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The plasma etching process was originally developed to produce antireflective (AR) nanostructures on polymer substrates. A common vacuum coating system equipped with a plasma source was used. The technique is now applied to deposit and etch organic films on glass. The resulting organic nanostructured layers exhibit a low effective refractive index that can be tuned down to approximately 1.1. Broadband AR coatings were developed by combining inorganic materials and organic nanostructured layers in such a way that the effective index decreases in a stepwise manner or gradually from the index of the substrate to that of the ambient medium. They exhibit AR properties from 400 nm to 1200 nm at normal and oblique light incidence.
© 2015 Optical Society of America
1. Introduction
Reducing the amount of light that is reflected in optical systems is one of the basic aims in photonics. The reflection of light causes a loss in the intensity of transmitted light and generates ghost images and stray light, which reduce the image quality of an optical system. Commonly used AR stacks that consist of 3 or 4 single layers are suitable for the visible spectral range (400-700 nm) and for normal light incidence on flat or slightly curved substrates [1]. However, a component’s AR coating must cover a broader spectral range if the component is designated for a broader range of light incidence angles or if tilted areas such as those on a curved lens must be considered. Both conditions cause a shift in the reflectance spectrum toward shorter wavelengths, which can be partially compensated by broadening the AR design to longer wavelengths.
It is clear from theoretical knowledge that broadband AR interference stacks on low-index substrates have certain limitations that cannot be overcome using the available thin-film materials. There is a consensus regarding the following limitations and properties [1–4]: (a) broader wavelength ranges typically require coatings with larger physical thicknesses; (b) for a given design and materials, the average residual reflectance increases with increasing bandwidth; (c) the lowest residual reflectance that can be achieved depends on the refractive indices (specifically, the ratio of the maximum to the minimum refractive index) of the materials used in the interference stack; and (d) the refractive index of the last low-index layer has a dominating influence on the residual reflectance achieved for a given bandwidth.
As a possible method of achieving broadband AR behavior, interference stacks that end in an “artificial” low-index layer (n < 1.3) have been demonstrated [5]. Alternatively, a low-index nanostructured layer of sufficient thickness would provide excellent AR performance for broadband and wide-angle requirements if the effective index decreases from the substrate index to that of air [6–8]. Many techniques can be used to obtain single low-index layers. These techniques, such as glancing angle deposition (GLAD), lithographic methods, replication, sol-gel routes and etching, have been summarized in recent reviews [9–11]. However, most single nanostructured layers do not provide the depth and aspect ratio required for broadband AR applications. For example, low-index nanostructures can be achieved on polymer substrates by applying low-pressure plasma etching [12,13]. The structure depth after etching has been found to be limited to approximately 100 ± 20 nm. These structured surfaces often behave similar to quarter-wave layers, with an approximately constant, low index ranging from 1.1 to 1.3. As a consequence, the reflectance of low-index substrates (glass and plastics) can be sufficiently reduced in the visible spectral range but not over a broader wavelength range.
Broadband AR coatings can be obtained by combining layers in such a way that the effective index decreases in a stepwise manner or gradually from the index of the substrate to that of the ambient medium. The combination should reach a total thickness of 200 nm at minimum. The principle has already been demonstrated using nanostructured layers. Stacks consisting of at least two nanostructured layers have been prepared using GLAD [14] and the wet-chemical processing of porous polymer layers [15]. Our recent work describes the multiple etching of polymer substrates and organic layers [16]. As the first step, a cycloolefin polymer substrate was etched to obtain a structure consisting of the polymer material. A double structure with a total thickness of approximately 220 nm was then produced via the deposition and etching of melamine. As a result, 1.4% residual reflection was achieved in the visible spectrum (400-800 nm) for the average of the spectra at 0°, 45° and 60° light incidence angles [16].
Several strategies have been pursued to achieve improved broadband AR performance on glass through the application of etched organic nanostructures. Examples are shown in Fig. 1(a)-1(c). Based on the process developed for plastics, two or more nanostructured organic layers may be useful for the formation of a thick gradient-index layer on a glass substrate. The principle is shown in Fig. 1(a). Experiments are underway toward the realization of this goal. However, the combination of several nanostructured layers is challenging because of the risk of higher scattering losses, which may occur when several structures are combined.
Fig. 1 Schemes for advanced broadband AR coatings on glass: (a) Combination of two organic nanostructured layers; (b) Inorganic step-down design combined with an organic nanostructure; (c) Organic nanostructure covered with inorganic material
This paper focuses on the systems depicted in Fig. 1(b) and 1(c). Figure 1(b) shows homogeneous inorganic layers in a step-down arrangement, which are covered with a low-index organic nanostructure. The arrangement illustrated in Fig. 1(c) uses an organic nanostructure as a starting layer, which is then embedded in silica. Both coatings exhibit a total thickness in the range of 220-250 nm, which should be sufficient to achieve AR properties in the spectral range of 400 nm to 1200 nm.
