Highly stain resistant polyvinyl acetal film, method of making and use thereof

Abstract

This invention relates to the fields of semiconductor manufacturing and microelectronics processing technology, and particularly to a highly antifouling polyvinyl acetal film, its preparation method, and its applications. This invention introduces a trace amount of fluorinated surfactant into the film-forming solution of polyvinyl acetal. Utilizing the surface self-migration effect of the surfactant during the film-forming process, a stable and dense hydrophobic antifouling barrier is constructed on the film surface. This barrier effectively repels and reduces the adsorption of organic gaseous pollutants and water vapor from the environment, maintaining the stability of the film's optical properties in cleanroom environments. Compared to films without the addition of fluorinated surfactant, its antifouling performance is significantly improved, which helps extend the service life of the protective film.

Description

Technical Field

[0001] This invention relates to the fields of semiconductor manufacturing and microelectronic processing technology, and in particular to a highly stain-resistant polyvinyl alcohol acetal film, its preparation method, and its application. Background Technology

[0002] In semiconductor photolithography, projection printing protective film is a key auxiliary material. This film is stretched and covered on the surface of the photomask. Its core function is to isolate dust in the air and prevent the mask pattern from being contaminated, which would affect the imaging quality on the wafer. This significantly improves product yield and mask lifespan.

[0003] An ideal photolithography protective film must meet extremely stringent performance requirements: it must have extremely high optical transmittance (usually >99%) in a specific ultraviolet working band (e.g., deep ultraviolet to far ultraviolet band); it must have excellent mechanical strength and dimensional stability to withstand the tension in the process; at the same time, the film itself must be extremely pure and must not introduce any foreign impurities or defects that affect the optical path.

[0004] Polyvinyl butyral (PVB) resin has been explored for the preparation of such protective films due to its good film-forming properties, optical transparency, and certain mechanical properties. PVB resin is typically prepared by coating a PVB resin solution using a coating process (such as spin coating). However, in actual semiconductor manufacturing environments, such PVB protective films face a significant challenge: surface contamination is a major issue.

[0005] Although lithography machines and chip production lines are located in high-level cleanrooms, molecular-level gaseous contaminants still exist in the environment that cannot be completely eliminated. These include organic amines, siloxanes, and plasticizer vapors released from equipment and materials. These substances gradually adsorb onto the surface of the protective film through physical or chemical means. Under strong ultraviolet light irradiation, the adsorbed organic matter may undergo photochemical reactions, forming a carbonaceous residue layer that is difficult to remove. This contamination layer irreversibly reduces the ultraviolet transmittance of the film and may introduce light scattering, leading to decreased imaging contrast and critical dimensional deviations. This affects the lifespan of the protective film, and the high frequency of film replacement also increases machine maintenance time and production costs.

[0006] However, current research on improving PVB photolithography protective films mainly focuses on enhancing their bulk optical properties and mechanical strength, while lacking effective solutions for surface anti-fouling stability during long-term use.

[0007] Therefore, developing a PVB protective film that can actively resist pollutant adsorption and extend service life while maintaining high light transmittance has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0008] Therefore, based on the technical defects of existing PVB photolithography protective films, which are easily contaminated and thus suffer from performance degradation and short service life, this invention provides a polyvinyl butyral protective film with durable high light transmittance and excellent anti-fouling properties.

[0009] The technical solution provided by this invention is as follows: A highly stain-resistant polyvinyl alcohol acetal film, which is made from the following raw materials: Polyvinyl acetal resin and fluorinated surfactants; The polyvinyl acetal resin is composed of the following repeating units: (I); (II); (III); The structural formula of the polyvinyl acetal resin is shown below: (IV); In Formula IV, x represents the molar percentage of repeating unit I, y represents the molar percentage of repeating unit II, and z represents the molar percentage of repeating unit III, x+y+z=1, where 0.18≤x≤0.22, 0.75<y≤0.80, and 0.01≤z<0.025. The polyvinyl acetal resin has a number-average molecular weight (Mn) of 20,000 to 80,000, a weight-average molecular weight (Mw) of 50,000 to 150,000, and a molecular weight distribution index (Mw / Mn) of 1.5 to 3.5.

