Transcript Perspectives Spintec 2007 - IMT

Thermally Assisted MRAM
How does it work ?
Cristian PAPUSOI, Bernard DIENY
SPINTEC, Grenoble, France
Acknowledgments: O.Redon, A.Astier, (LETI),
J.P.Nozières (CROCUS Tech.), S.Cardoso,
P.Freitas (INESC-MN Portugal)
SPINTEC – URA 2512
1
Introduction
1993 – 1998
EDUCATION
Ph. D., Faculty of Physics, “Al.I.Cuza” University, Iasi, Romania
1987 – 1992
Diploma in Physics, Faculty of Physics, “Al.I.Cuza” University, Iasi, Romania
2006 – 2007
ACADEMIC AND PROFESSIONAL EXPERIENCE
Postdoctoral Research Fellow, Grenoble, France
Investigation of RAM devices with thermal assisted switching
2004 – 2006
Postdoctoral Research Fellow, Center for Materials for Information Technology (MINT)
Center, University of Alabama, Tuscaloosa, USA
Fabrication and characterization of CPP (Current Perpendicular to the Plane) spin valves
2003 – 2004
Postdoctoral Research Fellow, RWTH University,2 Physikalisches Institut A, Aachen,
Germany
Role of non-magnetic defects inserted in metallic antiferromagnets on exchange bias
2000 – 2003
Postdoctoral Research Fellow, Information Storage Materials Laboratory, Toyota
Technological Institute, Nagoya, Japan
Thermal stability and recording performance of hard-disk media
1992 – 2000
Lecturer, Department of Electricity and Physical Electronics, Faculty of Physics, "Alexandru
Ioan Cuza" University, 11 Blvd. Carol I, 700 506 Iasi, Romania
2006
AWARDS
Outstanding REU student/postdoc mentor, University of Alabama (11/8/2006)
SPINTEC – URA 2512
2
SPINTEC - location
SPINTEC
MINATEC
LETI
SPINTEC – URA 2512
3
Main topics of research at SPINTEC
GOAL
Bridge fundamental research and advanced technology in spin electronics
Basic
phenomena
GMR
TMR
Spin transfer
Functional
materials
Modeling
Ferro/Antiferro
coupling
Analytical
models
Materials with
perpendicular
magnetization
Micromagnetism
Nanoparticles
CAD tools
Finite elements
Data storage
Spintronics
Patterned media
MRAM
GMR/TMR sensors
Microwave
oscillators
Recording heads
Thermally assisted
recording
Magnetic logic
gates
Created January 2002 as joint CEA/CNRS laboratory affiliated to MINATEC R&D
center
27 permanent staff members, 16 PhD students and 7 post-docs
SPINTEC – URA 2512
4
Why MRAM ?
• Non-Volatility of FLASH
• Density competitive with DRAM
• Speed competitive with SRAM
SPINTEC – URA 2512
5
Why Thermally Assisted MRAM ?
Problems in conventional MRAM
Selectivity –> difficulty in writing a single junction
Scalability –> electromigration in magnetic field lines with decreasing in-plane size
Thermal stability -> reduced life-time of written information
New approaches
1.
Thermally assisted MRAM (Spintec Patent + lab. demo)
- good thermal stability ensured by exchange coupling of the storage layer
with an Antiferromagnet;
- high selectivity;
- low power consumption during writing at high temperature.
2.
Current induced magnetization switching
- linear decrease of power consumption with decreasing junction in-plane area
3.
Possibility to integrate 1 and 2
SPINTEC – URA 2512
6
Outline
1.
TA-MRAM. Definition, structure and principle of operation.
2.
Electric characterization
3.
Regimes of operation. Power of the electric pulse PHP vs. junction
temperature TAF.
4.
Exchange bias as a temperature probe. Electric pulse width d vs.
junction temperature TAF.
5.
Conclusions
SPINTEC – URA 2512
7
Thermally Assisted MRAM (TA-MRAM) – principle



