Study of amorphous Ta2O5 thin films by DC magnetron reactive sputtering

The dc magnetron reactive sputtering deposition of tantalum pentoxide (Ta₂O₅) thin films was investigated. By combining Schiller's criterion and Reith’s “target preoxidation” procedure, high quality Ta₂O₅ thin films were prepared at a high deposition rate of about lOOÅ;/min. The deposited films were amorphous, with a refractive index around 2.07 and a dielectric constant of 20. An optical transmit-tance of 98.6% was obtained for a 4500Â thick film. The leakage current density is 5 × 10^?9 A/cm² at an electric field strength of 1 MV/cm and its breakdown field strength is above 2 MV/cm. The temperature coefficient of capacitance for capacitors fabricated using the deposited films is approximately +230 ppm/°C. X-ray photoelectron spectroscopy shows that the films are stoichiometric tantalum pentoxide, Ta₂O₅, and exhibit good stability.     

Low-Pressure chemical vapour deposition of tantalum pentoxide films for ulsi devices using tantalum pentaethoxide as precursor

A laminar flow low-pressure chemical vapour deposition (LPCVD) system (LAM Integrity^TM) has been used to deposit tantalum pentoxide (Ta₂O₅) from Ta(OEt)₅ films in the presence of oxygen (O₂) at 470 °C at a typical deposition rate of 4 nm min^?1. Uniformities of <1.5% (SD 1σ) over a 150 mm silicon substrate were obtained. The layers were annealed under different conditions. It was discovered that the films did not change their stoichiometry as determined by Rutherford backscattering (RBS). The as-deposited films were amorphous but became crystalline (β-Ta₂O₅) at temperatures > 700 °C. The transmission electron microscopy (TEM) results on crystallisation behaviour were supported by X-ray diffraction data. The electrical properties of the Ta₂O₅ films have been characterised using MIS (metal/insulator/silicon) capacitor structures. Leakage values of <10^?6 A cm^?2 at 6 MV cm^?1 equivalent applied electric field and breakdown strengths of >7 MV cm^?1 at 1.6 μA were obtained for annealed layers. Compound dielectric constants (native silicon oxide thickness of about 2.5 nm plus Ta₂O₅ of various thicknesses) between 14 and >30 have been measured. The electrical properties reveal the potential use of Ta₂O₅ as a storage capacitor dielectric in 64 and 256 Mbit DRAM (dynamic random access memory) devices.     

The effects of oxygen,nitrogen, and hydrogen annealing on Mg acceptors in GaN as monitored by electron paramagnetic resonance spectroscopy

Electron paramagnetic resonance (EPR) spectroscopy is used to study the unpassivated Mg-related acceptor in GaN films. As expected, the trends observed before and after O₂, N₂, or forming-gas anneals at temperatures <800°C are similar to those typically reported for electrical measurements. However, annealing at temperatures >850°C in O₂ or N₂ permanently removes the signal, contrary to the results of conductivity measurements. Approximately 10¹⁹ cm⁻³ Mg acceptors were detected in some GaN films grown by chemical vapor deposition (CVD) before acceptor activation, suggesting that it is possible to have electrically active Mg in as-grown CVD material.     

Role of high energy photons in dual spectral source rapid isothermal CVD

The use of tungsten halogen lamps and the deuterium lamp as the source of thermal and optical energies has been exploited to deposit thin films of Ta₂O₅ on Si and conducting substrates. The leakage current densities are as low as 10⁻¹⁰ A/cm² for gate voltage under 5V. Photons in visible, ultraviolet, and vacuum ultraviolet (λ<800 nm) regions provide higher bulk and surface diffusion coefficients as well as reduced activation energy for the chemical process involved in the chemical vapor deposition process. The low thermal mass of the substrate provides limited reaction processing capability. The photochemical and photophysical processes allow the participating atoms and molecules to adjust their bond geometries and occupy sites which result in overall reduction of stress and strain energy and provide materials with overall low microscopic defects at low processing temperature and with high throughput. New experimental results of Al-Ta₂O₅-Si₃N₄-poly Si-Al structure are presented. The leakage current-voltage characteristics are better than those reported by other researchers.     

