MgF2 Thin Films Research Articles

Investigation of deposition temperature effect on spatial patterns of MgF2 thin films.

In this work, the atomic force microscopy (AFM) technique was used to characterize 3D MgF2 thin film surfaces through advanced analysis involving morphological, fractal, multifractal, succolarity, lacunarity and surface entropy (SE) parameters, consistent with ISO 25178-2: 2012. Samples were synthesized by electron beam deposition, grown in three different temperatures. Three different temperatures of 25°C (laboratory temperature), 150 and 300°C were chosen. The temperature of 300°C is usually the highest temperature that can be deposited with the electron beam evaporation coating system. The substrates were made of glass (diameter 16 mm, thickness 3 mm), and the samples were prepared at a pressure of 5 × 10-5 Torr. The statistical results from the AFM images indicate that topographic asperities decrease with increasing deposition temperature, showing a decrease in roughness values. Regardless of the deposition temperature, all surfaces have a self-similar behavior, presenting a very linear PSD distribution, and, according to our results, the sample deposited at 300° had the highest spatial complexity. On the other hand, surface percolation is increasing when temperature increases, indicating that its low roughness and high spatial complexity play an important role on the formation of their most percolating surface microtexture. Our results demonstrate that the lower deposition temperature promoted the formation of less discontinuous height distributions in the MgF2 films.

A study of MgF2 thin film growth in the atomic layer deposition process by multi-scale simulations

Metal fluoride thin film enhances cyclability and capacity retention of a battery by protecting electrodes and reducing side reactions at the interface. In this work, growth mechanism for MgF2 thin film by ALD is investigated by multi-scale approach of density functional theory (DFT) and the kinetic monte carlo (KMC) method. Bis(ethylcyclopentadienyl)magnesium, Mg(EtCp)2, and hydrogen fluoride, HF, are selected as a precursor and reactive gas, respectively. DFT is adopted to reveal and find dominant reaction pathways calculating adsorption and activation energies of surface reactions. F on a clean MgF2 surface is a major reaction site for the precursor adsorption, and sequential and concurrent ligand decompositions are equally predicted during HF pulse. The KMC model is developed to reproduce the MgF2 deposition process with calculated energetics from DFT. Mass gain of the film, growth rate, and surface HF coverage calculated by KMC show a good agreement with experimentally measured values. Ligand molecules remaining on the surface are suggested to be a critical factor for a low surface HF coverage and carbon impurity. We believe the multi-scale simulation presented in this work can be a useful guide for the fundamental understanding of the reaction mechanism as well as the optimization of film quality.

Effect of Pre-annealing Treatment of Coating Material on MgF2 Thin Films Prepared by Thermal Evaporation Method

In the present study, we investigate the effect of substrate temperature, annealing temperature, and annealing time of the MgF2 coating material on the physical properties of the MgF2 thin film. The MgF2 thin film has been fabricated by MgF2 annealed coating material by thermal evaporation method. Our experimental results indicate that the oxidation of MgF2 in the air oven started at 800 ∘C. The optical transmittance of the MgF2 thin film decreases by increasing the annealing temperature and annealing time of MgF2 coating material. The results of the EDX test show that by increasing the annealing temperature and annealing time of MgF2 coating material, the content of oxygen element in the MgF2 thin film increases and the amount of fluorine element reduces.

Surface roughness and residual stress evolution in SiNx/ SiO2 multilayer coatings deposited by reactive pulsed magnetron sputtering

The surface roughness and residual stress behavior in two types of SiNx/SiO2 dielectric quarter-wave stacks was investigated experimentally. A reactive pulsed magnetron sputtering system was used to prepare the SiNx/SiO2 multilayer thin films. The results show that SiNx/SiO2 quarter-wave stack with a buffer layer of MgF2 thin film can reduce the residual stress. The effect of aging on the residual stress in two quarter-wave stacks was also studied. We found that the residual stresses in both SiNx/SiO2 multilayer coatings are changed from a compressive state to a tensile stress state with increasing the aging time. The root mean square (RMS) surface roughness of MgF2/(SiNx/SiO2)22 and (SiNx/SiO2)22 quarter-wave stacks are 2.23 ± 0.22 nm and 2.08 ± 0.20 nm, respectively.

