Optical analysis of coatings by variable angle spectrophotometry

Variable angle spectrophotometry (VAS) involves measurement of transmittance and reflectance as a function of the angle of incidence, wavelength and polarisation. This provides a means for analysis that yields the thickness and optical constants of the individual layers in multi-layer coatings, as well as other parameters that can be related to optical material properties. In the past decade, VAS has evolved into an accurate tool for optical characterisation of coatings, with many advantages over ellipsometry.The paper describes the tools for measurement and analysis developed by the author for the UV/Vis/NIR and IR wavelength ranges. Examples are given for single- and multilayer stacks with materials like ZnO, Cr, ITO and SiO₂.     

Low temperature sol-gel metal oxide and fluoride layer stacks for optical applications

MgF₂ and TiO₂ single layers and layer stacks were produced by a spin-coating sol-gel process. The final temperature treatment was carried out at 100 °C. The layers were deposited onto silicon and fused silica substrates and were analysed by means of atomic force microscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, ellipsometry, and UV-vis transmission spectroscopy. MgF₂ and TiO₂ single layers have morphological and optical properties comparable with physical vapour deposited layers. By using spectroscopic mapping ellipsometry, a good inter- and intra-sample homogeneity was confirmed. Multiple deposition steps result in a linear increase of layer thickness. Various films were deposited with thicknesses between 25 nm and 350 nm.It was shown that the low temperature sol-gel process results in films of optical quality. Anti-reflective and high reflective layer stacks consisting of MgF₂ and TiO₂ were designed and can be produced now by a sol-gel process, whereas the MgF₂ layers in the layer stacks contains also traces of MgF_2-2xO_x.     

Hybrid organic-dielectric thin film stacks for nonlinear optical applications

We propose an optical switching device that utilizes the third-order nonlinear optical effect in an organic material embedded into a superprism photonic crystal structure. A convenient way for the realization of the photonic crystal is using a multilayer thin film stack made of dielectrics. Following this approach, we face the challenge of building hybrid stacks containing both inorganic dielectric as well as organic materials. We evaluate strategies for the design and fabrication of hybrid thin film stacks to avoid the development of thin film cracking during the fabrication of hybrid PS-Ta₂O₅ (polystyrene-tantala) and PMMA-Ta₂O₅ (poly(methyl methacrylate)-tantala) stacks on glass substrates. Crack-free hybrid PS-Ta₂O₅ as well as PS-SiO₂ (polystyrene-silicon dioxide) and PMMA-SiO₂ (poly(methyl methacrylate)-silicon dioxide) samples on glass substrates were fabricated.     

Cut-on interference filter for the 8–11.7 μm region

The design and manufacture of a cut-on optical filter for the 8–11.7 μm region, fully blocked to uv, is described. The filter is composed of PbTe/ZnS symmetrical stacks deposited on two sides of a germanium substrate. Binder layers of Y₂O₃ between the multilayers and the substrate markedly improved the coating's environmental durability. A total of 30 layers was deposited and the average transmittance was 95% in the desired region.     

玻璃基掩埋式光波导堆栈的制作与表征

在硅酸盐光学玻璃基片上制作了光波导堆栈, 这种光波导堆栈通过Ag⁺/Na⁺熔盐离子交换和电场辅助离子扩散技术顺次制作了两层掩埋式光波导. 对光波导堆栈的横截面显微结构进行了观察, 并对堆栈中两层波导的损耗特性进行了测试. 所获得的光波导堆栈中的上、下两层波导芯部分别位于玻璃表面以下14和35 μm处; 上层光波导芯部尺寸约为12 μm×7 μm; 下层光波导芯部尺寸约为9 μm×8 μm. 通光测试显示两层波导在1.55 μm工作波长下均为单模光波导, 且两者之间没有相互耦合. 损耗测试分析结果显示: 堆栈中两层光波导的传输损耗均约为0.12 dB/cm,与单模光纤之间的耦合损耗分别为0.78和0.73 dB. 分析表明, 这种光波导堆栈在玻璃基集成光芯片的高密度集成方面具有很好的应用前景.     

