Preparation method of silica sol with low specific surface area

Abstract

The invention relates to the technical field of chemical mechanical polishing of semiconductors, in particular to a preparation method of low-specific-surface-area silica sol, which comprises the following steps: adding pure water into crude silica sol prepared by taking alkoxy silane as a silicon source, and stirring at low vacuum degree to obtain the low-specific-surface-area silica sol, the mass ratio of the added pure water is w, the vacuum degree is v / MPa, the temperature is T / DEG C, the stirring duration is t / h, and the following relational expression is met: 0.02 < = 300 * w * v / (t * logT) < = 0.04; wherein the mass ratio w of the added pure water is 6-35%, w = the mass of the pure water / the mass of the crude silica sol * 100%, the vacuum degree v is 0.005-0.03 MPa, the temperature T is 70-95 DEG C, and the stirring time t is 8-12 h. The post-treatment process is added, and the mass ratio w of the added pure water, the vacuum degree v, the temperature T and the stirring duration t are controlled to meet a certain relationship, so that the alcohol content of the system can be reduced on the basis of reducing the specific surface area of the silicon dioxide particles, and the silica sol with higher polishing efficiency and better polishing effect is obtained.

Description

Technical Field

[0001] This invention relates to the field of semiconductor chemical mechanical polishing technology, and more particularly to a method for preparing low specific surface area silica sol. Background Technology

[0002] Silica sol is an aqueous solution of nano-sized silica. It has a wide range of applications, widely used in the coatings industry due to its easy film formation, water resistance, and heat resistance. Silica sol can also be used to prepare polishing slurries for chemical mechanical polishing (CMP) processes in semiconductor integrated circuits. However, this type of silica sol has specific requirements regarding the shape, particle size, and association degree of the silica nanoparticles. The primary particle size is characterized by specific surface area, calculated as R from the specific surface area S using the empirical formula: R = 2727 / S. With similar apparent radii, a larger specific surface area generally indicates more porous structures on the particle surface and stronger adsorption capacity. Such particles are generally suitable for catalysis and other fields. However, in polishing, such a porous surface structure results in low polishing efficiency. Therefore, silica sols used in polishing typically require a minimum specific surface area.

[0003] Currently, the alkoxysilane sol-gel method is commonly used to prepare spherical silica sols. Silica particles prepared using this method have a particle size under an electron microscope that is much larger than the particle size calculated from the specific surface area, indicating a very large initial specific surface area. When these large-surface-area silica nanoparticles are formulated into a polishing slurry, the polishing rate is lower compared to those with smaller specific surface areas. In the prior art, patent CN113912070A discloses a dense and uniformly sized silica sol and its preparation method. This involves using organic and inorganic alkali compounds as a composite catalyst to hydrolyze silica esters, followed by high-temperature and high-pressure ripening in a pressure vessel to obtain silica sol particles. The temperature in the pressure vessel is 100-374.5℃, and the pressure is 0.1-22.5 MPa. This method uses a composite catalyst combined with high-temperature and high-pressure ripening, making the process relatively complex and energy-intensive. Therefore, a simpler and more effective method for obtaining low-surface-area silica sols is needed. Summary of the Invention

[0004] To address the aforementioned problems, this invention provides a method for preparing low specific surface area silica sol. Based on the alkoxysilane sol-gel method, post-processing is performed to obtain nano-silica particles with no significant change in particle size under an electron microscope, but with a significantly reduced specific surface area.

[0005] The present invention adopts the following technical solution:

[0006] A method for preparing a low specific surface area silica sol includes the following steps:

[0007] Pure water was added to the crude silica sol prepared using alkoxysilane as a silicon source, and the mixture was stirred under low vacuum to obtain the low specific surface area silica sol.

[0008] The mass percentage of pure water added is w, the vacuum degree is v / MPa, the temperature is T / ℃, and the stirring time is t / h, satisfying the following relationship: 0.02≤300*w*v / (t*logT)≤0.04;

[0009] The proportion of pure water added is 6-35%, where w = mass of pure water / mass of crude silica sol × 100%. The vacuum degree is 0.005-0.03 MPa, the temperature is 70-95℃, and the mass of pure water added is 20-100g relative to 300g of crude silica sol. The stirring time is 8-12h.

