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Innovative Memory workshop, June 29, 2011, Grenoble, France
Recent Progress inResistive Switching Memories
Yoshio NishiStanford UniversityStanford, California
U.S.A.
Motivations for New Nonvolatile Memories
• Scalability beyond 15nm nodes of existing memory in question both volatile memory and nonvolatile memory
• Increasing needs for less power consumption on chip
• Increasing demands for embedded memory size for memory access band width
• “Nano” materials evolution/revolutions have stimulated exploration of new memory opportunities
• Logic coupled with memory
Memory area will increase
Per
cen
tag
e o
f ar
ea in
So
C [
%]
2001 2004 2007 2010 2013 20160
20
40
60
80
100
Year
Memory
Logic
Due to design productivity, yield, and power
ITRS’ 2000
Traditional Memory Hierarchy
On CPU Embedded Off Package Memory Storage (HDD) Memory (SRAM cache) (DRAM)
Latency = 1x Latency ~= 100,000x
Normalized CPU Performance Normalized Media Access Time for 20K Read
20
180
160 Multicore CPU
140 CPU
120 Disk
100
80
60
More cores per chip will slow some 40 programs [red] unless there’s a big 20 boost in memory bandwidth [yellow]. 0
Source: Intel measurements
3 Source: “Multicore Is Bad News For Supercomputers”, IEEE Spectrum November 2008
3D Cross-point Memory
• Goal: Design a stackable cross-point memory array
(without switches or diodes).
• Must ensure that peripheral circuitry can fit to
accommodate a single bit line pitch
Memory elements
Word lines
Bit lines
Bit lines
With N-layer stack, can achieve effective cell size of 4F2/N
2011 ISSCCA 4Mb Embedded SLC Resistive-RAM Macro with
7.2ns Read-Write Random-Access Time and 160ns MLC-Access Capability
ITRI
2011 ISSCCA 4Mb Conductive-Bridge Resistive Memory with
2.3GB/s Read-Throughput and 216MB/s Program-ThroughputSony
May 18 2011
Resistive random-access memory (RRAM) is something various
electronics companies have been working on for years, but now
Panasonic seems to be ready to be the first to start mass-producing
the next-generation memory chips, according to a report in Japan’s
biggest business daily The Nikkei.
The new memory type can retain stored data over time even when it’s
not powered, it’s much faster and more eco-friendly than flash memory
chips, for example (using up to 90% less power). Panasonic says that
the energy consumed by TVs operating in standby, for example, could
be cut by 66% or more when using RRAMs.
The company plans to ship samples of 2Mbit RRAMs for use in TVs
and Blu-ray players by the end of this year, followed by mass
production in 2012.
Materials for Resistive Switching
Binary Metal Oxide TiO2,Al2O3, NiO, CuxO, ZrO2, MnO2, HfO2, WO3, Ta2O5, Nb2O5, VO2, Fe3O4…
Perovskite PCMO(Pr0.7Ca0.3MnO3), LCMO(La1-xCaxMnO3)BSCFO(Ba0.5Sr0.5Co0.8Fe0.2O3-δ), YBCO(YBa2Cu3O7-x)
(Ba,Sr)TiO3(Cr, Nb-doped), SrZrO3(Cr,V-doped), (La, Sr)MnO3Sr1-xLaxTiO3, La1-xSrxFeO3, La1-xSrxCoO3,
SrFeO2.7, LaCoO3, RuSr2GdCu2O3, YBa2Cu3O7…
Metal sulfide GexSe1-x(Ag,Cu,Te-doped), Ag2S, Cu2S, CdS, ZnS, CeO2,,
Others C(sp2-sp3)
What is the conduction mechanism for the “on” state?
• Metallic filament?
• Vacancy chain?
• Local density of states arising from
vacancy distribution or metallic behavior?
• Transport, electronic or ionic?
• Formation energy of conductive path?
• Macroscopic model vs atomic scale
model?
-1
0
1
2
3
4
ARZX ΓΓΓΓ
En
erg
y (
eV
)
M-1
0
1
2
3
4
ARZX ΓΓΓΓ
En
erg
y (
eV
)
M
-8 -6 -4 -2 0 2 4 6-150
-100
-50
0
50
100
150
DO
S (
arb
. u
nit
s)
E - EF (eV)
Spin up Spin down
• Even though we have more vacancies, lowering resistance is not so big as long as vacancies are randomly distributed.
Randomly Distributed Vacancies
[110]
Ti
Ti
0.1e/Å3
On –state
[001]
-8 -6 -4 -2 0 2 4 6-150
-100
-50
0
50
100
150
DO
S (
arb
. u
nit
s)
E - EF (eV)
-8 -6 -4 -2 0 2 4 6-150
-100
-50
0
50
100
150
DO
S (
arb
. u
nit
s)
E - EF (eV)
• Filament type conductive channel which is composed of Ti atoms can be formed in either [001] or [110] direction.
S.G. Park et al., EDL, 32, 197 (2011)
Ordering of Vacancies
Ti
0.1e/Å3
Stability of Vacancy Ordering
0.30
0.32
0.34
0.36
0.38
0.40
FR3R2
Evf (
eV/f
orm
ula
un
it.)
