Moisture resistant optical films: their production and characterization

Abstract

We have used an accelerated humidity test consisting of a 24-h cycle between 27–50°C at 95% relative humidity to rate the moisture resistance of typical protective films such as CaF₂, LaF₃, and fluorinated polymers (TFE, FEP) deposited on BaF₂ and NaCl substrates. The test life of BaF₂ specimens was increased from about 4 cycles to over 9, 26, and 42 cycles when protected by CaF₂, TFE, and LaF₃, respectively. Heat treating the as-deposited CaF₂ films further improved specimen life by a factor of 2. The lifetime of NaCI specimens was improved from a fraction of a cycle to 1 and 3 cycles, respectively, by CaF₂ and TFE coatings. No adherent LaF₃ films could be deposited on the salt. Microhardness measurements of the CaF₂ films showed a direct correlation with humidity resistance. The effect of heat treatment was apparently to sinter the fine CaF₂ particles composing the film, thus increasing film density, refractive index, and hardness while reducing moisture permeability. CaF₂ and TFE films function as antireflective, as well as protective layers, on BaF₂ and NaCl.