Takuya Sawada

Dielectric Activity and Ferroelectricity in Piezoelectric Semiconductor Li-Doped ZnO

Temperature dependences of dielectric constants, specific heat and D–E hysteresis loops of Li-doped zinc oxide ceramics were investigated. A new dielectric anomaly was found at 330 K in Zn1-xLixO with x=0.17. A cusp-like anomaly was also found in specific heat. A ferroelectric D–E hysteresis loop was successfully observed for the first time. These observations suggest that replacement of host Zn ions by small Li ions induces a ferroelectric phase in the wurtzite-type ZnO piezoelectric semiconductor. This material is a candidate for ferroelectric thin films in integrated ferroelectric devices.

Properties of the Elastic Anomaly in SrBi2Nb2O9-based Ceramics

A study was conducted on the effects of compositional modifications on the temperature coefficient of resonance frequency (TCF) in the disc planar vibration mode for the bismuth layer-structured compound SrBi2Nb2O9 (SBN)-based ceramics. The substitution of the strontium site by divalent and trivalent ions improved the TCF, accompanying the relaxation of elastic anomaly. The tolerance factor, t, was calculated for a pseudo cubic block to study the contribution of the structural factor to Curie temperature, T-C, and the temperature of elastic anomaly, T-ea, for each sample. T-ea showed no apparent tendency with t, whereas T-C increased as t decreased.

Fine-Tolerance Resonator Applications of Bismuth-Layer-Structured Ferroelectric Ceramics

The two-layered SrBi2Nb2O9 (SBN) ceramic resonator that generates second harmonics of the thickness extensional vibration is a strong candidate for fine-tolerance-resonator applications, because of its small electromechanical coupling coefficient and high temperature stability. The Nd substitution of Sr in SBN is effective in further decreasing the temperature coefficient of resonance frequency (TCF) while maintaining its high maximum value of electrical quality factor (Qemax) at a resonance frequency. On the other hand, a grain-oriented SBN-type ceramic resonator has an attractive property of thickness shear vibration. The temperature coefficient of oscillation frequency (TCO) could be less than 10 ppm/°C in an ordinary oscillation circuit with grain-oriented SBN-type ceramics. In this paper, we describe how a fine tolerance resonator can be realized using compositional and structural designs of the bismuth-layer-structured ferroelectric (BLSF) ceramic resonator.

ENERGY TRAPPING CHARACTERISTICS OF BISMUTH LAYER STRUCTURED COMPOUND CABI4TI4O15

Piezoelectric characteristics of bismuth layer structured compound CaBi4Ti4O15 ceramics were studied in terms of the energy trapping phenomenon of the second harmonic thickness extensional (TE2) vibration mode. An electrode-buried-type resonator was prepared by the co-firing technique to generate the TE2 mode, and frequency-lowering-type energy trapping was realized. Favorable resonant characteristics were obtained for piezoelectric filter and oscillator applications with narrow frequency tolerances, which require a small electromechanical coupling coefficient and a high mechanical quality factor. The electromechanical coupling coefficient for the TE2 vibration mode of the resonator was 13.9% and the mechanical quality factor Qm was 2560. The temperature coefficient of the resonance frequency was adequate for the applications. The microstructure of CaBi4Ti4O15 ceramic was also investigated. Marked shape anisotropy of the grain was observed; however, preferential grain orientation was not recognized when the ceramic was prepared by the ordinary firing process.

Piezoelectric Properties of SrBi2Nb2O9 Textured Ceramics

The textured ceramics of the bismuth layer structured ferroelectrics SrBi2Nb2O9 (SBN)-type compound were fabricated by the templated grain growth (TGG) method, and their characteristics and piezoelectric properties were studied. Platelike particles with a large shape anisotropy, which were needed as template particles in the TGG process, were obtained for the composition with 10% Nd substitution on the Sr site of SBN. Highly (00l) textured Sr0.9Nd0.1Bi2Nb2O9 (SNBN) ceramics were successfully obtained using the TGG process with platelike particles, and the orientation degree of the textured ceramics was 96%. The electromechanical coupling factor of the 96% orientation degree textured ceramics in the thickness shear vibration mode was three times as large as that for the randomly oriented ceramics. The temperature coefficient of the resonant frequency (TCF) of the textured ceramics was +20.9 ppm/°C, whose absolute value was two-thirds of that of the randomly oriented ceramics. The resonant frequency of the textured ceramics increased with temperature, while the resonant frequency of the randomly oriented ceramics decreased with increasing temperature. This result indicated the possibility of obtaining an excellent TCF by the optimization of grain orientation. Therefore, textured ceramics with orientation degrees from 54% to 96% were prepared using template particles of different average diameters, and an excellent TCF of -0.4 ppm/°C was obtained for the specimen with an orientation degree of 76%.

