# Ablation of Copper Metal Films by Femtosecond Laser Multipulse Irradiation
## 1 Introduction  
As the number of applications targeted at ultrafast laser processing increases, there is even greater need to grasp the physical phenomena involved in the laser-material interaction, in order to achieve better control and quality of the patterned features. The drilling or micromachining by femtosecond laser gamers much attention in scientific research because of its ablation precision compared to nanosecond or longer laser pulse durations [1; 2; 3; 4; 5]. In general, the physical mechanisms of metal ablation by femtosecond laser are still under study, where it has been observed that there are two different ablation phases during the transition from low to high fluence [3], gentle and strong ablation [6], respectively. Indeed, to produce ablation with high precision, one must apply the lowest above-threshold fluence possible to ensure that the damage of the material is minimized [2]. However, sometimes, this condition is not so practical from an industrial point of view. Several models have been proposed to understand ultrashort laser pulse ablation. [PERSON] et al. [7] suggested that matter is ejected by Coulomb Explosion (CE) after the creation of an intense electric field due to charge separation; however, the model presents an inconvenience concerning the nature of the solid material, with [PERSON] et al. [8] later clarifying that this type of explosion is dominant only in the case of dielectrics. [PERSON] et al. [9] applied a classical approach where they assumed that the energy transfer to the lattice occurs by electron-ion collisions instead of electron-phonon collisions. [PERSON] et al. [10] proposed that phase explosion is the mechanismresponsible for matter removal using Molecular Dynamics simulations (MD), also supported by the most widely-used Two Temperature Model (TTM) [11; 12]. If phase explosion is the dominant mechanism for ablation of a metal film irradiated by femtosecond laser pulses, the melted matter can be superheated without boiling, because the relaxation time is too short, such that a heterogeneous nucleation is produced. Consequently, when the temperature of the superheated liquids approaches the spinode (the limit of metastable phase heating [10]), i.e., when the temperature is close to the critical temperature of thermodynamic equilibrium \(T_{c}\), the liquid's tension rapidly drops to zero. Therefore, the homogeneous bubble nucleation occurs at an extremely high rate. The superheated liquid relaxes via the thermal explosion process, and the ejected matter is a mixture of vapor and liquid droplets [10; 13; 14]. [PERSON] et al. [15] simulated the ablation of a metallic target using bursts of 120-fs laser pulses. They found that under the same total fluence, a laser burst with a pulse separation time of 50 ps or longer than thermal relaxation time could significantly boost the ablation rate compared to the single pulse. We believe this effect may be due to a series of phase explosion processes during the irradiation with different rates of superheated liquid, because the pulse separation time is longer than the electron-phonon relaxation time. [PERSON] et al. [16] observed that the ablation of gold thin film irradiated by multiple femtosecond laser pulses, separated by a time of 280 ps, causes a peak in the temperature of electrons and the lattice. The lattice temperature after the arrival of a subsequent pulse is much higher, due to the residual heat energy already deposited by the first pulse. This enhancement in lattice temperature is due to thermal accumulation effect, because the time between pulses is shorter than the diffusion time of the thermal wave [17]. [PERSON] et al. [18] showed that this heat accumulation induced by high repetition rate causes a rapid increase in the mean temperature of the irradiated region, as subsequent pulses interact with a pre-heated volume. Therefore, less energy is necessary for ablation, and drilling efficiency increases significantly. Extremely high repetition rate (GHz) bursts have been proposed by [PERSON] et al.