MEMS Overview
SPEAKER
• Andrew Mason, Asst. Professor in Electrical and Computer Engineering
TOPIC
• Overview of Micro-Electro-Mechanical Systems (MEMS)
OUTLINE
• Overview of MEMS & Microsystems
Navid Yazdi
• Micromachining & MEMS process technology
Navid Yazdi
• Micro-electro-mechanical devices & microsensors
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Inertial sensors
Pressure sensors
Bio-sensors
Shock sensors
Navid Yazdi
Navid Yazdi
Andrew Mason
Andrew Mason
Integrated Microsystems
Andrew Mason
MEMS Overview, Prof. A. Mason
Page 1
What is MEMS?
• MEMS = MicroMicro-ElectoElecto-Mechanical Systems
– creation of 33-dimensional structures using integrated circuits
fabrication technologies and special micromachining processes
– typically done on silicon or glass (SiO2) wafers
• MEMS Devices and Structures
– transducers
• microsensors and microactuators
– mechanically functional microstructures
• microfluidics:
microfluidics: valves, pumps, flow channels
• microengines:
microengines: gears, turbines, combustion engines
• Integrated Microsystems
– integrated circuitry and transducers combined to perform a task
autonomously or with the aid of a host computer
– MEMS components provide interface to nonnon-electrical world
• sensors provide inputs from non-electronic events
• actuators provide outputs to non-electronic events
MEMS Overview, Prof. A. Mason
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Why Use MEMS?
• Motivation and Benefits
– Small Size
– Light Weight
– Enhanced Performance & Reliability
• high resolution devices
• array of devices
– Low Cost (from batch fabrication)
• Applications
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Automotive System
Health Care
Automated Manufacturing
Instrumentation
Environmental Monitoring & Control
Consumer Products
Aerospace
• MEMSMEMS-based
Microsystems
– highly integrated systems
– sensing
– actuation
– computation
– control
– communication
MEMS Overview, Prof. A. Mason
Page 3
Example MEMS-Based Microsystem
“Micro Cluster” Environmental Monitoring Microinstrument
(developed at UU-Mich in the 1990s, A. Mason, K. Wise, et. al.)
Humidity
Sensor
Power
Management
- Power Management
Accelerometer
RF Transmitter
Interface
Chips
Microcontroller
- Communication
35mm
Pressure
Sensor
Integrated Features
- Control
- Microcontroller
- RF Transceiver
- Sensing
- Pressure
- Humidity
- Temperature
- Vibration
I/O Connector
On/Off Switch
60mm
- Packaging
MEMS Overview, Prof. A. Mason
Page 4
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MEMS Fabrication Technologies
• Applying Micromachining to create 3-D structures using
2-D processing
2-D IC fabrication
technology
3-D structures
micromachining
• Micromachining Processes
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bulk and surface micromachining
isotropic etching
anisotropic etching
dissolved wafer process
deep reactive ion etching
anodic and fusion bonding
micromolding
MEMS Overview, Prof. A. Mason
Page 5
Surface vs. Bulk Micromachining
Bulk Micromachining:
Backside etch
Si Substrate
Bulk Micromachining:
Front-side Etch pit
typically polysilicon
Surface Micromachined
Structure
MEMS Overview, Prof. A. Mason
Si Substrate
Page 6
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Isotropic Etching of Silicon
• Isotropic etchant
– etches in all directions
– forms rounded pits in surface of wafer
• Most common solution
– HNA: Mixture of HF, HNO3, Acetic acid (CH3COOH)
With agitation:
Good reactant mass transport
Without agitation
MEMS Overview, Prof. A. Mason
Page 7
Anisotropic Etching of Silicon
• Anisotropic etchant
– directional-dependant etch; based on crystal planes
– forms flat-surface pits in surface of wafer
• Common anisotropic etchants
– EDP, KOH, TMAH
(111) Surface
orientation
(100) Surface orientation
54.74°
Silicon
Anisotropic wet etching using EDP, KOH: (100) surface
- Etch stop on (111) plane
MEMS Overview, Prof. A. Mason
