1. Introduction

One of the most important requirements for complex optical systems is the reduction of surface reflections. Common solutions to obtain antireflective (AR) properties on glass or plastic include the deposition of single layers and multilayer stacks that act as interference coatings [1, 2]. As an alternative to coatings, sub-wavelength structures have been a growing focus of research and development. The basic principle of AR structures or porous layers involves mixing the substrate material with regions of air on a sub-wavelength scale in such a way that the effective refractive index of the surface layer is reduced. Ideally, the modified surface would exhibit a gradual decrease in the effective refractive index from the substrate to air. In comparison to common interference coatings, gradient index surfaces are normally less sensitive to the angle of light incidence [3, 4].

AR structures were first discovered on the eye of a nocturnal moth [5]. Modern methods to produce AR structures or porous layers on optics include technologies such as holographic superposition of light-exposed photoresists [6], glancing angle deposition (GLAD) of oxide layers [7] and nanophase-separated polymer films [8]. Those and multiple further technologies are discussed in recent review articles [9, 10].

Our investigations to date have established that stochastically arranged nanostructures can be achieved on polymer substrates and on organic layers by using low-pressure plasma etching [11–13]. A broad range of different morphologies exhibiting AR properties can be generated on nearly any type of organic material. However, in some cases, AR structures produced by plasma etching do not show optimal gradient characteristics. Rather, they often behave as single homogeneous layers with a reduced effective refractive index (n_eff) ranging from 1.1 to 1.3. Furthermore, the depth achievable by single etching steps was found to be limited to approximately 100 ± 20 nm. As a consequence, the reflectance of various low-index substrates (glass and plastics) can be sufficiently reduced in the visible spectral range. However, for a wider range of incident light angles (0° to 60°), further improvements in performance are desirable.

The most sophisticated application is a curved lens in which the AR function is maintained throughout the visible spectral range and over an extended range of incident light angles. Extended broadband AR performance must be achieved on the inclined surfaces of the lens in the same quality as on the horizontal planar areas. Based on theoretical knowledge, it is widely accepted that interference stacks have limitations that cannot be overcome with currently available compact thin-film materials and that a gradient layer of sufficient thickness would provide the best possible AR performance for wideband or wide-angle AR applications [2–4].

To overcome the limitations of singe gradient layers and interference coatings, several thin film combinations have been investigated. Stacks consisting of at least two nanostructured layers were prepared by GLAD [14] and by the wet-chemical processing of porous polymer layers [15]. In this paper, a multiple etching process for polymer substrates and organic layers will be introduced.

2. Methods

A cycloolefin polymer (Zeonex E48R, Zeon Corp.) was used as the substrate material [16]. Zeonex flat substrates (thickness 1 mm, diameter 55 mm) with optical grade surfaces were produced by injection molding. 2,4,6-triamino-1,3,5-triazine (melamine, C₃H₆N₆, F_p 350 °C) was used as the coating material. Melamine was chosen as the coating material because it is easy to handle and forms self-organized nanostructures, as demonstrated previously [17].

Thin film deposition and etching was performed in a SyrusPro 1100 high vacuum chamber (Leybold-Optics) equipped with an advanced plasma source (APS) [18]. To produce nanostructures on Zeonex, an initial layer of TiO₂ with a thickness of 10 Ǻ to 20 Ǻ was deposited by electron beam evaporation immediately prior to etching [10]. The etching of the Zeonex substrate was carried out by applying the plasma emitted from the APS, which was a direct current (DC) plasma source. Oxygen (30 sccm) was used as the reactive gas and partially ionized by the argon plasma emitted directly from the source. The typical pressure while running the plasma source was 2x10⁻⁴ mbar. The ions were accelerated by a self-bias voltage to impinge on the substrate with a maximum energy of 120 eV (at a bias voltage of 120 V). The transmission during etching was controlled by in situ broadband monitoring in the wavelength range of 350 nm to 800 nm. The desired increase in transmission is normally obtained within a few minutes of etching. The Zeonex structure was covered with a 20 nm thick silica layer deposited by electron beam evaporation. The main function of this oxide interlayer was to prevent the modification of the underlying structure during the etching of the succeeding organic layers.

