Phase Change Memory: Device Physics, Reliability and Applications PDF
Preview Phase Change Memory: Device Physics, Reliability and Applications
Andrea Redaelli Editor Phase Change Memory Device Physics, Reliability and Applications Phase Change Memory Andrea Redaelli Editor Phase Change Memory Device Physics, Reliability and Applications Editor AndreaRedaelli MicronSemiconductorItaliaS.r.l. Vimercate,Italy ISBN978-3-319-69052-0 ISBN978-3-319-69053-7 (eBook) https://doi.org/10.1007/978-3-319-69053-7 LibraryofCongressControlNumber:2017959302 ©SpringerInternationalPublishingAG2018 Thisworkissubjecttocopyright.AllrightsarereservedbythePublisher,whetherthewholeorpartof the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilarmethodologynowknownorhereafterdeveloped. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publicationdoesnotimply,evenintheabsenceofaspecificstatement,thatsuchnamesareexempt fromtherelevantprotectivelawsandregulationsandthereforefreeforgeneraluse. Thepublisher,theauthorsandtheeditorsaresafetoassumethattheadviceandinformationinthis book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, express or implied, with respect to the material contained hereinor for anyerrors oromissionsthat may havebeenmade. Thepublisher remainsneutralwith regardtojurisdictionalclaimsinpublishedmapsandinstitutionalaffiliations. Printedonacid-freepaper ThisSpringerimprintispublishedbySpringerNature TheregisteredcompanyisSpringerInternationalPublishingAG Theregisteredcompanyaddressis:Gewerbestrasse11,6330Cham,Switzerland To my family, my friends, my colleagues, and to anyone that spends his life looking for progress of mankind... “...fatti non foste a viver come bruti, ma per seguir virtute e canoscenza.” Dante Alighieri Foreword Few scientists and engineers would have thought that it would have taken about 50 years to turn the ideas which S.R. Ovshinsky described in 1968 into a viable technology. In his seminal paper “Reversible electrical switching phenomena in disorderedstructures,”someofthefoundationsofanemergingmemorytechnology werecreated.Yet,asthepresentlookdescribesinpleasantdepthanddetail,itwas still a long journey before a mature technology base was developed. The mecha- nismexploitedinphasechangememoriesisthechangeofelectricalresistanceupon thetransitionfromtheamorphoustothecrystallinestate.Sucharesistivitychange uponcrystallizationisobservedinalmostallsolids.Nevertheless,therequirements the phase change memories have to meet are significantly more demanding. This canbeeasilyseenifjustafewoftheserequirementsarelisted.Foragoodmemory, weexpectthestoredinformationtobesecureforlongperiodsoftime,i.e.,ideally more than 10 years for a nonvolatile memory. This implies that the amorphous state,whichislessstablethanitscrystallinecounterpart,isstableatroomtemper- atureandslightlyaboveforthese10years.Yet,wealsowanttoeraseamorphous bits,ifneeded,withinafewtenstohundredsofnanosecondsuponapplyingshort heating pulses. Hence, upon heating the amorphous state should crystallize on a nanosecond time scale. Ultimate phase control is hence a prerequisite for a suc- cessful memory technology based on phase change materials. However, the requirements forphase changematerials gobeyond this.Apronounced resistance contrastisneeded,too.AsdiscussedinChap.2ofthebook,thisisenabledbythe unique transport properties which govern phase change materials. To realize the unusual crystallization kinetics, which were mentioned above, the concept of fragility is introduced and discussed in Chap. 3. Phase change materials are bad glass formers and show an unusual temperature dependence of their viscosity. Fortunately,inrecentyearstheunderstandingofmaterialpropertieshasadvanced considerably, so that even the behavior of memory cells can be simulated self- consistently, as discussed in Chap. 4, enabling a quantitative description of key features.Thishasalsohelpedstudiesofmemoryreliabilitywhichfocusondataloss by undesired crystallization and ways to avoid it. The two subsequent chapters vii viii Foreword describe material characterization and material engineering for device optimiza- tion. For a memory technology to be successful, scaling is a major challenge, discussedindetailinChap.8.Asecondmajorcharacteristicispowerconsumption, whichiscloselyrelatedtothedevicearchitectureemployed.Finally,thearchitec- tureofentirePCMarraysispresentedandtheimpactofselectorsisdiscussed.The