Bismuth Zinc Niobate Thin Films Research Articles

Pyrochlore structure and dielectric properties of bismuth zinc niobate thin films prepared by RF sputtering

Abstract Bi 1.5 Zn 1.0 Nb 1.5 O 7 (BZN) thin films with thickness from 60 nm to 200 nm were prepared by radio-frequency magnetron sputtering and post-annealed from 550 °C to 650 °C. The x-ray diffraction results indicated that the BZN thin films possessed a cubic pyrochlore phase. The BZN thin films exhibited thickness-independent dielectric properties with dielectric constant of ~180 and low loss tangent less than 1% at 10 kHz as the film thickness decreased to 60 nm. The BZN thin films with thickness of 200 nm and post-annealed at 650 °C had a tunability of 32.7% at a DC bias field of 1.5 MV/cm. The results suggest that the BZN thin films have promising applications on the embedded capacitors, tunable devices and energy storage devices.

Bismuth Zinc Niobate Thin Film Multilayer Capacitors with Cu Electrodes Fabricated at Low Temperature by RF Magnetron Sputtering

Thin film multilayer capacitors (MLCs) composed of amorphous Bi1.5Zn1.0Nb1.5O7 (BZN) dielectric layers with Cu internal electrodes were fabricated by radio‐frequency magnetron sputtering at a temperature below 150°C. Both BZN thin films and Cu internal electrodes were deposited in situ through a set of steel shadow masks at room temperature and postannealed at 150°C. The BZN dielectric layers used in the MLCs are amorphous and the thickness for each BZN layer is approximately 220 nm. Metallic Cu layer used as the internal electrode is about 50 nm thick. Auger electron spectroscopy analysis indicates that there are no diffusion between BZN films and Cu electrodes, as well as no oxidation of Cu electrodes during the fabrication process owing to room‐temperature deposition and low‐temperature postannealing (150°C). The thin film MLCs with different number of BZN layers were fabricated. The thin film MLCs with five BZN layers exhibit promising properties with dielectric constant of 72, capacitance density of 1600 nF/cm2, and loss tangent of 5.4% at 10 kHz. These results suggest that the BZN thin film MLCs have potential applications for the embedded PCBs.

Pressure-Dependent Etching Mechanism and Induced Dielectric Properties Variation of BZN Thin Films in SF6/Ar Plasma

Reactive ion etching was used to study bismuth zinc niobate (BZN) thin films etching in SF6/Ar plasma as a function of pressure. The etch rate increases as the pressure increases from 1 to 20 mTorr and decreases when the pressure exceeds 20 mTorr. The film surfaces were analyzed to determine the etching mechanism using X-ray photoelectron spectroscopy. Pressure is found to have an effect on etch reaction and non-volatile etch by-products removal through different ion density and energy, thus resulting in varying compositions, element chemical binding states on the film surface. Dielectric properties of the film, modified by SF6/Ar plasma, show an obvious variation with pressure. The permittivity shows a slight decrease in the range of 1–20 mTorr but a sharp decrease at 30 mTorr. Instead, the dielectric loss variation exhibits an opposite trend.

Effect of excess ZnO on structure and dielectric properties of Bi1.5Zn1.0Nb1.5O7 thin films grown at room temperature by RF magnetron sputtering

Bismuth zinc niobate (BZN) thin films with excess Zn contents were deposited from the ceramic targets containing various excess amounts of ZnO (0–20mol%) on Pt/TiO2/SiO2/Si substrate by RF magnetron sputtering at room temperature. As-deposited thin films were post-annealed at a temperature ≤200°C, which is compatible with PCB substrates. The effect of Zn deficiency and Zn excess on the microstructure, dielectric, and electrical properties of BZN thin films was studied. As-deposited and post-annealed BZN thin films at a temperature ≤200°C were amorphous in nature. An appropriate amount of excess zinc improves the dielectric and electrical properties of BZN thin films, while too much excess zinc leads to deteriorate the properties. BZN thin film with 5mol% excess Zn content exhibits the maximum dielectric constant of 64 with a very low loss of 0.6%, measured at 10kHz and the leakage current of less than 1µA at an applied electric field of 200kV/cm.

Structural and dielectric properties of calcium doped bismuth zinc niobate thin films prepared by pulsed laser deposition at room temperature

(Bi1.5Zn0.1Ca0.4)(Zn0.5Nb1.5)O7 (BZNCa) thin films were prepared by pulsed laser deposition on Pt/TiO2/SiO2/Si substrates at room temperature. The as-deposited films and films post-annealed from 100°C to 175°C for 10min in air are all amorphous. The influence of the post-annealing process on structural and electrical properties was investigated. The surface morphology of BZNCa films remains almost same with the annealing temperature. While the dielectric and leakage-current properties of the films are strongly influenced by the annealing temperature. The film deposited at O2 pressure of 4Pa and then post-annealed at 150°C shows the improved dielectric and leakage-current characteristics. The dielectric constant and loss tangent are 67.2 and 0.03at 100kHz, respectively. The leakage current density is less than 1µA/cm2 at an applied bias electric field of 400kV/cm. The results indicate that calcium doped BZN thin films have potential applications for embedded capacitors.