[0010] The fluorinated surfactant is selected as a nonionic polymeric fluorocarbon surfactant, and the amount of the fluorinated surfactant is 0.001% to 0.01% of the weight of the polyvinyl acetal resin.

[0011] Further, the number-average molecular weight (Mn) of the fluorinated surfactant is 2,000-10,000, preferably 5,000-6,000 g / mol.

[0012] If the molecular weight of the fluorinated surfactant is too low, the surfactant will migrate excessively to the surface during the film formation process, failing to form a stable anchoring layer, and may even volatilize; if the molecular weight is too high, the migration resistance will be too great, and it will not be able to be fully enriched on the surface in a short time during spin coating.

[0013] Based on the same inventive concept, the present invention also provides a method for preparing a highly stain-resistant polyvinyl acetal film, comprising the following steps: S1 dissolves polyvinyl acetal resin in a solvent to prepare a resin solution; S2. A fluorinated surfactant is added to the resin solution and stirred until homogeneous to obtain a film solution. S3 involves coating the film solution onto the substrate, allowing it to stand and dry at 20-30°C for at least 12 hours, and then peeling it off from the substrate to obtain a highly stain-resistant polyvinyl acetal film. The substrate is a clean, flat, rigid substrate, preferably a monocrystalline silicon wafer or a glass sheet, more preferably a monocrystalline silicon wafer. Before use, the substrate must be cleaned to remove surface oil and particles to ensure uniform wetting with the film solution.

[0014] Furthermore, the solvent used in step S1 is selected from amide solvents, ether solvents, or mixtures thereof.

[0015] Furthermore, the solvent used in step S1 is N,N-dimethylformamide, diethylene glycol dimethyl ether, or a mixture thereof.

[0016] Further, the concentration of polyvinyl acetal resin in the resin solution of step S1 is 5-30 wt%, preferably, the concentration of polyvinyl acetal resin in the resin solution of step S1 is 10-15 wt%.

[0017] Furthermore, in step S3, the film liquid is coated onto the substrate using a spin coating method.

[0018] Based on the same inventive concept, this invention provides the application of a highly stain-resistant polyvinyl acetal film in the preparation of a photolithographic protective film for protecting photomasks.

[0019] Furthermore, the thickness of the highly stain-resistant polyvinyl acetal film is 1.2–2.0 μm.

[0020] The beneficial effects achieved by this invention are as follows: (1) By introducing a trace amount of fluorinated surfactant into the film-forming solution of polyvinyl acetal, the present invention utilizes its surface self-migration effect during the film-forming process to construct a stable and dense hydrophobic anti-fouling barrier on the film surface. This barrier can effectively repel and reduce the adsorption of organic gaseous pollutants and water vapor in the environment, so that the optical performance of the film remains stable in the clean room environment. Compared with the absence of fluorinated surfactant, the ΔT (%) after DOP contamination is significantly increased, with an increase of more than 80%, which can achieve long-term resistance to contamination and significantly extend the service life of the protective film. (2) The amount of fluorinated surfactant introduced into the film solution in this invention is extremely low (<0.01 wt%). After film formation, it mainly accumulates on the surface of the film to form an extremely thin fluorocarbon molecular barrier layer. It has a negligible impact on the overall optical properties of the film, especially the transmittance in the key ultraviolet region (such as 254 nm) (usually a decrease of less than 0.1%). This makes the polyvinyl acetal film fully able to meet the stringent requirements of the photolithography process for the ultra-high transmittance of the protective film.

[0021] (3) The present invention can significantly improve the antifouling properties by introducing a trace amount of fluorinated surfactant into the film-forming solution of polyvinyl acetal. Its preparation process is simple, requires no equipment modification or complex post-processing, and is easy to achieve large-scale stable production with low industrialization risk.

[0022] (4) The surface of the polyvinyl acetal film of the present invention is covered with an antifouling layer containing fluorinated surfactants, which can also change the polyvinyl acetal film from hydrophilic to hydrophobic (the water contact angle can reach more than 110°). This is not only beneficial for antifouling, but also reduces the accumulation of static electricity on the surface, thereby reducing the static adsorption of fine particles to a certain extent, and further ensuring the cleanliness of the film during long-term use. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, 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 scope of protection of the present invention.

