SEMATECH 8th International Symposium
on Advanced Gate Stack Technology,Session 4: Spin Transfer Torque Memory
Bolton Landing, NY20, October 2011

http://www.csis.tohoku.ac.jp

Advanced magnetic tunnel junctions
based on CoFeB/MgO interfacial
perpendicular anisotropy
S. Ikeda1,2
M. Yamanouchi1, H. Sato1, K. Miura3,1,2, K. Mizunuma2, H. Yamamoto3, R. Koizumi2,H. D. Gan1, S. Kanai2, J. Hayakawa3, F. Matsukura1,2, H. Ohno1,2
1 Center for Spintronics Integrated Systems (CSIS), Tohoku Univ.
2 Laboratory for Nanoelectronics and Spintronics, RIEC, Tohoku Univ.
3 Central Research Laboratory, Hitachi, Ltd.

Acknowledgement: This work was supported by the project "Researchand Development of Ultra-low Power Spintronics-based VLSIs“ underthe FIRST Program of JSPS.

1

Spintronics devices(Expectation as low power consumption devices)

RecordingFree
BarrierPinnedReference Nonvolatility
 High speed operation
 Virtually unlimited
write endurance

Bit line

MTJ

SRAM/DRAM(Volatile)
Number of write cycles

 Magnetoresistive RAMs (MRAMs)
SPin transfer torque RAMs (SPRAMs)

1020

MRAM/SPRAM

1015

PRAM
FeRAM

1010

Word line

FlashGateSourceDrain Logic-in-memory architecture Reduction of leak current(static power) Reduction of interconnection delay
Mochizuki et al., IEICE Transactions on Fundamentals, E88-A (2005) 1408.
Matsunaga et al., Appl. Phys. Express, 1 (2008) 091301; ibid., 2 (2009) 023004.
Sekikawa et al., IEDM 2008, Suzuki et al., VLSI Technology 2009
Ohno et al., IEDM 2010
Matsunaga et al. VLSI Circuit 2011

105
1n

10n

1µ100nWrite time (s)10µMemoryProcessorcore

MTJCMOS

2MRAM development
1GToshiba

Hynix-Grandis

Toshiba- NECNECIBM-Infineon

10M32Mb SPRAMVLSI Circuit 2009

Everspin

Everspin

Takemura et al., IEEE J. Solid-State Circuits 45 (2010) 869.

TDK

Freescale

Hitachi-Tohoku Univ.
1M

NTHU-ITRI

Toshiba-NEC

128kb
Array
SA & Write

Cypress
100k

M4
M3
Cross section
SEM imageof SPRAM cell

Samsung
Y-Dec.

Sony

10k

MTJ cells

Control Circuits
Pre-Decoder
Word Driver/Dec.

Capacity (bits/chip)

100M

ITRI-TSMC-NTHU

M1
CMOS-Tr.

Sony
MRAM (demo)

Magnetic field

MTJ cellSpin transfer torque

MgObarrier

MRAM (product)IBM
1k

SPRAM (i-MTJ)
SPRAM (p-MTJ)

Freescale(Motorola)

Recording layer
pinned layer
Anti-ferromagnetic layer

Lower electrode

100
1998

2000

2002

2004

2006

2008

M22010

Year

2012

2014

50 nm

2Mb SPRAM
ISSCC 2007
Kawahara et al., IEEE J. Solid-State Circuits 43 (2008) 109.

Ikeda et al., IEEE Trans. Electron Devices, 54 (2007) 991.

3

Nonvolatile circuits for logic-in-memory

MTJ-Based
Nonvolatile Full
Adder
MTJ-Based Nonvolatile
Lookup-Table Circuit Chip

Matsunaga, …Hanyu et al.,Appl. Phys. Express 1 (2008) 091301.

Suzuki, …Hanyu,et al., VLSI
Circuit Symposium 2009.

Matsunaga, ... Hanyu et al., Appl. Phys.
Express 2 (2009) 023004.

2-kb
TCAM
cell array

Sense Amp.

Column Dec.

Row Dec.

MTJ-Based
Nonvolatile
Ternary ContentAddressable
Memory (TCAM)
cell

2kb-nonvolatile
TCAM chip
Matsunaga, …Hanyu,et al.,VLSI Circuit Symposium 2011.

174 µm4

Technology issues toward realization ofnonvolatile VLISs
To realize nonvolatile VLISs (MRAM and logic-in-memory)using the leading edge technology node, there are still issues
to be addressed.

