Wide-angle broadband antireflection coatings prepared by atomic

Applications of Polymer, Composite, and Coating Materials

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Wide-Angle Broadband Antireflection Coatings Prepared by Atomic Layer Deposition Kristin Pfeiffer, Lilit Ghazaryan, Ulrike Schulz, and Adriana Szeghalmi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03125 • Publication Date (Web): 14 May 2019 Downloaded from http://pubs.acs.org on May 14, 2019

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ACS Applied Materials & Interfaces

Wide-angle broadband antireflection coatings prepared by atomic layer deposition Kristin Pfeiffer,1,2 Lilit Ghazaryan,1,2 Ulrike Schulz,1 and Adriana Szeghalmi1,2,* 1Fraunhofer

Institute for Applied Optics and Precision Engineering, Albert- Einstein-Str. 7,

07747 Jena, Germany 2Institute

of Applied Physics, Abbe Center of Photonics, Friedrich Schiller University, Albert-

Einstein-Str. 15, 07745 Jena, Germany KEYWORDS: antireflection coatings, broadband, wide-angle, atomic layer deposition, nanoporous SiO2

ABSTRACT: A novel broadband antireflective coating with ultra-low residual reflectance for light incidence angles from 0° up to 60° is presented. The system consists of an interference multilayer coating made by atomic layer deposition (ALD) combined with a low-n nanoporous SiO2 top-layer obtained by wet-chemical etching of an atomically mixed SiO2:Al2O3 ALD composite. The average residual reflectance measured at normal incidence for double-sided coated B270 glass substrates is only 0.5% in a broad spectral range from 400 nm to 1100 nm. The average reflectance of the substrate considering both front and rear sides decreased in the visible spectral range of 420 – 680 nm from 9.9% and 15.8% to 0.4% and 1.8% at oblique angles of incidence (AOI) of 45° and 60°, respectively, by applying the ALD hybrid antireflection coatings. The measured average

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transmittance reaches 99.5% at AOI 6° in the 400 – 950 nm spectral range. Measurements three weeks after preparation show only a small reduction of the average transmittance to 99.3% in this spectral range spanning 550 nm. Ten months later, the average transmittance is still 99.0%, whereby the sample handling might have also affected the performance. The hybrid ALD system shows excellent conformal AR performance on a strongly curved B270 aspheric lens with a diameter of 50 mm and a height of 25 mm. The presented process is a promising route towards omnidirectional AR coatings on complex 3D optics, which are increasingly important for consumer and high performance optical systems.

INTRODUCTION Modern optical systems rely of a large number of lenses with different curvature radii, whereby the incoming light impinges on the lens surfaces under various angles of incidence. Antireflection (AR) coatings are applied to the optical surfaces to reduce reflectance losses in order to increase the efficiency of the optical systems and minimize ghost images and flare.1–3 Maximum transmittance values as desired for the optical components to prevent exceedingly large losses if multiple reflections can occur in the system. Even though highly curved substrates have often been addressed4–11, the application of wide-angle AR coatings to complex 3D surfaces remains challenging because of stringent requirements imposed on the film thickness precision. Antireflection coatings consisting of alternating high- and low-index materials are commonly prepared by physical vapor deposition (PVD) and chemical vapor deposition (CVD) techniques such as evaporation, sputtering, and plasma enhanced CVD.2,3 However, thickness gradients can occur on the inclined surfaces of strongly curved substrates due to shadowing effects. Because of film thickness variations, the AR coating will not meet the desired specification at steep inclinations of the substrate surface leading to increased reflectance. Several methods to address


