System scaling down in the viewpoint of both electrical blocks and physical interconnection is essential to wireless sensor nodes for layout-free and maintenance-free ubiquitous applications in wireless sensor networks (WSN). In this work, a piezoresistive planar MEMS sensor with the sensitivity of 0.2∼0.3 mV/V/g was developed for the monitoring of vibrations at low-frequency of a few Hz. To reduce power consumption of the system to the lowest limit as well as to reduce the quantity of electrical components, a fully-integrated low-power customized LSI with universal interface to sensors was designed and fabricated by using 0.18 µm 1.8V/3.3V 1P6M logic process for power management, data acquisition, data processing, and data transmission by RF signal. Power supply to the sensor can be managed by the customized LSI for reducing its power consumption. Output analog signal from the sensor can be obtained by the customized LSI through on-chip integrated amplifier and AD convertor. To reduce size of the system to the lowest limit, buried bump interconnection technology (B²it™) was introduced for the integration of above customized LSI, MEMS sensor, crystal oscillator, and passive components into a 3D structure. Those technologies enable the production of the world smallest class wireless sensor nodes with ultra-low power consumption for low-frequency vibration monitoring, and for temperature and humidity measurement.

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We developed a fabrication process of MEMS tunable grating supported by patterned polymer. The tunable grating was fabricated on single crystal silicon with thickness of 260 nm using membrane transfer bonding technique on glass. To obtain a good defined anchor structure of grating, two kinds of polymers were used for the fabrication of bonded wafer. The characteristic of the tunable grating ribbon was measured. The tunable grating ribbon was successfully actuated with the time to peak of 26 µs. Changing of diffraction pattern of the device was also demonstrated.

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A wireless node of strain sensor was developed for structural health monitoring. Surface acoustic wave (SAW) oscillators were used as sensor heads of the system, and can linearly detect the strain of 10^-5 to 10^-6 order. In the developed sensor node, a mixer chip and a communicator were integrated on the board for temperature compensation and construction of sensor mesh network. We practically applied the developed sensor nodes to measurement of the static deformation in the metal plate, and successfully detected the deformation distribution. In this experiment, power consumption of each node was 190 µW in sleep mode and 50 mW in data transmission mode.

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This paper presents an electrostatic vibration energy harvester with a conventional monopolar-charged and a novel bipolar-charged electret. The maximum output power of the -200 V monopolar-charged and ±100 V bipolar-charged harvester were 1.1 µW and 0.9 µW, respectively with a vibration amplitude of 350 µm_pp at 323 Hz with a 0.8 MΩ load resistance. The bipolar-charged harvester generates higher output power than the monopolar-charged harvester with an applied acceleration from 0 g to 0.7 g, because electrostatic force of the monopolar-chaged electret along the vibration direction prevents harvesting mass movement compared to the bipolar-charged electret. Further, we analyzed a SPICE model with electrostatic force and confirmed that the characteristics were according to the experiment results.

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