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Anisotropic Etching: Convex vs. Concave Corners
masking layer not attacked by Si etchant
Concave
corner
Convex
corner
Top View
Cantilever Beam
(100)
Masking layer
(111)
Buried etch stop layer
(SiO2 in SOI wafers)
Side View
Silicon
MEMS Overview, Prof. A. Mason
Page 9
Anisotropic Etching of Silicon: Example
bulk micromachined
silicon proof mass
MEMS Overview, Prof. A. Mason
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Dissolved Wafer Process
• Structure created by “diffusion masking layer”
• heavily p-dope silicon (p++)
• Dissolve bulk of silicon to release the p++ structure
Released p++ structure
P++
S ili
con
Dopant selective
etch (e.g. EDP)
Silicon
K.D. Wise, K. Najafi, Univ of Michigan
MEMS Overview, Prof. A. Mason
Page 11
Dissolved Wafer Process Example
• Shock Switch
– weighted cantilever beam with contacts that close by
acceleration (shock)
• Fabrication Flow
– create anchor, weight, support beam, and contact on Si
– create cavity and contact on glass
– bond wafers and then dissolve the Si wafer
LPCVD layer deposition
for beams
Anodic bonding glass to
silicon
Dissolve silicon bulk
and release structure
MEMS Overview, Prof. A. Mason
Page 12
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Deep Reactive Ion Etching (DRIE)
• Reactive Ion Etching = RIE
– mechanical (ion) etching in plasma for chemical selectivity
• Deep RIE
– creates high aspect ratio patterns, narrow and deep
Top view
A
A
A-A cross section
mask
• Trench-Refill process
Silicon
– can fill the etched “trench” with another material
MEMS Overview, Prof. A. Mason
Page 13
Glass-Si Anodic Bonding
• Bonding a glass wafer to a silicon wafer
– both wafer can (and generally are) patterned with structures
• Application
– creating sealed cavities on a wafer surface
• can be sealed in vacuum
– hermetic packaging
• Lead Transfer
– need to bring the metal leads out of from sealed cavities
Need sealing
Metal
Interconnect
Silicon
Glass
MEMS Overview, Prof. A. Mason
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Silicon-Silicon Fusion Bonding
• Two silicon wafer with/without SiO2 can be bonded
• Advantages: No thermal mismatch
• Needs contamination free, smooth, and flat wafers
(e.g. surface roughness ~5°A)
• Process Flow
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Clean wafers
Make the surfaces hydrophilic (e.g. dip in Nitric Acid)
Rinse-Dry
Place the wafers together apply pressure
H2 or N2 anneal at 800-1000°C
MEMS Overview, Prof. A. Mason
Page 15
Combined Bulk-Surface Process: Molding
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Etch silicon with high aspect ratio (e.g., DRIE)
Refill partially with sacrificial layer (e.g. silicon oxide)
Refill completely with structural layer (e.g. polysilicon)
Example: U-Mich Precision Inertial Sensor
Polysilicon Electrode with Damping Holes
Polysilicon
Electrode
Nitride
Standoff
Air Gap
Damping Holes
Air Gap
Silicon
Proof Mass
Air Gap
Nitride
Standoff
Electrode
Dimple
Vertical
Stiffener
Vertical
Stiffener
Silicon
Proof Mass
N. Yazdi & K. Najafi, Transducers’97.
MEMS Overview, Prof. A. Mason
Page 16
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Combined Bulk-Surface Process:
Precision Inertial Sensors
Dielectric Layer
Metal Pads
Metal Pad
A
A
Silicon Proof mass
Support
Rim
Damping Holes
N. Yazdi , A. Salian & K. Najafi, MEMS’99.
MEMS Overview, Prof. A. Mason
Page 17
LIGA Process
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LIGA: Lithographie, Galvanoformung, Abformung
Form high aspect ratio structures on top of wafer
Uses molding and electroplating
Synchrotron Radiation (X-Ray) used
• Features
– Aspect ratio: 100:1
– Gap: 0.25µm
– Size: a few millimeters
uses multiple
polymethyl
metharylate
(PMMA) layers
MEMS Overview, Prof. A. Mason
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LIGA Process: Example
Guckel, IEEE Proceedings, Aug. 1998.
MEMS Overview, Prof. A. Mason
Page 19
Monolithic Integration of MEMS and ICs
Why Monolithic?
