Fabrication Fluoride High Reflection Mirror by Thermal Evaporation in the ArF Excimer Laser

Abstract

ArF excimer laser is the main light resource for the microlithography technology. In the laser cavity, the optical components with lowest absorption and scattering loss are necessary. As a consequence only a few materials are promising candidates for 193nm coatings with high transmittance or high reflectance. Fluoride films exhibit relatively low optical loss as well as high laser induced damage thresholds. The potentiality of LaF3 and MgF2 is evaluated in respect of the production of improved optical coatings for applications. For this purpose, single layer is prepared by thermal evaporation at the deposited temperatures of 523K on Fused Silica. A first order bulk inhomogeneity model for extracting the optical constants of weak absorbing film is applied, which is based on spectrophotometry. Refractive index (n) and extinction coefficient (k) of the optimal LaF3 film are 1.678 and 2.24×10-3 at 193nm. In the case of the optimal MgF2 film, n and k are 1.443 and 4.72×10-4 at 193nm. High reflection (HR) LaF3/MgF2 coatings are designed and fabricated for normal incident on CaF2. The experimental results indicate that after coated and fluoride coatings deposit, according to optimal process, can have more suitable optical properties at deep ultraviolet (DUV), the reflectance of HR coatings reaches more than 98% at 193nm.