1

An Introduction to Microelectromechanical Systems(MEMS)

Bing-Feng Ju, ProfessorInstitute of Mechatronic Control Engineering,

College of Mechanical and Energy Engineering,Zhejiang UniversityP.R.China, 310027

Email: mbfju@zju.edu.cnTel: 86-571-8795-1730Fax: 86-571-8795-1941

Presented toGraduate students at

College of Mechanical and Energy EngineeringZhejiang University

February to May 2010

Who am I?Education: Ph.D. from Zhejiang University, 1999

MEng from Harbin Institute of Technology, 1996BEng from Harbin Institute of Technology, 1994(All degrees were in mechanical engineering; Ph.D. thesis in Precision Metrology)

Research and Teaching experiences:2006.12-Present

College of Mechanical and Energy Engineering, Zhejiang UniversityPosition: Professor (2007.12 Ph.D. Tutor Qualification)

2004.12-2007.04Department of Nanomechanics, Tohoku University(東北大学), Japan Position: Assistant Professor

2003.11-2004.11Department of Nanomechanics, Tohoku University(東北大学), JapanPosition: JSPS Young Foreigner Scientist

2002.05-2003.11DSO National Laboratories, SingaporePosition: Research ScientistAdjunct Assistant Professor of National University of Singapore (NUS)

2000.05-2002.05School of Mechanical & Aerospace Engineering, Nanyang Technological University (NTU), Singapore Position: Postdoctoral Research Fellow

Courses taught: BioMEMS, Precision & Nao Metrology

CONTENTSelf-introduction1. Overview of MEMS and Microsystems

Working Principles of Microsystems2. The Scaling Laws

Electromechanical Design of MEMS and Microsystems3. Material for MEMS and Microsystems

Part 1: Silicon and silicon compoundsPart 2: Piezoelectric and polymers

4. Microfabrication ProcessesPart 1: Photolithography, doping with ion implantation and diffusionPart 2: EtchingPart 3: Depositions: physical, chemical and epitaxy

5. MicromanufacturingAssembly, Packaging and Testing to Nanoscale Engineering

Part 1: MicroassemblyPart 2: Packaging with surface and wire bondingPart 3: Reliability and testing

6. Introduction to Nanoscale EngineeringPart 1: Overview of nanoscale engineeringPart 2: Material characterization and measurements

Textbooks:1. MEMS and Microsystems: design and manufacture, by Tai-Ran Hsu, McGraw-Hill Companies, Boston, 2002 (ISBN 0-07-239391-2)中译本：徐泰然，《 MEMS和微系统—设计与制造》 ，机械工业出版社，20042. Albert P. Pisano, An Introduction to Microelectromechanical Systems Engineering, ArtechHouse, 20003. Marc Madou, Fundamentals of Microfabrication, CRC Press, 20024. 庄达人，《VLSI制造技术》，高立图书有限公司，1996

Journals:1. Journal of MEMShttp://www.ieee.org/pub_preview/mems_toc.html2. Sensors Journal, IEEEhttp://ieeexplore.ieee.org/xpl/RecentIssue.jsp?puNumber=73613. Journal of Micromechanics and Microengineeringhttp://www.iop.org/Journals/jmCovering microelectronics and vacuum microelectronics, this journal focuses on fundamental work at the structural, devices and systems levels, including new developments in practical applications.4. Sensors and Actuators A: Physicalhttp://www.elsevier.com:80/inca/publications/store/5/0/4/1/0/3/

MEMS-ZJU

MEMS-ZJU

MEMS-ZJU

2

MEMS-Related Newsletters:• Micromachine Devices is a very thorough newsletter on MEMS and the MEMS industry. Contact

the editor Mr. Sid Marshall at:sid_marshall@msn.com for your free subscription. • R&D Magazine periodically contains articles on MEMS and related areas.

Alternative website: http://www.manufacturing.net/magazine/rd/index.htm

• MST News based in Germany is a newsletter (available in English) which focuses on European MEMS/MST activities. Also available from MST are "special reports" on US and Japanese MEMS/MST activities.

• Sensor Business Digest covers the sensor industry. Contact the editor Peter Adrian forsubscription information at: 415-345-7018.

• Microtechnology News published by the Business Communications Company, Inc. (BCC) offers an on-line sample issue. (http://www.vdivde-it.de/mst)

• Nanotech Alert is a newsletter from John Wiley & Sons Technical Insights, covering the MEMS and Nanotechnology industries. (http://www.wiley.com/technical_insights)

• Sensors Magazine is a general magazine covering all aspects of the sensors industry. (http://www.sensorsmag.com/)

Lecture 1（Part I）

Overview of MEMS and Microsystems

Unit

10-9m-nm:nano-meter10-6m-µm:micro-meter10-3m-mm:milli-meter10-2m-cm:centi-meter10-1m-dm: deci-meter

10-15m-fm:femto-meter10-12m-pm:pico-meter

10-18m-am:atto-meter

106m-Mm:mega-meter109m-Gm:giga-meter1012m-Tm:tera-meter

102m-hm:hecto-meter103m-km:kilo-meter

101m-dam:deca-meter100m-m:meter

WHAT IS MEMS?MEMS = MicroElectroMechanical System

Any engineering system that performs electrical and mechanical functions with components in micrometers is a MEMS. (1 µm = 1/10 of human hair)

Available MEMS products include:

Microsensors (To sense and detect certain physical, chemical, biological and optical quantity and convert it into electrical output signal)

Microactuators (to operate a device component, e.g., valves, pumps, electricaland optical relays and switches; grippers, tweezers and tongs; linear and rotary motors; micro gyroscopes, etc.)

Read/write heads in computer storage systems. Inkjet printer heads. Microdevices components (palm-top reconnaissance aircrafts, toy cars, etc.)

