Well-Aligned In-Situ Formed Open-End Carbon Nanotube for Device and Assembly

Applications

Lingbo Zhu and ChingPing Wong

Georgia Institute of Technology, Atlanta, GA, 30332

ABSTRACT

Carbon nanotubes (CNTs) have been proposed for applications in microelectronic

applications, especially for electrical interconnects, thermal management, and nanodevices, due

to their excellent electrical, thermal, and mechanical properties. In this paper, we reported a

simple process to achieve simultaneous CNT growth and opening of the CNT ends, while

keeping alignment of the original CNT films/arrays. The addition of relatively low reactivity

oxidizing agents (water) into the reaction furnace enables the feasibility. We proposed using

novel CNT transfer technology, enabled by open-ended CNTs, to circumvent the high carbon

nanotube (CNT) growth temperature and poor adhesion with the substrates that currently plague

CNT implementation. The process is featured with separation of high-temperature CNT growth

and low-temperature CNT device assembly. Field emission testing of the as-assembled CNT

devices is in a good agreement with the Fowler-Nordheim (FN) equation, with a field

enhancement factor of 4540.

INTRODUCTION

Carbon nanotubes (CNTs) have attracted great interest due to their extraordinary

structural, electrical, and mechanical properties, and their wide range of potential applications

[1]. The CNTs can be either metallic or semiconducting, depending upon how the graphite layer

is wrapped into a cylinder [2, 3]. For applications of the nanotubes in microelectronics, the most

interesting features are the ballistic transport of electrons and the extremely high thermal

conductivity along the tube axis [4]. Metallic CNTs show ballistic conductivity at room

temperature [5]. Based on these advantageous properties of CNTs, researchers have reported the

integration of CNTs into electrical interconnect applications [6-8].

Recent studies have demonstrated that the internal walls of MWCNTs can participate in

electrical transport, thereby enabling large current-carrying capacity [9]. Such achievements may

then allow CNTs to serve as conductive nanowires and thus replace copper and aluminum films

used in state-of-the-art circuits; such nanowires are less susceptible to electromigration under

high current density than are Cu and Al. Our intent is to develop a novel process to open the

nanotubes in-situ in order to study the corresponding CNT properties while maintaining CNT

film alignment. In this paper, we report a novel process for in-situ opening CNTs by water-

assisted selective etching. By taking advantage of such aligned open-ended CNT structure, we

build CNT architecture using novel CNT transfer technology. The success of this methodology is

reflected in the performance of the assembled CNT field emitters. This process may offer a new

paradigm for transferring and integrating CNTs onto integrated circuits (ICs) as well as other

moduli in microelectronic packaging systems, since the approaches used circumvent the high

CNT growth temperature and poor adhesion that currently plague CNT implementation.

EXPERIMENT

Ppm levels of water can etch the ends of the nanotubes as described previously [10]; only

a brief description of the process was presented here. The substrates used in this study were

(001) silicon wafers coated with SiO

(500nm) by thermal oxidation. The catalyst layers of

/Fe were formed on the silicon wafer by sequential e-beam evaporation. CVD growth of

CNTs was carried out at 775°C with ethylene as the carbon source, and hydrogen and argon as

carrier gases. The water vapor concentration in the CVD chamber was controlled by bubbling a

small amount of argon gas through water. Ethylene flowed into the CVD system for a preset

time, after which the flow was terminated; it followed by 5 min of only water, argon and

hydrogen flow, which was used to selectively etch the nanotube tips and carbon atoms at the

interfaces between the nanotubes and catalyst particles.

DISCUSSION

Open-ended CNT growth

Figure 1 shows that five-layered CNT stacks could be obtained by repeating the growth-

etching cycle five times. The CNT films in figure 1 were partially peeled off using tweezers to

demonstrate the layered structures of CNT films.

Figure 4. Cross-sectional SEM images of 5-layered CNT films, which were scratched to show

the layered structures of CNT films.

We believe that the relatively small amount of water etches the ends of the nanotubes

because more defect structures and thus high reactivity exist at the ends of the nanotubes, though

it may be possible that the water also attacks the defects along the nanotube walls. HRTEM

image shows CNTs with one open end as indicated in figure 2. We have examined CNTs from

numerous growth runs, with dozens of HRTEM images. Each image has shown open ended

structures of the as-grown CNTs.

(a)

Figure 2. HRTEM image of an open-ended nanotube.

CNT transfer technology

For electronic device applications, chemical vapor deposition (CVD) methods are

particularly attractive due to characteristic CNT growth features such as selective spatial growth,

large area deposition capability and aligned CNT growth. However, the CVD technique suffers

from several drawbacks. One of the main challenges for applying CNTs to circuitry is the high

growth temperature (>600°C). Such temperatures are incompatible with microelectronic

processes, which are typically, performed below 400-500 °C in back-end-of-line fabrication

sequences. Another issue is the poor adhesion between CNTs and the substrates, which will

result in long term reliability issues and high contact resistance. To fabricate microelectronics

devices that incorporate CNT blocks, the CNTs should be selectively positioned and

interconnected to other materials such as metal electrodes or bonding pads. To overcome these

disadvantages, we propose a methodology that we term “CNT transfer technology”, which is

enabled by open-ended CNT structures. This technique is similar to flip-chip technology as

illustrated schematically in figure 3 [11].

