microstructured vertically aligned carbon nanotube composites
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Microstructured Vertically Aligned Carbon Nanotube
Composites
Dr. Robert Davis & Group Brigham Young UniversityPhysics and Astronomydavis@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
Si
30 nm Al2O3 (Barrier Layer)
Pattern Fe by lift-off Heat to 750 °C in H2: Ar
Ar : H2 : C2H4 at 70 : 500 : 700 sccm
1-15 nm Fe
SEM - VACNT forest
30000X – edge
4500X – 3 µm holes
100X – 50 µm holes
Can top surface and edge roughness be improved?
Photoresist
VACNT growth process
Si
30 nm Al2O3 (Barrier Layer)
1-15 nm Fe
Light
PhotoresistPhotoresist
Photolithography & lift-off
Dependence on Fe Thickness15 nm
2 nm
5.5 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 Density (kg/m3)
Air
Silica aerogel: lowest density
Measured density
Silica aerogel: usual density range
Expanded polystyrene
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
Dried structureSubmerged nanotube structures
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
“floor layer”
VACNT Composite MEMS Process
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
t
K
Kt
KRsheet
1
2
1
R0
K1 = 42.6 Ω mK2 = 0.04 μm
Sheet conductivity versus thickness
0
100
200
300
400
500
600
0 5 10 15 20 25
Thickness t (um)1
/R
Properties: Resistivity
R0
ρforest ~ 4 Ω cm
ρpoly-Si alone ~ 10000 Ω m
Resistivity of Si-coated forests
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0 5 10 15 20
Thickness (μm)R
esis
tivi
ty (
Ω m
)
Properties: Resistivity
R0
ρforest ~ 4 Ω-cm
Resistivity of Si-coated (blue)and SiN-coated (red) forests
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0 5 10 15 20
Thickness (μm)R
esis
tivi
ty (
Ω m
)
Approximately the same resistivity as previously reported for other CNT-composites
Microstructure: Poly-Si
0.2 µm
Between tubes, dark field
Properties: Poly-Si
0.2 µm Si S
ubst
rate
Alum
ina
laye
r
Iron
Between tubes, dark field
Properties: Poly-Si
0.2 µm
Between tubes, dark field
Mechanical Characterization
Filled forests are solid and well adhered (can withstand the scotch tape test)
Beam Bending Measurement of Elastic Modulus
20um
1000um
38um
Young's Modulus of organ pipe vs deflection of end of singly clamped beam
000E+0
50E+9
100E+9
150E+9
200E+9
250E+9
000E+0 20E-6 40E-6 60E-6 80E-6 100E-6 120E-6
u'_op (m)
E'_
op
(P
a) Organ Pipe #1
Organ Pipe #2
Organ Pipe #2
Eop = 120 GPa
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
BYU Mechanical Engineering • Quentin Aten• Dr. Brian Jensen• Dr. Larry Howell
Nanocarbon Research Center
AIST Japan Dr. Kenji Hata
Funded by a Brigham Young University Environment for Undergraduate Mentoring Grant
Partial funding provided by Moxtek Inc.
High Aspect Ratio Micromachining
“High-aspect-ratio bulk micromachining of titanium,” Aimi et al., Nature Mat. 3, 103-5 (2004)
MARIO Process (Titanium)Deep Reactive Ion Etching (Si)
“C-MEMS for the Manufacture of 3D Microbatteries,” Wang et al., Electrochem.
Solid-State Lett. 7 (11) A435-8 (2004)
LIGA process (photoresist)SU-8 / C-MEMS (photoresist / carbon)
“Micromechanisms,” H. Guckel, Phil. Trans. R. Soc. Lond. 353, 355-66 (1995)
“Vertical Mirrors Fabricated by DRIE for Fiber-Optic Switching Applications,” C. Marx et al., J. MEMS 6, 277, (1997)
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 nmTitanium – 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
Alumina
0%
5%
10%
15%
20%
25%
30%
35%
40%
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)
Native Si Oxide
0%
5%
10%
15%
20%
25%
30%
35%
40%
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)
Thermal Si Oxide
0%
5%
10%
15%
20%
25%
30%
35%
40%
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)
Titanium
0%
5%
10%
15%
20%
25%
30%
35%
40%
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 annealReduces CNT diameter Increases CNT density and growth rateIndicates reduces Fe surface diffusion
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