A low-temperature curing PI photoresist suitable for flexible substrates and a preparation method thereof

Abstract

The application discloses a low-temperature curing PI photoresist suitable for a flexible substrate and a preparation method thereof, and belongs to the technical field of semiconductor electronic chemicals and flexible display manufacturing. The method comprises the following steps: dissolving a soluble polyimide resin containing an allyl group in 2-acetoxy-1-methoxypropane, adding a photo-crosslinking component containing a mercapto group, a photoinitiating component and a heteroaromatic base type low-temperature curing accelerator to obtain a coating liquid; coating the coating liquid on a flexible substrate, and performing pre-baking, ultraviolet exposure, development and rinsing, and then performing post-curing treatment at 180 DEG C to 230 DEG C to obtain a patterned polyimide film layer. The application adopts a soluble resin in combination with a mercapto-alkene crosslinking system and a curing accelerator, avoids the high-temperature closed-loop requirement of a traditional PI precursor, realizes low-temperature curing characteristics, reduces the risk of thermal damage to the flexible substrate, and significantly improves the bending resistance and strain adaptability of the film layer.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor electronic chemicals and flexible display manufacturing technology. Specifically, it relates to a low-temperature curing PI photoresist suitable for flexible substrates and its preparation method. Background Technology

[0002] Polyimide (PI) is widely used in microelectronic packaging, flexible display substrates, and advanced photoresists due to its excellent heat resistance, mechanical strength, and electrical insulation properties. With the development of flexible optoelectronic manufacturing technology, the flexible substrates used (such as PET, PEN, or thin-layer flexible PI) are highly sensitive to heat, and the processing and final curing temperature of the photoresist must usually be controlled at 200°C or even lower.

[0003] However, through searching and comparing existing technologies, conventional photosensitive polyimide (PSPI) materials and their supporting processes have the following significant drawbacks when adapted to flexible substrates: Traditional PSPI is mostly coated in the form of precursors such as polyamic acid, and after exposure and development, it must undergo high-temperature heat treatment at 300°C to 350°C to complete the dehydration and imidization ring-closure. This harsh high-temperature condition far exceeds the heat tolerance limit of most flexible substrates (such as PET and PEN), which not only leads to thermal degradation and severe yellowing of the flexible substrate, but also causes great thermal stress due to the mismatch of the coefficient of thermal expansion (CTE), resulting in warping, breakage, or even complete delamination of the multilayer structure. To avoid the above-mentioned high-temperature ring-closure process, some existing technologies have turned to using pre-closed "soluble polyimide" directly as the film-forming matrix. However, due to the limited solubility of such rigid polymers, existing systems often rely heavily on high-boiling-point, highly polar solvents such as N-methyl-2-pyrrolidone (NMP, boiling point about 202°C) or dimethylacetamide (DMAc). When the post-curing temperature is forcibly lowered to below 200°C, these high-boiling-point solvents are extremely difficult to completely evaporate. A large amount of solvent remaining inside the film layer acts as a plasticizer, severely degrading the electrical insulation, hardness, and chemical resistance of the cured film, and causing the fine patterns to swell or collapse during subsequent processing. Existing PSPI solutions, which aim to lower the curing temperature, have achieved low-temperature film formation, but their polyimide backbone and photocrosslinking network typically maintain high rigidity. In the dynamic usage scenarios faced by flexible devices, such as repeated bending and curling, this high-rigidity crosslinking network lacks strain dissipation and buffering capabilities. The interface between the cured film and the flexible substrate is prone to severe stress concentration, leading to the initiation and propagation of microcracks in the film layer, ultimately causing large-area peeling of the patterned layer, failing to meet the reliability requirements of flexible electronic products.

[0004] In summary, how to break free from dependence on high-boiling-point polar solvents, achieve complete solvent removal and high-density cross-linking and densification simultaneously in a low-temperature range below 200℃, and ensure that the cured fine patterned film has excellent bending strain adaptability to flexible substrates, is a technical challenge that urgently needs to be overcome in the fields of semiconductor electronic chemicals and flexible display manufacturing. Summary of the Invention

[0005] To address key issues in existing technologies, such as the excessively high post-curing temperature of traditional photosensitive polyimide precursors leading to thermal damage to flexible substrates, the over-reliance of soluble polyimide systems on high-boiling-point solvents like NMP resulting in significant solvent residue during low-temperature processing, and the lack of strain adaptability of conventional rigid crosslinked networks under dynamic bending, which easily leads to pattern layer peeling, this invention provides a low-temperature curing PI photoresist suitable for flexible substrates and its preparation method.