MEMS-ZJU

MEMS-ZJU

MEMS-ZJU

3

HOW SMALL ARE MEMS DEVICES?

They can be of the size of a rice grain, or smaller!

Three examples:

- Inertia sensors for airbag deployment systems in automobiles

- A microcar

- Robot musician

Inertia Sensor for Automobile “Air Bag” Deployment System

Micro inertia sensor (accelerometer) in place:

(Courtesy of Analog Devices, Inc)

Sensor-on-a-chip:(2 mm x 3 mm-smaller than a

rice grain)

Micro Cars(Courtesy of Denso Research Laboratories, Denso Corporation, Aichi, Japan)

Rice grains

Robot musician(Waseda University, Japan)

Over 100 micro-sensors and micro-actuators by MEMS technology

MEMS-ZJU

MEMS-ZJU

MEMS-ZJU

4

MEMS = a major milestone in Miniaturization –

A leading technology for the 21st Century, and

an inevitable trend in industrial products and systems development

MiniaturizationAn irresistible trend in the New Century

Miniaturization of Digital Computers- A remarkable case of miniaturization!

The ENIAC Computer in 1946A “Lap-top” Computer in 1996

A “Palm-top” Computer in 2003

Size: 106 downPower: 106 up

Size: 108 downPower: 108 up

This spectacular miniaturization took place in 50 years!!

The ENIAC computer- 50 years later

Principal Driving Force for the 21st Century Industrial Technology

There has been increasing strong market demand for:

“Intelligent,”

“Robust,”

“Multi-functional,” and

“Low-cost” industrial products.

Miniaturization is the only viable solution to satisfy such market demand

Market Demand for Intelligent, Robusting, Smaller, Multi-Functional Products - the evolution of cellular phones

Mobil phones 15 Years Ago: Current State-of-the Art:

Transceive voice only

Transceive voice+ others(Video-camera, e-mails, calendar, and access to Internet; and a PC

with key board; GPS and multimedia entertainment)

Size reduction

Palm-top Wireless PC

The only solution is to pack many miniature function components into the device

MEMS-ZJU

MEMS-ZJU

MEMS-ZJU

5

Miniaturization Makes Engineering Sense !!!

• Small systems tend to move or stop more quickly due to low mechanical inertia.It is thus ideal for precision movements and for rapid actuation.

• Miniaturized systems encounter less thermal distortion and mechanical vibrationdue to low mass.

• Miniaturized devices are particularly suited for biomedical and aerospace applications due to their minute sizes and weight.

• Small systems have higher dimensional stability at high temperature due tolow thermal expansion.

• Smaller size of the systems means less space requirements. This allows the packaging of more functional components in a single device.

• Less material requirements mean low cost of production and transportation.

• Ready mass production in batches.

Enabling Technologies for Miniaturization

Miniature devices(1 nm - 1 mm)

** 1 nm = 10-9 m ≈ span of 10 H2 atoms

Microsystems Technology(MST)

(1 μm - 1 mm)* Initiated in 1947 with the invention of transistors, but the term “Micromachining”was coined in 1982

* 1 μm = 10-6 m ≈ one-tenth of human hair

Nanotechnology (NT)(0.1 nm – 0. 1 μm)**

Inspired by Feynman in 1959, with active R&D began in around 1995There is a long way to building nanodevices!

A top-down approach

A bottom-up approach

The Lucrative Revenue Prospects for Miniaturized Industrial Products

Microsystems technology:$43 billion - $132 billion* by Year 2005( *High revenue projection is based on different definitions

used for MST products)

Nanotechnology:$50 million in Year 2001$26.5 billion in Year 2003(if include products involving parts produced by nanotechnology)

$1 trillion by Year 2015 (US National Science Foundation)

An enormous opportunity for manufacturing industry!!

There has been colossal amount of research funding to NT by governments of industrialized countries around the world b/cof this enormous potential.

Major Industrial Applications

1. Automotive Industry: Safety Engine and Power Trains Comfort and Convenience Vehicle Diagnostics and

Health Monitoring

2. Healthcare Industry: Diagnostics and Monitoring Testing Surgical Tools Drug Discovery and Delivery

3. Aerospace Industry: Instrumentations Safety Navigation and Control Micro Satellites

4. Information Technology Industry: Read/write Heads Inkjet Printer Heads Position Sensors Flat Panel Displays

5. Telecommunication Industry: Optical Switching for Fiber Optical

Couplings RF Switches Tunable Resonators, etc.

6. Industrial Products: Manufacturing Process Sensors Robotic Sensing Sensors for HVAC Systems Remote Sensing in Agriculture Environmental Monitoring

7. Consumer Products: Sporting Goods Smart Home Appliances Smart Toys and Games

MEMS-ZJU

MEMS-ZJU

MEMS-ZJU

6

342906807130331595TOTAL400.1530.006Microspectrometers

60602015Magnetoresistive sensors

360301506Gyroscopes

4309024024Accelerometers

8000.42200.01Infrared imagers

800400300100Chemical sensors

1300309600115Pressure sensors

2000711504Hearing aids

28004000450700In vitro diagnostics

37000.810000.5Heart pacemakers

100005004400100Inkjet printer heads

1200015004500530Hard disk drives

Revenue($ million)

2002 Units(million)

Revenue($ million)

1996 Units(millions)Product Types

MST Global Markets for Established Product Types

(Source: NEXUS 1998) $34 billion +4205104510733TOTAL

50.050.10.001Electronic noses

2020.50.01Anti-collision sensors

30301010Injection nozzles

7020101Inclinometers

80250.1Micromotors

100500.1Micro relays

1006001020Coil-on-chip

3001100.1Projection valves

50010010.01Magneto optical heads

100010000Lab-on-chip (DNA)

