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Pufall, THIC’05: 1
MRAM: Device Basics and Emerging Technologies
Matthew R. Pufall
National Institute of Standards and Technology325 Broadway, Boulder CO 80305-3337
Phone: +1-303-497-5206 FAX: +1-303-497-7364E-mail: [email protected]
Presented at the THIC Meeting at the National Center for Atmospheric Research, 1850 Table Mesa Drive, Boulder CO 80305-5602
July 19-20, 2005
Pufall, THIC’05: 2
Collaborators:
NIST:Bill RippardShehzu KakaSteve RussekTom Silva
Hitachi Global Storage:Jordan Katine
Freescale:Fred MancoffNick Rizzo
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Outline
• What is MRAM? What are its advantages?• When will we see MRAM?• How does MRAM work?
• Spintronics basics: Electron spin and Magnetoresistance• Anatomy of an MRAM bit• Magnetic Switching
• Engineering Challenges: Consistency and Thermal Stability• Freescale’s MRAM solution: “Toggle” MRAM• Big Problems in the Nano-Future: Scaling of bits• Emerging Solutions to Scaling: Spin Torque Switching
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What is MRAM?Magnetic-based Random Access Memory:
Uses small magnetic element to store 1,0 rather than electric charge
(Some) Other types of RAM:Storage Method Virtues
DRAM Charge on capacitor speed, size, costSRAM Multiple transistors speed, size, no refreshFRAM Ferroelectric capacitor nonvolatile, speedFlash Transistor w/ extra nonvolatile, cost, size
isolated gate
…All have advantages/tradeoffs: No “universal” solution
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MRAM advantages:
NonvolatileData don’t need refreshing
• “instant on”, low power• Data retention >10 yrs
FastRead/write symmetric
• 25 ns (10 ns in demo)• byte writeable
Unlimited cycling No “fatigue” after >1016 cycles
Viability Integrated into CMOS process
…If made (very) inexpensive and scalable, a potential “universal” solution
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When (and where) will we see MRAM?
Freescale:Demos out to vendors Shipping product end of 2005/early 2006
Others: IBM—In developmentToshiba/NECCypress, Honeywell: ?
Uses: Replace battery-backed SRAMCell phone/embedded memory
4 Mb chip, 180 nm process
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How does MRAM work?
Mx
y
zHysteresis Loop
-1 0 1Magnetic Field H
My
“1”
“0”-1
1
Field H
Ferromagnets have hysteresis: Information stored in state @H = 0
• Hard disks, tape storage
Magnetic fields used to change direction (state) of M
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How do you sense state of M?Magnetoresistance (MR): Resistance depends on
direction of Mcurrent I
Mfree
Mfixed
Low Resistance High Resistance
How? Electrons also have magnetic moments: Spin
“Spintronics”
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Magnetization Filters e- Spin:Electron spins become spin-polarized in direction of M: Spin filter
Magnetization M1
Transmitted spinsIncident e- current
Nonparallel spins scatter more: Higher resistance
∆R ~ cos(θM)
Sense M by Resistance:0.6x1.2µm bit at 300mV bias
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Mfree
7.5
8
8.5
9
9.5
10
10.5
11
-10 -7.5 -5 -2.5 0 2.5 5 7.5 10
MR=37%
7.5
8
8.5
9
9.5
10
10.5
11
-10 -7.5 -5 -2.5 0 2.5 5 7.5 10
Field H
7.5
8
8.5
9
9.5
10
10.5
11
-10 -7.5 -5 -2.5 0 2.5 5 7.5 10
Magnetic Field H
RA
(kΩµm
2 )
Mfixed
Current I
State of M determined by electrical measurement:“Magnetic Tunnel Junction” (MTJ)
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Anatomy of an MRAM bitBit Line
Line
BL Program Line
Tunnel Barrier
Pinning Layer
Digit Line
Bit Line
Pinned Ferromagnet
Free Ferromagnetic Layer
Heasy
M
-75 -50 -25 0 25 50 7505
10152025
3035
MR (%)Hhard = 0 Oe
Heasy (Oe)
RA = 10 kΩµm2
Hhard = 40 Oe
Hard axis field decreases Hc
Hhard
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Bit Selection Energetics
Eb
Eb
E
θ
“0” “1”
0 π
Eb
Hhard ≠ 0
(a)
(b)
Heasy≠ 0
Eb
Eb
E
θ
“0” “1”
0 π
Eb
Hhard ≠ 0
(a)
(b) Heasy > 0 orHhard > 0
Heasy = 0Unselected
½-SelectedLower barrier
Heasy≠ 0
(c)Selected (c) Heasy > 0 andHhard > 0
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Bit Addressing Challenges
IhardHhard
Heasy
MRAM bit
X
Programming Probabilityfrom 256k chip
ibit
idigit
Min. Fail bits
ibit
idigit
Min. Fail bits
Operating point
Ihard
IeasyData courtesy Freescale
Bits switched/addressed by two fieldsChallenge: Bit-to-bit variations make choosing
proper currents difficult/impractical
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Freescale’s Solution: “Toggle” MRAM
Bit LineBL Program LineBit LineProgram Line 2
Line
Free Tri-Layer
Tunnel Barrier
Line 1
Pinned Ferromagnet
Pinning Layer
Ferromagnetic layerCoupling LayerFerromagnetic layer
Program Coupled tri-layer programs more repeatably