ISSUES High output (TMR ratio>100%)
 Low switching current (IC < F μA)+ Scalability

 Thermal stability for nonvolatility (E/kBT > 40)
 Annealing stability
in back-end-of-line process (Ta > 350oC)• MTJs have to satisfy these requirements with scalability at the same time.
• In order to satisfy these requirements, we focus on the MTJs withperpendicular anisotropy electrodes.
5Progress of MTJs
700

CoFeB

bcc (001)

MgO

rock salt (001)

CoFeB

bcc(001)

604%@RT(1144%@5K)

TMR ratio (%) (%)
トンネル磁気抵抗比

600500

Tohoku Univ. & Hitachi

400

MgO-barrier MTJs

300

CanonANELVA & AISTIBMAIST

200
100

In-plane anisotropypseudo-SV
AIST

AIST

AlOx-barrier MTJs
Tohoku Univ.

Cr/Au
Ru (5)Ta(5)CoFeB(6) →MgO(2.1)
Ta(5)
Ru(10)Ta(5)Cr/Au
Ru (5)Ta(5)CoFeB(1.7) ↑MgO(0.9)
CoFeB(1 ) ↑Ta(5)Ru(10)
Ta(5)

SiO2/Si sub.

SiO2/Si sub.

CoFeB(5 ) →0
1994 1996 1998 2000 2002 2004 2006 2008 2010

Year
年

S. Ikeda et. al., Appl. Phys. Lett. 93, 082508 (2008).
Nat. Mat., 9, 721 (2010)

Perpendicular anisotropy
Cr/Au
pseudo-SV6

M-H curves of CoFeB/MgO stack samples
with different tCoFeB
MgO(1)
CoFeB(tCoFeB )SiO2/Si sub.

Ta=300 oC,
4 kOe, 1h

Ta=300oC

0M (T)Ta(5)
Ru(10)Ta(5)1tCoFeB = 2.0 nm

in-planeout-of-plane-11In-plane anisotropy

tCoFeB = 1.3 nm
Perpendicularanisotropy

Ta=300oC

0µ0Hs_in-plane = 0.34 T-1-0.5

Ms = 1.58 T
K = 2.1x105 J/m30.0

µ0H (T)

0.5

1.0

S. Ikeda et al., Nat. Mat. 9 (2010) 721.

7

tCoFeB dependence of KtCoFeB in CoFeB/MgO stack

MgO(1)
CoFeB(tCoFeB )Ta(5)Ru(10)
Ta(5)
SiO2/Si sub.

Ta= 300oC

K ⋅ tCoFeB (mJ/m2)

1
VSMFMR
Perpendicular

0-1

in-plane

0

1

2

tCoFeB (nm)KtCoFeB = (Kb-MS2/2µ0) tCoFeB+Ki•From the y-intercept, Ki = 1.3 mJ/m2.
•The CoFeB/MgO interfacial anisotropy Ki is dominant in the perpendicularanisotropy because Kb is negligible.
S. Ikeda et al., Nat. Mat. 9 (2010) 721.

8

tMgO dependence of Ms and K
Saturation magnetization Ms and effective magnetic anisotropy energy
density K are evaluated from the out-of-plane M-H curves.

tMgO = 0.6 nm

K (105 Jm-3)

Ms (T)

1.2

Mg

0.8O

0.4
0.0
2.0
1.5
1.0
0.5
0.0
0.0

0.42 nm

0.5

1.0

1.5

tMgO (nm)2.0

1 monolayer ofMgO is ~0.21 nm

More than 3 monolayers of MgO is required to stabilize the perpendicularanisotropy induced by CoFeB-MgO interface.
M. Yamanouchi et al., J. Appl. Phys. 109 (2011) 07C712.

9Possible factor of the perpendicular anisotropy
in CoFeB/ MgO stack
3d z 2 : in-plane anisotropy(m=0)

O 2 pzFe

3d z 2anti-bonding
Fe 3dz2 (m=0)

K. Nakamura et al., Phys. Rev. B, 81 (2010) 220409.
R. Shimabukuro et al., Physica E 42 (2010) 1014.

σ-coupling• 3dz2 band of Fe is pushed up above the Fermi energy.
• The contribution of 3dz2 orbital of Fe becomes small,resulting in appearance of perpendicular anisotropy.

O 2pZbonding

10

TMR properties in CoFeB-MgO p-MTJs

Al2O3Cr/AuRu (5)Ta(5)CoFeB(1.5 )MgO(0.9)

Ta = 300oC

V+

CoFeB(0.9 )Ta(5)Ru(10)
Ta(5)Parallel

40302010

SiO2/Si sub.

Antiparallel50

R (kΩ)

V--0.2-0.1

0.0µ0H (T)

0.1

0.2

(nm)

Clear hysteresis -> top and bottom
CoFeB electrodes have perpendicularanisotropy.
40 nm
S. Ikeda et al., Nat. Mat. 9 (2010) 721.

