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Special Issue "Applications of Optical Thin Film Coatings"

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A special issue of Coatings (ISSN 2079-6412).

Deadline for manuscript submissions: closed (30 December 2018) | Viewed by 81451

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Dr. Shigeng Song

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Guest Editor

Institute of Thin Films, Sensors & Imaging, School of Engineering and Computing, University of the West of Scotland, Paisley PA1 2BE, UK
Interests: deposition process; device simulation and modelling; optical interference filters; photovoltaics; SERS based on sculptured nano thin films; flexible electronics
Special Issues, Collections and Topics in MDPI journals

Prof. Dr. Desmond Gibson

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Guest Editor

School of Computing, Engineering and Physical Sciences, University of the West of Scotland, Paisley, Scotland PA1 2BE, UK
Interests: optical thin film and coatings, including medical device applications
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

The scope of this Special Issue, "Applications of Optical Thin Film Coatings", can range from thin film inorganic/organic materials studies for optimizing optical properties/novel materials, deposition process development, thin film characterizations, to the applications of optical thin films and coatings.

Optical thin film history can be traced back to 1817, when the antireflection effect of a single layer on glass was first discovered by Fraunhofer. Since then, significant progress has been made in optical thin films and coatings. Particularly modern optical thin films/coatings not only attract extensive attention for academic research; they are also integral to modern electronics (e.g., mobile devices) and command a significant share in the industry.

Demand for novel material development is driven by increasing application requirements: Novel properties, and better performance. For example, ultra-low absorption and mechanical loss optical coatings for high performance interferometry mirror for gravitational wave detection. Apart from traditional two-dimensional thin films/coatings, nano-structured films are also important in the following applications: Inorganic/organic hybrid coatings for the next generation LED, sculpted nano-metal film for Surface Enhanced Raman Scattering (SERS), heterogeneous nano-structure and quantum dots for photonics application, and much more.

There is a wide range of applications using optical thin films and coatings, just to name a few: Telecommunications, photovoltaics, photonic devices, sensors, lasers, displays/lighting, security devices, ophthalmics, automotive optics, building, and biological and biomedical applications. Due to increasing industrial demands, the range of applications is expected to expand.

This Special Issue will provide a forum for original research, critical reviews and perspectives in the area of optical thin films and coatings and its applications from both academia and industry. In particular, the topics of interest include, but are not limited to:

Optical thin film material developments
Thin film material characterization
Nano-structured thin films
Linear and nonlinear optical thin films
Optical Metamaterials
Novel coating materials
Multifunctional coatings (optical combined with other functions)
Optical thin film/coating process
Real-time process monitoring and control of optical coating processes
Coatings on polymers and special substrate materials
Optical coatings for telecommunications
Optical coatings for photovoltaic cells
Optical coatings for photonic and sensor devices
Optical coatings for displays and lighting
Optical coatings for laser applications; high power and/or short pulse duration control
Optical coatings for aerospace and space applications
Optical coatings for security devices
Optical coatings for automotive and building
Optical coatings for biological and medical applications

Dr. Shigeng Song
Prof. Dr. Desmond Gibson
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Coatings is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2600 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Published Papers (14 papers)

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Research

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Open AccessArticle

Optimised Performance of Non-Dispersive Infrared Gas Sensors Using Multilayer Thin Film Bandpass Filters

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Coatings 2018, 8(12), 472; https://doi.org/10.3390/coatings8120472 - 19 Dec 2018

Cited by 8 | Viewed by 4706

Abstract

In this work, performance improvements are described for a low-power consumption non-dispersive infrared (NDIR) methane (CH₄) gas sensor using customised optical thin film bandpass filters (BPFs) centered at 3300 nm. BPFs shape the spectral characteristics of the combined mid-infrared III–V based light emitting diode (LED)/photodiode (PD) light source/detector optopair, enhancing the NDIR CH₄ sensor performance. The BPFs, deposited using a novel microwave plasma-assisted pulsed DC sputter deposition process, provide room temperature deposition directly onto the temperature-sensitive PD heterostructure. BPFs comprise germanium (Ge) and niobium pentoxide (Nb₂O₅) alternating high and low refractive index layers, respectively. Two different optical filter designs are progressed with BPF bandwidths (BWs) of 160 and 300 nm. A comparison of the modelled and measured NDIR sensor performance is described, highlighting the maximised signal-to-noise ratio (SNR) and the minimised cross-talk performance benefits. The BPF spectral stability for various environmental temperature and humidity conditions is demonstrated. Full article