Performance:
- Reduce parasitics due to interconnecting devices
- Reduce noise & crosstalk
Size:
- Reduce pin count
- Reduce package volume
Cost:
- Integration with signal-processing better functionality
- Reduce packaging cost
- Self test & calibration at wafer level
MEMS Overview, Prof. A. Mason
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IC + MEMS Process Examples
UC Berekely Integrated CMOS & surface micromachining technology
• CMOS first and MEMS second
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CMOS circuit passivated using silicon nitride
Tungsten interconnects for CMOS
J. Bustillo, R. Howe, R. Muller, IEEE Proceedings Aug. 98
• Sandia Integrated CMOS & surface micromachining technology
– MEMS first in recessed cavity
– CMOS second after planarization
J. Smith et. al., IEDM’95
MEMS Overview, Prof. A. Mason
Page 21
MEMS Examples
Neural Recording Probes
• Monolithic
Integration of Wafer-Dissolved
Process and IC Technology
OUTPUT LEADS
P++ RIM
INTERCONNECTING
LEADS
SIGNAL PROCESSING
CIRCUITRY
RECORDING/STIMULATING
SITES
- Neural probes with signal processing circuitry
- P++ shanks
- P++ rim and timed etch to protect Najafi,
active
circuitry
Wise,
JSSC-21 (6), May 1986
SUPPORTING SILICON
SUBSTRATE
MEMS Overview, Prof. A. Mason
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Example: Capacitive Accelerometer
Suspension
Vertical
accelerometer
Suspension
Anchor
Mass
Anchor
Substrate
Substrate
Conductive
Electrode
(a)
Cs
Sense
Capacitance
(b)
One of the sense
Bidirectional capacitor elements
Sense Fingers
Anchors
Mass
Anchors
Mass
Mass
Cs
Lateral
Suspensions
accelerometer (a)
Bidirectional
Sense Fingers
Substrate
Suspensions
(b)
MEMS Overview, Prof. A. Mason
Page 23
Example: Z-Axis Torsional Accelerometer
Torque
produced by
the mass
Anchor
A'
Support
beam
Anchor
wm
T
T
C
Fixed
Sense
Fingers
Inertial Element
Support
posts on
glass
A
Glass Substrate
Selvakumar, Najafi, JMEMS 1998.
Anchor
Torsional suspension
Mass plate
Sense
fingers
MEMS Overview, Prof. A. Mason
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Capacitive Accelerometer
3-Axis Monolithic Surface Micromachined Accelerometer
Analog Devices ADXL50
Sense & Feedback
Interdigitated Fingers
Proofmass
Suspension
Courtesy of Lemkin, Boser, Trasnducers’97.
Courtesy of Kevin Chau, Analog Devices Inc.
MEMS Overview, Prof. A. Mason
Page 25
All-Silicon Micro-G Accelerometer
Metal Pads
Damping Holes
Anchor Dielectric
A
Silicon
Proofmass
Top View
Polysilicon
Top/Bottom Polysilicon
Electrode Stiffeners
A
Proofmass
Suspension
Beams
A-A Cross-Sectional View
Stiffened Polysilicon
Electrode
Proofmass
Frame
Stiffened Polysilicon
Electrode
Proofmass
MEMS Overview, Prof. A. Mason
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MEMS Gyroscopes
GM & UM Ring Gyroscope
Draper’s Tuning Fork Gyroscope
Perforated
masses (tines)
Drive
Combs
Suspension
Electrodes
Ring
structure
MEMS Overview, Prof. A. Mason
Page 27
Capacitive Pressure Sensors
Consists of two components:
• Fixed electrode
• Flexible diaphragm forming a moving electrode
• Sealed vacuum cavity between the two electrodes
Diaphragm (Upper electrode)
Lower electrode
Silicon
P++ Si
Ti/Pt/Au
Poly-Si
Tab Contact
SiO2/Si3N4/SiO2
External lead for
Glass Substrate
glass electrode
A. Chavan, K.D. Wise, Transducers’97
MEMS Overview, Prof. A. Mason
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Integrated Microsystems Architecture
• Flexible Architectures
– reconfigurable
• new/different sensors can be added
– sensor bus
Power
Management
Chip
Telemetry
Humidity
Sensor
Accelerometer
Transducer
Array
Pressure
Transducer
Array
EPROM
RAM
Crystal
SCI
Temp
Sensor
SPI
Wired
I/O Port
MCU
Timer
Analog
Interface
Analog
Interface
Bus
Interface
Bus
Interface
A/D
Digital
Interface
Bus
Interface
Battery
Capacitive Sensor
Interface Chips
Sensor Bus
MEMS Overview, Prof. A. Mason
Page 29
Microsystem Component: Interface Circuit
• Generic capacitive sensor interface
– sensor readout
– sensor bus communication
– programmable operation –useful for range of sensors
Element
Select
Cf
Digital Gain
Control
Charge
Integrator
Variable
Gain Amp
fs
Sensor1
MUX
Sensor2
Sample
& Hold
Temperature
Sensor
Sensor5
Programmable
Reference Capacitor
Sensor6
Self-Test
DAC
MUX
Digital
Control
b2
b1
b0
Element
Select
Bus Interface
Address Command Data
Register Decoder Latch
Self-Test
Command
Sensor Bus
MEMS Overview, Prof. A. Mason
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Microsystem Component: Shock switch
• System wake-up switch
– allows events to be captured while system is in sleep mode
– useful for system-level power management
– implements several shock thresholds
Suspension B eam
Silicon Anchor
Support
Altar
Air
Gap
Cantilevered
Mass
Iner tial
Mass
Metal Pads
& Interconnect
Symm etric
Contacts
MEMS Overview, Prof. A. Mason
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