CARBON NANOTUBE TEMPLATED MICROFABRICATION OF

HIGH ASPECT RATIO METAL STRUCTURES David McKenna

1, Brian D. Jensen

2,

Richard R. Vanfleet1, Robert C. Davis

1* , and David D. Allred

1**

1Department of Physics and Astronomy, Brigham Young University, USA 2Department 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 WorkshopNapa, California, August 8 - 10, 2011

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

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

f itting

Tungsten

Carbonyl

Cartridge heater

Heating Electrode

VACNTs in upright

conf iguration

Thermocouple

b) Cold-wall reaction Chamber,

Inverted Sample Configuration

Gas

Heated Source

Chamber

Ultra-torr

f itting

Cartridge heater

Heating Electrode

VACNTs in inverted

conf iguration

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

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.

2µm4µm

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