Q.X. Jia

Reactively sputtered RuO2 thin film resistor with near zero temperature coefficient of resistance

Ruthenium oxide thin film resistors were reactively sputtered onto SiO2/Si substrates using d.c. magnetron sputtering. Resistors with near zero temperature coefficient of resistance (TCR) were realized by optimizing the sputtering conditions. Experimental results demonstrated that the TCR was a strong function of substrate temperature during sputtering. A critical temperature around 80°C during sputtering was found at which the TCR changed signs, i.e. a temperature dependence of semiconductor-like behavior of the resistors compared with metal-like behavior. The dependence of the TCR on the material properties of the films was analyzed using scanning electron microscopy, X-ray diffraction, and energy dispersive X-ray analysis. Films with amorphous structure usually gave a negative TCR but a positive TCR for the films with a polycrystalline structure. The resistors with near zero TCR were believed to have a microcrystalline structure.

BaTiO3 thin film capacitors deposited by r.f. magnetron sputtering

Abstract Barium titanate, BaTiO3, thin film capacitors were deposited by r.f. magnetron sputtering using a perpendicular arrangement of the substrate with respect to the target. Capacitors with a single-layer amorphous or polycrystal structure, a bilayer structure of amorphous/polycrystal, and a trilayer structure of amorphous/graded polycrystal/polycrystal were extensively investigated. Comparatively, the single-layer amorphous BaTiO3 capacitor demonstrated the highest breakdown voltage (as high as 2.5 × 106Vcm-1) but lowest dielectric constant of around 16. The single-layer polycrystal BaTiO3 capacitor displayed a much higher dielectric constant of above 300 but also revealed high leakage current which in turn reduced the breakdown voltage. By combining the advantages of the amorphous and polycrystal BaTiO3 films, bilayer and trilayer capacitors were produced, yielding superior electrical properties. An optimum thickness for the amorphous layer minimizes the reduction in effective dielectric constant while providing a low d.c. conductivity of 7.0 × 10 -11 Ω -1 cm -1 at a bias of 4 V and a high breakdown voltage of 1.9 x 106Vcm-1.

A novel approach for evaluating the series resistance of solar cells

Abstract A new method is presented to evaluate the series resistance of a solar cell. The method not only takes into account the effects of light and temperature on the series resistance, but also takes the diode junction ideality factor as an output current dependent parameter such that the values of the resistance measured are more accurate compared with previous calculations in which the diode junction ideality factor is a constant parameter along the entire output I–V curve.

SiO2 and Si3N4 passivation layers on Y−Ba−Cu−O thin films

High‐Tc Y‐Ba‐Cu‐O superconductor thin films were passivated with thermally evaporated SiO2 or rf magnetron sputtered Si3N4. Thermal evaporation of SiO2 on the Y‐Ba‐Cu‐O thin‐film surface did not degrade the zero resistance temperature of Y‐Ba‐Cu‐O films. A seriously lowered zero resistance temperature of Y‐Ba‐Cu‐O films was found if Si3N4 was sputtered onto the Y‐Ba‐Cu‐O film surface. A one month exposure of the Y‐Ba‐Cu‐O film to the environmental air did not change the Tc onset of the superconductor thin film but gave a slight decrease of Tc zero with SiO2 as the surface passivation layer. High‐frequency capacitance‐voltage studies were made on metal‐insulator‐superconductor to investigate the interface and surface properties of Ya‐Ba‐Cu‐O thin films. The experimental data suggested that a very thin layer semiconductor phase Y‐Ba‐Cu‐O was produced between Y‐Ba‐Cu‐O films and the insulator films during the deposition of the passivation film onto the Y‐Ba‐Cu‐O surface.

High-performance barium titanate capacitors with double layer structure

High-performance barium titanate (BaTiO3) capacitors with excellent electrical and dielectric properties have been made by a two step deposition scheme using reactive rf magnetron sputtering. A novel double layer structure has been developed to reduce the pinholes and improve the electrical properties, such as higher dielectric constant, lower dissipation factor, higher breakdown fields and low leakage currents. Films deposited on a cooled substrate are amorphous whereas those deposited on a heated substrate are poly crystalline. Both polycrystalline and amorphous natures are verified by x-ray diffraction and scanning electron microscopy. Amorphous films have a low leakage current, a high breakdown voltage up to 2.5 x 106 V/cm, and a dielectric constant less than 20. Polycrystalline films yield a high dielectric constant of 330. However, these films also have large leakage currents. The capacitors with the two layer structures,i.e. amorphous layer on top of polycrystal layer, have been shown to be much superior to those prepared by either polycrystal or amorphous layer alone for practical applications. The dielectric constant and breakdown voltage of capacitors with a double layer are found to be as high as 220 and 1.2 x 106 V/cm, respectively. The leakage current is reduced to the same order as the amorphous films alone.

