Processing of Dielectric Optical Coatings by Nanosecond and Femtosecond UV Laser Ablation

Microprocessing of dielectric optical coatings by UV laser ablation is demonstrated. Excimer laser ablation at deep UV wavelengths (248 nm, 193 nm) is used for the patterning of thin oxide films or layer stacks. The layer removal over extended areas as well as sub-μm-structuring is possible. The ablation of SiO2, Al2O3, HfO2, and Ta2O5 layers and layer systems has been investigated. Due to their optical, chemical, and thermal stability, these inorganic film materials are well suited for optical applications, even if UV-transparency is required. Transparent patterned films of SiO2 are produced by patterning a UV-absorbing precursor SiOx suboxide layer and oxidizing it afterwards to SiO2. In contrast to laser ablation of bulk material, in the case of thin films, the layer-layer or layer-substrate boundaries act as predetermined end points, so that precise depth control and a very smooth surface can be achieved. For large area ablation, nanosecond lasers are well suited; for patterning with submicron resolution, femtosecond excimer lasers are applied. Thus the fabrication of optical elements like dielectric masks, pixelated diffractive elements, and gratings can be accomplished.