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these issues have been proposed, in particular substrate-rotational systems and application of shadowing masks.5,12–15 However, the capabilities of these methods are limited to constrained geometrical forms, i.e. cylinders or convex shaped lenses. A bare glass substrate typically has a reflectance loss of 8% for normal incidence of light. Under oblique incidence, the losses due to reflection are drastically increased, e.g. a plane B270 glass substrate exceeds a reflectance (considering front and rear side) of 15.8% at 60° incidence angle. The multilayer’s reflectance, and hence the AR performance, depends on the angle of incidence (AOI). For interference coatings, a blue-shift occurs for all interference features including the spectral region of low reflectance if the angle of incidence is increased. At steep angles of incidence, the reflectance in the long wavelength range increases. Therefore, AR coatings should be designed and produced to span a spectral range as broad as possible if incidence angles from normal to 60° and higher are required. At normal angle of incidence, these coatings are designed to possess high transmittance in a spectral range spanning more than twice the wavelength range required at the steep angle of incidence. As an example for photographic or illumination applications in the visible spectral range, in order to minimize the reflectance for a large angular acceptance space up to 60° AOI, it is necessary to extend the AR performance at normal incidence from 400 nm up to approximately 1100 nm wavelength. For many applications, single-layer antireflective coatings (SLAR) such as MgF2 (n=1.38) are used, because their reflectance never increases dramatically with the angle of incidence. However, their performance is limited to a relatively narrow spectral range for a reflectance below 0.5%. The residual reflectivity in a broad wavelength range can be lowered compared to a MgF2 single layer by applying materials with even lower refractive indices. Excellent results have been realized based on gradient index systems similar to moth’s eye nanostructures. Nanostructures or


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subwavelength structures and porous layers, which are solid refractive materials having pores filled with air, were developed for antireflection coatings with applications in photovoltaics16–22 or on glass substrates for optical components.4,23–31 Preferably, the maximum porosity, which corresponds to the lowest refractive index, is at the coating-air interface and the minimum near the substrate to imitate a moth’s eye type structure.32,33 The best possible solution for oblique light incidence is a gradient layer with sufficient thickness as already described by Minot fifty years ago.33 Unfortunately, the production of such nanostructures is still challenging specifically on complex shaped optics. In broadband multilayer antireflective coatings (BBAR), there is always a compromise between the width of the antireflective spectral range and the average residual reflectance, as the broadening of the spectral range will lead to an increase of the average residual reflectance. For example, a normal-incidence AR coating by means of a SiO2/TiO2 multilayer stack, with constant refractive indices of n=1.46 and n=2.40, respectively, reaches a theoretical minimum residual average of 0.01% in a 420-680 nm spectral range on a BK7 substrate, but only a 0.41% average reflectance in a 400-1100 nm spectral range (estimated with Amotchkina’s criterion34). Nevertheless, the broadband and wide-angle properties of interference stacks can be improved by using a material with sufficiently low n as outermost layer. In recent years, interference multilayers that couple PVD with nanostructures prepared by oxygen plasma etching of organic materials35,36, reactive ion etching37, glancing angle deposition38, or nanoimprint39 technologies have been reported. To counteract the decrease in coating thickness in PVD systems on inclined surfaces, the wavelength range needs to be significantly broadened. For instance, the expansion of the AR band to about 1400 nm is necessary to compensate a 50% thickness reduction on an inclined substrate surface with 60° tilt angle7,


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whereas the decrease of the coating thickness on the flank of the surface is determined according to the cosine dependence on the angle of vapor incidence. Thus, more complex and thicker AR systems are needed. Such super-broad PVD hybrid AR systems will work for inclinations up to roughly 60°. Consequently, this method is restricted to certain curved surfaces, in contrasts, half ball lenses or domes may not be covered. Here, we demonstrate a hybrid system that applies an AR system, which enables an excellent AR performance at highly curved surfaces. In the presented approach, both the interference stack and a nanoporous low-n outermost layer are prepared by atomic layer deposition (ALD). The key features of the ALD coatings, precise thickness control and conformal growth, promise a huge benefit for the use as optical coatings.40–48 ALD offers the advantage of producing highly conformal optical coatings also on more complex shaped surfaces.9 Similar to conventional techniques, the AR performance of an ALD multilayer system without a low refractive index top layer is strictly limited to a small angular range. In this work, the presented hybrid AR system shows a wide-angle broadband AR performance on simultaneously double-sided coated plane glasses as well as on highly curved aspheric glass lenses.

EXPERIMENTAL DETAILS Materials and Methods. All ALD layers were deposited in an Oxford Instrument remote plasma ALD tool (OpAL, Oxford Instruments Plasma Technology, Yatton, UK) equipped with an inductively coupled plasma (ICP) RF generator, operating at 13.56 MHz. Depositions were performed at 100°C and 150°C. The commercially available precursors titanium tetraisopropoxide (TTiP), trimethylaluminium (TMA) and tris(dimethylamno)silane (3DMAS) (Strem Chemicals, Newburyport, Massachusetts, United States) were used for TiO2, Al2O3 and SiO2, respectively.