[0010] In some preferred embodiments, the mass percentage of pure water added (w) is 8-20%, the vacuum degree (v) is 0.015-0.025 MPa, the temperature (T) is 80-95°C, and the stirring time (t) is 8-10 h.

[0011] In some specific embodiments, the crude silica sol weighs 300g, and the added pure water weighs 20-100g.

[0012] Silica sol is an aqueous solution of nano-sized silica. Theoretically, silica is a crystal formed by the long-range ordered arrangement of silicon and oxygen atoms. Silicon atoms are located at the center of a tetrahedron, and four oxygen atoms are located at the vertices. Many tetrahedra are connected by the oxygen atoms at the vertices, and each oxygen atom is shared by two tetrahedra. Its structure is illustrated below:

[0013]

[0014] However, when alkoxysilanes are used as silicon sources, the crude silica sol particles prepared contain a large number of residual alkyl chains, especially on the surface of nanospheres. Many Si-O-Si structures are actually Si-OR structures. The presence of the -OR structure causes the surface of the nanospheres to become a loose and porous structure, resulting in a low actual density of the silica sol, an increased nitrogen adsorption specific surface area, and a decrease in polishing efficiency.

[0015] The post-processing step of this invention is as follows: a certain amount of pure water is added to the crude silica sol prepared using alkoxysilane as the silicon source, and then the mixture is heated and stirred under low vacuum. By adding pure water to the obtained crude silica sol, the solid content of the system is kept stable. Simultaneously, by adjusting the system vacuum, temperature, and processing time, the conversion of the system from Si-OR to Si-OH can be controlled, thereby reducing the Si-OR structure in the silica sol product, thus reducing the specific surface area of ​​the silica sol and increasing its density. However, the inventors have found that during the above conversion process, the Si-OR structure will dechain into a Si-OH structure. Although the Si-OH structure has little effect on the specific surface area, it significantly affects subsequent polishing applications. Therefore, it is necessary to convert it into a Si-O-Si structure as much as possible. The inventors further discovered that by controlling the amount of pure water added, as well as the stirring temperature, time, and system vacuum degree, under certain conditions—specifically, 0.02 ≤ 300 * w * v / (t * log T) ≤ 0.04—the Si-OR structure can be dechained into a Si-OH structure, and further into a Si-O-Si structure, thereby obtaining a low specific surface area silica sol suitable for polishing. The entire process actually generates ROH alcohols and consumes a certain amount of H2O, which can be simply expressed as:

[0016] Si-O-R+H2O→Si-O-H+ROH→Si-O-Si

[0017] When 300*w*v / (t*logT) is greater than 0.04, it is not conducive to the transformation of Si-OR to Si-OH structure, and too much Si-OR remains in the system, resulting in a loose internal structure. When 300*w*v / (t*logT) is less than 0.02, too much ROH remains in the system, which on the one hand inhibits the transformation of Si-OR to Si-OH, and more importantly, leads to more surface active sites on silica nanospheres, which can easily interact with other components in the polishing slurry system, affecting the polishing effect.

[0018] In some embodiments, the crude silica sol is prepared using the sol-gel method.

[0019] In some preferred embodiments, the crude silica sol is prepared by the following method:

[0020] The alkoxysilane, solvent, and base catalyst are mixed and reacted at 20-60℃ for 1-10 hours. Then, the mixture is heated to 80-100℃ and concentrated by vacuum distillation to obtain the crude silica sol.

