Femtosecond and nanosecond laser sintering of silver nanoparticles on a flexible substrate

Pulsed-laser sintering of metal nanoparticles on flexible substrates has attracted increasing attention owing to its potential in direct printing of high-resolution patterns or fabricating flexible devices on thermally weak substrates. However, the physics of pulsed-laser sintering, including the microstructures, properties, and their correlations in different processing regimes, is not yet clearly understood. Especially, the characteristics of ultrafast laser sintering using femtosecond laser pulses without the heat accumulation effect and their comparison with those of nanosecond laser sintering are still unclear. This work analyzed the silver nanoparticle sintering process on polyethylene terephthalate substrates using a KrF nanosecond excimer laser and a Ti: sapphire femtosecond laser. The two laser sources with substantially different pulse widths were compared in terms of the sintering mechanisms, electrical conductivity, flexibility, and adhesion strength of the sintered silver films. Under optimal conditions, femtosecond laser sintering yielded better electrical conductivity and flexibility than nanosecond laser sintering. The nanosecond laser sintering occurred by solid-state diffusion (surface necking) or by full melting of the particles depending on the process condition. On the other hand, the femtosecond laser sintering took place only by inter-particle necking because the polymer substrate was damaged before melting of the particle began.