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The applied ALD tool has a showerhead type reactor where precursor vapor and reactant gases are introduced above the substrates. Vapor draw dose was used for the TMA and 3DMAS precursors, whereby the TMA was kept at room temperature and 3DMAS was heated to 30°C. The TTIP precursor was bubbled at 50°C temperature. The pulses of the organometallic precursors were 30 ms, 1.5 s, and 400 ms for the TMA, TTiP, and 3DMAS, respectively. Oxygen plasma with a flow rate of 50 sccm of oxygen and 300 W was applied as oxidizer for all materials. Plasma pulsing times of 5 s (Al2O3 at 100°C and 150°C), 6 s (TiO2 at 100°C and 150°C), 3 s (SiO2 at 100°C) and 10 s (SiO2 at 150°C) were used. Further details of the deposition processes have previously been reported.9 Nanoporous SiO2 was prepared by ALD and subsequent selective etching of an atomically mixed Al2O3:SiO2 layer as recently reported.27 In short, an Al2O3:SiO2 nanocomposite was deposited at 150°C from repeated ALD cycles with a ratio of 4:2 for Al2O3 and SiO2, respectively. After ALD deposition, selective wet chemical etching of the Al2O3 component in H3PO4 85% aqueous solution (Sigma-Aldrich, St. Louis, Missouri, United States) at 50°C temperature for 1 hour yields a nanoporous SiO2 layer with an average effective refractive index of about 1.15 at 633 nm wavelength. The final thickness of the nanoporous SiO2 is about 45% thinner than the thickness of the initially deposited Al2O3:SiO2 nanocomposite. Further optimization to reduce the etching time of the multilayer system is being considered in order to minimize the shrinkage of the nanoporous SiO2 compared to the nanocomposite. Single layer analysis of growth rates and optical constants was performed by ellipsometry (M2000, J. A. Woollam, Lincoln, United States). Deposition of SiO2, Al2O3 and TiO2 at 100°C resulted in growth rates of 1.18 Å/cycle, 1.22 Å/cycle and 0.33 Å/cycle, respectively. The deposition of the Al2O3:SiO2 nanocomposite at a deposition temperature of 150°C resulted in a


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growth rate of 1.05 Å/cycle. Refractive indices of Al2O3, SiO2 and the nanoporous SiO2 thin layers ranging from 50 nm to 100 nm were calculated using a Cauchy model. The refractive index of TiO2 thin layers were evaluated using the universal dispersion model proposed by Franta et al.49,50 The AR design was carried out with the OptiLayer thin film software (Version 12.12, OptiLayer GmbH, Garching, Germany). The AR performance under normal and oblique incidence was tested on plane B270 and BK7 glasses, both with a refractive index of n=1.52 at 633 nm wavelength. During the deposition, the glasses were positioned vertically using a home-build sample holder so that the front and rear sides are simultaneously coated in one single experiment. A photograph of the sample holder is shown in the Supporting Information. The reflectance and transmittance spectra were recorded ex situ at 6°, 45°, and 60° angle of incidence using a UV/VIS spectrophotometry (Lambda 950, Perkin Elmer, Waltham, Massachusetts, United States) equipped with a home-build attachment for absolute transmittance and reflectance measurements. Reflectance of unpolarized light is averaged from measured reflectance under s- and p-polarized light. The microstructure of the AR system was investigated in a scanning electron microscopy (SEM) top-view image (Hitachi S-4800 Field Emission SEM, Hitachi High-Technologies Corporation, Japan) and a scanning transmission electron microcopy (STEM) cross-sectional view image. The TEM lamella preparation was performed on a FEI Helios NanoLab G3 UC using Ga ions with an energy of 30 keV for cutting and pre-thinning of the lamella and subsequent low energy polishing with 5 keV and 2 keV. The STEM image was taken on the same machine using 30 keV electrons and a current of 12.5 pA. Additionally, the hybrid AR coating system was applied to a B270 aspheric lens with a diameter of 50 mm and a height of 25 mm in order to demonstrate its performance on highly curved


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surfaces. A micro-spectrophotometer (USPM-RU-W NIR, Olympus K. K, Tokyo, Japan) was used to measure reflectance under AOI 0° at different positions on the lens, whereby the lens is placed on a tilt stage and tilted to angles up to 60° in four different directions. The incident beam spot size is approximately 40 µm.

