A low specific surface area colloidal silica solution and a method for its preparation

Abstract

The application discloses a kind of low specific surface area colloidal silica solution and its preparation method, for the problems that the particle structure of the silica sol obtained by existing alkoxysilane hydrolysis method is loose, specific surface area is high, densification process depends on high temperature and high pressure, and impurities are easily introduced, the application uses ammonia / ammonium chloride buffer solution buffer system to stabilize reaction pH, combined with seed step growth process, with alkoxysilane as silicon source, drop by batch slowly, the system pH is regulated to 8.5~10 throughout the process, and the target product is prepared after incubation and maturation.The application can accurately control particle size, narrow particle size distribution, significantly reduce the specific surface area of product, the process is mild without high temperature and high pressure equipment, the product has high purity and good batch stability, and is suitable for high-end fields such as semiconductor chemical mechanical polishing.

Description

Technical Field

[0001] This invention relates to the field of aerogel materials technology, specifically to a low specific surface area colloidal silica solution and its preparation method. Background Technology

[0002] Colloidal silica (silica sol), as a dispersion system of nano-sized silica particles in aqueous or organic phases, has been widely used in catalyst supports, precision ceramics, coatings, and semiconductor manufacturing due to its excellent chemical stability, high specific surface area, and good mechanical grinding performance. Especially in semiconductor chemical mechanical polishing (CMP) processes, silica sol is the core abrasive component of the polishing slurry, and its physicochemical properties directly determine the planarization quality, defect rate, and material removal rate of the wafer surface.

[0003] As the feature size of semiconductor devices continues to shrink to the nanometer level, the performance requirements for silica sol abrasives in CMP polishing processes are becoming increasingly stringent. Studies have shown that, in addition to meeting high purity requirements (total metal impurity content typically below 300 ppb), the specific surface area and structural density of the silica particles used in CMP are also key factors affecting polishing efficiency and surface quality. Under conditions of similar apparent particle size, an excessively high specific surface area often indicates the presence of numerous micropores and incompletely condensed silanol groups (Si-OH) on the particle surface, while unhydrolyzed alkoxy groups (Si-OR) remain inside the particles. This loose and porous structure exacerbates the chemical erosion effect of the polishing slurry during CMP, making the particles prone to breakage or inelastic deformation under mechanical friction, thereby reducing the material removal rate and increasing the risk of defects such as scratches and pits on the wafer surface. Therefore, developing ultra-high purity silica sols with low specific surface area and high structural density has become an important research direction in the field of CMP abrasives.

[0004] Currently, the mainstream method for preparing high-purity silica sol is the alkoxysilane hydrolysis method. This method uses methyl orthosilicate, tetraethyl orthosilicate, etc., as silicon sources, and hydrolyzes and condenses them in an alkaline catalyst and an alcohol-water mixed solvent, effectively avoiding the problem of introducing impurities such as sodium ions into the water glass method. However, this method still has significant drawbacks in practical applications: on the one hand, hydrolysis and condensation occur simultaneously, and incomplete alkoxy hydrolysis results in a large amount of Si-OR residue inside and on the surface of the particles, making the nascent particles have a loose and porous structure with a high initial specific surface area, which is difficult to directly meet the requirements of efficient CMP polishing; on the other hand, to improve particle density, existing technologies usually use high-temperature and high-pressure curing treatment (such as CN113912070A) to promote the condensation of silanol groups inside the particles. Although the above methods reduce the specific surface area to a certain extent, they have problems such as high equipment requirements, high energy consumption, long production cycles, or the need to introduce additional metal ions / organic bases, which lead to difficulties in subsequent purification, thus restricting their application in the low-cost industrial production of high-quality polishing-grade silica sol. Summary of the Invention

[0005] To address the aforementioned shortcomings of existing technologies, this invention provides a low specific surface area colloidal silica solution and its preparation method. It employs an ammonia / ammonium chloride buffer solution catalytic system combined with a stepwise seed crystal growth process. Using alkoxysilane as the silicon source, the buffer system constructed from ammonia and ammonium chloride maintains a stable pH within a suitable range throughout the reaction process through its reversible dissociation equilibrium. This suppresses ammonia volatilization and pH drift under high-temperature conditions, preventing heterogeneous nucleation and abnormal particle growth. Simultaneously, by relying on pre-prepared highly monodisperse silica sol seed crystals to lock the particle count within the system, the silicon source is added in a controlled, batch-by-batch manner. The active silicic acid generated by hydrolysis is guided to be deposited and grown only on the surface of existing seed crystals, achieving the technical effects of precisely controlling the particle size of silica particles, narrowing the particle size distribution, densifying the internal pore structure of particles, and significantly reducing the BET specific surface area of ​​the product. Moreover, the preparation process described in this invention does not require high-temperature and high-pressure equipment, simplifies the subsequent purification process, ensures the high purity and batch reproducibility of the product, and solves the technical problems of existing alkoxysilane hydrolysis methods producing loose and porous silica sol particles with high specific surface area, and existing densification processes relying on high-temperature and high-pressure curing treatment, which easily introduces additional impurities, has high production costs, and is difficult to scale up for the production of high-end polishing-grade silica sol.

[0006] To achieve the above objectives, in a first aspect, the present invention provides a low specific surface area colloidal silica solution, wherein, by mass percentage, the silica content in the colloidal silica solution is at least 20%, the silica particle size is 25 nm to 50 nm, and the specific surface area of ​​the silica particles does not exceed 120 m². 2 / g.

[0007] Preferably, the colloidal silica solution contains 20%–30% silica by mass percentage, has a silica particle size of 25 nm–30 nm, and a specific surface area of ​​90–100 m² / g. 2 / g.

[0008] Preferably, the colloidal silica solution contains 20%–30% silica by mass percentage, has a silica particle size of 30 nm–35 nm, and a specific surface area of ​​80–90 m² / g. 2 / g.

[0009] Secondly, the present invention provides a method for preparing the above-mentioned colloidal silica solution, the specific steps of which are as follows:

[0010] Step 1: Mix the ammonia / ammonium chloride buffer solution and water thoroughly to obtain solution A; the amount of ammonia / ammonium chloride buffer solution in solution A is 2wt%~5wt%.

[0011] Step 2: Add the alkoxysilane to solution A in two portions and react at 50℃~80℃ for 3h~12h; the time interval between the two additions of alkoxysilane is 0.5h~1.5h; the mass ratio of the amount of alkoxysilane added each time to the mass of solution A is (0.07~0.13):1;

[0012] Step 3: Take a certain amount of solution A, water and ammonia / ammonium chloride buffer solution and mix them to obtain solution B; according to the mass percentage, the amount of solution A added is 12wt%~50wt%, the amount of ammonia / ammonium chloride buffer solution added is 0.7wt%~2wt%, and the balance is water, and the total amount of each component added is 100%;

[0013] Step 4: Slowly add alkoxysilane to solution B, stir the reaction, filter and concentrate to obtain the colloidal silica solution; the mass ratio of the amount of alkoxysilane added to solution B is (0.15~0.25):1.

