe impact on the overall pH and ionic strength of the polishing solution system, avoiding colloidal instability or secondary aggregation caused by local over-acidity or sudden changes in the electrolyte environment, thereby helping to maintain the dispersion stability and chemical consistency of the regenerated polishing solution. Furthermore, at this ratio, the fluorosilicate ions (SiF6)... 2- The formation rate of ) and the binding process of metal ions are more gradual, which is conducive to the formation of dense fluorosilicate crystals with uniform particle size.

[0035] In some embodiments, in step S2, the reaction is carried out at room temperature, and the mixing is achieved by stirring the reaction under stirring conditions at a speed of 100-300 rpm for a reaction time of 0.5 h to 3 h, preferably 1 h to 2 h, more preferably 1.5 h, to ensure that the hydrofluoric acid has sufficient time to fully erode the surface of the silica agglomerates and destroy their structure, while simultaneously allowing the generated fluorosilicate ions (SiF6) to... 2- The reaction proceeds by fully combining with metal ions to form complete crystals. Too short a reaction time may result in incomplete reaction, residual aggregates, or incomplete precipitation; too long a reaction time may cause secondary dissolution of the precipitate or slow instability of the colloidal system, increasing the uncontrollability of the process.

[0036] In some embodiments, the first filter medium has a mesh size of 2000-6000 mesh, or a pore size of 2.5μm-9μm; the second filter medium has a mesh size of 8000-15000 mesh, or a pore size of 0.8μm-2μm. The first filter medium first intercepts "coarse residue" composed of fluorosilicate precipitates, large silica agglomerates, and other impurities, preventing larger precipitates from directly entering the second filter medium and causing blockage. The second filter medium is specifically designed to retain residual fine crystals, nano-sized particles, and incompletely precipitated colloidal substances. The first filter medium (2000-6000 mesh) removes almost all visible agglomerates and precipitates, essentially eliminating the risk of scratches. The second filter medium (8000-15000 mesh) further filters out submicron-sized particles, restoring the particle size distribution of the abrasive in the polishing slurry to a stable state close to that of a fresh slurry, thereby ensuring consistent removal rates and surface finish quality in the regenerated polishing slurry.

[0037] It's worth noting that since the effective abrasive particles in the polishing slurry are typically smaller than 250 nanometers (0.25 μm), the pore size of the filter medium must be larger than this size to allow these small particles to pass through smoothly. For example, the mesh size of the second-stage filter medium is often chosen to be 8000~15000 mesh (corresponding to a pore size of 0.8 μm~2 μm), which is much larger than the effective abrasive particle size. This will not remove the effective abrasive components from the regenerated polishing slurry, but will only remove harmful large particle deposits. In semiconductor polishing processes, the large particle count (LPC) of the polishing slurry is a critical indicator. Through second-stage fine filtration, the number of particles ≥0.5 μm in the regenerated polishing slurry can be controlled within a safe range, thereby avoiding scratches on the wafer during the polishing process.

[0038] In some embodiments, the first filter medium has a mesh size of 4000 mesh (pore size approximately 3.4 μm), and the second filter medium has a mesh size of 10000 mesh (pore size approximately 1.15 μm). This not only efficiently removes fluorosilicate complex precipitates generated after chemical treatment, but also effectively prevents excessively large-particle precipitates from entering the second filter medium and causing blockage, ensuring the stable operation of the filtration system. The 10000 mesh second filter medium further intercepts any particles missed by the first filter medium, or some small, newly formed fluorosilicate crystal nuclei or colloids, thereby improving the quality of the regenerated polishing fluid.

[0039] In some embodiments, step S5 is followed by step S6: detecting the pH value and solid content of the fine filtrate, and adding deionized water or pH adjuster to adjust the fine filtrate to restore it to the preset polishing fluid process parameters, so that the chemical activity and abrasive concentration of the regenerated polishing fluid meet the processing requirements, and the regenerated polishing fluid can re-enter the production line in a clear and stable state, thereby supporting a true recycling model in terms of economy and reliability.