Design Considerations. The ALD hybrid AR system is aimed to minimize the residual reflectance of glass substrates for incidence angles up to 60° in the 420-680 nm spectral range. To reach the design target of an average residual reflectance Rav < 1% at AOI 60° at a B270 surface, an extension of the AR band to about 1100 nm wavelength is chosen for normal angle of incidence. The refractive indices of the ALD materials applied to the hybrid AR system are depicted in Figure 1. These have been determined from single layer experiments.

2.8 2.6

Refractive index

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2.4

TiO2

2.2 2.0 1.8 Al2O3

1.6

SiO2

1.4

nanoporous SiO2

1.2 1.0

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Figure 1. Refractive index versus wavelength determined from single layer experiments for the ALD materials used for the hybrid AR coating.


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refractive index. Moreover, the homogeneity of the nanoporous ALD thin film is very high as determined by ellipsometry and can be modelled as a layer with a constant effective refractive index. On the contrary, nanostructured layers with a gradient refractive index changing with thickness need somewhat more complex designs in order to approximate the system by discrete layers.37,38 The theoretical average reflectance of the designed AR coating is 0.6% in the 420-680 nm wavelength range at 60° AOI for both B270 or BK7 glass substrates. A relative deviation of the nanoporous SiO2 top layer of 5% in thickness and ± 0.02 in the refractive index still allows an average residual reflectance below 1.5%, see Supporting Information for a detailed error analysis of the design. The layer below the nanoporous SiO2, i.e. the last layer of the interference stack, is a dense SiO2 thin film (crosshatched area in Figure 2) with an intermediate refractive index between the underlying alumina and top nanoporous SiO2. This dense SiO2 layer serves as an etching stop layer because the ALD SiO2 has a high resistance against phosphoric acid. Previous etch experiments in a 50°C phosphoric acid were performed to ensure that the SiO2 layer is not etched. The etching rate of the SiO2 ALD thin film was found to be less than one nm/day. Due to the conformal nature of the ALD deposition technique, this SiO2 layer encapsulates the entire underlying interference system, in particular the underlying Al2O3 thin film.

RESULTS AND DISCUSSION Optical Properties on Planar Glass Substrates. In a first experiment, the ALD hybrid AR coating was deposited on both sides at the same time of a flat B270 glass substrate. The possibility of double-sided coatings is an advantage of ALD and can partly compensate the rather long


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porous SiO2 layer could successfully stop the underlying interference stack from being etched by the phosphoric acid (post deposition treatment). The sharp interface between the nanoporous SiO2 and the underlying compact SiO2 layer, as seen in Figure 3, also indicates negligible etching of the SiO2 layer. This also confirms the pinhole free and conformal growth of ALD.

Table 1. Experimental average reflectance (R) and transmittance (T) of the double-sided ALD hybrid AR system on B270 glass for different spectral ranges and light incidence angles (unpolarized reflectance from front and rear side). Transmittance measurements were not performed under oblique incidence since measurements under AOI 6° already revealed low optical losses. AOI

420 – 680 nm

400 – 950 nm

400 – 1100 nm

T

6°

99.5%

99.5%

99.4%

R

6°

0.4 %

0.4 %

0.5 %

45°

0.4 %

0.5 %

1.3 %

60°

1.8 %

3.3 %

5.0 %

The parallel shift of the calculated and measured spectra at 60° AOI, see Figure 4, is addressed to the accuracy of the absolute reflectance measurement using the home-built attachment that is in a ± 0.2% range. The authors attribute further minor deviations to the etching process as they could also see minor deviations in etch-to-etch reproducibility tests of nanoporous single layers.55 Additionally, the thickness uniformity of the ALD coating is limited by the plasma configuration used in this study. The non-uniformity NU% is approximately ± 2% over a 200 mm diameter area, with NU% calculated by (dmax –dmin)/2daverage. As the performance of the AR coating is highly


