Advanced key technologies for magnetron sputtering and PECVD of inorganic and hybrid transparent coatings

Besides classical multilayer systems with alternating low and high refractive indices, reactive pulse magnetron sputtering processes offer various possibilities of depositing gradient films with continuously varying refractive index. Using nanoscale film growth control it is possible to achieve optical filter systems with a defined dependency of refractive index on film thickness, e.g. by sputtering a silicon target in a time variant mixture of oxygen and nitrogen. Also reactive co-sputtering of different target materials such as silicon and tantalum in oxygen is suitable as well. Rugate filters made from SiO_xN_y or Si_xTa_yO_z gradient refractive index profiles find their application in spectroscopy, laser optics and solar concentrator systems.Furthermore polymer substrates are increasingly relevant for the application of optical coatings due to their mechanical and economical advantages. Magnetron PECVD (magPECVD) using HMDSO as precursor allows to deposit carbon containing films with polymer-like properties. Results show the suitability of these coatings as hard coatings or matching layers. Multifunctional coatings with antireflective and scratch-resistant properties were deposited on polymer substrates using a combined magPECVD and sputter deposition process.     

Multilayer antireflective and protective coatings comprising amorphous diamond and amorphous hydrogenated germanium carbide for ZnS optical elements

To effectively protect and improve the transmittance of ZnS optical elements in the far infrared band, combined amorphous diamond (a-D) and amorphous hydrogenated germanium carbide (a-Ge_1−xC_x:H) films have been developed. The optical interference coatings were designed according to the layer optics theory. The a-D films, of which refractive index and film thickness were controlled by changing substrate bias and deposition time respectively, were deposited by filtered cathodic vacuum arc technology. The a-Ge_1−xC_x:H films were prepared by radio frequency sputtering technology. During this process their refractive index was modulated by changing the gas flow rate ratio and their film thickness was controlled by the flow rate ratio and deposition time. It has been shown that the combined films are superexcellent antireflective and protective coatings for ZnS optical elements.     

Directional enhancement of refractive index and tunable wettability of polymeric coatings due to preferential dispersion of colloidal TiO2 nanorods towards their surface

We demonstrate the fabrication of nanocomposite coatings, of organic-capped colloidal TiO₂ nanorods dispersed into a poly(methyl methacrylate) matrix, with rising value of refractive index from the bottom to the top layers, and UV-induced surface wettability alteration, in a reversible manner. This behaviour is attributable to preferential dispersion of the TiO₂ nanoparticles towards the superficial layers of the coatings. Above a critical TiO₂ loading, the nanorods at the surface form aggregates deteriorating the optical and the surface properties of the nanocomposites. The optimal conditions for nanocomposite films preparation in terms of optimized nanorods dispersion, optical clarity, and surface smoothness are determined.     

Reactive pulse magnetron sputtered SiOxNy coatings on polymers

Amorphous SiO₂, Si₃N₄ and SiO_xN_y single layers have been deposited on silicon, glass and glycol modified polyethylene terephthalate substrates by reactive pulse magnetron sputtering. Apart from the expected correlation between refractive index, coating density and nitrogen content in the reactive gas mixture further results have been found regarding mechanical stress and the humidity barrier property of these thin films. The lowest compressive stress was observed in the coatings deposited with nitrogen contents of around 30% to 50% in the reactive gas mixture. The humidity barrier effect of the thin films already begins to increase significantly at low nitrogen contents of below 20% in the reactive gas. Additional investigations regarding chemical composition, coating adhesion and environmental stability complement this work with the main focus on optimizing these materials for optical multilayer systems on polymer substrates.     

Development of interference filters based on multilayer porous silicon structures

Porous silicon (PS) has a great potential in optical applications due to its tuneable refractive index. In particular, multilayer structures consisting of alternating PS layers with different refractive indices can be used as interference filters for applications in the field of optoelectronics and sensors. In the present work, the optical properties of PS single layers and multilayer structures were studied. Since the refractive index of PS varies depending on the air content of the porous matrix, the PS structures were modelled as an homogeneous mixture of silicon and air, according to the effective medium theories (EMTs). By adjusting the refractive index and thickness of each individual layer, we can obtain a stack of PS layers with the desired optical properties, resulting in interference filters of predetermined bandwidth.     

