NanoscaleResearchLettersNanoscaleResLett.9(1):526-526

OverviewofemergingnonvolatilememorytechnologiesJaganSinghMeena1,SimonMinSze1,UmeshChand1,Tseung-YuenTseng1

1.DepartmentofElectronicsEngineeringandInstituteofElectronics,NationalChiaoTungUniversity,Hsinchu30010,Taiwan

Copyright©2014Meenaetal.;licenseeSpringer.

DOI:10.1186/1556-276X-9-526

Publishedonline:25September2014

Abstract

NonvolatilememorytechnologiesinSi-basedelectronicsdatebacktothe1990s.Ferroelectricfield-effecttransistor(FeFET)wasoneofthemostpromisingdevicesreplacingtheconventionalFlashmemoryfacingphysicalscalinglimitationsatthosetimes.AvariantofchargestoragememoryreferredtoasFlashmemoryiswidelyusedinconsumerelectronicproductssuchascellphonesandmusicplayerswhileNANDFlash-basedsolid-statedisks(SSDs)areincreasinglydisplacingharddiskdrivesastheprimarystoragedeviceinlaptops,desktops,andevendatacenters.TheintegrationlimitofFlashmemoriesisapproaching,andmanynewtypesofmemorytoreplaceconventionalFlashmemorieshavebeenproposed.Emergingmemorytechnologiespromisenewmemoriestostoremoredataatlesscostthantheexpensive-to-buildsiliconchipsusedbypopularconsumergadgetsincludingdigitalcameras,cellphonesandportablemusicplayers.Theyarebeinginvestigatedandleadtothefutureaspotentialalternativestoexistingmemoriesinfuturecomputingsystems.Emergingnonvolatilememorytechnologiessuchasmagneticrandom-accessmemory(MRAM),spin-transfertorquerandom-accessmemory(STT-RAM),ferroelectricrandom-accessmemory(FeRAM),phase-changememory(PCM),andresistiverandom-accessmemory(RRAM)combinethespeedofstaticrandom-accessmemory(SRAM),thedensityofdynamicrandom-accessmemory(DRAM),andthenonvolatilityofFlashmemoryandsobecomeveryattractiveasanotherpossibilityforfuturememoryhierarchies.Manyothernewclassesofemergingmemorytechnologiessuchastransparentandplastic,three-dimensional(3-D),andquantumdotmemorytechnologieshavealsogainedtremendouspopularityinrecentyears.Subsequently,notanexaggerationtosaythatcomputermemorycouldsoonearntheultimatecommercialvalidationforcommercialscale-upandproductionthecheapplasticknockoff.Therefore,thisreviewisdevotedtotherapidlydevelopingnewclassofmemorytechnologiesandscalingofscientificproceduresbasedonaninvestigationofrecentprogressinadvancedFlashmemorydevices.

Review

BackgroundGeneraloverview

Theideaofusingafloatinggate(FG)devicetoobtainanonvolatilememorydevicewassuggestedforthefirsttimein1967byKahngDandSzeSMatBellLabs[1].ThiswasalsothefirsttimethatthepossibilityofnonvolatileMOSmemorydevicewasrecognized.Fromthatday,semiconductormemoryhasmadetremendouscontributionstotherevolutionarygrowthofdigitalelectronicssincea64-bitbipolarRAMchiptobeusedinthecachememoryofanIBMcomputerwasreportedin1969[2].Semiconductormemoryhasalwaysbeenanindispensablecomponentandbackboneofmodernelectronicsystems.Allfamiliarcomputingplatformsrangingfromhandhelddevicestolargesupercomputersusestoragesystemsforstoringdatatemporarilyorpermanently[3].Beginningwithpunchcardwhichstoresafewbytesofdata,storagesystemshavereachedtomultiterabytesofcapacitiesincomparativelylessspaceandpowerconsumption.Regardingapplicationaspects,thespeedofstoragesystemsneedstobeasfastaspossible[4].SinceFlashmemoryhasbecomeacommoncomponentofsolid-statedisks(SSDs),thefallingpricesandincreaseddensitieshavemadeitmorecost-effectiveformanyotherapplications[5].MemorydevicesandmostSSDsthatuseFlashmemoryarelikelytoserveverydifferentmarketsandpurposes.Eachhasanumberofdifferentattributeswhichareoptimizedandadjustedtobestmeettheneedsofparticularusers.Becauseofnaturalinherentlimitations,thelong-establishedmemorydeviceshavebeenshortedoutaccordingtotheirinventionstomatchwithportableelectronicdatastoragesystems.Today,themostprominentoneisthelimitedcapacityforcontinuedscalingoftheelectronicdevicestructure.ResearchismovingalongthefollowingpathsforembeddedFlashdevices:(i)scalingdownthecellsizeofdevicememory,(ii)loweringvoltageoperation,and(iii)increasingthedensityofstatepermemorycellbyusingamultilevelcell.Tosustainthecontinuousscaling,conventionalFlashdevicesmayhavetoundergorevolutionarychanges.Basically,itisexpectedthatanentireDVDcollectionbeinthepalmofahand.Noveldeviceconceptswithnewphysicaloperationingprinciplesareneeded.Itisworthwhiletotakealookatsemiconductormemoriesagainstthebackgroundofdigitalsystems.Thewaysemiconductordevicesareusedinasystemsenvironmentdetermineswhatisrequiredofthemintermsofdensity,speed/power,andfunctions.Itisalsoworthwhiletolookintotheeconomicsignificanceofsemiconductormemoriesandtherelativeimportanceoftheirvarioustypes.Forthepastthreeandahalfdecadesinexistence,thefamilyofsemiconductormemorieshasexpandedgreatlyandachievedhigherdensities,higherspeeds,lowerpower,morefunctionality,andlowercosts[3,6,7].Atthesametime,someofthelimitationswithineachtypeofmemoryarealsobecomingmorerealized.Assuch,thereareseveralemergingtechnologiesaimingtogobeyondthoselimitationsandpotentiallyreplaceallormostoftheexistingsemiconductormemorytechnologiestobecomeauniversalsemiconductormemory(USM).Inaddition,therewardsforachievingsuchadevicewouldbetogaincontrolofanenormousmarket,whichhasexpandedfromcomputerapplicationstoallofconsumerelectronicproducts.Lookingforwardtothefuture,therearewiderangesofemergingmemoryapplicationsforautomationand

informationtechnologytohealthcare.Thespecificationofnonvolatilememory(NVM)isbasedonthefloatinggateconfiguration,whichisthefeatureofanerasedgateputintomanycellstofacilitateblockerasure.Amongthem,designedFlashmemoriessuchasNORandNANDFlashhavebeendevelopedandthenproposedascommercialproductsintobulkmarket.Theyhavebeenconsideredasthemostimportantproducts.NORhashighoperationspeedforbothcodeanddatastorageapplications;ontheotherhand,NANDhashighdensityforlargedatastorageapplications[8].SincetheinceptionofFlashmemory,therehasbeenanexponentialgrowthinitsmarketdrivenprimarilybycellphonesandothertypesofconsumerelectronicequipment.While,today,integrationofasiliconchipisnoteconomical,toys,cards,labels,badges,valuepaper,andmedicaldisposablescouldbeimaginedtobeequippedwithflexibleelectronicsandmemory.Withgrowingdemandsforhigh-densitydigitalinformationstorage,memorydensitywitharrivingtechnologyhasbeenincreaseddramaticallyfromthepastcoupleofyears.Themaindrivetodeveloporganicnonvolatilememoryiscurrentlyforapplicationsofthin-film,flexible,orevenprintedelectronics.Oneneedsatechnologytotageverythingtoelectronicfunctionalitywhichcanbeforeseeninaverylargequantityandataverylowcostonsubstratessuchasplasticandpaper.Accessiblepopularizationofroll-to-rollmemorycommercializationisawaytomakeanencounterinterestingandchallengingtohavechargestoragedevicesofchoiceforapplicationswithenormousflexibilityandstrength.Recently,polymer(plasticmemory)andorganicmemorydeviceshavesignificantconsiderationbecauseoftheirsimpleprocesses,fastoperatingspeed,andexcellentswitchingability[9,10].Onesignificantadvantagepolymermemoryhasoverconventionalmemorydesignsisthatitcanbestackedvertically,yieldingathree-dimensional(3-D)useofspace[11].Thismeansthatinterabytesolid-statedeviceswithextremelylowtransistorcountssuchasdrivesaboutthesizeofamatchbook,thedatapersistsevenafterpowerisremoved.TheNANDFlashmarketiscontinuallygrowingbythesuccessiveintroductionofinnovativedevicesandapplications.Tomeetthemarkettrend,3-DNVMsareexpectedtoreplacetheplanarones,especiallyfor10-nmnodesandbeyond.Moreover,simple-structureorganicbistablememoryexhibitingsuperiormemoryfeatureshasbeenrealizedbyemployingvariousnanoparticles(NPs)blendedintoasingle-layeredorganicmaterialsandwichedbetweentwometalelectrodes[12,13].TheNPsactastrapsthatcanbechargedanddischargedbysuitablevoltagepulses.NPblendsshowpromisingdataretentiontimes,switchingspeed,andcyclingendurance,buttheon-statecurrentistoolowtopermitscalingtonanometerdimensions[10,14].Alotofthesegreatideastendtodiebeforereachingthispointofdevelopment,butthatisnottosaythatwewillbeseeingplasticmemoryonstoreshelvesnextyear.Therearestillmanyhurdlestogetover;softwarealoneisabigtask,asisthemanufacturingprocess,butitdoesbringthistechnologyonestepclosertoreality[15].Itisnotanexaggerationtosaythattheequivalentof400,000CDs,60,000DVDs,or126ÂyearsofMPGmusicmaybestoredonapolymermemorychipthesizeofacreditcard.

Thevisionofthisreview

Inthisreview,wefocusonelectricallyprogrammablenonvolatilememorychangesfromsiliconnanocrystalmemoryscalingtoorganicandmetallicNPmemorydevices.Further,thescalingtrendmovetowardstheemergingNVMtoflexibleandtransparentredox-basedresistiveswitchingmemorytechnologies.Thisreviewisintendedtogivean

overviewtothereaderofstoragesystemsandcomponentsfromconventionalmemorydevicesthathavebeenproposedinthepastyearsofrecentprogressincurrentNVMdevicesbasedonnanostructuredmaterialstoredox-basedresistiverandom-accessmemory(RRAM)to3-Dandtransparentmemorydevices.WedescribethebasicsofFlashmemoryandthenhighlightthepresentproblemswiththeissueofscalingtunneldielectricinthesedevices.Webrieflydescribeahistoricalchange,howtheconventionalFGnonvolatilememorysuffersfromachargelossproblemasthefeaturesizeofthedevicecontinuestoshrink.Adiscretepolysilicon-oxide-nitride-oxide-silicon(SONOS)memoryisthenproposedasareplacementoftheconventionalFGmemory.TheNCmemoryisexpectedtoefficientlypreservethetrappedchargeduetothediscretechargestoragenodewhilealsodemonstratingexcellentfeaturessuchasfastprogram/erasespeeds,lowprogrammingpotentials,andhighendurance.Wealsodiscusscurrentongoingresearchinthisfieldandthesolutionsproposedtosolvethescalingproblemsbydiscussingaspecificsolutionindetailwhichwouldbethecenterpieceinrecentmemoryworkprogress.Moreover,thisreviewmakesdistinctemergingmemoryconceptswithmorerecentmolecularandquantumdotprogrammablenonvolatilememoryconcepts,specificallyusingchargetrappinginconjugatedpolymersandmetalNPs.Weclassifyseveralpossibledevices,accordingtotheiroperatingprinciple,andcriticallyreviewtheroleofπ-conjugatedmaterialsinthedatastoragedeviceoperation.WedescribespecificationsforapplicationsofemergingNVMdevicesaswellasalreadyexistingNANDmemoryandreviewthestateoftheartwithrespecttothesetargetspecificationsinthefuture.Conclusionsaredrawnregardingfurtherworkonmaterialsandupcomingmemorydevicesandarchitectures.

Classificationofsolid-statememorytechnologiesDatastoragedevicescanbeclassifiedbasedonmanyfunctionalcriteria.Ofthem,silicon-basedsemiconductormemoriesarecategorizedintotwo:volatileandnonvolatile[3,16].Involatilememories,theinformationeventuallyfadeswhilepowersupplyisturnedoffunlessthedevicesusedtostoredatawillbeperiodicallyrefreshed.Ontheotherhand,nonvolatilememoriesretainthestoredinformationevenwhenthepowersupplyisturnedoff.Volatilememories,suchasstaticrandom-accessmemory(SRAM)anddynamicrandom-accessmemory(DRAM),needvoltagesupplytoholdtheirinformationwhilenonvolatilememories,namelyFlashmemories,holdtheirinformationwithoutone.DRAM(dynamicstandsfortheperiodicalrefresh)isneededfordataintegrityincontrasttoSRAM.ThebasiccircuitstructuresofDRAM,SRAM,andFlashmemoriesareshowninFigureÂ1.DRAM,SRAM,andFlasharetoday’sdominantsolid-statememorytechnologies,whichhavebeenaroundforalongtime,withFlashtheyoungest,at25Âyears.DRAMisbuiltusingonlyonetransistorandonecapacitorcomponent,andSRAMisusuallybuiltinCMOStechnologywithsixtransistors.Twocross-coupledinvertersareusedtostoretheinformationlikeinaflip-flop.Fortheaccesscontrol,twofurthertransistorsareneeded.Ifthewritelineisenabled,thendatacanbereadandsetwiththebitlines.TheFlashmemorycircuitworkswiththeFGcomponent.TheFGisbetweenthegateandthesource-drainareaandisolatedbyanoxidelayer.IftheFGisuncharged,thenthegatecancontrolthesource-draincurrent.TheFGgetsfilled(tunneleffect)withelectronswhenahighvoltageatthegateissupplied,andthenegativepotentialontheFGworksagainstthegateandnocurrentispossible.TheFGcanbe

erasedwithahighvoltageinreversedirectionofthegate.DRAMhasanadvantageoverSRAMandFlashofonlyneedingoneMOSFETwithacapacitor.ItalsohastheadvantageofcheapproductionaswellaslowerpowerconsumptionascomparedtoSRAMbutslowerthanSRAM.Ontheotherhand,SRAMisusuallybuiltinCMOStechnologywithsixtransistorsandtwocross-coupledinverters,andfortheaccesscontrol,twofurthertransistorsareneeded.SRAMhastheadvantageofbeingquick,easytocontrol,integratedinthechip,aswellasfastbecausenobusisneededlikeinDRAM.ButSRAMhasthedisadvantagesofneedingmanytransistorsandhenceexpensive,higherpowerconsumptionthanDRAM.IncomparisontoDRAMandSRAM,FlashmemoryhasFGbetweenthegateandthesource-drainareaandisolatedwithanoxidelayer.FlashmemorydoesnotrequirepowertostoreinformationbutisslowerthanSRAMandDRAM.

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Figure1.ThecircuitrystructuresofDRAM,SRAM,andFlashmemories.

BothtypesofmemoriescanbefurtherclassifiedbasedonthememorytechnologythattheyuseandbasedondatavolatilityasshownintheclassificationflowchartdepictedinFigureÂ2.VolatilememoriesconsistmostlyofDRAM[17],whichcanbefurtherclassifiedintoSDRAMandmobileRAMwhichonlyretaininformationwhencurrentisconstantlysuppliedtothedevice[18].AnothersmallbutveryimportantmemorydeviceisSRAM.ThemarketforDRAMdevicesfarexceedsthemarketforSRAMdevices,althoughasmallamountofSRAMdevicesisusedinalmostalllogicandmemorychips.However,DRAMusesonlyonetransistorandonecapacitorperbit,allowingittoreachmuchhigherdensitiesand,withmorebitsonamemorychip,bemuchcheaperperbit.SRAMisnotworthwhilefordesktopsystemmemory,whereDRAMdominates,butisusedforitscachememories.SRAMiscommonplaceinsmallembeddedsystems,whichmightonlyneedtensofkilobytesorless.ForthcomingvolatilememorytechnologiesthathopetoreplaceorcompetewithSRAMandDRAMincludeZ-RAM,TTRAM,A-RAM,andETARAM.Intheindustry,newuniversalandstablememorytechnologieswillappearasrealcontenderstodisplaceeitherorbothNANDFlashandDRAM.Flashmemoryispresentlythemostsuitablechoicefornonvolatileapplicationsforthefollowingreasons:Semiconductornonvolatilememoriesconsistmostlyoftheso-called‘Flash’devicesandretaintheirinformationevenwhenthepoweristurnedoff.Othernonvolatilesemiconductormemoriesincludemaskread-onlymemory(MROM),antifuse-basedone-timeprogrammable(OTP)memory,andelectricallyerasableread-onlymemory(EEPROM).Flashisfurtherdividedintotwocategories:NOR,characterizedbyadirectwriteandalargecellsize,andNAND,characterizedbyapagewriteandsmallcellsize.Nonvolatilememoryisacomputermemorythatcanretainthestoredinformationevenwhennotpowered[3,19,20].Nonvolatilesemiconductormemoriesaregenerallyclassifiedaccordingtotheirfunctionalpropertieswithrespecttotheprogramminganderasingoperations,asshownintheflowchartdescribedinFigureÂ2.Thesearefloatinggate,nitride,ROMandfuse,Flash,emerging,andothernewnext-generationmemorytechnologies.Today,thesenonvolatilememoriesarehighlyreliableandcanbeprogrammedusingasimplemicrocomputerandvirtuallyineverymodernelectronic

equipment,whichareexpectedtoreplaceexistingmemories.

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Figure2.Flowchartforthesemiconductormemoryclassificationaccordingtotheirfunctionalcriteria.

Amongthem,emergingnonvolatilememoriesarenowverycaptivating.Thenext-generationmemorymarketwillcoveruptheseemergingmemorytechnologies[21].Therearemainlyfivetypesofnonvolatilememorytechnology:Flashmemory,ferroelectricrandom-accessmemory(FeRAM),magneticrandom-accessmemory(MRAM),phase-changememory(PCM),andRRAM.Nonvolatilememory,specifically‘Flash’memory,whichischaracterizedbyalarge-block(or‘sector’)erasingmechanism,hasbeenthefastestgrowingsegmentofthesemiconductorbusinessforthelast10Âyears.SomeoftheseneweremergingtechnologiesincludeMRAM,FeRAM,PCM,spin-transfertorquerandom-accessmemory(STT-RAM),RRAMandmemristor.MRAMisanonvolatilememory[10,22].UnlikeDRAM,thedataisnotstoredinanelectricchargeflow,butbymagneticstorageelements.Thestorageelementsareformedbytwoferromagneticplates,eachofwhichcanholdamagneticfield,separatedbyathininsulatinglayer.Oneofthetwoplatesisapermanentmagnetsettoaparticularpolarity;theother’sfieldcanbechangedtomatchthatofanexternalfieldtostorememory.STT-RAMisanMRAM(nonvolatile)butwithbetterscalabilityovertraditionalMRAM.TheSTTisaneffectinwhichtheorientationofamagneticlayerinamagnetictunneljunctionorspinvalvecanbemodifiedusingaspin-polarizedcurrent.Spin-transfertorquetechnologyhasthepotentialtomakeMRAMdevicescombininglowcurrentrequirementsandreducedcostpossible;however,theamountofcurrentneededtoreorientthemagnetizationisatpresenttoohighformostcommercialapplications.PCMisanonvolatilerandom-accessmemory,whichisalsocalledovonicunifiedmemory(OUM),basedonreversiblephaseconversionbetweentheamorphousandthecrystallinestateofachalcogenideglass,whichisaccomplishedbyheatingandcoolingoftheglass.Itutilizestheuniquebehaviorofchalcogenide(amaterialthathasbeenusedtomanufactureCDs),wherebytheheatproducedbythepassageofanelectriccurrentswitchesthismaterialbetweentwostates.Thedifferentstateshavedifferentelectricalresistancewhichcanbeusedtostoredata.Theidealmemorydeviceortheso-calledunifiedmemorywouldsatisfysimultaneouslythreerequirements:highspeed,highdensity,andnonvolatility(retention).Atthepresenttime,suchmemoryhasnotbeendeveloped.Thefloatinggatenonvolatilesemiconductormemory(NVSM)hashighdensityandretention,butitsprogram/erasespeedislow.DRAMhashighspeed(approximately10Âns)andhighdensity,butitisvolatile.Ontheotherhand,SRAMhasveryhighspeed(approximately5Âns)butlimitedfromverylowdensityandvolatility.ItisexpectedthatPCMwillhavebetterscalabilitythanotheremergingtechnologies.RRAMisanonvolatilememorythatissimilartoPCM.Thetechnologyconceptisthatadielectric,whichisnormallyinsulating,canbemadetoconductthroughafilamentorconductionpathformedafterapplicationofasufficientlyhighvoltage.Arguably,thisisamemristortechnologyandshouldbeconsideredaspotentiallyastrongcandidatetochallengeNANDFlash.Currently,FRAM,MRAM,andPCMareincommercialproductionbutstill,relativetoDRAMandNANDFlash,remain

limitedtonicheapplications.ThereisaviewthatMRAM,STT-RAM,andRRAMarethemostpromisingemergingtechnologies,buttheyarestillmanyyearsawayfromcompetingforindustryadoption[23].Anynewtechnologymustbeabletodelivermost,ifnotall,ofthefollowingattributesinordertodriveindustryadoptiononamassscale:scalabilityofthetechnology,speedofthedevice,andpowerconsumptiontobebetterthanexistingmemories.TheNVSMisininspiringsearchofnovelnonvolatilememories,whichwillsuccessfullyleadtotherealizationandcommercializationoftheunifiedmemory.

Inprogress,anothernewclassofnonvolatilememorytechnologieswillofferalargeincreaseinflexibilitycomparedtodisks,particularlyintheirabilitytoperformfast,randomaccesses.UnlikeFlashmemory,thesenewtechnologieswillsupportin-placeupdates,avoidingtheextraoverheadofatranslationlayer.Further,thesenewnonvolatilememorydevicesbasedondeoxyribonucleicacid(DNA)biopolymerandorganicandpolymermaterialsareoneofthekeydevicesforthenext-generationmemorytechnologywithlowcost.NonvolatilememorybasedonmetallicNPsembeddedinapolymerhosthasbeensuggestedasoneofthesenewcross-pointmemorystructures.Inthissystem,traplevelssituatedwithinthebandgapofthepolymerareintroducedbytheNPs[24,25].Memorydevicesplayamassiveroleinallemergingtechnologies;assuch,effortstofabricateneworganicmemoriestobeutilizedinflexibleelectronicsareessential.Flexibilityisparticularlyimportantforfutureelectronicapplicationssuchasaffordableandwearableelectronics.Muchresearchhasbeendonetoapplytheflexibleelectronicstechnologytopracticaldeviceareassuchassolarcells,thin-filmtransistors,photodiodes,light-emittingdiodes,anddisplays[26-28].Researchonflexiblememorywasalsoinitiatedforthesefutureelectronicapplications.Inparticular,organic-basedflexiblememorieshavemeritssuchasasimple,low-temperature,andlow-costmanufacturingprocess.Severalfabricationresultsoforganicresistivememorydevicesonflexiblesubstrateshavebeenreported[29,30].Inaddition,withgrowingdemandforhigh-densitydigitalinformationstorage,NANDFlashmemorydensityhasbeenincreaseddramaticallyforthepastcoupleofdecades.Ontheotherhand,devicedimensionscalingtoincreasememorydensityisexpectedtobemoreandmoredifficultinabit-costscalablemannerduetovariousphysicalandelectricallimitations.Asasolutiontotheproblems,NANDFlashmemorieshavingstackedlayersareunderdevelopingextensions[31,32].In3-Dmemories,costcanbereducedbybuildingmultiplestackedcellsinverticaldirectionwithoutdevicesizescaling.Asabreakthroughforthescalinglimitations,various3-Dstackedmemoryarchitecturesareunderdevelopmentandexpectingthehugemarketof3-Dmemoriesinthenearfuture.Withlotsofexpectation,future-generationmemorieshavepotentialtoreplacemostoftheexistingmemorytechnologies.Thenewandemergingmemorytechnologiesarealsonamedtobeauniversalmemory;thismaygiverisetoahugemarketforcomputerapplicationstoalltheconsumerelectronicproducts.

MarketmemorytechnologiesbyapplicationsThesemiconductorindustryhasexperiencedmanychangessinceFlashmemoryfirstappearedintheearly1980s.ThegrowthofconsumerelectronicsmarketurgesthedemandofFlashmemoryandhelpstomakeitaprominentsegmentwithinthesemiconductorindustry.TheFlashmemorieswerecommerciallyintroducedintheearly1990s,andsince

thattime,theyhavebeenabletofollowMoore’slawandthescalingrulesimposedbythemarket.Thereareexpectedmassivechangesinthememorymarketoverthenextcoupleofyears,withmoredensityandreliabletechnologieschallengingthedominantNANDFlashmemorynowusedinSSDsandembeddedinmobileproducts.Server,storage,andapplicationvendorsarenowworkingonnewspecificationstooptimizethewaytheirproductsinteractwithNVM-movesthatcouldleadtothereplacementofDRAMandharddrivesalikeformanyapplications,accordingtoastoragenetworkingindustryassociation(SNIA)technicalworkinggroup[33,34].TheFlashmemorymarketplaceisoneofthemostvibrantandexcitinginthesemiconductorindustry,nottomentiononeofthemostcompetitive.Thecontinuousinventionofnewmemorytechnologiesandtheirapplicationsinthememorymarketalsoincreaseperformancedemands.Thesenewclassesofmemorieswiththelatesttechnologyincreasetheverticaldemandinthefuturememorymarket.Inthenextcomingyears,cumulativepricereductionsshouldbecomedisruptivetoDVDsandharddiskdrives(HDDs),stimulatehugedemand,andcreatenewFlashmarkets.

Thenonvolatilememoriesofferthesystemadifferentopportunityandcoverawiderangeofapplications,fromconsumerandautomotivetocomputerandcommunication.FigureÂ3showsNVSMmemoryconsumptionbyvariousapplicationsintheelectronicsindustrybymarketin2010extendingupwardsfromcomputersandcommunicationtoconsumerproducts[22].Itisnoticedthatthereisafastergrowthrateofthedigitalcellularphonesince1990;thevolumeofproductionhasincreasedby300times,e.g.,from5millionunitsperyeartoabout1.5billionunitsperyear.Nowadays,flexibilityandtransparencyareparticularlyofgreatsignificanceforfutureelectronicapplicationssuchasaffordableandwearableelectronics.Manyadvancedresearchtechnologiesareappliedtoflexibletechnologytobeusedinarealelectronicsarea[35].Althoughsilicon-basedsemiconductormemorieshaveplayedsignificantrolesinmemorystorageapplicationsandcommunicationinconsumerelectronics,now,therecentfocusisturningfromrigidsilicon-basedmemorytechnologyintoasoftnonvolatilememorytechnologyforlow-cost,large-area,andlow-powerflexibleelectronicapplications.Further,thememorymarketforthelongtermiscontinuouslygrowing,evenifwithsomeupsanddowns,andthisisexpectedtocontinueinthecomingyears[36].Sinceinnovationdrivesthesemiconductorindustry,anewtrendwithtransparencyaswellasflexibilityand3-Dtechnologieswillbeattractiveandmovetowardscontinuousgrowthinthenearfuture.

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Figure3.VariousNVSMapplicationsintheelectronicsindustrybymarketsizein2010.Reprintedfromref.[22].

SuccessivecreationofnewmobiledevicesleadstothecontinualgrowthofNANDproductsasshowninFigureÂ4.Tomeetthismarketdemand,earlythisyear,30-nmnodetechnologiesareinramping-upphase,20-nmnodetechnologiesareinthephaseoftransitiontomassproduction,anda10-nmnodetechnologyisunderdevelopment.Inaddition,thefuturemarketrequireshigh-speedoperationevenuptoapproximately1,500ÂMB/sinordertosatisfyalargeamountofdatacorrespondence[37].However,high-speedoperationscausehighpowerconsumptionandchiptemperatureincrease,

whichcandeteriorateNANDreliability.Hence,reductionofoperatingvoltageisinevitabletoachievethefutureNAND.Opportunitiesfortheuseof3-Daswellaspolymermemorydesigninmodernelectroniccircuitsarerapidlyexpanding,basedontheveryhighperformanceanduniquefunctionality.However,theirpracticalimplementationinelectronicapplicationswillultimatelybedecidedbytheabilitytoproducedevicesandcircuitsatacostthatissignificantlybelowthatneededtomanufactureconventionalelectroniccircuitsbasedon,forexample,silicon.Ifsuccessful,theselow-costfabricationprocesseswillultimatelyresultintheprintingoflarge-areaorganicelectroniccircuitsonasheetofplasticpaperusingaroll-to-rollmethod,wherelow-temperaturedepositionoforganicsisfollowedbymetaldepositionandpatterninginacontinuous,high-speedprocessanalogous,perhaps,toprocessesusedintheprintingofdocumentsorfabrics.

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Figure4.GrowthofNANDFlashmarketupto2014(iSuppli)andtheinterfacespeedofvariousNANDapplications.Reproducedfromref.[37].

Inrecentyears,IDTechExfindsthatthetotalmarketforprinted,flexible,andorganicelectronicswillgrowfrom$16.04billionin2013to$76.79billionin2023andthisgrowingtrendisexpectedtocontinueinthecomingyears(seeFigureÂ5a).ThemajorityofthatisOLEDs(onlyorganic,notprinted)andconductiveinkusedforawiderangeofapplications.Ontheotherhand,stretchableelectronics,logicandmemory,andthin-filmsensorsaremuchsmalleringredientsbuthavinghugegrowthpotentialastheyemergefromR&D[38].Thereportspecificallyaddressesthebigpicturethatover3,000organizationsarepursuingprinted,organic,flexibleelectronics,includingprinting,electronics,materials,andpackagingcompanies.Whilesomeofthesetechnologiesareinusenow-indeedtherearemainsectorsofbusinesswhichhavecreatedbillion-dollarmarkets-othersarecommerciallyembryonic.

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Figure5.Marketvolume(a)andglobalflexibledisplaymarketshipmentforecast(b).Reproducedfromrefs.[38,39].

Anotherkeypotentialmarketforprinted/flexibleelectronicsisnext-generationtransparentconductivefilmtoreplacebrittleandexpensiveindiumtinoxide(ITO)intouchscreensanddisplays,lighting,andphotovoltaics.Touchdisplayresearchsaysthatthemarketfornon-ITOtransparentconductorswillbeabout$206millionthisyearandgrowtosome$4billionby2020asshowninFigureÂ5b.‘HighdemandfortouchscreensfornotebookandPCsizedisplayshascreatedashortageofITOtouchsensorssincetheendoflastyeartodrivemoreinterestinthesetechnologies,andthemoreflexibleandpotentiallycheaperreplacementtechnologiesaregettingmoremature,notesJenniferColegrove,presidentandanalyst,whowillspeakattheFlexTechworkshopontransparentconductors.ShenotesthatAtmel,Fujifilm,UnipixelandCambriosareallinsomephaseofproductionâ

€™[39].Alargeamountofthesemiconductormarket(approximately20%)isgivenbythesemiconductormemories;thus,themarketforchipswilldevelopinthenextfewyears.Thisstudyreportsthatthereisananalysisoftheproductionprocessandthesubsequentvaluechain,whichcomprisesabenchmarkanalysisofthemainsegmentsofthesemiconductorindustry.

