UV-laser ablation of ultrathin dielectric layers

Ablation yields and thresholds for 193 nm UV laser ablation of ultrathin HfO2 layers on bulk SiO2 are presented. The single shot threshold fluence increases approximately linearly with HfO2 thickness from 28 nm to 120 nm. Due to the logarithmic dependence of ablation depth on fluence this results with increasing layer thickness in an exponential increase of fluence necessary for clean ablation of the whole layer. The observed ablation depth for fixed HfO2 thickness can be reproduced phenomenologically by taking ablation from the HfO2 film as well as the quartz substrate into account. As a first approach to a quantitative understanding we calculate numerically the heat evolution in the layered system and identificate ablation with the onset of melting of the absorbing layer. Whereas the ablation curve for a 74 nm thick film can be reproduced that way, this is not the case for the overall thickness dependence of the ablation threshold. This points to possible finite size effects for the phonon-phonon scattering rate in the thin dielectric layers.