[0014] Preferably, the mass ratio of water, ammonia, and ammonium chloride in the ammonia / ammonium chloride buffer solution is (4~8):(35~50):(180~220), the pH value of the ammonia / ammonium chloride buffer solution is 9~11, and the concentration of ammonia is 20wt%~28wt%.

[0015] Preferably, the alkoxysilane is one of methyl orthosilicate, ethyl orthosilicate, and methyltrimethoxysilane.

[0016] Preferably, in step 4, the alkoxysilane is added to solution B in multiple portions, with each addition time controlled between 3 and 7 hours, and the mass of alkoxysilane added each time is equal. During each addition, the pH value of the system is controlled between 8.5 and 10 using an ammonia / ammonium chloride buffer solution. After all the alkoxysilane has been added, the reaction is allowed to proceed for 3 to 7 hours to obtain the colloidal silica solution.

[0017] Preferably, in step 4, the temperature is controlled at 50℃~70℃, and the alkoxysilane is added in batches 4~5 times with a time interval of 4~7 hours.

[0018] Preferably, in step 4, a cross-flow membrane is used to concentrate the reaction product to obtain the colloidal silica solution.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] 1. This invention utilizes the synergistic effect of an ammonia / ammonium chloride buffer catalytic system and a stepwise seed crystal growth process to regulate silica particles. The buffer catalytic system composed of ammonia and ammonium chloride can maintain the pH of the reaction system stably within a suitable range of 9.5~10.5 through a chemical equilibrium mechanism. This system can replenish alkalinity by shifting the equilibrium to the right when hydroxyl ions are consumed during hydrolysis, and can also restore the pH by binding hydroxyl ions with ammonium ions when the addition of silicon source leads to excessive local alkalinity. At the same time, it suppresses the volatilization of ammonia under high temperature conditions, avoiding heterogeneous nucleation and abnormal particle growth caused by pH drift during the reaction. In this system, the chloride ions generated by ionization can form a stable ionic atmosphere around the particles, inducing the dense deposition of silica polymers on the seed crystal surface through electrostatic shielding and template guidance, reducing the internal microporosity and surface roughness of the particles. Moreover, this system does not have the problem of chemical adsorption of large molecular organic bases, which can simplify the subsequent purification process and ensure the high purity of the product.

[0021] 2. The stepwise seed crystal growth process designed in this invention adopts a nucleation-then-growth technical path. Highly monodisperse nanocrystals are pre-prepared in a buffer system to precisely control the number of particles within the system. Then, a silicon source is added in batches using the seed solution as the mother liquor, guiding the active silicic acid generated by hydrolysis to be directionally deposited on the surface of the existing seed crystals. This effectively inhibits secondary nucleation and gradually fills the micropores on the particle surface and residual defects inside, achieving precise control of particle size and narrowing of particle size distribution. This process is mutually supportive with the aforementioned ammonia / ammonium chloride buffer catalytic system. The stable pH environment provided by the buffer system provides uniform reaction conditions for the synchronous and uniform growth of the seed crystals, ensuring the batch reproducibility of the stepwise growth process. Furthermore, the ordered deposition mode of stepwise growth fully releases the structural guiding effect of the buffer system. Together, they achieve particle densification and significantly reduce the specific surface area of ​​the product.

[0022] 3. The entire preparation process of the method described in this invention can be completed in an aqueous phase system under normal pressure, without the need for high-temperature and high-pressure equipment and complex post-processing. The process is simple and has a wide operating window. The colloidal silica obtained has the characteristics of large particle size, low specific surface area, high purity and narrow particle size distribution, which can meet the requirements of high-end fields such as semiconductor chemical mechanical polishing for abrasive performance. It solves the technical problems of existing technologies where silica sol particles are loose and porous, have a high specific surface area, and require high-end equipment for densification, consume a lot of energy, and are prone to introducing impurities. Attached Figure Description

[0023] Figure 1 This is a TEM image of the colloidal silica solution prepared in Example 1. Detailed Implementation

[0024] This invention will be described clearly and completely with reference to the embodiments. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on this invention are within the scope of protection of this invention.

[0025] Unless otherwise specified in the specific context, the numerical ranges listed herein include upper and lower limits, as well as all integers and fractions within those ranges, and are not limited to the specific values ​​listed when the range is defined. While only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail. The terms “comprising,” “including,” “having,” “containing,” etc., as used herein are open-ended, meaning they include but are not limited to.

[0026] Unless otherwise specified, the experimental methods used in this invention are all conventional methods.

[0027] Unless otherwise specified, all materials and reagents used in this invention can be purchased or synthesized by known methods.

[0028] In the quantitative experiments of this invention, each experiment was repeated three times, and the average value of the results was taken.

[0029] I. A low specific surface area colloidal silica solution

[0030] To address the shortcomings of existing colloidal silica solution products, such as loose internal particle structure, numerous surface micropores and defects, high specific surface area, wide particle size distribution, and susceptibility to the introduction of metal ions or organic impurities, which make them unsuitable for the stringent performance requirements of high-end polishing applications, this invention addresses these issues by optimizing both the microstructure of the particles and the composition of the dispersion system. The design involves a colloidal silica solution in which nano-sized silica particles are uniformly dispersed in an aqueous medium. Pre-prepared monodisperse silica sol seed crystals serve as the substrate for particle growth. A dense tetrahedral network structure of silica is constructed within the particles through layer-by-layer deposition of silica-oxygen polymer components on the seed crystal surface, filling and repairing residual pores and surface micropores. Simultaneously, a buffering component composed of ammonia and ammonium chloride is used to maintain the pH and surface charge state of the dispersion system, controlling the uniformity of particle growth and preventing the adsorption and residue of metal ions and large-molecule organic bases on the particle surface. This results in a colloidal silica solution product with a specific solid content and particle size range. Finally, this invention yields a colloidal silica solution with a low specific surface area. By mass percentage, the colloidal silica solution contains at least 20% silica, with the remainder being water; the silica particles have a particle size of 25 nm to 50 nm, and the specific surface area of ​​the silica particles does not exceed 120 m². 2 / g.

[0031] Compared with existing technologies, the colloidal silica solution of this invention has many unexpected technical effects: Under the premise that the apparent particle size is basically the same or even slightly increased, the BET specific surface area of ​​the colloidal silica obtained by this invention is significantly reduced compared with conventional seedless hydrolysis products. When the particle size increases only from 33nm to 34nm, the specific surface area can decrease from 140m² / g to 97m² / g, a reduction of approximately 31%. This change significantly exceeds the theoretical decrease in specific surface area corresponding to only an increase in particle size, reflecting a substantial densification change in the pore structure and surface morphology inside the particles. Simultaneously, the entire densification process does not require additional post-processing steps such as high-temperature and high-pressure ripening, organic solvent replacement, or azeotropic distillation; it can be achieved solely through the control of reaction conditions within the aqueous system. Furthermore, the particle size distribution of the product does not deteriorate due to the increase in particle density, maintaining a highly monodisperse state, and the fluctuations in particle size and specific surface area parameters between different batches are at extremely low levels.

