Multilayer Thin Film Coatings Research Articles

Data-driven investigation of thickness variations in multilayer thin film coatings

Abstract Design and fabrication of multilayer thin film coatings for photonics applications require careful consideration of various parameters such as layer thickness, refractive indices and number of stacks. A growing trend uses machine learning for efficient navigation in the complex parameter space of photonics applications to efficiently extract valuable insights from the extensive datasets and to predict the optical performance. We developed an approach that combines Monte-Carlo and Finite-Difference Time-Domain simulations to model multilayer thin films. After conducting 95 200 runs, the data were analyzed using Neural Network fitting to explore how thickness variations influence the optical performance. An experiment validation on magnetron sputtered coated samples demonstrates the high accuracy of our method in predicting the optical performance of the thin film stacks (R 2 > 0.99), contributing to the understanding and enhancement of photonics stack properties for diverse optical applications using machine learning approaches.

COMPARATIVE CHARACTERISTICS OF COATINGS WITH SiO AND GeO ON LEUCOSAPPHIRE

The reasons for the sharp difference in the adhesion of multilayer coatings containing SiO or GeO together with Ge on a leucosapphire (Al2O3) plate have been established. It should be mentioned that Silicon(II) and Germanium(II) oxides are quite stable in the gaseous state and, contrary, are metastable in condensed state; at high temperature they disproportionate into ultra-dispersed composites of amorphous nature. A comparison is made of the surface properties of ultramicroscopic droplets formed on solid surfaces – a substrate or the previous layer – upon condensation of SiO, GeO, or Ge vapors on leucosapphire. A qualitative assessment of the ratio of the corresponding contact angles of wetting by the indicated melts, formed at the first moment of contact, has been carried out. In assessing the surface tension of SiO and GeO melts (or Si – SiO2 and Ge – GeO2 composites), we proceeded from the corresponding values for SiO2 and GeO2, which are 296 and 248 mJ/m2 near the crystallization temperatures. On this basis, it was established that the smallest value of the contact angle, and hence the best wetting, is observed for the GeO melt (somewhat less for the SiO melt) on the solid surface of Al2O3 or Ge; the solid surface of SiO or GeO (especially, the first of them) with molten germanium should be much weaker wetted. Hence, it follows that thin-film multilayer coatings obtained from Ge and GeO on a leucosapphire substrate should have a significantly higher climatic resistance due to higher adhesion compared to multilayer coatings from SiO and Ge. Indeed, a multilayer coating containing SiO on a leucosapphire substrate with a large surface can withstand storage in air for no more than 2–3 months and begins to peel off; at the same time, the GeO coating remains intact after 4 years of storage. Thus, the GeO film-forming material is a promising one for use in multilayer coatings such as cut-off filters in interference optics of the near and mid-IR spectral ranges.

Solving the inverse problem for determining the optical characteristics of materials

The paper describes a methodology for determining the optical and physical properties of anisotropic thin film materials. This approach allows in the future designing multilayer thin-film coatings with specified properties. An inverse problem of determining the permittivity tensor and the thickness of a thin film deposited on a glass substrate is formulated. Preliminary information on the belonging of a thin-film coating to a certain class can significantly reduce the computing time and increase the accuracy of determining the permittivity tensor over the entire investigated range of wavelengths and film thickness at the point of reflection and transmission measurement Depending on the goals, it is possible to formulate and, therefore, solve various inverse problems: o determination of the permittivity tensor and specification of the thickness of a thick (up to 1 cm) substrate, often isotropic; o determination of the permittivity tensor of a thin isotropic or anisotropic film deposited on a substrate with known optical properties. The complexity of solving each of the problems is very different and each problem requires its own specific set of measured input data. The ultimate results of solving the inverse problem are verified by comparing the calculated transmission and reflection with those measured for arbitrary angles of incidence and reflection.