11

Junction size dependence of Ic0200

Cr/Au
Ru (5)Ta(5)CoFeB(1.5)
MgO(0.9)
CoFeB(0.9 )Ta(5)Ru(10)
Ta(5)Ta=300oC
w/o Hex

IC0 (P to AP)
IC0 (AP to P)150

Critical current, IC0 (µA)

F = 40 ~ 76nmφ59 nm

70 nm
76 nm

Junction size

53 nm
44 nm

100500-50-100

SiO2/Si sub.

01000

2000

3000

4000

5000

6000

Recording layer volume, Vrec (nm3) = S・tCoFeB

50 nm
H. Sato et al., Appl. Phys. Lett. 99 (2011) 042501.

Ic0 linearly decreases with junction area S(volume Vrec).
We confirmed scalability of switching current
for spin transfer torque.
12

Thermal stability factor, E/kBT

Junction size dependence of E/kBT
80
Junction size6059 nm 70 nm 76 nm
53 nm
44 nm40200pulse current experimentpulse field experimentprevious report
01000

2000

3000

4000

5000

6000

Recording layer volume, Vrec (nm3) = S・t
E/kBT maintained almost constant values even though the junction diameter
was varied from 40 nm to 80.
IC0 reduces in proportion to the volume of the recording layer but the E/kBT
values are not affected much by the volume down to 40 nm in diameter.
H. Sato et al., Appl. Phys. Lett. 99 (2011) 042501.

13

Origin of the junction size dependence of IC0 and E/kBTNo clear reduction in E/kBT with different junction size
Nucleation type magnetization reversal
Effective diameter,
Nucleation
diameter,DDnn(nm)
(nm)

80

Junction area (diameter): S (D)60

Nucleation embryo
40200

Nucleation diameter: Dn02040

60

80

100

Junction size in diameter (nm)Dn showed almost constant values of 40 nm in diameter, which is almost
independent of junction size.
IC0=JC0S=JC0(π(D2-Dn2)/4)+π(Dn/2) 2)∝SE/kBT=Keffπ(Dn/2) 2/kBT ∝Dn2
H. Sato et al., Appl. Phys. Lett. 99 (2011) 042501.

14

Jc0 and E/kBT of MTJs with 40 nm diameter
Cr/Au

Al2O3

Ru (5)Ta(5)CoFeB(1.7)
MgO(0.85)
CoFeB(1.0 )Ta(5)Ru(10)Ta(5)w/o Hex
Ta=300oC
Al2O3w/o Hex
Ta=350oC

SiO2/Si sub.

40 nm

Ta(oC)
300350

TMR ratioRA(%)(Ωµm2)
124
18
113
16

Jc0(MA/cm2)
3.9
3.8

E/kBT
43.1
39.1

•Jc0 and E/kBT are maintained after annealing at 350oC.
•This MTJ system has a back end of the line (BEOL) compatibility.
S. Ikeda et al., Nat. Mat. 9, (2010) 721.
15K. Miura et al., MMM 2010, HC-02.

Comparison of MTJs
Type

Stack structure(nm)

Size(nm)

MR(%)

RA(Ωμm2)(MA/cm2)IC0(μA)

Δ=E/kBT

IC0/Δ

Ta (oC)

Ref.

i-MTJ

CoFeB(2)/Ru(0.65)/CoF
eB(1.8) SyF

100x200>130~102~400

65

~6.2

300350J. Hayakawa et al., IEEE
T-Magn., 44, 1962
(2008)

p-MTJ

L10FePt(10)/Fe(t)/Mg(0.4)/
MgO(1.5)/L10-FePt(t)Blanket120

11.8k----500

M. Yoshizawa et al.,IEEE T-Magn., 44, 2573
(2008)

p-MTJ

L10FePt/CoFeB/MgO(1.5)/
CoFeB/Co basedsuperlatticeBlanket------

H. Yoda et al.,Magnetics Jpn. 5, 184(2010) [in Japanese].

p-MTJ[Co/Pt]CoFeB/CoFe/Mg
OCoFe/CoFeB/TbFeCoBlanket

85-97

4.4-10----225

K. Yakushiji et al., APEX
3, 053033 (2010)

p-MTJ[CoFe/Pd]/CoFeB/MgO/
CoFeB/[CoFe/Pd]

800x800N

100(113)----350(325)