Recently,the3-DnonvolatilememorystructurehasalsoattractedconsiderableattentionduetoitspotentialtoreplaceconventionalFlashmemoryinnext-generationNVMapplications[37,40].3-Dmemoriesaregatheringincreasingattentionasfutureultra-high-densitymemorytechnologiestokeepatrendofincreasingbitdensityandreducingbitcost.TheNANDFlashmarketiscontinuouslygrowingbythesuccessiveintroductionofinnovativedevicesandapplications.Tomeetthemarkettrend,3-DNVMsareexpectedtoreplacetheplanarone,especiallyfor10-nmnodesandbeyond.Therefore,thefundamentalsandcurrentstatusofthe3-DNANDFlashmemoryarereviewedandfuturedirectionsarediscussed[41].3-DintegrationpromisestobeanexcellentreplacementofcurrenttechnologiesforthedevelopmentofNANDFlashmemory.TimeisrunningoutforplanarNANDtechnology.ItwillnotbelongthatplanarNANDwillbecompletelyreplacedby3-DNAND.3-DNANDpromisestosatisfythegrowingneedofNANDmemory[37].

Finally,NVMtechnologieshaveabrightfuturesinceeveryend-useapplicationneedstostoresomeparametersorsomeamountofanapplicationprogramintheon-boardNVMtoenableittofunction.TheupcomingNVMsarethebighopeforasemiconductormemorymarket,whichprovidesmemoriesforsystemstorunwithflexibility,reliability,highperformance,andlowpowerconsumptioninatinyfootprintinnearlyeveryelectronicapplication.Recentmarkettrendshaveindicatedthatcommercializedornear-commercializedcircuitsareoptimizedacrossspeed,density,powerefficiency,andmanufacturability.Flashmemoryisnotsuitedtoallapplications,havingitsownproblemswithrandom-accesstime,bitalterability,andwritecycles.Withtheincreasingneedtolowerpowerconsumptionwithzero-powerstandbysystems,observersarepredictingthatthetimehascomeforalternativetechnologiestocaptureatleastsomeshareinspecificmarketssuchasautomotivesmartairbags,high-endmobilephones,andRFIDtags.AnembeddednonvolatilememorywithsuperiorperformancetoFlashcouldseewidespreadadoptioninsystem-on-chip(SoC)applicationssuchassmartcardsandmicrocontrollers.

EmergingNVMtechnologiesforapplicationsThenewemergingnonvolatilerandom-accessmemoryproductsaddresstheurgentneedinsomespecificandsmall-formdevices.Therefore,iRAPfeltaneedtodoadetailedtechnologyupdateandmarketanalysisinthisindustry[42].Recently,YoleDéveloppementreportsdescribethatemergingmemorytechnologieshavegreatpotentialtoimprovefuturememorydevicestobeincreasinglyusedinvariousmarketsofindustryandtransportation,enterprisestorage,mobilephones,massstorage,andsmartcards[43].EmergingNVMapplicationsinvariousmarketsareshowninFigureÂ6.Buttherearenumerousopportunitiesexistingfornovelarchitecturesandapplicationsthattheseemergingmemorytechnologiescanenable.ThesenewemergingNVMproductsaddresstheurgentneedinsomespecificandsmall-formdevices.Therefore,emergingnonvolatilememoryproductsprovidemarketdataaboutthesizeofgrowthofthe

applicationsegmentsandthedevelopmentsofbusinessopportunities.Untilnow,onlyFeRAM,PCM,andMRAMwereindustriallyproducedandavailableinlow-densitychipstoonlyafewplayers.Thus,themarketwasquitelimitedandconsiderablysmallerthanthevolatileDRAM-andnonvolatileFlashNAND-dominantmarkets(whichenjoyedcombinedrevenuesof$50+billionin2012).However,inthenext5Âyears,thescalabilityandchipdensityofthosememorieswillbegreatlyimprovedandwillsparkmanynewapplicationswithNVMmarketdriversexplainedinmoredetail.

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Figure6.EmergingNVMapplicationsinvariousmarkets.

AccompaniedbytheadoptionofSTT-MRAMandPCMcachememory,enterprisestoragewillbethelargestemergingNVMmarket.NVMwillgreatlyimprovetheinput/outputperformanceofenterprisestoragesystemswhoserequirementswillintensifywiththegrowingneedforweb-baseddatasupportedbyfloatingmassservers.Inaddition,mobilephoneswillincreasetheiradoptionofPCMasasubstitutetoFlashNORmemoryinMCPpackagesto1-gigabyte(GB)chipsmadeavailablebyMicronin2012.Higher-densitychips,expectedin2015,willallowaccesstosmartphoneapplicationsthatarequicklyreplacingentry-levelphones.STT-MRAMisexpectedtoreplaceSRAMinSoCapplications,thankstolowerpowerconsumptionandbetterscalability.Smartcardsandmicrocontrollers(MCU)willlikelyadoptMRAM/STT-MRAMandPCMasasubstitutetoembedFlash.Indeed,Flashmemorycellsizereductionislimitedinthefuture.TheNVMcouldreducethecellsizeby50%andthusbemorecost-competitive.Additionalfeatureslikeincreasedsecurity,lowerpowerconsumption,andhigherendurancearealsoappealingNVMattributes.ThemassstoragemarketsservedbyFlashNANDcouldbeginusing3-DRRAMin2017to2018,when3-DNANDwillslowdownitsscalabilityaspredictedbyallofthemainmemoryplayers.Ifthishappens,thenamassiveRRAMramp-upwillcommenceinthenextdecadethatwillreplaceNAND;conditional3-DRRAMcost-competitivenessandchipdensityareavailable.ItisexpectedsurelythattheemergingNVMbusinesswillbeverydynamicoverthenext5Âyears,thankstoimprovementsinscalability/costanddensityofemergingNVMchips[44].AccordingtoarecentlypublishedreportfromYoleDéveloppement,EmergingNon-volatileMemoryTechnologies,IndustryTrendsandMarketAnalysis,theglobalmarketforemergingnonvolatilerandom-accessmemoryproductswasprojectedtohavereached$200millionin2012.Thismarketisexpectedtoincreaseto$2,500millionby2018atanaverageannualgrowthataCAGRof+46%throughtheforecastperiodwithmobilephones,smartcards,andenterprisestorageasmaingrowthdrivers(FigureÂ7).Marketadoptionofmemoryisstronglydependentonitsscalability.ThisYoleDéveloppementreportprovidesaprecisememoryroadmapintermsoftechnologicalnodes,cellsize,andchipdensityforeachemergingNVMsuchasFeRAM,MRAM/STT-MRAM,PCM,andRRAM.Amarketforecastisprovidedforeachtechnologybyapplication,units,revenues,andalsomarketgrowthasgivenadetailedaccountofemergingNVMmarketforecast(FigureÂ7).PCMdevices,thedensestNVMin2012at1ÂGB,willreach8ÂGBby2018,whichareexpectedtoreplaceNORFlashmemoryinmobilephonesandwillalso

beusedasastorageclassmemoryinenterprisestorage.MRAM/STT-MRAMchipswillreach8to16ÂGBin2018.TheywillbewidelysoldasastorageclassmemoryandpossiblyasaDRAMsuccessorinenterprisestorageafter2018.By2018,MRAM/STT-MRAMandPCMwillsurelybethetoptwoNVMonthemarket.Combined,theywillrepresenta$1.6billionbusinessby2018,andtheirsaleswillalmostdoubleeachyear,withdouble-densitychipslaunchedevery2Âyears.FeRAMwillbemorestableintermsofscalability,with8-to16-MBchipsavailableby2018;thedevelopmentofanewFRAMmaterialcouldraisescalability,butwedonotexpectittobewidelyindustrializedandcommercializedbefore2018.FeRAMwillgrowatasteadygrowthrate(10%peryear)andwillfocusonindustrialandtransportationapplicationsbecauseofthelow-densityavailability,whereasRRAMrevenueswouldnotreallysurgeby2018,withtheavailabilityofhigh-densitychipsofseveraltensofgigabytesthatcouldreplaceNANDtechnology.Meanwhile,ithasalsobeenconsideredbymemorytechnologistexpertsthatforlarge-volumemarketslikemassstorageNAND,onlyonetechnologywillbeadoptedinordertoreduceproductioncostandRRAMseemstobethebestcandidate.ButtherealmassiveadoptionofemergingNVMasareplacementforNANDandDRAMwillhappenafter2020.

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Figure7.EmergingNVMmarketforecastbyapplicationsfrom2012to2018(inM$).Reproducedfromref.[43].

AdvancesinFlashmemorytechnologiesFlashmemoryisbasicallyaMOSFETnonvolatiledevicethatcanbeelectricallyerasedandreprogrammed[3,45].ItisatechnologythatisprimarilyusedinmemorycardsandFlashdrivesforgeneralstorageandtransferofdatabetweencomputersandotherdigitalproducts.Sincetheinventionofthetransistor,NVSMhadbeenthemostimportantinventionintheelectrondevicefield.Thefloatinggatememorywasusedtostoretheinformationandatunnelingcurrentforprogramminganderasingoperations.Thechargeisinjectedintoorremovedfromthefloatinggateandthefloatinggateremainsinthatstate,evenafterpowerisremoved,whichmeansthatFlashmemoryisnonvolatile.TheinventionofNVSMfurthergaverisetoanewclassofmemorydevicesandhencebroadeneditsapplicationstobecomeubiquitous.TherearealargenumberofproductsinthemarketnowwhichuseFlashdevicesexclusivelyassecondarystorage.Fewexamplesoftheirapplicationsincludemedicaldiagnosticsystems,notebookcomputers,digitalaudioplayers,digitalcameras,mobilephones,personaldigitalassistants,digitaltelevisions,universalserialbus(USB)Flashpersonaldisks,GlobalPositioningSystems,andmanymore.Semiconductorstoragedevicesstoredataintinymemorycellsmadeofverysmalltransistorsandcapacitorsmadeofsemiconductormaterialssuchassilicon.Eachcellcanhold1bitofinformationandanarrayofcellsstoresalargechunkofinformation.Flashdevicesaregainingpopularityoverconventionalsecondarystoragedeviceslikeharddisks.TheFlashmemoryfabricationprocessiscompatiblewiththecurrentCMOSprocessandisasuitablesolutionforembeddedmemoryapplications.A

FlashmemorycellissimplyaMOSFETcell,exceptthatapolysiliconfloatinggate[46](orasiliconnitridechargetraplayer)issandwichedbetweenatunneloxideandaninter-polyoxidetoformachargestoragelayer[47].AlthoughFlashmemoryislikelythestandardchargestoragedeviceforthenextgeneration,scalingmayeventuallybelimitedbythetunneloxidelimit[8].Intermsoftheoperationspeedofprogramanderase,Flashmemoryrequiresathintunneloxidetoenhancethecarriertransportbetweenthefloatinggateandthesiliconsubstrate.However,theverythintunneloxidesuffersfrommanyreliabilityissueslikereductioninoperationvoltage,andafteraconsiderablenumberofprogramanderasecycles,thetunneloxideundergoesdeteriorationloss[48].Thus,researchershavefocusedonpossiblesolutionsandproposedalternatetechnologies,includingnitride-basedmemory,nanocrystalmemory,andswitchingmemory.AllothernonvolatilememoriesrequireintegrationofnewmaterialsthatarenotascompatibleastheconventionalCMOSprocess.

NORandNANDFlashmemorytechnologies

NORandNANDFlash,twomajorFlashtypes,aredominantinthememorymarket.NORFlashhaslowerdensitybutarandom-accessinterface,whileNANDFlashhashigherdensityandinterfaceaccessthroughacommandsequence[49].TheircorrespondingstructuresareshowninFigureÂ8.NORandNANDFlashcomefromthestructureusedfortheinterconnectionsbetweenmemorycells.Intelisthefirstcompanytointroduceacommercial(NORtype)Flashchipin1988,andToshibareleasedtheworld’sfirstNANDFlashin1989[50].Dependingonhowthecellsareorganizedinthematrix,itispossibletodistinguishbetweenNANDFlashmemoriesandNORFlashmemories.InNORFlash,cellsareconnectedinparalleltothebitlines,whichnotablyallowthecellstobereadandprogrammedindividually.TheparallelconnectionofNORFlashcellsresembletheparallelconnectionoftransistorsinaCMOSNORgatearchitecture.Ontheotherhand,inNANDFlash,thecellsareconnectedinseries,resemblingaNANDgate.Theseriesconnectionsconsumelessspacethantheparallelones,reducingthecostofNANDFlash.Itdoesnot,byitself,preventNANDcellsfrombeingreadandprogrammedindividually.Mostoftheengineersandscientistsarenotsofamiliarwiththedifferencesbetweenthesetwotechnologies.Generally,theyusuallyrefertotheNORarchitectureas‘Flash’andareunawareoftheNANDFlashtechnologyanditsmanybenefitsoverNOR[51].ThiscouldbeduetothefactthatmostFlashdevicesareusedtostoreandruncodes(usuallysmall),forwhichNORFlashisthedefaultchoice,althoughweareprovidingsomemajordifferencesbetweenNORandNANDFlashtechnologiesbytheirarchitectureandtheinternalcharacteristicfeaturesoftheindividualFlash.

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Figure8.ComparisonofNORFlasharrayandNANDFlasharrayarchitectures.

NORFlashisslowerineraseoperationandwriteoperationcomparedtoNANDFlash[52].ThismeansthatNANDFlashhasfastereraseandwritetimes.Moreover,NANDFlashhassmallereraseunits,sofewererasesareneeded.NORFlashcanreaddataslightlyfasterthanNANDFlash.NORFlashofferscompleteaddressanddatabusesto

randomlyaccessanyofitsmemorylocations(addressabletoeverybyte).ThismakesitasuitablereplacementforolderROMBIOS/firmwarechips,whichrarelyneedstobeupdated.Itsenduranceis10,000to1,000,000erasecycles.NORFlashishighlysuitableforstoringcodesinembeddedsystems.Mostoftoday’smicrocontrollerscomewithbuilt-inFlashmemory[53].

NANDFlashoccupiesasmallerchipareapercell.ThismakesNANDFlashavailableingreaterstoragedensitiesandatlowercostsperbitthanNORFlash.ItalsohasuptotentimestheenduranceofNORFlash.NANDismorefitasstoragemediaforlargefilesincludingvideoandaudio.USBthumbdrives,SDcards,andMMCcardsareofNANDtype[54].NAND’sadvantagesarefastwrite(program)anderaseoperations,whileNOR’sadvantagesarerandomaccessandbytewritecapability.NOR’srandomaccessabilityallowsforexecuteinplace(XiP)capability,whichisoftenarequirementinembeddedapplications.NANDisslowrandomaccessible,whileNORishamperedbyhavingslowwriteanderaseperformance.NANDisbettersuitedforfilingapplications.However,moreprocessorsincludeadirectNANDinterfaceandcanbootdirectlyfromNAND(withoutNOR).However,NANDcannotperformreadandwriteoperationssimultaneously;itcanaccomplishtheseatasystemlevelusingamethodcalledshadowing,whichhasbeenusedonPCsforyearsbyloadingtheBIOSfromtheslowerROMintothehigh-speedRAM.