(This article belongs to the Special Issue Applications of Optical Thin Film Coatings)

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Open AccessArticle

Comparison of the Physicochemical Properties of TiO₂ Thin Films Obtained by Magnetron Sputtering with Continuous and Pulsed Gas Flow

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Coatings 2018, 8(11), 412; https://doi.org/10.3390/coatings8110412 - 20 Nov 2018

Cited by 44 | Viewed by 5334

Abstract

In this paper, a comparison of TiO₂ thin films prepared by magnetron sputtering with a continuous and pulsed gas flow was presented. Structural, surface, optical, and mechanical properties of deposited titanium dioxide coatings were analyzed with the use of a wide range of measurement techniques. It was found that thin films deposited with a gas impulse had a nanocrystalline rutile structure instead of fibrous-like anatase obtained with a continuous gas flow. TiO₂ thin films deposited with both techniques were transparent in the visible wavelength range, however, a much higher refractive index and packing density were observed for coatings deposited by the pulsed gas technique. The application of a gas impulse improved the hardness and scratch resistance of the prepared TiO₂ thin films. Full article

(This article belongs to the Special Issue Applications of Optical Thin Film Coatings)

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Open AccessArticle

Design and Fabrication of Moth-Eye Subwavelength Structure with a Waist on Silicon for Broadband and Wide-Angle Anti-Reflection Property

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Coatings 2018, 8(10), 360; https://doi.org/10.3390/coatings8100360 - 09 Oct 2018

Cited by 21 | Viewed by 5867

Abstract

Reflection loss on the optical component surface is detrimental to performance. Several researchers have discovered that the eyes of moths are covered with micro- and nanostructured films that reduce broadband and wide-angle light reflection. This research proposes a new type of moth-eye subwavelength structure with a waist, which is equivalent to a gradient refractive index film layer with high–low–high hyperbolic-type fill factor distribution. The diffraction order characteristics of a moth-eye subwavelength structure are first analyzed using a rigorous coupled wave analysis. The moth-eye structural parameters are optimized within the spectral range of 2–5 μm using the finite-difference time-domain method. The experimental fabrication of the moth-eye structure with a waist array upon a silicon substrate is demonstrated by using three-beam laser interferometric lithography and an inductively coupled plasma process. The experimental and simulation results show good agreement. The experimental results show that the reflectivity of the moth-eye structure with a waist is less than 1.3% when the incidence angle is less than 30°, and less than 4% when the incidence angle is less than 60°. This research can guide the development of AR broadband optical components and wide-angle applications. Full article

(This article belongs to the Special Issue Applications of Optical Thin Film Coatings)

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Open AccessArticle

Online Characterization Algorithms for Optical Coating Production with Broadband Monitoring

Alexander Tikhonravov

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Coatings 2018, 8(9), 323; https://doi.org/10.3390/coatings8090323 - 14 Sep 2018

Cited by 1 | Viewed by 2853

Abstract

Algorithms for the online determination of thicknesses of already-deposited layers are important for the reliable control of optical coating production. Possible ways of constructing such algorithms in the case of coating production with direct broadband monitoring are discussed. A modified triangular algorithm is proposed. In contrast to the well-known triangular algorithm, the new algorithm does not determine all thicknesses of previously deposited layers but only those for which an increase in the accuracy of their determination is to be expected. The most promising algorithms are compared in terms of their accuracy and operational speed. It is shown that the modified triangular algorithm is much faster than the triangular algorithm, and both algorithms have close accuracy. The operational speed of the modified triangular algorithm can be a decisive factor for its use in modern broadband monitoring systems. Full article

(This article belongs to the Special Issue Applications of Optical Thin Film Coatings)

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Open AccessArticle

Preparation and Spectrum Characterization of a High Quality Linear Variable Filter

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Coatings 2018, 8(9), 308; https://doi.org/10.3390/coatings8090308 - 31 Aug 2018

Cited by 12 | Viewed by 3945

Abstract

To meet the requirements for lightweight, miniaturized dispersive optical systems for space applications, linear variable filters with a high transmittance and spatial dispersion coefficient are proposed. The filters were produced with dual ion beam sputtering, where a single layer thickness variation was achieved with a deposition rate adjustment based on a linear variable correction formula. A linear variable trend matching method was used to correct the film thickness based on the reduction of the mismatch error between two materials: Ta₂O₅ and SiO₂. The influence of the spectral and spatial measuring average effects was addressed by sampling the spot size optimization. This paper presents an all-dielectric linear variable filter that operates between 520 and 1000 nm, with an excellent linear dependence of 40 nm/mm over 12 mm. The linear variable filter possessed a 2.5% bandwidth, and its transmittance was found to be >80% at the central wavelength of the band, with a 0.1% transmittance in the cut-off region. These results indicate great potential for optical devices for space applications, and the developed process has good reproducibility and stability. Full article

