Functionalising complex shaped optics

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The shape of modern optical components is becoming increasingly complex, which can make them more difficult to coat.

               

 

Dr Kristin Pfeiffer, Pallabi Paul, and Dr Adriana Szeghalmi have developed conformal coating processes for functionalising strongly curved optics

Optical components in numerous applications rely on functional coatings to obtain various levels of performance. Such coatings are used in applications ranging from unique astronomical systems to high-volume devices and consumer products such as eyeglasses, cameras and lidar systems.

Many of these functional coatings, including anti-reflection (AR) coatings, dichroic mirrors, beam splitters and filters, consist of multiple layers of high and low refractive index oxides, including TiO2, HfO2, Al2O3 and SiO2.

According to publicly available information, the global market volume of AR coatings alone is around $4bn and increasing.

Light incident on any surface is partially reflected. These reflections must be suppressed to enhance the performance of optical devices. In a multilayer AR coating, there is a reflected beam at each interface. The interference of these reflected beams enables the propagation of light to be manipulated as desired.

Destructive interference makes it possible to nearly eliminate the reflection from transparent surfaces, such as glass or plastics, for specific spectral ranges. Therefore, the coatings must have precisely defined film thickness compositions; individual layers have thicknesses in the range of a few nanometres up to several hundred nanometres, depending on the target AR property.

Plastic optics pose a challenge

Thermoplastics such as poly(methyl methacrylate) (PMMA), polycarbonate and polystyrene are widely used for producing various optical elements such as freeform surfaces, aspheric lenses, Fresnel lenses and many other diffractive optical elements. In general, these substrates can be manufactured in complex shapes with significantly reduced cost compared to glass substrates by the injection moulding method. Thanks to being lightweight, optical components made of plastics are an important substitute for glass optics.

PMMA in particular, which has a high transmission (approximately 92 per cent) in the visible spectrum (400 to 700nm), excellent hardness, and a high Abbe number, is extensively used in precision optical manufacturing. However, precision coatings on plastics are rather challenging because of the crack formation and low adhesion of the dielectric coatings to the polymer surface. Since the parameters of optimised processes for glass substrates cannot be directly transferred to plastics, explicit polymer-specific research is required to functionalise these materials.

Existing approaches

Different deposition methods based on wet chemistry, physical vapour deposition and chemical vapour deposition techniques have been applied on PMMA and other plastic substrates to try to enhance their optical functionalities.

These include the sol-gel method, ion- and plasma-assisted evaporation and sputtering processes, and plasma-enhanced chemical vapour deposition.

Previous research also suggests that applying a special direct current glow discharge plasma pre-treatment, or creating a vacuum ultraviolet protection layer by boat evaporation prior to the plasma ion-assisted depositions, can improve the adhesion of thin films on PMMA.

Several other approaches, involving moth-eye structures by plasma-assisted etching, full wafer and roll-to-roll nano-imprint lithography (NIL), layer-by-layer assembly of hollow silica nanoparticles and porous quarter wave coating with colloidal nanospheres, have also been demonstrated for AR coatings on plastics.

The environmental stability of nanostructured AR coatings however is rather limited, restricting their usage to the inner surfaces of optical systems.

Our solution

Nowadays the shape of numerous optics is becoming increasingly complex (see lead image). For such strongly curved substrates, atomic layer deposition (ALD) becomes the most promising coating technology to achieve a conformal coating with excellent thickness control1. With it, multilayer thin film coatings can be applied to manipulate the transmittance and reflectance of the optics based on the optical interference effect between the reflected beams at each interface.

We have developed various high and low refractive index oxides that can be applied widely in optical coatings. Thermal and plasma-enhanced ALD (PEALD) processes have been optimised successfully for Al2O3‚ TiO2, and SiO2 materials for coating plastics. We have also analysed the influence of process parameters on the uniformity‚ optical and mechanical properties‚ and environmental stability of the resultant coatings.

It is challenging to optimise ALD processes towards adhesive and crack-free films on PMMA. A low deposition temperature at around 60°C is preferential for temperature-sensitive PMMA substrates, as the polymer starts degrading at around 80°C. Additionally, these films should be homogeneous, uniform in thickness, and dense for application in optical coatings.

