Comparative study of ALD SiO₂ thin films for optical applications

We have investigated the suitability of atomic layer deposition (ALD) for SiO2 optical coatings and applied it to broadband antireflective multilayers in combination with HfO₂ as the high refractive index material. SiO₂ thin films were successfully grown using tris[dimethylamino]silane (3DMAS), bis[diethylamino]silane (BDEAS) with plasma activated oxygen as precursors, and the AP-LTO 330 precursor with ozone, respectively. The amorphous SiO₂ films show very low optical losses within a spectral range of 200 nm to 1100 nm. Laser calorimetric measurements show absorption losses of 300 nm thick SiO₂ films of about 1.5 parts per million at a wavelength of 1064 nm. The films are optically homogeneous and possess a good scalability of film thickness. The film surface porosity - which correlates to a shift in the transmittance spectra under vacuum and air conditions - has been suppressed by optimized plasma parameters or Al₂O₃ sealing layers.