mram 4 dieny.pdf

[email protected] July 2013 Part 4 inMRAMinMRAM2013

Part 4: Advanced MRAM concepts

A

P


•Ultrafast precessional STTRAM

•Race track memories

•3-terminal devices

•Voltage controlled MRAM

•Comparison of STTRAM with resistive RAM


OUTLINE








OUTLINE


Several families of MRAM

Thermally Assisted (TAS) STT-TAS

Hx

Hy

Field-driven STT (STT MRAM)

Perpendicular

Precessional

Planar

DW motion

Spin-orbit torque(spin-Hall, Rashba)


5

Idea: elaborate a Spin Transfer Torque ‐MRAM with an ultrafast switching to achieve the sub nano second‐scale writing time

Storage disk drive

TB~ms

Main memoryDRAMGB

50‐10 ns

CacheSRAMkB – MB~0.5 à 2 ns

Register

Processor

2 GHz

~100 MHz

Density

Speed

Memory Hierarchy

• Replacing SRAM memory• Building non volatile logic circuits with low‐energy consumption

Ultrafast MRAM :

p-STTRAM Need for a sub-ns switching MRAM

Need for ultrafast STTRAM for SRAM-like applications


Stochastic switching in conventional STTRAM approaches

Tra

nsm

itte

d

volt

age

(mV

)

Time after pulse (ns)

Referencelayer

MgO

In-plane Out-of-plane

→→→→

××=Γ )( MPMa j

Spin-transfer torque:

Mr

Mr

Pr

Pr

•Stochastic reversal • incubation time preceding a large thermal fluctuation•Slows down the STTRAM writing

Devolder et al. Phys. Rev. Let. vol 100 (2008)

MgO based in-plane MTJ


7

MRAM with orthogonal polarizer and analyzer

O. Redon, B. Dieny andB. Rodmacq, US Patent FR0015893 (2000),

US6532164B2

A.Kent et al, APL 84, 3897 (2004)

Free Layer

Reference Layer

(Analyzer)

Perpendicular Polarizer

→→→→→→→

→→→→

××+××+×+×−= )()(0 MPMaMAMadt

MdM

MHM

dt

MdjPerpJLong

S

effαγ

Equation of motion

→ STT from Perpendicular polarizer:Out‐of‐plane oscillations of free layer

magnetization

Switching in less than 1ns

No incubation delay (PM)

→ STT from in‐plane analyzer:Bipolar switching of free layer

magnetization

2 STT contributions

MgO

Ar

Pr

Mr

MgO (lower RA) or Cu


RF oscillator with perpendicular to plane polarizer

(SPINTEC patent + Lee et al, Appl.Phys.Lett.86, 022505 (2005) )

D.Houssamedine et al,Nat.Mat 2007

Injection of electrons with out-of-plane spins;Steady precession of the magnetizationof the soft layer adjacent to the tunnel barrier.

Precession (2GHz-40GHz) + Tunnel MR ⇒ RF voltageInteresting for frequency tunable RF oscillators ⇒ Radio opportunism

Cu

PtMn

Cu

CoFeCoFe

Al2O3

(Pt/Co)

J


Free Layer

Reference Layer

(Analyzer)

Perpendicular Polarizer

Depending on the relative influence of STT from perpendicular polarizer and in-plane analyzer, different switching behaviors can be observed:

-If STT from perpendicular Polarizer dominates→ Oscillatory switching probability versus current pulse duration. Switching whatever the current direction.More difficult to control in STTRAM devices since requires controlling the current pulse duration at ±50ps. Requires read before write.

→→→→→→→

→→→→

××+××+×+×−= )()(0 MAMaMPMadt

MdM

MHM

dt

MdjLongjPerp

S

effαγ

Ar

Pr

How to conveniently tune the relative influence of these two STT contributions?

-If STT from in-plane Analyzer dominates→Non-oscillatory switching probability vs pulse duration. Final state determined by current direction. No need to read before write.



