mram 2 dieny.pdf
TRANSCRIPT
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Part 2: Spin Transfer Torque RAM
(STTRAM)
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In-plane magnetized STTRAM
Reliability issues in STTRAM
Out-of-plane magnetized STTRAM
Downsize scalability of STTRAM
Part 2: Spin Transfer Torque MRAM
OUTLINE
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Thermally Assisted (TAS) STT-TAS
Hx
Hy
Field-driven STT (STT MRAM)
Perpendicular
Precessional
DW motion
Planar
Spin-orbit torque
(spin-Hall, Rashba)
Several families of MRAM
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Current through cell proportional to MTJ area
jwrite SST in-plane ~ 8.105A/cm quasistatic
~ 3.106A/cm @10ns
STT MRAM scalability of write current
Writing 0 Writing 1
+
= K
M
P
te
jSF
planeinWR 22
22
0
h
Huai et al, Appl.phys.Lett.87, 222510 (2005) ;
Hayakawa, Jap.Journ.Appl.Phys.44 (2005) L1246
ONjSTT
Vdd
0
freepinned
ONjSTT
0
Vdd
Writing determined by a current density :
Field written
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STT MRAM Write influence of current pulse width
Thermally activatedSTT switching
PrecessionalSTT switching
In-plane magnetized MTJ
Hosomi et al, Sony, 4Kbit demo (2005)
In the thermally activated regime:
Ic0= extrapolated STT switching current at 1ns pulse width
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Courtesy Grandis
STT MRAM compact cell
Vertical transistor technology may allow even smaller cell
Below 45nm, transistor does not limit cell size
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STT MRAM Distribution consideration
~1.4-1.8V~0.4-0.7V~0.15-0.2V
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STT MRAM Key parameters
(thermal stability factor) data retention, read disturb, operating temperature range,downsize scalability
( should be between 60 and 80 depending on chip density)
TMR (read signal) read speed, sense margin, tolerance on RA dispersion
TMR above 150% to achieve fast (sub 10ns) read-out.
Jc0 (write current density) cell size, write speed, write consumption, reliability
J c below 1.106A/cm are desirable to insure select transistor size smaller than MTJ
Vbd (MTJ breakdown voltage) reliability, endurance
Vbd should be above 3 times write voltage (typically Vbd~1.5V at 10ns and Vwrite~0.5V)
Key challenge is to achieve low write current and good retention at the sametime
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RETENTIONWRITABILITY
Write current (in-plane MTJ):
Thermal stability factor (in-plane magnetized MTJ)
(A=area, =STT efficiencyHd=demag field=0Ms)=dampingMs=magnetizationtF=thickness of storage layer
assuming shape anisotropy dominates
Classical dilemma in memory technology :
STT MRAM Key parameters
Increasing the retention at small size requires to increase Ms and or tF but penaltyon write current.Importance to play on parameters such as Gilbert damping .
