Dielectric coatings can be used for several reasons – as dielectric barrier film on top of some conductive structure like electrode in liquid crystal display and can be used for entirely optical purpose as dielectric mirror. Moreover, they can be used as part of bandgap filter.
Dielectric mirrors and filters are composed of multiple thin film stacks, carefully designed and deposited to achieve wavelength selective effect. If for antireflective coatings, interference is used to cancel reflections, in this case interference is designed to have opposite effect. Well designed dielectric mirrors can have reflective values over 99%.
Current approach to readily available range of commercial compounds would be utilization of magnetron-sputtered thin films. Magnetron-sputtered silicon oxide SiOx thin films exhibit high transmittance and high resistance to dielectric breakdown.
Moreover, it is possible to adjust the refractive index of magnetron-sputtered films by varying deposition parameters. For example, silicon oxynitride SiOxNy is very suitable dielectric material for optical diffuser applications, since it’s refractive index can be varied from 1.5 to 1.8. Particular advantage is that refractive index is linearly dependent on nitrogen content, see figure below.
The disadvantage of silicon oxide films can be the highly active surface due to unfinished silicon oxide bonds and in case of porous coating structure, the total effect can be very significant. Silicon nitride coating can be more appropriate.