Accurate prediction of multilayered residual stress in fabricating a mid-infrared long-wave pass filter with interfacial stress measurements.

We present an accurate approach to predict the residual stress in a multilayered mid-infrared long-wave pass filter (MIR-LWPF) by using interfacial stress measurements. Magnesium fluoride (MgF2) and zinc sulfide (ZnS) thin films were used to fabricate 7-layer (MgF2/ZnS)3/MgF2 MIR-LWPF devices by electron-beam evaporation with ion-assisted deposition technique. The interfacial stress between the high-index of ZnS and low-index of MgF2 thin film materials was obtained from the residual stress measurements based on Twyman-Green interferometer and fast Fourier transformation (FFT) method. The modified Ennos formula was used to estimate the residual stress in the (MgF2/ZnS)3/MgF2 multilayered thin films. The difference between the predicted stress value and the measured value is 28 MPa by the proposed method. In the MIR-LWPF design of (MgF2/ZnS)3/MgF2 multilayer structure, the optical transmittance at a near-infrared wavelength of 1.0 µm to 2.5 µm is less than 10%, and the transmittance at a mid-infrared wavelength of 2.5 µm to 7.5 µm is greater than 93%. The proposed method can accurately evaluate and predict residual stress in fabricating mid-infrared long-wave pass filter device which possesses low residual stress as well as lower surface roughness.

Effect of SF6 flow ratio on microstructure and properties of MgF2 thin films prepared by magnetron sputtering

In order to suppress F- deficiency of MgF2 film prepared with magnetron sputtering, the effects of SF6: Ar gas flow ratio on the structure and properties of the film were investigated. F: Mg ratio increased rapidly first then decreased with the increase of SF6 flow ratio, got 2.13 at 7%, closest to the stoichiometric ratio 2:1; while the film's refractive index first decreased and then increased, got a minimum at 550 nm 1.409 at 7%. MgF2 film increased the glass substrate transmittance within 300–1100 nm to 94.24% from 93.2%, and improved photoelectric conversion efficiency of GaAs solar cell by 2.1%.

Magnesium fluoride as low-refractive index material for near-ultraviolet filters applied to optical sensors

This article describes the fabrication of MgF2 and MgO thin-film-based optical filters and compares the optical transmission of the filters over UV. The MgF2 thin-films were deposited by use of an e-beam technique and their optical properties were characterised by ellipsometry. The effect of substrate temperature on the optical properties was studied. The MgF2 optimum refractive indices were obtained with a substrate temperature between 200 °C and 300 °C. Optical simulations were performed to compare the performance of MgF2 and MgO in the fabrication of near-UV narrow bandpass optical filters. While MgO-based optical filters result in a higher transmittance peak intensity, especially at 350 nm, the MgF2 optical filters are narrower, present lower values of FWHM, a mean value of 20 nm. This feature could be especially relevant for specific applications on fluorescent optical sensors. Finally, a Fabry-Perot based on a MgF2/TiO2 optical filter was deposited, using an e-beam technique for the MgF2 thin-films and RF-sputtering technique for the TiO2 thin-films. The MgF2/TiO2 optical filter peak transmittance is approximately 70% close to 400 nm, as expected. The results are discussed with focus on applications in fluorescent optical sensors for peaks at 350, 370, 380 and 400 nm, respectively.

A New Strategy of Constructing a Highly Fluorinated Solid‐Electrolyte Interface towards High‐Performance Lithium Anode

AbstractLithium fluoride (LiF)‐rich solid‐electrolyte interface (SEI) layers are reported as an effective method to suppress lithium dendrites growth and capacity fading for Li‐plating/Li‐stripping. Herein, a new pre‐fluorination strategy is proposed by constructing an MgF2 thin film on pristine copper foil via the magnetron sputtering. This MgF2 film proves a lithiophilic substrate and shows highly irreversible conversion reaction to LiF in the first cycle. The resulting uniform LiF‐rich dual‐layered SEI layer helps homogenize lithium ions flux, and remaining magnesium metal particles help reduce the overpotential of lithium nucleation, which regulates the behavior of Li‐plating/Li‐stripping. As a result, the half‐cells (with a fixed plating capacity of 0.5 mAh cm−2 at 0.5 mA cm−2) with the electrode after 3 min sputtering treatment show a long cycle life and a high average Coulombic efficiencies (98.3% for 600 cycles). This pre‐fluorination strategy paves an effective and facile way for the application of lithium metal batteries.

