Silicon Monoxide Protected Front-Surface Mirrors*

Abstract

An evaporation method is described for producing front-surface mirrors with good abrasion, corrosion, and reflection qualities. Aluminum is used as the reflecting material because evaporated aluminum films have high reflectivity in all useful spectral ranges and show, in the electron microscope, a much finer grain and smoother surface than similar silver coatings. Since direct evaporation of SiO₂ is difficult and yields loose layers, a lower oxide of silicon, silicon monoxide, was investigated and found to produce good quality protective films. Silicon monoxide evaporates at a much lower temperature than silicon or silicon dioxide and deposits on the mirror surface in uniform and adherent layers which, when exposed to the atmosphere, partially oxidize to SiO₂. The visual reflectivity of aluminum mirrors protected with an optimum thickness (about 1500A) of SiO is about 89 percent, or only one percent lower than that of the unprotected aluminum surface. In the infra-red from 2μ to 8μ these layers do not show any appreciable absorption; however, the absorption band characteristic of silicon oxides appears between 8μ and 10μ. In the ultraviolet the SiO layer shows absorption which varies with the evaporation conditions. This absorption can be decreased by an oxidizing treatment. The SiO-protected mirror is unaffected by prolonged heating in air at 400C, and is highly resistant to sea water, sodium hydroxide, and most acids. The SiO-protected mirror also shows good abrasion resistance. To produce mirrors on metallic bases it is advisable to use an SiO film as an intermediate layer because it improves adherence and prevents diffusion from base metal to reflecting layer. With the SiO evaporation method it is also possible to make protected front-surface mirrors on plastic materials. Evaporated SiO layers are also successfully used as replica and support films in the electron microscope.