A study of incubation effects in femtosecond laser irradiation of silicon and copper

Abstract

Multi-shot ablation thresholds, $F_{\text {th}}(N)$ for N number of shots, were investigated for polycrystalline copper and single crystalline silicon using a Near-Infrared femtosecond laser, with wavelength of 800nm and pulse duration of 130fs. Fluences, F, above and below the single-shot threshold $F_1$ were used in the study. To better understand the incubation effects, the results are compared to two existing incubation models. The first one is the widely used power law and the second one includes the effects of absorption change and critical fluence, $F_\infty$. No ablation would result for a material even with infinite number of laser shots if $F < F_\infty$. From the data generated by $F>F_1$, $F_{\text {th}}(N)$ were determined. The single-shot ablation threshold, $F_1$, for polycrystalline copper and single crystalline silicon were determined to be 0.87J/cm$^2$ and 0.34J/cm$^2$ respectively. From the data generated by $F < F_1$, $F_{\text {th}}(N)$ were also determined and $F_\infty$ for polycrystalline copper and single crystal silicon were estimated to be 0.18J/cm$^2$ and 0.21J/cm$^2$, respectively. For copper, $F_{\text {th}}(N)$ data from $F > F_1$ and $F < F_1$ are consistent with each other, and the power law fit the experimental data reasonably well until the $F_\infty$ effect sets in when $N > 1000$. For silicon, we found values of $F_{\text {th}}(N)$ from $F < F_1$ are significantly higher than those of $F > F_1$. This study provides important information for the femtosecond laser nanomilling technique when nanometer depth resolution can be made possible by using multiple pulses with $F < F_1$.