[0021] This invention primarily addresses the problem of poor polishing performance caused by the large specific surface area of ​​nano-silica particles in silica sol. Analysis reveals that when using alkoxysilanes as the silicon source to prepare silica sol, the silica particles exhibit a Si-OR structure, affecting their density and becoming a key factor influencing the application effect of the silica sol. This Si-OR structure is due to the alkoxy structure in the silicon source; therefore, the aforementioned problem exists regardless of the alkoxysilane used as the silicon source. The post-processing method of this invention directly treats the obtained crude silica sol product. Therefore, the specific selection of the alkoxysilane as the silicon source is not limited, and its selection does not affect the processing effect of this invention. In other words, this invention is a post-processing improvement based on the conventional alkoxysilane sol-gel method for preparing silica sol. Therefore, the specific parameters of the preparation process can be used as long as they are within the conventional range. For example, the temperature for preparing silica sol using the sol-gel method can be 20℃, 25℃, 30℃, 35℃, 40℃, 45℃, 50℃, 55℃, 60℃, and specific values ​​between these values; the reaction time can be 1h, 2h, 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h, and specific values ​​between these values; the temperature for vacuum distillation concentration can be 80℃, 85℃, 88℃, 90℃, 92℃, 96℃, 100℃, and specific values ​​between these values; for space considerations, this invention will not exhaustively list all the specific values ​​included in the range.

[0022] In some embodiments, the alkoxysilane includes one or more of methyl orthosilicate, ethyl orthosilicate, tetrapropoxysilane, tetrabutoxysilane, methyltrimethoxysilane, and methyltriethoxysilane.

[0023] In some embodiments, the solvent includes one or more of water, methanol, ethanol, isopropanol, n-butanol, and isobutanol.

[0024] In some embodiments, the alkaline catalyst includes one or more of ammonia, potassium hydroxide, sodium hydroxide, and ammonium bicarbonate. In some preferred embodiments, ammonia is preferably used as the alkaline catalyst for controlling the metal ion content.

[0025] In some embodiments, the solid content of the concentrated crude silica sol is 8-12%.

[0026] In some embodiments, the SEM apparent particle size of the silica particles in the crude silica sol is 30-50 nm.

[0027] In some embodiments, the crude silica sol contains silica particles with an apparent SEM particle size of 35 nm and a specific surface area greater than 100 m².2 / g; In the low specific surface area silica sol, the specific surface area of ​​silica particles with an apparent particle size of 35nm by SEM is less than or equal to 85m². 2 / g.

[0028] In some embodiments, the alcohol content of the low specific surface area silica sol system is less than 1 wt% as detected by gas chromatography; in some preferred embodiments, the alcohol content of the low specific surface area silica sol system is less than 0.5 wt% as detected by gas chromatography.

[0029] Compared with the prior art, the beneficial effects of the present invention are:

[0030] (1) This invention, based on the preparation of silica sol by the alkoxysilane sol-gel method, performs post-processing by adding pure water to the crude silica sol and heating and stirring under a certain vacuum, thereby obtaining a silica sol product with a smaller specific surface area. Furthermore, by controlling the mass ratio w of pure water added, the vacuum degree v, the temperature T, and the stirring time t during the post-processing to meet certain relationships, this invention can reduce the specific surface area of ​​silica particles and also reduce the alcohol content of the system, thereby obtaining a silica sol with higher polishing efficiency and better polishing effect.

[0031] (2) The method for preparing low specific surface area silica sol of the present invention is applicable to the existing alkoxysilane sol-gel method for preparing silica sol, and has no special requirements for the selection of alkoxysilane, alkaline catalyst and solvent. It has a wide range of applications and is simple to operate, making it suitable for industrial production applications. Detailed Implementation

[0032] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of the present invention.

[0033] A method for preparing a low specific surface area silica sol according to the present invention includes the following steps:

[0034] Pure water was added to the crude silica sol prepared using alkoxysilane as a silicon source, and the mixture was stirred under low vacuum to obtain the low specific surface area silica sol.

[0035] The mass percentage of pure water added is w, the vacuum degree is v / MPa, the temperature is T / ℃, and the stirring time is t / h, satisfying the following relationship: 0.02≤300*w*v / (t*logT)≤0.04;

[0036] The proportion of pure water added (w) is 6-35%, where w = mass of pure water / mass of crude silica sol × 100%, the vacuum degree (v) is 0.005-0.03 MPa, the temperature (T) is 70-95℃, and the stirring time (t) is 8-12 h.

