Light Induced Damage of Silicon Nitride with femtosecond laser

This report focuses on studying optical damage of thin Silicon Nitride (20nm) on Silicon (bulk), and Silicon
Nitride (120nm) on Silicon Dioxide (18nm) on Silicon (bulk). These samples are damaged with a 45fs single
pulse at 400nm central wavelength (pump) and measured with a 45fs 800nm central wavelength (probe) in
a pump-probe setup.
For the Silicon Nitride on Silicon sample, it’s found that for pulses below the ablation threshold, there’s likely
a crystal/amorphous phase change underneath the Silicon Nitride layer (Silicon or Silicon native oxide). For
pulses near the ablation threshold, complete delamination is observed for the Silicon Nitride on Silicon
sample. Aditonally, microstructural changes at the center of delamination and at the parts where Silicon
Nitride is not yet delaminated is seen with SEM and AFM. For pulses above the ablation threshold, multiple
damage regimes with different morphology are identified. In these regimes, Silicon removal is observed with
”pit” morphology, as well as planar delamination.
For the Silicon Nitride on Silicon Dioxide on Silicon sample, similar behavior as for the Silicon Nitride
on Silicon sample is found for pulses below the ablation threshold. Meaning, a phase change is observed.
However, not only the phase of the buried silicon but the silicon nitrate itself can change. For Pulses near
the ablation threshold, morphology of bulging and ”donuts” (circular areas where height increases at the
edges) are observed. For pulses above the ablation threshold, cascading craters are seen. The thickness of
these craters suggest that damage in Si substrate does not play a dominant role (compared to other possible
mechanisms) due to Silicon Nitride not being removed from the Silicon substrate upwards.