Fluorine-Free, Low-Dielectric Photosensitive Polyimides Based on Methyl Methacrylate-Functionalized Bulky Diamines for High-Fidelity UV Patterning

Redistribution layers (RDLs) are key structural components in advanced semiconductor packaging, where materials must combine low dielectric constant (Dk), photopatternability, and PFAS-free composition under low-temperature processing conditions. Conventional photosensitive polyimides (PSPIs), however, generally require thermal imidization above 350 °C and exhibit Dk above 3.0, limiting their integration with fine-pitch RDL architectures. Herein, we report a series of designed soluble PSPIs synthesized from hydroxyl-containing diamines─3,3,3′,3′-Tetramethyl-1,1′-spirobisindane-5,5′-diamino-6,6′-diol (DHSBI), 9,9′-Bis(4-aminophenyl)-9H-fluorene-2,7-diol (APFD), and 2-Amino-4-[1-(3-amino-4-hydroxyphenyl)cyclohexyl]phenol (AACP)─modified with methyl methacrylate (MMA) groups to introduce both photoreactivity and dielectric tunability. The incorporation of bulky, low-polarity MMA-functionalized diamines increases free volume and reduces chain packing, leading to significantly lower permittivity (Dk = 2.67 @ 40 GHz) while enabling direct UV patterning without additional thermal imidization. These PSPIs exhibit excellent pattern fidelity down to 20 μm, strong Cu adhesion, and a low curing temperature of 250 °C. The rational design of MMA-grafted diamine monomers demonstrates a viable route to low-Dk, photocurable, and environmentally sustainable PI systems for next-generation high-density semiconductor packaging.