[0006] The present invention adopts the following technical solution: a method for preparing a low-temperature curable PI photoresist suitable for flexible substrates, characterized in that, by weight, it includes the following steps: S1, under inert gas protection, 24.0-32.0 parts of soluble polyimide resin containing allyl structural units and 0-10.0 parts of soluble polyimide resin containing dimethylsiloxane flexible segments are added to 60.0-70.0 parts of 2-acetoxy-1-methoxypropane and stirred at 20-30°C to dissolve, obtaining a resin solution; S2, 1.5-3.0 parts of a mercapto photocrosslinking component, 0.5-1.2 parts of a free radical photoinitiating component, and 0.1-0 parts of other components are added to the resin solution. S1. Mix 6 parts of a heteroaryl base-type low-temperature curing accelerator under light-protected conditions, and after degassing and filtration, obtain a low-temperature curing PI photoresist coating solution; S2. Coat the low-temperature curing PI photoresist coating solution onto the surface of a flexible substrate, and after pre-baking, UV exposure, development, rinsing and drying, cure at 180-230°C for 0.5-2 hours in an inert gas environment to obtain a patterned low-temperature curing polyimide film layer; wherein, the mercapto-containing photocrosslinking component and the soluble polyimide resin containing allyl structural units undergo a mercapto-olefin radical addition reaction under UV exposure, and the heteroaryl base-type low-temperature curing accelerator is used to promote the stabilization of the crosslinking network and the removal of residual solvents in the post-curing stage.

[0007] Preferably, the soluble polyimide resin containing allyl structural units comprises 24.0 to 30.0 parts, and the soluble polyimide resin containing dimethylsiloxane flexible segments comprises 2.0 to 8.0 parts, which together constitute the soluble polyimide resin component.

[0008] Preferably, the soluble polyimide resin containing allyl structural units comprises aromatic structural units derived from magnolol and allyl structural units that can participate in free radical addition reactions, and the soluble polyimide resin is an imidized polyimide resin before exposure, rather than a polyamic acid precursor that requires high-temperature dehydration and ring closure.

[0009] Preferably, in the soluble polyimide resin containing dimethylsiloxane flexible segments, the dimethylsiloxane flexible segments are introduced as copolymer structural units or side chain connecting structural units in the polyimide backbone to reduce inter-chain stacking of polyimide and mitigate strain mismatch between the patterned low-temperature curing polyimide film and the flexible substrate.

[0010] Preferably, the mercapto-containing photocrosslinking component is pentaerythritol tetra(3-mercaptopropionic acid) ester, and the free radical photoinitiator is 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-1-butanone.

[0011] Preferably, the heteroaryl base type low-temperature curing accelerator is 2,2′-bipyridine, and its dosage is 0.2 to 0.4 parts, and the heteroaryl base type low-temperature curing accelerator does not replace the mercapto-containing photocrosslinking component in the photocrosslinking reaction during the exposure stage.

[0012] Preferably, the 2-acetoxy-1-methoxypropane is used as both the main solvent in step S1 and the developer in step S3, the rinsing is performed using ethanol, and N-methyl-2-pyrrolidone is not used as the developer in the developing step.

[0013] Preferably, in step S3, the flexible substrate is a PET, PEN, or flexible PI film; the pre-baking conditions are 60–90°C for 1–5 minutes; and the ultraviolet exposure uses a light source in the wavelength range of 350–420 nm, with an exposure dose of 50–200 mJ / cm². 2 The developing time is 10-60 seconds, and the rinsing time is 5-30 seconds.

[0014] Preferably, in step S3, the post-curing is carried out in a nitrogen atmosphere, the post-curing temperature is 180-200℃, the post-curing time is 1-1.5h, the dry film thickness of the obtained patterned low-temperature cured polyimide film is 8-15μm, the minimum resolution is not greater than 12μm, and the pattern retention rate after bending is not less than 90%.

[0015] A low-temperature curing PI photoresist suitable for flexible substrates, wherein the low-temperature curing PI photoresist is prepared by the aforementioned preparation method; the low-temperature curing PI photoresist comprises a soluble polyimide resin containing allyl structural units, a mercapto-containing photocrosslinking component, a free radical photoinitiator component, a heteroaryl base low-temperature curing accelerator, and 2-acetoxy-1-methoxypropane, and is capable of forming a patterned polyimide film layer on a flexible substrate by post-curing at a temperature not exceeding 230°C.