TableÂ1highlightsthemajordifferencesbetweenNORandNAND.ItshowsthatNANDisidealforhigh-capacitydatastoragewhileNORisbestusedforcodestorageandexecution,usuallyinsmallcapacities.Therearemanyotherdifferencesbetweenthesetwotechnologieswhichwillbefurtherdiscussedindividually.However,thoselistedinthetableareenoughtostronglydifferentiatethetypesofapplicationsusingthem:NORistypicallyusedforcodestorageandexecution.This,mainlyincapacitiesupto4ÂMB,iscommoninapplicationssuchassimpleconsumerappliances,low-endcellphones,andembeddedapplications,whilerawNANDisusedfordatastorageinapplicationssuchasMP3players,digitalcameras,andmemorycards[55-57].ThecodesforrawNAND-basedapplicationsarestoredinNORdevices.

Seefulltable

Table1.ComparisonbetweenNORandNANDFlashmemories[55-57]

ScalingandchallengesofFlashmemorytechnologies

Currently,therehavebeenincreasingdemandsonreducingthefeaturesizeinmicroelectronicproductsandmoreinterestinthedevelopmentofFlashmemorydevicestomeetthegrowingworldwidedemand.AconventionalFGmemorydevicemusthaveatunneloxidelayerthicknessof8Ânmtopreventchargelossandtomake10Âyears’dataretentioncertain.ThisnecessitywilllimitscalabilityforFlashmemorydevices[8,58].Thus,inordertomeettechnologyscalinginthefieldofmemoryanddatastoragedevices,

mainstreamtransistor-basedFlashtechnologieswillbedevelopedgraduallytoincorporatematerialandstructuralinnovations[59].Dielectricscalinginnonvolatilememorieshasbeenreachedneartothepointwherenewapproacheswillberequiredtomeetthescalingrequirementswhilesimultaneouslymeetingthereliabilityandperformancerequirementsforfutureproducts.High-dielectric-constantmaterialsarebeingexploredaspossiblecandidatestoreplaceboththetraditionalSiO2andoxide/nitride/oxide(ONO)filmsusedinFlashmemorycells.FlashcellscalinghasbeendemonstratedtobereallypossibleandtobeabletofollowMoore’slawdowntothe90-nmtechnologygenerations.Thetechnologydevelopmentandtheconsolidatedknow-howareexpectedtosustainthescalingtrenddowntothe50-nmtechnologynodeandbelowasforecastedbytheInternationalTechnologyRoadmapforSemiconductors(ITRS)inFigureÂ9,whichindicatesthatthesiliconMOSFETwasalreadyinthenanoscale.TheminimumfeaturesizeofanindividualCMOSFEThasshrunkto15Ânmwithanequivalentgateoxidethickness(EOT)of0.8Ânmin2001[13].However,semiconductorFlashmemoryscalingisfarbehindCMOSlogicdevicescaling.Forexample,theEOTofthegatestackinsemiconductorFlashmemoryisstillmorethan10Ânm.Moreover,semiconductorFlashmemorystillrequiresoperationvoltagesofmorethan10ÂV,whichisstillfarfromtheoperationvoltageofCMOSlogicdevices.ItisimportanttoscaletheEOTofthegatestacktoachieveasmallmemorycellsizeandalsoprolongbatterylife.

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Figure9.ThetrendofMOSFETscalingfromITRS.ReproducedfromITRSCorp.

AnotherlimitationofFGtechnologyisthattunneloxidescalingislimitedbystress-inducedleakagecurrent(SILC)relatedtochargetransferproblemasindicatedinFigureÂ10[60,61].TheSILKincreaseswithdecreasingoxidethickness.Thiscanbeattributedtotunnelingassistedbythetrapsinthebulkofthedielectric.Trap-assistedtunnelingcantakeplaceatverylowelectricfields.Ifthedensityoftrapsisincreased,theleakagewillalsoincrease.Electricalstresscanincreasethenumberofthesetraps.Soitbecomesanimportantlimitationofscalingdownthememorydevice[62].ForEOT < 8Ânm,asingleoxidetrapwillcausetocompletethechargelossintheFGFlashcell.Thescalingofthegatestacksandoperationvoltagesareoftenrelatedtoeachother.Atunneloxidethicknessofmorethan8ÂnmiscurrentlyusedinthecommercialFlashmemorychiptomeetthe10Âyears’dataretentiontimerequirement.Ifthetunneloxideweretobescaledbelow2Ânm,theoperationvoltagecouldbereducedfrommorethan10ÂVtobelow4ÂV[63].Unfortunately,theretentiontimewouldalsobereduced,from10Âyearstoseveralseconds.Thisphysicaldamagetothetunneloxideduringthecyclingprocesscausesdataretentionproblems,programdisturbance,readdisturbance,anderraticcharacteristicbehavioroftheFGmemorycell.Suchproblemsseverelylimitthereliabilityandmultilevelcelloperation.Thisbasiclimitationofthetunneloxidethicknessbecomesincreasinglyimportantwithscaling.Newstoragenodeconceptsarealsobecomingattractiveasanalternativeapproachtoaddresssomeofthedielectricscalinglimitations.Flashmemoryadoptsachargestoredinasiliconnitrideasthetrappinglayer,whichexhibitssignificantlyreduceddefect-relatedleakagecurrentandverylowSILCas

comparedtoSiO2withasimilarEOT[64].Sucharelentlessreductionofdevicedimensionshasmanychallengeslikeretention,endurance,reductioninthenumberofelectronsintheFG,dielectricleakage,cell-to-cellcrosstalk,thresholdvoltageshift,andreductioninmemorywindowmargins[65,66].ThekeyconceptofrealscalingissuessuchasmaterialandstructuralchangesinFlashmemorytechnologiesisprovidedindetailinthenextdistinctpart.

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Figure10.SchematicplotsofaFlashmemorycellandthedegradationofitstunneloxide.ThedegradationleadstotheformationofpercolationpathsresponsiblefortheFGchargeloss,hencethelossofthestoredinformation.Thepresenceoftrapsintheenergybarrieryieldsthetrap-assistedtunneling

mechanismandoriginatesthestress-inducedleakagecurrent(SILC).

FGFlashmemorytechnology

TheFGNVmemoryisabasicbuildingblockofFlashmemory,whichisbasedonFGthin-filmstorage(TFS)memoriesthathavebeendevelopedwiththeadditionofanerasegateconfiguration.TheconventionalFGmemory(FigureÂ11a)consistsofaMOSFETconfigurationthatismodifiedtoincludepolysiliconasachargestoragelayersurroundedbyaninsulatedinnergate(floatinggate)andanexternalgate(controlgate).ThiswhatmakesFlashmemorynonvolatileandallfloatinggatememoriestohavethesamegenericcellstructure.Chargeistransferredtoorfromthefloatinggatethroughathin(8to10Ânm)oxide[1,67].Becausethefloatinggateiselectricallyisolatedbytheoxidelayer,anyelectronsplacedonitaretrappedthere.Flashmemoryworksbyadding(charging)orremoving(discharging)electronstoandfromafloatinggate.Abit’s0or1statedependsuponwhetherornotthefloatinggateischargedordischarged.Whenelectronsarepresentonthefloatinggate,currentcannotflowthroughthetransistorandthebitstateis‘0’.Thisisthenormalstateforafloatinggate.Whenelectronsareremovedfromthefloatinggate,currentisallowedtoflowandthebitstateis‘1’.TheFGmemoryhasachievedhighdensity,goodprogram/erasespeed,goodreliability,andlowoperatingvoltageandpromotesenduranceforFlashmemoryapplication.

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Figure11.SchematicsoftheconventionalFGmemoryandSONOS.Schematicsof(a)floatinggateandthin-filmstorage-basedembeddednonvolatilememorybitcells,dependingonthechargestoredinsidethegatedielectricofaMOSFET,and(b)thenitride

traps(SONOS),embeddedintothegateoxideofaMOSFET.

SONOSmemorytechnology

InordertosolvethescalingissueoftheFGmemory,theSONOSmemoryhasbeenproposedasaFlashtechnologysincethe1980s[68,69].TheacronymSONOSisderivedfromthestructureofthedeviceasshowninFigureÂ11b.TheSONOSdeviceisbasicallyaMOSFET,wherethegatehasbeenreplacedbyanONOdielectric.TheSONOSmemoryhasabetterchargeretentionthantheFGmemorywhentheFGbitcell’stunnelingoxidelayerisbelow10Ânm[70].Moreover,theSONOSmemoryexhibitsmanyadvantages,e.g.,easytofabricate,highprogram/erase(P/E)speed,lowprogrammingvoltageand

powerconsumption,andbetterpotentialforscalabilitybelowthe70-nmnode,accordingtotheITRS[71].Thecharge,holesorelectrons,areinjectedintothenitridelayerusingdirecttunnelingthroughthetunneloxidelayer.Thenitridelayeriselectricallyisolatedfromthesurroundingtransistor,althoughchargesstoredonthenitridedirectlyaffecttheconductivityoftheunderlyingtransistorchannel.SincetheSONOSmemorypossessesspatiallyisolateddeep-leveltraps,asingledefectinthetunnelingoxidewillnotcausedischargeofthememorycell.ThethicknessofthetopoxideisimportanttopreventtheFowler-Nordheimtunnelingofelectronsfromthegateduringerase.Whenthepolysiliconcontrolgateisbiasedpositively,electronsfromthetransistorsourceanddrainregionstunnelthroughtheoxidelayerandgettrappedinthesiliconnitride.Thisresultsinanenergybarrierbetweenthedrainandthesource,raisingthethresholdvoltageVth(thegate-sourcevoltagenecessaryforcurrenttoflowthroughthetransistor).Moreover,thenitridelayeriselectricallyisolatedfromthesurroundingtransistor,althoughchargesstoredonthenitridedirectlyaffecttheconductivityoftheunderlyingtransistorchannel.Theoxide/nitridesandwichtypicallyconsistsofa2-nm-thickoxidelowerlayer,a5-nm-thicksiliconnitridemiddlelayer,anda5-to10-nm-thickoxideupperlayer[72,73].However,SONOS-typeFlashmemorieshaveseveraldrawbackssuchasshallowtrapenergylevel,erasesaturation,andverticalstoredchargemigration[74].Theprogrammingspeedandoperatingvoltageproblemscanbesolvedbyreducingthetunneloxidethickness.Atlowtunneloxidethickness,theissuesthatimpactSONOS-typememoriesincludeerasesaturationandverticalchargemigration,whichseriouslydegradetheretentioncapabilityofthememory[75].Thus,manyconcernsstillremainfortheSONOStypeofmemories,whichwillbediscussedinthenextsection.

LimitationsofFGandSONOSmemorytechnologies

Scalingdemandsverythingateinsulatorsinordertokeepshortchanneleffectsandcontroltheshrinkageofthedevicesizeandmaximizetheperformance.Whenthetunnelingoxidethicknessisbelow10Ânm,thestoragedchargeintheFGiseasytoleakduetoadefectinthetunnelingoxideformedbyrepeatedwrite/erasecyclesordirecttunnelingcurrent.

ThetunnelinggateoxidethicknessinaconventionalFlashmemorycannotbescaleddowntosub-7Ânmbecauseofchargeretention[76].TheSONOSFlashmemorycanrelievetheproblembutstillhasarelativelythickgatedielectricthicknessofabout7Ânm.Therefore,conventionalSONOSFlashmemoryalsohasascaling-downproblem.ManystudieshaveshownthatthechargeretentioncharacteristicsinscaledSONOSnonvolatilememorydeviceswithalowgateoxidethicknessandathightemperatureareproblematicwithshallow-leveltraps[48,77,78].FortheconventionalSONOSmemory,erasesaturationandverticalstoredchargemigration[79,80]arethetwomajordrawbacks;themostchallengingtasksarehowtomaintainanacceptablechargecapabilityofthediscretestoragenodesandhowtofabricatenanocrystalswithconstantsize,highdensity,anduniformdistributions[81].Whenthetrapenergylevelisshallow,erasesaturationandverticalmigrationoccurandtheelectronchargedecayrateincreasesduetolowtunneloxidethickness,issuesthatimpactSONOS-typememoriesasshowninFigureÂ12.ThiserasesaturationmakesSONOSeraselessastheerasevoltageorthetunneloxidethicknessisincreased.SincetheSONOSmemoryusessiliconnitrideasachargetrappinglayer,theelectronsintheSisub-conductionbandwilltunnelthroughthetunnelingoxide

andaportionofthenitride,andthisconsequentlydegradestheprogramspeed.Besidesthis,theconductionbandoffsetofnitrideisonly1.05ÂeVandback-tunnelingofthetrappedelectronmayalsooccur.Althoughapplyingaveryhighelectricfieldmayacceleratethede-trappingrate,thegateelectroninjectioncurrentexceedsthede-trappingbutresultinginpracticallyanincreaseinchargeandnoerasing.Usinganultra-thin(<2Ânm)tunneloxideoffersanefficientchargedirecttunnelingeraseandopensamemorywindow.However,thedirecttunnelingcannotbeturnedoffatalowelectricfield,leadingtopoorretentionandreaddisturb.Thus,theSONOSmemorycannotbeusedforNANDFlashwithoutfurtherinnovationofnewmemorytechnologies.ThemainreasonforthegrowthofemergingNVMtechnologiesisthatscalinghasnowbecomeaseriousissueforthememoryindustry.Notonlyaremanyofthesenewtechnologiesinherentlymorescalable,butalsotheyseemwellsuitedtothenextgenerationofmobilecomputingandcommunicationsthatwilldemandhigh-capacitymemoriescapableofstoringandrapidlyaccessingvideoandalargedatabasewithoutoverburdeningbatterypowersources.

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Figure12.Fowler-Nordheim(FN)tunnelingofelectronsfromthegateduringeraseanderasesaturationinSONOSnonvolatilememory.Thisindicatesthereducedmemorywindowastheerasevoltageisincreased.Reproducedfromref.[74].

Manyalternatedevicestructuresareproposedtohopefullycircumventthesescalingchallengesandtoimprovethedeviceperformance.InanefforttocontinueMoore’slawandovercometheultimatelimitationsofMOS-basedmemorydevices,otherstorageconceptshavebeenproposedinsearchofthe‘unifiedmemory’.Theidealmemorydeviceortheso-called‘unifiedmemory’wouldsatisfysimultaneouslythreerequirements:highspeed,highdensity,andnonvolatility.Atthepresenttime,suchanidealmemoryhasnotbeendeveloped.FGNVSMhashighdensityandnonvolatility,butitsP/Espeedislow.DRAMhashighspeed(approximately10Âns)andrelativelyhighdensity,butitisvolatile.SRAMhasveryhighspeed(approximately5Âns),butitsuffersfromverylowdensityandvolatility.Manynonvolatilememorydeviceshavebeenproposedonthebasisofchangingchargestoragematerialsandnewdeviceconceptsforthe‘unifiedmemory’.Thesestructureswillbeconsideredinthenextsections.Inlightofsuchissues,emergingmemorysolutionsseemtobeakeytechnology.

