Research Reports: Coating of plastic optics

The Fraunhofer IOF offers a wide range of functionalizations for surfaces and coatings - and among them also for optical components made of plastic. For customers from industry and research, we develop plastic coatings with a wide variety of properties such as antireflection, scratch resistance, antifingerprint, antifog, and more.

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Coating ot plastic optics

Below you will find research reports on our recent developments in the field of coating plastic optics:

Antireflection of polycarbonate for lidar applications

In the age of autonomous driving, vehicles need a variety of optical systems for distance control and monitoring, including miniaturized cameras and LIDAR systems. Transparent polycarbonate (PC) is used, i.e., for optical lenses, displays, and protective covers. In most cases, high transmittance is required, which can only be guaranteed by antireflective (AR) functionalization. In a project funded by the AiF, Fraunhofer IOF investigates and compares polycarbonates and alternative transparent polymers with respect to their surface properties relevant for AR coating deposition. Different combinations of parameters were evaluated to bond multilayer AR systems to surfaces by means of plasma-ion-assisted  deposition (PIAD) with high adhesive strength. In general, a polymer specific optimization of the deposition parameters is essential for each material type. Typical polycarbonates may only be coated under very mild plasma conditions to prevent subsequent defect formation. Excellent coating adhesion has already been achieved on alternative high refractive index polymers such as OKP-1 and EP6000. For dome-shaped components in LIDAR applications, the AR design targeted in PIAD must be extensively broadband. Figure 1 shows the reflection on such a PC component. In the cladding area up to an inclination angle of 60°, the residual reflection is below 1 %. Components with complex geometries can be coated even better using the ALD technology as conformal coatings can be achieved in this way.

With AR-plas^® nanostructures, excellent AR functionality can be obtained for higher angles of light incidence (AOI) (Fig. 2). For LIDAR wavelengths, samples with corresponding AR nanostructures were developed and produced for various industrial customers. In summary, the expertise of Fraunhofer IOF comprises extensive knowledge of surface optimization of polymers as well as several polymer-compatible antireflective coating processes such as PIAD, ALD and AR-plas^® plasma etching.

Acknowledgement

The results were obtained within the framework of the IGF project "Polymers 2020" (20663 BR of EFDS e.V., funded by BMWI) and in the
in the fo⁺ project (growth core, funded by BMBF).

Authors: Ulrike Schulz, Kristin Pfeiffer, Adriana Szeghalmi, Nancy Gratzke, Tina Seifert, Friedrich Rickelt

AR-plas^® antireflection of 3D-printed hybrid polymers

Additive manufacturing exhibits many advantages compared to conventional manufacturing because it enables maximum freedom of design, high individuality, and sustainability. There is a growing interest to extend this to the field of optics, more precisely, to fabricate highly individualized transparent optical components. Until now, the printing of macroscopic optics is still an especially challenging requirement. Fraunhofer IOF has already achieved good results with inkjet printing technology of Ormocers^® for highly transparent micro and macro optical elements (Fig. 1).

In addition to shape accuracy, the resulting optical quality is affected strongly by the final functionalization of the surface. Plasma etching of various polymer types has shown excellent antireflective properties with a transmittance > 98 % (VIS) by the formation of stochastic nanostructures (AR-plas^®).

The latest investigations have focused on the surface functionalization of organic-inorganic hybrid polymers based on OrmoComp^®, which are used for 3D-inkjet printing. OrmoComp^® is transparent in the visible spectral range and shows – caused by its high inorganic proportion – an increased  chemical, mechanical, and thermal stability compared to pure organic materials. With the help of the AR-plas^® technology, a broadband antireflective structure could be realized on surfaces of OrmoComp^®-derivates, whereas pure OrmoComp^® samples did not show any measurable change of transmittance.

After etching the printed samples with an argon/oxygen plasma for various etching times, stochastic “sponge-like” nanostructures were observed. Structure depth and pore size are correlated with etching time. It was proved that the structure formation is primarily based on a degradation of carbon-hydrogen compounds during the etching process. Thus, the structured part consists mostly of an inorganic Si-O network. With precise control of the etching parameters, the structure depth and pore size can be varied. A structure depth of approx. 100 nm results in good antireflective properties for the visible range whereas deeper structures of approximately 250 nm are necessary for the near-infrared range. By controlling the structure formation, an adjustable antireflective performance from the visible to the near-infrared range can be realized on printed surfaces (Fig. 2 and 3).

Authors: Sabrina Wolleb, Falk Kemper, Erik Beckert, Ulrike Schulz

Read more: 3D inkjet printing of optical components

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Hybrid materials for the preparation of stochastic nanostructures

Stochastic nanostructures, produced by low-pressure plasma etching, have been applied for antireflection (AR) coatings of polymer substrates for several years. Broadband AR-properties can also be achieved on glass if an organic layer is deposited by evaporation before etching.

Evaporation processes for organic materials have therefore been developed. Small organic molecules with multiple conjugated bonds such as derivatives of purines und pyrimidines are particularly suitable. An essential step of technology is the top-coating of generated structures
with silica.

It has recently been shown that plasma treatments as an additional last step are able to create chemical bonds between the oxide and organic groups. Furthermore, a large part of the organic content can be decomposed and removed. As a result, nano-porous hybrid materials are created which contain only traces of organic substances. This opens up a number of new applications. The AR-coatings then show in general a better stability in humid conditions. Some alternative organic precursors can be used which would tend to crystallize at higher humidity (Fig. 1).

