Molecular topology-driven benzocyclobutene-based ultralow dielectrics with copper-matched low thermal expansion†

Abstract

To address the critical challenge of balancing ultralow dielectric constant (k) with low coefficient of thermal expansion (CTE) in high-frequency electronic applications, this study develops a series of tri-armed benzocyclobutene (BCB)-based resins via rational molecular design. Five functional monomers (Ph-BCB, Ph-ene-BCB, Ph-yne-BCB, TPA-yne-BCB, TPB-yne-BCB) were synthesized through Suzuki, Heck, and Sonogashira coupling reactions, followed by thermal curing to form crosslinked polymers. The introduction of branched architectures and rigid conjugated cores effectively enhanced free volume fraction while suppressing molecular chain mobility, achieving synergistic optimization of dielectric and thermomechanical properties. The cured resins exhibited exceptional performance: dielectric constants as low as 1.83 (TPA-yne-BCB) at 1 kHz, dielectric loss below 0.0015, and CTE values ranging from 19.23–34.63 ppm °C⁻¹, closely matching copper (16 ppm °C⁻¹). The SAXS and WAXS analyses confirmed that enlarged free volume and reduced polarization from optimized topology were key to low-k performance. Additionally, the materials demonstrated outstanding thermal stability (5% weight loss >500 °C), high mechanical strength (elastic modulus up to 10 GPa), and hydrophobicity (water absorption <2%). This work provides a groundbreaking strategy for designing high-performance dielectric materials for 5G millimeter-wave packaging, flexible electronics, and 3D heterogeneous integration.

Keywords: Benzocyclobutene; Ultralow dielectric constant; Low thermal expansion; Tri-armed monomer; Branched polymers.