fundamentals of oxide resistive random access memory (rram)

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Fundamentals of

Oxide Resistive Random Access Memory

(RRAM)

David Gilmer

*Nano & Giga Challenges Forum 2017

Memory Technology Trends: New Drivers

26% CAGR

37% CAGR

Future may have different needs:

1. Mobile: smaller, lower power, working memory

than SRAM & lower power storage than NAND

2. Data Center: faster storage than HDD

3. SOC: smaller, lower power working memory

than SRAM & embedded memory

2

3

[1] W. Y. Choi, and T.-J. King Liu IEDM p. 603 (2007).

[2] A. Driskill-Smith, Y. Huai Future Fab p. 28 (2007).

[3] J. E. Green Nature v. 445 p. 414 (2007).

[4] B. Yu IEEE Trans on Nanotech 7, p. 496 (2008).

[5] Kryder, et. al. IEEE TRANS ON MAGNETICS, 45, NO. 10, (2009)

• MLC like NAND

• Cost structure like NAND

• Function, reliability like NAND

• Density > NAND

• Speed > NAND.

Benchmarking Sub 2Xnm Memory Candidates

Goals:

BL1

BL2

BL3

WL1 WL2 WL3

1F Unit cell = 4F2

F= minimum litho CD

A=multiplier of min CD (cell size)

4

NV memory options

MRAM, STT-RAM RRAM

[high density, low power,

“immune” to scaling issues]

Flash (NAND)

Increasingly difficult scaling

Alternative

Storage

Mechanisms

…Changing e- Spins …To Moving Atoms:

Incumbents :

From: Trapping Electrons…To: Moving Atoms

Micron Technologies

http://blogs.zdnet.com/

Aug. 2009

Toshiba T. Kamigaichi,

EDM 2008.

SAMSUNG

2009

25nm

34nm Floating gate

Trap electrons…

Incumbents

*Hard to maintain charge

When aggressively scaled

WL

BL

O

*Hard to maintain polarization

When aggressively scaled

http://blogs.zdnet.com/

What is Resistive Random Access Memory?

(RRAM)

• For any memory system to work, it needs at least

two different detectable states

• RRAM detectable memory states are “high” or

“low” resistance…. A resistance change memory

5

V

V

V

V

V

VV

V

V

V

V

V

V

VV

V

Electric

Current

Low

High

Resistance

+

-

Electric

Current

High

Low

Resistance

+

-

Why metal-oxide filament-based RRAM?

6

V

V

V

V

V

VV

V

V

V

V

V

V

VV

V

V

V

V

V

V

V

VV

V

= =

Challenge for filament-based RRAM

7

*Filament forming is a random processControl conductive filament

Uniformity

Key Understand operating mechanisms

8

Dielectric

Breakdown

Filament formation is key

to well behaved switching

Dielectric Fresh State:

starting defect profile

important

Features of filamental Bipolar RRAM

I-compliance: limit with

internal FET for low

parasitics, overshoot

I

On

(LRS)

Off (HRS)

9

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

-2 -1 0 1 2 3

Resistive switching requires

Metal-Oxide defects

Vg

I (A)

HfO2 HfO2

Ar

1.E-11

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

-2 -1 0 1 2 3

No Ion-Bombardment

Ar bombarded of HfO2

Oxygen gettering for sub-stoichiometry

*National Synchrotron Light Source (NSLS) at (BNL)

“Oxygen exchange layer (OEL) for

Generating O-deficiency in HfO2”

Evidence of HfO2 Reduction by OEL

10

12

Filament Mechanism: Grain boundaries are weak point;

likely location for filaments

• Different conduction mechanisms

along GB and Grains due to

accumulated defects at GB’s

over GB

over grains

C-AFM data

SiO2

Si

HfO2

C-AFM

*YEW, K. S. et al., Proceedings of the International Conference on Solid State

Devices and Materials (SSDM), Japan, 2009.

