A polyimide film, a method for preparing the same, and an application thereof

Abstract

The application discloses a polyimide film and a preparation method and application thereof. The preparation method comprises the following steps: carrying out a polycondensation reaction on diamine monomers and dianhydride monomers in an organic solvent to obtain a polyamide acid solution; adding an aqueous solution of tetramethylammonium hydroxide as a low-temperature imidization promoter to obtain a precursor solution; coating the precursor solution on a substrate and carrying out a thermal imidization treatment to obtain the polyimide film. The aqueous solution of tetramethylammonium hydroxide has strong electron-donating ability, small steric hindrance and low boiling point, can be completely volatilized at about 200 DEG C after playing a low-temperature imidization promoting role, and can make the obtained polyimide after imidization maintain good mechanical properties and dielectric properties, and the product performance is basically the same as that of intrinsic polyimide obtained by high-temperature imidization at 350 DEG C. The low-temperature imidization promoter can be used in various polyimide systems, and the low-temperature imidization method for preparing the polyimide is universal and can be applied to the preparation of various polyimides.

Description

Technical Field

[0001] This invention relates to the field of polyimide materials technology, and in particular to a polyimide film, its preparation method, and its application. Background Technology

[0002] Polyimide (PI) is a polymer material whose main chain contains repeating units of imide rings. Its highly conjugated structure and high aromaticity give it excellent comprehensive properties, such as excellent high and low temperature resistance, mechanical properties, electrical insulation properties, and chemical stability. It can be used for a long time in a temperature range of -200℃ to 260℃, making it one of the polymer materials with the best comprehensive performance.

[0003] Given its excellent thermal stability, mechanical properties, chemical stability, and electrical insulation, polyimide (PI) is widely used for electrical insulation in harsh environments such as railway locomotive traction and oil spill response. It is also applied in high-temperature cables, electronic products, nuclear power plants, solar photovoltaic and wind power, as well as national defense, nuclear energy, and aerospace technology. With technological advancements, PI films, in addition to meeting the physical property requirements of various products, possess special properties such as high strength, high toughness, wear resistance, high temperature resistance, and corrosion resistance, meeting the design requirements of being lightweight, thin, short, small, and highly reliable. In recent years, it has found even wider applications in microelectronics manufacturing and packaging.

[0004] The synthesis of polyimide generally involves reacting diamine and dianhydride to generate polyamic acid, a polyimide precursor, which is then imidized at 350°C to form a stable polyimide. However, high-temperature imidization limits its application in semiconductor devices. For example, when used as an alignment film in a color liquid crystal display containing thin-film transistors (TFTs), excessively high processing temperatures may cause the color filter to decolorize, resulting in the TFT losing its function. In wafer-level packaging, high temperatures may cause warping and chip damage due to the mismatch in thermal expansion coefficients between the polyimide (PI) and epoxy molding compound. The solder joints of low-melting-point solder in plastic-encapsulated circuits may also experience cracking, detachment, and recrystallization, severely damaging the performance of the plastic-encapsulated devices.

[0005] Lowering the imidization temperature of polyimides is generally controlled by adjusting the molecular composition of the polyimide, the synthesis process, or by adding low-temperature imidization accelerators. Increasing the flexibility of the molecular chain can lower the imidization temperature, but this may negatively impact its mechanical and thermal properties. Low-temperature imidization can also be achieved through one-step solution polymerization, but the commonly used high-boiling-point solvents often contain carcinogens, which is detrimental to large-scale production of films or fibers. Currently, the common method is to add low-temperature imidization accelerators to promote low-temperature imidization. The advantage of this method is its wide applicability in different polyimide systems, which is significant for improving the overall imidization efficiency of polyimide materials and reducing energy consumption.

[0006] Patent application CN102336910A discloses a low-temperature imidized polyimide resin and its preparation method. This method involves dissolving diamine and dianhydride in a specific low-boiling-point solvent (130-180℃) and catalyzing the reaction with a specific low-boiling-point catalyst (60-120℃) to convert the diamine and dianhydride into polyimide at a lower temperature, thus achieving low-temperature imidization. However, this method directly generates polyimide without producing polyamic acid intermediates, making it difficult to apply in the field of photosensitive polyimides. Patent application CN109946925A discloses the application of a compound in promoting the low-temperature imidization of polyimide. This compound is a nitrogen heterocyclic compound, an amino acid, or an aromatic compound containing a nitrogen, carbonyl, or at least two hydroxyl groups outside the ring, capable of forming a complex with hydrogen ions. The polyimide prepared by this method has a low imidization temperature, but the amount of imidization accelerator added is relatively large, which easily leads to imidization accelerator residue and the risk of continuous volatilization. Furthermore, the tensile strength of the prepared polyimide film is only up to 75% of that of intrinsic polyimide. Patent application CN112321827A discloses the application of a compound as a polyimide imidization accelerator, a polyimide precursor, its preparation method, and its application. By adding 5-aminobenzimidazole, the imidization reaction is promoted, and it also acts as a capping agent, preventing volatilization in subsequent operations, thereby optimizing the application of polyimide. However, the resulting film samples still show differences in mechanical properties compared to intrinsic polyimide.