100040501Optical switches

1000100101Drug delivery systems

Revenue($ millions)

2002 Units(millions)

Revenue($ millions)

1996 Units(millions)

Product Types

MST Global Market for Emerging Products

Source: NEXUS 1998

0

1 0

2 0

3 0

4 0

5 0

6 0

2000 2 001 200 2 2003 2 004 20 05

Yea r

Rev

enue

, $bi

llion

Market Growth of MST Products

Source: NEXUSSource: Nexus=Network of Excellence in Multifunctional Microsystems of European Community

$50 Billion

MEMS and Microsystems Devices and Products

Micro Sensors:

Acoustic wave sensorsBiomedical and biosensorsChemical sensorsOptical sensorsPressure sensorsStress sensorsThermal sensors

Micro Actuators:

Grippers, tweezers and tongsMotors - linear and rotaryRelays and switchesValves and pumpsOptical equipment (switches, lenses & mirrors, shutters, phase modulators, filters, waveguide splitters, latching & fiber alignment mechanisms)RF MEMS (oscillators, varactors, switches)

Microsystems = sensors + actuators + signal transduction:

Airbag deployment systems• Microfluidics, e.g. Capillary electrophoresis (CE)• Drug delivery systems with micropumps and valves

MEMS-ZJU

MEMS-ZJU

MEMS-ZJU

7

MicroSensingElement

InputSignal

TransductionUnit

OutputSignal

PowerSupply

MEMS as a Microsensor

Micro pressure sensors

MicroActuatingElement

OutputAction

TransductionUnit

PowerSupply

MEMS as a Microactuator

Micromotor produced by a LIGA Process

Components of Microsystems

Sensor

Signal Transduction &

ProcessingUnit

Actuator

PowerSupply

Microsystem

Microsystems = sensors + actuators + signal transduction

Typical Microsystems Products

MEMS-ZJU

MEMS-ZJU

MEMS-ZJU

8

Inertia Sensor for “Air Bag” Deployment System(Courtesy of Analog Devices, Inc.)

Inertia Sensor for Automobile “Air Bag” Deployment System

Micro inertia sensor (accelerometer) in place:

(Courtesy of Analog Devices, Inc)

Sensor-on-a-chip:(the size of a

rice grain)

Unique Features of MEMS and Microsystems (1)- A great challenge to engineers

•Components are in micrometers with complex geometryusing silicon, si-compounds and polymers:

25 µm

25 μm

A microgear-train bySandia National Laboratories

Capillary Electrophoresis (CE) Network Systems for Biomedical Analysis

A simple capillary tubular network with cross-sectional area of 20-30 μmis illustrated below:

AnalyteReservoir,A

Analyte WasteReservoir,A’

BufferReservoir,B

WasteReservoir,B’

Injection Channel

Sepa

ratio

n C

hann

el

Silicon Substrate

“Plug”

Work on the principle of driving capillary fluid flow by applying electric voltages at the terminals at the reservoirs. Fast separation of species in analyte sample for biomedicalApplications.

MEMS-ZJU

MEMS-ZJU

MEMS-ZJU

9

Commercial MEMS and Microsystems Products

Micro Sensors:

Acoustic wave sensorsBiomedical and biosensorsChemical sensorsOptical sensorsPressure sensorsStress sensorsThermal sensors

Micro Actuators:

Grippers, tweezers and tongsMotors - linear and rotaryRelays and switchesValves and pumpsOptical equipment (switches, lenses & mirrors, shutters, phase modulators, filters, waveguide splitters, latching & fiber alignment mechanisms)

Microsystems = sensors + actuators + signal transduction:

• Microfluidics, e.g. Capillary electrophoresis (CE)• Micro accelerometers (inertia sensors)

INPUT:Desired

Measurements or

functions

Sensing and/oractuatingelement

Transductionunit Signal

Conditioner& Processor

Controller Actuator

SignalProcessor

MeasurementsComparator

OUTPUT:Measurements

or Actions

MEMS

Package on a single “Chip”

Intelligent Microsystems - Micromechatronics systems

Evolution of Microfabrication

There is no machine tool with today’s technology can produce any device or MEMS component of the size in the micrometer scale.

The complex geometry of these minute MEMS components can only be produced by various physical-chemical processes – the microfabrication techniques originallydeveloped for producing integrated circuit (IC) components.

Significant technological development towards miniaturization was initiated with the invention of transistors by three Nobel Laureates, W. Schockley, J. Bardeen and W.H. Brattain of Bell Laboratories in 1947.

This crucial invention led to the development of the concept of integrated circuits (IC) in 1955, and the production of the first IC three years later by Jack Kilby of Texas Instruments.

ICs have made possible for miniaturization of many devices and engineering systems in the last 50 years.

The invention of transistors is thus regarded as the beginning of the 3rd Industrial Revolution in human civilization.

Evolution of IC Fabrication

MEMS-ZJU

MEMS-ZJU

MEMS-ZJU

10

Comparison of Microelectronics and MicrosystemsMicroelectronics Microsystems (silicon based)

Primarily 2-dimensional structures Complex 3-dimensional structureStationary structures May involve moving componentsTransmit electricity for specific electrical functions Perform a great variety of specific biological, chemical,

electromechanical and optical functionsIC die is protected from contacting media Delicate components are interfaced with working mediaUse single crystal silicon dies, silicon compounds,ceramics and plastic materials

Use single crystal silicon dies and few other materials,e.g. GaAs, quartz, polymers, ceramics and metals

Fewer components to be assembled Many more components to be assembledMature IC design methodologies Lack of engineering design methodology and standardsComplex patterns with high density of electricalcircuitry over substrates

Simpler patterns over substrates with simpler electricalcircuitry

Large number of electrical feed-through and leads Fewer electrical feed-through and leadsIndustrial standards available No industrial standard to follow in design, material

selections, fabrication processes and packagingMass production Batch production, or on customer-need basisFabrication techniques are proven and welldocumented

Many microfabrication techniques are used forproduction, but with no standard procedures

Manufacturing techniques are proven and welldocumented

Distinct manufacturing techniques

Packaging technology is relatively well established Packaging technology is at the infant stagePrimarily involves electrical and chemicalengineering

Involves all disciplines of science and engineering

Natural Science:Physics, Chemistry

Biology

Mechanical Engineering• Machine components design.• Precision machine design.• Mechanisms & linkages.• Thermomechanicas:

solid & fluid mechanics, heattransfer, fracture mechanics.

