A method for improving the bending resistance of nanoimprint photoresist on flexible substrates

Abstract

This invention discloses a method for improving the bending resistance of nanoimprint photoresist on flexible substrates, belonging to the fields of photoresponsive azobenzene polymer materials and nanoimprint technology. By selecting azobenzene polymers with a specific molecular weight range as the imprinting material, this invention leverages the property that higher molecular weight can effectively enhance the bending resistance of the resulting microstructures, thereby directly fabricating fine structures with high mechanical reliability on flexible substrates. This method provides a practical solution for manufacturing devices with extremely high structural integrity requirements, effectively addressing the core problem of pattern damage during dynamic bending, leading to device performance degradation. It is particularly suitable for manufacturing flexible optoelectronic devices with stringent structural integrity requirements, such as flexible optical waveguides, and can significantly improve the durability of patterns under dynamic bending environments.

Description

Technical Field

[0001] This invention belongs to the field of photoresponsive azobenzene polymer materials and nanoimprint technology, specifically relating to a method for improving the bending resistance of flexible substrate nanoimprint photoresist. Background Technology

[0002] Nanoimprint lithography is a high-resolution, high-efficiency, and low-cost micro / nano pattern replication technology. It achieves pattern transfer by using a mold imprinting method, rather than traditional optical exposure, thus overcoming the limitations of optical diffraction and enabling the replication of feature sizes <10 nm. The core process involves pressing a mold with a nanoscale pattern onto a thin layer of imprinting adhesive that is sensitive to ultraviolet light or heat. The adhesive is then cured by ultraviolet light irradiation or heating. Finally, the mold is removed, leaving a patterned structure on the substrate that is the opposite of the mold. In recent years, with the rise of flexible electronics, wearable devices, and biomimetic sensing, applying nanoimprint lithography to flexible substrates has become a clear and urgent technological trend.

[0003] Flexible substrates typically have low surface energy, a large difference in coefficient of thermal expansion compared to the mold, and are prone to elastic deformation under pressure. This leads to process defects during imprinting, such as uneven filling of the imprinting adhesive, localized pattern deformation, and adhesion or tearing during demolding, severely impacting pattern fidelity and yield. In dynamic applications such as flexible electronics, devices need to withstand repeated bending, stretching, or torsion, making them highly susceptible to microcrack initiation and propagation, structural layer peeling, or permanent plastic deformation, resulting in rapid functional degradation or failure. Therefore, there is a significant contradiction between the "formability" and "durability" of patterns.

[0004] While photoresponsive azobenzene polymers have shown promise in flexible nanoimprinting, current technological development focuses primarily on optimizing their photochemical reactivity, such as improving photoresponse speed and isomerization efficiency. However, for flexible optoelectronic devices (such as flexible waveguides) that require repeated bending in practical applications, the structural integrity and shape fidelity of the imprinted microstructure under dynamic bending stress remains a crucial but underdeveloped issue.

[0005] The molecular weight of a polymer is a fundamental parameter that directly affects the mechanical properties of its thin film. However, in existing technologies, the selection and control of molecular weight have not established a clear and predictable correlation with the bending resistance and defect resistance of the imprinted structure on a flexible substrate. This makes it difficult to improve the durability of the pattern under bending conditions through material design, resulting in a bottleneck in the manufacture of high-performance, high-reliability flexible optical waveguides. Summary of the Invention

[0006] This invention addresses the shortcomings of existing technologies by providing a method to improve the bending resistance of nanoimprint photoresists on flexible substrates. By selecting azobenzene polymers with a specific molecular weight range as the imprinting material and employing a high degree of polymerization design strategy, this invention effectively enhances the bending resistance of the resulting microstructures, thereby directly fabricating fine structures with high mechanical reliability on flexible substrates. This method provides a practical solution for manufacturing flexible micro / nano devices with extremely high structural integrity requirements. It effectively solves the core problem of micro / nano structures being easily damaged during dynamic bending, leading to device performance degradation. It is particularly suitable for manufacturing flexible optoelectronic devices with stringent structural integrity requirements, such as flexible optical waveguides, and can significantly improve the durability of patterns under dynamic bending environments.

[0007] The present invention provides a method for improving the bending resistance of nanoimprint photoresist on flexible substrates by using azobenzene polymer material as the imprint photoresist and performing nanoimprinting on a flexible substrate.