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Important parameters to reduce Jswitching :
-Gilbert damping as low as possible (NiFe, CoFeB ~0.007-0.01)
-Magnetization Ms as low as possible(Co75Fe25 Ms=1600emu/cm3, Co: Ms=1400emu/cm3, CoFeB: Ms~600emu/cm3,But must remain compatible with sufficient thermal stability factor
-Thickness of the switching layer as small as possibleBut not too small because TMR amplitude degrades when magnetic electrode thinner than~1.5nm and Gilbert damping increases
-Current polarization as large as possible (bccCoFe/MgO ~80%)
- Dual stacks with two antisymmetric pinned layers
- Increase perpendicular anisotropy in the storage layer
Reducing current density for switching with in-plane magnetized MTJ
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Gilbert Damping
Insulators: Yttrium iron garnet
1x10-4Gilbert damping Material
Y3Fe5O12
Semi metals: Heusler alloy
2x10-3Gilbert damping Material
NiMnSb
Metals:
0.01Co0.9
Fe0.1
0.007Ni0.8Fe0.2
0.10-0.15(Co/Pd) ML
0.2-0.3
Gilbert damping Material
(Co/Pt)
Out-of-planeanisotropy butlarge damping
Take advantage of the interfacial anisotropy at magnetic metal/oxide interface
Mostcommonly
used (CoFeB)
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Concept later used by Grandis
Dual MTJ
In-plane STT RAM: Dual stack with antiparallel pinned layers
MTJ with partial
out-of-plane
anisotropy
Proposed by SPINTEC in 2001: FR2832542 filed 16th Nov.2001, US6385082
Lower RA
higher RA
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In-plane STT RAM: In-plane magnetized MTJ with
partial out-of-plane anisotropy in the storage layer
Idea: Hd and Hk of opposite sign, Hd tends to bring back the magnetization in-planewhereas Hk tends to pull it out-of-plane
PinnedReference
layer
Storagelayer bcc
Fcc bilayer withPerpendicular
anisotropy
Pd/Co
Ta 0.2nm structuraltransition layer
CoFeB 2nm
MgO
CoFeB 3nmRu 0.7
CoFe 2nm
IrMn 7nm
However, concern with thermalstability at sub-45nm dimension
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2013: STT RAM product
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Summary for in-plane STTRAM
STT RAM development based on major breakthrough discoveries:-Giant TMR of MgO based MTJ-Spin transfer torque phenomenon
Giant TMR of MgO based MTJ relies on spin-filtering of electrons according tothe symmetry of their wave functions. Requires very good bcc crystallinity ofboth the MgO barrier and adjacent CoFe magnetic layers.
Low STT write current implies low Gilbert damping, low magnetization (at thecost of reduced thermal stability with in-plane shape anisotropy), high currentpolarization.
Toggle MRAM good for robust, low density NVM applications (automotive,spatial)
Inplane STT RAM: difficulty to achieve good retention and low write current atthe same time. Nevertheless some routes exist to reach dimensions ~ 40nm usingdual stack or structures with partial out of plane anisotropy.
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In-plane magnetized STTRAM
Reliability issues in STTRAM
Out-of-plane magnetized STTRAM
Downsize scalability of STTRAM
Part 2: Spin Transfer Torque MRAM
OUTLINE
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Example of cross-
sectional TEM viewof MgO MTJ
TMR = 130 %R.A = 30.mMgO barrier: 1.1nm
1.1nm
V
E ~ 5 108
V/m
Typical EBreakdown inoxides ~ 109V/m
~0.5V
STTRAM endurance
At each write event, the tunnel barrier is exposed to an electrical stress. How manycycles can it resist before electrical breakdown?
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Reliability of the tunnel barrier - endurance
t variable
30ns30ns
Vapplied= 1.25V
t =70nsExample:
Application of repeated pulses of voltage across the barrier until breakdown occurs
Accelerated conditions:
1.2Volt
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=
ttF exp1)(
Cumulative distribution of deadMTJ after N pulse of duration spacedby t follows a Weibull distribution:
with t=N and function oft
=30ns=30nst =70nsVapplied=1.3V
: Nb of cycles after which 1-1/e=63% of junctions have experienced breakdown
Extremely long endurance can be obtained in MTJ under specific working conditions.Different tunneling regime compared to standard CMOS working conditions
(thinner oxide, lower voltage, direct tunneling versus Fowler-Nordheimtunneling)
Reliability of the tunnel barrier - endurance
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STT RAM endurance
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Dramatic increase in endurance to breakdown for an intermediate delay time ~80ns.Understood in terms of trapping/detrappingof electrons on traps inside the MgObarrier with escape~80ns and stress induced by electrostatic forces between trappedelectrons and electrodes
Nbofcyclesto63%
bre
akdown
Pulses ofsame polarity
Pulses ofalternating polarity
30ns30ns
n
po
n Large number of electrons trapped inthe barrier (no time to escape). Large steadystress on the barrier.
p At each pulse, some electrons gettrapped but then escape betweenpulses. Alternating stress on the barrier.
