tEchnology SputtEr coAtingS
The high performance of sputtered coating is achieved only when used with high- precision metrology. Wolfgang Ebert is CEO and owner of Laseroptik (Hanover, Germany), a company that offers IBS, magnetron, and evaporated coatings. ‘Sputtering is more complex than other PVD processes,’ he says. ‘You can buy a sputtering system off- the-shelf and be happy enough with [the results], even with a low level of in-house knowledge, particularly if you produce high volumes of just a few standard products. This, however, wouldn’t work with our business at Laseroptik, because we produce a mix of both high- and low-volume components on our sputtering machines. In our situation we need good experience of our own, and ambitious in-house experts with plenty of expensive metrology equipment. IBS is capable of producing coatings with outstanding characteristics, such as low particle [impurity] numbers and the best stochiometry, but this performance doesn’t just come naturally,’ he says, adding that the ion guns used in sputtering machines require particularly special attention. ‘IBS is the cutting-edge technique, with the results in terms of total losses sometimes only limited by the quality of the substrates, but I would not have started using IBS if I hadn’t have had the right experts with many years of experience in this field.’ When it comes to monitoring the deposition
process, techniques that were suitable for an evaporative process may not be suitable for IBS, as Rob Beeson, VP of Engineering at Idex (Rochester, New York, USA), explains: ‘We
Sputter machines represent a significant investment, and one which can only be fully realised alongside in-house metrology expertise. Image courtesy of REO
use optical techniques to monitor our IBS coatings; although this can be used for soft- coated [evaporated] filters, it’s used that often.’ When producing evaporated filters, he adds, it’s more common to use crystal monitoring, in which a vibrating crystal is placed in the chamber alongside the substrate. This crystal vibrates at a frequency that depends on how much mass has accumulated on it. ‘It’s a more noisy process, and it doesn’t give a direct optical measurement,’ he says. ‘The crystal gives a measurement of the thickness of the coating, but thickness is not the whole picture; we want to know the index and the optical path, and optical monitoring gives us both of those parameters.’ The optical measurement equipment shines light through the coating itself as it is
being deposited, explains Beeson, and the resulting signals allow the deposition machine to determine when to stop depositing layers, even incorporating a degree of auto-correcting. For companies producing optical coatings, a unique selling point often stems from the degree of control they can guarantee, rather than the kind of machine they’re using. ‘The entire optical monitoring system is developed in-house,’ says Beeson, ‘and most high-end coating companies will have their own optical monitoring systems, or at least their own algorithms. Most companies don’t develop their own coaters or coating processes (although a few do), and mostly it’s about what you do with the coater, how you run it, and how you control it that makes a difference.’ l
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MAY 2011 l ElEctro opticS
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