Influence of Relevant Gas Pressure on the Properties of Ionplated Ta2O5 Films. Einfluss relevanter Gasdrücke auf die Eigenschaften ionenplattierter Ta2O5 Schichten

Ta₂O₅ films were deposited onto unheated fused silica substrates (Suprasil®) by reactive low voltage ion plating (RLVIP). From these films of about 200 nm thickness the optical properties (refractive index n and the absorption coefficient k) and also the mechanical properties (density ρ and intrinsic stress σ) were investigated in dependence of the working gas pressure (Ar) and the reactive gas pressure (O₂). The experiments show a reasonable correlation between refractive index, density and intrinsic stress of the films. With low total pressure high refractive indices (up to n₅₅₀=2.25), high compressive film stress and high relative film density were found. However the film density, the refractive index and also the intrinsic stress decreased with films prepared under raising total gas pressure. The optical absorption depends on the amount of oxygen in the gas phase during deposition. By adding more oxygen to the Ar/O₂ gas mixture primarily the absorption could clearly be decreased.     

ITO‐coatings by reactive low‐voltage ion plating: film properties and plasma analysis

Transparent conductive oxides (TCO) are widely used materials for multifarious applications. According to today's state of knowledge, indium‐tin‐oxide (ITO) still offers the best electrical properties among numerous TCOs. However, ITO films produced by ion plating have only rarely been reported to be investigated. For most coating processes, ITO films need to be deposited under high temperature conditions (some 100 °C substrate heating) or require post‐deposition heat treatment in order to obtain high film quality. In this study, reactive low‐voltage ion plating (RLVIP) was used, which allows ‐ due to plasma assistance during the coating process ‐ deposition of ITO films at temperatures below 100 °C. Essential film properties, i.e. resistivity and optical transmission, were optimised by variation of arc current, gas pressure and deposition rate. These quantities ‐ particularly arc current and gas pressure ‐ have huge influence on the characteristics of the supporting plasma. This was shown by analysing the plasma with a mass‐spectrometric plasma monitoring system and with a Langmuir probe. In comparison with formerly studied coating materials (Ta₂O₅,Nb₂O₅,HfO₂), different plasma compositions regarding the presence of metal oxide ions were determined, which could be attributed to elemental and molecular energy properties (ionisation and binding energies).     

Effects of ion energy on internal stress and optical properties of ion-beam sputtering Ta2O5 films

Abstract

The effects of ion-beam energy on the internal stress and optical properties of tantalum pentoxide (Ta₂O₅) thin film have been investigated. Ta₂O₅ thin films were deposited on unheated glass substrates by ion-beam sputter deposition (IBSD) with different ion-beam voltage V _b. The mechanical properties, internal stress and surface roughness, and the optical properties, refractive index and absorption, were studied directly after deposition. The refractive index, extinction coefficient and surface roughness were found to depend on the ion-beam energy. The internal stresses were measured by the phase-shifting interferometry technique. The film stress was also found to be related to V _b, and a high compressive stress of -0.560 GPa was measured at V _b = 750 V. Ta₂O₅/SiO₂ multilayer coatings had smaller average compressive stress than single-layer Ta₂O₅ film.     

Optische und mechanische Eigenschaften von RLVIP HfO2‐Schichten

Optical and mechanical properties of RLVIP HfO₂ films In this paper HfO₂‐films were deposited on unheated fused silica, borosilicate glass, and silicon wafer substrates by reactive low voltage ion plating (RLVIP). Optical film properties, i. e. refractive index and absorption as well as mechanical properties, particularly film stress, were investigated. Their dependence on deposition parameters, i. e. arc current and oxygen partial pressure was studied. The film refractive index was calculated from spectrophotometric measurements. The low absorption was determined by photothermal deflection spectrometry. Stress measurements were performed by bending disc method with uncoated and coated silicon wafer substrates.     

