Photosensitive polyimide with high pattern stability and low thermal expansion coefficient based on poly (amic acid) containing dual cross-linked groups

Abstract

To obtain negative photocured polyimides with clear photopolymerized patterns and a low thermal expansion coefficient, we synthesized a type of photosensitive polyamic acid (PAA). Using 3,3',4,4'-Biphenyltetracarboxylic dianhydride (BPDA), p-Phenylenediamine (PPD), and 3,5-Diaminobenzoic acid (DABA) containing an active carboxyl group as monomers, we synthesized a polymer chain, then grafted the photo-crosslinking groups 2-(Dimethylamino)ethyl methacrylate (DMAMA) and Cinnamyl alcohol (CA) onto different carboxyl groups in the polymer backbone, with DMAMA formed ionic bonds and CA formed ester bonds, resulting in a photosensitive polyamic acid (PSPAA) with two types of photo-crosslinking groups. After UV curing, this formed a high-density cross-linked network and was converted into polyimide through high-temperature imidization at 330°C. The CA ratio was controlled to synthesize four different PSPAAs (1, 2, 3, 4). The photolithography process parameters of the four PSPAAs and their corresponding PIs were systematically studied. The photocuring properties and thermal properties of PSPAAs and PIs were characterized using Fourier transform infrared spectroscopy (FT-IR), proton nuclear magnetic resonance (¹H-NMR), scanning electron microscopy (SEM), and other techniques. The tensile strength of PI4 could reach 118 MPa, and the thermal expansion coefficient was as low as 7.8 × 10⁻⁶°C⁻¹. The photolithographic pattern resolution of PSPAA2 was 18 μm, while the line spacing could be as low as 3 μm; PSPAA4 demonstrated good solubility resistance of the exposed area and maintained pattern integrity even after high-temperature processing. This research provides a foundation for the development of new photosensitive polyimides.

Keywords

Photosensitive polyimide polyamic acid photo-crosslinking polymer film

Introduction

Polyimide (PI) is a polymer material that integrates excellent heat resistance, dielectric properties, chemical resistance, and mechanical performance. Due to its outstanding properties and widespread applications in electronics, packaging, and coatings, it has attracted significant attention over the past decades.¹ Polyimide has been used as a passivation and protective material and an intermetallic dielectric in integrated circuits (ICs) and thin-film multilayer electronic packaging.² In these applications, polyimide offers distinct advantages, including high thermal stability, high resistivity, high breakdown voltage, and a low dielectric constant.^3,4

Traditional PI materials, however, lack photosensitivity. Therefore, packaging dielectric layers requires a photoresist coating to form patterns, followed by processes like etching and photoresist removal to create PI patterns, resulting in a relatively complex procedure. Photosensitive polyimide (PSPI) was developed as a solution.⁵ It has photosensitive properties, which allow it to retain the dielectric properties of PI while also functioning as a photolithographic patterning medium, significantly simplifying the process.⁶ PSPI is an organic material with imide rings and photosensitive groups incorporated into its molecular chain. It provides outstanding thermal stability, mechanical properties, and photosensitivity.⁷ PSPI is highly sensitive to ultraviolet light, electron beams, ion beams, and X-rays. These photosensitive groups can enhance polyimide’s performance through a cross-linked network and somewhat simplify the photolithographic process.⁸

Polyamic acid (PAA) is a polyimide precursor. Among them, photosensitive polyamic acid (PSPAA) esters or salts are formed by connecting photosensitive small molecules to the main chain of polyamic acid at the carboxyl group as an active site.⁹ The photo-induced cross-linking between photosensitive small molecules builds bridges between polyamic acid molecular chains.^10,11 This method effectively reduces the polymer’s solubility after exposure, and the commonly used photosensitive small molecules contain acrylate derivatives.¹² The key to polyamic acid salt systems is designing an appropriate photosensitive tertiary amine to form salts with polyamic acid. In contrast, the synthesis of polyamic acid esters is relatively more complex.¹³ Early polyamic acid esters were synthesized through a three-step reaction of esterification, acylation, and amidation, and later, chlorine-free synthesis methods were developed, such as amidation catalyzed by N,N′-Dicyclohexylcarbodiimide.¹⁴

Ultraviolet (UV) curing technology utilizes medium and short wavelength UV light (300-800 nm) to excite the photoinitiators in liquid UV materials, converting them into free radicals or cations, which in turn initiate the polymerization of polymer materials (resins) containing active functional groups into insoluble and non-meltable solid coatings.¹⁵ This technology offers advantages such as environmental friendliness, fast curing, high efficiency, solvent-free formulations, and low energy consumption.^10,16 Its excellent solution processability and relatively low curing temperature make it ideal for solving related problems.

