<1> IEDM 2007 Perpendicular ST-MRAM U. Klostermann

A Perpendicular Spin Torque Switching based MRAM for the

28 nm Technology Node U.K. Klostermann1, M. Angerbauer1, U. Grüning1, F. Kreupl1,

M. Rührig2, F. Dahmani3, M. Kund1, G. Müller1

1 Qimonda AG

3 Altis Semiconductor

2 Siemens AG

<2> IEDM 2007 Perpendicular ST-MRAM U. Klostermann

Perpendicular Spin Torque (P-ST) based MRAM o A New Concept

Assessment for 28 nm Node o Data Retention o Low Switching Currents o Cell to Cell Interaction o Barrier Reliability

Cell Layout

Read Analysis

Outline

<3> IEDM 2007 Perpendicular ST-MRAM U. Klostermann

Conventional MRAM

“1”

“0”

Magnetic Field H

Cel

l Res

ista

nce

R

Low

WRITE: Word/Bit line field used to set magnetic free layer READ: Electrical determination of R by sense amplifiers

Magnetic Hysteresis: Anti- Parallel

Parallel

0

High

<4> IEDM 2007 Perpendicular ST-MRAM U. Klostermann

Spin Torque Select-Based MRAM

Writing is done by a critical select current

Bit Line

-1000 -500 0 500 10000

20406080

100120140160

"0"

"1"

Set "0"Set "1"

resi

stan

ce c

hang

e M

R [%

]

switching voltage Vc [mV]switching voltage Vc [ mV ] mag

neto

resi

stan

ce [

% ]

S

pinned barrier

free layer

D

<5> IEDM 2007 Perpendicular ST-MRAM U. Klostermann

In-Plane Magnetization

Perpendicular Anisotropy

reference layer barrier

free layer

shape Hk

interface Hk

Perpendicular Magnetization

Perpendicular anisotropy is very high

<6> IEDM 2007 Perpendicular ST-MRAM U. Klostermann

Realization

Feasibility of concept is demonstrated MTJ stack engineering is important

Source: “Spin transfer switching in TbCoFe / CoFeB / MgO / CoFeB / TbCoFe magnetoresistive tunneling junctions with perpendicular magnetic anisotropy” , M. Nakayama et al. , BB-09, 52nd Magnetism and Magnetic Materials Conference (MMM) in Tampa, Nov. 2007

CoFeTb

CoFeTb

<7> IEDM 2007 Perpendicular ST-MRAM U. Klostermann

Scalability of Activation Energy

High anisotropy ensures scaling below 20 nm

ksa HMVolE ∗∗=21

MTJ size Material

0 20 40 60 80 1000

100

200

300

400

500

Product Target: 85 kBT

activ

atio

n en

ergy

Ea [

k BT ]

MTJ width w [nm]

P-ST I-ST

MTJ width w [ nm ]

activ

atio

n en

ergy

Ea [

kBT

]

Anisotropy

Perpendicular

In-Plane

<8> IEDM 2007 Perpendicular ST-MRAM U. Klostermann

Scalability of Switching Current

Absence of demagnetization fields reduces required switching current Ic ~ 30 µA

0 20 40 60 80 1000

1530

150300450600750

Ic ~ w1.5

Ic ~ const.

sw

itchi

ng c

urre

nt I c [

µA]

MTJ width w [nm]

Ic P-ST Ic I-ST

MTJ width w [ nm ]

switc

hing

cur

rent

I c [

µA ]

Perpendicular

In-Plane

<9> IEDM 2007 Perpendicular ST-MRAM U. Klostermann

Cell to Cell Interaction

Significantly reduced stray field interaction

0 1 2 3 40

20

40

60

80

100

p

dist

urb,

x o

r z [

]

distance from center x [F = 28 nm]

P-ST I-ST

distance from center x [ F = 28 nm ] field

com

pone

nt H

(x o

r z) [

Oe

]