Long-term performance and durability of Ir/B4C multilayer x-ray mirrors: focusing on composition, structure, and reflectivity properties

Multilayer (ML) thin film coatings have shown promise in achieving hard x-ray nanofocusing with high reflectivity and high resolution. The chemical, structural, and long-term stability of Ir/B4C MLs, which are of great interest to the synchrotron and astrophysics communities, are not yet fully understood. The evolution of the x-ray performance of Ir/B4C ML mirrors was monitored over 5 years, and the chemical and structural properties were investigated in depth. Reflectivity scans reveal significant alteration in the energy range of 3.4 to 10 keV over this period. Furthermore, thickness and density degradation of B4C layers were observed in scanning electron transmission microscopy results. The oxidation of B4C occurs only for the top layers, whereas the buried B4C layers go through various complex chemical modifications. The x-ray reflectivity model of Ir/B4C structure was modified, based on the experimental findings, and resulted in good understanding of the long-term reflectivity performance of the x-ray mirror coatings.

Evaluation of Multilayer Thin Film Coatings for Solar Thermal Applications

The multilayer thin film coatings are one of the proven technologies for improvement in solar thermal and optical applications. In current solar thermal and optical systems, multilayer thin film coatings find application in many functions such as photovoltaics, heat exchangers, filters, sensor technologies, laser windows, mirrors, reflectors and optics for digital projections. The solar absorptance and thermal emittance of the multilayer thin film coatings are one of the leading factors for its applicability in said areas. The processing conditions, morphology and surface finish influence the solar absorptance and thermal emittance behavior of multilayer thin film coating. Therefore, emphasis has given in this paper to deposit multilayer thin film coatings with an increased solar absorptance and decreased thermal emittance to improve its applicability in solar thermal applications. Multilayer thin film coatings (Al2O3/Ni/W–Al2O3/W) were deposited using DC/RF magnetron sputtering on the stainless steel substrate to improve its applicability in solar thermal receiver tube for power generation. The performance of this multilayer thin film was investigated by measuring the absorptance and emittance using 410 Solar and ET 100 in the solar spectrum region at a variable incident angle from 20° to 60°. The effect of optical properties, microstructure and morphology of the multilayer thin film coatings was also investigated. The maximum absorptance 0.92 and minimum emittance < 0.1 were observed in deposited multilayer thin film coating with the combination of Tungsten, Al2O3 and Nickel, respectively. The observed values indicate the practical applicability of the multilayer coatings in medium-to-high-temperature range of solar thermal receiver tubes.

Multilayer thin films of Zinc (II) Phthalocyanine loaded Poly(D, L-lactide-co-glycolide) nanocapsules by layer-by-layer self assembly

Systematic design and controlled distribution of drug loaded nanoparticles in molecular level are highly challenging by traditional techniques. In this study, encapsulation of photodynamically active hydrophobic molecule, zinc (II) phthalocyanine, by biocompatible and biodegradable copolymer poly (D, L- lactide-co-glycolide) is achieved by co-solvent evaporation method. The average size of the drug-loaded nanoparticles is obtained below 100 nm for the first time for thin film assembly with 86% yield which is stated as crucial for the efficiency of drug delivered. These nanospheres are further studied for multilayer thin film coatings by dip-spin layer-by-layer self-assembly. As a scalable and controlled method, LbL offered a uniform distribution of hydrophobic photosensitizers ZnPc loaded PLGA capsules below 10 nm precisely. Light absorbance-transmittance spectroscopy and quartz-crystal microbalance measurements are performed for thin film growth and adsorption kinetics, respectively. Besides, the thickness of layers, the stability of thin film structures and homogeneous distribution of nanoparticles on the surface are controlled and investigated by surface profiler, dynamic light scattering, and scanning electron microscopy measurements respectively. Prepared multilayer thin film coatings are promising candidates for tumors on the skin or just under the skin by photodynamic therapy applications.

Enhancing the spectral reflectance of refractory metals by multilayer optical thin-film coatings