In recent decades, many photosensitive cross-linking polymers with terminal double bonds have been widely studied and applied in UV-curable coatings.^17,18 Among them, PSPI with various photosensitive monomers has been widely studied. Guo et al.¹⁹ synthesized a series of negative photosensitive polyimides containing chalcone groups in the main chain and cinnamic acid or acetate groups in the side chain. These PSPIs with two photo-crosslinking groups exhibited good thermal stability and solubility in polar organic solvents. All the synthesized polyimides underwent photoinduced ring-opening reactions under UV light, and the synthesized photosensitive polyimides could induce 5CB liquid crystal molecules to align uniformly in the liquid crystal unit under UV irradiation, showing promising potential in the field of liquid crystal displays.

After that, the cross-linked sites on the main polyimide chain are continuously expanded, such as by introducing cross-linking groups at the end of the polymer chain. Liu et al.²⁰ used monamine-grafted photosensitive monomers with carboxylic acid groups to synthesize a series of negative photosensitive polyimides and studied the properties of UV-cured coatings, such as double bond conversion rate, transparency, and water absorption. The results showed that these polyimides exhibited high double bond conversion rates and transparency while maintaining low water absorption, demonstrating potential applications in optoelectronics, aerospace, and microelectronics.

At the same time, polyimide’s photo-crosslinking technology is also applied in many fields. Chung et al.⁷ synthesized a series of photosensitive polyimides incorporating photo-crosslinking groups and electro-optic chromophores within their side chains. The resultant electro-optic films demonstrated excellent solubility across various solvents. Additionally, the electro-optic coefficient of these films remained stable for up to 500h at 150°C and for 1h at 200°C. Consequently, the thermal and chemical stability of the electro-optic polyimide were significantly enhanced through the photo-crosslinking reaction, suggesting that this material is suitable for photolithographic processes.

However, as far as we know, many dual cross-linked groups type PSPIs require solubility in organic solvents; additionally, introducing two cross-linking groups necessitates lower curing temperatures to prevent the disruption of cross-linked segments. These conditions require the introduction of unique monomers, which not only increases the cost but also reduces the mechanical properties.

To address the issue of monomer selection while maintaining the stability of photosensitive groups, we developed a novel synthesis method for PSPIs, introducing two cross-linked groups during the polyamic acid stage to achieve UV-curing, which eliminates the requirement for low imidization temperatures and high solubility in organic solvents, thus providing greater flexibility in choosing monomers. We employed 3,3′,4,4′-Biphenyltetracarboxylic dianhydride, and p-Phenylenediamine to provide good mechanical properties to the material, with 3,5-Diaminobenzoic acid as a carrier for Cinnamyl alcohol grafting and 2-(Dimethylamino)ethyl methacrylate grafted onto the inherent carboxyl groups of polyamic acid, preparing a dual cross-linked group type photosensitive polyamic acid. The polyimides were synthesized through high-temperature imidization, and their thermal and optical properties were evaluated.

Experiment

Materials

3,3′,4,4′-Biphenyltetracarboxylic dianhydride (BPDA), 3,5-Diaminobenzoic acid (DABA)and p-Phenylenediamine (PPD) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Cinnamyl alcohol (CA), 2-(Dimethylamino)ethyl methacrylate (DMAMA), N-N-Dimethylacetamide (DMAc), N,N'-Dicyclohexylcarbodiimide (DCC), 4-Dimethylaminopyridine (DMAP) and Diphenyl (2,4,6-trimethylbenzoyl)phosphine oxide (TPO) were purchased from Shanghai Macklin Biochemical Co., Ltd.

