FEATURE OPTICAL COATINGS
One of the interferometer optics at the LIGO observatory in the USA. The site first detected gravitational waves in 2015, opening-up an entirely new field of astronomy through which vibrations in spacetime are measured
a target of the desired coating material, causing target atoms to ‘sputter’ off. The target atoms carry significant kinetic energy, which causes them to form a dense, hard, and smooth film on the surface of optical components. The process allows for the precise monitoring and control of parameters such as layer growth rate, oxidation level and energy input. Using a variation of the sputtering
process, called plasma-assisted reactive magnetron sputtering (PARMS), a glow discharge plasma is generated, but a magnetic field confines it near the target instead of filling the entire coating chamber. The plasma accelerates positive ions onto the target, ejecting target atoms that then deposit onto optical surfaces. Because of the confinement of the plasma, PARMS operates at a relatively low chamber pressure, which reduces set-up time and allows for the more economical coating of high-volume optics. Thin-film coatings formed by PARMS are hard and dense and the process is highly repeatable, although less so than IBS. However, PARMS has a higher throughput, making it an appealing compromise between the high price and performance of IBS with more economical coating technologies such as IAD e-beam evaporative deposition. “A lot of the standard processes have to be carried out at high temperature, but sputtering can be carried out at room temperatures, so there’s an immediate economic benefit,” says Gibson. “You can have higher throughput systems compared with thermal or electron beam evaporation.” Today, both IBS and PARMS remain relatively expensive processes to carry
24 Electro Optics February 2024
out, however. At UWS, Gibson and his colleagues are working on a process called microwave plasma-assisted sputtering (MPAS), which could offer a six-fold increase in production throughput compared with current high-temperature electron-beam deposition production processes. Moreover, MPAS enables the use of a wider range of thermally sensitive/ strategic substrates. For certain applications, particularly small components with complex shapes, chemical vapour deposition (CVD) processes are starting to be used to create optical coatings. Atomic layer deposition
“The optical coatings industry is probably 50 years behind the electronics industry, but it’s going through the same evolution”
(ALD), for instance, is a coating technology based on self-limiting surface reactions between precursors and reactants. During the process, the substrate to be coated is placed in a vacuum chamber, into which gaseous precursors and reactants are pulsed alternately. They react with the functional groups (atomic groups in a compound) on the surface of either the substrate or a previously deposited layer to create new reactive groups. The reaction is self-limited by the number of functional groups and comes to a halt after a pulse finishes. Between pulses, the vacuum chamber is purged so that no gas phase-
reactions can occur. As such, an ALD cycle typically comprises four consecutive steps: precursor pulse; purge step; reactant pulse; purge step. The ALD cycle is repeated, depositing films of the same sub-nanometre thickness sequentially. The thickness of the deposited film can therefore be defined precisely by the number of ALD cycles. This precision ensures that high-quality optical coatings are created on the substrate, no matter its shape. Using evaporation or sputtering based technologies, by contrast, coatings of variable thickness can be created on curved surfaces. Gibson explains: “A good analogy is
the semiconductor industry, which is a very heavy user of thin-film technology for electronics and optics. It started with thermal evaporation, then moved through sputtering as the volumes increased the benefits, and then moved on to CVD. The optical coatings industry is probably 50 years behind the electronics industry, but it’s going through the same evolution.” While this evolution is happening,
the electronics industry takes a highly standardised approach to manufacture. Oliver points out that the processes chosen to produce optical coatings are defined by the nature of the application for which they are to be used. He says that, for instance, sputtering is unsuitable for fusion laser optics: “We can get better uniformity out of evaporation techniques. We can get spatial control, we can reduce thin film stresses, and we don’t bend the optic like we would using an energetic technique. So, it’s not that we’re doing evaporation because it’s cheap, easy and old. It’s because it’s the best technology for that application.” EO
www.electrooptics.com
Caltech/MIT/LIGO Lab
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