[0016] Compared with the prior art, the present invention has at least the following beneficial effects: (1) Breaking the high-temperature curing barrier and realizing truly non-destructive processing of flexible substrates. The present invention uses soluble polyimide resin containing magnolol structure directly as film-forming matrix, and supplements it with heteroaryl base curing accelerator (such as 2,2′-bipyridine), which greatly reduces the traditional dehydration and ring-closure curing temperature to a safe range of 180℃~230℃. This completely avoids the thermal degradation, yellowing and thermal stress warping deformation caused by traditional high-temperature processes above 300℃ to flexible substrates such as PET and PEN. (2) Abandoning traditional high-boiling point solvents and eliminating the "plasticization" hidden danger of residual solvents from the root. Thanks to the excellent solubility brought by the non-coplanar aromatic skeleton, this solution completely eliminates high-boiling point solvents such as N-methyl-2-pyrrolidone (NMP) that are difficult to volatilize, and uses low-toxicity, low-boiling-point 2-acetoxy-1-methoxypropane as a unified coating and developing solution. The solvent can be fully removed under low temperature heat treatment, ensuring the density, high hardness and electrical insulation reliability of the film. (3) The combination of rigidity and flexibility in molecular and network design gives the film excellent bending resistance and durability. The present invention cleverly introduces polydimethylsiloxane (PDMS) flexible segments into the crosslinking network, which greatly alleviates the strain mismatch between the cured rigid film and the flexible substrate. After more than 1,000 extreme bends, the patterned film can still maintain a high adhesion of grade 0 and a pattern integrity rate of more than 90%, which perfectly meets the complex dynamic deformation requirements of flexible electronic devices. (4) The introduction of the "thiol-ene" click crosslinking mechanism achieves high photolithography resolution and transmittance. This scheme adopts the thiol free radical addition reaction that is not affected by oxygen inhibition, and the photosensitive response is extremely fast; combined with the mild homologous solvent development process, the excessive swelling of the pattern is effectively suppressed, achieving a high precision micro-fine linewidth of 10.0μm, and the cured film has excellent high transmittance in the visible light region (transmittance of 450nm exceeds 88%). Attached Figure Description

[0017] Figure 1 This is the infrared spectrum of resin A prepared according to the present invention.

[0018] Figure 2 This is the infrared spectrum of resin A-1 prepared according to the present invention.

[0019] Figure 3This is the infrared spectrum of the low-temperature curing PI photoresist composition prepared in Example 1 of the present invention. Detailed Implementation

[0020] The present invention will be further described below with reference to embodiments. It should be noted that the following embodiments are for illustrative purposes only and are not intended to limit the scope of protection of the present invention. For those skilled in the art, equivalent substitutions or conventional adjustments made to the source of raw materials, equipment model, operating sequence, and process details without departing from the concept of the present invention should be considered as falling within the scope of protection of the present invention. Furthermore, for parts by mass and mass ratios, grams are chosen as the default unit. Simultaneously, the treatment of residues from the synthesis method steps is performed using default impurity removal or residual removal based on common knowledge in the art.

[0021] I. Raw Materials, Equipment and General Instructions

[0022] Unless otherwise specified, all raw materials used in the following examples are commercially available reagents. Among them: (1) Resin A: Soluble polyimide resin containing aromatic structural units and allyl structural units derived from magnolol, which is a pale yellow solid resin prepared in advance by the applicant and dried under vacuum, and dried in a vacuum oven at 80°C for 12 hours before use. (2) Resin A-1: ​​Soluble polyimide resin with a flexible segment of dimethylsiloxane introduced into the molecular structure of resin A, which is a pale yellow solid resin prepared in advance by the applicant and dried under vacuum, and dried in a vacuum oven at 80°C for 12 hours before use. (3) 2-acetoxy-1-methoxypropane (CAS: 108-65-6), electronic grade, used as a solvent and developer. (4) Pentaerythritol tetra(3-mercaptopropionic acid) ester (CAS: 7575-23-7), used as a mercapto-containing photocrosslinking component. (5) 2-Benzyl-2-dimethylamino-1-(4-morpholinophenyl)-1-butanone (CAS: 119313-12-1), as a free radical photoinitiator. (6) 2,2′-Bipyridine (CAS: 366-18-7), as a low-temperature curing accelerator. (7) Anhydrous ethanol, as a rinsing solution. (8) N-Methyl-2-pyrrolidone, used in some comparative examples.

