Chapter 14:Fundamentals of Microelectromechanical Systems

Jon MahEric Wilson

14.1 What are MEMS?

Micro-electro-mechanical systemsExamplesBenefitsNeed for fabrication technologies

What are Sensors and Actuators?

Sensors Physical input Weak Signal

Actuator Output or

processing Some physical

change

14.2 What are MEMS Applications?

NOW Accelerometer Pressure and

chemical flow analysis

Inkjet print heads mm-μm

FURURE Medical

diagnostics Drug delivery

(No more Medellin cartel!!!)

(Just kidding, different drugs)

μm-nm

Fundamentals of MEMS Devices

Silicon Already in use Manipulatable conductivity Allows for integration

Thin-Film Materials Silicon dioxide Silicon nitride

Micromachining Fabrication

Thin Films Layers (μm) put on

Si Photomask

Positive or negative

Wet Etching Isotropic Anisotropic KOH

Micromachining Fabrication II

Dry Etching RIE DRIE

Rate-Modified Etching Cover with Boron Wet etch with KOH

Lift-Off ProcessLift-off process Noble metals For unetchable

materials Acetone

Excimer laser technique Burn with UV

Surface MicromachiningGrow silicon dioxideApply photoresistExpose and developEtch silicon dioxideRemove photoresistDeposit polysiliconRemove silicon dioxide

Bulk micromachining Same, except not

LIGA Technique

Lithographie, Galvanoformung, and Abformung Or, lithography,

plating and molding

High aspect ratioMany materialsX-Rays

MEMS PackagingWafer stack thicknessWafer dicing concerns Before After

Thermal managementUnique considerationsProtective coating

Hermetic Packaging andDie Attach Process

Hermetic packaging Prevents diffusion of water Materials No organics of plastics

Die Attach Process Thermal considerations Cracking or creep

Wiring and Interconnects and Flip Chip

Wiring and interconnects Gold > Aluminum Thermocompression Bonding Thermosonic Gold Bonding

Flip Chip Intimate attachments Cram everything together

MEMS PackagingPurposes Reduce EMI Dissipate Heat Minimize CTE Deliver Required Power Survive Environment

Types of MEMS Packages

Ceramic Packaging Hermetic when sealed High Young’s Modulus Flip Chip or

Wirebonding

Plastic Packaging Not Hermetic Postmolding Premolding

Metal Packaging Hermetic when sealed Easy to assemble Low Pin Count

Typical MEMS Devices

Sensors Pressure Sensors Accelerometers

Actuators Gyroscopes High Aspect Ratio Electrostatic

Resonators Thermal Actuators Magnetic Actuators Comb-drives

Pressure Sensors

Gauge Pressure SensorsDifferential Pressure SensorsAbsolute Pressure Sensors

AccelerometersApplications: Air bag crash sensors Active suspension

systems Antilock brake

systems Ride control systems

Units of mV/g

ActuatorsHigh aspect ratio electrostatic resonatorPiezoelectric crystalsThermal actuatorsComb-drivesMagnetic actuators

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

MEMS Accelerometer

Mass, Spring, Damper Model

MEMS Accelerometer (cont’d)

MEMS Accelerometer (cont’d)

MEMS Gyroscopes

Typically Vibratory Gyroscopes Utilize Coriolis Acceleration (“fictional force”) Due to rotating reference frame

Types of Vibratory Gyroscopes

Vibrating Beam, Vibrating Disk, Vibrating Shell

Vibrating Ring GyroscopeCapacitive drive and sense uses perturbations to the resonance of the ring structure to measure rate

Vibrating Ring Gyroscope (cont’d)

qsense – amplitude of secondary flexural modeAg – angular gain of ring structureQ – quality factor of the structureω0 – angular flexural resonance frequencyqdrive – vibration amplitude of the primary flexural mode

Ωz – rotation rate around the normal axis

Flexural Modes of Vibrating Ring Gyro

First Mode Second Mode

Polysilicon Ring Gyro

80μm thick, 1mm wide with 1.2μm gapcapacitance changes on order of 10-18F!

Fabrication of HARPSS

High Aspect ratio combined poly- and single-crystal siliconUtilizes Deep RIE of Si

Interface and Control Electronics for Vibrating Ring Gyro

Open Loop gyros have bandwidth of a few hertzClosed Loop gyros bandwidth limited by readout and control electronics

Brownian Noise

Due to Brownian motion of ring structure Random movement caused by

molecular collisions Fundamental limit on resolution

Microstructures with large mass and high resonance frequencies reduce Brownian noise in vibratory gyros

Summary and Future Trends

Current MEMS devices are used most in automotive, medical, consumer, industrial and aerospace applicationsBulk micromachining, microfabrication, and surface micromachining technologies drive MEMS size and shapesPackaging requires design for environment (i.e. pressure sensors in oil)Mechanical fatigue, stiction, and hermeticity are main failure mechanismsVibrating ring gyro case study (fabrication, operation, control electronics, and Brownian noise)