View
246
Download
3
Category
Tags:
Preview:
Citation preview
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)
Recommended