[0037] The transformation process of the silica sol structure is as follows: Si-O-R + H₂O → Si-O-H + ROH → Si-O-Si. Therefore, Si-OR needs to be converted to Si-OH first, and then further converted to Si-O-Si. Since the conversion of Si-OR to Si-OH requires the participation of residual alkali in the system, if the vacuum degree v and temperature T are too high, the vacuuming rate will be too fast, and the alkali substance (ammonia) in the system will volatilize too quickly, which is not conducive to this process. If the process time, i.e., the stirring time t, is too short, the conversion will be incomplete. Conversely, if the vacuum degree v is too low and the temperature T is insufficient, the ROH generated during the structural transformation cannot be removed in time, which will inhibit this step of the transformation, resulting in a relatively large amount of residual Si-OR. The presence of water in the process mainly serves to maintain the stability of the solid content in the system.

[0038] Specifically, as one embodiment of the present invention, the vacuum degree v can be 0.005MPa, 0.008MPa, 0.01MPa, 0.012MPa, 0.015MPa, 0.018MPa, 0.02MPa, 0.022MPa, 0.025MPa, 0.03MPa, and specific values ​​between these values; the temperature T can be 70℃, 75℃, 80℃, 85℃, 90℃, 95℃, and the values ​​between these values. Specific point values ​​between the values; when the crude silica sol is 300g, the mass of pure water added can be 20g, 25g, 30g, 40g, 50g, 60g, 70g, 80g, 90g, 100g, and specific point values ​​between the aforementioned point values; the stirring time t can be 7h, 7.5h, 8h, 8.5h, 9h, 9.5h, and specific point values ​​between the aforementioned point values; for space considerations, this invention will not exhaustively list the specific point values ​​included in the aforementioned range.

[0039] In one embodiment of the present invention, the crude silica sol is prepared by the following method:

[0040] The alkoxysilane, solvent, and base catalyst are mixed and reacted at 20-60℃ for 1-10 hours. Then, the mixture is heated to 80-100℃ and concentrated by vacuum distillation to obtain the crude silica sol.

[0041] This invention does not impose any particular limitations on the selection of the alkoxysilane used as the silicon source, as well as the specific choice of solvent and catalyst. For example, the alkoxysilane can be one or more of methyl orthosilicate, tetraethyl orthosilicate, tetrapropoxysilane, tetrabutoxysilane, methyltrimethoxysilane, and methyltriethoxysilane. The solvent can be one or more of water, methanol, ethanol, isopropanol, n-butanol, and isobutanol. The alkaline catalyst includes one or more of ammonia, potassium hydroxide, sodium hydroxide, and ammonium bicarbonate. The amounts of alkoxysilane, solvent, and alkaline catalyst are also not particularly limited. In one embodiment of this invention, tetraethyl orthosilicate is used as the silicon source, methanol as the solvent, and 14 wt% ammonia as the raw material, with a mass ratio of 1:2-5:10-20.

[0042] Example 1

[0043] I. Preparation of crude silica sol

[0044] 489g of 14wt% ammonia solution was poured into a mixture of 2022g of tetraethyl orthosilicate and methanol, and reacted at 40℃ for 3h. Then the temperature was raised to 90℃ and the solution was distilled under reduced pressure to reduce the total weight of the solution to 300g to obtain crude silica sol with a SEM particle size of 35nm.

[0045] II. Post-processing

[0046] Pure water was added to the crude silica sol prepared in step one, and the mixture was stirred under low vacuum to obtain a silica sol with low specific surface area.

[0047] The added pure water mass ratio was w (w = pure water mass / crude silica sol mass × 100%), the vacuum degree was v / MPa, the temperature was T / ℃, and the stirring time was t / h. Specific preparation-post-treatment conditions are shown in Table 1.

[0048] Examples 2-12 and Comparative Examples 1-44

[0049] Examples 2-12 and Comparative Examples 1-44 follow the same preparation-post-treatment methods as the examples, except that the specific preparation-post-treatment conditions are shown in Table 1.

