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.
© 2007 Optical Society of America
Modern optical applications require polymer surfaces with antireflective properties. In most cases, optical interference coatings deposited by vacuum evaporation processes are used . One of the advantages of thermoplastic polymers such as polymethylmethacrylate (PMMA), polycarbonates and polycycloolefins as optical materials is the opportunity to form aspheric shaped lenses and Fresnel lenses by applying injection molding or hot-embossing . However, on diverse inclined surfaces thin films cannot be deposited with the nanometer-precision required. As an alternative, antireflective properties for polymers can be obtained using so-called “moth-eye structures” [3,4]. The basic principle is to mix the polymer material with air on a sub-wavelength scale to decrease the effective refractive index. Our previous work describes the self-organized formation of stochastically arranged antireflective structures using a low-pressure plasma etching process that is restricted to PMMA and leads to stochastically arranged antireflective nanostructures [5,6].
The effect of spontaneous structuring by ion bombardment processes has been described in the literature for a wide range of materials. Depending on the substrate and the ion beam parameters, the size of such structures ranges from tens to thousands of nanometers. In most cases the organization of the nanostructure relies on generating a regular or non-regular pattern on the surface . According to the physical model proposed by Bradley and Harper (BH theory), ion bombardment acts as a driving force to roughen the surface by random removal of atoms, and surface diffusion acts to restore the surface to a flat equilibrium state. The balance between roughening due to sputtering and various smoothing mechanisms can lead to a wide range of morphologies . Spontaneous structure formation during plasma treatment or ion beam etching of polymers has been described for many cases . However, the aspect ratio is normally far from being useful for antireflection. Therefore, the spontaneous formation of more or less regular-shaped surface features with an aspect ratio of 2 and higher on PMMA was surprising (Fig. 1(a)). With a structure depth of typically 200– 300 nm and a distance between surface features of approximately 70–100 nm, excellent antireflection properties for light of normal and oblique incidence have been achieved. Our 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.
Flat samples with optical grade surfaces (thickness 1 mm) were prepared by injection molding from PMMA 7N (Roehm), Zeonex E48R (Zeon Nippon), Trogamid CX (Degussa) and Ultrason E2010 (Degussa). Cleaning before etching or coating was performed merely by blowing the samples with ionized dry air. The etching process was carried out in an APS904 vacuum deposition chamber (Leybold-Optics) equipped with an advanced plasma source (APS) . Oxygen used as the reactive gas is partly ionized by the argon plasma emitted from the DC plasma source. Argon and oxygen ions are accelerated by a self-bias voltage to impinge on the substrate with energy of up to approximately 120 eV. Initial layers and top layers (TiO2, SiO2) were deposited by electron beam evaporation immediately prior to etching with the ion source APS. A Phillips XL40 scanning electron microscope (SEM) was used to visualize the nanostructures. A thin Au layer (< 10 nm) was deposited to achieve electrical conductivity. The samples optical characterization was carried out using a Lambda 900 spectrophotometer (Perkin Elmer) for light of normal incidence and an MCS 400 spectrophotometer (Carl Zeiss) at higher angles of incident light. The gradient index design and the coating design have been evaluated by using the software OptiLayer, Version 5.22.
3. Results and discussion
3.1 Structure formation and optical properties on PMMA
Chemical modifications of PMMA caused by plasma emissions may play an important role in forming a self-organized etching mask that promotes this deep structure. On the basis of this assumption, efforts have been concentrated to create useful artificial etching masks and to generate defined antireflective structures on the surfaces of various other polymers. Thin “initial” layers were deposited by electron beam deposition within a thickness range from 0.5 to 2.5 nm followed by an etching step. It was found that different dielectric materials work very well as an initial layer, whereas metallic layers did not show any useful effect. The structure formation can be controlled by the thickness and deposition conditions of the initial layer. Further parameters that can be to manipulate the structure are the etching time, plasma power, working pressure and gas composition. The basic theory to explain the mechanism of structure formation was that the deposited material grows as a non-continuous film forming islands. A second assumption is that the continuous initial layer disperses into pieces during the plasma treatment caused by the much higher thermal expansion of the substrate compared with that of the initial layer material. Thus a mask formation could take place when the sample is heated by the plasma. However both ideas could not be proved up to now. AFM images of surfaces with initial layer did not indicate an island growth. Substantial further investigations including transmission electron microscopy are in progress to understand the effect of initial layer to the structure formation.
On PMMA, a highly efficient antireflection structure was achieved after approximately 300 s etching without an additional etching mask. By using a dielectric initial layer and the same etching parameter, the optimal etching time is reduced to 150 s. The self-organized structure shown in Fig. 1(a) consists of equal cone-shaped bumps in contrast to the evenly distributed cavities and holes after applying an initial layer (Fig. 1(b)). Even though the surface topography differs for both cases, both structures exhibit similar antireflection properties.
To impart antireflective properties in a certain spectral range using surface structures, the spacing of a suitable array or structure has to be much smaller than the wavelength concerned, and the optical thickness of the modified surface layer has to be at least one quarter of that wavelength. With increasing structure depth, the transmittance range naturally shifts to longer wavelength.