• Intelligent control.• Micro process equipment

design and manufacturing.• Packaging and assembly design.

Quantum physicsSolid-state physicsScaling laws

Electrical Engineering• Power supply.• Electric systems

design in electro-hydrodynamics.

• Signal transduction,acquisition,condition-ing and processing.

• Electric & integratedcircuit design.

• Electrostatic & EMI.

Materials Engineering• Materials for device

components & packaging.• Materials for signal

transduction.• Materials for fabrication

processes.

Process Engineering• Design & control of

micro fabricationprocesses.

• Thin film technology.

Industrial & Systems Engineering• Process implementation.• Production control.• Micro packaging & assembly.

Electromechanical-chemical Processes

MaterialScience

(Multidiscipline of MEMS.Slide presentation)HSU

Commercialization of MEMS and Microsystems

Major commercial success:

Pressure sensors and inertia sensors (accelerometers) with worldwide market of:

• Airbag inertia sensors at 2 billion units per year.• Manifold absolute pressure sensors at 40 million units per year.• Disposable blood pressure sensors at 20 million units per year.

Recent Market DynamicsOld MEMS New MEMS

Pressure sensorsAccelerometersOther MEMS

BioMEMSIT MEMS for Telecommunication:(OptoMEMS for fiber optical networksRF MEMS for wireless)

Application of MEMS and Microsystemsin

Automotive Industry

52 million vehicles produced worldwide in 1996There will be 65 million vehicle produced in 2005

Principal areas of application of MEMS and microsystems:

• Safety• Engine and power train

• Comfort and convenience Vehicle diagnostics and health monitoring

Telematics, e.g. GPS, etc.

MEMS-ZJU

MEMS-ZJU

MEMS-ZJU

11

(1)(7)

(10)(2)

(3)(4)

(5)

(6)

(8)(9)

(2) Exhaust gas differential pressure sensor

(1) Manifold or Temperature manifold absolute pressure sensor

(3) Fuel rail pressure sensor(4) Barometric absolute pressure sensor(5) Combustion sensor

(7) Fuel tank evaporative fuel pressure sensor

(6) Gasoline direct injection pressure sensor

(8) Engine oil sensor

(9) Transmission sensor

(10) Tire pressure sensor

Principal Sensors Silicon Capacitive Manifold Absolute Pressure Sensor

Application of MEMS and Microsystemsin

Aerospace Industry

Cockpit instrumentation. Wind tunnel instrumentation Microsattellites Command and control systems with MEMtronics Inertial guidance systems with microgyroscopes, accelerometers and fiber optic gyroscope. Attitude determination and control systems with micro sun and Earth sensors. Power systems with MEMtronic switches for active solar cell array reconfiguration, and

electric generators Propulsion systems with micro pressure sensors, chemical sensors for leak detection, arrays

of single-shot thrustors, continuous microthrusters and pulsed microthrousters Thermal control systems with micro heat pipes, radiators and thermal switches Communications and radar systems with very high bandwidth, low-resistance radio-

frequency switches, micromirrors and optics for laser communications, and micro variable capacitors, inductors and oscillators.

Sensors and actuators for safety - e.g. seat ejection Sensors for fuel efficiency and safety

Application of MEMS and MicrosystemsIn Biomedical Industry

Disposable blood pressure transducers

Catheter tip pressure sensors

Biosensors

Pace makers

Respirators

Lung capacity meters

Barometric correction instrumentation

Medical process monitoring

Kidney dialysis equipment

Micro bio-analytic systems: bio-chips, capillary electrophoresis, etc.

Drug delivery systems

MEMS-ZJU

MEMS-ZJU

MEMS-ZJU

12

Application of MEMS and Microsystemsin

Consumer Products

Scuba diving watches and computers

Bicycle computers

Sensors for fitness gears

Washers with water level controls

Sport shoes with automatic cushioning control

Digital tire pressure gages

Vacuum cleaning with automatic adjustment of brush beaters

Smart toys, e.g., fish, dogs, etc.

Application of MEMS and Microsystemsin the

Telecommunication Industry

Optical switching and fiber optic couplings

RF relays and switches

Tunable resonatorsMicro lenses: Micro switches:

Micro Optical Switches

2-Dimensional

3-Dimensional

Concluding Remarks

1. Miniaturization of machines and devices is an inevitable trend in technological development in the new century.

2. There is a clear trend that microsystems technology will be further scaled down to the nano level. (1 nm = 10-3 μm = 10 shoulder-to-shoulder H2 atoms).

3. Despite the fact that many IC fabrication technologies can be used to fabricate silicon-based MEMS components, microsystems engineering requires the application of principles involving multi-disciplines in science and engineering.

4. Team effort involving multi-discipline of science and engineering is the key to success for any MEMS industry.

MEMS-ZJU

MEMS-ZJU

MEMS-ZJU

13

Lecture 1（Part II）

Working Principles of MEMS and Microsystems

Due to the minute sizes, microactuators work on radically different principles than the conventional electromagnetic means, such as solenoids and ac/dc motors.

Instead, electrostatic, thermal, piezoelectric and shape-memoryalloys are extensively used in microactuations.