Overview about the optical properties and mechanical stress of different dielectric thin films produced by reactive-low-voltage-ion-plating

Thin films of Ta₂O₅, Nb₂O₅, and HfO₂ were deposited by reactive-low-voltage-ion-plating (RLVIP) on unheated glass and silicon substrates. The film thickness was about 200 nm. Optical properties as well as mechanical film stress of these layers were investigated in dependence of various deposition parameters, i.e. arc current and oxygen partial pressure. For an arc current in the range between 40 and 50 A and an oxygen partial pressure of at least 11 · 10^− 4 mbar good results were obtained. The refractive index and film thickness were calculated from spectrophotometric transmission data using the Swanepoel theory. For example at 550 nm wavelength the refractive index for thin RLVIP-Nb₂O₅-films was found to be n₅₅₀ = 2.40. The optical absorption was obtained by photo-thermal deflection spectrometry. For the investigated materials absorption coefficients in the range of k = 5 · 10^− 4 at 515 nm wavelength were measured. The mechanical film stress was determined by measuring the difference in bending of silicon substrates before and after the deposition process. For dense films, i.e. no water vapour sorption on atmosphere, the mechanical film stress was always compressive with values of some hundred MPa. In case of films deposited with higher arc currents (I_arc > 60A) and lower oxygen pressure (< 15 · 10^− 4 mbar) the influence of a post deposition heat treatment at 350 °C for 4 h on air was also investigated. For these films the properties could clearly be improved by such treatment. However, by using lower arc currents and higher oxygen partial pressure during the ion plating process, immediately dense and environmental stable films with good optical as well as mechanical properties could be achieved without post deposition heat treatment. All the results obtained will be presented in graphs and diagrams.     

Properties of cosputtered SiO2-Ta2O5-mixtures

In this work, different mixtures of SiO₂ and Ta₂O₅ films were investigated for application in Rugate-filter-design. Not only optical properties were considered but also the mechanical and stoichiometric behavior especially with increasing thermal strain. It could be shown that different compositions of SiO₂ und Ta₂O₅ show different behavior with increasing temperature. Furthermore, it was detected that the requirement of oxygen strongly correlates with the ratio of the two layer materials. Latter results in changes of the optical properties, which can entail the destruction of the optical performance of a Rugate filter. At the end of this article, some suggestions for Rugate design calculations are presented taking the results of this work into account.     

Laser treatment of optical Nb2O5‐ and HfO2‐films

Optical thin films have to fulfil high quality requirements, which can be achieved for example by reactive low voltage ion plating (RLVIP). But especially for applications in precision optics, additional treatments are necessary to reduce residual optical absorption and compressive stress arising in the coatings, and to enhance the stability of the coatings – specifically for laser applications. In practice, post deposition heat treatment and backside coatings are mostly used to overcome these problems. In order to provide alternative methods to handle the disadvantages of the RLVIP‐process, the idea was to replace the mentioned steps by a laser treatment. This means that a laser beam is directed onto the sample after deposition or even during the coating process. In this study, the influence of a high power CO²‐laser beam on thin Nb²O⁵‐ and HfO²‐films was investigated. The effects on the refractive index and the film thickness are presented for different energy densities of a TEA‐CO²‐laser beam (10.59μm). For Nb²O⁵‐films a thickness increase up to 12.2nm (6.4 %) and a refractive index decrease of 0.074 (3.1 %) were found. In case of HfO² the values were 2.3nm (1.2 %) in thickness and 0.007 (0.3 %) in refractive index. From the observed changes also distinct impacts on the film stress can be expected. One intention of this research was also to call attention to an alternative technique for enhancement of thin film properties.     

Gravitational waves detector mirrors: Spectroscopic ellipsometry study of Ta2O5 films on SiO2 substrates

A stack of Ta₂O₅/SiO₂ layers is presently used as coating layer of mirrors in interferometric detectors for gravitational waves. The sensitivity of these detectors is limited in the 50-300 Hz frequency range by the mirror thermal noise, and it was suggested that mechanical losses in the Ta₂O₅ are the dominant source of noise. We focus here on Spectroscopic Ellipsometry (SE) results (in the 0.75 ÷ 5 eV spectral range) obtained on high quality Ta₂O₅ films deposited on SiO₂ substrates by Double Ion Beam Sputtering at the Laboratoire des Matériaux Avancés (Lyon, France). The films are extremely flat as indicated by the 0.2 nm RMS roughness determined by Atomic Force Microscopy (AFM) on (20 × 20) μm² areas. The comparison of the optical properties determined by SE with literature data, corroborated by X-ray Photoelectron Spectroscopy (XPS) data, suggests that the films present a non-ideal bulk stoichiometry and/or some degree of nanoporosity. The possible influence of an interface layer is also discussed.     

