Scaling Projections for Resistive Switching Memories Rainer Waser JARA-FIT @ FZJ Forschungszentrum Jülich & RWTH Aachen University Outline Introduction - Features and Classification of Resistive RAM Electronic switching effects - including FTJ (brief) Fuse-Antifuse switching effect (brief) Ionic switching effects - cation migration redox systems (electrochem. metallization cells) Ionic switching effects – anion migration redox systems Evolution and limts of memory (Geometry, c-based vs r- based) General aspects of RRam, Classification Cation based .. Anion based .. Passive arrays (diodes / Flocke, reference cells) Center of Nanoelectronic Systems for Information Technology Forschungszentrum Jülich

Introduction - Features and Classification of Resistive RAM 1 Introduction - Features and Classification of Resistive RAM - 2

Evolution of the memory density – geometry aspects 2. Beispiel. Erinnern wir uns an die Entwicklung der Speicher mit wahlfreiem Zugriff. Schon die ersten Magnetkernspeicher waren matrix-organisiert – eine Matrix aus Spalten und Zeilen mit den binären Informationsspeichern an den Kreuzungspunkten. Hier ein Speicher mit einer Kapazität von 1 kBit aus dem Jahr 1965 – 1 kBit reicht gerade einmal um 2 Zeilen Text zu speichern. 1970 kam der DRAM, der Speicher auf Si-Chips und er war bis 1985 auf ein 1 Mbit Kapazität gewachsen. Ermöglicht wurde dies durch die Entwicklung der Lithographietechnology, die es erlaubte kleinste Strukturgrößen, minimum feature sizes, von 1 Mikrometer Größe zu fertigen. Heute werden die ersten DRAM-Speicher mit einer Kapazität von 1 Gbit gefertigt – 50 000 Textseiten Kapazität, mehr als der gesamte Textteil der Encyclopedia Britannica. Wie weit wird die Entwicklung gehen? Rein geometrisch sollte es möglich sein 1 Tbit auf einem cm^2 unterzubringen – bei einer Feature Size von nur noch 6 nm. Kann man dies noch realisieren – oder gibt es vorher physikalische Grenzen? R. Waser (ed.), Nanoelectronics and Information Technology, 2nd ed. Wiley, 2005 - 3

Random Access Memories Phase Change Electrochem. Metallization Redox-based Oxides Molecular Switches Random Access Memories Challenges Fundamentals: Physics & Chemistry Resistive Ferroelectric  Superparaelectric Limit Interface & Scaling Effects DRAM Limit: Dielectric Area Mega-Bit Era Giga-Bit Era Tera-Bit Era - 4

Basic Definitions of Resistive RAM Operation Electrical switching between ON(LRS) and OFF(HRS) state „write“-Operation by large voltage pulses (typically with current compliance) „read“ operation by small (sensing) voltage pulses Polarity modes of RRAM Unipolar (symmetrical) - URS Bipolar (antisymmetrical) - BRS I V Read RESET SET I RESET CC V CC SET CC Read Oder: - Device Integration Speicher, die ihre Information nicht verlieren, wenn sie die Betriebsspannung ausgeschaltet wird - der Traum von der Festplatte auf dem Chip – mit einer Zugriffsgeschwindigkeit heutiger RAMs. Memory cell Two-terminal element between electrodes Active Matrix: 6...10 F2 , 1 cell / 1 T Passive Matrix: array size e.g. 8 x 8, approx 4 F2 History many reports since the 1960s mainly binary oxides, mainly unipolar switching - 5

Requirement of bipolar switching asymmetry of the system (e.g. different electrode materials) Forming process electrical stress as a precondition for switching sometimes: formation during first cycle(s) Location of the switching event - In the electrode area homogeneously distributed effect? effect confined to filaments? Location of the switching event - Between the electrodes along the entire path / in the middle area? at one of the interfaces? - 6

Classification of the switching mechanisms Resistive Switching Thermal effect Electronic effect Ionic effect Phase Change Effect (well known) Charge trap – Coulomb Barrier Cation migration - electrometallization Fuse-Antifuse effect Charge injection – IMT transition effect Anion migration - redox effect Ferroelectric Tunneling barrier R. Waser and M. Aono, Nature Materials, 2007 - 7

Electronic Switching Effects - 3 Electronic Switching Effects - including ferroelectric tunnel junctions - 8

