Home Resource Center Newsletters Newsletter Archives Coating Materials News 2000 to 2010 Silicon Dioxide SiO2 for Optical Coating Silicon Dioxide, SiO2, Applications Silicon Dioxide, SiO2, is the low-index, low absorption material used in combination with high-index oxide layer coatings that operate in the UV (~200 nm) to IR (~3 μm) regions. Typical applications include antireflection coatings for near-UV laser optics, all-dielectric mirrors, beam-dividers, bandpass filters, and polarizers. Silica can be used in combination with specific high-index layers, for example Hafnia, Zirconia, and Tantala, to form multilayer structures with high damage thresholds for specialized UV laser applications. Silica films sometimes are useful in promoting adherence between two dissimilar materials, especially oxide-compositions. In contrast to the parent quartz form, Silica films are amorphous and never obtain the equivalent density, hardness or water impermeability of the crystal form. Film Properties Completely oxidized silica films are absorption-free over the range below ~250 nm to at least 5 μm. Film layers are amorphous and smooth. High mechanical compressive stress limits the thickness the single layer thickness. When starting from Silica pieces, little dissociation and oxygen loss occurs during evaporation, and it is not always necessary to provide a background pressure of oxygen to obtain low-absorbing films. Adhesion is good to glass, most other oxides, and some polymers. The films generally grow with an amorphous structure and relatively high packing density so they exhibit minimum index changes when vented to moist air. The appearance of water absorption bands near 2.9 and 6.2 μm indicates less than perfect packing density. The refractive index is maximized and water band absorption is minimized with the use of high energy deposition techniques such as IAD or sputter deposition and high substrate temperature. Low absorption SiO2 films can be produced by oxidizing Silicon Monoxide in a reactive oxygen background. Evaporation would proceed from a baffled box and therefore the possibility of generating micro-particulates is eliminated. Alternatively, evaporation can proceed from flat surfaces of large pieces of Silicon Monoxide that are swept by a low-power e-beam. Films so deposited exhibit low optical absorption, but the possibility exists for particulate emission. Refractive Index The refractive indices are dependent on the degree of oxidation, the substrate temperature, and the deposition energy. The curve below shows typical values. They can be slightly higher than values for fused Silica. Material Behavior Recommended preconditioning consists of slow sweeping of the electron beam to fuse the top surfaces of suitably sized pieces. Prevent hole drilling by using a rapid large area sweep. Replace the charge when it becomes dark colored. Alternatively, start with SiO from a baffled box source and provide a highly reactive oxygen atmosphere ~1-2 E-04 Torr. Physical Properties of Solid Material Molecular Weight 60 Melting Point 1700° C Color Clear to white (see item description) Crystal Density 2.17g/cc Evaporation Parameters Evaporation temperature ~1200° C Source Container No liner for E-beam Rate 2 Å/sec. Partial pressure of oxygen 1 x 10-5 Torr Substrate temperature 200° C to 300° C Quartz crystal monitor Z-ratio 1 Forms and SizesAvailable Materion Advanced Chemicals offers materials for evaporation as well as sputtering targets.