Hee Bum Hong

Voltage tunable dielectric properties of rf sputtered Bi2O3-ZnO-Nb2O5 pyrochlore thin films

Abstract The Bi2O3-ZnO-Nb2O5 (BZN) pyrochlore thin films were prepared on platinized Si substrates using a reactive rf magnetron sputtering. The structures, surface morphologies, dielectric properties and voltage tunable dielectric properties of films with deposition parameters were investigated. The sputter deposited BZN pyrochlore thin films have a cubic pyrochlore phase and secondary phases of zinc niobate and bismuth niobate when crystallized at 600∼800 °C. The dielectric constant and tunability of thin films changed with an O2/Ar ratio and post-annealing temperature. The BZN pyrochlore thin films sputtered in 15% O2 and annealed at 700 °C had a dielectric constant of 153, tan δ of ∼0.003 and maximum tunability of 14% at 1000 kV/cm of dc bias field under 1 MHz.

Structural Transition and Microwave Dielectric Properties of ZnNb2O6–TiO2 Sintered at Low Temperatures

The addition of CuO to ZnNb2O6–TiO2 significantly lowered the sintering temperature to ~900°C in comparison to ~1200°C for samples without CuO. The effect of CuO additions on the phase transitions and microwave dielectric properties of 0.42ZnNb2O6–0.58TiO2 has been investigated using X-ray powder diffraction and a network analyzer. The structural transition from columbite and rutile to ixiolite was influenced by both the sintering temperature as well as the amount of CuO. These influences, however, were competing. Therefore, it is assumed that the different constitutions of crystal structure of ZnNb2O6–TiO2 samples due to increase either the sintering temperature or the amount of CuO lead to considerable variation in the microwave dielectric properties. The τf was modified to ~0 ppm/°C, as a result of the variation in phase constitutions as a function of composition.

The Reversible Phase Transition and Dielectric Properties of BaNb2O6 Polymorphs

Barium metaniobate, BaNb2O6 with polymorphic orthorhombic and hexagonal forms was prepared by solid-state reaction. Hexagonal BaNb2O6 is the low temperature phase, and can be transformed to the orthorhombic phase over a broad temperature range above 1150°C. It was observed that the hexagonal-to-orthorhombic phase transition is reversible due to the structural similarity of these two polymorphs. The difference in transition kinetics and associated mechanisms could be confirmed from the analysis of time dependence of transformed phase. We also found the dielectric properties of each phase to be significantly different. In particular, orthorhombic BaNb2O6 has good microwave dielectric properties: Q×f=43,000 GHz, er=30, τf=-45 ppm/°C.

Microwave Dielectric Properties of Bi2(Zn1/3Ta2/3)2O7 Polymorphs

Polymorphic phases of Bi2(Zn1/3Ta2/3)2O7 pyrochlore were studied. A metastable monoclinic structure was observed at temperatures below 950°C. This low-temperature phase possesses highly temperature-stable characteristics in the microwave frequency range (5–7 GHz) with a temperature coefficient of resonant frequency (τf)=-8.8 ppm/°C. The addition of B2O3 to Bi2(Zn1/3Ta2/3)2O7 significantly lowered the sintering temperature, to 850°C, without an associated structural transition. Low-temperature sintering mechanisms have been investigated using dilatometry and transmission electron microscopy. Bi2(Zn1/3Ta2/3)2O7 with 0.5 wt% B2O3 can be suitable for application in low-temperature cofired ceramics (LTCC), due to its reduced sintering temperature and good microwave dielectric properties: quality factor (Q×f)=3500 GHz at 6.7 GHz, dielectric constant (er)=63.9 and τf=-14.1 ppm/°C.

RF Sputtered BZN Pyrochlore Thin Films for Voltage Tunable Dielectric Device Applications

The BZN pyrochlore thin films were prepared on platinized Si substrates using a reactive RF magnetron sputtering. The structures, surface morphologies, dielectric properties and voltage tunable properties of films with deposition parameters were investigated. The BZN thin films have a cubic pyrochlore phase and secondary phases of zinc niobate, bismuth niobate when crystallized at 6001C - 8001C. The dielectric constant and tunability of thin films are O2/Ar ratio and post-annealing temperature dependent. The BZN thin films sputtered in 15% 02 and annealed at 700°C had a dielectric constant of 153, tan 8 of -0.003 and maximum tunability of 14% at 1,000kV/cm.