Microstructured Vertically
Aligned Carbon Nanotube
Composites
Dr. Robert Davis & Group
Brigham Young University
Physics and Astronomy
davis@byu.edu
(presented by Prof. David
Allred, BYU)
Outline
• Vertical aligned carbon nanotube (VACNT) growth
• Dense nanotube structures
• Filling in with Si by LPCVD
• Resistivity
• Crystallinity of filler material
• Density, Elastic Modulus
• Constraints/Engineering Design Rules
Vertical Nanotube Growth
VACNT growth details
Pattern Fe by lift-off
 Heat to 750 °C in H2: Ar
 Ar : H2 : C2H4 at 70 : 500 : 700 sccm

1-15 nm Fe
30 nm Al2O3
Si
(Barrier Layer)
SEM - VACNT forest
Can top surface and edge
roughness be improved?
100X – 50 µm holes
4500X – 3 µm holes
30000X – edge
VACNT growth process

Photolithography & lift-off
Light
Photoresist
30 nm Al2O3
Si
1-15 nm Fe
(Barrier Layer)
Dependence on Fe Thickness
15 nm
5.5 nm
2 nm
< 1 nm
Good repeatability
Rate strongly dependant on thickness
2 nm Fe on Al2O3
Small voids (< 200 nm across)
 Sharp features (few stray tubes);
Sidewall roughness < 200 nm
 High growth rate ~50 µm/min

TEM Grid
Bistable Mechanisms
Nanotube “forest” growth
•
•
•
•
Height: up to 1mm+
Feature size: a few microns
Speed: 10-100µm/min
Density:
Material
Air
Silica aerogel: lowest density
Measured density
Silica aerogel: usual density range
Expanded polystyrene
Density (kg/m3)
1.2
1.9
9.0
5 – 200
25 – 200
Low density, weakly bound
material
As-grown forests are flimsy and tear off the surface at the slightest touch
Dense Nanotube
Structures
Liquid Induced Densification
Submerged nanotube structures
Dried structure
Vapor Condensation Induced
Densification -- Lines
Longer exposure to vapor Shorter exposure to vapor
Surface forces dominates
Unequal angles become equal angles.
Final structure depends on initial structure surface forces
Difficult to control what results!
Horizontal Aligned CNT films
AIST Japan group working on in-plane
aligned CNT MEMS
Yuhei Hayamizu, Takeo Yamada, Kohei Mizuno, Robert C. Davis, Don N. Futaba, Motoo
Yumura, & Kenji Hata Nature Nanotechnology 3, 241 (2008).
Microstructured VACNT
Composites
Leave the nanotubes vertical?
Filling in with Si by LPCVD
VACNT Composite MEMS Process
“floor layer”
Solid High Aspect Ratio
Structures
A variety of materials?
Filled with
amorphous Si:
Filled with
amorphous C:
High aspect ratio structures in
a variety of materials?
FROM: Chemical Vapor Deposition, ed. Jong-Hee Park, ASM International (2001)
Properties: Resistivity
•
•
•
•
Isolated nanotubes: Can exhibit ballistic
conduction over distances of several microns
Undoped poly-Si: ρ ~ 102 Ω cm
Si-coated nanotubes: ρ ~ ?
Coat tubes with insulator  Conductive
MEMS made from insulating materials?
Properties: Resistivity
Sheet conductivity versus thickness
600
500
1/R
400
300
200
100
R0
0
0
5
10
15
20
25
Thickness t (um)
Rsheet 
K1
K
 1
t  K2
t
K1 = 42.6 Ω m
K2 = 0.04 μm
Properties: Resistivity
Resistivity of Si-coated forests
0.08
Resistivity (Ω m)
0.07
0.06
0.05
0.04
0.03
0.02
0.01
R0
0
0
5
10
15
20
Thickness (μm)
ρforest ~ 4 Ω cm
ρpoly-Si alone ~ 10000 Ω m
Properties: Resistivity
Resistivity of Si-coated (blue)
and SiN-coated (red) forests
0.08
Resistivity (Ω m)
0.07
0.06
0.05
0.04
0.03
0.02
0.01
R0
0
0
5
10
15
20
Thickness (μm)
ρforest ~ 4 Ω-cm
Approximately the same resistivity as previously reported for other CNT-composites
Microstructure: Poly-Si
Between tubes, dark field
0.2 µm
Properties: Poly-Si
Between tubes, dark field
0.2 µm
Properties: Poly-Si
Between tubes, dark field
0.2 µm
Mechanical Characterization
Filled forests are solid and well adhered (can withstand the scotch tape test)
Beam Bending Measurement of
Elastic Modulus
1000um
20um
38um
Young's Modulus of organ pipe vs deflection of end of singly clamped
beam
250E+9
Eop = 120 GPa
E'_op (Pa)
200E+9
150E+9
Organ Pipe #1
Organ Pipe #2
Organ Pipe #2
100E+9
50E+9
000E+0
000E+0
20E-6
40E-6
60E-6
80E-6
100E-6
120E-6
u'_op (m)
Reported bulk polySi modulus ~ 140-210 GPa,
dependent on deposition conditions
Actuated device:
thermomechanical in-plane
microactuator (TIM)
Developing Engineering
Design Rules