Preparation of wide optical spectrum and high antireflection MgF2 thin film with SF6 as reactive gas

In order to suppress F-deficiency, SF6 was added into the working gas Ar2 as the reaction gas to deposit MgF2 thin film on quartz glass substrate with radio frequency (RF) magnetron sputtering, and the effects of working pressure on the chemical compositions, microstructure and property of MgF2 thin film were investigated. The results show that with the working pressure increase from 1.0 to 2.5 Pa, the atomic ratio of F: Mg decreased continuously, and reached 2.02 at 2.0 Pa, very close to the ideal stoichiometric ratio of 2:1; the crystallinity of MgF2 film improved first then decreased, and finally changed into amorphous state; the particles’ profile of MgF2 film became clearer and their size increased significantly at first, but finally their profile became blurred. The refractive index of MgF2 film decreased first and then increased, and got the lowest value at 2.0 Pa, 1.384, almost equal to that of MgF2 bulk crystal. The transmittance of the coated glass within 300–1100 nm (hereinafter referred to as the transmittance of the thin film) increased first and then decreased. The transmittance of all films was higher than that of the bare glass substrate (93.2%), that is to say, all films had antireflection effect. The film transmittance reached 94.99% at 2.0 Pa, higher than that of the bare glass substrate by 1.79%, and higher than the simulated value calculated with G-Solver software by about 0.5%.

Influences of Oxygen Ion Beam on the Properties of Magnesium Fluoride Thin Film Deposited Using Electron Beam Evaporation Deposition

Magnesium fluoride (MgF2) materials are commonly used for near/medium infrared anti-reflection optical coatings due to their low refractive index and wide-range transparency. Ion assistant deposition is an important technique to increase the density of MgF2 and reduce absorption around 2.94 µm caused by high porosity and moisture adsorption. However, the excessive energy of Argon ion will induce a color center and; therefore, lead to UV/Visible absorption. In this paper, oxygen ion was introduced to reduce the color center effect in MgF2 thin film deposited using electron beam evaporation with ion assistant. The films were deposited on Bk7 and single crystal silicon substrates under various oxygen ion beam currents. The microstructure, optical constant, film density, stress, and adhesion are investigated, including the absorption properties at the infrared hydroxyl (–OH) vibration peak. The results show that as the oxygen ion beam current increases, the absorption value at the position of the infrared OH vibration, defects, and stress of the film decrease, while the refractive index increases. However, MgF2 has poor adhesion using oxygen ion-assisted deposition. Thin MgF2 film without ion beam assistant was used as adhesive layer, high density, and low absorption MgF2 film with good adhesion was obtained.

Combining super-resolution microscopy with neuronal network recording using magnesium fluoride thin films as cover layer for multi-electrode array technology

We present an approach for fabrication of reproducible, chemically and mechanically robust functionalized layers based on MgF2 thin films on thin glass substrates. These show great advantages for use in super-resolution microscopy as well as for multi-electrode-array fabrication and are especially suited for combination of these techniques. The transparency of the coated substrates with the low refractive index material is adjustable by the layer thickness and can be increased above 92%. Due to the hydrophobic and lipophilic properties of the thin crystalline MgF2 layers, the temporal stable adhesion needed for fixation of thin tissue, e.g. cryogenic brain slices is given. This has been tested using localization-based super-resolution microscopy with currently highest spatial resolution in light microscopy. We demonstrated that direct stochastic optical reconstruction microscopy revealed in reliable imaging of structures of central synapses by use of double immunostaining of post- (homer1 and GluA2) and presynaptic (bassoon) marker structure in a 10 µm brain slice without additional fixing of the slices. Due to the proven additional electrical insulating effect of MgF2 layers, surfaces of multi-electrode-arrays were coated with this material and tested by voltage-current-measurements. MgF2 coated multi-electrode-arrays can be used as a functionalized microscope cover slip for combination with live-cell super-resolution microscopy.

