Micromachines that work!

DN Staff

September 22, 1997

9 Min Read
Micromachines that work!

MEMS Micro gearmotor

Clearwater, FL--Records are meant to be broken; champions dethroned. A few short months after reporting what was claimed to be the "world's smallest dc brushless motor" (see DN 5-5-97), along comes an even smaller motor.

Measuring a mere 1.9 mm in diameter by 5.5 mm long, this brushless dc motor offers a 0.5V dc nominal voltage with a no-load current of 73 mA. No-load speed is rated at 150,000 rpm with a stall torque of 7 muNm. Developed by Micro Mo(R) Electronics Inc. in cooperation with sister company Faulhaber GmbH, Schonaich, Germany, and its research partner IMM, Mainz-Hechtsheim, Germany, the micromotor features specially fabricated bearings, a rare-earth-magnet system, and proprietary lubricants.

Planetary gearheads, produced by LIGA technology, add only 2-4 mm to motor length, depending on the selected ratio (3.6:1 to 47:1). Based on deep X-ray lithography and electroplating, LIGA permits molded micro-range gears with very high aspect ratios.

"Comparative semiconductor-based techniques of surface and bulk machining," says Stephen O'Neil, Micro Mo vice president of Advanced Research & Planning, "are more appropriate for two-dimensional devices with low aspect ratios and little output power or load-bearing capabilities."

Gearhead numbers: Maximum output torques up to 300 muNm intermittent and 150 muNm continuous; efficiencies from 50% to 80% depending on ratio. Target and beta test applications: Medical diagnostic, treatment, and surgical devices; cell biology research tools; chemical analysis apparatus; micro-recording and data storage devices; reconnaissance and security equipment; and pumping/dispensing systems.


Fast printing head

Neuchatel, Switzerland--High-density arrays of microelectromagnets form the basis of a 700 page-per-minute magnetographic printing machine. Not only do they improve print speed, these silicon magnet arrays increase resolution to 480 dots per inch (dpi)--twice that of classical magnet arrays made from individually manufactured electromagnets.

An array of electromagnets writes magnetic pixels on a drum. These pixels pick up magnetic toner which is brought to the paper. The system then recovers residual toner and demagnetizes the drum.

Electromagnets integrated on a silicon chip (1,000 devices/cm2) have been successfully used in high-performance magnetographic printing heads producing a maximum induction of about 1 Tesla.

Developed by CSEM SA (Centre Suisse d'Electronique et de Microtechnique SA), each electromagnet consists of a flat multi-turn coil surrounding a magnetic circuit. A magnetic writing pole and a large, low-reluctance back plate form the circuit. The writing pole concentrates and guides the magnetic flux generated by the coil while the back plate provides a flux return path. Since the number of devices per chip is too high for addressing each inductive coil individually, an on-chip integrated diode matrix sequentially addresses the coils.

Magnetic poles pass through holes in the silicon chip. Coils magnetize the poles. On-chip diode matrices demultiplex the driving signals, directing information to the proper pixels.

Batch fabrication lowers costs. Starting with the silicon substrate: 1) Standard microelectronic processes build the addressing electronics; 2) Microelectrochemical deposition lays down the gold coils; and 3) Silicon bulk micromachining, followed by electrodeposition of the FeNi back plate and writing pole, form the magnetic circuit.

High-speed printing heads are presently in production at CSEM SA for Nipson Printing Systems, Belfort, France.


In-pipe micro inspection machine

Aichi, Japan--An eddy current sensor, multi-layer actuator, countermass, and three clamps comprise this miniature "pipe crawler," developed by DENSO Research Laboratories. Components reside in a 60"-mm-thick shell weighing 0.084g. Shell construction involves the same processes applied to integrated circuit manufacture: precision machining, plating, and etching.

When voltage increases slowly, the countermass moves, but the clamps do not. Decreasing voltage quickly moves the clamps, but not the countermass.

Each actuator layer comprises a piezoelectric element and elastic plate, arranged like a bi-metal. Because shell volume of the piezoelectric element is constant, the piezoelectric element expands in the direction of applied voltage, but contracts in the perpendicular direction. This bends the elastic plate, amplifying displacement. Layer number determines actuator force.

