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A. D. Yarbrough, PhDEngineering & Technology Group

The Aerospace CorporationEl Segundo, CA20 March, 2001

Sponsored byIEEE Distinguished Lecturer Series

&The Aerospace Corporation New Technology Overview Series

MEMS: MicroelectromechanicalSystems

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Outline• Introduction and Context• MMIC and MEMS Synergy

– Heritage & Definitions– Processing

• Structures Motivated by Space Applications• Advantages and Disadvantages• Summary & Conclusion• References, Resources & Acknowledgments

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Introduction• MEMS (MicroElectroMechanical Systems):

miniature devices with elements achieved by 2-and 3-dimensional silicon micromachining

• Identify MEMS applications relevant to satelliteand mobile communication systems– Highly integrated circuits leveraging

» RFIC/MMIC (RF and monolithic microwave integratedcircuit) technology

» Mature semiconductor industry fabrication techniquesdeveloped over 30+ years

– Innovative packaging– Greater emphasis on reliability

» Characterization of MEMS materials properties and testing

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• Leverage and combine technology strengths to synthesizehigh-performance, compact, cost-effective subsystems

• Apply MEMS technologies to achieve– higher levels of integration in millimeterwave

communications components– miniaturization AND integration of electronic and

mechanical components on same chip» leads to reduced subsystem volume, weight and cost

– increase performance per mm2 of wafer surface area• Develop semiconductor processes and capabilities to

enable fabrication of novel MEMS structures• Leverage in-house capabilities to advance the state of the

art in MEMS for space applications

Motivation

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The Nanosatellite Concept

From work of S. W. Janson, H. Helvajian & E. Y. Robinson, The Aerospace Corporation

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Aerospace/DARPA MEMS PicoSat Mission

PicoSat1

PicoSat2

PicoSat3

OPAL (Orbiting PicoSatAutomated Launcher)

150-ft diameter TrackingEarth Station AntennaSRI, International, Palo Alto, CA

Two communicatingon-orbitspace stations

100-ft tether

Comm link betweeneach PicoSat& ground station

Data Acquisition& Control PC

Comm link betweenPicoSats

NOTES:1. Drawing Not to Scale2. Three-week space station battery life3. Space stations referred to as “PicoSats”4. 902-928 MHz, 100mW max transmit power

750 kmAltitude

PicoSats ejectfrom OPAL

On-boardMEMS switchexperiments

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PICOSAT Goals and Applications

• PICOSAT as a Space Test Vehicle– MEMS and microsystems space technology– Testing, development, demonstration,

insertion• PICOSAT as part of operational

architecture– MEPSI (MEMS based PICOSAT Inspector)– Communications networks– Remote sensing networks– Cooperative constellations and phased arrays– On orbit robotics developments

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Merits of PICOSATs and Nanosats• Benefits

– Lower cost» Reduced fabrication and testing costs» Reduced integration and launch costs

– Rapid development and adaptation to payloads– Easily accommodated (ride-along) on any host

vehicle– Frequent flights (target 2 - 3 flights per year or

more)• Current Limitations

– Power, downlink (trade power for large groundantenna)

» Power source, data transfer rates

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First PicoSat Flight

• Jan 26, 2000 OSP1/Minotaur launch from VAFB– Two Minuteman stages, two Pegasus stages (first flight)

• JAWSAT– Successful deployment of OPAL, ASUSat1, FalconSAT

• Feb 6 deployment of PICOSATs from OPAL– Two tethered PICOSATs: 1 x 3 x 4 in., 270 gms each– Followed by STENSAT, Artemis PICOSAT deployments

• PICOSAT three-day mission (Feb 6-9)– Successful tracking and acquisition strategy– Successful two-way command and communications– MEMS switch operation data returned to Earth

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OSP (Orbital SubOrbital Program)/Minotaur* Launch Vehicle