[0040] Furthermore, this application also provides a chemical mechanical polishing method for wafers, specifically, when polishing the wafer, a regenerated polishing slurry prepared as described above is used, and the surface scratches and other defects of the wafer polished with the regenerated polishing slurry are controlled within the process safety threshold.

[0041] Furthermore, the chemical mechanical polishing method for wafers described in this application is particularly suitable for silicon carbide (SiC) single crystals. This is mainly because silicon carbide polishing has extremely stringent requirements for the dispersion stability and chemical cleanliness of abrasive particles in the polishing slurry; any micron-sized hard agglomerates can cause fatal scratches on the substrate surface. This method, through the aforementioned regeneration step, ensures that the polishing slurry is free of harmful large particles and has extremely low metal impurity content. Therefore, polishing with the regenerated polishing slurry prepared by this method can meet the extreme requirements of SiC devices for ultra-smooth substrate surfaces and ultra-low damage.

[0042] The following examples further illustrate the method for regenerating silicon oxide polishing slurry provided in this application. The method provided in this application is not limited to the SiC substrate polishing scenario described in the following examples. For chemical mechanical polishing processes of other hard, brittle, or difficult-to-process materials involved in existing semiconductor and integrated circuit manufacturing, and in situations where colloidal silicon dioxide-based polishing slurries are used and the slurry may fail due to particle agglomeration or crystallization, the regeneration method of this application can be referred to or adapted for application.

[0043] Example 1 This example provides a method for regenerating silicon oxide polishing waste liquid, with the following specific steps: Step S1: Collect the waste liquid generated in the silicon carbide (SiC) CMP polishing process. The original waste liquid is an alkaline silica sol system containing nano-sized silica particles and potassium hydroxide (KOH) as a pH adjuster and stabilizer. Due to prolonged use and water evaporation, obvious white flocculent crystals and hard agglomerates have appeared in the waste liquid, and it contains trace amounts of metal impurity ions introduced during the polishing process. The content of metal cations in the waste liquid was measured to be approximately 1000 ppm.

[0044] Step S2: Prepare a 1% (w / w) hydrofluoric acid (HF) aqueous solution as a fluorine-containing complexing agent solution. Add the collected silica polishing waste liquid and the HF aqueous solution to the reactor at a volume ratio of 10:1. Turn on the stirring device, control the speed at 200 rpm, and stir and mix at room temperature (25℃) for 1.5 hours. Reaction mechanism explanation: In this process, the added HF first reacts with the unstable, highly reactive amorphous silica crystals and agglomerates in the waste liquid to generate fluorosilicic acid; subsequently, the fluorosilicic acid rapidly reacts with the abundant potassium ions in the waste liquid, the stabilizer from the original polishing liquid, and other metal impurity cations to generate water-insoluble potassium fluorosilicate precipitates and other fluorosilicate precipitates. This step transforms the originally viscous, difficult-to-intercept colloidal agglomerates into dense precipitate particles.

[0045] Step S3: Primary coarse filtration. The reaction mixture is pumped into a primary filter using a diaphragm pump. The primary filter is equipped with a 4000-mesh (approximately 3.4 μm pore size) nylon filter screen. In this step, most of the fluorosilicate precipitate generated in step S2 and the original coarse particles in the waste liquid are retained, resulting in a semi-transparent pre-filtrate.

[0046] Step S4: Secondary Fine Filtration. The primary filtrate is pumped into the secondary filter. The secondary filter is equipped with a 10,000-mesh (approximately 1.15 μm pore size) nylon filter. This step further removes residual microcrystal nuclei and fine precipitates from the primary filtrate, resulting in a clear and transparent fine filtrate.

[0047] Step S5: Collect the fine filtrate after secondary filtration as a regenerated silica polishing slurry for later use. Testing showed that the solid content of the regenerated silica polishing slurry was 25%, and the metal cation content was less than 500 ppm. In this embodiment, the original silica polishing slurry had a solid content of 30%, meaning that compared to before crystallization, the silica content in the polishing slurry decreased somewhat after regeneration, but it still met the polishing requirements.