(This article belongs to the Special Issue Applications of Optical Thin Film Coatings)

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Open AccessArticle

Automated Control of Plasma Ion-Assisted Electron Beam-Deposited TiO₂ Optical Thin Films

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Coatings 2018, 8(8), 272; https://doi.org/10.3390/coatings8080272 - 05 Aug 2018

Cited by 4 | Viewed by 4425

Abstract

A hollow cathode plasma source has been operated automatically, demonstrating independent control of plasma ion energy and ion current density for plasma ion-assisted electron beam-deposited titania (TiO₂). The lanthanum hexaboride hollow cathode design described in this work utilizes both the interior and exterior cathode surfaces, with the additional electrons generated removing the need for a separate neutralizing source. Automatic feedback control of plasma source cathode-to-anode accelerator voltage (AV—via argon gas flow to the anode and/or cathode plasma source areas) and accelerator current (AC—via an external high-current power supply) provides independent control of the ion energy distribution function and ion current density, respectively. Automated run-to-run reproducibility (over six separate deposition runs) in TiO₂ refractive index (550 nm) was demonstrated as 2.416 ± 0.008 (spread quoted as one standard deviation), which is well within the required refractive index control for optical coating applications. Variation in refractive index is achievable through control of AV (ion energy) and/or AC (ion current density), directly influencing deposited TiO₂ structural phase. Measured dependencies of TiO₂ refractive index and extinction coefficient on AV and AC are described. Optimum plasma source parameters for assisted electron beam deposition of TiO₂ optical thin-film applications are highlighted. Full article

(This article belongs to the Special Issue Applications of Optical Thin Film Coatings)

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Open AccessFeature PaperArticle

Investigation of TiO₂ Thin Film Deposited by Microwave Plasma Assisted Sputtering and Its Application in 3D Glasses

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Coatings 2018, 8(8), 270; https://doi.org/10.3390/coatings8080270 - 02 Aug 2018

Cited by 13 | Viewed by 3727

Abstract

TiO₂ deposition using separate regions for sputtering and oxidation is not well investigated. We optimized process parameter for such as oxygen flow and microwave power to produce high quality TiO₂ filters for Stereo/3D imaging applications. This deposition technique was chosen for its unique advantages: high deposition rates while increasing the probability of obtaining stoichiometric oxides, reduces possibility of target poisoning and provides better stability of process. Various characterization methods, such as scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman, X-ray diffraction (XRD), transmission spectroscopy, were used in compliment to simulations for detailed analysis of deposited TiO₂ thin films. Process parameters were optimized to achieve TiO₂ films with low surface scattering and absorption for fabricating multi-passbands interference filter for 3D glasses. From observations and quantitative analysis of surfaces, it was seen that surface roughness increases while oxygen flow or microwave power increases. As the content of anatase phase also increases with higher microwave power and higher oxygen flow, the formation of anatase grains can cause higher surface roughness. Optical analysis of samples validates these trends and provided additional information for absorption trends. Optimized parameters for deposition process are then obtained and the final fabricated 3D glasses filters showed high match to design, within 0.5% range for thickness error. Full article

(This article belongs to the Special Issue Applications of Optical Thin Film Coatings)

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Open AccessArticle

Design and Preparation of a Micro-Pyramid Structured Thin Film for Broadband Infrared Antireflection

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Coatings 2018, 8(5), 192; https://doi.org/10.3390/coatings8050192 - 21 May 2018

Cited by 18 | Viewed by 5905

Abstract

A micro-pyramid structured thin film with a broad-band infrared antireflection property is designed and fabricated by using the single-point diamond turning (SPDT) technique and combined with nano-imprint lithography (NIL). A structure with dimensions of 10 μm pitch and 5 μm height is transferred from the copper mold to the silicon nitride optical film by using NIL and proportional inductively-coupled plasma (ICP) etching. Reflectance of the micro-optical surface is reduced below 1.0% over the infrared spectral range (800–2500 nm). A finite-difference-time-domain (FDTD) analysis indicates that this micro-structure can localize photons and enhance the absorption inside the micro-pyramid at long wavelengths. As described above, the micro-pyramid array has been integrated in an optical film successfully. Distinguishing from the traditional micro-optical components, considering the effect of refraction and diffraction, it is a valuable and flexible method to take account of the interference effect of optical film. Full article

