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M EM EM S - Z -Z JU JU Who am I? An Introduction to Microelectromechanical Systems(MEMS) Bing-Feng Ju, Professor Institute of Mechatronic Control Engineering, College of Mechanical and Energy Engineering, Zhejiang University P.R.China, 310027 Email: [email protected] Tel: 86-571-8795-1730 Fax: 86-571-8795-1941 Presented to Graduate students at College of Mechanical and Energy Engineering Zhejiang University February to May 2010 Education: Ph.D. from Zhejiang University, 1999 MEng from Harbin Institute of Technology, 1996 BEng 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 University Position: Professor (2007.12 Ph.D. Tutor Qualification) 2004.12-2007.04 Department of Nanomechanics, Tohoku University(東北大学), Japan Position: Assistant Professor 2003.11-2004.11 Department of Nanomechanics, Tohoku University(東北大学), Japan Position: JSPS Young Foreigner Scientist 2002.05-2003.11 DSO National Laboratories, Singapore Position: Research Scientist Adjunct Assistant Professor of National University of Singapore (NUS) 2000.05-2002.05 School of Mechanical & Aerospace Engineering, Nanyang Technological University (NTU), Singapore Position: Postdoctoral Research Fellow Courses taught: BioMEMS, Precision & Nao Metrology CONTENT Self-introduction 1. Overview of MEMS and Microsystems Working Principles of Microsystems 2. The Scaling Laws Electromechanical Design of MEMS and Microsystems 3. Material for MEMS and Microsystems Part 1: Silicon and silicon compounds Part 2: Piezoelectric and polymers 4. Microfabrication Processes Part 1: Photolithography, doping with ion implantation and diffusion Part 2: Etching Part 3: Depositions: physical, chemical and epitaxy 5. Micromanufacturing Assembly, Packaging and Testing to Nanoscale Engineering Part 1: Microassembly Part 2: Packaging with surface and wire bonding Part 3: Reliability and testing 6. Introduction to Nanoscale Engineering Part 1: Overview of nanoscale engineering Part 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,2004 2. Albert P. Pisano, An Introduction to Microelectromechanical Systems Engineering, Artech House, 2000 3. Marc Madou, Fundamentals of Microfabrication, CRC Press, 2002 4. 庄达人,《VLSI制造技术》,高立图书有限公司,1996 Journals: 1. Journal of MEMS http://www.ieee.org/pub_preview/mems_toc.html 2. Sensors Journal, IEEE http://ieeexplore.ieee.org/xpl/RecentIssue.jsp?puNumber=7361 3. Journal of Micromechanics and Microengineering http://www.iop.org/Journals/jm Covering 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: Physical http://www.elsevier.com:80/inca/publications/store/5/0/4/1/0/3/ 1 M EM EM S - Z -Z JU JU MEMS-Related Newsletters: • Micromachine Devices is a very thorough newsletter on MEMS and the MEMS industry. Contact the editor Mr. Sid Marshall at:[email protected] 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 for subscription 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/) Unit 10-18m-am:atto-meter 10-15m-fm:femto-meter 10-12m-pm:pico-meter 10-9m-nm:nano-meter 10-6m-µm:micro-meter 10-3m-mm:milli-meter 10-2m-cm:centi-meter 10-1m-dm: deci-meter 100m-m:meter 1 10 m-dam:deca-meter 102m-hm:hecto-meter 103m-km:kilo-meter 106m-Mm:mega-meter 109m-Gm:giga-meter 1012m-Tm:tera-meter Lecture 1(Part I) Overview of MEMS and Microsystems 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, electrical and 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.) 2 M EM EM S - Z -Z JU JU Inertia Sensor for Automobile “Air Bag” Deployment System Micro inertia sensor (accelerometer) in place: 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 Sensor-on-a-chip: (2 mm x 3 mmsmaller than a rice grain) - A microcar - Robot musician (Courtesy of Analog Devices, Inc) Micro Cars Robot musician (Courtesy of Denso Research Laboratories, Denso Corporation, Aichi, Japan) (Waseda University, Japan) Rice grains Over 100 micro-sensors and micro-actuators by MEMS technology 3 M EM EM S - Z -Z JU JU Miniaturization An irresistible trend in the New Century Miniaturization of Digital Computers - A remarkable case of miniaturization! MEMS = a major milestone