[0023] Specifically, the raw materials and preparation method of resin A are as follows: 6FDA: 4,4'-(hexafluoroisopropylidene) bisphthalic anhydride, CAS: 1107-00-2. This monomer is used in the polymerization reaction. Source of goods: Zhongke (Tianjin) Chemical Co., Ltd. 6FODA: 2,2'-bis(trifluoromethyl)-4,4'-diaminodiphenyl ether, CAS: 344-48-9. This monomer is used in the polymerization reaction. Source of goods: Zhongke (Tianjin) Chemical Co., Ltd. TMA: trimellitic anhydride, CAS: 552-30-7. This substance is used as a capping agent. Source of goods: Bid Pharmaceutical. DDBA: 5,5'-diallyl-[1,1'-biphenyl]-2,2'-dimethylbis(4'-amino-[1,1'-biphenyl]-4-carboxylate). NMP: N-methylpyrrolidone, CAS: 872-50-4. Anhydrous solvent used in the polymerization stage. Product source: Bailingwei Technology. Acetic anhydride, CAS: 108-24-7. Dehydrating agent used for chemical imidization ring closure. Pyridine, CAS: 110-86-1. Catalyst used in conjunction with acetic anhydride. Product source: Bailingwei Technology. Raw material for the synthesis of DDBA monomer (core comonomer of resin A): Magnolol, CAS: 528-43-8. Constitutes the core skeleton of DDBA. Product source: Aladdin Reagent. 4-Bromobenzoyl chloride, CAS: 586-75-4. Participates in the first step of esterification reaction. Product source: Aladdin Reagent. 4-Aminophenylborate salt, CAS: 80460-73-7. Participates in coupling reaction to introduce amino groups. Product source: Aladdin Reagent. Potassium phosphate, CAS: 7778-53-2. As a base for coupling reaction. Product source: Bid Pharmaceutical. Triethylamine, CAS: 121-44-8. As a catalyst / acidifying agent for esterification reaction. Source of goods: Bid Pharmaceutical. Tetra(triphenylphosphine)palladium (Pd(PPh3)4), CAS: 14221-01-3. As a catalyst for Suzuki coupling. Source of goods: Bid Pharmaceutical. Tetrahydrofuran (THF), CAS: 109-99-9. Reaction solvent. Source of goods: Bailingwei Technology. Synthesis of novel diamine monomer DDBA: Esterification reaction to generate DDBr: At room temperature, magnolol and 4-bromobenzoyl chloride were dissolved in tetrahydrofuran (THF) solvent, and triethylamine (TEA) was added for reaction. The mass ratio of magnolol, 4-bromobenzoyl chloride, tetrahydrofuran, and triethylamine was 1:1.8:20:0.9, and the reaction time was 24 h, yielding the bromine-containing intermediate product DDBr.Suzuki coupling to generate DDBA: The intermediate DDBr obtained above was mixed with 4-aminophenylborate acid salt in THF solvent, and tetrakis(triphenylphosphine)palladium (Pd(PPh3)4) and potassium phosphate (K3PO4) were added. The mass ratio of DDBr, 4-aminophenylborate acid salt, THF / water mixed solvent (mass ratio of the two is 10:1), tetrakis(triphenylphosphine)palladium and potassium phosphate is 1:0.70:20:0.09:1.68. The reaction was carried out under nitrogen protection and at 75°C for 24 h (oil bath temperature 75°C, reaction solution is under reflux), and finally the allyl-containing diamine monomer DDBA was obtained. Polymerization of resin A (soluble polyimide): Gel preparation (forming polyamic acid PAA): Under room temperature and argon protection, the self-made DDBA monomer (5.0000 g) and 6FODA (2.5596 g) were dissolved together in 57 mL of anhydrous NMP solvent. Subsequently, 6.3410 g of dianhydride monomer 6FDA and 0.3655 g of TMA (trimeric triphthalic anhydride) as a capping agent were added to control the solid content of the solution at 20 wt%. The mixture was stirred continuously at room temperature for approximately 12 h to generate a polyamic acid (PAA) solution. Chemical dehydration and ring closure: 4.02 mL of acetic anhydride and 1.73 mL of pyridine were added sequentially to the above PAA solution. The reaction was continued with stirring at room temperature for 24 h to complete chemical imidization. Precipitation and washing: The mixture after the reaction was completed was added dropwise to 2 L of a mixture of deionized water and ethanol (water to ethanol volume ratio of 7:3) for precipitation. After precipitation, the precipitate was washed three times with ethanol. Drying into powder: The obtained solid was placed in a vacuum oven and dried at 80 °C to obtain a soluble allyl polyimide resin (i.e., resin A), the infrared spectrum of which is shown below. Figure 1 As shown.

