M' Meyyappan

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Novel One-dimensional Nanostructures
CNTs by PECVD
M. Meyyappan Center for Nanotechnology NASA Ames
Research Center Moffett Field, CA
94035 mmeyyappan_at_mail.arc.nasa.gov web
http//www.ipt.arc.nasa.gov
Acknowledgement Alan Cassell, Jun Li, Jing Li,
Quoc Ngo, Cattien Nguyen, Jeff Sun, and Bin Yu
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Outline
Carbon Nanotubes - Properties and Potential
Applications - Growth Results - SPM
Applications - Interconnects, chip
cooling Inorganic Nanowires - Properties -
Growth Results - Applications
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Carbon Nanotube
CNT is a tubular form of carbon with diameter as
small as 1 nm. Length few nm to microns. CNT
is configurationally equivalent to a two
dimensional graphene sheet rolled into a tube
(single wall vs. multiwalled).
See textbook on Carbon Nanotubes Science and
Applications, M. Meyyappan, CRC Press, 2004.
CNT exhibits extraordinary mechanical properties
Youngs modulus over 1 Tera Pascal, as stiff as
diamond, and tensile strength 200 GPa. CNT can
be metallic or semiconducting, depending on
(m-n)/3 is an integer (metallic) or not (semicon).
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CNT Properties
The strongest and most flexible molecular
material because of C-C covalent bonding and
seamless hexagonal network architecture Strengt
h to weight ratio 500 times greater than Al,
steel, titanium one order of magnitude
improvement over graphite/epoxy Maximum
strain 10 much higher than any
material Thermal conductivity 3000 W/mK in
the axial direction with small values in the
radial direction Very high current carrying
capacity Excellent field emitter high aspect
ratio and small tip radius of curvature are
ideal for field emission Other chemical groups
can be attached to the tip or sidewall (called
functionalization)
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CNT Applications
Sensors, Bio, NEMS CNT based microscopy AFM,
STM Nanotube sensors bio,
chemical Molecular gears, motors,
actuators Batteries (Li storage), Fuel Cells,
H2 storage Nanoscale reactors, ion
channels Biomedical - Nanoelectrodes for
implantation - Lab on a chip - DNA sequencing
through AFM imaging - Artificial
muscles - Vision chip for macular degeneration,
retinal cell transplantation
Electronics CNT quantum wire
interconnects Diodes and transistors for
computing Data Storage Capacitors Field
emitters for instrumentation Flat panel
displays
Challenges
Challenges
Controlled growth Functionalization
with probe molecules, robustness Integration,
signal processing Fabrication techniques
Control of diameter, chirality Doping,
contacts Novel architectures (not CMOS
based!) Development of inexpensive
manufacturing processes
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CNT Applications Structural, Mechanical
High strength composites Cables, tethers,
beams Multifunctional materials Functionaliz
e and use as polymer back bone - plastics with
enhanced properties like blow molded
steel Heat exchangers, radiators, thermal
barriers, cryotanks Radiation
shielding Filter membranes, supports Body
armor, space suits
Challenges
- Control of properties, characterization - Disper
sion of CNT homogeneously in host
materials - Large scale production - Application
development
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CNT Synthesis
CNT has been grown by laser ablation
(pioneered at Rice) and carbon arc process
(NEC, Japan) - early 90s. - SWNT, high
purity, purification methods
CVD is ideal for patterned growth
(electronics, sensor applications) - Well
known technique from microelectronics - Hydr
ocarbon feedstock - Growth needs catalyst
(transition metal) - Growth temperature
500-950 deg. C. - Numerous parameters
influence CNT growth
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CNTs on Patterned Substrates
L. Delzeit et al., Chem. Phys. Lett., Vol. 365,
p. 368 (2001) J. Phys. Chem. B, Vol. 106, p.
5629 (2002).
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Plasma Reactor for CNT Growth
Certain applications such as nanoelectrodes,
biosensors would ideally require individual,
freestanding, vertical (as opposed to towers or
spaghetti-like) nanostructures The high
electric field within the sheath near the
substrate in a plasma reactor helps to grow such
vertical structures dc, rf, microwave,
inductive plasmas (with a biased
substrate) have been used in PECVD of such
nanostructures
Cassell et al., Nanotechnology, 15 (1), 2004
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CNT in Microscopy
Atomic Force Microscopy is a powerful technique
for imaging also CD metrology, nanomanipulation,
as platform for sensor work, nanolithography... C
onventional silicon and other tips wear out
quickly. CNT tip is robust, offers amazing
resolution.
2 nm thick Au on Mica imaged with SWNT
Simulated Mars dust
Written using multiwall tube
Nguyen et al., Nanotechnology, 12, 363 (2001)
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MWNT Scanning Probe
Profilometry in Semiconductor Manufacturing
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High Resolution Imaging of Biological Materials
DNA
PROTEIN
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Imaging in Aqueous Environments
The hydrophobic nature of the CNT graphitic
sidewall is chemically incompatible with
aqueous solutions. Probes are unstable when
submerged in solution. The CNT probe is
treated with a ethylene diamine coating,
rendering it hydrophilic.
DNA on mica in 20 mM Tris HCl and 10 mM magnesium
chloride solution (near physiological conditions)
R.M. Stevens et al., IEEE Trans.
Nanobioscience, Vol. 3, pp. 56-60 (2004).
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Cu Damascene Interconnects
G. Steinlesberger, et al., Microelectronic
Engineering, 64, 409 (2002).
H. H. Hwang, M. Meyyappan, G.S.Mathad, and
R.Ranade, J. Vac. Sci. Technol., B 20(6), 2199
(2002).

