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September 20, 2007
Technical Overview of Magnetoresistive Random Access Memory (MRAM)DISKCON USA 2007 , Santa Clara, California
Jon SlaughterManager, MRAM Magnetic Materials and Process Integration
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Outline
Overview of Toggle MRAM WritingReadingReliabilityExtended temperaturesScalingSummary
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MRAM Advantages
Nonvolatile
Fast
Unlimited Cycling
Modular Integration
Highly Reliable
Extended Temperatures
Data Retention - ≥ 20 years
Symmetrical Read/Write – 35ns
Unlimited Endurance - ≥ 1016
Easily Integrates in Back EndCompatible with Embedded Designs
-40 ºC < T < 150 ºC Operation Demonstrated
Intrinsic Reliability Exceeds 20 Year Lifetime at 150 ºC Continuous Use
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First Commercial MRAM now in Volume Production
4 Mb Toggle MRAM • 35ns symmetrical read/write• Unlimited endurance• Data retention > 20 Years• 256Kx16bit organization• 3.3V single power supply• Fast SRAM pinout• Consumer temperature range(0 °C – 70 °C)
• Extended Industrial temperature range(-40 °C – 105 °C)
Available now
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How MRAM Works
ON for sense OFF for
program
MTJ
iref
Write Line 2
Write Line 1
i
i
isense
Information stored as magnetic polarization
• Detected as a resistance state
Isolation transistor can be logic device, no high on/off ratio needed
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4 Mb MRAM bit cell
Metal 4
MTJ
MRAM module
Contact
Via 1
Metal 2
Via 2
Metal 4
Via 3
Metal 5
Metal 3
Bit cell
Metal 1
Cu
Cu
Al
Al
Al
Metal 4
Metal 5
MTJ
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Outline
Overview of Toggle MRAM WritingReadingReliabilityExtended temperaturesScalingSummary
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Free Layer Field Response
H=0H=0
Conventional MRAMSingle Layer
H≠0
Toggle MRAMCoupled Trilayer
H=0 H≠0
Aligns with applied field Rotates perpendicularto applied field
pinned pinnedpinned pinned
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Toggle Write Operation
I1
I2
High-RState
Low-RState H1
I1
H1 + H2
I2
H2
I2
I1
MTJ
pinned pinned
Advantages: Eliminates disturb - Large operating window
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Toggle-bit Array Characteristics
4Mb, March-6N Toggle Map
No switch
100% toggle
Cur
rent
1
Cur
rent
1
No switching
No
switc
hing
SwitchingRegion
Bit SaturationCurrent 2Current 2
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Outline
Overview of Toggle MRAM WritingReadingReliabilityExtended temperaturesScalingSummary
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Read Distribution within an Array
14 15 16 17 18 190
500
1000
1500
2000
2500
#bits
Bit Resistance [kΩ]
Low State High State
σ~0.8%
∆R/σ = 30typical
Resistance of bits in a 4Mb array
10 nm
Optimized for MRAM
Critical Factors:1. Tunnel barrier quality2. Pattern fidelity
Critical Factors:1. Tunnel barrier quality2. Pattern fidelity
V1/2~+0.7/-0.55 V
TMFreescale™ and the Freescale logo are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners. © Freescale Semiconductor, Inc. 2006.
Outline
Overview of Toggle MRAM WritingReadingReliabilityExtended temperaturesScalingSummary
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Tunnel Junction Reliability: Read
LowState
HighState
Necessarymargin for
spec. read- out speed
Margin beforedrift
MarginafterdriftN
umbe
r of B
its @
RBit Resistance
1 10 100 1000 10000 1000000
1000
2000
3000
4000
5000
6000
7000
0.6V
0.8V1.0VT=175 C
Res
ista
nce
[Ω]
Time [s]
Dielectric Breakdown(catastrophic)
Resistance Drift(gradual)
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Intrinsic Reliability vs. TemperatureMTJ failure modes as a function of address cycles
1E+15
1E+16
1E+17
1E+18
1E+19
1E+20
75 85 95 105 115 125
Average Junction Temperature (C)
Max
Cyc
les
/ Add
rMagnetic Bridging
20 year Continuous operation
Res DriftTJ TDDBEMMagn Bridging
1E+15
1E+16
1E+17
1E+18
1E+19
1E+20
75 85 95 105 115 125
Average Junction Temperature (C)
Max
Cyc
les
/ Add
rMagnetic Bridging
20 year Continuous operation
Res DriftTJ TDDBEMMagn Bridging
Res DriftTJ TDDBEMMagn Bridging
• Resistance Drift = Change in resistance of the magnetic cell materials (2% resistance change criteria)• TJ TDDB = Time Dependant Dielectric Breakdown of the Tunnel Junction Oxide• EM = Time to Failure by Electromigration in the copper write lines (10% resistance change criteria)• Magn bridging = Interlayer bridging in the magnetic materials that would cause the magnetic properties to change
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MRAM Endurance Cycling
0%
100%
1 100 10000 1000000 1E+08 1E+10 1E+12 1E+14
Number of Read/Write Cycles
Pass
ing
%
MRAM: Unlimited Read/Write Endurance
MRAM Endurance Tested to 58 Trillion Cycles with No Change in Critical Parameters.
