Copyright © 2005 American Scientific Publishers
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Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 5, 1745–1748, 2005
Fabrication of a One-Dimensional Array of Nanopores
Horizontally Aligned on a Si Substrate
Hongguo Zhang,1 Zhi Chen,1 ∗ Tianxiang Li,2 and Kozo Saito2
1
Department of Electrical and Computer Engineering and Center for Nanoscale Science and Engineering,
University of Kentucky, Lexington, KY 40506, USA
2
Department of Mechanical Engineering, University of Kentucky, Lexington, KY 40506, USA
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A one-dimensional array of nanopores horizontally aligned on a silicon substrate was successfully
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Libraries
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fabricated by anodic aluminum oxidation (AAO) using aof
modified
two-step
procedure.
pictures
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show clear nanostructures of well-aligned one-dimensional nanopore arrays without cracks at the
Thu,
01processes
Sep 2005
interfaces of the sandwiched structures.
The
are13:17:18
compatible with the planar silicon integrated circuit processing technology, promising for applications in nanoelectronics. The formation
mechanism of a single nanopore array on Si substrates was also discussed.
Keywords: Nanopore Arrays, Anodization, AAO, 1-D Array, Nanotemplate.
∗
Author to whom correspondence should be addressed.
J. Nanosci. Nanotech. 2005, Vol. 5, No. 10
fabrication of 1-D AAO templates using a sandwiched
structure on a glass substrate (Al2 O3 /Al/Glass). However,
they have only achieved very limited success, because no
follow-up research has ever been carried out and the pore
structures are hardly distinguishable in their scanning electron microscope (SEM) pictures.10 In addition, in their
process, the Al2 O3 film and glass substrates were used,
which are not compatible with the silicon integrated circuit
processes.
We attempted to improve their process so that it may
be compatible with the Si integrated circuit processes. We
found that the major difficulty for realization of the 1-D
AAO nanopore array using conventional anodization was
the cracks occurring at the interface between the SiO2
and the AAO layer. The presence of cracks suggests that
this approach is unreliable and substantial improvement is
needed. This might be why it is hard to view Masuda’s
SEM images. In this communication, we will present
successful fabrication of horizontal 1-D AAO templates
without any cracks at the two interfaces using a modified two-step process, based on two types of structures,
SiO2 /Al/SiO2 /Si and SiO2 /Al/Si. The fabrication processes
are completely compatible with the Si integrated circuit
processes.
The fabrication processes of the two structures, SiO2 /Al/
SiO2 /Si and SiO2 /Al/Si, are illustrated schematically in
Figure 1. First, For the SiO2 /Al/SiO2 /Si structure, a
100-nm layer of SiO2 was thermally grown on an undoped
1533-4880/2005/5/1745/004
doi:10.1166/jnn.2005.157
1745
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In the past decade, as an effective self-assembly template,
nanoscale anodic aluminum oxide (AAO) has attracted
much interest for fundamental scientific research as well
as industrial applications due to the high aspect ratio
(∼1000), high pore density (∼1011 pores/cm2 , high level
of ordering and uniformity.1–6 It is a very simple and
facile approach because AAO templates are well-defined
and their lengths and diameters are easily controlled. If
the AAO template fabrication can be integrated into the
mature Si integrated circuit processing technologies, it
will be more useful for fabrication of nanoscale electronic
devices. Up to now, it is still very difficult to utilize these
AAO templates for fabrication of nanoelectronic devices,
because all the AAO templates were vertically grown in
two-dimensions (2-D) on substrates, not compatible with
the mainstream Si planar processing technology. If a onedimensional (1-D) AAO array with nanopores horizontally
aligned on a Si substrate can be formed, it will be much
more promising for fabrication of nanoelectronic devices
and nano-electromechanical systems (NEMS) using the
planar processing technology. Although another material,
mesoporous silica with a sub-10 nm pore structure, shows
promising as a self-organized template,7–9 it is still very
difficult to fabricate a single array of nanopore channels using mesoporous silica. Masuda et al.10 attempted
Fabrication of a One-Dimensional Array of Nanopores Horizontally Aligned on a Si Substrate
SiO2
SiO2
Al
Zhang et al.
(a)
(b)
(c)
(d)
Al
SiO2
Si substrate
Si substrate
Hexagonal
Cells
Hexagonal
Cells
SiO2
Pores
Al
Pores
SiO2
Al
SiO2
Si substrate
(a)
Si substrate
(b)
Fig. 1. Fabrication procedures of one-dimensional horizontal nanopore
arrays based on two types of structures: (a) SiO2 /Al/SiO2 /Si and
(b) SiO2 /Al/Si.
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Si substrate. For the SiO2 /Al/Si structure, a non-oxidized
Si substrate was used. Then, an aluminum layer with a
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and a silicon
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oxide layer of ∼2 m were deposited on the oxidized
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and non-oxidized Si substrates using an e-beam evaporaIP
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tion apparatus from an aluminum source of 99.99% and
Thu, respectively.