In the next step, a 200 nm melamine layer was deposited by thermal evaporation from a molybdenum boat at a base pressure of 5x10⁻⁶ mbar and a deposition rate of 0.5 nm/s. Monoclinic melamine crystals start sublimating at a temperature of approximately 220 °C. The second etching process was carried out as described previously, but without using an initial layer and by applying a bias voltage of only 80 V (maximum ion energy 80 eV). It was essential to reduce the ion energy for the second etch step to prevent any influence on the first structure. Another SiO₂ layer with a thickness of 20 nm was deposited on top of the melamine structure to improve the mechanical and environmental stability of the system.

For the simulation of optical properties and for the design calculation, the design tool OptiLayer was used [19]. Ex-situ optical measurements of reflection and transmission at incidence angles of 6°, 45° and 60° were performed using a Lambda 900 spectrophotometer (Perkin Elmer). The spectral transmission T_a and reflection R_a at 45° and 60° was obtained by averaging the spectra measured with s-polarized and p-polarized light. The structured surfaces were visualized using a Zeiss SIGMA scanning electron microscope (SEM).

3. Results and discussion

3.1 Single AR structures on Zeonex polymer and on a melamine thin film

Zeonex E48R is an important thermoplastic used for optical lenses. The preparation of AR structures by plasma etching was described in an earlier paper [12]. Zeonex undergoes homogeneous material removal when treated with oxygen ions under a 10⁻⁴ mbar pressure range. Structure formation occurs only if a thin oxide layer is deposited prior to the ion bombardment. The etching rate is 0.8 nm/s for a bias voltage of 120 V. Depending on the initial thin oxide layer, the ion energy applied and the etch time, the structure can be modified, resulting in a shift in its reflectance minimum. For common applications in the visible spectral range at normal light incidence, the structure can be covered with up to 40 nm silica. Figure 1(a) shows the reflectance spectra of a direct-etched single AR structure on Zeonex measured at light incidence angles of 6°, 45° and 60°. The sponge-like structure was able to markedly reduce the reflectance in the visible spectral range (400 nm to 700 nm) at normal light incidence. However, the performance was insufficient at 60° light incidence. A typical structure without a silica top-layer is shown in Fig. 1(b) (SEM top-view) and Fig. 1(c) (SEM on edge, sample inclined at 45°).

 

Fig. 1 Reflection spectra of Zeonex (without backside reflection) before and after etching measured at 6°, 45° and 60° light incidence (a) and SEM micrographs of structured Zeonex from the top-view (b) and at a viewing angle of 45° (c).

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Melamine layers produced by evaporation on glass are transparent. During plasma etching, pillar structures are formed in a self-organized way without an additional initial layer. Again, the reflection minimum can be shifted within the visible range by tailoring the etch conditions [17]. Furthermore, the filling factor of the structure and its effective refractive index varies over a certain range, and the structure may exhibit more or less gradient character. Figure 2 shows the optical performance of an etched melamine layer on glass and SEM-images to illustrate the structure morphology. The depth of the typical pillar structure covered with silica was approximately 110 nm [Fig. 2(c)]. The spectral bandwidth is broadened, and the behavior at large light incidence angles is better compared with etched Zeonex due to the larger structure depth. However, the residual reflectance at normal light incidence is slightly increased. The modelling in section 3.2 will show that the filling factor of this structure and the remaining effective refractive index are very low and therefore not optimal for a single AR-layer.

 

Fig. 2 Reflection spectra (without backside reflection) of an etched melamine layer on BK7 glass measured at 6°, 45° and 60° light incidence angles compared to the spectrum of uncoated BK7 (a); SEM micrograph of structured Zeonex from the top-view (b) and at a viewing angle of 45° (c).

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3.2 Preparation and properties of double-structures

Deeper structures can be obtained in some cases by increasing the etch time. However, the size of the single elements (pores, pillars) comprising the structure will increase along with the depth. As a result, the scattering of the surfaces would increase and the transmission enhancement would be limited. Therefore, our aim to obtain a thicker gradient layer was to arrange various narrow-size structures successively in multiple steps.