[0048] The silicon carbide wafer was polished using the regenerated silicon oxide polishing slurry prepared by the above method. The specific steps are as follows: (1) Polishing slurry preparation: A liquid: the obtained regenerated silicon oxide polishing slurry; B liquid: commercially available polishing activator, density 0.95-1.15g / cm3, viscosity <100, pH value 8.5-10.5; solid content 6%~12%; wherein, the polishing slurry preparation used in this application is from Shanghai Qinrui Technology Co., Ltd.; oxidant: 30% hydrogen peroxide; ratio: A liquid: B liquid: oxidant = 1:1:0.1 (volume ratio).

[0049] (2) Polishing process: Equipment: Chuangji 50GPAW-TD single-sided polishing machine; Workpiece: 6-inch conductive SiC substrate wafers (20 wafers); Polishing process: The SiC wafers are attached to the ceramic carrier with liquid wax and fixed to the polishing machine; The prepared working polishing fluid is added to the polishing system to perform single-sided pressure polishing on the Si surface of the wafer; Process parameters: Carrier: Ceramic material, thickness 16 mm, diameter 485 mm; Polishing pad: White velvet damping cloth; Wafer mounting configuration: 4 ceramic carriers are used for a single polishing, 5 wafers are mounted on each carrier, and the total number of wafers processed is 20; Pressure: The total pressure applied by a single polishing head (PP head) is 380 kg; Rotation speed: The rotation speed of the lower polishing pad is 34 rpm, and the rotation speed of the polishing head (PP head) is 32 rpm; Polishing fluid supply: It is supplied in a circulating manner, and the flow rate is controlled at 8±2 L / min; Temperature control: The processing temperature is controlled at 30±2°C and cooled by a chiller set to 22°C; Time: The polishing duration is 4 hours.

[0050] Polishing results: Material removal from wafers after polishing: 0.6-0.8 μm; Surface roughness Ra<0.1 nm.

[0051] Using the above-mentioned polishing slurry, multiple batches of SiC were polished continuously, and the average total length of the scratches after polishing of each batch of SiC was recorded. The results are recorded in Table 1.

[0052] Example 2 differs from Example 1 in that the concentration of the added HF aqueous solution in step S2 is 3%, while the other steps are the same. Testing revealed that the solid content of the regenerated silica polishing slurry was 22%, and the content of metal cations was less than 500 ppm.

[0053] Example 3 differs from Example 1 in that the concentration of the added HF aqueous solution in step S2 is 5%, while the other steps are the same. Testing showed that the solid content of the regenerated silica polishing solution was 18%, and the content of metal cations was less than 500 ppm.

[0054] Example 4 differs from Example 1 in that the concentration of the added HF aqueous solution in step S2 is 0.5%, while the other steps are the same. Testing revealed that the solid content of the regenerated silica polishing solution was 26%, and the content of metal cations was less than 500 ppm.

[0055] Example 5 differs from Example 1 in that, in step S2, the mixture is stirred and mixed at room temperature for 3 hours, while the remaining steps are the same. Testing revealed that the solid content of the regenerated silica polishing slurry was 27%, and the content of metal cations was less than 500 ppm.

[0056] Example 6 differs from Example 1 in that, in step S2, the mixture is stirred and mixed at room temperature for 0.5 hours; all other steps are the same. Testing showed that the solid content of the regenerated silica polishing slurry was 23%, and the content of metal cations was less than 500 ppm.

[0057] Example 7 differs from Example 1 in that a 4000-mesh nylon filter is used in step S3, and a 15000-mesh nylon filter is used in step S4; the remaining steps are the same. Testing showed that the solid content of the regenerated silica polishing slurry was 23%, and the content of metal cations was less than 500 ppm.