bookcloseswithdiscussionofapplicationopportunitiesandperformanceimprove- mentsenabledbyPCMs.Hence,thebookisideallysuitedforengineerswhoplanto utilize the potential of PCMs. Yet it is also an interesting testimony to the many challenges that had to be overcome in realizing a memory technology based on a highlyunconventionalclassofsolids. RWTHUniversity,Aachen,Germany MatthiasWuttig Preface In the last decades, the use of electronic systems expanded in many areas of the modernsociety:networking,mobiledevices,personalcomputers,cloudstorage,as well as social media are part of everyone’s life, becoming strong drivers for the semiconductormarketincrease.Amongthesemiconductorcomponents,neededto realize the electronic systems, memories are becoming key elements being today oneofthemaincontributorsindefiningtheoverallcostandsystemperformance.In mobile applications, for example, the smartphones and tablets, the memory capa- bility is often used as a key feature to justify the system cost to the costumer. Furthermore,theotherkeyfeaturethatthecostumerconsidersisthedurationofthe battery that is related to the system power consumption being again strongly impacted by the memory chips. In computer applications, the transition from magnetic hard drive to NAND-based solid-state disk boosted significantly the systemperformancebothintermsofpowerconsumptionandspeed.Fordatacenter server applications (the reader can think, e.g., to search engines and social media servers),theaccessoflargeamountofinformationrequireshighmemorydensities operated at very high speed. Today this is done mainly through volatile DRAM chips that need continuous refresh, thus adding very high power consumption on topoftheonerelatedtotheCPUs. Up to now, memory demand has been satisfied by the optimization and miniaturization of standard memory technologies, i.e., DRAM and NAND flash. However, with the increasing demand of enhanced cell performance and thereductionofthecelldimensionmuchmoredifficultthaninthepast,theusual scaling approach is becoming insufficient. On the one hand, the scaling of DRAM is more and more critical, thus translating in a cost increase; on the other hand, NAND flash technology performance is often poor especially when the application needs a fast communication between the CPU and the memory element. In this framework, new alternative memory technologies have the potential to bring faster data storage and higher read throughput than NAND whilestillmeetingthenonvolatilityrequirementsandprovidingasignificantcost advantageoverDRAM. ix x Preface Furthermore,followingthesamesuccessfulpaththatwealreadyobservedwith the introduction of NAND solid-state disks in the computer memory hierarchy, where the nonvolatility and a fast memory access with respect to magnetic disk translatedtoasignificantboostingofsystemlevelperformances,inthesameway thisnewmemoryelement(oftenreferredintheliteratureasstorageclassmemory, SCM)canboosttheoverallsystemperformances.Despiteoriginallyconsideredas a flash memory replacement, today the phase change memory (PCM) has already aroseastheidealcandidatetobeusedasSCMfeaturingtherequiredintermediate performances, thus being able to fill the gap between NAND and DRAM in the system. Despite some books already exist that discuss phase change material and their interesting properties, no books have been published focusing on PCM and its devicephysics.Theaimofthisbookisthustoreviewandsummarizethelearning thathasbeendevelopedonPCMfromadeviceperspective. In the first chapter, a short introduction of PCM is reported. Starting from a historicalreview,themanufacturedmemorychipswithdifferentarchitecturesand densities have been reviewed through the years. The basic working principle and the device physics are then discussed pointing out the peculiar nature of this innovative device based on a phase transformation between a crystalline phase andanamorphousone.Thetransitionisthermalinnatureandnotconventionalfor thestandardoperationofmicroelectronicdevices.Inthelastpartofthechapter,the cell performances are compared to the ones of standard memories highlighting advantagesanddisadvantages. The second chapter summarizes the fundamental electrical properties of PCM devices in both the amorphous and crystalline states. First, the band structures of crystallineandamorphousphasechangematerialsarestudiedbasedontheanalysis ofthinfilmsofactivematerials.Then,thedevicecharacteristicsinaPCMdevice including conduction and threshold switching phenomena are shown. Finally, the