CurrentemergingmemorytechnologiesRecentstudieshaverevealedthatthereisaclosecorrelationamongexistingandemergingmemorytechnologiesinviewofscalability.Thescalingtrendofmemorytransitionleadstosmallerandsmallermemorydevices,whichhavebeenroutinelyobserved.Tofurthersupportthisassertion,anothersetofcurrentprogressinmemorytechnologyisdescribedtotheincreasingimportanceofmemorytousers’experienceandtheimportanceofmemorytosystemperformance.Therearemanyemergingmemorytechnologieswhicharetryingtoreplaceexistingmemorytechnologiesinthemarket.ThesenewmemorydevicessuchasRRAM,PCM,andSTT-RAMhaveread/write/retention/endurancecharacteristicsdifferentfromthoseofconventionalSRAM,DRAM,andFlash[82].But

theidealcharacteristicsofnewemergingmemorytechnologieshavetobemeetingtheperformanceofSRAMandthedensityofNANDFlashintermsofstability,scalability,andswitchingspeed.Thus,goingbeyondthetraditionalbistablememory,thepossibilitiesofmultilevel,high-performancememorydevicessuitableformarketmustbeexplored.Currently,thereareseveraltechnologiesthatshowsomepromise;someofthesenewemergingtechnologiesareMRAM,FeRAM,PCM,STT-RAM,nano-random-accessmemory(NRAM),racetrackmemory,RRAMandmemristor,molecularmemory,andmanyothers[10,83].Eachofthesememorytechnologieswillbebrieflyoutlinedanddiscussedinthefollowingsections.Inviewofthecommercialproduction,currently,MRAM,FeRAM,andPCMareincommercialproductionbutstillremainlimitedtonicheapplicationsrelativetoDRAMandNANDFlash.Thereisaprospectthatamongtheemergingmemorytechnologies,MRAM,STT-RAM,andRRAMarethemostpromisingones,buttheyarestillmanyyearsawayfromcompetingforindustryadoption[84].Itisnecessaryforanynewtechnologytobeabletodelivermostforindustryadoption.Forindustryadoptiononamassscale,someparametersmustbematchedwithexistingmemorytechnologies.Inconsiderationofnewtechnologyforindustryapplication,thescalabilityofthetechnology,speedofthedevice,powerconsumptiontobebetterthanexistingmemories,endurance,densities,betterthanexistingtechnologiesandmostimportantlythecost;iftheemergingtechnologycanonlyrunoneortwooftheseattributes,then,atmostdesirable,itislikelytoberesignedtonicheapplications.

MRAM

MRAMormagneticRAMisanonvolatileRAMtechnologyunderdevelopmentsincethe1990s.RRAMmethodsofstoringdatabitsusemagneticchargesinsteadoftheelectricalchargesusedbyDRAMandSRAMtechnologies.MRAM,firstdevelopedbyIBMinthe1970s[85],isexpectedtoreplaceDRAMasthememorystandardinelectronics.MRAMisbasicallybasedonmemorycellshavingtwomagneticstorageelements,onewithafixedmagneticpolarityandanotherwithaswitchablepolarity.ThesemagneticelementsarepositionedontopofeachotherbutseparatedbyathininsulatingtunnelbarrierasshowninthecellstructureinFigureÂ13.Moreover,scientistsdefineametalasmagnetoresistiveifitshowsaslightchangeinelectricalresistancewhenplacedinamagneticfield.BycombiningthehighspeedofstaticRAMandthehighdensityofDRAM,proponentssaythatMRAMcouldbeusedtosignificantlyimproveelectronicproductsbystoringgreateramountsofdata,enablingittobeaccessedfasterwhileconsuminglessbatterypowerthanexistingelectronicmemories.Technically,itworkswiththestateofthecell,whichissensedbymeasuringtheelectricalresistancewhilepassingacurrentthroughthecell.Becauseofthemagnetictunneleffect[86],ifbothmagneticmomentsareparalleltoeachother,thentheelectronswillbeabletotunnelandthecellisinthelowresistance‘ON’state.However,ifthemagneticmomentsareantiparallel,thecellresistancewillbehigh.ThememorycharacteristicsofMRAMofwritinganderasingarefulfilledbypassingacurrentthroughthewritelinetoinduceamagneticfieldacrossthecell.MRAMhasbeenslowlygettingoffthegroundbuthasnowenteredthemarketandwillbecomeincreasinglyavailableformassproductioninthecoupleofyearsandbeyond.Currently,ithasreachedsomelevelofcommercialsuccessinnicheapplications[87].VariouscompaniessuchasSamsung,IBM,HitachiandToshiba,andTSMCareactivelydevelopingvarianttechnologiesofMRAMchips.Inviewof

powerconsumptionandspeed,MRAMcompetesfavorablythanotherexistingmemoriessuchasDRAMandFlash,withanaccesstimeofafewnanoseconds[88-90].Althoughithassomelimitationduringthe‘write’operation,thesmallercellsizecouldbelimitedbythespreadofthemagneticfieldintoneighboringcellsandneedanamendmenttocompetecompletelyasauniversalmemory.ThepriceofMRAMisalsoanotherissueandconsideredalimitingfactor,withpricesfarinexcessofallthecurrentlyestablishedmemoriesatapproximately£2to£3($3to$5)permegabyte[91].Accordingtothispricelevel,MRAMisinexcessof1,000timesthepriceofFlashmemoryandover10,000timesthepriceofharddiskdrives.Itisexpectedthatofthenext-generationmemorytechnologies,MRAM,inthefuture,willhavethebiggestmarket,followedbyFeRAM,PCRAM,andmemristors.

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Figure13.BasicMRAMcellstructure.

STT-MRAM

STT-MRAMisamagneticmemorytechnologythatexertsthebaseplatformestablishedbyanexistingmemorycalledMRAMtoenableascalablenonvolatilememorysolutionforadvancedprocessnodes[92,93].ItisanewkindofmagneticRAMwiththefollowingfeatures:fastreadandwritetimes,smallcellsizes,potentiallyevensmaller,andcompatibilitywithexistingDRAMandSRAM.Aswehavediscussedintheprevioussection,MRAMstoresdataaccordingtothemagnetizationdirectionofeachbitandthenanoscopicmagneticfieldssetthebitsinconventionalMRAM.Ontheotherhand,STT-MRAMusesspin-polarizedcurrents,enablingsmallerandlessenergy-consumingbits.ThebasiccellstructureofSTT-RAMisdepictedinFigureÂ14.Inaddition,STT-RAMwritingisatechnologyinwhichanelectriccurrentispolarizedbyaligningthespindirectionoftheelectronsflowingthroughamagnetictunneljunction(MTJ)element.Datawritingisperformedbyusingthespin-polarizedcurrenttochangethemagneticorientationoftheinformationstoragelayerintheMTJelement[94].TheresultantresistancedifferenceoftheMTJelementisusedforinformationreadout.STT-RAMisamoreappropriatetechnologyforfutureMRAMproducedusingultra-fineprocessesandcanbeefficientlyembeddedinsubsequentgenerationsofsuchsemiconductordevicesasFPGAs,microprocessors,microcontrollers,andSoC.AspecialbonusforembeddeddesignersisthefactthattheinternalvoltageSTT-RAMrequiresisonly1.2ÂV.ThedifferencebetweenSTT-MRAMandaconventionalMRAMisonlyinthewritingoperationmechanism;thereadsystemisthesame.ThememorycellofSTT-MRAMiscomposedofatransistor,anMTJ,awordline(WL),abitline(BL),andasourceline(SL)[95].Currently,STT-RAMisbeingdevelopedincompaniesincludingEverspin,Grandis,Hynix,IBM,Samsung,TDK,andToshiba.However,forSTT-RAMtobeadoptedasauniversalmainstreamsemiconductormemory,somekeychallengesshouldberesolved:thesimultaneousachievementoflowswitchingcurrentandhighthermalstability.Itmustbedense(approximately10ÂF2),fast(below10Ânsofreadandwritespeeds),andoperatingatlowpower[96].

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Figure14.BasicSTT-RAMcellstructure.

FeRAM

FeRAMisanonvolatileRAMthatcombinesthefastreadandwriteaccessofDRAMcells,consistingofacapacitorandtransistorstructureasshowninFigureÂ15.Thecellisthenaccessedviathetransistor,whichenablestheferroelectricstateofthecapacitordielectrictobesensed.Inspiteofitsname,FeRAMdoesnotcontainiron.Thepolarizationpropertiesofaferroelectricsubstanceareusedasamemorydevice.Today’sFeRAMusesleadzirconatetitanate(PZT);othermaterialsarebeingconsidered.ThemaindeveloperofFeRAMisRamtronInternational.FeRAMisthemostcommonkindofpersonalcomputermemorywiththeabilitytoretaindatawhenpoweristurnedoffasdoothernonvolatilememorydevicessuchasROMandFlashmemory[97].InaDRAMcell,thedataperiodicallyneedrefreshingduetothedischargingofthecapacitor,whereasFeRAMmaintainsthedatawithoutanyexternalpowersupply.Itachievesthisbyusingaferroelectricmaterialintheplaceofaconventionaldielectricmaterialbetweentheplatesofthecapacitor.Whenanelectricfieldisappliedacrossdielectricorferroelectricmaterials,itwillpolarize,andwhilethatfieldisremoved,itwilldepolarize.Buttheferroelectricmaterialexhibitshysteresisinaplotofpolarizationversuselectricfield,anditwillretainitspolarization.OnedisadvantageofFeRAMisthathasadestructivereadcycle.Thereadmethodinvolveswritingabittoeachcell;ifthestateofthecellchanges,thenasmallcurrentpulseisdetectedbyindicatingthatthecellwasintheOFFstate.However,itisafastmemorythatcanendureahighnumberofcycles(e.g.,1014)[98],meaningthattherequirementforawritecycleforeveryreadcyclewillnotresultinshortproductliveswithaverylowpowerrequirement.Itisexpectedtohavemanyapplicationsinsmallconsumerdevicessuchaspersonaldigitalassistants(PDAs),handheldphones,powermeters,andsmartcards,andinsecuritysystems.FeRAMisfasterthanFlashmemory.ItisalsoexpectedtoreplaceEEPROMandSRAMforsomeapplicationsandtobecomeakeycomponentinfuturewirelessproducts.EvenafterFeRAMhasachievedalevelofcommercialsuccess,withthefirstdevicesreleasedin1993[99,100],currentFeRAMchipsofferperformancethatiseithercomparabletoorexceedingcurrentFlashmemories[98,101],butstillslowerthanDRAM.

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Figure15.BasicstructureofaFeRAMcell.Thecrystalstructureofaferroelectricandanelectricpolarization-electricfieldhysteresiscurvearealsoshown.

PCRAM

PCRAM,alsoknownasPCM,perfectRAM(PRAM),OUM,andchalcogenideRAM(CRAM),isatypeofnonvolatileRAMbasedonaclassofmaterialcalledchalcogenideglassesthatcanexistintwodifferentphasestates(e.g.,crystallineandamorphous)[102,103].ThebasicPCRAMcellstructureisdepictedinFigureÂ16.Mostphase-change

materialscontainatleastoneelementfromgroup6oftheperiodictable,andthechoiceofavailablematerialscanbefurtherwidenedbydopingthesematerials[104-107].Inparticular,themostpromisingaretheGeSbTealloyswhichfollowapseudobinarycomposition(betweenGeTeandSb2Te3),referredtoasGST.Thesematerialsareinfactcommonlyusedasthedatalayerinrewritablecompactdisksanddigitalversatiledisks(CD-RWandDVD-RW)wherethechangeinopticalpropertiesisexploitedtostoredata.Thestructureofthematerialcanchangerapidlybackandforthbetweenamorphousandcrystallineonamicroscopicscale.Thematerialhaslowelectricalresistanceinthecrystallineororderedphaseandhighelectricalresistanceintheamorphousordisorderedphase.ThisallowselectricalcurrentstobeswitchedONandOFF,representingdigitalhighandlowstates.Thisprocesshasbeendemonstratedtobeontheorderofafewtensofnanoseconds[108],whichpotentiallymakesitcompatiblewithFlashforthereadoperation,butseveralordersofmagnitudefasterforthewritecycle.ThismakesitpossibleforPCMtofunctionmanytimesfasterthanconventionalFlashmemorywhileusinglesspower.Inaddition,PCMtechnologyhasthepotentialtoprovideinexpensive,high-speed,high-density,high-volumenonvolatilestorageonanunprecedentedscale.Thephysicalstructureisthree-dimensional,maximizingthenumberoftransistorsthatcanexistinachipoffixedsize.PCMissometimescalledperfectRAMbecausedatacanbeoverwrittenwithouthavingtoeraseitfirst.PossibleproblemsfacingPCRAMconcernthehighcurrentdensityneededtoerasethememory;however,ascellsizesdecrease,thecurrentneededwillalsodecrease.PCMchipsareexpectedtolastseveraltimesaslongascurrentlyavailableFlashmemorychipsandmayprovecheaperformassproduction.WorkingprototypesofPCMchipshavebeentestedbyIBM,Infineon,Samsung,Macronix,andothers.Also,theproductionofPCMhasbeenannouncedrecentlybybothcollaborationsbetweenIntelandSTMicroelectronicsaswellaswithSamsung[109,110].

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Figure16.BasicPCRAMcellstructure.ReproducedfromIBM-Macronix-Qimonda.

ComparisonofprimarycontendersforMRAM,STT-RAM,FeRAM,andPCMtechnologies

Beforegoingtootheremergingmemories,wehereinprovideacomparisonamongMRAM,FeRAM,andPCM.ThespecificfeaturesofthesememorydevicesareprovidedinTableÂ2.Relativelymature,new-materialmemoriessuchasMRAM,STT-RAM,FeRAM,andPCMcanofferavarietyoffeaturesthathavepotentialtobethecandidatesfornext-generationnonvolatilememorydevices.Brand-newconceptssuchasRRAM,molecular,organic/polymer,andothernanowire-basedmemorytechnologieshavealsobeenproposed.Thesearediscussedindetailinthefollowingsection.

Seefulltable

Table2.SummaryofprimarycontendersforMRAM,FeRAM,STT-RAM,andPCMtechnologies

RRAM

RRAMisadisruptivetechnologythatcanrevolutionizetheperformanceofproductsinmanyareas,fromconsumerelectronicsandpersonalcomputerstoautomotive,medical,military,andspace.Amongallthecurrentmemorytechnologies,RRAMisattractingmuchattentionsinceitiscompatiblewiththeconventionalsemiconductorprocesses.Memristor-basedRRAMisoneofthemostpromisingemergingmemorytechnologiesandhasthepotentialofbeingauniversalmemorytechnology[111].Itoffersthepotentialforacheap,simplememorythatcouldcompeteacrossthewholespectrumofdigitalmemories,fromlow-cost,low-performanceapplicationsuptouniversalmemoriescapableofreplacingallcurrentmarket-leadingtechnologies,suchasharddiskdrives,random-accessmemories,andFlashmemories[112].RRAMisasimple,two-terminalmetal-insulator-metal(MIM)bistabledeviceasshowninthebasicconfigurationinFigureÂ17.Itcanexistintwodistinctconductivitystates,witheachstatebeinginducedbyapplyingdifferentvoltagesacrossthedeviceterminals.RRAMusesmaterialsthatcanbeswitchedbetweentwoormoredistinctresistancestates.Manycompaniesareinvestingmetaloxidenanolayersswitchedbyvoltagepulses.Researchersgenerallythinkthatthepulses’electricfieldsproduceconductingfilamentsthroughtheinsulatingoxide.HPLabsplanstoreleaseprototypechipsthisyearbasedon‘memristors’inwhichmigratingoxygenatomschangeresistance[113].Xuetal.havealsodefinedthatamongallthetechnologycandidates,RRAMisconsideredtobethemostpromisingasitoperatesfasterthanPCRAMandithasasimplerandsmallercellstructurethanmagneticmemories(e.g.,MRAMorSTT-RAM)[114].IncontrasttoaconventionalMOS-accessedmemorycell,amemristor-basedRRAMhasthepotentialofformingacross-pointstructurewithoutusingaccessdevices,achievinganultra-highdensity.Thisdeviceisbasedonthebistableresistancestatefoundforalmostanyoxidematerial,includingNiO,ZrO2,HfO2,SrZrO3,andBaTiO3[115-119].Currently,SamsungandIBMareactivelyinvestigatingRRAM.