oBalance between trapping anddetrapping:Moderate trapped charge and moderatetime-modulation of trapped charge
Reliability of the tunnel barrier - endurance
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-- - -+ + + +
MgO
Need to minimize density of trapping sites in MgO:
Oxygen vacancies in MgO Interfacial traps at BO/MgO interface if formation of BO. Dislocations in MgO due to lattice mismatch between MgO and CoFe (~4%)
Trapped electrons
CoFeB
CoFeB
Screening charges appearing in the metallic electrodes
Large stress generated on the barrier
Selma Amara, Applied Physics Letters 99 (2011) 083501
Reliability of the tunnel barrier - endurance
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Reliability of the tunnel barrier endurance
Dislocations as possible electron trapping sites
Possibility of improvement with ~10%V addition in CoFeB
06.4a287.2a FeFe ==Possibility to reduce the
crystallographic mismatch and
thereby the density of
dislocations by some V alloying21.4aMgO =
38.4a210.3a VV ==
TMR(%)
Hoogefac
tor(amplitudeof1/fnoise)
Herranz et al,
Appl. Phys. Lett. 96, 202501 (2010)
1/f noise measurement may become
a technique for characterizing the
endurance of MTJ without stressingthem
Optimum magnetic electrode composition: Co18Fe54V8B20)
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In-plane magnetized STTRAM
Reliability issues in STTRAM
Out-of-plane magnetized STTRAM
Downsize scalability of STTRAM
Part 2: Spin Transfer Torque MRAM
OUTLINE
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Thermally Assisted (TAS) STT-TAS
Hx
Hy
Field-driven STT (STT MRAM)
Perpendicular
Precessional
Planar
Several families of MRAM
DW motion
Spin-orbit torque
(spin-Hall, Rashba)
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+
=Tk
VM
pAg
Tkej
B
sBc
planein
2
)0(
4
h
In-plane versus out-of-plane STT switching
In-plane magnetized MTJ Out-of-plane magnetized MTJ
Thermal stability determined by in-
plane anisotropy (shape anisotropy)Simpler materials but additional penaltyin jc due to out-of plane precession
More complex materials but lower jcexpected thanks to direct proportionalitybetween J c and thermal stability
=pAg
Tkej Bc
perp
)0(
4
h
= damping
P = polarization
A = Area
g(0)~1
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STT switching in perpendicular MTJ
TbFeCo
TbFeCoCoFeB
CoFeBMgO
TMR = 10%, jc(30ns) = 5 106A/cm, (d=130nm) = 107
Toshiba (2008):
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A key point for p-STT MRAM is to be able to increase
the perpendicular anisotropy without increasing the Gilbert damping
Difficulty:Large anisotropy often implies large spin-orbit coupling (provided for instance by
Pt, Pd, Au) which yields large Gilbert damping .
In p-MTJ , current for switching by STT (Ic) proportional to Gilbert damping and tothermal stability factor
Material issue in perpendicular STT-MRAM
( )Tk
VMK
B
s 2/2
0
=
Solution found thanks to the existence of a large perpendicular anisotropy
at magnetic metal/oxide interface
=pAg
Tkej Bc
perp
)0(
4
h
= damping
P = polarization
A = Area
g(0)~1
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Surprisingly large perpendicular anisotropy at magnetic metal/oxide interface(Monso et al APL 2002)
S.Monso, et al, APL 80 (2002), 4157-9. First observation of PMA at Co/AlOx
B.Rodmacq et al,Journ.Appl.Phys.93, (2003), 7513. PMA at Co or CoFe/MgO, CrO, TaO
A.Manchon et al, Journ. Appl. Phys. 104, 043914 (2008). XPS, XAS, interpretation of PMA at M/Ox
B.Rodmacq et al, Phys.Rev.B79, 024423 (2009) Influence of annealing on PMA at Co/AlOxL.Nistor et a, IEEE Trans Mag., 46 (2010), 1412 Correlation PMA -TMR at CoFe(B)/MgO
Very general phenomenon ofperpendicular anisotropy observed at awide variety of M/Ox interfaces withM=Co, CoFe, CoFeBand Ox= AlOx, MgO, TaOx, CrO2,
Due to hybridizationbetween Co dz andO sp orbitals
Co-dz
O-pz
