1. Investigation into the Performance of CNT-Interconnects by Spin Coating Technique
Wei-Chih Chiu and Bing-Yue Tsui
Department of Electronics Engineering and Institute of Electronics
National Chiao-Tung University, Hsinchu, Taiwan, R.O.C.
scsa.ee94g@nctu.edu.tw
Abstract
In this work, we proposed a simple fabrication process,
spin coating and dry etching, to construct CNT-
interconnects. The CNT-interconnects formed by the slow
rate spin coating method have conductivity of more than
102
(S/cm) and the CNT-interconnects with 102
square
numbers possess about 30% conductive probability. In
addition, we inserted metal bridges into 1000-μm-long
CNT-interconnects to effectively facilitate the performance.
Introduction
Since the late 1990s, copper has been adopted to be the
main material for interconnect in VLSI technology due to
the exceptional conductivity, 5.88x105
(S/cm). However,
with the scaling of integrated circuit, the occurrence of size
effect and electromigration has been deeply influencing the
performance of Cu-interconnect [1-2]. Besides, the
fabrication difficulty of Cu-interconnects constructed by
electroplating becomes more and more challenging [3].
New interconnect technology should be developed.
Since 1991, the discovery of carbon nanotubes
revolutionizes the post-Si technology. Carbon nanotubes
have been adopted for various applications due to the
excellent electrical properties. In theory, the current
capability of a single metallic CNT with diameter 1nm,
2.4x108
(A/cm2
) of current density, is one thousand times
greater than that of copper without any adverse effects [4].
In this study, we used spin coating and dry etching to
fabricate various sizes of CNT-interconnects and
investigated the characteristics of the CNT-interconnects.
Fabrication
Two sets of CNT-interconnects were designed for
investigation. The width varying set has the width ranging
from 5μm to 500μm with fixed length at 100μm, and the
length varying set has the length ranging from 5μm to
1000μm with fixed width at 5μm as illustrated in Fig.1. For
better performance of the 1000-μm-long CNT-interconnects,
Pd/Ti metal bridges were inserted at every specified
distance. The conductance of each CNT-interconnect is
measured by the four-point bridge resistors as shown in
Fig.2. The starting material, silicon wafer, was first capped
by a 200-nm-thick wet oxide grown in high temperature
furnace and by a 1-nm-thick Al2O3 by an atomic-layer
deposition system (ALD). The CNT solution was prepared
by dissolving 2mg arc-discharge grown CNTs provided by
Carbolex in dimethylformamide (DMF) and then uniformly
dispersing by 24hr sonication. The first type of CNT film is
formed by total 400 drops of the CNT solution with spin
speed 500 rpm for 30 sec per cycle. This sample is named
the normal rate (NR) sample. The other type is formed by
100 rpm for 10 sec per cycle and is named the slow rate
(SR) sample. A 120o
C baking after each cycle was adopted
for the entire evaporation of the DMF solution. Next, the
Pd/Ti metal was deposited by a sputtering system with the
ratio 8/1 and patterned by a lift off process. Finally, the
CNT-interconnects were patterned by O2 plasma etching.
Results and Discussions
Figure 3 shows the conductance distribution of the width
varying CNT-interconnect set. From the statistical results,
the NR samples with less than 6 square numbers are
probable to be conductive while every CNT-interconnect of
the SR sample has at least 70% conductive probability. As
for the CNT-interconnect conductance distribution of the
length varying set shown in Fig.4, the CNT- interconnects
of the SR samples also demonstrate higher conductivity and
conductive probability than the NR sample. The results
interpret that slow rate spin coating and heating process
could effectively keep most of the well suspended CNTs
deposited on the wafer and possibly form more conductive
paths within the interconnect regime. In addition,
the size of 5μmx500μm CNT
-interconnects has about 30% conductive probability.
