F. Kreupl et al. 1
Carbon-based Resistive Memory
Franz Kreupl, Rainer Bruchhaus+, Petra Majewski , Jan B. Philipp,
Ralf Symanczyk, Thomas Happ*, Christian Arndt*, Mirko Vogt*,
Roy Zimmermann*, Axel Buerke*, Andrew P. Graham*, Michael Kund
Qimonda AG,
+Qimonda North America, *Qimonda Dresden GmbH & Co. OHG [email protected]
F. Kreupl et al. 2
Outline of Presentation • Introduction and Motivation • Basic Switching in Carbon • Carbon Allotropes • Carbon Nanotubes • Graphene-like Conductive Carbon • Insulating Carbon • Conclusions
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Introduction and Motivation • Resistive Memories like
– Phase Change (GexSbyTez,.... ) – Nano-Ionic, (CBRAM-like, e.g. Ag2S, Ag-GeSe, Cu2S....) – Transition-Metal-Oxide ( NiO, TiO2,.....) are very promising memory technologies
• but are binary, ternary or even more complex materials: – what about scalability – how do they respond to volume/surface ratio – what about variability, if scaled
• Are there other, simpler materials available?
F. Kreupl et al. 4
Introduction and Motivation • The available current in a memory cell
is given by the select device (FET, diode) An upper limit may be estimated by:
j= e*Nd*vsat= e*1019*107 = 16 MA/cm2
• Typical currents in phase change
memories are ~10 MA/cm2 Are there options or materials
which enable switching at low currents (some µA)?
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Introduction: Carbon Memory sp3 sp2
I2 > I1
high conductance low conductance
I1
sp2 to sp3 conversion of disordered graphitic carbon (phase change of carbon) inherently scalable to atomic scale (no phases of different materials)
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Basic Switching in Carbon I1
TEM image by courtesy of J. Huang et al., Nano Letters 2006 Vol. 6, No. 8 pp. 1699-1705
• Current changes structure and resistance • Resistance changes by a factor of ~ 100 • High Low well known from e-fuses • New: switch to disorder by short pulse
I + -
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Short Laser Pulses on Graphite
Bonelli et al., Laser-irradiation-induced structural changes on graphite, Phys. Rev. B 59, 13513 (1999)
• Short laser pulse induces disorder (D-band) • D-band overlaps with sp3-peak at 1332 cm-1 • Diamond cubic phase observed by
e-beam diffraction Disordered, quenched state by short energy pulse
0 1000 2000 30000
5001000150020002500
Inte
nsity
(a.u
)
Raman shift cm-1 0 1000 2000 30000
500
1000
1500
Inte
nsity
(a.u
.)
Raman shift cm-1
pulse 20 ns
1580 cm-1 G=1580 D=1355
before after overtones
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3 ps Laser Pulse on Graphite
source: Anming Hu, PhD Thesis, U Waterloo, 2008
sp3 sp
632 nm 325 nm
sp3
• The shorter the laser pulse the more disorder Disordered, quenched state by short energy pulse
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Short Energy Pulses
• Fluence of the laser pulse at graphite ablation threshold =~ 285 mJ/cm2
Energy density (energy/volume) = 95 KJ/cm3 • Energy density for a wire subjected to a current:
E/V = j2ρt = 1GA/cm2 ·1mΩcm · 20 ns = 2 MJ/cm3
Disordered, quenched state by short current pulse
K. Sokolowski-Tinten et al., CLEO 2000
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Short Current Pulse
temperature distribution in carbon filament after 1 ns current pulse with 1.7 GA/cm2 : Tpeak ~ 3900 K; rapid cool down (0.05 ns)
carbon cell
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Carbon Nanotubes
• Nanotubes have a length dependent switching current • 25 nm long tubes need ~ 100µA @ 1.5 V
and have no phonon-limited transport • tubes > 200 nm need ~ 30 µA @ 4V (phonon-limited) Select device needs to handle ~ 30 µA and 4-8 Volt
0 1 2 3 40
20µ
40µ
60µ
80µ
100µ
200 nm 25 nm
voltage [V]
curre
nt [A
]
local modification of sp2 structure by
current pulse
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Carbon Nanotubes In vacuum ~ 12 µA current possible
