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Rising to the ion challenge

In order to obtain the properties required from complex optical coatings, companies running ion beam sputtering machines have had to improve the control of the deposition process. Greg Blackman looks at the technology involved

Optical coating companies are building advanced monitoring and metrology into their production to meet the demands for specialised coatings. Vendors, including Advanced Thin Films in the US and Laseroptik in Germany, are now focusing on implementing monitoring mechanisms within their machinery to get better control over the deposition process, and also investing in metrology equipment to test the finished product.

‘You really need metrology tools to characterise your coating designs and enable process development,’ commented Nick Traggis, CTO of Advanced Thin Films, which is owned by Idex.
Ion beam sputtering (IBS) is generally considered the method of choice for low-loss, high damage threshold coatings, and other complex or high-precision coatings. It is in their IBS machines that both Advanced Thin Films and Laseroptik have deployed systems to monitor the sputtering process for greater control in order to make more spectrally complex coatings.

‘We’ve [Advanced Thin Films] put a lot of effort into developing better optical monitoring schemes for higher resolution filters for lasers,’ said Traggis. ‘We’re now able to make more spectrally complex coatings than we could in the past. We still have the same goals with regards to losses and damage threshold, but now we’re able to make higher resolution filters. That’s really helpful for a lot of the laser customers wanting to combine beams or resolve certain lines out of a laser.’

The control mechanism in Advanced Thin Films’ IBS machines is a servo loop, whereby the machine monitors the spectral performance of the coating in situ while it’s being deposited. It allows the machine to make minute adjustments to the coating design layer by layer. This technology was originally invented to make wavelength-division multiplexing (WDM) filters for telecommunications in the late 90s; Advanced Thin Films is applying it to higher power laser applications.

‘Low-loss coatings are something that is not easy to produce. You need a certain amount of experience and you need the right coating technology,’ commented Dr Wolfgang Ebert, CEO of Laseroptik.

Laseroptik has also developed control mechanisms for its IBS machines. ‘With full control of IBS, you can reduce the light losses to some parts per million,’ Ebert continued. ‘That’s what customers are aiming for; if they order highly reflective mirrors then they ask for losses less than 10ppm. That includes absorption, residual transmission and scatter.’

Scatter can come from surface roughness, and from particle contamination in the coating. So, super polished substrates are often used for the manufacture of low-loss mirrors and the entire process is carried out in a clean room. High-resolution microscopes are then used to scan the coatings for defects that can lead to stray light.

Ion beam sputtering can deposit more layers, and thinner layers, with greater control over the deposition process than alternative methods like electron beam evaporation. The technology originates from the US, from the military initially, where a lot of investment went into developing the ion beam sputtering process. The radio frequency ion gun was invented in Germany, initially for space propulsion.

‘The coating technique can achieve reflectivity of 99.99 per cent,’ commented Gemma Micklewright, technical sales engineer at Laser Components. This is compared to 99.9 per cent for other technologies. The extra precision is not necessary for standard antireflection coatings, but does become important when applying low-loss coatings or those for femtosecond lasers, for instance.

Laser Components provides a range of high-power coatings, typically deposited with ion beam sputtering. Micklewright said that a really popular design using IBS is one that works for the first (1,064nm), second (532nm) and third harmonic (355nm) of Nd:YAG.

The downside of IBS is cost and the process also takes longer. ‘Putting down 30 layers with standard vapour deposition only takes a couple of hours, whereas, with IBS, because of the control the machine has over the process and the computing power needed, a complex coating cycle can take eight hours,’ said Micklewright.

High power continuous wave laser beams input high irradiance into the optics. The low bulk absorption in an IBS coating is most suitable for high average power applications, explained Traggis, because here the damage threshold is caused by thermal effects. If the coating is absorbing too much energy, the substrate can’t dissipate the heat away, which over time will damage the coating.

‘We have a tool in-house to measure the absorption of our coatings directly, which gives us some advantages in developing processes and new materials,’ Traggis said. Advanced Thin Films uses a photothermal technique for measuring bulk absorption in the coating. The company’s target for these high average power CW systems is coatings that have less than 1-2ppm absorption.

The other application for high power would be high peak power, or pulsed laser systems. Here, the worry is the breakdown of the electric field of the coating, rather than thermal effects. Low absorption and good film integrity matter, but the purity of the materials is also important. ‘We still prefer ion beam sputtering for these types of coating, although a very well applied e-beam coating could also do the same thing,’ stated Traggis. IBS provides more repeatability and control, he said.

Optics used in femtosecond lasers have very complex coating designs. ‘The mirrors for femtosecond lasers not only must have a certain reflectivity, but also well-defined phase characteristics (group delay dispersion, GDD) to keep the duration of the laser pulses under control,’ explained Ebert.

Femtosecond mirrors may have either low GDD, or a tailored non-zero GDD, Ebert said. There are broadband chirped mirrors with moderate GDD, or highly dispersive narrow bandwidth mirrors, so-called GTI mirrors. There might be mirrors that compensate for the normal dispersion introduced by other components like the laser crystal in a laser cavity, for instance, or they could serve as compressors in chirped pulse amplification systems. ‘Especially the topmost layers of such dispersive coatings are terribly sensitive to manufacture errors,’ said Ebert. ‘Thus, the coating method, and the film thickness control need to be as precise as can be.’

