Gen Hashiguchi

Silicon Nanotweezers With Subnanometer Resolution for the Micromanipulation of Biomolecules

We describe electrostatically actuated silicon nanotweezers which are intended for the manipulation and characterization of filamentary molecules. The microelectromechanical system consists of a pair of opposing tips whose distance can be accurately adjusted by means of an integrated differential capacitive sensor. The fabrication process is based on silicon-on-insulator technology and combines KOH wet anisotropic etching and deep reactive ion etching of silicon to form sharp nanotips and high aspect ratio microstructures, respectively. In the designed prototype, the initial gap between the tips was around 20 mum. The device showed a maximum displacement of about 2.5 mum, and we could achieve a resolution better than 0.2 nm (in static mode). We measured a resonant frequency of 2.5 kHz and a quality factor (Q factor) of 50 in air. The instrument was used to perform static and dynamic mechanical manipulations on DNA molecules, and we could distinctly observe the viscoelastic behavior of DNA bundles from these experiments.

Fabrication of Array of Hollow Microcapillaries Used for Injection of Genetic Materials into Animal/Plant Cells

We have proposed a micromachined system with the aim of controlling injection of DNA into cells. The injection system is composed of two components: hollow microcapillaries for injection and microchambers for trapping cells. The hollow microcapillary array, the most important part of the system has been fabricated. In this paper, we present a micromachined DNA injection system and the fabrication of hollow microcapillary array. Bosch deep reactive ion etching (RIE) etching was used to etch small, deep holes, approximately 5 µm in diameter and 100 µm in depth, on a silicon substrate, and enabled the fabrication of microcapillaries with microchannels inside. The fabricated hollow microcapillaries are 1 µm in thickness, 30 µm in length and 5 µm in diameter, and are made of SiO2. The height of the microcapillaries can be easily controlled. Silicon substrate with microcapillaries is bonded anodically with a processed glass substrate, and enables the silicon membrane to endure pressure from outside during DNA injection. By back-side silicon etching in TMAH solution and making holes on tips of microcapillaries, the fabrication of hollow microcapillaries is completed. Hollow microcapillary arrays can also be used in some applications other than DNA injection, such as microchannels in fluid delivery systems.

Microelectromechanical digital-to-analog converters of displacement for step motion actuators

This paper presents a novel micromechanism for precise positioning by using an N-bit digital code. The mechanism is an N-stage network of connected suspensions, in which an electrostatic actuator is attached to the longer suspensions of compliance 2C, and N of such unit structures are connected side by side with the shorter suspensions of compliance C. Each actuator is an electrostatic shuttle moving back and forth between the driving electrodes, and is operated by the corresponding digit of the input code. The N-bits of local displacement accumulate in the suspension network to synthesize an analog output, which is proportional to the analog value coded with the N-bit input. The output displacement is independent of the fluctuation of the driving voltage since the traveling distance of the shuttle is clipped by mechanical stoppers. We call the mechanism a microelectromechanical digital-to-analog converter (MEMDAC) since the function is equivalent to the electrical digital-to-analog converter known as the R-2R resistor network. Three different types of MEMDACs are compared. Preliminary results of a silicon micromachined 4-bit MEMDAC successfully showed a total stroke of 5.8 /spl mu/m with a step of 0.38 /spl mu/m. The positioning resolution can be made finer by simply increasing the number of chained units.

Fabrication of Silicon Quantum Wires Using Separation by Implanted Oxygen Wafer

A new fabrication method of silicon quantum wires using separation by implanted oxygen (SIMOX) wafer has been proposed, which combines Si KOH anisotropic etching and local oxidation of silicon (LOCOS) techniques. In the fabrication, the cross-sectional dimensions of Si wires are determined solely by the thickness of the SOI layer. Using this novel method, we have fabricated a Si quantum wire with dimensions less than 100 nm and confirmed its great potential for making ultrafine structures without the need for high-resolution lithography or etching.

Monolithic Fabrication and Electrical Characteristics of Polycrystalline Silicon Field Emitters and Thin Film Transistor.