[0007] As mentioned above, some existing methods for promoting the low-temperature imidization of polyimide materials by adding low-temperature imidization accelerators have limited application ranges, while others show a significant reduction in performance (especially tensile properties) compared to high-temperature imidized products. To address these shortcomings, there is an urgent need to research and develop a universal method for preparing polyimide by low-temperature imidization, which can maintain the excellent comprehensive properties of polyimide while reducing the imidization temperature. Summary of the Invention

[0008] This invention aims to solve at least one of the technical problems existing in the prior art. To this end, this invention proposes a polyimide film, its preparation method, and its applications.

[0009] In a first aspect, the present invention provides a method for preparing a polyimide film, comprising the following steps:

[0010] S1. A polyamic acid solution is prepared by polycondensation of diamine monomer and dianhydride monomer in an organic solvent.

[0011] S2. A low-temperature imidization accelerator is added to the polyamic acid solution to obtain a precursor solution; the low-temperature imidization accelerator is an aqueous solution of tetramethylammonium hydroxide;

[0012] S3. The precursor solution is coated onto the substrate and subjected to thermal imidization treatment to obtain a polyimide film.

[0013] The method for preparing polyimide films according to embodiments of the present invention has at least the following beneficial effects: The method uses tetramethylammonium hydroxide aqueous solution as a low-temperature imidization accelerator, which has the characteristics of strong electron-donating ability, low steric hindrance, and low boiling point. After exerting its low-temperature imidization accelerator effect, it can completely volatilize at around 200°C. The polyimide obtained after imidization can maintain good mechanical properties (especially tensile properties), dielectric properties, and thermal properties. The product performance is basically the same as that of intrinsic polyimide obtained by high-temperature imidization at 350°C, exhibiting excellent overall performance. Furthermore, this low-temperature imidization accelerator can be used in various polyimide systems, thus proving that this method for preparing polyimide by low-temperature imidization has universality and can be applied to the preparation of various polyimides.

[0014] Among them, the low-temperature imidization accelerator tetramethylammonium hydroxide aqueous solution promotes low-temperature imidization. Specifically, tetramethylammonium hydroxide ionizes in water to release nucleophilic ions, which attack the hydrogen atoms on the amide bond. This causes the hydroxyl group to transfer and capture the hydrogen on the amide bond, accelerating the ring-closure dehydration process, promoting the occurrence of the low-temperature imidization reaction, and increasing the degree of imidization. Furthermore, it has low steric hindrance and strong water absorption, allowing it to react rapidly in the imidization reaction, promoting the formation of acyl cations, and absorbing the small water molecules removed during imidization, thus promoting the forward reaction. In addition, because the boiling point of tetramethylammonium hydroxide aqueous solution is low (approximately 110°C), it is easily decomposed during thermal imidization treatment (such as gradient heating to 200°C), and the decomposition products can escape from the reaction system. Therefore, the addition of the above-mentioned low-temperature imidization accelerator has a shorter time to affect the polyimide (PI) molecules, reducing the damage to the originally neatly arranged molecular chains of PI, thereby reducing the impact on the performance of PI film products.

[0015] In some embodiments of the present invention, in step S2, the molar fraction of tetramethylammonium hydroxide in the precursor solution is 12.5% ​​to 17.5% of the polyamic acid, preferably 15%. At this relatively small addition amount, tetramethylammonium hydroxide can promote low-temperature imidization. After exerting its effect, it is essentially removed at 200°C without affecting the subsequent material properties, making the product properties essentially identical to those of intrinsic polyimide obtained by high-temperature (350°C) imidization.

[0016] In some embodiments of the present invention, in step S2, the molar ratio of tetramethylammonium hydroxide to solvent water in the tetramethylammonium hydroxide solution is 1:(1.5-3); for example, the molar ratio of tetramethylammonium hydroxide to solvent water in the tetramethylammonium hydroxide solution is 1:1.5, 1:2, 1:2.5 or 1:3, preferably 1:2. The tetramethylammonium hydroxide aqueous solution can be prepared by mixing tetramethylammonium hydroxide with solvent water and then dissolving it by ultrasonic stirring.

[0017] In some embodiments of the present invention, in step S2, after the low-temperature imidization accelerator is added to the polyamic acid solution, it is stirred until it is uniformly mixed; the stirring can be done by magnetic stirring or mechanical stirring, and the stirring time can be controlled to be more than 1 hour to ensure uniform mixing.