Puls Magnetron Sputtern von optischen Mehrlagenschichten auf Kunststofffolien

Plastic foils coated with thin layers or layer stacks gain more and more importance in different kinds of industrial applications. Sputtering is preferred in all cases in which an excellent uniformity and reproducibility is required. It is necessary to apply the Dual Magnetron Sputtering Technology (DMS) for an effective deposition of optical layer stacks on large areas. These stacks consist in most cases of electrically insulating single layers. The industrial like pilot sputter roll coater FOSA 600 at FEP site in Dresden was installed in order to meet the growing demand of research and development service in this field. The main present applications for sputtered layers on foils are presented. Additionally the potential of optical filter systems on plastic foils are discussed. These kind of products can be made in the FOSA 600 as small scale production.     

Method based on the use of transient assemblies for the synthesis of broad band semitransparent dielectric reflectors

A new method for the synthesis of broad band semitransparent dielectric reflectors with high reflectance based on the use of two materials was developed. Its main feature is that the starting design consists of a number of quarter-wave stacks with the same even number of layers and that the wavelengths for these stacks are quarter-waves are spaced uniformly on a logarithmic scale. After the first optimization the stacks are split into smaller identical fragments and a second optimization, in a different spectral region, takes place. The reflectors obtained in this way have very flat characteristics in a broad spectral region. The calculations were performed for the combination Sb₂O₃/MgF₂.     

Refractive index profile modelling of dielectric inhomogeneous coatings using effective medium theories

The coatings having refractive index changing with the thickness present interesting optical performance, improved mechanical properties and smaller light scattering in comparison with classical multilayer stacks. Lot of theoretical work and experimental advances have been done for designing and production of mixture layers with such particular performances. The effective refractive index of the mixture coatings can be calculated by the use of effective medium theories. The refractive index profile characterization of inhomogeneous films that are mixtures of SiO₂ and Nb₂O₅ is presented. The composition is linearly changed through the thickness of the layers. Ex-situ spectrophotometric measurements, i.e. reflectance and transmittance at different incidence angles, are used for the precise characterization of the refractive index profiles. Linear, Maxwell-Garnet, Bruggeman and Lorentz-Lorenz effective medium theories are applied and quality and differences of the results are studied and analyzed. It is shown that the Lorentz-Lorenz model is the most appropriate for the given mixture, suggesting components are well mixed and there are no separated phases.     

Tin oxide-black molybdenum photothermal solar energy converters

Efficient photothermal solar converters exhibit high solar absorptance and low thermal emittance. The spectral selectivity of both tin oxide (SnO₂) and black molybdenum (BlMo) has been demonstrated in the literature. Tin oxide is transparent in the visible yet becomes reflective in the infrared. Black molybdenum is absorptive in the visible while exhibiting only moderate infrared reflectance, if post-deposition annealing is avoided. Coating BlMo with SnO₂ increases absorptance due to antireflection in the visible and decreases emittance due to the higher infrared reflectance of SnO₂. Typical $\text{SnO}{}_2\text{BlMo}$ stacks exhibit an absorptance of 91 % (of the air mass 2 solar input) and a normal emittance of 13 % (of the 200 C blackbody curve). Both layers are produced under one atmosphere at temperatures as high as 500 C. Exposure of the stacks to air at 400 C for over 48 hours has resulted in no observable optical changes.     

Band Structure Engineering of Interfacial Semiconductors Based on Atomically Thin Lead Iodide Crystals

To explore new constituents in two‐dimensional (2D) materials and to combine their best in van der Waals heterostructures is in great demand as being a unique platform to discover new physical phenomena and to design novel functionalities in interface‐based devices. Herein, PbI₂ crystals as thin as a few layers are synthesized, particularly through a facile low‐temperature solution approach with crystals of large size, regular shape, different thicknesses, and high yields. As a prototypical demonstration of band engineering of PbI₂‐based interfacial semiconductors, PbI₂ crystals are assembled with several transition metal dichalcogenide monolayers. The photoluminescence of MoS₂ is enhanced in MoS₂/PbI₂ stacks, while a dramatic photoluminescence quenching of WS₂ and WSe₂ is revealed in WS₂/PbI₂ and WSe₂/PbI₂ stacks. This is attributed to the effective heterojunction formation between PbI₂ and these monolayers; type I band alignment in MoS₂/PbI₂ stacks, where fast‐transferred charge carriers accumulate in MoS₂ with high emission efficiency, results in photoluminescence enhancement, and type II in WS₂/PbI₂ and WSe₂/PbI₂ stacks, with separated electrons and holes suitable for light harvesting, results in photoluminescence quenching. The results demonstrate that MoS₂, WS₂, and WSe₂ monolayers with similar electronic structures show completely distinct light–matter interactions when interfacing with PbI₂, providing unprecedented capabilities to engineer the device performance of 2D heterostructures.     

