Silicon Nanoelectronics And Beyond: An Overview And Recent .

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OverviewSilicon NanoelectronicsSilicon Nanoelectronics and Beyond:An Overview and RecentDevelopmentsN.M. Ravindra, Vishal R. Mehta, and Sudhakar ShetThis year marks the 40th anniversaryof the invention of the first beam-leaddevice by Lepselter et al. Lepselterand coworkers proposed a method offabricating a new semiconductor devicestructure and its application to highfrequency silicon switching transistorsand ultra-high-speed integrated circuits.Beam-lead technology, also known asair-bridge technology, has establisheditself for its unsurpassed reliability inhigh-frequency silicon switching transistors and ultra-high-speed integratedcircuits for telecommunications andmissile systems. The beam-lead devicebecame the first example of a commercial microelectromechanical structure(MEMS). Since its inception, MEMS hastaken advantage of the evolving silicontechnology, resulting in today’s nanoelectromechanical structure and nanooptomechanical structure. In this paper,an overview of recent developments ofsilicon nanoelectronics is presented.INTRODUCTIONIn April 1965, Lepselter and colleagues1 proposed a technique of fabri-cating a structure consisting of depositingan array of thick contacts on the surfaceof a slice of standard planar-oxidizeddevices. The excess semiconductor fromunder the contacts was removed, therebyseparating the individual devices andleaving them with semi-rigid beam leadscantilevered beyond the semiconductor.The contacts served as electrical leads inaddition to also serving the purpose ofstructural support for the devices. Thesedevices were called beam-lead devices.In Figure 1, a cut-away cross section ofa high-frequency beam-lead switchingtransistor, proposed by Lepselter et al.,1is presented, while Figure 2 shows anisolated monolithic integrated circuit(isolith) fabricated by Lepselter et al.The circuit is a four-input direct-coupledtransistor logic (DCTL) gate and consistsof four n-p-n transistors.2Figure 3 shows a summary of sensordevelopment activities in the UnitedStates since their beginnings in the1950s.3 This figure takes into accountthe materials-oriented research at BellTelephone Laboratories, Honeywell,and Westinghouse. As part of theFigure 1. A cut-away cross section of high frequency beam-lead switching transistor.116development of Lepselter’s beam-lead(air-isolated) integrated circuits at BellTelephone Laboratories in the 1960s,precision silicon etching technologywas developed. By the mid-1970s, thistechnology had been utilized in importantways by the sensor community and hadbeen called “micromachining.” Many ofthese new devices were micro-actuators,micro-sensors, and micro-motors. Theintegration of these devices led to microinstrumentation systems on a singlechip. The term “microelectromechanicalstructures (MEMS)” was born in the late1980s to describe one of the results ofthe sensor-actuator field.EVOLUTION OFNANOELECTRONICSBy definition, the word nano simplyrefers to a nanometer or one billionthof a meter. A red blood cell measures 5,000 nm while ten hydrogen atoms,lined up side by side, is equal to 1nm. One thousand nanometers in anydimension has been accepted to represent nanotechnology. As the physicaldimensions approach nano-scale, thematerial behavior and properties aregoverned by quantum physics. Examplesof the discrepancy in material propertiesbetween macro- and nano-scale canbe illustrated by gold, which appearsyellow at the macro-scale and is seen asred at the nano-scale. Another popularexample is carbon; at the macro-scale itis soft and malleable; it becomes harder,stronger, and more rigid than steel at thenano-scale. Further, at the macro-scale,carbon is a poor conductor of electricity.It is a better conductor of electricity thansilicon or copper at the nano-scale.4While nanoscience is pure research,nanotechnology is the application ofresearch for the purpose of solving problems and manufacturing new materials.JOM June 2005

improved hard disks.Nanoelectronics holds promise fordeveloping electronic componentsbeyond silicon. But the impact of nanoelectronics will reach much further thanabFigure 2. The top and bottom view of a four-input DCTL gate.2Advanced ProcessesSensor-Circuit IntegrationNew Materials, ModelingSurface MicromachiningFirst MicroactuatorsAdvanced Sensing SystemsImplantable Biomedical Sensors 1980sIntegrated SensingSystems 1970s 1960sTechnology Expansion More Complex DevicesNew ApplicationsCommercialization Bell Telephone LaboratoriesFirst Micromachined Sensors First MicromachinedDevices 1950sHoneywell MaterialsResearch Pfann, ThurstonMason, Smith Materials ResearchNova SensorIC SensorApplications inFord Automotive ControlMotorola Industrial AutomationAnalogGM DelcoDevices Health Care Consumer ProductsFoxboro ICT Rosemount Burr-BrownKulite Endevco Various Other Companies NathansonMIT