Preparation method of high-purity silica sol and application thereof

Abstract

The application discloses a preparation method of high-purity silicon dioxide sol and application thereof, and belongs to the technical field of functional inorganic nanometer material manufacturing. The method comprises the following steps: firstly, performing preliminary impurity removal treatment on elemental silicon powder by using dilute acid, then performing water washing and drying, and then performing two-stage gradient temperature rising reaction on the elemental silicon powder, water, a catalyst and a recyclable solid defoaming agent to obtain silicon sol primary products; the solid defoaming agent in the silicon sol primary products is recovered, filtration is performed, and the filtered silicon sol is subjected to oxidation treatment by adding hydrogen peroxide for two-stage temperature rising oxidation treatment or ultraviolet-ozone combined oxidation treatment, and then the silicon sol is further subjected to impurity removal and concentration by using a mixed bed ion exchange resin and an ultrafiltration process to obtain silicon dioxide sol. The silicon dioxide sol has low conductivity and good polishing effect, and has a good application prospect in the field of electronic polishing.

Description

Technical Field

[0001] This invention belongs to the field of functional inorganic nanomaterial manufacturing technology, specifically relating to a method for preparing high-purity silica sol and its application. Background Technology

[0002] Silica sol is a milky white or white liquid, a dispersion of nano-sized silica particles in water or solvent, possessing high specific surface area, good adsorption capacity, and stability. As an important inorganic functional material, silica sol plays an irreplaceable role in semiconductor polishing, catalyst support, and electronic coatings. Silica sol can be prepared through methods such as hydrolysis of orthosilicates (e.g., Chinese patent document CN109665531A), polymerization growth of water glass after ion exchange (e.g., Chinese patent document CN110980745A), and hydrolysis and condensation of silicon powder (e.g., Chinese patent document CN108716005A). Among these methods, the hydrolysis of elemental silicon has attracted much attention due to the readily available raw materials and simple process; however, its industrialization still faces the following technical bottlenecks:

[0003] (1) Residual elemental silicon: Incomplete reaction of silicon powder will result in the presence of extremely hard elemental silicon nanoparticles in the silica sol. When used as a polishing slurry, these nanoparticles can easily cause scratches on the wafer surface, which will seriously affect the performance of semiconductor devices.

[0004] (2) Low reaction efficiency and yield: Hydrogen foam during the reaction process hinders the contact between silicon powder and solution. Traditional defoamers (such as organosilicon) are easy to remain and difficult to recover, resulting in a yield of less than 80%.

[0005] (3) Insufficient purity: Alkali metal ions (Na+) introduced by alkaline catalysts (such as NaOH) + K + It is difficult to completely remove, and its conductivity is often higher than 50 μS / cm, which cannot meet the requirements of electronic grade silica sol.

[0006] Chinese patent document CN108716005A discloses a method for preparing silica sol by electrolysis of silica micropowder slurry. This method involves mixing silica micropowder with an alkaline solution to form a slurry, and then separating the slurry using a bipolar membrane electrolytic cell. Silicate ions in the slurry pass through an anion exchange membrane into the anode chamber, where they combine with hydrogen ions generated during anodic electrolysis to form a silicic acid solution. Cations in the slurry pass through a cation exchange membrane into the cathode chamber, where they mix with hydroxide ions generated during cathodic electrolysis to obtain an alkaline solution. As the concentration of the silicic acid solution in the anode chamber increases, silica sol is obtained. However, this method may suffer from problems such as membrane fouling and high purity requirements for the silica micropowder.

[0007] Chinese patent document CN118495542A discloses a method for preparing narrow-distribution, spherical silica sol using silicon powder. The method includes: dispersing silicon powder in water and activating the silicon powder by heating to 55-75°C; adding an alkaline catalyst to the activated silicon powder dispersion and conducting a high-temperature, high-pressure reaction under sealed conditions, with a reaction pressure of 0.1-5 MPa, a reaction temperature of 130-200°C, and a reaction time of 5-10 hours; the mass ratio of silicon powder, water, and alkaline catalyst is 50-300:500-3000:0.5-5. While this invention simplifies the process of preparing silica sol by silicon powder hydrolysis under normal pressure, it also suffers from high equipment requirements and high water consumption.