[0024] For the following experiments, please provide a detailed experimental procedure (including the source and acquisition of experimental materials; if purchased externally, please provide the source). The fluorinated surfactant used in the following examples was a nonionic polymeric fluorocarbon surfactant based on perfluorobutane sulfonate (3M FC-4432), with Mn≈5,200 g / mol and PDI≈1.19 as determined by GPC. Polyvinyl acetal: structural parameters x≈0.19, y≈0.8, z≈0.01, with a number-average molecular weight (Mn) of 45,000–50,000 g / mol.

[0025] In all the following embodiments, the only variable in the preparation process of the highly stain-resistant polyvinyl acetal film is the amount of fluorinated surfactant added (as a percentage by mass relative to the PVB resin), and all other steps are the same as below (hereinafter referred to as the basic preparation steps): S1 Weigh 12.25 g of polyvinyl butyral (PVB) resin, place the polyvinyl butyral resin in a clean container, add 100 mL of N,N-dimethylformamide solvent, and mechanically stir at room temperature for 4 hours to completely dissolve the resin, obtaining a clear PVB resin solution.

[0026] S2. Add a fluorinated surfactant (the amount added varies depending on the specific embodiment) to the resin solution, stir and mix evenly to obtain a film solution; S3 filters the membrane solution through a 0.22 μm polytetrafluoroethylene needle filter, and then spin-coates the membrane solution onto a clean 6-inch silicon wafer using a spin coater with a program of 500 rpm / 5 seconds (spreading) followed by 2800 rpm / 60 seconds (film formation); the coating amount is 2-6 ml, preferably 4 ml.

[0027] The coated substrate was then left to stand in a Class 100 clean environment at room temperature (23 ± 2℃, relative humidity <30%, without forced convection to avoid airflow disturbance affecting surfactant migration and surface layer formation) for at least 12 hours. The solvent (DMF) was gradually removed by slow evaporation, allowing sufficient time for the fluorinated surfactant to migrate and spontaneously accumulate at the film-air interface, forming a dense, low surface energy layer. After the solvent had completely evaporated, the film was carefully peeled off from the silicon wafer, yielding a film with a thickness of approximately 1.8 ± 0.1 μm.

[0028] Example 1: A method for preparing a highly stain-resistant polyvinyl acetal film, which is prepared by following the above-described standard preparation steps. Specifically, in step S2, 0.1225 mg of a fluorinated surfactant is added to the resin solution (the amount of fluorinated surfactant added is 0.001% of the weight of the polyvinyl acetal resin).

[0029] Example 2: A method for preparing a highly stain-resistant polyvinyl acetal film, which is prepared using the above-described standard preparation steps. Specifically, in step S2, 0.3675 mg of a fluorinated surfactant is added to the resin solution (the amount of fluorinated surfactant added is 0.003% of the weight of the polyvinyl acetal resin).

[0030] Example 3: A method for preparing a highly stain-resistant polyvinyl acetal film, which is prepared using the above-described standard preparation steps. Specifically, in step S2, 0.6125 mg of a fluorinated surfactant is added to the resin solution (the amount of fluorinated surfactant added is 0.005% of the weight of the polyvinyl acetal resin).

[0031] Example 4: A method for preparing a highly stain-resistant polyvinyl acetal film, which is prepared using the above-described standard preparation steps. Specifically, in step S2, 0.9800 mg of a fluorinated surfactant is added to the resin solution (the amount of fluorinated surfactant added is 0.008% of the weight of the polyvinyl acetal resin).

[0032] Example 5: A method for preparing a highly stain-resistant polyvinyl acetal film, which is prepared using the above-described standard preparation steps. Specifically, in step S2, 1.2250 mg of a fluorinated surfactant is added to the resin solution (the amount of fluorinated surfactant added is 0.010% of the weight of the polyvinyl acetal resin).