[0032] In some embodiments of the present invention, the colloidal silica solution contains 20%–30% silica by mass percentage, has a silica particle size of 25 nm–30 nm, and a specific surface area of ​​90–100 m² / g. 2 / g. The silica content can be 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, or 30%, or any range or sub-range between any two specific values ​​mentioned above; the silica particle size can be 25nm, 26nm, 27nm, 28nm, 29nm, or 30nm, or any range or sub-range between any two specific values ​​mentioned above; the specific surface area of ​​the silica particles can be 90m². 2 / g、91m 2 / g、92m 2 / g、93m 2 / g、94m 2 / g、95m 2 / g、96m 2 / g、97m 2 / g、98m 2 / g、99m 2 / g or 100m 2 / g, or can be any range and subrange between any two specific values ​​mentioned above; it should be understood that, in specific implementations, any of the above ranges can be combined with any other ranges, as long as the technical effect of the present invention can be achieved.

[0033] In some embodiments of the present invention, the colloidal silica solution contains 20%–30% silica by mass percentage, has a silica particle size of 30 nm–35 nm, and a specific surface area of ​​80–90 m² / g. 2 / g. The silica content can be 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, or 30%, or any range or sub-range between any two specific values ​​mentioned above; the silica particle size can be 30nm, 31nm, 32nm, 33nm, 34nm, or 35nm; the specific surface area of ​​the silica particles can be 80m². 2 / g、81m 2 / g、82m 2 / g、83m 2 / g、84m 2 / g、85m 2 / g、86m 2 / g、87m 2 / g、88m 2 / g、89m 2 / g or 90m 2 / g, or can be any range and subrange between any two specific values ​​mentioned above; it should be understood that, in specific implementations, any of the above ranges can be combined with any other ranges, as long as the technical effect of the present invention can be achieved.

[0034] II. A method for preparing the above-mentioned colloidal silica solution

[0035] This invention addresses the issue from two perspectives: regulating the pH stability of the reaction system and controlling the growth path of the particles. Firstly, it employs a buffer system formed by ammonia and ammonium chloride as the alkaline catalyst, replacing traditional single ammonia or inorganic alkaline catalysts. The reversible equilibrium of the buffer pair allows for pH control throughout the reaction. Secondly, it utilizes a stepwise seed crystal growth process. First, uniform silica sol crystals are catalyzed in the buffer system to determine the initial particle count. Then, a portion of the seed solution is used as the mother liquor. By adding alkoxysilane silicon sources in batches at a controlled rate, the active silicic acid generated from hydrolysis is guided to deposit and grow only on the surface of the existing seed crystals. This allows for the construction of a controlled particle structure growth path under mild reaction conditions. Therefore, the specific steps of the method described in this invention are as follows:

[0036] Step 1: Mix the ammonia / ammonium chloride buffer solution and water thoroughly to obtain solution A; the amount of ammonia / ammonium chloride buffer solution in solution A is 2wt%~5wt%.

[0037] Step 2: Add the alkoxysilane to solution A in two portions and react at 50℃~80℃ for 3h~12h; the time interval between the two additions of alkoxysilane is 0.5h~1.5h; the mass ratio of the amount of alkoxysilane added each time to the mass of solution A is (0.07~0.13):1;

[0038] Step 3: Take a certain amount of solution A, water and ammonia / ammonium chloride buffer solution and mix them to obtain solution B; according to the mass percentage, the amount of solution A added is 12wt%~50wt%, the amount of ammonia / ammonium chloride buffer solution added is 0.7wt%~2wt%, and the balance is water, with the total amount of each component being 100%;

[0039] Step 4: Slowly add alkoxysilane to solution B, stir the reaction, filter and concentrate to obtain the colloidal silica solution; the mass ratio of the amount of alkoxysilane added to the mass of solution B is (0.15~0.25):1.

[0040] The preparation method described in this invention uses silica sol seed solution as initial particles to synthesize colloidal silica through a stepwise growth process. This technical solution enables precise and controllable particle size growth, effectively avoiding the uneven particle size distribution problem caused by explosive nucleation in traditional homogeneous nucleation processes. By adding silicon sources in a stepwise and differentiated manner, it is ensured that all active silicic acid is used for the growth of existing seed crystals rather than secondary nucleation, thereby obtaining a highly monodisperse colloidal silica product.

[0041] In some embodiments of the present invention, the mass ratio of water, ammonia, and ammonium chloride in the ammonia / ammonium chloride buffer solution is (4~8):(35~50):(180~220), the pH value of the ammonia / ammonium chloride buffer solution is 9~11, and the ammonia concentration is 20wt%~28wt%. The ammonia / ammonium chloride buffer system is chosen as the alkaline catalytic system in this invention to match the process requirements for the stepwise hydrolysis growth of alkoxysilanes to prepare low specific surface area, high-purity silica sol. Considering multiple aspects such as pH stability control, dense particle growth guidance, product purity assurance, and industrial feasibility, it has significant compatibility advantages compared to various conventional alkaline substances. While ammonia water, commonly used in the process, can provide an alkaline reaction environment, it is a volatile weak base. At reaction temperatures of 50°C to 80°C, ammonia molecules will continuously evaporate from the system, causing the pH of the system to gradually decrease during the reaction, making it impossible to maintain a stable alkaline environment. At the same time, during the batch feeding of silicon source, ammonia water alone cannot quickly offset local alkalinity fluctuations, which can easily cause local pH to be too high or too low, inducing explosive nucleation or abnormal particle growth. Ultimately, this results in a wide particle size distribution and poor batch reproducibility, making it difficult to support the stepwise growth process to achieve orderly densification of particles. The conjugate acid-base system composed of ammonia and ammonium chloride can achieve a reversible dissociation equilibrium between ammonium ions and free ammonia. When hydrolysis consumes hydroxide ions, the alkalinity is replenished by shifting the equilibrium to the right. When the local alkalinity is too high, the pH is adjusted by ammonium ions binding hydroxide ions. At the same time, it effectively inhibits the volatilization of ammonia at high temperatures, and keeps the pH of the system stable within the optimal range of 9.5~10.5 for silica sol growth, providing a uniform and stable reaction environment for the stepwise growth of crystal seeds. Meanwhile, the chloride ions generated by the complete ionization of ammonium chloride can form a stable ionic atmosphere around the particles. Through electrostatic shielding and template-guided effects, it induces the dense deposition of silica polymers on the surface of the crystal seeds, further helping to fill the particle pores and reduce the specific surface area. This effect cannot be achieved by a single ammonia water system. Meanwhile, this invention also found that while inorganic strong bases such as sodium hydroxide and potassium hydroxide can provide a strongly alkaline environment, they lack buffering capacity. Even small fluctuations in acidity and alkali can cause drastic changes in the pH of the system, failing to meet the stringent pH stability requirements during stepwise growth and easily inducing secondary nucleation and particle aggregation. More importantly, these strong bases introduce metal cation impurities such as sodium and potassium. For high-purity silica sols used in semiconductor CMP applications, metal ions can severely affect the surface quality of the polished wafers. Furthermore, these metal ions are difficult to completely remove through conventional filtration and concentration processes, resulting in extremely high subsequent purification costs and failing to meet the purity requirements of electronic-grade products.While organic bases such as triethylamine and tetramethylammonium hydroxide can avoid the introduction of metal impurities, their large molecular size makes them prone to adsorption onto the surface of silica particles during particle growth via hydrogen bonding or electrostatic interactions, forming difficult-to-remove organic residues. This not only increases the difficulty and cost of subsequent purification processes but also alters the charge and wetting properties of the particle surface, affecting the dispersion stability and polishing performance of the silica sol in the polishing solution. Furthermore, most organic bases have limited buffering capacity and cannot maintain a stable pH range throughout the reaction cycle. Some organic bases are also prone to decomposition at high temperatures, producing byproducts that further introduce impurities. In addition, the cost of organic base raw materials is generally high, and some varieties pose environmental and toxic risks, which is not conducive to large-scale industrial production. As for buffer systems composed of weak acid and weak base salts such as ammonium acetate and ammonium citrate, their effective buffering pH range is mainly concentrated in the neutral or weakly acidic range, which is poorly matched with the weakly alkaline environment required for silica sol growth and cannot provide sufficient buffering capacity in the range of 9-11. Moreover, the organic acid ions introduced by such systems will, on the one hand, generate complexation or adsorption during particle growth, interfering with the dense deposition process of siloxane polymers on the seed surface, and on the other hand, the residual organic acid ions will also affect the purity and application performance of the product. Therefore, they are also unsuitable for the preparation of low specific surface area high-purity silica sol in this invention. In contrast, the ammonia / ammonium chloride system does not have the problem of adsorption of large molecular organic components, has high particle surface cleanliness, simple subsequent purification steps, and low raw material cost and high availability, making it economical for industrial scale-up. At the same time, it does not contain metal impurities, and excess ammonia and ammonium salts can be easily removed by subsequent heating or membrane separation processes, making it fully suitable for the preparation requirements of high-purity silica sol.

