The role of microstructure on the optical performance of neutron irradiated dielectric mirrors

Dielectric mirrors of HfO2/SiO2 and Al2O3/SiO2 designed for optimum reflectivity at 248 nm with 11 and 30 bi-layer coatings, respectively, survived irradiation to 0.1 dpa at 448 K without film cracking or delamination from their sapphire substrates. Subsequent annealing of the irradiated samples resulted in a loss of reflectivity in the HfO2/SiO2 mirror, while the Al2O3/SiO2 type remained unaffected. Microstructural changes that correlate to optical property changes of the mirror are investigated. The amorphous layers of the Al2O3/SiO2 mirror provide greater stability despite increased Al and Si interdiffusion across the film interfaces with increasing dose and post-irradiation annealing temperature. This interdiffusion may have limited the densification of SiO2 in the Al2O3/SiO2 mirror, but no interdiffusion was observed for the HfO2/SiO2 mirror. The thickness changes in the SiO2 layers of the HfO2/SiO2 mirror resulted in a shifting of the peak reflectivity to lower wavelengths. The formation of an amorphous Al-O layer within the substrate is observed in the 0.1 dpa irradiated HfO2/SiO2 mirror, which on further annealing at 573 and 673 K resulted in a buckling-type delamination failures in the mirrors producing a loss in reflectivity.