1. Introduction

Improving the light transmission of optical systems by reducing of the Fresnel reflections on glass and plastic surfaces in the visible spectral range has been an ongoing task for more than 80 years. For example, if oblique light incidence occurs on shaped lens surfaces, interference antireflection (AR) coatings consisting of bulk oxide and fluoride layers may reach theoretical limits regarding the residual reflectance achievable in the visible spectral range [1, 2]. Gradient AR coatings with a decreasing refractive index from the substrate surface to the outermost medium (air) can help to improve the situation [3]. In addition, low-index layers with sub-wavelength structure size, which provide a much lower effective refractive index n_eff than bulk layers, can improve the performance broadband AR coatings if they are used as last layer of suitable layer stacks [4–6].

A manifold of methods has been developed to produce nanostructured low-index layers, including, for example sol-gel-coatings, glancing angle deposition, embossing and imprinting procedures, as summarized in a number of review articles [7–9]. Many of them are useful as low-cost replacements for single layer AR coatings on large flat areas, i.e. for solar application. However, the criteria for a low-index layer to be suitable as the last layer of AR interference coatings are specific. The layer has to be well defined regarding its effective refractive index n_eff in the range 1.05 <n_eff< 1.25 and its thickness. At the same time, absorption and scattering losses have to be low to ensure high transmission. In particular, the criterion of scattering is challenging. The size of the single features of the nanostructures has to be tuned in the range below approximately 100 nm. While used in stacks together with underlying layers, it would be an advantage if the low-index layer is achievable within the same or with compatible procedures.

Our previous work has described the formation of antireflective nanostructures on bulk polymer substrates and on small-molecule organic layers by treating the surfaces with a special ion-source in a commercially available box-coater [10, 11]. A desired n_eff in a range from 1.2 down to 1.11 is achieved after covering the etched nanostructures with a thin inorganic layer (mostly silica). Despite modifying the overall refractive index, this cupping layer is essential to obtain a high environmental stability and stable properties over time [12]. While looking for suitable organic base materials, more than fifty polymers and organic layers have been evaluated during the last ten years. Only a few of them were found which showed the formation of a bumpy structure by etching in a complete self-organized way, this means, without deposition of thin layers who initiate the inhomogeneous etching [12]. The self-organized grown bumpy nanostructured layers exhibited the lowest n_eff and at the same time the highest layer thickness. For example, Polymethylmethacrylate (PMMA) or melamine nanostructures can provide a well-controllable very low effective refractive index in that way. Both have been already successfully applied as low-index last layers of multilayer AR coatings [6, 13].

In this paper the mechanism for the self-organized structure formation will be discussed. Based on the proposed mechanism, a new group of materials is identified which should potentially be tested for its evaporation and structure formation properties.

2. Material selection for self-organized structure formation

Self-organized structure formation on inorganic materials caused by ion bombardment has been comprehensively discussed in literature. Bumpy protuberances (“cone” or “pillars”) had already been observed and investigated after sputter bombardment on silicon targets, on metals, glass and silica [14]. The discussions to explain the structure formation by physical parameters took into account surface contaminations and seed layers in the start phase followed by inhomogeneous sputtering, surface migration and re-deposition [15, 16]. Bradley and Harper’s theory has explained the influence of sputtering yield on growth and the increase of already existing initial structures [17]. Further factors mentioned in literature include defect structures in the bulk surface, and a certain crystal micro structure and surface roughness effects of the bombarded surface [14].

The dominating effect during plasma and ion treatment of polymers and vacuum-deposited organic layers is chemical decomposition and removal of the material. However, following the mechanism discussed in literature for inorganic materials, a chemical surface modification, and its fast appearance and disappearance in an early state of ion bombardment, may initiate the inhomogeneous etching on some organic surfaces. Such a chemical fluctuation should be initiated by interaction of the organic surface with the plasma species during the first seconds of etching. Indeed, on PMMA a modified surface composition could be detected after a short ion bombardment which disappeared later [18]. For melamine, the general possibility of aggregate formation by condensation (i.e by forming melam or melem) is assumed [19].

Following that experience, we were looking for organic molecules that are able to form dimers or higher aggregates by chemical bonding or coordinative bonding (Van der Waals or hydrogen bonds), preferably initiated under plasma conditions. Suitable basic building blocks are purine and pyrimidine-based nucleobases in nature, such as thymine, guanine, cytosine, adenine, xanthine, uracil and others. They are known to form base-pairs with other nucleobases resulting in long-chain nucleic acids like i.e. deoxyribonucleic acid (DNA) but also to build dimers only [20]. Various purine and pyrimidine-based oligonucleotides have been investigated to form self-assembled flower-shaped supramolecular structures from solutions [21]. Adenine and uracil thin films have been prepared by electrospray in vacuum to investigate their electronic structure and ionization energies [22]. No information was found about vacuum evaporation and optical properties of this kind of materials.