Jin et al., nature 18 nanotechnology | VOL 3 | JANUARY 2008 |
on-state off-state 12 uA
switch on 6 uA, 1.6V
on-state
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Allotropes of Carbon (investigated)
• Carbon Nanotubes - sp2-type - difficult to integrate - high conductivity
• Graphene or Conductive Carbon - sp2-type - easy to integrate
- high conductivity
• Insulating carbon - sp3-type, diamond-like
- easy to integrate - high resistivity
graphene layers
SiO2
nanotube
source: J. Robertson
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Conductive Carbon (CC)
• Conductive Carbon is – graphene-like – easily deposited (CVD) – can be used as interconnect
material (highly conductive) – easy to pattern
SiO2
conductive carbon in via
R. Seidel. et al., Chemical Vapor Deposition Growth of Single-Walled Carbon Nanotubes at 600 °C and a Simple Growth Model J. Phys. Chem. B, (2004) G. Aichmayr et al., Carbon-high-k Trench Capacitor for the 40nm DRAM Generation, VLSI Technology, (2007)
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Conductive Carbon: Memory Cell
• Carbon memory cells with varying diameter
30 nm
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Conductive Carbon: Critical Current
• Critical current density of 350 MA/cm2 observed • Appropriate cell diameter ~ 6 nm for I < 100 µA Use spacer, cladding or self-assembled nano-pores
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Conductive Carbon (CC): Switching
• Shmoo-plot of 40 nm diameter CC memory cell smaller diameter, current compliance and optimized
pulses required
1 2 3 4 5 6 7 8 9 10 11 12102103104105106107108109
1010
Reset 1 Set 1 Reset 2
Resi
stan
ce (O
hm)
Pulse Amplitude (V) 1 2 3 4103104105106107108109
Resi
stan
ce (O
hm)
Cycle
Reset Set
5 ns reset 300 ns set pulse
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Insulating Carbon (IC): Memory Cell
• Insulating diamond-like carbon film • First switching occurs now from high to low state
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Insulating Carbon (IC): Critical Field
• Quasi-static switching curves determine critical field • Switching power is about 50µW with leakage currents. • Very low power levels: 5 µA @ 1.5 V (P= 7.5 µW)
350 nm wide
150 nm wide
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Ins. Carbon: Read Endurance & Speed
• Read endurance at 75 degree C: 2.3*1013 read cycles at 0.1 V.
• Switching speed is faster than 11 ns.
0 100 200 300 400 500 6000
2
4
switching event
11 ns
puls
e he
ight
(V)
t (ns)10M 100M 1G 10G 100G 1T 10T0
25p50p75p
9µ
10µon-state
off-state
re-adjustmentof the probes
curre
nt [A
] @ 0
.1 V
number of read cycles
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Insulating Carbon: Switching
• I(V) curves show similar behavior after pulses • Resistance level can be trimmed by individual
voltage pulses (multi-level capability)
0.02 0.04 0.06 0.08 0.10
10p
100p
1n
10n
100n
1µ
after 5 V 500 µs after 5 V 100 ns after 5 V 500 µs after 5 V 100 ns
curre
nt [A
]
Voltage [V]0 1 2 3 4 5 6 7 8
6k8k
10k12k14k16k18k20k22k
w=100 ns w=5 ns
Resi
stan
ce [Ω
]
Voltage [V]
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Insulating Carbon: Filament size
crater with ~10 nm diameter
10 V pulse evaporates metal filament ~ 10 nm @ 10 V use carbon as current spreader
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Conclusions • New carbon memory proposed based on
sp2 to sp3 conversion • Inherently fast: reset ~ ns, set ~ ns • Nanotubes need ~30 µA @ 8V • Graphene-like Conductive Carbon
needs pores < 6 nm • Insulating Carbon shows lowest switching
power: 5 µA @ 1.5 V (P= 7.5 µW) • Pulses and cell design needs to be optimized • Should also work with Fullerens, Graphene and
Diamonds
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Acknowledgement Qimonda Fab Dresden Roland Thewes, Werner Pamler, Walter Weber, Uli Klostermann, Klaus-Dieter Ufert, Henning Riechert Stanford University Questions?
Thank you!
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