Influence of ex-situ annealing on the properties of MgF2 thin films deposited by electron beam evaporation

We report on the properties of magnesium fluoride (MgF2) thin films deposited by electron beam evaporation as a function of substrate deposition temperature and ex-situ annealing temperature. In particular, we report on the dependence of refractive index on annealing temperature, which can be used as a tuning parameter of the optical properties. Mechanical and structural properties of the films influenced by the annealing are also examined. Changing the substrate temperature from 50 °C to 240 °C caused a decrease of the refractive index and the lowest value of 1.36 (measured at 632.8 nm) was achieved for the substrate temperature of 240 °C. Rapid thermal annealing further decreased the refractive indices to slightly below 1.32. This could indicate increase in the film porousness and removal of adsorbed water molecules. Prior annealing the film surfaces were very smooth with root mean square and mean roughness below 1 nm. Annealing above 700 °C changed the structure of the films drastically, as they started to form a granular structure, while an annealing temperature of 1000 °C increased the refractive index to a value as high as 1.5. Using X-ray photoelectron spectroscopy we show that the surface of the films consist mainly of Mg and F atoms, but also small traces of C and O are present. The Mg:F ratio remained essentially the same (43:57) between different deposition temperatures. To demonstrate the need for post-deposition annealing treatment, we have also studied the aging effect in the MgF2 based anti-reflective coatings.

Refractive Index Sensor Using Long-Range Surface Plasmon Resonance with Prism Coupler

Long-range surface plasmon resonance (LRSPR)-based sensors exhibit high sensitivity as compared to the conventional SPR sensors due to low losses. A high refractive index prism and low refractive index dielectrics are required to excite long-range surface plasmon (LRSP) mode. In this work, a symmetrical structure consisting of metal (Au) layer sandwiched between the two similar refractive index dielectrics has been utilized to excite LRSPR modes. Two dielectrics, i.e., LiF and MgF2 thin films have been chosen for glucose sensing. Theoretical simulations for optimizing the thickness of two dielectrics have been carried out. Highly sensitive refractive index sensors based on LRSPR have been developed using the two dielectric thin films and their sensitivities have been compared.

Influence of physical plasma etching treatment on optical and hydrophilic MgF2 thin film

In this study, MgF2 thin film was prepared on glass substrate by thermal evaporation method. After that surface of the MgF2 thin film was improved using physical plasma etching treatment by plasma enhanced chemical vapor deposition (PECVD) equipment. Structural properties and surface morphology and roughness of the thin films were evaluated by GIXRD, FESEM and AFM, respectively. Also optical properties were investigated throughout measurement of transmission spectra and refractive index in visible region by use of UV/vis spectrometer and ellipsometer, respectively. Water contact angles of the surfaces were measured by water contact analyzer. Results of the research indicated that transparency of the MgF2 thin films remind without change and hydrophilic properties of the MgF2 thin films turned to superhydrophilic properties after physical plasma etching treatment.

(Invited) ALD Metal Fluorides for Ultraviolet Filter and Reflective Coating Applications