2. Methods
Plasma ion-assisted deposition (PIAD) is a well-established technique for the deposition of inorganic and organic thin films [17]. A Leybold SyrusPro 1100 coating chamber equipped with e-beam guns, boat evaporation arrangements for organic substances and the Advanced Plasma Source (APS) were used in all of the experiments described here.
The organic small-molecule compounds 2,4,6-triamino-s-triazine (melamine, CAS 108-78-1) and 5,5′-bis(4-phenylyl)-2,2′-bithiophene (BP2T, CAS 175850-28-9) were evaporated from a molybdenum boat at a base pressure of 5x10−6 mbar and a deposition rate of 0.5 nm/s. Inorganic layers (MgF2 and SiO2) were deposited by e-beam evaporation. All coatings were deposited at room temperature. Ion assistance from the APS was applied for the deposition of SiO2 only. The typical pressure during the operation of the plasma source was 2x10−4 mbar. The ion energy was adjusted between approximately 80 eV and 120 eV, corresponding to an APS bias voltage in the range of 80 V to 120 V.
The etching step was also performed by applying the APS plasma using the same ion energy range. Oxygen (30 sccm) was used as the reactive gas and was partially ionized by the argon (15 sccm) plasma emitted from the source. The etching was controlled by in situ broadband monitoring of the sample’s transmission (OptiMon) [18]. The desired increase in transmission was typically obtained within a few minutes of etching. Ex situ optical characterization of the samples was performed using a Lambda 900 spectrophotometer (Perkin Elmer). The spectral reflection at 45° was obtained by averaging the spectra measured with s-polarized and p-polarized light. The structured surfaces were visualized using a Zeiss SIGMA SEM 3. For the simulation of the optical properties and the design calculations, the design tool OptiLayer was used [19].
3. Results and discussion
3.1 Inorganic step-down design combined with a low-index organic nanostructure
Most organic materials can be etched to form low-index nanostructures. Certain materials, i.e., PMMA, Pleximid, Teflon (PTFE) and melamine thin films, form “bump” structures that are completely self-organized if the appropriate plasma parameters are applied [12, 20]. The final etched structures are typically covered with a thin silica layer for protection.
For melamine covered with 20 nm silica, the etch procedure was aligned to produce a layer with an average effective refractive index as low as possible. In result, a structure exhibiting an effective refractive index of approximately 1.1 and a thickness of about 125 nm was achieved. The thickness was determined by evaluating the scanning electron micrograph (SEM) while the refractive index was obtained by using a fixed thickness value of 125 nm and minimizing the deviation from the three angle reflectance measurements by automated routines and also by trial and error. Figure 2 shows the measured and calculated reflectance spectra and the two-layer model that was used to describe the optical properties. An SEM image of a cross section of the structure is shown in Fig. 2(c). We also tested the technical reproducibility and found thickness values in a range of 115 nm to 130 nm and while the average refractive index varied from 1.08 to 1.15.
Fig. 2 Reflection spectra (without backside reflection) of an etched melamine layer on BK7 glass measured at 0°, 45° and 60° (a); the simulated refractive index profile (b); and an SEM image of the structured melamine at a viewing angle of 45° (c).
To obtain a step-down arrangement of higher thickness, layers of SiO2 (n = 1.46 @ 400 nm) and MgF2 (n = 1.38 @ 400 nm) were arranged between the glass substrate (n = 1.53 @ 400 nm) and the melamine/silica structure (n = 1.1). The target function was defined for the spectral range from 400 nm to 1200 nm and for incidence angles from 0° to 45°. The optimal thicknesses of the inorganic layers were determined through design optimization with the thickness of the structured layer fixed to 125 nm. The resulting design is presented in Fig. 3(b). The coating was produced in a closed vacuum process. Melamine was deposited with a thickness of 200 nm on top of the inorganic layers.
Fig. 3 Reflectance spectra (without backside reflection) of the SiO2/MgF2/melamine coating on B270 glass (n = 1.53) (a), the simulated refractive index profile (b), and an SEM micrograph at a viewing angle of 45° (c).
Next, the etching step was performed; the etching was terminated after 250 s, when the monitoring system showed the highest transmission. As the final step, a 20-nm silica layer was deposited, which further improved the transmission. A thicker silica layer would not be appropriate because it would cause scattering losses to increase, thereby causing transmission to decrease markedly. Figure 3(a) shows the reflectance spectra for light incidence angles of 0° and 45°. The residual reflectance was markedly reduced in comparison with the single melamine structure.
