T. Shimamura

NTT DOCOMO, Edo, Tōkyō, Japan

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Publications (32)18.64 Total impact

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    ABSTRACT: In this paper, we describe a novel structure of a vibrational micro-electro-mechanical system (MEMS) device for power generation enhancement. A synchronized multiple-array vibrational device, in which movable plates are connected by rods, increases the area of the movable plate in the energy conversion region and couples the phase of movement. The fabricated device resonates at approximately 1430 Hz with an acceleration amplitude of 6 m/s2 and nanoampere-order AC current is generated. These results confirm that this MEMS vibrational device will contribute to the progress in energy harvesting.
    Japanese Journal of Applied Physics 05/2012; 51(5). · 1.07 Impact Factor
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    ABSTRACT: This paper describes an analysis and a performance limit of a vibrational energy harvester with a novel slit-and-slider structure. This structure has a separable electret and microelectromechanical systems (MEMS) parts. In the MEMS parts, movable electrodes slide due to external vibration and receive electrical field that is periodically modulated by slits of fixed electrodes. The structure was fabricated based on MEMS technology and produced an ac current of 170 pA with an external vibration of amplitude of 1 m/s2 at a frequency of 1166 Hz. Since the structure is separable, individual characterization of the electret and movable electrodes was performed. On the basis of their quantitative analyses, a structural model was constructed and validated. The model showed a way to optimize structural and material parameters for enhancement of output power and predicted a performance limit of 2.5 × 10-3 μW and 6.1% as output power and harvester effectiveness, respectively. This value of effectiveness is comparable to that of conventional non-MEMS-based large energy harvester around 1 cm3, which indicates feasibility of MEMS-based small energy harvesters around 0.01 cm3 by appropriate designing.
    Journal of Microelectromechanical Systems 01/2012; 21(5):1218-1228. · 2.13 Impact Factor
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    ABSTRACT: This paper describes a 1-cm3 event-driven wireless sensor node (WSN) consisting of a solar battery with a capacitor, vibration sensor, wireless circuit, and antenna. The WSN transmits data using energy in the millimeter-size capacitor charged by the solar battery. The concept of the wireless transmission scheme is to not only reduce 1-bit energy while transmitting data continuously but also to reduce all energy of “one-time data transmission” from circuit start-up to the end of data transmission. We have already fabricated and evaluated a wireless chip that uses ring oscillators as clocks and transmits OOK pulse trains. In this paper, we show that these architectures are suitable for reducing energy of “one-time data transmission” and for WSN operation by limited energy because they can reduce the start-up time and activated time in 1-bit symbol of RF circuits. The fabricated sensor node can transmit 36-bit data 3 m by charged energy of 8 μJ in the capacitor when the sensor detects the event. This enables us to achieve 1-cm3 WSN and paves the way to the future millimeter-size battery less WSNs.
    Communication Systems (ICCS), 2012 IEEE International Conference on; 01/2012
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    ABSTRACT: This paper describes techniques for an impedance-sensing circuit integrated into a capacitive fingerprint sensor to prevent spoofing with a fake finger. We have reported a sensor chip with an embedded impedance-sensing function. We proposed an impedance-sensing circuit that features current-to-voltage conversion using a unity gain buffer. Here, the design of the sensing circuit is discussed. The detectable impedance range and the sensitivity are analyzed within the impedance range for various human fingers. A test chip with the proposed circuit was fabricated using 0.5-$\mu{\hbox {m}}$ CMOS/sensor processes. The results confirm that the difference in impedance between a real finger and a fake finger is detected without any degradation of the original characteristics of the fingerprint sensor chip.
    IEEE Sensors Journal 01/2012; 12(5):1393-1401. · 1.85 Impact Factor
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    ABSTRACT: Design and Performance of a Sub-Nano-Ampere Two-Stage Power Management Circuit in 0.35-µm CMOS for Dust-Size Sensor Nodes
    IEICE Transactions. 01/2011; 94-C:1206-1211.
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    ABSTRACT: A power management scheme using a MEMS switch is proposed as a way to accumulate charge even when the energy-harvesting device generates minute current of
    01/2011;
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    ABSTRACT: This paper describes a millimeter-sized electret vibrational energy harvester fabricated by a MEMS-based process for power generation enhancement. To increase current generation, we developed a new “slit-and-slider” structure with horizontal arrays and vertical protrusions. This structure enables us to increase an area and to narrow a gap of energy conversion in the limited size. The structure was fabricated by gold-electroplating. A lower chip with a movable part and an upper chip with a fixed part were aligned by chip-on-chip mounting. The device resonated around 1430 Hz with an acceleration amplitude of 6 m/s 2 . With an ethylene-tetrafluoroethylene
    01/2011;
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    ABSTRACT: This paper describes a new capacitive-sensing circuit technique that improves the quality of images captured with capacitive fingerprint sensor LSIs. The quality of the captured image depends on the surface condition of the finger. When the finger is dry, the electrical resistance of the finger surface is high. The finger surface resistance induces a voltage drop in the electrical potential of the finger surface (which should be grounded), which leads to poor image quality. To capture clear images even when the finger is dry, the circuit technique improves the image quality using the series resistance caused at the finger surface. The potential of the finger surface is controlled by an enhancement plate and a voltage control circuit. A test chip implementing this technique was fabricated on a 0.5 ¿m CMOS process and a sensor process. The chip captures a clear fingerprint image of a dry finger, confirming the effectiveness of the circuit technique for capturing clear fingerprint images independent of the finger surface condition with the capacitive fingerprint sensor LSIs.