Effects of annealing in O2 and N2 on the electrical properties of tantalum oxide thin films prepared by electron cyclotron resonance plasma enhanced chemical vapor deposition

The tantalum oxide thin films with a thickness of 14 nm were deposited at 95°C by electron cyclotron resonance plasma enhanced chemical vapor deposition (ECRPECVD), and annealed at various temperatures (700∼850°C) in O₂ and N₂ ambients. The microstructure and composition of the tantalum oxide thin films and the growth of interfacial silicon oxide layer were investigated and were related to the electrical characteristics of the film. Annealing in an O₂ ambient led to a high dielectric constant (ε_r(Ta₂O₅) = 24) as well as a small leakage current (E_bd = 2.3 MV/cm), which were due to the improved stoichiometry and the decreased impurity carbon content. Annealing in an N₂ ambient resulted in poor and nonuniform leakage current characteristics. The as-deposited tantalum oxide films were crystallized into δ-Ta₂O₅ after annealing at above 750°C regardless of the ambient. The leakage current of the film abruptly increased after annealing at 850°C probably because of the stress caused by thermal expansion or contraction.     

Synthesis of the PZT films deposited on pt-coated (100) Si substrates for nonvolatile memory applications

Ferroelectric lead-zirconate-titanate (PZT) thin films were deposited by the pulsed laser deposition technique on Pt-coated (100) Si substrates. This study was focused on the investigation of the PZT film growth on (100) Si substrate at varying deposition parameters and electrical characterization of the films including hysteresis loop and fatigue properties by RT66A Standardized Ferroelectric Test System. PZT deposited at higher temperature (575°C in 450 mTorr O₂ partial pressure) showed the best crystalline structure. The remnant polarization and the retained polarization of the ferroelectric capacitors were 13 μC/cm² and 20 μC/cm², respectively. The crystallographic properties of the films were determined using the x-ray diffractometer method. The cross-sectional transmission electron microscope results showed very smooth interfaces among different layers of films.     

Properties of Flame Sprayed Ce0.8Gd0.2O1.9‐δ Electrolyte Thin Films

Thin films of Ce_0.8Gd_0.2O_1.9‐δ (CGO) are deposited by flame spray deposition with a deposition rate of about 30 nm min^?1. The films (deposited at 200 °C) are dense, smooth, and particle‐free and show a biphasic amorphous/nanocrystalline microstructure. Isothermal grain growth and microstrain are determined as a function of dwell time and temperature and correlated to the electrical conductivity. CGO films annealed for 10 h at 600 °C present the best electrical conductivity of 0.46 S m^?1 measured at 550 °C. Reasons for the superior performance of films annealed at low temperature over higher‐temperature‐treated samples are discussed and include grain‐size evolution, microstrain relaxation, and chemical decomposition. Nanoindentation measurements are conducted on the CGO thin films as a function of annealing temperature to determine the hardness and elastic modulus of the films for potential application as free‐standing electrolyte membranes in low‐temperature micro‐SOFCs (solid oxide fuel cells).     

Influence of annealing on the structural,optical and electrical properties of indium oxide films deposited on c-sapphire substrate

Indium oxide (In₂O₃) films were prepared on Al₂O₃ (0001) substrates at 700 °C by metal-organic chemical vapor deposition (MOCVD). Then the samples were annealed at 800 °C, 900 °C and 1 000 °C, respectively. The X-ray diffraction (XRD) analysis reveals that the samples were polycrystalline films before and after annealing treatment. Triangle or quadrangle grains can be observed, and the corner angle of the grains becomes smooth after annealing. The highest Hall mobility is obtained for the sample annealed at 900 °C with the value about 24.74 cm²·V^-1·s^-1. The average transmittance for the films in the visible range is over 90%. The optical band gaps of the samples are about 3.73 eV, 3.71 eV, 3.70 eV and 3.69 eV corresponding to the In₂O₃ films deposited at 700 °C and annealed at 800 °C, 900 °C and 1 000 °C, respectively.     