[0008] The azobenzene polymer material, abbreviated as P(NB-Azo-C4), has the following general structural formula:

[0009]

[0010] Formula I

[0011] Where n represents the degree of aggregation (DP), which ranges from 5 to 400.

[0012] The sample with DP=5 had poor film-forming properties and could not be imprinted. Therefore, the subsequent experiments used values ​​of 8-400. The low degree of polymerization samples could be compared with the high degree of polymerization samples in terms of bending resistance and defect resistance.

[0013] Further optimization is to select a degree of polymerization of 250-400.

[0014] The preparation method of the azobenzene polymer P(NB-Azo-C4) of the present invention includes the following steps:

[0015] Step 1: 4-(4-Butylphenylazo)phenol, represented by Formula I-a, reacts with 6-chloro-1-hexanol, represented by Formula I-b, to form the compound represented by Formula I-c.

[0016]

[0017] Step 2: The compound shown in Formula I-d, cis-5-norbornene-exo-2,3-dicarboxylic anhydride, is reacted with 6-aminohexanoic acid shown in Formula I-e to form the compound shown in Formula I-f.

[0018]

[0019] Step 3: The compound shown in Formula I-c undergoes an esterification reaction with the compound shown in Formula I-f to form the azobenzene monomer NB-Azo-C4 shown in Formula I-g.

[0020]

[0021] Step 4: The Grubbs second-generation catalyst shown in Formula I-h reacts with anhydrous pyridine shown in Formula I-i to form the Grubbs third-generation catalyst shown in Formula I-j, wherein Mes is mestrimethylphenyl and PCy3 is tricyclohexylphosphine.

[0022]

[0023] Step 5: The azobenzene monomer NB-Azo-C4 shown in Formula I-g undergoes ring-opening metathesis polymerization under the catalysis of the Grubbs third-generation catalyst shown in Formula I-j to form the azobenzene polymer P (NB-Azo-C4) shown in Formula I. The degree of polymerization of the synthesized azobenzene polymer P (NB-Azo-C4) shown in Formula I is controlled by controlling the molar ratio of the azobenzene monomer shown in Formula I-g to the Grubbs third-generation catalyst shown in Formula I-j.

[0024] The molar ratio of the azobenzene monomer to the Grubbs third-generation catalyst is 5-400:1.

[0025]

[0026] The nanoimprinting process includes the following steps:

[0027] The azobenzene polymer material P (NB-Azo-C4) is dissolved in cyclopentanone to form a solution. This solution is spin-coated onto a flexible substrate, and the solvent is evaporated to obtain a spin-coated nanoimprint film. A transparent polydimethylsiloxane (PDMS) template with a porous microstructure is attached to the polymer film, air bubbles are removed, and pressure is applied for imprinting. The film is then irradiated with ultraviolet light to liquefy the azobenzene polymer P (NB-Azo-C4). Pressure is applied to fill the template with the liquefied azobenzene polymer P (NB-Azo-C4), and then visible light is used to solidify the imprinted structure. Finally, the imprinted PDMS mold is removed.

[0028] The azobenzene polymer material P(NB-Azo-C4) is dissolved in cyclopentanone to form a solution. The concentration of the azobenzene polymer material in cyclopentanone is 30 mg / mL. The spin coating speed is 1000-2000 r / min. The film thickness of the azobenzene polymer with different degrees of polymerization is controlled to be 200±10 nm.

[0029] The flexible substrate measures 2.5 × 2.5 × 0.01 cm.3 Transparent polyimide (PI).

[0030] The pressure imprinting involves loading a 1.5 kg weight.

[0031] The ultraviolet light has a wavelength of 365 nm and an intensity range of 1.7-70 mW·cm. -2 .

[0032] The visible light wavelength is 530 nm, and the light intensity range is 1.5-55 mW·cm. -2 .

[0033] This invention selects azobenzene polymers with a specific molecular weight range as imprinting materials, leveraging the property that higher molecular weights can effectively enhance the bending resistance of the resulting microstructures. This allows for the direct fabrication of fine structures with high mechanical reliability on flexible substrates. The method of this invention provides a practical solution for manufacturing devices with extremely high structural integrity requirements (such as flexible optical waveguides). It effectively addresses the core problem of patterns being easily damaged during dynamic bending, leading to device performance degradation. It is particularly suitable for manufacturing flexible optoelectronic devices with stringent structural integrity requirements, such as flexible optical waveguides, and can significantly improve the durability of patterns under dynamic bending environments. Attached Figure Description

[0034] The accompanying drawings, which form part of this application, are provided to further illustrate the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention, but do not constitute an undue limitation thereof. Obviously, the drawings described below are merely some embodiments, and those skilled in the art can obtain other drawings based on these drawings without any creative effort.