[0058] Example 8 differs from Example 1 in that a 4000-mesh nylon filter is used in step S3, while an 8000-mesh nylon filter is used in step S4; the remaining steps are the same. Testing showed that the solid content of the regenerated silica polishing slurry was 25.5%, and the content of metal cations was less than 500 ppm.

[0059] Comparative Example 1 differs from Example 1 in that step S2 is omitted, while the remaining steps are the same.

[0060] Tests showed that the solid content of the regenerated silica polishing slurry was 27%, and the content of metal cations was approximately 1500 ppm.

[0061] The difference between Comparative Example 2 and Example 1 is that untreated polishing waste liquid was directly applied to the CMP process to polish the wafer.

[0062] Table 1

[0063] It is worth mentioning that in the CMP process, in order to evaluate the surface quality of the wafer after polishing, the process safety threshold of the average scratch length of the wafer is usually set to less than 10 mm as a key performance indicator.

[0064] As shown in the table above, the regenerated polishing fluid generated in Example 1 of this application can be continuously and effectively recycled at least 5 times (the scratch lengths for the 1st to 5th cycles are 3.4 mm, 3.5 mm, 3.7 mm, 6.8 mm, and 9.2 mm, respectively). After the 5th cycle, the scratch length (9.2 mm) is still below the threshold of 10 mm, ensuring an extremely high product qualification rate.

[0065] Within the process safety threshold range, the cycle life and wafer product yield of the regenerated polishing slurry in other embodiments are not as good as those in Embodiment 1 of this application. For example, Embodiment 2 (HF concentration of 3%) can only be effectively cycled twice; Embodiment 3 (HF concentration of 5%) can only be applied once; although Embodiment 4 (HF concentration of 0.5%) can be cycled four times, the scratch length each time is higher than that in Embodiment 1 (e.g., it reaches 7.77 mm in the fourth time). The wafer scratch length is close to the process safety threshold, indicating that its quality consistency is slightly worse. Example 5 (stirring and mixing for 3 hours) can only be effectively applied once; Example 6 (stirring and mixing for 0.5 hours) can be effectively applied three times, but the wafer scratch length is close to the process safety threshold, and the quality consistency is slightly poor; Example 7 (using 4000 mesh and 15000 mesh filters in sequence) and Example 8 (using 4000 mesh and 8000 mesh filters in sequence) can be applied three times, but the scratch length of the wafer in the third test is 7.7 mm in Example 7 and as high as 8.9 mm in Example 8, and the quality consistency is slightly poor.

[0066] Within the process safety threshold range, the comparative examples showed extremely poor recycling ability, or even no recycling ability at all. The first scratch length of Comparative Example 2 (directly using crystallization waste liquid) was as high as 24.9 mm, with a failure rate of 100%. Compared with other examples, the content of metal cations in Comparative Example 1 was about 1000 ppm, which could only be effectively used once, indicating that the excessively high content of metal cations in polishing waste liquid would seriously limit its recycling performance.

[0067] Furthermore, the key performance indicators of the polishing slurry after regeneration by the method of this application are optimized and balanced: the solid content of the regenerated polishing slurry is basically the same as that of the original silica polishing slurry (25% in Example 1). At the same time, the concentration of metal cations in the waste liquid, which is the core cause of colloidal instability, is reduced by orders of magnitude. This allows the regenerated polishing slurry to synergistically ensure the stability and functionality of the regeneration system from both composition and structure levels, so as to achieve more than five high-quality cycles.

[0068] It is worth noting that traditional views hold that the recycling of silica polishing slurries faces several inherent technical biases: First, polishing slurries are complex systems containing silica particles and various functional additives. After crystallization, the physicochemical properties of each component change to varying degrees, making it difficult to achieve overall simultaneous regeneration through a single method. Second, attempting to reverse agglomeration by controlling the crystallization process (such as adjusting supersaturation) involves precise thermodynamic and kinetic control, resulting in a narrow process window and a high risk of introducing secondary pollution. Finally, any cumbersome regeneration scheme may lose its industrialization value due to excessive costs. These perceptions collectively constitute the technical bias that "crystallized polishing slurries are difficult to regenerate economically and effectively."