Perpendicular

In-Plane

<10> IEDM 2007 Perpendicular ST-MRAM U. Klostermann

Impact of Interaction on Ea

High data retention at dense spacing

Correct Ea by

0 5 10 15 20 25 30

60

70

80

90 Product Target: 85 kBT

activ

atio

n en

ergy

Ea [

k BT ]

structural cell size [F2 with F = 28 nm]

P-ST I-ST

activ

atio

n en

ergy

Ea [

kBT

]

structural cell size [ F ² with F = 28 nm ]

5.1~

1

−×

k

disturbH

H

Perpendicular

In-Plane

<11> IEDM 2007 Perpendicular ST-MRAM U. Klostermann

Reliability Estimates

P-ST allows to use high RA for reliable operation

0.1 1 10 1000.0

0.2

0.4

0.6

0.8

1.0

VBD at Product Life Time

vo

ltage

Vc o

r VBD

[V]

RA of MTJ barrier [Ωµm2]

Vc P-ST Vc I-ST

0.1 1 10 1000.0

0.2

0.4

0.6

0.8

1.0

VBD at Product Life Time

vo

ltage

Vc o

r VBD

[V]

RA of MTJ barrier [Ωµm2]

Vc P-ST Vc I-ST

Perpendicular In-Plane

sw

itchi

ng v

olta

ge [

V ]

RA of MTJ barrier [ Ωµm² ]

<12> IEDM 2007 Perpendicular ST-MRAM U. Klostermann

Cell Layout at 28 nm Node

6 F² layout ensures sufficient current drivability

2F

3F

6 F² @ 28 nm

<13> IEDM 2007 Perpendicular ST-MRAM U. Klostermann

0 50 100 1501E-301E-261E-221E-181E-141E-10

sing

le b

it er

ror p

er 1

acc

ess

temperature T [°C]

γ = 0.5 γ = 0.4 γ = 0.3

Read Disturb

At Ic ~ 30 µA a read current of Ir ~ 10 µA (γ ~ 0.3) is feasible without read disturb

sing

le b

it er

ror p

er 1

acc

ess

temperature T [ °C ]

γ := ratio (read / write) current

Ic ~ 30 µA

<14> IEDM 2007 Perpendicular ST-MRAM U. Klostermann

MTJ Stack Performance

I-ST demonstrated high MR at low RA P-ST will require similar stack performance

1 10 100100

150

200

250

300

M

R =

( R1 -

R0 )

/ R 0

[%]

RA of MTJ [Ωµm2]

Measured magneto resistance (MR) for in-plane systems

M

R [

% ]

RA of MTJ barrier [ Ωµm² ]

MR := ( R1 - R0 ) / R0

<15> IEDM 2007 Perpendicular ST-MRAM U. Klostermann

Read Circuit

Current compliance avoids read disturb

EQL

V_READ

READ_EN

CSL

V_READ

I_REF

SA_I

N

SA_R

EFMTJ

MBL

BL

OUT

cWL

Source Line

select transistor

1.1 V potential

reference current

voltage compliance for MTJ: controlling

MBL potential by V_READ = 0.95 V

optimized SA

current compliance Ir

Typical: R0 = 6 kΩ R1 = 12 kΩ Rpara = 14 kΩ

<16> IEDM 2007 Perpendicular ST-MRAM U. Klostermann

Read Operation Simulation

Fast random array read access time ~ 30 ns demonstrated

0

10

20

30

0 10 20 30 40 50 60 70 80 900,0

0,4

0,8

1,2 SA_IN SA_REF

sign

al [V

]

time [ns]

MBL

0,00,40,81,2

sign

al [V

]

READ_EN OUT

read

cur

rent

[µA]

Ir

sign

al [

V ]

time [ ns ]

sign

al [

V ]

read

cur

rent

[ µA

]

<17> IEDM 2007 Perpendicular ST-MRAM U. Klostermann

Perpendicular Spin Torque has been studied targeting the 28 nm node.

Expected benefits are: long data retention (> 10 yrs @ 85°C) low write current (~ 30 µA) small cell sizes (~ 6 F²) high write endurance and no read disturb

Random access speeds are 30 ns for read and 10 ns for write.

Summary