S.D. SenturiaOther Universities WestinghouseUniversity of Wisconsin H. Guckel HoneywellTufte, SteltzerNamuraUC-Berkeley R. Muller, R. White, R. HoweUniversity of Michigan K.D. Wise, K. Najafi Klein, D’StefanLepselterStanford University J.B. Angell, J.D. MeindlCase-Western Reserve University W.H. Ko Silicon EtchingMicrostructures 2005 June JOMelectronics include carbon nanotubesthat can be used in both electroniccomponents and displays, and nanomaterials that can be used in films thatmake smaller, more flexible displays and From a historical point of view, RichardFeynman5 first wrote about the potentialfor nanoscience in an influential 1959 talk“There’s Plenty of Room at the Bottom.”Feynman argued in support of studyingconcepts to build equipment needed towork at atomic dimensions. In 1981, in apaper titled “Molecular Engineering: AnApproach to the Development of GeneralCapabilities for Molecular Manipulation,” Eric Drexler6 built a frameworkfor the study of devices that were ableto move molecular objects and positionthem with atomic precision. In 1989, ascientist in IBM’s Almaden ResearchCenter moved individual xenon atomsto form the company’s logo on a nickelplate.7Since its inception just about sixdecades ago, silicon material, as wellas device and circuit technology, hasrapidly progressed, nearing the ultimate barrier in the micro-electronicand chip level of development. Thus,science has entered into the new eraof the atomic realm. Nanotechnologyis revolutionizing electronics throughthe development of nano-enabled systems. These systems incorporate novelnanostructures that integrate functionalcomplexity directly into each individualnanoparticle, enabling the low-costfabrication of revolutionary high-value,high-performance applications in a broadrange of industries from life and physicalsciences to information technology andcommunications to renewable energy todefense. These nanostructures includenanowires, nanorods, nanotetrapods,and nanodots formed from elementaland compound semiconductors. Thesedevices, circuits, and systems exploitthe fundamentally new and uniqueelectronic, optical, magnetic, interface,and integration properties associatedwith materials on the nanometer scale.Possible applications include electronics and information technology, healthcare, environmental protection, energy,anti-terrorism, and homeland defense.7Nanoelectronics refers to electronicsat the sub-micrometer scale. Today, manyintegrated-circuit components in production already consist of device featuresizes at the nanoscale. Nanoelectronicsalso includes molecular electronics,which utilizes individual molecules inelectronics. Recent advancements innanotechnology with applications in1990sFigure 3. A summary of sensor development activities in the United States since theirbeginnings in the 1950s.317

Figure 4. High-resolution transmissionelectron micrographs of the Si/SiO 2interface for samples of different oxidethicknesses. The oxidation temperatureis 800 C.13the next-generation integrated circuits. Itis the key to hard disks with large capacity; new forms of nonvolatile memory;smaller, more flexible displays; strongerbatteries and power sources; more efficient networks; quantum computing; andmore. The most commonly studied nanomaterial today is the carbon nanotube,which comes in both single-walled ormulti-walled (tubes within tubes) varieties. Carbon nanotubes are tiny cylindersof carbon atoms. In addition to beingstronger than steel, these nanotubes areexcellent conductors of electricity. Manyexperts are of the opinion that photolithography, the process currently usedto make chips, will be unable to keepup with the ever-decreasing dimensionsof chip features. It is possible that analternative to photolithography will bebased on nanotechnology. Three competing technologies—x-ray lithography,e-beam lithography, and nano-imprintlithography—will allow the creation ofpatterns down to 100 nm.8Silicon microelectronics has transitioned to silicon nanoelectronics due tocost-performance correlations:9 With decreasing feature sizes, thedevice cost decreases while itsperformance increases New markets are created byenhanced performance Research and development andcapital investment are supportedby reduced costsSilicon enjoys natural abundanceaccompanied by a very mature andreliable technology in the semiconductor industry. The complementary metaloxide semiconductor field effect transistor (CMOS FET), which is the currentbasis of ultra-large-scale integrationcircuits, has begun to show fundamentallimits associated with the laws of quantum mechanics and the limitations offabrication techniques. The Semiconductor Industry Association’s InternationalTechnology Roadmap for Semiconductors shows no known solutions in theshort term for a variety of technologicalrequirements including gate dielectric,gate leakage, and junction depth. Therefore, it is anticipated that entirely newdevice structures and