in Miniaturization – Size: 106 down Power: 106 up A leading technology for the 21st Century, and A “Lap-top” Computer in 1996 The ENIAC Computer in 1946 108 Size: down Power: 108 up an inevitable trend in industrial products and systems development The ENIAC computer - 50 years later A “Palm-top” Computer in 2003 This spectacular miniaturization took place in 50 years!! Principal Driving Force for the 21st Century Industrial Technology There has been increasing strong market demand for: Market Demand for Intelligent, Robusting, Smaller, Multi-Functional Products - the evolution of cellular phones Mobil phones 15 Years Ago: “Intelligent,” Current State-of-the Art: Size reduction “Robust,” “Multi-functional,” “Low-cost” and Palm-top Wireless PC industrial products. Transceive voice+ others Miniaturization is the only viable solution to satisfy such market demand Transceive voice only (Video-camera, e-mails, calendar, and access to Internet; and a PC with key board; GPS and multimedia entertainment) The only solution is to pack many miniature function components into the device 4 M EM EM S - Z -Z JU JU 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 vibration due to low mass. Enabling Technologies for Miniaturization Microsystems Technology • Miniaturized devices are particularly suited for biomedical and aerospace applications due to their minute sizes and weight. Initiated in 1947 with the invention of transistors, but the term “Micromachining” was coined in 1982 Miniature devices (1 nm - 1 mm) • Small systems have higher dimensional stability at high temperature due to low thermal expansion. • Smaller size of the systems means less space requirements. This allows the packaging of more functional components in a single device. A top-down approach (MST) (1 μm - 1 mm)* A bottom-up approach Inspired by Feynman in 1959, with active R&D began in around 1995 There is a long way to building nanodevices! Nanotechnology (NT) (0.1 nm – 0. 1 μm)** • Less material requirements mean low cost of production and transportation. • Ready mass production in batches. 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) $1 trillion by Year 2015 (US National Science Foundation) An enormous opportunity for manufacturing industry!! ● There has been colossal amount of research funding to NT by governments of industrialized countries around the world b/c of this enormous potential. * 1 μm = 10-6 m ≈ one-tenth of human hair ** 1 nm = 10-9 m ≈ span of 10 H2 atoms Major Industrial Applications 1. Automotive Industry: ● Safety ● Engine and Power Trains ● Comfort and Convenience ● Vehicle Diagnostics and Health Monitoring 2. Healthcare Industry: ● Diagnostics and Monitoring ● Testing ● Surgical Tools ● Drug Discovery and Delivery 3. Aerospace Industry: ● Instrumentations ● Safety ● Navigation and Control ● Micro Satellites 4. Information Technology Industry: ● Read/write Heads ● Inkjet Printer Heads ● Position Sensors ● Flat Panel Displays 5. Telecommunication Industry: ● Optical Switching for Fiber Optical Couplings ● RF Switches ● Tunable Resonators, etc. 6. Industrial Products: ● Manufacturing Process Sensors ● Robotic Sensing ● Sensors for HVAC Systems ● Remote Sensing in Agriculture ● Environmental Monitoring 7. Consumer Products: ● Sporting Goods ● Smart Home Appliances ● Smart Toys and Games 5 M EM EM S - Z -Z JU JU MST Global Markets for Established Product Types Product Types 1996 Units Revenue 2002 Units Revenue (millions) ($ million) (million) ($ million) MST Global Market for Emerging Products Product Types 1996 Units Revenue 2002 Units Revenue (millions) ($ millions) (millions) ($ millions) Hard disk drives 530 4500 1500 12000 Drug delivery systems 1 10 100 1000 Inkjet printer heads 100 4400 500 10000 Optical switches 1 50 40 1000 Heart pacemakers 0.5 1000 0.8 3700 Lab-on-chip (DNA) 0 0 100 1000 In vitro diagnostics 700 450 4000 2800 Magneto optical heads 0.01 1 100 500 4 1150 7 2000 Projection valves 0.1 10 1 300 115 600 309 1300 Coil-on-chip 20 10 600 100 Chemical sensors 100 300 400 800 Micro relays 0.1 50 100 Infrared imagers 0.01 220 0.4 800 Micromotors 