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a sintered silicon oxide source of 99.95%
After formation of the lead contact on the aluminum layer
with conductive epoxy, the subsequent sealing of the specFig. 2. Scanning electron microscope (SEM) images of AAO arrays
imen except for the fractured surface with positive photo
based on SiO2 /Al/SiO2 /Si structure with perfect interfaces for 600-nm
thick aluminum film (a) (b) and 140 nm thick aluminum film (c) (d):
resist was carried out. The clean specimen was anodized
(a) 4–5 arrays of nanopores at a lower magnification and (b) their details
in an oxalic acid solution of 0.20 M at 5–25 C at 40 V
at a higher magnification; (c) One array of nanopores at a lower magnififor 3–7 minutes (the first step). Then the anodized oxide
cation and (d) its details at a higher magnification. The modified two-step
layer was removed by dipping the specimen into a mixture
procedure is as follows: 5 min for the first step, 20 min for the second
of phosphoric acid (6 wt%) and chromic acid (1.8 wt%)
step, and 20 min for pore widening.
at 55 C for 2–4 minutes. The reanodization (the second
cross section is near the surface. It is well known that the
step) was performed for 10–20 minutes under the same
pores should be better organized deep inside the structure.1
conditions as the first step. Finally, the specimens were
When the thickness of the Al film is about 140 nm, we
dipped into a 5 wt% phosphoric solution at 20 C for
obtained a well-aligned 1-D nanopore array (See Figs. 2c
20 minutes for pore widening. Field-emission scanning
and 2d). The pore diameter is about 50 nm. The pore depth
electron microscope (FE-SEM, JEOL JSM-6100) was used
is about 500 nm according to the general anodization rate
to examine all the specimens.
at the same voltage and the same electrolyte solution.
The basic assumption is that if the thickness of the aluminum film in the sandwiched structure is equivalent to the
Cracks at interfaces are a great challenge during fabrisize of 4 cells, there should be 4 arrays of nanopores, and
cation of the multi-layer structure, especially for our samif the thickness of the aluminum film in the sandwiched
ples, which were dipped and etched in the acid solutions
structure is equivalent to the size of a single cell, one array
for a long time. For the conventional two-step anodizaof nanpores should be expected. Table I shows the cell
tion method, the first step lasts for a very long time (up
sizes of AAO anodized in various parameters. The porous
to 10 hrs.) and the second step lasts for a short time (less
structures of an Al film with a thickness of 600 nm were
than 30 min.)11 Figures 3a and 3c show clearly cracks
shown in Figures 2a and 2b. There are about 4–5 arrays
occur at the SiO2 /AAO interface using the conventional
of cells. The pores were not well ordered because the
two-step anodization method, where the first step lasts
Table I.
Pore diameter, wall thickness, and cell size of AAO formed in various parameters.
Electrolyte
0.2
0.3
0.3
0.3
1746
M
M
M
M
Oxalic acid, 32 F
Sulfuric acid, 75 F
Sulfuric acid, 75 F
Sulfuric acid, 75 F
Voltage V (V)
Pore diameter P (nm)
Wall thickness T (Å/V)
Cell size W (Å)
40
20
10
8
50
30
15
10
120
100
90
80
1460
700
330
228
J. Nanosci. Nanotech. 5, 1745–1748, 2005
Zhang et al.
Fabrication of a One-Dimensional Array of Nanopores Horizontally Aligned on a Si Substrate
based on the SiO2 /Al/SiO2 /Si structure using the conventional anodization procedure showing cracks at the AAO/SiO2 interface (a)(c) and
those using the modified two-step procedure showing no cracks at the
AAO/SiO2 interface (b)(d). The conventional two-step method:8 >10 min
for the first step, 20 min for the second step, and 20 min for pore widening. The modified two-step procedure: 5 min for the first step, 20 min
for the second step, and 20 min for pore widening.
J. Nanosci. Nanotech. 5, 1745–1748, 2005
Acknowledgments: This work is supported by National
Science Foundation (ECS 0304129), Department of Energy
(DE-FG02-00ER4582 and DE-FG26-04NT42171), Army
Research Laboratory (W911NF-04-2-0023), and Kentucky
Science & Engineering Foundation (KSEF-148502-03-78).
1747
of
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longer than 10 min. In order to solve this problem, optimal
anodization parameters for 1-D nanopore templates were
explored based on references.1 11–14 We have developed
a modified two-step anodization procedure, i.e., a properly short first anodization followed with a relatively long
Si
second anodization. For our samples, the anodization was
carried out in 0.20 M oxalic acid solution with a DC voltage of 40 V at 5 C (rather than room temperature) for
5 minutes for the first step and 20 minutes for the second step respectively. Using this modified two-step procedure, we succeeded in solving the crack problem based
the SiO2 /Al/SiO2 /Si structure, seen in Figures 3b and 3d.
