Morphology and structure of tantalum oxide deposit prepared by KrF excimer laser CVD

Tantalum oxide films have been deposited on substrates by decomposing Ta(OCH₃)₅ photolytically in the beam of a KrF excimer laser under various conditions of laser fluence (200–450 J m^–2), repetition rate (20–120 Hz), supply rate of Ta(OCH₃)₅ (50–400 mg h^–1) and substrate temperature (403–723 K). The deposits were highly oriented when produced at laser fluences of 350 and 450 J m^–2. Their XRD patterns suggested the formation of -Ta₂O₅. The (1 1 1 0) planes were preferentially oriented parallel to the substrate surface when produced at lower repetition rates, higher supply rates of Ta(OCH₃)₅, and lower substrate temperatures; whereas (1 1 1 1) planes were similarly oriented when the conditions were reversed. The preferred orientation may be explained in terms of supersaturation. The deposits produced at a fluence of 200 J m^–2 were, however, rather amorphous.     

Rutherford backscattering investigation of thermally oxidized tantalum on silicon

The Rutherford backscattering technique utilizing 2 MeV He⁺ ions was used for studying Ta and thermally grown Ta oxide films on Si substrates. Significant impurity effects were observed for the as-deposited Ta films and are attributed to gettering during deposition. Partially oxidized Ta films exhibit a surface Ta₂O₅ layer with substantial oxygen incorporation in the underlying Ta film. In contrast with anodic Ta₂O₅ films on tantalum, there is no sharp boundary between Ta and Ta₂O₅. Tantalum oxide films on silicon are, to a first approximation, stoichiometric. Their apparent density, as determined from the areal density of Ta atoms, increases with thickness (from 4.7 to 7.3 g cm^-3) as do their refractive indices. This supports the contention that incorporation of silicon is responsible for these effects and that they are not merely due to a change in stoichiometry.     

Nucleation and growth of aluminium oxide on silicon in the CVD process

Films of aluminium oxide have been formed on single crystal silicon substrates using AlCl₃-CO₂-H₂ gas mixtures in a cold-walled chemical vapour deposition (CVD) reactor. The nucleation and subsequent growth of the deposit have been observed under the varying process parameters. It is found that the nucleation and growth of the Al₂O₃ are dependent on the H₂O flux and H₂O supersaturation. An activation energy of 34.8 Kcal mol^–1 is obtained for the growth rate indicating that the CVD of Al₂O₃ on silicon is a thermally activated process and limited by surface reaction. Scanning electron micrographs (SEM) show that the deposited films are amorphous at low temperature, 850° C, but change to fine grained polycrystalline structure at high temperature, 1000° C.     

Reduction of droplet of tantalum oxide using double slit in pulsed laser deposition

We developed a double slit method in order to reduce droplets that were a problem in pulsed laser deposition (PLD) of tantalum oxide. The tantalum oxide films were deposited using KrF excimer laser ablation of a Ta target in 5 mass% O₃ at a pressure of 10 Pa. Stoichiometric and dense films with few droplets were obtained at a pressure of 10 Pa. Without double slit, as the laser pulses increased, the number of droplets containing rich metal on the film gets increased and its size became larger, and the surface morphology of the target also became more and more rugged. It was found that the number of droplet could be controlled by changing the initial roughness of the target. The number of droplets with a diameter size of under 1 μm was decreased to 1/10 of their sizes. Droplets larger than 3 μm, which mostly affect the corrosion resistance and hardly increase even beyond the laser pulses of 100 000. It is evident from this study that the double slit is a very effective method for reducing the droplets, which are otherwise a problem often seen in film production by PLD.     