Switching at grain boundaries induced by AFM tip

amorphous HfO2 crystalline HfO2

Switching observed only on GBs oxygen deficient regions

Forming

13Lanza et al., Appl. Phys. Lett. 101, 193502 (2012)

14

• Bond breakage followed by oxygen

out-diffusion• Vacancy diffusion

Alternative description

IfIf

Metal-Oxide RRAM Material Model;

Form a Conductive Filament

15

kT

EEETG

oxact exp),(

Trap generation rate:

Heat dissipation increases deeper in the oxide

larger O-deficient region near the forming anode

Forming process and filament geometry

Cone-shaped filament

0.E+00

2.E+02

4.E+02

6.E+02

8.E+02

1.E+03

1.E+03

02468

power [W/m]

z[nm]

Higher power

dissipation

closer to anode

anode cathode

eCF

anode cathode

eCF eCF

e

ehw

hwO

defect generation

by Temp/Eox

*G. Bersuker et al, IEDM 2010; G. Bersuker et al, JAP 2011

(-)(+)

16

GB

O2-

O2-

O2-

TiN

HfO2

Ti

TiN

HfO2GB

O2-

O2-

O2-

O2-

TiN

O2-

O2-

O2-

TiN

Ti

Filament growth model • Initial: O-vacancies along grain

boundary

• Two-step process for each O atom

removal during filament growth:

1) Breakage of Hf-O bond

2) Diffusion of released O ion

Breakage of Hf-O bonds by thermo-

electrical stress:

O-ion diffusion by E-field assisted

substitutional hopping, :

G. Bersuker, ME 2001; J. McPherson, APL 2003

),,(

),,(exp),,( 0

zyxkT

zyxEEGzyxG

oxA

𝑫 = 𝝊 ∗ 𝒆−

𝑬𝑫−𝑸𝝀𝟐 𝑭𝒆𝒙𝒕

𝑲𝒃𝑻

Bersuker & Gilmer, CH.9; Metal oxide “RRAM” technology in; “Advances in Non-

Volatile Memory and Storage Technology” 2014, Pages 288–340

17

formingFilament reset mechanism

• Most voltage drops across the narrow filament tip

-1.0 -0.5 0.0

5.0x10-5

1.0x10-4

1.5x10-4

2.0x10-4

2.5x10-4

3.0x10-4

I [A

]

Vg [V]

ResetModelOhmic

*Need ~1nm

Tunnel barrier to fit

*Tunnel gap ~ 1nm

• Current deviates from Ohmic initially due to Resistance from Temp

• Reset occurs when temp sufficient for oxidation and oxygen is available

• Asymmetric structure and switching supports model

*G. Bersuker et al, IEDM 2010

_

+

ReSet process

*G. Bersuker et al, IEDM 2010; G. Bersuker et al, JAP 2011

Reset process

O(2-) hopping between interstitial

positions and Hf oxidation at

oxide/filament interface

18

TiN

O2-

O2-

TiN

O2-

O2-

O2-

O2-O2-

O2- O2-

O2-

O2-

O2-

O2-O2-

O2-

HfO2

O2-O2-

Eox

.

Radial T-profile

Re-oxidation of the filament tip

driven by O-ion Coulomb repulsion

and density gradient

18



Positive forming/set, negative reset

Reset has Polarity Dependency!!

FormingReset

Set

Asymmetric defect/vacancy profile “metal-OEL”/HfO2-x dielectrics ;

Vg [V]

I g[A

]

Oxygen Poor near the reset anode prevents barrier No reset

Oxygen Rich near the reset anode barrier forms Reset

Bias to TE

Barrier forms

*D. Gilmer, et al, IMW 2012

X

Negative forming/set, positive reset

Barrier does

NOT form

Bias to TE

Oxidation for Asymmetric Defect Profile

20

Relative O% profile DC switching showing 100% yield for a 10-cycle

set/rest dc sweep for 35 die (350 total sweeps)

[Inset; characteristic endurance]

200ns pulsed endurance to 108

• Operating the Asymmetric O-vacancy stack in preferred polarity = 100% yield

• Operating the Asymmetric O-vacancy stack in NON-preferred polarity = 0% yield

TE

BE

HfO2

HfOx


for various

asymmetric O-vacancy HfOx

Reset Anode against “Good” vs “Bad” dielectic

• Even non-preferred bias polarity has similar current deviation

near reaching the forming compliance during reset sweep….