In this lecture we will learn the working principles of many microsensors and actuators in MEMS and microsystems.

Minute sensors are expected to detect a variety of signals associated with:

Accelerations (velocity and forces)Pressure, Chemical, Optical, Thermal (temperatures), humidity,Biological substances, etc.

Input samples may be: motion of a solid, pressurized liquids or gases,biological and chemical samples.

Working Principles for Microsensors

MicroSensingElement

InputSignal

TransductionUnit

OutputSignal

PowerSupply

BioMEMS

The term “BioMEMS” has been a popular terminology in the MEMS industry inrecent years due to the many breakthroughs in this emerging technology, which many believe to be a viable lead to mitigate the sky-rocketing costs in health care In many industrialized countries in which aging population is a common problem.

BioMEMS include the following three major areas:

(1) Biosensors for identification and measurement of biological substances,

(2) Bioinstruments and surgical tools, and

(3) Bioanalytical systems for testing and diagnoses.

MEMS-ZJU

MEMS-ZJU

MEMS-ZJU

14

Major Technical Issues in BioMEMS Production:

(1) Functionality for the intended biomedical operations.

(2) Adaptivity to existing instruments and equipment.

(3) Compatibility with biological systems of the patients.

(4) Controllability, mobility, and easy navigation for operations such as those required in laparoscopy surgery.

(5) Functions of MEMS structures with high aspect ratio(defined as the ratio of the dimensions in the depth of the structure to the dimensions of the surface)

Note: Almost all bioMEMS products are subjected to the approvalfor marketing by the FDA (Food and Drug Administration)of the US government.

Biomedical Sensors and Biosensors

These sensors are extensively used in medical diagnosis, environmental protection, drug discovery and delivery, etc.

Biomedcial Sensors

For the measurements of biological substances in the sample and also for medical diagnosis purposes.

Input signal: Biological sample (typically in minute amount in µL or nL)

Microsensing element: a chemical that reacts with the sample.

Transduction unit: the product of whatever the chemical reactions between the sample and the chemical in the sensing element will convert itself into electrical signal (e.g. in millivolts, mV).

Output signal: The converted electrical signal usually in mV.

Example of a biomedical sensor:

A sensor for measuring the glucose concentration of a patient.

Pt electrode

Ag/AgCl Reference electrode

Polyvinyl alcohol solutionBlood sample

H+ H+H+

H+ H+V

i

Working principle:

The glucose in patient’s blood sample reacts with the O2 in the polyvinyl alcohol solution and produces H2O2.

The H2 in H2O2 migrates toward Pt film in a electrolysis process, and builds up layers at that electrode.

The difference of potential between the two electrodes due to the build-up ofH2 in the Pt electrode relates to the amount of glucose in the blood sample.

Powersupply

Biosensors

These sensors work on the principle of interactions between the biomolecules in the sample and the analyte (usually in solution)in the sensor.

Signal transduction is done by the sensing element as shownbelow:

B B BB

ANALYTE

BB

B

Sensor

ChemicalOpticalThermalResonantElectrochemicalISFET (Ion SensitiveField Effect Transducer)

OutputSignals

Biomolecule Supply

B

Biomolecule Layer

MEMS-ZJU

MEMS-ZJU

MEMS-ZJU

15

Chemical Sensors

Work on simple principles of chemical reactions between the sample, e.g. O2and the sensing materials, e.g. a metal.

Signal transduction is the changing of the physical properties of the sensing materials after the chemical reactions.

There are four (4) common types of chemical sensors:

(1) Chemiresistor sensors.(2) Chemicapacitor sensors.

ChemicallySensitive Polyimide

Metal Insert

Metal Electrodes

Input currentor voltage

Output:Change of Resistance

Input Voltage Output:Capacitance Change

Measurand Gas

Chemical Sensors-Cont’d

(3) Chemimechanical sensors:Work on certain materials (e.g. polymers) that change shapes when they are exposed to chemicals. Measuring the change of the shape of the sensing materials to determine the presence of the chemical.

(4) Metal oxide gas sensors:Sensing materials: certain semiconducting materials, e.g. SnO2 changetheir electrical resistance when exposed to certain chemicals.

SnO2

SiO2

Electric Contact

Silicon Substrate

Measurand Gas

Chemical Sensors-Cont’d

O3NoneIn2O3

NO2, CONoneMoO3

COAuGa2O3

COTi-doped + AuFe2O3

NH3PtWO3

Halogenated hydrocarbonsV, MoZnO

H2SCuOSnO2

H2, O2, H2SSb2O3SnO2

AlcoholsPtSnO2

COPt + SbSnO2

CO2La2O3, CaCO3BaTiO3/CuO

Gas to be DetectedCatalyst AdditivesSemiconducting Metals

Available metal oxide gas sensors:

Optical Sensors These sensors are used to detect the intensity of lights.

It works on the principle of energy conversion between the photons in the incident light beams and the electrons in the sensing materials.

The following four (4) types of optical sensors are available:Photon Energy

Semiconductor B

Semiconductor AR

Photon Energy

ΔR

(a) Photovoltaic junction (b) Photoconductive device

Vout

_+

RPhoton Energy

p-Material

n-Material

BiasVoltage

ReverseBias

Voltage

Junction

p n

Photon Energy

Leads

(c) Photodiodes

A is moretransparent tophoton energy in incident lightthan B.

⇒

MEMS-ZJU

MEMS-ZJU

MEMS-ZJU

16

Optical Sensors-Cont’d

np p

Base

Photon Energy

np p

Base

Collector Emitter Collector Emitter

Phot

on E

nerg

y

(d) Phototransistors

Silicon (Si) and Gallium arsenide (GaAs) are common sensing materials. GaAs has higher electron mobility than Si- thus higher quantum efficiency.

