This document describes a process for infiltrating carbon nanotube templates with tungsten to create mechanically robust microstructures. Carbon nanotubes are grown vertically in a pattern and then infiltrated with tungsten using chemical vapor deposition of tungsten hexacarbonyl. Parameters like temperature, geometry, and precursor amount influence the infiltration. Near full infiltration and capping of the carbon nanotube structures is achieved at temperatures above 300°C in an inverted sample geometry close to the precursor source. This process allows creation of tungsten-carbon nanotube composites for applications requiring high temperature or corrosion resistance.

1. CARBON NANOTUBE TEMPLATED MICROFABRICATION OF
HIGH ASPECT RATIO METAL STRUCTURES
David McKenna1
, Brian D. Jensen2
,
Richard R. Vanfleet1
, Robert C. Davis1*
, and David D. Allred1**
1
Department of Physics and Astronomy, Brigham Young University, USA
2
Department of Mechanical Engineering, Brigham Young University, USA
ABSTRACT
Carbon nanotube templated microfabrication (CNT-M) is a recently developed method for fabrication of high aspect ratio
structures. In CNT-M, a 3-D framework is created using patterned growth of vertically aligned carbon nanotubes. The
framework is then transformed into a mechanically robust structure by chemical vapor infiltration. We are applying CNT-M
to fabricate structures from several materials including metals and ceramics for a variety of applications including chemical
separations, x-ray windows, micromechanical systems, and energy storage. Here we present a new tungsten infiltration
process for CNT-M using a tungsten carbonyl W(CO)6 precursor.
INTRODUCTION
Microfabrication of three dimensional structures is very challenging in metals and ceramics. Dry etching is used extensively
for deep silicon micromachining, but in metal and ceramic materials, low etch rates and low etch selectivity limit patterning
by vertical etching to relatively thin films. LIGA allows fabrication of high aspect ratio structures in electroforming metals
but requires a synchrotron facility.
Carbon nanotube templated microfabrication is a recently developed alternative method for the fabrication of high aspect
ratio structures [1-4] and is compatible with a wide variety of materials. In CNT-M, 3-D forests of patterned vertically-
aligned carbon nanotubes (VACNTs) are grown as a low-density, high-aspect ratio framework (figure 1). The nanotube
framework is then infiltrated with a secondary material by chemical vapor infiltration transforming it into a structurally
robust structure. CNT-M has been used to fabricate microsensors and actuators[1][2] using silicon and silicon nitride as the
infiltration materials. Figure 2 (a-f) shows a process diagram for CNT-M fabrication and figure 2(f) shows a scanning
electron microscopy (SEM) image of CNT-M structures fabricated with silicon nitride as the infiltration material. CNT-M
can also produce porous hierarchically structured materials with precise control over three dimensional structure on the
microscale and control over porosity on the nanoscale [3]. These porous CNT-M structures have been used as media for high
performance chemical separations, specifically for thin layer chromatography.
Although CNTs are known for their tensile strength, the very low density (9 mg/cc which corresponds to ~99% void
space)[1] structures formed through VACNT growth are easily damaged with a light touch or even a burst of air as seen
figure 1b. The low density CNT structures
however form a three-dimensional framework
which can be infiltrated to create mechanically
robust structures consisting almost entirely of the
infiltration material [1].
Tungsten is a highly desirable material for
microstructures. It has the highest melting point
(3422 °C), highest yield strength (550 MPa) and
lowest coefficient of thermal expansion (4.5 μm
m -1
K-1
) of all elemental metals. It is chemically
resistant to corrosion by acids, alkalis, and
oxygen.
Chemical vapor deposition (CVD) of
tungsten is well developed and has been used in
several applications ranging from use as chemical
diffusion barriers to interconnects in silicon
devices. Tungsten CVD is done primarily with
tungsten halides such as tungsten hexafluoride
(WF6) and a reducing gas such as SiH4 . A
challenge of this process is that the byproducts of
these reactions, such as HF, are corrosive and
toxic. Organometallic CVD is a non-corrosive
substitute. For this project we used tungsten
carbonyl W(CO)6. The W(CO)6 CVD process
Figure 1 Patterned vertically
aligned carbon nanotube growth.
a) The high aspect ratio structure
shown consists of patterned 3 µm
pores separated by 2 µm walls in a
400 µm tall structure. Inset at
higher magnification shows the
nanoscale porosity of the walls b)
the low density VACNT structures
are very fragile, breaking at the
slightest touch
a b
978-0-9743611-7-8/MFG2011/$20©11TRF-0002 33 Technologies for Future Micro-Nano Manufacturing Workshop
Napa, California, August 8 - 10, 2011

2. does not produce pure tungsten films but instead a tungsten oxicarbide in the following reaction: W(CO)6 + Heat →WOxCy +
5 CO. This unbalanced equation does not represent a stoichiometric reaction[4]. For the remainder of this paper CVD
tungsten will refer to the tungsten oxycarbide film produced by the CVD process. The CVD tungsten can be reduced to a
pure tungsten film by annealing it at 800 °C in the presence of hydrogen gas.