Electronic Switching Mechanisms General Effect Charge injection or movement of displacement charges  modification of the electrostatic barrier, bipolar switching Variants Charge trap effects / Coulomb repulsion Trapping at an interface – e. g. Taguchi Sensor effect or at internal traps (compare: Flash) – e.g. polymer - embedded metal nanoclusters Insulator-Metal Transition effects Charge doping  change of band structure Ferroelectric tunneling barrier effects tunneling parallel / antiparallel FE polarization  modification of the tunnel barrier (thickness or height) Geometry Homogeneous current density  scaling limits (?) - 9

Ferroelectric Tunnel Junction Basic Effect Different Tunneling currents parallel and antiparallel to the ferroelectric polarization (no lateral confinement required) References: Esaki (1968), Tsymbal, Kohlstedt et al, Science (2006)  No reliable experimental evidence yet ! Results

Projections – Ferroelectric Tunnel Junction Memory State-of-knowledge No experimental verification yet; competition by redox-based processes. Scaling limits worse than MTJ, because the ferroelectricity fades below approx. 2 nm thickness (ab-initio theory & HRTEM experiment) Speed no inherent speed limits because polarization reversal in ferroelectrics < 1ns Energy dissipation Recharging of a tiny FE capacitor (much smaller than in FeRAM) low energy switching expected Challenges 1. realization of a pronounced effect unlikely 2. expected scaling limits not favorable

Fuse-Antifuse Switching Effect 2 Fuse-Antifuse Switching Effect - 12

Fuse-Antifuse Switching Mechanism Materials MIM thin film stack with I = transition metal oxide showing a slight conductivity e. g. Pt/NiO/Pt SET process Controlled dielectric breakdown e. g. by thermal runaway  formation of a conducting filament RESET process Thermal dissolution of the filament (fuse blow)  disconnected filament I. G. Baek et al. (Samsung Electronics), IEDM 2004 - 13

Temperature profile - Thermal effect assisting other switching types? FEM simulation (Ansys ®) of metallic TiO filament (3 nm) in TiO2 matrix Pt TE Pt BE TiO2 27 nm 54 nm 2 nm Pt TE Pt BE TiO2 27 nm 54 nm 2 nm 5 nm 1 filament 3 filaments 390 K 1100 K Relationship to other switching effects Toggle between bipolar and unipolar switching has been possible by adjusting the current compliance; demonstrated for TiO2 thin films (Jeong et al. 2006) and Cu:TCNQ (Kever et al. 2006) High current compliance  unipolar fuse/antifuse switching - 14

FEM Simulation of the RESET process 160 nm thick NiO film on n-Si with Au top electrodes U. Russo et al., IEDM 2007 - 15

Transition from the BRS to the URS mode Pt/TiO2(27nm)/Pt stack, sputter deposited, electroformated at 1mA for BRS operation Current compliance Higher current compliance (~ 3 mA) leads to transition of the switching mode (BRS  URS) This transition to URS is irreversible: (URS  BRS) LRS of a stable URS Details: see talk H. Schroeder & D.-S. Jeong (F6) and Jeong, Schroeder, Waser, APL89, 082909 (2006)

Projections – Fuse-antifuse effect State-of-knowledge - thermo-chemical effect; - filamentary nature confirmed; - nature of ON-state filament still unknown (Ni? NiOx<1? else?) Scaling limits equal or larger than filaments in redox systems (perhaps larger because of the thermal nature of the effect) – 10 nm diameter ?? Speed approx. 10 ns (speculative value) Energy dissipation Relatively high, because of thermal nature; no scaling expected because of filamentary nature; material optimization will lead to reduction Challenges better understanding required in order to optimize material and processing

Ionic switching effects - Cation-migration induced redox systems 4 Ionic switching effects - Cation-migration induced redox systems - 18

Cation Migration Redox Systems Names: Electrochemical Metallization Cell (ECM); PMC; CBRAM Working principle ON-switching: Reduction @ cathode  Ag filament formation Ag+ + e‘  Ag or OFF-switching: Oxidation @ anode Ag  Ag+ + e‘ or M. Faraday (1834) Required ingredients Electrolyte: Ag+ ion conductor; Question: is the presence of Ag+ ions required? What concentration? Bei x Ladungsneutral, ‘ negativ, . positiv Type of cations: M/Mz+ in a moderate area of the electrochemical potential series high exchange current density (Butler-Volmer equation) Examples: Ag, Cu, Ni, (and few more) - 19

