三田 信

We have successfully fabricated a tunneling tip integrated with a silicon micromachined electrostatic actuator of high aspect ratio. Tip sharpness has been investigated by scanning over carbon graphite as an atom scale.

This paper describes the fabrication and actuation of a novel device composed of twin nano probes. The size of the probes are 200nm-high, 280nm-wide and 5 mum-long, which are formed by silicon anisotropic etching. The initial gap of about 400nm between the probes become 84nm when 101mW input power was given to the thermal expansion micro actuators integrated with the probes. Precise motion down to 4nm/mW was confirmed by simultaneous TEM observation.

A silicon shadow mask for evaporation or sputtering of any shape of patterns was fabricated by Al-Delay Masking Process (DMP). Using DMP, a Si high aspect ratio structure having arbitrary number of multiple-depth can be fabricated. By applying two layers of DMP, Si shadow mask with a mechanical alignment structure was fabricated. A Si chip on which deposition should be performed was successfully inserted to the mechanical alignment structure by tweezer, ensuring the high alignment precision less than ±5μm of the shadow mask aperture to the chip. With three layers of DMP, a shadow mask for deposition of a doughnuts-like shape was fabricated. The successful deposition of the doughnuts-like shape showed the large freedom of deposition patterns.

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 MEMDAC's are compared, Preliminary results of a silicon micromachined 4-bit MEMDAC successfully showed a total stroke of 5.8 mu m with a step of 0.38 mu m. The positioning resolution can be made finer by simply increasing the number of chained units.

A new micromachining process is described that guarantees highly accurate optical alignment in the free-space optics of matrix switches. This process is demonstrated using a 2 x 2 elementary cell, which is easily scalable to a larger M x N matrix. Promising performances such as low insertion loss (0.30-0.70dB) and a fast switching time (300 mu s) are obtained.

This paper reports a new and low cost method of fabrication for M*N matrix switch using wet etching of silicon: a self-aligned batch process allowing the fabrication of vertical mirrors and V-grooves is performed in one-level of mask in (100) silicon wafer. The feasibility of a self-latching system with electromagnetic force is shown for the actuation of switch. Promising performances such as insertion loss lower than 0.5 dB, submillisecond switching time (0.4 ms) and reliable operation (20&gt million cycles) are achieved.

2.7-Dimensional Si micromachining methods are presented: Al-DMP (delay-masking-process) and LOCOS-Al-DMP. Silicon structures with an arbitrary number of thickness could be obtained. Wet anisotropic etching technology and dry high-aspect-ratio-technology are combined by means of LOCOS. The general idea is to embed masking layers on one plane and use them one after another. Surface micromachined structures can also be embedded, providing 2+2.7-dimensional structures.

A Scratch-Drive-Actuator array is fabricated in an inverted way. According to the applied square wave of ±90V from 100Hz to 100kHz, the inverted SDAs successfully conveyed an object placed on it to the designed direction. Since SDA is a microactuator with several ten μN of force and several ten nm of actuation step, the inverted SDA will be a key device of precise, powerful conveyance system over a large area.

Simultaneous visual control of tip position is indispensable for in-situ observation of nanoscopic phenomena at the tunneling gap. In this paper we propose a one-chip tunneling control device, which is small enough to load on a standard sample holder of the transmission electron microscope (TEM). Tunneling probes and micro actuators have been successfully integrated on a 2.4 x 2.4-mm(2)-chip by silicon micromachining technique. A pair of sharp silicon tips is obtained by the combination of stress-induced oxidation and selective etching of silicon oxide. Typical dimensions of the tips are 10 nm in radius and I mum in length with a 200-nm-initial gap. An electrostatically operated comb-drive actuator is used to close the gap with a voltage around 100 V. The tunneling tips are suspended over a through hole in the base substrate, and the tunneling gap can be observed by TEM. We found that clear images could be obtained without distortion or shift due to the driving voltage applied to the integrated actuator.

We have newly invented a micromechanism for precise position control, which is equivalent to that of an electrical digital-to-analog converter(DAC) of the R-2R resistor network. The micromechanical C-2C compliance suspension network has been fabricated on a silicon-on-insulator wafer by a micromachining technique. When N-bit digital driving voltages are applied to the micromechanism, cascaded electrostatic microactuators produce a set of displacements of +/-1 mu m. These displacements accumulate in the suspension network with binary weights to synthesize DAC-like displacement output. A traveling displacement of 6 mu m is obtained while the positioning resolution is 0.38 mu m in the case of 4 bit input. Resolution can be improved further by increasing the addressing size of the input.

This paper describes the application of micromachine technology to fabricate nanoscale structures for the observation of quantum phenomena, such as atomic motion and electron transport in molecules, tunneling gaps and quantum nanowires. An ultra-high resolution TEM (transmission electron microscope) will visualize the tunneling gap or the nanowires and give us the direct information of geometries, motion and electromagnetic field distribution with atomic level resolution. The TEM, which is under development, is expected to resolve a single atom because it detects the phase shift of electron waves by electron interferometry. A micromachine scanning tunneling microscope (micro-STM) will be placed in the TEM to control the tunneling current. The overview of the project, micromachining process for fabricating nanowires with microactuators, and some fabrication results are described. We made a nano wire with triangular cross section of 80 nm in side length and of 5 micrometers in length successfully. The wire was overhanging between the edge of an observation aperture. The wire was integrated with a rigid microactuator to complete a micro tunneling current device.

With the promotion of SPM technology, scientific interest has expanded from visual observation of static nanometric structures to in-situ investigation of mesoscopic phenomena, such as quantized electrical conductance of atom bridges. Bringing two probes or more into a tunneling distance is needed to determine the transfer function of the nanometric specimen. Technical difficulties lie in reducing the tip radius and the total size of the tip-positioning mechanism. For this reason we have previously proposed lateral tunneling unit that is a tunneling probe integrated with an electrostatic actuator produced by silicon micromachining technique. Size reduction of the whole SPM systems was found to work in favor of improving the control accuracy of tip-positioning as well as suppressing the effects of thermal drift and external vibration. Based upon these results we have recently developed such one-chip STM (scanning tunneling microscope) which is small enough to be loaded in the chamber of the transmission electron microscope. A pair of pre-aligned sharp tunneling tips has been successfully fabricated by the combination of deep dry etching and stress induced oxidation of silicon. The mutual distance of the tips is controlled by using the integrated electrostatic microactuators. The device enables us to in-situ observe atom scale phenomena in the tunneling gap.

A 2×2 microoptical switch was designed with a simple mechanism based on electrostatic actuation. Fabrication involved patterning and insulating the driving electrodes, and forming the trenches by wet etching in a (110) substrate and covering with a sacrificial oxide. Polysilicon was deposited and patterned in the actuator and mirror, and four grooves were dry etched from the backside. The mirror was metallized by evaporation, and the actuator was raised out-of-plane and held there by plastic deformation resulting from Joule heating. The distance between fibers can be as close as a few tens of micrometers, and fiber alignment is assured by the grooves. Fabrication can be batch processed except in the final reshaping step.