[0024] Meanwhile, the raw materials and preparation methods of resin A-1 are as follows: DDBA (self-made bio-based diamine): 5,5'-diallyl-[1,1'-biphenyl]-2,2'-dimethylbis(4'-amino-[1,1'-biphenyl]-4-carboxylate). 6FODA (fluorinated rigid diamine): 2,2'-bis(trifluoromethyl)-4,4'-diaminodiphenyl ether, CAS: 344-48-9. Siloxane diamine (introducing oligomeric siloxane flexible segments): 1,3-bis(3-aminopropyl)tetramethyldisiloxane, CAS: 2469-55-8. 6FDA (dianhydride): 4,4'-(hexafluoroisopropylidene)bisphthalic anhydride, CAS: 1107-00-2. TMA (capping agent): trimellitic anhydride, CAS: 552-30-7. NMP (polymerization solvent): N-methyl-2-pyrrolidone, CAS: 872-50-4. Dehydrating agent: Acetic anhydride, CAS: 108-24-7. Catalyst: Pyridine, CAS: 110-86-1. Formulation (forming siloxane-containing polyamic acid PAA): Under room temperature and argon protection, self-made allyl diamine DDBA (5.000 g), fluorinated diamine 6FODA (2.048 g), and siloxane diamine 1,3-bis(3-aminopropyl)tetramethyldisiloxane (0.378 g) were added to a dry three-necked flask, along with 56.5 g of anhydrous NMP solvent, and stirred until the diamine was completely dissolved. Subsequently, dianhydride monomer 6FDA (6.341 g) and end-capping agent TMA (0.366 g) were slowly added in batches. The solid content of the system was controlled at 20 wt%. After continuous mechanical stirring at room temperature for approximately 12 hours, a uniform and viscous polyamic acid (PAA) copolymer solution containing oligomeric siloxane flexible segments was generated. Chemical dehydration and ring-closure (imidization): 4.02 mL of acetic anhydride and 1.73 mL of pyridine were slowly added dropwise to the PAA solution. The reaction was continued at room temperature for 24 hours to allow the polyamic acid to undergo sufficient dehydration and ring-closure, completing the chemical imidization. Precipitation and washing: The polymer mixture after the reaction was completed was slowly added dropwise to 2 L of a mixture of deionized water and ethanol (water to ethanol volume ratio 7:3) under vigorous stirring. The polymer precipitated as a pale yellow filamentous or powdery form. After complete precipitation, the precipitate was filtered and thoroughly washed with pure ethanol, then filtered three times to remove residual solvent and small molecule byproducts. Drying into powder: The washed solid polymer was transferred to a vacuum oven and continuously dried under vacuum at 80°C for 12 hours until constant weight was achieved. The final product is a pale yellow solid powder, which is the soluble polyimide resin (resin A-1) with the introduction of dimethylsiloxane flexible segments. It is placed in a desiccator for later use. Its infrared spectrum is shown below. Figure 2 As shown. With Figure 1 In comparison, it exhibits the classic infrared characteristics (i.e., 1000-1100 cm⁻¹) after the introduction of PDMS flexible segments. -1The broad and strong peak, 1260cm -1 and 800cm -1 (the sharp peak).

[0025] The main equipment used in the following examples is as follows: electronic balance (Mettler ME204E), magnetic stirrer (IKAC-MAGHS7), planetary degassing machine (THINKYARE-310), spin coater (KW-4A), hot plate (CHEMAT), vacuum oven (DZF-6050), LED exposure machine (main wavelength 380nm), Fourier transform infrared spectrometer (ThermoNicoleti S50), ultraviolet-visible spectrophotometer (Shimadzu UV-3600iPlus), viscometer (Brookfield DV2T), step tester (KLAP-7), pencil hardness tester, cross-cut adhesion tester, and bending test device.

