Femtosecond laser sintering of silver nanoparticles for conductive thin-film fabrication

Femtosecond laser sintering of metal nanoparticles for fabricating thin metal films/patterns has drawn substantial attention owing to its potential in fabrication of thin-film electrodes in various applications. In such applications, the electrical conductivity and the adhesion strength of the sintered film need to be optimized by forming desired microstructures. However, the physical mechanisms of sintering by ultrashort laser pulses without the effect of inter-pulse heat accumulation, especially microstructure formation, have not yet been clarified. Also, the relation between the microstructure of various sintered films and their properties are largely unknown. In this work, the microstructures and the properties (electrical conductivity and adhesion strength) of sintered film are analyzed by varying the process conditions for Ti:sapphire femtosecond laser sintering of silver nanoparticles. Sintering occurred mainly by surface necking at low laser fluences while by melting of the particles at high laser fluences. When laser fluence was further increased, the balling phenomenon occurred by capillary instability of the molten pool. These sintering phenomena were largely similar to those observed in typical thermal sintering. Both the electrical conductivity and the adhesion strength increased with the laser fluence to reach maximum when melting of the particles began before triggering the balling phenomenon. The maximum electrical resistivity ~ 8.7 μΩ cm obtained in this work was similar to that obtained by other thermal-sintering processes.