Calcium phosphate thin film processing by pulsed laser deposition and in situ assisted ultraviolet pulsed laser deposition

Calcium orthophosphates (CaP) and hydroxyapatite (HA) were intensively studied in order to design and develop a new generation of bioactive and osteoconductive bone prostheses. The main drawback now in the CaP and HA thin films processing persists in their poor mechanical characteristics, namely hardness, tensile and cohesive strength, and adherence to the metallic substrate. We report here a critical comparison between the microstructure and mechanical properties of HA and CaP thin films grown by two methods. The films were grown by KrF* pulsed laser deposition (PLD) or KrF* pulsed laser deposition assisted by in situ ultraviolet radiation emitted by a low pressure Hg lamp (UV-assisted PLD). The PLD films were deposited at room temperature, in vacuum on Ti–5Al–2.5Fe alloy substrate previously coated with a TiN buffer layer. After deposition the films were annealed in ambient air at 500–600 °C. The UV-assisted PLD films were grown in (10^–2–10^–1 Pa) oxygen directly on Ti–5Al–2.5Fe substrates heated at 500–600 °C. The films grown by classical PLD are crystalline and stoichiometric. The films grown by UV-assisted PLD were crystalline and exhibit the best mechanical characteristics with values of hardness and Young modulus of 6–7 and 150–170 GPa, respectively, which are unusually high for the calcium phosphate ceramics. To the difference of PLD films, in the case of UV-assisted PLD, the GIXRD spectra show the decomposition of HA in Ca₂P₂O₇, Ca₂P₂O₉ and CaO. The UV lamp radiation enhanced the gas reactivity and atoms mobility during processing, increasing the tensile strength of the film, while the HA structure was destroyed.     

Synthesis and characterization of plasma assisted chemically vapor deposited tantalum

This study focuses on the synthesis of tantalum (Ta) coatings on high strength steel by plasma assisted chemical vapor deposition using tantalum pentachloride (TaCl₅) as a preferred precursor and hydrogen (H₂) as a reducing agent. The interrelationships governing the growth kinetics, compositions, and coating properties are discussed as a function of deposition temperature, total pressure, and gas composition. The synthesized tantalum coatings are shown to be essentially pure with trace amounts of oxygen, carbon, and chlorine. The coatings are found to be dense and to exhibit conformal coverage. Preferential formation of the α-Ta phase is noted to occur when coatings are grown sequentially and in-situ on a TaN_x seed layer.     

Effects of the reaction parameters on the deposition characteristics in ZrO2 CVD

Zirconium dioxide (ZrO₂) films have been deposited on to silicon wafers by the chemical vapour deposition (CVD) technique involving the application of gas mixtures of ZrCl₄, CO₂, and H₂. The relationships between the deposition rate and various reaction parameters, such as the gas flow rate, the deposition temperature, and the composition of reactant gases, were studied. The film was identified as nearly stoichiometric monoclinic ZrO₂ by using X-ray photoelectron spectroscopy, infrared transmission, and X-ray diffraction. Zirconium tetrachloride (ZrCl₄) is the only species acting as zirconium donor which results from thermodynamic calculations in the present system. The CVD of ZrO₂ is a thermally activated process and the activation energy is about 80 kJ mol^–1 at the surface chemical reaction controlled region. The deposition mechanism, initially a kinetic process controlled by diffusive mass transfer, becomes a kinetic process governed by the surface chemical reactions with increasing total flow rate above 700°C. The dependence of the deposition rate on the reactant gas composition is mainly affected by the relative contents of the zirconium donor and the oxygen donor. At ZrCl₄ mole fractions lower than 2.0 × 10^–3, the deposition rate increases with the ZrCl₄ mole fraction; however, at ZrCl₄ mole fractions higher than that the deposition rate is mainly influenced by the H₂O-forming reaction between CO₂ and H₂.     

Thermodynamics of the Chemical Vapor Deposition of Carbides in the System TaBr5–CCl4–Cd

Thermodynamic analysis of the chemical vapor deposition (CVD) of tantalum carbides in the TaBr₅–CCl₄–Cd system is carried out. The equilibrium composition in the system is calculated as a function of temperature and ratios of reagents. The optimal CVD conditions for tantalum carbides with particular deviations from stoichiometry are assessed.     

Pulsed laser deposition and photoluminescence measurements of ZnO thin films on flexible polyimide substrates

In order to assess the potential of zinc oxide (ZnO) in flexible electronics applications, we created continuous ZnO films on polymeric substrates for evaluation of structural and optical properties. Specifically, we have used pulsed laser deposition to deposit ZnO films with thickness of several microns on flexible free-standing polyimide substrates. A KrF excimer laser (248 nm) operated at fluences of 3.0-6.2 J/cm² was used. ZnO films were deposited at temperatures between room temperature and 300 °C under O₂ atmosphere at a pressure of 50 Pa. Good flexibility characterizes the obtained layers and X-ray diffraction measurements show that films present all reflections of hexagonal ZnO. We discuss luminescence measurements on the films in relation to the complex interface phenomena expected in our samples.     