Piezoelectric and Ferroelectric Properties of the Modified SrBi 2 Nb 2 O 9 Ceramics

Piezoelectric properties of Bismuth layer compound ferroelectrics SrBi 2 Nb 2 O 9 (SBN) were investigated. Their piezoelectric properties were evaluated from m 150°C to 450°C. Good resonant characteristics were obtained throughout the whole temperature range. Temperature coefficients of the resonance frequency (TCfs) are small, around 20ppm/°C. These good resonant characteristics, especially the good TCfs, are strongly related to the elastic anomaly observed in the temperature range approximately between 250°C and 350°C. When Ba ion substitutes Sr in SBN, the elastic anomaly becomes smoother, and the TCf decreases. SBN also shows a relaxor characteristics with increasing the Ba substitution.

Measurements of SRAM sensitivity against AC power noise with effects of device variation

SRAM exhibits the sensitivity of false operation against static and sinusoidal supply voltage variation. A measurement system combines direct radio frequency (RF) power injection, on-chip monitoring of voltage variation on power supply lines, and built-in self test of memory read/write operations. The bit error rate (BER) of an SRAM core exponentially increases when the lowest instantaneous voltage on the power supply line of SRAM cells during RF injection linearly decreases. Test dice on wafers at five different process corners in a 1.5 V 90 nm CMOS technology were tested. The minimum allowable voltage with BER of less than a single bit failure in average becomes smaller, thus more tolerant, when n-channel devices are at the slow corner in a conventional 6-transistor SRAM cell. The measurement technique enables to experimentally evaluate dynamic noise margin of SRAM cores in a given technology.

Piezoelectric Properties in Textured Ceramics of SrBi2Nb2O9

A study has been done for the piezoelectric properties in texured ceramics of SrBi2Nb2O9 (SBN) fabricated by the templated grain growth (TGG) method. Remanent polarization along to the stacking direction increased with the orientation degrees. That corresponds to the increases in electro-mechanical coupling coefficient with orientation degree. The temperature coefficient of the resonant frequency (TCF) in thickness sheer vibration mode changed from negative to positive with increasing orientation degrees, and an excellent TCF of -0.4 ppm/°C was obtained for the specimen with an orientation degree of 76%.

Temperature Dependence of Piezoelectric Properties for Textured SrBi 2 Nb 2 O 9 Ceramics

Temperature dependences of piezoelectric properties were studied from -50 to 250degC for the textured ceramics of bismuth layer-structured ferroelectrics, SrBi2Nb2O9. Excellent stability of resonance frequency was obtained over a wide temperature range for 76% textured ceramics. The temperature variation of resonance frequency was slight even in a high temperature region above 150degC, and was smaller than that of typical quartz crystal oscillator. Textured SBN ceramics are major candidate materials for resonator applications in a high temperature range.

Temperature dependence of piezoelectric properties for textured SBN ceramics

Temperature dependences of piezoelectric properties were studied for (001) textured ceramics of bismuth layer-structured ferroelectrics, SrBi2Nb2O9 (SBN). The textured ceramics with varied orientation degrees were fabricated by templated, grain-growth method, and the temperature dependences of resonance frequency were estimated. Excellent temperature stability of resonance frequency was obtained for the 76% textured ceramics. The resonance frequency of the 76% textured specimens varied almost linearly over a wide temperature range. Therefore, the variation was slight, even in a high temperature region above 150degC. Temperature stability of a quartz crystal oscillator is generally higher than that of a ceramic resonator around room temperature. The variation of resonance frequency for the 76% textured SrBi2Nb2O9 was larger than that of oscillation frequency for a typical quartz oscillator below 150degC also in this study. However, the variation of the textured SrBi2Nb2O9 was smaller than that of the quartz oscillator over a wide temperature range from -50 to 250degC. Therefore, textured SrBi2Nb2O9 ceramics is a major candidate material for the resonators used within a wide temperature range.