Ovonic Unified

8/6/2019 Ovonic Unified

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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




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




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

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