Laser Ablation of Metal Films Using Molecular Dynamics Simulations

Arun Kumar Upadhyay

Trennlinie

 

Using molecular dynamics simulation, we study the response of thin metal films (Cu) to ultrafast laser irradiation. This is modeled as a homogeneous and instantaneous energization.

 

Processes below the spallation threshold: (Melting)

Above an energy density of E0=0.4 eV/atom, the slab melts. Fig. 1 shows a representative case E0=0.5 eV/atom. It is seen how after around 0.7 ps, the potential energy starts increasing considerably at the cost of the kinetic energy. This is evidence of the melting process, where the latent heat of melting shows up as potential energy of the system. Potential and kinetic energy in film equilibrate swiftly, within 0.2 ps.

Figure 1

Processes at the spallation threshold: (Void Formation)

Void formation occurs at the onset of spallation. For laser intensities below but close to the ablation threshold, the strong tensile pressure in the metal film leads to the opening of large voids. Fig. 2 shows the series of processes occurring in the film at an intensity corresponding to E0=0.8 eV/atom. In the middle of the slab a void nucleates at a time of roughly 4 ps. It grows and reaches its maximum diameter of around 26 at a time of 13 ps, but then collapses again, until it vanishes at 29 ps.

Figure 2

 

Processes above the spallation threshold of metals: (Spallation)

Above a critical energy per atom, Ecr=1.0 eV/atom, the slab fragments, as shown in Fig. 3. Fragmentation occurs when the pressure in the molten film becomes strongly tensile, such that bubbles nucleate in the middle of the slab at a time of around 1.6 ps. The temperature, which has been quite low in this region, increases due to the breaking of bond in the void formation process. The voids grow until 3 ps and have coalesced at 4 4.3 ps. At this time, fracture occurs in the system, and the top and the bottom layers are separated and drift away from each other. This mechanism is reminiscent of cavitation in liquids or spallation in solids.

Figure 3

 

Processes above the spallation threshold: (Cluster Distribution)

At small energy, large fragments are found at the expansion front of the system, while the inner region is filled with a two-phase mixture of vapor and droplets. At higher energization, the cluster distribution is homogeneous throughout the expansion volume. The cluster formation process takes only a relatively short time. However, since the cluster density is quite high and the cluster temperature is large, small changes in the cluster distributions occur until the end of the simulation time, due to evaporation from clusters and cluster agglomeration. The cluster distribution shows two regimes: (i) a quickly decaying peak centered at monomers, (ii) a decaying large-cluster tail. This two-regime distribution can be fitted by a bi-exponential law. Alternatively, the low size-size part is equally well described by a power law.

 

Figure 4

 

 

Effect of laser pulse width on materials phenomena under ultrafast laser irradiation:

The simulation algorithm combines molecular dynamics for the atom motion supplemented by an analytical solution of the time dependence of the homogeneous electron dynamics. The results show that the laser-pulse duration affects at fixed laser fluence virtually all material processes in the irradiated film: Melting, build-up of compressive and tensile pressure, spallation, and cluster formation. For higher fluences, the target fragments and a vapor cloud form which contains clusters with broad size distribution. With increasing laser pulse width, this distribution becomes narrower and the probability of forming large clusters is diminished. If the laser-pulse duration is smaller than the electron-phonon time of the material, the latter quantity becomes dominant in limiting the materials processes.

 

Figure 5

 

Movies:

  1. Melting process
  2. Void formation process
  3. Spallation process