Introduction

Calcium fluoride (CaF₂) has many useful properties for optical and electronic technologies. The preparation of thin films of CaF₂ has been investigated for optical coatings and devices, insulating epitaxial films on Si, and as a high temperature solid lubricant. Formation of stable, dense crystalline calcium fluoride films by standard vapor deposition methods has required high substrate temperatures or high temperature post-deposition anneals. Recent efforts to reduce the deposition temperatures include: low rate molecular beam epitaxy (300°C), ionized-cluster-beam deposition (420°C), ion-beam-assisted laser evaporation (20°C), and magnetron sputtering with off-axis substrates (< 100°C). Environmentally stable polycrystalline and epitaxial films with properties comparable to bulk CaF₂ can be achieved at deposition rates 20-300 Å/m. Resistivities are ~ 10¹⁶ λ cm, and the refractive index at 633 nm is ~ 1.44.