Effect of ultrarapid quenching on the structure and mechanical properties of Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> and Ta<Subscript>2</Subscript>O<Subscript>5</Subscript>

We have studied the detailed structure and mechanical properties of the Nb₂O₅ and Ta₂O₅ pentoxides after ultrarapid quenching in comparison with the properties of Nb₂O₅ and Ta₂O₅ ceramics prepared by a conventional ceramic processing technique and using high-intensity light (HIL) in an optical furnace. The results demonstrate that high-energy processing (HIL and ultrarapid quenching) improves the hardness and strength of Nb₂O₅ and Ta₂O₅. At the same time, HIL processing and quenching lead to structural disordering of the Nb₂O₅ and Ta₂O₅ pentoxides.     

Enhanced dielectric properties of modified Ta2O5 thin films

 Tantalum oxide (Ta₂O₅) is a promising high dielectric constant material for the DRAM applications because of its ease of integration compared to other complex oxide dielectrics. The dielectric constant and thermal stability characteristics of bulk Ta₂O₅ samples were reported to enhance significantly through small substitutions of Al₂O₃. However, this improvement in the dielectric constant of (1-x)Ta₂O₅-xAl₂O₃ is not clearly understood. The present research attempts to explain the higher dielectric constant of (1-x)Ta₂O₅-xAl₂O₃ by fabricating thin films with enhanced dielectric properties. A higher dielectric constant of 42.8 was obtained for 0.9Ta₂O₅–0.1Al₂O₃ thin films compared to that reported for pure Ta₂O₅ (25–30). This increase was shown to be closely related to a-axis orientation. Pure Ta₂O₅ thin films with similar a-axis orientation also exhibited a high dielectric constant of 51.7, thus confirming the orientation effect. The leakage current properties and the reliability characteristics were also found to be improved with Al₂O₃ addition. Received: 24 November 1998 / Reviewed and accepted: 7 December 1998     

Electrical properties of Ta2O5 films deposited on ZnO

High dielectric constant (high-k) Ta₂O₅films have been deposited on ZnO/p-Si substrate by microwave plasma at 150°C. Structure and composition of the ZnO/p-Si films have been investigated by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) for chemical composition. The electrical properties of the Ta₂O₅/ZnO/p-Si metal insulator semiconductor (MIS) structures were studied using high frequency capacitance-voltage (C-V), conductance-voltage (G-V) and current-voltage (I-V) characteristics. Charged trapping properties have been studied by measuring the gate voltage shift due to trapped charge generation under Fowler-Nordheim (F-N) constant current stressing.     

Substrate bias voltage influenced structural, electrical and optical properties of dc magnetron sputtered Ta2O5 films

Tantalum oxide (Ta₂O₅) films were formed on silicon (111) and quartz substrates by dc reactive magnetron sputtering of tantalum target in the presence of oxygen and argon gases mixture. The influence of substrate bias voltage on the chemical binding configuration, structural, electrical and optical properties was investigated. The unbiased films were amorphous in nature. As the substrate bias voltage increased to −50 V the films were transformed into polycrystalline. Further increase of substrate bias voltage to −200 V the crystallinity of the films increased. Electrical characteristics of Al/Ta₂O₅/Si structured films deposited at different substrate bias voltages in the range from 0 to −200 V were studied. The substrate bias voltage reduced the leakage current density and increased the dielectric constant. The optical transmittance of the films increased with the increase of substrate bias voltage. The unbiased films showed an optical band gap of 4.44 eV and the refractive index of 1.89. When the substrate bias voltage increased to −200 V the optical band gap and refractive index increased to 4.50 eV and 2.14, respectively due to the improvement in the crystallinity and packing density of the films. The crystallization due to the applied voltage was attributed to the interaction of the positive ions in plasma with the growing film.     