Synthesis of the photosensitive polyamic acid (PSPAA)

Firstly, PPD (0.270 g, 2.5 mmol) was added into a three-necked round bottom flask and 8.48 g DMAc. Add DABA (0.380 g, 2.5 mmol) and stir for 10 minutes. BPDA (1.471 g, 5 mmol) was slowly dropped into the above solution with vigorous mechanical stirring under nitrogen for 12h. Secondly, CA (0-0.335 g, 0-2.5 mmol), the catalyst DMAP (0.097 g, 0.8 mmol), and the dehydrating agent DCC (0.124 g, 0.6 mmol) were slowly added into the above solution, and continue the reaction for 24 hours. Then, add DMAMA (1.572 g, 10 mmol) into the flask and react for 2 hours. Add TPO (0.115 g) and react for 2 hours. Finally, obtain a photosensitive polyamic acid solution with varying CA contents. Depending on the CA grafting amount, the prepared photosensitive polyamic acids are PSPAA1, PSPAA2, PSPAA3, and PSPAA4 (Table 1).

Table 1.

The molar ratios of each component in the reaction mixtures (mmol).

Samples	BPDA	DABA, PDA	DMAMA	CA
PSPAA1	5	2.5	10	0
PSPAA2	5	2.5	10	0.5
PSPAA3	5	2.5	10	1.5
PSPAA4	5	2.5	10	2.5

Preparation of UV-cured coatings

PSPAA was filtered through a 0.45 μm polytetrafluoroethylene membrane to remove the impurities. The solution was then placed under a vacuum to remove the remaining bubbles. A certain amount of PSPAA was then applied to a circular glass sheet using a pipette for film coating. The spin coater was used with the following parameters: a low speed of 800 r/min for 10 s, followed by a high speed of 4500 r/min for 20 s. The film was then placed in an 80°C oven for pre-baking for 2.5 minutes to remove most of the solvent, causing the polymer surface to lose its tackiness and adhere to the circular glass surface. The polymer film was exposed to 365 nm UV light under a photomask for 5 seconds, and the exposure dose was 750 mJ/cm². It was then developed in DMAc solution for a certain period (with varying development times for different samples), followed by washing off the surface of DMAc with deionized water. The film was then baked at 80°C for 8 minutes to remove moisture, producing a polymer film with a defined pattern (Figure 1).

Preparation of polyimide (PI) coatings

To further evaluate the performance of the polymer after imidization, PSPAA was spin-coated onto a glass sheet with parameters of 500 r/min for 10 seconds and 1000 r/min for 15 seconds, followed by heating at 90°C for 30 minutes to obtain a thick film. The polymer was then exposed to UV light for 30 seconds, placed in an oven, and heated to 250°C for 10 minutes, followed by heating to 330°C for 10 minutes. Based on the different types of PSPAA, polyimide films PI1, PI2, PI3, and PI4 were obtained (Figure 2).

The characterizations of PSPAA and PI

Fourier transform infrared spectroscopy (FTIR) was carried out using a Nicolet IS50 instrument. It uses ATR mode to measure polymer films, with a scan range of 500 to 4000 cm⁻¹ and a resolution of 4 cm⁻¹; The Bruker Avance 600 ¹H-NMR spectrometer used tetramethylsilane (TMS) as the internal standard, dimethyl sulfoxide as the solvent, and PSPAA after solvent removal was used as the solute.

Used the Mettler Toledo STAR Thermogravimetric Analyzer (TGA1100SF, Mettler Toledo, Switzerland), TGA was performed on PI samples (about 5 mg), with a heating rate of 15°C/min, temperature range of 25 to 800°C, nitrogen as the purge gas, and a flow rate of 50 mL/min; A TMA450 thermomechanical analyzer from TA Instruments, USA, was used to analyze the thermal expansion properties of PI films, under conditions of heating to 50°C in a nitrogen atmosphere, then increased the temperature at a rate of 10°C/min to 500°C, with the aspect ratio of the test sample was 15 mm × 5 mm; and the thermal expansion coefficient was calculated according to the following formula:

α = \frac{1}{L} \frac{d l}{d t}

In the formula, α is the thermal expansion coefficient (°C⁻¹), L is the original length of the PI film, and ΔL is the relative elongation of the sample as the temperature rises from t₁ (°C) to t₂ (°C).