Deposition of thin refractory MIS structures on InP

In the present investigation, we report on significant increases in the refractory-InP Schottky barrier by process modification of the metal-semiconductor interface. Refractory-InP MIS structures were investigated as pos-sible gate metallizations. These structures consisted of Ta/Ta₂}O₅}/InP and TiW/TiO₂} /InP. A thin layer of Ta (100å) was electron beam vapor aeposited followed by the in situ formation of Ta₂}O₅} at 300?C and an oxygen overpressure. The remainder of the Ta film was deposited at 90?C substrate temperature. The TiW/TiO₂} system was formed by sputter deposition of Ti (100A?), in situ oxidation to form TiO₂} followed by deposition of TiW (88 wt % W, 12 wt % Ti). Effective barrier height for the TiW/TiO /InP structure was measured to be 0.65 eV and for the Ta/Ta₂}O₂} /InP, 0.5 eV. Leakage current at-5V was less than 3 A/cm²} for both refractory-InP MIS structures. The TiW/TiO₂} /InP structure was stable up to 400-450°C, while the Ta/Ta₂ O₅/InP system degraded to 0.2 eV at 300-350?C.     

Fabrication and Electrochemical Properties of Epitaxial Samarium‐Doped Ceria Films on SrTiO3‐Buffered MgO Substrates

Thin films of samarium‐oxide‐doped (20 mol%) ceria (SDC) are grown by pulsed‐laser deposition (PLD) on (001) MgO single‐crystal substrates. SrTiO₃ (STO) prepared by PLD is used as a buffer layer on the MgO substrates to enable epitaxial growth of the fluorite‐structured SDC film; the STO layer provides a proper crystalline match between SDC and MgO, resulting in highly crystalline, epitaxial SDC films grown in the (001) orientation. Film conductivity is evaluated by electrochemical impedance spectroscopy measurements, which are performed at various temperatures (400–775 °C) in a wide range of oxygen partial pressure (pO₂) values (10^?25?1 atm) in order to separate ionic and electronic conductivity contributions. At 700 °C, SDC/STO films on (100) MgO exhibit a dominant ionic conductivity of about 7 × 10^?2 S cm^?1, down to pO₂ values of about 10^?15 atm. The absence of grain boundaries make the SDC/STO/MgO heterostructures stable to oxidation‐reduction cycles at high temperatures, in contrast to that observed for the more disordered SDC/STO films, which degraded after hydrogen exposure.     

Preparation of high Tc (Yb,Y)Ba2Cu3O7-δ thin films by co-evaporation from effusion cells

We have prepared superconducting thin films of (Yb,Y)Ba₂Cu₃O_7-δ by evaporation of copper, ytterbium or yttrium, and barium fluoride from Knudsen effusion cells. A simple two chamber vacuum system produced stable evaporation rates of 5–10 nm/min using various Knudsen cells without realtime feedback control. Excellent stoichiometry was obtained in the films by optimizing the deposition of Cu from a dual filament cell, Yb from a single filament cell and Y and BaF₂ from high temperature cells. Films were deposited mainly on SrTiO₃ substrates at temperatures ranging from 120 to 600° C and O₂ partial pressure up to 1.5 ⋻ 10⁻⁵ Torr. Post deposition anneals in O₂ and O₂ + H₂O produced films with room temperature resistivities as low as 227μΩ-cm andT _c (R = 0) at 91 K. Films were characterized using x-ray diffractometry, Rutherford backscattering spectrometry, energy dispersive x-ray analysis, scanning electron microscopy as well as electronic transport measurements.     