Tuning the relative STT influence of Tuning the relative STT influence of perpperp polarizer and polarizer and inin--plane analyzer by playing on the cell aspect ratioplane analyzer by playing on the cell aspect ratio

→ STT from perpendicular polarizer:

→ STT from in‐plane analyzer:

2g(0)

M 2

2g(0)

M 2 2s0s0 μαμα Fs

KFLong

c

teMH

tej ⎟

⎠⎞

⎜⎝⎛≈⎟

⎠⎞

⎜⎝⎛ +⎟

⎠⎞

⎜⎝⎛=

hh

2/2)g(

M 2 s0 KFPerpc

Htej

πμ

⎟⎠⎞

⎜⎝⎛=h

To get bipolar non-oscillatory switching probability versus pulse duration:

Longc

Perpc jjj >> implying

Hk = in-plane shape anisotropy field

1<Perpc

Longc

j

ji.e ( )

( ) 1

02 <

K

s

H

M

g

g απ

The easiest way to fulfill this relationship is to increase HK by increasing the cell aspect ratio (AR)

Typically,

7.0)0(/)2/( ,01.0~ ,6.1~0.01,~ 00 ≈ggTHTM Ks πμμα with AR~2 so that ratio~1.1

J.Sun, PRB 62, 570 (2000) (SI units)

K.J.Lee et al, APL 86, 022505 (2005)


Elliptical nanopillars270x80 nm

TMR~65% and RA~16 Ω.µm2

Annealing 300oC; 90 minutes under in‐plane field

→Synthetic antiferromagnetic layers reduce the magnetostatic interactions between layers



time (ns)Mag

neto

resi

stan

ce(a

.u.)

Current density (1010 A/m2)0 5 10 15F

requ

ency

(G

Hz)

0

10

20

Cur

rent

dens

ity(x

1010

A/m

2 )

Pulse width (ns)0%

50%

100%Out-of-plane precession of the free layer magnetization due to STT from perpendicular polarizer

Cell size : 80nm*160nm Macrospin simulation:Proba of switching under STT from perp polarizer only

PAP

Dynamics dominated by the STT contributionfrom perpendicular polarizer (AR=2)


AP and P reference signals measured under sufficiently large magnetic field to maintain the AP and P configurations during the current pulse. Field switched off afterwards.

As expected, the perpendicular polarizer induces a large amplitude precession around the normal to the plane

P stateCell size : 80nm*160nm

AP initial state

Single-shot transmitted signal (AR=2)


2.00E-008 2.50E-008 3.00E-008 3.50E-008 4.00E-008 4.50E-008-0.08

-0.07

-0.06

-0.05

-0.04

-0.03

-0.02

-0.01

0.00

0.01

Single pulse Average of 100 traces Reference signal

Vo

ltag

e (

V)

time after pulse2.00E-008 2.50E-008 3.00E-008 3.50E-008 4.00E-008 4.50E-008

-0.08

-0.07

-0.06

-0.05

-0.04

-0.03

-0.02

-0.01

0.00

0.01


0,6GHz

Vol

tage

(V

)

time after pulse (s)

2.00E-008 2.50E-008 3.00E-008 3.50E-008 4.00E-008 4.50E-008

-0.07

-0.06

-0.05

-0.04

-0.03

-0.02

-0.01

0.00

0.01


Vol

tage

(V

)

time after pulse (s)2.00E-008 2.50E-008 3.00E-008 3.50E-008 4.00E-008 4.50E-008

-0.07

-0.06

-0.05

-0.04

-0.03

-0.02

-0.01

0.00

0.01


Vol

tage

(V

)

time after pulse (s)

1GHz

500mV

708mV630mV

563mV

Precession frequency depends linearly on the

current density

7 8 9 10 11 120.7

0.8

0.9

1.0

1.1

1.2

Fre

qu

ency

(G

Hz)

Current Density (x1010 A/m²)

Lee et al, APL86, 022505 (2005)

Frequency vs current density (AR=2)


15

0 1 2 3 4 5 6 7 8 9 10

Single shot traces at 1.4V

Time after pulse (ns)

De-phasing of precessionalmotion explains the probability amplitude decay observed in probability measurements

Acknowledgement to T. Devolder at IEF in Paris for real time measurements

Pulse duration (ns)

Sw

itchi

ng p

roba

bilit

y (%

)