(This article belongs to the Special Issue Applications of Optical Thin Film Coatings)

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Open AccessArticle

Improvement of Aluminium Film Adhesion on PMMA Using Pre-Layer and Specific Annealing

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Coatings 2018, 8(5), 186; https://doi.org/10.3390/coatings8050186 - 15 May 2018

Cited by 6 | Viewed by 5924

Abstract

During the electron beam evaporation for deposition of an aluminium film, high-speed electrons decelerate when they bombard the material and part of the electron energy contributes to radiation. Due to the high sensitivity of polymethyl methacrylate (PMMA) to such radiation (in UV), the PMMA surface is degraded. This results in a weak surface of PMMA layer and decreases film adhesion. Based on the film interface adhesion mechanism and the relationship between film structure and stress, this research proposed and investigated a method for producing high quality Al reflective thin film on a PMMA surface with good adhesion. This was done by depositing a pre-layer of 20 nm Al using resistant evaporation to protect the PMMA surface from radiation damage, followed by the deposition of 80 nm Al using e-beam evaporation with ion assisted deposition. Using this method, an average reflectance of 88.6% was achieved in the wavelength range of 400–800 nm. The elastic modulus and hardness were tested by nanoindentation for the calculation of the thermal stress of the film. Adhesion was tested using the pressure strip peeling test and meets the military national standard. Full article

(This article belongs to the Special Issue Applications of Optical Thin Film Coatings)

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Open AccessArticle

Semitransparent Decorative Coatings Based on Optical Interference of Metallic and Dielectric Thin Films for High Temperature Applications

Enrique Carretero

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Rafael Alonso

Coatings 2018, 8(5), 183; https://doi.org/10.3390/coatings8050183 - 11 May 2018

Viewed by 4742

Abstract

This paper introduces a thin film multilayer structure composed of dielectric and metallic layers that allows for a wide range of aesthetic appearances using the phenomenon of optical interference. In addition, this multilayer structure allows the reflection and transmission coefficients to be controlled independently. The application of these decorative coatings to induction stoves is also studied. The aim is to provide an attractive aesthetic appearance for the transparent glass-ceramic, and allow the visualization of blue and white lighting systems. Moreover, degradation of these decorative coatings is studied at high temperatures, so as to ensure that the coating does not change its aesthetic appearance during normal operation of the stove. It has been found to be necessary to use dielectric materials with low diffusion coefficients of oxygen, or not containing oxygen, to prevent oxidation of the metal layers when subjecting the coating to high temperatures. Full article

(This article belongs to the Special Issue Applications of Optical Thin Film Coatings)

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Open AccessArticle

Effects of Annealing on Residual Stress in Ta₂O₅ Films Deposited by Dual Ion Beam Sputtering

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Coatings 2018, 8(4), 150; https://doi.org/10.3390/coatings8040150 - 20 Apr 2018

Cited by 15 | Viewed by 6938

Abstract

Optical coatings deposited by the dual ion beam sputtering (DIBS) method usually show high compressive stress, which results in severe wavefront deformation of optical elements. Annealing post-treatment has been widely used to control the residual stress of optical coatings. However, the effect of annealing on the stress of Ta₂O₅ films deposited by the IBS method has not been reported in detail. In this study, different thicknesses of Ta₂O₅ films were deposited by IBS and annealed at different temperatures from 473 to 973 K in air, and the effect of annealing on the stress of Ta₂O₅ films was investigated. The as-deposited Ta₂O₅ films deposited by IBS show high compressive stress, which are about 160 MPa. The compressive stress decreases linearly with the increasing temperature, and the wavefront deformation of Ta₂O₅ films increases linearly with film thickness (within 20 μm) at the same annealing temperature. When the temperature rises to 591 K, Ta₂O₅ films with zero-stress can be obtained. Ta₂O₅ films show tensile stress instead of compressive stress with further increasing annealing temperature, and the tensile stress increases with increasing temperature. Meanwhile, with the increasing annealing temperature, the refractive index of Ta₂O₅ film decreases, indicating the decreasing packing density. The atomic force microscope (AFM) test results show that surface roughness of Ta₂O₅ films slowly increases with the increasing of annealing temperature. Moreover, X-ray photoelectron spectroscopy (XPS) analysis shows that the Ta in Ta₂O₅ films can be further oxidized with increasing annealing temperature, namely, the absorption of Ta₂O₅ film can be reduced. X-ray diffraction (XRD) analysis shows that the annealing temperature should be below 923 K to maintain the amorphous structure of the Ta₂O₅ film. Full article

