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FOCUS ON NANOMANUFACTURING
Conformal dielectric films on silicon nanowire arraysby plasma enhanced chemical vapor deposition
J. Fronheiser Æ J. Balch Æ L. Tsakalakos
Received: 11 November 2007 / Accepted: 16 March 2008 / Published online: 16 April 2008
� Springer Science+Business Media B.V. 2008
Abstract In this article, we describe the coating of
silicon nanowire arrays with thin dielectric layers
using Plasma Enhanced Chemical Vapor Deposition
(PECVD). The impact of deposition pressure, tem-
perature, and nanowire array density on the silicon
oxide coating thickness uniformity was assessed using
a detailed electron microscopy observations of the
nanowire arrays. Deposition rates were found to vary
along the nanowire length as a function of the above
process parameters, and ranged from 0 to 35 nm/min.
The coating thickness was found to be most uniform at
higher pressures and temperatures, and high-density
nanowire arrays with smaller nanowire diameters and
larger lengths led to the deposition of coating with a
smaller thickness gradient along the wire length.
Keywords Nanowire � Silicon � Silicon oxide �Thin film � Coating � PECVD � Nanomanufacturing
Introduction
In recent years there has been a significant interest in
the fundamental science of nanowire (NW) and
nanotube (NT) arrays, including their synthesis
(Cui et al. 2001 and Lew et al. 2004) and properties,
as well as applications based on nanostructures.
Many devices and applications have been demon-
strated based on vertically aligned arrays, including
field-effect transistors (Goldberger et al. 2006), solar
cells (Baxter et al. 2006; Law et al. 2005), lasers
(Huang et al. 2001), super-hydrophobic surfaces
(Rosario et al. 2004), biotemplating surfaces (Dong
et al. 2006), and others. A critical factor in making
useful structures from NW/NT arrays is the develop-
ment of coating strategies that allow additional
functionality for the array and/or to assist in improving
their properties. This includes dielectric layers, active
electronic films, layers to impart biofunctionality, and
layers to enhance mechanical robustness.
In order to help the deposition processes for such
coatings to be manufacturable, several major require-
ments must be attained: (a) the process must be
inherently scalable to large areas; (b) relatively low
processing times are required to coat large area, dense
arrays; (c) the processing temperatures should be low
to minimize damaging or changing the structure and
thus properties of the arrays; and (d) the coatings
should be uniform along the wire length and across
the substrate. Furthermore, the deposition of such
coatings on quasi- or one-dimensional nanostructure
arrays should preferably be accomplished with stan-
dard processes such that adoption in a future
manufacturing setting can be facilitated.
While there have been several reports in the
literature regarding coating of NW/NT arrays, it is
J. Fronheiser � J. Balch � L. Tsakalakos (&)
General Electric - Global Research Center, Niskayuna,
NY 12309, USA
e-mail: [email protected]
123
J Nanopart Res (2008) 10:955–963
DOI 10.1007/s11051-008-9381-4
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evident that more work is required to elucidate the
variables and mechanisms that control the process
parameters described above.
Atomic Layer Deposition (ALD) is the method that
has been explored in greatest depth for coating of NW/
NT arrays. There are several benefits of ALD that
have been identified; these include (a) a high degree of
conformality; (b) thickness control at the sub-nm level
due to the self-limiting nature of the reactions
involved; (c) deposition within structures that are
laterally confined; (d) flexibility to potentially deposit
multiple compositions ranging from binary and multi-
component oxides to sulfides, arsenides, etc. (Ritala
and Leskala 1999). A particularly well-studied coating
that has been applied by ALD is Al2O3. Min et al.
(2003) deposited alumina on ZnO nanorods on Si
substrates at 300 �C using trimethylaluminum and
water. They showed uniform, conformal layers on the
ZnO nanorods without the presence of an interfacial
alloy. The growth rate was *0.19 nm/cycle (22 s total
cycle time). This method was subsequently used to
fabricate alumina nanotubes by selectively wet etching
the ZnO nanorod core (Hwang et al. 2004).
ALD was also used to fabricate multi-layer coatings
on carbon nanotube (CNT) arrays (Hermann et al.
2005). It was shown that some very uniform alumina/
W/alumina coatings could be deposited on CNTs so as
to create a co-axial cable-type structure that allows for
subsequent functionalization of the outer alumina
layer. This allowed for attachment of perfluorinated
molecules to the structure that rendered the CNTs
hydrophobic. Other applications have also been
demonstrated, including multi-layer deposition of
Ta2O5–NbxZryOz multi-layer films, TiO2 nanotubes
formed using cellulose nanofibers as templates,
alumina nanotubes using electrospun poly(vinyl)
pyrroline nanofibers, titania deposition on Ni nanorod
arrays, and Ru film deposition within microporous Si
(Leskela et al. 2007).