According to percolation theory, the conductivity of
randomly distributed CNT sticks, as shown in Fig.5, has a
power law dependent relation with the density. For a two
dimensional conductive plane, the exponent of the power
function can be theoretically calculated as 1.33 [5]. In this
study, we interpret the size variation of CNT- interconnects
under the same CNT density as CNT density varying at a
designated interconnect regime. Figures 6, 7 show the
characteristics and fitting curves of the average
conductance as a function of the square numbers of the
CNT-interconnects. The high exponents of the power fits
for both NR samples, about 1.5, means that there are only
few CNT connected path within the interconnect regime
and these interconnects are sorted into percolation region in
this study. The SR samples in width varying set have the
exponent 1.45 close to 1.33 implies that part of
CNT-interconnects had two dimensional CNT connected
conductive planes, while in the length varying set, all of the
CNT-interconnects are classified into power region based
on 1.34 of the exponent. The size effect occurred in CNT-
interconnect conductance shown in Fig.8 arises from the
bundling effect between CNT sticks during baking process.
Figure 9 shows the conductance distribution of the
5μmx1000μm CNT-interconnects with metal bridges
crossed at every specified distance. The result shows that
inserting metal bridges within the CNT-interconnects is
indeed able to improve the conductive probability of long
interconnect. However, too many bridge metals may
240978-1-4673-4842-3/13/$31.00 c 2013 IEEE

2. conversely exacerbate the conductivity owing to the contact
resistance from Pd/Ti to CNTs.
Summary
In summary, the investigation into performance of
CNT-interconnects by spin coating technique has been
completed. Through slow rate spin coating method, the
conductivity as well as conductive probability obtained
great enhancement. Next, the size-dependently percolative
characteristics of the CNT-interconnect conductance were
demonstrated. At final, the conduction of 1000-μm-long
CNT-interconnects with the metal bridges inserted
approves the potential for future VLSI interconnects.
Acknowledgement
This work was supported in part by the Ministry of Education
in Taiwan under ATU Program, and was supported in part by the
National Science Council, Taiwan, R.O.C. under the contract
No. NSC 100-2221-E-009- 010-MY2.
References
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P. C. Andricacos, et al., Soc.Interface, (1999) 32
B. Q. Wei, et al., Appl.Phys.Lett., 79 (2001) 1172
L. Hu, et al., Nano Lett., 4 (2004) 2513.
Fig.1 Schematic width and length varying
sets of CNT-interconnects with four-probe
bridge resistors designed for this work.
Fig.4 Conductance distribution of the length
varying set of CNT-interconnects (5μm in
width). The SR samples have higher
conductivity and conductive probability.
Fig.7 Power fit for average conductance and
square number of the length varying set of
CNT-interconnects (5μm in width) All NR
samples locate in percolation region, while
all SR samples are in power region.
Fig.2 Schematic layout of the four-probe
bridge resistor with metal bridges. The
length and width of the metal bridge is 7μm
and 4μm, respectively. The distance
between metal bridges ranges from 25μm to
500μm. The 5μm space between two probes
is for precise measurement of CNT
-interconnect conductance.
Fig.5 Top-view SEM image of CNT
networks formed by 200 drops CNT
solution and spin coating at 500rpm for
30sec. The 1-nm-thick ALD Al2O3 layer is
used to improve the uniformity.
Fig.8 The distribution of the average
conductance of width and length varying
sets of the SR samples within the same
range of square number. The difference of
conductance results from the size effect
caused by bundling phenomena.
Fig.3 Conductance distribution of the width
varying set of CNT-interconnects (100μm in
length). The SR samples have higher
conductivity and conductive probability.
Fig.6 Power fit for average conductance and
square number of the width varying set of
CNT-interconnects (100μm in length). The
entire NR samples belong to percolation
region, while the SR samples with less than
15 square numbers are around power
region.
Fig.9 Conductance distribution of
CNT-interconnects (1000μm in length)
made by slow rate spin coating with various
square numbers between Pd/Ti metal
bridges.
2013 IEEE 5th
International Nanoelectronics Conference (INEC) 241