Dynamics of double-pulse laser printing of copper microstructures

Abstract

Laser induced forward transfer process can be implemented in a double-pulse scheme where a solid thin film deposited on a transparent donor substrate is irradiated by two synchronized lasers. In a recently demonstrated methodology, a long pulse is first applied to melt the film and an appropriately delayed ultrashort laser pulse initiates material transfer in the liquid phase toward a receiver substrate. This provides a versatile method to print high-resolution (<2 µm) patterns with a long working distance (>40 µm). In this paper we focus on the study of the dynamical aspects associated with these printing performances. The temperature evolution of the thin copper film during irradiation with a quasi-continuous wave (QCW) pulse is calculated. By combining the calculations with time-resolved imaging experiments, we reveal the influence of the copper film temperature and molten metal diameter on the ejection dynamics. Characterization of the transferred materials shows that the delay between the two laser pulses is a control parameter for the shape and volume of the printed structures. This is finally exploited to demonstrate high-precision printing of different debris-free microstructures onto a Si receiver substrate set as far as 60 µm away from the donor film.

Laser-Induced Forward Transfer Applications in Micro-engineering

“…A microsecond laser pulse is used to melt the relatively thick copper donor film (1 μm thick), while a delayed ultrashort pulse is then used to propel the material onto the receiving substrate below. The ejection dynamics of the process are studied and reveal that the delay between the subsequent pulses can be used to control both the shape and volume of the printed voxels (Li et al 2019). A high aspect ratio copper microstructure can be seen in Fig.…”

Jetting regimes of double-pulse laser-induced forward transfer

“…We have shown in a previous work how the local temperature of the copper film can evolve from the room temperature to its vaporization point, and the corresponding molten pool diameter is changed during the irradiation by the QCW pulse. In the present investigations, we have chosen to deliver the femtosecond pulse with a fixed delay of 275 µs after the rising edge of the QCW pulse so that the molten pool diameter is always maintained at 20 µm and the temperature at the center of the molten pool is 1590 K according to calculations [36]. Under these conditions, liquid copper jetting behaviors induced by different femtosecond laser fluences are studied.…”

“…This method appears to be a versatile method to print high-resolution (< 2 µm) patterns with a long working distance (> 40 µm). It has successfully generated liquid nano-jets without debris from solid donor films of various materials (Cu, Au), leading to well-defined and clean printed structures [35,36].…”

“…In this study, a QCW laser pulse with a duration of 383 µs and a pulse energy of 770µJ ± 70µJ is systematically applied. With this irradiation, after 150 µs, donor film starts melt from irradiation the center [36]. Even if such a long pulse is not needed to melt the whole film thickness, these conditions guarantee a clear separation between the slow melting process and the fast ejection process.…”

“…Even if such a long pulse is not needed to melt the whole film thickness, these conditions guarantee a clear separation between the slow melting process and the fast ejection process. Here, we arbitrary trigged the fs pulse with a fixed delay of 275 µs that ensures a constant diameter of the melted copper area [36]. The remaining energy delivered by the QCW pulse between 275 µs and 383 µs has no effect of ejection dynamics.…”