[0026] Unless otherwise specified, all flexible substrates are made of 25.0 μm thick polyimide flexible film. Before coating, the substrates are wiped and cleaned once each with acetone and isopropanol, then treated in an oxygen plasma cleaner for 45 seconds, and then immediately proceed to the coating process.

[0027] II. Example 1

[0028] Example 1 is a preferred embodiment of the present invention.

[0029] (I) Step (1) Prepare the photoresist composition

[0030] Table 1 lists the specific amounts of each component used in Example 1.

[0031] Table 1. Raw material formula for step (1) of Example 1

[0032] The specific procedure is as follows: Add 26.0 g of resin A and 6.0 g of resin A-1 to a 250 mL brown three-necked flask, purge with nitrogen for 15 min, and then add 64.8 g of 2-acetoxy-1-methoxypropane. Stir at 400 r / min for 2.5 h at 25 °C until the resin is completely dissolved to obtain a resin solution.

[0033] Subsequently, 2.0 g of pentaerythritol tetra(3-mercaptopropionic acid) ester, 0.8 g of photoinitiator, and 0.4 g of 2,2′-bipyridine were added, and the mixture was stirred at 300 rpm for 40 min under light-protected conditions. The mixture was then degassed at 2000 rpm for 5 min using a planetary degassing machine, and then filtered through a 0.20 μm polytetrafluoroethylene filter membrane to obtain the low-temperature curing PI photoresist composition of Example 1. The infrared spectrum is shown below. Figure 3 As shown.

[0034] (ii) Step (2) Coating, pre-baking, exposure, development and post-curing

[0035] Table 2 lists the key process parameters for step (2) of Example 1.

[0036] Table 2 Process conditions for step (2) of Example 1

[0037] After following the procedures in Table 2, a patterned polyimide film with a smooth surface and clear boundaries was obtained. The dry film thickness was measured to be 12.3 μm using a profilometer.

[0038] III. Examples 2 to 5

[0039] Examples 2 to 5 were all carried out according to the method of Example 1, with the only difference being that some formulas and process parameters were adjusted. Parameters not listed were the same as in Example 1.

[0040] (a) Formula adjustment

[0041] Table 3. Formula adjustments for Examples 2 to 5

[0042] Table 4. Remaining components and process adjustments in Examples 2 to 5

[0043] Among them, Example 2 was used to investigate the effect near the lower limit of the amount of low-temperature curing accelerator; Example 3 was used to investigate the change in flexibility after increasing the amount of dimethylsiloxane flexible segment introduced; Example 4 was used to investigate the change in crosslinking density after increasing the amount of mercapto-containing photocrosslinking component; and Example 5 was used to investigate the combined effect of a lower amount of dimethylsiloxane flexible segment introduced and extended heat preservation time.

[0044] IV. Comparative Examples 1 to 6

[0045] Comparative Examples 1 to 6 all used Example 1 as a reference, with only single-factor or a few-factor variations, in order to observe the influence of key components and key process conditions on the results.

[0046] Table 5. Formulation or process changes of Comparative Examples 1 to 6

[0047] Comparative Example 1 illustrates the necessity of low-temperature curing accelerators for low-temperature post-curing. Comparative Example 2 illustrates the effect of the flexible segments of dimethylsiloxane on flexibility and adaptability. Comparative Examples 3 and 4 investigate the effects of post-curing conditions below or above the defined window, respectively. Comparative Example 5 illustrates the effect of replacing the solvent with a higher boiling point on the developing window, residual solvent, and film appearance. Comparative Example 6 illustrates the necessity of mercapto-olefin photocrosslinking for pattern retention.

[0048] Test method:

[0049] To ensure comparability between different embodiments and comparative examples, all samples were taken from the same batch of flexible substrates and placed at 23°C and 50% relative humidity for 24 hours before testing.

[0050] Table 6 Test Items and Test Methods

[0051] The test results are as follows.