Development and fabrication of RuO2 thin film resistors

Abstract Ruthenium oxide, RuO2, thin film resistors on SiO2Si or ceramic alumina substrates were fabricated using reactive d.c. magnetron sputtering. The realization of both negative and positive temperature coefficient of resistance (TCR) of the resistors made it feasible to obtain zero TCR resistors. Properties of resistors with negative or positive TCR could be easily controlled by either changing the substrate temperature or oxygen pressure during sputtering. In situ annealing of the resistors at a temperature of 250°C in oxygen atmosphere proved to be an effective way to stabilize and also to tune the TCR of the resistors which were deposited at a substrate temperature of 25°C. RuO2 thin film resistors with a TCR in the range of 0 ± 20 ppm ° C −1 were reproducibly obtained using this approach.

Interactions between ferroelectric BaTiO 3 and Si

Ferroelectric BaTiO3 thin films were deposited on single crystal Si substrates by radio-frequency magnetron sputtering. Bilayer structures of BaTiO3 thin films, either amorphous on polycrystalline (A/P) or polycrystalline on microcrystalline (P/M), were utilized to reduce the leakage current and to enhance the dielectric constant of the films compared to single polycrystalline and amorphous layer structures, respectively. Relatively lower charge density, determined by the capacitance-voltage measurement on the capacitors with a configuration of Au/ BaTiO/p-Si/Al, was detected for the BaTiO3 thin film with a structure of P/M. The current-voltage characteristics of the Al/SiO2(~1.8 nm)/p-Si/Al diodes fabricated on the Si after removing BaTiO3 layers gave direct evidence of the preservation of the Si surface crystal by using a P/M instead of a A/P structure. This was further confirmed by the Auger electron spectroscopy analysis on the samples.

Effect of barrier layers on BaTiO 3 thin film capacitors on Si substrates

The choice of the bottom electrode or barrier layer plays an important role in determining the electrical and structural properties of metal/ferroelectric/metal thin film capacitors. A substantial improvement of the electrical and structural properties of the capacitors was found by using RuO2 as a bottom electrode. Electrical measurement on a capacitor with a structure of BaTiO3(246 nm)/RuO2 (200 nm)/SiO2/Si, where the BaTiO3 thin film was deposited at room temperature, showed a dielectric constant of around 15, leakage current density of 1.6 × 10−7A/cm2 at 4 V, a dc conductivity of 9.8 × 1014S/cm, and a capacitance per unit area of 5.6 × 104pF/cm2. A similar structure but with polycrystalline BaTiO3 (273 nm) as the dielectric deposited at 680°C showed a dielectric constant of 290, leakage current density of 1.7 × 10−3A/cm2 at 4 V, a dc conductivity of 1.2 × 10−8 S/cm, and a capacitance per unit area of 9.4 × 105 pF/cm2. Scanning electron microscopy analysis on the films showed differences in the microstructure due to the use of different bottom electrode materials, such as RuO2 or Pd.

Characterization of the Ag/YBa2Cu3O7-x contact in thin films

Silver (Ag) contacts to very thin superconducting YBa2Cu3O7−x films were prepared by thermal evaporation. The nature of the Ag/YBa2Cu3O7−x contact during thermal treatment was in situ investigated by a combination of three‐ and four‐terminal resistance measurements. The experimental results suggested that the interaction between Ag and the YBa2Cu3O7−x film began at a temperature of around 370 °C. The contact resistance measurement for different films also demonstrated that the contact property was a strong function of the film quality and surface conditions. The lack of reproducibility in forming a low‐resistance contact to very thin YBa2Cu3O7−x films and the high probability of degrading the film quality after thermal treatment of the contact might be due to the excess Ag doping in YBa2Cu3O7−x. Ag island formation, as revealed by scanning electron microscopy after thermal treatment of the contact, is a limitation of Ag for use as a good contact electrode for very thin superconducting films.

Microstructural analysis and modeling of RuO2 thin film resistors

Abstract The microstructural properties of RuO 2 thin film resistors with different temperature coefficients of resistance (TCRs) were analyzed using X-ray diffraction, scanning electron microscopu and Auger electron spectroscopy. A model based on microstructure of the films was developed to explain the experimental results, such as the negative, positive and near-zero TCRs of the resistors. The grain size controlled the resistivity of the resistors. However, the chemical composition of the film was more likely to control the TCR of the resistors. A layer-like structyre was required to fabricate near-zero-TCR resistors. The modeling based on the electrical measurement of the resistors provided valuable guidance in designing and fabricating near-zero-TCR resistors.