METHODS
Patterned CNT Growth
The VACNT growth process is as described previously [1-3]. First a pattern or “footprint” of the structure is created in
photoresist on a substrate (silicon wafer) through standard photolithography. The surface is then coated with a 30 nm thin
film of alumina coating by e-beam evaporation. Next a 2-6 nm layer of iron is added (Figure 2a); this bilayer acts as the
catalyst for VACNT growth. The photoresist (and the bilayer on its surface) is removed leaving the catalyst pattern. The
CNTs grow vertically from the catalyst forming sharp boundaries, this type of growth is called vertically aligned carbon
nanotubes (figure 2b and figure 1a ). The vertical growth transforms the 2-D footprint into a 3-D structure.
Tungsten Infiltration
The infiltration reactor configuration is illustrated in figure 3. The tungsten carbonyl [W(CO)6] source compound was placed
in a cylindrical aluminum block (called the heated source chamber) which was inserted into the reaction chamber via a 2 inch
Ultra-Torr fitting. A cartridge heater was used to heat the source chamber; a K-type thermocouple and Omron E5K
temperature controller was used to regulate the temperature. The source chamber (containing the carbonyl) was heated to
160 °C which causes the [W(CO)6]to volatilize. It was held at this temperature for the duration of the deposition process to
ensure a constant rate of carbonyl vapor entering the reaction chamber. The carbonyl vapor was carried into the cold-wall
reaction chamber by preheated hydrogen gas (50-150 sccm). The hydrogen gas was preheated by passing through the heated
source chamber. The reaction chamber was held at low pressure (3-6 torr) with a roughing pump. The VACNT samples were
heated on a graphite susceptor plate which was heated electrically. The electrical power was supplied by a Variac
autotransformer connected to a 100:1 step down transformer which delivers current in excess of 200 amps to the susceptor
plate during heating. The VACNTs were heated to above the decomposition temperature of the carbonyl vapor. When the
Figure 2 CNT-M process (a) A catalyst bilayer consisting of a 2 nm Fe film on a 30 nm Al2O3 film is patterned by lift-off
on a silicon wafer. (b) A forest of VACNT’s is grown from the patterned catalyst. (c) Chemical vapor infiltration coats the
individual nanotubes, filing in the patterned carbon nanotube framework with secondary material. A floor layer (indicated
by the white circle) of the secondary material is deposited on the exposed substrate (d) The floor layer is removed by
reactive ion etching(RIE). (e) A sacrificial layer is etched to release part or all of the structure from the substrate. (f)
Electron micrograph of structures fabricated in silicon nitride by the CNT-M process.
500 µm
a
ed
cb
f
34

3. carbonyl vapor contacted the heated VACNTs it decomposed and the tungsten film was deposited on the nanotubes. The
byproducts are evacuated and exhausted through the vacuum pump in gaseous form.
The primary goal of this project was to uniformly infiltrate patterned VACNT forests with tungsten. In an effort to
maximize infiltration, various geometries were explored. Initially the substrate was placed upright in the middle of a graphite
susceptor plate, approx. 6 inches from the source of heated carbonyl (Figure 3a). A inverted sample mount was also created
which allowed the sample to be much closer to the source in an inverted geometry (within 2 inches). This inverted mount
consisted of a block of aluminum with an angle on one end and placed on the suseptor plate (Figure 3b). A clip was used to
hold the sample at the angle. Other parameters that were varied were the gas flow rate and the temperature.
RESULTS
Figure 4 shows scanning electron microscope (SEM) images of samples infiltrated in different geometries and temperatures.
All three samples were infiltrated with an H2 carrier gas at 50 sccm and the same initial amount of carbonyl (3 g). The
inverted geometry places the sample closer to the source as seen in figure 3b and results in more deposition as seen in figure
4a (some deposition in interior) relative to figure 4b(no detectable deposition on interior). Higher temperature (530 °C vs
290°C) also results in increased deposition and a significant capping layer as seen in figure 4c.
Figure 5 shows SEM images of a sample was infiltrated in the inverted position at 300 °C. The initial amount of
carbonyl was 6 grams. There is significantly more infiltration than at 290 °C resulting in a robust structure that appears to be
more than 50% solid. A thick solid cap has formed on the surface and is clearly visible. The coated VACNTs below the cap
have an average diameter of 100 nm. The cap is 3 microns thick.
The sample in figure 6 was done at 300 C while inverted. The initial amount of carbonyl was 5 grams. The images are of
a section of the VACNTs that have been broken to reveal the inner nanotubes. The average diameter of the CNTs is 100 nm.
A 2 micron cap also formed on top of the VACNTs.