Water absorption test: Cut the PI film into 20 mm × 20 mm standard samples, dried at 140°C for about 1 hour, then measured its dry weight G₁. The samples were submerged in deionized water at room temperature for 48 hours, then dried off and weighed to measure the wet weight G₂. The formula for the water absorption rate (W) was:

W (wt %) = \frac{G_{2} - G_{1}}{G_{1}} \times 100 %

The thickness of the films before and after development was measured using the ET-150 profilometer made by Kosaka Corporation, Japan, with a development time set at 3 minutes after exposure; Contact angle goniometer (OCA40 Micro): droplet volume of 3 μL, testing speed at medium; Scanning electron microscopy (SEM) analysis used the S-3400 model from Hitachi, Japan, to observe patterns after development, with test conditions including an acceleration voltage of 15 kV and gold sputtering of the polymer patterns beforehand.

Results and discussion

Discussion of the adding amount of DABA

Before conducting the aforementioned experiments, it was necessary to determine the amount of DABA added. Because DABA itself contains a nucleophilic carboxyl group, which has strong electron-withdrawing properties, leading to a lower molecular weight of polyamic acid synthesized at room temperature. The diamine monomers used in this experiment are DABA and PPD, where DABA provides cross-linking sites for CA, and PPD enhances the mechanical properties of the polymer chains. Based on the addition levels of DABA in the diamines—30%, 50%, and 70%—the three samples were named PI30, PI50, and PI70, respectively. As shown in Figure 3, with the increase in DABA content, the elongation at the break of PI decreases continuously while the thermal degradation rate increases. Hence, to maintain adequate cross-linking sites for PI, elongation at break above 5%, and a thermal decomposition temperature (5% weight loss) over 400°C, this study determined the optimal DABA addition ratio to be 50%.

Synthesis of PSPAA and PI

The synthesis of PSPAA and PI is shown in (Figure 2). The carboxyl group of DABA provides an active site for incorporating the photoreactive unit CA, while BPDA and PPD impart excellent mechanical properties to the film. PSPAA is the polymer chain with photoreactive groups, and PI is the polymer chain that loses its photoreactive groups after imidization. FT-IR structural tests were conducted to verify the formation of the polymer chain and the incorporation of the two photoreactive cross-linkers. The PSPAA and PI FT-IR spectra are shown in Figure 4(a)/(b). From Figure 4(a), the characteristic absorptions of the amide groups can be observed: the C = O stretching vibration at 1664 cm⁻¹ and the C-N stretching vibration at 1558 cm⁻¹, indicating the successful preparation of polyamic acid. The characteristic absorption band of the C = C double bond on CA appears at 1608 cm⁻¹, confirming the presence of a cinnamyl group on the polyamic acid. The bending vibration absorption of = CH₂ appears at 894 cm⁻¹, indicating the successful grafting of DMAMA onto the polyamic acid backbone. From Figure 4(b), three characteristic absorptions of the imide ring in PI can be observed: the C = O stretching vibration at 1778 cm⁻¹, the C-N-C bond stretching vibration at 1378 cm⁻¹, and the imide ring deformation’s vibration at 725 cm⁻¹. Moreover, the C = O stretching vibration around 1660 cm⁻¹ for polyamic acid has disappeared in the figure, indicating that the polymer underwent most imidization after thermal treatment. In Figure 4(c), PSPAA1/2/3/4 grafted with the photoreactive monomer DMAMA shows the characteristic peaks of -C = CH₂ at δ = 5.69/6.04 ppm, indicating the successful grafting of the photoreactive unit onto the polyamic acid molecule. Compared to PSPAA1, PSPAA2/3/4 shows the characteristic peaks of -C = C- at δ = 6.34/6.55, indicating that the CA photoreactive unit had been successfully grafted onto the polyamic acid molecule.

Thermal analysis of PI films

Polyimide must possess high resistance to thermal deformation and decomposition to meet the application requirements for heat-resistant coatings, packaging materials, and buffer layers. This study analyzed the thermal properties of polyimide using TGA and TMA.