[0042] In some embodiments of the present invention, the alkoxysilane is one of methyl orthosilicate, ethyl orthosilicate, and methyltrimethoxysilane.

[0043] In some embodiments of the present invention, in step 4, alkoxysilane is added to solution B in multiple portions, with each addition time controlled between 3 and 7 hours, and the mass of alkoxysilane added each time is equal. During each addition, the pH value of the system is controlled between 8.5 and 10 using an ammonia / ammonium chloride buffer solution. After all the alkoxysilane has been added, the reaction is allowed to proceed for 3 to 7 hours to obtain the colloidal silica solution. When adding alkoxysilane in batches, the temperature is controlled between 50°C and 70°C, and the number of batches of alkoxysilane added is 4 to 5. In the present invention, the design of adding alkoxysilane in multiple equal portions with a single addition time controlled between 3 and 7 hours is primarily intended to precisely maintain the instantaneous concentration of active silicic acid in the reaction system below the critical nucleation threshold. The monosilicic acid generated by the hydrolysis of alkoxysilane in an alkaline environment is the raw material for particle growth. When the concentration of monosilicic acid in the system exceeds the adsorption and consumption capacity of the seed crystal surface, homogeneous nucleation will spontaneously occur to generate new particles, i.e., secondary nucleation. This invention utilizes a slow, batch-wise, and equal-volume feeding method to allow the silicon source to hydrolyze at a uniform rate and low concentration, releasing active silicic acid. The monosilicic acid generated by hydrolysis can be promptly adsorbed onto the surface of existing seed crystals and participate in condensation growth. This ensures that the silicon source generation rate matches the seed crystal growth consumption rate, fundamentally suppressing secondary nucleation and ensuring that all silicon sources are used for the synchronous growth of the original seed crystals. Ultimately, this achieves precise control of particle size and a highly narrow particle size distribution. If conventional one-time feeding or rapid dripping operations are used, the instantaneous feeding will cause a sharp increase in local silicon source concentration, generating a large amount of active silicic acid exceeding the seed crystal consumption capacity in a short period of time. This triggers explosive secondary nucleation, resulting in a wide particle size distribution and uneven particle size, failing to meet the stringent requirements for particle size uniformity in semiconductor CMP abrasives. At the same time, the rapidly generated silicon-oxygen polymers will deposit on the particle surface in a loose and disordered manner, failing to form a dense silicon-oxygen network structure. This results in a large number of micropores and a high specific surface area inside the particles, making it difficult to achieve the core goal of particle densification.

[0044] In some embodiments of the present invention, in step 4, the design of using an ammonia / ammonium chloride buffer solution to stably control the pH of the system between 8.5 and 10 during each dropwise addition provides a uniform and stable reaction kinetic environment for the entire particle growth process. The hydrolysis rate of alkoxysilanes and the condensation rate of siloxy groups are highly sensitive to the pH of the system. Within the weakly alkaline range, the higher the pH, the faster the hydrolysis and condensation rates. At the same time, the hydrolysis process continuously consumes hydroxide ions in the system. If dynamic control is not performed, the pH of the system will continuously decrease as the reaction proceeds, resulting in a gradual slowdown in the particle growth rate and significant differences in the density of the surface structure of particles grown at different stages. By using a buffer solution to regulate and maintain pH stability in real time, the hydrolysis and condensation rates can be kept constant throughout the dropwise growth process, and the deposition rate of seed crystals at each growth stage can be uniform. This avoids both rapid loose deposition caused by excessively high local pH and growth stagnation and particle size unevenness caused by excessively low pH, ensuring that the particles grow densely layer by layer, improving surface smoothness, and reducing micropore defects. If the conventional one-time addition of alkali without continuous pH control is adopted, the pH will be too high in the early stage of the reaction, resulting in a fast particle growth rate but a loose and porous structure. In the later stage of the reaction, the pH will be too low, resulting in insufficient growth rate and increased particle surface defects. This will not only lead to insufficient overall particle density and high specific surface area, but also cause significant differences in particle size and performance between different batches due to fluctuations in the initial alkali amount. The process reproducibility is poor and it is difficult to meet the needs of stable industrial production.

