Reflectance and durability of Ag mirrors coated with thin layers of Al(2)O(3) plus reactively deposited silicon oxide

A technique for producing adherent and well-protected front surface Ag mirrors with an evaporated Al(2)O(3) + SiO(x) overcoat and an evaporated Al(2)O(3) underlayer is described. A thin Al(2)O(3) layer promotes adhesion between the Ag film and its substrate. The optimum thickness of each outer layer necessary to give good adhesion and protection to the Ag surface with minimal loss in reflectance due to ir absorption was found to be about 300 A for the Al(2)O(3) layer and between 1000 A and 2000 A for the SiO(x) film. Ag surfaces coated in this fashion retained a normal incidence reflectance in excess of 95% over the wavelength region from 450 nm to the far ir, even when exposed to harsh sulfide and humidity environments. Calculations demonstrate the advantage of using protected front surface Ag in comparison to more durable metal reflectors, such as Al or Rh, in terms of high reflectance and low visible region polarization. However, a very pronouncedrestrahlen reflectance and absorption effect in the thicker SiO(x) film is responsible for a calculated drop in reflectance from 98.5% to about 65% at lambda = 8.1 mum and 45 degrees incidence for the Ag + Al(2)O(3) + SiO(x) coating.