Physical vapor deposition, as well as other chemical
vapor deposition have also been used to coat NW/NT
structures. CNTs were coated with W using a physical
vapor deposition process in which a W filament was
used as the source (T = 2473 K) and the sample was
held at a relatively high temperature of 973 K (Zhang
et al. 2000). Boron nitride coatings were applied to
SiC NWs using B and SiO2 precursors heated in a BN
crucible at 1400–1500 �C (Tang et al. 2002). SiC
nanowires were also coated with a carbon layer for
improving the strength of mechanical composites
using a chemical vapor infiltration (CVI) method at
1223 K (Yang et al. 2005). These methods, while
effective in coating the nanostructures of interest, are
also too high temperature to be suitable for most
device-related applications. A relatively low-temper-
ature method to coat NWs and NTs with oxides using
an acid pre-treatment method has also been demon-
strated (Gomathi et al. 2005). While these
aforementioned processes have been shown to effec-
tively coat NWs/NTs, it is desirable to use well-
established processes that are inherently scalable and
applicable to arrays on a substrate.
Plasma-enhanced chemical vapor deposition
(PECVD) is a process that meets many of the
requirements outlined above. It is regularly used in
the electronics and solar energy industries and large-
scale tools are available. Many compositions can be
deposited by PECVD (much like ALD), such as silicon
nitride, silicon oxide, and amorphous silicon, as well as
crystalline semiconductors and conductors. (Reif 1984).
While PECVD generally results in less conformal films
compared to ALD, we will show that it is possible to
form relatively conformal coatings on nanowire arrays
using PECVD, on par with results in the literature
discussed above for ALD coatings. The processing
temperatures are typically less than 400 �C, making
this process a strong candidate for future nanomanu-
facturing of coatings on NW/NT arrays. This is
enabled by the fact that the plasma effectively
dissociates precursor molecules, thus reducing the
required process temperature. While PECVD has been
used to synthesize nanowires (Hofmann et al. 2003)
and nanotubes (Teo et al. 2002), to our knowledge
there has been little or no work reported on coating of
NW/NT arrays using PECVD. Here, we present an in
depth analysis of a prototypical system, namely silicon
oxide films on silicon nanowires arrays, to highlight
both the advantages of this process as well as
opportunities for process improvements.
Experimental procedure
Silicon nanowire arrays were grown on h111ioriented silicon substrates. Following deposition
of a 50 A thick Au film, catalytic CVD employing
the vapor–liquid–solid (VLS) growth mechanism
(Wagner and Ellis 1964; Cui et al. 2001) was used
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123
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to grow p-type Si nanowires with silane, hydrochloric
acid, and trimethylboron (Lew et al. 2004). Both low-
and high-density nanowire arrays were synthesized.
High-density arrays were grown at 650 �C for
30 min, whereas the low-density arrays were grown
at 650 �C for 30 min without the use of HCl.
The arrays were subsequently coated with silicon
oxide films using a plasma-enhanced chemical vapor
deposition (PECVD) system (Oxford Plasma100) The
PECVD process consisted of flowing silane (SiH4)
and nitrous oxide (N2O) at a flow rate of 15 and
710 sccm, respectively operating at 13.56 MHz.
Table 1 summarizes the specific conditions employed
for each type of sample.
The nanowire arrays were characterized using
scanning electron microscopy (SEM) with a LEO
VP1200 field emission system.
Results and discussion
Figure 1 shows typical scanning electron micrographs
(SEM) of the low- and high-density nanowire arrays
prior to deposition of the PECVD silicon oxide. The
low-density wires have a mean diameter of 182 ±
81 nm and a length of *6 microns. Figure 2 shows
the nanowire diameter distribution for a typical low-
density and high-density sample. The nanowires in the
high-density array have a mean diameter of 84 ±
17 nm (Fig. 2) and a length of *22 microns. The
high-density samples also show a bimodal length
distribution, in which a population of shorter nano-
wires with lengths of 2–5 microns are observed (see
Fig. 1).
Figure 3 shows SEM images of low-density
nanowires arrays coated with PECVD silicon oxide
under the size conditions outlined in Table 1. It is
evident that the thickness of the coating on the
nanowires is not constant along the wire length.
