Prospects and Challenges of Redox-based RRAM...

The Microelectronics Training Center, IMEC v.z.w.www.imec.be/mtc

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MTC 2008 : Prospects and Challenges of Redox-based RRAM ConceptsIMEC© 2008

Rainer WaserPage 1

Rainer WaserJülich-Aachen Research Alliance JARA,

Section Fundamentals of Future Information TechnologyFZJ Jülich & RWTH Aachen University

Outline1 Introduction2 Electrochemical metallization effect

- cation migration redox systems3 Valence change switching effect

– anion migration redox systems4 Thermochemical switching effect5 Conclusion

1. 행사 기본 개요IMST Tutorial 2008

Prospects and Challenges of Redox-based RRAM Concepts

Resistive Switching Group * Kristof Szot

* Regine Dittmann* K. Shibuya (Humboldt fellow)

* Tobias Menke

Acknowledgement* S. Blügel, G. Bihlmeyer, K. Urban, C. L. Jia

(JARA-FIT & IFF, FZ Jülich)* Stan Williams et al. (HP labs)* Hasegawa (Aono group)* R. Bruchhaus, M. Kund (Qimonda)* M. Kozicki (ASU)

* HGF, DFG * bmbf, Qimonda, EU EMMA, Intel Corp.

Funding

* Christine Schindler (now Siemens)

* Ruth Münstermann

- 2

* Shen Wan

* Carsten Kügeler (head of the NAL)

* Herbert Schroeder


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- 3

• often an initial electrical formation (defined stress) needed• „write“-Operation by large voltage pulses

(typically with current compliance) • „read“ operation by small (sensing) voltage pulses

Operation Electrical switching between ON(LRS) and OFF(HRS) state

Polarity modes of RRAM

I

V

Read

RESET

RESET

SET

SET

I

V

Read

RESET

SET

Unipolar (symmetrical) - URS Bipolar (antisymmetrical) - BRS

Basic Definitions of Resistive RAM

History • many reports since the 1960s • mainly binary oxides, mainly unipolar switching• Stan Williams et al. (2008):

memristor / memristive devices according to L. Chua

CC

CC

CC


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Resistive SwitchingMemories

MolecularC

onfi-guration

Mem

ory

Electrostatic/ FE

Effects

Phase Change

Mem

ory

Thermochem

icalEffect

ValencyC

hangeEffect

Electrochemical

metallization

cell

Magnetoresistive

Mem

oryEffects

Ferroelectric/ M

F Tunneling

Emerging Memories: Classification of the mechanisms

Nanom

echanicalM

emory

Redox-basedswitching

phenomenain chalcogenides

Resistive SwitchingMemories

Electrostatic/ FE

Effects

Phase Change

Mem

ory

Thermochem

icalEffect

ValencyC

hangeEffect

Electrochemical

metallization

cell

Magnetoresistive

Mem

oryEffects

Ferroelectric/ M

F Tunneling

Emerging Memories: Classification of the mechanisms

Nanom

echanicalM

emory

MolecularC

onfi-guration

Mem

ory


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Confinement of the Switching Event

Important issue

• homogeneouslydistributed effect?

Location of the switching event - In the electrode area

• effect confined to filaments?

Waser & Aono, Nat.Mat. (2007) A. Sawa, Mat.Today (2008)

- 8


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Electrochemical Metallization Memory

OperationON-switching:Reduction @ cathode→ Ag filament formation

Ag+ + e‘ → Ag

OFF-switching:Oxidation @ anode

Ag → Ag+ + e‘ or

M. Faraday (1834)

Names: ElectrochemicalMetallization Memory (ECM); PMC; CBRAM

Electrolyte* amorphous GeSe2+x and GeS2+x* Disordered and amorphous

sulfides and oxides

Ag/GeSex

Qimonda group (2006)

Hirose & Hirose(1976)

+10 μA

-10 μA

Aono et al (2005)

-1 0 +1 V

Kozicki (1997)

SiO2 – an unconventional ECM electrolyte

States and ProcessesLooking at the elementary steps

C. Schindler et al. (2007)

Pt

Cu Cu

Cu

Pt

Pt

SiO2 SiO2

SiO2


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ECM operation – in search of the rate-limiting step

Anodic dissolution• electrochemical electron transfer

reaction (Butler-Volmer)• no overpotential expectedAg

Pt

Cation transport• drift /diffusion transport• high fields → drift dominant

SiO2 Cathodic deposition• electrochemical electron transfer

reaction (Butler-Volmer)• crystallization overpotential expected

Filament growth• one “winning” filament, because of

field confinement


Formation process

• significantly higher SET voltage• thickness dependence of SET voltage→ drift is rate-limiting during formation→ drift is not(!) rate-limiting during switching



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Kinetics of the SET process

• SET voltage Von depends exponentially on 1/ramp rate(over > 8 orders of magnitude!!)

