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Barry Chin Li Cheung Publications Published Research - Department of Chemistry
March 2006
Fabrication of nanopillars by nanosphere lithography Fabrication of nanopillars by nanosphere lithography
Chin Li Cheung
University of Nebraska at Lincoln
, ccheung2@unl.edu
R. J. Nikolic
Center for Micro and Nano Technology, Lawrence Livermore National Laboratory, Livermore, CA
C. E. Reinhardt
Center for Micro and Nano Technology, Lawrence Livermore National Laboratory, Livermore, CA
T. F. Wang
Directorate of Chemistry and Materials Sciences, Lawrence Livermore National Laboratory, Livermore, CA
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Cheung, Chin Li; Nikolic, R. J.; Reinhardt, C. E.; and Wang, T. F., "Fabrication of nanopillars by nanosphere
lithography" (2006).
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1. Introduction
Device physics is known to change when structures of ma-
terials are engineered at the nanoscale [1, 2]. Nanospheres
made of silica or organic polymer have been applied to gener-
ate various functional nanostructures, from photonic band gap
materials [3]and surface plasmon sensor arrays [4]to nanome-
ter-sized magnetic domains with orientation dependent mag-
netic anisotropy [5]. Nanosphere lithography (NSL) is an eco-
nomical technique to generate single-layer hexagonally close
packed or similar patterns of nanoscale features. Generally,
NSL applies planar ordered arrays of nanometer-sized latex or
silica spheres as lithography masks to fabricate nanoparticle
arrays. NSL has been reported to generate nanowell arrays by
conventional reactive ion etching [6, 7]. Though NSL and re-
active ion etching has been recently used to pattern nanome-
ter-sized metal masks to produce columnar and spike struc-
tures [8–10], high aspect ratio pillar features with various
separation spacing have not yet been demonstrated using this
technique. Nanopillar patterned surfaces are usually made by
deep ultraviolet lithography, electron beam lithography or na-
noimprinting. These structures have been exploited for appli-
cations ranging from microuidic chips for separation of long
DNA molecules [11]and adhesion substrates for cell growth
[12]to the investigation of hydrophobicity and hydrophilicity
on nanostructured surfaces [13].
Here we present the application of a low-cost NSL
method to generate high aspect ratio silicon nanopillars by
high density plasma reactive ion etching. The size and sepa-
ration of these nanopillars were controlled by using polysty-
rene nanospheres of different sizes and etching these spheres
with oxygen plasma to tailor the diameter of the nanosphere
“lithography mask.” Since these “masks” are densely packed
with separation at the nanoscale, their packing density drasti-
cally affects the ow of reactive ion etchants and efuent ion
species during the etching process. Thus, both the separation
of etched “resists” and the etch time control the etch rates of
silicon. An investigation of the as-fabricated structures with
scanning electron microscopy (SEM) indicates that the etch
rates of the masked silicon substrates decrease linearly with
increasing aspect ratio of the resulting etched structures.
2. Experiment
Our strategy to fabricate the nanopillars consists of three ma-
jor steps: spin coating of polystyrene beads on silicon, reactive
ion etching (RIE) with oxygen to tailor the size of the polysty-
rene beads, and deep RIE of silicon to etch the pillar structures
Published in Nanotechnology 17:5 (March 14, 2006), pp. 1339–1343; doi:10.1088/0957-4484/17/5/028
Copyright © 2006 IOP Publishing Ltd. Used by permission. http://stacks.iop.org/Nano/17/1339
Submitted November 19, 2005; revised January 8, 2006; published online February 10, 2006.
Fabrication of nanopillars by nanosphere
lithography
C. L. Cheung
1,*
, R. J. Nikolić
2
, C. E. Reinhardt
2
, and T. F. Wang
3
1
Department of Chemistry and Center for Materials Research and Analysis, University of Nebraska–Lincoln,
Lincoln, NE 68588, USA
2
Center for Micro and Nano Technology, Lawrence Livermore National Laboratory, 7000 East Avenue,
Livermore, CA 94550, USA
3
Directorate of Chemistry and Materials Sciences, Lawrence Livermore National Laboratory, 7000 East Avenue,
Livermore, CA 94550, USA
* Correspondence: C. L. Cheung, email: ccheung2@unl.edu
Abstract
A low cost nanosphere lithography method for patterning and generation of semiconductor
nanostructures provides a potential alternative to the conventional top-down fabrication techniques.