[0052] (a) Examples 1 to 5

[0053] Table 7. Rheological, graphical, and curing results of Examples 1 to 5

[0054] Table 8. Film performance results of Examples 1 to 5

[0055] As shown in Tables 7 and 8, Example 1 achieved a relatively balanced result in terms of resolution, low-temperature curing degree, adhesion, and pattern retention rate after bending. In Example 2, the curing degree and adhesion both decreased after reducing the amount of 2,2′-bipyridine, indicating that insufficient low-temperature curing accelerator at 180°C to 190°C would lead to inadequate reaction progression in the post-curing stage. In Example 3, the pattern retention rate after bending increased to 97.2% after increasing the amount of resin A-1, but the minimum resolution changed from 10.0 μm to 12.0 μm, indicating that increasing the flexible segments is beneficial for mitigating bending stress, but an excessively high proportion of flexible segments would slightly reduce edge support after development. In Example 4, the hardness and density of the imidized film improved after increasing the amount of mercapto-containing photocrosslinking component, but the transmittance decreased slightly, indicating that increased crosslinking density would lead to some optical loss. In Example 5, a high degree of curing and good pattern accuracy could still be maintained by appropriately extending the post-curing time, even with a lower amount of resin A-1.

[0056] (ii) Comparative Examples 1 to 6

[0057] Table 9. Rheological, graphical, and curing results of Comparative Examples 1 to 6.

[0058] Table 10. Membrane performance results of Comparative Examples 1 to 6

[0059] The comparative results show that the key components and key process conditions in this invention all play a clear role. In Comparative Example 1, after deleting 2,2′-bipyridine, under the same post-curing conditions at 180°C, the degree of curing decreased from 97.8% in Example 1 to 88.6%, while the adhesion changed from grade 0 to grade 2. This indicates that the low-temperature curing accelerator is necessary to reduce the reaction resistance in the post-curing stage and improve the density of the structure at low temperatures. In Comparative Example 2, without the introduction of resin A-1, although the minimum resolution was slightly better than in Example 1, the pattern retention rate after bending decreased from 94.6% to 71.6%. This indicates that the simple rigid polyimide structure is more conducive to maintaining the edge of the pattern, but after repeated bending on a flexible substrate, microcrack propagation and local peeling are more likely to occur. Therefore, the introduction of the flexible segment of dimethylsiloxane has a substantial effect on flexibility and adaptability. In Comparative Example 3, when the post-curing temperature was reduced to 150°C, the curing degree was only 82.3%, and the hardness dropped to HB. This indicates that when the post-curing temperature is below the window specified in the claims, the post-curing is insufficient, resulting in more residual solvent and unreacted structures inside the film, leading to a simultaneous decrease in mechanical stability and adhesion. In Comparative Example 4, when the post-curing temperature was increased to 250°C, although the curing degree increased somewhat, the transmittance at 450nm dropped to 84.2%, and the pattern retention rate after bending also decreased to 83.0%. This indicates that exceeding the specified temperature window increases the thermal stress between the film and the flexible substrate, and enhances the tendency for local yellowing, which is detrimental to flexible lithography applications. In Comparative Example 5, after replacing 2-acetoxy-1-methoxypropane with N-methyl-2-pyrrolidone, the minimum resolution deteriorated to 16.0μm, and the transmittance dropped to 82.6%. This indicates that the removal and swelling control of high-boiling-point solvents during pre-baking and development processes are not as good as the solvent system used in this invention, which easily dulls the edges of the pattern and brings a higher risk of residue. In Comparative Example 6, although the degree of curing after heat treatment could still reach 96.8% after the removal of the mercapto-containing photocrosslinking component, a complete pattern could not be formed after development. This indicates that the mercapto-olefin crosslinking pair used in this invention is a necessary condition for the stable formation of the insoluble region after exposure and cannot be omitted.

[0060] Based on the above embodiments and comparative examples, it can be seen that the present invention uses a soluble polyimide resin A containing allyl structural units as the main resin, and incorporates a mercapto-containing photocrosslinking component, a free radical photoinitiator component, 2,2′-bipyridine, and 2-acetoxy-1-methoxypropane into the system. This enables the production of a polyimide photoresist film layer with a clear pattern, high adhesion, and high pattern retention rate after bending at a lower post-curing temperature. The dimethylsiloxane flexible segment provided by resin A-1 directly contributes to the bending stability on the flexible substrate, while 2,2′-bipyridine directly affects the degree of completion of low-temperature post-curing. In the technical solution of the present invention, the mercapto-containing photocrosslinking component, the low-temperature curing accelerator, the amount of dimethylsiloxane flexible segment introduced, and the post-curing temperature window work together to determine the pattern retention capability, the degree of low-temperature curing, and the flexibility adaptability. The absence of any key component, or the deviation of key process conditions from the defined window, will lead to a significant decrease in at least one core performance characteristic.