[0018] In some embodiments of the present invention, in step S3, the thermal imidization treatment is carried out in an oxygen atmosphere.

[0019] In some embodiments of the present invention, the thermal imidization treatment is a gradient temperature rise heat treatment.

[0020] In some embodiments of the present invention, the highest temperature of the gradient heating heat treatment is 195 to 205°C, for example, the highest temperature of the gradient heating heat treatment is 200°C.

[0021] In some embodiments of the present invention, the gradient heating heat treatment is performed by first heating from room temperature to 75-85°C and holding for 50-70 minutes; then heating to 145-155°C and holding for 50-70 minutes; then heating to 175-185°C and holding for 50-70 minutes; and finally heating to 195-205°C and holding for 20-30 minutes. Preferably, the heating rate is 3-5°C / min.

[0022] In some embodiments of the present invention, in step S3, before coating the precursor solution onto the substrate, the precursor solution is first subjected to a defoaming treatment. Specifically, the defoaming treatment can be performed in a vacuum drying oven; the defoaming time can be controlled between 0.5 and 1.5 hours, preferably 1 hour. The precursor solution is coated onto the substrate by means of coating or other methods.

[0023] In some embodiments of the present invention, step S3, after the thermal imidization treatment, further includes peeling the polyimide film off the substrate. Specifically, the substrate with the polyimide film on its surface can be immersed together in deionized water at 80-90°C to detach the polyimide film from the substrate, and then the polyimide film can be dried.

[0024] In some embodiments of the present invention, in step S1, the polycondensation reaction is carried out in an ice-water bath.

[0025] In some embodiments of the present invention, step S1 includes: dissolving the diamine monomer in an organic solvent under ice-water bath conditions, and then adding the dianhydride monomer in portions to carry out a polycondensation reaction. Specifically, the diamine monomer can be dissolved in the organic solvent under ice-water bath conditions by mechanical stirring; the dianhydride monomer can be divided into multiple equal portions (e.g., 3 portions, 4 portions, etc.), with each portion added at intervals of 10-20 minutes. The molar ratio of the diamine monomer to the dianhydride monomer is controlled at 1:1.01-1.02; the polycondensation reaction time is generally controlled at 5-7 hours to ensure uniform solution viscosity.

[0026] In some embodiments of the present invention, in step S1, the diamine monomer is selected from at least one of 4,4'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 4,4'-diaminobiphenyl, p-phenylenediamine, m-phenylenediamine, 1,4-bis(4-aminophenoxy)-2-tert-butylbenzene, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenyl sulfide, and 2,2-bis(4-aminophenyl)hexafluoropropane.

[0027] In some embodiments of the present invention, the dianhydride monomer is selected from at least one of 1,2,4,5-pyromellitic dianhydride, 2,3,3',4'-diphenyl ether tetracarboxylic acid dianhydride, 3,3',4,4'-biphenyltetracarboxylic acid dianhydride, 3,3',4,4'-diphenyl ether tetracarboxylic acid dianhydride, and 3,3',4,4'-benzophenone tetracarboxylic acid dianhydride.

[0028] In some embodiments of the present invention, the organic solvent is selected from at least one of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide.

[0029] In some embodiments of the present invention, in step S1, the mass concentration of the polyamic acid solution is 10% to 20%.

[0030] In some embodiments of the present invention, step S2 includes: adding a photosensitive compound and a low-temperature imidization accelerator to a polyamic acid solution to obtain a precursor solution; the polyimide film obtained in step S3 is a photosensitive polyimide film.

[0031] In a second aspect, the present invention provides a polyimide film prepared by any of the polyimide film preparation methods proposed in the first aspect of the present invention.

[0032] In a third aspect, the invention provides the application of one or more methods for preparing polyimide films in the fabrication of semiconductor devices. Attached Figure Description

[0033] The present invention will be further described below with reference to the accompanying drawings and embodiments, wherein:

[0034] Figure 1 Infrared spectra of the polyimide films prepared in Example 1 and Comparative Examples 1, 2, and 13;

[0035] Figure 2 The graph shows a comparison of the tensile strength test results of the polyimide films prepared in Examples 1 and 2 and Comparative Examples 1, 2, 4, 6, and 9.

[0036] Figure 3 The graph shows a comparison of the dielectric strength test results of the polyimide films prepared in Examples 1 and 2 and Comparative Examples 1, 2, 4, 6, and 9. Detailed Implementation

[0037] The following will describe the concept and technical effects of the present invention clearly and completely with reference to embodiments, so as to fully understand the purpose, features and effects of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are all within the scope of protection of the present invention.

