Antireflective coatings are essential to improve transmittance of optical elements. Most research and development of AR coatings has been reported on a wide variety of plane optical surfaces; however, antireflection is also necessary on nonplanar optical surfaces. Physical vapor deposition (PVD), a common method for optical coatings, often results in thickness gradients on strongly curved surfaces, leading to a failure of the desired optical function. In this work, optical thin films of tantalum pentoxide, aluminum oxide and silicon dioxide were prepared by atomic layer deposition (ALD), which is based on self-limiting surface reactions. The results demonstrate that ALD optical layers can be deposited on both vertical and horizontal substrate surfaces with uniform thicknesses and the same optical properties. A Ta2O5/Al2O3/ SiO2 multilayer AR coating (400-700 nm) was successfully applied to a curved aspheric glass lens with a diameter of 50 mm and a center thickness of 25 mm.
A basic requirement for many optical applications is the reduction of Fresnel-reflections. Besides of interference coatings, nanostructures with sub-wavelength size as known from the eye of the night-flying moth can provide antireflective (AR) properties. The basic principle is to mix a material with air on a sub-wavelength scale to decrease the effective refractive index. To realize AR nanostructures on polymers, the self-organized formation of stochastically arranged antireflective structures using a low-pressure plasma etching process was studied. An advanced procedure involves the use of additional deposition of a thin oxide layer prior etching. A broad range of different structure morphologies exhibiting antireflective properties can be generated on almost all types of polymeric materials. For applications on glass, organic films are used as a transfer medium. Organic layers as thin film materials were evaluated to identify compounds suitable for forming nanostructures by plasma etching. The vapor deposition and etching of organic layers on glass offers a new possibility to achieve antireflective properties in a broad spectral range and for a wide range of light incidence.
This study focuses on the atomic layer deposition (ALD) of high quality SiO2 thin films for optical application. One of the challenges for the application of dielectric ALD layers in optical coatings is the realization of low absorption and scattering losses. Furthermore the layers have to be prepared with a precise controlled thickness and repeatable optical properties. SiO2 films were deposited using tris[dimethylamino]silane (3DMAS) and oxygen plasma on Si(100)substrates, quartz and BK7 glass substrates at temperatures between 100 °C and 300 °C. Film growth rate and refractive indices of SiO2 thin films were studied as function of deposition temperature. A linear growth behavior of SiO2 ALD films is confirmed, allowing a scalability of film thickness just by counting ALD cycles. The grown films are resistant to abrasion and possess good adhesion to glass substrates. The optical losses of the films are negligible in the investigated spectral range from 250 nm to 1100 nm. An antireflective (AR) coating was prepared by atomic layer deposition using SiO2 as low refractive index material and HfO2 as high refractive index material.
Antireflective (AR) coatings for lenses with extreme curvature must be designed for use in the visible and the near-infrared ranges to ensure sufficient AR performance in the visible range on the inclined surfaces of such lenses. In this work, multiple inorganic and nanostructured organic layers are considered for use in AR coatings with alternating high-index and low-index layers and in arrangements with effective indices that decrease from the substrate side to the surrounding medium. An AR coating effective in the spectral range from 400 nm to 1400 nm was realized by combining a multilayer interference system with a plasma-etched organic layer. Step-down index systems or gradient layers with sufficient thickness are especially promising for lenses.
A small molecule is a low molecular weight organic compound which is by definition not a polymer. Therefore, physical
vapor deposition by evaporation as common for inorganic oxides is often possible. Organic layers can be useful as
components of interference stacks for different functions. A number of organic compounds have interesting UV
absorption characteristics and can be used to protect UV-sensitive polymers such as polycarbonate. In addition, organic
layers can be applied to generate nanostructured thin films with a very low effective refractive index, as shown recently
for polymers. A structured organic single layer can be applied as an antireflective (AR) coating for a glass lens. The
applicability of several small molecule compounds will be discussed in this paper.