We have developed a novel monolithic fabrication method for polycrystalline silicon (poly-Si) field emitters and a poly-Si thin film transistor (TFT). Poly-Si field emitters were fabricated using a mold transfer process, which allows a TFT to be fabricated under the field emitters and saves space for emitter fabrication. We have also demonstrated successful control of the field emission using the monolithically fabricated TFT.

New Fabrication Method and Electrical Characteristics of Conical Silicon Field Emitters

A new fabrication method for a silicon field emitter has been proposed, which combines the techniques of Si KOH anisotropic etching and local oxidation of silicon (LOCOS) to form the emitter tip. The fabricated conical silicon emitter had an extremely well-defined structure indicative of good reproducibility and uniformity of the fabrication process. The emission characteristics were examined under DC and AC conditions, and indicated high performance of the emitter. Successful application to a digit display has been demonstrated.

Single-DNA-molecule trapping with silicon nanotweezers using pulsed dielectrophoresis

DNA manipulation based on dielectrophoresis between microfabricated electrodes is one of the most efficient methods for the physical handling of molecules. Dielectrophoresis is routinely used for stretching and trapping DNA molecules between the opposing tips of silicon nanotweezers. However, the precise number of trapped molecules is difficult to predict, as a continuous application of ac voltage continually attracts the molecules while the electric-field-induced fluid flow prevents them from bridging the tips. To circumvent this difficulty, the dielectrophoresis signal is applied during very short intervals. In this pulsed mode, the electrohydrodynamic fluid flow is lessened and the molecule trapping success rate is greatly enhanced. A fluorescently labeled single λ-DNA molecule was successfully stretched and captured by the silicon nanotweezers with 50 ms pulses of a 1 MHz, 1 MV m−1 ac dielectrophoresis voltage. This single-molecule trapping between the tweezers tips is monitored, in real time, under fluorescence imaging.

A multiple Degrees of Freedom Equivalent Circuit for a Comb-Drive Actuator

We present an equivalent circuit of a comb-drive actuator which takes account of mechanical motion in multiple degrees of freedom (DOF). The circuit is derived by a purely theoretical procedure; A lagrange function of the comb-drive actuator is first set up, and following linearization of the Lagranges equation, results in a linear motion matrix. The obtained linear matrix is then divided into a mechanical matrix, an electrical matrix, and an interacting matrix which is composed of non-diagonal elements and denotes electro-mechanical energy conversion. The mechanical matrix and the electrical matrix are expressed by simple combination of LCR elements, while the interacting matrix is configured by capacitances and dependent source elements. From the equivalent circuit, we can understand interaction, not only between the electrical system and the multi mechanical systems, but also between the different mechanical freedom systems. The equivalent circuit model in the 2DOF system is verified by comparing the experimental result of electrical admittance with simulation, using the developed equivalent circuit, and shows good agreement merely by fitting parasitic capacitance and mechanical resistances. A multi-DOF equivalent circuit is essential when simulating MEMS devices, such as gyro sensors in which at least two-dimensional mechanical motions must be taken in account.

SiO2 Electret Generated by Potassium Ions on a Comb-Drive Actuator

Silicon dioxide electret generated by doping potassium ions will be demonstrated by forming it on a comb-drive actuator. The comb-drive actuator made of silicon on an insulator substrate is oxidized with bubbling a stream of KOH solution to form silicon oxide film including potassium ions uniformly on the etched side walls of comb electrodes. After a bias-temperature procedure at about 900–1000 K and 100 V was applied to the device, we confirmed a 40 V built-in potential difference between the opposing comb electrodes. The gradual decay of the potential was observed, but 35 V was maintained even after 1 month.

Fabrication and Emission Characteristics of Polycrystalline Silicon Field Emitters

A fabrication method and emission characteristics of polycrystalline silicon (poly-Si) gated field emitters have been presented. The fabrication combines mold emitter fabrication and anodic bonding techniques to transfer the emitters to a Pyrex glass substrate. The tantalum gated structures were fabricated by an etch-back technique using a photoresist process, and a gate opening of about 0.7 µ m has been successfully obtained. The emission characteristics of the fabricated poly-Si field emitters were also presented, showing high current drivability.