Electrochemical deposition/dissolution Faraday´s law Phase generation / dissolution rate Growth speed of a (cylindrical) nanofilament Example: Ag filament of 10nm diameter at I = 1 mA - 20

Working Principle of the ECM Cell Ag Ag Ag Ag Ag+ Pt Pt Pt Pt Oxidation of top electrode Ion migration Reduction at bottom electrode Electrodeposit formation Non-volatile conductive connection Dissolution of the conductive path under reverse bias

Prototypical Electrochemical Cell – Pt / H2O / Ag I-V: bipolar switching Time evolution during switching on: formation of a Ag dendrite tree at the Pt electrode Ag Ag Ag Pt Pt Pt X. Guo, C. Schindler, S. Menzel, R. Waser, APL (2007) - 22

Example: Ag+ electrochemical GeSex cells (M. Kozicki, 1997) Electrolyte: GeSex (or GeSx ) – dissolution of Ag+ by photoassisted or thermally assisted oxidation of adjacent Ag phase; high driving force in case of excess Se in GeSex (as polyanions). Structure: Ag2Se rich nanocrystals in amorphous GeSex matrix What is special?:  stable reservoir of Ag+ ions („buffer“)  high Ag+ mobility & activation energy (easy migration path in amorphous matrix?) Bei x Ladungsneutral, ‘ negativ, . positiv Ag 50nm GeSe on W GeSex+Ag2Se Pt or W - 23

Special case – the “Atomic Switch” Quantized conductance atomic switch – tunneling gap as an electrode Der genaue Mechanismus ist offen, ähnlich zu GeSe K. Terabe, M. Aono, et al., Nature (2005) - 24

Aono group: Qimonda: Cu/Cu2S X-bar Mbit demonstrator Quimonda material: Multilevel – mehrere „ Schalter “ - 25

Low-Write Currents in Ag+ electrochemical GeSex cells Ag/Ag-Ge-Se/Pt cells fabricated in vias of a Si3N4 layer. Via diameter 2.5 m by photolithogrpahy. I-V curves with current compliance set to 1 nA (C. Schindler, R. Waser, et al., NVMTS 07) Latest results (unpublished): 10 pA ! - 26

Prospect of Multi-Bit Data Storage 2.5 µm via 2.5 µm via The write current determines the ON resistance and erase current. By varying the write current over 5 orders of magnitude, the ON state is variable over the same range increasing the opportunity for multi-bit data storage. Im „2 Bit“ Betrieb geringe Leistungsaufnahme weiterer Vorteil: breiter Betriebsbereich bzgl der Schreibströme ermöglicht, Pfade unterschiedlicher Dicke zu schreiben: Multi-Bit Speicherung über 5 Größenordnungen von Schreibströmen im gezeigten Betriebsbereich Ierase < I write: weiterhin können thermische Prozesse ausgeschlossen werden C. Schindler et al., NVMTS 07

Cell Size Dependence Electrodeposition starts at the point of highest E-field. Tip of the growing metallic path is the most probable site for further electrodeposition. Pt Ag Although the cross sectional area varies by a factor of 400, the ON resistance does not vary by a factor of more than 5 between the smallest and the largest cell. Kleine Ströme – geringe Wärmeentwicklung – hohe Speicherdichte möglich: Skalierungspotential hoch, da unabhängig von Zellgröße wird gezeigt für alle Schreibströme von 1 nA bis 100 µA und Zellen, deren Querschnittsflächen um einen Faktor 400 variieren The ON state is cell size-independent. C. Schindler et al., NVMTS 07

Unconventional electrolyte: SiO2 Sputtered SiO2 thin film For comparison Ag-Ge-Se Von = 400 mV Voff = -100 mV -100 mV < Vread < 400 mV Iwrite = 25 µA Ierase = -12 µA Similar switching characteristics as observed in Ag-Ge-Se! C. Schindler, M. Kozicki, R.Waser, submitted

Modeling: MD approach to Browian Dynamics 40 nm S. Menzel et al., to be published N: #ions in simulated region Eplate: applied field Gamma: friction coefficient Delta v, x: gaussian distributed location, velocity No Yes Yes No

