Makoto Mita

A micromachined impact microactuator driven by electrostatic force

This paper presents a novel micromachined actuator which is developed to produce precise and unlimited displacement. The actuator is driven by impact force between a silicon micro-mass and a stopper. The suspended silicon micro-mass is encapsulated between two glass plates and driven by electrostatic force. When the mass hits the stopper which is fixed on glass plates, impact force is generated to drive the whole actuator in a nano size step (/spl sim/10 nm). The overall dimension of the device is 3 mm /spl times/3 mm. The driving voltage is 100 V and average speed is 2.7 /spl mu/m/s. The total thickness is 600 /spl mu/m.

A MEMS piggyback actuator for hard-disk drives

This paper reports a new fabrication process and designing method to integrate MEMS piggyback actuators on a silicon-on-insulator (SOI) wafer with magnetic read/write heads of hard-disk drives. Large bandwidth of the tracking servo system is designed by reducing the load mass for the tracking microactuator to be around 40 /spl mu/g. A prototype electrostatic MEMS actuator (2 mm /spl times/ 3 mm /spl times/ 0.6 mm) of multiple parallel plates has been successfully integrated by using high-aspect ratio microstructures (gap opening 2 /spl mu/m into 50-/spl mu/m-SOI wafer) patterned by deep reactive-ion-etching (DRIE). A dc displacement of 0.5 /spl mu/m, which is almost the same size as data track width, has been obtained at a driving voltage of dc 60 V and the fundamental resonance is found at 16 kHz. An analytical model of the MEMS piggyback actuator has been proposed to predict electromechanical performance. The fabrication method proposed here is very simple and straightforward to put the head-element-drive mechanism into practice.

Embedded-mask-methods for mm-scale multi-layer vertical/slanted Si structures

Complicated deep-etched structures having multiple heights and vertical/slanted walls have realized by fully-silicon-based batch fabrication process, which only needs lithography on a flat surface. Arbitrary numbers of mask layers were laminated on the initial surface of a substrate and deep-RIE, LOCOS or anisotropic wet etching were performed to make microstructures using each mask layer subsequently.

Self-aligned micromachining process for large-scale, free-space optical cross-connects

A new micromachining process for large-scale optical cross-connects is presented. It satisfies the high-accuracy optical alignment required for free-space optics. A self-aligned batch-process allowing the simultaneous fabrication of vertical mirrors and fiber guides is performed with only one-mask. This process is based on bulk micromachining of [100] silicon. A first demonstration is performed on a 2/spl times/2 elementary cell then, it is extended to the fabrication of larger mirror arrays. Promising performances such as insertion loss lower than 0.5 dB, sub-millisecond switching time (0.3 ms) and reliable operation (more than 20 million cycles) are demonstrated on a bypass switch. An improved fabrication process, leading to an increase of integration density is also presented. It is based on the combination of deep dry-etching and anisotropic wet-etching.

Micropore x-ray optics using anisotropic wet etching of (110) silicon wafers

To develop x-ray mirrors for micropore optics, smooth silicon (111) sidewalls obtained after anisotropic wet etching of a silicon (110) wafer were studied. A sample device with 19 microm wide (111) sidewalls was fabricated using a 220 microm thick silicon (110) wafer and potassium hydroxide solution. For what we believe to be the first time, x-ray reflection on the (111) sidewalls was detected in the angular response measurement. Compared to ray-tracing simulations, the surface roughness of the sidewalls was estimated to be 3-5 nm, which is consistent with the atomic force microscope and the surface profiler measurements.

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.

Electrostatic impact-drive microactuator

A fully packagable micromachined actuator was developed for generating precise but unlimited displacement. A suspended silicon mass is encapsulated between glass plates and driven by electrostatic force. By hitting a stopper, it generates impact force to drive the whole actuator in a small step (/spl sim/10 nm). It is a micromachined and electrostatic version of the impact-drive actuator.

Switched-Layer Design for SOI Bulk Micromachined XYZ Stage Using Stiction Bar for Interlayer Electrical Connection

A simple and new technique for interlayer electrical interconnection for silicon-on-insulator microelectromechanical systems (SOI-MEMS) micromachining was developed without using additional device layers or patterning processes. A part of the SOI microstructure was shaped into a slender cantilever and intentionally brought into contact onto the substrate surface by surface stiction force after sacrificial release. The contact resistance between the stiction bar and the substrate was studied with and without a subsequent metallization process. The stiction bar was found to improve the MEMS design flexibility in allocating electrical components (comb-drive electrodes and interconnection tethers) and mechanical components (suspensions and frame) to the SOI and the substrate layer and to make a high-density complex double-deck structure in a small footprint. As an example of the high-density design, we developed a micro-XYZ stage with the lateral and vertical comb-drive mechanisms and compared the design with the conventional bulk micromachined structures.

An equivalent-circuit model for MEMS electrostatic actuator using open-source software Qucs

We report an equivalent circuit model for MEMS (microelectromechanical systems) electrostatic actuator using open-source circuit simulator Qucs (quite universal circuit simulator). Electrostatic force, equation of motion, and Kirchhoffs laws are implemented by using the EDD (equation defined device) function of Qucs. Mathematic integral operation in the equation of motion is interpreted into electrical circuits by using an ideal electrical capacitor that read input signal as current and returns accumulation result in terms of voltage. Seamless multi-physics mixed signal simulation between micro mechanics and electronics has become possible on the single platform of the circuit simulator.

A Silicon Micromachined

In this paper, we report an array of f- thetas microlens optical scanners developed for 3-D optical cross connect (OXC) system using the bulk-silicon micromachining technique. An electrostatic XY-stage mechanism for the 2-D lens scanner was designed to have a small footprint (2 mmtimes2 mm), compared with an integrated silicon lens (diameter 1 mm) due to the newly developed double-deck actuator design; all the mechanical parts (suspensions and frames) were made in the substrate layer of a silicon-on-insulator (SOI) wafer, and the electrostatic actuation mechanism (electrodes and electrical interconnection) was made in the SOI layer. A silicon lens was integrated on top of the XY-stage by transferring the spherical profile of a thermal-reflow photoresist pattern into the SOI layer by reactive-ion etching. The XY lens scanner was found to operate at lateral displacement of 19 mum in the X-directions and 23 mum in the Y-directions at drive voltages of 110 and 60 V, respectively. For an optical assembly of the OXC, we used additional lenses in a telescope formation to double the beam angle that was steered by the f- thetas microlens scanner by which the lens displacement could be designed to be smaller by a factor of 1/2