1. Introduction

Commonly, interference coatings are used to reduce the Fresnel-reflections of optical lenses [1,2]. As an alternative, nanostructures with sub-wavelength size, as found on the eye of the night-flying moth (“moth-eye structures”), can provide antireflective (AR) properties [3]. The basic principle is to mix the substrate material with air on a sub-wavelength scale in such a way that the effective refractive index of the surface layer is reduced. In an ideal case the modified surface shows a gradual fall of the effective refractive index from the substrate to air [4]. In comparison to common interference coatings, gradient index surfaces are normally less sensitive to the angle of light incidence [5].

Various methods have been described to produce gradient index layers, among them sol-gel and replication procedures [6–8]. Our previous work described the self-organized formation of stochastically arranged antireflective structures on polymers using a low-pressure plasma etching process [9]. An advanced procedure involves the use of the additional deposition of a thin oxide layer prior to etching [10]. A broad range of different morphologies exhibiting antireflective properties can be generated on almost all types of polymeric materials. To find an alternative solution for glass lenses, organic “base” layer materials have been investigated. The “small molecule” materials were deposited on glass and plasma-etched thereafter. In this study the results for 2,4,6-triamino-1,3,5-triazine (melamine) and N,N´-di(1-naphthyl)-N,N´-diphenyl benzidine (NPB, also reported as “α-NPD”) and will be presented.

2. Methods

The chemical composition of melamine and NPB is shown in Fig. 1 . Melamine was chosen because it is an easy to handle, inexpensive compound [11]. Melamine possesses very good fire and abrasion resistance, and excellent chemical inertness.Thin films of melamine are commonly used as gate dielectrics in organic field effect transistors [12]. NPB is well known as an evaporable layer material for organic light emitting diodes (OLEDs) [13]. The thin films were prepared in a BAK 640 high vacuum chamber (Balzers). Two thermal evaporators (CreaPhys GmbH) were used for evaporation of the organic substances. These consist of conic alumina crucibles heated by a filament. In addition, the vacuum chamber is equipped with an electron beam source.

 

Fig. 1 Chemical structure and melting temperature of N,N´-di(1-naphthyl)-N,N´-diphenyl benzidine (NPB) and 2,4,6-triamino-1,3,5-triazine (melamine).

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The organic layers were deposited at a base pressure of 5x10⁻⁶ mbar and a deposition rate of 0.2 nm/s. Substrates were fused silica, B270 glass and silicon. Process control of the deposition rate and film thickness was performed using a quartz oscillator.The etching process was carried out in an APS904 vacuum deposition chamber (Leybold-Optics) equipped with an advanced plasma source (APS) [14]. In the case of NPB, a thin layer of TiO₂ was deposited by electron beam evaporation immediately prior to etching [10]. The etch step was carried out by applying the plasma emitted from the APS which is a direct current (DC) plasma source. Oxygen (30 sccm) was used as the reactive gas and partly ionized by the argon (15 sccm) 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 80 eV (at bias voltage 80 V) or 110 eV (at bias 110 V). In some cases a SiO₂ layer with a thickness ranging from 25 nm to 45 nm was deposited with a deposition rate of 1.0 nm/s by ion-assisted deposition after etching in the same vacuum process.

The etching was controlled by in situ broadband monitoring (350 nm – 800 nm) of the sample’s transmission (OptiMon) [15]. Ex-situ optical characterization of samples was carried out using a Lambda 900 spectrophotometer (Perkin Elmer). The refractive index n and extinction coefficient k were derived from transmission and reflection spectra of the thin films deposited on fused silica by applying OptiChar software [16]. IR transmission spectra of organic films deposited on silicon were measured using a Varian 3100 FT-IR spectrometer over the range 500–4000 cm–1. The structured surfaces were visualized using a Zeiss SIGMA SEM.3.

3. Results and Discussion

3.1 Structure formation on melamine-layers

Monoclinic melamine crystals start sublimating at a temperature of about 220°C in a vacuum of 5x10-6 mbar. Typically the colorless and transparent layers show a scatter loss of about to 3% to 4% at 500 nm, which is caused by the columnar growth structure of the thin film (Fig. 2a ). The refractive index of melamine is 1.81 at 500 nm and 1.75 at 800 nm [17].

 

Fig. 2 SEM images of 200 nm melamine thin film on glass (a) prior to etching and after etching for (b) 60 s, (c) 160 s and (d) 200 s.

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A condition for a subsequent structure formation is an adequate etch rate. The layer thickness of homogeneous layers can normally be derived from simulation of the spectral curves in the visible range. In the case of melamine a structure formation occur during etching and the layers after etching appear not homogeneous. Therefore, the change of layer thickness during etching was determined by measuring the absorption bands in the infrared (IR) region (for IR band assignment see [13]). The absorbance calculated from the transmission spectra of the thin films deposited on silicon decreases linearly with the layer thickness according to the Lambert–Beer law (Fig. 3 ). An average etching rate for melamine of 1.3 ± 0.1 nm/s for an APS bias voltage of 110 V was determined.

 

Fig. 3 Left: detail of FTIR absorbance spectra of melamine measured before and after etching and, right: absorbance values at 3130 cm-1 (band of interacting NH groups [13]) as a function of etch time and the evaluation of the etch rate.