The theory of "classical" optical interference coatings is based on assumptions like ideal homogeneity and
isotropy of the materials, as well as absolutely smooth and infinitesimally thin interfaces between the
individual coating materials. Within the framework of these assumptions, there exists an elaborated theoretical
apparatus for solving design and characterization tasks for optical coatings. At the same time, coating
deposition techniques have been perfected in order to match with the requirements of homogeneity and
smoothness of these coatings in practice.
Remaining discrepancies between the theoretically predicted and practically achieved coating performance can
- at least partially - be attributed to the violation of the above-mentioned ideal assumptions. But a closer look
on this matter reveals a more differentiated picture: Nanostructure effects can be tackled as additional degrees
of freedom for coating design, and can lead to useful property combinations that are inaccessible to "classical"
coatings prepared on the basis of the traditionally available coating materials.
This presentation deals with practical examples, where explicit violations of the usually assumed perfect
homogeneity and smoothness of the coatings have resulted in novel and innovative coating material properties
or coating designs. Examples include:
- Effects of noble metal islands embedded in semiconductor films: applications in photovoltaics
- Antireflection effects of nanostructured surfaces: motheye-structures
- Effects of nanoporosity in oxide films on refractive index, thermal shift and mechanical stress:
balanced coating properties
The examples demonstrate the possible benefits of the exploitation of nanostructure-caused effects in
interference coating science and technology.
Due to their electrical conductivity and transparency in the visible spectral range, transparent conductive oxides
(TCOs) are suitable as transparent front electrodes for multiple cell concepts. One promising device is a
semiconductor-insulator-semiconductor (SIS) solar cell, in which the TCO induces the pn juntion and acts as a
transparent electrode at the same time. Due to its work function, the thin TCO layer leads to the inversion of the
subsurface region. The high refractive index of transparent conductive oxides enables antireflection coating in a
limited spectral range. One approach to achieve broadband antireflection properties with effective light coupling
into the absorber is a nanostructured silicon interface. For large area modifications of silicon, the inductive
coupled plasma (ICP) etching technology is a possible technique.
The combination of the nanostructured surface and the SIS system leads to a novel solar cell concept with
promising properties and a simple production process. In our study, we used pulsed dc magnetron sputtering
for the deposition of thin ITO films on p-doped unstructured and ICP-structured silicon substrates. Optical
and structural properties have been analyzed. Furthermore, the solar cell performance of the first devices under
AM1.5G illumination will be shown and discussed.
Highly transparent thin films of indium tin oxide are important for different kinds of optical and electrical
applications. So far, deposition of these materials has been limited to high temperature processes. This study
describes a plasma ion-assisted evaporation process with substrate temperatures below 100°C and correlates the
structural and electrical properties of the coatings with the process parameters. The influence of gas-mixture,
mean ion energy and temperature has been investigated by
four-point-measurement, atomic force microscopy,
scanning electron microscopy and x-ray spectroscopy. The coatings exhibit mean extinction coefficients of 7•10-3 in the VIS range and specific resistivities in the range of 4.0 μΩm.
A new technology based on plasma etching has been developed to produce antireflective surface structures. By choosing thin initial layers and variable plasma conditions, a broad range of nanostructures can be obtained on various polymers. A broadband antireflective effect can be achieved that is less sensitive to the incident angle of light compared to multilayer interference coatings. Thin layers of silica help in mechanical protection, especially if the structured surface is nearly enclosed by the protection layer. In addition, surfaces that show both antireflective properties and an antifogging effect have been prepared. Combinations of SiO2 and fluorine-containing layers were found to be useful in obtaining
super-hydrophobic behavior. This advanced plasma etching is not limited to a special plasma source and the suitability of different plasma sources is discussed.
Stochastic, self-organized nanostructures are produced by a low-pressure plasma treatment on the polymer polymethylmetacrylate (PMMA). The phenomena obtained by plasma treatment (structure formation and antireflective effect) are investigated regarding surface modifications, structure growth, and chemical modifications. Optically, the structure acts like a gradient layer with decreasing effective refractive index towards air, which is suitable for antireflection of PMMA.