[0033] Example 6: A method for preparing a highly stain-resistant polyvinyl acetal film, which is prepared using the above-described standard preparation steps. Specifically, in step S2, 6.125 mg of a fluorinated surfactant is added to the resin solution (the amount of fluorinated surfactant added is 0.050% of the weight of the polyvinyl acetal resin).

[0034] Comparative Example 1: A method for preparing a polyvinyl acetal film. Compared with Examples 1 to 4, step S2 is omitted in this example, i.e., no fluorinated surfactant is added. The specific preparation steps are as follows: S1 Weigh 12.25 g of polyvinyl butyral (PVB) resin and place it in a clean container. Add 100 mL of N,N-dimethylformamide solvent and stir mechanically at room temperature for 4 hours to completely dissolve the resin, obtaining a clear PVB resin solution.

[0035] S2 filters the PVB resin solution through a 0.22 μm polytetrafluoroethylene needle filter, and then spin coats the film solution onto a clean 6-inch silicon wafer using a spin coater with a program of 500 rpm / 5 seconds (spreading) followed by 2800 rpm / 60 seconds (film formation). The coated substrate was then left to stand at room temperature in a Class 100 clean environment for more than 12 hours until the solvent had completely evaporated. The film was then carefully peeled off from the silicon wafer to obtain a film with a thickness of approximately 1.8 ± 0.1 μm.

[0036] The properties of the polyvinyl acetal films prepared in Examples 1 to 6 and Comparative Example 1 were characterized, and the characterization results are shown in Table 1.

[0037] The following are the testing standards or methods for characterizing the performance of the polyvinyl acetal films prepared in Examples 1-6 and Comparative Example 1.

[0038] 1. UV-Vis transmittance test (254nm): Tested according to GB / T 2410-2008 "Determination of transmittance and haze of transparent plastics" standard, using a UV-Vis spectrophotometer. 2. The haze test shall be conducted in accordance with the procedures in GB / T 2410-2008 "Determination of transmittance and haze of transparent plastics", using a haze meter. 3. The water contact angle test was conducted according to the method in GB / T 30693-2014 "Measurement of the contact angle between plastic films and water". The instrument used was a contact angle measuring instrument. 4. The surface energy testing method is the OWRK two-liquid method (Owens-Wendt-Rabel-Kaelble method). Based on the measured contact angles of water and diiodomethane, the polar and dispersive components of the solid surface are solved through a system of equations. The specific procedures are: a) measuring the contact angle of water; b) measuring the contact angle of diiodomethane; c) calculating the surface energy. 5. Test on the change in light transmittance after DOP contamination: The test steps are as follows: Initial transmittance measurement: The initial transmittance of the thin film sample at 254 nm was measured using the method described above and denoted as T0.

[0039] Contamination exposure: Add approximately 50 mL of dioctyl phthalate (DOP) to the bottom of the desiccator (to a height of approximately 1 cm). Suspend the film sample inside the desiccator, ensuring the sample does not touch the liquid surface. Seal the desiccator and incubate at 25 ± 1°C for 24 hours.

[0040] Post-exposure measurement: After removing the sample and allowing it to stabilize at room temperature for 30 minutes, without cleaning the surface, immediately measure the transmittance at 254 nm using the same method, and record it as T1.

[0041] Data processing: ΔT (%) = T1 - T0. Three parallel samples were tested for each sample, and the average value was taken.

[0042] 6. Tensile strength: The test shall be conducted in accordance with GB / T 1040.3-2006 "Determination of tensile properties of plastics". The instrument used is a universal testing machine.

[0043] Table 1: Performance of Polyvinyl Acetal Film As can be seen from the table above, the contact angles of Examples 1 to 6 are significantly higher than those of Comparative Example 1. Moreover, with the increase of FC-4432 addition, the surface energy of the film is significantly reduced and the water contact angle is significantly increased. This shows that the fluorinated surfactant can migrate to the surface of the film during the film formation process, forming a low surface energy layer and giving the film excellent hydrophobicity. Compared to Comparative Example 1, the antifouling performance of Examples 1 to 4 was significantly improved. Furthermore, with increasing FC-4432 content, the absolute value of the transmittance change (ΔT) after DOP contamination decreased, indicating enhanced antifouling performance. This demonstrates that the introduction of fluorinated surfactants makes it difficult for oily contaminants (DOP) to adhere to the film surface, thereby reducing the decrease in transmittance caused by contamination. However, in Examples 5 and 6, with further increases in FC-4432 content, the change in the absolute value of the transmittance change (ΔT) after DOP contamination was very small, indicating that the effect of FC-4432 content exceeding 0.1% on further improving the antifouling performance of the film is limited.