[0035] Figure 1 The image shows the proton NMR spectrum of the azobenzene small molecule Ig (NB-Azo-C4).

[0036] Figure 2 GPC curves for azobenzene polymer P(NB-Azo-C4) with different degrees of polymerization (DP=5-400).

[0037] Figure 3The UV-Vis absorption spectra of the trans-cis isomerization process (Figure a) and the cis-trans isomerization process (Figure b) of the azobenzene polymer P(NB-Azo-C4) film with DP=50 are shown. The initial azobenzene polymer P(NB-Azo-C4) film contains a large number of aggregated and substrate-oriented azobenzene groups. During photoisomerization under UV irradiation, the azobenzene undergoes deaggregation and random orientation. The trans-azophenyl group has a π-π* absorption band at approximately 350 nm and a cis-azophenyl group has an n-π* absorption band at approximately 445 nm. After irradiation with ultraviolet light, the trans-azophenyl group's π-π* absorption band decreases, while the cis-azophenyl group's n-π* absorption band increases, indicating that the azophenyl group has undergone a trans-cis isomerization process. After irradiation with green LED light, the trans-azophenyl group's π-π* absorption band increases, while the cis-azophenyl group's n-π* absorption band decreases, indicating that the azophenyl group has undergone a cis-trans isomerization process.

[0038] Figure 4 The photoinduced solid-liquid transition behavior of azobenzene polymer P(NB-Azo-C4) with DP=50 under ultraviolet light irradiation is shown in Figure (a). Figure (b) is a microscopic photograph of solid powder of azobenzene polymer P(NB-Azo-C4), and Figure (c) is a microscopic photograph after ultraviolet light irradiation.

[0039] Figure 5 Imprint height maps of azobenzene polymer P(NB-Azo-C4) with different degrees of polymerization were obtained by atomic force microscopy (AFM). (a) shows the imprint height map for DP=8, (b) for DP=15, (c) for DP=25, (d) for DP=50, (e) for DP=100, (f) for DP=150, (g) for DP=250, (h) for DP=300, and (i) for DP=400.

[0040] Figure 6 The imprint height is statistically analyzed. Monomers and samples with DP=5 have poor film-forming properties and cannot be imprinted. Samples with DP=15 to DP=250 have relatively high imprint heights, exceeding 200 nm. Samples with DP=300 to DP=400 have poor flowability, resulting in imprint heights less than 100 nm, and the columns may collapse.

[0041] Figure 7-16 Imprint SEM images of azobenzene polymer P(NB-Azo-C4) with different degrees of polymerization.

[0042] Figure 7The images show SEM images of the azobenzene polymer P(NB-Azo-C4) with a DP=8. Images (a) to (e) are SEM images of five randomly selected regions. The corresponding defect rate statistics are shown in Table 2.

[0043] Figure 8 The images show SEM images of the azobenzene polymer P(NB-Azo-C4) with a DP=15. Images (a) to (e) are SEM images of five randomly selected regions. The corresponding defect rate statistics are shown in Table 3.

[0044] Figure 9 The images show SEM images of the azobenzene polymer P(NB-Azo-C4) with a DP=25. Images (a) to (e) are SEM images of five randomly selected regions. The corresponding defect rate statistics are shown in Table 4.

[0045] Figure 10 The images show SEM images of the azobenzene polymer P(NB-Azo-C4) with a DP=50. Images (a) to (e) are SEM images of five randomly selected regions. The corresponding defect rate statistics are shown in Table 5.

[0046] Figure 11 The images show SEM images of the azobenzene polymer P(NB-Azo-C4) with a DP=100. Images (a) to (e) are SEM images of five randomly selected regions. The corresponding defect rate statistics are shown in Table 6.

[0047] Figure 12 The images show SEM images of the azobenzene polymer P(NB-Azo-C4) with a DP=150. Images (a) to (e) are SEM images of five randomly selected regions. The corresponding defect rate statistics are shown in Table 7.