Formation of aluminum oxide films from aluminum hexafluoroacetylacetonate at 350–450° c

A study of the thermally activated decomposition of Al(hfa)₃ (aluminum hexafluoroacetylacetonate) from the gas phase to form Al₂O₃ on silicon substrates is reported. The decomposition process was carried out in an open tube atmospheric pressure reactor in either argon or oxygen/argon mixtures in the temperature range, 350–450° C. The chemical vapor deposition process resulted in the formation of aluminum oxide films in all instances. The dielectric strength of Al/Al₂O₃/Si capacitors which received a post-metal anneal, but did not receive a high temperature annealing treatment, with aluminum oxide films prepared from Al(hfa)₃ in argon, was found to be in the range 2–6 MV/cm. The difference between the flatband voltage of the MOS structures and the metal-silicon work function difference was positive, indicative of a net negative oxide charge with a density of approximately 3 × 10¹¹ – 3 × 10¹² cm^-2, assuming the charge is located at the oxide-silicon interface. Decomposition of Al(hfa)₃ was also carried out in oxygen/argon mixtures with the oxygen concentration in the range 10–60 vol %. This process led to the deposition of aluminum oxide films with breakdown fields in the range 8–9 MV/cm. However, the flatband voltages of the Al/Al₂O₃/Si capacitors were even more positive than those obtained with Al₂O₃ formed in pure argon. High temperature (800–1000° C) oxygen or nitrogen annealing treatments of alumina films deposited in either argon or oxygen/argon mixtures were evaluated from the point of view of their influence on the oxide film properties. In particular, an annealing process in oxygen at 1000° C for 15 min was found to result in a reduction of the net negative oxide charge, and an improvement of the dielectric strength of films deposited in argon. Films formed in oxygen/argon mixtures did not change appreciably following oxygen annealing, as far as breakdown fields are concerned, but the oxide net negative charge was reduced. As in an earlier study by the authors, of copper film deposition from Cu(hfa)₂, it was found that essentially carbon free films could be obtained under appropriate conditions.     

Preparation of (Pb0.88La0.12)TiO3 thin films for dynamic random access memory by low pressure-metalorganic chemical vapor deposition

La-modified lead titanate (PLT) thin films were prepared by hot-wall type low pressure-metalorganic chemical vapor deposition method. Pb(dpm)₂, La(dpm)₃, and titanium tetraisopropoxide were used as source materials. The films were deposited at 500°C under the low pressure of 1000 mTorr and then annealed at 650°C for 10 min in oxygen ambient. Sputter-deposited platinum electrodes and 180 nm thick PLT thin films were employed to form MIM capacitors with the best combination of high charge storage density (26.7 μC/cm² at 3V) and low leakage current density (1.5 × 10^-7 A/cm² at 3V). The measured dielectric constant and dielectric loss were 1000∼1200 and 0.06∼0.07 at zero bias and 100 kHz, respectively.     

Temperature dependence of refractive index of Ta2O5 Dielectric Films

The temperature dependence of refractive index of tantalum pentoxide (Ta₂O₅) dielectric films was investigated experimentally. The films were formed by a magnetron radio frequency sputtering technique on the Si substrates. After deposition, the film was fabricated into an antiresonant reflecting optical waveguide using a novel wet etching technique. The thermal variation of refractive index of Ta₂O₅ was characterized by measuring the index-vs-temperature coefficient of the antiresonant reflecting optical waveguide with a Mach-Zehnder interferometry system. The measured result was 2.3 × 10^-61/K at 632.8 nm from 298 to 328K. This result indicates that index-vs-temperature coefficient of the Ta₂O₅ dielectric films is about ten times less than those of conventional III-V semiconductors.     