Projections – ECM effect State-of-knowledge - reasonably well understood; - filamentary nature confirmed; - some open questions with respect to nanoclusters in GeSe & OFF switching Scaling limits Very thin filaments conceivable (< 2 nm ??); i. e. min. F = 5 nm ?? Speed < 30 ns ? (perhaps even faster; compare to fuse-antifuse) Energy dissipation ON switching: electrochem. reduction of Ag+ within one filament (or fraction of) + migration energy (Faraday transfer number = 1 assumed) OFF switching: ?? Challenges improvement of the voltage – speed bargain;

Ionic switching effects - Anion-migration induced redox systems 5 Ionic switching effects - Anion-migration induced redox systems - 32

Examples of Resistive Switching in Oxides Redox-based resistive switching type Examples of Resistive Switching in Oxides Capacitor-like structure with ► Cr-doped SrZrO3 thin films ► (Ba,Sr)TiO3 thin films ► SrTiO3 Single crystals as resistive element 1 SrTiO3 (100) Pt SrRuO3 300 nm SrZrO3(0.2 %Cr) Characteristics after forming process: reversible switching between stable impedance states non-volatile non-destructive read-out multilevel memory BST gehört zur Materialklasse der Perovskite. Als kandidat für zukünftige nicht flüchtige Speicher …wurde von der IBM Gruppe (Beck) resistives Schalten gezeigt an (mit Übergangsmetallen) dotierten Perovskiten. Hier ist SrZrO3 als Isolatormaterial in einer einfachen Kondensatorstruktur mit SrRuO3 und Platin als Elektroden gezeigt. An der Kondensatorstruktur wurde diese gezeigt I(V) Kurve gemessen. Beginnend vom Ursprung mit steigender Spannung steigt die Kurve auf dem niedrigohmigen Ast und kommt auf einer hochohmige Bahn zurück. Im Ursprung kreuzt der hochohmige Ast den niedrigohmigen Ast. Informationen werden als „Widerstand“ durch Spannungs- oder Strom-pulse gespeichert. Diese Struktur gilt als ein möglicher Kandidat für zukünftige nicht-flüchtige Speicher Resistive Speicher sind geeignet für hohe Integrationsdichten, brauchen keine mindest Kapazität wie bei Kapazitiven Speicher und sind über 48 Monaten nichtflüchtig. Blom et al. Au/PbTiO3, PRL 94.10 Watanabe et al. Au PZT Kohlstedt et al. BaTiO3 Schmehl et al., Appl 82 3077 (2003) Von der IBM-Gruppe wurde auch „Multilevel switching“ and SZO gezeigt. Man kann also in so eine einfache Kondensatorstruktur (bei 4 Levels) 2 Bit (oder mehr) speichern. (Bei 77 K „multilevel memory“ im Zustand niedriger Impedanz, durch Länge und Amplitude des Schreibimpulses steuerbar.) Vorgeschlagenes Model: Stromtransport gesteuert durch Ladungstransfer-Prozesse über Störstellen. (Low temperatur favorable for systematic studies) .(possible candidate for future highly integrated nonvolatile memory devices.) A. Beck, J. G. Bednorz, Ch. Gerber, C. Rossel and D. Widmer, Appl. Phys. Lett. 77, 139 (2000). - 33

High temperature defect equilibria Example: SrTiO3:1at% La, T=1100°C Schottky equilibrium (for SrO lattice) [D] Redox equilibrium law of mass action These curves have been interpreted consistently in the 70ies already, based on the Schottky equilibrium established at very high temperatures and the oxygen exchange equilibrium which is alive down to 600 … 700 C. As a result,you see the evolution of the electron concentration with the plateau region and the change over to cation vacancy compensation. The redox process is controlled by the combination of these two equilibria. Preconditions local electroneutrality homogeneous material - 34

Electrical characteristics of dislocations: Preferential conductivity paths in SrTiO3 single crystals 8nA 4nA 2nA 0nm 25nm 8nA 7pA U=3.5V 5mm NC-AFM of etched (100) surface of strontium titanate (estimated density of dislocations of 4*109/cm2 ) c-AFM mapping of local conductivity of (100) surface of thermally reduced strontium titanate -> hot spots density of strong hot spots ~5*1010/cm2 density of weak hot spots ~5*1011/cm2 Trala lala K. Szot, 1999, 2004 - 35