Precession decoherence (AR=2)


Cu

PtMn

Cu

CoFeCoFe

Cu

(Pt/Co)

JD.Houssamedine et al,Nat.Mat 2007

F re q u e n c y vs Tim e

f (H z )

t (s

)

7 . 1 7 .2 7 .3 7 .4 7 .5 7 .6 7 .7 7 .8

x 1 09

2

4

6

8

1 0

1 2

x 1 0-7

0.90 mA1.25 µs

Time evolution of spectrumTime domain measurement

5 1 0 1 5

x 1 0-8

-0 .0 2

-0 .0 1 5

-0 .0 1

-0 .0 0 5

0

0 .0 0 5

0 .0 1

0 .0 1 5

0 .0 2

t (s )

V (

V)

0.90 mASlidingwindow

FFT

0 150 nstime f(GHz)7.1 7.80

1

t(μs)

Δf~200MHz

nsf 5/1 ≈Δ≈τ

Precession decoherence also seen in STT oscillators: contribution to linewidth


Pulse width (ns)

0%

50%

Cur

rent

dens

ity(x

1010

A/m

2 )

100%Switching probability (initial=P)

When the condition is fulfilled:

Non-oscillatory bipolar switching can be achieved.If the current is too large, oscillatory switching

probability is recoveredUltrafast reversal without incubation time

Longc

Perpc jjj >>

Cell size = 50nm*250nm

Pulse width (ns)

Switching probability (initial=AP)

P AP

AP P

Cur

rent

dens

ity(x

1010

A/m

2 )

Dynamics dominated by STT contribution from in-plane analyzer (AR=5)


0.0 0.2 0.4 0.6 0.8 1.0 1.2

AP

mag

neto

resi

stan

ce (

a.u.

)

time (ns)

4.5e10 A/m2 5.5e10 A/m2 6.5e10 A/m2 7.5e10 A/m2P

0.0 0.2 0.4 0.6 0.8 1.0 1.2

AP

Pma

gnet

ore

sist

anc

e (a

.u.)

time (ns)

Without perpendicular polarizer(STT from in-plane analyzer only)

J = 25 1010 A/m²

Incubationtime

STT from both in-plane and perpendicular polarizer

No stochastic incubation timeReduced switching currentSwitching time is reduced to 300ps by increasing the current density



0 2 4 6 8 10

0.0

0.2

0.4

0.6

0.8

1.0

Sw

itch

ing

Pro

bab

ility

Pu lse w idth (ns)

500m V 562m V 631m V centered b ias 631m V A P b ias 631m V P b ias 708 794m V

Fullfilling the condition allows for bipolar non-oscillatory switching of the storage layer magnetization suitable for SRAM type of applications

⇒ precession stops at stable state after half a precession period.

Switching with 90fJ

Similar results:Liu et al, APL97, 242510 (2010)

Cell size = 50nm*250nm

Longc

Perpc jjj >>


Marins de castro Sousa et al,

Journal of Applied Physics 111 (2012) 07C912


• Ultrafast STT switching with combined STT influences from perpendicular polarizer and in-plane analyzer

• Successful integration of a perpendicular polarizer in an in-plane magnetized MTJ with good TMR signal (~60-70%).

• Oscillation of the switching probability associated with dominant STT influence from perpendicular polarizer

• Bipolar non-oscillatory switching can be achieved for ultrafast reliable writing in STTRAM by using elongated cells. Drawback is larger footprint.

• Sub-ns switching and low energy consumption can be achieved (90fJ range)

Summary on ultrafast STTRAM with orthogonal polarizers








OUTLINE


Field driven DW motion Current driven DW motion

Magnetic fieldH = -20mT

Positive current Negative currentH=0

(a)

(b) (c) (d)

Current Induced Domain Wall (DW) motion


Current Induced Domain Wall (DW) motionSTT used to push domain walls:


Reading

Writing

Vertical racetrack

Horizontal racetrack

Racetrackstorage array

A

B

C

D

E

Race-track memories

Parkin et al, IBM (2004)

Shift register based on coherent domain wall displacements induced by current


Race-track memories

Injection of domain walls:

Injection by field

Injection by STT through a tunnel barrier can also be used or using the fringing field from a domain wall in an underlying wire (see previous slide).