We found that employing longer purge times and extra pump down steps during the creation of a thermal Al2O3 protection layer, along with the proper tuning of plasma parameters, resulted in adhesive and crack-free films on PMMA substrates. Upon achieving the desired optical properties of the films – dispersion behaviour, optical losses and mechanical stability – a possible route for optical coatings for the visible spectral range was established via thermal ALD and PEALD processes2.

Uncoated (left) and AR-coated dome (right) having significantly reduced residual reflection from each surface of the dome.

While uncoated PMMA substrates have a reflectance of nearly 8 per cent in the visible range, this has been reduced to below 1.2 per cent for the 420 to 670nm spectral range by applying a double-sided AR coating with an average reflectance of 0.7 per cent2. The optimised ALD coatings show good adhesion to the PMMA substrates even after a climate test in a high-humidity and high-temperature chamber. These results enable a possible route by ALD to deposit uniform, crack-free, adhesive and environmentally durable thin film layers on sensitive thermoplastics such as PMMA.

Subsequently, the 3D conformal growth of ALD films was exploited on PMMA domes – highly demanding complex shaped substrates. Moreover, both surfaces can be coated simultaneously by ALD, which compensates for the relatively slow growth rate in ALD technology. ALD provides the flexibility to functionalise arbitrary surface geometries, even without having any in situ monitoring or complex substrate rotation.

The above image shows the visual appearance of an uncoated and an AR-coated dome, with the reflections clearly visible at both outer and inner surface. This AR coating is designed to maintain a blue colour independent of the viewing angle3. The reflectance becomes faint after coating. In order to make the substrates nearly invisible, a higher performance AR functional coating can be applied. Such broadband and angle insensitivity, however, rely on ultra-low refractive index, nanoporous or nanostructured top layers4, which are relatively fragile. The reflectance is reduced to below 0.5 per cent in the visible spectral range at 0° to 45° angles of incidence. Even at steep viewing angles, the reflectance must be very low.

Conclusion and future steps

Our conformal coatings can be applied to various optical specifications on a range of substrates, both glass and polymer. They can be realised for highly curved substrates such as freeform components, ball and half-ball lenses, domes and high aspect ratio nanostructures such as gratings or photonic crystal fibres.

Domes especially are difficult to coat with conventional technologies, due to their highly curved convex outer surface and deeply concave inner surface. Hemispherical and hyper-hemispherical dome-shaped windows are applied to protect equipment for optical remote sensing, such as automotive and terrestrial lidar systems, and optical communication, such as laser communication systems. Wide-angle dome ports are ideal for applications in surveillance and security camera systems, as well as submersible camera systems for underwater photography and underwater photogrammetry. For all these purposes, the dome-shaped windows act as an additional optical element, for which high transmission is required in the desired wavelength range. This high transmission can be realised by conformal AR or filter coatings.

The perspective of ALD for such optical applications is promising, since high efficiency on complex substrates has been demonstrated; however, scalability and throughput are critical aspects towards future applications. In the next phase, we will gradually scale up the optimised ALD technology and evaluate the application in industrial use with prospective project partners. Higher throughput and process transfer will be evaluated in corresponding industry-related ALD batch tools. Further challenges involve improving the mechanical stability of optical components, and coating development for various polymer materials.

Dr Kristin Pfeiffer is a research associate at Fraunhofer IOF and the Friedrich Schiller University Jena 

Pallabi Paul is a research associate at the Friedrich Schiller University Jena

Dr Adriana Szeghalmi is a group lead at Fraunhofer IOF and the Friedrich Schiller University Jena

References

[1] Coatings, Volume 7, Issue 8: ‘Antireflection Coatings for Strongly Curved Glass Lenses by Atomic Layer Deposition’; www.mdpi.com/2079-6412/7/8/118
[2] Coatings, Volume 10, Issue 1: ‘Antireflection Coating on PMMA Substrates by Atomic Layer Deposition’; https://doi.org/10.3390/coatings10010064
[3] Optics Letters, Volume 44, Issue 13: ‘Antireflection coating with consistent near-neutral color on complex-shaped substrates prepared by ALD’; https://doi.org/10.1364/OL.44.003270
[4] ACS Applied Material Interfaces, Volume 11, Issue 24: ‘Wide-Angle Broadband Antireflection Coatings Prepared by Atomic Layer Deposition’; https://doi.org/10.1021/acsami.9b03125

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