• a threshold voltage exists

→ RRAM application:guarantees fast switching andlong retention times

→ Understandingprocess is cationic electrontransfer reaction limited!


for Cu / SiO2(15nm) / Ir cells


Kinetics of the cathodic reaction

1. possibility: Electron transfer Butler-Volmer-equatione-

⎥⎦

⎤⎢⎣

⎡⎟⎠⎞

⎜⎝⎛ −−−⎟

⎠⎞

⎜⎝⎛=

kTze

kTzeii ηαηα )1(expexp 0

0( )( , ) exp c

cN zei K Z N

kTα η+⎡ ⎤= ⎢ ⎥⎣ ⎦

2. possibility: nucleation overpotential

Z0: areal density of nucleation sites

Nc: number of atoms of a critical nucleus

C. Schindler et al. (submitted)


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( ) ( ) ( ) ( )+ Δ = + Δ + Δ + Δ2i i 1 2

Compute location G

i i i

x

x t t x t c t v t c t a t x

( )= ≠

+ Δ = − − ∑1,

compute forces on ions2 N

plate Coulombi

Cu Cuj i j

eE Fa t tm m

( )( )

( )

=

= = Δ

= = −

0 distributed uniformly

02

0

iG

i i

platei

Cu

x t

v t veE

a tm

iCompute velocity v

ion docked ?

dendrite complete ?Finish

= + Δt t t

= + Δt t t

drop new ion

No

No

Yes

Yeselectrode rct(Butler-Volmer)

40 nm

ECM – Molecular Dynamics Modeling

S. Menzel, R. Waser, to be published

Growth speed of a (cylindrical) nanofilament

A2

A 0

[cm/s] [A]Mh Ir zN eπ ρ

=&

Example: Ag filament of 10nm diameterat I = 1 μA

1.3m/sh→ &

Location of the RESET eventAfter SET metal filament (e.g. Ag)RESET where will the filament will open ?

Experimental / theoretical finding: at Ag filament / Ag electrode contact

General questionSituation after RESET – chemically symmetrical cell ! How can a symmetrical cell switch in a bipolar fashion?Possible answer Morphological effect (?)

Impact on long-term reliability (?)

RESETb) late ON-statec) Early OFF-StateV = 200mV(steps 10 mV)

X. Guo, C. Schindler,S. Menzel, R. Waser, APL (2007)

- 16

ECM operation – RESET issue Ag

Pt


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Capacitor-like structure with Cr-doped SrZrO3 thin films PCMO thin films (Ba,Sr)TiO3 thin films TiO2 thin films SrTiO3 Single crystalsas resistive element

A. Beck, J. G. Bednorz, Ch. Gerber, C. Rossel and D. Widmer, Appl. Phys. Lett. 77, 139 (2000).

SrZrO3(0.2 %Cr)

SrTiO3 (100)

Pt

SrRuO3

300 nm

Examples

after forming process: reversible bipolar switching between stableimpedance states

- 18

Bipolar resistive switching in transition metal oxides

Characteristics


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Basics of ionic defects and valencies of SrTiO3

Point defects in compensated SrTiO3

HT: annealing at a High Temperatureequilibrium (e.g. 1000 K)

LT: quenching (at each point) toa Low Temperature (e. g. 400 K)

(bar)-20 +10

(calculated)

- 20

Basics of ionic defects and valencies of SrTiO3

HT conductivity of epi-SrTiO3PLD grown thin films

C. Ohly et al., JAmCerSoc (2006)


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Extended defects in SrTiO3 & their electronic structure

Grain boundaries... and their impact on conductivity

R. Hagenbeck et al. (1999)


R. De Souza et al. (2003)

Low angle grain boundaries ...