K. Mizunuma et al.,
MMM&INTERMAG2010

p-MTJ

CoFeB (1)/ TbCoFe (3)130 φ~15

4.7

650

107

6.08-

M. Nakayama et al., APL
103, 07A710 (2008)

p-MTJL10-alloy

50-55 φ---

49

56

0.88-

T. Kishi et al., IEDM
2008

p-MTJ

Fe based L10(2)/CoFeB (0.5)----9---

H.Yoda et al., Magnetics
Jpn. 5, 184 (2010) [inJapanese].

p-MTJ

CoFeB/MgO/
CoFeB

40 φ

113
(124)

16(18)

3.8(3.9)48(49)

39(43)

1.23

350
(300)

S. Ikeda et al., Nat.
Mat. 9 (2010) 721.
K. Miura et al.,
MMM2010,HC-02(CIPT)

202(CIPT)(CIPT)

18.7k(20.2k)JC0(1.14)

These p-MTJ technologies will be a promising building block for nonvolatile VLSIs
using spin-transfer torque switching.
16

Issue for p-MTJs
Bit Information “0”FreeReferenceSNSN

Bit Information “1”Different
polarity=StableSParallel state
Low resistanceNN

The samepolarity=Unstable

S
Antiparallel stateHigh resistance

The thermal stability ∆p=E/kBT of anti-parallel state in p-MTJs becomes
low by comparison with parallel state.
Slide 1717

Enhancement of thermal stability∆P, AP = ∆ [1±Hs/Hc0]2
Conventional structure
Free

∆AP

Hs

dipole interlayer
coupling field“1”E/kBT“0”∆P

Reference

Stepped structure

E/kBTFree“1”Hs∆AP“0”∆P

Reference

Hs decreases by employing step structure with large reference layer.
∆AP increases with decreasing diameter of reference layer.

18

Two types of p-MTJs
Conventional structure

100 nm

100 nmF = 100 nm

F = 100 nm
1.5 nm CoFeB
0.9 nm MgO
0.9 nm CoFeB

Stepped structure

Free layer
Reference layer

F : Feature size

CoFeB
MgO
CoFeB

1.5 nm0.9 nm0.9 nm

300 nmSlide 19

K. Miura et al., 2011 VLSI Technology, 11B-3.

19

4

Conventional structure

3Resistance (kΩ)

Resistance (kΩ)

Hs in two types of p-MTJs
Hs

2
1-100-50050100
Magnetic field (mT)4

Stepped structure

3Hs2
1-100-50050
100
Magnetic field (mT)Conventional structure

Stepped structure

TMR ratio (%)

10097RA (Ωµm2)

1313

Hs (mT)22

5

Hs can be reduced by using stepped structure.

Slide 20

K. Miura et al., VLSI Technology 2011, 11B-3.

20

∆P and ∆AP in two types of p-MTJs
Conventional structure

Stepped structure“AP”->”P”“AP”->”P”

10.8
0.6
0.4
0.2
0-150

“P”->”AP”Probability

Probability

∆P,AP

-75
075
Magnetic field (mT)

150

1
0.8
0.6
0.4
0.2
0-150

“P”->”AP”-75075Magnetic field (mT)Conventional structure

Stepped structure

71.2

72.9

46.5

70.1

∆P∆AP

∆AP in stepped structure increases.
K. Miura et al., VLSI Technology 2011, 11B-3.

150

21

Cell area of stepped structure
Cell area (µm2)

10F = 100 nm

SRAM 100F2(F: feature size)

CoFeB
MgO
CoFeB

132 nm

300 nm0.1

F = 100 nm

0.01
020 nm

50100

Technology node (nm)

150

CoFeB
MgO
CoFeB
200 nmCell area of 0.09 µm2 (300nmφ) in the stepped structure corresponds toSRAM cell area at 32 nm technology node.
Cell area can be down to 0.04 µm2 (200nmφ) without degrading theretention time over 10 years, which corresponds to SRAM cell area at
20 nm technology node.
K. Miura et al., VLSI Technology 2011, 11B-3.

22

Summary We have demonstrated that the critical current Ic0 in CoFeB/MgO
perpendicular anisotropy MTJs (p-MTJs) can be scaled down withdecreasing recording layer volume.
 The thermal stability factor E/kBT can be maintained at about 40
even though the recording volume was reduced to 40 nm. Themagnetization reversal in CoFeB/MgO p-MTJs is dominated bynucleation type magnetization reversal (nucleation diameter ~40nm).
 CoFeB/MgO p-MTJs show the high TMR ratio of more than 100%,high thermal stability at dimension as low as 40 nm diameter and a
low switching current of 49 μA at the same time .
 CoFeB/MgO p-MTJ with step structure shows the enhancement ofthermal stability in antiparallel state, which achieves thermal stability
for data retention time over 10 years.
23