Hw
Iw
IHP
ON
OFF
Heating
Status 0

Hex
PHP = 0
Hw = 0
Writing

R
H
0
T = Room Temperature
SPINTEC – URA 2512

Hw
T  Blocking Temperature
Status 1
Cooling
R
PHP = 0
Hw < 0
OFF
OFF
ON
R 1
Hex
H
PHP = 0
Hw = 0
T = Room Temperature
8
H
Thermally Assisted MRAM (TA-MRAM) – structure
TMR = 30%
450 nm
Hard mask
Vbias = 40 mV
Ta (90nm)
(thermal barrier)
Al (20nm)
Storage layer
Tunnel barrier
Reference layer
IrMn (5nm)
NiFe (3nm)
CoFe (1.5nm)
AlOX (0.5 nm)
CoFe (3nm)
Ru (0.8 nm)
CoFe (2.5nm)
PtMn (20nm)
Hard mask
(thermal barrier)
SPINTEC – URA 2512
Ta (50nm)
9
Thermally Assisted MRAM (TA-MRAM) – electric characterization
I MR Tester
H
(I>60 mA, |H|<1000 Oe)
Equivalent electric circuit of the MRAM device
(by network analyser)
Trigger
Pulse Generator
(d>1.5 ns, UPG<3.8 V)
1
2
0
MTJ
OSC
PPB
54 pH
2UPG
~
Switch
4.2 Ω
ZPG = 50 Ω
UAPPL
54 pF
RMTJ
≈ 800 Ω
ZOSC = 50 Ω
No amplitude attenuation (UAPPL=UMTJ) for d > 1 ns
pulse width
What do we measure ?
UMTJ = 2UPG – (ZPG+ZOSC)I
U OSC (mV)
50
40
30
I = UOSC / ZOSC
20
10
0
0
50
Time (ms)
SPINTEC – URA 2512
100
UMTJ
PHP = UMTJ I
10
Thermally Assisted MRAM (TA-MRAM) – measurement procedure
YES
WRT
PHP (d)
SPINTEC – URA 2512
Electric
pulse
PHP
HSET
6
SAT
800
-3
W)
H EX
750
SAT
H EX
700
WRT
H EX
WRT
H
EX
5
= 0 Oe
4
3
2
1
650
-500
P HP (x10
WRT
HEX<HEX ?
NO
850
WRT
Quasi-Static MR cycle (HEX,
HC, R)
Time
R ( )
Attempt to reverse HEX
(positive magnetic field pulse
HSET and heating pulse of
power PHP)
d
Increase heating pulse power PHP
Decrease pulse width d. Set PHP to PHP,min
Saturate
HEX
(negative
magnetic field pulse -HSET and
heating pulse of power PHP, MAX)
Magnetic field pulse
How do we measure ?
Set pulse width d to dMAX. Set
pulse power PHP to PHP, MIN
0
0
H (Oe)
500
-9
-8
-7
-6
-5
-4
-3
-2
-1
10 10 10 10 10 10 10 10 10
d (s)
11
0
10
Thermally Assisted MRAM (TA-MRAM) – operation regimes
TAF
Stationary regime
(TAF=TRT+PHP)
PHP
Transient regime
TAF
Temperature T
1-D model of heat diffusion in the MTJ stack

 T
kTB
PHP
Magnetic stack (ci, i, di)
  ciidi  cTB TB dTB 



2
T

T

RT

 t
dTB
S
i


Thermal barrier 1 Thermal barrier 2
(cTB, TB, dTB)
(cTB, TB, dTB)