[0008] Chinese patent document CN101857236A discloses a method for preparing high-concentration silica sol from ultrafine silica powder. The method includes: taking 20-25 parts by weight of silica powder with a purity of 97%-99% and a particle size of 500-2000 mesh, adding 100-200 parts by weight of deionized water, and heating at 50-60℃ for 10-30 minutes; then, at a temperature of 70-90℃, adding a sodium hydroxide aqueous solution with a mass percentage concentration of 0.16%-0.96% in portions to the silica powder and water mixture while stirring, controlling the pH of the reaction solution to 8.5-10.0, and the entire stirring reaction time is 6-8 hours; then, after standing at room temperature for 8-12 hours, filtering under reduced pressure to obtain the high-concentration silica sol. This method has the problem of requiring high purity silica powder.

[0009] Therefore, research is needed on the preparation of silica sol by hydrolysis of elemental silicon to overcome the aforementioned bottlenecks. Summary of the Invention

[0010] To overcome the problems of difficulty in removing small amounts of elemental silicon microparticles and insufficient purity of silica sol during the preparation of silica sol from elemental silicon, this invention provides a method for preparing high-purity silica sol, which yields silica sol with low electrical conductivity and good polishing effect.

[0011] The specific technical solution adopted is as follows:

[0012] A method for preparing high-purity silica sol includes the following steps:

[0013] (1) After the elemental silicon powder is subjected to preliminary impurity removal treatment with dilute acid, it is washed with water and dried to obtain pretreated silicon powder; wherein the particle size of the elemental silicon powder is 50-500 mesh and the purity is ≥95%;

[0014] (2) The reaction system consisting of pretreated silica powder, water, catalyst and recyclable solid defoamer was subjected to a two-stage gradient heating reaction. The parameters of the first stage were 55-65℃ for 50-70 min and the parameters of the second stage were 85-95℃ for 4-8 h to obtain silica sol primary product.

[0015] (3) Let the silica sol stand, recover the solid defoamer, then filter it, and oxidize the filtered silica sol. The oxidation treatment is carried out by adding hydrogen peroxide for two-stage heating oxidation treatment or ultraviolet ozone combined oxidation treatment.

[0016] (4) The silica sol obtained in step (3) is treated with a mixed bed ion exchange resin to make the silica sol conductivity <10μS / cm (alkali metal ion content <1ppm).

[0017] (5) The silica sol treated by ultrafiltration concentration step (4) is adjusted to pH 9.5 to 10.0 and a stabilizer is added during the concentration process to obtain silica sol with silica content ≥ 30 wt% and silica average particle size of 10 to 50 nm.

[0018] Specifically, the silica sol prepared by the method of the present invention has high purity, extremely low content of elemental silicon microparticle impurities (surface roughness Ra < 0.2 nm after wafer polishing), and low content of impurity ions (alkali metal ions, etc.) (conductivity < 10 μS / cm), and good storage stability.

[0019] Preferably, in step (1), after the elemental silicon powder is subjected to preliminary impurity removal treatment with dilute acid, the acid solution is separated by sedimentation, and the silicon powder is washed with deionized water until the conductivity is ≤50μS / cm to ensure that the acid residue meets the standard.

[0020] Optionally, in step (1), the dilute acid may be selected from at least one of dilute hydrochloric acid, dilute sulfuric acid, dilute nitric acid or hydrofluoric acid.

[0021] Specifically, preliminary purification with dilute acid can remove impurities such as iron, aluminum, and calcium. The concentration and number of acid washes can be adjusted according to the purity of the silicon powder. For elemental silicon powder with a particle size ≤200 mesh, 1%–5% hydrofluoric acid is added to the 2%–6% dilute hydrochloric acid, dilute sulfuric acid, or dilute nitric acid cleaning solution to break down the surface oxide layer. The cleaning time is 5–15 minutes. During the acid washing process, ultrasonic cleaning with a frequency of 30–50 kHz and a power of 500–800 W is applied to improve cleaning efficiency and reduce acid consumption.

