Kazuyoshi Ono

Nano-watt power management and vibration sensing on a dust-size batteryless sensor node for ambient intelligence applications

Nanowatt-power circuit techniques that could overcome the critical bottlenecks preventing the realization of dust-size battery-less sensor nodes are reported. Sensor networks deploying large numbers of nodes are anticipated for “ambient intelligence” [1]. For such networks, the sensor nodes should be miniaturized to the size of dust and be maintenance-free because battery-powered matchbox-sized nodes would be difficult to distribute in large numbers (100 to 1000, for instance) in rooms, and replacing the batteries would be very time-consuming. When the size of a power source is miniaturized to a few mm3, generated power is lowered to the nano-watt-level according to energy density limitations [2]. When the generated power is smaller than the power used for radio, sensor nodes have to accumulate energy [1]. The basic concept of a sensor node with functions for energy accumulation is shown in Fig. 27.10.1. The node contains the function blocks for power generation, power management, sensing, and radio. The power-management block accumulates energy to the accumulation capacitor, which is large enough for radio. When the voltage monitor detects that the voltage of the accumulated energy has reached the voltage needed for radio, the sensing block is activated. Then, the radio block transmits the sensed data when an event occurs. In this mode of operation, the voltage monitor for controlling the switch should operate with sub-nanowatt-level power dissipation because the monitor operates continuously during energy accumulation. Moreover, the power for sensing needs to be reduced to the sub-nanowatt level since the sensing function needs to be active after energy accumulation. On the other hand, the previous works for capacitive-sensing [3–4] and voltage reference [5] consume more than sub-microwatt since they use amplifiers and DC-biased resistors. To solve these problems, we use a voltage-monitoring circuit for power management and a vibration-sensing circuit that can operate even when the power source generates only nanoampere-level currents.

Monolithic Integration Fabrication Process of Thermoelectric and Vibrational Devices for Microelectromechanical System Power Generator

This paper describes a fabrication process to integrate different types of energy-converting microelectromechanical system (MEMS) devices for a power generator. MEMS structures and a fabrication process for the monolithic integration are proposed for the purpose of accumulating small amounts of energy from various surrounding environments. For the MEMS power generator, thermocouples and movable plates are simultaneously fabricated in thick-film formation processes with deep reactive ion etching of silicon and gold electroplating. The fabricated thermoelectric devices of gold and silicon produced voltages in accordance with the temperature difference at the thermocouples. The vibrational devices with gold movable plates resonated with external vibrations. These results confirm that the monolithic integration process of MEMS devices can be established for harvesting thermal and vibrational energies from their surroundings.

Surface Cleaning of Gold Structure by Annealing during Fabrication of Microelectromechanical System Devices

We describe a technique for cleaning a gold surface using a dry process during the fabrication of microelectromechanical system (MEMS) devices. After exposure to oxygen plasma for ashing of the organic contaminants or etching of a sacrificial-layer film, the gold surface is oxidized. On such an oxidized surface, there are different incubation periods at different places, which give rise to nonuniform thickness in electroplating as well as in electrodeposition. A surface analysis by X-ray photoelectron spectroscopy (XPS) revealed that annealing at a temperature of over 260 °C causes oxygen to desorb from the gold oxide. The application of this cleaning technique before electroplating or electrodeposition leads to uniform growth.

Analysis of Electret-Based MEMS Vibrational Energy Harvester With Slit-and-Slider Structure

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.

Synchronized Multiple-Array Vibrational Device for Microelectromechanical System Electrostatic Energy Harvester

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.

Nonlinear oscillation for a millimeter-sized electrostatic energy harvester

This paper describes a demonstration of power enhancement by nonlinear oscillation in a millimeter-sized electrostatic vibrational energy harvester for the future Internet of Things. To attain nonlinearity in the microelectromechanical-system (MEMS) device, a gold spring, which has a lower value of Youngs modulus than conventional materials, is adopted as a component of the MEMS structure. The nonlinear oscillation for the millimeter-sized ethylene tetrafluoroethylene (ETFE) electret energy harvester was confirmed experimentally by applying external vibration. The normalized harvesters effectiveness for the nonlinear oscillation is 9.6 times higher than that for the linear one.