 t 

TAF  TRT  PHP 1  exp 


 TR 

d 
TR  TB   ciidi  cTB TB dTB 
2kTB  i

d
  TB
2kTB S
Layer position z
c
k

d
- specific heat capacity J/(Kg K)
– thermal conductivity W/(K m)
- density Kg/m3
- layer thickness m
SPINTEC – URA 2512
Time
12
Thermally Assisted MRAM (TA-MRAM) – transient regime
dPUMP  dPROB
W)
WRT
P HP (x10
-3
SPINTEC – URA 2512
120 MHz
maximum writing
frequency
WRT
P
HP
= 4.6 mW
5
d = 10 ns
WRT
P
4
HP
= 2.8 mW
3
2
0
5
10
-9
15
20
d (x10 s)
-3
W)
5
WRT
P PROB (x10
Magnetic field pulse
PPROB
Electric pulse
Electric pulse
PPUMP
HSET
Time
TR=2.7 ns
d = 3 ns
6
WRT
4
P
3
P
HP
(d = 3 ns)
WRT
HP
TR=2.7 ns
(d = 10 ns)
2
0
20
40
60
80
100
-9
 (x10 s)
13
Thermally Assisted MRAM (TA-MRAM) – stationary regime
200
Measurement time per
point is 50 ms >> TR ->
always in stationary regime
H EX , H C (Oe)
H
C
100
0
=105 K/W
H
-100
EX
-200
20
70
T
AF
o
120
170
( C)
  - measured as a function of temperature
  - measured as a function of PHP and converted into temperature according to
TAF = TRT +  PHP
SPINTEC – URA 2512
14
Thermally Assisted MRAM (TA-MRAM) – consistency of results
Material
c*
J/(K kg)
k**
W/(K m)
16327
144
4.3
PtMn
12479
247
4.9
CoFe
8658
446
37
kTB = 4.38 W/(K m)
Ru
12370
239
120
c d  c
Ta***
Al
Theory – 1D model of heat diffusion

kg/m3
(b)
AlOx
3900
900
27
IrMn
10181
316
5.7
NiFe
8694
447
37
Al
2700
904
235
SiO2
2200
730
1.4
*for metallic alloys calculated according to
Dulong-Petit law
**for metallic alloys calculated according to
Widemann-Franz law
***confirmed
by
electrical
resistivity
measurements (170 mxcm) and XRD scan
TB = 15846 kg/m3
cTB = 154.1 J/(K kg)
i
TB TB dTB
i i i
 0.306 J/(K m2 )
Experiment
( = 105 K/W, TR = 2.7 ns)
kTB  dTB 2S  4.53 W/(K m)
c d  c
i
i i i
TB TB dTB
 TR S  0.315
J/(K m2 )
SPINTEC – URA 2512
15
Thermally Assisted MRAM (TA-MRAM) – consistency of results
3-D simulations of heat diffusion in the MTJ stack
(measured write power is used as input)
Electric pulse
300 oC
junction stack
200 oC
100 oC
25 oC
electrodes
 = 0.84 x
200
Ta
SiO2
105
250
K/W
T AF ( C )
o
o
150
100
231 nm
TR = 2.3 ns
200
T AF ( C )
SiO2
Ta
150
100
50
50
0
0
1
P
HP
(x10
SPINTEC – URA 2512
2
-3
W)
0
3
6
9
12
-9
 (x10 s)
16
15
Thermally Assisted MRAM (TA-MRAM) – conclusions
Two temperature regimes of the MTJ evidenced:
 transient temperature regime for pulse widths d < 9 ns;
 stationary temperature regime for longer pulse widths; in this
regime, the relationship between the temperature of the
storage layer TAF and the power of the electric pulse PHP is
linear: TAF = TRT + 105 (K/W) PHP.
Use of thermal barrier layers reduces the electric power density
required for writing but also decreases the writing frequency
The writing power density increases with decreasing pulse width
from 0.6 mW for d = 1 s up to 6 mW for d = 2 ns; even in the range
of pulse widths 1 s - 10 ns, where the storage layer reaches the
stationary temperature regime by the end of the electric pulse,
the writing power shows a 500 % increase.
SPINTEC – URA 2512
17
Thermally Assisted MRAM (TA-MRAM) – exchange bias as temperature
probe
AFM layer = collection of
non-interacting single domain
grains
(-)
(+)
FM layer = single domain
mAF  m AF D a0
D
HE  JINT mAF
mAFM

S
HE
JINT  J0 mF m AF
2
E / S  KAF tAF sin   JE cos 
2KAFtAF > JE


peq  1 1  e 2JE S kBT 
1



 f0 e eb JE S kBT   e eb  JE S kBT 

eb  K AF t AF 1  1 4 JE K AF t AF 
(-)
2
JE  JINT a0D 
(+)
E/S

p t   peq 1  e t   pini e t 
eb
JE
JE
2KAFtAF < JE
0
45
90
135
180
 (deg)
SPINTEC – URA 2512
18