Several experiments were performed to determine the necessary parameters for obtaining single structures exhibiting the desired optical properties for a later combination. For Zeonex, the shorter etch time used for the single-layer solution described in section 3.1 was applied. Therefore, the reflectance minimum shifted to a lower wavelength [Fig. 3(d); gray line indicates the measured reflectance of the Zeonex structure]. The effective refractive index profile can be described as a 3-layer arrangement [Fig. 3(a)]. In a first approximation, the single structure melamine (deposited on glass) could be modeled as a two-layer arrangement [Fig. 3(b)]. That corresponds in the upper part to the pillar profile visible in the SEM picture [Fig. 2(c)] exhibiting a filling factor less than 15% and an effective refractive index of 1.09.

 

Fig. 3 Refractive index profiles simulated to describe the optical properties of the single structures on Zeonex (a) and melamine (b) and of the double-structure (c); the measured (gray lines) and calculated (dashed black lines) indicate the reflection spectra without backside reflection of the substrates.

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Both structures were processed one after another but separated by a 30 nm silica layer. Another 20 nm silica layer was deposited on top to protect the surface. The optical properties and an SEM image of the double-structure are shown in Fig. 4.

 

Fig. 4 Reflection spectra (without backside reflection) of the double-structure on Zeonex measured at 6°, 45° and 60° light incidence angles compared to the spectrum of the Zeonex substrate (a); SEM micrograph of the double-structured Zeonex at a viewing angle of 45° (b).

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The refractive index profile of the double structure was approximated by taking into account the refractive index profiles of the single structures, the measured reflection spectra of the double structure and its depth obtained from SEM. Figure 3(d) shows that the experimental data were in very good agreement with those obtained from the calculations using the refractive index profiles. However, the refractive index profile of the double-structure [Fig. 3(c)] was not merely a combination of the profiles of the two single structures; obviously, there was an additional effect from the silica interlayer as well as intermixing effects.

The double-structure exhibited reduced reflection for all light incidence angles, as expected. The structure depth and average transmission and reflection values are summarized in Table 1 for the single structures [Figs. 1 and 2] and the double-structure [Fig. 4]. The double-structure achieved a residual reflection of 1.3% by averaging the spectra measured at 0°, 45° and 60° light incidence angles in the visible spectral range (400 nm – 750 nm) and of 1.7% in the range from 350 nm to 1000 nm. From the SEM investigations, a total structure depth of approximately 220 nm was achieved for the double structure. Figure 5 shows the transmission, T, and the sum R + T to reveal the losses due to scattering in the wavelength range below 550 nm. The scattering loss for the double structure was approximately 1.5% at 400 nm but less than 0.5% at 480 nm and for longer wavelengths. Notably, absorption from the Zeonex substrate appears at longer wavelengths.

Table 1. Average reflection in the visible spectral range (400 nm to 750 nm) and structure depth.

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Fig. 5 Transmission, T, and the sum of R + T at normal light incidence for the double-structure on Zeonex.

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4. Conclusion

Nanostructured surfaces can provide broadband and wide-angle AR performance if a sufficient structure depth of at least 200 nm can be achieved. Scattering can be minimized by applying two successive layers instead of one thick layer. The first material used should have a lower etch rate than the material used for the outer layer. The lower etch rate of the first material, together with a thin silica interlayer on top of the first structure, can ensure the stability of the first structure during subsequent etch steps. The entire stack must exhibit a decreasing effective refractive index going from the substrate to the surrounding air.

In contrast to other technologies used to produce nanostructures, the plasma etching of organic materials can be easily controlled. A common box coater equipped with a plasma source for ion-assisted deposition can be used for all process steps, which are applied in a single closed vacuum process. An area of one square meter (typical calotte size) or more can be used for substrates at the same time. Moreover, the procedure works on flat substrates, on curved lenses and on microstructured surfaces (such as Fresnel lenses).

Broadband AR characteristics were achieved on a cycloolefin polymer in our initial experiments. It is obvious that all types of optical polymers can be treated in a similar manner by using a melamine layer to generate the second structure. For applications on glass, the evaporation and etching of several organic materials is under development.