    IEEE Journal of Solid-State Circuits 06/2010; · 3.06 Impact Factor
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    ABSTRACT: Nano-watt-power circuit techniques that accumulate energy from nanoampere-level currents and continuously sense vibration are used for dust-sized batteryless sensor nodes. A chip is fabricated using a 0.35 ¿m CMOS process. The vibration is sensed with 0.7 nW and the energy is accumulated from a 1 nA current source with voltage monitoring.
    Solid-State Circuits Conference Digest of Technical Papers (ISSCC), 2010 IEEE International; 03/2010
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    ABSTRACT: This paper presents experimental proof of energy harvesting with MEMS-based vibrational devices. To clarify the effect of the coupling of vibration of MEMS devices and electrical field of electrets for current generation, a slit-and-slider structure was developed. This structure enables to combine the MEMS devices and electrets after their vibrational and electrical properties have been characterized separately. In the fabrication, the movable gold electrodes were released and then aligned by chip-on-chip mounting to face the fixed electrodes. A 1-mm-square electrode resonated around 1166 Hz with external vibration of a magnitude of acceleration of 1 m/s<sup>2</sup>. The surface and rear potentials of the electret were around -150 and +100 V, respectively. The coupling of the mechanical and electrical properties produced an AC current of 170 pA. The results pave the way to designing and fabricating small-sized vibrational energy harvesters.
    Solid-State Sensors, Actuators and Microsystems Conference, 2009. TRANSDUCERS 2009. International; 07/2009
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    ABSTRACT: A fingerprint sensor that integrates fraud detection and fingerprint sensing to prevent spoofing with a fake (artificial) finger is presented. Fingerprint identification using capacitive fingerprint sensing provides small user-authentication systems. For systems that need a higher level of security, fraud detection, which determines whether the sensed finger is alive or not, is necessary. Integrating fraud detection capability into a capacitive sensor is important because attempted fraud has to be detected at the same time that the fingerprint is captured. Various methods that use information about a finger, such as its electrical characteristics, optical characteristics or elastic characteristics, have been tried. Impedance-sensing is suitable from the viewpoint of using electrical signals. An impedance-sensing circuit should not increase the chip size nor degrade the quality of the captured fingerprint image. To meet these requirements, we propose an impedance-sensing scheme built into a capacitive sensor and implemented as a circuit and electrode without changing the chip size.
    Solid-State Circuits Conference, 2008. ISSCC 2008. Digest of Technical Papers. IEEE International; 03/2008
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    ABSTRACT: In this paper, we describe a new capacitive-sensing scheme that detects the displacement of a movable structure for the control of a microelectoromechanical-systems (MEMS) device stacked on a complementary metal-oxide-semiconductor (CMOS) LSI. The problem is that the small capacitance of the air gap between the movable element and a sensor plate needs to be detected in spite of the large parasitic capacitance caused by LSI interconnections. The solution is a sensing circuit that features a shield plate and a ramp detection circuit. The shield plate separates the sensed capacitance from the parasitic capacitance. A ramp detection circuit enhances the detection sensitivity using a gradient signal generated by the parasitic capacitance. To check the effectiveness of the scheme, a sensing circuit and a MEMS variable capacitor were fabricated in a 0.6-mum CMOS process and a MEMS process. This scheme enhanced the sensitivity, which is the ratio of the output voltage to the sensed capacitance, by a factor of six. These results demonstrate that this scheme is suitable for the control of a CMOS-MEMS device.
    Japanese Journal of Applied Physics 01/2008; 47:3418-3422. · 1.07 Impact Factor
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    ABSTRACT: The authors performed a scratch test in which a weighted needle applies scratch stress to the surface of a capacitive fingerprint sensor large scale integration (LSI), which has a grounded wall (GND wall) structure where each sensor plate is surrounded by a lattice-like wall. The scratch stress degrades not only the sensor's surface but also the metal interlayer. Increasing the thickness of the surface passivation film and that of the interlayer reduce the degradation of the surface and inner structures. To confirm the influence of thick passivation film on the electrostatic discharge (ESD) hardness and the intensity of sensing of the fingerprint sensor LSI, the authors performed an air discharge test and fingerprint identification. The tests show that a thick passivation film and a thick interlayer are effective in preventing scratch stress with ESD hardness and the intensity of sensing of the fingerprint sensor LSI.