In the TGA of Figure 5(a)/(b), the temperatures of 5% weight loss (Td₅) and 10% weight loss (Td₁₀), as well as the residual weight (R_w) at 800°C for PI1/2/3/4 are analyzed, and the results are summarized in Table 2. The Td₅ of the PI films are 368°C, 361°C, 317°C, and 328°C, respectively. Td₁₀ values reached 432°C, 413°C, 373°C, and 382°C, respectively; the residual weight R_w at 800°C exceeded 53%; and the maximum weight loss rates (DTG) are observed at 631°C, 618°C, 614°C, and 613°C, respectively. Overall, the materials exhibit excellent thermal performance.

Table 2.

TG and DTG data of PI1/2/3/4.

Materials	T_d5/°C	T_d10/°C	800°C residual rate/%	D_TG/°C
PI1	368	432	57.3	631
PI2	361	413	56.1	618
PI3	317	373	53.2	614
PI4	328	382	56.7	613

The lower Td₅ could be due to the residual DMAMA, with a significant weight loss rate observed at 300∼350°C. The increase in weight loss rate between 350 and 500°C could be caused by decarboxylation cross-linking reactions between benzoic acids on the polymer backbone, as illustrated in Figure 6.²¹ After 600°C, the DTG reaches its peak, corresponding to the degradation of the polyimide backbone.

Thermal stability of materials refers to their ability to maintain shape and size during heating or cooling. As shown in Figure 7, TMA was used to measure the linear coefficient of thermal expansion (CTE) and the glass transition temperature (T_g) of four PI films. The CTE values of PI1/2/3/4 at 100 °C–250°C are 21.92 × 10⁻⁶, 33.22 × 10⁻⁶, 24.91 × 10⁻⁶, and 7.85 × 10⁻⁶°C⁻¹, respectively, indicating that PI4 has lower CTE values compared to PI1/2/3. This phenomenon might result from the higher incorporation of the highly cross-linked side group CA in its precursor PSPAA4, leaving more residues after imidization and thus improving dimensional stability. With the increase in CA content, the T_g values of PI1/2/3/4 are 300.68°C, 333.24°C, 297.91°C, and 333.05°C, respectively, indicating little change in T_g for PI2/3/4 compared to PI1. For typical photocurable polyimide materials, the T_g of the polyimide prepared in this study has been significantly improved, indicating excellent thermal resistance.

In the tensile test shown in Figure 8(a)/(b), the elongations at the break for PI1/2/3/4 are 2.61%, 2.81%, 3.95%, and 3.57%, respectively, with break stress values of 90.28 MPa, 80.85 MPa, 111.65 MPa, and 118.79 MPa. It can be seen that after grafting the photosensitive monomer CA, the elongation of PI2/3/4 films has slightly improved compared to PI1. At the same time, the decrease in the tensile strength of PI2 may be attributed to the addition of catalysts, dehydrating agents, and photoinitiators, with the small amount of CA added having a minimal effect on the strength enhancement. The Young’s modulus of PI1/2/3/4 are similar, indicating that the incorporation of the photosensitive monomer CA has little effect on the Young’s modulus.

Development image analysis of PSPAA and PI

During development, it is crucial to ensure adequate time for development to remove any residual material between the lines while ensuring that the lines are not overly damaged. To demonstrate the solubility resistance of the exposed area, we used a step profiler to measure the thickness of the films, as shown in Figure 9(a), the thickness changes of PSPAA1/2/3/4 films before and after development were measured, with change rates of 19.33%, 11.98%, 8.83%, and 6.23%, respectively. This result is attributed to introducing more carbon-carbon double bonds in the polymer chain, resulting in a higher cross-linking density after UV exposure. Figure 9(b) shows the water absorption angle and water contact angle of PI films. The water absorption rates of PI1/2/3/4 are 5.7%, 6.0%, 3.9%, and 1.1%, and the contact angles are 59°, 67°, 58°, and 69°, respectively. These results may be due to the addition of CA, which alters PI’s pore structure and surface roughness.