Good thermomechanical properties of refractory metals, including tungsten, tantalum, molybdenum, and niobium, make them attractive candidates for operation in extreme environments. In addition, their ability to reflect thermal radiation in the infrared region beyond 1.5&#x00A0;&#x03BC;m makes them attractive for use as reflective surfaces in extreme environments. These refractory metals, however, have relatively poor reflectivity in the visible and near-infrared spectral regions, making them absorb incident thermal radiation. Average absorption percentages of these metals at the 300&#x2013;1500&#x00A0;nm spectrum is in the range of 40%&#x2013;50%. In this paper, we propose and demonstrate that by using periodic thin-film dielectric coatings deposited over refractory metals, the absorption of thermal radiation can be drastically reduced. Various thin-film optical filter designs are investigated to engineer and improve the spectral reflectivity of refractory metals in the visible and near-infrared spectral regions without deteriorating the performance beyond 1.5&#x00A0;&#x03BC;m. TiO2, Al2O3, and SiO2 are used as materials, which are dielectrics that do not absorb incident radiation in the visible and near-infrared with high melting points and Young&#x2019;s modulus. Our results indicate that a combination of several periodic segments, designed at different wavelengths around which high reflectivity is desired, can be utilized to generate high reflectivity in the broadband spectrum. Bandwidth and magnitude of the reflectivity over the spectrum are highly dependent on materials, number of segments, and number of layers in the segments of the filters.

Nanostructured Stealth Surfaces for Visible and Near-Infrared Light.

So far, all previous attempts to apply nanostructures for perfect transmission have not achieved maximum transmittance beyond 99.5% due to the limited regularity of the nanoscale surface geometry: too low for many high-end applications. Here we demonstrate a nanostructured stealth surface, with minimal reflectance (<0.02%) and maximal transmittance (>99.8%) for a wavelength range, covering visible and near-infrared. Compared to multilayer thin film coatings for near-infrared applications our antireflective surfaces operate within a much broader wavelength range, are mechanical stable to resist human touch or contamination, show a 44% higher laser-induced damage threshold, and are suitable for bended interfaces such as microlenses as well.

Features of the Photoalignment of Disodium Bis-[(4-Hydroxy-3-Carboxylate-6-Methyl)Phenylazo]-5,5′-Dioxobenzothiophene in Thin Films

UDC 539.23:547-304.4 E. A. Gracheva, and V. S. Mikulich Thin fi lms and multilayer thin-fi lm coatings based on the new dichroic azo dye disodium bis[(4-hydroxy-3carboxylate-6-methyl)phenylazo]-5,5�c -dioxobenzothiophene (FtF-1) were produced by the spin-coating method. The optical properties and morphology of the FtF-1 fi lms depended on the formation conditions and used solvents. Reversible photo-induced dichroism was observed in fi lms obtained by spin-coating of FtF-1 solutions in DMF and aqueous polyvinylpyrrolidone on quartz substrates upon irradiation with plane-polarized light (�� = 450 nm, I = 15 mW/cm 2 ). The anisotropic properties of the FtF-1 fi lms were caused by trans-cis-isomerization of the irradiated dye molecules.

Near-ultraviolet absorption annealing in hafnium oxide thin films subjected to continuous-wave laser radiation

Hafnium oxide (HfO2) is the most frequently used high-index material in multilayer thin-film coatings for high-power laser applications ranging from near-infrared to near-ultraviolet (UV). Absorption in this high-index material is also known to be responsible for nanosecond-pulse laser-damage initiation in multilayers. In this work, modification of the near-UV absorption of HfO2 monolayer films subjected to irradiation by continuous-wave (cw), 355-nm or 351-nm laser light focused to produce power densities of the order of ∼100 kW/cm2 is studied. Up to a 70% reduction in absorption is found in the areas subjected to irradiation. Temporal behavior of absorption is characterized by a rapid initial drop on the few-tens-of-seconds time scale, followed by a longer-term decline to a steady-state level. Absorption maps generated by photothermal heterodyne imaging confirm the permanent character of the observed effect. Nanosecond-pulse, 351-nm and 600-fs, 1053-nm laser-damage tests performed on these cw laser–irradiated areas confirm a reduction of absorption by measuring up to 25% higher damage thresholds. We discuss possible mechanisms responsible for near-UV absorption annealing and damage-threshold improvement resulting from irradiation by near-UV cw laser light.