0.1 5 2 80 24 240 90 430 Inclinometers 1 10 20 70 Gyroscopes 6 150 30 360 Injection nozzles 10 10 30 30 Magnetoresistive sensors 15 20 60 60 Anti-collision sensors 0.01 0.5 2 20 Electronic noses Hearing aids Pressure sensors Accelerometers Microspectrometers TOTAL 0.006 3 0.15 40 1595 13033 6807 34290 0.001 0.1 0.05 5 33 107 1045 4205 TOTAL $34 billion + (Source: NEXUS 1998) Source: NEXUS 1998 MEMS and Microsystems Devices and Products Market Growth of MST Products 60 Micro Sensors: $50 Billion 50 Revenue, $billion 40 30 Acoustic wave sensors Biomedical and biosensors Chemical sensors Optical sensors Pressure sensors Stress sensors Thermal sensors Micro Actuators: Grippers, tweezers and tongs Motors - linear and rotary Relays and switches Valves and pumps Optical equipment (switches, lenses & mirrors, shutters, phase modulators, filters, waveguide splitters, latching & fiber alignment mechanisms) RF MEMS (oscillators, varactors, switches) 20 Microsystems = sensors + actuators + signal transduction: 10 0 2000 2 001 200 2 2003 2 004 Yea r 20 05 ● Airbag deployment systems • Microfluidics, e.g. Capillary electrophoresis (CE) • Drug delivery systems with micropumps and valves Source: NEXUS Source: Nexus=Network of Excellence in Multifunctional Microsystems of European Community 6 M EM EM S - Z -Z JU JU MEMS as a Microsensor MEMS as a Microactuator Power Supply Power Supply Input Signal Micro Sensing Element Output Action Transduction Unit Output Signal Micro Actuating Element Transduction Unit Micro pressure sensors Micromotor produced by a LIGA Process Components of Microsystems Power Supply Typical Microsystems Products Signal Transduction & Processing Unit Sensor Actuator Microsystem Microsystems = sensors + actuators + signal transduction 7 M EM EM S - Z -Z JU JU Inertia Sensor for “Air Bag” Deployment System Inertia Sensor for Automobile “Air Bag” Deployment System (Courtesy of Analog Devices, Inc.) Micro inertia sensor (accelerometer) in place: Sensor-on-a-chip: (the size of a rice grain) (Courtesy of Analog Devices, Inc) Unique Features of MEMS and Microsystems (1) - A great challenge to engineers •Components are in micrometers with complex geometry using silicon, si-compounds and polymers: Capillary Electrophoresis (CE) Network Systems for Biomedical Analysis A simple capillary tubular network with cross-sectional area of 20-30 μm is illustrated below: Buffer Reservoir,B Analyte Reservoir,A Injection Channel Analyte Waste Reservoir,A’ A microgear-train by Sandia National Laboratories 25 μm Separation Channel 25 µm “Plug” Waste Reservoir,B’ Silicon Substrate 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 biomedical Applications. 8 M EM EM S - Z -Z JU JU Commercial MEMS and Microsystems Products Micro Sensors: Acoustic wave sensors Biomedical and biosensors Chemical sensors Optical sensors Pressure sensors Stress sensors Thermal sensors Intelligent Microsystems - Micromechatronics systems Package on a single “Chip” Micro Actuators: Grippers, tweezers and tongs Motors - linear and rotary Relays and switches Valves and pumps Optical 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/or actuating element Transduction unit MEMS Signal Conditioner & Processor OUTPUT: Controller Actuator Measurements or Actions Signal Processor Comparator Measurements Evolution of IC Fabrication Evolution of Microfabrication 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. ● There is no machine tool with today’s technology can produce any device or MEMS component of the size in the micrometer scale. 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. ● The complex geometry of these minute MEMS components can only be produced by various physical-chemical processes – the microfabrication techniques originally developed for producing integrated circuit (IC) components. 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. 