We report on the development of atomic layer deposition (ALD) processes for integrating optical filter coatings directly onto back-illuminated silicon sensors for ultraviolet applications. These devices, including CCD imaging arrays and large-area avalanche photodiodes, are modified with a two-dimensional doping method utilizing molecular beam epitaxy to provide high internal quantum efficiency in the UV. Detector integrated coatings are then required to recover additional external losses due to natural silicon reflectance at these short wavelengths. This includes the development of broadband AR coatings, multilayer narrowband coatings, and metal-dielectric filters for solar-blind devices. The use of ALD is attractive for this purpose because the low deposition temperature and thermal surface reactions do not degrade the 2D doping layers, and therefore do not result in reduced detector quantum efficiency. Applications implementing these coated detectors related to terrestrial and space astronomy, astrophysics missions, and particle physics experiments will be discussed. Cosmology is experiencing a very exciting time in exploration. Communities of scientists and technology innovators are studying four concepts for flagship future missions selected and supported by NASA. Two of these four missions are Earth-orbiting large aperture telescope observatories that will include UV capabilities. Currently, these ongoing studies have identified high efficiency detectors and high reflectivity mirror coatings as high priority technologies, particularly those that can extend and enhance system performance for UV wavelengths shorter than those previously explored by missions like the Hubble Space Telescope. At wavelengths below 200 nm, typical ALD dielectric oxide materials like TiO2, HfO2, SiO2, and Al2O3 exhibit large optical losses; we have pursued the development of new processes for ALD metal fluorides to extend coating performance into this far UV wavelength range. Thin films of MgF2, AlF3, and LiF are demonstrated using anhydrous hydrogen fluoride (HF) as the fluorine-containing precursor. This is in contrast to alternate ALD methods for fluoride materials which often utilize reactive metal fluorides as the fluorine precursors, and can result in thin films with components of residual metal oxide contamination. The use of HF has allowed for films with good UV optical properties at deposition temperatures as low 100 °C. This enables compatibility with a variety of 2D-doped silicon sensor formats as well as optical components like polymeric diffraction gratings. The deposition rate of these fluoride materials has a more significant substrate temperature dependence than other common ALD chemistries that often display a ‘window’ of uniform film growth per ALD cycle. Characterization of these coatings is performed with measurements of far UV reflectance and transmittance, x-ray photoelectron spectroscopy, x-ray diffraction, ellipsometry, and atomic force microscopy. These ALD fluoride materials are also effective protective coatings for reflective aluminum optics. Protected UV Al mirrors with ALD AlF3 overcoats are shown to be competitive with state-of-the-art commercial mirrors made with conventional physical vapor deposition (PVD) methods. The use of ALD in these components holds promise in reducing the thickness of the overcoat relative to PVD approaches. This has significant performance implications for far UV mirrors and the ultimate short wavelength cutoff of Al mirrors for potential future NASA astrophysics missions. The large scale coating uniformity, and corresponding reflectance uniformity of ALD overcoats is also attractive for the same applications. Additionally, the use of HF-based ALD chemistries can also allow for the thermal atomic layer etching (ALE) of a variety of materials including aluminum oxide. By combining these ALE methods with ALD encapsulation, it is possible to chemically strip the oxidation that occurs on metallic aluminum and effectively replace it with thin fluoride layers. Stable Al mirrors with protective fluoride coatings approximately 3 nm thick are demonstrated with this combined ALE/ALD approach. The environmental stability of these coatings will be described in the context of resistance to high-humidity conditions. Figure 1

Multilayer AR coatings of TiO2/MgF2 for application in optoelectronic devices

A multilayer system of 10 alternating TiO2 and MgF2 thin film was prepared on BK7 glass substrate by electron beam and resistive thermal evaporation techniques respectively. The substrate with 20mm diameter and thickness of 1mm were placed in a vacuum chamber at partial pressures of 10–5mbar for TiO2 and 10–6mbar for MgF2 film respectively. The as-deposited films were annealed at 400°C for an hour in ambient environment. The structural, optical and morphological properties were studied by XRD, UV–vis spectrophotometer, ellipsometery and scanning electron microscopy (SEM) respectively. The structural analysis confirms that all the thin films samples were polycrystalline in nature with tetragonal and rhombohedra phases. SEM analysis confirms the uniformity of surface and quite stable multilayer thin film structure without any micro cracks. Ellipsometery studies were carried out and refractive index and extinction coefficient as function of wavelength are obtained. The samples are found highly transparent in the range of 400–800nm, whereas the overall absorption and reflection of the multilayer thin film was decreased for the annealed samples. It has been demonstrated that the residual reflectance of the AR coating can be significantly reduced by combining the multilayer systems with an outermost low index layer made of MgF2. The realization of such hybrid multilayer systems are extremely useful as front surface of the photovoltaic cells, optoelectronic devices and front mirror for laser.