Rapid thermal chemical vapor deposition of in-situ boron doped polycrystalline SIxGe1-x

In-situ doped polycrystalline Si_xGe_1-x (x = 0.7) alloys were deposited by rapid thermal chemical vapor deposition (RTCVD) using the reactive gases SiH₂Cl₂, GeH₄ and B₂H₆ in a H₂ carrier gas. The depositions were performed at a total pressure of 4.0 Torr and at temperatures 600° C, 650° C and 700° C and different B₂H₆ flow rates. The conditions were chosen to achieve high doping levels in the deposited films. Our results indicate negligible effect of B₂H₆ flow on the deposition rate. The depositions follow an Arrhenius type behavior with an activation energy of 25 kcal/mole. Boron incorporation in the films was found to follow a simple kinetic model with higher boron levels at lower deposition rates and higher B₂H₆ flow rates. As-deposited resistivities as low as 2 mΩ-cm were obtained. Rapid thermal annealing (RTA) in the temperature range 800-1000° C was found to reduce the resistivity only marginally due to the high levels of boron activation achieved during the deposition process. The results indicate that polycrystalline Si_xGe_1-x films can be deposited by RTCVD with resistivities comparable to those reported for in-situ doped polysilicon.     

SnS?Sno2 conversion of chemically deposited SnS thin films

The thermal decomposition of chemically deposited SnS thin films to SnO₂ films by air annealing at temperatures up to 400°C is discussed. The conversion of a 0.7 μm thick SnS thin film to an SnO₂ film involves the creation of non-stoichiometric SnS, SnS + SnS₂ mixed phase and non-stoichiometric SnO₂ (i.e. SnO_{2 ? x}), as concluded from X-ray diffraction patterns, optical transmission spectra and electrical characteristics. The SnO₂ thin films obtained in this manner are photoconductive, with a lowest sheet resistance (in the dark) of about 10⁵ Ω/□ and an activation energy (E_a) of 0.1 eV for the electrical conductivity observed for the SnS films annealed at 325°C. This was found as the onset temperature for conversion of the SnS + SnS₂ phase to the non-stoichiometric SnO_{2 – x} film. Elevation of the annealing temperature to 400°C results in an elevation of the sheet resistance to about 10⁹ Ω/□ with the value of E_a at 1.3 eV, indicating an improvement in the degree of stoichiometry.     

Optimized La0.6Sr0.4CoO3–δ Thin‐Film Electrodes with Extremely Fast Oxygen‐Reduction Kinetics

La_0.6Sr_0.4CoO_3–δ (LSC) thin‐film electrodes are prepared on yttria‐stabilized zirconia (YSZ) substrates by pulsed laser deposition at different deposition temperatures. The decrease of the film crystallinity, occurring when the deposition temperature is lowered, is accompanied by a strong increase of the electrochemical oxygen exchange rate of LSC. For more or less X‐ray diffraction (XRD)‐amorphous electrodes deposited between ca. 340 and 510 °C polarization resistances as low as 0.1 Ω cm² can be obtained at 600 °C. Such films also exhibit the best stability of the polarization resistance while electrodes deposited at higher temperatures show a strong and fast degradation of the electrochemical kinetics (thermal deactivation). Possible reasons for this behavior and consequences with respect to the preparation of high‐performance solid oxide fuel cell (SOFC) cathodes are discussed.     

Influence of the metal electrode on the characteristics of thermal Ta2O5 capacitors