Data from >2800 bits from 900 devices 8 orders of magnitude more cycles than current Flash technologyNo known failure modes are seen or expected.
Flash Capability
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Outline
Overview of Toggle MRAM WritingReadingReliabilityExtended temperaturesScalingSummary
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OperatingRegion
T = 125 ºC
Non-SwitchingRegion
Bit Line Current
Dig
it Li
ne C
urre
nt
OperatingRegion
T = 125 ºC
Non-SwitchingRegion
Bit Line Current
Dig
it Li
ne C
urre
nt
Fiel
d (O
e)
0
50
100
150
200
250
300
0 50 100 150Temp (°C)
Hsat
Hsw
Write Operating Region at 125 °C
•Hsat, Hsw decrease linearly w/Temp, reducing window
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Read Signal vs. Temperature
0.9 1 1.1 1.2 1.3Normalized Rcell
150 °C25 °C
“0” “1”
Less read margin at 150 °C, but distributions are still well separated
0
5
10
15
20
25
30
35
-50 0 50 100 150
MR
(%)
MR decreases with temperature
T (°C)
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Read Speed with Temperature
Read access time strobe for temperatures-40 to 125 °C
Meets specifications over industrial temperature range
26 28 30 32 3410-1
100
101
102
103
104
105
106
107
Fail
bits
Access Time Strobe (ns)
0
T = -40 C
T = 125 C
TMFreescale™ and the Freescale logo are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners. © Freescale Semiconductor, Inc. 2006.
Outline
Overview of Toggle MRAM WritingReadingReliabilityExtended temperaturesScalingSummary
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Historical Trend of TMRRoom temperature TMR values reported in the literature
050
100150200250300350400450500
1994 1996 1998 2000 2002 2004 2006 2008
Year
TMR
at R
T (%
)
TohokuMIT
AnelvaSonyTohokuIBMFujitsu CNRS
MPI CSIC
AIST
AIST
AnelvaAlOx
Tunnel Barriers
MgOTunnel Barriers
Freescale
Tohoku & Hitachi
Freescale
IBMAIST
IBM
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Read Margin with MgO Tunnel Barrier
0
5000
10000
15000
20000
25000
0.8 1 1.2 1.4 1.6 1.8 2Normalized Rcell
Num
ber o
f Bits
σ~1.5%
49 σ
“0” “1” Much higher MR with MgO replacing AlOx in tunnel barrier
• MR/σ = 2X AlOx
Demonstration in 4Mb circuit
AlOx → MgO
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MRAM Cell Integration in 90nm BEOL
Full integration of MgO-based MRAM devices with 90nm front end CMOS.
MRAM process with clad Cu write lines.
8 kb arrays of memory cells
Cell Size 0.29 µm2 –Linear shrink from 180nm
MTJ resistance of 1kohm-µm2
Toggle write characteristics
90nm CMOS
MRAM
0.29µm 2Bit CellIEDM 2005
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Large Operating Window at 90nm
Effective operating
region
Operating region where the bit toggles when both write currents are over their threshold values.
Scalability of the toggle MRAM concept to 90nm is demonstrated.
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Spin-Torque MRAM
∆S
IDC
Each spin flipproduces ∆S = ħ
free magnettunnel barrier
∆S∆t
= Torquefixed magnet
Jc ≈ 107 A/cm2Isolationtransistor
Advantages: Smaller cell size, lower write current
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Distribution Considerations
Vswitch VbVread
σbσsσr
6*(σs+σb)
6*(σs+σr)
Spin-transfer MRAM
+/- Vswitch
Iswitch
Applied Voltage
• For Small Cell – Iswitch = Vswitch/Rcell must be <<1mA• Vswitch << Vbreakdown for good Endurance• Vread << Vswitch to avoid Disturb• Vread >> transistor mismatch requirement• 10 Yr Data Retention requires Eb / kbT ≥ 50
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High MR at Low RA MTJ Material
0
20
40
60
80
100
120
0 1 2 3 4 5 6 7RA (Ω-µm2)
MR
(%)
2 nm
X-TEM: MgO-based MTJ Stack
Fixed
Free
Best process gives good MR down to < 2 Ω-µm2
TMR vs. RA for Optimized Lowest-RA MTJ Material
MgO
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Batch Fabrication of MgO Tunnel Junction Nanopillars
•200 mm Si wafer• Optical lithography: • 100 nm x200 nm bits
50 nm
SiO2
Top electrode
Bottom electrode
100nm
500nm 2 nm
RA (Ω−µm2) MR (%) IcP→AP (mA)
JcP→AP: ~4.5x106 A/cm2Quasistatic
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Summary
• Advances in magnetic materials and devices have enabled in the first commercial MRAM
• Toggle MRAM is the only MRAM technology shipping today• High-performance, high-endurance and highly reliable non-
volatile memory• Extendable to industrial and automotive applications
-40 °C – 105 °C operation available now
• New materials and devices show the potential for advanced scaling and further performance improvements
• High TMR from MgO enables high-speed and scaling• Spin-Torque has potential to achieve high densities
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