Influence of pulse separation on the ablation mechanism of silicon: Spallation and phase explosion vs. melt ejectionWednesday (24.06.2020) 15:10 - 15:30 Room 1
Ultrafast burst material processing with repetition rates of several MHz to GHz is a common technique to effectively leverage the high pulse energies provided by state of the art laser systems. Burst processing promises to increase the ablation volume compared to processing with single pulses, while at the same time it seems to decrease the residual heat remaining in the sample in the so called “ablation cooling” regime. However, this technique has the drawback of losing precision and ablation quality due to significant melt burr formation at the ablation crater edges.
In this study we focus on the melt dynamics of electronic grade silicon <100> after irradiation with ultrafast near-infrared pulses with varying inter-pulse spacing. First the melt dynamics, recorded after subjecting the sample to single pulses with varying fluence were determined using a pump-probe microscopy setup. Based on the measurement of the temporal evolution of the sample reflectivity, the re-solidification times were measured to be 8 ns and 75 ns for fluences of 0.47 J/cm2 and 4.73 J/cm2, respectively. In a second experiment the sample was irradiated with energetically equal pulses, exhibiting inter-pulse spacing of 10 µs and 12 ns.
We observe two different domains of burst laser material processing. In one domain, subsequent pulses arrive after solidification, thus a solid surface is targeted and spallation as well as phase explosion are the driving ablation mechanisms. In the other domain subsequent pulses arrive before solidification takes place and consequently irradiate a liquid surface, resulting in melt ejection. Therefor, the ablation volume increase in the burst mode can be attributed to ablation in the liquid phase. This is well known from nanosecond pulse ablation, which exhibits high ablation volumes with the drawback of melt ejection.