In‐Situ‐Charakterisierung optischer Mehrschichtsysteme in Großflächen‐ Produktionsanlagen

The paper discusses approaches to the insitu analysis of optical multi‐layer coating stacks such as mirror coatings, AR coatings or lowE layer stacks on large‐area substrates in production environments. The stable production of complex layer systems requires in‐situ analysis systems that are able to provide spectral information and optical performance data, but also yield thickness information for individual layers and thus aide the operating staff in detailed analyzing deviations from the production target stack. Solutions incorporating in‐situ optical reflectance, transmittance and ellipsometry measurements wi^th optical data being collected not only for the completed layer stack, but also at intermediate coating stages, are discussed and the accuracy, robustness and stability of different measurement systems and computation strategies are compared.     

Mechanical integrity analysis of multilayer insulator coatings on flexible steel substrates

The cohesive and adhesive properties, and related critical radius of curvature of thin multilayer insulator coatings on a 152 μm thick flexible steel substrate were investigated using tensile experiments carried out in-situ in an optical microscope. This method was found to be well adapted for the two types of coatings studied: SiO₂ single layers with different thickness and SiO₂/SiN_x/polyimide stacks. Special attention was paid to the influence of surface roughness and yielding of the steel substrate. Coating delamination and spallation was observed at low strain in case of SiO₂ coatings on unpolished steel, resulting from roughness-induced stress concentrations and slippage of grain boundaries. Polishing the steel substrate, or using a polyimide interlayer, was found to be useful to avoid premature delamination of the layers. In all investigated cases, a critical radius of curvature for layer damage of approximately 5 mm was found.     

Processing of CdTe thin films by the stacked elemental layer method: compound formation and physical properties

Cadmium telluride (CdTe) thin films have been deposited using the stacked elemental layer (SEL) technique. This process consists of sequentially depositing tellurium and cadmium layers and then annealing the stacks in order to synthesize the compound. The films were characterized using X-ray diffraction (XRD), optical transmittance and reflectance, scanning electron microscopy (SEM) and atomic force microscopy (AFM). The evolution of the thin film reaction and compound formation were studied using X-ray data. The results show that the growth is diffusion-controlled and the activation energy is (82±2) kJ/mol. The effect of the conventional post-synthesis CdCl₂ heat treatment on the physical properties of the films produced is also reported.     

Effective medium theory applied to photonic crystals composed of cubic or square cylinders

Using the effective medium theory, I interpret the band-gap opening in photonic crystals with simple geometries as an interference effect between alternating layers of high and low optical indices and introduce the interesting concept of multidimensional quarter-wave stacks. The interpretation provides a simple insight into band-gap opening processes. For several simple crystal geometries, I analyze the variations of the gap width and depth with respect to the light polarization, the incident angle, and contrast inversion. For two- and three-dimensional structures composed of cubic and square cylinders, I show that the effective medium theory can be used to predict accurately the gap width, the central wavelength, and the attenuation at the central wavelength. The validity domain of the effective medium theory predictions is checked with results from rigorous computations.     

Fabrication process of intrinsic Josephson junction stacks in Bi2Sr2CaCu2O8+x crystals by double-sided patterning process using dilute hydrochloric acid

We have developed a new double-sided patterning (DSP) process that employs dilute acid (pH = 1.65) to fabricate Bi₂Sr₂CaCu₂O_8+x (Bi-2212) stacks of intrinsic Josephson junctions. These stacks, which were fabricated from a single crystal of Bi-2212, were surrounded by an acid-treated product. The critical aspect of this process is that the Bi-2212 surrounding the photoresist pattern was converted into a transparent material, BiOCl, which was connected with the Bi-2212 crystal. Consequently, this process provides a simple way to achieve DSP of the surfaces of Bi-2212 crystals. This new DSP process realizes a remarkably improved reproducibility in fabricating Bi-2212 stacks that exhibit good current–voltage characteristics with a large hysteresis and multiple branches at T = 77 K.     