One of the candidates is uracil (2,4-Pyrimidindion). Base-pairs containing uracil are connected by the formation of hydrogen bonds between the amine and carbonyl groups on the complementary bases. As known from literature, uracil is also able to form intermolecular bonded dimers as shown in Fig. 1 [20]. Taking into account the very low health risk, good availability at low price and promising thermal properties, uracil was chosen to test the evaporation and thin-film forming properties followed by etching experiments.

 

Fig. 1 Uracil chemical structure obtained from thymine by hydrolysis (a) and one of the possible uracil dimer structures (b) where two molecules are bonded via hydrogen bonds.

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3. Experimental

A Buehler high-vacuum chamber SyrusPro 1100 equipped with thermal evaporators for the organic materials, an electron beam gun evaporator for oxide materials and the advanced plasma source (APS) was used in all of the experiments described here. The organic compound uracil (CAS N66-22-8, empirical formula C₄H₄N₂O₂, molecular weight 112.09 g/mol, m_p >300°C) was purchased from Sigma-Aldrich. The powder was used as received for evaporation from a molybdenum boat. The deposition rate of uracil was 0.4 nm/s at a base pressure of 5x10⁻⁶ mbar. Single layers with thicknesses of 20 nm and approximately 240 nm were deposited on glass (B270), and fused silica. AR-hybrid layer was deposited on glass and on polymer substrate zeonex (Zeonex E48R). Silica and MgF₂ layers were deposited by e-beam evaporation. Ion assistance from the APS was applied for the deposition of SiO₂ only.

Etching experiments to produce nanostructures were performed by applying the Ar/O₂ plasma emitted from the APS plasma source. The ion energy was adjusted in a range between about 80 eV and 120 eV corresponding to an APS-bias voltage range between 80 V and 120 V [13]. The typical pressure while running the plasma source was 2x10⁻⁴ mbar. The deposition processes and etching steps were controlled by in situ broadband monitoring (WBOMS, Buehler) of the sample’s transmission.

Optical properties in the ultraviolet and visible (UV-VIS) spectral range were derived from spectral transmission and reflection measurements of glass and fused silica in the 200-1200 nm spectral range (Lambda 900 spectrophotometer, Perkin Elmer). Scanning electron micrograph (SEM) images were performed using a Zeiss SIGMA SEM. Software OptiLayer was used for design calculations and for modeling the optical properties of uracil layers.

4. Results and discussion

4.1 Properties of evaporated uracil thin films

Uracil layers deposited on glass and quartz appeared smooth, colorless and highly transparent after preparation for at least two hours (Fig. 2(a)). However, the microcrystalline structure changed dramatically during storage. Figures 2(b) and 2(c) show SEM images of the surfaces after 24 hours and after 26 days of storage time at normal laboratory conditions. Another part of the same sample is shown in Fig. 2(d) after 24 h climatic testing at 85 °C and 85% humidity. It is proposed that larger crystals grow as a result of Ostwald ripening (growth of large particles at the expense of smaller particles, driven by surface energy reduction) [23]. Other structural transitions are also possible to be responsible for the overall increase of volume. A hydration shell containing up to eleven molecules water around a single uracil molecule is discussed in literature [24, 25]. Recently, hydrogen bonded fibrillary aggregates from uracil films prepared by adsorption from a liquid solution have been mentioned [26].

 

Fig. 2 Top-view SEM images of uracil thin film as deposited (a), after 24 h (b) and 26 (c) days’ storage at room temperature and normal humidity, and after 24 h storage at 85 °C and 85% relative humidity (d); initial film thickness: 240 nm.

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Optical constants n and k (Fig. 3 left) were calculated from the transmission and reflection spectra of the deposited films (thickness 20 nm and 240 nm), which were measured shortly after preparation. The 240 nm thick layer was used to determine n and k in the range from 350 nm to 800 nm while the 20 nm thin layer was more suitable to achieve the optical constants in the ultraviolet range because of the high absorption in this spectral range. Both films showed increased scattering after 24 h storage at room temperature, which corresponds to the change of the morphology as mentioned before. Figure 3 (right) shows the effect for the 20 nm layer. The low absorption down to 300 nm qualifies the material for application in the UV range.

 

Fig. 3 Optical properties of Uracil thin film in terms of real (n) and imaginary (k) parts of complex refractive index (left) and total losses (100-R-T) of a layer with 20 nm thickness as deposited and after 24h storage at room temperature (right).

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4.2 Plasma-etching and structure formation

The change in morphology of a vapor deposited uracil thin film may not be an issue for its application as low-index layer if two conditions are satisfied. The thin film deposition and the preparation of a desired nanostructured layer should follow up quickly one after the other, and the nanostructure itself has to be stable over time. The first condition can easily be satisfied because the plasma-etching process is applied immediately afterwards and in the same vacuum chamber as the layer deposition. The stabilization of plasma-etched organic structures can typically be accomplished by depositing a thin oxide layer (mostly silica or alumina) on top of the organic materials [12].