Perpendicular Magnetic Anisotropy (PMA)
at magnetic metal/oxide interface
-0.2
-0.1
0
0.1
0.2
-8 -6 -4 -2 0 2 4 6 8
Mz
Out-of-plane magnetic field (kOe)
3'30
1'30
3'002'302'00
Time of exposureto oxygen plasma
-0.8 -0.4 0 0.4 0.8
(T)
Underlayer/Co/Al
z
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1 3
2
CoFe
buffer/CoFe 1/Mg_x natural oxidation/Ru 5 (nm) Ta=330C
Ta=330Cbuffer/CoFe 1.6/Mg x natural oxidation/CoFe 2.5/Ru 0.8/NiFe 5/IrMn 10 (nm)
Very good correlation between max TMR and max PMA (tMg=1.2 nm)
PMA at magnetic metal/oxide interface
Correlation PMA energy TMR amplitude
Interfacial anisotropy energy atCoFe/MgO interfaceKs~1.5erg/cm=1.5mJ /mas large as at Co/Pt interfacedespite weak spin-orbitcoupling in Co, Fe, Mg, O
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Explanation of the correlation between PMA & TMR
Co(Fe)-O interfacial bond formation influences both TMR amplitude and PMA
Co
O
Optimumoxidation
hybridization between O sp orbitals andCo dz2 orbital combined with spin-orbitinteraction yields the interfacial PMA[A. Manchon et al., J.Appl.Phys. 104, 043914, 2008]
TMR ofCo/MgO/Co MTJ
TMRPMA
Mg
O
Co
O
Mg
Co
>
[Butler et al.,, J.Lee e t a l . Poster EV09 IntermagMMM 2010]
Perpendicular underlayer/Co/MgO
Better penetration of1 electronsthrough the barrier if O terminated
TMR TMR
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PMA at magnetic metal/oxide interface
Confirmation by ab-initio calculations
Ks=2.93 10-3J /m for two Fe/MgO interfacesi.e. 1.46 10-3J /m per Fe/MgO interfaceLower for under or over-oxidized interface
H.Yang, M.Chshiev, B.Dieny, Phys.Rev.B 2011.
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Ikeda et al, Nat.Mat.2804 (2010)
TMR=124%RA=18.m
=43(70 required for 1Gbit)
P-MTJ based on interfacial PMA (i-PMA)
No Pt nor Pd in the stack
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Perpendicular STT Demo Chips
Toshiba 64 Mb perpendicular STT
K.Tsuchida et al.ISSCC 2010
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50ns
10ns
253 cells
Perpendicular STT Demo Chips
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Thermal stability in p-MTJ
Two possible magnetization reversal mechanisms
1) Coherent rotation (preferred at small dimensions)
2) Nucleation of a reversed domain at pillar edge and propagation of domainwall across the pillar (preferred at larger dimensions)
In between Curlingmode as described by A.Aharoni
Domain wall width comparable to pillar diameter
The chosen switching mechanism will be the one with lowest energy barrier
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Thermal stability factor in p-MTJ
Thermal stability factor essentially determined by effective anisotropy and reversal
volume which can be limited to the nucleation volume if nucleation/propagation reversal:
Si-sub/Ta(5)/Ru(10)/Ta(5)/Co20Fe60B20(0.9)/MgO(0.9-1.0)/Co20Fe60B20(1.5)/Ta(5)/Ru(5)
Sato el al, Tohoku, Hitachi, Intermag 2011, CC04
Required for32Mbit
Dnucleation~45nm
J c MTJ area nucleation area
Coherent
rotation
Nucleation-propagation
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Thermal stability factor in p-MTJ
To favor coherent rotation rather than nucleation at edges and propagation ofdomain wall, it is preferable:
-To use magnetic material with large stiffness constant i.e. high Curie temperaturesuch as Co rich CoFe alloys (Tcurie Co~1400K whereas Tcurie Fe~1043K)
-To use material with low magnetization(weaker demagnetizing field at edges of the pillar
-To avoid strond reduction of stiffness constantat the edges of the pillar due to etching damages B
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In-plane magnetized STTRAM
Reliability issues in STTRAM
Out-of-plane magnetized STTRAM
Downsize scalability of STTRAM
Part 2: Spin Transfer Torque MRAM
OUTLINE
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How small can we go with p-STT MRAM?