Self-consistent optical constants of MgF_2, LaF_3, and CeF_3 films

Various fluorides are materials in nature that extend their transparency range to the shortest wavelengths in the far ultraviolet (FUV, 100nm<λ<200 nm). These are relevant materials to prepare multilayer coatings in the FUV for demanding applications such as space instrumentation for astrophysics, solar physics and atmosphere physics, as well as free electron lasers, plasma diagnostics, synchrotron radiation, lithography, spectroscopy, etc. Multilayer design requires an optical constant of the coating materials. Multilayers optimally alternate two transparent materials with contrasting refractive indices. The optical constants of a low-index material, MgF2, and of two high-index materials, LaF3 and CeF3, have been determined in a wide spectral range and are presented here. Thin films of MgF2, LaF3, and CeF3 were deposited by boat evaporation onto substrates at 523 K. Transmittance, reflectance, and ellipsometry measurements were performed in ranges jointly covering the 30-950-nm spectral range. This range was extended with literature data and extrapolations to obtain self-consistent optical constants using the Kramers-Kronig (KK) analysis. An iterative, double KK analysis procedure (successive reflectance-phase and k-n KK analyses) was carried out to obtain a self-consistent set of optical constants per material. With the final data sets, the experimental measurements were satisfactorily reproduced. Global self-consistency of the data sets was successfully evaluated through sum rules; additionally, local self-consistency at each photon energy range was also evaluated through a novel sum-rule method which involves window functions. The new sets of optical constant extend the data availability mainly to the FUV and beyond, particularly for CeF3, where few data had been reported.

Effective graded refractive-index anti-reflection coating for high refractive-index polymer ophthalmic lenses

The reduction of optical reflection is important for many applications such as solar cells and lenses. The effective graded refractive-index (graded-RI) anti-reflection (AR) coating is designed a tri-layer structure with SiO2, MgF2, and ITO thin films for high refractive-index polymer ophthalmic lenses. The effective graded-RI AR coating is fabricated using the E-beam evaporation system. The mean reflectance of the effective graded-RI AR coating lens was measured to 2.9% (400–800nm), which was lower than that of the commercial multi-layer AR coating lens (3.9%) for high RI polymer lenses (n=1.67). Also, the blocking of the high energy visible light is more effective than the commercial multi-layer AR coating lens. It is known that the high energy visible light caused retinal damage. The ability to change the maximum reflectance band of an AR coating by simply changing the film thickness is advantageous for other potential applications.

Improving Anti-Reflection MgF2 Thin Films by Laser Shock Peening and Investigation of its Laser Damage Threshold

This study is divided into two parts. In the first part, the influence of laser shock peening (LSP) on the surface properties of MgF2 thin films is studied. The MgF2 monolayer films with thickness of 100 nm were prepared on a BK7 glass substrate by physical vapor deposition (PVD). An ArF excimer laser with 110 mJ energy, 53 J/cm2 power density, 20 ns pulse duration and wavelength of 193 nm in TEM00 mode was used for the LSP experiments at different (20, 50, 90, and 120) number of the pulses. Transmittance of MgF2 thin films, before and after the LSP treatment, was evaluated by a spectrophotometer and the surface morphology of samples was examined by scanning electron microscope. In the second part, the effect of the LSP treatment on the laser induced damage threshold of a sample irradiated by 90 pulses of the ArF excimer laser was examined.

Sol–gel preparation of TiO2 and MgF2 multilayers

TiO2 and MgF2 thin films were prepared by sol–gel processing. Their microstructure was investigated by scanning electron microscopy, X-ray diffraction and ellipsometric porosimetry as a function of the number of coating-firing cycles with different single layer thicknesses. TiO2/MgF2 multilayers were processed in different stacking sequences; the nucleation of the subsequent material was correlated to the underlying crystal structure and the respective film morphology. It was found that dense crystalline MgF2 films on glass can be manufactured by homoepitaxial growth of multiple thin layers. On an underlying TiO2 layer the effect of densification and crystallization is increased. In the reverse film order no such effect could be observed.