[0045] In some embodiments of the present invention, in step 4, after all the silicon sources are added, the reaction continues for 3 to 7 hours for heat preservation and ripening. This is to optimize the particle structure and enhance the stability of the system. After the addition is completed, a small amount of unreacted active silicic acid remains in the system, and there are a large number of incompletely condensed silanol groups and residual alkoxy groups on the particle surface and near the surface, and the internal micropores are not fully filled. Through the subsequent heat preservation and ripening process, the residual active silicic acid can be completely deposited on the particle surface, while promoting the further condensation reaction of silanol groups on the particle surface and inside, closing the surface micropores, repairing internal pore defects, and increasing the crosslinking degree of the particle silicon-oxygen network, making the particle structure more compact, thereby further reducing the BET specific surface area of ​​the product; at the same time, the ripening process can eliminate highly active sites on the particle surface, improve the storage stability of the sol system, and avoid gel agglomeration during subsequent concentration and storage. If the ripening process is omitted and the conventional operation of stopping the reaction immediately after feeding is adopted, a large number of active silanol groups and uncondensed groups will remain on the particle surface. The internal pores cannot be effectively filled. Not only will the specific surface area of ​​the product not be reduced to the ideal level, but the mechanical strength of the particles will also be insufficient. They will be easy to break and cause surface scratches during CMP polishing. At the same time, the active silicic acid remaining in the system will continue to condense during subsequent concentration and storage, causing particle agglomeration or system gelation, resulting in poor product storage stability.

[0046] In some embodiments of the present invention, in step 4, a cross-flow membrane is used to concentrate the reaction product to obtain the colloidal silica solution.

[0047] In some embodiments of the present invention, the above-mentioned process designs in step 4 do not act independently, but rather work synergistically to form a complete technical closed loop: slow batch feeding to suppress secondary nucleation is the basis for achieving synchronous particle growth; stable pH control throughout the process provides the necessary reaction conditions for uniform and dense particle deposition; and the final heat preservation and curing completes the densification of the particle structure. The three work together to achieve the technical effects of precise control of colloidal silica particle size, narrow particle size distribution, low specific surface area, and good batch stability.

[0048] III. Examples and Comparative Examples

[0049] Example 1

[0050] Step 1: Preparation of initial seed solution: Add 5g of ammonia / ammonium chloride buffer solution with pH 10 to 205g of pure water and stir at 60℃ and 600 rpm. Weigh 22g of tetraethyl orthosilicate and add it to the above solution all at once. After stirring for 30min, add another 22g of tetraethyl orthosilicate and react for 4h.

[0051] Step 2: Preparation of colloidal silica: Take 100g of the above seed solution, 300g of high-purity water, and 5g of ammonia / ammonium chloride buffer solution, and stir at 60℃ and 600 rpm. Add 87g of tetraethyl orthosilicate dropwise to the above solution over 5 hours. Repeat this step 5 times. Monitor the pH throughout the reaction process and adjust it to 8.5-10 using the ammonia / ammonium chloride buffer solution. After the addition is complete, continue the reaction for 5 hours to obtain a low specific surface area silica sol.

[0052] Step 3 Post-processing: The obtained low specific surface area silica sol is filtered through three stages of 0.45μm, 0.2μm and 0.1μm, and then concentrated to a solid content of 20~30% through a cross-flow membrane.

[0053] Example 2

[0054] An improvement upon Example 1 was made, differing in that: Step 2 involved preparing colloidal silica: 150g seed solution, 350g high-purity water, and 5g ammonia / ammonium chloride buffer solution were mixed and stirred at 60°C and 600 rpm. 87g of tetraethyl orthosilicate was added dropwise to the solution over 5 hours, and this step was repeated 5 times. After the addition was complete, the reaction was allowed to proceed for 5 hours to obtain a low specific surface area silica sol. All other steps were identical to those in Example 1.

[0055] Example 3

[0056] Step 1: Preparation of initial seed solution: Add 5g of ammonia / ammonium chloride buffer solution with pH 10 to 207g of pure water and stir at 60℃ and 600 rpm. Weigh 22g of methyl orthosilicate and add it to the above solution at once. After stirring for 30min, add another 22g of methyl orthosilicate and react for 4h.

[0057] Step 2: Preparation of colloidal silica: Take 100g of the above seed solution, 300g of high-purity water, and 5g of ammonia / ammonium chloride buffer solution, and stir at 60℃ and 600 rpm. Add 87g of methyl orthosilicate dropwise to the above solution over 5 hours. Repeat this step 5 times. Monitor the pH during the reaction and adjust it to 8.5-10 using the ammonia / ammonium chloride buffer solution. After the addition is complete, react for 5 hours to obtain a low specific surface area silica sol.

[0058] Step 3 Post-processing: The obtained low specific surface area silica sol is filtered through three stages of 0.45μm, 0.2μm and 0.1μm, and then concentrated to a solid content of 20~30% through a cross-flow membrane.

[0059] Example 4

[0060] Step 1: Preparation of initial seed solution: Add 5g of ammonia / ammonium chloride buffer solution with pH 10 to 205g of pure water and stir at 60℃ and 600 rpm. Weigh 22g of tetraethyl orthosilicate and add it to the above solution all at once. After stirring for 30min, add another 22g of tetraethyl orthosilicate and react for 4h.

[0061] Step 2: Preparation of colloidal silica: Take 100g of the above seed solution, 300g of high-purity water, and 7g of ammonia / ammonium chloride buffer solution, and stir at 60℃ and 600 rpm. Add 87g of tetraethyl orthosilicate dropwise to the above solution over 5 hours. Repeat this step 5 times. Monitor the pH during the reaction and adjust it to 8.5-10 using the ammonia / ammonium chloride buffer solution. After the addition is complete, react for 5 hours to obtain a low specific surface area silica sol.

[0062] Step 3 Post-processing: The obtained low specific surface area silica sol is filtered through three stages of 0.45μm, 0.2μm and 0.1μm, and then concentrated to a solid content of 20%~30% through a cross-flow membrane.

[0063] Example 5

[0064] Step 1: Preparation of initial seed solution: Add 5g of ammonia / ammonium chloride buffer solution with pH 10 to 205g of pure water and stir at 60℃ and 600 rpm. Weigh 15g of tetraethyl orthosilicate and add it to the above solution at once. After stirring for 30min, add another 22g of tetraethyl orthosilicate and react for 4h.

[0065] Step 2: Preparation of colloidal silica: Take 75g of the above seed solution, 300g of high-purity water, and 7g of ammonia / ammonium chloride buffer solution, and stir at 60℃ and 600 rpm. Add 87g of tetraethyl orthosilicate dropwise to the above solution over 5 hours. Repeat this step 5 times. Monitor the pH during the reaction and adjust it to 8.5-10 using the ammonia / ammonium chloride buffer solution. After the addition is complete, react for 5 hours to obtain a low specific surface area silica sol.

[0066] Step 3 Post-processing: The obtained low specific surface area silica sol is filtered through three stages of 0.45μm, 0.2μm and 0.1μm, and then concentrated to a solid content of 20~30% through a cross-flow membrane.

[0067] Comparative Example 1

[0068] The following adjustments were made to Example 1, with the following differences: Preparation of colloidal silica: 400g of high-purity water, 5g of ammonia / ammonium chloride buffer solution, stirred at 60°C and 600 rpm. 91g of tetraethyl orthosilicate was added dropwise to the above solution over 5 hours, and this step was repeated 5 times. After the addition was complete, the reaction was allowed to proceed for 5 hours to obtain a colloidal silica solution. Post-treatment: Same as in Example 1.