MMFS

M56A

MMSS

SR19

Orion50XL

JAWSATOrion 38

*Modified Minute Man Missile

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PicoSat Mission Photos

PicoSats PicoSats under test in anechoicchamber at Aerospace Corporation

OPAL: Orbiting PicoSat Automated Launcherunder test at Stanford University’s Space SystemsDevelopment Laboratory

PicoSats being loaded into OPAL

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PicoSat Mission Photos

PicoSat ground station:SRI (Palo Alto) 50-m diameterdish

Ground PicoSat mounted on antenna feed

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MEMS/MMIC Synergy• Silicon micromachining has emerged as an

extension of IC fabrication technology• 30+ years of experience and process technology to

build upon• Recipes highly consistent with capabilities of

techniques and equipment present in a modestIC/MMIC fab facility

• Responds to need to integrate varied circuittopologies or interface miniaturized mechanicalcomponents with electronics

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• MEMS (MicroElectroMechanical Systems):miniature devices with elements achievedby 2- and 3-dimensional siliconmicromachining

• Bulk micromachining: use of wet & dryetching techniques, etch masks and etchstops to sculpt microelectromechanicaldevices from silicon substrate

Definitions and Terminology

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• Sacrificial layer: spacer layer in which structuralparts of device are encased; subsequentlydissolved away; used to produce released* or free-standing structures

• Surface micromachining: use of successivedeposition and patterning of structural andsacrificial materials on a substrate to fabricatemicroelectromechanical components (no substrateetching)

*released: structures suspended over a substrate by etchingaway the intermediate supporting layer on which they rest

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• Anisotropic etch : preferential etching ofsingle crystal silicon along specific crystalplanes

• ODE (orientation dependent etch): same asanisotropic etch

• LIGA (LIthografie, Galvanik, Abformung):process by which high aspect ratio platingmolds are fabricated– German words for lithography, electroforming,

injection molding

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Major IC and MEMS Fabrication Steps

FilmDeposition

Lithography

Etching

Dicing &Packaging Testing

Doping

Wafer

Masks

bulk etchingsurface micromachiningrelease processing }

alter conductivityetch stop{

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• Available for shared, low-cost micromachiningprojects– MUMPS = Multi-User MEMS Process (Cronos)– MOSIS = Metal Oxide Silicon Implementation Service

(Information Sciences Institute-USC)– Sandia National Laboratories SUMMiT Process– Advanced Micro Devices

• Aerospace resources– Extensive diagnostic tools– Center for Microtechnology

» “Silicon Etcher” laser processing

Foundry Services

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• Millimeterwave antenna array concept– Array to be combined with feed line structure as

part of an integrated “receiver on a chip” frontend concept

» Combine MEMS and MMIC technologies

Sample applications for satellite and mobile communication systems:

Device & Process Development

Integrated feed network

Antennas

LNALO

Mixer

RF IFBPFEmbedded Interconnects

Embedded Interconnects

BPF

BPF

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• Begin development of constituent elements for integratedreceiver front end– Form antenna reflector surface in thick oxide or SOG (spin on glass)

film– Metallize surface– Form deployed, hinged beam for antenna feed and support

Aerospace Corporation Patent:MICROMACHINED REFLECTOR ANTENNA SYSTEM AND METHOD

U.S. 6,008,776 (December 28, 1999) & U.S. 6,045,712 (April 4, 2000)

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• Eventual component forintegrated receiver front end– Electrostatically-driven,

micromachined switchmechanism combined with filter

– When actuated, one arm ofrotating switch contacts inputfeed for one of two nearby filterstructures

Aerospace Corporation Patent:MICROMACHINED ROTATING INTEGRATED SWITCH

U.S. 6,072,686 (June 6, 2000)

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• Build capabilities for the design, fabrication andtesting of MEMS structures

» Explore bulk & surface silicon micromachining techniques» * Develop release processes not vulnerable to stiction» Characterize process for deposition of thick films» Establish repeatability of wafer bonding process