... perl chains ofdislocations

Dislocations

K. Szot (2007)

... made visible by etch pits on the surface


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Edge dislocations in SrTiO3 crystal (stacking fault)

Jia et al PRL.(2006)

TiO2

TiO2

SrO/SrO

SrOTiO2SrO

SrO

TiO2 /TiO2

I/V1

I/V2

-0.5 0.0 0.5Applied bias (V)

0.01

0.001

100

10

1

1000

0.1

metallicsemicond.insulating

I/V1

I/V2

1.0

0.1

10

0.01

50nm1nm

I ~ 1.2 nA I ~0.009nA

Current (nA)

(nA)

Thermal preformation by reduction annealing:conductive Tip AFM Mapping – types of I-V Characteristics

SrTiO3 s.c. thermally reduced at 850 C, pO2 ~ 10-20 bar

K. Szot, W.Speier, G. Bihlmeyer, R. Waser, Nature Materials, 2006 - 24


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K. Szot (2008)

Dislocation exit ...... at a center of an etch pit

is highly conducting (!)3.6 nA

0.002 nA

70 nm

70 nm

• Surface chemically etched

• reduction anneal at 1000 K

• simultaneous AFM topography andLC-AFM current scan

Procedure

- 26

Emulating the formation in homogenous SrTiO3

Electrochemical concentration polarization... based on drift-diffusion in STO:Fe as a mixedionic-electronic solid electrolyte Pt/STO:Fe/Pt cell

... for low polarization voltages

O2 Vnx x

⎡ ⎤∂∂ ⎣ ⎦=∂ ∂

Analytical solutionfor t →∞


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Emulating the formation in homogenous SrTiO3

Electrochemical concentration polarization... based on drift-diffusion in STO:Fe as a mixedionic-electronic solid electrolyte Pt/STO:Fe/Pt cell

... for higher polarization voltages

Waser et al. (1990)

1-D numerical calculationassuming partially ion blockingelectrodes

Results:• extremely pronounced

concentration polarization• enhanced n-conductivity

near the cathode

• enhanced p-conductivitynear the anode

• extension of the regionsdepends on theblocking coefficients

OV

- 28

Emulating the formation in homogenous Pt/STO:Fe/Pt

Concentration polarization- made visible byelectrocolorationtrue color transmission picturesof a STO:0.3at Fe crystal slab

Time

Waser et al. (1991)


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Emulating the formation in homogenous Pt/STO:Fe/Pt

Concentration polarization- made visible byelectrocoloration.... and simulated increase inthe overall conductance

Time

Waser et al. (1991)

SrTiO3

Optical micrograph and CAFM (above)and AFM tapping mode (right)

Microscopic view on electrocoloration / formation

2000 nm

K.Szot, W.Speier,G.Bihlmayer & R.Waser (Nature Mat. 2006)


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I-Source

Iconst

Electrometer

t(h)0.5 1.0 1.5 2.00.00.0

1.0

2.0

3.0

4.0U(V)

ElectroreductionSrTiO3 (100)

T=450°C, Iconst,=1mAVacuum

300 600 900 T(K)100

200

R (Ω)

Electro-reduction

Thermal-reduction

a

b

50nm

50nm

0nm

3nm

50nm

50nm

0nm

3nm

Applied bias (V)0 1 2 3 4 5

n1

n2

n3

n4

n5

n6

n7800

600

400

200 0

n15

Cur

rent

(nA

)

1000

Res

ista

nce

(Ω)

1.4×1010Ω

106

1010

108

3.2×106Ω

800 40Distance (nm)

1012c

d

non-metallic

metallic

„on“

„off“

K. Szot et al., Nature Materials, 2006

Tip-induced switching of dislocations in SrTiO3


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LC-AFM write/erase processes onepi-STO (10nm) / SRO / STO

R. Dittmann, K. Szot, R. Waser rrl-pss, 2007

- 33

Switching of epitaxial STO (10nm) thin films

Formation of localized metallically conducting sub-oxides by electroreduction

N V

O

1022 cm-3

A-B

Ti+2

Ti+3

Ti+4

Ti+2

Ti+3

Ti+4

A B

Extended defects after electroreduction

- 34

Redox reaction at the electrode


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Redox reaction at the electrode - Interconnected network of extended defects- Switching ON – oxygen vacancy accumulation near the surface;

conduction through the Ti(4-x)+ sublattice

K. Szot et al., Nature Materials, 2006

I(mA) I(mA) I(mA) I(mA) I(mA)

Φ1 Φ2 Φ3 Φ4 Φ5 & Φ6

V(V

)

Switching of SrTiO3 (100), Potential distribution RT, p=10-8mbar

Interface I Interface II

I(mA) I(mA) I(mA) I(mA) I(mA)

V(V

)

