Plasma Technology

The process of Ion beam sputter deposition produces sputtered atoms with high average energy. Films made from these atoms show improved properties and adhesion when compared to conventional deposition techniques such as evaporative or ion assisted deposition. A high quality optical coating should have the same optical thickness over a variety of conditions. Conventional deposited films have a porous microstructure making them moisture sensitive. When these films collect moisture (e.g. when exposed to air) the optical thickness of the film changes due to the difference between the refractive index of water and air. This causes a spectral shift in the reflectance or transmittance of the film. Ion beam sputtering produces films with a very densely packed surface structure – reducing the sensitivity to moisture and resulting in little or no spectral shift.

High Quality Optical Coatings: SiO₂, TiO₂, Ta₂O₅

	Due to sputtered atoms high energy, Ion beam sputtered films have a density very similar to the bulk density of the materials used. This density is extremely reproducible from run to run. With care it is possible to deposit many materials in an amorphous or semi-crystalline state. This is often highly advantageous for optical coatings, as it reduces the effect of birefringence.
	A major feature of an optical coating is it's surface quality. The quality of the surface determines the performance of the optical device itself. High quality optical coatings start with a smooth super-polished optical substrate with roughness typically of 0.05 nm rms. A conventionally deposited film will add roughness to the surface of the optical substrate, the degree of the roughness dependant upon the technique used. For example a film deposited by evaporative techniques produces a surface roughness of typically 1 nm rms, wile ion assisted deposition techniques produce a surface roughness of typically 0.4 nm rms. Ion beam sputter deposition produces films with a surface roughness equal to that of the super-polished substrate, 0.05 nm rms.
	A high quality optical coating should have low optical loss. Losses inside an optical coating arise from scatter and absorption. Ion beam sputtering produces films with total losses so low that sophisticated devises are needed to measure them. Data for the measurement of mirrors with losses less than 2ppm have been published by two independent groups. Both groups made their mirrors on Oxford Instruments Ion beam sputter deposition systems.