Alumina Thin Films as Optical Waveguides

Amorphous AI₂O₃, fabricated by reactive rf magnetron sputtering, has been evaluated as a planar waveguide material. The microstructure and optical properties of planar waveguides were examined as a function of deposition (substrate temperature, O₂ flow rate) and annealing conditions. X‐ray and electron diffraction verified that as‐deposited films were amorphous for substrate temperatures up to 500°C and for a wide range of O₂ flow rates. This amorphous phase was stable through anneals up to 800°C, but crystallized to “y‐Al₂O₃ at 1000°C and to α‐Al₂O₃ at 1200°C. The amorphous films had transmission windows that extended from 200 nm to 7 (Am with an average refractive index of 1.65 and reproducible losses as low as ∼1 dB/cm at 632.8 nm. The refractive index increased with substrate temperature, but was independent of O₂ flow rate. The losses decreased with substrate temperature and increased as a function of O₂ flow rate. As a final check on the amorphous structure, Cr‐doped films were prepared by codeposition. Fluoresence was detected only in annealed crystalline films.