• Challenges
• Etching high aspect ratio features
• Void-free filling
• Surface and grain boundary scattering
• Electromigration

Chen et al., IEEE Elec. Dev. Lett., 19, 508(1998)
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Carbon Nanotube Interconnects ?

• CNT advantages
• Small diameter, high aspect ratio
• High current carrying capacity
• Highly conductive along the axis
• High mechanical strength

Infineon
Question How to integrate this into
device processing?
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Process Flow for PECVD-Grown CNFs
As-grown CNF array

• CNFs, with their vertical orientation, have
  the capability to fulfill both size and
  performance requirements for next generation ICs
• In contrast, SWNTs in spite of their better
  conductivity are not ideal since filling a via
  with spaghetti-like structures is not useful.

STEM showing CNF morphology
CNF array embedded in SiO2
Li et al., Appl. Phys. Lett, 82, 2491 (2003)
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I-V Characteristics of a Pd-Catalyzed CNF
Parallel nature of CNF walls is better for
current transport.
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Reliability Measurement of CNF Via
No degradation of CNF via was observed over
several days of high current density stress
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21 nm diameter, 4 ?m tall via
Copper vs CNF via

• Measurement (Pd-catalyzed CNF)
• 50 ??-cm
• R 5.8 k?

Theoretical Estimate

• 2.7 ??-cm
• R 312 ?

Practical measurements, when done, likely to
resistance
Opportunities exist to improve and resistance
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Nanotube Materials for Hubble Space Telescope
(HST)
Current Problem Hubble Space Telescope Imaging
Spectrograph overheats, causing data
degration Solution Carbon Nanotube (CNT) as
thermal interface greatly improves HSTs ability
to dissipate excess heat This technology has
been licensed to industry for computer chip
cooling.
PI Alan Cassell Team Members Jun Li, Brett
Cruden, and Quoc Ngo
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Carbon Nanotubes for Chip Cooling
Material MWNTs intercalated with Cu
(electrochemical approach)
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Comparison to Real Thermal Budget
Normalizing to Area
RCNT/Cu0.404 K/W
Sample 1
RCNT/Cu0.358 K/W
Sample 2
Sample 3
RCNT0.42 K/W
R. Viswanath et. Al, Intel Tech. Jour., Q3 (2000)
Best recent result RCNT/Cu 0.098 cm2.K/W
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Mechanical Stability of CNT/Cu Film
Before compressive stress
After compressive stress
Fiber integrity is maintained up to 60 psi
(normal pressure values for packaging)
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Various Inorganic Nanowires
(INWs)
All these have been grown as 2-d thin films in
the last three decades Current focus is to
grow 1-d nanowires
Down to 0.4 eV
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Growth Methods
Approach
Comments
- Template removal could pose problems - Device
integration not straight forward - Uniform pore
size? - Large area possible - Not
scalable - Not suited for incorporating in device
fabrication sequence - CVD-like - Patterned
growth - Large area possible - Amenable to
integrate with device fabrication
schemes - Diameter control? Need lithography?
Template based Laser
Ablation Vapor-Liquid-Solid Approach
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Nanowire Growth Experimental Setup

• Quartz tube furnace with gas inlet
• Controlled inlet of carrier/buffer gas and/or
  precursor gas
• Constituent vapor from heated solid precursor
  material in the furnace
• Heated substrate for nanowire growth
• Pressure control
• Pump with adjustable valve to and pressure gauge
  to adjust pressure
• Avoid oxygen contamination
• Perform nanowire growth at various pressures (50
  mTorr to 760 Torr)
• Temperature control
• Programmable high-temperature oven with
  temperature measurement
• Perform nanowire growth at various substrate
  temperatures

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CVD Type Synthesis of Nanowires
Vapor - Liquid - Solid (VLS) Technique
Example ZnO nanowire growth

• Reaction
• ZnO is reduced to Zn vapor and COx by the
  graphite powder
• Zn vapor dissolves into the gold nanoparticles
• Once the Zn/Au solution saturates, Zn grows out
  and gets oxidized, resulting in ZnO nanowires.

