Electrical and microstructure characteristics of SU8–Cu composite thin film fabricated using femtosecond laser direct writing

Abstract

Through photo-patterning by a femtosecond laser direct writing on a spin-castable composite, this study presents a novel approach for fabricating highly conductive composite microstructures. Composite material was developed using SU8 (glycidyl ether of bisphenol A) and copper precursors, particularly copper (II) nitrate trihydrate and copper (II) chloride dehydrate. Copper was employed as the filler due to its excellent electrical conductivity and cost-effectiveness. The synthesized composite solution was first spin-coated on a substrate, and the dried thin film was then subjected to femtosecond laser direct writing to form pattern microstructures. Femtosecond laser irradiation induced two-photon absorption that led to polymerization of SU8, reduction of copper ions and sintering of copper particles. The resulting surface morphology of microstructures was affected by scanning speed, as low speed subsequently led to heat accumulation and ablation, while the line width was mainly determined by laser power. Electrical resistance decreases with the increase in scanning speed until an optimum scanning speed was achieved. On the other hand, as laser power increased, line width increased, while resistance decreased. Electrical conductivity of 365.5 S/m was achieved, which is a leap of advancement as compared to pure SU8 with conductivity of 10–14 S/m.