Synthesis of a fluorinated photoresist for optical waveguide devices

A linear fluorinated bis-phenol-A novolac resin (LFAR) for optical waveguide was synthesized based on 4,4′-(hexafluoro-isopropylidene)diphenol, epoxy chloropropane and formaldehyde. Negative fluorinated photoresist (FP) was made by composing the LFAR, diphenyl iodonium salt and solvent. The film, which was made by spin-coating FP, had good UV light lithograph sensitivity, large hardness and high glass transition temperature (Tg>170°C, after crosslinking). Low-loss optical waveguides with very smooth top surface were fabricated from the resulting FP by direct UV exposure and chemical development. For waveguides without upper cladding, the propagation loss of the channel waveguides was measured to be 0.21 dB/cm at 1550 nm.