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How does Toggle work? TimingAnti-parallel-coupled layers respond differently to fields:
HY
HY HY
LowResistance
State
HighResistance
State
HX HX
HX
Figure courtesy Freescale
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What does Toggle do?• Moves “1/2 select” error horizon:
H 2
toggling
toggling
I
IIIII
IV
No disturb
No disturb
No disturb
No disturb
toggling
toggling
I
IIIII
IV
No disturb
No disturb
No disturb
No disturb
i bit
idigit
Operatingregion
0% switching region(no disturbs)
H1
Also increases bit volume (& thermal stability)
4Mb, March 6N Toggle Map
Data courtesy Freescale
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Freescale 4Mb MRAM Layer structure
Program Path: Dashed green lineSense Path: Red line (isolated)
Chip process at 180 nm node: Design ported to 90 nm (May ‘05)
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Future Difficulties: Scaling
2003 2004 2005 2006 2007
DRAM ½ pitch (nm) 100 90 80 70 65MPU Physical Gate Length (nm) 45 37 32 28 25
From ITRS roadmap, 2003
…MRAM must compete with this (aggressive!) scaling to be viable
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Scaling Problem: Thermal StabilityAs bits get smaller: more susceptible to thermal fluctuations
Energy barrier proportional to volume, anisotropy: Must increase anisotropy to keep constant
Eb
E
θ
“0” “1”
0 π
Eb
E
θ
“0” “1”
0 π
But, bigger anisotropy—Need bigger fields to switch bit! Engineering problem…
Thermal fluctuations: Problem in hard disk media, read headsGeneral concern in nanoscale devices!
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Possible Solution: Spin TransferElectron spins become spin-polarized in direction of M: M exerts torque
Transmitted spins
Magnetization M1
Incident e- current
Reverse also happens: polarized spins exert torque on M
Torque
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Spin-Transfer-Driven SwitchingSign of torque depends on direction of current: Causes magnetizationmotion
-4 -2 0 2 4
7.35
7.40
7.45
7.50
7.55
7.60
7.65
dV/d
I (Ω
)
Current (mA)
Current-driven hysteretic switching
Free layer
Polarizer
Electron current
Bistable device: Current throughdevice drives switching
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High Speed ST-Switching
100 1000 10000
0.0
0.5
1.0
Sw
itchi
ng P
roba
bilit
y
Pulse Duration (ps)
7.8 6.2 4.9 3.9 3.1 2.5 2.2 2.0 1.8
Pulse Amplitude Current (mA)
-4 -2 0 2 47.1
7.2
7.3
7.4
7.5
dv/d
I (Ω
)
I (mA)
µ0H = 66.7 mT
30 nm
Katine HGST
0 -1 -2 -3 -4 -5 -6 -7 -8-0 .5
0 .0
0 .5
1 .0
1 .5
2 .0
2 .5
3 .0
3 .5
Q u a s i- s ta t ics w itc h in g c u r re n t
1/τ 95
(ns-1
)
I (m A )
<300 psswitching time!
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Spin Transfer Advantages
IhardHhard
Heasy
MTJ bit
X• Removes X-point field lines—
simpler lithography, two terminal devices
• Becomes more efficient as devices shrink: Scalable
Spin Transfer: ~1/d2Fields: ~1/d
ddd
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Spin-Transfer-Driven Oscillators
Monostable device (high fields): Current-tunable, Coherent, microwave
magnetization precession
applied field H16.00 16.25 16.50
0100200300400500600700800900
100011001200
Pow
er (p
W)
Frequency (GHz)
Cur
rent
(mA
)
8
3
5
µ0H = 0.9 TElectron current
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SummaryMRAM is possible “universal” memory solution
• “Spintronic” device: Uses e- charge and spin• Fast (3-10 ns switching times)• Nonvolatile (magnetic storage)• Low power (no refresh)• Rad-hard (though supporting electronics aren’t!)
Toggle MRAM solves ½-select problemProblem: Must scale competitively with Si technology
• Nanomagnetic elements sensitive to temperature• Complicated lithography
Emerging technology solution: Switching with Spin-polarized electron currents
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A Brief History of MRAM
BIWB Core memory, first disk drive, flat film, bubble, plated wire
1984-86 A. V. Pohm and Honeywell investigating radiation hard memory based on anisotropic magnetoresistance (AMR)
1987 GMR Discovered: Binash, Grunberg et al, PRB B 39, 4828 (1989); Baibich, Fertet al. PRL 61, 2472 (1988).
1989 NVE formed to develop AMR based MRAM
1990 IBM Spin Valve (B. Dieny, V Speriosou, S. S. Parkin, B Gurney et al.)
1994 IBM Spin valve MRAM patent (D. D. Tang et al. IBM)
1995 Magnetic Tunnel Junctions (MTJ) (Modera et al PRL 74, 3273, 1995)
1996 DARPA MRAM program: Honeywell (PSV), Motorola (PSV⇒MTJ) IBM (MTJ)
1999 IBM, Motorola MRAM working demos??
2002 Motorola goes to cladded write lines & tToggle write
2004 Motorola samples 4M MRAM chip
2004/2005 230% TMR in MgO MTJs; Spin transfer switched MRAM
1991 IBM fir
st hard-disk
MR head
1998 IBM intro
duces first
GMR head
1984 IBM fir
st tape M
R head