Ng et al, Appl. Phys. Lett., Vol. 82, p. 2023
(2003).
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Catalyst Metal Selection
Gold modulates carrier recombination in both
n-type and p-type materials because high
mobility interstitial gold atoms can transform
into electrically active low mobility
substitutional sites. Several other metals
were tried in growing SnO2 and silicon
wires. Group IV Group V Group VI Group VII
Ti Nb Ta Al Cr
Mo W Fe Pd Ir
Co Ag Pt Ni Au
Cu Generally, growth density decreases with
increasing melting point
since catalyst serves as soft template, catalyst
formation from the thin film and fluidity of the
molten nanoparticle are factors.
X
X
P. Nguyen et al. Adv. Mat. 17, p. 1773 (2005).
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Vertically-Aligned Nanowires for Device
Fabrication
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Ge Quantum Wires

• Quantum Effect - Ge has larger Bohr radius 24.3nm
  (Si 4.7nm) more prominent quantum effect than
  Si.
• Potential Application quantum wire computing,
  tunable light detector.

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Ge Nanowires-On-Insulator
Directly assembled on SiO2 substrate
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Silicon Nanowires
SiCl4 H2 system Nanowires 40-80 nm
diameter and 1-2 micron tall Very narrow set
of conditions yield vertical wires High SiCl4
concentrations (gt 0.25º C) and high T (gt 950º
C) yield thin, curvy wires
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What Nanotechnology Can Do for Electronics

• Advanced technologies that would far exceed what
  current industrial top-down approach can do.
• Devices that exhibit 101103X improved
  performance metrics
• Devices that allow for 10100X higher packing
  density
• Devices that operate at 102104X reduced power
  dissipation
• And, devices with comparable endurance
  reliability, and simple manufacturing

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Vertical Surround-Gate Field Effect Transistor
A process flow outlining the major fabrication
steps of a VSG-FET.
Ng et al., Nano Letters, Vol. 4 (7), p. 1247
(2004)
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Vertical Surround-Gate Field Effect Transistor
In both n-type (normally on device) and
p-type, Ids with Vds threshold voltages -
3.5 V and 0.25 V respectively Ion/Ioff 104,
103 transconductance per nanowire 50 nS, 35
nS.
ZnO NW
Ng et al., Nano Letters, Vol. 4 (7), p. 1247
(2004)
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Why 1-D Phase-Change Nanowire?

• Low Thermal Energy for Programming
• Reduced melting point at 1-D
• Reduced programmable element volume
• Reduced activation energy at 1-D
• Device Scalability
• Ultra-low current / voltage / power operation
• Reduced thermal interference between neighboring
  memory cells

Top electrode
Bottom electrode
2-D Thin film PRAM
1-D Nanowire PRAM
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GeTe Nanowires TEM, SAED, and EDS
GeTe11
lt110gt
40 nm
(a) TEM image of an individual GeTe nanowire with
a diameter of about 40 nm. The inset shows an
SAED pattern of fcc cubic lattice structure. (b)
EDS spectrum of the same GeTe nanowire.
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GeTe Nanowires Melting Experiment and In-Situ
Monitoring by TEM
Liquid GeTe
In-situ Tm measurement of GeTe nanowire under TEM
image monitoring (a) The GeTe nanowire is under
room temperature. (b) The GeTe nanowire is heated
up to 400?C when the nanowire is molten and its
mass is gradually lost through evaporation. The
remaining oxide shell can be seen from the image.
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GeTe Nanowires Melting Process (video records
under TEM)
Video camera records of the melting and
evaporation process in TEM
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GeTe Nanowires Melting Point
Tm of bulk GeTe 725oC
46 reduction!
Tm of GeTe nanowires 390oC
The melting temperature of the nanowire is
identified as the point at which the electron
diffraction pattern disappears and the nanowire
starts to be evaporated. Lower Tm is translated
into potentially much reduced thermal programming
energy of data storage device.
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Nanowire-based IR Detector
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Application Summary for Nanowires
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Summary
Carbon nanotubes and inorganic nanowires will
impact electronics and computing, materials and
manufacturing, energy, transportation. The
field is interdisciplinary but everything starts
with material science. Challenges
include - Novel synthesis techniques - Charac
terization of nanoscale properties - Large
scale production of materials - Application
development Opportunities and rewards are
great and hence, tremendous worldwide interest