Other materials, e.g. Lithium (Li), Sodium (Na), Potassium (K) and Rubidium (Rb) are used for this purpose.

Working Principle of Silicon Solar Photovoltaic (PV)

n-silicon (excessive electrons)

p-silicon (atoms with “holes” by unbalanced electrons)

Junction (weak dielectric)

-+ +++++++ + + + +------- - - - - -

Electric field with 2 electrodes (a battery):

- -- - - - - -- -Extra electrons:

Photons from Sun

MigratingElectrons

Cur

rent

Pressure Sensors

Micro pressure sensors are used to monitor and measure minute gas pressure in environments or engineering systems, e.g. automobile intake air pressure to the engine.

They are among the first MEMS devices ever developed and produced for“real world” applications.

Micro pressure sensors work on the principle of mechanical bending ofthin silicon diaphragm by the contacting air or gas pressure.

Cavity Cavity

Silicon Diewith

Diaphragm

ConstraintBase

Measurand Fluid Inlet

Measurand Fluid Inlet

(a) Back side pressurized (b) Front side pressurized

Pressure Sensors-Cont’d

The strains associated with the deformation of the diaphragm aremeasured by tiny “piezoresistors” placed in “strategic locations” on the diaphragm.

Silicon Diaphragm

Pyrex GlassConstrainingBase or Metal

Header

MetalCasing

Passage forPressurized

Medium

Silicone gel

Wire bondMetal film

Dielectric layer

Piezoresistors

DieAttach

Interconnect

R1

R3

R4

R2

Metal Pad Metal Pad

R1, R2, R3, R4 = Piezoresistors

Top view of silicon die

VoVin

R1(+ve)

R2(-ve)

R3 (+ve)

R4(-ve)

+

-a

b

Wheatstone bridge for signal transduction

These tiny piezoresistors are made from doped silicon. They work on the same principle as “foil strain gages” with much smaller sizes (in µm) with much higher sensitivities and resolutions.

MEMS-ZJU

MEMS-ZJU

MEMS-ZJU

17

Pressure Sensors-Cont’d

Other ways of transducing the deformation of the diaphragm to electronic output signals are available, e.g.,

CavityConstraint

Base

Measurand Fluid Inlet

V

MetallicElectrode

MetallicElectrode

Silicon Die

Silicon Cover

Silicon diaphragm1200 μm sq.x 100 μm thick

Vibrating beam:(n-type Si wafer,40 μm widex 600 μm long x 6 μm thick)

Silicon die(400 μm thick)

Constraint basePressurized medium

Diffused p-type electrode

by capacitance changes(for higher temperature applications)

by resonant vibration(for higher resolutions)

MEMS microphones work on this principle.

Major problems in pressure sensors involve the packaging and protection of the diaphragm from the contacting air or gas pressure.

Pressure Sensors-Cont’d

Thermal Sensors

Thermal sensors are used to monitor or measure temperature in anenvironment or an engineering systems.

Common thermal sensors involve thermocouples and thermopiles.

Thermal sensors work on the principle of the electromotive forces (emf)generated by heating the junction made by dissimilar materials (beads):

V

Heat

Bead

Metal Wire A

Metal Wire B

Voltage Output

(a) A thermocouple

V

Voltage Output

Metal Wire A

Metal Wire B

ColdJunction

HotJunction

Heat

(b) A dual junction thermocouple

ii

ii

TV Δ= βThe generated voltage (V) by a temperature rise at the bead (ΔT) is:

where β = Seebeck coefficient

Thermal Sensors-Cont’d

The Seebeck coefficients for various thermocouples are:

-0.23 to 21.11-50 to 176811.35 at 600oCPt (13%)-Rh/PtS

-6.26 to 20.87-270 to 40038.74 at 0oCCopper/constantanT

-0.24 to 18.70-50 to 176810.19 at 600oCPlatinum (10%)-Rh/PtR

-6.55 to 54.87-270 to 137239.48 at 0oCChromel/alumelK

-8.10 to 69.54-210 to 120050.37 at 0oCIron/constantanJ

-9.84 to 76.36-270 to 100058.70 at 0oCChromel/constantanE

Range (mV)Range (oC)Seebeck Coefficient(μV/oC)

Wire MaterialsType

Common thermocouples are of K and T types

MEMS-ZJU

MEMS-ZJU

MEMS-ZJU

18

Thermal Sensors-Cont’d

Thermopiles are made of connecting a series of thermocouples in parallel:

ΔV

Hot

Jun

ctio

nR

egio

n, T

h

Cold JunctionRegion, Tc

Thermocouples

The induced voltage (ΔV) by the temperature change at the hot junction (ΔT) is:

TNV Δ=Δ β

with N = number of thermocouple pairs in the thermopile.

Thermal Sensors-Cont’d

A micro thermal sensor:

HotJunctionRegion

32 Thermocouples16 μm wide

3.6

mm

3.6 mm

Diaphragm: 1.6 mm dia x 1.3 μm thick

Cold JunctionRegion

20 μ

m

Hot Junction Region

Thermocouples

DiaphragmSilicon Rim

Support

Top view

Elevation

32 polysilicon-gold thermocouples

dimension of thermopile is: 3.6 mm x 3.6 mm x 20 µm thick

Typical output is 100 mV

Response time is 50 ms.

Working Principles for Microactuators

MicroActuatingElement

OutputAction

TransductionUnit

PowerSupply

Power supply: Electrical current or voltage

Transduction unit: To covert the appropriate form of power supply to the actuating element

Actuating element: A material or component that moves with power supply.

Output action: Usually in a prescribed motion.

Actuation Using Thermal Forces

Solids deform when they are subjected to a temperature change (∆T)

A solid rod with a length L will deform in length by ∆L = α∆T, in whichα = coefficient of thermal expansion (CTE) – a material property.