[0048] Figure 13 The images show SEM images of the azobenzene polymer P(NB-Azo-C4) with a DP=250. Images (a) to (e) are SEM images of five randomly selected regions. The corresponding defect rate statistics are shown in Table 8.

[0049] Figure 14 The images show SEM images of the azobenzene polymer P(NB-Azo-C4) with a DP=300. Images (a) to (e) are SEM images of five randomly selected regions. The corresponding defect rate statistics are shown in Table 9.

[0050] Figure 15 The images show SEM images of the azobenzene polymer P(NB-Azo-C4) with a DP of 400. Images (a) to (e) are SEM images of five randomly selected regions. The corresponding defect rate statistics are shown in Table 10.

[0051] Figure 16 The imprinting defect rate of azobenzene polymer P (NB-Azo-C4) with different degrees of polymerization is statistically summarized. The imprinting defect rate is relatively high for low degree of polymerization (DP=8, DP=15), and very low for high degree of polymerization.

[0052] Figure 17 This is a diagram illustrating bending using a tensile testing machine.

[0053] Figure 18 The images show SEM images of azobenzene polymer P(NB-Azo-C4) with DP=25 after being bent 100 times with a tensile testing machine to control displacement and compress a 2 cm pattern to 0.5 cm. Figures (a) to (c) are SEM images of the bending area gradually magnified.

[0054] Figure 19 To increase the number of bends and the degree of bending, a tensile testing machine was used to bend the azobenzene polymer P(NB-Azo-C4) 200 times, controlling the displacement. The 2 cm pattern was compressed to 0.3 cm. SEM images of the DP=25 azobenzene polymer P(NB-Azo-C4) are shown. Figures (a) to (c) are SEM images of the bending area gradually magnified.

[0055] Figure 20 This is a diagram illustrating bending the weight.

[0056] Figure 21 The images show SEM images of the azobenzene polymer P(NB-Azo-C4) with a DP=8 after bending with weights. Images (a) through (c) are SEM images of the bending area that are progressively magnified.

[0057] Figure 22 The images show SEM images of the azobenzene polymer P(NB-Azo-C4) with a DP=15 after bending with weights. Images (a) through (c) are SEM images of the bending area that are progressively magnified.

[0058] Figure 23 The images show SEM images of the azobenzene polymer P(NB-Azo-C4) with a DP of 25 after being bent with weights. Images (a) through (c) are SEM images of the bent area that are progressively magnified.

[0059] Figure 24 The images show SEM images of the azobenzene polymer P(NB-Azo-C4) with a DP of 50 after being bent with weights. Images (a) through (c) are SEM images of the bent area that are progressively magnified.

[0060] Figure 25The images show SEM images of the azobenzene polymer P(NB-Azo-C4) with a DP of 100 after being bent with weights. Images (a) through (c) are SEM images of the bent area that are progressively magnified.

[0061] Figure 26 The images show SEM images of the azobenzene polymer P(NB-Azo-C4) with a DP of 150 after being bent with weights. Images (a) through (c) are SEM images of the bent area that are progressively magnified.

[0062] Figure 27 The images show SEM images of the azobenzene polymer P(NB-Azo-C4) with a DP of 250 after being bent with weights. Images (a) through (c) are SEM images of the bent area that are progressively magnified.

[0063] Figure 28 The images show SEM images of the azobenzene polymer P(NB-Azo-C4) with a DP of 300 after being bent with weights. Images (a) through (c) are SEM images of the bent area that are progressively magnified.

[0064] Figure 29 The images show SEM images of the azobenzene polymer P(NB-Azo-C4) with a DP of 400 after being bent with weights. Images (a) through (c) are SEM images of the bent area that are progressively magnified. Detailed Implementation

[0065] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments will be clearly and completely described below with reference to the accompanying drawings. The following embodiments are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0066] Example 1: Synthesis of compound I-c

[0067] In a 250 mL flask, compound I-a (2.50 g, 9.83 mmol) was dissolved in 13.78 mL of N,N-dimethylformamide solution, and potassium carbonate (1.79 g, 12.95 mmol) was added. The mixture was stirred at 30 °C for 30 min. Then, potassium iodide (0.11 g, 2.76 mmol) and compound I-b (1.62 g, 11.84 mmol) were added to the flask, and the reaction mixture was stirred vigorously at 110 °C for 24 h. After the reaction mixture cooled to room temperature, it was extracted three times with 100 mL of ethyl acetate and 100 mL of distilled water. The organic phase was collected, washed three times with saturated brine, dried over anhydrous magnesium sulfate, filtered to remove magnesium sulfate, and the solvent was removed by rotary evaporation to obtain the crude product. The crude product was dissolved in 10 mL of dichloromethane, recrystallized by slowly adding 250 mL of petroleum ether, filtered, and dried under vacuum at 45 °C for 12 h to obtain compound I-c in 87.4% yield.

