Kiyotaka Yamashita

Bistable nanowire for micromechanical memory

We present a micromechanical device designed to be used as a non-volatile mechanical memory. The structure is composed of a suspended slender nanowire (width: 100 nm, thickness: 430 nm, length: 8 to 30 ?m) clamped at both ends. Electrodes are placed on each side of the nanowire to (1) actuate the structure during the data writing and erasing mode and (2) determine its position by measuring the capacitive bridge in the reading mode. The structure is patterned by electron beam lithography on a pre-stressed thermally grown silicon dioxide layer. When later released by plasma etching, the stressed material relaxes and the beam buckles by itself to a position of lower energy. These symmetric bistable Euler beams exhibit two stable deformed. This paper presents the microfabrication process and analysis of the static buckling of nanowires. Snapping of these nanowires from one stable position to another by mechanical or electrical means will also be discussed.

rf microelectromechanical system device with a lateral field-emission detectora)

We propose a micromachined device that utilizes the field-emission (FE) phenomenon as a mean to modulate signal for radio-frequency microelectromechanical system applications. In this article, we present the stationary reference (SR) device and the resonator-embedded (RE) device and compare their field-emission performances. The SR device contains no moving part and is used to examine the conditions to excite field emission. The RE device has an embedded microresonator of bandpass filter characteristic. Due to enhanced tip sharpness and closer gap, initial results show that compared to the SR device, the FE current of the RE device has been increased by 192 times under the same anode-cathode potential difference of 240V and 2×10−8Torr vacuum level.

In-plane bistable nanowire for memory devices

We present a micromechanical device designed to be used as a non-volatile mechanical memory. The structure is composed of a suspended slender nanowire (width: 100 nm, thickness: 430 nm, length: 8 to 30 mum) clamped at both ends. Electrodes are placed on each side of the nanowire to 1) actuate the structure during the data writing and erasing mode and 2) determine its position by measuring the capacitive bridge in the reading mode. The structure is patterned by electron beam lithography on a pre-stressed thermally grown silicon dioxide layer. When later released by plasma etching, the stressed material relaxes and the beam buckles by itself to a position of lower energy. These symmetric bistable Euler beams exhibit two stable deformed. This paper presents the microfabrication process and the analysis of the static buckling of nanowires. Snapping of these nanowires from one stable position to another by mechanical or electrical means will also be discussed.

Influences of Uniform Electrical Fields on Burning Rate Constant of Ethanol Droplet Combustion

The burning rate constants of ethanol droplet flame under uniform electrical fields were investigated experimentally and numerically. The droplet was burned between two flat electrodes and 1–7 kV DC Volts were applied. The combustion chamber was dropped from the top of a drop tower and the experiment was carried out under microgravity environment to eliminate the effect of buoyancy. Direct photographs of the droplet flame were captured and the change in droplet diameter was measured to obtain burning rate constants. A two-dimensional numerical simulation was also conducted on the droplet combustion in a uniform electrical field. A chemical reaction model with some elementary reactions of ion species was applied. The deformation of the electrical field was predicted by solving the Poisson equation. The predictions show that the burning rate constant increases and the flame deformation becomes large as the applied voltage increases, which is in qualitatively agreement with the experimental results. The relationship between the burning rate constant and the induced flow velocity by Coulomb force is discussed in the analogy of the burning behavior in a convective flow.

Vibration-driven MEMS energy harvester with vacuum UV-charged vertical electrets

Our target is to realize a vibration-driven MEMS generator with vertical electrets for energy harvesting applications. With the aid of a novel high-speed charging method using vacuum UV irradiation, we have realized vertical electrets on the sidewall of comb fingers. An early prototype of in-plane electret generator with parylene electrets has been successfully microfabricated through a single-wafer approach. Uniform surface potential on the vertical electrets of the comb fingers is confirmed with a Kelvin force microscopy (KFM) measurement. Power generation with the prototype has been also demonstrated.