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Figure17.BasicRRAMcellstructure.Aschematicdiagramofthemechanismoftheresistiveswitchinginametal/oxide/metal-structuredmemorycellisalsoshown.Reproducedfromref.[123].

Kamiyaetal.haverevealedbyatheoreticalmechanismthatRRAMshowsfilamentary-typeresistiveswitching,wheretheoxygenvacancyisconsideredtoformconductivefilamentsintheresistivematerialasshowninFigureÂ17[120].TheformationanddisruptionofthesefilamentsarethusthemechanismsresponsiblefortheON-OFFswitchinginRRAMdevices.Thekeyissueis,therefore,torevealelectronicrolesinthe

formationanddisruptionofthevacancyfilaments.RRAMcanbeswitchedbetweenthelowresistancestate(LRS)andthehighresistancestate(HRS)oftheresistivematerialbyapplyingvoltagestotheelectrodes.LeehasexplainedthatduringtheSETprocess,thecurrentlevelincreasesfromHRStoLRSasthevoltageincreasesfrom0ÂVtothecriticalpointwhichiscalledthesetvoltage(Vset),whilethecurrentlevelabruptlydecreasesfromLRStoHRSattheresetvoltage(Vreset)undertheRESETprocess.TheSETandRESETprocessesarerepeatedlycarriedoutbysweepingthegatevoltagewiththebinarystatesLRSandHRS[121].WangandTsengandLinetal.haveindicatedthattheinterfaceplaysanimportantroleinenhancingtheperformancesofRRAM[122,123].Recently,Gouxetal.haveexplainedthatusingastackedRRAMstructurehasbeenshowntobeoneofthemostpromisingmethodstoimprovethememorycharacteristics[124].Althoughbeingamostpromisingmemoryelement,criticalissuesforthefuturedevelopmentofRRAMdevicesarereliable,suchasdataretentionandmemoryendurance[125].Adataretentiontimeofover10Âyearscanbeextrapolatedfromretentioncharacteristicsmeasuredathightemperaturesandamemoryenduranceofover106Âcycles[126].Therefore,astatisticalstudyofreliability,availability,andmaintainabilityisessentialforthefuturedevelopmentofRRAM.

Polymermemory

Throughoutthelastfewyears,polymershavefoundgrowinginterestasaresultoftheriseofanewclassofnonvolatilememories.Inapolymermemory,alayerconsistsofmoleculesand/ornanoparticlesinanorganicpolymermatrixissandwichedbetweenanarrayoftopandbottomelectrodesasillustratedinFigureÂ18.Moreover,polymermemoryhastheadvantageofasimplefabricationprocessandgoodcontrollabilityofmaterials[127].Polymermemorycouldbecalleddigitalmemorywiththelatesttechnology.Itisnotpossibleforasilicon-basedmemorytobeestablishedinlessspace,butitispossibleforpolymermemory.Lingetal.explainedthatpolymermaterialshavesimplicityinstructure,freereadandwritecapability,betterscalability,3-Dstackingability,low-costpotential,andhugecapacityofdatastorage[128].Theyrevealedthatapolymermemorystoresinformationinamannerthatisentirelydifferentfromthatofsilicon-basedmemorydevices.Ratherthanencoding‘0’and‘1’fromthenumberofchargesstoredinacell,apolymermemorystoresdataonthebasisofhighandlowconductivitywhilerespondingtoanappliedvoltage.Amongthelargenumberofemergingmemorytechnologies,polymermemoryistheleadingtechnology.Itismainlybecauseofitsexpansioncapabilityin3-Dspace[129]sincemostpolymersareorganicmaterialsconsistingoflongchainsofsinglemolecules.Priortopolymermemoryfabrication,depositionofanorganiclayerisusuallydonebythesol-gelspincoatingtechnique.Alltheothernecessaryconstituentmaterialsaredissolvedinasolventwhichisthenspin-coatedoverasubstrate.Whenthesolventisevaporated,athinfilmofmaterialwith10-to100-nmthicknessissuccessfullydepositedatbottomelectrodes.Topelectrodesaredepositedasthefinalstep.Theconductivityoftheorganiclayeristhenchangedbyapplyingavoltageacrossthememorycell,allowingbitsofdatatobestoredinthepolymermemorycell.Whenthepolymermemorycellbecomeselectricallyconductive,theelectronsareintroducedandremoved.Eventhepolymerisconsideredasa‘smart’materialtotheextentthatfunctionalityisbuiltintothematerialitselfofswitchabilityandchargestore.Thiswillopenuptremendousopportunitiesinthe

electronicsworld,wheretailor-madememorymaterialsrepresentanunknownterritory.Thenonvolatilenessandotherfeaturesareinbuiltatthemolecularlevelandoffersveryhighadvantagesintermsofcost.Butturningpolymermemoryintoacommercialproductwouldnotbeeasy.Memorytechnologiescompetenotonlyonstoragecapacitybutonspeed,energyconsumption,andreliability.‘Thedifficultyisinmeetingalltherequirementsofcurrentsiliconmemorychips,’saysThomas,theDirectorofPhysicalSciencesatIBM’sWatsonResearchCenterinYorktownHeights,NY.Theyarelikelytobelimitedtonicheapplications[130].

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Figure18.Structureofapolymermemorydevice.

Racetrackmemory

Inaracetrackmemory,informationisstoredonaU-shapednanowireasapatternofmagneticregionswithdifferentpolarities.TheU-shapedmagneticnanowireisanarrayofkeys,whicharearrangedverticallyliketreesinaforestasshowninFigureÂ19.AchievingcapacitiescomparabletoverticalRMorharddriveswouldrequirestacksofthesearrays.Thenanowireshaveregionswithdifferentmagneticpolarities,andtheboundariesbetweentheregionsrepresent1or0Âs,dependingonthepolaritiesoftheregionsoneitherside[131,132].Themagneticinformationitselfisthenpushedalongthewire,pastthewriteandreadheadsbyapplyingvoltagepulsestothewireends.Themagneticpatterntospeedalongthenanowire,whileapplyingaspin-polarizedcurrent,causesthedatatobemovedineitherdirection,dependingonthedirectionofthecurrent.AseparatenanowireperpendiculartotheU-shaped‘racetrack’writesdatabychangingthepolarityofthemagneticregions.Aseconddeviceatthebaseofthetrackreadsthedata.Datacanbewrittenandreadinlessthanananosecond.Aracetrackmemoryusinghundredsofmillionsofnanowireswouldhavethepotentialtostorevastamountsofdata[133,134].Inthisway,thememoryrequiresnomechanicalmovingofpartsandithasagreaterreliabilityandhigherperformancethanHDDs,withtheoreticalnanosecondoperatingspeeds.Foradeviceconfigurationwheredatastoragewiresarefabricatedinrowsonthesubstrate,conventionalmanufacturingtechniquesareadequate.However,forthemaximumpossiblememorydensity,thestoragewiresareproposedtobeconfiguredrisingfromthesubstrateina‘U’shape,givingrisetoa3-Dforestofnanowires.Whilethislayoutdoesallowhighdatastoragedensities,italsohasthedisadvantageofcomplexfabricationmethods,withsofar,only3-bitoperationofthedevicesdemonstrated[133].Astheaccesstimeofthedataisalsodependentonthepositionofthedataonthewire,thesewouldalsobeperformancelossesiflongwiresareusedtoincreasethestoragedensityfurther.Thespeedofoperationofthedeviceshasalsobeenanissueduringdevelopment,withmuchslowermovementofthemagneticdomainsthanoriginallypredicted.Thishasbeenattributedtocrystalimperfectionsinthepermalloywire,whichinhibitthemovementofthemagneticdomains.Byeliminatingtheseimperfections,adatamovementspeedof110Âm/shasbeendemonstrated[133].

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Figure19.RacetrackmemorydiagramshowinganarrayofU-shapedmagneticnanowires.Thenanowiresarearrangedverticallyliketreesinaforestandapairoftinydevicesthatreadandwritethedata.AdoptedfromIBM.

OthernewmemorytechnologiesResearchersarealreadyworkinghardonseveralemergingtechnologies,asdiscussedinprevioussections,topursuestorage-classmemorieswithamoretraditionaldesignthanthatoftheracetrackmemory,whichplacesthebitsinhorizontalarrays.

Molecularmemory

Amolecularmemoryisanonvolatiledatastoragememorytechnologythatusesmolecularspeciesasthedatastorageelement,ratherthan,e.g.,circuits,magnetics,inorganicmaterials,orphysicalshapes[135].Inamolecularmemory,amonolayerofmoleculesissandwichedbetweenacross-pointarrayoftopandbottomelectrodesasshowninFigureÂ20.Themoleculesarepackedinahighlyorderedway,withoneendofthemoleculeelectricallyconnectedtothebottomelectrodeandtheotherendofthemoleculeconnectedtothetopelectrode,andthismolecularcomponentisdescribedasamolecularswitch[136].Langmuir-Blodgett(LB)depositionisideallysuitedfordepositingthemolecularlayerforthefabricationofmolecularmemorydevices[137,138].Then,regardingthemolecularmemoryoperation,byapplyingavoltagebetweentheelectrodes,theconductivityofthemoleculesisaltered,enablingdatatobestoredinanonvolatileway.Thisprocesscanthenbereversed,andthedatacanbeerasedbyapplyingavoltagetotheoppositepolarityofthememorycell.Theincreasingdemandfornonvolatileelectronicmemorieswillgrowrapidlyinordertokeeppacewiththerequirementsforsubsystemsinvolvedinflightdemonstrationprojectsanddeepspaceoperations.Atthesametime,mass,volume,andpowermustbeminimizedformissionaffordabilityconcerningtheserequirements;molecularmemorycouldbeaverypromisingcandidatetofillthisneed.

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Figure20.Cellstructureofamolecularmemorydevice.

Recently,Plafkehasrevealedclearlyviaanarticlethatlikemostexperimentaltechnologythatsoundssoamazingthatwewantitrightnow,themolecularmemorycelldoesnotprovideenoughpowerforacommercialdevice[139].Thisiscurrentlyonlyabletoproducea20%jumpinconductivity.However,theareaofmolecularswitchingmemoryispromising,havingeliminatedtheneedfornear-absolutezerotemperaturesandremovedsomeoftheconstraintsoftheshapeandnumberoflayersofthemoleculesheetswhichintendtoconveythattwoofthebiggestbarriersaretakenaway.Thus,molecularmemoryrequiresstrongattentiontoworkoversuchissuesandneedsimmediateamendmenttoseethepossibilityofauniversalmemoryinthefuture.

MNW

Inthelasttwodecades,anincreasinginterestisobservedforelectronics-relateddevicesandthesearchforauniversalmemorydatastoragedevicethatcombinesrapidreadandwritespeeds,highstoragedensity,andnonvolatilityisdrivingtheinvestigationofnewmaterialsinthenanostructuredform[140].AsanalternativetothecurrentFlashmemorytechnology,anoveltransistorarchitectureusingmolecular-scalenanowirememorycellsholdsthepromiseofunprecedentlycompactdatastorage.Themolecularnanowirearray(MNW)memoryisfundamentallydifferentfromothersemiconductormemories;informationstorageisachievedthroughthechannelofananowiretransistorthatisfunctionalizedwithredox-activemoleculesratherthanthroughmanipulationofsmallamountsofcharge.Itisrelativelyslowandlackstherandomaccesscapability,whereindatathatcanberandomlyreadandwrittenateverybytearebeingactivelypursued.FigureÂ21showstheschematicdesignofaMNWmemorycell.Lieber,andAgarwalandLieberhaverevealedthatthenanowire-basedmemorytechnologyisapowerfulapproachtoassembleelectronic/photonicdevicesatultra-smallscalesowingtotheirsub-lithographicsize,defect-freesingle-crystallinestructure,anduniquegeometry[141,142].Nanowiressynthesizedbychemicalorphysicalprocessesarenearlyperfectsingle-crystalstructureswithasmallgeometryandperfectsurface.Thechannelofananowiretransistorisfunctionalizedwithredox-activemolecules.Duringprogramming,controlofthevoltageactingonthesubstrateispossibletochangetheoxidationandreductionstatesoftheactivemolecules.Finally,bymeasurementoftheconductanceofthenanowirewiththegatebiasfixedat0ÂVorasmallvoltageandfromthehysteresis,thetwostatescanbedefinedasahigh-conductanceONstateandalow-conductanceOFFstate.TheMNWmemoryhasadvantagesoflowpowerdissipation,ultra-highdensity,simplefabricationprocess,3-Dstructure,andmultilevelstorage,anditfunctionsatthenanoscalewithafewelectronsbutlimitedbylowretentiontimeparameter[143,144].Moreover,thedepositionofmetalsontoamonolayerofmolecularwirescanleadtolowdeviceyield,andthisproblemremainsamajorchallenge[145].However,mentioningthetermemergingclassmemory,itcouldbeexpectedthattheMNWmemoryrepresentsanimportantsteptowardsthecreationofmolecularcomputersthataremuchsmallerandcouldbemorepowerfulthantoday’ssilicon-basedcomputers.

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Figure21.AMNWmemorycellstructure.

SNW

Semiconductormemoryisessentialforinformationprocessingasakeypartofsilicontechnology;semiconductormemoryhasbeencontinuouslyscaledtoachieveahigherdensityandbetterperformanceinaccordancewithMoore’slaw[146].Flashmemorymayreachfundamentalscalinglimits,however,becauseathicktunnelingoxideisrequiredtopreventchargeleakageandachieve10Âyears’retention.AsFlashmemoryapproachesitsscalinglimit,severalalternativestrategieshavebeenproposedtoextendorreplacethecurrentFlashmemorytechnology[147].Theseapproachesarerevolutionary,butmajorchallengesmustbeovercometoachievesmallmemorysizeandaggressivetechnologydesignarchitecture.Inadditiontotheengineeringoftrappinglayers,thedevice

performancecanalsobeimprovedbyusinginnovativenonplanarchannelgeometries.Amongthevariousnanostructurematerials,semiconductornanowirememory(SNW)hasinducedgreatscientificinterestaspossiblebuildingblocksforfuturenanoelectroniccircuitry.InaSNWmemorydevice,nanowiresareintegratedwithSONOStechnology.ThebasicschematicdesignofSNWisdepictedinFigureÂ22.TheSNWmemoryshowshighmobility,lesspowerdissipation,andhighperformance.Moreover,being3-D-stacked,theSNWmemoryenhancescelldensityanddatacapacitywithoutrelyingonadvancesinprocesstechnology.Thenanowire-basedmemorydevicecanstoredataelectricallyandisnonvolatile,meaningitretainsdatawhenthepoweristurnedoff,likethesilicon-basedFlashmemoryfoundinsmartphonesandmemorycards[148],withminimalincreaseinchipsize.Inaddition,theSNWdeviceexhibitsreliablewrite/read/eraseoperationswithalargememorywindowandhighon-to-offcurrentratio,whicharehighlyadvantageousforapplicationsinnonvolatilememory[149].TheSNWmemorycannotholddataaslongastheexistingFlash,butitisslowerandhasfewerrewritecyclesanditcouldpotentiallybemadesmallerandpackedtogethermoredensely.Anditsmainadvantageisthatitcanbemadeusingsimpleprocessesatroomtemperature,whichmeansthatitcanbedepositedevenontopofflexibleplasticsubstrates[150].TheSNWcould,forinstance,bebuiltintoaflexibledisplayandcouldbepackedintosmallerspacesinsidecellphones,MP3players,plasticRFIDtags,andcreditcards.