[0068] Example 2: Synthesis of compound I-f

[0069] Compound I-d (3.28 g, 20.00 mmol), compound I-e (2.62 g, 20.00 mmol), triethylamine (0.28 mL, 2.00 mmol), and 40 mL of toluene were added to a 250 mL round-bottom flask equipped with a magnetic stir bar and a reflux condenser. The mixture was stirred at 110 °C for 19 h. After the reactants cooled to room temperature, the solvent was removed by rotary evaporation. The resulting solid was extracted three times with 100 mL of dichloromethane and 100 mL of distilled water. The organic phase was collected, washed three times with saturated brine, dried over anhydrous magnesium sulfate, filtered to remove the magnesium sulfate, and the solvent was removed by rotary evaporation to obtain compound I-f in 86.4% yield.

[0070] Example 3: Synthesis of small azobenzene molecule I-g (NB-Azo-C4)

[0071] Compound I-f (3.50 g, 12.70 mmol), compound I-c (3.75 g, 10.60 mmol), 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride (2.45 g, 12.70 mmol), N,N-diisopropylethylamine (2.10 mL, 12.70 mmol), and 4-dimethylaminopyridine (0.13 g, 1.06 mmol) were added to a 250 mL round-bottom flask and dissolved in 100 mL of dichloromethane. The mixture was stirred at room temperature for 24 h. The reaction solution was washed three times with 100 mL of distilled water, then three times with saturated brine. The solution was dried over anhydrous magnesium sulfate, and the magnesium sulfate was filtered off. The solvent was removed by rotary evaporation to obtain the crude product, which was dissolved in 10 mL of tetrahydrofuran. The crude product was recrystallized by slowly adding 250 mL of methanol, filtered, and then dried under vacuum at 45 °C for 12 h to obtain azobenzene molecule I-g in 86.6% yield. Figure 1 The image shows the 1H NMR spectrum of the azobenzene small molecule I-g (NB-Azo-C4).

[0072] Example 4: Synthesis of Grubbs Third Generation Catalyst I-j

[0073] Grubbs second-generation catalyst (0.50 g, 0.59 mmol) was added to a glove box, followed by the addition of anhydrous pyridine (5 mL). After stirring thoroughly for 30 min, a large amount of n-pentane (100 mL) was added to obtain a green precipitate. The precipitate was rapidly filtered and washed three times with ice-cold n-pentane, then filtered again under vacuum and dried in a vacuum drying oven at room temperature for 12 h. It was then quickly transferred to a glove box and stored at -10 °C.

[0074] Example 5: Synthesis of azobenzene polymer P (NB-Azo-C4)

[0075] Grubbs third-generation catalyst I-j is very unstable in air. Therefore, the ring-opening metathesis polymerization of azobenzene polymer P (NB-Azo-C4) was carried out in a glove box. A small azobenzene molecule I-g (0.50 g, 0.83 mmol) was placed in a 20 mL glass bottle, a magnetic shovel was added, and after three nitrogen purgings in the transition chamber, it was placed in the glove box. Dichloromethane (1.25 mL) was added to dissolve the monomer. The pre-calculated mass of Grubbs third-generation catalyst I-j was weighed into a 10 mL glass bottle in the glove box. The molar ratio of monomer to catalyst was 5:1, 8:1, ..., 300:1, 400:1, etc., corresponding one-to-one with the expected degree of polymerization. Dichloromethane (1.25 mL) was added to a glass bottle containing a weighed Grubbs third-generation catalyst I-j to dissolve the catalyst. The catalyst solution was then drawn into a 2 mL syringe and rapidly added to a solution of azobenzene small molecules I-g. After reacting for 5 min, 5 drops of vinyl ether were added to terminate the reaction. The mixture was stirred for another 10 min and then removed from the glove box. The reaction solution was diluted with 2 mL of tetrahydrofuran and rapidly passed through an alkaline alumina chromatography column to remove residual Ru metal salt. After removing the solvent by rotary evaporation, the polymer was dissolved in 2 mL of tetrahydrofuran and added dropwise to 40 mL of rapidly stirred methanol. The mixture was centrifuged at 8500 rpm for 5 min and the supernatant was discarded. The dissolution-precipitation step was repeated 3 times. The solid was then vacuum dried in an oven at 45 °C for 12 h to obtain a series of azobenzene polymers P(NB-Azo-C4) with different degrees of polymerization.