1) From magnetic point of view
MgO
MgO
or
Reference layer
Additional polarizing layer toincrease STT efficiency(US6950335B2, Fig.8 (2001)) andcompensate stray field fromreference layer
RA~2-5.m (provides the TMR)
RA70
Magnetic stack optimization:
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K.Yakushiji et al, APL97, 232508(2010)
Optimal structure: (Co/Pt) or (Co/Pd) ordered ML
for reference and additional polarizing layers
Post-annealing stability of up to 370 C, provide a very large PMA Ku~ 39.106 ergs/cm3
.
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= .)0(
4Tk
Pg
eI B
perp
write
h
The total current to write by STT is related to the thermal stability factor by
For 1Gbit chip, 1 FIT in 10years withproba
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IEDM2011, paper24.1Sub-20nm STT MRAM
65nm
40nm
20nm
17nm
There is a clearly improved process B (?)
Ta/Co20Fe60B20/MgO/pinned ref
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Asymmetry likely due to stray field from reference layer
Distribution from dot to dot not reported
Ic=45A for size 22nm
IEDM2011, paper24.1Sub-20nm STT MRAM
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Edge damages due to etching : IBE
Etching of the MTJ pillar can have mainly electrical impacts but also magnetic impacts
IBE : Avoid redepositionof metallicspecies on the side of the barrier.
Amorphizationof MgO may take placeat edges locally changing RA and TMR(may be cured by post-etchinganneals)
Tapering can affect the magneticproperties (not a strong effect)
Alloying at edges may affect anisotropy
and exchange stiffness (a reduction by2 orders of magnitude of exchangestiffness is acceptable).
MgO
Ta
Ta
PtMn
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Post etching anneal may help curing the damages at edges. Encapsulation required.Need to recrystallize the barrier and neighboring electrodes around the edges andrestaure the strong interfacial anisotropy, the right RA and TMR amplitude.
Post-etching annealing
Fe or Fe rich CoFeB
Ti/Ta
Fe or Fe rich CoFeBMgO
Co60Fe10Cr10B
MgO
Co10Fe70B
(Co2/Pd2) ML
Ti/Ta
Co10Fe70BTi/Ta
(Co2/Pd2) ML
46264
152
40
15
2
40
Co60Fe10Cr10BThanks to the insertion betweentwo MgO barriers, should bequite thermally stable uponannealing
Proposed structure should withstand high annealing at least up to T ~370C:
K.Yakushiji et al, APL97, 232508(2010)Withstand annealing T of 370C
K.Yakushiji et al, APL97, 232508(2010)Withstand annealing T of 370C
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100nmMTJ pillar etched by RIE
Edge damages due to etching : RIE
Still under development.See tutorial on MRAM processing (JP Nozieres) .
Main issues:Avoid corrosion of the magnetic materialsVarious types of materials requiring different etchgas chemistry
Conclusion on p-STT RAM
In p-MTJ s, taking advantage of the interfacial anisotropy at CoFe/MgOinterface allows to circumvent the issue of combining large PMA with lowGilbert damping.
Switching by coherent rotation below 25nm diameter MTJ pillar.
Optimal structure is a double barrier MTJ with oppositely magnetizedpolarizing layers and a multilayered structure of the storage layer.
Can be scaled down to 10nm diameter from magnetic and electricalviewpoints.
Edge damages produced by etching must be carefully addressed.Significant progresses lately according to Samsungs report at IEDM
2011.