Surface tension,coexistence curve,and vapor pressure of binary liquid-gas mixtures

The objective of this paper is to present measurements of the vapor pressure, capillary coefficient, and refractive index of four binary mixtures, CO₂-SF₆, R14-SF₆, SF₆-R13B1, and SF₆-R22, at liquid-vapor equilibrium at different average concentrations. The measuring temperature range covered the entire liquid-vapor region from the triple line up to the critical point. The capillary coefficient was determined by means of the capillary rise method; the refractive index, by measuring the angle of refraction of a light beam passing through a prism and the sample. In order to obtain the liquid-vapor densities of pure substances the Lorentz-Lorenz relation can be used. However, in applying this relation to calculate the liquid-vapor densities of a mixture, one may need the concentrations of both the liquid and the vapor phase, which are, for the most part, quite different from the average concentration of the mixture. Calculating the concentrations of both fluid phases with the aid of an equation of state and comparing with measurements, we could show that the molar refraction coefficient of the mixtures can be simply determined from the average concentration and the molar refraction coefficients of their pure components. The surface tension of the mixtures could then be calculated from the measured capillary coefficient and the refractive index with the aid of the Lorentz-Lorenz relation.Paper presented at the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.     

Optical characterization of hybrid antireflective coatings using spectrophotometric and ellipsometric measurements

A hybrid antireflective coating combining homogeneous layers and linear gradient refractive index layers has been deposited using different techniques. The samples were analyzed optically based on spectrophotometric and spectroscopic ellipsometry measurements under different angles of incidence in order to precisely characterize the coatings. The Lorentz-Lorenz model has been used to calculate the refractive index of material mixtures in gradient and constant index layers of the coating. The obtained refractive index profiles have been compared with the targeted ones to detect errors in processes of deposition.     

Optical characterization of thin chalcogenide films by multiple-angle-of-incidence ellipsometry

Optical properties of thin chalcogenide films from the systems As-S(Se) and As-S-Se were investigated as a function of the film composition, film thickness and conditions of illumination by light using multiple-angle-of-incidence ellipsometry. Thin films were deposited by thermal evaporation and exposed to white light (halogen lamp) and to monochromatic light from Ar⁺ — (λ = 488, 514 nm) and He-Ne- (λ = 632.8 nm) lasers. The ellipsometric measurements were carried out at three different angles of light incidence in the interval 45-55°, at λ = 632.8 nm. An isotropic absorbing layer model was applied for calculation of the optical constants (refractive index, n and extinction coefficient, k) and film thickness, d. The homogeneity of the films was checked and verified by applying single-angle calculations at different angles. It was shown that the refractive index, n of As-S-Se films is independent of film thickness in the range of 50 to 1000 nm and its values varied from 2.45 to 3.05 for thin layers with composition As₂S₃ and As₂Se₃, respectively. The effect of increasing in the refractive index was observed after exposure to light which is related to the process of photodarkening in arsenic containing layers. The viability of the method for determining the optical constants of very thin chalcogenide films with a high accuracy was confirmed.     

Sorption and optical properties of sol-gel thin films measured by X-Ray Reflectometry and Ellipsometric Porosimetry

Oxide thin films synthesized using the sol-gel technique have the advantages of low cost, high thickness control, tunable refractive index and silicon technology compatibility, properties that make them potential materials for optoelectronic applications. For very thin films with low porosity, the determination of sorption and optical properties is quite complex because of the small sample size. Thus, there is a need to use especially designed techniques to obtain reliable results. In this work, a comprehensive study on the porosity evolution of SiO₂ and TiO₂ thin films using X-Ray Reflectometry and Environmental Ellipsometric Porosimetry is presented. For sol-gel SiO₂ thin films, it was found that the effective refractive index increases with thermal treatment as the porosity decreases. However, the refractive index of the walls was found constant. For sol-gel TiO₂ films, crystallized in anatase phase, both the effective refractive index and the wall refractive index increase with thermal treatment. SiO₂ and TiO₂ thermal oxides were also characterized for comparison.     

Antireflecting coating from Ta<Subscript>2</Subscript>O<Subscript>5</Subscript> and SiO<Subscript>2</Subscript> multilayer films

Sol-gel method is important for depositing antireflective coating that allows control over thickness as well as the index of refraction. Antireflective coatings which are produced from Ta₂O₅ and SiO₂ multi-layer thin films using sol-gel spin coating method are presented. The refractive index and the thickness are controlled by the composition and the concentration of the solution respectively. The thickness, refractive index and extinction coefficient of the films were calculated through transmission and reflection measurement by an NKD analyser. Mechanical properties of the films were checked by the cross tape test and dry sun test at 760 W/m². The result shows that the sample heat treated at 450^∘C for 15 min approaches a reflectance with less than 0.5% at around 840 nm.     

The theory of double-layer antireflection coatings

The theory of double-layer antireflection coatings using non-quarter-wave films is reviewed and extended. Normal incidence only is considered.The spectral reflectance of a double-layer coating depends on the refractive indices of the layers and the adjacent media, and on Ø₁ and Ø₂, the phase thickness of the layers. It is known that the reflectance is theoretically zero if the refractive indices satisfy certain inequalities and if the phase thicknesses are those calculated from certain equations.To obtain a fuller appreciation of double-layer coatings, the reflectance throughout the whole (Ø₁, Ø₂) plane is studied analytically. The pattern formed by the lines of constant reflectance exhibits translational symmetry, and the unit pattern which generates the whole pattern has a twofold axis of symmetry. The shape of the unit pattern depends on the refractive indices of the four media. Numerical examples are given to illustrate the similarities and differences between the unit patterns of some double-layer systems.     