Forests of silicon pillars of sub-500 nm diameter and with an aspect ratio up to 10 were fabricated
using a combination of the nanosphere lithography and deep reactive ion etching techniques. The
nanosphere etch mask coated silicon substrates were etched using oxygen plasma and a time-
multiplexed “Bosch” process to produce nanopillars of different length, diameter and separation.
Scanning electron microscopy data indicate that the silicon etch rates with the nanoscale etch masks
decrease linearly with increasing aspect ratio of the resulting etch structures.
1339
1340 Cheu ng, niko l ić, Rei n haR d t, & Wa ng i n N a N o t e c h N o l o g y 17 (2006)
(gure 1). For the NSL step, 2 × 2 cm
2
pieces of silicon or 2
inch silicon wafers (Prime grade, 2 mm thick, single and dou-
ble sided polished silicon wafers, Recticon Enterprises, Inc.,
Pottstown, PA) were rst cleaned by heating the samples in a
100 ml RCA solution (an aqueous mixture of 30% hydrogen
peroxide (Mallinckrodt Baker, Inc., Paris, KY), concentrated
ammonium hydroxide (FisherScientic, Pittsburgh, PA), and
DI water with the volume ratio of 1:1:5.) at 70 °C for 30 min,
followed by a thorough rinse with DI water and drying in a
stream of nitrogen. Then, a polystyrene bead solution pre-
pared by a modied procedure in the literature [14] (50 µl of
Triton X-100 diluted with methanol by 1:400 (Sigma-Aldrich,
Milwaukee, WI) and 350 µl of polystyrene beads with diam-
eter 500 nm (5050A, Duke Scientic, Palo Alto, CA)) was
spin-coated on these samples with a commercial spin coater.
The spin-coating program consists of three stages: (i) 400 rpm
for 10 s to spread the beads solution evenly; (ii) 800 rpm for
2 min to spin away the excess bead solution; (iii) 1400 rpm
for 10 s to spin off the excess materials from the edges. The
surfaces of the resulting substrates are strongly diffracted un-
der room light to reect a purple opal color. The diameters of
the polystyrene bead coated substrates were then tailored by
a parallel plate RIE etcher with 200 sccm of oxygen and 8.4
sccm of tetrauoromethane at a pressure of 200 mTorr and ra-
dio frequency (RF) power of 100 W with various etch periods
(0, 30, 60, 90, 120 s) (gure 2, row A).
Nanopillars were fabricated with the polystyrene bead
coated silicon by applying the “Bosch” time-multiplexed RIE
process [15, 16] in a commercial inductive coupled plasma
etcher. Briey, in the process, alternated cycles of etching in
a ow of SF
6
(12 sccm, 12s)and passivation in a ow of C
4
F
8
(85 sccm, 9 s) were used to etch the unprotected areas and to
deposit uorinated polymer to protect the side walls of the re-
sulting etched structures. The plasma with these chemical spe-
cies was generated with an RF power of 600 W and platen
power of 14 W at a pressure of 4.5 mTorr. The temperature
of the substrate was kept at 25 °C by cooling with a stream of
helium. The samples were etched with different numbers of
etch cycles and examined by scanning electron microscopy to
investigate the relationship between the etch rate and aspect
ratio of the etched structures.
3. Results and discussion
The three-step fabrication scheme has been successfully car-
ried out to yield “forests” of silicon nanopillars of diame-
ters from 250 to 350 nm. After the NSL in step 1, SEM data
showed that the spin-coated beads were packed mostly as
monolayers of hexagonally closed packed (HCP) domains
of 5–150 µm in domain size on the substrates. Occasionally,
holes with diameters of 5–20 µm of empty space were found
between the domains. Though dip-drying is one of the more
common techniques for NSL [8], we found that spin-coating
was a more controllable process, especially for patterning a
monolayer of beads on wafer-scale substrates.
The tailoring of the size of polystyrene bead masks by
oxygen RIE shrunk the diameters of these beads from 500
to 250 nm linearly with etch time (gure 2, Row A & gure
3). Since the position of each bead remained the same after
the oxygen RIE, the separations between each bead mask in-
creased as the sizes of the beads decreased. Though the etch-
ing process caused a minor increase in surface roughening on
the etched beads, the beads in general were etched evenly in
the lateral dimension within the tested time periods of 0–150
s. The heights of the beads were also etched gradually dur-
ing the RIE process. However, RIE runs with 180 s or longer
often resulted in near to total removal of the beads, and thus
they were used not in our experiments.
The fabrication of silicon nanopillars from the bead
masks of different sizes and separation led to pillars of diame-
ters from 200 to 350 nm and height of about 400 nm–2 µm by
applying the “Bosch” time-multiplexed silicon etch process.