Race-track memories

Race-track: a multibit MRAM with extended storage layer.


Parkin et al, IEDM 2011

Race-track memories

Successful demonstration on a 8μm long shift register:


Race-track memories

Remaining challenges:

-Use perpendicular-to-plane materials for narrower domain walls and weak pinning energies.

-Avoid pinning defects which locally trap domain walls. One single pinning defect along a given track may prevent that whole track to properly work.

-Homogeneity of properties if using vertical dimension.








OUTLINE




Hx

Hy


Perpendicular

Precessional

Planar

DW motion



3-terminal MRAM cell based on current-induced domain wall propagation

Advantages : -Less electrical stress on the barrier during write (improved reliability)-Less current required to write since thickness<<width-Multibit possible

referencebarrier

Storage stripe

Disadvantage : Larger cell

0 1 2 30

100

200

300

400

DW

vel

oci

ty (

m/s

)

Current density (x 1012 A/m2)

V ~ 250m/s100nm ~ 400ps



NEC

The memory cell has a shape such that a magnetic wall is necessarily exist.Domain wall moved by STT influence from in-plane current.

Potential of 0.1-mA and 2-ns writing with sufficient thermal stability,



Multibit MRAM (shift register) based on DW propagation




Hx

Hy


Perpendicular

Precessional

Planar

DW motion



AlOx 2 nmCo 0.5 nmPt 3 nm

2 2

1 1

1 /v

c vB E

c= ×

−

r rr

Breaking of inversion symmetry – Rashba effect

Co-dz²

O-pz Charge transfer at Co/MOxinterface⇒Interfacial E field

Er

Jr

Jr

Er

Rashba effect in MTJ electrodes with in-plane current


36

M.Miron et al, Nature 476,189 (2011)

Magnetization reversal by Rashba effectin MTJ electrode with in-plane current

Writing with in-plane current

Reading with current through barrier

Switching induced by spin-orbit torque: Rashba or Spin-Hall








OUTLINE


Voltage controlled MRAM

STTRAM are written by a pulse of current flowing through the MTJ.The main source of energy consumption during write is the Jouledissipation:

tRIE 2=In STTRAM, the energy per write event is in the range 0.1pJ-10pJ (depending on the retention).

If we could control the magnetic properties of the storage layer by voltage without significant current flow through the MTJ, then the energy consumption could be reduced to

2

2

1CVE = Could be in the range of fJ provided

V is below 1V



Several approaches:

-Using multiferroïc materials : materials which exhibit coupled structural, electrical and magnetic properties (piezoelectric, magneto-electric, magneto-elastic). Example: BiFeO3

-Using synthetic multiferroïc materials :

Piezo-electric

Magneto-elastic

-Using the influence of interfacial electric field on the perpendicular anisotropy at magnetic metal/oxide interface



At metal/oxide interface, density of states at Fermi energy can be tuned by electrical field. However very short penetration depth of electrical field in metal (<2Å): interfacial effect. Can be used to tune the CoFeB/MgO anisotropy by electrical field.Can very much reduce the power consumption compared to STT if operates at V<1V

Intrinsically weak effect. Large manifestation if close to condition of anisotropy reorientation (for instance compensation between in-plane demagnetizing energy and perp interfacial energy). But difficult to use in actual device because condition fulfilled at only one temperature.

Endo et al, APL96, 212503(2010)








OUTLINE


STT-MRAMReRAM

ITRS ERD Roadmap 2010

Vwr1~0.9V@5nsVwr0~-0.9VVread~0.3V




STT-MRAM ReRAM

COMPARISON MRAM / ReRAM

Statistical phenomenon associated with migration of vacancies or metallic ions

J.Lee et al, Gwanju IST, Korea (IEDM2010)

eVEF

0EF

F1 F2

Spin-dependent quantum mechanical tunneling of electrons (as polarizer/analyzer in optics).Switching of magnetization described by LLG equation:

θ

( )2

cos1min

θ−Δ+= RRR

( ) ( )dt

dMMMMMaIMbIHM

dt

dMppeff ×+××++×−= αγγ . .

