11_-_mems_world

MICROSYSTEMS’WORLD AN INTRODUCTIONMEMS is a process technology used to create tinyintegrated devices or systems that combinemechanical and/or sensor with electricalcomponents.They are preferably fabricated usingintegrated circuit(IC)batch processing techniquesand can range in size from a few micrometers tomillimetres.These devices(or systems)have the ability to sense, control and actuate on the micro scale, and generate effects on the macro scale.The interdisciplinary nature of MEMS utilizes design,engineering and manufacturing expertise from a wide and diverse range of technical areas including integrated circuit fabrication technology,mechanical engineering,materials science,electrical engineering,chemistry and chemical engineering,as well as fluid engineering,optics, instrumentation and packaging.The complexity of MEMS is also shown in the extensive range of markets and applications that incorporate MEMS devices. MEMS can be found in systems ranging across automotive,medical,electronic,communication and defence applications.Current MEMS devices include accelerometers for airbag sensors,inkjet printer heads, computer disk drive read/write heads,projection display chips,blood pressure sensors,optical switches,microvalves,biosensors and many other products that are all manufactured and shipped in high commercial volumes.MEMS(Micro Electro-Mechanical System) is an acronym that originated in the United States, and is also referred to as Microsystems Technology(MST) in Europe.Regardless of terminology, the uniting factor of a MEMS device is in the way it is made. While the device electronics are fabricated using‘computer chip’IC technology, the micromechanical components are fabricated by sophisticated manipulations of silicon and other substrates using micromachining processes.While integrated circuits are designed to exploit the electrical properties of silicon, MEMS takes advantage of either silicon’s mechanical properties or both its electrical and mechanical properties.In the most general form, MEMS consist of mechanical microstructures,microsensors, microactuators, and microelectronics,all integrated onto the same silicon chip.Pressure SensorNovaSensorHOW LARGE ARE MEMS?Wafer (10 cm)Chip (~1 cm)Virus (~10 nm)Transistor(~1 µm)Molecules(~1 nm)Hair(~100 µm)Advantages of MEMSSCALING ISSUES•Scaling involved with MEMS is tremendous(often >1000X smaller than the macroscopic counterpart)–our experience and intuition of macro-scale phenomena and designs will not transfer directly to the microscale•Must have a good understanding for the scaling properties of the key physical phenomena involved with the MEMS–Mass–Pressure–Friction–…•Scaling properties of any one phenomena could:–present a barrier to performance or feasibility–require design strategies that are different than that used for macro-scale versions–require specialized fabrication processes and materials•High surface-to-volume ratio of MEMS require more attention in controlling the surface characteristics.SCALING PHYSICAL PHENOMENAReducing Dimensions by s causes:-volume forces (mass) to be reduced by s3-surface forces (pressure, friction, electrostatic, ...) to be reduced by s2-line forces (surface tension) to be reduced by s3)2)1)Log DimensionLogForceSCALING OF MECHANICAL SYSTEMS •Mechanical Stiffness of a Cantilever Beam:F x= k x x[N]k x= w t3 E/ (4 L3)[N/m]w:width of beamt:thickness of beamE:elastic modulus (Single Crystal Silicon ~200 GPa)L:length of beam–Scales down linearly with dimensions (mass and inertial force scale as s-3)–Micromechanical systems are stiffer than macromechanical systems (relatively speaking), so can withstand tremendous accelerations without breaking or being disturbed•Negative Consequence of Rapidly Scaled Mass–Inertial sensor performance is proportional to the mass of the proof mass •MEMS inertial sensors must be able to resolve smallerdeflections than macroscopic inertial sensorsSCALING OF FLUIDIC SYSTEMS •Reynolds number is a function of sizeRe= (ρV D) / µ[unitless]ρ:densityV:characteristic velocityD:characteristic length or diameterµ: viscosity–Re> 4000 = Turbulent (macroscopic flow)–Re< 2000 = Laminar (microscopic flow)•makes it difficult to mix two liquids in microscopic fluidicsystems (mixing dominated by simple diffusion)•Bubbles can be a BIG problem!!! (or a useful tool…)SCALING THERMAL SYSTEMS •Miniaturizing thermal systems–Increase surface-to-volume ratios•heat is conducted out more quickly •With micromachining it is possible to make excellent thermally isolated (or conductive) structures•One of the most successful MEMS applications is the inkjet printer head, superseding even automotive and medical pressure sensors.•Inkjet printers use a series of nozzles to spray drops of ink directly on to a printing medium.Droplets of ink can be formed in 2different ways:thermally or piezoelectrically.