[0050] The preparation methods of the crude silica sol used for post-processing in Examples 7 and 8 are the same as those in Example 1, except that 600g and 150g of crude silica sol are prepared respectively for post-processing. Compared with Example 1, the raw materials are reduced / expanded proportionally in the preparation stage of the crude silica sol.

[0051] The preparation methods of the crude silica sol used for post-processing in Examples 9 and 10 are the same as those in Example 1, except that the solid content of the crude silica sol is different, at 8% and 12% respectively.

[0052] The preparation methods of the crude silica sol used for post-processing in Examples 11 and 12 are the same as those in Example 1, except that the silicon source selected in the preparation stage of the crude silica sol is methyl orthosilicate and methyltrimethoxysilane, respectively.

[0053] Comparative Example 43 directly used the crude silica sol obtained in Example 1 without post-processing.

[0054] Comparative Example 44 used the crude silica sol obtained in Example 1 for post-processing, the only difference being that pure water was not added during the post-processing.

[0055] Table 1

[0056]

[0057]

[0058] The crude silica sols prepared in Examples 1-12 and Comparative Examples 1-44, as well as the silica sol products obtained after post-processing, were tested using the following methods:

[0059] SEM apparent particle size: measured using an electron microscope.

[0060] Specific surface area of ​​silica particles with an apparent particle size of 35 nm obtained by SEM analysis using a nitrogen adsorption specific surface area analyzer.

[0061] Primary particle size: calculated using the empirical formula R = 2727 / S, where S is the specific surface area.

[0062] Alcohol content in the system: determined using gas chromatography.

[0063] Particulate hydroxyl content titration consumption: Crude silica sol and post-processed silica sol products were used as samples. The hydroxyl content of the samples was tested using the following titration method: the volume of 0.1 mol / L NaOH solution consumed per 1.5 g of silica material was less than 9 ml. Take 1.5 g of silica sol particles, add pure water to a final volume of 90 mL, then add hydrochloric acid until the pH becomes 3.6, then add 30 g of sodium chloride, add water to completely dissolve the sodium chloride, and finally bring the volume to 150 mL to obtain the test solution. Add the test solution to an automatic titration apparatus, add 0.1 mol / L sodium hydroxide aqueous solution, and determine the titration amount of sodium hydroxide aqueous solution required to reach pH 9.0.

[0064] Application effect test: A 5% solids content solution of silica sol was prepared, and 1.5g of 5% NaOH solution was added to 100g of this solution. This solution was used to polish a Si wafer with a 100nm thick SiO2 film. The application effect was evaluated based on the change in film thickness and surface roughness after polishing. A film removal rate of 2.5–3.5nm / min was considered an acceptable application effect.

[0065] The test results are shown in Table 2.

[0066] Table 2

[0067]

[0068]

[0069] As shown in Table 2, this invention, based on the preparation of silica sol using the alkoxysilane sol-gel method, involves post-processing. By adding pure water to the crude silica sol and heating and stirring under a certain vacuum, a silica sol product with a smaller specific surface area is obtained. Furthermore, by controlling the mass ratio of pure water (w), vacuum level (v), temperature (T), and stirring time (t) during the post-processing to satisfy certain relationships, this invention can reduce the specific surface area of ​​silica particles and also decrease the alcohol content of the system, thereby obtaining a silica sol with higher polishing efficiency and better polishing effect.

