| Yttrium Fluoride, YF3, produces low-index film layers that exhibit good transparency in the UV through infrared regions. The films are insoluble and show low stress, making YF3 a useful substitute for ThF in many IR applications. It can be combined in multilayers with other fluoride compounds and with higher-index ZnS and ZnSe layers for AR and filter coatings out to wavelengths >10 µm. Its absorption at 10.6 µm is low, but limits its use to laser applications that do not require the highest damage thresholds. Film Properties: Yttrium Fluoride films can be deposited from the melt by resistance-heated or electron beam evaporation. Film density and refractive index increase with substrate temperature. Thicknesses greater than 2 µm will adhere to Zinc Sulfide, glass, and Germanium substrates heated to 250°C. Yttrium Fluoride is insoluble, therefore making it suitable for humid conditions when applied hot. Water absorption bands of depth 2-5% are present at 2.8-3.2 µm and 5.6 -7.3 µm on colder substrates. When used in multi-layer combination with ZnS or ZnSe, the substrate temperature must be reduced to ~175°C to avoid the low sticking coefficients that those materials exhibit at higher temperatures. Amorphous films exhibiting low scatter can be deposited below 150°C substrate temperature, but adhesion and refractive index properties are compromised. Above ~250°C, the films become crystalline and harder, but they exhibit noticeable scatter and are more highly stressed. Information provided by Materion. |