Channel cracks in atomic-layer and molecular-layer deposited multilayer thin film coatings

Metal oxide thin film coatings produced by atomic layer deposition have been shown to be an effective permeation barrier. The primary failure mode of such coatings under tensile loads is the propagation of channel cracks that penetrate vertically into the coating films. Recently, multi-layer structures that combine the metal oxide material with relatively soft polymeric layers produced by molecular layer deposition have been proposed to create composite thin films with desired properties, including potentially enhanced resistance to fracture. In this paper, we study the effects of layer geometry and material properties on the critical strain for channel crack propagation in the multi-layer composite films. Using finite element simulations and a thin-film fracture mechanics formalism, we show that if the fracture energy of the polymeric layer is lower than that of the metal oxide layer, the channel crack tends to penetrate through the entire composite film, and dividing the metal oxide and polymeric materials into thinner layers leads to a smaller critical strain. However, if the fracture energy of the polymeric material is high so that cracks only run through the metal oxide layers, more layers can result in a larger critical strain. For intermediate fracture energy of the polymer material, we developed a design map that identifies the optimal structure for given fracture energies and thicknesses of the metal oxide and polymeric layers. These results can facilitate the design of mechanically robust permeation barriers, an important component for the development of flexible electronics.

Multilayered nanocrystalline CrN/TiAlN/MoS2 tribological thin film coatings: preparation and characterization

Nanocrystalline multilayer thin film coatings, composed of nanometer-scale thick CrN, TiAlN and MoS2 tri-layer systems, were prepared by reactive co-sputtering processes. The self-lubricated multilayer coating structures were deposited by one-fold oscillating movement of substrates in front of the sputter sources. Three independently operated direct current (dc) excited unbalanced magnetrons (UM) with rectangular cathodes of TiAl alloy (50/50%), pure chromium and MoS2 were used as sputter sources. The reactive sputtering process was performed in a mixture of Ar-N2 atmosphere. Hardened high-speed-steel (HSS) and thin oxide covered Si (100) wafers were used as substrates for tribological- and microstructure investigations, respectively. According to results of the chemical composition evaluated by Auger-electron spectroscopy (AES) and microstructure investigation by cross sectional transmission electron microscopy (XTEM), the CrN, TiAlN and the MoS2 phases form practically continuous layers with large gradient transition of composition. The as-deposited CrN/ (Al,Ti)N/MoS2 coatings have shown good friction behaviour, tested at room temperature in dry sliding condition with a ball-on-disk tribometer.

Multilayer thin film coatings capable of extended programmable drug release: application to human mesenchymal stem cell differentiation.

The promise of cellular therapy lies in healing damaged tissues and organs in vivo as well as generating tissue constructs in vitro for subsequent transplantation. Adult stem cells are ideally suited for cellular therapies due to their pulripotency and the ease with which they can be cultured on novel functionalized substrates. Creating environments to control and successively driving their differentiation toward a lineage of choice is one of the most important challenges of current cell-based engineering strategies. In recent years, a variety of biomedical platforms have been prepared for stem cell cultures, primarily to provide efficient delivery of growth or survival factors to cells and a conducive microenvironment for their growth. Here, we demonstrate that repeating tetralayer structures composed of biocompatible poly(methacrylic acid) (PMAA)/poly(acryl amide) (PAAm)/poly(methacrylic acid) (PMAA)/poly(ethylene oxide)-block-poly(ε-caprolactone) (PEO-b-PCL) micelles arrayed in layer-by-layer (LbL) films can serve as a payload region for dexamethasone (dex) delivery to human mesenchymal stem cells (MSCs). This architecture can induce MSC differentiation into osteoblasts in a dose-dependent manner. The amount of dex loaded in the films is controlled by varying the deposition conditions and the film thickness. Furthermore, release of dex is also controlled by changing the amount of covalent crosslinking of multilayers via thermal treatments. The multilayer architecture including payload and cell-adhesion region introduced here are well suited for extended cell culture thus affording the important and protective effect of both dex release and immobilization. These films may find applications in the local delivery of immobilized therapeutics for biomedical applications, as they can be deposited on a wide range of substrates with different shapes, sizes, and composition.

Use of Multilayer Thin-Film Coatings for Complete Restoration of Integument in Its Deep Damage (Experimental Study)

The effect of multilayer thin-film wound coatings upon regeneration of skin in its deep damage was studied 18 Chinchilla rabbits. Full-layer skin wounds 40 x 40 mm were inflicted on the back of the animals. In experimental group of animals the wounds were closed by multilayer coatings consisting of 50 aluminum metalized lavsan films. In control group one layer coating with thin aluminum plate was used. In all control animals the pyonecrotic wound processes developed within 1 — 2 weeks after intervention. In experimental rabbits the wound healing process showed no clinical signs of inflammation. In 6 weeks a full value regenerate in the zone of deep skin damage was formed. Its thickness and structure was typical of normal skin. Principle of multilayer coatings is perspective for the elaboration of coatings at treatment of deep and small wounds of various etiology when high quality of dermal regenerate is required.