Figure 4 shows higher resolution SEM micrographs
of the top and bottom of the low-density NW arrays.
Deposition on the Au nanocatalyst particle at the tip,
which is typically associated with the VLS mechanism,
is clearly observed, and the fact that there is no
re-growth of nanostructures from these particles is
important. It is also evident that the silica layer is also
fully deposited between wires on the thin polycrystal-
line Si–Au layer that typically accompanies nanowires
Table 1 PECVD process
parameters used to coat Si
nanowires arrays in this
study
Run
ID
SiH4
(sccm)
N2O
(sccm)
Pressure
(mtorr)
Power
(W)
Power
density
(mW/cm2)
Temp
(C)
Time
(min)
1 15 710 1000 15 37 370 14
2 15 710 500 15 37 370 14
3 15 710 1500 15 37 370 14
4 15 710 1000 15 37 200 14
5 15 710 500 15 37 200 14
6 15 710 1500 15 37 200 14
Fig. 1 SEM images of representative (a) low and (b) high
density nanowires arrays before PECVD oxide deposition
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growth using nanocatalyst that are not templated or
patterned (Lombardi et al. 2006).
As we are interested in the manufacturabity of this
process, we studied the key geometrical parameters
of the thin film coating on the nanowires. Of
particular interest is the variation of the coating
thickness with nanowire length. Therefore, the gra-
dient of the nanowires thickness was measured as a
function of length for various process parameters;
array density and pressure are the parameters that are
analyzed in this work. We define the slope, or taper,
of the PECVD oxide film at a given position along
the nanowire length as the change in thickness of the
film along the given segment divided by the length of
the segment. This was measured for multiple nano-
wires in the array such that both an average taper and
the standard deviation of the taper for the particular
array was evaluated. Measurements were conducted
by evaluating the coating thickness at several loca-
tions along the length of the wire. Figure 5 shows a
higher magnification SEM image with arrows speci-
fying measurement points. The diameter of the coated
nanowires were measured in these locations and
analyzed as described above. The film deposited on
the top of the NWs at the location of the Au catalyst
nanoparticle was not included in these measurements.
The taper of a PECVD oxide film deposited on the
low-density array is shown in Fig. 6. The slope
steadily increases upon approaching the top of the
NWs, and the smallest change in slope along the NW
length is observed for run 3 and 6. These two were
0
5
10
15
20
25
30
35
40
50 100 150 200 250 300 350 400 450 500 More
Wire Diameter (nm)
Fre
quen
cy
0
510
1520
25
3035
40
30
Wire Diameter (nm)
Fre
quen
cy
45 60 75 90 105 120 135 More
(a)
(b)
Fig. 2 Diameter distributions of (a) low and (b) high density
nanowires arrays
Fig. 3 SEM images of low-density Si NW arrays coating with
silicon oxide by PECVD using the conditions in runs 1–6 (a–f,respectively). Note the magnification is not the same for all
images. The magnification bar for each image is for 1 micron,
except for image (e) for which it is 2 microns
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processed at the highest pressure used in this study,
i.e. 1500 mTorr and suggests increasing pressure
changes the plasma and film properties such that
smaller thickness gradients occur along the wire
length. For comparison, the total nanowire thickness
(Si NW plus oxide coating) as a function of position
along the nanowires is shown in Fig. 7. While we do
not have exact data on the initial–individual nanowire
diameters that were measured, based on pre-PECVD
diameter statistics we estimate that the deposition rate
of the silicon oxide layers varies from 15 to 35 nm/
min along the nanowires length.
Deposition of silicon oxide on high-density silicon
nanowires arrays shows features different from those
noted above. The measured films are thinner and the
value for the slope is less than that of the low-density
wires. Figure 8 shows typical SEM micrographs from
the high-density arrays. Figure 9 contains higher
magnification images showing that at the bottom of
the array there may be regions that were not coated
by the PECVD oxide; indeed, it is estimated, based
on morphological observations, that the bottom
2–5 microns of the wires were not coated.
Since the wires compared here are of considerably
different lengths, 6 lm and 22 lm for the low and
high density wires, respectively, only the top portions
of the longer wires should be used to compare with
the shorter low-density wires. Analysis of the slope of
the coatings on the high-density NW arrays in this
Fig. 4 SEM images of the
(a) top and (b) bottom
regions of low-density Si
NW arrays coated using run
#1. The magnification bar
for each image is for
1 micron
Fig. 5 Higher magnification showing measurement method.