The mechanism of reactive sputtering

The reactive sputtering of tantalum in mixed argon/oxygen atmospheres at a total pressure of 3.0×10^–4 torr has been investigated by means of measurements on deposition and growth rates, density, electrical properties and electron diffraction. The main controlling factor on all of the parameters was found to be the partial pressure of oxygen.The deposition rate was determined by the partial pressure of oxygen and assumed one of two values — either that associated with a clean tantalum target or that characteristic of an oxidised target. In the former region the oxygen content of the sputtered film was mainly dependent on the partial pressure of oxygen in the sputtering atmosphere and could range from zero to 100%. In the latter region, the films were always oxidised, but were deposited at a rate which was a fifth of that of the oxidised films sputtered under the former conditions.It is concluded that there is a critical oxygen pressure, below which tantalum metal is sputtered and undergoes reaction at the substrate, and above which tantalum oxide is sputtered from an oxide surface as the result of reaction at the target.     

Preparation of amorphous TiS x thin film electrodes by the PLD method and their application to all-solid-state lithium secondary batteries

Titanium sulfide thin film electrodes were prepared by the pulsed laser deposition method using a KrF excimer laser. Thin films of various compositions were prepared under several deposition conditions such as Ar gas pressure, laser fluence, and target-substrate distance. The thickness of the titanium sulfide thin film prepared under Ar gas pressure of 0.01 Pa, the pulse energy of 200 mJ/pulse, and the distance of 5 cm between the target and the substrate was ca. 400 nm. The films prepared at room temperature showed no peaks in the XRD pattern and no periodic lattice fringe in high-resolution transmission electron microscopic images, suggesting that they were amorphous. An all-solid-state cell using a TiS_4.0 thin film electrode formed on a pelletized Li₂S–P₂S₅ glass–ceramic electrolyte showed the reversible capacity of 543 mAh g⁻¹, which was higher than that of a cell using a TiS_1.7 film. The former solid-state cell retained higher capacity for 20 cycles at room temperature.     

Chemical vapour deposition of superconductors

Chemical vapour deposition processes (CVD) can produce metastable fine-grained materials as well as epitaxial coatings and can have a very large throwing power depending on the process parameters. Therefore, CVD is an prospective method to deposit high-temperature superconducting materials withT _c⩾10 K. One of the first superconductors which were produced was Nb₃Sn on tapes and single wires. This superconducting material is, however, today produced by metallurgical methods. Since the detection of Nb₃Ge, CVD has become for these coatings the main method of production for the following reasons: high deposition rates, possibility to dope the material by addition of further doping gases to the CVD-process, continuous process. These coatings were deposited on tapes. For the first time the large throwing power of the CVD process was utilized for the deposition of B1 -NbC _x N _y , on carbon fibre bundles. This opens the possibility to produce multifilamentary structures used for magnetic applications. The structure of the coating can be varied by changing the gas properties, by addition of further gases, by an ultrasonic field, by ignition of a gas discharge and by multi-layering. CVD could also be a prospective method for producing the new class of superconductors withT _c⩾30 K.     

Experimental and theoretical study of the role of the gas composition and plasma-surface interactions on the SnOx films stoichiometry prepared by DC magnetron reactive sputtering

In this paper, we have studied the tin oxide films deposition by DC magnetron reactive sputtering. We have investigated the discharge parameters such as discharge voltage and deposition rate and the discharge composition as a function of the input oxygen partial pressure. We have compared these results with the deposited films stoichiometry. In the constant current discharge mode, we observe, with increasing oxygen partial pressure, a decrease of the discharge voltage followed by a slight increase, and a drop of the deposition rate. For each experimental conditions, we measure the gas composition by mass spectrometry (glow discharge mass spectrometry mode and residual gas analysis mode (RGA)) and the deposited films stoichiometry by X-ray photoemission spectroscopy. The results are fitted by means of a model, taking into account the plasma-surface interactions. All the data are fitted by the same equation, with only four fitting parameters, namely the sticking-reaction coefficients of O and O₂ on Sn and SnO surfaces. Our results show that the main reaction is the reaction between the atomic oxygen and the metallic part (Sn) of the substrate. This reaction is characterized by a sticking coefficient value (α₁⁰) of 0.96.     