Thermal preformation by reduction annealing: conductive Tip AFM Mapping – types of I-V Characteristics 10 SrTiO3 s.c. thermally reduced at 850 C, pO2 ~ 10-20 bar 1.0 Current (nA) 1000 0.1 I/V1 metallic 100 semicond. insulating 0.01 10 I/V2 (nA) 1 I/V2 0.1 1nm I ~ 1.2 nA I ~0.009nA 50nm I/V1 0.01 0.001 -0.5 0.0 0.5 Applied bias (V) K. Szot, W.Speier, G. Bihlmeyer, R. Waser, Nature Materials, 2006 - 36

„off“ „on“ a c b d 50nm Resistance (Ω) 1.41010Ω 106 1010 108 3.2106Ω 80 40 Distance (nm) 1012 non-metallic „off“ metallic 50nm 1000 b 50nm 0nm 3nm d n15 800 n7 n6 „on“ 600 n5 Current (nA) 400 n4 n3 200 n2 n1 1 2 3 4 50nm 5 Applied bias (V) K. Szot et al., Nature Materials, 2006 - 37

Edge dislocations in SrTiO3 crystal (stacking fault) SrO/SrO SrO TiO2 /TiO2 Jia et al PRL.(2006)

Extended defects after reduction A B A-B N VO 1022 cm-3 A-B Ti+2 Ti+3 Ti+4 Dies wurde indirekt bestimmt (Mott-Kriterium mit Annahmen eps=200, meff=10 für STO): der Ausbaus von nur wenig (10^14 cm^-3) O führt schon zu metall. Verhalten -> nur möglich, wenn hoch lokal angeordnet => Kanäle Formation of localized metallically conducting sub-oxides by electroreduction - 39

Redox Reactions at the Electrodes - Interconnected network of extended defects - Switching ON – oxygen vacancy accumulation near the surface; conduction through the Ti(4-x)+ sublattice - 40

Learning from lateral cells K. Szot, etc. Nature Materials, 2006 SrTiO3 X-ray absorption near edge spectroscopy (XANES); EXAFS Optical micrograph and CAFM (above) and Cr K-edge XANES (right) mapping M. Janousch et al. Adv. Mater. 19, 2232 (2007)

Switching of SrTiO3 (100), Potential distribution I(mA) Φ1 Φ2 Φ3 Φ4 Φ5 & Φ6 V(V) Switching of SrTiO3 (100), Potential distribution RT, p=10-8mbar Interface I Interface II Φ1 – Φ2 Φ2 – Φ3 Φ3 – Φ4 Φ4– Φ5 Φ5 – Φ6 Emax >104V/cm Emax ~30V/cm Emax ~20V/cm Φ6 Φ5 I-Source Generator Electrometer SrTiO3 crystal 3mm K. Szot, to be published

LC-AFM characterization of epi-STO (10nm) / SRO / STO K. Szot, R. Dittmann, R. Waser, rrl-pss, 2007 - 43

Write / erase patterning LC-AFM write/erase processes on epi-STO (10nm) / SRO / STO Write / erase patterning K. Szot et al., rrl-pss, 2007 - 44

Temperature dependence K. Szot, R. Dittmann, et al., rrl-pss, 2007 - 45

Formation and resistive switching of redox-active dislocations Current questions Switching model Formation and resistive switching of redox-active dislocations Fast transport tracks ? Electronic charge injection mode (Schottky emission, FN ?) Local redox equilibria Continuity equation Poisson equation Concentration profiles, field profiles, space charge profiles 3-D network of extended defects? Nanoscale effects ? Phase formation or frozen kinetics? N VO 1022 cm-3 A-B Ti+2 Ti+3 Ti+4 K. Szot, 2004 - 46

Switching model – former results Δx i = 1 i = n i = 2 T. Baiatu, K. H. Härdtl, R. Waser, J. Am. Cer. Soc. (1990)

Switching model – recent results D.-S. Jeong et al., to be published Pt/TiO2(27nm)/Pt stack, sputter deposited, electroformated at 1mA for BRS operation

Projections – redox-type memory effect State-of-knowledge - redox mechanism suggested during formation; - details of the switching mechanism need to be studied (thermal assisted?) - nature of ON-state filament: conducting (sub-)oxide? Scaling limits estimated filament diameter: 2 – 10 nm ? Speed < 30 ns ? (perhaps even faster; compare to fuse-antifuse); literature reports of values of approx. 10 ns Energy dissipation perhaps similar to ECM; possibly thermal assisted, i.e. additional dissipation Challenges Much better understanding required in order to optimize material and processing (and to make more precise projections)

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