[0022] Specifically, in step (2), the catalyst is tetramethylammonium hydroxide, or a mixture of tetramethylammonium hydroxide and tetrapropylammonium hydroxide. The selection of the above catalyst can avoid the introduction of impurity ions.

[0023] Furthermore, when tetramethylammonium hydroxide is selected as the catalyst, the catalyst addition mass is 0.1% to 3% of the mass of elemental silica powder; when a mixture of tetramethylammonium hydroxide and tetrapropylammonium hydroxide is selected as the catalyst, the tetramethylammonium hydroxide addition mass is 0.1% to 0.5% of the mass of elemental silica powder, and the tetrapropylammonium hydroxide addition mass is 0.05% to 0.1% of the mass of elemental silica powder.

[0024] Preferably, the ratio of elemental silica powder to water is 1 kg: 4-6 L, and the mass of solid defoamer is 0.1% to 5% of the mass of elemental silica powder.

[0025] Specifically, the solid defoamer is a hydrophobic and water-insoluble defoaming particle with a particle size of 10nm to 500mm, selected from hydrophobically modified silica, alumina, calcium carbonate, titanium dioxide or barium sulfate.

[0026] Preferably, the parameters for the two-stage heating oxidation treatment are: the first stage is 90-100℃ and 0.5-1MPa pressure for 50-70 min, and the second stage is 150-200℃ and 1-2MPa pressure for 30 min-5 h.

[0027] More preferably, when adding hydrogen peroxide, the mass of the active ingredient hydrogen peroxide in the hydrogen peroxide is 0.5% to 5% of the mass of SiO2 in the filtered silica sol, and the hydrogen peroxide is added three times in a mass ratio of 1:0.4 to 0.6:0.4 to 0.6.

[0028] Preferably, the parameters for the combined ultraviolet and ozone treatment are: ozone concentration of 40-60 ppm, ultraviolet wavelength of 254 nm, reaction temperature maintained at 80-120℃, and treatment at normal pressure for 1-3 hours.

[0029] Preferably, the ultrafiltration process uses a multi-stage ultrafiltration membrane series for concentration, with each ultrafiltration membrane having a molecular weight cutoff of ≥10kDa and maintaining a concentration rate of ≤5L / h.

[0030] Preferably, the stabilizer is selected from polyvinyl alcohol or hydroxypropyl methylcellulose, and the mass of the stabilizer is 0.1% to 0.5% of the mass of SiO2 in the silica sol.

[0031] The present invention also provides a polishing fluid, the components of which include silica sol prepared by the method described above.

[0032] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0033] (1) In the process of preparing high-purity silica sol, the silica sol is oxidized to effectively reduce the impurities of elemental silicon particles in the silica sol, which increases its application prospects as a semiconductor polishing liquid and reduces the polishing defects it causes to the polishing surface. When the silica sol is used for wafer polishing, the surface roughness Ra after polishing is <0.2nm.

[0034] (2) The silica sol prepared by the method of the present invention has good stability and long storage period, and contains few impurity ions, and the silica particles have uniform particle size and good dispersibility.

[0035] (3) The present invention uses elemental silicon as raw material to prepare silica sol. The production process is simple, the production cycle is short and the efficiency is high. It has good application prospects in the field of electronic polishing. Attached Figure Description

[0036] Figure 1 This is a scanning electron microscope image of the silica sol prepared in Example 1.

[0037] Figure 2 This is a scanning electron microscope image of the silica sol prepared in Example 2.

[0038] Figure 3 This is a scanning electron microscope image of the silica sol prepared in Example 3.

[0039] Figure 4 The image shows a scanning electron microscope (SEM) image of the silica sol prepared in Comparative Example 1.

[0040] Figure 5 The image shows a scanning electron microscope (SEM) image of the silica sol prepared in Comparative Example 2. Detailed Implementation

[0041] To make the objectives, features, and advantages of this invention more apparent and understandable, a detailed description is provided below through specific embodiments. Many specific details are set forth in the following description to provide a thorough understanding of the invention. However, the invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below. Technical features in various embodiments of the invention can be combined appropriately without mutual conflict.