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Figure22.Abottom-gateFET-basednonvolatileSNWmemorydevice.

NRAM

NRAMisacarbonnanotube(CNT)-basedmemory,whichworksonananomechanicalprinciple,ratherthanachangeinmaterialproperties[151].NRAMusescarbonnanotubesforthebitcells,andthe0or1isdeterminedbythetube’sphysicalstate:upwithhighresistance,ordownandgrounded.NRAMisexpectedtobefasteranddenserthanDRAMandalsoveryscalable,abletohandle5-nmbitcellswheneverCMOSfabricationadvancestothatlevel.Itisalsoverystableinits0or1state.ProducedbyNantero,thesememoriesconsistofthestructureshowninFigureÂ23awithanarrayofbottomelectrodescoveredbyathininsulatingspacerlayer[152].CNTsarethendepositedonthespacerlayer,leavingthemfreestandingabovethebottomelectrodes.UnwantedCNTsareremovedfromtheareasaroundtheelectrode,withtopcontactsandinterconnectsdepositedontopofthepatternedCNTlayer.DuringthetimethattheCNTsarefreestanding,thereisnoconductionpathbetweenthebottomandtopelectrodesandhencethememorycellisintheOFFstate.However,ifalargeenoughvoltageisappliedoverthecell,thenanotubesareattractedtothebottomelectrodewheretheyareheldinplacebyvanderWaalsforces[153].DuetotheconductivenatureoftheCNTs,theelectrodesarenowconnectedandthecellreadsthelowconductivityONstateasshowninFigureÂ23b.TheOFFstatecanbereturnedbyrepellingthenanotubeswiththeoppositeelectrodepolarity.NonvolatilityisachievedduetothestrengthofthevanderWaalsforcesovercomingthemechanicalstrainofthebentnanotubes,henceholdingthecellintheONstate.NRAMoffersthepossibilityofasimplecellarchitecture,whichcouldoperateat

muchhigherspeedsthantheconventionalFlashandwithlowpoweruse.Cuietal.reportedCNTmemorydevicesexhibitinganextraordinarilyhighchargestoragestabilityofmorethan12Âdaysatroomtemperature[154].However,asNRAMisbasedonCNTs,itsuffersfromfabricationproblemsthatareinherentincarbonnanotube-baseddevices.TheissuesincludethecostandfabricationcomplexityofproducingtheCNTs,ensuringuniformdispersionsofnanotubes,anddifficultiesinremovingnanotubesfromtheunwantedpositionsonthesubstrate.

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Figure23.NRAMstructurewith(a)OFFstateand(b)ONstate.

Millipedememory

In2002,IBMdevelopedapunchcardsystemknownasMillipede,whichisanonvolatilecomputermemorystoredinathinpolymersheetwithnanoscopicholestoprovideasimplewaytostorebinarydata[155].Itcanstorehundredsofgigabytesofdatapersquarecentimeter.However,thepolymerrevertstoitspre-punchedformovertime,losingdataintheprocess.Millipedestoragetechnologyisbeingpursuedasapotentialreplacementformagneticrecordinginharddrives,atthesametimereducingtheformfactortothatofFlashmedia.Theprototype’scapacitywouldenablethestorageof25DVDsor25millionpagesoftextonapostagestamp-sizedsurfaceandcouldenable10ÂGBofstoragecapacityonacellphone.Millipedeusesthousandsoftinysharppoints(hencethename)topunchholesinathinplasticfilm.Eachofthe10-nmholesrepresentsasinglebit.Thepatternofindentationsisadigitizedversionofthedata.Thelayoutofthemillipedecantilever/tipincontactwiththedatastoragemediumisshowninFigureÂ24.AccordingtoIBM,Millipedecanbethoughtofasananotechnologyversionofthepunchcarddataprocessingtechnologydevelopedinthelatenineteenthcentury[156].However,therearesignificantdifferences:Millipedeisrewritable,anditmayeventuallyenablestorageofover1.5ÂGBofdatainaspacenolargerthanasingleholeinthepunchcard.StoragedevicesbasedonIBM’stechnologycanbemadewithexistingmanufacturingtechniques,sotheywillnotbeexpensivetomake.AccordingtoP.Vettiger,headoftheMillipedeproject,thereisnotasinglestepinfabricationthatneedstobeinvented.VettigerpredictsthatananostoragedevicebasedonIBM’stechnologycouldbeavailableasearlyas2005[155].Now,researchersatIBM’sZurichResearchLaboratoryinSwitzerlandhaveclockedtherateofdataloss.Theyhavecalculatedthatat85°C,atemperatureoftenusedtoassessdataretention,itwouldlosejust10%to20%ofinformationoveradecade,comparabletoFlashmemory[157].

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Figure24.Schematiclayoutofthemillipedecantilever/tipincontactwiththedatastoragemedium.Adoptedfromref.[157].

WORMmemorybasedonDNAbiopolymernanocomposite

TheuseofDNAiswellknownasagoodmodelformetalNPsynthesisduetoitsaffinitytothemetalions[158].Inrecentyears,DNAhasalsobeenshowntobeapromisingopticalmaterialwiththematerialprocessingfullycompatiblewithconventionalpolymerforthin-filmoptoelectronicapplications[159,160].ResearchersfromNationalTsingHuaUniversityinTaiwanandtheKarlsruheInstituteofTechnologyinGermanyhavecreatedaDNA-basedmemorydevice,thatis,write-once-read-many-times(WORM),thatusesultraviolet(UV)lighttoencodeinformation[161].Thedeviceconsistsofasinglebiopolymerlayersandwichedbetweenelectrodes,inwhichelectricalbistabilityisactivatedbyinsituformationofsilvernanoparticlesembeddedinabiopolymeruponlightirradiation(FigureÂ25).ThedevicefunctionallyworkswhenshiningUVlightonthesystem,whichenablesalight-triggeredsynthesisprocessthatcausesthesilveratomstoclusterintonanosizedparticlesandreadiesthesystemfordataencoding.Forsomeparticularinstance,theteamhasfoundthatusingDNAmaybelessexpensivetoprocessintostoragedevicesthanusingtraditional,inorganicmaterialslikesilicon,theresearcherssay[161,162].TheysaidthatwhennovoltageorlowvoltageisappliedthroughtheelectrodestotheUV-irradiatedDNA,onlyalowcurrentisabletopassthroughthecomposite;thiscorrespondstothe‘OFF’stateofthedevice.ButtheUVirradiationmakesthecompositeunabletoholdachargeunderahighelectricfield,sowhentheappliedvoltageexceedsacertainthreshold,anincreasedamountofchargeisabletopassthrough.Thishigherstateofconductivitycorrespondstothe‘ON’stateofthedevice.Theteamfoundthatthischangefromlowconductivity(‘OFF’)tohighconductivity(‘ON’)wasirreversible:oncethesystemhadbeenturnedon,itstayedon,nomatterwhatvoltagetheteamappliedtothesystem.Onceinformationiswritten,thedeviceappearstoretainthatinformationindefinitely.Theresearchershopethatthetechniquewillbeusefulinthedesignofopticalstoragedevicesandsuggestthatitmayhaveplasmonicapplicationsaswell.Consequently,WORMmemoriesbasedonDNAabiopolymernanocompositehaveemergedasanexcellentcandidatefornext-generationinformationstoragemediabecauseoftheirpotentialapplicationinflexiblememorydevices.ThisworkcombinesnewadvancesinDNAnanotechnologywithaconventionalpolymerfabricationplatformtorealizeanewemergingclassofDNA-basedmemory.

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Figure25.SchematicdesignofamemorydeviceconsistingofathinDNAbiopolymerfilmsandwichedbetweenelectrodes.Thememoryswitchingeffectisactivateduponlightirradiation.Adoptedfromref.[161].

QDmemory

Memorymadefromtinyislandsofsemiconductors-knownasquantumdots-couldfillagapleftbytoday’scomputermemory,allowingstoragethatisfastaswellaslonglasting.Researchershaveshownthattheycanwriteinformationintoquantumdotmemoryinjustnanoseconds.Memoryisdividedintotwoforms:DRRAMandFlash[163,164].ComputersuseDRAM,forshort-termmemory,butdatadoesnotpersistforlongandmustberefreshedover100timespersecondtomaintainitsmemory.Ontheotherhand,Flashmemory,likethatusedinmemorycards,canstoredataforyearswithoutrefreshingbutwritesinformationabout1,000timesslowerthanDRAM.Newresearchshowsthatmemorybasedonquantumdotscanprovidethebestofboth:long-termstoragewithwrite

speedsnearlyasfastasDRAM.Atightlypackedarrayoftinyislands,eacharound15Ânmacross,couldstore1terabyte(1,000ÂGB)ofdatapersquareinch,theresearcherssay.DieterBimbergandcolleaguesattheTechnicalUniversityofBerlin,Germany,withcollaboratorsatIstanbulUniversity,Turkey,demonstratedthatitispossibletowriteinformationtothequantumdotsinjust6Âns[165,166].Thekeyadvantagesofquantumdot(QD)NVMsarethehighread/writespeed,smallsize,lowoperatingvoltage,and,mostimportantly,multibitstorageperdevice.However,thesefeatureshavenotbeenrealizedduetovariationsindotsizeandlackofuniforminsulatorcladdinglayersonthedots[167].IncorporatingQDsintothefloatinggateresultsinareductioninchargeleakageandpowerdissipationwithenhancedprogrammingspeed.ResearchersinIndiaandGermanyhavenowunveiledthememorycharacteristicsofsiliconandsilicon-germaniumQDsembeddedinepitaxialrare-earthoxidegadoliniumoxide(Gd2O3)grownonSi(111)substratesasshownintheDQMstructureinFigureÂ26.MultilayerSiaswellassingle-layerSi1−xGex(wherex = 0.6)QDsshowexcellentmemorycharacteristics,makingthemattractivefornext-generationFlash-floating-gatememorydevices[168,169].

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Figure26.Structureofquantumdotmemory.Adoptedfromref.[168].

3-Dcross-pointmemory

Memoryproducersarealsotryingtodevelopalternativetechnologiesthatmaybescalablebeyond20-nmlithography.Fortruescalabilitybeyond20-nmtechnologynodes,itisnecessarytodesignacross-pointmemoryarraywhichdoesnotrequirediodesforaccesselements[170].Thecross-pointmemoryarchitecturecouldbedesignedsuchthatitcanbeeasilyfabricatedinmultiplelayerstoformastacked3-Dmemory[171].The3-DtechnologyhasbroughttohighvolumeanNVMwherearraysofmemorycellsarestackedabovecontrollogiccircuitryinthethirddimension,andstacking3-DmemorydirectlyoverCMOSallowsforhigharrayefficiencyandverysmalldiesize[172].The3-Dtechnologyusesnonewmaterials,processes,orfabricationequipment,whichcontrollogiccircuitrycomposedoftypicalCMOS.Thememoryconstructionusestypicalback-endprocessingtools,andeachmemorylayerisarepeatofthelayersbelowit.Thebasicdesignofthe3-DcellconsistsofaverticaldiodeinserieswithamemoryelementasshowninFigureÂ27.Buildingintegratedcircuitsverticallyallowsforareducedchipfootprintwhencomparedtoatraditional2-Ddesign,byanapproximatefactorofthenumberoflayersused.Thisofferssignificantadvantagesintermsofreducedinterconnectdelaywhenroutingtoblocksthatotherwisewouldhavebeenplacedlaterally.Theprocessforthe2-Dcross-pointarraycanbebuiltintoamultilayer3-Darchitecture.Traditionally,a3-Dintegratedcircuit(3-D-IC)hasusedmorethanoneactivedevicelayer.Whileresistance-changememorycellsarenotactivedevices,theyfunctionasrectifyingdevicesindesign.Furthercharacterizationoftheresistance-changematerialisalsonecessaryin

ordertoguaranteethatthe3-Dcross-pointmemorywillbepracticalfordatastorage.Also,thescalabilityofmetal-oxideresistance-changematerialsbeyond20-nmtechnologynodesstillneedstobestudied.Moreover,theprogrammingoperationisexpectedtobecompetitivewithbothNANDandNORFlashintermsofspeedbecauseoftherelativelylowvoltagerequirementsofresistance-changematerials.Iftheperipheralcircuitryforaccommodatingthewriteoperationcanbemadesufficientlycompact,thenthe3-Dcross-pointmemorywillindeedbeaviablereplacementforNANDandNORFlashinfutureprocessgenerations.

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Figure27.Thebasicdesignofa3-Dcellthatconsistsofaverticaldiodeinseries.(top)Sideview,(bottomright)topview,and(bottomleft)3-Dview.

TFM

Transparentandflexibleelectronics(TFE)is,today,oneofthemostadvancedtopicsforawiderangeofdeviceapplications,wherethekeycomponentistransparentconductingoxides(TCOs),whichareuniquematerialsthatoxidesofdifferentoriginplayanimportantrole,notonlyasapassivecomponentbutalsoasanactivecomponent[173].TFEisanemergingtechnologythatemploysmaterials(includingoxides,nitrides,andcarbides)andadevicefortherealizationofinvisiblecircuitsforimplementingnext-generationtransparentconductingoxidesinaninvisiblememorygeneration[174].Ingeneral,theTF-RRAMdeviceisbasedonacapacitor-likestructure(e.g.,ITO/transparentresistivematerial/ITO/transparentandflexiblesubstrate),whichprovidestransmittanceinthevisibleregion[175].Forsuchnewclassofmemorytechnology,dataretentionisexpectedtobeabout10Âyears.Thebasicstructuraldesignofthenewmemorychipsisconfigured,namelywithtwoterminalsperbitofinformationonatransparentandflexiblesubstrateratherthanthestandardthreeterminalsperbitonarigidandopaquesubstrate(FigureÂ28).Theyaremuchbettersuitedforthenextrevolutioninelectronic3-DmemorythanFlashmemory.Thesenewmemorychipsthataretransparent,areflexibleenoughtobefoldedlikeasheetofpaper,shrugoff1,000°Ftemperaturestwiceashotasthemaxinakitchenoven,andsurviveotherhostileconditionscouldusherinthedevelopmentofnext-generationFlash-competitivememoryfortomorrow’skeychaindrives,cellphones,andcomputers,ascientistreportedtoday.Speakingatthe243rdNationalMeetingandExpositionoftheAmericanChemicalSociety,theworld’slargestscientificsociety,hesaidthatdeviceswiththesechipscouldretaindatadespiteanaccidentaltripthroughthedrierorevenavoyagetoMars.Andwithaunique3-Dinternalarchitecture,thenewchipscouldpackextragigabytesofdatawhiletakinguplessspace[176].DespitetherecentprogressinTF-RRAM,itneedslotsofworktosatisfythedualrequirementsofresistancetorepeatedbendingstressandtransparentproperties.Thus,itissupposedthatanachievementofsuchTF-RRAMdevicewillbethenextsteptowardstherealizationoftransparentandflexibleelectronicsystems.WehopethatFT-RRAMdeviceswillmarkamilestoneinthecurrentprogressofsuchuniqueandinvisibleelectronicsystemsinthenearfuture.

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Figure28.AschematicdesignofFT-RRAMandaflexible,transparentmemorychipimagecreatedbyresearchersatRiceUniversity.ReproducedfromTourLab/RiceUniversity.