– Explore stress, strain and fatigue mechanisms of filmsand materials used in MEMS structures

Aerospace Corporation Patent :METHOD OF HF VAPOR RELEASE OF MICROSTRUCTURES*

Notice of Allowance January 23, 2001

PolysiliconOxideSubstrate

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Sample Aerospace MEMS Structures

•Suspended polysilicon bridge test structure•~20 µm l x 2.5 µm w x 1500 A t•Confirms complete removal ofsacrificial oxide layer

•Released polysilicon cantilever beams•~40 µm l x 2 µm w x 1500 A t •No stiction•Anchors (supports) visible

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Sample AerospaceMEMS Structures

•Irregularly shaped released beams•~1500 A thick•Simulates meandering transmissionlines and circuit interconnects

•Close-up view of released beams•Smooth sidewalls and completelyclean substrate below

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Sample Aerospace MEMS Structures

Silicon Micromotor Lateral resonantmicroactuator

Vibrometer

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Advantages of MEMS Si Processing• Material is abundant, inexpensive, now can be

produced and processed controllably tounparalleled standards of purity and perfection

• Silicon processing based on thin deposited films andshapes highly amenable to miniaturization

• Circuits are batch fabricated, making them viablefrom a practical and commercial point of view

• Feature sizes readily achievable with modestprocessing capabilities

• Expanded data gathering and sensing• Reduced weight and volume

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Disadvantages of MEMS Si Processing• IC processing considerations restrict design

geometries in direction normal to wafer surface• Understanding film mechanical properties• Component reliability• Compatibility of surface and bulk micromachining

processes, which differ significantly• Translate achievements in Silicon to Gallium

Arsenide environment• No widely accepted space qualification plan• New paradigm; need to overcome perception as

risky technology

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• Develop capability for metal deposition on vertical-walled structures(planetary evaporation)

• Enhance techniques for reliable deposition of films thicker than 1 mmand processing thick wafers

• Produce electrostatically-driven cantilever beams– switches, millimeterwave component structures

• Evaluate strengths and areas of application for LIGA (LIthografie,Galvanik, Abformung) process by which high aspect ratio platingmolds are fabricated

• Use MEMS to add capability to existing systems• Combine MEMS and MMIC technologies• Demonstrate components for receiver-on-a- chip• Characterize film properties: stress, strain & fatigue mechanisms• Develop novel MEMS for new space communications concepts

Aerospace Future Plans

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Technology challenges• IC processing considerations restrict design

geometries in direction normal to wafersurface

• Issues of pattern definition and mechanicalproperties

• Compatibility of materials for processing• Surface and bulk micromachining processing

significantly differ• Wafer bonding and handling• Resistance to new technology and paradigm

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Conclusions• For many electromechanical systems, miniaturization is

neither practical, nor desirable• Nonetheless, micromechanical structures for novel

millimeterwave components, sensing and actuation can oftenimprove the overall performance of the macroscopic system

• Be prepared to evaluate and support its integration intocustomer’s space systems at the appropriate time

• Need for compact, highly-capable, cost-effective subsystemsfor mobile and satellite communications can be met by thefusion of MEMS and microwave integrated circuittechnologies

• Novel millimeterwave concepts emerging include:– integrated receivers– micromachined antenna arrays and switches– embedded interconnects and waveguides

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References, Resources&Acknowledgements

• Electronic Systems Division, Microelectronics Technology Department, Center forMicrotechnology, The Aerospace Corporation

• National Nanofabrication Facility, Cornell University• Berkeley Sensor and Actuation Center, UC-Berkeley• Jet Propulsion Laboratory and Cal Tech• Case Western Reserve University• Center for Integrated Sensors and Circuits, University of Michigan• Sandia National Laboratory• Microelectronics Center of North Carolina (MCNC)-Cronos• Air Force Research Lab• Rockwell Science Center• Stanford University• SMC/TE Space Test Program• Space Command Space Surveillance Network (SSN)