    Reliability physics symposium, 2007. proceedings. 45th annual. ieee international; 05/2007
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    ABSTRACT: This paper describes a novel RF CMOS-MEMS switch that integrates RF MEMS switches and CMOS control circuits. A single-pole 8-through RF CMOS-MEMS switch was fabricated and its operation at 3.3 V supply voltage was achieved. The switch was encapsulated with a thin film at wafer level to prevent destruction during packaging. Experimental results confirm that the switch has mechanical reliability for more than 1 billion cycles
    Electron Devices Meeting, 2006. IEDM '06. International; 01/2007
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    ABSTRACT: This paper describes logic and analog test schemes that improve the testability of a pixel-parallel fingerprint identification circuit. The pixel contains a processing circuit and a capacitive fingerprint sensor circuit. For the logic test, we propose a test method using a pseudo scan circuit to check the processing circuits of all pixels simultaneously. In the analog test, the sensor circuit employs dummy capacitance to mimic the state of a finger touching the chip. This enables an evaluation of the sensitivity of all sensor circuits on logical LSI tester without touching the chip with a finger. To check the effectiveness of the schemes, we applied them to a pixel array in a fingerprint identification LSI. The pseudo scan circuit achieved a 100&percnt; failure-detection rate for the processing circuit. The analog test determines that the sensitivities of the sensor circuit in all pixels are in the proper range. The results of the tests confirmed that the proposed schemes can completely detect defects in the circuits. Thus, the schemes will pave the way to logic and analog tests of chips integrating highly functional devices stacked on a LSI.
    01/2007;
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    ABSTRACT: In this paper, we propose a technique of preventing both wet-release-related and in-use sticking of actuators in microelectromechanical system (MEMS) devices. The technique involves the electrodeposition of a water-repellent organic dielectric film that renders the microstructure surface inactive towards the water used for rinsing. The source material is a core/shell emulsion, which consists of sulfonium cations with epoxy groups containing water-repellent silicone polymers. Applying this technique to the encapsulation of a microstructure confirms its effectiveness in preventing both release-related sticking and in-use sticking of a MEMS structure.
    Japanese Journal of Applied Physics 01/2007; 46:6454-6457. · 1.07 Impact Factor
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    ABSTRACT: This paper presents fingerprint image enhancement and rotation schemes that improve the identification accuracy with the pixel-parallel processing of pixels. In the schemes, the range of the fingerprint sensor is adjusted to the finger state, the captured image is retouched to obtain the suitable image for identification, and the image is rotated to the correct angle on the pixel array. Sensor and pixel circuits that provide these operations were devised and a test chip was fabricated using 0.25-μm CMOS and the sensor process. It was confirmed in 150,000 identification tests that the schemes reduce the false rejection rate to 6.17&percnt; from 30.59&percnt;, when the false acceptance rate is 0.1&percnt;.
    01/2006;
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    ABSTRACT: In this paper, we describe the application of electrodeposition to microelectromechanical system (MEMS) devices made of gold. After the release ashing of an organic sacrificial layer by oxygen plasma exposure, the thickness of the organic dielectric film electrodeposited on gold surfaces was found to be nonuniform. A surface analysis by X-ray photoelectron spectroscopy revealed that the causes of the nonuniformity were the differences in chemical states and amounts of gold oxide that arose from oxygen plasma exposure at different places. A uniform coating was obtained by removing the gold oxide by hydrochloric acid dipping before electrodeposition on gold comb-shaped electrodes in a vibrational MEMS device. This uniform coating prevents electrical shorts and in-use sticking between the gold vibrator and electrode in the device, ensuring high durability.
    Japanese Journal of Applied Physics 01/2006; 45:5646-5649. · 1.07 Impact Factor
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    ABSTRACT: In this paper, we describe a novel packaging technology for use in microelectromechanical-system (MEMS) device fabrication and its application to an optical MEMS micromirror array that consists of a mirror chip and an electrode chip that controls the micromirrors. Chip-on-chip technology, which involves the selective deposition of silver paste by screen printing, is used to join the two chips. The experimental results ensure a high production yield and heat cycle test results indicate good reliability.
    Japanese Journal of Applied Physics 01/2005; 44:8177-8181. · 1.07 Impact Factor
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    ABSTRACT: This paper describes a novel fabrication process for control electrodes which enables a micromirror array to be fabricated on LSIs. Electrodes designed to reduce drive voltage of the micromirror can be fabricated flexibly. Novel anti-sticking technology, in which a polyimide film is deposited only on control electrodes, prevents sticking between mirrors and electrodes. We fabricated a 100-ch micromirror array. The maximum rotational angle of 1° at 30 V was achieved using our developed technology.
    Electron Devices Meeting, 2003. IEDM '03 Technical Digest. IEEE International; 01/2004