[0076] Example 6: Gel permeation chromatograms of a series of azobenzene polymers P (NB-Azo-C4) with different degrees of polymerization from Example 5.

[0077] Figure 2 The following are gel chromatograms of azobenzene polymer P(NB-Azo-C4) with different degrees of polymerization in Example 5. The specific number-average molecular weight, degree of polymerization, and polymerization distribution index (PDI) are shown in Table 1 below.

[0078]

[0079] Example 7: Ultraviolet-Visible Spectroscopy Test

[0080] Take 30 mg of solid azobenzene polymer P (NB-Azo-C4) with different degrees of polymerization (DP=50 as an example), dissolve it in 1 ml of cyclopentanone to form a solution, and spin-coat the solution onto a 2.5 × 2.5 × 0.1 cm plate. 3 On a quartz plate, the ultraviolet-visible spectrum in the wavelength range of 200-800 nm was measured using an ultraviolet spectrometer.

[0081] Example 8: Photoinduced solid-liquid transition test of azobenzene polymer P (NB-Azo-C4)

[0082] Take 2 mg of solid powder of azobenzene polymer P (NB-Azo-C4) with different degrees of polymerization, taking DP=50 as an example, place it on a glass slide, cover it with a coverslip, and irradiate it in situ with an ultraviolet lamp (wavelength 365 nm, light intensity 3.73 mW·cm). -2 After irradiating for 10 minutes, the azobenzene polymer solid powder was found to liquefy. Then, a green LED lamp (wavelength 530 nm, light intensity 13.48 mW·cm²) was used. -2 ) for 10 minutes.

[0083] Example 9: Preparation of spin-coated thin films of azobenzene polymer on flexible substrates (PI)

[0084] Take 30 mg of solid azobenzene polymer P (NB-Azo-C4) with different degrees of polymerization, dissolve each in 1 ml of cyclopentanone to form a solution, and spin-coat the solution onto a 2.5 × 2.5 × 0.01 cm plate. 3 A 200±10 nm thick azobenzene polymer film was formed on a transparent polyimide (PI) film by rotating at 1000-2000 r. The film was then dried in an oven at 45 °C for 12 h to allow the solvent to evaporate, resulting in an azobenzene polymer spin-coated film on a flexible PI substrate.

[0085] Example 10: Nanoimprinting experiment

[0086] Take azobenzene polymer P (NB-Azo-C4) films with different degrees of polymerization from Example 9 and spin-coat them. A polydimethylsiloxane (PDMS) template is attached to the film. The PDMS template consists of square micropillars arranged in a 530 nm periodic pattern, with a pillar diameter of 275 nm and a pillar height of 300 nm. Air bubbles are removed by hand, and a 1.5 kg weight is added and pressed for 5 min. The weight is then removed, and the film is exposed to ultraviolet light (wavelength 365 nm, light intensity 3.15 mW·cm²). -2 Irradiation for 5 minutes liquefies the azobenzene polymer, filling the mold. A 1.5 kg weight is then added and pressed for another 5 minutes to ensure better filling of the template. A green LED light (wavelength 530 nm, light intensity 13.48 mW·cm²) is used. -2 Irradiate for 5 minutes to solidify the imprinted structure, then slowly peel off the PDMS with tweezers to obtain the microstructure pattern nanoimprinted on the flexible substrate.

[0087] Example 11: Defect Rate Statistics

[0088] The azobenzene polymer P (NB-Azo-C4) spin-coated films with different degrees of polymerization, nanoimprinted on a flexible substrate in Example 10, were used. Five imprinted areas were randomly selected for SEM scanning. The defect rate was calculated using the number of points and defects in the SEM images. The number of columns counted in each imprinted area ranged from 14,000 to 23,000. Detailed defect rate statistics are shown in Tables 2-10 below.

