Synthesis and characterization of photosensitive benzocyclobutene‐functionalized siloxane thermosets

Two methylphenylsiloxane monomers with crosslinkable benzocyclobutene functionalities at the terminal positions, 1,1,5,5‐dimethyldiphenyl‐1,1,5,5‐di[2′‐(4′‐benzocyclobutenyl)vinyl]‐3,3‐diphenyltrisiloxane (BCB‐1) and 1,1,3,3‐dimethyl‐diphenyl‐1,1,3,3‐di[2′‐(4′‐benzocyclobutenyl)vinyl]disiloxane (BCB‐2) were prepared and characterized. By heating the solution of BCB‐1 and BCB‐2 in mesitylene, two partially polymerized resins of BCB‐1B and BCB‐2B with high molecular weight were also achieved. The monomers and their oligomers fully cured at temperatures above 250 °C. Cured BCB‐1 and BCB‐2 exhibited high T g (257 and 383 °C) and good thermal stability ( T 5% > 472 °C both in N 2 and in air). They also demonstrated low dielectric constants (2.69 and 2.66), low dissipation factors (2.36 and 2.23), and low water absorptions (0.20% and 0.17%). Moreover, a negative photosensitive formulation derived from BCB‐1B in combination with 2,6‐bis(4‐azidobenzylidene)‐4‐methylcyclohexanone (BAC‐M) as a photosensitive agent has been developed. The photosensitive composition, BCB‐1B containing 5 wt % BAC‐M, showed a sensitivity of 550 mJ/cm 2 and a contrast of 1.96 when it was exposed to a 365 nm light (i‐line) and developed with cyclohexanone at 25 °C. A fine negative image of 10 μm line‐and‐space pattern was also printed in a film which was exposed to 700 mJ/cm 2 of i‐line by contact‐printing mode. The negative image can be maintained without any pattern deformation in the curing process.