[0044] In Examples 1 to 5, with the increase of the amount of fluorinated surfactant added, the transmittance at 254 nm decreased slightly, and the haze increased slightly. This may be due to slight scattering caused by the trace aggregation of the additive inside the film or changes in the surface microstructure. However, within the addition range of 0.001% to 0.010%, the transmittance remained above 98.90%, and the haze was below 0.17%, indicating excellent optical performance that met the high transmittance requirements of the photolithography protective film. In Example 6, when the amount of fluorinated surfactant added was 0.5%, the transmittance of the film decreased to 98.5%, and the haze increased significantly.

[0045] In Examples 1 to 6, the tensile strength decreased slightly with increasing dosage. This is likely because the addition of the fluorinated surfactant may weaken the intermolecular forces of the polymer to some extent, leading to a slight decrease in mechanical strength. However, within the dosage range of 0.001% to 0.010%, the tensile strength remained above 66.5 MPa, indicating good mechanical properties.

[0046] In Example 6, the surface energy and contact angle of the film prepared reached their optimal levels, but the decrease in optical properties (transmittance and haze) and mechanical properties was significantly increased. This indicates that while excessive addition can further reduce surface energy, it sacrifices the optical clarity and mechanical strength of the film. Therefore, the addition of the fluorinated surfactant FC-4432 can significantly improve the hydrophobicity and antifouling properties of the film. However, considering both the improvement in surface properties and the preservation of optical and mechanical properties, the optimal addition range for FC-4432 is 0.001 wt% to 0.010 wt%. This range achieves significant surface modification while maximizing the preservation of the film's optical and mechanical properties. Example 3, in particular, better preserves optical and mechanical properties with minimal sacrifice in antifouling effect and surface properties, achieving the best balance between significantly improved antifouling performance and near-perfect core optical and mechanical properties. Therefore, the optimal addition amount of FC-4432 is considered the optimal implementation.

[0047] This invention selects polyvinyl butyral (PVB) resin with a specific structural ratio as the matrix material to prepare a thin film that meets the stringent requirements of photolithography protective film. The hydroxyl groups in the molecular chain are strongly polar groups, providing polarity and hydrogen bonding, which affect solubility, compatibility with solvents, and cohesive strength of the film. A suitable hydroxyl content ensures that PVB resin can dissolve in polar aprotic solvents (such as DMF and diethylene glycol dimethyl ether), while also giving the film moderate hydrophilicity. Hydrogen bonds can form between the hydroxyl groups, which is beneficial for improving the film's mechanical strength, modulus, and heat resistance. If the x value is too low, the resin's solubility deteriorates, making it impossible to form a uniform film solution and negatively impacting the film's mechanical and thermal properties. However, an excessively high hydroxyl content makes the film hygroscopic, affecting dimensional stability. When PVB resins with different hydroxyl compositions are dissolved in DMF organic solvent, if x < 0.18, the resin lacks sufficient polarity, making it difficult to dissolve in polar solvents like DMF and preventing the preparation of a uniform solution. If x > 0.22, there are too many hydroxyl groups, increasing the film's hydrophilicity and making it prone to hygroscopic absorption, leading to dimensional instability.

[0048] The butyral group in the molecular chain is a characteristic structural unit and main part of PVB resin, determining the material's basic flexibility, film-forming properties, and thermoplasticity. A high y value ensures that the film has good mechanical toughness, impact resistance, and a low glass transition temperature, allowing it to be tensioned on the frame without easily cracking. When y ≤ 0.75, the degree of butyralization is insufficient, and the film is brittle (low elongation at break). When y > 0.80, the degree of butyralization is too high, the film strength decreases, and it cannot withstand the tension force in the lithography machine. This indicates that increasing the acetal group content can improve flexibility.