Based on the process parameters mentioned earlier, the SEM images of the four photosensitive resins, PSPAA1/2/3/4, after exposure and development are shown in Figure 10. In the Figure 10(a), corresponding to PSPAA1, some patterns have fused. In the magnified image, it can be observed that the lines have been significantly eroded, with edges that should theoretically be right angles turned into obtuse ones. In Figure 10(b), corresponding to PSPAA2, the lines are not noticeably damaged, and there is no residual colloid between the lines. The resolution has significantly improved, reaching 18 μm, with line spacing of 3 μm. This improvement may be due to the addition of CA, which increases the cross-linking density in the exposed area, making it more resistant to dissolution in the developer. In Figure 10(c), corresponding to PSPAA3, the increase in CA content enhances the polymer’s cross-linking density, resulting in line profiles closer to rectangles after development. In Figure 10(d), corresponding to PSPAA4, there is no adhesion between the lines, and the line corners are closer to right angles. In the magnified image, it is clear that the line roughness is lower, indicating an improvement in the surface’s resistance to erosion. The introduction of CA significantly enhances the cross-linking density in the exposed area, making the pattern more resistant to damage by the developer.

The corresponding polyimide patterns are obtained by heating PSPAA1 and PSPAA4 from Figure 10 at 330°C for 10 minutes. As shown in Figure 11(a)/(b), the particle precipitation and cracking of PI1 are observed, along with the rupture of some impurities, which damage the pattern’s surface. Compared to PSPAA4, the pattern of PI4 shows little change. It is evident that the introduction of CA dramatically improves the thermal stability of polyamic acid.

Conclusion

A series of photosensitive polyamide acids containing two kinds of double bonds were synthesized and characterized. The structure of polyamide acids and polyimides, mechanical properties, and SEM morphology were investigated. The results showed that 3,5-Diaminobenzoic acid provides cross-linking sites for introducing Cinnamyl alcohol. However, it also led to a decrease in the mechanical properties of the polymer chain and an increase in the rate of thermal weight loss. The introduction of Cinnamyl alcohol enabled the PI film to demonstrate excellent thermal expansion coefficient and water absorption properties. Additionally, SEM images revealed that the introduction of more cross-linking groups sharpens the pattern lines and enhances thermal stability. Therefore, it is an efficient and straightforward way to obtain outstanding lithographic properties by incorporating Cinnamyl alcohol in polyamide acids. It is highly plausible that UV-cured coatings based on these double bonds have potential applications in advanced microelectronics and optoelectronic fields.

Supplemental Material

Supplemental Material - Photosensitive polyimide with high pattern stability and low thermal expansion coefficient based on poly (amic acid) containing dual cross-linked groups

Supplemental Material for Photosensitive polyimide with high pattern stability and low thermal expansion coefficient based on poly (amic acid) containing dual cross-linked groups by Zhou, Lei, Chen Shengyu, Fei Chentao and Yonggang Min in High Performance Polymers

Footnotes

Acknowledgments

The authors would like to acknowledge support received from Guangdong University of Technology.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Yonggang Min

Data availability statement

The datasets generated during and analyzed during the current study are available from the corresponding author on reasonable request.

Supplemental Material

Supplemental material for this article is available online.

References

Chae

Yang

Lee

. Photo-induced aminobenzoate group formation on a polymeric photobase generator bearing N-phenylcarbamate groups and its application to color imaging material. Polymer 2014; 55: 2362–2368. DOI: 10.1016/j.polymer.2014.03.037.

Fukukawa

Shibasaki

Ueda

. Direct patterning of poly(amic acid) and low-temperature imidization using a photo-base generator. Polym Adv Technol 2006; 17: 131–136. DOI: 10.1002/pat.661.

Higashihara

Ueda

. Recent progress in thermally stable and photosensitive polymers. Polym J 2018; 50: 57–76. DOI: 10.1038/pj.2017.46.

Tseng

Lin

Kuo

, et al. An ultra heat-resistant polyimide formulated with photo-base generator for alkaline-developable, negative-type photoresist. Reactive Funct Polym 2020; 157: 1. DOI: 10.1016/j.reactfunctpolym.2020.104760.

Kim

, et al. Fabrication of highly flexible electromagnetic interference shielding polyimide carbon black composite using hot-pressing method. Compos B Eng 2021; 221: 1. DOI: 10.1016/j.compositesb.2021.109010.

Gao

Wang

Jia

, et al. Intrinsically negative photosensitive polyimides with enhanced high-temperature dimensional stability and optical transparency for advanced optical applications via simultaneous incorporation of trifluoromethyl and benzanilide units: preparation and properties. Polymers 2022; 14: 1. DOI: 10.3390/polym14183733.