[0042] Unless otherwise specified, the operating methods in the following examples are generally performed under conventional conditions or as recommended by the manufacturer. Contents not described in detail in this specification are prior art known to those skilled in the art. Unless otherwise specified, the experimental materials used in the examples below can be purchased from conventional biochemical reagent companies.

[0043] Example 1

[0044] Step 1: Silicon powder pretreatment

[0045] Take 1 kg of elemental silicon powder with a particle size of 200 mesh and a purity of ≥95%, add 5% dilute hydrochloric acid and wash for 30 min to remove impurities such as iron and aluminum. Add 3% hydrofluoric acid and wash for 10 min. During the acid washing process, apply ultrasonic cleaning with a frequency of 40 kHz and a power of 500 W. During the acid washing process, adopt a multi-stage countercurrent acid washing process, with multiple acid washing tanks connected in series, and circulate acid solution with an acid-resistant ceramic membrane (pore size 0.1 μm). The acid reuse rate is ≥85%. After the initial impurity removal treatment by acid, separate the acid solution by sedimentation, and wash the silicon powder with deionized water until the conductivity is 45 μS / cm. Then, further vacuum dry for later use.

[0046] Step 2: Preparation of silica sol

[0047] The pretreated silicon powder from step 1 was added to a reactor along with 5 L of deionized water and stirred at 300 rpm. Then, tetramethylammonium hydroxide (0.5% of the mass of elemental silicon powder) and hydrophobic silica (0.3% of the mass of elemental silicon powder) were added as defoamers. The reaction system was then subjected to a two-stage gradient temperature increase reaction. The first stage was held at 60°C for 60 min, and the second stage was held at 90°C for 6 h to obtain the initial silica sol product.

[0048] Step 3: Purification and Oxidation Treatment

[0049] The silica sol obtained in step 2 was allowed to stand for 8 minutes to recover the defoamer. Then, the silica sol was filtered using a 0.1 μm ceramic membrane to remove unreacted silica powder with larger particle sizes. The filtered silica sol was then subjected to oxidation treatment. The oxidation treatment method was a two-stage heating oxidation treatment with hydrogen peroxide. The total amount of hydrogen peroxide added was calculated as 2% of the mass of SiO2 in the filtered silica sol. The hydrogen peroxide was added in three batches at a mass ratio of 1:0.5:0.5. After the first addition, the mixture was stirred for 10 minutes, and after the second addition, it was stirred for 30 minutes. The two-stage heating oxidation treatment was as follows: the first stage parameters were 95℃ and 1MPa pressure for 60 minutes, and the second stage parameters were 180℃ and 1.8MPa pressure for 3 hours.

[0050] Step 4: Ion exchange and concentration

[0051] The silica sol after oxidation in step 3 was treated with a mixed-bed ion exchange resin to achieve a conductivity of 8 μS / cm. Subsequently, it was concentrated using a three-stage ultrafiltration membrane series (molecular weight cutoff 50 kDa → 30 kDa → 10 kDa) at a concentration rate of 4 L / h. During concentration, the pH was adjusted to 9.8, and polyvinyl alcohol (PVA) was added as a stabilizer. The mass of PVA was 0.3% of the SiO2 mass in the silica sol. After concentration, a high-purity silica sol with a silica content of 32 wt% and an average silica particle size of 40 nm was obtained (SEM image shown). Figure 1 (as shown), and it did not gel after 6 months of storage.

[0052] Performance testing: The wafer was polished using the high-purity silica sol as a polishing slurry, and the surface roughness of the polished wafer was measured to be Ra = 0.18 nm (AFM).

[0053] Example 2

[0054] The difference between this embodiment and Embodiment 1 is that:

[0055] (1) The particle size of elemental silicon powder is 100 mesh. The silicon powder is washed with deionized water until the conductivity is 35 μS / cm.