[0038] Example 1

[0039] This embodiment prepares a polyimide film, and the specific preparation method is as follows:

[0040] S1. Under ice-water bath conditions, 0.8009 g (0.004 mol) of ODA (4,4'-diaminodiphenyl ether) was dissolved in 11.2738 g of DMF (dimethylformamide). After being mechanically stirred and mixed evenly, 1.1886 g (0.004 mol) of BPDA (3,3',4,4'-biphenyltetracarboxylic acid dianhydride) was added to the solution in three portions at 15 min intervals to carry out the polycondensation reaction. The reaction was stopped after 6 h to ensure that the solution viscosity was uniform, and a polyamic acid solution was obtained.

[0041] S2. Dissolve 0.1086 g (0.0006 mol) of tetramethylammonium hydroxide in 0.0216 g (0.0012 mol) of water and stir evenly under ultrasonic conditions to obtain a tetramethylammonium hydroxide solution as a low-temperature imidization accelerator, wherein the molar ratio of tetramethylammonium hydroxide to water is 1:2; then mix the low-temperature imidization accelerator with the polyamic acid solution prepared in step S1 and stir magnetically for 2 h at room temperature to obtain a precursor solution, wherein the molar fraction of tetramethylammonium hydroxide in the precursor solution is 15% of the polyamic acid;

[0042] S3. The precursor solution obtained in step S2 is placed in a vacuum drying oven for defoaming treatment for 1 hour. Then, the defoamed precursor solution is coated onto a substrate glass plate with a coating thickness of 400 μm, and then placed in a muffle furnace containing an oxygen atmosphere for thermal imidization treatment. The thermal imidization process employs a gradient heating technique. First, the temperature is raised from room temperature to 80°C at a rate of 5°C / min, and held at 80°C for 60 minutes. Then, the temperature is raised from 80°C to 150°C at a rate of 5°C / min, and held at 150°C for 60 minutes. Next, the temperature is raised from 150°C to 180°C at a rate of 5°C / min, and held at 180°C for 60 minutes. Finally, the temperature is raised from 180°C to a maximum of 200°C and held at a rate of 3°C / min for 30 minutes, ensuring complete imidization of the polyimide film. After completing the above thermal imidization treatment, the polyimide film, i.e. the substrate glass sheet, is cooled to room temperature in a muffle furnace. Then, it is immersed in deionized water heated to 90°C to detach the polyimide film from the substrate glass sheet. After drying, a polyimide film is obtained, which is light yellow, 10cm×8cm in size, and 20μm thick, and is denoted as PI-T15-2-200.

[0043] Example 2

[0044] This embodiment prepared a polyimide film. The difference between this embodiment and Example 1 is that in step S2, 0.1086 g (0.0006 mol) of tetramethylammonium hydroxide was dissolved in 0.0324 g (0.0018 mol) of water and stirred evenly under ultrasonic conditions to obtain a tetramethylammonium hydroxide solution with a tetramethylammonium hydroxide to water molar ratio of 1:3. This solution was used instead of the tetramethylammonium hydroxide to water molar ratio of 1:2 obtained in step S2 of Example 1 as a low-temperature imidization promoter. It was mixed with the polyamic acid solution obtained in step S1 and magnetically stirred at room temperature for 2 hours to obtain a precursor solution with a tetramethylammonium hydroxide molar fraction of 15% in polyamic acid. Other operations were the same as in Example 1 to obtain a polyimide film, which was light yellow, 10 cm × 8 cm in size, and 20 μm thick, denoted as PI-T15-3-200.

[0045] Comparative Example 1

[0046] This comparative example prepared a polyimide film. The difference between this comparative example and Example 1 is that the operation of step S2 in Example 1 is omitted in this comparative example. After obtaining the polyamic acid solution according to the operation of step S1 in Example 1, it is used as the precursor solution. No low-temperature imidization accelerator is added, and then the operation of step S3 in Example 1 is directly performed to obtain the polyimide film (denoted as PI-200).

[0047] Comparative Example 2

[0048] This comparative example prepared a polyimide film. The difference between this comparative example and Example 1 is that step S2 in Example 1 is omitted. After preparing the polyamic acid solution according to step S1 in Example 1, it is used as the precursor solution without adding a low-temperature imidization accelerator. Furthermore, in step S3, the thermal imidization treatment employs a gradient heating process: first, the temperature is raised from room temperature to 80°C at a rate of 5°C / min, and then held at 80°C for 60 min; subsequently, the temperature is raised from 80°C to 150°C at a rate of... The temperature was increased at a rate of 10°C / min, and the temperature was held at 150°C for 60 minutes. Then, the temperature was increased from 150°C to 250°C at a rate of 10°C / min, and the temperature was held at 250°C for 60 minutes. The temperature was then increased from 250°C to the maximum temperature of 350°C at a rate of 10°C / min, and the temperature was held at 350°C for 30 minutes to ensure complete curing of the polyimide film. Other operations were the same as in Example 1 to obtain a polyimide film (denoted as PI-350).