•Invented in 1979by Hewlett-Packard, MEMS thermal inkjet printer head technologyuses thermal expansion of ink vapour.Within the printer head there is an array of tiny resistors(heaters)that can be fired under microprocessor control with electronic pulses of a few milliseconds(usually less than3microseconds).Ink flows over each resistor,which when fired,heat up at 100millionºC/s,vaporizing the ink to form a bubble.As the bubble expands, some of the ink is pushed out of a nozzle.When the bubble collapses, a vacuum is created which pulls more ink into the print head from the reservoir in the cartridge.It is worth noting there are no moving parts in this system (apart from the ink itself)illustrating that not all MEMS devices are mechanical.•A piezoelectric element can also be used to force the ink through the nozzles. In this case, a piezoelectric crystal is located at the back of the ink reservoir of each nozzle. The piezoelectric crystal element receives a very small electric charge causing it tovibrate.When it vibrates inwards it forces a tiny amount of ink out of the nozzle.As the element vibrates back out,it pulls some more ink into the reservoir to replace the ink that was sprayed out.Epson patented this technology but it is also used by the majority of the other leading printer companies.•MEMS has enabled more and more heating elements and piezoelectric crystals to be incorporated into a printer head.Early printers had12nozzles with resolutions of up to92dpi possible.Today,modern inkjet printers have up to600nozzles which can all fire a droplet simultaneously enabling4800dpi.Epson,Lexmark,Hewlett-Packard, Olivetti,Xerox and Canon all use a form of these MEMS in their inkjet printers. Over 350million units were sold in 2000.DROPLET EJECTION PRINCIPLE ACTUATOR MICRO SECTION2Texas Instruments Digital Micromirror DeviceMEMS sensormicroseismometerlow power interfacemicropower analog CMOSADCDSP and controlmicropowerDSPRF network interfacemicropower RF CMOS VCOSYSTEMSAPPLICATIONSToday, high volume MEMS can be found in a diversity of applications across multiple markets:A MEMS device may find numerous applications across a diversity of industries.For example, the MEMS inkjet printer head nozzle in widespread use today has developed from a nozzle originally used in nuclear separation.AUTOMOTIVE APPLICATIONS…•First MEMS product fully integratedwith circuits…Surface Micromachined Inertial SensorsSEM photograph of a vertical accelerometer; planar view (top) and a close up of the membrane structure (bottom) (MOTOROLA).MOTOROLA VERTICAL ACCELEROMETERMore than 10million units have been shipped !!!The first wave of MEMS was automotive and the killer app was airbag triggers.The second wave of MEMS was consumer electronics and portrait-to-landscape auto-switching was the killer app.Now the third wave of MEMS has begun, according to several MEMS experts, namely medical applications of MEMS, with the killer app likely tobe point-of-care diagnostics.Pressure SensorNovaSensorBIOMEDICAL APPLICATIONS ……Biomedical Microactuators Micropumps, valves, filters, needles …•Precisely controlling gases and fluids is critical–diagnostic systems –surgical systems –therapeutic systems•Many efforts exist to develop reliable low-cost high-precision–microneedles –microfilters –microvalves–micropumpsKeller (UCB)Pisano (UCB)…Penetrating Microelectrodes …•Many configurations–# of shanks, electrodes, dimension•For recording or stimulationKensallWise (Univ. Michigan)Richard Norman (Utah)Greg Kovacs (Stanford)Kensall Wise (Univ. Michigan)…Microgrippersand MicrotweezersC.-J. Kim (UCLA / MAE)C. Keller (UCB)Texas Instruments Digital Micromirror Device•Digital Mirror Device–Individual tilting Al mirrors –Electrostatic Forces–Microfabricated on CMOS–Metal / Polymer MEMS ProcessOPTICAL APPLICATIONSMEMS Can Miniaturize Systems …Micro-Optical SystemsPresent•Micro-optics and optomechanical structures are made separately •Assembly and alignment required10’s cmFuture•MEMS micropositioners and actuators are monolithically fabricated with micro-optics •On-chip micro alignment1 mmMing Wu (UCLA/EE)L. Fan and M. C. Wu, TRANSDUCERS 97Integrated Photonics LaboratoryMicrophotonics300-µm Ball LensMicro-XYZ Stages • 3 degrees of freedom•Independent movement of x, y, and z•Large travel distance ( > 150 µm)•Fine positioning resolution (~ 30 nm)•Fully self-assembledMicroactuatorsMEMS Art & Style …MEMS Can Miniaturize Systems …Triple-Piston Microsteam Engine -Water inside of three compression cylinders is heated by electric current and vaporizes,pushing the piston out.Capillary forces then retract the piston once current is removed (Sandia Lab.)The world's smallest guitar is 10micrometers long --about the size of a single cell --with six strings each about 50nanometers, or 100atoms,wide.Made by Cornell University researchers from crystalline silicon and with electron-beam lithography and wet etching.MAD MEMS !ELECTRO-DISCHARGE MACHININGE-BEAM LITHOGRAPHYSURFACE MICROMACHINING AND CAPILLARITY。