Navid Yazdi. • Micromachining & MEMS process technology. Navid Yazdi. • Micro
-electro-mechanical devices & microsensors. – Inertial sensors. Navid Yazdi.
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 – – – –
•
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
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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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1
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 – – – – – – –
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
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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
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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 – – – – – – –
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
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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
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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
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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
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Anisotropic Etching of Silicon: Example
bulk micromachined silicon proof mass
MEMS Overview, Prof. A. Mason
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5
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
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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
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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
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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 – – – – –
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
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Combined Bulk-Surface Process: Molding • • • •
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
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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
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LIGA Process • • • •
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
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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 • •
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
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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
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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
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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
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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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