Phase Change Memory: Device Physics, Reliability and Applications

342 Pages·2018·15.206 MB·English

by  Andrea Redaelli (eds.)

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
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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.