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Reliability ofReliability ofOvonicOvonic
Unified MemoryUnified Memory
Neal MielkeNeal Mielke Intel CorporationIntel CorporationStephen HudgensStephen Hudgens Ovonyx IncOvonyx Inc
Brian JohnsonBrian Johnson Intel CorporationIntel Corporation
TylerTylerLowreyLowrey Ovonyx IncOvonyx Inc
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AgendaAgenda
Introduction: OUM MemoryIntroduction: OUM Memory
Reliability CapabilityReliability Capability
Degradation MechanismsDegradation Mechanisms
Future workFuture work
ConclusionsConclusions
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Chalcogenide MaterialChalcogenide Material
Chalcogenide is the general class ofChalcogenide is the general class of
switching media in CDswitching media in CD--RW and DVDRW and DVD--RWRW
In high volume production and low costIn high volume production and low cost
Laser beam energy is used to control theLaser beam energy is used to control the
switching between crystalline andswitching between crystalline andamorphous phasesamorphous phases
Higher energyHigher energy --> amorphous> amorphous
Medium energyMedium energy --> crystalline> crystalline
Low energy laser beam to readLow energy laser beam to read
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AmorphousPhase CrystallinePhase
Short Range Atomic Order
Low Free Electron Density
High Activation Energy
High Resistivity
Long Range Atomic Order
High Free Electron Density
Low Activation Energy
Low Resistivity
0.2 microns
Electron Diffraction Patterns
Material Characteristics
Scale:
Amorphous vs Crystalline PhasesAmorphous vs Crystalline Phases
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Ovonics Unified Memory (OUM)Ovonics Unified Memory (OUM)
Instead of using laser beam, useInstead of using laser beam, use
electric current to heat the materialelectric current to heat the material High current, high temperature:High current, high temperature:
amorphous phase, high resistanceamorphous phase, high resistance
Medium current, lower temperature:Medium current, lower temperature:crystalline phase, low resistancecrystalline phase, low resistance
Low current to sense resistanceLow current to sense resistance
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Amorphous orCrystalline Chalcogenide
Crystalline Chalcogenide
Memory StructureMemory Structure
Resistiv
eHeater
ThermalInsulator
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Bit line
Word line
Array Element: Junction Diode selection
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Memory array operation showing select and deselect conditions
BL n-1 BL n+1
WL n
WL n+1
WL n-1
BL n
BLn
BLn-1
BLn+1
WLn
WLn-1
WLn+1
Ireset
0V
0V
0V
Vdd
Vdd
Iset
0V
0V
0V
Vdd
Vdd
Iread
0V
0V
0V
Vdd
Vdd
Reset Set Read
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Time
Tempera
ture
Ta
T
T
m
x
AmorphizingRESET Pulse
Crystallizing
(SET) Pulse
t1
t2
Basic Device Operation:Basic Device Operation:
Set/Reset PulsesSet/Reset Pulses
Curre
nt
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IV Curve of Chalcogenide ElementIV Curve of Chalcogenide Element
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RRsetset and Rand Rresetreset as Function of Cell Currentas Function of Cell Current
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Reliability Considerations
Endurance: Withstand set/reset cycles
Data retention: Retain data over
time/temperature Disturb Immunity: Ability of cell to
retain data in face of voltage transients
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AgendaAgenda
Introduction: OUM MemoryIntroduction: OUM Memory
Reliability CapabilityReliability Capability
Degradation MechanismsDegradation Mechanisms
Future workFuture work ConclusionsConclusions
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RRsetset and Rand Rresetreset as Function of Cyclesas Function of Cycles
Capability: Stable window beyond 1012 cycles
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Endurance
Capability: Stable programming characteristics
1.E+03
1.E+04
1.E+05
1.E+06
0 0.2 0.4 0.6 0.8 1
Pulse Current (A.U.)
DeviceResistance
(Ohms)
1E2 Cycles
1E9 Cycles
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Retention CharacteristicsRetention Characteristics
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Retention at 70Retention at 70C after 10C after 1077 CyclesCycles
Capability: Many years data retention
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Disturb Immunity
Concern (left): Heat in cycled cell could spreadto adjacent cell, converting reset to set
Capability (right): No disturb over > 109
pulses Ah, but what about scaling?
BL n-1 BL n+1
WL n
WL n+1
WL n-1
BL n
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Disturb Scaling
Heat spread limited by:
1. Diffusion:
2. Steady State:
Radial: 1/R
3-D resistive divider
Main limit is steady state:
. >0.3-5 m in previousexample (0.18 m tech)
Heat equation scales:
adjacent cell temperatureunchanged with scaling
Capability: Disturb not an
issue with future scaling
Dt
Dt
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AgendaAgenda
Introduction: OUM MemoryIntroduction: OUM Memory
Reliability CapabilityReliability Capability
Degradation MechanismsDegradation Mechanisms
Future workFuture work ConclusionsConclusions
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Endurance: Reset Migration
Walk-in of R-I characteristic with cycles Some migration always present in 1st two cycles
(virgin chal has slightly different microstructure)
Severe migration (above) occurs with non-optimized electrodes & interface quality
1.E+01
1.E+02
1.E+03
1.E+04
1.E+05
1.E+06
1.E+07
0.0 0.5 1.0
Current (A.U.)
Resistan
ce
1E5 Cycles
3E7 Cycles
0 Cycles
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Endurance: Stuck Reset
Often caused by physical separation of chalfrom electrode in non-optimized devices
Example above is unpassivated cell
0
1
10
100
1.E+05 1.E+06 1.E+07 1.E+08 1.E+09 1.E+10 1.E+11
Cycles
R
esistance(K
Ohms)
Reset
Set
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Endurance: Stuck Set
Stuck set is more common failure mode (above) Endurance scales with energy per pulse
Can occur when chal intermixes with adjacent
materials Strongly dependent on electrode & dielectric materials
1.0E+03
1.0E+04
1.0E+05
1.0E+06
1.0E+071.0E+08
1.0E+09
1.0E+10
1.0E+11
1.0E+12
1.0E+13
1 10 100 1000
Energy per Pulse (A.U.)
CyclesUn
tilFailure
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Retention for Non-Optimized Device
Retention can fall short of capability withnon-optimized processes
Post-Bake:
Process B
Post-Bake:
Process A
Pre-Bake
Equiv 106 years
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Future Work
Atomic-level models for effects of continuedhigh-J stressing of chalcogenide
Dynamics of crystallization: seeding,nucleation, etc.
Chalcogenide-electrode interactions:Chemical/mechanical stability, effect onelectrical characteristics
Dependence of above effects onstoichiometry of the chalcogenide
Improved reliability acceleration models for
endurance degradation mechanisms
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Conclusions Optimized OUM can possess strong
endurance, retention, and disturbcapability
Degradation mechanisms clearlyobservable on non-optimized devices Window and reset-current instability with
endurance cycling
Degraded retention (reset to set)
All mechanisms depend on purity and
compatibility of the chalcogenide andsurrounding materials
Detailed acceleration and atomic-levelmodels are areas for future work