Kazuya Niki

Silicon Beam Resonator Utilizing the Third-Order Bending Mode

A silicon beam resonator utilizing the third-order bending mode is designed and fabricated. It has three driving electrodes for increasing the amplitude of the third-order mode. The mechanical vibration modes of the beam are measured using a laser-Doppler vibrometer, and the electrical characteristic is evaluated with a network analyzer. Because the in-plane vibration is caused by the electrostatic force exerted on a gap between the beam and each driving electrode, the amplitude of the third-order mode in the in-plane vibration can be enhanced by placing three driving electrodes along a resonant beam. The measured resonant frequencies well agree with the simulated ones. From the measurement of the third-order mode in the in-plane vibration with a network analyzer, it has been shown that resonant frequency decreases by 2.3 kHz as DC voltage increases from 30 to 70 V owing to the spring softening effect. The DC bias dependence agrees well between the electrical and mechanical measurements. Finally, the mechanism of inducing an out-of-plane vibration is discussed from a viewpoint of the influence of the electric field generated on a substrate.

Microwave Detector Using Granular-Type YBCO Superconductors

A highly sensitive microwave detector has been developed for the first time by granular-type bulk YBCO superconductors with fine particles. Particles with a size of about 1 µm, having a SNS-like grain boundary, were controlled by a coprecipitation method, followed by a sintering process. A new fabrication method for bulk superconductors was also developed to produce a precise microbridge-type structure. Sensitivity to microwave power below -40 dBm was obtained, and a direct mixing effect was obtained at a frequency of 8.6 GHz at the temperature of 77 K. It was confirmed that the Josephson effect of the detector at the SNS grain boundary formed in the sintering process for ceramics is the most responsive for microwave detection.

Microelectromechanical Systems Resonator Utilizing Torsional-to-Transverse Vibration Conversion

A silicon microelectromechanical systems (MEMS) resonator utilizing the torsional-to-transverse vibration conversion is designed, fabricated and evaluated. The resonant frequency for the torsional modes mostly depends on only beam length, providing a large tolerance in the fabrication process. It has been, however, a critical issue to investigate the mechanism for generating the torsional vibration and the reduction of motional resistance. We propose a new beam structure, in which four torsion beams are vibrated by twist force generated by a transverse beam. The novel process for fabricating resonators provides a narrow gap surrounded by flat surfaces, which can reduce the motional resistance. The fabricated resonators are measured with a laser-Doppler (LD) vibrometer. The scanning function of the LD vibrometer confirms the torsional-to-transverse vibration conversion has been successfully achieved. The measured resonant frequency, 10.96 MHz, is in good agreement with the simulated one. The Q-factor has been also measured to be as high as 2.2 ×104 in vacuum. The electrical characteristic is evaluated with an impedance analyzer. At the resonant frequency, the extracted motional resistance for the 0.5-µm-gap resonator is 2.0 MΩ, which is greatly reduced, owing to the narrow gap effect, from that of the 1-µm-gap resonator. The temperature coefficient of the resonant frequency between -40 and 85 °C, has been measured to be -24.4 ppm/deg. The resonant frequency linearly decreases as the temperature rises.

YBCO-BiO Composite Superconductor for Granular-Type Josephson Microwave Detector

We have developed a new YBCO-BiO composite superconductor for granular-type Josephson microwave detectors. This composite superconductor was prepared by doping YBa2Cu3Ox with Bi2O3 and it was found that the normal state resistance increased with the amount of Bi2O3 doping up to 15 wt.%, without deterioration at the zero-resistance temperature. BaBiO3 was segregated at the interface of the YBCO particles during sintering at a temperature of about 940°C. This seemed to enhance the resistance of the grain boundary and improve the microwave detection responsivity. Practical responsivity (RMv) of 180 V/W and noise equivalent power (NEP) of about 1.4×10-14 W/√Hz were obtained using a YBCO-BiO detector with 15 wt.% Bi2O3 doping.

Heterodyne Detection of Microwaves Using Granular-Type Y1Ba2Cu3O7-x Superconductors

A heterodyne mixing experiment of microwaves at about 20 GHz was performed using a microwave detector made from granular-type Y1Ba2Cu3O7-x (YBCO) superconductors. Although the bridge included many boundaries, voltage steps corresponding to the Josephson frequency and step height dependence for microwave power similar to those of a single weak-link bridge were clearly observed. The IF signal showed peak amplitudes corresponding to the peak dynamic resistantance due to nonlinearity resulting from the Josephson effect. It was confirmed for the first time that this detector operates stably for the heterodyne mixer at 60 K or below and has a potential for high sensitivity due to its large dynamic resistance.

