Influence of defects on extreme ultraviolet laser ablation of LiF

We study the influence of defects (neutral Li${}^0$ and F${}^0$) on the extreme ultraviolet (XUV) laser-induced ablation of 10-nm-thick LiF films. Our method combines a molecular-dynamics scheme for LiF and a set of equations describing the temporal evolution of the conduction-electron density and temperature. We find that defects may decrease the ablation threshold by a factor of around 4. This is caused by the lattice destabilization and the tensile pressure the defects induce upon creation. Metallic colloids form due to the high mobility of Li${}^0$ in the heated crystal. Inhomogeneous defect distributions are shown to be even more effective for crack formation and ablation. In the extreme case, when the thermal heating induced by the laser is negligible and only the effect of defect formation is considered (“cold ablation”), LiF spalls in the solid state.

Figure 1

Interaction potentials, Eq. (2), of (a) Li${}^0$ and F${}^0$ with the F${}^{-}$ and Li${}^{+}$ ions constituting the LiF crystal lattice and (b) neutral Li${}^0$ atoms interacting with each other.

Figure 2

Temporal evolution of the laser power density [lower (black) line], the number of free electrons [upper (red) line], and the defects created [rectangular (blue) line]. Data for the number of free electrons calculated for a laser fluence of $F=10$ mJ/cm${}^2$.

Figure 3

Snapshots of the thermal ablation occurring at the ablation threshold; laser fluence $F=38$ mJ/cm${}^2$. Red points, Li${}^{+}$ ions; blue points, F${}^{-}$ ions.

Figure 4

Snapshots of the defect-supported ablation process. Laser fluence $F=10$ mJ/cm${}^2$; defect concentration, 0.45%. Red points, Li${}^{+}$ ions; blue points, F${}^{-}$ ions. Green points, Li${}^0$; black points, F${}^0$.

Figure 5

(a) Pressure and (b) temperature evolution for thermal ablation (laser fluence $F=38$ mJ/cm${}^2$) and for defect-supported ablation (laser fluence $F=10$ mJ/cm${}^2$; defect concentration, 0.45%).

Figure 6

(a) Pressure and (b) temperature evolution for three processes at a laser fluence of $F=10$ mJ/cm${}^2$: thermal processes (“without defects”) and defect-supported processes at two defect concentrations: 0.39% and 0.45%.

Figure 7

Snapshots of the defect-supported ablation process with inhomogeneously distributed defects in a thin layer in the middle of the crystal. Laser fluence $F=10$ mJ/cm${}^2$; defect concentration, 0.20%. Color code as in Fig. 4.

Figure 8

Top view of the ablation process shown in Fig. 7. Only defects are shown: (green) circles, Li${}^0$; (black) squares, F${}^0$.

Figure 9

(a) Pressure and (b) temperature evolution for the defect-supported process with inhomogeneously distributed defects in a thin layer in the middle of the crystal. Laser fluence $F=10$ mJ/cm${}^2$, defect concentration: 0.20%. Black: reference process without defects. The red and blue lines coincide until the time of 10 ps.

Figure 10

Snapshots of “cold ablation” processes. Green points are Li${}^0$ ions and black points are F${}^0$ ions. Color code as in Fig. 4. (a) Defect concentration 0.54%, temporary crack formation. (b) Defect concentration 0.57%, solid state ablation. (c) Defect concentration 0.54%, but reduced recombination time (4 ps), solid state ablation.

Figure 11

(a) Pressure and (b) temperature evolution for “cold ablation” processes. Ablation occurs for a defect concentration of 0.57% (“increased defect concentration”) but not for a reduced recombination time. Black line: subthreshold process for a defect concentration of 0.54% (''reference''). Before 1 ps all three lines coincide; between 1 and 14.5 ps the red and black lines coincide.