a,b
a
a
c a
b
c
We have investigated the nanopore formation utilizing SiC/Si (001) heteroepitaxial growth. Inverse pyramidal pits were produced by {111} faceted on the top of Si layer of Silicon on Insulator (001) substrate after SiC growth by using
CH
3
SiH
3
pulse jet chemical vapor deposition. Randomly distributed nanopores with the size of

10 nm were obtained after dipped into BHF solution for etching the buried oxide layer through the top of the pit. It was found that the densities of the pits and the nanopores strongly depend on the initial SiC nucleation density which can be controlled by the pulse frequency and number of CH
3
SiH
3
pulse jets.
Keywords: Nanopore, SOI, SIMOX, Supersonic jet CVD, SiC, Si, Heteroepitaxial growth, Pit formation.
Nanopore DNA sequencing is one of the most promising technologies being developed as a cheap and fast alternative
1
.
Mainly there are two kind of nanopores are being fabricated. They are biomaterial where a-hemolysin, a transmembrane protein inserted in a lipid bilayer aluminum oxide
7
2
and solid-state domain such as silicon nitride
3,4
. For silicon nitride ion beam sculpting is used
3,4
, SiO
2
5
, multiwall carbon nanotube
6
while high energy electron beam is for SiO
2
5
and
. However, these techniques are not suitable for mass production. Chemical vapor deposition (CVD) is widely used in the semiconductor industry to produce thin films. It is suitable for fabricating samples in mass production. We have previously developed a new CVD technique using pulsed supersonic free jets control the film thickness by adjusting the pulse width and frequency
9 multilayer structures
10-13
8
. Since this method makes it possible to
, it is suitable for growing very thin SiC/Si
. During the formation thin SiC, Si atoms are consumed from the Si substrate
14
. The SiC/Si interface has been plagued with the concurrent formation of voids in the Si substrate. The voids or interfacial pits are randomly formed in hollow inverted pyramids surrounded by {111} facet lying just beneath the film
14,15
. Recently, we reported that the nanopore can be obtained by using these {111} faceted pits in the top Si layer of Silicon on Insulator
(SOI) substrates during the SiC/Si(100) heteroepitaxial growths utilizing CH
3
SiH
3
pulse jet CVD
16
. In this paper, we further investigated the influence on CH
3
SiH
3
pulse jet numbers and frequencies of the nanopore formation.
We used Separation by IMplanted OXygen (SIMOX) (001) substrates with the top Si and buried oxide (BOX) layer of
180 nm and 100 nm, respectively as shown in Figure 1(a). They were cut into 25

25mm
2
pieces. Conventional RCA cleaning
17 was carried out in order to remove metal and organic contamination on the substrate surface and then the substrates were introduced into the chamber immediately after being dipped in aqueous 5% buffered HF (BHF) solution to remove the native oxide. The SiC growth on SIMOX (001) was carried out utilizing pulse jet CVD system which described elsewhere
9
.
* hafizal@zaiko8.zaiko.kyushu-u.ac.jp; phone +81-92-802-2966; fax +81-92-802-2990;
(b)

(a)
180 nm
100 nm
(b) (c) Nanopore
SIMOX (001) SiC growth and pit formation BHF dip
Figure 1. Schematic diagram of the nanopore formation using SIMOX (001) substrate (a) Bare Si surface, (b) SiC growth and pit formation by CVD process and (c) after being dipped into BHF solution.
Electronic grade CH
3
SiH
3
was introduced into the CVD chamber by utilizing a Parker Hannifin Corporation General
Valve with a nozzle diameter of 0.8 mm. The distance between the substrate and the valve was approximately 20 cm.
During the SiC growth and pit formation [Figure 1(b)], the substrate temperature and the pulse CH were set at 900 °C and 100
 s, respectively. It should be noted that a pulse beam produced by the valve nozzle is 1.7 x
10
17
3
SiH
3
pulse width
molecules and the total number of molecules arriving on the substrate is estimated to be