During each of the “Bosch” etch cycles, the exposed silicon
areas were rst etched with SF
6
and then passivated with a
polymer formed by C
4
F
8
. Since the bombardment of the reac-
tive ions was directional and perpendicular to the surface of
the substrate in the deep RIE etcher, the bottoms of the etched
wells were preferentially etched during the etch phase of the
etch cycle. The side walls were etched only minimally due to
the parallel directionality of the reactive ions to the side walls
and the protection of deposited uorinated polymers. Con-
sequently, with appropriate etch masks and etch parameters,
this etch process had been shown to produce silicon structures
with high aspect ratio features of 40 and above [17].
For silicon etch periods shorter than 22 cycles, the fab-
rication scheme yields short silicon pillars with structures as
predicted. After ve etch cycles, the diameters of the as-fabri-
cated pillars were similar to the size of the ve different bead
etch masks used (gure 2, row B and row C). The “Bosch”
process was shown to effectively etch the silicon substrate
though the gaps between the “bead masks,” which are of the
order of 20–150 nm. This further demonstrates that the chosen
etch process for the fabrication of small-size features is crucial
over the ordinary RIE process which had been used to fabri-
cate only low aspect ratio wells and pillars [7, 8]. Though the
size of the bead mask shrunk about 5–10% in diameter during
the silicon etch, the diameters of the pillars still matched the
size of the corresponding masks because the diameter was de-
ned during the beginning of the etch process. Moreover, the
structures and arrangements of ‘bead masks’ remained mostly
intact after the silicon etch.
The shapes of the cross sections along the silicon pillars
in general conform to the shapes of the bead masks. Along
the length of a silicon pillar, small but periodic variations of
the pillar diameter indicate the etching and passivation cycles
Figure 1. Schematic diagram of nanopillar fabrication by nanosphere li-
thography. Step 1: spin coat a hexagonally close packed monolayer of
polystyrene beads on substrates. Step 2: tailor the size of the beads’ “re-
sist” by oxygen plasma etching. Step 3: etch the exposed semiconductor
areas by deep reactive ion etching using the “Bosch” process.
FabR iCati o n o F nan opil laRs by nano s phe R e l i tho g Raph y 1341
during the “Bosch” process. This “scalloping” effect is usu-
ally a function of polymer quantity deposited for etch protec-
tion during the passivation period and the strength of the RF
power in the etching period.
The etch rate of the silicon substrate was found to be a
strong function of the aspect ratio of the etched well geome-
try. In our case, the etched areas are of a complicated and pe-
riodic pattern. To simplify the estimation, we dene the aspect
ratio as the depth of the etched well (length of the pillar) di-
vided by the average of the minimum separation between the
bead masks and ignore the effect of the etch mask pattern on
the etch rate. Using the data from gure 2, a graph of silicon
substrate etch rates versus the aspect ratio (gure 4)indicates
that the silicon etch rate drops as the aspect ratio increases.
The aspect ratio etch rate dependence (ARDE) phe-
nomenon in deep RIE etching is commonly observed in the
micrometer and sub-micrometer scales when the aspect ratio
of the etched wells is above 1. The ARDE had been exten-
sively studied for trench etching in the µm range; however,
little has been reported for high density nanoscale patterns.
Various factors such as Knudsen transport of neutrals, ion
shadowing and charging effects could contribute to the phe-
nomenon [16]. In our case, since the separation between the
bead masks is of the order of 10–150 nm, the transport of
etchants and as-etched gaseous products is the major etch rate
limiting factor to etch these pillars. This is further supported
by the typical linear relationship between the silicon depth
etch rate and the aspect ratio, which is commonly observed in
the ARDE analysis for micrometer-scale trench etching exper-
iments limited by chemical transport (Figure 4). Since lower
chamber working pressure will increase the mean free path of
the molecular species used and produced in the etch process,
the etch experiments were carried out at 4.5 mTorr to facil-
itate the transport and to increase the directionality of these
species.