Actuator expansion/contraction moves the inspection machine forwards and backwards within a pipe. During expansion, static friction force between the clamps and pipe exceeds inertial force of the counter mass; inertial force overcomes stiction force during contraction.

Able to detect cracks approximately 10 mum in width, the micro inspection machine operates in both air and liquid.


Rotating micromirror

Dallas, TX--Video is now following in audio's footsteps by going digital for ease of storage and transmission. Digital light processing (DLP) enables digital displays for digital video formats, thus eliminating the digital-to-analog processing step. It works by transmitting a burst of light that the eye interprets as a color analog image.

Digital micromirror device (DMD) uses electrical signals to move a 16-mum-square aluminum mirror to reflect light. DMD arrays can produce black and white or with filters, color dispalys as part of a digital light processing system.

Key to DLP is a MEMS device from Texas Instruments called a DMD (digital micromirror device) light switch. Each DMD has a 16-mum-square aluminum mirror that can reflect light in one of two directions, depending on the state of the underlying CMOS SRAM memory cell. With the memory cell in the (1) state, the mirror rotates to +10 degrees; with the cell in the (0) state, the mirror rotates to -10 degrees.

The mirror is rigidly connected to an underlying yoke. The yoke connects via two mechanically compliant torsion hinges to support posts attached to the substrate. Address electrodes for the mirror and yoke are connected to the complementary sides of the memory cell.

Electrostatic fields develop between the memory cell and the yoke and mirror, creating an electrostatic torque. This torque works against the restoring torque of the hinges to rotate the mirror. The mirror and yoke rotate until the yoke comes to rest against mechanical stops that are at the same potential as the yoke.

By combining a DMD with a light source and projection optics, the mirror reflects light either into or out of the pupil of the projection lens. Thus, the (1) state appears bright and the (0) state appears dark. Binary pulse-width modulation of the incident light results in shades of gray. Stationary or rotating color filters in combination with one, two, or three DMD chips result in color.


Carbon monoxide sensor

Phoenix, AZ--Using silicon-based technology to detect gases has many advantages: sensors smaller than traditional devices, consistent performance, and lower device cost.

A silicon micromachined carbon monoxide sensor is available now from Motorola. Applications include home fire, smoke, and gas alarms; industrial monitoring and maintenance systems; and automotive emission and sensing.

A gas-sensitive, thin-film, tin-oxide layer sits on top of a micro-machined silicon diaphragm in Motorola's carbon monoxide sensor. An embedded heater raises the temperature of the tin oxide to be sensitive to the gas; the silicon diaphragm reduces power consumption.

Motorola makes the sensors using both bulk and surface micromachining technologies licensed from Switzerland-based Microsens S.A. Bulk micromachining involves single crystal silicon etching. Micromechanical structures developed this way are made either of silicon crystal or deposited or grown layers on silicon. Surface micromachining involves depositing or growing layers on top of the substrate to build micromechanical devices.

A thin film of tin oxide senses concentration of carbon monoxide. A heater sits below the film, embedded in a silicon oxide layer. The entire structure rests on silicon bulk-micromachined to 2 mm. This base layer decreases thermal mass and power needed to heat the structure to its 100 to 450C operating temperature.

Motorola plans to offer a line of chemical sensors for different gases. Soon to be available: a methane sensor and sensors to measure hydrogen sulfide.


Blood pressure sensor

Milpitas, CA--The highest-volume application for MEMS products in the medical industry is for measuring a patient's blood pressure in hospital intensive-care units.

One such sensor is the Model 1620 disposable blood-pressure sensor from EG&G IC Sensors. Here's how it works: Fluid from a saline bag passes through tubing into the MEMS sensor pack-age and then into the patient's arm. As the heart beats, a pressure wave moves up the fluid path and is detected by the sensor, which sends the information to a monitor.

Four resistors form a Wheatstonebridge to reflect the pressure seen by the silicon diaphragm.

The MEMS sensor itself consists of a pressure-sensing element mounted on a ceramic substrate. The element comprises a silicon diaphragm that moves under pressure and thick-film resistors that form a Wheatstone bridge. The resistors are arranged so that as the diaphragm moves, opposite sides of the bridge will change by similar magnitudes and in the same direction. When a constant current or voltage is applied to the bridge, the unbalance causes a change in the sensor's output voltage. Thus, the sensor's voltage output directly reflects the pressure.

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