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sensitive to the refractive index and the film thickness of the top layer, it requires further investigation to increase the accuracy of the nanoporous layer. Additionally, the determined refractive index of the single layer might slightly vary from the actual top nanoporous SiO2 layer of the AR system. The low optical losses of the ALD thin films including the nanoporous top layer enable high transmittance in a broad spectral range spanning more than 600 nm (see Figure 5, which also depicts the sum of transmittance and reflectance). Even after storage in air for three weeks, the transmittance is only slightly reduced, possibly due to the filling of subjacent pores with water or due to a reorganization of the porous network.27 The transmittance decreases mainly around 650 nm wavelength by approximately 0.4% within three weeks after preparation. Notably, in all cases the average transmittance is higher than 99.3% in the investigated spectral range from 400950 nm. Notably, under vacuum, the transmittance spectrum (thick blue curve shown in Figure 5) indicates an even better antireflection performance of the multilayer system than under ambient conditions. The transmittance increases to above 99% throughout the wavelength range. Further measurements have been carried out to study the long-term stability of the AR coatings. The average transmittance of a sample aged for 10 months is above 99.0%. The sample was stored under ambient conditions and repeatedly analyzed. As shown in Figure 5 the maximum change occurred around 650 nm wavelength. The transmittance at this wavelength decreased from 99.5% to 99.0% within three weeks after preparation and further to 98.5% after ten months. Hence, the major degradation seems to occur in a short time after preparation and long-term application of such hybrid AR coatings is feasible. The average optical loss, determined from 100%-T-R, is 0.57% at 400 nm but below 0.06% in the range of 500-1100 nm light. Optical losses below 400 nm, indicated by lower sum of


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transmittance and reflectance, of the presented AR coating arise due to the light absorption in the applied TiO2 material and the BK7 glass substrate itself.

Transmittance, Transimttance + Reflectance, T+R (%)

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100.0

99.5

99.0

directly after preparation in air in air (T+R) aged for 3 weeks in air in vacuum in vented chamber aged for 10 months in air

98.5

98.0

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Wavelength (nm)

Figure 5. Transmittance and sum of transmittance and reflectance (T+R) of hybrid AR coating applied on both sides of a plane BK7 glass substrate, measured after preparation, 3 weeks later, under air and vacuum conditions, and 10 months later. Stability Aspects. Similar to nanostructures, the nanoporous top layer is fragile. Thus, the AR hybrid system may only be applied to protected surfaces, for example inner lenses of objectives. It is, however, important that the optical performance is not deteriorated by environmental factors, in particular thermal conditions. Therefore, stability tests of the AR hybrid system were performed in warm and humid conditions and further in vacuum conditions in order to verify how the system varies according to external environmental factors. First, a climate test was performed. The samples were kept for 48 h at 85°C under a relative humidity of 85%. Reflectance spectra before and after climate test show only minor deviations


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below 0.2% that is within the limits of accuracy of the measurements, pointing out the stability of the hybrid ALD coating in a warm humid atmosphere. Second, the transmittance spectra of an unheated double-sided glass measured in air (Perkin Elmer) and under vacuum conditions at a base pressure of approximately 5x10-5 mbar (vacuum evaporation coating tool with optical broadband monitoring) were compared, see Figure 5. Measurement under vacuum conditions were executed within the first 10 min after chamber pump down. Under ambient conditions, the pores of the nanoporous material are filled with water. Under vacuum conditions, when the voids are no longer filled with water from the ambient environment, the effective refractive index of the porous top layer is reduced. The reduced refractive index has a positive effect on the overall performance of the hybrid AR system, as can be seen from an increased transmittance when measured under vacuum. After venting the chamber with air, the pores of the top-layer are assumed to be again filled with water, leading to a reduction of the transmittance of the hybrid AR system. The vacuum-to-air shift might possibly be prevented by the encapsulation with ALD Al2O3,46,56 as it shows good water vapor barrier properties due to its dense and pin-hole free deposition. It is worth to discuss here a short survey of various AR systems reported in the literature on glass substrates, although the antireflection target is not necessarily identical with our aim. Some of the most performant AR systems on glass have been realized using nanopillars etched directly in the glass substrate28, hybrid AR systems with top low-n layer sub-wavelength structures prepared by by etching of organic materials with oxygen plasma34,36, reactive ion etching of SiO2 thin films37, glancing angle deposition38, and nanoimprint followed by dry etching.39 The height and shape of the sub-wavelength structures have a great impact on the AR performance since scattering losses might impair transmittance of the optics. For optimal performance in the visible to near IR spectral