Figure 3 Tungsten infiltration reactor a) The tungsten infiltration process takes place in a cold-wall reaction chamber. A
heated source chamber was connected to the reaction chamber via an Ultra Torr fitting. Sample is shown in upright
configuration. b) sample is shown in inverted configuration.
Thermocouple
a) Cold-wall reaction Chamber,
Upright Sample Configuration
Gas
Heated Source
Chamber
Ultra-torr
fitting
Tungsten
Carbonyl
Cartridge heater
Heating Electrode
VACNTs in upright
configuration
b) Cold-wall reaction Chamber,
Inverted Sample Configuration Heated Sourc
Chamber
Ultra-torr
fitting
Cartr
Heating Electrode
VACNTs in inverted
configuration
Aluminum Heat
Transfer Block
a) Low Temp Inverted c) High Temp Uprightb) Low Temp Upright
30µm 20µm 2µm
Figure 4. Scanning electron micrograph cross sectional views of samples infiltrated in different geometries at low
and high temperatures. a) infiltrated at 290 °C in the inverted geometry b) infiltrated at 290 °C in the upright
geometry c) infiltrated at 530 °C in the upright geometry.
35

4. DISCUSSION
The above results show that
tungsten infiltration can vary from almost
no infiltration to near full (but still porous)
infiltration. Geometry and temperature are
both critical variables in tuning infiltration.
Of the two geometries, the inverted (fig 4a)
geometry seems best for infiltration.
However at lower temperature, even the
inverted geometry gave non-uniform
infiltration; the sample in figure 4a seems
to have been partially infiltrated, with the
inside of the sample being more infiltrated
than the region near the surface. This non-
uniformity may be caused by a temperature
gradient across the CNTs where the bottom
of the VACNTs were hotter than the tops
resulting in preferential deposition on the
bottom. Samples at slightly higher
temperatures (such as those in figure 5 and
6) have a much more uniform infiltration
top to bottom. The upright geometry at
high temperature (fig 4. right) resulted in a
cap (although incomplete). The incomplete
cap formation is probably due to the
limited material in the source chamber as
the deposition was dome until the source
material was depleted. The sample in figure 6 was done on shorter VACNT growth, this may explain the full infiltration and
subsequent cap. Also, it appears that the sample broke due to stress and infiltration began to take place at the bottom of the
sample as well, this resulted in the encapsulation of the CNTs.
CONCLUSIONS
We have explored conditions for infiltrating patterned VACNTs for CNT-M. We have observed regimes for both uniform
infiltration and capping of forest features depending on temperature and reactor geometry. The metal infiltrated VACNT
structure forms a tungsten-carbon nanotube matrix capable of being manipulated and strong enough to be used in applications
such as MEMS. This tungsten-CNT composite may be used in conjunction with CNT-M to create metal MEMS that have the
high temperature and anticorrosive properties of tungsten. Other possible applications of the tungsten-CNT composites may
take advantage of its porosity in catalysis or as a micro-filter material. Future work is needed to determine the composition,
mechanical, andelectrical properties of this microstructured composite.
[1] D. N. Hutchison, Q. Aten, B. W. Turner, N. B. Morrill, L. L. Howell, B. D. Jensen, R. C. Davis, and R.R.
Vanfleet “High aspect ratio microelectromechanical systems: A versatile approach using carbon nanotubes as a
framework,” Solid-State Sensors, Actuators and Microsystems Conference, 2009. TRANSDUCERS 2009.
International, p. 1604-1607, 2009.
[2] D. N. Hutchison, N. Morrill, Q. Aten, B. Turner, L. L. Howell, B. D. Jensen, R. R. Vanfleet, and R. C.
Davis “Carbon Nanotubes as a Framework for High-Aspect-Ratio MEMS Fabrication,” Journal of Micro
Electromechanical Systems, vol. 19, no. 1, pp. 75-82, (2010)
[3] Jun Song, David S. Jensen, David N. Hutchison, Brendan Turner, Taylor Wood, Andrew Dadson, Michael
A. Vail, Matthew R. Linford, Richard R. Vanfleet, and Robert C. Davis “Carbon Nanotube Templated
Microfabrication of Porous Silicon Carbon Materials with Application to Chemical Separations,” Advanced
Functional Materials, p. 1-8, 2011.
[4] K. K. Lai and H. H. Lamb, “Tungsten chemical vapor deposition using tungsten hexacarbonyl - microstructure of as-
deposited and annealed films,” Thin Solid Films, vol. 270, p. 114-121, Jun. 2000.
* davis@byu.edu
** allred@byu.edu
Figure 5 Inverted configuration tungsten deposition at 300 °C. a) Top down
image of where the sample broke. b) Cross sectional view at the broken edge.
2µm
4µm
a b
10µm 500 nm
a b
Figure 6. Inverted depositonat 600 C from 5 grams of W(CO)6 a) A cross
sectional view of broken sample. b) A close-up of the infiltrated CNTs.
36