[0069] This invention represents a groundbreaking shift in approach: instead of pursuing the overall restoration of complex systems, it focuses on removing crystalline aggregates that lead to performance degradation. To this end, a low-concentration hydrofluoric acid (HF) is creatively introduced to target and chemically transform crystalline silicon oxide into easily separable fluorosilicate complexes. Simultaneously, based on a precise understanding of the effective abrasive particle size (less than 250 nm) in the original polishing slurry, a two-stage gradient filtration scheme of 4000 mesh and 10000 mesh is specifically designed. This ensures efficient retention of transformation products and residual large particles while fully preserving functional nano-abrasives. Furthermore, it achieves high-quality regeneration of the failed polishing slurry in a simple manner, overcoming long-standing technical biases and economic bottlenecks.

[0070] The basic principles, main features, and advantages of this application have been described above. Those skilled in the art should understand that this application is not limited to the above embodiments. The embodiments and descriptions in the specification are merely the principles of this application. Various changes and modifications can be made to this application without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claims. The scope of protection claimed by this application is defined by the appended claims and their equivalents.

Claims

1. A method for regenerating silicon oxide polishing waste liquid, characterized in that, Includes the following steps: S1. Collect the waste liquid of silicon oxide polishing solution that has crystallized or agglomerated, the waste liquid containing metal cations; S2. Add hydrofluoric acid aqueous solution to the waste liquid and mix evenly to carry out the reaction, the mass concentration of the hydrofluoric acid aqueous solution is 0.5%~5%, in step S2, the volume ratio of the hydrofluoric acid aqueous solution to the waste liquid is 1:8~1:12, the hydrofluoric acid is used to erode the bonding points on the surface of the agglomerated silicon oxide particles, and convert some of the silicon oxide into mobile fluorosilicate ions, the fluorosilicate ions combine with the metal cations in the waste liquid to form fluorosilicate precipitates; S3. The mixture after step S2 is filtered through a first filter medium to remove particles with a particle size larger than a first threshold, to obtain the initial filtrate; S4. The initial filtrate is subjected to secondary filtration through a second filter medium to remove particles with a particle size larger than a second threshold, thereby obtaining a fine filtrate, wherein the pore size of the second filter medium is smaller than that of the first filter medium. S5. Collect the fine filtrate as a regenerated silica polishing slurry.

2. The method according to claim 1, characterized in that, In step S2, the mass concentration of the hydrofluoric acid aqueous solution is 0.5% to 3%.

3. The method according to claim 2, characterized in that, In step S2, the volume ratio of the hydrofluoric acid aqueous solution to the waste liquid is 1:

10.

4. The method according to claim 1, characterized in that, In step S2, the method for achieving uniform mixing is as follows: the reaction is carried out under stirring conditions, with a stirring speed of 100-300 rpm and a reaction time of 0.5h-3h.

5. The method according to claim 1, characterized in that, In step S3, the first filter medium has a mesh size of 2000-6000 mesh or a pore size of 2.5μm-9μm; the second filter medium has a mesh size of 8000-15000 mesh or a pore size of 0.8μm-2μm.

6. The method according to claim 1, characterized in that, In step S3, the first filter medium has a mesh size of 4000 mesh, and the second filter medium has a mesh size of 10000 mesh.

7. The method according to claim 1, characterized in that, In step S1, the metal cations in the waste liquid include Na. + K + The content of the metal cation is 1000ppm~3000ppm.

8. The method according to claim 1, characterized in that, The process includes step S6 after step S5: detecting the pH value and solid content of the fine filtrate, and adding deionized water or pH adjuster to adjust the fine filtrate to restore it to the preset polishing solution process parameters.

9. A chemical mechanical polishing method for wafers, characterized in that, When polishing the wafer, the regenerated polishing fluid prepared by any one of the methods described in claims 1-8 is used.

10. The method according to claim 9, characterized in that, The wafer is a silicon carbide single crystal.

Images