Interactions in water–THF binary mixture by contact angle, FTIR and dielectric studies

Contact angle, dielectric and IR studies of aqueous tetrahydrofuran (THF) are carried over the entire concentration range. IR studies indicate the formation of blue-shifted hydrogen bonds between water and THF molecules over the entire concentration region. On addition of small quantities of THF to water, THF molecules enter into the hydrogen bonded water network forming clathrate like complexes. Concentration dependent wetting of aqueous THF over various substrates is also reported. The effect of clathrate formation on the wetting properties of the substrate has also been explored. The clathrate structures formed are seen to influence the wetting properties on moderately hydrophobic substrates like Indium tin Oxide (ITO), Cubic Bismuth Zinc Niobate thin film on sapphire substrate (CBZN), Yttrium-stabilized zirconia (YSZ), Strontium Titanate (STO) and Si wafer. Contact angles decreases with an increase in THF concentration except in the region (0.03≤XTHF≤0.05 and 0.07≤XTHF≤0.09) where clathrate structure appears and hence no change of contact angle is seen.

Microwave dielectric properties of bismuth zinc niobate thin films deposited on alumina by pulsed laser deposition

Abstract Bi2Zn2/3Nb4/3O7 thin films were prepared on Al2O3 substrates by pulsed laser deposition. The phase compositions and microstructures were characterized by X-ray diffraction and atomic force microscopy. The as-deposited films were all amorphous in nature. All films were crystallized after the post annealing at the temperature range of 700–900 °C for 30 min in air. The texture characteristics change with annealing temperature. A split post dielectric resonator method was used to measure the microwave dielectric performance at the resonant frequencies of 10, 15 and 19 GHz. For the films annealed at 900 °C, the preferential orientation is similar to the monoclinic BZN bulk. The microwave dielectric constants at 10, 15 and 19 GHz are 69.4, 58.9 and 47.9, respectively, which are closer to these of the monoclinic BZN bulk.

A novel antifuse structure based on amorphous bismuth zinc niobate thin films

A novel antifuse structure with amorphous bismuth zinc niobate (a-BZN) dielectrics was proposed. The characteristics of the a-BZN antifuse were investigated. Programming direction of up to down was chosen to rupture the a-BZN antifuse. The breakdown voltage of the a-BZN antifuse was obtained at a magnitude of 6.56 V. A large off-state resistance of more than 1 GΩ for the a-BZN antifuse was demonstrated. The surface micrograph of the ruptured a-BZN antifuses was illustrated. Programming characteristics with the programming time of 0.46 ms and on-state properties with the average resistance value of 26.1 Ω of the a-BZN antifuse were exhibited. The difference of characteristics of the a-BZN antifuse from that of a cubic pyrochlore bismuth zinc niobate (cp-BZN) antifuse and gate oxide antifuse was compared and analyzed.

Effects of SF6/Ar gas-mixing ratio on the etching behavior and properties of BZN thin films

Bismuth zinc niobate (BZN) thin films were etched as a function of the SF6/Ar gas-mixing ratio in a reactive-ion etching (RIE) system. The etching characteristics of surface etch rate and compositions were investigated. Within the ratio range of choice, the etch rate of BZN films is found to decrease first and then increase as the ratio decreases. The small error bars shown in the line graph present a high reliability of the etch rate data. BZN films surfaces were investigated by X-ray photoelectron spectroscopy (XPS). Metal fluorides were found to remain in the surface, resulting in varying relative atomic percentages with gas-mixing ratio. Zn-rich surfaces were formed for ZnF2 residues were hard to remove. Bi and Nb can be removed easily through chemical reactions because of their high volatility, while BiF and NbF can be still detected using narrow scan spectra, which were thought to be present in the form of a metal-oxy-fluoride (Metal-O-F). Gas-mixing ratio is found to have an effect on etch reaction and the removal of residues through different plasma ratios etch process, therefore, leading to varying compositions and element chemical binding state, and resulting in the AFM surface morphology and dielectric property variation. The minimum value of F atomic concentration is achieved at SF6/Ar ratio of 32/13, while the lower surface roughness and the greater dielectric constant both appear at the ratio of 40/5.

Realization of Tunable Varactors Using Bi<sub>1.5</sub>Zn<sub>1.0</sub>Nb<sub>1.5</sub>O<sub>7</sub> Thin Films

Cubic pyrochlore bismuth zinc niobate thin films are known to exhibit voltage dependent dielectric properties. In this paper, we are demonstrating the fabrication and characterization of interdigital (IDC) and circular patch (CPC) capacitors using the pulsed laser deposited Bi1.5Zn1.0Nb1.5O7 (BZN) thin films on sapphire and platinised silicon substrates respectively. The IDCs fabricated are having 12 fingers of width 12 m each and separated by a gap of 8 m. The CPC are having circular patches with inner radius of 100 m and concentric ground plane with a radius of 300m. The electrical properties of these capacitors were characterized both at low frequencies and at microwave frequencies. The CPC varactors were having a tunability of 25% at 15 Volts. The calculated capacitance of the IDC varactor at 5.3 GHz with 0 V dc bias was 1.1 pF, which has got changed to 0.99 pF by the application of 30 Volts exhibiting a tunability of 10%.