Both steps, evaporation of uracil and etching, were controlled by in situ measurement of transmission. An increase of transmission as indication for a self-organized formation of nanostructures was observed during the first etching tests. Optimal parameters with respect to gas composition and ion energy (Bias Voltage of APS source) have been determined experimentally. Figure 4 shows SEM-images of the structure development (initial thickness 240 nm) depending on time. Starting with small holes, meandering walls and bumps are formed. After 550 s, a 140 nm thick effective layer is formed where no bulk uracil is seen on the ground between the protuberances.

 

Fig. 4 SEM images of uracil thin film after different etching times (APS plasma source, 110V Bias, Ar/O₂) (see Visualization 1).

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Figure 5 shows some of the transmission spectra of partly etched uracil-layers corresponding to the structures shown in Fig. 4, and a calculated curve to simulate the optical properties of the structure after 550 s. The optical properties of this structured layer correspond to those of a homogeneous layer with a thickness of 140nm and a refractive index of 1.17.

 

Fig. 5 Transmission spectra of uracil-layer (including rear side reflectance) measured in situ during etching. Black dotted curve: calculated spectrum for a layer with a refractive index of 1.17, thickness 140 nm.

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4.3 AR-coating with uracil-nanostructure

Uracil-nanostructures covered with silica can be tuned to achieve any effective refractive index in a range 1.15< n_eff <1.3 by variation of the etching parameters and start thickness. The average total losses caused by scattering are below 0.25% in the wavelength range 450 nm to 1000 nm (0.4%@ 500 nm). There are many possibilities to develop broadband AR-systems by combining such a structured hybrid layer with inorganic layer stacks. In this work, we used a three-layer stack consisting of silica/ magnesium fluoride/ silica together with the uracil hybrid layer and optimized the design for the wavelength range 450 nm to 1000 nm. The second silica layer was introduced to separate the magnesium fluoride from the organic part. Evaporated MgF₂ is a porous and hygroscopic material which may attract humidity, while silica can be densified by ion assistance. The inorganic layers were deposited by electron beam gun followed by the thermal evaporation of a 220 nm uracil-layer. Immediately after, plasma-etching was carried out until a desired transmission curve was reached. Finally, 45 nm of silica was deposited on top of the structure followed by a second plasma-etching step for stabilization purposes. The calculated and experimentally achieved reflectance is provided in Fig. 6(a). The average reflectance is approximately 0.17% in a spectral range from 450 nm to 1000 nm. In addition, the average reflectance for light incidence angles from 0° to 60° in the visible spectral range (450 nm to 700 nm) is below 0.25%, and therefore much lower than obtainable with conventional “bulk” AR-coatings (Fig. 6(b)). Figure 6(c) shows the SEM cross section of the coating. Even if a silica layer deposited by e-beam evaporation in a standard PVD coating machine is not able to cover the pillars in a uniform way, the walls of pillars typically obtain a thin coverage (5-10 nm) which is sufficient to preserve the overall structure. The silica thickness reaches up to about 40 nm on top of the bumpy hills. The coating appeared unchanged after climatic tests at 85 °C and 85% relative humidity.

 

Fig. 6 Properties of hybrid AR coating on zeonex substrate consisting of silica, magnesium fluoride and uracil: (a) calculated (blue line) and measured (dashed black line) reflectance spectra without rear side reflectance; (b) calculated reflectance for selected wavelengths at oblique light incidence in the visible spectral range; (c) SEM cross section.

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5. Summary and outlook

Nanostructured films exhibiting a very low effective refractive index are extremely useful for broadband antireflective systems. In particular, a technical way to produce those films together with other optical interference layers in a closed process opens the way for cost-effective production.

Self-organized structure formation in plasma- and ion-etching processes seems to follow the rule that a chemical inhomogeneity occurring during initial etching is essential for the growth of useful structures. Following this model, a group of biomaterials based on purines and pyrimidines has been identified because of its general ability to form dimers and other aggregates. Uracil as a first candidate has been studied in more detail. Vacuum deposited uracil layers initially showed a high transparency but suffered from recrystallization and scattering after short storage times on air and especially in contact with humidity. However, stable low-index nanostructures were obtained by plasma-etching immediately after layer deposition and by depositing protective oxide layers and further plasma treatments afterwards. After melamine, uracil is only the second material which can be evaporated and which has the ability to develop nanostructures by plasma-etching in a self-organized way.

All process steps can be carried out in a box coater equipped with ion source as commonly applied for the production of optical interference coatings. AR-coatings combining inorganic layers and uracil have demonstrated the expected very low reflectance in a broad spectral range. They are especially useful for oblique light incidence. In the next step, further derivatives from purine and pyrimidine will be investigated for their advantages as optical thin-film materials.