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Home > Coatings Section > Dielectric Coatings

Coatings consist of layers with different refractive indices. There are three major techniques used for dielectric coating: electron-beam deposition (E-beam), ion-assisted electron-beam (IAD) and ion beam sputtering (IBS). All of these processes are quite similar in their principle. They consist in evaporating some coating material on the substrate. The difference lies in the deposition energy. Because of low energies involved when using electronbeam deposition, thin film material contains bubbles and micropores, like a sponge. These will eventually fill with water, which will change the refractive index of the coating and thus the properties of the optics. (This is known as environmental shifting). The presence of water also lowers the damage threshold of the optics: when submitted to an intense light, the water will tend to vaporise and scrap off bits of the coating. Finally, even in the absence of water, inhomogeneities of coating layers lower the theoretical damage threshold. The positive points about this technology is that it is cheap, widespread and very versatile. The coating itself is also slightly flexible, which makes the optic more resistant to mechanical stress. Some of the major optics manufacturer only have access to that type of coating at the moment and outsource IBScoated optics. Ion-assisted electron-beam is an intermediate technique, between ion-beam sputtering and e-beam. So are its results. Ion beam sputtering involves energies 100 times higher than e-beams. As a result the molecules of the coating layers form covalent bound when deposited. The result is free from bubbles or pores, more homogenous, more durable, have higher damage threshold and is more repeatable and controllable. They also show lower scattering and absorption properties, and overall higher specifications (more broadband, steeper transitions when needed, better spectral stability...). This is high precision coating, and the surface roughness can be controlled at better than 1 Å RMS (!), that is <λ/5000. Of course, this comes at a higher cost (atom-by-atom removal is very slow), and even worse, it is limited in the types of coatings it can handle: most of the UV coatings for instance involve fluorides which dissociate when sputtered. In this case, e-beam is the only option.

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