Longer etch progress experiments were also performed to
determine the effect of a prolonged etch period on the mor-
phology of the pillar structures and the durability of the bead
masks. HCP bead masks on silicon prepared by the above
procedure were tailored by oxygen plasma to yield average
etched bead diameters of 250 nm and minimum bead separa-
tions of 250 nm. Both 15 and 22 cycles of the silicon “Bosch”
etch process were used to etch these samples to generate pil-
lar structures of different aspect ratio (Figure 5). After 15 cy-
cles of etching, the bead masks remained intact and the di-
ameter of the as-etched pillars is similar to those of the bead
masks. However, as the number of etch cycles increases from
15 to 22, the bead masks started to degrade. The tops of the
Figure 2. Scanning electron micrographs of nanospheres-on-silicon samples after the second and third step of the nanopillar fabrication scheme as
shown in gure 1. Row A: top views of the nanospheres’ “resist” on silicon substrates etched by oxygen plasma treatment of periods of 0–120 s. Row B
and row C: top and side views of samples in row A etched by deep reactive ion etching using the “Bosch” process for ve cycles. All scale bars are 750
nm.
Figure 3. A plot of the diameters of oxygen RIE etched polystyrene
beads on silicon substrates versus the RIE etch time. The error bar is ± 3
standard deviation of each etched bead diameter measurement.
Figure 4. A plot of etch rates of silicon substrates patterned with poly-
styrene bead masks pre-etched with 0–120 s oxygen RIE etch periods
versus the measured aspect ratio of the corresponding etched wells using
the data in gure 2, row C.
1342 Cheu ng, niko l ić, Rei n haR d t, & Wa ng i n N a N o t e c h N o l o g y 17 (2006)
pillars were also etched 5–10% more due to their better ac-
cessibility to etchants in the process. Nevertheless, nearly uni-
form silicon pillars of about 5 and 10 aspect ratio can be fab-
ricated using the present process parameters without further
optimization (Figure 5). Moreover, as the etch time increases,
the side walls of the as-etched pillars become smoother with-
out the “scallop features,” probably due to a local heating ef-
fect in the etching process. The tapering effect at the bottom
of the etched areas is found to be minimal. The degrading of
the polystyrene beads during the processing limits the unifor-
mity of the as-fabricated pillars. We are currently exploring
the use of more robust beads made of cross-linked polymer
and silica and optimized etching conditions to make forest of
pillars with higher aspect ratios.
4. Conclusion
In short, a fabrication scheme that combines nanosphere li-
thography and reactive ion etching (both standard and deep
RIE) was developed to produce hexagonally close packed
semiconductor nanopillars. The etch rates of these structures
were found to decrease with an increasing aspect ratio and
thus the etch process is chemical transport rate limited. Pil-
lars of smaller diameters and of different materials can be po-
tentially fabricated using bead masks of smaller sizes with the
present scheme.
Since the patterns generated by nanosphere lithography
depend on the geometry of the sphere packing, the reported
scheme allowed only a single layer of patterned features to be
generated on a substrate at a time. Also, the presented proce-
dure yields only hexagonally close packed pillar patterns of
limited domain size with diameters of tens of microns. How-
ever, applications which do not require perfect hexagonal pat-
terns such as adhesion growth substrates for cell growth [12]
and the production of superhydrophobic surfaces [13] can be
envisaged with further chemical functionalization of the pillar
surface. Moreover, epitaxially grown semiconductor wafers
with different semiconductor layers can be used directly to
fabricate nanopillars with self-aligned and well-dened homo
or hetero junction properties. Such structures can be used for
investigating proof of principle device architecture such as
the vertical surround-gate eld-effect transistors [18]. Unlike
other gas phase nanowire synthesis, our process does not re-
quire the patterning of a metal catalyst and thus minimizes the
potential for unintentional doping of the wires.
The challenge to generate a large domain size of packed
nanospheres depends on the control of the phase of colloi-
dal packing in the solution media and the interaction between
the spheres and the substrates. Recently, Jiang et al have re-
ported their progress in generation of nearly single-domain
HCP nanosphere patterns on 4 inch wafers by adjusting the
viscosity of the silica nanosphere solution with appropriate
polymer content [7]. This provides a possible route to extend
the limitation of the present procedure to generate nanopillar
patterns with longer range order. Hence, the present demon-
strated scheme provides an alternative inexpensive method for
nanoscale, self-aligned pillar fabrication.
Acknowledgments
The authors are grateful for the support of the Laboratory Di-
rected Research and Development grant from the Lawrence
Livermore National Laboratory (grant # 04-ERD-107). CLC
thanks the support of the start up research fund from the Uni-
versity of Nebraska–Lincoln. This work was performed under
the auspices of the US Department of Energy by the Univer-
sity of California, Lawrence Livermore National Laboratory
under Contract No. W-7405-Eng-48.
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Figure 5. Scanning electron micrographs of nanopillars fabricated
from a polystyrene bead patterned silicon surface pre-etched with a
120 seconds oxygen RIE etch period, followed by (a) 15 or (b) 22 cy-
cles of the silicon etch process.
