Aromatic polyimides have excellent thermal stability, mechanical strength and toughness, high electric insulating properties, low dielectric constants and dissipation factors, and high radiation and wear resistance, among other properties, and can be processed into a variety of materials, including films, fibers, carbon fiber composites, engineering plastics, foams, porous membranes, coatings, etc. Aromatic polyimide materials have found widespread use in a variety of high-tech domains, including electric insulating, microelectronics and optoelectronics, aerospace and aviation industries, and so on, due to their superior combination characteristics and variable processability. In recent years, there have been many publications on aromatic polyimide materials, including several books available to readers. In this review, the representative progress in aromatic polyimide films for electronic applications, especially in our laboratory, will be described.

Fluorinated aromatic polyimide (FAPI) films with rigid polymer backbones have been prepared by chemical imidization approach. The polyimide films exhibited excellent mechanical properties including elastic modulus of up to 8.4 GPa and tensile strength of up to 326.7 MPa, and outstanding thermal stability including glass transition temperature (T_g) of 346.3–351.6 °C and thermal decomposition temperature in air (T_d5) of 544.1–612.3 °C, as well as high colorless transmittance of >81.2% at 500 nm. Moreover, the polyimide films showed stable dielectric constant and low dielectric loss at 10–60 GHz, attributed to the close packing of rigid polymer backbones that limited the deflection of the dipole in the electric field. Molecular dynamics simulation was also established to describe the relationship of molecular structure and dielectric loss.

The thermal and mechanical properties of the chemically imidized polyimide (CIPI) films and thermally imidized polyimide (TIPI) films were investigated systematically. Experimental results indicated that the CIPI films show dramatically enhanced tensile strength and modulus with obviously reduced coefficient of thermal expansion (CTE) in comparison with TIPI films. These enhancements results from the high in-plane orientation and close packing of the CIPI backbones. Compared with thermal imidization which starts at about 140 °C, the chemical imidization activated by acetic anhydride and isoquinoline initiates the cyclization even at room temperature. The resulting imide rings restrict the mobility of polymer chains and lead to the in-plane orientation with solvent evaporation. Additionally, fewer small molecules remain in the films after treated at 120 °C by chemical imidization than by thermal imidization. The polymer chain plasticization caused by the evaporation of small molecules at high temperature is obviously restricted. Moreover, the partially imidized polymer inhibits the decomposition of mainchains that occurs at subsequent high temperature process, being beneficial to the formation of high molecular weight PI films. Hence, chemical imidization pathway shows apparent advantage to produce PI films with great combined properties, including high modulus, strength and toughness, as well as high thermal dimension stability etc.

Low-temperature curable negative-tone photosensitive polyimide (n-LTPI) viscous solutions were prepared by dissolving photo-crosslinkable poly (amic ester) (pc-PAE) resin, photophotocrosslinker, photoinitiator, and the heteroaromatic base as curing catalysts, and other additives in organic solvents. Among them, the pc-PAE resin was synthesized by polycondensation of aromatic diacid chloride and diester of 2-ethoxymathacrylate, aromatic diamines in aprotic solvents. After being spun-coated on a silicon wafer surface, soft-baked, exposed to UV light, and developed, the n-LTPI with 2% of imidazole (IMZ) as a curing catalyst produced high-quality photo-patterns with line via resolution of 5 μm at 5 μm film thickness. The photo-patterned polymer films thermally cured at 230 °C/2 h in nitrogen showed 100% of the imidization degree (ID) determined by in situ FT-IR spectroscopy. The thermally cured polymer films exhibited great combined mechanical and thermal properties, including mechanical properties with tensile strength of as high as 189.0 MPa, tensile modulus of 3.7 GP, and elongation at breakage of 59.2%, as well as glass transition temperature of 282.0 °C, showing great potential in advanced microelectronic packaging applications.

Carborane-containing aromatic polyimide (CPI) films with ultrahigh thermo-oxidative stability at 700 °C have been prepared by casting poly(amic acid) (PAA) resin solution on a glass surface, followed by thermal imidization at elevated temperatures. The PAA solution was prepared by copolymerization of an aromatic dianhydride and an aromatic diamine mixture, including carborane-containing aromatic diamine in an aprotic solvent. The CPI films showed excellent thermo-oxidative stability at 700 °C due to the multilayered protection layers formed on the film surface by thermal conversion of the carborane group into boron oxides. The boron oxide layer enhanced the degradation activation energy and suppressed the direct contact of inner polymer materials with oxygen molecules in a high-temperature environment, acting as a “self-healing” skin layer on the polyimide materials. The CPI-50 film was still flexible and maintained 50% retention of mechanical strength even after thermo-oxidative aging at 700 °C/5 min. The mechanism of thermo-oxidative degradation was proposed.

Solution-processable polyimide (PI) materials are attractive for applications in future flexible optoelectronics, owing to their excellent thermal and chemical resistance, inherent flexibility, and superior dielectric properties. They are widely integrated and investigated in various semiconductor devices. Here, the unique characteristics of PIs and their applications in optoelectronics are analyzed and summarized. A general introduction from the material aspect is given, followed by detailing the PI applications in multiple devices, including field-effect transistors, capacitors, photodetectors, and memory devices. In this review, the uniqueness and potential of PIs for next-generation optoelectronics are fully demonstrated, and the underlying challenges are proposed.