Multilayer ultraviolet reflective coating based on atomic layer deposited aluminum oxide and fluoride

Ultraviolet optical coatings employ wide bandgap dielectric materials due to their characteristic low absorption. High-reflectivity and antireflective coatings are essential for optical devices, which can be achieved by alternately depositing two dielectrics with different refractive indices. In this research, a multilayer high-reflectivity coating has been designed for middle UV wavelengths using Al2O3 and AlF3 layers on a sapphire (0001) substrate, and the initial two-layer structure has been fabricated by atomic layer deposition. The surface morphology and roughness of the coating was measured by atomic force microscopy after each deposition step. Ultraviolet spectroscopy and spectroscopic ellipsometry were used to characterize the optical performance of the single and multilayer coatings. Monochromatic x-ray photoemission spectroscopy was used to study the film composition, bonding, and impurities. A bilayer reflective coating was demonstrated, with a smooth surface (Rq < 1 nm) and peak reflectance of 25%-30% at a wavelength of 196 nm. The measured reflectance deviated from the simulations in the middle UV range, and an analysis of the AlF3 layer prepared by plasma enhanced atomic layer deposition indicated the presence of Al-rich clusters, which were associated with the UV absorption. A thermal atomic layer deposition process for AlF3 deposition showed reduced absorption, which could be more effective for shorter wavelength designs.