Low-loss tunable capacitors fabricated directly on gold bottom electrodes

At microwave frequencies, conductor losses due to the bottom electrode resistance severely limit the performance of metal-insulator-metal capacitors that employ tunable dielectric thin films. Here we demonstrate that a novel tunable dielectric, bismuth zinc niobate (BZN), can be integrated directly with low-resistivity Au bottom electrodes. The favorable crystallization kinetics allowed for a low thermal budget process compatible with Au electrodes. BZN thin films on Au bottom electrodes showed low dielectric loss tangents of ∼0.0005 and high dielectric tunabilities of ∼50%. The Au/BZN interface was abrupt and free of reaction phases. At high frequencies (>1MHz) the total Au/BZN capacitor device loss was reduced compared to capacitors with Pt bottom electrodes. The low device losses of Au/BZN capacitors revealed a device geometry-dependent loss mechanism that contributed significantly to the device loss at high frequencies.

BZN THIN FILM CAPACITORS FOR MICROWAVE LOW LOSS TUNABLE APPLICATIONS

ABSTRACT Bismuth zinc niobate (BZN), a high dielectric constant material that combines low dielectric losses with an electric-field dependent permittivity, was implemented as the dielectric in metal-insulator-metal (MIM) capacitors. Dielectric properties up to 20 GHz were evaluated by measuring reflection coefficients with a vector network analyzer. The dielectric constant was around 180 and the quality factor remained greater than 100 up to 5 GHz for 64 μ m2 devices. No onset of dielectric relaxation, as seen in bulk ceramic data, could be detected in the measured frequency range. The results show that BZN thin films have great potential for low loss, tunable microwave devices.

Temperature dependence of the dielectric tunability of pyrochlore bismuth zinc niobate thin films

The change in permittivity of bismuth zinc niobate (BZN) films with the cubic pyrochlore structure under an applied electric field was measured as a function of temperature. Dielectric measurements were performed using parallel-plate capacitor structures with Pt electrodes on sapphire substrates. The electric field tunability of the permittivity was weakly temperature dependent and increased with decreasing temperature up to the onset of dielectric relaxation. At temperatures below the onset of the dielectric relaxation (∼150K at 1MHz), larger electric fields were required to achieve the highest tunabilities. A simple model of hopping, noninteracting dipoles was not suited to describe the dielectric tunability of BZN thin films. A better description of the experimentally observed behavior at temperatures above the onset of the dielectric relaxation was obtained using a simple random-field model with hopping dipoles in a uniform distribution of random fields.

Influence of strain on the dielectric relaxation of pyrochlore bismuth zinc niobate thin films

Bi 1.5 Zn 1.0 Nb 1.5 O 7 (BZN) films were deposited by rf magnetron sputtering on different substrates to systematically vary the film stress due the thermal mismatch between BZN and the substrate. Substrates included Pt/SiO2 covered silicon, vycor glass, magnesium oxide, and sapphire. The BZN film microstructures (orientation, grain size, and roughness) were similar on the different substrates. Measurements of the permittivity and dielectric loss tangent were carried out between 80 and 300 K at frequencies between 10 kHz and 10 MHz. Films that were under a moderate tensile stress showed a low-temperature dielectric relaxation, associated with a dielectric loss peak and drop in permittivity, at ∼100 K. In contrast, the dielectric relaxation was shifted to temperatures below 80 K in films on vycor that were under a large tensile stress. This shift reflected a lowering of the activation energy of the dielectric relaxation processes due to tensile stress. It is expected that films under large tensile stress require higher frequencies than bulk BZN to shift the dielectric relaxation to room temperature, which makes these films attractive for low-loss high-frequency applications.

Composition control and dielectric properties of bismuth zinc niobate thin films synthesized by radio-frequency magnetron sputtering

Radio-frequency sputtering was used to deposit near-stoichiometric Bi1.5Zn1.0Nb1.5O7 (BZN) films. Composition, crystallinity, and phase purity of the films were investigated by Rutherford backscattering spectrometry, x-ray diffraction, and transmission electron microscopy (TEM). X-ray diffraction detected cubic pyrochlore in films that were annealed above 400 °C. Films were fully crystallized at 750 °C. TEM and electron diffraction confirmed the cubic pyrochlore structure of the grains. Dielectric constant and loss were measured using planar Si/SiO2/Pt/BZN/Pt and Al2O3/Pt/BZN/Pt capacitor structures, respectively. After annealing at 750 °C, films on Pt coated silicon wafers showed a permittivity of 170, low dielectric losses, and a large electric field tunability of the dielectric constant at a measurement frequency of 1 MHz. Dielectric loss tangents improved when substrates were moderately heated during deposition.