Thermally Assisted MRAM (TA-MRAM) – exchange bias as temperature
probe
Ni80Fe20/Ir20Mn80 Ferromagnetic/Antiferromagnetic bilayer
TNeel=350 oC, a0=2.7 Å, l=0.33
JINT = 8.11x10-15 erg
KAF = 2.44x105 erg/cm3
 T T  d
B
R.White et al, J.Appl.Phys. 92, 4828 (2002)
d = 2 ns
d = 100 s
tAF = 6 nm
300
400
1.0
300
tAF = 12 nm
0.6
100
0
0.4
T B /T N
 t
0.8
o
200
TB ( C)
tAF = 10 nm
200
0.6
100
0.4
0
-100
-100
0.2
0.2
TB = 50 oC
0.0
15
20
25
30
35
40
45
AF grain size D (nm)
TB = 275 oC
-200
0.0
15
20
25
30
35
40
-200
45
AF grain size D (nm)
Decreasing the pulse width d may require a considerable increase of the
writing temperature !!!
SPINTEC – URA 2512
19
o
tAF = 8 nm
0.8
T B /T N
tAF = 4 nm
TB ( C)
1.0
400
Thermally Assisted MRAM (TA-MRAM) – exchange bias as temperature
probe
100
1.0
H EX , H C (a.u.)
2
-100
HEX
-200
Probability (a.u.)
H EX , H C (Oe)
HC
0
1
0
10
20
30
d=
() 1 s
() 10-3 s
(▲) 10-7 s
H
C
0.5
0.0
H
EX
-0.5
40
-1.0

AF grain size (nm)
0
50 100 150 200
Temperature (oC)
25
200 nm
100
175
250
o
( C)
T
AF
TAF = TRT + 8.97 x103 PHP
1.0
KAF = 7.3 x 105 erg/cm3
JINT = 8.5 x 10-15 erg
0.5
H EX (a.u.)
KF = 1.3 x 104 erg/cm3
PHP=
() 0.38 mW -> 58 oC
() 0.69 mW -> 85 oC
(▲) 0.92 mW -> 105 oC
(▼) 1.24 mW -> 133 oC
(◄) 1.47 mW -> 153 oC
(►) 1.7 mW -> 173 oC
() 2.24 mW -> 220 oC
0.0
-0.5
-1.0
-7
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
10
d (s)
SPINTEC – URA 2512
20
HP
Write power P
3
2
1
0
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
10 10 10 10 10 10 10 10 10 10
Pulse width d (s)
o
AF
(x10
WRT
W
4
( C)
HEX = 0 Oe
5
AF temperature T
WRT
6
-3
W)
Thermally Assisted MRAM (TA-MRAM) – exchange bias as temperature
probe
300
250
200
150
100
50
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
10 10 10 10 10 10 10 10 10 10
Pulse width d (s)
WRT
 3-D simulation of heat diffusion – experimental write power PHP (d) is used as input
WRT
Exchange bias model - temperature required to set HEX = 0 for heating time equal
to the pulse duration d; temperature pulse shape is assumed rectangular.
WRT
Exchange bias model - temperature required to set HEX = 0 for heating time equal
to the pulse duration d; temperature pulse shape is calculated by 3-D simulation of
heat diffusion.
SPINTEC – URA 2512
21
Conclusion
Writing temperature increases with decreasing pulse width d as a
consequence of thermal relaxation in the Antiferromagnetic storage
layer and approaches the Neel temperature in the limit d -> 0.
Exemple: writing with 2 ns pulses imply heating at about 300 oC with
possible negative effects on the integrity of tunnel barrier and
storage layer antiferromagnet.
Solution:
decrease
the
writing
temperature
by
using
antiferromagnets of lower Nèel temperature than IrMn (TN ≈ 350 oC)
for pinning the storage layer.
SPINTEC – URA 2512
22