Sensors and Actuators最新文献

A description is given of the design, fabrication and analysis of an electrostatically-driven micro-actuator in which a planetary-motion rotor rolls inside a cylindrically shaped stator cavity. The design has four primary advantages: (1) the motor geometry and the rolling motion enable very small gaps to be achieved, which are accurate and stable, and across which electrostatic forces act, leading to high forces on the rotor; (2) relative motion is achieved by rolling rather than sliding, thus obviating the concern over internal friction; (3) higher output torques can be traded for lower rotor speeds, due to immediate planetary reduction; and (4) the power output should be higher than for systems constructed using two-dimensional silicon fabrication approaches, since woble motor lengths are not limited by such fabrication methods. The stator segment recruitment logic can range from simple, open-loop stepping to full servo-controlled commutation using rotor position sensors.

Two-dimensional analytical and finite-element simulations that estimate motor torque generated by electrostatic fields have been used to determine the influece of: (1) rotor and stator radii; (2) stator segment angular width and position with respect to the contact point; and (3) dielectric properties and dimensions (e.g., insulator thickness on rotor) of motor materials. Dynamic modelling is being used in the comparison of predicted and observed motor behavior, and for the study of the effects of stator segment recruitment logic.

A number of eccentric-motion micromotors constructed via different fabrication techniques, have been operated. Electro-discharge machining (EDM) is the fabrication method of choice for the prototypes presently used for experimental studies. Typical rotor diameters for the EDM motor are about 500 μm, with lengths of 500 μm. Motor operation has been achieved with commutation rates in excess of 120 000 r.p.m.

Interdigitated finger (comb) structures are demonstrated to be effective for exciting electrostatically the resonance of polysilicon microstructures parallel to the plane of the substrate. Linear plates suspended by a folded-cantilever truss and torsional plates suspended by spiral and serpentine springs are fabricated from a 2 μm-thick phosphorus-doped low-pressure chemical-vapor-deposited (LPCVD) polysilicon film. Resonance is observed visually, with frequencies ranging from 18 kHz to 80 kHz and quality factors from 20 to 130. Simple beam theory is adequate for calculating the resonance frequencies, using a Young's modulus of 140 GPa and neglecting residual strain in the released structures.

We describe the design, fabrication and operation of several micromotors that have been produced using integrated-circuit processing. Both rotors and stators for these motors, which are driven by electrostatic forces, are formed from polycrystalline silicon 1.0–1.5 μm thick. The diameters of the rotors in the motors tested are between 60 and 120 μm. Motors with several friction-reducing designs have been fabricated using phosphosilicate glass (PSG) as a sacrificial material and either one or three polysilicon depositions.

This paper describes two operational harmonic electrostatic motors. A cylindrical rotor is placed inside a hollow cylindrical hole of slightly larger diameter. Electrodes on the circumference of the hole attract the rotor electrostatically and cause it to roll inside the stator. The harmonic motion of the rotor produces a ‘gear reduction’ between the electrical drive frequency and the shaft rotation rate. This motor design has the advantage of a gear reduction that increases the torque of the motor, and laso has several other advantages. First, it uses the clamping force, normally larger than the tangential force used by most electrostatic motor designs, to generate the motion. Secondly, the sliding friction between the rotor and stator, a source of hindrance for most microelectrostatic motors, helps by keeping the rotor and stator from slipping. Thirdly, this motor uses rolling surfaces that dissipate less energy in friction than sliding surfaces.

Resonant-structure micromotors are a class of mechanism that operates on the principle of mechanical resonance, so that the armature is driven for maximum displacement and power. The application of resonance in motors is placed in historical perspective and scaling issues for such resonant systems are identified and quantified. Along with a conventional, tensile-element resonant structure, a novel, ‘crab-leg’ flexure system is described and flexural spring constant and stress formulae derived.

