Download Carbon Nanotube Memory

Carbon Nanotube Memory
Yong Tang
04/26/2005
EE 666 Advanced Solid State Device
Outline
Introduction to Carbon Nanotube
Multi-Walled and Single-Walled
Metallic and Semiconducting
CNT Memories
CNT FET Memory
Bulky Ball NMD
Bi-layer CNT RAM
NRAM
Summery
Two types of Carbon Nanotubes
Single-Walled CNT1
Multi-Walled CNT2
Source:1. http://www.photon.t.u-tokyo.ac.jp/~maruyama/agallery/nanotubes.
2. "Helical microtubules of graphitic carbon", S. Iijima, Nature 354, 56 (1991)
Conductance of SWNT
E
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Carbon Nanotubes are intrinsically p-type semiconductors
Interactions with metal electrodes
Impurities induced during synthesis
Interaction with oxygen in the atmosphere
Two Categories
Attempt to use various transistor like
electrical properties of the nanotubes to
emulate semiconductor memories
Attempt to use the mechanical properties
of nanotube to create bistable devices
which can be used as memories.
Advantage
Great potential for storage memory (116 Gb/cm2 )
Small size offers faster switching speeds (100GHz ) and
low power
Easy to fabricate: standard semiconductor process
Bistability gives well defined on & off states
Nonvolatile nature: no need to refresh.
Faster than SRAM, denser than DRAM, cheaper than
flash memory.
Have an almost unlimited life, resistant to radiation and
magnetism—better than hard drive.
CNT FET Memory (1)
RTL SRAM with CNT FETs.
The storing of logical state, 0 and 1,
are shown after the switch is opened.
Source: Adrian Bachtold, et al., “Logic Circuits with Carbon Nanotube Transistors”;
Science; Vol 294; P-1317; November 9, 2001.
CNT FET Memory (2)
Semiconducting SWNT
The reversibility of switching between the high conductance (ON)
and low conductance (OFF) states within the SWCNT device
both the ON and OFF state turned out to be stable over a period
of at least 12 days.
A threshold voltage shift of ~1.25 V.
atomic force microscopy
image of the nanotube
between electrode lines
separated by ~150 nm.
Source: J. B. Cui, et al. “Carbon nanotube memory devices of high charge
storage stability”, 2002 Appl. Phys. Lett.
CNT FET Memory (2)
Memory effects observed at room temperature in an individual
SWNT with a diameter of 2 nm. The bias voltage Vbias is 10 mV.
Source: J. B. Cui, et al. “Carbon nanotube memory devices of high charge
storage stability”, 2002 Appl. Phys. Lett.
Problem
Difficulty in fabricating precisely the
nanotube circuitry.
Properly contact to the electrodes.
Better ways to manufacture are being
researched.
Contact resistance an issue with CNT
devices. Theoretical limit of 6Kohms is
high and will limit max. current.
NanoMemory Device
A new carbon structure, the buckyball (C60), was discovered in
1985.
A single-wall carbon nanotube would contain a charged ( K+)
buckyball. That buckyball will stick tightly to one end of the tube or
the other.
Source: M. Brehob “The Potential of Carbon-based Memory Systems”, IEEE 1999
NanoMemory Device
Assign the bit value of the device depending on which side of
the tube the ball is. The result is a high-speed, non-volatile bit
of memory.
Source: M. Brehob “The Potential of Carbon-based Memory Systems”, IEEE 1999
NanoMemory Device
In general the amount of voltage which needs to be applied depends
upon the length of the capsule.
A field of 0.1 volts/cm is sufficient to move the shuttle from one side
of the tube to the other.
Write speed: 20 picoseconds
Source: M. Brehob “The Potential of Carbon-based Memory Systems”, IEEE 1999
Problem: How to read???
Three-wire detection
Monitor conductance
Hard to make middle wire
connection
Current detection
Done with writing
Use more shuttles
Long capsule
I
Bi-layer CNT RAM
The clever thing is it combines both electronic and
mechanical properties of single-wall nanotubes.
Metallic nanotubes will bend toward a perpendicular
semiconducting nanotube when electrically charged.
When a metallic nanotube is one to two nanometers
away from a semiconducting nanotube, the electrical
resistance at the junction is low, creating an ON state.
When the nanotubes are apart the resistance is
much higher, creating an OFF state.
Structure
Nonconductive spacers keep
the higher nanotubes flat and
raised above the lower level.
These spacers can be
between five and ten
nanometers in height to
separate the layers of
nanotubes.
These spacers must be tall
enough to separate two layers
of nanotubes from each other
when both are at rest, yet short
enough to allow small charges
to attract and cause bends in
the nanotubes.
Source: Thomas Rueckes, et al.,”Carbon Nanotube Based Nonvolatile Random Access Memory
for Molecular Computing”, SCIENCE, VOL 289, 7 JULY 2000.
Working Principle
Bistable at NT crossing:
Top NT Suspended:
potential energy minimum
Top NT contacting lower NT:
van der Waals attraction
Source: Thomas Rueckes, et al.,”Carbon Nanotube Based Nonvolatile Random Access Memory
for Molecular Computing”, SCIENCE, VOL 289, 7 JULY 2000.
I-V Characteristic
The touching of two nanotubes
decreases resistance between
the two wires dramatically,
yielding different I-V
characteristics.
Experimental results show 10X
higher resistance for off state
Bit value can be sensed by
determining resistance with low
voltage applied at electrodes
Once a bend is made, it will
remain until opposite charges
are placed at the intersection.
Source: Thomas Rueckes, et al.,”Carbon Nanotube Based Nonvolatile Random Access Memory
for Molecular Computing”, SCIENCE, VOL 289, 7 JULY 2000.
Problem
The distance between the crossed wires has to
be controlled fairly precisely: one to two
nanometers
Assemble and aligning a large number of these
cross-wires. To make this pattern of nanotubes
with precise control of distance is going to be
the difficulty.
Not yet a reliable way to produce separate sets
of metallic and semiconducting nanotubes.
NRAMTM by Nantero
Applied charge make CNT ribbons bend down to touch
the substrate or bend up back to its original state.
Ribbon-up gives 'zero' and ribbon-down is 'one'.
Source: http://www.nantero.com/nram.html
Structure
Fabricated on a silicon wafer, CNT ribbons are suspended
100 nanometers above a carbon substrate layer.
Source: http://www.nantero.com/nram.html
Bistable State
Source: http://www.nantero.com/nram.html
Bistable State
Source: http://www.nantero.com/nram.html
Read-out
Source: http://www.nantero.com/nram.html
Read-out
Source: http://www.nantero.com/nram.html
Problem
A production chip would require millions of
these ribbons manufactured cleanly and
consistently and long enough to bend.
Extremely difficult to align them.
Summary
CNT Memory devices based on
electrical and mechanical properties.
Although have some problems, more
advantages.
A promising “Universal Memory”.