Height to width ratio for
dimensional stability

Maximum feature width
for filling of forest interior
Role of geometry
LPCVD fill-factor

Researchers
BYU Physics
• David Hutchison
• Brendan Turner
• Katherine Hurd
• Matthew Carter
• Nick Morrill
• Dr. Richard Vanfleet
Funded by a Brigham Young
University Environment for
Undergraduate Mentoring Grant
Partial funding provided by Moxtek
Inc.
BYU Mechanical Engineering
• Quentin Aten
• Dr. Brian Jensen
• Dr. Larry Howell
Nanocarbon Research Center
AIST Japan Dr. Kenji Hata
High Aspect Ratio Micromachining
Deep Reactive Ion Etching (Si)
“Vertical Mirrors Fabricated by DRIE for Fiber-Optic Switching
Applications,” C. Marx et al., J. MEMS 6, 277, (1997)
SU-8 / C-MEMS (photoresist / carbon)
“C-MEMS for the Manufacture of
3D Microbatteries,” Wang et al., Electrochem.
Solid-State Lett. 7 (11) A435-8 (2004)
MARIO Process (Titanium)
“High-aspect-ratio bulk micromachining of titanium,”
Aimi et al., Nature Mat. 3, 103-5 (2004)
LIGA process (photoresist)
“Micromechanisms,” H. Guckel,
Phil. Trans. R. Soc. Lond. 353, 355-66 (1995)
VACNT Growth Studies
Objective: Dimensional Control
Stable uniform features
 Smooth straight sidewalls and top surface
 Minimum void dimension

Variables:
Barrier layer material
 Fe catalyst thickness

Barrier layer study:
 Al2O3 –
30 nm
 Native - SiO2
 Thermally grown SiO2 – 30 nm
 Titanium – 30 nm
Early growth
2 sec. of C2H4 flow
Barrier Layer Study
TEM analysis of barrier layers
Both oxides form barrier to Fe diffusion, Al2O3 results in smallest particles
Quantitative particle size analysis of
barrier layers
Native Si Oxide
Alumina
40%
40%
35%
35%
30%
30%
25%
25%
20%
20%
15%
15%
10%
10%
5%
5%
0%
0%
1.5
4.5
7.5 10.5 13.5 16.5 19.5 22.5 25.5 28.5 31.5 34.5 37.5 40.5 43.5
1.5
4.5
Particle size (nm)
Particle size (nm)
Thermal Si Oxide
Titanium
40%
40%
35%
35%
30%
30%
25%
25%
20%
20%
15%
15%
10%
10%
5%
5%
0%
7.5 10.5 13.5 16.5 19.5 22.5 25.5 28.5 31.5 34.5 37.5 40.5 43.5
0%
1.5
4.5
7.5 10.5 13.5 16.5 19.5 22.5 25.5 28.5 31.5 34.5 37.5 40.5 43.5
Particle size (nm)
1.5
4.5
7.5 10.5 13.5 16.5 19.5 22.5 25.5 28.5 31.5 34.5 37.5 40.5 43.5
Particle size (nm)
Alumina has far more of the smallest particles.
Effects of Al2O3 barrier layer

Reduce Fe loss through diffusion

Controls Fe particle size during anneal
Reduces CNT diameter
 Increases CNT density and growth rate
 Indicates reduces Fe surface diffusion