[0049] Example 3

[0050] This embodiment prepared a polyimide film. The difference between this embodiment and Example 1 is that in step S1, 0.8724 g (0.004 mol) of PMDA (1,2,4,5-pyromellitic dianhydride) was used instead of 1.1886 g (0.004 mol) of 3,3',4,4'-biphenyltetracarboxylic acid dianhydride in Example 1. Other operations were the same as in Example 1. A polyimide film was obtained, which was light yellow, 10 cm × 8 cm in size, and 20 μm thick, and was denoted as PI'-T15-2-200.

[0051] Comparative Example 3

[0052] This comparative example prepared a polyimide film. The difference between this comparative example and Example 3 is that: in this comparative example, step S2 in Example 3 is omitted; after preparing the polyamic acid solution according to step S1 in Example 3, it is used as the precursor solution without adding a low-temperature imidization accelerator; and in step S3, the thermal imidization treatment adopts a gradient heating process, first heating from room temperature to 80°C at a heating rate of 5°C / min, and holding at 80°C for 60min; then heating from 80°C to 150°C at a heating rate of 10°C / min. The temperature is increased at 10°C / min and held at 150°C for 60 minutes. Then, the temperature is increased from 150°C to 250°C at a rate of 10°C / min and held at 250°C for 60 minutes. Next, the temperature is increased from 250°C to the maximum temperature of 350°C at a rate of 10°C / min and held at 350°C for 30 minutes to ensure complete curing of the polyimide film. Other operations are the same as in Example 3 to obtain the polyimide film, which is the intrinsic polyimide film, denoted as PI'-350.

[0053] Comparative Example 4

[0054] This comparative example prepared a polyimide film. The difference between this comparative example and Example 1 is that in step S2, the amount of water used in the preparation of the tetramethylammonium hydroxide solution was adjusted to 0.0108 g (0.0006 mol), that is, the molar ratio of tetramethylammonium hydroxide to water in the prepared tetramethylammonium hydroxide solution was 1:1; other operations were the same as in Example 1, and a polyimide film was obtained, denoted as PI-T15-1-200.

[0055] Comparative Example 5

[0056] This comparative example prepared a polyimide film. The difference between this comparative example and Example 1 is that in step S2, the amount of tetramethylammonium hydroxide used in the preparation of the tetramethylammonium hydroxide solution was adjusted to 0.0724 g (0.0004 mol), and the amount of water was adjusted to 0.0072 g (0.0004 mol), that is, the molar ratio of tetramethylammonium hydroxide to water in the prepared tetramethylammonium hydroxide solution was 1:1. The molar fraction of tetramethylammonium hydroxide in the precursor solution obtained according to the method in Example 1 was 10% of the polyimide. Other operations were the same as in Example 1, and a polyimide film was obtained, denoted as PI-T10-1-200.

[0057] Comparative Example 6

[0058] This comparative example prepared a polyimide film. The difference between this comparative example and Comparative Example 5 is that in step S2, the amount of water used in the preparation of the tetramethylammonium hydroxide solution was adjusted to 0.0144 g (0.0008 mol), that is, the molar ratio of tetramethylammonium hydroxide to water in the prepared tetramethylammonium hydroxide solution was 1:2; other operations were the same as in Comparative Example 5, and a polyimide film was obtained, denoted as PI-T10-2-200.

[0059] Comparative Example 7

[0060] This embodiment prepared a polyimide film. The difference between this embodiment and Comparative Example 5 is that in step S2, the amount of water used in the preparation of the tetramethylammonium hydroxide solution was adjusted to 0.0216 g (0.0012 mol), that is, the molar ratio of tetramethylammonium hydroxide to water in the prepared tetramethylammonium hydroxide solution was 1:3; other operations were the same as in Comparative Example 5, and a polyimide film was obtained, denoted as PI-T10-3-200.

[0061] Example 4

[0062] This embodiment prepares a polyimide film. The difference between this embodiment and Example 1 is that in step S2, the amount of tetramethylammonium hydroxide used in the preparation of the tetramethylammonium hydroxide solution is adjusted to 0.0905 g (0.0005 mol), and the amount of water is adjusted to 0.0180 g (0.0010 mol). That is, the molar ratio of tetramethylammonium hydroxide to water in the prepared tetramethylammonium hydroxide solution is 1:2. The molar fraction of tetramethylammonium hydroxide in the precursor solution obtained according to the method in Example 1 is 12.5% ​​of the polyimide. Other operations are the same as in Example 1, and a polyimide film is obtained, denoted as PI-T12.5-2-200.