Φ1 – Φ2 Φ2 – Φ3 Φ3 – Φ4 Φ4– Φ5 Φ5 – Φ6

Emax >104V/cm Emax ~30V/cm Emax ~20V/cm Emax ~20V/cm Emax >104V/cm

Φ1 Φ2 Φ3 Φ4 Φ6Φ5I-Source

Generator

Electrometer

SrTiO3 crystal

3mmK. Szot, to bepublished


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ohmiccontact

Schottkycontact

Nb-STO substrate

epi-STO (750nm)

Pt top electrode

Electroded epi-SrTiO3 thin film

System (example)

FormingFormation of a virtual cathode

Impedance spectroscopy

Menke et al. (2008)

0 2.5e7 5.0e7 7.5e7 1.0e8

-1.0e8

-7.5e7

-5.0e7

-2.5e7

0

Z'Z'

'

leads bottom R_bulk

C_bulk

R_interface

C_interface

unformed

formed

leads bottom R_bulk

C_bulk

Switching• Reversible switch between two

formed states (ON / OFF)

• Contact point of conductive filamentis modified by voltage-driven accumulation / dispersion of

Redox-based Oxide Memory – Write time < 10 ns

Pt/TiO2(40nm)/Pt cells preparedby e-beam lithographyand lift-off technique

500 nm

Nauenheim, Kügeler, Waser,(submitted)

64 x 64 bit Crossbar array


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Switching model –recent results

D.-S. Jeong, Thesis (2008)

Pt/TiO2(27nm)/Pt stack, sputter deposited, electroformed at 1mA for BRS operation

HP Memristor –asymmetry by graded system

D. Strukov, J. Borghetti, R. S. Williams, Small (2008. accepted)

J. Yang, D. Pickett, X. Li, Ohlberg, D. Stewart, R. S. Williams, Nature Nanotech. (2008)


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- 41

Materials

Thermochemical (Fuse-Antifuse) Switching Mechanism

MIM thin film stack with I = transitionmetal oxide showing a slightconductivity

e. g. Pt/NiO/PtSET process

Controlled dielectric breakdowne. g. by thermal runaway

formation of a conductingfilament

RESET processThermal dissolution of the filament(fuse blow)

disconnected filament I. G. Baek et al. (Samsung Electronics), IEDM 2004

- 42


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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

54 nm

2 nm5 nm

Pt TE

Pt BE

TiO2

27 nm

54 nm

54 nm

2 nm

1 filament 3 filaments

390 K 1100 K

Toggle between bipolar and unipolar switching has been possibleby 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

Relationship to other switching effects

- 43

FEM Simulation of the RESET process

160 nm thick NiO film on n-Si with Au top electrodes

U. Russo et al., IEDM 2007- 44


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• Functions beyond pure memory

Prospects

... from FPGA type logic to neural functions

• Ultimately high scaling potential.... of both, cation- and anion-migration memristive concepts

• Technologically compatible to CMOS interface

Challenges

• Highly scaled interconnect lines... and reliable electrode contacts

• Long-term reliability... in particular with respect to retention (at 85 C)

• Design rules not yet known... to guide search in the material´s „treasure map“

• Defect engineering... just at ist very beginning


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Sr2TiO4(001)

TiO2-termianted SrTiO3 (001)

[ ]0013

42TiOSra

2D dislocation effects on resistive switching

Sr2TiO4 / SrTiO3

APL83, 2315

Vicinal surface: dislocation density controlled by a miscutangle of substrates.

Bottom electrode

Top electrode

47

ex.: Bi4Ti3O12/SrTiO3

Aim: tailoring structural filaments

Anti‐phase boundaries (APBs) as dislocation centers working as conduction filaments ??

Redox-based nanoscale memristors – defect engineering?

..inspired by ...


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Bipolar resistive switching in transition metal oxides

-4,0

-3,5

-3,0

-2,5

-2,0

-1,5

-1,0

-0,5

0,0

0E+0 1E-6 2E-6 3E-6 4E-6 5E-6xg [cm]

EC, E

D , E

Imre

f , EA

, EV [e

V] Ec(k) [eV]

Ed(k) [eV]

Imref new (c2) [eV]

Ea(k) [eV]

Ev(k) [eV]

Purely electronic charge trap mechanism?

ϕ(Pt) = 5.35eV; ϕoffset (0V)=1.35eV2 Dead layers of 2nm each; KDL=22;Acceptors VSr: NA=1017cm-3; Donors VO: ND=1017cm-3;

Pt/STO(50nm)/Pt, V =1V, 300 K

General resultNo trap site suitable to match retention timeSimmon Verderber model fails

H. Schroeder, S. Schmitz APL (2003)

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Prospects and Challenges of Redox-based RRAM...

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