Claims

1. A method for preparing a low-temperature curable PI photoresist suitable for flexible substrates, characterized in that: The method comprises the following steps, by weight: S1, under an inert gas atmosphere, 24.0–32.0 parts of a soluble polyimide resin containing allyl structural units and 0–10.0 parts of a soluble polyimide resin containing a dimethylsiloxane flexible segment are added to 60.0–70.0 parts of 2-acetoxy-1-methoxypropane and stirred at 20–30°C to dissolve, thereby obtaining a resin solution; wherein the soluble polyimide resin containing allyl structural units comprises aromatic structural units derived from magnolol and allyl structural units capable of participating in free radical addition reactions, and the soluble polyimide resin is an imidized polyimide resin before exposure, rather than requiring high-temperature dehydration and sealing. A polyamic acid precursor containing a ring; in the soluble polyimide resin containing a dimethylsiloxane flexible segment, the dimethylsiloxane flexible segment is introduced as a copolymer structural unit or side chain connecting structural unit in the polyimide backbone to reduce inter-chain stacking of polyimide and mitigate strain mismatch between the patterned low-temperature curing polyimide film and the flexible substrate; S2, 1.5-3.0 parts of a mercapto-containing photocrosslinking component, 0.5-1.2 parts of a free radical photoinitiator component, and 0.1-0.6 parts of a heteroaryl base type low-temperature curing accelerator are added to the resin solution, mixed under light-protected conditions, and after degassing and filtration, a low-temperature curing PI photoresist coating solution is obtained; wherein, the mercapto-containing photocrosslinking component... The film is composed of pentaerythritol tetra(3-mercaptopropionic acid) ester, wherein the free radical photoinitiator is 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-1-butanone; the heteroaryl base type low-temperature curing accelerator is 2,2'-bipyridine, and its dosage is 0.2-0.4 parts, and the heteroaryl base type low-temperature curing accelerator does not replace the mercapto-containing photocrosslinking component in the photocrosslinking reaction during the exposure stage; S3, the low-temperature curing PI photoresist coating solution is coated on the surface of a flexible substrate, and after pre-baking, ultraviolet exposure, development, rinsing and drying, it is cured at 180-200℃ for 1-1.5h in a nitrogen atmosphere to obtain a patterned low-temperature curing polyimide film layer; wherein, the 2-Acetoxy-1-methoxypropane is used as both the main solvent in step S1 and the developer in step S3. The rinsing is performed using ethanol, and N-methyl-2-pyrrolidone is not used as the developer in the developing step. The resulting patterned low-temperature curing polyimide film has a dry film thickness of 8–15 μm, a minimum resolution of no more than 12 μm, and a pattern retention rate of no less than 90% after bending. The mercapto-containing photocrosslinking component undergoes a mercapto-olefin radical addition reaction with the allyl-containing soluble polyimide resin under ultraviolet exposure. The heteroaryl base type low-temperature curing accelerator is used to promote the stabilization of the crosslinking network and the removal of residual solvent in the post-curing stage.

2. The method for preparing low-temperature curable PI photoresist suitable for flexible substrates according to claim 1, characterized in that: The soluble polyimide resin containing allyl structural units comprises 24.0 to 30.0 parts, and the soluble polyimide resin containing dimethylsiloxane flexible segments comprises 2.0 to 8.0 parts, which together constitute the soluble polyimide resin component.

3. The method for preparing low-temperature curable PI photoresist suitable for flexible substrates according to claim 1, characterized in that: In step S3, the flexible substrate is PET, PEN, or a flexible PI film; the pre-baking conditions are 60–90°C for 1–5 minutes; the ultraviolet exposure uses a light source in the wavelength range of 350–420 nm, and the exposure dose is 50–200 mJ / cm². 2 The developing time is 10-60 seconds, and the rinsing time is 5-30 seconds.

4. A low-temperature curing PI photoresist suitable for flexible substrates, characterized in that: The low-temperature curing PI photoresist is prepared by the preparation method according to any one of claims 1 to 3; the low-temperature curing PI photoresist includes a soluble polyimide resin containing allyl structural units, a mercapto photocrosslinking component, a free radical photoinitiator component, a heteroaryl base low-temperature curing accelerator, and 2-acetoxy-1-methoxypropane, and can form a patterned polyimide film layer on a flexible substrate by post-curing at a temperature not exceeding 230°C.

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