High transparent thermoplastics have the capability to put glass out of business, especially in everyday life's optics. Their diverse nature gives rise to different antireflection principles. The reduction of surface reflection losses in polymethylmethacrylate (PMMA) is demonstrated by means of argon/oxygen plasma treatment. Since the presented reduction of reflection occurs in a wide spectral range, the technique may be applied for omnidirectional devices or curved substrates. The etching process creates a self-organized stochastic subwavelength structure at the substrate itself. The decrease in reflection is described by effective medium theory (EMT), converting the surface topology into a depth-dependent filling factor profile. In a second step this nano-scaled structure is used as the initial point for a broadband absorber by coating it with a nontransparent metal layer. A high-efficient absorber can be obtained, if the metal acts as backside coating of the double-sided plasma-treated substrate and steady-going transitions between the materials eliminating the Fresnel reflections. In practice, the magnitude of absorption depends on depth of structure as well as on the complex refractive index of the metal.
Evaporation and reactive pulse magnetron sputtering are two methods to deposit broadband antireflection coatings at low temperatures. The performance of broadband antireflective coatings on polycarbonate (PC) deposited by these methods is shown. Plasma-treatment of the plastic substrates has an important effect on the adhesion of the coating system on the substrate. For sputtering different parameter sets for the pre-treatment as well as the deposition pressure and their effect on adhesion were examined for polycarbonate substrates. Furthermore evaporation and reactive pulse magnetron sputtering were compared with regard to the adhesion of broadband antireflective coatings on PC.
Improvements of the environmental stability of inorganic optical coatings on polymer substrates are expected from a slight variation of the chemical composition of SiO2 films by organic modification through the addition of gaseous hexamethyldisiloxane (HMDSO) to a classical plasma ion-assisted deposition process. The influence of several process parameters on the chemical composition of these coatings has been studied on polycarbonate, quartz glass and silicon substrates to define an optimum range of process parameters with regard to the useful properties obtained of the coated polymer optical component.
Antireflection (AR) properties are required for optical surfaces to avoid disturbing reflections as well as to improve the transmission of optical systems. The common method to reduce the reflection on optics is vacuum deposition of interference coatings. However, special efforts are required for each type of plastic to develop polymer-capable vacuum coating processes due to the manifold chemical and physical properties of optical polymers. An alternative procedure for the antireflection of polymers is the generation of surface structures that decrease the index of refraction in a surface region. In this paper, the suitability of the miscellaneous thermoplastic polymers for plasma-ion assisted deposition processes is evaluated. This comprises the study of damage effects caused by the contact with plasma and high-energy radiation as well as the development of special coating designs and of suitable process conditions. Coating properties achieved are discussed for PMMA and poly-cycloolefines. The same ion source arrangement as used for coating has been applied for etching an antireflective sub-wavelength surface structure into PMMA. In summary, the paper shows the practical application fields for both technologies.
The performance of the high temperature resistant polymers Pleximid, APEC and Ultrason as substrate materials in plasma-assisted physical vapor deposition processes was studied and compared with well-known thermoplastics for optical applications. Different effects of UV irradiation and plasma exposure on the polymers' optical features, surface energy and adhesion properties for oxide layers, typically used for interference multilayer coatings, are shown.
A theoretical approach to design multicycle AR coatings with predetermined residual reflectance, bandwidth, and cycle number is presented. The approach uses a novel step-index concept involving symmetric or asymmetric quarter-wave layer sequences to substitute refractive indices which are less than a given low refractive index. These substitutions result in a chosen step-down index profile matching the refractive index of the substrate to air. Each step of the index profile is the origin of a cycle of the synthesized AR design. Multicycle AR designs for the visible region with bandwidths of 1.6, 2.0, and 3.0 are presented as examples.