3.2 Organic nanostructure covered with silica
Organic materials adhere to inorganic surfaces only through weak van der Waals forces. Moreover, organic layers inside bulk films tend to be disrupted or broken. Therefore, AR coatings with an organic nanostructure on top can be easily destroyed by abrasion or during adhesion tests. Improved mechanical stability is thus desired for many applications. Therefore, procedures have to be investigated to reduce the amount of organic material in the coatings and to increase the thickness of the protective inorganic coating instead.
Some of the organic small-molecule compounds that have been investigated for nanostructured layers tend to form crystalline films rather than amorphous layers. The concept is to modify the naturally grown structures to obtain an aspect ratio that is as high as possible while simultaneously creating spaces between the nanocrystallites. This template provides the basis for the formation of an AR coating after the deposition of a silica layer in the thickness range of 70 nm to 120 nm. The total structure size must be adjusted to account for the limitations imposed by scattering. Higher scattering losses are expected if the size of the nanocrystallites reaches dimensions comparable to the wavelength.
A suitable base material for this procedure is BP2T, which is a p-type organic semiconductor [21]. Evaporated BP2T films exhibit a typical microcrystalline nanostructure as presented in Fig. 4(a). The naturally grown structure can be modified by plasma etching as shown in Fig. 4(b) and 4(c). The structure shown in Fig. 4(c) was achieved after applying oxygen plasma for 350 s (APS 120 V) shows spaces on the glass substrate that are free of organic material. BP2T is not completely free of absorption in the visible range. However, after etching, the amount of organic material that remains in the coating is very low.
Fig. 4 SEM images of a BP2T layer (viewing angle 45°): the naturally grown structure of an evaporated film (a) and the surface after 300 s (b) and after 350 s (c) of plasma etching.
In the subsequent step, SiO2 was deposited until the optical monitoring system indicated a transmission maximum. A total coating thickness of approximately 230 nm was achieved by depositing 90 nm of silica. Figure 5(a) shows the reflectance spectra for light incidence angles of 0° and 45°. A step-down or gradient index model as shown in Fig. 5(b) is appropriate to describe the optical properties. An SEM image of a cross section of the material is presented in Fig. 5(c). It shows a “bump” structure consisting predominantly of silica. The amount of organic material was estimated to be less than 15%.
Fig. 5 Reflectance spectra of the BP2T/SiO2 coating on B270 glass for 0° and 45° light incidence angles (a), the simulated refractive index profile (b), and an SEM micrograph at a viewing angle of 45° (c).
3.3 Comparison of coating properties
Figure 6 compares the transmission spectra of the two coatings. The higher losses of the BP2T-based coating at 400 nm are attributed to this coating exhibiting both higher absorption and higher scattering losses compared with the melamine-based coating.
Fig. 6 Transmission spectra of the SiO2/MgF2/melamine coating (a) and the BP2T/SiO2 coating (b) on B270 glass (both sides coated).
The optical properties and the results of durability, adhesion and abrasion tests for both coatings are summarized in Tab. 1. Both coatings demonstrated suitable AR properties for the visible/NIR range (400-1200 nm). For the visible range only, their performance was also excellent at greater light incidence angles.
Table 1. Optical properties and results of durability tests.
Both coatings passed the applied climatic tests. As expected, the coating with embedded BP2T was more stable against moderate abrasion. Fingerprints on this coating were easily removed with cloth and ethanol. The coating also passed a standardized test procedure wherein the surface was exposed to a soft cloth under a load of 500 g and was found to resist peeling in a tape test, whereas the melamine-based coating was damaged during these tests.
4. Conclusion
Plasma-etched organic layers are useful as components of various types of broadband AR coatings on glass. They may be used not only as low-index final layers but also as structural templates to achieve a desired low index after coating with silica. Coatings with an organic nanostructure as the outermost layer suffer from limitations in their mechanical properties. By contrast, the coating tested here that consisted of an organic layer embedded in a silica layer showed promising mechanical stability.
The coatings described here are simple to produce using common coating equipment. In contrast to other technologies that are used to produce nanostructures, the plasma etching of organic materials can be readily controlled. A common box coater equipped with a plasma source for ion-assisted deposition can be used for all processing steps, which can be applied in a single closed vacuum process. A substrate area of one square meter (i.e., the size of a typical calotte) or more can be simultaneously treated.
In addition, the vapor deposition and etching of organic layers on glass provides a route for achieving antireflective properties on strongly curved lenses. On the inclined surface of a lens, a vacuum deposited layer appears thinner, and therefore, the interference stacks are shifted and distorted. A coating with a gradient or step-down index profile and of sufficient thickness will provide broadband AR behavior in the center of a curved lens while still offering adequate performance in the visible spectrum on the inclined areas.
Acknowledgments
This research was supported by Bundesministerium für Bildung und Forschung (BMBF, FKZ 13N12160).
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