available, however. DSI predominantly coats optics and films using magnetron sputtering. Compared to IBS, the energy of magnetron sputtering is lower, and so the films are not as dense, and cannot be produced to the same high precision. They are, however, produced quickly – meaning that this technique is used for coating windows and films.
Mirrors produced with sputtered coatings are capable of achieving up to 99.999 per cent reflection. Image courtesy of DSI
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in the coefficient of thermal expansion between the two. It is bit of a problem, and DSI and other coating companies tend to stress-compensate coatings. The most common way to do this is to flip the optic over and put an anti-reflective coating or some kind of sacrificial coating on the reverse side, so if we’ve curled it in one direction we then flip it over and curl it back again,’ she explains. Annealing and other techniques are also employed to varying degrees in order to obtain a flat optic. ‘The nice thing about sputter- coated optics,’ says Black, ‘is that the flatness doesn’t change; whatever it is when the coating is completed, it will stay that way. The flatness of an optic with an evaporated coating, on the other hand, will actually change as the coating relaxes and absorbs humidity.’
plastic options
‘The other exciting advantage of sputter coating,’ say Black, ‘is that it can be done at 100°C, give or take a few degrees, and so we can sputter onto plastic substrates, or those that are temperature sensitive. Although we could evaporate a coating onto a substrate at a lower temperature, it would usually lead to poor adhesion, and so most people [using an evaporative process] would heat the substrate within the chamber so as to increase the energy and reduce the porosity of the coating.’ REO’s Edmond adds that even where the substrate isn’t deliberately heated to increase bonding, evaporative processes themselves tend to heat the optic substantially. ‘Because IBS is a momentum-transfer process, rather than a boiling process, there is very little substrate heating, and so the technique is suitable for plastic substrates.’ Plastic optics are important in weight-sensitive applications. IBS is not the only kind of sputter coating
26 ElEctro opticS l MAY 2011
the limitations The high energies used in IBS do mean that some coating materials cannot be sputtered, as Black explains: ‘Pretty much the only limitation for sputtering is that the target material has to be a metal, an oxide, or a nitride, and customers working in the far-IR or deep-UV parts of the spectrum are usually looking for fluoride coatings.’ When sputtering a metal fluoride material, the fluoride ion will dissociate from the metal ion, leading to an ineffectual coating and a build-up of dangerous fluorine gas. Edmond elaborates: ‘Metals and metal oxides are fine, but there’s so much energy in the IBS process that sulphides, selenides, and fluorides will dissociate. Fluoride materials are typically used in coatings because they’re transparent right down into the deep UV, whereas oxides absorb sharply below 266nm.’ As such, sputter-coated optics are not suitable for applications such as excimer laser microlithography at 193nm.
the importance of control Gregory Fales, product line manager at US- based Edmund Optics, says that the company’s
i would not have started using iBS
if i hadn’t have had the right experts with many years of experience in this field
development work is going into improving the blocking density of the filters produced. Much of the performance of the final optic depends, he says, on the accuracy of the measurements the company is able to make during the deposition of the coating. ‘Newer metrology will allow us to produce coatings with blocking densities of up to 8x easily. Right now we could design a coating with a theoretical blocking density of 10 to 14x, but achieving this relies on measuring the thickness of coatings during deposition, and that has always been the technical challenge. Coating technology has historically been a few steps ahead of the metrology technology, so they’re only just starting to catch up with each other,’ he says.
www.electrooptics.com
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