Kazuo Yokoyama

Electromigration in Aluminum Film Stripes Coated with Anodic Aluminum Oxide Films

The effect of an anodic-Al2O3-coating on the electromigration lifetime of Al stripes is investigated. Experiments show that the mean time to failure (MTF) of Al stripes is significantly increased by the presence of the anodic Al2O3 on stripe-surfaces. In particular the most effective structure of anodic oxide film for MTF is constructed by double layers of porous and composite oxides. It is also observed that coating with the double layers of alumina films has an effect greater than that with chemical-vapor deposited SiO2 films, and that the Al stripes coated with either alumina or SiO2 films have the same activation energy as Al stripes without coating. The observed difference in MTF-improvement can be explained by the difference of mechanical characteristics of the coating films.

(11n)-oriented BiSrCaCuO thin film formed on SrTiO3(110) substrate by RF magnetron sputtering

A superconducting BiSrCaCuO thin film was grown on a SrTiO3(110) substrate at 660°C by sputtering a single target. The RHEED observation showed that the c-axis of the film meets at an angle of about 42° with the substrate surface. The XRD measurement showed that the film has a main peak at 2 θ=31.2°. From these results, the film is considered to chiefly consist of a 2212 phase with (117) orientation and/or a 2212/2223 superlattice phase with (118) orientation. The film surface has a stripelike texture which probably corresponds to the film orientation. The zero-resistivity temperature of the film was lower by 20 K than that of the film formed on the SrTiO3(100) substrate.

Surface morphology and crystal structures of as-grown BiSrCaCuO thin films prepared by RF magnetron sputtering from three targets

As-grown superconducting BiSrCaCuO thin films were grown at 650°C on MgO(100) substrates by RF magnetron sputtering from three targets. The substrate was rotated over the three targets: Bi3Sr2Ca2Cu3Ox, Bi2Sr3Ca3Cu3Ox and Bi2Sr2Ca2Cu4.5Ox. By adjusting the staying time of the substrate over each target, it was possible to precisely control the chemical composition of the thin film. This paper describes the superconducting properties, surface morphology and crystal structures of sputtered films formed by varying the staying time. X-ray diffraction measurement showed that the crystal structure could be changed by varying the staying time. The film with the optimum composition showed zero resistivity at 75 K and had an exceedingly smooth surface. This film had the mixed phase which is considered to be a stacked structure consisting of a high-Tc phase and a low-Tc phase.

Bi2(Sr, Ca)3Cu2Ox and Bi2(Sr, Ca)4Cu3Ox Thin Films with (11n) Orientation

As-grown Bi2(Sr, Ca)3Cu2Ox and Bi2(Sr, Ca)4Cu3Ox thin films with the (11n) orientation were formed on SrTiO3(110) substrates by consecutive sputtering. RHEED patterns and XRD analysis indicated that the c-axis of the Bi2(Sr, Ca)3 Cu2Ox and Bi2(Sr, Ca)4Cu3Ox films are tilted against the substrate surface by approximately 41° and 45°, respectively. The Bi2(Sr, Ca)3Cu2O3 film was found to have mainly a (117) orientation, whereas the Bi2(Sr, Ca)4Cu3Ox film is considered to have mainly a (1110) orientation.

Superconductivity of Bi system thin films formed by consecutively sputtering three Pb-doped targets of slightly different compositions

Superconducting Bi system thin films have been formed on MgO(100) substrates by RF magnetron sputtering from three lightly Pb-doped targets: Bi2.4Pb0.6Sr2Ca2Cu3Ox, Bi1.6Pb0.4Sr3Ca3Cu3Ox and Bi1.6Pb0.4Sr2Ca2Cu4.5Ox. To improve the superconductivity of the as-grown films, the films were kept in a sputtering chamber after deposition. The film, formed at 660°C and kept at the same substrate temperature and gas pressure as the sputtering conditions for 5 h after deposition, showed zero resistivity at 83 K and a resistivity drop at 115 K. The critical current density of the film was 4×105 A/cm2 at 77 K and 1×107 A/cm2 at 52 K.

