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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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