High Quality Factor 80 MHz Microelectromechanical Systems Resonator Utilizing Torsional-to-Transverse Vibration Conversion

A silicon microelectromechanical systems (MEMS) resonator utilizing torsional-to-transverse vibration conversion with quarter-wavelength torsional support beams is designed, fabricated, and evaluated. The resonant frequency for torsional modes mostly depends only on beam length, providing a large tolerance in the fabrication process. However, the following have remained critical issues: the increase in the quality factor (Q-factor) and the reduction in the motional resistance. We propose a new beam structure, in which the MEMS resonator utilizing torsional-to-transverse vibration conversion is anchored by four quarter-wavelength torsional support beams. First, the fabricated resonators are measured with a laser-Doppler (LD) vibrometer. The measured resonant frequency of 78.224 MHz has been in good agreement with the simulated one. The Q-factor has also been measured to be as high as 3.0×104 in vacuum. Then, the electrical characteristic is evaluated with an impedance analyzer. The Q-factor has been electrically measured to be as high as 3.1×104 in vacuum, which agrees well with the mechanically measured one of 3.0×104. The Q-factor has also been electrically measured to be as high as 1.3×104 at atmospheric pressure. In the measurement, a spring softening effect has been clearly observed. By increasing the DC bias voltage from 20 to 40 V, the resonant frequency has decreased by 640 Hz. The extracted motional resistance for a 0.1-µm-gap resonator has been greatly reduced to 0.039 MΩ at 5 V DC, owing to the narrow-gap effect, from that of a 0.25-µm-gap resonator. The tolerance in the fabrication process has also been evaluated and successfully verified from the measurement of the fabricated MEMS resonators.

Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub /spl delta// thin films on MgO annealed in a sealed capsule

High-quality c-axis oriented Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub /spl delta// thin films have been prepared on MgO, which is more suitable for high frequency applications than LaAlO/sub 3/ because of its lower dielectric constant (/spl epsi/) and smaller dielectric loss tangent (tan /spl delta/). Annealing of the thin films was carried out in an alumina capsule sealed with a gold gasket. It was revealed that preannealing of the pellets for Tl compensation is effective in improving the structural properties of the thin films. X-ray diffraction (XRD) rocking curve scans of Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub /spl delta// (0 0 12) reflection revealed that a full width of half maximum (FWHM) of thin films annealed with an unpreannealed pellet is 0.89/spl deg/, while that of thin films annealed with a preannealed pellet is 0.22/spl deg/. The highest critical temperature (T/sub c/) is 108 K and the highest critical current density J/sub c/ is 2/spl times/10/sup 6/ A/cm/sup 2/ at 77 K.

Ka-band performance of step-edge Josephson junctions using Tl2Ba2CaCu2Ox films on MgO substrates

Abstract Tl 2 Ba 2 CaCu 2 O x (Tl-2212) step-edge Josephson junctions (SEJJs) on MgO substrates have been frabricated and tested for millimeter wave applications. Although the single SEJJs exhibited clear Shapiro steps under Ka-band (27 to 40 GHz) irradiation and a Fraunhofer-like pattern under a magnetic field, their responsivities as Ka-band detectors were too small to construct a mixing device. This is due to their small normal resistance ( R n ) of 0.2 to 0.9 Ω, which determines the impedance matching for the microstrip signal lines. Therefore, series-connected six SEJJs (6-SEJJs) have also been fabricated to improve the impedance matching. Their responsivities are 2–3 orders of magnitude larger than those of single SEJJs. Furthermore, efficient IF (intermediate frequency) output of fundamental mixing with low LO (local oscillation) power of a few μW is observed up to 95 K.

Enhanced mode detection for microwaves using a high-impedance YBCO-BiO composite superconductor

Abstract We have developed a Josephson type microwave detector using a YBCO-BiO composite superconductor which uses the non-linear effect produced by a complex network of Josephson weak links at the grain boundary (enhanced mode detection). This composite superconductor was prepared by doping YBa2Cu3Ox with Bi2O3, which causes high normal state resistance in the composite and enhances the Josephson effect at the grain boundary. It was found that the measured responsivity (RMV) improved by increasing the normal resistance (Rn) of the detector through enhanced matching of the incident microwaves and the detector. An RMV of 180 V/W and NEP of 1.4×10−14 W/Hz 1 2 were obtained with a detector having an Rn of 13 Ω. These results suggest the feasibility of a practical detector having an RMV of more than 104 V/W which operates at 60 K by the enhanced mode detection of a high-TC superconductor.

Effects of Bi2O3 Incorporation into YBCO Superconductors for Microwave Detector

The micro-structure of YBCO-BiO composite superconductors for high- impedance microwave detectors has been studied. The results of XRD measurement and EPMA showed that both Ba2YBiO6 and CuO coexisted in the YBa2Cu3O7−δ (abbreviated to YBCO) grain boundaries. Ba2YBiO6 has a similar structure to BaBiO3, and an insulating-resistivity which would appear to confer high normal resistance and high sensitivity on microwave detectors in which these YBCO-BiO composite superconductors are used. As the Bi203 doping content increased, a Fraunhofer-like pattern in a relation between the Ic and the magnetic field was observed in device samples having a micro-bridge with 100 µm thick, 100 µm wide and 300 µm long. This suggested that the proportion of Josephson junction pass to super-current pass in conduction networks of YBCO-BiO superconductors was increased by Bi2O3 doping.