effectsofnonuniformresistivity,thetransientphenomenaafterswitching,suchas recoveryanddrift,willbeillustrated. Thethirdchapterisdevotedinthefirstparttotheinsightsintotherelevantcell thermalparameters.Inthesecondpart,thechapterisfocusedonthediscussionof advanced properties involving the phase change materials in terms of fragility behavior and ionic motion effects. The comprehension of all those pieces is of great importance for a successful implementation of PCM through proper engi- neering of heat generation and thermal resistances as a very important step for powerconsumptionoptimizationandprogramlatency. In the development of a new technology, the availability of a tool able to simulate the cell behavior is key for proper device engineering as extensively discussed in Chap. 4. In fact, the main physical ingredients at the basis of PCM operation are uncommon for conventional electronics, and the available commer- cialtoolscannotbethusemployed.Inthischapter,anumericalmodelisdescribed that couples the conduction properties of crystalline and amorphous Ge Sb Te 2 2 5 with a local nucleation and growth algorithm to account for the phase transition dynamics.Theproposedmodelcansimulatethree-dimensionalPCMdevices,and Preface xi itiscapableofquantitativelyreproducingthekeyfeaturesofchalcogenidephysics whenintegratedinanelectronicmemorycell. Chapter 5 gives a review of the reliability mechanisms that are key metrics to enable commercialization of PCM. For PCM the primary risk of data loss is undesiredcrystallizationoftheamorphousmaterialphase.Thisprocessisstochas- tic in nature and is accelerated by both temperature and electrical bias. Write endurance in PCM is dependent upon both the chalcogenide and the surrounding electrodematerialsandmanufacturingscheme.Duetothepotentialofthematerials tointeractatthehighprogrammingtemperature,thesematerialsmustbecarefully chosen and integrated to enable a highly reliable cell. With proper device and manufacturing design, the ability to achieve these requirements has been demon- strated in commercial memory products. In addition, as PCM data storage is not basedontrappedcharge(asintheprevailingfloatinggatememorydevices),itcan supportapplicationswithradiation-tolerantrequirements. Performances of PCM are intimately linked to the microscopic properties of the phase change materials. The outstanding properties of chalcogenides are reviewed in Chap. 6. The structural and physical properties of the amorphous andcrystallinestatesareillustratedbyfocusingontwoprototypicalphasechange alloys (Ge Sb Te and GeTe). The origin of the electrical contrast between the 2 2 5 amorphousandcrystallinestatesofphasechangematerialsandthenatureofthe crystallizationmechanismsarediscussedatmicroscopiclevel.Thedopanteffects onthestructureoftheamorphousphaseandthewaytheyenhanceitsstabilityare discussed in detail. Furthermore, the link between the resistance drift of the amorphous state and the structural changes occurring during its ageing is also analyzed. InChap.7,chalcogenidematerialoptimizationguidelinesforcellperformance enhancement are provided, with focus on automotive applications where higher requirement in terms of thermal stability is usually required. Because of some conflicting device performances, for example, the thermal stability of the programmed states and the programming speed, the phase change material needs to be properly engineered. Stoichiometric compounds like Ge Sb Te and GeTe 2 2 5 offershortprogrammingtimes(intheorderofafewtensofnanoseconds),butthe thermalstabilityoftheprogrammedstatesislimited(about10yearsat100(cid:1)C).The additionoflightelementsintothesestoichiometricalloys,forexample,nitrogenor carbon,allowsdecreasingtheprogrammingcurrentandextendingthetemperature range in which the programmed states are stable, thus achieving the specification required for automotive applications (at least 10 years at 150 (cid:1)C). The use of nonstoichiometric Ge-Sb-Te alloys enables to further extend the performance of the PCM devices. On the onehand, germanium-rich alloys exhibita high thermal stability of the programmed states. On the other hand, antimony-rich alloys can ensureaveryhighprogrammingspeedsuitableforstorageclassmemoryapplica- tions.Meanwhile,othermaterialssystems,such asGa-Ge-Sb,have recently dem- onstratedbothahighthermalstabilityandahighspeed,attheexpensehoweverofa highprogrammingcurrent.
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