Chung

Seo

Lim

, et al. A photo-functional electro-optic polyimide with excellent high-temperature stability. Dyes Pigments 2019; 163: 547–552. DOI: 10.1016/j.dyepig.2018.12.049.

Dong

Shen

, et al. Microfabrication of functional polyimide films and microstructures for flexible MEMS applications. Microsystems & Nanoengineering 2023; 9: 1. DOI: 10.1038/s41378-023-00503-5.

Dick

Bell

Luke

, et al. High aspect ratio patterning of photosensitive polyimide with low thermal expansion coefficient and low dielectric constant. J Nanolithogr MEMS, MOEMS 2016; 15: 1. DOI: 10.1117/1.Jmm.15.3.033503.

10.

Yeh

Ueda

Hsu

. Alkaline-developable positive-type photosensitive polyimide with high mechanical strength and high resolution based on chain extendable poly(amic acid), thermally degradable cross-linker and photoacid generator. J Polym Sci 2020; 58: 948–955. DOI: 10.1002/pol.20190230.

11.

Zhu

Wang

. Preparation of a novel ternary photosensitive polyimide for liquid crystal alignment films. Liq Cryst 2021; 48: 2200–2208. DOI: 10.1080/02678292.2021.1936233.

12.

Croft

Guo

, et al. Recent development of polyimides: synthesis, processing, and application in gas separation. J Polym Sci 2021; 59: 943–962. DOI: 10.1002/pol.20210001.

13.

Haruki

Oda

Wasada

, et al. Deposition of fluorinated polyimide consisting of 6FDA and TFDB into microscale trenches using supercritical carbon dioxide. J Supercrit Fluids 2017; 119: 238–244. DOI: 10.1016/j.supflu.2016.10.001.

14.

Chang

Tseng

Liu

, et al. Investigating the structure-sensitivity relationship of photosensitive polyimide formulated by using a photobase generator. J Polym Sci 2023; 61: 2122–2132. DOI: 10.1002/pol.20230230.

15.

Ogura

Higashihara

Ueda

. Direct patterning of poly(amic acid) and low-temperature imidization using a crosslinker, a photoacid generator, and a thermobase generator. J Polym Sci Polym Chem 2009; 47: 3362–3369. DOI: 10.1002/pola.23411.

16.

Ogura

Mizoguchi

Ueda

. Development of negative-type photosensitive polyimide, based on poly(amic acid)s, photo base generator and thermal base generator. J Photopolym Sci Technol 2008; 21: 125–130. DOI: 10.2494/photopolymer.21.125.

17.

Jang

Suk

Kim

, et al. Direct photo-patternable, low-temperature processable polyimide gate insulator for pentacene thin-film transistors. Org Electron 2012; 13: 1665–1670. DOI: 10.1016/j.orgel.2012.05.024.

18.

Song

Heo

Baek

, et al. The improved photosensitivity of photosensitive polyimides containing o-nitrobenzyl ether groups induced by the addition of photoacid generator. J Polym Sci 2021; 59: 340–352. DOI: 10.1002/pol.20200851.

19.

Guo

Wang

. Polyimides with main-chain photosensitive groups: synthesis, characterization and their properties as liquid crystal alignment layers. Eur Polym J 2009; 45: 888–898. DOI: 10.1016/j.eurpolymj.2008.11.038.

20.

Liu

Wang

Lin

, et al. Synthesis and property of fluorinated polyimides with double bond end groups for UV-cured coating. Prog Org Coating 2016; 99: 103–109. DOI: 10.1016/j.porgcoat.2016.05.010.

21.

Liu

Qiu

Quan

, et al. Fine-tuned thermally cross-linkable 6FDA-based polyimide membranes for aggressive natural gas separation. J Membr Sci 2021; 635: 1. DOI: 10.1016/j.memsci.2021.119474.

Supplementary Material

Please find the following supplemental material available below.

For Open Access articles published under a Creative Commons License, all supplemental material carries the same license as the article it is associated with.

For non-Open Access articles published, all supplemental material carries a non-exclusive license, and permission requests for re-use of supplemental material or any part of supplemental material shall be sent directly to the copyright owner as specified in the copyright notice associated with the article.

0.00 MB

0.11 MB