computationalparadigms will be required to augmentand/or replace standard planar CMOSdevices. Two promising beyond-CMOStechnologies that each take a very different fabrication approach are molecularelectronics and silicon-based quantumelectronic devices.Molecular electronics is based onbottom-up fabrication paradigms, whilesilicon-based nanoelectronics are basedupon the logical continuation of thetop-down fabrication approaches utilized in CMOS manufacturing. Thesetwo approaches bracket the possiblemanufacturing techniques that will beutilized to fabricate future nanoelectronicdevices. In addition, electronic devicesfabricated with organic materials forma dramatically emerging technologytargeting applications such as printablelarge-area displays, wearable electronics, paper-like electronic newspapers,low-cost photovoltaic cells, ubiquitousintegrated sensors, and radio-frequencyidentification tags. These applicationsare challenging to implement in conventional CMOS technology.10 In addition,the primary difficulties facing nanodevice fabrication are making contacts todevices on a nanometer scale, interconnecting the nanodevices massively, andproviding a means to input and read outdata.11NANOELECTRONICSAPPLICATIONS ANDOPPORTUNITIESNanoelectronic devices beingattempted today for logic and processing applications include nanotubes,nanowires, molectronics, spintronics,single-electron transistors, quantumcellular automata, quantum computing, and alternative architectures. Formemory applications, magnetic drivesand tapes, optical disks, holographicmedia, magnetic random-access memory(RAM), charge-driven phase change,molecular charge base memory, nanotube RAM, scanning probe systems,MEMS cantilever switch, ferroelectricRAM, and polymer memory are beingTable I. A Summary of Funding Opportunities for Nanotechnology15Fiscal 3Enact/Actual2004Req./Enact2005National Science FoundationDepartment of DefenseDepartment of EnergyNational Institutes of HealthNASANISTEPAHomeland Security (TSA)Department of AgricultureDepartment of /39.620/2210/33.4— /5.8—/——0/1.5— /110/11.4/13052762118935535151TOTAL270422/465 72%600/697 50%770/862 24%819/961982* All in millions of dollars18JOM June 2005

Millions /Yearattempted. Key industry players focusingon the development of nanoelectronicdevices include IBM, Intel, Interuniversity Microelectronics Center, HewlettPackard Company, Motorola, Leti,STMicroelectronics, MicroelectronicsTechnology Laboratory, the FrenchNational Center for Scientific Research,Nanotube Manufacturers, Nanosys,Carbon Nanotechnologies, Helix Materials Solutions, and Nanodynamics.It is anticipated that nanoelectronicswill play a major role in the developmentof the following devices/subsystems:logic/processing, memory/storage,interconnects, thermal management, anddisplays. In the end-user market, the following sectors seem to be of importance:mobile computing, home computingand consumer electronics; enterprisecomputing and telecommunications; cellphones, global positioning systems andother communication devices; portable4,0003,5003,0002,5002,0001,5001,0005000— W. Europe— Japan— USA— Others— TotalFigure 5. Past government investments fornanotechnology.1519971998199920002001recording and display/playback devices;control systems and embedded computing; sensors, smart cards, radio frequencyidentification, and other disposableproducts; and military and homelandsecurity.Biological and biomedical applications will continue to deploy nanotechnology in drug delivery systems andimaging applications. An example of200220032004a revolutionary approach to imaging isbeing utilized by Given Imaging. GivenImaging12 is redefining gastrointestinaldiagnosis by developing, producing, andmarketing innovative, patient-friendlyproducts for detecting gastrointestinaldisorders. The company’s technologyplatform is the Given DiagnosticSystem, featuring the PillCam SBvideo capsule, a disposable capsule thatTable II. Possible Device Applications of esDisadvantagesRemarksSingle-ElectronTransistors (SET)Logic elementSmall size,low powerSensitive to background charge instability.High resistance and low drive current. Cannotdrive large capacitive (wiring) loads. Requiresgeometries 10 nm for room-temperatureoperationUse of Coulomb blockade innanocrystal “floating-gate”-typenonvolatile memory demonstrated.May improve retention time.Quantum Dot(Quantum CellularAutomata)Logic elementSmall sizeMultiple levels of interconnection across longdistance difficult. Room-temperature operationdifficult. New computation algorithmsrequired. Method of setting the initial state ofthe system not available. Single defect in lineof dots will stop propagation.Devices demonstrated at lowtemperatures. QCA architecturesextensively investigatedResonant TunnelingDiode (RTD)Logic elementdynamicmemorySmall sizeTunneling process sensitive to small filmthickness (tunneling distance variation, leadingto process control difficulties. Requires