When two materials with distinct CTE bond together and subject to atemperature change, the compound material will change its geometryas illustrated below with a compound beam:

Heat

α1

α2

α1 > α2

These compound beams are commonly used as microswitches and relaysin MEMS products.

MEMS-ZJU

MEMS-ZJU

MEMS-ZJU

19

Actuation Using Shape Memory Alloys (SMA)

SMA are the materials that have a “memory” of their original geometry (shape)at a typically elevated temperature of production.

These alloys are deformed into desired geometry at typically room temperature.

The deformed SMA structures at room temperature will return to their original shapes when they are heated to the elevated temperature of their production.

Ti-Ni is a common SMA.

A micro switch actuated with SMA:

Constraint Base

Shape Memory Alloy Stripe.g. TiNi or Nitinolor

Resistance Heating Strip

Silicon Cantilever Beam

Actuation Using Piezoelectric Crystals

A certain crystals, e.g. quartz, exhibit an interesting behavior when subjectedto mechanical deformation or electric voltage.

This behavior may be illustrated as follows:

V

MechanicalForces

App

lied

Vol

tage

, V

Induced Mechanical Deformation

Mechanical force induced electric voltage

Electric voltage inducedmechanical deformation

This peculiar behavior makes piezoelectric crystals ideal candidate formicro actuation as illustrated in the following case:

Actuation Using Piezoelectric Crystals-Cont’d

Constraint Base

VPiezoelectric

Electrodes

Silicon Cantilever Beam

Actuation Using Electrostatic Forces

Electrostatic Force between Two Particles – The Coulomb’s Law:

(with charge q)

(with charge q’)

A

B

Distance, r

Attractio

n F

Repulsion F

2

'4

1rqqF

πε=The attraction or repulsive force:

where ε = permittivity of the medium between the two particles= 8.85 x 10-12 C2/N-m2 or 8.85 pF/m in vacuum (= εo)

r = Distance between the particles (m)

MEMS-ZJU

MEMS-ZJU

MEMS-ZJU

20

Electrostatic Force Normal to Two Electrically Charged Plates:

Actuation Using Electrostatic Forces-Cont’d

VGap, d

Length, L

Width, W

The induced capacitance, C is:d

WLdAC oror εεεε ==

The induced normal force, Fd is:

222

1 Vd

WLF ord

εε−=

in which εr = relative permittivity of the dielectric material between the two plates(see Table 2.2 for values of εr for common dielectric materials).

Actuation Using Electrostatic Forces-Cont’d

Electrostatic Force Parallel to Two Misaligned Electrically Charged Plates:

WdL

Fd Fw

FLV

Force in the “Width” direction:2

21 V

dLF or

wεε

−=

Force in the “Length” direction:

2

21 V

dWF or

Lεε

−=

Applications of Micro Actuations

Micromotors

Unlike traditional motors, the driving forces for micro motors is primarily the parallel electrostatic forces between pairs of misaligned electrically charged plates(electrodes), as will be demonstrated in the following two cases:

Linear stepping motors:

Two sets of electrodes in the form of plates separated by dielectric material (e.g. quartz film).

One electrode set is fixed and the other may slide over with little friction. The two sets have slightly different pitch between electrodes

Fixed set electrodes:

Moving set electrodes:

A

A’ B’ C’ D’

Pitch:w+w/3

W/3

Dielectric material

B C D

W W

W Step Movements

Applications of Micro Actuations-Cont’d

Energize the set A-A’ will generate a force pulling A’ over A due to initial misalignment.

Once A and A’ are aligned, the pair B and B’ become misaligned.

Energize the misaligned B-B’ will generate electrostatic force pulling B’ over B.

It is now with C’ and C being misaligned.

Energize C’ and C will produce another step movement of the moving set over thestationary set.

Repeat the same procedure will cause continuous movements of the moving sets

The step size of the motion = w/3, or the size of preset mismatch of the pitch between the two electrode sets.

Fixed set electrodes:

Moving set electrodes:

A

A’ B’ C’ D’

Pitch:w+w/3

W/3

Dielectric material

B C D

W W

W Step Movements

MEMS-ZJU

MEMS-ZJU

MEMS-ZJU

21

Applications of Microactuations-Cont’d

Rotary stepping motors:

Involve two sets of electrodes - one set for the rotor and the other for the stator. Dielectric material between rotor and stator is air. There is preset mismatch of pitches of the electrodes in the two sets.

Applications of Microactuations-Cont’d

Working principle of this rotary motor is similar to that in linear motors.

Stator

RotorGear for

transmittingtorque

A micro motor produced by Karlsruhe Nuclear Research Center, Germany:

Microvalves

Electric ResistanceHeating Rings

Flexible Silicon Diaphragm

SiliconBase

Constraint Base

INLET FLOW

FLOW OUTLET Centerline

A special microvalve designed by Jerman in 1990. Circular in geometry, with diaphragm of 2.5 mm in diameter x 10 μm thick. The valve is actuated by thermal force generated by heating rings. Heating ring is made of aluminum films 5 µm thick. The valve has a capacity of 300 cm3/min at a fluid pressure of 100 psig. Power consumption is 1.5 W.

Micropumps

Electrode

InletCheckValve

OutletCheckValve

Pumping Chamber

DeformableSilicon

Diaphragm

ConstraintBase

V

Low PressureFluid Inlet

High PressureFluid Outlet

Electrostatically actuated micropump:

An electrostatic actuated pump in 1992. The pump is of square geometry with 4 mm x 4mm x 25 μm thick. The gap between the diaphragm and the electrode is 4 µm. Pumping rate is 70 μL/min at 25 Hz.