An RF-MEMS device with a lateral field-emission detector

We propose a new device that utilizes the field-emission effect as a signal-detecting mechanism for RF-MEMS (radio-frequency micro electro mechanical systems) applications. The device consists of a micro electromechanical resonator of BPF (band-pass filter) characteristics and a pair of silicon tips for field-emission, both of which are monolithically integrated by the silicon micromachining technology. The present paper will show a complete set of experimental results on the field-emission current that was controlled by the micromechanical structure, including the latest report on improved fabrication processes for sharper emission tips.

EFFECTS OF FUEL VAPOR IN AMBIENCE ON SPONTANEOUS IGNITION OF ISOLATED FUEL DROPLET

Experiments have been carried out on the effects of pre-vaporized fuel in the ambience on spontaneous ignition behavior of isolated fuel droplets. An isolated droplet of n-heptane or n-dodecane, which is sustained by a quartz fiber, is inserted into a high-temperature gas field including fuel in the electric furnace. Methane and n-heptane are selected as fuels of the ambience. The ambient pressure and the temperature are set to be 0.3 MPa and 700 K, respectively, in order to prevent the consumption of methane and oxygen in the ambience. The condition of the ambience including fuel is elucidated using a numerical analysis of chemical reactions. The results indicate that the ignition delay time increases due to methane included in the ambience. In the case of the ambience where n-heptane vapor is included, both n-heptane and oxygen are consumed by chemical reactions prior to the droplet insertion. The ignition delay time of n-heptane droplet in the reacted n-heptane and air mixture is larger than that in air and chemical reactions at the reduced oxygen concentration seems to be slow. The ignition delay time is shorter in the reacted n-heptane and air mixture than in an air with the same amount of oxygen as the reacted mixture.

A Phase Shifter by LTCC Substrate with an RF-MEMS Switch

12.5 GHz band digital phase shifters to control a beam direction for an Active Integrated Phased Array Antenna (AIPAA) are designed and fabricated an RF-MEMS device and a LTCC substrate. We designed and fabricated low loss RF-MEMS wave guide (Lt0.5 dB) and 4-bit LTCC phase shifters with small-size (10 mm times 13 mm times 0.55 mm). From the experiment data, the insertion loss of RF-MEMS device and LTCC phase shifter is measured to be about 0.5 dB and about 1 dB at 12.5 GHz, respectively. And an integrated RF-MEMS SPDT switches on an LTCC phase shifter by using flip-chip bonding are measured the insertion loss is about 2.0 dB at 12.5 GHz per 1 bit.

A dual-SPDT RF-MEMS switch on a small-sized LTCC phase shifter for Ku-band operation

Integration of Low Temperature Co-fired Ceramics (LTCC) technology and RF-MEMS is applicable for the realization of a small and cost-effective phase shifter. In this paper, a line-switched-type phase shifter with 3D waveguide structures and a novel design of RF-MEMS switch was demonstrated. The design method and fabrication process were reported with experimental results. The insertion loss and phase error of the 4-bit digital phase shifter on LTCC substrates with the size of 8mm × 8mm × 0.55mm was measured to be around 0.5dB/bit and less than 5% at 12.5GHz, respectively. The Dual Single-Pole-Double-Throw (D-SPDT) MEMS switch showed driving voltage of 40V, insertion loss of 0.9dB, return loss of 12dB and isolation of −29dB at 12.5GHz.

A 16-element power AIPAA with a 4-bit digital LTCC phase shifter operating at the 5.8 GHz-band for wireless communication and power transmission

5.8 GHz band digital phase shifters to control a beam direction for an Active Integrated Phased Array Antenna (AIPAA) are designed. The 4-bit phase shifters with integrated GaAs FET SPDT switches are integrated with an LTCC substrate. We realized a low loss 4-bit LTCC phase shifter with the size of 10 mm × 13 mm × 0.55 mm. From the simulation results, the insertion loss of LTCC phase shifter without the switch is simulated with about 0.5dB/bit. The insertion loss of the LTCC phase shifter integrated with the GaAs FET SPDT switches and phase error are measured to be about 2.3dB/bit and less than 15°, respectively. In addition, beam scanning with angles of ±30 degrees was successfully achieved with 10 Mbps MSK signal from the 16-element active integrated phased array antenna.