Effects of substrate temperatures and deposition rates on properties of aluminum fluoride thin films in deep-ultraviolet region

摘要

Aluminum fluoride (AlF_3) is a low-refractive-index material widely used in coatings for deep-ultraviolet (DUV) optical systems, especially 193 nm laser systems. Low optical loss and stability are essential for film application. In this study, AlF_3 thin films were prepared by thermal evaporation with a resistive heating boat. The effects of substrate temperatures and deposition rates on the optical properties in vacuum and in air, composition, and microstructures were discussed respectively. In vacuum the deposition parameters directly influenced the microstructures that determined the refractive index. When the films were exposed to air, aluminum oxide (Al_2O_3) formed in the films with water adsorption. Thus the refractive index increased and a nonmonotonic changing trend of the refractive index with substrate temperature was observed. The Al_2O_3 was also found to be conductive to reducing absorption loss. AlF_3 films prepared at a high substrate temperature and deposition rate could yield stable structures with large optical loss.