MEMS-ZJU

MEMS-ZJU

MEMS-ZJU

22

Piezoelectrically actuated pump:

An effective way to pump fluid through capillary tubes. Tube wall is flexible. Outside tube wall is coated with piezoelectric crystal film, e.g. ZnO

with aluminum interdigital transducers (IDTs). Radio-frequency voltage is applied to the IDTs, resulting in mechanical

squeezing in section of the tube (similar to the squeezing of toothpaste) Smooth flow with “uniform” velocity profile across the tube cross section.

F V

Flexible Tube Wall

Piezoelectric coatingwith transducer

Flow

Micro Heat Pipes- a viable solution to cooling in molecular electronics

Evaporator

Adiabatic Section CondenserHeat Source

Heat Sink

Heat Source VAPOR

LIQUIDHeatSink

Cross-Sections

Elevation

Heat Source Heat SinkVapor

WickCondensed

liquid

WickHeat Source Heat Sink

Conventional Heat Pipes

Micro Heat Pipe

Sharp corners of microconduitprovide capillary driving pressurefor the return of condensed liquid

- no wicks is necessary

Microaccelerometers

Accelerometers are used to measure dynamic forces associatedwith moving objects.

These forces are related to the velocity and acceleration of the movingobjects.

Traditionally an accelerometer is used to measure such forces. A typical accelerometer consists of a “proof mass” supported by a spring and

a “dashpot” for damping of the vibrating proof mass:

Springk

MassM Dashpot

with damping

CVibratingSolid Body

The accelerometer is attached to the vibrating

solid body

Micro Accelerometers-Cont’d

Springk

MassM Dashpot

with damping

CVibratingSolid Body

The accelerometer is attached to the vibrating

solid body

The instantaneous displacement of the massy(t) induced by the attached moving solid

body is measured and recorded with respectto time, t.

The associated velocity, V(t) and the acceleration α(t) may be obtained by the following derivatives:

2

2 )()()()()(dt

tyddt

tdytanddt

tdytV === α

The associated dynamic force of induced by the moving solid is thus obtained by using the Newton’s law, i.e. F(t) = M α(t), in which M = the mass of the moving solid.

♦ In miniaturizing the accelerometers to the microscale, there is no room for thecoil spring and the dashpot for damping on the vibrating mass.

♦ Alternative substitutes for the coil spring, dashpot, and even the proof massneed to be found.

MEMS-ZJU

MEMS-ZJU

MEMS-ZJU

23

Microaccelerometers-Cont’d

There are two types microaccelerometers available.

(1) The cantilever beam accelerometer:

Mass, M

PiezoresistorSilicon Cantilever

Beam

Constraint Base

Casing

Constraint Base

Vibrating Base

In this design: Cantilever beam = coil spring; Surrounding viscous fluid = dashpot for damping of the proof mass

The movement of the proof mass is carried out by the attached piezoresistor.

Microaccelerometers-Cont’d

(2) Balanced force microaccelerometer: This is the concept used in the “air-bag” deployment sensor in automobiles

In this design: Plate beam = proof mass;Two end tethers = springsSurrounding air = dashpot

Stationaryelectrodes

Moving electrode

The movement of the proof mass is carried out by measuring the change of capacitances between the pairs of electrodes.

Beam MovementAcceleration

Inertia Sensors – Unique Micromechatronics DevicesWorking Principle of Balanced-

Force Accelerometers:Balanced-Force Accelerometers:

• The need for integrating microelectronics (ICs)and moving microstructures – A great challenge!

Acceleration3 mm

2 mm

Diaphragm:≈ 1µm thick Backplate (≈2 µm)

Acoustic holes

Pressure equalization hole

Air gap (≈2 µm)

Acoustic Wave Input(air pressure wave)

dB→ MPa

ΔC

Electrical signal output:

MEMS MicrophonesApplications: Beamforming microphone arrays for wind tunnels Beamforming microphone arrays for smart hearing aids Mobile telephones Notebook and palm-top computers

MEMS-ZJU

MEMS-ZJU

MEMS-ZJU

24

x

y

z

x

y

zx

y

z

x

y

z

x

y

z

x

y

z

x

y

z

x

y

z

VFc

Ω

V Fc

Ω

V

Fc

Ω ΩV

Fc

(a) (b)

(c) (d)

Microgyroscopes- for precision motion control

The Vectorial Representation of Coriolis Motion:

ProofMassx x

y

y

Gyro Framey-Spring for

Force MeasurementsResonator

for Linear MotionGeneration

x-Position

x-Spring

y-Position

A Microgyroscope Actuated by Electrostatic Forces

SUMMARY

MEMS and microsystems consist of sensors, actuators, power supply and signal transducers.

Microsensors work on the principle of change of sensing material propertiesin response to the substances to be sensed and detected.

Actuating forces for MEMS and microsystems are radically different fromtraditional electromagnetic forces.

Microactuating forces include: Electrostatic forces – low in magnitudes, but fast responsransductioes Thermal forces – larger in magnitudes, but slow in response Piezoelectric forces – flexible in magnitudes, fast responses,

but with limited lasting power.

Signal transduction and power supply are two major challenging factors inthe design of microsystems.

总结性的小问题

• 1. 微机电系统的英文全称（PPT第7页）

• 2. MEMS作为Sensor的的结构框图（PPT第24页）

• 3. MEMS作为Actuator的结构框图（PPT第25页）

• 4. Microsystem的结构框图（PPT第26页）

• 5. 从听课内容请举出任意一种MEMS器件的工作原理（Actuator(linear motor, miropump)、Sensor(Biosensor, Gas sensor, Pressure sensor)或Microsystem）（PPT第55-93页）

• 5. 请你介绍两种以上你所知道的MEMS器件或系统

MEMS-ZJU

MEMS-ZJU

MEMS-ZJU