Coating of plastics for optical applications is mostly intended to improve their mechanically durability and to provide a desired optical function. A potential layer system to realize an antireflection function and to improve the mechanical properties simultaneously in a combination of hard coating SiO2 with classical 4-layer antireflection system consisting of SiO2 and Ta2O5. Plasma-Ion Assisted Deposition (Plasma-IAD) at room temperature has been applied for coating experiments on PMMA and Polycarbonate. The temperature on a polymer substrate can reach a critical value in respect to its low thermal stability, if thick layers Ta2O5 have to be deposited. The increase of temperature is mainly determined by the radiation heat of electron beam evaporator. With needle optimization, an alternative coating design has been developed. The design is characterized by low thickness of high refractive index layers, which add up to less than about 5% of the total multilayer thickness. The high refractive index layers are almost evenly distributed over the whole stack. The total thickness of this kind of coating is adjustable between 800 nm and about 2600 nm. The mechanical properties of the coating deposited on plastics corresponds to that of a single SiO2 laser of the same thickness.
Fresnel lenses for projection optics require a reduction of reflection losses in a wide wavelength and angular range. Their surface relief consisting of inclined microstructures with increasing inclination angle causes a reduction in AR- coating thickness form center to the edges and hence a spectral shift.
Most Fresnel lenses for projection systems are made from Polymethylmethacrylate (PMMA). Such lenses, e.g. for LCD- projectors, are flat discs whereas one surface is plane and the other microstructured. Special coatings are necessary to reduce stray light and 'hot spots' in the projected image, finally. Plasma-ion Assisted Deposition has been applied to realize dielectric multilayers on PMMA. Additionally, a low pressure plasma pre-treatment has been carried out as the first step of the coating process which inducing an irreversible modification of surface composition that leads to excellent adhesion conditions for coating. The special problem for coating the microstructured surface is the inclination angle of the small Fresnel facets which increases from the center to the outer area of microstructured lens. Consequently, the vapor incidence angle changes in the same way. Another problem is to keep the temperature at the polymer surface low during coating process. Thin microstructured polymer lenses show lower dimensional stability compared to flat or compact samples. Several coating designs have been invested to get a uniform moderate antireflective effect and in result an optimized light impression for the observer.
Properties of single-layer and multilayer Al2O3/SiO2 coatings deposited by Plasma Ion Assisted Deposition (PIAD) and Low Loss Reactive Evaporation (LL-RE) have been studied with emphasis on their use in the UV and VUV spectral region. The influence of significant deposition parameters, mainly the bias voltage in the case of PIAD and the substrate temperature in the case of LL-RE, on the optical and structural properties as well as on the film stress is investigated by spectrophotometry, IR- spectroscopy, light scattering, atomic force microscopy, and laser beam deflection stress measurements. Laser photon interaction with single-layer films and multilayer coatings was studied for the different wavelengths of excimer lasers (ArF (193 nm), KrF (248 nm), XeCl (308 nm) and the 3rd harmonic (355 nm) of the Nd:YAG solid state laser. High laser damage resistance and environmentally stable optical characteristics have been accomplished for multilayer coatings, especially for KrF (248 nm) excimer laser. The influence of the surface roughness of the substrates on the surface topography and the related scatter losses of the coatings has been investigated by integrated light scattering and atomic force microscopy measurements.
A coherent heterodyne detector was built that provides high angular resolution and near-specular angle measurements of the bidirectional scattering distribution function (BSDF). It extends the range of covered spatial frequencies by more than one order of magnitude towards lower frequencies.
Plasma ion assisted deposition was used to deposit SiO2 coatings on polycarbonate and on silicon. To study the influence of ion bombardment during the deposition process the bias voltage as an equivalent for ion energy was varied. At a definite bias value surface microtopography is changed abruptly. Coatings deposited at higher bias voltages show a stable chemical composition and therefore constant optical properties can be expected. It was also found that the barrier effect of coatings against moisture uptake, measured for polycarbonate substrates, increases with bias voltage.
2 September 2008 | Glasgow, Scotland, United Kingdom
Advancements in Polymer Optics Design, Fabrication, and Materials
31 July 2005 | San Diego, California, United States
SC730: Optical Coating on Plastics
This course provides attendees with a basic knowledge of transparent polymer materials for optical applications. The course concentrates on material properties, vacuum coating processes, interactions between polymers and plasma, coating stress and testing procedures. Many useful examples for antireflection of polymer surfaces and for multifunctional coatings are included throughout.