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Carbon Nanotube Memory

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Title: Carbon Nanotube Memory


1
Carbon Nanotube Memory
  • Yong Tang
  • 04/26/2005
  • EE 666 Advanced Solid State Device

2
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

3
Two types of Carbon Nanotubes
Multi-Walled CNT2
Single-Walled CNT1
Source1. http//www.photon.t.u-tokyo.ac.jp/maruy
ama/agallery/nanotubes. 2. "Helical
microtubules of graphitic carbon", S. Iijima,
Nature 354, 56 (1991)
4
Conductance of SWNT
  • Carbon Nanotubes are intrinsically p-type
    semiconductors
  • Interactions with metal electrodes
  • Impurities induced during synthesis
  • Interaction with oxygen in the atmosphere

5
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.

6
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 magnetismbetter than hard drive.

7
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.
8
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.
9
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.
10
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.

11
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
12
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
13
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
14
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

15
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.

16
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.
17
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.
18
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.
19
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.

20
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
21
Structure
  • Fabricated on a silicon wafer, CNT ribbons are
    suspended 100 nanometers above a carbon substrate
    layer.

Source http//www.nantero.com/nram.html
22
Bistable State
Source http//www.nantero.com/nram.html
23
Bistable State
Source http//www.nantero.com/nram.html
24
Read-out
Source http//www.nantero.com/nram.html
25
Read-out
Source http//www.nantero.com/nram.html
26
Problem
  • A production chip would require millions of these
    ribbons manufactured cleanly and consistently and
    long enough to bend.
  • Extremely difficult to align them.

27
Summary
  • CNT Memory devices based on electrical and
    mechanical properties.
  • Although have some problems, more advantages.
  • A promising Universal Memory.
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