Multilayer thin-film coatings based on chromium nitride and aluminum nitride: Comparative depth profiling by secondary ion mass spectrometry and glow-discharge optical emission spectrometry

Round-robin characterization is reported on the sputter depth profiling of CrN/AlN multilayer thin-film coatings on nickel alloy by secondary ion mass spectrometry (SIMS) and glow-discharge optical emission spectrometry (GD-OES). It is demonstrated that a CAMECA SIMS 4550 Depth Profiler operated with 3 keV O2+ primary ions provides the best depth resolution and sensitivity. The key factor is sample rotation, which suppresses the negative influence of the surface topography (initial and ion-induced) on the depth profile characteristics.

Optical coatings with an integral FTIR air layer

The use of solid frustrated total internal reflection (FTIR) layers to control the polarization effects in optical coatings usually results in large substrate sizes and complicated designs. To overcome this problem, it is proposed to incorporate an air FTIR layer into the multilayer thin film coatings. The low refractive index of air not only helps to reduce the substrate sizes, but also simplifies coating designs or improves the performance. The principle and layer structures of the proposed multilayers are described. Examples of polarizing- and non-polarizing beam-splitters for the infrared spectral region are given. Some practical manufacturing issues are also discussed.

Focus on materials

Focus on materials

Performance evaluation of multilayer thin film coatings under mixed rolling–sliding dry contact conditions

This paper focuses on the application of advanced finite element analysis to simulate the response of complex multilayer coatings when subjected to mixed rolling–sliding dry contact conditions, typical of those experienced by heavily loaded gear tooth flanks. In this study, a versatile model is developed to investigate the response of a variety of complex coatings. Through the investigation of three advanced surface coatings the developed model is shown to offer detailed subsurface stress and strain information, quantifying the benefits provided by the application of each coating. A number of rolling contact fatigue tests were also conducted to complement the computational simulations. An adapted twin disc testing machine was used to investigate the progressive wear rates of the three advanced surface coatings when subjected to mixed rolling–sliding contact at high load and in dry conditions. The result is a clear indication that advanced surface coating techniques can provide significant improvements in wear resistance and surface durability. From the rolling contact fatigue experiments conducted in this study, a multilayer tungsten carbide/carbon coating was shown to provide the most significant increase in surface durability with a 16-fold reduction in measured weight loss compared to the uncoated substrate over the 145,000 cycle test duration investigated.

Rigorous analysis of the spatial dispersion effect in multilayer thin-film coatings

We demonstrate a wavelength-dependent spatial lateral beam shift of a reflective Gaussian beam on the surface of multilayer thin-film coatings. Numerical analysis of the behavior of a Gaussian-form light beam incident on the surface of the coatings is performed. The theoretical analysis is for a two-dimensional light sheet. The spatial lateral beam dispersion can be determined rigorously from the numerical wave field in the region of the incident medium. The coatings present a spatial dispersion effect much larger than in previous reports. The thin-film coatings can act not only as positive spatial dispersion devices but in a proper case, also as negative spatial dispersion devices. As such, the devices produce a wavelength-dependent negative lateral shift of the reflective beam.

Versatile Multilayer Thin Film Preparation Using Hydrophobic Interactions, Crystallization, and Chemical Modification of Poly(vinyl alcohol)

Multilayer thin film coatings were prepared on silicon substrates. Poly(vinyl alcohol) was adsorbed from aqueous solution to propyldimethylsilyl-modified silicon wafers. This thin semicrystalline coating was chemically modified using acid chlorides to form thicker, hydrophobic coatings. The products of the modification reactions allowed adsorption of a subsequent layer of poly(vinyl alcohol) that could subsequently be hydrophobized. This two-step process (adsorption/chemical modification) allows layer-by-layer deposition to prepare coatings with thickness, chemical structure, and wettability control.