An Electret-Augmented Low-Voltage MEMS Electrostatic Out-of-Plane Actuator for Acoustic Transducer Applications.

Despite the development of energy-efficient devices in various applications, microelectromechanical system (MEMS) electrostatic actuators yet require high voltages to generate large displacements. In this respect, electrets exhibiting quasi-permanent electrical charges allow large fixed voltages to be integrated directly within electrode structures to reduce or eliminate the need of DC bias electronics. For verification, a biased electret layer was fabricated at the inner surface of a silicon on insulator (SOI) structure facing a 2 µm gap owing to the high compatibility of silicon micromachining and the potassium-ion-electret fabrication method. A electret-augmented actuator with an out-of-plane motion membrane reached a sound pressure level (SPL) of 50 dB maximum with AC input voltage of alone, indicating a potential for acoustic transducer usage such as microspeakers. Such devices with electret biasing require only the input signal voltage, thus contributing to reducing the overall power consumption of the device system.

Development of a Cantilever-Type Electrostatic Energy Harvester and Its Charging Characteristics on a Highway Viaduct.

We have developed a micro-electro-mechanical systems (MEMS) electrostatic vibratory power generator with over 100 µ W RMS of (root-mean-square) output electric power under 0.03 G RMS (G: the acceleration of gravity) accelerations. The device is made of a silicon-on-insulator (SOI) wafer and is fabricated by silicon micromachining technology. An electret built-in potential is given to the device by electrothermal polarization in silicon oxide using potassium ions. The force factor, which is defined by a proportional coefficient of the output current with respect to the vibration velocity, is 2.34 × 10 - 4 C/m; this large value allows the developed vibration power generator to have a very high power efficiency of 80.7%. We have also demonstrated a charging experiment by using an environmental acceleration waveform with an average amplitude of about 0.03 G RMS taken at a viaduct of a highway, and we obtained 4.8 mJ of electric energy stored in a 44 µ F capacitor in 90 min.

Small single-crystal silicon cantilevers formed by crystal facets for atomic force microscopy.

We have developed a batch fabrication method of small cantilevers formed by crystal facets of single-crystal silicon for improving the sensitivity of atomic force microscopy. In order to realize a small cantilever with a very sharp tip, we have employed KOH anisotropic etching and local oxidation of silicon. We have made two types of small cantilevers, the V-shaped triangular type and the bulk triangular type. The length of each cantilever is 20 microm. The tip of the V-shaped type is bridged by two wires with thickness of 0.6 mum. The bulk triangular type has a thickness of 1.5 microm. The frequency characteristics of the cantilevers vibrated using photothermal excitation were measured by laser Doppler velocimetry. The resonance frequency of the V-shaped type and the bulk triangular type were 687 kHz and 8.42 MHz, and their spring constants are estimated to be 0.7 N/m and 370 N/m, respectively.

AFM picking-up manipulation of the metaphase chromosome fragment by using the tweezers-type probe.

We have studied the development of a new procedure based on atomic force microscopy (AFM) for the analysis of metaphase chromosome. The aim of this study was to obtain detailed information about the specific locations of genes on the metaphase chromosome. In this research, we performed the manipulation of the metaphase chromosome by using novel AFM probes to obtain chromosome fragments of a smaller size than the ones obtained using the conventional methods, such as glass microneedles. We could pick up the fragment of the metaphase chromosome dissected by the knife-edged probe by using our tweezers-type probe.

Humidity dependence of charge transport through DNA revealed by silicon-based nanotweezers manipulation.

The study of the electrical properties of DNA has aroused increasing interest since the last decade. So far, controversial arguments have been put forward to explain the electrical charge transport through DNA. Our experiments on DNA bundles manipulated with silicon-based actuated tweezers demonstrate undoubtedly that humidity is the main factor affecting the electrical conduction in DNA. We explain the quasi-Ohmic behavior of DNA and the exponential dependence of its conductivity with relative humidity from the adsorption of water on the DNA backbone. We propose a quantitative model that is consistent with previous studies on DNA and other materials, like porous silicon, subjected to different humidity conditions.

Single DNA molecule isolation and trapping in a microfluidic device.

This paper presents a systematic method to isolate and trap long single DNA segments between integrated electrodes in a microfluidic environment. Double stranded lambda-DNA molecules are introduced in a microchip and are isolated by electrophoretic force through microfluidic channels. Downstream, each individual molecule is extended and oriented by ac dielectrophoresis (900 kHz, 1 MV m(-1)) and anchored between aluminium electrodes. With a proper design, a long DNA segment (up to 10 microm) can be instantly captured in stretched conformation, opening way for further assays.

DNA manipulation and retrieval from an aqueous solution with micromachined nanotweezers.

We have demonstrated DNA handling with micromachined nanotweezers that consist of a pair of opposing nanoprobes and integrated thermal expansion microactuators for changing the probe gap. The probe tips coated with a thin Al layer were dipped into a droplet of a solution containing lambda-DNA molecules labeled with fluorescence dye, and then an ac electric field was applied between probes for several seconds. DNA molecules were then captured between the probe tips and retrieved from the solution to the air. The DNA capture between the probe tips could be performed more successfully on the droplet surface than in the underwater region. We also conducted an observation of the retrieved DNA molecules by transmission electron microscope and found that the thickness of the retrieved DNA molecules under the condition of this experiment was approximately 21 nm when the time of the applied ac power (1 MHz, 20 Vpp) was 20 s.