0 100 200 300 400 500 600 700 800 900 100010

-6

10-5

10-4

10-3

10-2

10-1

100

Resistance

No

rmal

ise

dC

oun

t

0 100 200 300 400 500 600 700 800 900 100010

-6

10-5

10-4

10-3

10-2

10-1

100

10-1

100

Resistance

No

rmal

ise

dC

oun

t

Rmin Rmax

>25σ

0 100 200 300 400 500 600 700 800 900 100010

-6

10-5

10-4

10-3

10-2

10-1

100

Resistance

No

rmal

ise

dC

oun

t

0 100 200 300 400 500 600 700 800 900 100010

-6

10-5

10-4

10-3

10-2

10-1

100

10-1

100

Resistance

No

rmal

ise

dC

oun

t

Rmin Rmax

>25σ

1Mbit chip TA-MRAM

Resistance (kΩ)0 10.5

8kbit ReRAM(K.Kit, SAIT, SamsungIEDM 2010)

R distributions:

(2011)


Multilevel capabilityMemristor functionality with large ΔR amplitude

J.Lee et al, Gwanju IST, Korea (IEDM2010)

0

40

80

120

160

TM

R(%

)

-300 -200 -100 0 100 200 300

H(Oe)

MgO based MTJ

•Binary resistance levels•Multilevel and memristor possible but with much less ΔR amplitude than with ReRAM. Not so easy to implement (R(θ), DW or stacking of several MTJ)

Multilevel capability:

Cyclability:

W.C.Chien et al, Macronix, Hsinchu, Taiwan, (IEDM2010)>1016 cycles

10 6 10 7 10 8 10 9 10 10 10 11 10

120

140

160

180

200

220

240

RmaxRmin

Number of pulses

12

Res

ista

nce

(Ω)

Vwrite

COMPARISON MRAM / ReRAM (2011)


W.C.Chien et al, Macronix, Hsinchu, Taiwan, (IEDM2010)

Retention:

Speed:

Y.T.Cui et la, PRL104, 097201(2010)

STT switching in MTJ

W.C.Chien et al, Macronix, Hsinchu, Taiwan, (IEDM2010)

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛−−−−=

WR

cell

Bchip I

I

Tk

EtmF 1expexp1

0τ

Takemura et al, IEEE Journ of Solid State Circuits, 45, 869 (2010)

E/KBT>67

OK with perpendicular MTJOK with TAS



Possibility of crossbar architecture:

Difficulty is the in-stack diode

MTJ+diode

http://www.unitysemi.com/

M.-J. Lee et al., Samsung, IEDM 2007



Summary of differences MRAM / ReRAM

“Unlimited” cyclability (>1016 cycles) Cyclability ~ 108 cycles, more than FLASH

Narrow distribution of Rmin / Rmax

5kΩ±0.3kΩ1Mbit: / 12kΩ±0.8kΩ

Larger distribution of Rmin / Rmax

3-6kΩ /6Kbit: 10kΩ - 300kΩ

Bilevel resistanceMultilevel possible but not straighforward

“Natural” continuous change of RMultilevel capability easier to implement

Moderate ΔR : Rmax/Rmin ~2-3 Large ΔR : Rmax/Rmin ~5 - 50

Applications

DRAM, SRAM, e-SRAM, logic-in-memory,Embedded-NVM

e-SRAM, Embedded-NVM, FLASH,Memristor, neuromorphic architecture


R.SousaS.BandieraY.Hadj-LarbiB.RodmacqS.AuffretM.SouzaL.NistorJP Nozieres

B.DelaetM.T.DelayeM.C.Cyrille

L.Buda-PrejbeanuM.ChshievH.BeaS.AmaraV.BaltzJ.MoritzP.Y.ClementC.BaraducL.CuchetB.LacosteQ.StainerG.Vinai

B.CambouI.L.PrejbeanuK.MackayL.LombardE.GapihanC.DucruetY.ConrauxC.Portemont

Work partly supported by the projects

NANOINNOV SPIN (2009)

HYMAGINE (ERC2009)

Acknowledgements

mram 4 dieny.pdf

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