Chemomechanical effects in optical coating systems

The major in-service failure mechanisms of modern optical coatings for architectural glass can be mechanical (e.g. scratch damage). Many of these coatings are multilayer structures of less than 100 nm thickness and different coating architectures are possible (i.e. different layer materials, thickness and stacking order). These coatings are exposed to different types of climatic conditions. In such circumstances it has been shown that chemomechanical effects can lead to changes in the hardness as well as the fracture resistance of bulk oxides. High performance glass is coated with anti-reflection coatings (e.g. ZnO, SnO₂) and barrier layers (e.g. TiO_xN_y) which are also expected to suffer from such chemomechanical effects. In this study we have demonstrated the chemomechanical behaviour of a range of optical coatings exposed to water. Water exposure tends to reduce the hardness and increase the fracture resistance of the coating making it more vulnerable to plastic deformation during scratching. The susceptibility of different coatings to chemomechanical effects is discussed.     

Ellipsometric study of thermally induced redistribution and crystallization of Ge in Ge:SiO2 mixture layers

Mixture layers of Ge:SiO₂ of 40:60 mol% respectively, have been prepared by co-sputtering. The thermally induced change of optical properties of the layers was studied by variable angle spectroscopic ellipsometry. The mixture was modelled as an unknown material with optical constants described by multiple oscillators. The optical parameters determined from ellipsometric measurements can be well correlated with structural changes in the mixture. The results indicate that Ge in the mixture deposited or annealed up to 600 °C is in an amorphous state and it redistributes with increase of temperature, changing refractive index through the layer. The crystallization starts between 600 and 650 °C, at first next to the substrate. Crystallites size grows with temperature. Results were compared with findings of grazing incidence wide angle X-ray scattering measurements and a good agreement was found. Ellipsometry has been shown to be an appropriate non-invasive technique for characterization of this kind of layers.     

Design of multi-layer anti-reflection coating for terrestrial solar panel glass

To date, there is no ideal anti-reflection (AR) coating available on solar glass which can effectively transmit the incident light within the visible wavelength range. However, there is a need to develop multifunctional coating with superior anti-reflection properties and self-cleaning ability meant to be used for solar glass panels. In spite of self-cleaning ability of materials like TiO₂ and ZnO, these coatings on glass substrate have tendency to reduce light transmission due to their high refractive indices than glass. Thus, to infuse the anti-reflective property, a low refractive index, SiO₂ layer needs to be used in conjunction with TiO₂ and ZnO layers. In such case, the optimization of individual layer thickness is crucial to achieve maximum transmittance of the visible light. In the present study, we propose an omni-directional anti-reflection coating design for the visible spectral wavelength range of 400–700 nm, where the maximum intensity of light is converted into electrical energy. Herein, we employ the quarter wavelength criteria using SiO₂, TiO₂ and ZnO to design the coating composed of single, double and triple layers. The thickness of individual layers was optimized for maximum light transmittance using essential Mcleod simulation software to produce destructive interference between reflected waves and constructive interference between transmitted waves.     

Variable refractive index optical coatings

The discrete and restricted values of refractive index of the bulk optical materials at present available are a serious limitation on the usefulness of these materials for optical coatings. This limitation can be overcome by utilizing the atom-by-atom condensation feature of the growth of vapour-deposited thin films, which allows the homogeneous mixing of different materials irrespective of their solubility restrictions. We have used this feature of co-deposition of different materials to form mixed optical materials of variable refractive index, the variation being determined by the composition of the source material. Measurements of the optical constants of these films, prepared by co-evaporation of mixtures of ZnS and MgF₂ of various compositions, have been made. The refractive index of the mixed films is found to be in good agreement with the values predicted on the basis of the Lorentz-Lorentz theory. In addition, the optical equivalence of alternate layers of ZnS and MgF₂ with step thicknesses ranging from 5 to 250 Å has been studied. For step thicknesses less than 100 Å, the optical properties of the composite films are equivalent to those for the homogeneously mixed films. For larger step thicknesses, considerable and complicated interference effects are observed. Thus, variable refractive index composite films can be prepared by (a) co-deposition and (b) deposition of alternate discrete layers of different materials as long as the step thickness does not exceed about 100 Å. Furthermore, these techniques of obtaining optical films of mixed materials can be extended to any combination of materials and therefore they open up a new field in materials technology.     