[0070] Comparing Example 1 and Comparative Examples 1-42, it can be seen that the proportion of pure water added in the post-treatment process, the vacuum degree of the system, the temperature, and the processing time (i.e., the stirring time) all affect the quality of the final silica sol product. Pure water is mainly used in this invention to maintain the solid content of the system; further control of the vacuum degree, temperature, and stirring process helps to control the alkaline environment of the system, thereby facilitating the conversion of Si-OR to Si-OH. In the post-treatment method of this invention, a silica sol product with good polishing application effect can only be obtained when the proportion of pure water added (w), the vacuum degree (v), the temperature (T), the stirring time (t), and the relationship between the four all meet the requirements. When any of the parameters—pure water mass ratio, vacuum degree, or system temperature—is too high or too low, even if the pure water mass ratio w, vacuum degree v, temperature T, and stirring time t satisfy 0.02≤300*w*v / (t*logT)≤0.04, the required membrane removal rate of 2.5–2.5 nm / min cannot be met due to particle size and the consumption of alkali by residual hydroxyl groups, resulting in poor silica sol application performance. Furthermore, when the stirring time is short, the conversion of Si-OR to Si-OH is incomplete, and the silica sol product still contains a large amount of Si-OR structure with a large specific surface area, making it unsuitable for polishing applications. When the stirring time is long, there will be issues such as low alkali consumption, excessive particle polishing rate, and severe polishing damage. Conversely, excessively long stirring times can lead to severe solid precipitation, making it impossible to obtain a sample.

[0071] The present invention has been further described above with reference to specific embodiments. However, it should be understood that the specific description herein should not be construed as limiting the nature and scope of the present invention. Various modifications made to the above embodiments by those skilled in the art after reading this specification are all within the scope of protection of the present invention.

Claims

1. A method for preparing a low specific surface area silica sol, characterized in that, Includes the following steps: Pure water was added to the crude silica sol prepared using alkoxysilane as a silicon source, and the mixture was stirred under low vacuum to obtain the low specific surface area silica sol. The mass percentage of pure water added is w, the vacuum degree is v / MPa, the temperature is T / ℃, and the stirring time is t / h, satisfying the following relationship: 0.02≤300*w*v / (t*logT)≤0.04; The proportion of pure water added (w) is 6-35%, where w = mass of pure water / mass of crude silica sol × 100%, the vacuum degree (v) is 0.005-0.03 MPa, the temperature (T) is 70-95℃, and the stirring time (t) is 8-12 h.

2. The method for preparing low specific surface area silica sol according to claim 1, characterized in that, The mass percentage of pure water added is 8-20%, the vacuum degree is 0.015-0.025MPa, the temperature is 80-95℃, and the stirring time is 8-10h.

3. The method for preparing low specific surface area silica sol according to claim 1, characterized in that, The crude silica sol was prepared by the sol-gel method.

4. The method for preparing low specific surface area silica sol according to claim 3, characterized in that, The crude silica sol was prepared using the following method: The alkoxysilane, solvent, and base catalyst are mixed and reacted at 20-60℃ for 1-10 hours. Then, the mixture is heated to 80-100℃ and concentrated by vacuum distillation to obtain the crude silica sol.

5. The method for preparing low specific surface area silica sol according to claim 4, characterized in that, The alkoxysilane includes one or more of methyl orthosilicate, ethyl orthosilicate, tetrapropoxysilane, tetrabutoxysilane, methyltrimethoxysilane, and methyltriethoxysilane; and / or, The solvent includes one or more of water, methanol, ethanol, isopropanol, n-butanol, and isobutanol; and / or, The alkaline catalyst includes one or more of ammonia, potassium hydroxide, sodium hydroxide, and ammonium bicarbonate.

6. The method for preparing low specific surface area silica sol according to claim 4, characterized in that, The solid content of the crude silica sol obtained by concentration is 8-12%.

7. The method for preparing low specific surface area silica sol according to claim 4, characterized in that, The SEM apparent particle size of the silica particles in the crude silica sol is 30-50 nm.

8. The method for preparing low specific surface area silica sol according to claim 1, characterized in that, In the crude silica sol, the silica particles with an apparent SEM particle size of 35 nm have a specific surface area greater than 100 m². 2 / g; In the low specific surface area silica sol, the specific surface area of ​​silica particles with an apparent particle size of 35 nm as determined by SEM is less than or equal to 85 m². 2 / g.

9. The method for preparing low specific surface area silica sol according to claim 8, characterized in that, Gas chromatography analysis showed that the alcohol content of the low specific surface area silica sol system was less than 1 wt%.

10. The method for preparing low specific surface area silica sol according to claim 9, characterized in that, Gas chromatography analysis showed that the alcohol content of the low specific surface area silica sol system was less than 0.5 wt%.

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