Meanwhile, using the same procedure, a well-aligned 1-D
(a)
(b)
nanopore template without cracks at interfaces from the
Fig. 4. (a) Scanning electron microscope (SEM) images of 1-D horiSiO2 /Al/Si structure was also fabricated (See Fig. 4).
zontal nanopore array based on the SiO2 /Al/Si sandwiched structure and
The mechanisms for formation of only a single array
(b) a typical cross-sectional image of the structure. The modified two-step
of nanopores instead of two arrays of half-pores may be
procedure is as follows: 5 min for the first step, 20 min for the second
understood based on the self-adjusting effect.15 Initially
step, and 20 min for pore widening.
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barrier layer (oxide) is formed. In a corrosive solution,
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after the barrier
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pores
to be
Al/barrier-layer
interface).15(cid
The70025377),
growth rate University
of oxide
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initiated. The hexagonalKentucky
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are 292596),
formed firstUniversity
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then pores are initiated at the centers of the cells. The
face and the rate of dissolution depends on the local elec01 Sep tric
2005
13:17:18
pore and cell diameters and the barrier layer Thu,
thickness
field
at the solution/barrier interface. This local field
increase and a number of major pores eventually propais
inversely
proportional to the curvature of the pore base.
gate and certain of incipient pores cease to function.15 16
There exists an equilibrium curvature of pore base in
In the steady state, the field-assisted dissolution rate of
the steady state. If the curvature is less than its equilibthe pore base (the solution/barrier-layer interface) equals
rium value, the local field at the solution/barrier interface
the field-assisted growth rate of oxide at cell base (the
will increase, leading to stronger dissolution of the barrier layer. This tends to increase the curvature of the pore
(b)
(a)
base. If the curvature is large than its equilibrium, the local
field will decrease, leading to less dissolution of the barrier layer. This will reduce the curvature of the pore base.
The equilibrium value will be maintained in the steady
state due to the self adjustment. The same argument can
be applied to the cell base. If for some reason, two cells
are cramped into the space for only a single cell in equilibrium, the curvature of both cells is much less than its
equilibrium value. This may result in strong dissolution of
No Crack
Crack
the barrier layer due to much stronger local electric field.
Thus two cells may spread toward each other until eventually a single cell emerges.
(c)
(d)
In summary, the well-ordered 1-D horizontal nanopore
arrays with pore diameters of ∼50 nm in sandwiched
Crack
No Crack
structures on Si substrates were fabricated using the modified two-step anodization procedure without any cracks at
interfaces. SEM pictures show clear nanostructures of the
well-aligned 1-D nanopore arrays. The processes are compatible with the planar silicon integrated circuit processing
technology, promising for applications in nanoelectronics.
Fig. 3. Scanning electron microscope (SEM) images of AAO arrays
Fabrication of a One-Dimensional Array of Nanopores Horizontally Aligned on a Si Substrate
References and Notes
1. H. Masuda and K. Fukuda, Science 268, 1466 (1995).
2. O. Jessensky, F. Muller, and U. Gosele, Appl. Phys. Lett. 72, 1173
(1998).
3. W. B. Choi, E. J. Bae, J. W. Lee, and J. O. Lee, Appl. Phys. Lett.
79, 22 (2001).
4. S.-H. Jeong, H.-Y. Hwang, and K.-H. Lee, Appl. Phys. Lett. 78, 14
(2001).
5. D. Crouse, L. Yu-Hwa, E. Miller, and M. Crouse, Appl. Phys. Lett.
76, 49 (2000).
6. A. Cai, H. Zhang, H. Hua, and Z. Zhang, Nanotechnology 13, 627
(2002).
7. H. Miyata and K. Kuroda, Chem. Mater. 12, 49 (2000).
8. A. Okabe, T. Fukushima, K. Ariga, and T. Aida, Angew. Chem. Int.
41, 3414 (2002).
Zhang et al.
9. V. R. Koganti and S. E. Rankin, J. Phys. Chem. B 109, 3279 (2005).
10. H. Masuda, K. Nishio, and N. Baba, Appl. Phys. Lett. 63, 3155
(1993).
11. H. Masuda and M. Satoh, Jpn. J. Appl. Phys. Part 2. 35, L126
(1996).
12. W. C. Hu, D. W. Gong, Z. Chen, L. M. Yuan, K. Saito, C. A. Grimes,
and P. Kichambare, Appl. Phys. Lett. 79, 3083 (2001).
13. J. W. Diggle, T. G. Downie, and C. W. Goulding, Chem. Rev. 69,
365 (1969).
14. J. W. Diggle, T. G. Downie, and C. W. Goulding, in Modern Aspects
of Electrochemistry, edited by J. O. M. Bokris, R. E. White, B. E.
Conway, Plenum Press, New York (1989) Vol. 23, p. 401.
15. J. P. O’Sullivan and G. C. Wood, Proc. Royal Soc. of London 317,
511 (1970).
16. F. Keller, M. S. Hunter, and D. L. Robinson, J. Electrochem. Soc.
100, 411 (1953).
Received: 16 February 2005. Accepted: 1 March 2005.
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J. Nanosci. Nanotech. 5, 1745–1748, 2005