Vacancy configurationR 1
R1 : random config.
R2, R3 : random on (110)
F : Filament on (110)
S.G. Park et al., EDL, 32, 197 (2011)
Evf = E(TiO2-x) – E(TiO2) +n/2E(O2)
E(TiO2-x) : The total energy of a supercell containing oxygen vacancies
E(TiO2) : The total energy of a perfect TiO2 in the same size of supercell
E(O2) : The energy of oxygen molecule
n : The number of oxygen vacancy
Ti3+ - Ti3+ pair chain(missing two oxygen chains)
Ti3+
Ordering of bipolaron chains
Rutile (TiO2)
High Vo concentration
Vo ordering
The formation of Bipolaron (Ti3+ - Ti3+)
Magnéli phase (Ti4O7)
Rutile (TiO2) ���� Magnéli (Ti4O7)
Vacancy Ordering in Magnéli Phase
• High Vo concentration in rutile TiO2 results in the transition to Magneliphase by forming bipolaron chains.
• Bipolaron chain composed of successive Ti3+ atoms in one direction���� associated with vacancy ordering in TiO2
• “ON”-state conduction is related to vacancy ordering.
Vo chain
Ti
O
Vo
-8 -6 -4 -2 0 2 4 6-150
-100
-50
0
50
100
150
DO
S (
arb
. u
nit
s)
E - EF (eV)
-6 -4 -2 0 2 4 6 8-150
-100
-50
0
50
100
150
DO
S (
arb
. u
nit
s)
E - EF (eV)
Vo2+ chain
Vo2+
• Even though electrons are removed from vacancies, many defect levels are still in the band gap with strong interactions.
Charged Vacancy (Vo2+)
Vo
Vo2+
Partial charge densityELF
0.1e/Å3
0.1e/Å3
• The ordering of positively charged vacancies can also make a conductive channel as neutral vacancies.
Charged Vacancy (Vo2+)
Ti
O
Vo
0.1e/Å3
Ti
0.1e/Å3
S.G. Park et al., VLSI (2011) (accepted)
Rupture of Conductive Channel
2Vo off[001] chain
Vo
• The conductive channel is disconnected by the diffusion of oxygen into the channel.
Vo chain Vo + H chain
Ti
O
Vo
Vo+H
• Substitutional Hydrogen atoms remove some of defect states which were induced by oxygen vacancies.
-1
0
1
2
3
4
ARZX ΓΓΓΓ
En
ergy
(eV
)
M-1
0
1
2
3
4
ARZX ΓΓΓΓ
En
ergy (
eV)
M
Hydrogen in Conductive Channel
Vo chain Vo + H complex chain
Ti
H+Vo
0.1e/Å3 0.1e/Å3
Ti
Vo
H
Vo
S.G. Park et al., VLSI (2011) (accepted)
Hydrogen in Conductive Channel
• Hydrogen segregated to the vacancy sites - results in the rupture of the conductive channel by localizing electrons in those sites.
Initial (Insulator) On (LRS) Off (HRS)
Electro
forming
Vacancies in random Vo ordered domains Disruption of Vo ordering
Reset
Set
• Vo concentration Increases locally ���� Vo are ordered. (LRS)• Thermal heating by high current density ���� Vo diffuse out (HRS)• The resistance of each state might be determined by the amount of
vacancy ordered domains. (It doesn’t have to be Magneli phase.)
Vo
Switching Modeling
Understandings from ab-initio study of conduction mechanism
• The multi-oxygen vacancy configuration is linked to the formation of a metallic filament
• The chain like vacancy configurations may account for the higher conductivity observed in oxygen deficient TiO2 and NiO
• Filament rapture can take place by oxygen or hydrogen at substitutional sites
• Electron transport and interface effects also contribute to the formation of vacancy configurations –future directions.
Possible Elimination of “Forming”
• Adequately designed “vacancy pool” built
in device structure
• In-situ processing to avoid Schottky junction formation
• Demonstrated possibility for reduced
switching power consumption and multi-
state bit
Feasibility Demonstration for Scalability
• LRS would scale beyond certain geometry
• On/off ratio sustainable
• Variability decrease with geometry scaling,
partly helped by bit/word line resistance
Status Quo for ReRAM
• In-depth understanding of resistive switching mechanism, formation energy for nano-conductive filament: Good progress made
• On-state and off-state conduction mechanisms: Progress for on-state, but still a lot of room for off-state
• Effects of electrode and additional element doping: active electrode vsinert electrode, but less systematic work for doping
• Variability of switching characteristics: better in smaller device area• Programming power vs data retention: subject for filament formation
energy
• Effort toward scalability: geometry driven vs MLC and/or MLS• Demonstration of cross point memory: selection device becoming a
focal point
• Product introduction: Productization has started with small scale integration
Ending CommentFuture of Resistive Switching
Memory?• Replacement of Flash
• Nonvolatile RAM, given that programming speed
and set/reset endurance are enough for applications
• Embedded memory with least area penalty, particularly coupled with 3D stacking
• Controllability and reproducibility with additional doping
• Materials compatibility with Si based CMOS