Laseroptik’s femtosecond optics are coated with ion beam sputtering, on machines that can directly feed in the theoretical designs and control the machine online, in situ. This capability has been developed by Laseroptik to make its coating machines extremely accurate. The company also measures the GDD in-house after the coating has been applied.

Traggis at Advanced Thin Films commented: ‘The key to making good ultrafast coatings for femtosecond lasers is in metrology and being able to measure the coating performance. Five years ago we didn’t have a good way to measure our coating performance other than to give it to the customer and ask them to put it in their system and tell us what happens.’

Advanced Thin Films uses commercial metrology tools based on white light interferometry to measure the direct dispersion performance of the coating. This means the company knows how the coating will behave before the optic is shipped to the customer. ‘From the commercial side, that’s a lot more desirable, both from process development and being able to develop a quality product,’ said Traggis.

Large optics

The lenses for astronomy can reach up to two metres in diameter, which in itself makes coating such optics challenging. Firstly, the coating chamber has to be large and there are a whole host of engineering and tooling difficulties with applying coatings to large optics.

‘The optics are very high-value and are also very heavy,’ commented Jim Hooker, director of business development at L-3 Communications Applied Optics Center (AOC). ‘Very few of the larger optics can be handled in traditional ways; they have to be carried and loaded into the coating chamber using mechanised processes and specialised carriers. We have to manufacture unique tooling to hold the optic prior to and during the coating process.’

Some of the optics L-3 AOC coats can be larger than 30 inches in diameter, which are typically for ground-based observatories or optics for satellites.

Along with the handling of the optic and the tooling, Hooker explained that there is also the engineering process, which involves theoretical design of the coating, validation of the coating, and characterisation of the surface of the optic. This latter stage is to map the wavefront to ensure defects are not imparted into the optic. Then there’s the coating process itself.

‘For large area coatings, customers need a defined transmission or reflection across the entire surface area. That’s the art of coating, to make the coating uniform,’ commented Ebert at Laseroptik.

Laseroptik has three machines coating large optics: electron beam evaporation, ion assisted deposition (IAD) and ion beam sputtering (IBS), although next year the e-beam machine for large areas will be transformed into IAD, as the films are denser. The two IAD machines can coat optics up to 1.3 metres in diameter (or maximum length for rectangular substrates). ‘A large optic has to be rotated and that makes the chambers very voluminous,’ said Ebert. ‘It takes a long time to evacuate and heat these vacuum box coaters.’

Laseroptik’s largest coating machine can coat bar-like lenses up to two metres in length using IBS. It has been designed and built by the company, because no such machine was commercially available.

Apart from astronomy, other applications for large optics include laser systems for display manufacture in microelectronics – the larger the laser optics can be made, the larger the displays. On the scientific side, there are a lot of petawatt laser fusion projects using these large optics, such as Laser Mégajoule in France. These machines have round mirrors that can be one metre in diameter.

‘To keep the coating uniform for larger optics, the inner architecture of the machine has to be altered,’ commented Ebert. ‘This can take the form of using shields in order to get a homogeneous thin film covering the optic. Also, the coating design can help to make the coating less sensitive to layer thickness variations.’

There is also the time taken to coat large optics. Ebert stated: ‘The process development, to design the machine architecture and the coating itself, can take half a year or even longer, to get to less than 0.5 per cent uniformity deviation over the entire surface area.’

E-beam still has a role to play

While ion beam sputtering is the method of choice for complex, high-spec designs, electron beam evaporation is still the basis for many optical coatings – 12 of Laseroptik’s 25 coating machines are e-beam machines. ‘Electron beam evaporation is still so versatile,’ commented Ebert. Laseroptik uses e-beam for all its fluoride coatings for ultraviolet optics, as well as high laser induced damage threshold (LIDT) coatings.

‘If 99.8 per cent reflectivity is sufficient then the customer can save a lot of money using e-beam technology with high-capacity chambers that are easy to load and quick to coat. Sputtering can take many times longer,’ Ebert added.

At the same time, IBS is growing – ‘I’m doubling my capacity [of IBS] next year,’ commented Ebert, increasing from four IBS machines to eight. ‘The main driver for the growth of IBS is coming from industry, where laser optics are needed in serial production with demanding spectral requirements,’ he said. ‘Here, the coating design is so complex that only IBS can do it, for example for multi-wavelength coatings. There are also European universities and scientific institutes that would have had to go to the US for IBS 10 years ago, but can now come to companies in Europe to get their optics coated with IBS.’

There is also growth in the market for more high-performance UV coatings, according to Micklewright at Laser Components, and also applying coatings to the tips of fibres, which might be standard antireflection designs, but also more complex beam splitter or polarising coatings.
Hooker at L-3 AOC noted a trend for more specialised coatings for the management of different wavelengths. ‘Everybody wants specific performance from an optic, whereas 20 years ago companies were content to use more generic-type coatings such as broadband antireflective coatings. Now people want very specific wavelengths targeted and higher surface quality – a scratch/dig of 20/10 or tighter,’ he said.

For the high-end coating companies, Traggis at Advanced Thin Films said suppliers all need to develop in-house measurement capabilities for all performance aspects of the coatings.
‘Traditionally, I’d coat an optic and then send it to some external laboratory to measure its damage threshold, for example,’ he continued. ‘We’re building our own laser damage threshold testing setup so that we can test our own coatings. We already have the capability to measure bulk absorption and dispersion of the coatings. Having the capability to measure damage threshold in-house gives us a much faster path to coating development, because we don’t have to wait two weeks for an external laboratory.’