Optics and Multicoatings

Currently the two leading candidates for mirror substrates are epoxy replicated aluminum foils (ERAFs) and a relatively new type of thin glass manufactured by the DESAG division of Schott.  ERAFs are produced by depositing a thin film of gold onto a glass surface and bonding an aluminum foil, between 0.1 and 0.5mm to the gold with epoxy.  Since gold does not adhere well to glass, the gold-epoxy-aluminum foil can be easily separated from the glass, and the gold surface of the ERAF should have surface roughness comparable to that of the glass.  The DESAG glass is intended for use in flat panel displays ranging in thicknesses from 0.1mm to over 1mm, but it has recently been demonstrated that this glass can be thermally formed into mirror sections of the desired radius of curvature.

Both substrates will be coated with a depth-graded multilayer to achieve maximum reflectivity of hard x-rays, therefore the winning candidate will be chosen for the quality of its surface based on measurements of the interfacial roughness of the deposited multilayer.

A multilayer is a thin film coating consisting of many alternating layers of high and low refractive index materials.  The multilayer diffracts X-rays in a fashion analogous to a crystal. Alternating layers of high-Z and low-Z materials create a periodic structure of differing electron densities, like the atomic planes in crystals. If the phases of the waves scattered from each layer coincide then the multilayer will achieve maximum reflectivity.  Multilayers can be made by several vacuum deposition techniques including magnetron sputtering.

In a magnetron sputtering device, the cathode (target) and anode are coaxially arranged, and a coaxial cylinder magnet is placed behind the cathode.  The substance to be deposited is turned into plasma by applying a large voltage, and directed to the target by adjusting the magnetic field.