[0089]

[0090]

[0091]

[0092]

[0093]

[0094]

[0095]

[0096]

[0097]

[0098] Example 12: Bending Resistance Test

[0099] Take the spin-coated films of azobenzene polymer P (NB-Azo-C4) with different degrees of polymerization nanoimprinted on a flexible substrate in Example 10. Taking DP=25 as an example, the films were bent multiple times using a tensile testing machine, pausing for 2 seconds after each bend to control displacement. A 2 cm pattern was pressed to 0.5 cm / 0.3 cm. After bending 100 times with controlled displacement, a 2 cm pattern was pressed to 0.5 cm. The number and degree of bending were then increased, with 200 bends performed using the tensile testing machine, and the 2 cm pattern was pressed to 0.3 cm. Scanning electron microscopy (SEM) was used to observe the imprinted surface. No cracks appeared on the surface of the sample with lower polymerization degree, DP=25.

[0100] Take the azobenzene polymer P (NB-Azo-C4) spin-coated films with different degrees of polymerization nanoimprinted on a flexible substrate as in Example 10, attach one end of the PI film to the bottom of a 500 g weight, flip it over and bend it, with the distance between the raised end of the weight and the tabletop being h = 1.5 cm, and hold for 30 s. Observe the imprinted surface using a scanning electron microscope (SEM). The sample with a lower degree of polymerization showed larger cracks and more severe structural damage after bending. The cracks in the sample with DP=150 began to decrease, while the samples with higher degrees of polymerization (DP=250, DP=300, DP=400) showed no cracks when bent under the same conditions.

[0101] In summary, this invention provides a method for optimizing the nanoimprinting effect of azobenzene polymer P(NB-Azo-C4) on flexible substrates by controlling its degree of polymerization (DP value). Compared with the prior art, the advantages of this invention are specifically reflected in:

[0102] By adjusting a single parameter of the polymer—the degree of polymerization (DP value)—the bending resistance and pattern integrity of the embossed structure can be systematically and significantly improved. The method is direct and the control target is clear.

[0103] While ensuring high-resolution pattern transfer, it effectively solves the core problem that flexible imprinted structures are prone to cracks and defects under dynamic bending, and is especially suitable for the manufacture of flexible optoelectronic devices (such as optical waveguides) with extremely high requirements for structural fidelity.

Claims

1. A method for improving the bending resistance of nanoimprint photoresist on flexible substrates, characterized in that: Nanoimprinting was performed on a flexible substrate using azobenzene polymer material as an imprinting photoresist. The azobenzene polymer material, abbreviated as P(NB-Azo-C4), has the following general structural formula: ； Where n represents the degree of aggregation, with a value ranging from 250 to 400.

2. The method according to claim 1, characterized in that... The nanoimprinting process includes the following steps: The azobenzene polymer material P (NB-Azo-C4) is dissolved in cyclopentanone to form a solution. This solution is spin-coated onto a flexible substrate, and the solvent is evaporated to obtain a spin-coated nanoimprint film. A transparent polydimethylsiloxane template with a microstructure is attached to the imprint film, air bubbles are removed, and pressure is applied for imprinting. The film is then irradiated with ultraviolet light to liquefy the azobenzene polymer P (NB-Azo-C4). Pressure is applied to fill the template with the liquefied azobenzene polymer P (NB-Azo-C4), and then visible light is used to solidify the imprint structure. Finally, the mold is removed.

3. The method according to claim 2, characterized in that... The azobenzene polymer material P(NB-Azo-C4) is dissolved in cyclopentanone to form a solution, and the concentration of the azobenzene polymer material in cyclopentanone is 30 mg / mL.

4. The method according to claim 2, characterized in that... The spin coating speed is 1000-2000 r / min, and the thickness of the imprinted film is controlled at 200±10 nm.

5. The method according to claim 2, characterized in that... The flexible substrate is a transparent polyimide.

6. The method according to claim 2, characterized in that... The pressure imprinting involves loading a 1.5 kg weight.

7. The method according to claim 2, characterized in that... The ultraviolet light has a wavelength of 365 nm and an intensity range of 1.7-70 mW·cm. -2 .

8. The method according to claim 2, characterized in that The visible light wavelength is 530 nm, and the light intensity range is 1.5-55 mW·cm. -2 .

Images