Use of metastable, dissociated and charged gas species in synthesis: a low pressure analogue of the high pressure technique

Oxidation of silver using microwave-induced oxygen plasma and oxygen-ozone gas mixture was studied as a function of temperature and partial pressure. The oxide Ag₂O was formed at temperatures well above its normal decomposition temperature in oxygen plasma at a pressure of 5 Pa. The higher oxide AgO_1–x was formed in O₂₊O₃ gas mixtures at lower temperatures. The oxygen chemical potentials for the oxidation of Ag to Ag₂O, Ag₂O to AgO_1–x and AgO to Ag₂O₃ were evaluated from thermodynamic data and compared with the experimental results to obtain information on the chemical potential of oxygen in microwave plasma and gases containing ozone. The oxygen potential of the gas phase in microwave plasma operating at a pressure of 5 Pa was found to be in excess of 36 kJ/mol at 750 K. This is equivalent to a pressure of diatomic oxygen gas greater than 3 × 10⁷ Pa. In the O₂₊O₃ mixture at ambient pressure containing 5 mole percent O₃, the oxygen potential is 112 kJ/mol at 465 K. The equivalent pressure of diatomic oxygen is 4 × 10¹⁷ Pa. Thus, metastable species such as O₃ or charged species such as O^– present in plasma can be used as a powerful reagent for the syntheses of metastable oxides. Similar techniques can be used for other metastable inorganic solids such as nitrides for functional applications.     

Effects of oxygen gas flow rate and ion beam plasma conditions on the opto-electronic properties of indium molybdenum oxide films fabricated by ion beam-assisted evaporation

The purpose of the present work is to experimentally study the effects of the oxygen gas flow rate and ion beam plasma conditions on the properties of indium molybdenum oxide (IMO) films deposited onto the polyethersulfone (PES) substrate. Crystal structure, surface morphology, and optoelectronic properties of IMO films are examined as a function of oxygen gas flow rate and ion beam discharge voltage.Experimental results show that the IMO films consist of a cubic bixbyite B-In₂O₃ single phase with its crystal preferred orientation alone B(222). Mo⁶⁺ ions are therefore considered to partially substitute In³⁺ sites in the deposit. Under-controlled ion bombardment during deposition enhances the reaction among those arriving oxygen and metal ion species to condense into IMO film and facilitates a decreased surface roughness of IMO film. The film with ultimate crystallinity and the lowest surface roughness is obtained when the oxygen flow rate of 3 sccm and the discharge voltage of 110 V are employed. This results in the lowest electrical resistivity due mainly to the increased Hall mobility and irrelevant to carrier concentration. The lowest electrical resistivity of 8.63 × 10^− 4 ohm-cm with a 84.63% transmittance at a wavelength of 550 nm can be obtained, which satisfies the requirement of a flexible transparent conductive polymer substrate.     

Absorption and emission spectra of the YBCO laser plume

The plasma produced during laser ablation deposition of thin film YBCO has been studied by optical emission spectroscopy. There is evidence of increased YO band emission in the range 590–625 nm as the ambient oxygen gas pressure confining the plume is increased in the range 30–200 m Torr. Temporal profiles show that close to the target the plume is insensitive to ambient oxygen pressure. It is deduced that the optical emission here is excited by electron impact excitation. Further away from the target there is evidence that two distinct processes are at work. One is again electron excitation; the emission from this process decreases with distance because the expanding plume cools and collisions become less frequent in the expanding gas. The second is driven by oxidation of atomic species expelled at high speeds from the target. The main region of this activity is in the plume sheath where a shock front ensures heating of ambient O₂ and reaction of monatomic plasma species to form oxide in an exothermic reaction. Spatial mapping of the emission demonstrates clearly how increasing oxygen gas pressure confines the plasma and enhances the emission intensity from the molecular YO species ejected from the target in a smaller region close to the target. Ba⁺ is observed as a dominant species only very close to (within 1 mm of) the target. Absorption spectra have been taken in an attempt to examine ground state and cool species in the plume. They reveal the quite surprising result that YO persists in the chamber for periods up to 1 msec. This suggests an explanation for the recent report of off-axis laser deposition in terms of simple condensation. Previously, quasi-ballistic transfer of material from target to substrate has been considered the only significant process.     