[0069] Comparative Example 2

[0070] Preparation of initial seed solution: Add 5g of ammonia / ammonium chloride buffer solution with pH 10 to 207g of pure water and stir at 60℃ and 600rpm. Weigh 40g of tetraethyl orthosilicate and add it to the above solution at once. After stirring for 30min, add another 40g of tetraethyl orthosilicate and react for 4h.

[0071] Preparation of colloidal silica: Take 100g of the above seed solution, 300g of high-purity water, and 5g of ammonia / ammonium chloride buffer solution, and stir at 60℃ and 600 rpm. Add 87g of tetraethyl orthosilicate dropwise to the above solution over 5 hours. Repeat this step 5 times. Monitor the pH during the reaction and adjust it to 8.5-10 using the ammonia / ammonium chloride buffer solution. After the addition is complete, react for 5 hours to obtain a low specific surface area silica sol.

[0072] Post-processing: The obtained low specific surface area silica sol was filtered through three stages of 0.45μm, 0.2μm and 0.1μm, and then concentrated to a solid content of 20-30% through a cross-flow membrane.

[0073] Comparative Example 3

[0074] The method was modified from Example 1, with the difference that in the post-processing, the resulting low specific surface area silica sol was filtered through three stages of filtration (0.45 μm, 0.2 μm, and 0.1 μm), and then concentrated using a conventional distillation process. During the concentration process, local instability occurred, resulting in gelation. The other steps were exactly the same as in Example 1.

[0075] Comparative Example 4

[0076] The method is based on Example 1, but with the following adjustment: the ammonia / ammonium chloride buffer solution in Example 1 is replaced with concentrated ammonia water; the other steps are exactly the same as in Example 1.

[0077] Comparative Example 5

[0078] The method is based on Example 1, but with the following adjustment: the ammonia / ammonium chloride buffer solution in Example 1 is replaced with tetramethylammonium hydroxide; the other steps are exactly the same as in Example 1.

[0079] IV. Performance Testing

[0080] 1. Detection Method

[0081] (1) Apparent particle size: The sample was observed using a high-resolution transmission electron microscope. 100 particles were randomly selected from the electron microscope images and their arithmetic mean was calculated as the apparent particle size of the sample.

[0082] (2) Specific surface area: The BET (Brunauer-Emmett-Teller) nitrogen adsorption method was used for determination.

[0083] (3) Solid content: determined by gravimetric method

[0084] A certain mass of sample was dried at 120℃ to constant weight, and the change in mass before and after drying was calculated. The calculation formula is as follows:

[0085] Solid content = (m1 ÷ m2) × 100%; where m1 is the mass after drying and m2 is the mass before drying.

[0086] (4) Polishing rate test

[0087] To compare the performance differences between the examples and comparative examples, the products prepared in the examples and comparative examples were used for polishing rate testing. Specifically, polishing was performed on 4-inch wafers using the following process parameters: the mixing ratio was set to 1:30 (the mass ratio of silica sol and pure water prepared in the examples and comparative examples before use), the pressure of the reaction system was controlled at 2 psi, and the material flow rate was set at 150 mL / min. Under these conditions, the drive speed (H / P) was maintained at 87 / 93 rpm, and the processing time for this step lasted for 2 minutes.

[0088] Polishing rate calculation formula: RR = (D1 - D2) ÷ t1;

[0089] In the formula, D1 is the initial film thickness of the wafer before polishing (unit: Å); D2 is the final film thickness of the wafer after polishing, cleaning, and drying (unit: Å); and t1 is the polishing time (unit: min).

[0090] 2. Test Results

[0091] Table 1

[0092] Example 1 Tetraethyl orthosilicate 22 100 Ammonia / ammonium chloride buffer solution 5 435 Crossflow membrane 34 97 25 291 Example 2 Tetraethyl orthosilicate 22 150 Ammonia / ammonium chloride buffer solution 5 435 Crossflow membrane 31 105 24.6 241 Example 3 Methyl orthosilicate 22 100 Ammonia / ammonium chloride buffer solution 5 435 Crossflow membrane 30 103 23.6 238 Example 4 Tetraethyl orthosilicate 22 100 Ammonia / ammonium chloride buffer solution 7 435 Crossflow membrane 37 89 25 295 Example 5 Tetraethyl orthosilicate 15 75 Ammonia / ammonium chloride buffer solution 7 435 Crossflow membrane 39 80 25 301 Comparative Example 1 Tetraethyl orthosilicate 22 0 Ammonia / ammonium chloride buffer solution 5 435 Crossflow membrane 33 140 24.6 210 Comparative Example 2 Tetraethyl orthosilicate 40 100 Ammonia / ammonium chloride buffer solution 5 435 Crossflow membrane 30 150 23 142 Comparative Example 3 Tetraethyl orthosilicate 40 100 Ammonia / ammonium chloride buffer solution 5 435 Ordinary distillation 34 97 Gel formation during distillation / Comparative Example 4 Tetraethyl orthosilicate 22 100 ammonia 5 435 Crossflow membrane 15 411 20.3 85 Comparative Example 5 Tetraethyl orthosilicate 22 100 Tetramethylammonium hydroxide 5 435 / 13 650 gel /

[0093] Analysis of the examples, comparative examples, and results in Table 1 shows that:

[0094] (1) Based on the performance test results, the products prepared using Examples 1 to 5 of this application can all stably obtain colloidal silica solutions with a solid content of more than 23%, with TEM apparent particle size distribution in the range of 30 to 39 nm and BET specific surface area controlled in the range of 80 to 105 m². 2 Within the range of / g, the polishing rate for 4-inch wafers can reach 238~301 Å / min; while the comparative examples show varying degrees of performance degradation in terms of particle size uniformity, structural compactness, specific surface area, polishing performance, and colloidal stability, with some comparative examples even failing to produce qualified colloidal products. This result directly demonstrates that the technical solution of this application can prepare high-purity colloidal silica with precise and controllable particle size, narrow particle size distribution, dense particle structure, low specific surface area, and excellent polishing performance under ambient pressure and mild aqueous phase reaction conditions. At the same time, the preparation process of this application has good batch reproducibility and scale-up feasibility, effectively solving the technical problems of existing alkoxysilane hydrolysis methods producing silica sol particles that are loose and porous with high specific surface area, and existing densification processes that rely on high temperature and high pressure curing, easily introduce additional impurities, have high production costs, and are difficult to scale up for the production of high-end polishing-grade silica sol.