Plasma-etched organic layers for antireflection purposes

Organic layers can be used to realize special functions in optical interference coatings. Suitable compounds for such layers were thermally evaporated and characterized. A plasma etching procedure was applied to produce nanostructures on top of the organic layers to reduce their effective refractive indices. Broadband antireflective coatings were obtained by combining these artificial low-index layers with conventionally prepared interference stacks.     

CZTS thin films on transparent conducting electrodes by electrochemical technique

We have fabricated single phase Cu₂ZnSnS₄ (CZTS) films using a specially designed 3-stage electrochemical system. Sequential electrodepositon of constituent metallic layers was carried out on SnO₂/F coated glass substrates using a platinum counter electrode and a saturated calomel reference electrode. Unique bath compositions were formulated for each of these constituents. Sequentially deposited tri-layer stacks were annealed in sulfur environment to get CZTS phase. Detailed structural, morphological and optical characterization experiments were performed using several techniques including x-ray diffraction, Raman and UV-visible spectroscopy, scanning electron microscopy and atomic force microscopy. All characterization experiments indicated that the films are single phase with a measured direct band gap of 1.5 eV.     

Thermal gain of multilayer stacks

A theory for the spectral emittance and thermal gain of multilayer stacks is presented. The theory only requires knowledge of the optical constants and thicknesses of different layers for computation of the spectral emittance and thermal gain. Numerical results show that the optical properties and thicknesses of various layers and the substrate of the stack play a vital role in determining the spectral emittance and thus the total thermal gain of the stack.     

In situ and ex situ stress measurements of YF3 single layer optical coatings deposited by electron beam evaporator

Yttrium fluoride (YF₃) is a material with good potential as a single-layer anti-reflection (AR) coating deposited onto glass substrates. YF₃ is a possible candidate to replace more studied fluorides such as MgF₂ or ThF₄ the latter being radioactive. For thin-film photovoltaic solar cells, depositing such layers could be a cost-effective way of improving the transmission of light to the p–n junction. However, long-term stability of these AR coatings is an important issue to be considered. This paper is concerned with the residual stress of YF₃ single layers deposited onto glass substrates, being a potential failure mechanism. The measured stress values are correlated to the structure of the layers, using ex situ characterization techniques, such as X-ray diffraction (XRD) and atomic force microscopy (AFM), in an attempt to explain the observed changes. Three substrate temperatures were investigated during deposition, namely 25, 115, and 210 °C. Intrinsic stress of up to 197 MPa has been observed in amorphous YF₃ single layers deposited by electron beam evaporation, at ambient temperature. At a substrate temperature of 210 °C, the intrinsic stress decreased to 67 MPa in the YF₃ layer, developing an orthorhombic structure. Adsorptive stress is an important issue encountered in YF₃ layers, directly related to a low packing density. The in situ stress measurements were carried out using a novel optical approach with a laser-fiber system, briefly introduced here. The residual stress values measured with this novel optical system were compared to two ex situ stress measurement techniques. The optical performance of the YF₃ single layers was also assessed using a spectrometer ex situ and in situ by the interferometry capability of the novel in situ monitoring device.     

Fabrication of KTa0.65Nb0.35O3 film by pulsed laser deposition on glass substrate with various buffer layers

KTa_0.65Nb_0.35O₃ (KTN) thin films were deposited on amorphous glass substrates using a range of single buffer layers such as indium tin oxide (ITO), zinc oxide (ZnO), 3 at% Al-doped ZnO (AZO), and 3 at% Ga-doped ZnO (GZO), as well as a variety of multi-buffer layers such as SrTiO₃ (STO)/ITO, STO/ZnO, STO/AZO, and STO/GZO using a pulsed laser deposition system. All films showed a polycrystalline perovskite phase with the exception of all single buffer layers and STO/ITO multi-buffer layers. The STO buffer layer is important for crystallizing KTN films due to the similar lattice constant and same crystal structure. The optical transmittance of all films exhibited a transmittance ?90% in the wavelength range.