Arrows represent measurement points. The magnification bar is
for 1 micron
0
0.05
0.1
0.15
0.2
0.25
0.3
0
Position from NW base (nm)
Tap
er
SiO2-1 LDSiO2-2 LDSiO2-3 LDSiO2-4 LDSiO2-5 LDSiO2-6 LD
7000600050004000300020001000
Fig. 6 Silicon oxide slope for various deposition conditions on
low-density nanowire arrays
0
200
400
600
800
1000
1200
Dia
met
er (
nm)
SiO2-1 LDSiO2-2 LDSiO2-3 LDSiO2-4 LDSiO2-5 LDSiO2-6 LD
0
Position from NW base (nm)7000600050004000300020001000
Fig. 7 Total diameter (nanowire + oxide coating) for various
deposition conditions on low-density nanowire arrays
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region reveals that the slope is approximately an
order of magnitude lower than for the low-density
arrays (Fig. 10). Also there is little variation in taper
of the oxide coating with nanowire length; the
variation observed in Fig. 10 is within the experi-
mental error of *15%. Clearly, the higher-density
wires, with smaller diameters and greater length,
have a strong impact on the transport of excited
reactant species within the NW array and subsequent
film deposition on the nanowire sidewalls. The total
nanowire thickness (Si NW plus oxide coating) as a
function of position along the high-density nanowires
is shown in Fig. 11. Based on estimates from pre-
PECVD deposition NW diameter statistics, the
deposition rate varies from 0 to 35 nm/min, though
this is distributed more uniformly along the upper
regions of the longer nanowires.
Once again, the effect of pressure dominates the
observed taper in the high-density NW arrays, giving
the lowest values. The global change in oxide
thickness was also determined as a function of both
pressure and temperature (Fig. 12). It was indeed
found that the total average slope was lowest at
higher pressures and at higher temperatures. These
trends were held true for both low and high-density
arrays.
In order to understand the mechanisms associated
with the observed processing trends, we analyze the
key factors that may influence the PECVD film
uniformity. According to our results, the pressure,
Fig. 8 SEM images of high-density Si NW arrays coating
with silicon oxide by PECVD using the conditions in runs 1–6
(a–f, respectively). Note the magnification is not the same for
all images. The magnification bar for each image is for 2
microns except for images (a) and (b) for which it is
10 microns
Fig. 9 SEM images of the
(a) top and (b) bottom
regions of high-density Si
NW arrays coated using run
#1. The magnification bar
for each image is for
1 micron
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temperature, and array density are major parameters
to be considered. Figure 13 shows a model of the
structures explored and key features. It is assumed
that the factors influencing deposition within the
inverse structure, namely within nanochannels of a
template array (Lew and Redwing 2003), are similar
to the nanowire array structure.
Pressure effects play a dominant, yet somewhat
counterintuitive, role in the deposition process.
Assuming classical diffusion principles the pressure
has an inverse effect on the diffusion rate constant and
therefore the concentration of gas phase reactant
species at the bottom of the wires should decrease
with increasing pressure (Plawsky 2001). In plasma
deposition, there is some concentration gradient of
neutral species, however, the majority of reactive
species are charged molecules that gain momentum by
the alternating AC-field. Therefore, classical diffusion
principles do not directly apply. A more detailed
analysis considering the flux of reactive species
impinging on the surface is required to explain our
results. For a given temperature the number of
impinging molecules that strike the surface is directly
proportional to the operating pressure (Maissel and
Glang 1970). During the deposition process the density
of adatoms on the surface is related to the molecular
impingement rate, the absorption and desorption rates,
and the sticking coefficient. The sticking coefficient
accounts for the fraction of atoms that do not adsorb on
the surface. It generally depends on the fraction of
surface sites covered with the adsorbed species, the gas
and surface temperatures, as well as surface features
such as roughness, defect sites, and exposed bonds or
vacancies (Lieberman and Lichtenberg 1994). We
postulate that the increased pressure changes the make-
up and energy of the plasma gas phase species. It is
0
0.05
0.1
0.15
0.2
0.25
0.3
0 5000
Position from NW base (nm)
Tap
er
SiO2-1 HDSiO2-2 HDSiO2-3 HDSiO2-4 HDSiO2-5 HDSiO2-6 HD
25000200001500010000
Fig. 10 Silicon oxide slope for various deposition conditions
on high-density nanowire arrays
0
100
200
300
400
500
600
700
800
900
1000
Dia
met
er (
nm)
SiO2-1 HDSiO2-2 HDSiO2-3 HDSiO2-4 HDSiO2-5 HDSiO2-6 HD
0 5000
Position from NW base (nm)25000200001500010000
Fig. 11 Total diameter (nanowire + oxide coating) for vari-
ous deposition conditions on high-density nanowire arrays
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
400
Deposition Pressure (mtorr)
Slo
pe (
Del
ta T
hick
ness
/ W
ire L
eng)
LD-200CLD-370CHD-200CHD-370C