The effect of various electrodes (Al, W, TiN) deposited by evaporation (Al) and sputtering (W, TiN) on the electrical characteristics of thermal thin film (15-35 nm) Ta₂O₅ capacitors has been investigated. The absolute level of leakage currents, breakdown fields, mechanism of conductivity, dielectric constant values are discussed in the terms of possible reactions between Ta₂O₅ and electrode material as well as electrode deposition process-induced defects acting as electrically active centers. The dielectric constant values are in the range 12-26 in dependence on both Ta₂O₅ thickness and gate material. The results show that during deposition of TiN and Al a reaction that worsens the properties of Ta₂O₅ occurs while there is not an indication for detectable reduction of Ta₂O₅ when top electrode is W, and the leakage current is 5-7 orders of magnitude lower as compared to Al and TiN-electroded capacitors. The high level of leakage current for TiN and Al gate capacitors are related to the radiation defects generated in Ta₂O₅ during sputtering of TiN, and damaged interface at the electrode due to a reaction between Al and Ta₂O₅, respectively. It is demonstrated that the quality of the top electrode affects the electrical characteristics of the capacitors and the sputtered W is found to be the best. The sputtered W gate provides Ta₂O₅ capacitors with a good quality: the current density <7 × 10⁻¹⁰ A/cm² at 1 V (0.7 MV/cm, 15 nm thick Ta₂O₅). W deposition is not accompanied by an introduction of a detectable damage leading to a change of the properties of the initial as-grown Ta₂O₅ as in the case of TiN electrode. Damage introduced during TiN sputtering is responsible for current deterioration (high leakage current) and poor breakdown characteristics. It is concluded that the sputtered W top electrode is a good candidate as a top electrode of storage capacitors in dynamic random access memories giving a stable contact with Ta₂O₅, but sputtering technique is less suitable (favorable) for deposition of TiN as a metal electrode due to the introduction of radiation defects causing both deterioration of leakage current and poor breakdown characteristics.     

Oxygen and carbon incorporation in low temperature epitaxial Si films grown by rapid thermal chemical vapor deposition (RTCVD)

Using SIMS analysis, we have measured oxygen and carbon concentrations in epitaxial Si films grown between 550 and 900° C. The films were grown by rapid thermal chemical vapor deposition from SiH₄ as well as several different SiH₂Cl₂ sources. We have found that at low deposition temperatures (∼750° C or lower), oxygen incorporation is first dictated by source gas impurities and then by residual chamber gases. For the case of SiH₂Cl₂, which can have substantial oxygen content due to its reactivity with H₂O, oxygen concentrations of about 10²⁰ cm^-3 are typical at low deposition temperatures. SiH₄, however, can be obtained in higher purity, and oxygen concentrations of 10¹⁸ cm^-3 can be realized at low temperatures. At higher deposition temperatures (750-900° C), SiO volatilizes, leaving the films grown from all sources with low oxygen concentrations, typically less than 5 × 10¹⁷ cm^-3. Carbon incorporation is much less of a problem since it is present to a lesser extent both in the chamber background and in the source gases. Carbon levels less than or equal to 10¹⁸ cm^-3 can be obtained at all deposition temperatures greater than about 650° C. The performance ofp/n junctions is shown to degrade significantly for junctions grown below 850° C. We conclude that for growth of long lifetime Si films in the temperature range <800° C, that low residual H₂O partial pressures (<10^-10 Torr) are desired. Therefore, CVD chambers should be loadlocked and also capable of base pressures as low as about 10^-9 Torr.     

Preparation and properties of Ta2O5 film capacitors using TiSi2 bottom electrode

Thin Ta₂O₅ films were grown in a low pressure chemical vapor deposition(LPCVD) reactor. The Al/Ta₂O₅/p-Si and Al/Ta₂O₅/TiSi₂/p-Si capacitors were fabricated and their capacitor characteristics were investigated. During a dry O₂ annealing process of the Ta₂O₅, the oxidation of Si substrate and TiSi₂ layer underneath the Ta₂O₅ layer was observed resulting in formation of SiO₂ and TiO₂ on their surfaces, respectively, due to the oxygen diffusion through the Ta₂O₅ layer. As-deposited 100 nm thick Ta₂O₅ film was found to be crystallized to δ-Ta₂O₅ at a temperature of about 700°C. The crystallized Ta₂O₅ film showed a higher leakage current density for the Al/Ta₂O₅/p-Si capacitor, compared to an amorphous Ta₂O₅ film. For the Al/Ta₂O₅/TiSi₂/p-Si capacitor, on the other hand, the leakage current characteristic was improved as the annealing temperature increased. Capacitance of the capacitor was found to also increase as the annealing temperature increased. The Al/Ta₂O₅/TiSi₂/p-Si capacitor, however, has failed to show the better capacitor characteristics over the conventional Al/Ta₂O₅/p-Si capacitor.