Negative-tone photosensitive polyimide with high transparency

Negative-tone photosensitive polyimides (n-PSPI) with high transparency at visible wavelength region (400–700 nm) were prepared by dissolving photosensitive polyimide precursor -poly(amide ester)(PAE) resins, photoinitiator, photocrosslinker and other additives in organic solvents. The PAE resins were synthesized by polycondensation of a fluorinated aromatic diamine with an aromatic diester dichloride or its mixture. The n-PSPI coating solutions showed good photolithographic performance with resolution of 8 µm (via) and 7 µm (line) at 5 µm of the thermally cured film thickness. After thermally cured at 350 °C/1h, the typical PSPI film exhibited excellent thermal properties with Tg of 344 °C and Td5 of 525 °C, respectively. The mechanical properties with elongation at breakage of as high as 42.2% and tensile strength of 133.7 MPa were also measured. Additionally, the thermally cured PSPI films exhibited excellent transparency with UV cuttoff wavelength of 362 nm, transmittance of 90.5% at 450 nm, and 95.5% at 550 nm, as well as yellowness indices of less than 6.55. Furthermore, theoretical simulations were used to prove that trifluoromethyl group in the backbone structure of PSPI precursor resin can effectively improve the transparency of PSPI films.