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range, nanopillars28 with a height of 780 nm reached maximum transmittance of 99.8% and average transmittance above 99% over a spectral range spanning approximately 450 nm. These nanopillars have also successfully been applied on both sides of semispherical fused silica shells, although the performance is slightly reduced on the inner surface. Unfortunately, increasing the nanopillar height resulted in increased scattering and absorption losses in the visible spectral range while the average reflectance could be reduced to below 0.3% in the spectral range from 200 nm to 2600 nm. In comparison, the AR system presented here, can be easily extended to the UV spectral range by applying UV transparent oxides (such as HfO257) and is the scope of future experiments. Preliminary results revealed minimal optical losses down to 193 nm wavelength for the nanoporous ALD SiO2, whereby a single layer AR coating with 99.4% transmittance has been realized.58 In the case of hybrid AR systems consisting of interference coatings combined with a low-n top layer, the various low refractive index layers have a refractive index of around 1.15. A porous SiO2 GLAD layer with a porosity of 68% corresponds to a refractive index of 1.1519 similar to our nanoporous SiO2.27 Both GLAD19 or reactive ion etching37 and the wet chemical etching process discussed here can easily reach a thickness of around 150 nm for such highly porous layers. Lower refractive indices are in general obtained in graded index layers, whereby the top region has a refractive index down to 1.02.37 In contrast, the method proposed here, is limited to a refractive index of approximately 1.13, but it has been shown that this relatively high refractive index is also sufficient for high performance AR hybrid systems. Previous more complex hybrid broadband wide-angle AR systems using GLAD MgF2 layers with a refractive index down to 1.16 and a total of 15 layers have achieved an AR performance with R < 0.5% spanning a spectral range from 400 nm to 1700 nm.38 The graded top MgF2 layer had a thickness of approximately 160 nm. Hence,


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even broader wavelength ranges are feasible using the nanoporous SiO2 prepared by ALD and wet chemical etching and are targeted in following projects. Optical Properties on Highly Curved Aspheric Lens. Finally, the ALD hybrid AR coating was applied to a strongly curved aspheric B270 lens. The AR performance is tested on an inclined surface by 20°, 40°, and 60° tilting, respectively. For each angle of inclination, the reflectance spectra were measured at four points evenly spaced around the lens and compared with the reflectance spectra at the center position of the lens, see Figure 6. This cross-curvature measurement revealed a low residual average reflectance of less than 0.3% on the center and on the inclined surfaces in a 400-1000 nm spectral range. The reflectance spectra along the samples’ surface are consistent with each another, confirming the conformal nature of the ALD deposition technique. One measured point is slightly deviating, with an average residual reflectance of 0.5%, which is attributed to a defect on the lens surface, e.g. scratch or dust particles. 5 center position tilt of 20° tilt of 40° tilt of 60°

4

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3 25 mm

Reflectance (%)

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60°

2 50 mm

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Wavelength (nm)

Figure 6. Reflectance (without sample backside) across an AR-coated lens surface measured under light at normal incidence (AOI=0°) for different tilt angles of the lens. Four points, each along each latitude circles for the lens’ tilt angle of 20°, 40° and 60° were measured.


