4. MEMS Sensor

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MEMS Sensor

MECHENG 405K.C. Aw

Note: Some of the materials are based on Prof. Mark Bachman of UCI

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Bulk micromachined pressure sensor

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Surface micromachined pressure sensor

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Commercial Pressure Sensor

Surface micromachined pressure sensor from Integrated Sensing System, Inc

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Capacitive based MEMS pressure sensors

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Optical based MEMS pressure sensors

Optical patch changes with membrane deflection and is measured using spectrometer.

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MEMS Microphone

MEMS microphones that uses capacitive sensing.

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MEMS microphone (Akustica)

TechnologyArray of Teflon-like diaphragms on Si substrateUses FM modulation for read-out

IssuesClose spacing of diaphragms is difficult for beam forming methods to obtain directionality (at 1kHz, wavelength = 85 mm)

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Commercial MEMS Microphone

Commercial MEMS microphone – Knowles SiSonic SP0103N

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MEMS microphone (piezoresistive)

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MEMS microphone (piezoelectric)

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Accelerometer

Automotive application requirements:Anti-lock braking system ± 1 gVertical body motion ± 2 gWheel motion ±40 gAir bag deployment ±50 gSteering feedback ± 100o/sShock survivability 500 g, 1 m drop to concreteFrequency response

0 - 5 Hz vertical0.5 - 50 Hz, horizontal1 kHz, air bags

Temperatures -40 C to 85 C-40 C to 125 C under hood

Miscellaneous 200 V/m, hermetic seal

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Accelerometer – Basic Principles (Spring-mass device)

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Accelerometer with strain gauge

Modern accelerometer with strain gauge. Note use of bulk etched siliconand glass (bonded by anodic bonding). Lucas NovaSensor.

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Bulk etched silicon and pyrex glass bonded together. Used for monitoringheart wall accelerations. Roylance and Angell (1979)

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Accelerometer with piezo-electric sensing

Piezo-electric output using ZnO piezoelectric material

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Accelerometer with capacitive sensing

Asymmetric plate construction of accelerometer with capacitive pickup

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Typical calculations

Typical calculation for bending cantilever style accelerometer

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MEMS AccelerometersSurface micromachining: Analog Devices, e.g. ADXL-50

Introduced in 1991, and in volume production by 1993 for car airbags.

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Force balanced-capacitive accelerometer ADXL-50

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Force balanced-capacitive accelerometer ADXL-50

Electronics uses balanced signal and electrostatic force for feedback.

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3-axis accelerometer

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Another spring-mass system: Resonators

Capacitive accelerometer can act as a resonator if damping is removed.

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MEMS Resonator

If device is small the resonant frequency can be very high.

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Resonator

E – Young’s modulusρ - densitytc – cantilever thicknesslc – cantilever length

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Resonator

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MEMS AccelerometerSilicon Designs, Inc

Silicon Design's accelerometers use capacitance change due to acceleration force as the sensed parameter.

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MEMS Accelerometer

The sense element wing is a flat plate of nickel supported above the substrate surface by two torsion bars attached to a central pedestal.The structure is asymmetrically shaped so that one side is heavier than the other, resulting in a center of mass that is offset from the axis of the torsion bars.When an acceleration force produces a moment around the torsion bar axis, the plate or wing is free to rotate, constrained only by the spring constant of the torsion bars. On the substrate surface, beneath the sense element wing, two conductive capacitor plates are symmetrically located on each side of the torsion bar axis. The upper wing and the two lower capacitor plates on the substrate form two air-gap variable capacitors with a common connection. This creates a fully active capacitance bridge. When the wing rotates about the torsion bar axis, the average distance between the wing and one surface plate decreases, increasing the capacitance for that plate, while the distance to the other plate increases, decreasing its capacitance.

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Optical MEMS

ApplicationScannersProjectors

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MEMS microphone (optical)

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What is a Gyroscope?

A device that can measure angular motion or displacementApplications

Aerospace:Inertial guidance systems

Automotive:Angular rate sensors (for traction control, etc.)

Entertainment/consumer:Virtual reality sensors, pointing devices, etc.

Industrial automation:Motion control, robotics

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Gyroscope Principles

Principle of operationThe simplest gyroscopes use a high speed, rotating inertial disk that is loosely coupled to the frame holding it.A rotation in the frame imparts a torque (rotation) on the spinning disk, which precesses(rotates) as a result (conservation of angular momentum).

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Gyroscope Principles

Practical uses usually limit the movement to measure only one axis of rotation (roll, pitch or yaw).The induced torque is monitored by a meter which counteracts the torque with springs or a similar restoring force.

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Problems for MEMS gyroscopes

MEMS processes cannot produce devices with large inertial masses, nor can they produce freely “spinning” disks.Even the best MEMS motors still quickly slow down and stop if not externally actuated. The inertial mass of the wheels is very, very small. Furthermore, they cannot be made so that they process freely in 3D.

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MEMS Gyroscope principles

Coriolis effectMotion in a rotating reference frame leads to “sideways” movement.Can’t walk a straight line in a rotating reference (merry-go-round) without exerting a sideways force (or acceleration).

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Constrained motion means a force is imparted.By measuring the imparted force (or its effect on an oscillator), we can measure the angular velocity. Almost all MEMS gyroscopes use this feature.

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Tuning fork gyroscopeA tuning fork is simple example of the Corioliseffect and how it can be used to monitor angular motion.By measuring the amplitude of oscillation in the sideways direction, the angular motion can be deduced. Used in Daimler Benz AG MEMS gyroscope.

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Vibrating ring gyroscopeA ring is flexured back and forth in resonant mode. The Coriolis effect induces flexure that is sideways (and out of phase) with the driving flexure.Since the Coriolis force vibrates the ring sideways, it produces a second mode of vibration which adds to the first. The result is a “rotation” of the mode pattern of the ring. Most MEMS gyros use this method in closed-loop mode.

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MEMS Gyroscope-Examples

Delco Electronics Corp.

Vibrating ring assembly electroformed on CMOS substrate.Device is freestanding metal. High precision capacitance circuits monitor ring vibration and provide electrostatic actuation for closed loop operation.

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Silicon Sensing Systems(Formerly British Aerospace Systems) VSG ring sensor.Device is freestanding metal silicon with metal traces. External magnetic field is applied and current loops pass through the device initiating movement. Other metal loops are used to measure induced current.

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Daimler Benz AGTuning fork sensor (process flow).Device is fabricated from silicon with piezoelectric actuator (Al Nitride) and piezoresistive (diffused) sensor. SOI wafers are fusion bonded together to form final device.

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Roger Bosch GmbHTuning fork with lateral accelerometer.Device is fabricated from silicon using surface and bulk micromachining methods. On top of large bulk micromachined oscillator is surface micromachinedaccelerometer similar to the ADXL series. Actuation is by external magnetic field and inductive current loops.

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cont/-Process flow

Device is fabricated from silicon using surface and bulk micromachining methods. Deep silicon etch processes and MEMS level packaging.

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University of California, Irvine

Device made at UCI and at MicrofabricaDeep etched micromachined vibrating ring gyroscope.

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MEMS Gyroscope-Example Applications

SegwaySilicon Sensing Systems VSG ring sensor “Dynamic Stabilization”Five sensors used to monitor orientation of the scooter, sampled at 100 times/second. Sensors include VSG ring gyroscopes and liquid-filled tilt sensors.

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MEMS Gyroscope-Example Applications

Human motion sensing

Virtual Reality, Human/Machine interface/GamingConsumer and game markets require lower performance specs and lower costs.These represent an emerging market for inertial MEMS devices.

4. MEMS Sensor

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