Kei Kuwabara

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.

Novel Structure and Fabrication Process for Integrated RF Microelectromechanical-System Technology

This paper describes a novel structure and fabrication process for the integration of several types of RF microelectromechanical-system (MEMS) device, such as switches and varactors having different structures. It also describes an encapsulation technique suitable for the integrated devices to protect movable parts during packaging. An adaptable multilayer structure and its fabrication process, which includes planarization with photosensitive polyimide, are proposed for integration. A capsule structure fabricated using spin-coating film transfer and hot-pressing technology is also proposed for protection. Several types of RF MEMS device were simultaneously fabricated on the same substrate using these techniques. The results confirm that these techniques will pave the way for the development of single-chip RF transceivers with integrated RF MEMS devices.

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.

Electrodeposition of Organic Dielectric Film and Its Application to Vibrational Microelectromechanical System Devices

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.

Selective Removal of Dry-Etching Residue Derived from Polymer Sacrificial Layer for Microelectromechanical-System Device Fabrication

A selective removal of dry-etching residue using hydrofluoric acid (HF) vapor is described in relation to the fabrication of microelectromechanical-system (MEMS) devices. Auger electron spectroscopy (AES) analysis of residue after dry etching of polymer sacrificial layers reveals that the residue is mainly composed of silicon oxides. HF vapor removes the dry-etching residue, and raising the vapor temperature enables the selective removal of the residue without damaging silicon dioxide (SiO2), which is often used as the insulator for MEMS devices. Direct-contact-type MEMS switches with SiO2 insulators fabricated using polymer sacrificial layers demonstrate the effectiveness of removing the dry-etching residue selectively.

RF MEMS switches integrated with sealed suspended coplanar waveguides for reconfigurable RF circuits

This paper describes an RF-MEMS switch structure and its fabrication process for developing low-loss multiport RF switches that integrate multiple RF MEMS switches and CMOS control circuits. In our structure, RF MEMS switches and coplanar waveguides are seamlessly integrated, and they are suspended above a CMOS LSI to reduce the loss due to the lossy Si substrate. A gold multilayer stacking technique was used to fabricate the structures, and the STP technique was used to seal them for damage-free packaging. Switching operation of RF MEMS switches was achieved and low insertion loss of 0.07 dB/mm at 5 GHz was obtained for the suspended coplanar waveguides.

A Capacitive Sensing Scheme for Control of Movable Element with Complementary Metal-Oxide-Semiconductor Microelectoromechanical-Systems Device

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-µm 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.

Electrodeposition of water-repellent organic dielectric film as an anti-sticking coating on microelectromechanical system devices

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.