1.4 x 10
14
molecules / cm
2
18
. The pulse frequency and pulse number were changed at 1~10 Hz and 600~18000 pulses respectively. After the SiC growth, the nanopores were formed by dipping into BHF solution for 10 min in order to remove BOX layer under the
{111} faceted pits [Figure 1(c)]. Then the films were characterized by using scanning electron microscopy (SEM).
Figures 2(a) and 2(b) show the SEM images of samples of as-grown and after being dipped into BHF solution, respectively. Square pits with the <011> oriented sides with the density of 2.1x10
8
cm
-2
are homogeneously distributed over the surface as shown in Figure 2(a). On the other hand, pits with shadow of circular patterns are clearly observed after being dipped into BHF solution for 10 min [Figure 2(b)]. The circular patterns have

1
 m in diameter and the density of 6 x10
5
cm
-2
. This is due to the BHF etching process where BOX layer was partly removed from the top of the pits to form an air gap. This air gap corresponds to circular patterns which are indicating the formation of nanopore
16
.
SiC Growth After BHF dip
(a)
2.1x10
8
cm
-2
(b)
6.0x10
5
cm
-2
10 Hz
Figure 2. SEM surface of SiC growth (pits) (a) and after BHF dipped for 10 min (nanopores) (b) at pulse number of
18000 pulses of CH
3
SiH
3
jets irradiation. The pulse frequency was set at 10 Hz.

In order to investigate the dependence of nanopore formation on CH
3
SiH
3
pulse numbers, we carried out the SiC growths by introducing various pulse numbers at frequency of 5 Hz. Figure 3 shows the SEM image samples of as SiC growth
[Figures 3(a), 3(c) and 3(e)] and after BHF dipped [Figures 3(b), 3(d) and 3(f)]. When the pulse number is increased from 6000 pulses to 18000, randomly distributed pits was formed with constant density at

3x10
8
cm
-2
[Figure 3(a), 3(c) and 3(e)]. On the other hand, nanopore was formed with significantly increased in density from 0 to 4.2×10
6
/cm
2 respectively on the Si surface after dipped into BHF solution [Figures 3(b), 3(d) and 3(f)]. The result can be explained through pit formation and SiC nucleation mechanism reported by Li et al
14
. Pits were named as voids or trenches and their formation strongly depends on the SiC nucleation density. When high pulse number is introduced into the sample surface, SiC nucleation exhibit significantly higher density on the top Si surface and the size of the produced pits becomes deep enough to reach the top of BOX layer. This result shows that the pit density remains constant, while the density of nanopore increases with increasing of pulse number.
6000 pulses 9000 pulses 18000 pulses
SiC
Growth
After
BHF dip
(a)
(b)
3.0x10
8
cm
0 cm
-2
-2
(c)
(d)
3.3x10
1.7x10
6
8
cm
cm
-2
-2
(e)
(f)
3.1x10
4.2x10
6
8
cm
cm
-2
-2
Figure 3. SEM surface of pits (SiC growth) and nanopores (after BHF dipped) at frequency of 5 Hz with different pulse numbers. 6000 pulses for (a) and (b), 9000 pulses for (c) and (d), 18000 pulses for (e) and (f).
It should be noted that the influences of pulse frequency on nanopore formation can be investigated by the samples with pulse irradiation numbers at 18000 pulses for 10 Hz and 5 Hz as shown in Figures 2(b) and 3(f). At 18000 irradiation pulses where the frequency was dropped from 10 Hz to 5 Hz, we found that the pit density was increased from 2.1x10
8 cm
-2
to 3.1x10
8
cm pattern from 6x10
-2
5
, respectively [see Figures 2(a) and 3(e)], while the nanopore density was shown the same increasing
cm
-2
to 4.2x10
6
cm
-2
, respectively [see Figures 2(b) and 3(f)]. This result shows that the pit and nanopore densities are remarkably increased as the frequency of pulses decreased.
We have investigated the SEM images surface of nanopores formation on SOI substrates utilizing SiC/Si(001) heteroepitaxial growth by using supersonic jet CVD. It was found that nanopores can be obtained in the top Si layer by etching the BOX layer under the pits. We have shown that the density of nanopore can be increased by decreasing the pulse frequency and increasing the number of pulse jet. The results indicate that the density of the nanopore formation can be controlled by utilizing pulse frequency and number of CH
3
SiH
3
pulse jets.

One of the authors (Y.I.) appreciates the support of Grant-in-Aid for Young Scientists (B) No. 20760459 from the
Ministry of Education, Science, Sports and Culture, Japan.
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