9 M EM EM S - Z -Z JU JU Natural Science: Comparison of Microelectronics and Microsystems Microelectronics Physics, Chemistry Biology Microsystems (silicon based) Primarily 2-dimensional structures Stationary structures Transmit electricity for specific electrical functions IC die is protected from contacting media Use single crystal silicon dies, silicon compounds, ceramics and plastic materials Fewer components to be assembled Mature IC design methodologies Complex patterns with high density of electrical circuitry over substrates Large number of electrical feed-through and leads Industrial standards available Mass production Fabrication techniques are proven and well documented Manufacturing techniques are proven and well documented Packaging technology is relatively well established Primarily involves electrical and chemical engineering Complex 3-dimensional structure May involve moving components Perform a great variety of specific biological, chemical, electromechanical and optical functions Delicate components are interfaced with working media Use single crystal silicon dies and few other materials, e.g. GaAs, quartz, polymers, ceramics and metals Many more components to be assembled Lack of engineering design methodology and standards Simpler patterns over substrates with simpler electrical circuitry Fewer electrical feed-through and leads No industrial standard to follow in design, material selections, fabrication processes and packaging Batch production, or on customer-need basis Many microfabrication techniques are used for production, but with no standard procedures Distinct manufacturing techniques Packaging technology is at the infant stage Involves all disciplines of science and engineering Quantum physics Solid-state physics Scaling laws Electromechanical -chemical Processes Electrical Engineering • Power supply. • Electric systems design in electrohydrodynamics. • Signal transduction, acquisition,conditioning and processing. • Electric & integrated circuit design. • Electrostatic & EMI. Material Science Mechanical Engineering • Machine components design. • Precision machine design. • Mechanisms & linkages. • Thermomechanicas: solid & fluid mechanics, heat transfer, fracture mechanics. • Intelligent control. • Micro process equipment design and manufacturing. • Packaging and assembly design. Process Engineering • Design & control of micro fabrication processes. • Thin film technology. Materials Engineering • Materials for device components & packaging. • Materials for signal transduction. • Materials for fabrication processes. Industrial & Systems Engineering • Process implementation. • Production control. • Micro packaging & assembly. (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 Dynamics Old MEMS Pressure sensors Accelerometers Other MEMS New MEMS BioMEMS IT MEMS for Telecommunication: (OptoMEMS for fiber optical networks RF MEMS for wireless) Application of MEMS and Microsystems in Automotive Industry 52 million vehicles produced worldwide in 1996 There 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. 10 M EM EM S - Z -Z JU JU Silicon Capacitive Manifold Absolute Pressure Sensor Principal Sensors (7) (6)(1) (4) (3) (2) (9) (5) (10) (8) (1) Manifold or Temperature manifold absolute pressure sensor (2) Exhaust gas differential pressure sensor (3) Fuel rail pressure sensor (4) Barometric absolute pressure sensor (5) Combustion sensor (6) Gasoline direct injection pressure sensor (7) Fuel tank evaporative fuel pressure sensor (8) Engine oil sensor (9) Transmission sensor (10) Tire pressure sensor Application of MEMS and Microsystems in Aerospace Industry Application of MEMS and Microsystems In Biomedical Industry Disposable blood pressure transducers Catheter tip pressure sensors ● 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 radiofrequency 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 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 11 M EM EM S - Z -Z JU JU Application of MEMS and Microsystems in Consumer Products Application of MEMS and Microsystems in the Telecommunication Industry Scuba diving watches and computers ● Optical switching and fiber optic couplings Bicycle computers ● RF relays and switches Sensors for fitness gears ● Tunable resonators Micro lenses: Washers with water level controls Micro switches: 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. Micro Optical Switches Concluding Remarks 2-Dimensional 3-Dimensional 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 multidisciplines in science and engineering. 