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Differences in wavelength positions of the AR interference ripples on the B270 lens compared to the B270 glass (Figure 4 and 6) are due to the thickness non-uniformity (NU%) of values between ±1.5% (Al2O3, SiO2) and ±2.0% (TiO2) across the 200 mm diameter substrate table in the used research ALD tool. Further improvement of thickness uniformity is essential to be competitive with other technologies. Nevertheless, thickness non-uniformities below 1% on areas of around 200 mm and larger were reported for Al2O3, SiO2, and TiO2 using other ALD tools.44,59,60 As discussed previously, similarly low reflectance values were also obtained by other reported hybrid AR technologies on plane surfaces. Obviously, those technologies combining PVD with nanostructures may be applied to highly curved lenses as well, subject to the requirement of a complex PVD multilayer system that compensates the thickness loss at the inclined surfaces. The advantage of our approach, which utilizes ALD, is that the multilayer system can be less complex and thinner in thickness. The thinner the AR stack and the less numbers of layers are applied, the simpler its control will be. Furthermore, a thinner film is accompanied by lower residual film stress and generally lower depositing times. The used ALD research tool can hold several aspheric lenses with a height up to 30 mm on a 200 mm diameter area, that can be coated at the same time and then etched in parallel resulting in a higher throughput; however, it is restricted to small-series coatings on prototypes. Scalability of the process is being considered using large plasma ALD tools using a Planar Triple Spiral Antenna (PTSA) as large planar inductively coupled (IC) plasma source that enables excellent uniformity on large areas60 and batch ALD tools61,62. Note, the presented deposition route is not limited to plasma ALD processes, but may also be adapted by thermal H2O or O3-based ALD processes. Finally, the ALD technology is promising towards coatings of concave shaped components (e.g. domes, inner surfaces of tubes), which is currently under investigation.


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CONCLUSION In conclusion, a novel conformal broadband and wide-angle AR coating based on a combination of a multilayer ALD coating with a nanoporous SiO2 top layer prepared by ALD and subsequent wet chemical etching has been demonstrated. A high AR performance with an average reflectance of 0.5% was realized in the spectral range from 400-1100 nm at normal incidence (front plus rear side). In the visible spectral range of 420-680 nm, the average reflectance (including backside reflectance) for a double-sided coated glass has been only 1.8% at 60° angle of incidence. This corresponds to a single side reflectance below 1% even at steep light incidence. An average transmittance of 99.5% (400-950 nm) under air and vacuum conditions is achieved for a doublesided coated glass substrate as the hybrid AR system exhibits low optical losses of less than 0.6% even at 400 nm wavelength. Three weeks after preparation, the double-sided coated glass substrate only showed a minor reduction of the average transmittance to 99.3% in the 400-950 nm spectral range. The main advantage of this ALD coating concept is that the ALD hybrid AR system may use a less complex and thinner multilayer system compared to PVD hybrid AR systems in order to achieve very low residuals reflectance (< 0.2% per side) even on strongly inclined surfaces in a broad wavelength range. The complete AR system can be deposited in one process followed by one post deposition treatment. In addition, lenses and other parts with different geometry can be functionalized in the same coating run. This all ALD preparation route can be extended for AR applications in the UV spectral ranges and may be applied on different shaped objects such as half domes or ball lenses. The mechanical stability of nanoporous SiO2 thin films with high porosity as well as the low deposition rates of current ALD processes remain the main challenges towards large-scale applications. These challenges can be addressed by the development of batch coating tools and spatial ALD


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technologies. Furthermore, the preparation route can be applied also on polymer substrates in the case numerous components are required.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Error analysis of the hybrid AR design, Substrate holder for plane glass substrates, Transmittance and optical losses of a 10 month aged sample, Reflectance at oblique incidence of a 10 month aged sample (PDF)

AUTHOR INFORMATION Corresponding Author * [email protected]; Tel.: +49-3641-807-320 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This work is funded within the Deutsche Forschungsgemeinschaft (DFG) (Emmy Noether Grant SZ253/1-1), Fraunhofer Society (Attract Grant 066-601020) and BMWi ZF4309604SY8 projects. K. Pfeiffer is grateful to the Carl Zeiss Foundation for supporting her doctoral studies.


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ACKNOWLEDGMENT The authors thank David Kästner for technical support.

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FOR TABLE OF CONTENTS ONLY


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TOC Image Antireflection coatings are essential for both consumer and high performance optical components in order to increase the transmittance of the optics and to reduce ghost images or stray light. Atomic layer deposition can make high performance antireflection coatings even on steeply curved substrates 451x309mm (72 x 72 DPI)


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Wide-angle broadband antireflection coatings prepared by atomic

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