A selective chemical vapor deposition (CVD) tungsten process is used to fabricate three-dimensional micromechanical structures on a silicon substrate. Patterned structures are formed in silicon dioxide trenches by selective nucleation and growth of tungsten from the bottom of the trench. Examples are shown for selective growth of tungsten on single-crystal silicon, on thin films of silicon and on silicon-implanted, silicon dioxide layers. This high deposition-rate selective CVD tungsten process had been used to fabricate tungsten micromechanical structures greater than 4 μm thick. Tungsten patters with features of 0.3 μm × 0.3 μm in 0.6 μm of tungsten are fabricated using electron beam lithography. As an example of an electromechnical structure, cantilever beams have been fabricated to make micromechanical tweezers that move in two dimensions by the application of potential differences between the beams (lateral motion), and between the beams and the silicon substrate (vertical motion). Tungsten microtweezers 200 μm in length with a cross-section of 2.7 μm × 2.5 μm closed with an applied voltage of less than 150 V.

The internal stress and Young's modulus of thin films are determined by measuring the deflection versus pressure of rectangular membranes made of them. In order to reduce the measurement error for Young's modulus due to the unknown Poisson's ratio, a 2 mm × 8 mm rectangular membrane is adopted. Measurements are made by using a computerized measurement system. Low-pressure chemical vapor deposition (LPCVD) silicon nitride films are characterized and found to have an internal stress of 1.0 GPa and Young's moduls of 290 GPa, showing that the rectangular membrane load-deflection technique could be utilized to measure the internal stress and Young's modulus of films deposited onto LPCVD silicon nitride membranes. By using this composite membrane technique, a LPCVD polysilicon film and a plasma-CVD silicon nitride film are characterized. The internal stress and Young's modulus are found to be −0.18 GPa and 0.11 GPa for the LPCVD polysilicon film and 0.11 GPa and 210 GPa for the plasma-CVD silicon nitride film, respectively.

Microsensors based on active polysilicon resonant microstructures are attractive because of their wide dynamic range, high sensitivity and frequency shift output. In this paper, we discuss processing issues for integrating electrostatically-driven and -sensed polysilicon microstructures with on-chip nMOS device. Surface-micro-machining using sacrificial spacer layers is used to obtain relased microstructures. A novel feature is the use of rapid thermal annealing (RTA) for strain relief of the ion-implanted, phosphorous-doped polysilicon. Resonance frequencies of cantilever beams indicate a lower-bound Young's modulus of about 90 GPa and an upper-bound compressive residual strain of only 0.002%, indicating that RTA is potentially useful for strain relief.

A normally closed microvalve and a micropump are fabricated on a silicon wafer by micro-machining techniques. The normally closed microvalve has a movable silicon diaphragm and a small piezoactuator to drive it. The controllable gas flow rate is from 0.1 ml/min to 85 ml/min at a gas pressure of 0.75 kgf/cm². The micropump is a diaphragm-type pump, which consists of two polysilicon one-way valves and a diaphragm driven by a small piezoactuator. The maximum pumping flow rate and pressure are 20 μ/min and 780 mmH₂O respectively.

In recent years, substantial effort has been devoted to the design and fabrication of a new class of silicon sensors, the accelerometer. A number of companies have been working in the area to produce, for the first time, an accelerometer that is substantially more cost effective and with higher performance than previously possible. Careful electromechanical design and micromachining process development has allowed silicon accelerometers to be fabricated in volume.

Two questions that arise in ultra-high reliability applications, such as safe-and-arming, are whether the aceelerometer is free and working and whether the device is broken. A unique solution to these questions has been designed and implemented in a piezoresistive accelerometer; this approach allows the device to be tested by electrostatic deflection of the mass. A number of key advantages result from this configuration. Even though the spring constants of the device may vary from unit to unit or over temperature, and even though the piezoresistive coefficients vary over temperature, as long as the voltage and initial separation gap are held constant, the output will be proportional to a given acceleration. Applications for the self-testing technique are in temperature compensation, testability and uni-directional force-balance applications.

This approach of building testability into the sensor bridges the gap between the open-loop sensors now in production and the much more complex closed-loop force-balance devices.