[0095] (2) This application uses an ammonia / ammonium chloride buffer catalytic system composed of ammonia water and ammonium chloride to replace the single ammonia water, inorganic strong base or organic base catalyst commonly used in traditional processes. This system relies on the reversible dissociation equilibrium of the conjugate acid-base of ammonium ions and free ammonia, and can stably maintain the pH of the system in the suitable range of 8.5~10 throughout the entire cycle of alkoxysilane hydrolysis and condensation: when the hydrolysis reaction consumes hydroxide ions and causes the alkalinity to decrease, the equilibrium shifts to the right to replenish free ammonia to maintain the alkalinity; when the silicon source feed causes local alkalinity fluctuations, ammonium ions can combine with hydroxide ions to achieve pH correction. At the same time, this buffer system can significantly suppress the volatilization and escape of ammonia under high temperature reaction conditions and avoid continuous pH drift of the system. The direct technical effect of this application's technical feature is the elimination of heterogeneous nucleation and abnormal particle growth induced by pH fluctuations, providing a uniform and stable reaction dynamic environment for particle growth and ensuring consistent particle growth rates at different stages and in different regions. Simultaneously, the chloride ions generated by the complete ionization of ammonium chloride in the system can form a stable ionic atmosphere around the silica particles. Through electrostatic shielding and template-guided effects, this induces the orderly and dense deposition of silica polymers on the particle surface, reducing microporous defects and surface roughness within the particles and further enhancing the particle's structural compactness. Furthermore, this buffer system does not contain sodium, potassium, or other metal cations, nor does it suffer from the chemical adsorption problem of large-molecule organic bases. Excess ammonia and ammonium salts after the reaction can be easily removed through subsequent membrane separation or simple heating, ensuring high product purity without complex purification processes. This technical feature of the application specifically addresses a series of technical problems in existing technologies, such as poor pH stability of single-base catalysts leading to uneven particle growth, the easy introduction of metal impurities by inorganic strong bases resulting in high purification costs, and the tendency of organic bases to produce surface adsorption residues and disrupt colloidal stability.

[0096] (3) This application adopts a seed-step growth process, with a process design of slow, equal-volume addition of silicon source in batches, dynamic pH control throughout the process, and end-point heat preservation and ripening, to construct an ordered particle growth path of "nucleation first, growth later". In this process, highly monodisperse silica sol seed crystals are prepared in the buffer system in advance, and the number of seed crystals is used to accurately lock the final total number of particles in the system; in the subsequent growth stage, the instantaneous concentration of active silicic acid in the system is strictly controlled below the critical nucleation threshold by slow, batch, and equal-volume addition of silicon source, so that all the active silicic acid generated by hydrolysis is directionally deposited on the surface of the existing seed crystals, rather than spontaneously forming new particles. This technological improvement fundamentally suppresses secondary nucleation, enabling precise control of particle size and narrowing of particle size distribution. Simultaneously, the layer-by-layer, orderly deposition pattern gradually fills residual pores and surface micropore defects within the particles. Combined with the post-droplet curing process, it further promotes the cross-linking reaction between the incompletely condensed silanol groups on the particle surface and within, closing surface micropores, repairing internal pore defects, and improving the overall cross-linking degree of the silicon-oxygen network. Ultimately, this results in a denser particle structure and a significant reduction in the BET specific surface area. This process design specifically addresses the technical challenges of traditional homogeneous nucleation processes, which involve wide particle size distribution, loose and porous internal structure, high specific surface area, and the need for additional high-temperature, high-pressure curing to improve density.

[0097] (4) The stable pH environment provided by the ammonia / ammonium chloride buffer system in this application is a prerequisite for the stepwise seed growth process to achieve the expected results: the constant pH ensures that the hydrolysis rate and condensation rate of each batch of silicon source are consistent, so that all seed crystals in the system can maintain a synchronous and uniform growth state. This avoids the rapid loose deposition of silicon-oxygen polymers caused by excessively high local pH, and also avoids the stagnation of particle growth and uneven particle size caused by excessively low pH, thus fully ensuring the batch reproducibility of the stepwise growth process. Conversely, the orderly deposition mode of stepwise seed growth also provides ample room for the structure-guiding role of the buffer system: the chloride ion-mediated dense deposition effect can continue to act on the surface and internal structure of the particles along with the layer-by-layer growth of the particles, rather than being dispersed and offset in the disordered homogeneous nucleation process, further enhancing the technical effects of particle densification and specific surface area reduction. The two support and work together to achieve the goal of simultaneously controlling particle size and densifying structure under normal pressure and mild conditions. This eliminates the need for additional high-temperature and high-pressure curing processes, which reduces equipment requirements and energy consumption, and avoids the risk of introducing impurities during post-processing.

[0098] (5) The performance data of each comparative example can further verify the above technical effects: Comparative example 4 uses a single ammonia solution to replace the ammonia / ammonium chloride buffer solution, and the final product has an apparent particle size of only 15 nm and a BET specific surface area of ​​up to 411 m². 2The polishing rate was only 85 Å / min, and the performance degradation was due to the fact that ammonia alone is a volatile weak base. At the reaction temperature, the continuous volatilization of ammonia caused the pH of the system to drop continuously, leading to uncontrolled hydrolysis and condensation rates. This resulted in explosive homogeneous nucleation, generating a large number of fine and loosely structured particles. While the specific surface area increased significantly, the polishing performance was severely degraded. This result also indirectly confirms the crucial role of the ammonia / ammonium chloride buffer system in maintaining pH stability and ensuring normal, dense particle growth. Comparative Example 5 used tetramethylammonium hydroxide as a catalyst, but gelation occurred directly during the reaction, failing to yield an effective colloidal product. This is because large-molecule organic bases easily adsorb onto the surface of silica particles through hydrogen bonding or electrostatic interactions, altering the surface charge characteristics of the particles and disrupting the dispersion stability of the colloid. Furthermore, the buffering capacity of the organic base is limited and cannot support pH stability during the stepwise growth process, ultimately leading to system instability. This result demonstrates the advantages of the ammonia / ammonium chloride system in terms of product purity and colloidal stability, solving the problem of organic base catalysts easily introducing residues and disrupting system stability.

