Optical Coatings

ION-ASSISTED BOMBARDMENT
Ion-assisted bombardment is a coating technique that can offer
unique benefits under certain circumstances. Ion assist during
coating leads to a higher atomic or molecular packing density in the
thin-film layers. This results in a higher refractive index and, most
important, superior mechanical characteristics.
Specifically, the lack of voids in the more efficiently packed
film means that it is far less susceptible to water-vapor absorption.
Water absorption by an optical coating can change the index of
refraction of layers and, hence, the optical properties. Water absorption
can also cause mechanical changes that can ultimately lead to
failure.
Ion-assisted coating can also be used for cold processing.
Eliminating the need to heat parts allows cemented parts, such as
achromats, to be safely coated.
MONITORING AND CONTROLLING LAYER THICKNESS
A chamber set up for multilayer deposition has several sources
that are preloaded with various coating materials. The entire
multilayer coating is deposited without opening the chamber.
A source is heated, or the electron gun is turned on, until the
source is stable. The shutter above the source is opened to expose
the chamber to the vaporized material. When a particular layer is
deposited to the correct thickness, the shutter is closed and the source
is turned off. This process is repeated for the other sources.
The most common method of monitoring the deposition process
is optical monitoring. A monitor beam of light passes through the
chamber and is incident on a blank monitor substrate. Reflected
light is detected using photomultiplier and phase-sensitive detection.
As each layer is deposited onto the reference blank, the intensity
of reflected light from it oscillates in a pseudo sine wave (rectified).
The turning points represent quarter- and half-wave thicknesses at
the monitor wavelength, with intermediate thicknesses between.
Deposition is automatically stopped as the reflectance of the reference
surface passes through the appropriate point.
SCATTERING
Reflectance and transmittance are usually the most important
optical properties specified for a thin film, closely followed by
absorption. However, the degree of scattering caused by a coating
is often the limiting factor in the ability of coated optics to perform
in certain applications. Scattering is quite complex. The overall
degree of scattering is determined by imperfections in layer interfaces
and interference between photons of light scattered by these
imperfections as shown in figure 5.17. It is also a function of the
granularity of the layers. This is difficult to control as it is an inherent
characteristic of the materials used. Careful modification of deposition
conditions can make a considerable difference to this effect.
The most notable examples of applications where scattering is
critical are intracavity mirrors for low-gain lasers, such as certain
helium neon laser lines and continuous-wave dye lasers.
TEMPERATURE AND STRESS
A major problem with thin films is caused by inherent mechanical
stresses. Even with careful control of the vacuum, source
temperature, and optimized positioning of the optics being coated,
many thin-film materials do not deposit well on cold substrates.
This is particularly true of involatile materials. Raising the substrate
temperature a few hundred degrees improves the quality of these
films, often making the difference between usable and useless film.
The elevated temperature seems to allow freshly condensed atoms
(or molecules) to undergo limited surface diffusion.
Optics that have been given a multilayer thin-film coating at an
elevated temperature require very slow cooling to room temperature.
Thermal expansion coefficients of substrate and film materials are
likely to be somewhat different. As cooling occurs, the coating
contracts and produces stress in the layers. Many pairs of coating
materials do not adhere particularly well to each other owing to
different chemical properties and bulk packing characteristics.
Temperature-induced stress and poor interlayer adhesion are
the most common thickness limitations for optical thin films. Until
new technologies, such as ion-assisted deposition, are developed
into true production tools, stress must be reduced by minimizing
overall coating thickness and by carefully controlling the production
process.
incident light
Figure 5.17 Interface imperfections scattering light in a
multilayer coating
Fundamental Optics Gaussian Beam Optics Optical Specifications Material Properties Optical Coatings
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