[0056] (2) The catalyst is a mixture of tetramethylammonium hydroxide (0.3% of the mass of elemental silica powder) and tetrapropylammonium hydroxide (0.08% of the mass of elemental silica powder).

[0057] (3) The silica sol after oxidation was treated with mixed bed ion exchange resin to make the conductivity of silica sol 7 μS / cm.

[0058] The remaining steps and parameters are the same as in Example 1.

[0059] In this embodiment, a high-purity silica sol with a silica content of 30 wt% and an average silica particle size of 30 nm was obtained after concentration (SEM image shown). Figure 2 (As shown).

[0060] Performance testing: The wafer was polished using the high-purity silica sol as a polishing slurry, and the surface roughness of the polished wafer was measured to be Ra = 0.15 nm (AFM).

[0061] Example 3

[0062] The difference between this embodiment and Embodiment 1 is that:

[0063] (1) The oxidation treatment method is to add hydrogen peroxide. The total amount of hydrogen peroxide added is calculated as 3% of the mass of SiO2 in the filtered silica sol.

[0064] (2) After adding hydrogen peroxide, a two-stage heating oxidation treatment was carried out: the first stage parameters were 95℃ and 1MPa pressure for 60min, and the second stage was 200℃ and 2MPa pressure for 2h.

[0065] (3) The silica sol after oxidation was treated with mixed bed ion exchange resin to make the conductivity of silica sol 6 μS / cm.

[0066] The remaining steps and parameters are the same as in Example 1.

[0067] In this embodiment, a high-purity silica sol with a silica content of 31 wt% and an average silica particle size of 35 nm was obtained after concentration (SEM image shown). Figure 3 (As shown).

[0068] Performance testing: The wafer was polished using the high-purity silica sol as a polishing slurry, and the surface roughness of the polished wafer was measured to be Ra = 0.14 nm (AFM).

[0069] Comparative Example 1

[0070] The only difference between this comparative example and Example 1 is that no oxidation treatment is performed; all other steps and parameters are the same as in Example 1.

[0071] After concentration, this comparative example yielded a silica sol with a silica content of 31 wt% and an average silica particle size of 40 nm (SEM image shown). Figure 4 (As shown).

[0072] Performance testing: The wafer was polished using the silica sol as a polishing slurry, and the surface roughness of the polished wafer was measured to be Ra = 0.7 nm (AFM).

[0073] Comparative Example 2

[0074] The difference between this comparative example and Example 1 is as follows:

[0075] (1) No additional hydrofluoric acid was used for cleaning and no ultrasonic-assisted cleaning was applied during the pickling process.

[0076] (2) No recyclable solid defoamer was added.

[0077] (3) No oxidation treatment was performed.

[0078] (4) A conventional ultrafiltration system was used, the concentration rate was not controlled, the pH was not adjusted during the concentration process, and no stabilizer was added.

[0079] The remaining steps and parameters are the same as in Example 1.

[0080] After concentration, this comparative example yielded a silica sol with a silica content of 35 wt% and an average silica particle size of 55 nm (SEM image shown). Figure 5 (As shown).

[0081] Performance testing: The wafer was polished using the silica sol as a polishing slurry, and the surface roughness of the polished wafer was measured to be Ra = 1.3 nm (AFM).

[0082] Sample Analysis

[0083] Table 1 Comparison of the performance of silica sol obtained from the examples and comparative examples.

[0084] index Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Electrical conductivity (μS / cm) 8 7 6 8 10 Polishing roughness Ra (nm) 0.18 0.15 0.14 0.7 1.3 <![CDATA[SiO2 content (wt%)]]> 32 30 31 31 35

[0085] As shown in Table 1, the results of Examples 1-3 illustrate the significant advantages of the present invention in controlling residual impurities in elemental silicon microparticles, improving purity, and increasing process efficiency. Among them, oxidation treatment, especially two-stage heating oxidation treatment with hydrogen peroxide, is crucial for the deep removal of impurities from elemental silicon microparticles. Specific types of catalysts, mixed-bed ion exchange resins, and process control work together to reduce the content of impurity ions, especially alkali metal ions.