[0099] (6) Regarding the stepwise seed crystal growth process, Comparative Example 1 did not add seed solution; instead, silicon source was directly added dropwise to the buffer system for reaction. The apparent particle size of its product was 33 nm, which is close to 34 nm in Example 1, but the BET specific surface area was as high as 140 m². 2 The yield was significantly higher than that of Example 1, with a corresponding decrease in polishing rate to 210 Å / min. This result indicates that without the guiding effect of seed crystals, the silicon source hydrolyzes and grows randomly via homogeneous nucleation. The particles contain numerous micropores and uncondensed silanol groups, resulting in a loose and porous structure. Therefore, with similar apparent particle sizes, the specific surface area is significantly higher. Furthermore, the loosely structured particles are prone to breakage or inelastic deformation during polishing, failing to provide stable mechanical grinding and leading to a decrease in material removal rate. Although Comparative Example 2 retained the seed crystal process, the initial silicon source feed rate was too high, resulting in an excessive number of nuclei and a loose seed crystal structure. The final product had a particle size of 30 nm but a BET specific surface area as high as 150 nm. 2The polishing rate was only 142 Å / min, indicating that the number and structural quality of seed nuclei are fundamental to achieving densification through stepwise growth. Improper seed preparation cannot achieve the expected particle densification effect. The two comparative examples above jointly demonstrate that the seed-based stepwise growth process of this application can significantly improve particle density and reduce specific surface area under the premise of comparable particle size, thereby improving polishing performance and effectively solving the problems of loose particles and high specific surface area in seedless homogeneous growth processes. Comparative Example 3 used a conventional distillation process instead of cross-flow membrane concentration, resulting in localized instability and gelation during concentration, failing to obtain a qualified product. This is because the distillation process suffers from excessively high local concentrations and uneven temperature distribution, easily damaging the double-layer structure of colloidal particles and causing particle aggregation and gelation. In contrast, the cross-flow membrane concentration used in this application can gently increase the solid content of the system at room temperature while maintaining the stability of the colloidal dispersion system, ensuring product performance and storage period, and solving the problem of colloidal instability and gelation easily caused by traditional concentration methods. Furthermore, the performance variations within the embodiments also demonstrate the adjustability of the process: with adjustments to the amount of seed crystals and catalyst, the product particle size can be precisely controlled within the range of 30~39nm, corresponding to a specific surface area that varies within the range of 80~105m² / g, while maintaining excellent polishing performance overall. This indicates that the preparation method of this application can be adapted to different application scenarios through parameter adjustments, demonstrating good process flexibility.

[0100] In summary, this invention utilizes an ammonia / ammonium chloride buffer catalytic system in synergy with a stepwise seed crystal growth process. The buffer system stabilizes the reaction pH range and creates an electrostatically shielded ion atmosphere for the particles, guiding the dense deposition of silicon-oxygen polymers while simultaneously avoiding metal impurities and organic macromolecular residues to ensure high product purity. The stepwise seed crystal growth process pre-prepares dense crystal nuclei, adds silicon sources in batches at low rates to suppress secondary nucleation, precisely controls the particle size distribution, and fills the internal pores of the particles layer by layer. The cross-flow membrane gentle concentration process, as a complementary technical feature, achieves high solids content concentration without damaging the microscopic dense structure of the particles, ensuring the complete retention of the microscopic properties obtained from the preceding control. The buffer system provides a uniform and stable weakly alkaline reaction environment, which is a necessary reaction condition for the synchronous and orderly deposition of seeds in the stepwise seed growth process. The ordered particle growth path formed by the stepwise seed growth can fully utilize the dense template guiding effect of chloride ions in the buffer system. The two mutually support and amplify the particle densification control effect, synergistically achieving the core technical effects of significantly reducing the BET specific surface area and narrowing the particle size distribution. The low specific surface area and highly dense particle microstructure further solve the application problems of easy particle breakage, low grinding efficiency, and numerous wafer surface defects in traditional silica sol polishing processes, bringing about an application layer with improved wafer polishing rate. The surface technology enhances the overall performance; the cross-flow membrane concentration process continuously stabilizes particle dispersion, ensuring that the performance advantages of low specific surface area and high polishing efficiency are not diminished throughout the concentration and storage process. The above-mentioned multiple technologies have a synergistic effect, mutually promoting each other, and jointly solving a series of technical problems existing in the alkoxysilane hydrolysis preparation process, such as loose and porous particles, high specific surface area, wide particle size distribution, high energy consumption and cost of high temperature and high pressure ripening, easy introduction of impurities by the catalytic medium, easy gel instability in the post-processing, and poor polishing performance. At the same time, the ambient pressure and mild reaction conditions simplify the configuration of production equipment, reduce the difficulty of purification, and improve the batch reproducibility and industrial production adaptability of the product.

[0101] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit the technical solutions. Those skilled in the art should understand that any modifications or equivalent substitutions to the technical solutions of the present invention without departing from the spirit and scope of the present invention should be covered within the scope of the claims of the present invention.

Claims

1. A low specific surface area colloidal silica solution, characterized in that, Based on mass percentage, the colloidal silica solution contains at least 20% silica, the silica particles have a particle size of 25nm~50nm, and the specific surface area of ​​the silica particles does not exceed 120m². 2 / g.

2. The colloidal silica solution according to claim 1, characterized in that, Based on mass percentage, the colloidal silica solution contains 20%–30% silica, with a silica particle size of 25 nm–30 nm and a specific surface area of ​​90–100 m². 2 / g.

3. The colloidal silica solution according to claim 1, characterized in that, Based on mass percentage, the colloidal silica solution contains 20%–30% silica, with a silica particle size of 30 nm–35 nm and a specific surface area of ​​80–90 m². 2 / g.

4. A method for preparing the colloidal silica solution according to any one of claims 1 to 3, characterized in that, The specific steps are as follows: Step 1: Mix the ammonia / ammonium chloride buffer solution and water thoroughly to obtain solution A; the amount of ammonia / ammonium chloride buffer solution in solution A is 2wt%~5wt%. Step 2: Add the alkoxysilane to solution A in two portions and react at 50℃~80℃ for 3h~12h; the time interval between the two additions of alkoxysilane is 0.5h~1.5h; the mass ratio of the amount of alkoxysilane added each time to the mass of solution A is (0.07~0.13):1; Step 3: Take a certain amount of solution A, water and ammonia / ammonium chloride buffer solution and mix them to obtain solution B; according to the mass percentage, the amount of solution A added is 12wt%~50wt%, the amount of ammonia / ammonium chloride buffer solution added is 0.7wt%~2wt%, and the balance is water, with the total amount of each component added being 100%; Step 4: Slowly add alkoxysilane to solution B, stir the reaction, filter and concentrate to obtain the colloidal silica solution; the mass ratio of the amount of alkoxysilane added to the mass of solution B is (0.15~0.25):

1.

5. The method according to claim 4, characterized in that, The mass ratio of water, ammonia, and ammonium chloride in the ammonia / ammonium chloride buffer solution is (4~8):(35~50):(180~220), the pH value of the ammonia / ammonium chloride buffer solution is 9~11, and the concentration of ammonia is 20wt%~28wt%.

6. The method according to claim 4, characterized in that, The alkoxysilane is one of methyl orthosilicate, ethyl orthosilicate, and methyltrimethoxysilane.

7. The method according to claim 4, characterized in that, In step 4, alkoxysilane is added to solution B in multiple portions, with each addition time controlled between 3 and 7 hours, and the mass of alkoxysilane added each time is equal. During each addition, the pH value of the system is controlled between 8.5 and 10 using an ammonia / ammonium chloride buffer solution. After all the alkoxysilane has been added, the reaction is allowed to proceed for 3 to 7 hours to obtain the colloidal silica solution.

8. The method according to claim 7, characterized in that, In step 4, the temperature is controlled at 50℃~70℃, and the alkoxysilane is added in 4~5 batches.

9. The method according to claim 4, characterized in that, In step 4, the reaction product is concentrated using a cross-flow membrane to obtain the colloidal silica solution.

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