[0063] Example 5

[0064] This embodiment prepared a polyimide film. The difference between this embodiment and Example 1 is that in step S2, the amount of tetramethylammonium hydroxide used in the preparation of the tetramethylammonium hydroxide solution was adjusted to 0.1267 g (0.0007 mol), and the amount of water was adjusted to 0.0252 g (0.0014 mol). That is, the molar ratio of tetramethylammonium hydroxide to water in the prepared tetramethylammonium hydroxide solution was 1:2. The molar fraction of tetramethylammonium hydroxide in the precursor solution obtained according to the method in Example 1 was 17.5% of the polyimide. Other operations were the same as in Example 1, and a polyimide film was obtained, denoted as PI-T17.5-2-200.

[0065] Comparative Example 8

[0066] This comparative example prepared a polyimide film. The difference between this comparative example and Example 1 is that in step S2, the amount of tetramethylammonium hydroxide used in the preparation of the tetramethylammonium hydroxide solution was adjusted to 0.1448 g (0.0008 mol), and the amount of water was adjusted to 0.0144 g (0.0008 mol), that is, the molar ratio of tetramethylammonium hydroxide to water in the prepared tetramethylammonium hydroxide solution was 1:1. The molar fraction of tetramethylammonium hydroxide in the precursor solution obtained according to the method in Example 1 was 20% of the polyimide. Other operations were the same as in Example 1, and a polyimide film was obtained, denoted as PI-T20-1-200.

[0067] Comparative Example 9

[0068] This comparative example prepared a polyimide film. The difference between this comparative example and Comparative Example 8 is that in step S2, the amount of water used in the preparation of the tetramethylammonium hydroxide solution was adjusted to 0.0288 g (0.0016 mol), that is, the molar ratio of tetramethylammonium hydroxide to water in the prepared tetramethylammonium hydroxide solution was 1:2; other operations were the same as in Comparative Example 8, and a polyimide film was obtained, denoted as PI-T20-2-200.

[0069] Comparative Example 10

[0070] This comparative example prepared a polyimide film. The difference between this comparative example and Comparative Example 8 is that in step S2, the amount of water used in the preparation of the tetramethylammonium hydroxide solution was adjusted to 0.0432 g (0.0024 mol), that is, the molar ratio of tetramethylammonium hydroxide to water in the prepared tetramethylammonium hydroxide solution was 1:3; other operations were the same as in Comparative Example 8, and a polyimide film was obtained, denoted as PI-T20-3-200.

[0071] Comparative Example 11

[0072] This comparative example prepared a polyimide film. The difference between this comparative example and Example 1 is that in step S2, the amount of tetramethylammonium hydroxide used in the preparation of the tetramethylammonium hydroxide solution was adjusted to 0.1810 g (0.0010 mol), and the amount of water was adjusted to 0.0360 g (0.0020 mol). That is, the molar ratio of tetramethylammonium hydroxide to water in the prepared tetramethylammonium hydroxide solution was 1:2. The molar fraction of tetramethylammonium hydroxide in the precursor solution obtained according to the method in Example 1 was 25% of the polyimide. Other operations were the same as in Example 1, and a polyimide film was obtained, denoted as PI-T25-2-200.

[0073] Comparative Example 12

[0074] This comparative example prepared a polyimide film. The difference between this comparative example and Example 1 is that in this comparative example, step S2 uses an equal amount of tetrabutylammonium hydroxide instead of tetramethylammonium hydroxide used in Example 1. Other operations are the same as in Example 1. A polyimide film was obtained, denoted as PI”-T15-2-200.

[0075] Comparative Example 13

[0076] This comparative example prepared a polyimide film. The difference between this example and Example 1 is that in step S2, 0.1086 g (0.0006 mol) of tetramethylammonium hydroxide was directly used as a low-temperature imidization accelerator, that is, the molar ratio of tetramethylammonium hydroxide to water in the low-temperature imidization accelerator was 1:0; other operations were the same as in Example 1, and a polyimide film was obtained, denoted as PI-T15-0-200.

