1. Scaling Projections for Resistive Switching Memories
Rainer Waser
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

2. Introduction - Features and Classification of Resistive RAM1
Introduction - Features and
Classification of Resistive RAM
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3. 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 – 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
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4. 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
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5. 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: 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
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6. 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?
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7. 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
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8. Electronic Switching Effects - 3
Electronic Switching Effects -
including ferroelectric tunnel junctions
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9. 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 (?)
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10. 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

11. 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

12. Fuse-Antifuse Switching Effect2
Fuse-Antifuse Switching
Effect
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13. 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
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14. 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
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15. FEM Simulation of the RESET process
160 nm thick NiO film on n-Si with Au top electrodes
U. Russo et al., IEDM 2007
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16. 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, (2006)

17. 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

18. Ionic switching effects - Cation-migration induced redox systems4
Ionic switching effects -
Cation-migration induced redox systems
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19. Cation Migration Redox Systems
Names:
Electrochemical Metallization Cell (ECM); PMC; CBRAM
Working principle
ON-switching: cathode  Ag filament formation
Ag+ + e‘  Ag or
OFF-switching: 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)
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20. 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
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21. Working Principle of the ECM CellAg 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

22. 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)
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23. 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
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24. 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)
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25. Aono group: Qimonda: Cu/Cu2S X-bar Mbit demonstrator
Quimonda material: Multilevel – mehrere „ Schalter “
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26. 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 !
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27. 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

28. 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

29. 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

30. 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

31. 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;

32. Ionic switching effects - Anion-migration induced redox systems5
Ionic switching effects
- Anion-migration induced redox systems
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33. 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 (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).
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34. 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
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35. 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
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36. Thermal preformation by reduction annealing: conductive Tip AFM Mapping – types of I-V Characteristics 10
SrTiO3 s.c. thermally reduced at 850 C, pO2 ~ 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
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37. „off“ „on“ a c b d 50nm Resistance (Ω) 1.41010Ω 106 1010 108 3.2106Ω80 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
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38. Edge dislocations in SrTiO3 crystal (stacking fault)
SrO/SrO
SrO
TiO2 /TiO2
Jia et al PRL.(2006)

39. 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
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40. 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
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41. 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)

42. 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

43. LC-AFM characterization of epi-STO (10nm) / SRO / STO
K. Szot, R. Dittmann, R. Waser, rrl-pss, 2007
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44. 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

45. Temperature dependence
K. Szot, R. Dittmann, et al., rrl-pss, 2007
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46. 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
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47. Switching model – former resultsΔx
i = 1
i = n
i = 2
T. Baiatu, K. H. Härdtl, R. Waser, J. Am. Cer. Soc. (1990)

48. 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

49. 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)

50. Thank You!