4. Team effort involving multi-discipline of science and engineering is the key to success for any MEMS industry. 12 M EM EM S - Z -Z JU JU In this lecture we will learn the working principles of many microsensors and actuators in MEMS and microsystems. Lecture 1(Part II) Working Principles of 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. ● 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-memory alloys are extensively used in microactuations. Working Principles for Microsensors BioMEMS Power Supply Input Signal Micro Sensing Element Transduction Unit The term “BioMEMS” has been a popular terminology in the MEMS industry in recent 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. Output Signal 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. 13 M EM EM S - Z -Z JU JU Biomedical Sensors and Biosensors Major Technical Issues in BioMEMS Production: These sensors are extensively used in medical diagnosis, environmental protection, drug discovery and delivery, etc. (1) Functionality for the intended biomedical operations. Biomedcial Sensors (2) Adaptivity to existing instruments and equipment. For the measurements of biological substances in the sample and also for medical diagnosis purposes. (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 approval for marketing by the FDA (Food and Drug Administration) of the US government. Example of a biomedical sensor: Pt electrode Power supply V i 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. Biosensors A sensor for measuring the glucose concentration of a patient. H+ Input signal: Biological sample (typically in minute amount in µL or nL) H+ Blood sample Polyvinyl alcohol solution H+ H+ H+ 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 shown below: ANALYTE Ag/AgCl Reference electrode Working principle: Biomolecule Layer ● 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. Output Signals B B B B B Sensor B B Biomolecule Supply B Chemical Optical Thermal Resonant Electrochemical ISFET (Ion Sensitive Field Effect Transducer) ●The difference of potential between the two electrodes due to the build-up of H2 in the Pt electrode relates to the amount of glucose in the blood sample. 14 M EM EM S - Z -Z JU JU Chemical Sensors Chemical Sensors-Cont’d Work on simple principles of chemical reactions between the sample, e.g. O2 and 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: (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 change their electrical resistance when exposed to certain chemicals. (1) Chemiresistor sensors. (2) Chemicapacitor sensors. Chemically Sensitive Polyimide Input current or voltage Input Voltage Measurand Gas Metal Insert Output: Change of Resistance Metal Electrodes Output: Capacitance Change Electric Contact SnO2 SiO2 Silicon Substrate Measurand Gas Optical Sensors Chemical Sensors-Cont’d ● These sensors are used to detect the intensity of lights. Available metal oxide gas sensors: Semiconducting Metals ● It works on the principle of energy conversion between the photons in the incident light beams and the electrons in the sensing materials. Catalyst Additives Gas to be Detected BaTiO3/CuO La2O3, CaCO3 CO2 SnO2 Pt + Sb CO SnO2 Pt Alcohols SnO2 Sb2O3 H2, O2, H2S SnO2 CuO H2S ZnO V, Mo Halogenated hydrocarbons WO3 Pt NH3 Fe2O3 Ti-doped + Au CO Ga2O3 Au CO MoO3 None NO2, CO In2O3 None O3 ● The following four (4) types of optical sensors are available: Photon Energy A is more transparent to photon energy in incident light ⇒ than B. R Photon Energy Semiconductor A Junction Semiconductor B (a) Photovoltaic junction (b) Photoconductive device Bias Voltage Reverse _ Bias + Voltage ΔR Photon Energy Photon Energy R Vout p-Material n-Material p n Leads (c) Photodiodes 15 M EM EM S - Z -Z JU JU Optical Sensors-Cont’d Working Principle of Silicon Solar Photovoltaic (PV) Photon Energy p n p Emitter Junction (weak dielectric) Collector Base p n p Emitter