It is now possible to produce AR-coatings on glass and quartz for the ultraviolet spectral range. Additional high-index oxide layers can be applied to cover the structures and to construct quarter wave systems such as dielectric mirrors and filters (Fig. 3). Surfaces with higher mechanical resistance are obtainable by depositing silica layers on top of small structures. In addition, these stable small structures are useful to achieve hydrophobic or hydrophilic surface properties.

A patent which describes the advanced production route has been granted.

Authors: Ulrike Schulz, Peter Munzert, Friedrich Rickelt, Nancy Gratzke

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Antireflection coating AR-plas2 for glass- and plastic lenses

There is an increasing demand for high performance antireflection (AR) coatings applied to complexly shaped surfaces. The application of such coatings on strongly curved lenses remains a challenge, particularly when a wide range of light incidence angles is required. PVD deposition processes, commonly used in industry, are not suitable for conformal coating resulting in a shift to shorter wavelength of the antireflection band on inclined areas. Conformal coating deposition has recently been made possible at Fraunhofer IOF using Atomic Layer Deposition pilot plants. This technology is being developed for use in the optics industry; however, it can only partly solve the challenges.

To achieve excellent antireflection properties over the full area of a curved lens with well-established evaporation technology, it is necessary to compensate the spectral shift by widening the AR bandwidth. The crucial factor for minimal remaining reflection is a low effective refractive index n < 1.15 for the final, nanostructured layer. At the same time, requirements for high angles of light incidence can be achieved more easily than before.

Fraunhofer IOF has identified new organic materials suitable for the generation of nanostructures with refractive indices in a range from 1.08 < n < 1.25. The antireflection coating AR-plas2^®, which is based on these materials, can be manufactured cost effectively and on an industrial scale. These multilayer coatings consist of only two to three elements and are distinguished by their excellent environmental stability. On a plastic lens surface, it is possible to etch the first structure directly into the substrate. A nanostructure is formed by etching an organic layer with the use of a patented procedure. This organic structure determines the topography of the subsequently deposited inorganic layer (Fig. 2)

After special post-treatments, the final coating contains only residuals of the organic material and is subsequently stable and UV-transparent. These new low-loss coatings allow a color neutral reduction of the reflectance for curved lenses with an average remaining reflectance of < 0.2 % over the visible wavelength range (Fig. 3).

Authors: Ulrike Schulz, Peter Munzert, Friedrich Rickelt, Heiko Knopf

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Organic-inorganic functional coatings by co-evaporation (ORKO)

Transparent optical coatings play an important role in tailoring light. Through the use of thin films, it is possible to modify the surface depending on the environmental conditions. Fogging, which is a disturbing effect, occurs when small droplets adhere to the surface. It reduces the efficiency of optical surfaces and can also pose a significant safety risk (e. g. visors and glasses).

An extensive analysis of commercially available antifogging products concluded that hydrophilic or water-absorbing materials are theoretically suitable to counteract fogging. However, these are too soft or not long-lasting enough for most practice-orientated applications. The chemical components that achieve the antifogging effect must be buried deep in the composite material. It should therefore affect the mechanical stability, but not the optical properties of the surface.

In the course of the evaluation of different low molecular hydrophilic polymers, for the first time cellulose acetate (CA) could be deposited by thermal evaporation. The homogeneous layers have a contact angle lower than 10° which emphasizes the good hydrophilic property even in long-term tests. Subsequently, first experiments of combined evaporation of CA and SiO₂ have been carried out and lead to good approaches for functional surfaces.

To protect the hydrophilic components, the organic compounds can also be incorporated in porous oxide layers. Such a reservoir-system can be obtained, for example, by co-evaporation of two materials (e. g. Al₂O₃/SiO₂) followed by dissolving and removing one component. Alternatively, porous SiOx-layers produced by Combustion Chemical Vapor Deposition (CCVD) can be filled. For filling experiments, antifogging lacquers provided by the industrial partners and some alternative hydrophilic materials will be tested.

Eight industrial partners of various research areas such as ophthalmic lenses, automotive engineering and specialty chemicals collaborate for this current project of the European Society for Thin Films (EFDS) supported by the Industrial Research Associations (grant number: 18778 BR).

Authors: Heike Müller, Christiane Weber, Heiko Knopf, Eric Henker, Ulrike Schulz

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Broadband antireflection coating for optical lenses

Reducing the reflected light in optical systems is a basic aim of photonics. Reflected light causes losses in the intensity of transmitted light and can generate ghost images and stray light. To reduce these aberrations, antireflection (AR) interference multilayers are typically used. The application of sub-wavelength structures represents an alternative approach. Suitable nanostructures can be produced via plasma etching on surfaces consisting of organic materials. The combination of interference coatings with nanostructured layers has been further developed within the scope of the BMBF joint project FIONA.

The application of AR coatings on the surface of strongly curved lenses is a challenge. Vapor deposited layers aregenerally thinner on inclined areas. As a consequence, the reflectance spectrum in the inclined regions is shifted to shorter wavelengths which can increase the reflectance in the visible spectral range. To ensure sufficient performance in this range on inclined surfaces, the spectral range of an AR coating can be extended to include the near infrared region, thus enabling coverage of the visible range over the entire lens.

To achieve broadband AR performance, several coatings comprising inorganic layers and organic nanostructures have been developed. One of the newly developed broadband AR designs consists of alternating high-index and low-index layers accomplished by a plasma-etched nanostructured organic layer (see scanning electron micrograph Fig. 2). A coated lens is shown in Figure 1. A residual average reflectance below 0.3 % was achieved in the spectral range from 400 nm to 1500 nm, which is significantly lower than that achievable with classical interference systems.

Authors: Ulrike Schulz, Friedrich Rickelt, Peter Munzert, Christiane Weber

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