directcurrent bias, large standby power consumption.Multivalue logic sensitive to noise margin.Speed of RTD circuits likely to be determinedby the conventional devices required in thecircuit.Small- to medium-scalecircuits demonstrated. Mostdemonstrations on III-Vcompound semiconductorsRapid Single-FluxQuantum (RSFQ)DeviceLogic elementVery highspeedpossibleRequires liquid helium temperature. Lacks ahigh-density random-access memory. Requirestight process tolerance.Very-high-speed (THz) circuitsdemonstrated.Two-TerminalMolecular DevicesLogic elementmemorySmall sizeNo inherent device gain. Scaling to largememory size may be difficult without gain.Placement of molecules in a circuit difficultand not yet demonstrated. Temperaturestability of organic molecules may beproblematic.Sixteen-bit cross-point memorydemonstrated.Carbon NanotubeFETLogic elementBallistictransport(high speed)small sizePlacement of nanotubes in a circuit difficultand not yet demonstrated. Control of electricalproperties of carbon nanotube (size, chirality)difficult and not yet achieved.Device scaling properties notyet explored. Inverter circuitdemonstrated.DNA ComputingLogic elementHighparallelismImperfect yield. General-purpose computingnot possible.2005 June JOM19

as much as many of the countries in theworld.A summary of various possible deviceapplications of nanotechnology is presented in Table II. The challenges posedby each of the device applications arealso described in the table. In Figures 6and 7, some of the examples of siliconnanoelectronic devices are illustrated.CONCLUSIONSabFigure 6. (a) An optical microscope photograph of a signal inverter circuit fabricated witha silicon crystal layer of nanoscale thickness and with control gate electrodes fabricatedboth above and below the active silicon layer. (b) A scanning electron microscope imageof the silicon wafer from which the circuit shown in (a) is fabricated. The electronicallyactive part of the structure is the thin silicon layer sandwiched between the back-gate andfront-gate layers in the image.16captures video after it is ingested by thepatient. The PillCam SB video capsuleis the only naturally ingested method fordirect visualization of the entire smallintestine. It is currently marketed inthe United States and in more than 60other countries and has benefited morethan 122,000 patients worldwide. Thecompany is developing a complete lineof PillCam video capsules for detectingdisorders throughout the gastrointestinaltract. The PillCam ESO video capsule forvisual examination of the esophagus iscurrently under review by the U.S. Foodand Drug Administration, and capsulesfor visualization of the stomach and colonare under development.A well-known example of the use offilms of nano-dimensions is illustrated inFigure 4. In this figure, high-resolutiontransmission-electron micrographs ofthe Si-SiO2 interface for various oxidethicknesses in the 2 nm to 20 nm rangeare presented.13 Another well knownexample of devices of nano-dimensionshas to do with shallow junction formation in CMOS technology. The Nanoelectronic Device Metrology (NEDM)project of the U.S. National Institute ofStandards and Technology14 is developing metrology for three specific areas ofnanotechnology: silicon-based quantumelectronics, molecular electronics, andorganic electronics. In addition to thisproject, various federal agencies havecommitted funds to support research innanotechnology. A summary of thesefunding opportunities is presented inTable I. Past worldwide governmentinvestments in nanotechnology areillustrated in Figure 5. As can be seen inthis figure, the United States has investedProperty-structure correlations willcontinue to drive nanotechnology. Healthand biosciences will dictate the terms ofgrowth in relation to betterment of humanliving conditions. In order for siliconnanoelectronics to thrive, it will haveto seek compromise with other semiconductors. Free-standing nano deviceswould be difficult to manufacture due tolimitations of materials stability, contactproblems, and long-term reliability.References1. M.P. Lepselter et al., “Beam-Lead Devices andIntegrated Circuits,” Proceedings of the IEEE, 53 (4)(1965), p. 405.2. (04/04/05).3. 1.htm (04/04/05).4. l(04/03/05).6. K.E. Drexler, “Molecular Engineering: An Approachto the Development of General Capabilities forMolecular Manipulation,” Proc. Natl. Acad. Sci. USA,78 (1981), pp. 5275–5278.7. (04/03/05).8. ources/faq/ (04/02/05).10. (04/01/05).11. (04/03/05).12. (04/04/05).13. N.M. Ravindra et al., “Silicon Oxidation and Si-SiO2Interface of Thin Oxides,” J. Mater. Res.,

NANOELECTRONICS By defi nition, the word nano simply refers to a nanometer or one billionth of a meter. A red blood cell measures ~5,000 nm while ten hydrogen atoms, lined up side by side, is equal to 1 nm. One thousand nanometers in any dimension has been accepted to repre-sent nanotechnology. As the physical dimensions approach nano-scale, the