Controlled synthesis and characteristics of antireflection coatings of TiO2 produced from a organometallic colloid

Antireflection titanium dioxide (TiO₂) coatings have been developed on monocrystalline silicon by a sol–gel spin-coating process using titanium di-isopropoxidebis(acetylacetonate) colloidal precursor solution. The effect of titanium content in the precursor, spin rate, sintering duration and temperature have been studied and their effect on coating thickness and optical properties (i.e., refractive index and reflectivity) were investigated. The influence of post-deposition sintering temperature on the optical characteristics, composition and the microstructure of the coatings have been evaluated by UV–vis spectroscopy, ellipsometry, X-ray photoelectron spectroscopy, atomic force microscopy and X-ray diffraction techniques. Solar cells made on silicon wafers with TiO₂ as antireflection layer showed enhancement of more than 20% in short circuit current density in comparison to a cell devoid of the TiO₂ coating.     

A note on the use of ellipsometry for studying the kinetics of formation of self-assembled monolayers

Ellipsometry is currently one of the most important techniques for characterization of the deposition and growth mode of ultra thin organic films. However, it is well known that for thicknesses normally encountered in organic monolayer films, as would occur for example in self-assembled monolayers, ellipsometry cannot be used to simultaneously determine the thickness and refractive index of the monolayer film. Current practice is to assume a reasonable value for the film refractive index and calculate an effective ‘ellipsometric thickness’. This communication seeks to show that the alternative approach of assuming a thickness for the monolayer (determined by the length of the molecule) and calculating the effective film refractive index lends itself to easier and more meaningful physical interpretation. The Lorentz-Lorenz formula is then used to transform the effective refractive index into a surface coverage and hence to an effective mass coverage. The methodology advanced is applied to the kinetics of formation of a self-assembled monolayer of a well-studied molecule, octadecanethiol on Au.     

Optical properties of amorphous carbons and their applications and perspectives in photonics

Amorphous carbon exhibits a wide variety of optical properties and, thus, offers substantial opportunities for various applications in photonics. The main optical properties, which should be taken into account for the design of new photonic devices, are the refractive index n, the fundamental gap E_g and the E₀₄ gap. In this work, the optical properties of the various forms of amorphous carbon films grown by plasma-enhanced chemical vapor deposition, pulsed laser deposition, sputtering and vacuum cathodic arc deposition and the crucial structural and chemical factors that determine n, E_g, and E₀₄ are reviewed. The knowledge of the optical properties of such films is exploited in order to design and implement various photonic devices such as: 1) anti-reflection (AR) coatings for various uses including photovoltaic modules, 2) interferometric sensors and indicators based on carbon-based AR layers, and 3) laser patterning of amorphous carbons and study of its photosensitivity for holographic applications.     

Formation of silicon dioxide layers at low temperatures (150-400 °C) by atmospheric pressure plasma oxidation of silicon

The formation of silicon dioxide (SiO₂) layers at low temperatures (150-400 °C) by atmospheric pressure plasma oxidation of Si(0 0 1) wafers have been studied using a gas mixture containing He and O₂. A 150 MHz very high frequency (VHF) power supply was used to generate high-density atomic oxygen in the atmospheric pressure plasma. Oxidation rate, structure, and thickness and refractive index profiles of the oxidized layers were investigated by ellipsometry and infrared absorption spectroscopy. Atomic force microscopy was also employed to observe atomic-scale morphologies of the layer surface and wafer Si surface, after chemical removal of the oxidized layers. It was found that stoichiometric SiO₂ layers were obtained at higher oxidation rates than conventional dry O₂ thermal oxidation and radical oxidation processes, even at a very low substrate temperature of 150 °C. Although thickness variations were observed in the plasma region, the refractive index was independent of both substrate temperature and VHF power. In addition, the SiO₂ surface and SiO₂/Si interface roughnesses were comparable to those obtained in conventional dry oxidation at high temperatures.     

Dense outer layers formed by plasma treatments of silica coatings produced by the sol–gel method

Silica gel coatings prepared by the sol-gel method were subjected to low-temperature plasma treatments in air and argon. This was found to give rise to the formation of a dense outer layer, whose thickness increased with the duration of the treatment and decreased when the pressure in the plasma chamber was reduced. The formation of a dense layer in the coatings was confirmed from measurements of the overall thickness and the refractive index by ellipsometry, and also from TEM examination and light transmission experiments. The coatings were found to contain an uppermost layer of alumina, which was formed through secondary sputtering of aluminium from the electrodes in the plasma chamber. The concentration of alumina in the mixture with silica decreased through the thickness of the coating and became zero at a distance slightly smaller than the overall thickness of the dense layer. The thickness of the dense layer was found to be slightly higher with argon than with air plasma treatments. Whereas the aluminium concentration through the thickness of the coating was the same for the two types of treatment, a carbon residue was found only in the case of argon plasma treatments. The composition of the underlying silica layers was found to correspond to SiO_1.6. This revised version was published online in November 2006 with corrections to the Cover Date.