Substrate-heated femtosecond sintering of densified Cu nanoparticle films

Femtosecond (fs) laser sintering of metallic nanoparticles offers a powerful route for additive manufacturing of conductive layers, but its application to copper nanoparticles (Cu NPs) is constrained by hot-electron-driven ablation and a narrow sintering window. Our prior studies showed that double-pulse fs irradiation can mitigate these effects by lowering the peak electron temperature. Building on this foundation, the present work demonstrates that integrating controlled substrate heating with double-pulse femtosecond sintering improves film quality by suppressing ablation and enhancing densification. Systematic evaluation under four thermal conditions—ambient, 60 °C, 120 °C, and sequential 60→120 °C—shows that substrate heating promotes more uniform microstructures and reduces roughness from Sa = 0.385 μm to 0.101 μm. The sequential heating strategy delivers the best film quality, achieving 97–98% coverage, < 3% porosity, and the highest hardness of 189.7 ± 54.8 MPa. Substrate heating also strengthens reductive sintering, increasing the measured Cu/O ratio to 23.40 at 60 °C and reducing oxygen content relative to the non-heating case. Overall, this combined thermal-ultrafast approach enables Cu films with improved oxidation resistance, higher film density and reduced porosity, although a dedicated long-term oxidation study was not performed.