However, the non-preferred biasing does not reset, it breaks

21*D. Gilmer, et al, IMW 2012

Asymmetric Defect Profile observed for

Best Behaved Bipolar RRAM

Increasing O-VACANCY

Gradient

Various RRAM stacks with O-vacancy asymmetry

• O-vacancy asymmetric stacks Preferred Reset polarity

with the “Good” dielectric against the Reset Anode (+)

• Opposites attract….O- (+); V00

(-); M+ (-)

Preferred

Reset Polarity!

+

-TE

TaOx

Ta2O5

Vo

VoVo

TiN

Vo

Vo

BE

TE

TiOx

TiO2

Vo

VoVo

TiNVo

BE

Ti

HfO2

Vo

Vo

Vo

BE

TE

HfOx

HfO2

Vo

Vo

Vo

Vo

Vo

Vo

TiN

BE

TE

Zr

HfO2

Vo

Vo

Vo

BE

TE

HfO2

TaOx

TaVo Vo

VoVo

Vo

VoVoVo

VoVo

BE

TE

22*D. Gilmer, et al, IMW 2012

Symmetrical Bias does not similarly break the

tunnel barrier Due to Asymmetric defect profile

Oscilliscope

monitored

80us pulsed

SetSet

ReSet

• Asymmetry and

Preferred

ReSet Bias *Slower to change,

‘fluctuating’

Competition between

breakdown and oxidation

23

and

pulsed Reset

• Asymmetry and

Preferred Set Bias

*Immediate and singular

resistance change

Barrier breakdown


RRAM operations: From grain boundaries to

conductive filament

Forming event

CF

TE w/ strong O-1

affinity

MeOx

Reset

e-

HRS

barrier

e-e-e-

LRS

e-

induced heating/field bond breakage

Initial leakage path along GB

Set

V

V

V

V

V

V

Model-based simulations reproduce the entire forming-switching phenomenon *G.Bersuker et al, JAP 2011

switching

Vacancy

Oxygen Ion

24

Mechanism of Forming transient: Run-away

Hf-O bond breakage process

xyz

xyzoxact

oxxyzkT

EEGETG

,

0 exp),(

Bond breakage rate:

*L. Vandelli et al, IRPS’12

Time-to-Forming: Weibull statistics

breakdown event

Breakage of next-neighbor

bonds

hw

OV+

25



5nm

Program Description (3D Map creation)

HfOx-Based RRAM fully ModeledSimulated unit cell

Electron transport via vacancies power dissipation T+E field

new vacancies generation + oxygen diffusion electron transport

increases power dissipation increases … new vacancy generation

+ oxygen diffusion ……filament formed

Main loop: numerically

solving Fourier heat-transfer,

charge continuity equations in

each unit volume

• Resistivity– local

values depending on

# of vacancies

• Potential

• Power dissipation

• Temp. / E-Field

3D Map Update

26Bersuker & Gilmer, CH.9; Metal oxide “RRAM” technology in; “Advances in Non-


Why oxygen ions stay near filament?



28

There is an optimum density of vacancies

out-diffused

only few generated

No vacancies

Low Density

Medium Density

High Density



many initial

vacancies not

enough oxygen

ions generated

Too few

vacancies

lack of oxygen ions

in the filament

vicinity

Insufficient reset in:

Stoichiometric hafnia: lack of oxygen ions due to recombination w/ vacancies

Highly substoichiometric hafnia: low number of generated oxygen ions

Both Stoichiometric and High vacancy density

films demonstrate poor reset



A

A’

A”

‘Moderate’

vacancy

density

results in best

switching



Simulated set/reset cycle



Summary

32

The goal; creating vacancy profiles advantageous for the filament oxidation during reset and barrier break in set

• Model identifies material properties and operation conditions controlling

RRAM performance (supported by O-vacancy Asymmetry Device data)

• Asymmetric stacks have preferred bipolar operation polarity

• O-vacancy Asymmetry Assists the Set-ReSet Mechanism

• Many techniques can introduce vacancies; ‘OEL’, Plasma, Reactive PVD, etc…

Asymmetric TiN\HfOx\Ti\TiN

40nmx40nm x-bar80ns ±1.5V pulse,

25uA compliance

HRS

LRS

Vo Vo Vo

Vo VoHfHf

Vo Vo Vo

Vo VoHfHf

+ Set bias

-

Vo Vo Vo

Vo VoHfHf

ReSet Set

fundamentals of oxide resistive random access memory (rram)

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