Evaporative coatings are, however, easy and quick to produce, and relatively inexpensive compared to other coating types. Evaporative techniques can be used with a wide range of coating materials to produce filters for use in the UV, visible, or IR regions of the spectrum. ‘For many applications, evaporated coatings are still appropriate and still meet performance requirements, but there are a significant number of demanding and high-power applications in which e-beam coatings fall short,’ notes Damery.
Aiming denser Where e-beam coatings fall short, many customers opt for the intermediate technique of ion-assisted deposition (IAD). ‘In IAD we take the same target and e-beam set up, but then we shoot high- energy ions into the chamber,’ explains Damery. ‘These act as little peening hammers to pound the coating material onto the substrate with more energy – pounding it into a state of higher density so as to reduce porosity,’ notes Damery. IAD coatings are less porous than those produced by e-beam only, but they are not fully-densified. ‘In a fully densified coating, there is utterly no porosity – there is no space whatsoever for any moisture, cleaning solvents, or dirt to get into the coating.’
Defining sputtering So what is sputtering? Ian Edmond, senior thin film engineer at REO, explains: ‘Sputtering means taking a target material and using energetic ions to knock materials off. Instead of bulk heating the material to boil little bits off [as in e-beam], we use the accelerated ions as little bullets to knock bits off at the atomic or molecular level. If you set it up right, and bombard the target with these very high energy ions, the fragments that come off also do so with high energy – five or 10 times the energy that they would have in an evaporative process.’ Environmental durability aside, coatings
produced by ion beam sputtering also boast significant optical benefits, explains Edmond: ‘They have the lowest scatter and absorption properties of any coating technology. It’s possible to produce low-loss mirrors with less than 1ppm scatter and absorption. We can make a mirror that’s 99.9997 per cent reflecting, which is an enabling technology in itself for devices such as cavity ring-down spectrometers or ring-laser gyroscopes.’ Such low loss and scatter stems from the
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fact that an IBS coating can be deposited with approximately twice the precision of an equivalent evaporated coating, and when designing coatings to fractions of a wavelength, precision means performance. ‘The reason for the greater precision is that rather than melting and boiling the material and then having faith that it’s going to condense with the right refractive index to the right thickness, when we’re ripping atom by atom from a target it is much more accurate and deliberate. The repeatability and controllability of the process is far superior, leading to about twice the precision in terms of both the accuracy of refractive indices, the consistency of those indices, and the accuracy of the physical thickness.’
coating technology has
historically been a few steps ahead of metrology technology, so they’re only just starting to catch up with each other
One further advantage of sputter-coating is
that it allows optics manufacturers to control the stoichiometry of the coatings they produce, particularly when alloy-based coatings are used. ‘If the target is an alloy, we don’t get dissociation of the two metals with sputter coating. In evaporative processes, on the other hand, the two metals have different melting points and so they would evaporate at different temperatures, and so we would not get a thin film of the alloy,’ says REO’s Edmund.
Stress reduction
‘Coatings always have an internal stress,’ notes REO’s Damery, ‘so that they are literally pulling on the substrate material below them, and in some cases they can pull it out of shape. When you’re talking about an optic with a flatness of λ/20, it doesn’t take much stress to pull that optic away from its proper shape. For this reason, the flatness of most optics is specified by its manufacturers prior to the coating process.’
Jessica Black, sales engineer at Deposition Sciences Inc, (DSI, also in Boulder, Colorado), confirms that minimising the stresses in coatings is an important consideration: ‘Increasing the energy of the coating process makes the great, non-shifting substrates we’re looking for, but the energy also leads to more stress in the coating. As the material condenses on the substrate, there can also be a difference ➤
MAY 2011 l ElEctro opticS 25
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