Effect of dilution gases in methane on the deposition of diamond-like carbon in a microwave discharge II: Effect of hydrogen

The effect of hydrogen as a dilution gas on the deposition of diamond-like carbon by the decomposition of methane in a microwave discharge was studied from surface analysis of the substrate and from plasma diagnostics. When carbon deposited from a CH₄-Ar plasma and consisting of large amounts of graphite and small amounts of diamond, was placed in the hydrogen plasma chemical sputtering of carbon to form hydrocarbons and adsorption of hydrogen on the carbon substrate were observed. The reaction occured only on graphite and not on diamond. The effects of hydrogen as a dilution gas on the deposition of diamond-like carbon from CH₄-H₂ plasma are to cause the formation of CH₃ radicals in the plasma, the removal of graphite from the deposit and the adsorption of atomic hydrogen on the deposit as an active participant in the diamond crystallization process.     

Deposition of nickel oxide by direct current reactive sputtering: Effect of oxygen partial pressure

Nickel oxide thin films were deposited by Direct Current magnetron reactive sputtering from Ni target onto SnO₂:F conductive glass substrates. The process was carried out without intentional heating, in an argon/oxygen gas mixture with various oxygen contents and discharge currents. The polycrystalline NiO thin films were deposited with controlled growth of the structure along [111] and [200] crystallographic directions for chosen conditions. Morphology of as-deposited films was found to depend on the preferentially oriented NiO crystals. Moreover, on the basis of discharge voltage as a function of the O₂ partial pressure for a constant discharge current, we present here the method to estimate the deposition conditions allowing us to achieve the desired preferential growth of transparent p-type semiconductor NiO, by Direct Current magnetron reactive sputtering.     

Deposition rates of titanium nitride plates prepared by chemical vapour deposition of TiCl4+NH3 system

Titanium nitride plates (TiN_x,x = 0.74–1.0, about 2 mm thick maximum) were prepared by chemical vapour deposition (CVD) using TiCI₄, NH₃ and H₂ as source gases. The effects of CVD conditions, i.e. gas molar ratio (m _N/Ti = NH₃/TiCI₄) and deposition temperature (T_dep), on deposition rates and surface morphology were examined, and the deposition mechanism of the CVD-TiN_x plates was discussed. The relationship between m_N/Ti and deposition rates showed a maximum peak at certainm _N/Ti, and this maximum peak shifted to lowerm _N/Ti with increasing_{T dep}. The activation energy for the formation of CVD-TiN_x plates was about 80 kJ mol^–1 in the lower temperature range. The decomposition reaction of NH₃ gas could be associated with the rate-controlling step. At higher temperatures, the diffusion process may be the rate-controlling step, and a large amount of powder (mainly NH₄Cl) was formed in the gas phase. The highest deposition rate obtained in the present work was 1.06×10^–7 ms^–1 (0.38 mmh^–1) atT _dep = 1773 K andm _N/Ti = 0.87.     

Critical oxygen content in porous anodes of solid tantalum capacitors

X-ray diffraction analysis of sintered porous anodes of solid tantalum capacitors and the current-voltage (I–V) characteristics of Ta2O5 amorphous layers formed on the anode surface have been performed. A strong correlation between a sharp increase of direct current in the I–V characteristics at some critical oxygen content and the creation of a saturated solid-phase solution of oxygen in tantalum was found. The appearance of a crystalline oxide Ta2O5 phase was detected in porous anodes at oxygen contents above the critical oxygen level. The decrease of effective radius below 1 m of Ta powder particles used in sintering leads to the size effect: the oxygen content in the porous anode after sintering exceeds the solubility limit. © 1998 Kluwer Academic Publishers