Dual-step sintering of Cu nanoparticles with femtosecond laser
Selective laser sintering with femtosecond (fs) lasers and metal nanoparticles (NPs) can achieve high precision and dense sub-micron features with lower porosity, low oxidation and reduced residual stress, due to the high peak power and highly localized heat affected zone. Successful sintering of metal NPs with fs laser is challenging due to the ablation caused by hot electron effects, such as blast and Coulomb explosion. Electron firstly absorbs the laser energy and becomes a hot electron gas with a temperature of several thousand Kelvins. These hot electrons can ablate nanoparticles within a couple of picoseconds, way before the energy is passed to the lattice, and hence hinder the sintering process. In this study, we proposed a dual-step sintering strategy that deposits the laser energy into metal NPs via multiple steps to lower the electron temperature significantly and while still maintaining a high enough lattice temperature for sintering in order to avoid the ablation. To demonstrate this concept, we utilized a pair of fs laser pulses with a time delay at picosecond scale to sinter Cu NPs. The delay time is chosen to be comparable with the electron-phonon coupling time so that the highest electron temperature would not be too high to cause ablation, while the final lattice temperature is still enough for sintering. When the delay time is slightly longer than the electron-phonon coupling time of Cu NPs, it is shown that the ablation area is drastically reduced and the power window for successful sintering can be extended by about 2 times. At the same time, the heat affected zone can be reduced by 66% (area). Our results showed that comparing with sintering with single femtosecond laser pulse train, with this dual-step sintering strategy: (i) the ablation could be suppressed significantly and the sintering window could be extended by twice; (ii) the heat affected zone in the glass substrate can be reduced by more than half; (iii) there is clear improvement on surface smoothness. This new strategy can be adopted for all the SLS processes with fs laser and unlock the power of fs-SLS for future applications.