Mechanical properties of Nb2O5 and Ta2O5 prepared by different procedures

We have studied the mechanical properties of niobium pentoxide and tantalum pentoxide ceramics prepared by a conventional ceramic processing technique and by exposure to high-intensity light (HIL). The results demonstrate that, after HIL exposure in an optical furnace, the niobium pentoxide and tantalum pentoxide ceramics possess enhanced microhardness and improved mechanical properties (strength, fracture toughness, and brittle microstrength) owing to the formation of fractal micro- and nanostructures. With increasing exposure intensity, the strength of the Nb₂O₅ and Ta₂O₅ ceramics increases.     

Conducting and topographic AFM analysis of Hf-doped and Al-doped Ta2O5 films

A conductive atomic force microscopy (C-AFM) has been used to study conductivity and electrical degradation of ultrathin (4 nm) Hf- and Al-doped Ta₂O₅ at the nanometer scale. The hardness testing has been also performed using the force measuring ability of the AFM. Since the size of the analyzed area is very small, features which are not visible by macroscopic tests are observed: extremely low leakage current (~ pA) up to significantly higher than the fields during standard current-voltage measurements; charge trapping/detrapping processes manifesting as current peaks at pre-breakdown voltages. Hf and Al addition improves the local conductivity of Ta₂O₅, provokes modification of the leakage current mechanism, and is effective in extending the potential of pure Ta₂O₅ as a high-k material at the nanoscale. The results point to a decisive role of the type of the dopant on the electrical and mechanical properties of the films and their local response to short term microwave irradiation. Hf-doped Ta₂O₅ exhibits excellent electrical stability and high hardness. Al doping provides more plastic films with large electrical inhomogeneities; the microwave treatment at room temperature is a way to improve these parameters to a level comparable to those of Hf-doped films.     

Ion conducting properties of hydrogen-containing Ta2O5 thin films prepared by reactive sputtering

Hydrogen-containing Ta₂O₅ (Ta₂O₅:H) thin films are considered to be a candidate for a proton-conducting solid-oxide electrolyte. In this study, Ta₂O₅:H thin films were prepared by reactively sputtering a Ta metal target in an O₂ + H₂O mixed gas. The effects of sputtering power and post-deposition heat treatment on the ion conducting properties of the Ta₂O₅:H thin films were studied. The ionic conductivity of the films was improved by decreasing the RF power and a maximum conductivity of 2 × 10⁻⁹ S/cm was obtained at an RF power of 20 W. The ionic conductivity decreased by heat-treatment in air, and no ion-conduction was observed after treatment at 300 °C due to the decrease in hydrogen content in the films.     

Inherent ductility in amorphous Ta2O5 films

Thin films (30 to 80 nm) of refractory tantalum metal were successfully sputter-deposited on uniformly deformable fluoropolymer and polyimide substrates in stress free form. These films were later anodized into amorphous Ta₂O₅ which is a non-porous (barriertype) oxide with excellent corrosion resistant properties. X-ray photo-emission spectroscopy studies were carried out on tantalum and Ta₂O₅ to determine the chemical composition and oxidation states of elements. Thin tantalum and Ta₂O₅ films on fluoropolymer substrates contained fluorine as an impurity while similar films on polyimide substrate contained no fluorine and, in general, fewer impurities. Both thin tantalum films and the corresponding anodic oxides, when deformed in tension to 10% strain, exhibited the expected ductile behaviour of metals where slip bands were observed in the electron microscope. In some cases, minor cracks were observed in the deformed anodic films due to suspected local detachment of the film from the substrate.     