[0049] The carbonyl group in the acetyl group of the molecular chain has a strong absorption peak in the far ultraviolet region (especially 200-300 nm). Inappropriate content will affect its application in deep ultraviolet lithography. Its content needs to ensure that the protective film has extremely high ultraviolet transmittance (>99%). When z ≥ 0.03, the transmittance begins to fall below 99%. Although the acetyl content can enhance the water resistance of the PVB film, it will impair its ultraviolet transmittance. When z < 0.01, although the transmittance is higher, the industrial purification cost increases sharply, reducing the cost-effectiveness.

[0050] The present invention ensures that the prepared membrane has excellent flexibility and film-forming properties, while having high ultraviolet transmittance, good solubility and mechanical properties, by selecting a specific structural ratio of polyvinyl butyral (PVB).

[0051] The present invention and its embodiments have been described above. This description is not restrictive, and the embodiments shown are only one of the embodiments of the present invention. The actual structure is not limited to this. In conclusion, if those skilled in the art are inspired by this description and design similar structures and embodiments without departing from the spirit of the present invention, they should all fall within the protection scope of the present invention.

Claims

1. A highly stain-resistant polyvinyl acetal film, characterized in that, It is made from the following raw materials: Polyvinyl acetal resin and fluorinated surfactants; The polyvinyl acetal resin consists of the following repeating units. composition: (I)； (II); (III); The structural formula of the polyvinyl acetal resin is shown below: (IV); In Formula IV, x represents the molar percentage of repeating unit I, y represents the molar percentage of repeating unit II, and z represents the molar percentage of repeating unit III, x+y+z=1, where 0.18≤x≤0.22, 0.75<y≤0.80, and 0.01≤z<0.

025. The fluorinated surfactant is selected as a nonionic polymeric fluorocarbon surfactant, and the amount of the fluorinated surfactant is 0.001% to 0.01% of the weight of the polyvinyl acetal resin.

2. The highly stain-resistant polyvinyl alcohol acetal film according to claim 1, characterized in that, The average number-average molecular weight (Mn) of the fluorinated surfactant is 2,000-10,000 g / mol.

3. A method for preparing a highly stain-resistant polyvinyl acetal film according to claim 1 or 2, characterized in that, Includes the following steps: S1 dissolves polyvinyl acetal resin in a solvent to prepare a resin solution; S2. A fluorinated surfactant is added to the resin solution and stirred until homogeneous to obtain a film solution. After coating the substrate with the film solution, S3 allows it to stand and dry for at least 12 hours before peeling it off from the substrate to obtain a high-fouling-resistant polyvinyl alcohol acetal film.

4. The method for preparing a highly stain-resistant polyvinyl acetal film according to claim 3, characterized in that, The solvent used in step S1 is an amide solvent, an ether solvent, or a mixture thereof.

5. The method for preparing a highly stain-resistant polyvinyl acetal film according to claim 4, characterized in that, The solvent used in step S1 is N,N-dimethylformamide, diethylene glycol dimethyl ether, or a mixture thereof.

6. A method for preparing a highly stain-resistant polyvinyl acetal film according to claim 4 or 5, characterized in that, The concentration of polyvinyl acetal resin in the resin solution of step S1 is 5-30 wt%.

7. The method for preparing a highly stain-resistant polyvinyl acetal film according to claim 3, characterized in that, Step S3 involves allowing the mixture to stand and dry at 20-30°C.

8. The method for preparing a highly stain-resistant polyvinyl acetal film according to claim 3, characterized in that, Step S3 involves spin coating the film solution onto the substrate.

9. The application of the high-fouling-resistant polyvinyl alcohol acetal film prepared by the preparation method of the high-fouling-resistant polyvinyl alcohol acetal film according to claim 1 or 2, or the high-fouling-resistant polyvinyl alcohol acetal film according to claims 3 to 8, in the preparation of photolithographic protective films for protecting photomasks.

10. The application according to claim 9, characterized in that, The thickness of the highly stain-resistant polyvinyl acetal film is 1.2–2.0 μm.

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