Ulrike Schulz

Review of modern techniques to generate antireflective properties on thermoplastic polymers.

Modern optical applications need solutions for providing polymer surfaces with antireflective properties. The problems involved in coating comprise thermal limitations, incompatible mechanical properties of coating and substrate materials, and interaction between polymers and plasma. As an alternative for coating, antireflective properties on polymers can also be obtained by hot embossing or by ion etching of surface structures. My objective is to provide the criteria for choosing suitable deposition or structuring methods based on an understanding of plasma-, radiation-, and ion-induced surface phenomena; material compatibility; mechanical and environmental performance; and cost issues. The potential to produce antireflective interference coatings is documented for plasma-enhanced physical- and chemical-vapor-deposition methods, including modern hybrid techniques, as well as for solgel wet-chemical processes. The review about state-of-the-art coatings focuses on the thermoplastic acrylic, polycarbonate, and cycloolefin polymers.

Surface modification of PMMA by DC glow discharge and microwave plasma treatment for the improvement of coating adhesion

PMMA shows poor adhesion for evaporated inorganic coatings. The DC and microwave plasma treatments can considerably increase free surface energy but an improvement of coating adhesion is, according to our results, only possible by the DC process. ATR spectroscopy has been applied to investigate the changed chemical structure of DC plasma-treated PMMA. Samples treated by microwave plasma did not show ATR alteration while XPS analysis indicated surface oxygen enrichment.

Antireflection of transparent polymers by advanced plasma etching procedures.

Self-organized nanostructures that provide antireflection properties grow on PMMA caused by plasma ion etching. A new procedure uses a thin initial layer prior to the etching step. Different types of antireflective structures can now be produced in a shorter time and with fewer limitations on the type of polymer that can be used. The durability of the structured surfaces can be improved by the deposition of additional thin films.

Antireflection coating design for plastic optics

The coating of plastics for optical applications is intended to improve the mechanical durability of soft polymers and to serve an antireflection function. Usually a classic four-layer antireflection system is added on top of a single-layer hard coating. With needle optimization, an alternative coating design has been developed. The design is characterized by thin high-refractive-index layers that are almost evenly distributed over the whole stack. Plasma ion-assisted deposition was used to deposit coatings upon poly(methyl methacrylate), polycarbonate, and cyclo-olefin copolymer. Uniform antireflection and high scratch resistance have been achieved.

Symmetrical periods in antireflective coatings for plastic optics

Plastic optical parts require antireflective as well as hard coatings. A novel design concept for coating plastics combines both functions. Symmetrical three-layer periods with a phase thickness of 3/2pi are arranged in a multilayer to achieve a step-down refractive-index profile. It is shown mathematically that the equivalent index of symmetrical periods can be lower than the lowest refractive index of a material used in the design, if the phase thickness of the symmetrical period is set equal to 3/2pi instead of the usual pi/2. The straightforward application of the concept to the design of antireflection coatings in general is demonstrated by example.

Transparent thermoplastic polymers in plasma-assisted coating processes

Abstract The performance of polymethylacrylate, polycarbonate, Zeonex ® and Topas ® as substrate materials in plasma-assisted physical vapour deposition processes has been studied. Different effects of UV radiation, plasma and ion bombardment on the polymers’ optical features, surface energy and adhesion properties for oxide layers are shown. As a consequence, conditions for the deposition of optical coatings on these substrates are discussed.

Antireflection coating with UV-protective properties for polycarbonate

Polycarbonate is the chosen material for covers of automotive displays because it combines high transparency with high breaking strength. The requirements for coatings include an improvement of the scratch resistance and antireflection properties as well as high stability for challenging environmental conditions. A coating that involves all required properties has been developed and deposited onto polycarbonate by plasma-ion assisted deposition.

Wideband antireflection coatings by combining interference multilayers with structured top layers.

The residual reflectance obtained for a broad wavelength range depends mainly on the refractive index of the last layer. Using interference layer stacks composed of naturally available low- and high-index materials, the residual reflection for a broad range cannot be adjusted below a certain limit. However, nanostructured (gradient) and porous layers are effective media with a refractive index lower than that of natural materials. Results demonstrate that an interference layer stack combined with a structured layer as the last layer yields better antireflection properties owing to the low effective index of the structure.

Plasma-etched organic layers for antireflection purposes

Organic layers can be used to realize special functions in optical interference coatings. Suitable compounds for such layers were thermally evaporated and characterized. A plasma etching procedure was applied to produce nanostructures on top of the organic layers to reduce their effective refractive indices. Broadband antireflective coatings were obtained by combining these artificial low-index layers with conventionally prepared interference stacks.

Low temperature deposition of indium tin oxide films by plasma ion-assisted evaporation

Coatings of transparent conductive oxides, especially indium tin oxide (ITO), are important in different fields. So far, application of these materials has been limited to substrates with high thermal stability. We describe an improved coating process for ITO based on plasma ion-assisted evaporation at a substrate temperature below 100 °C, which is suitable for organic substrates. In characterizing the thin films, we used the classical Drude theory to calculate the resistivity from optical film properties and compared the data with linear four-point measurements. X-ray diffraction spectroscopy was used to determine the structural properties of the thin films.