[0077] Performance testing

[0078] (I) Infrared Testing

[0079] Fourier transform infrared spectroscopy was used to analyze the polyimide films synthesized in Examples 1, 3, Comparative Examples 1-3, and Comparative Example 13. The scanning range was 4000 cm⁻¹. -1 ~400cm -1 The resolution is 4cm. -1 The results obtained are as follows: Figure 1 As shown. Due to the reaction of polyamic acid to form polyimide, approximately 1660 cm... -1 The characteristic absorption peak of C=O in the -CONH- region disappears at approximately 1378 cm⁻¹. -1 The tensile vibrations of the C–N ring in the imide ring can be observed at the point where the polyamic acid is imidized to obtain polyimide. Therefore, the degree of imidization, or the degree of imidization, can be expressed as D% = S%. 1378 / S 1500 ×100% yields, where S 1378S represents the tensile vibration of C–N in the imide ring. 1500 The representative peak is the absorption peak of the benzene ring, S. 1378 The size gradually increases as the reaction proceeds, S 1500 The magnitude remains constant during the reaction; therefore, the value of this ratio indicates the extent of the reaction. Furthermore, according to the degree of imidization D% = S 1378 / S 1500 Calculated by multiplying by 100%, the thermal imidization degrees of the polyimide films PI-200 (Comparative Example 1), PI-350 (Comparative Example 2), PI-T15-2-200 (Example 1), PI-T15-0-200 (Comparative Example 13), PI-T15-0-200 (Comparative Example 3), and 99.8% (Comparative Example 3) were 92.3%, 99.4%, 99.8%, 93.3%, 99.7%, and 99.8%, respectively. Therefore, it can be concluded that... It is known that Comparative Example 2, imidization at 350°C, can achieve near-complete imidization; in Example 1, tetramethylammonium hydroxide aqueous solution was added as a low-temperature imidization promoter, and complete imidization could be achieved at 200°C. This is mainly because the ions ionized from tetramethylammonium hydroxide in water can participate in the imidization reaction, thereby promoting the degree of reaction; while in Comparative Example 13, pure tetramethylammonium hydroxide was added, which could not ionize to produce the required ions to promote imidization, resulting in a relatively low degree of imidization.

[0080] (II) Tensile Test

[0081] The polyimide films prepared in Examples 1-5 and Comparative Examples 1-12 were cut into strips of 8cm × 2cm and subjected to tensile tests on a tensile testing machine at a tensile rate of 2mm / min. The test results are as follows. Figure 2 As shown in Table 1.

[0082] Table 1

[0083]

[0084]

[0085] Tests have shown that the tetramethylammonium hydroxide solution of a specific concentration used in this application, as a low-temperature imidization promoter, and mixed with polyamic acid in a specific ratio to prepare a precursor solution, allows tetramethylammonium hydroxide to ionize into nucleophilic ions in water, thereby promoting the low-temperature imidization reaction and achieving a higher degree of imidization. However, if the water content in the tetramethylammonium hydroxide solution is too high, it may disrupt the polyimide molecular chain arrangement and reduce tensile strength. Therefore, through research and experimentation, it has been determined that the molar ratio of tetramethylammonium hydroxide solution to water solvent can be controlled at 1:(1.5-3), and the molar fraction of tetramethylammonium hydroxide in the precursor solution of polyamic acid can be controlled at 12.5%-17.5%.

[0086] Furthermore, the tensile strength of the polyimide film prepared in Comparative Example 12 was 120±3MPa, which was inferior to that of Example 1. The main reason for its poor performance was that its chemical structure with greater steric hindrance gave it a strong steric hindrance effect, which was not conducive to the reaction. In addition, the reaction process required heating to about 200°C to achieve complete evaporation of the solvent and complete synthesis of the product. However, tetrabutylammonium hydroxide has a lower boiling point than tetramethylammonium hydroxide and can only undergo the imidization-promoting reaction at 100°C. After that, it could not continue to play a role due to evaporation, resulting in the polyimide product having lower performance than that of Example 1.

[0087] (ii) Dielectric strength

[0088] The dielectric strength of the polyimide films PI-T15-2-200, PI-T15-3-200, PI-200, PI-350, PI-T15-1-200, PI-T10-2-200, and PI-T20-2-200 prepared in Examples 1 and 2 and Comparative Examples 1, 2, 4, 6, and 9 was tested. Specifically, gold was deposited on the surface of the polyimide film to improve conductivity, with a deposition thickness of 120 nm. The dielectric strength was then tested using a dielectric strength tester. The results are as follows: Figure 3 As shown. By Figure 3 It is known that the polyimide films prepared in Examples 1 and 2 (especially Example 1) have good dielectric properties. Specifically, under the control of the raw material dosage, especially the molar ratio of tetramethylammonium hydroxide to solvent water in the tetramethylammonium hydroxide aqueous solution, and the molar fraction of tetramethylammonium hydroxide in polyamic acid in the precursor solution, the degree of imidization can be increased, the imidization ratio of polyamic acid increases, the polarity decreases, the dielectric constant decreases, and thus the dielectric properties are closer to those of the intrinsic polyimide film.

