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Chapter 14:Fundamentals of

MicroelectromechanicalSystems

Jon MahEric Wilson

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14.1 What are MEMS?

Micro-electro-mechanical systems

Examples

BenefitsNeed for fabrication technologies

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What are Sensors and Actuators?

Sensors

Physical input

Weak Signal

Actuator

Output or processing Some physical

change

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14.2 What are MEMS Applications?

NOW

Accelerometer

Pressure andchemical flowanalysis

Inkjet print heads

mm-m

FURURE

Medical diagnostics

Drug delivery (No more Medellin

cartel!!!)

(Just kidding,different drugs)

m-nm

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Fundamentals of MEMSDevices

Silicon

Already in use

Manipulatable conductivity

Allows for integration

Thin-Film Materials

Silicon dioxide

Silicon nitride

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Micromachining Fabrication

Thin Films Layers (m) put on

Si

Photomask Positive or negative

Wet Etching Isotropic

Anisotropic

KOH

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Micromachining Fabrication II

Dry Etching RIE

DRIE

Rate-Modified

Etching Cover with Boron

Wet etch with KOH

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Lift-Off Process

Lift-off process

Noble metals

For unetchable materials

Acetone

Excimer laser technique

Burn with UV

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Surface MicromachiningGrow silicon dioxide

Apply photoresist

Expose and develop

Etch silicon dioxideRemove photoresist

Deposit polysilicon

Remove silicon dioxide

Bulk micromachining

Same, except not

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LIGA Technique

Lithographie,Galvanoformung,

and Abformung Or, lithography,

plating and molding

High aspect ratio

Many materialsX-Rays

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MEMS Packaging

Wafer stack thickness

Wafer dicing concerns

Before

After

Thermal management

Unique considerations

Protective coating

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Hermetic Packaging andDie Attach Process

Hermetic packaging

Prevents diffusion of water

Materials

No organics of plastics

Die Attach Process

Thermal considerations

Cracking or creep

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Wiring and Interconnects andFlip Chip

Wiring and interconnects

Gold > Aluminum

Thermocompression Bonding

Thermosonic Gold Bonding

Flip Chip

Intimate attachments Cram everything together

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MEMS PackagingPurposes

Reduce EMI

Dissipate Heat

Minimize CTE Deliver Required Power

Survive Environment

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Types of MEMS Packages

Ceramic Packaging

Hermetic when sealed

High Youngs Modulus

Flip Chip or Wirebonding

Plastic Packaging Not Hermetic

Postmolding

Premolding

Metal Packaging Hermetic when sealed

Easy to assemble

Low Pin Count

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Typical MEMS Devices

Sensors

Pressure Sensors

Accelerometers

Actuators

Gyroscopes

High Aspect Ratio Electrostatic Resonators

Thermal Actuators

Magnetic Actuators

Comb-drives

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Pressure Sensors

Gauge PressureSensors

Differential PressureSensors

Absolute PressureSensors

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Accelerometers Applications:

Air bag crash sensors

Active suspension

systems Antilock brake systems

Ride control systems

Units of mV/g

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Actuators

High aspect ratioelectrostaticresonator

Piezoelectric crystals

Thermal actuators

Comb-drives

Magnetic actuators

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Failure Mechanisms

Failure by Stiction and Wear Cause of most MEMS failures Microscopic adhesion Corrosion

Delamination Due to bonding between dissimilar materials

Environmentally Induced Failures Thermal cycle, shock, vibration, humidity, radiation

Cyclic Mechanical Fatigue Critical for comb and membrane MEMS Causes changes in elasticity

Mechanical Dampening Effect Moving parts at resonance

Loss of Hermeticity

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MEMS Accelerometer

Mass, Spring, Damper Model

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MEMS Accelerometer (contd)

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MEMS Gyroscopes

Typically Vibratory Gyroscopes

Utilize Coriolis Acceleration (fictional force)

Due to rotating reference frame

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Types of Vibratory Gyroscopes

Vibrating Beam, Vibrating Disk, Vibrating Shell

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Vibrating Ring GyroscopeCapacitive drive and sense uses perturbations tothe resonance of the ring structure to measurerate

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Vibrating Ring Gyroscope (contd)

qsense amplitude of secondary flexural mode

Ag angular gain of ring structure

Q quality factor of the structure

0 ± angular flexural resonance frequencyqdrive ± vibration amplitude of the primary flexural mode

z ± rotation rate around the normal axis

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Flexural Modes of Vibrating Ring Gyro

First Mode Second Mode

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Polysilicon Ring Gyro

80m thick, 1mm wide with 1.2m gap

capacitance changes on order of 10-18F!

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Fabrication of HARPSS

High Aspect ratiocombined poly- and

single-crystal siliconUtilizes Deep RIE of Si

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Interface and Control Electronics for Vibrating Ring Gyro

Open Loop gyros have bandwidth of a few hertz

Closed Loop gyros bandwidth limited by readout and control electronics

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Brownian Noise

Due to Brownian motion of ring structure

Random movement caused by molecularcollisions

Fundamental limit on resolution

Microstructures with large mass and highresonance frequencies reduce Brownian

noise in vibratory gyros

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Summary and Future Trends

Current MEMS devices are used most inautomotive, medical, consumer, industrial andaerospace applicationsBulk micromachining, microfabrication, and

surface micromachining technologies driveMEMS size and shapesPackaging requires design for environment (i.e.pressure sensors in oil)

Mechanical fatigue, stiction, and hermeticity aremain failure mechanisms Vibrating ring gyro case study (fabrication,operation, control electronics, and Browniannoise)

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