1T1R-RRAM

One-transistorone-resistor(1T1R)-RRAMisalsooneclassofemergingmemorytechnologywithimpressivecharacteristics.Itmeetsthedemandsfornext-generationmemorysystems.Itisexpectedthat1T1R-RRAMcouldbeabletomeetthedemandofhigh-speed(e.g.,performance)memorytechnology.The1T1Rstructureischosenbecausethetransistorisolatescurrenttocells,whichareselectedfromcellswhichdonot.Thebasiccellstructureof1T1RisdepictedinFigureÂ29.1T1R-RRAMconsistsofanaccesstransistorandaresistorasastorageelement.ZangenehandJoshihavealsomentionedthatthe1T1RcellstructureissimilartothatofaDRAMcellinthatthedataisstoredastheresistanceoftheresistorandthetransistorservesasanaccessswitchforreadingandwritingdata[177,178].Inreferencetothis,theyrevealedthe1T1RcellasthebasicbuildingblockofaNVRRAMarrayasitavoidssneakpathproblemtoensurereliableoperation.Moreover,the1T1Rstructureismorecompactandmayenableverticallystackingmemorylayers,ideallysuitedformassstoragedevices.But,intheabsenceofanytransistor,theisolationmustbeprovidedbya‘selector’device,suchasadiode,inserieswiththememoryelement,orbythememoryelementitself.Suchkindsofisolationcapabilitieshavebeeninferiortotheuseoftransistors,limitingtheabilitytooperateverylargeRRAMarraysin1T1Rarchitecture.1T1Rmemorypolaritycanbeeitherbinaryorunary.Bipolareffectscausepolaritytoreverseresetoperationtosetoperation.Unipolarswitchingleavespolarityunaffectedbutusesdifferentvoltages.

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Figure29.Thebasiccellstructureof1T1R-RRAM.

MTM,PFRAM,SPBMM,andCMORRAM-futurealternateNVMs

Otherpotentialemergingclassesofmemorytechnologies,wearedescribinginshort,aremoleculartunnelmemory(MTM),polymericferroelectricRAM(PFRAM),spin-polarizedbeammagneticmemory(SPBMM),lightmemory,andcomplexmetal-oxideRRAM(CMORRAM).Wecansaythatthesearesistermemorytechnologiesofmolecularmemory,ferroelectric/polymermemory,magneticmemory,andmetal-oxideRRAM,respectively.Althoughthesenewtechnologieswillalmostcertainlyresultinmorecomplexmemoryhierarchiesthantheirfamilymemories,theyarelikelytoallowtheconstructionofmemorychipsthatarenonvolatile,havelowenergy,andhavedensityanddevelopmentclosetoorbetterthanthoseofDRAMchips,withimprovedperformanceandallowingmemorysystemstocontinuetoscaleup.

ConclusionsThisarticlereviewedthehistoricaldevelopmenttotherecentadvancementonmemoryarchitectureandscalingtrendofseveralconventionaltypesofFlashwithintheMOSfamilyandprojectedtheirfuturetrends.Withgreatprogressbeingmadeintheemergingmemorytechnologies,currenttrendsandlimitationswerediscussedbeforeleadingtosomeinsightintothenextgenerationofmemoryproducts.Forthepastthreeandahalfdecadesinexistence,thefamilyofsemiconductormemorieshasexpandedgreatlyandachievedhigherdensities,higherspeeds,lowerpower,morefunctionality,andlowercosts.Inthepast40to50Âyears,NVSMhasgrownfromtheFGconcepttoFAMOS,SAMOS,Flashmemory,multilevelcells,RRAM,3-Dstructures,andTF-RRAM.Since1990,NVSMisaninspiredtechnology,whichhasusheredinthedigitalage,enabledthedevelopmentofallmodernelectronicsystems,andbroughtunprecedentedbenefittohumankind.Atthesametime,someofthelimitationswithineachtypeofmemoryarealsobecomingmorerealized.Asthedevicedimensionisreducedtothedeca-nanometerregime,NVSMfacesmanyseriousscalingchallengessuchastheinterfaceofneighboringcells,reductionofstoredcharges,andrandomtelegraphnoise.Assuch,wehopeandareconfidentthatthereareseveralemergingtechnologiesaimingtogobeyondthoselimitationsandpotentiallyreplaceallormostoftheexistingsemiconductormemorytechnologiestobecomeaUSM.Despitetheselimitations,thefieldofconventionalsemiconductormemorieswouldcontinuetoflourishandmemorydevicescientistswillfindthewaytomeetthesechallengesandmayevendevelopa‘unifiedmemory’withlowcost,highperformance,andhighreliabilityforfutureelectronicsystems.Progresstowardsaviablenewresistivememorytechnologyreliesonfullyunderstandingthemechanismsresponsibleforswitchingandchargetransport,thefailuremechanisms,andthefactorsassociatedwithmaterialsreliability.Moreover,thedevelopmentofcurrentredox-basedresistiveswitchingwillhelptoimproveouroldtechnologies,andfurtherresearchwillproducemoreimpressiveresultsthatwillbenefitindustriesandsocietytoimprovethequalityoflifeforbillionsofpeoplearoundtheworld.

CompetinginterestsTheauthorsdeclarethattheyhavenocompetinginterests.

Authors’contributionsJSMdesignedthestructureandmodifiedthemanuscript.SMSandTYTparticipatedinthesequencealignmentandeditingthemanuscript.UCparticipatedinitsdesignandcoordinationandhelpedtodraftthemanuscript.Allauthorsreadandapprovedthefinalmanuscript.

Authors’informationJSMreceivedhisbachelor’sdegree(PhysicsHonors)fromtheDepartmentofPhysics,AligarhMuslimUniversity,Aligarh,India,andMaster’sdegreeinsolid-statetechnologyfromtheDepartmentofPhysicsandMeteorology,IndianInstituteofTechnology(IIT),Kharagpur,India,in2007.HereceivedhisPh.D.degreeinNanotechnologyfromNationalChiaoTungUniversity(NCTU),Taiwan,inFebruary2012.FromMarch2012toDecember2013,hehasbeenaPostdoctoralResearchAssociateintheDepartmentofPhotonicsandDisplayInstitute,NCTU,Taiwan.HeiscurrentlyaPostdoctoralResearchAssociateintheDepartmentofElectronicsandtheInstituteofElectronics,NCTU,Taiwan.Hiscurrentresearchinterestsincludedesigning,fabrication,andtestingofatransparentandflexiblerandom-accessmemory(RRAM)forapplicationininvisibleandrollablenonvolatilememorydevices.Hehaspublishedvariousresearchpapersinreputedjournalsandpresentedhisresearchininternationalconferencesoverflexiblesubstrate-basedthin-filmtransistorsandcapacitordevicesfortheirapplicationsindisplayandRFidentificationtags.

SMSreceivedhisB.S.degreefromNationalTaiwanUniversity,M.S.degreefromtheUniversityofWashington,andPh.D.degreefromStanfordUniversity,allinElectricalEngineering.HewaswithBellTelephoneLaboratoriesfrom1963to1989asamemberoftheTechnicalStaff.HejoinedNationalChiaoTungUniversity(NCTU)from1990to2006asaDistinguishedProfessor.Atpresent,heisaNationalEndowedChairProfessoratNCTU.HehasservedasaVisitingProfessororConsultingProfessortomanyacademicinstitutionsincludingtheUniversityofCambridge,DelftUniversity,BeijingJiaotongUniversity,TokyoInstituteofTechnology,SwissFederalInstituteofTechnology,andStanfordUniversity.Hehasmadefundamentalandpioneeringcontributionstosemiconductordevices,especiallymetal-semiconductorcontacts,microwavedevices,andsubmicronMOSFETtechnology.Ofparticularimportanceishiscoinventionofthenonvolatilesemiconductormemory(NVSM)whichhassubsequentlygivenrisetoalargefamilyofmemorydevicesincludingFlashmemoryandEEPROM.TheNVSMhasenabledthedevelopmentofallmodernelectronicsystemssuchasthedigitalcellularphone,ultrabookcomputer,personaldigitalassistant,digitalcamera,digitaltelevision,smartICcard,electronicbook,portableDVD,MP3musicplayer,antilockbrakingsystem(ABS),andGlobalPositioningSystem(GPS).Hehasauthoredorcoauthoredover200technicalpapers.Hehaswrittenandedited16books.HisbookPhysicsofSemiconductorDevices(Wiley,1969;2nded,1981;3rded,2007)isoneofthemostcitedworksincontemporaryengineeringandappliedsciencepublications(over24,000citationsaccordingtoISIPress).HehasreceivedtheIEEEJ.J.EbersAward,theSunYat-senAward,theNationalEndowedChairProfessorAward,andtheNationalScienceandTechnologyPrize.HeisaLifeFellowofIEEE,anAcademicianoftheAcademiaSinica,aforeignmemberoftheChineseAcademyofEngineering,andamemberoftheUSNationalAcademyofEngineering.

UCreceivedhisMSdegreeinSolidStateElectronicsfromtheIndianInstituteofTechnology,Roorkee,India,in2010.HeiscurrentlyaPh.D.candidateoftheInstituteofElectronics,NationalChiaoTungUniversity,Taiwan.Heiscurrentlyworkingonthe

fabricationandcharacterizationofresistiveswitchingmemorydeviceswithafocusonmemorystructureinnovationsandreliability.

TYTisnowaLifetimeChairProfessorintheDepartmentofElectronicsEngineering,NationalChiaoTungUniversity.HewastheDeanoftheCollegeofEngineeringandViceChancelloroftheNationalTaipeiUniversityofTechnology.Hereceivednumerousawards,suchastheDistinguishedResearchAwardfromtheNationalScienceCouncil,AcademicAwardoftheMinistryofEducation,NationalEndowedChairProfessor,andIEEECPMTOutstandingSustainedTechnicalContributionAward.

AcknowledgementsFirstofall,theauthorswouldliketothankandgratefullyacknowledgeallcorrespondingpublishersforthekindpermissiontoreproducetheirfiguresandrelateddescription,usedinthisreviewarticle.ThisworkwassupportedbytheNationalScienceCouncil,Taiwan,underProjectNo.NSC102-2221-E-009-134-MY3.

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Figure1.ThecircuitrystructuresofDRAM,SRAM,andFlashmemories.

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Figure2.Flowchartforthesemiconductormemoryclassificationaccordingtotheirfunctionalcriteria.

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Figure3.VariousNVSMapplicationsintheelectronicsindustrybymarketsizein2010.Reprintedfromref.[22].

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Figure4.GrowthofNANDFlashmarketupto2014(iSuppli)andtheinterfacespeedofvariousNANDapplications.Reproducedfromref.[37].

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Figure5.Marketvolume(a)andglobalflexibledisplaymarketshipmentforecast(b).Reproducedfromrefs.[38,39].

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Figure6.EmergingNVMapplicationsinvariousmarkets.

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Figure7.EmergingNVMmarketforecastbyapplicationsfrom2012to2018(inM$).Reproducedfromref.[43].

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Figure8.ComparisonofNORFlasharrayandNANDFlasharrayarchitectures.

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Table1.ComparisonbetweenNORandNANDFlashmemories[55-57]

Features NOR NAND

Memorysize ≤512Mbit 1to8Gbit

Sectorsize Approximately1Mbit Approximately1Mbit

Programtime 9Âμs/word 400Âμs/page

Erasetime 1Âs/sector 1Âms/sector

Readaccesstime <80Âns 20Âμs

Writeparallelism 8to16words 2Kbyte

Outputparallelism Byte/word/dword Byte/word

Readparallelism 8to16words 2Kbyte

Accessmethod Random Sequential

Price High Verylow

Reliability Standard Low

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Figure9.ThetrendofMOSFETscalingfromITRS.ReproducedfromITRSCorp.

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Figure10.SchematicplotsofaFlashmemorycellandthedegradationofitstunneloxide.ThedegradationleadstotheformationofpercolationpathsresponsiblefortheFGchargeloss,hencethelossofthestoredinformation.Thepresenceoftrapsintheenergybarrieryieldsthetrap-assistedtunnelingmechanismandoriginatesthestress-inducedleakagecurrent(SILC).

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Figure11.SchematicsoftheconventionalFGmemoryandSONOS.Schematicsof(a)floatinggateandthin-filmstorage-basedembeddednonvolatilememorybitcells,dependingonthechargestoredinsidethegatedielectricofaMOSFET,and(b)thenitridetraps(SONOS),embeddedintothegateoxideofaMOSFET.

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Figure12.Fowler-Nordheim(FN)tunnelingofelectronsfromthegateduringeraseanderasesaturationinSONOSnonvolatilememory.Thisindicatesthereducedmemorywindowastheerasevoltageisincreased.Reproducedfromref.[74].

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Figure13.BasicMRAMcellstructure.

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Figure14.BasicSTT-RAMcellstructure.

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Figure15.BasicstructureofaFeRAMcell.Thecrystalstructureofaferroelectricandanelectricpolarization-electricfieldhysteresiscurvearealsoshown.

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Figure16.BasicPCRAMcellstructure.ReproducedfromIBM-Macronix-Qimonda.

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Table2.SummaryofprimarycontendersforMRAM,FeRAM,STT-RAM,andPCMtechnologies

Features FeRAM MRAM STT-RAM PCM

Cellsize(F2)Large,approximately40to20

Large,approximately25

Small,approximately6to20

Small,approximately8

Storagemechanism

Permanentpolarizationofaferroelectricmaterial(PZTorSBT)

PermanentmagnetizationofaferromagneticmaterialinaMTJ

Spin-polarizedcurrentappliestorqueonthemagneticmoment

Amorphous/polycrystalphasesofchalcogenidealloy

Readtime(ns) 20to80 3to20 2to20 20to50

Write/erasetime(ns) 50/50 3to20 2to20 20/30

Endurance 1012 >1015 >1016 1012

Writepower Mid Midtohigh Low Low

Nonvolatility Yes Yes Yes Yes

Maturity Limitedproduction Testchips Testchips Testchips

Applications Lowdensity Lowdensity Highdensity Highdensity

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Figure17.BasicRRAMcellstructure.Aschematicdiagramofthemechanismoftheresistiveswitchinginametal/oxide/metal-structuredmemorycellisalsoshown.Reproducedfromref.[123].

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Figure18.Structureofapolymermemorydevice.

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Figure19.RacetrackmemorydiagramshowinganarrayofU-shapedmagneticnanowires.Thenanowiresarearrangedverticallyliketreesinaforestandapairoftinydevicesthatreadandwritethedata.AdoptedfromIBM.

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Figure20.Cellstructureofamolecularmemorydevice.

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Figure21.AMNWmemorycellstructure.

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Figure22.Abottom-gateFET-basednonvolatileSNWmemorydevice.

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Figure23.NRAMstructurewith(a)OFFstateand(b)ONstate.

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Figure24.Schematiclayoutofthemillipedecantilever/tipincontactwiththedatastoragemedium.Adoptedfromref.[157].

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Figure25.SchematicdesignofamemorydeviceconsistingofathinDNAbiopolymerfilmsandwichedbetweenelectrodes.Thememoryswitchingeffectisactivateduponlightirradiation.Adoptedfromref.[161].

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Figure26.Structureofquantumdotmemory.Adoptedfromref.[168].

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Figure27.Thebasicdesignofa3-Dcellthatconsistsofaverticaldiodeinseries.(top)Sideview,(bottomright)topview,and(bottomleft)3-Dview.

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Figure28.AschematicdesignofFT-RRAMandaflexible,transparentmemorychipimagecreatedbyresearchersatRiceUniversity.ReproducedfromTourLab/RiceUniversity.

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Figure29.Thebasiccellstructureof1T1R-RRAM.

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