p-silicon (atoms with “holes” by unbalanced electrons) Photons from Sun Base Electric field with 2 electrodes (a battery): (d) Phototransistors Silicon (Si) and Gallium arsenide (GaAs) are common sensing materials. GaAs has higher electron mobility than Si- thus higher quantum efficiency. Extra electrons: - -- - + + - + - + - - - - + + - - - - - -- - - - + + + + + Current Collector Photon Energy n-silicon (excessive electrons) Migrating Electrons + Other materials, e.g. Lithium (Li), Sodium (Na), Potassium (K) and Rubidium (Rb) are used for this purpose. Pressure Sensors Pressure Sensors-Cont’d ● 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. ● The strains associated with the deformation of the diaphragm are measured by tiny “piezoresistors” placed in “strategic locations” on the diaphragm. R1, R2, R3, R4 = Piezoresistors ● They are among the first MEMS devices ever developed and produced for “real world” applications. R4 Metal Pad R1 ● Micro pressure sensors work on the principle of mechanical bending of thin silicon diaphragm by the contacting air or gas pressure. Wire bond Silicon Die with Diaphragm Piezoresistors + Metal film Dielectric layer Cavity Constraint Base Die Attach Pyrex Glass Constraining Base or Metal Header (b) Front side pressurized Interconnect Passage for Pressurized Medium R3 (+ve) Vin R2(-ve) Metal Casing R1(+ve) a Vo - Silicone gel Silicon Diaphragm Measurand Fluid Inlet (a) Back side pressurized Metal Pad Top view of silicon die Measurand Fluid Inlet Cavity R3 R2 ● 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. b R4(-ve) Wheatstone bridge for signal transduction 16 M EM EM S - Z -Z JU JU Pressure Sensors-Cont’d Pressure Sensors-Cont’d ● Other ways of transducing the deformation of the diaphragm to electronic output signals are available, e.g., Metallic Electrode Silicon Cover by capacitance changes Metallic Electrode V (for higher temperature applications) Silicon Die MEMS microphones work on this principle. Cavity Constraint Base Measurand Fluid Inlet ● Major problems in pressure sensors involve the packaging and protection of the diaphragm from the contacting air or gas pressure. Diffused p-type electrode Vibrating beam: (n-type Si wafer,40 μm wide Silicon diaphragm x 600 μm long x 6 μm thick) 1200 μm sq.x 100 μm thick Silicon die by resonant vibration (400 μm thick) (for higher resolutions) Pressurized medium Constraint base Thermal Sensors Thermal Sensors-Cont’d ● Thermal sensors are used to monitor or measure temperature in an environment or an engineering systems. ● Common thermal sensors involve thermocouples and thermopiles. Type ● Thermal sensors work on the principle of the electromotive forces (emf) generated by heating the junction made by dissimilar materials (beads): Metal Wire A Heat Metal Wire A i i Bead Cold Junction Metal Wire B (a) A thermocouple Heat i V Voltage Output V Voltage Output i Metal Wire B (b) A dual junction thermocouple The Seebeck coefficients for various thermocouples are: Hot Junction E J K Wire Materials Chromel/constantan Iron/constantan Chromel/alumel Seebeck Coefficient (μV/oC) Range (oC) Range (mV) 58.70 at 0oC -270 to 1000 -9.84 to 76.36 50.37 at 0oC -210 to 1200 -8.10 to 69.54 39.48 at 0oC -270 to 1372 -6.55 to 54.87 600oC -50 to 1768 -0.24 to 18.70 R Platinum (10%)-Rh/Pt 10.19 at T Copper/constantan 38.74 at 0oC -270 to 400 -6.26 to 20.87 S Pt (13%)-Rh/Pt 11.35 at 600oC -50 to 1768 -0.23 to 21.11 The generated voltage (V) by a temperature rise at the bead (ΔT) is: V = β ΔT where β = Seebeck coefficient Common thermocouples are of K and T types 17 M EM EM S - Z -Z JU JU Thermal Sensors-Cont’d Thermal Sensors-Cont’d Thermopiles are made of connecting a series of thermocouples in parallel: A micro thermal sensor: 3.6 mm Thermocouples 3.6 mm Hot Junction Region, Th 32 Thermocouples 16 μm wide ● Response time is 50 ms. Top view ΔV The induced voltage (ΔV) by the temperature change at the hot junction (ΔT) is: Hot Junction Region 20 μm Thermocouples with N = number of thermocouple pairs in the thermopile. ● dimension of thermopile is: 3.6 mm x 3.6 mm x 20 µm thick ● Typical output is 100 mV Diaphragm: 1.6 mm dia x 1.3 μm thick Cold Junction Region, Tc ΔV = N β ΔT Cold Junction Region Hot Junction Region ● 32 polysilicon-gold thermocouples Silicon Rim Support Diaphragm Elevation Working Principles for Microactuators Power Supply 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 which Output Action Micro Actuating Element α = coefficient of thermal expansion (CTE) – a material property. Transduction Unit ● When two materials with distinct CTE bond together and subject to a temperature change, the compound material will change its geometry as illustrated below with a compound beam: 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. α1 > α2 α1 Heat α2 ● These compound beams are commonly used as microswitches and relays in MEMS products. 18 M EM EM S - Z -Z JU JU Actuation Using Shape Memory Alloys (SMA) Actuation Using Piezoelectric Crystals ● 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. ● A certain crystals, e.g. quartz, exhibit an interesting behavior when subjected to mechanical deformation or electric voltage. ● The deformed SMA structures at room temperature will return to their original shapes when they are heated to the elevated temperature of their production. ● This behavior may be illustrated as follows: ● A micro switch actuated with SMA: V Shape Memory Alloy Strip e.g. TiNi or Nitinolor Induced Mechanical Deformation Applied Voltage, V Mechanical Forces ● Ti-Ni is a common SMA. Resistance Heating Strip Silicon Cantilever Beam Constraint Base Mechanical force induced electric voltage Electric voltage induced mechanical deformation ● This peculiar behavior makes piezoelectric crystals ideal candidate for micro actuation as illustrated in the following case: Actuation Using Electrostatic Forces Actuation Using Piezoelectric Crystals-Cont’d ● Electrostatic Force between Two Particles – The Coulomb’s Law: Electrodes V Piezoelectric Silicon Cantilever Beam Constraint Base r e, nc s ta n F i D o cti F tra At on lsi pu Re A (with charge q) B (with charge q’) The attraction or repulsive force: F = 1 qq ' 4πε r 2 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) 19 M EM EM S - Z -Z JU JU Actuation Using Electrostatic Forces-Cont’d ● Electrostatic Force Normal to Two Electrically Charged Plates: Actuation Using Electrostatic Forces-Cont’d ● Electrostatic Force Parallel to Two Misaligned Electrically Charged Plates: Fd Length, L V Gap, d dth Wi C = εr εo ● The induced capacitance, C is: ,W d W ● Force in the “Width” direction: 1 εr εo L 2 V 2 d Fw = − ● The induced normal force, Fd is: Fd = − 1 ε r ε o WL 2 V 2 d2 FL L V A WL = εr εo d d Fw ● Force in the “Length” direction: FL = − 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). 1 εr εo W 2 V 2 d Applications of Micro Actuations-Cont’d Applications of Micro Actuations Micromotors W W/3 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: Moving set electrodes: Fixed set electrodes: Pitch: w+w/3 A’ Step Movements C’ B’ D’ Dielectric material B A W C D W Linear stepping motors: ● Energize the set A-A’ will generate a force pulling A’ over A due to initial misalignment. ● 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 ● Once A and A’ are aligned, the pair B and B’ become misaligned. W W/3 Moving set electrodes: Fixed set electrodes: Pitch: w+w/3 A’ ● It is now with C’ and C being misaligned. Step Movements C’ B’ ● Energize the misaligned B-B’ will generate electrostatic force pulling B’ over B. D’ ● Energize C’ and C will produce another step movement of the moving set over the stationary set. Dielectric material B A W W C D ● 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. 