Microstructure and Optical Properties of Tantalum Modified TiO<Subscript>2</Subscript> Thin Films Prepared by the Sol–Gel Process

Tantalum doped TiO₂ thin films ((TiO₂)_1−x (Ta₂O₅) _x , x=0, 0.1%, 0.3%, 0.5%, 0.8%) were prepared on ITO-coated substrates by means of the sol–gel method and spin coating technology followed by rapid thermal annealing treatment (RTA). The effects of various processing parameters, including Ta content (x=0–0.8%) and annealing temperature, on the growth and properties of thin films were investigated. Structural characteristics by X-ray diffraction analysis indicated that the doping of Ta₂O₅ in the TiO₂ without change the anatase structure of TiO₂ thin films. The optical transmittance of (TiO₂)_1−x (Ta₂O₅) _x thin films decrease from 50% down to 20% with increasing the Ta₂O₅ concentrations from x=0.00 to x=0.8%. The absorption coefficient shows energy gap were decreased with increasing Ta₂O₅ content from 2.932 eV for x=0.00 to 2.717 eV for x=0.8%. Doping TiO₂ with Ta₂O₅ can lower its band gap and shift its optical response to the visible region.     

Preparation and properties of tantalum pentoxide (Ta2O5) thin films for ultra large scale integrated circuits (ULSIs) application – A review

Tantalum pentoxide (Ta₂O₅) thin films have rapidly evolved into an important field of research/development for both basic and applied science with the promise of creating a new generation of advanced micro devices for electronics applications. This paper provides a broad review of the current status and future trends of _Ta2O₅ thin films as a highly promising storage dielectric material for high-density dynamic random access memory applications. Various methods of thin film material processing are briefly reviewed. The physical, electrical and dielectric characteristics of Ta₂O₅ thin films with specific examples from recent literature and the associated conduction mechanisms are summarized and discussed. Some suggestions to improve the electrical properties of the films are finally included. ©1998Kluwer Academic Publishers     

Effects of annealing in O2 and N2 on the microstructure of metal organic chemical vapor deposition Ta2O5 film and the interfacial SiO2 layer

Ta₂O₅ films were made by metal organic chemical vapor deposition (MOCVD) and annealed at various temperatures under N₂ and O₂ ambients in a conventional furnace and a rapid thermal reactor (RTR). The microstructure and composition of the Ta₂O₅ film and the interfacial SiO₂ layer before and after various annealing treatments were studied using X-ray diffraction (XRD), atomic force microscopy (AFM), transmission electron microscopy (TEM), and Auger electron spectroscopy (AES). The as-deposited Ta₂O₅ film has an amorphous structure. The surface topology of the as-deposited Ta₂O₅ film is smooth without any apparent features. Annealing of the as-deposited film results in crystallization to an orthorhombic structure with (1 0 0) preferred orientation, and an increase in surface roughness, with the appearance of grain boundaries under AFM. The crystallization temperature varies in the various annealing treatments. An interfacial SiO₂ layer was found between the as-deposited/annealed Ta₂O₅ films and silicon substrate. The annealing treatments result in an increase in thickness of the SiO₂ layer and roughness changes of the Ta₂O₅/SiO₂/Si interfaces, which are discussed in terms of element diffusion and thermodynamic stability.     

Structural and electrical properties of Ta<Subscript><Emphasis Type="Bold">2</Emphasis></Subscript>O<Subscript><Emphasis Type="Bold">5</Emphasis></Subscript> thin films prepared by photo-induced CVD

Tantalum oxide (Ta ₂ O ₅ ) films and Al/Ta ₂ O ₅ /Si MOS capacitors were prepared at various powers by ultraviolet photo-inducing hot filament chemical vapour deposition (HFCVD). Effects of ultraviolet light powers on the structure and electrical properties of Ta ₂ O ₅ thin films were studied using X-ray diffraction (XRD) and atomic force microscopy (AFM). The dielectric constant, leakage current density and breakdown electric field of the samples were studied by the capacitance–voltage (C–V) and current–voltage (I–V) measurements of the Al/Ta ₂ O ₅ /Si MOS capacitors. Results show that the Ta ₂ O ₅ thin films grown without inducement of UV light belong to amorphous phase, whereas the samples grown with inducement of UV-light belong to δ-Ta ₂ O ₅ phase. The dielectric constant and leakage current density of the Ta ₂ O ₅ thin films increase with increasing powers of the UV- lamps. Effects of UV- lamp powers on the structural and electrical properties were discussed.