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The growth of a fine grained structure begins shortly after etching (Fig. 2b). The self-organized nanostructure is gradually destroying or overlapping the columnar growth structure (Fig. 2c, d). A maximal structure depth of about 130 nm was reached after about 200 s when applying 110 V bias. The transmission spectra of a 270 nm layer, as deposited on B270 glass before and after different etching times, are shown in Fig. 4 . The scatter loss (100%-R-T) decreases with etch time, as estimated. The minimum of the reflectance in the range from 350 nm to about 770 nm is adjustable by using different etching parameters (bias voltage, etch time), as shown in Fig. 5 .

 

Fig. 4 Transmission T and total losses (100- R + T) of a 300 nm melamine thin film before etching and after 100s etching (dashed lines) and after 200 s etching (dotted lines). The transmission of the uncoated B270 glass is shown for comparison.

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Fig. 5 Reflectance of etched melamine layers on glass including 30 nm silica top layer (without back side reflectance).The reflectance of the uncoated B270 glass is shown for comparison.

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3.2 Structure formation on NPB-layers

Transparent thin films of NPB were produced by thermal evaporation from the melt of the microcrystalline raw material at about 280 °C. NPB thin films do not show losses in the visible spectral range. They are highly absorbing in the UV-range, with a steep absorption edge at about 400 nm. The refractive index is about 1.83 @ 500 nm and 1.74 @ 700 nm, which is in agreement with the literature [13]. The surfaces of the thin films are smooth. The SEM images did not show any significant surface roughening even after an etching time of up to 1000 s. The layer thickness after different etching times was derived from transmission spectra measured online and by simulation of the spectral curves using the OptiChar software. An etching rate of 0.47 ± 0.03 nm/s was determined, which is about a half the melamine value. In our next experiment, a thin layer of TiO₂ (about 2-4 nm) was deposited prior to etching. In this case the etch rate decreased a little, to 0.34 ± 0.02 nm/s. Starting with a 270 nm layer thickness the highest transmission was achieved after about 430 s. Figure 6 shows SEM images of the typical “sponge-like” structures after different etch times. The structure depth was typically less than 80 nm. The final structure of the NPB shows more large holes compared to that of melamine, as shown in Fig. 6d. A reduced reflection can be obtained in restricted spectral ranges. The spectral position of the minimum reflectance is shifting to shorter wavelength with increasing etch time caused by the decreasing total layer thickness. The structured coating after 240 s acts like a typical two-layer AR-coating because of the remaining layer of high index NPB between the low-index glass and the low-index structured zone. A residual reflectance < 1% was only achieved over a very small spectral region of about 500 nm ± 50 nm after 430 nm etching. However, the antireflective function could be broadened significantly by adding a silica layer on top of this sample. The silica layer modifies the filling factor and increases the thickness of the low-n-layer. The top-layer is also useful to improve the chemical and mechanical stability of the surface. Figure 7 shows the reflectance of etched NPB layers after 240 s and 430 s and the effect of a silica top-layer as well.

 

Fig. 6 SEM images of a 270 nm NPB thin film on glass after coating with 4 nm TiO₂ and etching for (a) 60 s, (b) 180 s, (c) 240 s, and (d) 430 s.

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Fig. 7 Reflectance of etched NPB layers on glass without and with 36 nm silica top layer (without back side reflectance).The reflectance of the uncoated B270 glass is shown for comparison.

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5. Conclusion

Antireflective nanostructures have been obtained on both materials. On melamine, a structure grows in a self-organized way so no artificial mask was used. The anisotropic etching is probably caused by a passivation layer formed in an initial chemical reaction of the melamine in the plasma zone. The structure formation starts if ions penetrate the passivation. However, the etch rate is probably dominated by the chemical degradation of the organic material rather than by physical etching (ion bombardment). The structure depth on melamine is about 110 nm to 130 nm (Fig. 8 ). The structured zone consists of small needles which exhibit only a modest and gradual change of filling factor from the bulk to the ambient.

 

Fig. 8 SEM images of structured melamine (a) and NPB (b) on silicon, viewing angle 45°.

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NPB layers show isotropic etching. A structure formation can be initiated by a thin oxide layer, which probably operates like a passivation layer. It is surprising and not completely understood yet that the etch rate is nearly the same, independent of whether an initial layer was applied or not. Hence the layer does not act as an etching mask during the entire process. Probably the initial layer is etched away after a certain time but the nanostructure remains stable afterwards and can be transferred into the depth until the substrate is reached. However, the thickness of the structured zone never exceeded about 80 nm. The structure is characterized by holes like a sponge, as shown in Fig. 8.

For application on glass lenses, melamine seems to be more suitable than NPB. The essential advantage is the considerable structure depth. Thin films of silica deposited on top of the structures are helpful to improve the chemical and mechanical stabilities of organic structured surfaces in both cases. In addition, a notable broadening of the antireflective performance was observed for NPB. It is assumed by considering interference effects that the antireflective performance of both structured layers can be further improved by using at least one underlying oxide layer [17].

The reflection of light not only causes a loss in intensity but also generates ghost images and stray light which reduce the image quality in an optical system. The vapor deposition and etching of an organic layer on glass offers a new possibility to achieve homogeneous antireflective properties especially on curved lenses. On inclined areas of a lens a vacuum deposited layer will occur thinner but also the etch rate is lower than on a horizontal area. The problem of the disturbance of antireflective interference stacks on curved lenses caused by different layer thicknesses on top in on the inclined surfaces can be overcome in this way. A reduction of ghost images and stray light can be estimated therefore by applying the method for imaging systems.