(This article belongs to the Special Issue Applications of Optical Thin Film Coatings)

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Open AccessArticle

Study on the Properties of 1319 nm Ultra-High Reflector Deposited by Electron Beam Evaporation Assisted by an Energetic RF Ion Source

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Coatings 2018, 8(2), 74; https://doi.org/10.3390/coatings8020074 - 14 Feb 2018

Cited by 2 | Viewed by 3493

Abstract

Ultra-high reflectors, working as a critical optical component, has been widely applied as a cavity mirror in fine optical systems such as laser gyro, F-P interferometer, etc. For decades, ion beam sputtering (IBS) technology, which can deposit ultra-low loss and dense layers, has been commonly believed to be the only and irreplaceable method to fabricate ultra-high reflectors. Thus, reports on other methods are rare and a reflectivity above 99.99% obtained by evaporation technology (including ion assisted evaporation) has not been seen yet. In the present study, an energetic radio frequency (RF) ion source was introduced during the electron beam evaporation process, which improved the layer quality dramatically. An ultra-high reflector at 1319 nm with reflectivity of 99.992% (measured by cavity-ring down method) was successfully deposited on a φ100 mm × 25 mm single crystal silicon substrate whose surface roughness was approximately 0.420 nm. The surface figure of the reflector was accurately controlled superior to 1/6λ (λ = 632.8 nm). The measured absorption was approximately 3–5 ppm and the calculated scatter based on surface roughness measurement was approximately 6.64 ppm. Total loss of the reflector was systematically discussed. This study showed that it is possible to apply electron beam evaporation in ultra-high reflector manufacture and the method is capable of depositing reflectors with an aperture larger than φ600 mm which is the maximum capacity of current IBS technology. Full article

(This article belongs to the Special Issue Applications of Optical Thin Film Coatings)

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Open AccessArticle

Surface Modification of Sol-Gel Silica Antireflective Coatings by F-PMHS: A Simple Method for Improvement of Amphiphobicity

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Coatings 2018, 8(2), 57; https://doi.org/10.3390/coatings8020057 - 05 Feb 2018

Cited by 12 | Viewed by 5319

Abstract

Sol-gel silica antireflective coatings (ARCs) with improved amphiphobicity were simply fabricated on BK7 glass substrates via fluorinated-poly(methylhydrogen)siloxane (F-PMHS) surface modification by the dip-coating method. The results of Fourier Transform Infrared (FTIR) and X-ray Photoelectron Spectroscopy (XPS) showed that F-PMHS were covalently bonded to the surface of ARCs. F-PMHS modification significantly improved hydrophobicity and oleophobicity of silica ARCs by increasing their water contact angles from 27° to 105° and oil contact angles from 17° to 45°. In addition to the improved amphiphobicity, the modified ARCs also possessed excellent transmittance. Most importantly, it was found that with increasing F-PMHS content the atom amounts and porous property of modified ARCs were almost unchanged. This result had been shown to be associated with the changes of optical property and amphiphobicity for silica ARCs, and the details were discussed. Full article

(This article belongs to the Special Issue Applications of Optical Thin Film Coatings)

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Review

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Open AccessReview

Simulation and Optimization of Film Thickness Uniformity in Physical Vapor Deposition

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Coatings 2018, 8(9), 325; https://doi.org/10.3390/coatings8090325 - 16 Sep 2018

Cited by 23 | Viewed by 15911

Abstract

Optimization of thin film uniformity is an important aspect for large-area coatings, particularly for optical coatings where error tolerances can be of the order of nanometers. Physical vapor deposition is a widely used technique for producing thin films. Applications include anti-reflection coatings, photovoltaics etc. This paper reviews the methods and simulations used for improving thin film uniformity in physical vapor deposition (both evaporation and sputtering), covering characteristic aspects of emission from material sources, projection/mask effects on film thickness distribution, as well as geometric and rotational influences from apparatus configurations. Following the review, a new program for modelling and simulating thin film uniformity for physical vapor deposition was developed using MathCAD. Results from the program were then compared with both known theoretical analytical equations of thickness distribution and experimental data, and found to be in good agreement. A mask for optimizing thin film thickness distribution designed using the program was shown to improve thickness uniformity from ±4% to ±0.56%. Full article

(This article belongs to the Special Issue Applications of Optical Thin Film Coatings)

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