Elimination of Curvature in Microelectromechanical-System Membrane

The SOI (silicon-on-insulator)-based MEMS (micro electro-mechanical systems) are attracting a great deal of interest from the viewpoints of increasing fabrication accuracy, process simplicity, and device performance [1]. The recent rapid performance advances of MEMS devices, which frequently require nanometer-scale control of movable-membrane shape, have focused attention on the problem of the membrane deformation in microfabrication [2]. In optical MEMS devices, which frequently employ a free-standing membrane structure to reflect or diffract light, the out-of-plane deformation sensitively causes critical degradation of optical characteristics, such as coupling loss and crosstalk. Therefore, the deformation must be small in comparison to the optical wavelength of interest to avoid compromising device performance. This paper presents a process in which the curvature in the membrane structure can be eliminated by annealing and describes the mechanism involved. In MEMS device fabrication, oxygen plasma exposure is often used to ash off an organic sacrificial layer and clean the surface. However, the process often cause the oxidization of the silicon membrane surface, which results in unexpected movement of the membrane due to the charge accumulated in the oxidized surface. Therefore, the oxidized surface has to be removed to prevent from such charge drift. Argon plasma exposure is usually used for this purpose, as shown in Fig. 1. In this process, we measured the shape of membrane with 4.5-m thickness with white-light microscope-based interferometer (NewView TM 200; Zygo). The obtained shape is out-ofplane deformation, which is derived from the compressive stress in the membrane, and the curvature, , was -3.4 m -1 , a value larger than before exposure, -1.0 m -1 . To suppress the curvature enhanced by argon plasma exposure, we examined an annealing treatment. Figure 2 shows the relationship between the curvature and annealing time at 500 °C in nitrogen ambient. The curvature decreased with annealing time, and after a two-hour annealing it saturated,∞ = -0.6 m -1 , which is slightly smaller than that of before argon plasma exposure. This difference might be related to the compressive stress due to the oxide layer. To investigate the difference in the surface state before and after annealing, we carried out a total-reflection X-ray fluorescence (TXRF) analysis. Figure 3 shows that there is argon on silicon surface before annealing and that there is no argon after it. This indicates that argon is implanted into the crystal lattice of silicon by argon plasma exposure and that there is a correlation between the curvature and implanted argon. Figure 4 shows the relationship between implanted argon concentration, obtained from the peak intensity in TXRF spectra, and the curvature change with the saturated one ( = ∞ -). It is found that there is a linear relationship between them. These results mean that curvature change of membrane increases in response of the argon concentration, and that the desorption of implanted argon by annealing causes the decrease of membrane curvature. In summary, we clarified the relationship between curvature change in membrane and implanted argon concentration and demonstrated the elimination of curvature by annealing.

Removal of Gold Oxide by Low-Temperature Hydrogen Annealing for Microelectromechanical System Device Fabrication

A technique for cleaning a gold surface with a dry process is described for the fabrication of microelectromechanical system (MEMS) devices. Thermal desorption spectroscopy (TDS) analysis of a gold surface exposed to reactive oxygen revealed that annealing causes oxygen to desorb from the gold oxide. To examine the effectiveness of dry-process surface cleaning, the surfaces were annealed in a vacuum, with nitrogen as an inert ambient, and hydrogen as a reductive ambient. For the annealing in a vacuum and nitrogen ambient, a temperature of over 260 °C is necessary for the oxide removal. Annealing in hydrogen ambient drastically lowers the cleaning temperature from 260 to 60 °C.

Vacuum Annealing of Gold Electrodes for Surface Cleaning in MEMS Device Fabrication

We developed a dry process for the removal of gold oxide formed during oxygen plasma exposure. This technique features vacuum annealing at over 270degC to desorb oxygen from the gold oxide. This vacuum annealing eliminates the incubation period to achieve the conformal film growth.However, when gold and HCl(aq) soluble material such as aluminum coexist on the same surface or when there are fragile microstructures for wet processes after release, this dipping cannot be applied because it can damage the surface or microstructures in the MEMS fabrication process. The profile for vacuum annealing was found to be consistent with one for HCl(aq) dipping. Moreover, there was no incubation period, meaning that vacuum annealing indeed removes the gold oxide. We can conclude that the vacuum annealing of the gold oxide surface is a promising way to obtain the pure gold surface.