[0089] (III) Coefficient of thermal expansion

[0090] The coefficient of thermal expansion (CTE) is also considered the most reliable factor influencing chain orientation changes; generally, a decrease in CTE is accompanied by an increase in in-plane chain orientation. The inventors further tested the CTE of the polyimide films prepared in Example 1 and Comparative Example 2. Specifically, a thermomechanical analyzer was used to measure the CTE of the PI films. The test was conducted in stretching mode, with the temperature program set to increase from room temperature to 300°C at a rate of 5°C / min. Stretching was performed within the temperature range, and the tensile load was a fixed value of 0.2 N. The results showed that the CTE of PI-350 prepared in Comparative Example 2 was 32.9 ppm / K, while the CTE of PI-T15-2-200 prepared in Example 1 decreased to 20.9 ppm / K. This may be because the high-temperature imidization process for preparing polyimide films may involve side reactions such as crosslinking and isomer formation, or even reactions such as anhydride hydrolysis and molecular chain breakage, which negatively affect chain orientation and thus result in a higher CTE. In contrast, the CTE values ​​of the films prepared by adding tetramethylammonium hydroxide aqueous solution in Example 1 were all reduced. On the one hand, during the reaction at a lower temperature, the residual solvent in the film that did not evaporate in time can increase the mobility of molecular chains, which is conducive to the orderly arrangement of molecular chains and further promotes the formation of an ordered structure. At the same time, the added tetramethylammonium hydroxide aqueous solution can also promote the imidization reaction to proceed rapidly, thereby playing a positive role in the arrangement of molecular chains and in-plane orientation. Therefore, the reduction in CTE can also support the reason for the increase in tensile strength.

[0091] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A method for preparing a polyimide film, characterized in that, Includes the following steps: S1. A polyamic acid solution is prepared by polycondensation of diamine monomer and dianhydride monomer in an organic solvent. S2. A low-temperature imidization accelerator is added to the polyamic acid solution to obtain a precursor solution; the low-temperature imidization accelerator is an aqueous solution of tetramethylammonium hydroxide; the molar ratio of tetramethylammonium hydroxide to water in the aqueous solution of tetramethylammonium hydroxide is 1:(1.5~3), and the molar fraction of tetramethylammonium hydroxide in the precursor solution of polyamic acid is 12.5%~17.5%; S3. The precursor solution is coated onto the substrate and subjected to thermal imidization treatment to obtain a polyimide film.

2. The method for preparing the polyimide film according to claim 1, characterized in that, In step S3, the thermal imidization treatment is a gradient temperature rise heat treatment.

3. The method for preparing the polyimide film according to claim 2, characterized in that, The highest temperature of the gradient heating heat treatment is 195~205℃.

4. The method for preparing the polyimide film according to claim 3, characterized in that, The gradient heating heat treatment is as follows: first, the temperature is raised from room temperature to 75~85℃ and held for 50~70 min; then, the temperature is raised to 145~155℃ and held for 50~70 min; then, the temperature is raised to 175~185℃ and held for 50~70 min; finally, the temperature is raised to 195~205℃ and held for 20~30 min.

5. The method for preparing the polyimide film according to claim 4, characterized in that, The heating rate is 3~5℃ / min.

6. The method for preparing the polyimide film according to claim 2, characterized in that, In step S3, before coating the precursor solution onto the substrate, the precursor solution is first subjected to a defoaming treatment.

7. The method for preparing the polyimide film according to claim 1, characterized in that, In step S1, the polycondensation reaction is carried out in an ice-water bath.

8. The method for preparing the polyimide film according to claim 7, characterized in that, Step S1 includes: dissolving the diamine monomer in an organic solvent under ice-water bath conditions, and then adding the dianhydride monomer in portions to carry out a polycondensation reaction.

9. The method for preparing the polyimide film according to any one of claims 1 to 8, characterized in that, In step S1, the diamine monomer is selected from at least one of 4,4'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 4,4'-diaminobiphenyl, p-phenylenediamine, m-phenylenediamine, 1,4-bis(4-aminophenoxy)-2-tert-butylbenzene, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenyl sulfide, and 2,2-bis(4-aminophenyl)hexafluoropropane.

10. The method for preparing the polyimide film according to claim 9, characterized in that, The dianhydride monomer is selected from at least one of 1,2,4,5-pyromellitic dianhydride, 2,3,3',4'-diphenyl ether tetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride, and 3,3',4,4'-benzophenone tetracarboxylic dianhydride.

11. The method for preparing the polyimide film according to claim 10, characterized in that, The organic solvent is selected from at least one of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide.

12. The method for preparing the polyimide film according to claim 11, characterized in that, In step S1, the mass concentration of the polyamic acid solution is 10%~20%.

13. A polyimide film, characterized in that, The polyimide film is prepared by any one of claims 1 to 12.

14. The application of the method for preparing the polyimide film according to any one of claims 1 to 12 in the preparation of semiconductor devices.

Images