[0086] The embodiments described above provide a detailed explanation of the technical solutions of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, additions, or similar substitutions made within the scope of the principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing high-purity silica sol, characterized in that, Includes the following steps: (1) After the elemental silicon powder is subjected to preliminary impurity removal treatment with dilute acid, it is washed with water and dried to obtain pretreated silicon powder; wherein the particle size of the elemental silicon powder is 50-500 mesh and the purity is ≥95%; (2) The reaction system consisting of pretreated silica powder, water, catalyst and recyclable solid defoamer was subjected to a two-stage gradient heating reaction. The parameters of the first stage were 55-65℃ for 50-70 min and the parameters of the second stage were 85-95℃ for 4-8 h to obtain silica sol primary product. (3) Let the initial silica sol stand, recover the solid defoamer, then filter it, and oxidize the filtered silica sol. The oxidation treatment is carried out by adding hydrogen peroxide and performing a two-stage heating oxidation treatment. The parameters of the two-stage heating oxidation treatment are: the first stage is 90-100℃, 0.5-1 MPa pressure for 50-70 min, and the second stage is 150-200℃, 1-2 MPa pressure for 30 min-5 h. (4) Treat the silica sol obtained in step (3) with a mixed bed ion exchange resin to make the silica sol conductivity <10 μS / cm; (5) The silica sol treated by ultrafiltration concentration step (4) is adjusted to pH 9.5-10.0 and a stabilizer is added during the concentration process to obtain silica sol with silica content ≥30wt% and average silica particle size of 10-50 nm. The ultrafiltration process uses a multi-stage ultrafiltration membrane series for concentration. The molecular weight cutoff of the ultrafiltration membranes is ≥10kDa, and the concentration rate is maintained at ≤5 L / h.

2. The method for preparing high-purity silica sol according to claim 1, characterized in that, In step (1), after the elemental silicon powder is subjected to preliminary impurity removal treatment with dilute acid, the acid solution is separated by sedimentation, and the silicon powder is washed with deionized water until the conductivity is ≤50μS / cm.

3. The method for preparing high-purity silica sol according to claim 1, characterized in that, In step (1), the dilute acid is selected from at least one of dilute hydrochloric acid, dilute sulfuric acid, dilute nitric acid or hydrofluoric acid.

4. The method for preparing high-purity silica sol according to claim 1, characterized in that, The catalyst is tetramethylammonium hydroxide, or a mixture of tetramethylammonium hydroxide and tetrapropylammonium hydroxide; And / or, when tetramethylammonium hydroxide is selected as the catalyst, the catalyst addition mass is 0.1% to 3% of the mass of elemental silica powder; when a mixture of tetramethylammonium hydroxide and tetrapropylammonium hydroxide is selected as the catalyst, the tetramethylammonium hydroxide addition mass is 0.1% to 0.5% of the mass of elemental silica powder, and the tetrapropylammonium hydroxide addition mass is 0.05% to 0.1% of the mass of elemental silica powder.

5. The method for preparing high-purity silica sol according to claim 1, characterized in that, Solid defoamers are hydrophobic and water-insoluble defoaming particles with a particle size of 10 nm to 500 mm, selected from hydrophobically modified silica, alumina, calcium carbonate, titanium dioxide, or barium sulfate.

6. The method for preparing high-purity silica sol according to claim 1, characterized in that, When adding hydrogen peroxide, the mass of the active ingredient hydrogen peroxide in the hydrogen peroxide should be 0.5% to 5% of the mass of SiO2 in the filtered silica sol. The hydrogen peroxide should be added three times in a mass ratio of 1:0.4 to 0.6:0.4 to 0.

6.

7. The method for preparing high-purity silica sol according to claim 1, characterized in that, The stabilizer is selected from polyvinyl alcohol or hydroxypropyl methylcellulose, and the mass of the stabilizer is 0.1% to 0.5% of the mass of SiO2 in the silica sol.

8. A polishing liquid, characterized in that, The components include silica sol prepared by the method described in any one of claims 1-7.

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