20 M EM EM S - Z -Z JU JU Applications of Microactuations-Cont’d 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. ● Working principle of this rotary motor is similar to that in linear motors. A micro motor produced by Karlsruhe Nuclear Research Center, Germany: Rotor Stator Micropumps Microvalves Electrostatically actuated micropump: ● 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. Flexible Silicon Diaphragm Gear for transmitting torque Electric Resistance Heating Rings ● 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. Deformable Silicon Diaphragm V Electrode INLET FLOW Inlet Check Valve Pumping Chamber Silicon Base Outlet Check Valve Constraint Base FLOW OUTLET Centerline Low Pressure Fluid Inlet Constraint Base High Pressure Fluid Outlet 21 M EM EM S - Z -Z JU JU Micro Heat Pipes- a viable solution to cooling in molecular electronics 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. Piezoelectric coating with transducer F Flow V Flexible Tube Wall Conventional Heat Pipes Heat Sink Heat Source Condensed liquid Wick Vapor Heat Source Wick Heat Sink ink Heat S urce Heat So Micro Heat Pipe Conde ion tic Sect Adiaba Sharp corners of microconduit provide capillary driving pressure for the return of condensed liquid - no wicks is necessary nser rator Evapo Cross-Sections Heat Sink LIQUID Heat Source VAPOR Elevation Microaccelerometers Micro Accelerometers-Cont’d ● Accelerometers are used to measure dynamic forces associated with moving objects. ● These forces are related to the velocity and acceleration of the moving objects. ● 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: The accelerometer is attached to the vibrating solid body Spring k Mass M Vibrating Solid Body Dashpot with damping C The accelerometer is attached to the vibrating solid body ● The instantaneous displacement of the mass y(t) induced by the attached moving solid body is measured and recorded with respect to time, t. ● The associated velocity, V(t) and the acceleration α(t) may be obtained by the following derivatives: V (t ) = dy (t ) dt and α (t ) = Spring k Mass M Vibrating Solid Body Dashpot with damping C dy (t ) d 2 y (t ) = dt dt 2 ● 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 the coil spring and the dashpot for damping on the vibrating mass. ♦ Alternative substitutes for the coil spring, dashpot, and even the proof mass need to be found. 22 M EM EM S - Z -Z JU JU Microaccelerometers-Cont’d Microaccelerometers-Cont’d ● There are two types microaccelerometers available. (2) Balanced force microaccelerometer: ● This is the concept used in the “air-bag” deployment sensor in automobiles (1) The cantilever beam accelerometer: Silicon Cantilever Beam Casing ● In this design: Plate beam = proof mass; Two end tethers = springs Surrounding air = dashpot Piezoresistor Mass, M Be am Constraint Base Constraint Base Stationary electrodes Moving electrode Vibrating Base Ac c Mo ve me nt ele ra tio n 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. ● The movement of the proof mass is carried out by measuring the change of capacitances between the pairs of electrodes. Inertia Sensors – Unique Micromechatronics Devices Working Principle of BalancedForce Accelerometers: Balanced-Force Accelerometers: MEMS Microphones Applications: ● Beamforming microphone arrays for wind tunnels ● Beamforming microphone arrays for smart hearing aids ● Mobile telephones ● Notebook and palm-top computers Acoustic Wave Input (air pressure wave) dB→ MPa 2 mm 3 mm • The need for integrating microelectronics (ICs) and moving microstructures – A great challenge! Ac ce ler a ti on Electrical signal output: Diaphragm: ≈ 1µm thick Air gap (≈2 µm) Backplate (≈2 µm) ΔC Acoustic holes Pressure equalization hole 23 M EM EM S - Z -Z JU JU Microgyroscopes - for precision motion control A Microgyroscope Actuated by Electrostatic Forces The Vectorial Representation of Coriolis Motion: Ω V Fc V x y Fc z Ω (b) z y V y z x x V y Fc x Proof Mass x-Spring x z Ω Fc x Resonator for Linear Motion Generation x (a) y-Spring for Force Measurements y y x x-Position Gyro Frame z Ω x y y z y y-Position x z z (c) (d) y SUMMARY ● MEMS and microsystems consist of sensors, actuators, power supply and signal transducers. ● Microsensors work on the principle of change of sensing material properties in response to the substances to be sensed and detected. ● Actuating forces for MEMS and microsystems are radically different from traditional 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 in the 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 24