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-LTO330 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.