1600140012001000800600
Fig. 12 Mean slope of the oxide coating (total change in
thickness/nanowire length) on low and high-density nanowire
arrays as a function of pressure for two different temperatures
plasmaPrecursor flow
Precursor radical velocity
Reaction rate constant & sticking coefficient
Reaction product diffusion coefficient
boundary layerCb
C (x,y,z)
Precursor diffusion flow
Fig. 13 Schematic of the structure considered and the key
parameters influencing PECVD oxide deposition. A three-
dimensional model is under development to fully determine the
impact of nano-array structural parameters on deposition
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known that the sticking coefficient changes with gas
species, where even changing the concentration of one
particular species may affect the ability of atoms to
diffuse along the wire length. The mean free path is
inversely proportional to the gas pressure where it is
assumed that additional atomic collisions are largely
responsible for the change in plasma behavior (Maissel
and Glang 1970). In this way the pressure likely
changes the surface chemistry by increasing surface
diffusion as well as decreasing the sticking coefficients
of reactive species, which may assist in yielding a more
uniform oxide deposition along the wire length.
The temperature is also a critical control parameter
because the sticking coefficients typically decrease
with increasing temperature. This is evident in the
data presented above, where the increased tempera-
ture yields a more uniform coating along the wire
length, since there is greater probability for molecules
to leave the surface and diffuse or drift (in the local
electric field) to other locations of the nanowire.
The least significant factor affecting the coating
uniformity is the nanowire array density. One potential
explanation for this relates again to the sticking
coefficient. SEM and TEM observations generally
show that the surface morphology of Si nanowires in
the low-density nanowire arrays is more faceted and
they contain a higher concentration of gold particles on
the surface compared to nanowires in the high-density
arrays. We hypothesize that these provide additional
defect sites that effectively increase the sticking
coefficients, especially near the top of the wires as
seen by the large increase in taper at their tips. As such,
a balance between temperature and pressure is required
to allow transport of species through the interstices of
the array with uniform reaction on the sidewalls. More
detailed calculations are required to better quantify
these relationships and optimize the growth para-
meters, while also taking into account the impact of
activated species created in the plasma environment.
Additional studies correlating the surface defect
structure of nanowires in the low and high density
arrays to the observed film thickness taper will help to
further shed light on the mechanisms underlying the
deposition of activated species on nanowire/tube
arrays by PECVD.
Finally, a potential problem with PECVD pro-
cessing of nanowires is the potential for damage of
the wire surfaces by the plasma. However, we note
that nanowires processed with PECVD have been
observed by transmission electron microscopy
(TEM) in our lab (data not shown) and no obvious
damage to the nanowires has been observed. Indeed,
deposition of thin films on Si has been widely
reported with minimal damage to the Si surface,
e.g., PECVD films are typically used to passivate
surface states on single crystal Si (Aberle 2000).
Conclusions
In conclusion, the deposition of silicon oxide films on
silicon nanowire arrays using plasma-enhanced
chemical vapor deposition has been studied with
respect to process uniformity. The effect of pressure,
temperature, and nanowire array density on the
coating uniformity as a function of position along
the nanowire length was quantified. It was observed
that higher pressure and higher temperatures lead to a
more uniform coating. Higher density arrays lead to
both a smaller gradient in the coating thickness with
NW length, as well as a more uniform change of the
gradient with length. For long nanowire arrays
(*22 microns), the PECVD does not deposit near
the base of the nanowires. This work shows that
PECVD deposition on NW/NT is dependent not only
on the PECVD process parameters, but also on the
nature of the array being coated. A more thorough
mechanistic understanding, particularly as relates to
precursor transport within the arrays, coupled with
experimental optimization of the process, is required
to achieve better control of the deposition rates and
thickness uniformity. The use of PECVD to coat
nanowire arrays with silicon oxide and other mate-
rials is shown to be a viable candidate for future
nanomanufacturing of materials and devices using
such structures.
Acknowledgments The authors wish to thank T. Vandenbriel
and S. Klopman for technical support with the nanowire growth
and PECVD deposition and B.A. Korevaar, G. Dalakos, R.R.
Corderman and R. Rohling for helpful discussions.
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