In a first approximation, the structured polymer zone for the structures shown in Fig. 1 can be understood as a single layer with an effective refractive index neff that is lower than the polymer refractive index ns. The ideal refractive index ni to achieve zero reflection for a defined wavelength and for air as the ambient medium is given by . In the case of PMMA, with ns= 1.49, an ideal single layer for antireflection should have a refractive index of 1.22. In fact, an ideal refractive index has been yielded by approximation from the scanning electron micrographs showing a volume filling factor for the polymer around 0.45. Both structured surfaces show a tendency to have a lower volume filling factor on top and a larger one at the bottom. The result is a transition zone with a graded refractive index that increases with depth. The self-organized structure shows a rather distinct gradient with a slightly larger thickness than the structure formed by applying initial layers. However, both structure zones are characterized by a considerably greater thickness than the “quarterwave” optical thickness that leads to a broad antireflection performance. A good modeling of the optical properties of the structured surfaces can be achieved using sequences of thin effective layers as demonstrated for structure 2 in Fig. 2(a).
Figure 2(b) shows for comparison the refractive index profile of a common broadband antireflective coating that consists of 4 singe layers. The residual reflection obtainable by such a coating and by the structure is shown in Fig. 2(c). In particular, the structured surface shows a more efficient antireflective function than the coating at higher angles of light incidence. In addition, curved surfaces with a gradient antireflective structure are less sensitive to disturbances than coatings as long as the effective thickness on the tilted area is not considerably smaller than the quarterwave thickness. Optical coatings consisting of multiple layers appear always thinner at inclined surfaces and can completely fail their target function thereby.
3.2 Other polymer substrates and protective coatings
This new procedure means that many polymers can be used to achieve antireflective properties in this way. Both low reflection and high transmission in the visible spectral range have been obtained for different types of cycloolefine polymers (Zeonex, Zeonor), polycarbonates (Makrolon, CR39), polyethersulfone (Ultrasone), and polyamide (Trogamid), as well as triacetate cellulose film (TAC) and polyethylene terephthalate (PET) film. Some examples are shown in Fig. 3.
Cycloolefin polymers and copolymers are an important new group of thermoplastic materials that have entered the market for optics during the last ten years. Zeonex as its representative provides a useful combination of properties such as high transparency, low water absorption and comparatively high thermal stability. Without applying an initial layer, the topography of Zeonex is nearly unaffected by ion etching. However, an excellent antireflection effect has been achieved by applying a 1.5 nm thick initial layer prior the etching (Fig. 4(a)).
Typically, antireflective surface structures are mechanically weak and can be destroyed easily by rubbing the surface. This was observed for etched structures, as well as for replicated antireflective structures produced by embossing. An improvement in durability is anticipated if the structures can be stabilized with additional hard coatings. Such over-coating experiments have been carried out by depositing silica layers of 20–80 nm in thickness on top of the structures. Both, the thickness of the protective layers and the plasma treatment time have to be balanced to avoid deterioration of the spectral properties. The deposited films do not completely close the surface, as shown in Fig. 4(b) for the Zeonex sample. The scattering at the surface can be suppressed below a value of 0.5% in the visible spectral range.
In result of similar tests with other polymers it was found that the protective layers work very well on polymers that show cross-linking rather than degradation during the plasma treatment. Even acrylic polymers such as PMMA show a tendency to form degradation products of lower molecular weight after plasma and ion treatments . On the other hand, cross-linking reactions have been reported after plasma treatment for polyolefins and polyamides . Moreover, better stabilization can be achieved for structures containing holes than a more open structure consisting of high bumps. Different levels of durability have therefore been achieved. PMMA surfaces with a self-organized structure and a protective layer can be touched and handled carefully, but are damaged after rubbing with a cloth. Many other polymers such as polycycloolefines, polyethersulfone, polyamide and PET have been stabilized very well. The protected structure on Zeonex (Fig. 4(b)) can be touched by hand and fingerprints can be cleaned using ethanol and cloth.
Most antireflective surface structures used in industry today are periodically arranged nanostructures generated by replicating lithographically produced master structures . Up to now, it has been very complicated to apply this top-down technique to complex-shaped surfaces. Moreover, periodically arranged nanostructures are susceptible to optical limitations and color effects for light of oblique incidence caused by high diffraction orders. In contrast, antireflective surface structures produced by plasma etching can easily be applied to optical components that already contain structures at millimetre or micrometer scale and to sharp curved lenses. Moreover, using initial layers prior etching, the technology is not longer limited for the application of a special ion source. The investigations using initial layers showed that the etching process can be conducted under various plasma conditions. Controlled structure formation using initial layers should always be possible if the plasma is able to ablate the polymer. Therefore, efforts are in progress to investigate plasma techniques that can be scaled up to etch larger areas or that can operate in an in-line mode.
This research is supported by the Bundesministerium für Wirtschaft und Technologie (BMWi) and the Arbeitsgemeinschaft industrieller Forschungsvereinigungen (AiF) under contract numbers 15091 BR and 14580 BR and by the Bundesministerium für Wissenschaft und Forschung (BMBF) under contract number 13N9160.
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