CNTs

Flip

CNTs

Substrate

UBM

Substrate

UBM

Sn/Pb solder

CNTs

Substrate

UBM

Sn/Pb solder

Reflow & Remove Si chip

CNTs

Substrate

UBM

Sn/Pb solder

CNTs

Flip

CNTs

Substrate

UBM

Substrate

UBM

Substrate

UBM

Sn/Pb solder

Substrate

UBM

Substrate

UBM

Sn/Pb solder

CNTs

Substrate

UBM

Sn/Pb solder

Substrate

UBM

Substrate

UBM

Sn/Pb solder

Reflow & Remove Si chip

CNTs

Substrate

UBM

Sn/Pb solder

CNTs

Substrate

UBM

Sn/Pb solder

Substrate

UBM

Substrate

UBM

Sn/Pb solder

Figure 3. Schematic diagram of “CNT transfer technology”. UBM: under bump metallization.

See the text for detailed explanations.

The substrates can be FR-4 boards coated with copper foil or other materials and moduli,

such as heat sinks. To improve the adhesion and wetting of solder on the substrates, the under

bump metallization (UBM) layers are sputtered onto the substrate metallization. The eutectic tin-

lead paste is then stencil-printed on the UBM. The silicon substrates with CNTs are flipped and

aligned to the corresponding copper substrates, and reflowed in a seven-zone BTU reflow oven

at higher temperatures (peak temperature at 270 °C) than those typically used (220 °C) to

simultaneously form electrical and mechanical connections. This process is straightforward to

implement and offers a strategy for both assembling CNT devices and scaling up a variety of

devices fabricated using nanotubes (e.g., flat panel displays). This process could overcome the

serious obstacles of integration of CNTs into integrated circuits and microelectronic device

packages by offering low process temperatures and improved adhesion of CNTs to the

substrates. Figure 4 indicates that the entire CNT film (1.5 cm × 1.5 cm) is transferred to the

substrate (2.54 cm × 2.54 cm), since no trace amount of CNTs are evident on the silicon chip.

Figure 4. (a) Photograph of open-ended CNT film transferred onto the copper substrate coated

with eutectic tin-lead solder.

Figure 5 shows the demarcation between the broken CNTs and the intact and connected

ones. When pulled from the substrate, the CNTs break along the axis rather than at the CNT-

solder interface. The excellent mechanical bonding strength of CNTs on the substrate anchors the

CNTs and thereby makes decreasing electrical/thermal contact resistance possible.

Figure 5. SEM of the copper substrates on which the CNTs were assembled after some CNTs

were pulled from the surface by tweezers; this shows the excellent mechanical bond strength of

CNTs transferred to the copper substrate by the solder reflow process.

(a)

To explore the electrical properties of CNTs connected by solders on the copper

substrates, field emission characterization of the as-prepared assembly has been performed. The

height of the nanotube films is ~323

m with diameters in the range of 10 to 20 nm. We

measured the (cathodic) electron emission from 1.5 cm by 1.5 cm well-aligned open-ended CNT

films shown in figure 6a at room temperature and in a vacuum chamber below 10

-5

Torr. The

spacing between the CNT tip and the anode (phosphor-coated ITO glass) was ~180

m and was

maintained by a poly (tetafluoroethylene) (PTFE) spacer. The measured current density

(mA/cm

) as a function of electric field (V/ m) is shown in figure 6b. A typical turn-on field,

which produces a current density of 10

A/cm

, is ~1.8V/ m, while the emission current density

of 1 mA/cm

requires an applied field of ~2.74 V/ m. The small turn-on field is consistent with

literature data of 1.5-2 V/

m observed in CVD-grown dense CNT films [12]. At an electric field

of 3.4 V/

m, the assembled CNT field emitters emit a current density of 5 mA/cm

. A plot of

ln(I/V

) versus 1/V yields a straight line in a good agreement with the Fowler-Nordheim (FN)

expression. Furthermore, the quality of fit to the Fowler-Nordheim expression implies good

nanotube/substrate electrical contact. The slope of the FN plot can be used to calculate the field

enhancement factor

. The Fowler-Nordheim equation can be written as [13]:

)exp()

4.10

exp()(

1042.1

5.1

−

(1)

where I is the emission current (A), A the emission areas (m

), V applied voltage, d the distance

between CNT tips and anode (m),

the work function (eV), and B constant (6.44×10

, VeV

1.5

-1

). When ln(I/V

) is plotted versus 1/V, the slope of this linear formulation is given by

βφ

5.1

dB− . Assuming that the work function is 5.0 eV [14], the derived field enhancement

factor is calculated to be 4540, which is sufficient for application in field emission displays.

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

1.21.41.61.82.02.22.4

-16

-15

-14

-13

-12

-11

Ln(I/V

)

1000/V

Experimental data

Linear Fitting by FN equation

R=0.999

Current Density (mA/cm

)

Electric Field (V/µm)

Figure 6. (a) Emission pattern of the as-assembled CNTs by applying electrical field of 3.0

m. (b) Field emission measurements of CNT films in (a) at room temperature. The inset

shows a Fowler-Nordheim plot, which indicates that the transferred CNTs demonstrate good

field emission characteristicse.

(a) (b)

CONCLUSIONS

We reported an in-situ method to manufacture well-aligned open-ended CNTs. The open-

ended structures are the key to the successful assembly of CNTs on substrates by a solder reflow

process. This process is compatible with current microelectronics fabrication sequences and

technology. The distinctive CNT-transfer-technology features are separation of high-temperature

CNT growth and low-temperature CNT device assembly. Field emission testing of the as-

assembled CNT devices indicates good field emission characteristics, with a field enhancement

factor of 4540. CNT transfer technology shows promising applications for positioning of CNTs

on temperature-sensitive substrates, and for the fabrication of field emitters, electrical

interconnects, thermal management structures in microelectronics packaging.

ACKNOWLEDGMENTS

We would like to thank NSF for funding support (DMI-0422553). We also thank Dr.

Yong Ding for HRTEM examinations.

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