Formation of As-grown BiSrCaCuO thin films by sputtering three targets heavily doped with Pb in atmospheres of O2 and N2O/O2 mixtures

As-grown BiSrCaCuO thin films have been formed on MgO(100) substrates at various temperatures by RF magnetron sputtering from three targets in atmospheres of neat O2 and N2O/O2 mixtures (1:1). The three targets had slightly different compositions heavily doped with Pb: Bi3Pb3Sr2Ca2Cu3Ox, Bi2Pb2Sr3Ca3Cu3Ox and Bi2Pb2Sr2Ca2Cu4.5Ox. Both films formed in O2 and N2O/O2 mixtures at 660°C showed zero resistivity at 77–78 K. A film obtained at 540°C by N2O/O2 mixtures showed superconductivity, while a film obtained at the same substrate temperature by neat O2 did not. Gas mixtures of N2O and O2 used instead of neat O2 as a sputtering gas were effective in lowering the growth temperature by 20–40 degrees.

Formation of As-Grown YBaCuO Thin Films by RF Magnetron Sputtering from Three Targets

As-grown YBaCuO thin films with 500 A thickness have been formed at 620°C on MgO(100) substrates by RF magnetron sputtering from three targets: Y1.5Ba2Cu3Ox, Y1Ba3Cu3Ox and Y1Ba2Cu4.5Ox. The chemical composition can be precisely controlled by this method. A smooth surface was obtained by adjusting the chemical composition. Zero resistivity was observed at 81 K for the best superconducting film. The films with a slightly Cu excessive composition showed somewhat higher Tc(zero) than the stoichiometric film. X-ray diffraction spectra of sputtered films did not change distinguishably upon varying of the chemical composition when the value of Ba/Y ranged from 1.5 to 2.3 and the value of Cu/Y ranged from 1.8 to 3.3. The c-axis of the film was highly oriented normal to the substrate surface, and the c-axis lattice constant was equal to that of a bulk sample.

Effect of Growth Temperature on Superconducting Phases of As-Grown BiSrCaCuO Thin Film Formed by RF Magnetron Sputtering

The effect of growth temperature on superconducting phases has been studied for BiSrCaCuO thin films grown on MgO(100) substrates at temperatures in the range 500–660°C by RF magnetron sputtering from three targets. The chemical compositions of the metal elements of the films formed at each substrate temperature were adjusted to Bi:(Sr+Ca):Cu=2:3:2, 2:3.5:2.5 and 2:4:3. Low-Tc phases (232 phases), 232/243 phases and high-Tc phases (243 phases) were obtained above 580°C with the 2:3:2 composition, above 590°C with the 2:3.5:2.5 composition, and above 620°C with the 2:4:3 composition.

Preparation of Ca-Doped YBaCuO Thin Films Containing 248 Phase by RF Magnetron Sputtering

Ca-doped YBaCuO thin films containing predominantly a 248 phase were prepared on SrTiO3(100) substrates by sputtering three Ca-doped targets at 500°C and annealing at 750°C. The as-deposited films formed at above 500°C predominantly had a 123 phase, and those at 300°C were amorphous. On the other hand, the as-deposited films obtained at 400–500°C had an unidentified major phase with a lattice spacing of 7 A, which changed into a 248 phase by annealing, whereas the phase of amorphous films and the 123 phase did not.

Formation of BiSrCaCuO Thin Films with (11n) Orientation on SrTiO3 (110) Substrates by RF Magnetron Sputtering

As-grown BiSrCaCuO thin films were grown on SrTiO3(110) substrates at 660 °C by consecutively sputtering. XRD measurement showed that the films with a low-T c phase composition and a high-T c phase composition had main peaks at 2Ө of 31.0° and 31.9°, respectively. The film with a low-T c phase composition is considered to mainly consist of a low-T c phase with (117) orientation.