研究計畫名稱：先進奈米壓印顯微術之研究
論文名稱：利用高分子膠體奈米顆粒製造大範圍週期性奈米壓印之
模具
作者：卓奕弘(Yihong Cho)、陳培菱(Peilin Chen)
聯絡人：陳培菱
通訊地址：中央研究院
應用科學及工程研究所籌備處
電話：(02)27898000
傳真：(02)27826680
e-mail：peilin@gate.sinica.edu.tw
1
Fabrication of Large-Area Periodic Nanopillar Arrays for Nanoimprint
Lithography Using Polymer Colloid Masks**
Yi-Hong Cho, Peilin Chen
Institute of Applied Science and Engineering Research, Academia Sinica. 128, Section 2
Abstract
A novel scheme for the fabrication of large-area nanoimprint stamps has been developed
based on the utilization of a combination of nanosphere lithography and reactive ion etching.
Both single and double layer polystyrene beads have been employed to construct well-ordered,
periodic silicon nanopillar arrays. The nanopillar arrays fabricated by this method have been
successfully used as the stamps for nanoimprint lithography. Our result indicates that this
approach is capable of producing large-area sub-50 nm periodic nanostructures.
Introduction
Periodic nanostructures are of great
research interests, because of their potential
applications in photonic crystals [1-3], data
storages [4-6] and biosensors [7-8]. To realize
such opportunity, it requires the development
of lithography techniques, which are capable
of fabricating large-area periodic
nanostructures with reasonable control of
their size and periodicity. Present
commercial optical lithography allows
mass-replication of nanostructures with ~130
nm resolution and is approaching its limit set
by diffraction (~110 nm for UV light with a
wavelength of 193 nm)[9]. To fabricate
nanostructures in the sub-100 nm region,
different techniques such as
electron-projection and extreme-ultraviolet
lithography are under development.
However, the cost and progress of these
systems may prevent their immediate
utilization. Therefore, lots of recent research
efforts have been focused on the development
of alternative cost-effective, high-throughput,
high-resolution lithography techniques for
both industrial applications and fundamental
studies. Among these approaches,
2
nanoimprint lithography is one of the most
promising schemes for large-area
mass-replications. It has been demonstrated
[10-12]
that nanoimprint technique is capable of
replicating large-area nanostructures with
feature size smaller than 10 nm and the
imprint time can be as fast as a few hundred
nanoseconds. Despite of these promising
results, there are still technical challenges
remained to be solved such as sticking
between stamp and polymer layer [13] and the
influence of visco-elasticy behavior of
polymer on the imprinted results [14].
Therefore, it is very important to conduct
researches on many fundamental aspects of
nanoimprint lithography. However, the
fabrication of stamps for nanoimprint
lithography requires e-beam lithography,
which limits the accessibility of nanoimprint
technique. Here we report a simple stamp
fabrication technique that is capable of
producing large-area, well-ordered, periodic
nanopillars for nanoimprinting with sufficient
size control in the sub-50 nm region.
Experimental
The detailed procedure of fabricating
large-area periodic nanopillar arrays as stamps
for nanoimprint lithography is illustrated
schematically in figure 1. Silicon substrates
are first coated with monodispersed
polystyrene solution to form large-area close
packed structures on the surfaces.
Depending on the concentration and speed of
spin-coater, single layer or double layer
polystyrene template can be obtained. Since
the double layer template produces periodic
structures that are more suitable for
commercial applications (fig 1a), we will
focus our discussion on the features created
by the double layer templates. In the second
step, a metal film is deposited on the double
layer template and the polystyrene beads can
be dissolved in CH2Cl2 solution leaving
periodic metal arrays on the surface (fig 1b).
These periodic metal arrays then serve as the
etching masks in the reactive ion etching
process. By placing the silicon substrates in
a reactive ion etcher, large-area silicon
nanopillar arrays can be obtained (fig. 1c).
To conduct nanoimprinting experiment, the
silicon nanopillar array is pressed at high
pressure against a poly (methyl methacrylate)
(PMMA) film on a silicon wafer at
temperature higher than the glass transition
temperature of PMMA (fig1. d). After
removing the nanopillar stamp, the silicon
wafer is first etched in oxygen plasma to
remove residual PMMA in the imprinted
holes and further deposited with desired
materials (fig1. e). After lift-off process,
periodic arrays of desired materials can be
obtained (fig1. f)
To demonstrate that the nanopillar arrays
prepared by nanosphere lithography is
suitable for nanoimprint lithography, we have
utilized 150 nm tall silicon nanopillar arrays
fabricated by 350 nm polystyrenes as the
stamps for nanoimprinting.
In this
experiment, a 200 nm thick 950K PMMA
film was spun on a silicon substrate and baked
3
in a vacuum oven at 1200C for 10 minutes.
Both PMMA and nanopillar arrays were
heated to 2000C and then compressed in a
hydraulic press at a pressure about 135
kg/cm2 for 5 min. The press was released
when the temperature dropped below the glass
transition temperature of PMMA, 1050C.
Each stamp has been used for nanoimprinting
at least for twenty times. No noticeable
pattern degradation or damages on the stamps
were observed.
Figure 2a shows the SEM image of the
nanoimprint stamp prepared by nanosphere
lithography using 350 nm polystyrene beads.
The lateral dimension of the triangle
nanopillars was around 65 nm. The result of
nanoimprinting is shown in fig 2b. Then a 20
nm thick chromium film was deposited on the
top of PMMA. The lift-off process was
performed by placing the silicon substrate in
acetone and sonicating for 3 to 5 minutes.
Fig 2c and fig 2d exhibit the SEM images of
well-ordered periodic nanodots after lift-off
process. The diameters of nanodots were
slightly less than 50 nm.
This result
confirms that our method is capable of
fabricating large-area sub-50 nm periodic
nanostructures. It is interesting to note that
the nanoparticles formed by nanosphere
lithography are triangular in their shape while
the subsequent template produces circular
nanodots through nanoimprinting. This is
probably due to the fact that the nanopillars
fabricated by this scheme are cone-like pillars
with round tips. One remarkable feature of
our approach is that the size of nanopillars
could be reduced by various techniques such
as ashing and trimming [15]. Therefore, the
size and separation of periodic nanostructures
can be independently adjusted. Shown in
figure 2e is one example of using
size-reduced nanopillar for nanoimprinting.
In this case, the size of nanopillars was
reduced by chromium etchant. The lateral
dimension of imprinted holes was less than 30
nm. Other approaches for size reduction are
currently under investigation in our group.
Conclusions
we have developed a low-cost, high
throughput fabrication process for producing
large-area, well-order periodic nanopillars for
nanoimprinting with feature size less than 50
nm that would allow easily accessing the
nanoimprinting technique without the need of
e-beam lithography. When these stamps
were used in nanoimprint lithography,
large-area periodic nanostructures with lateral
dimension less than 30 nm have been
obtained. The size and separation of the
fabricated periodic nanostructures can be
independently adjusted by selecting different
diameters of polystyrene beads in the
nanosphere lithography and trimming the
nanopillars with various size reduction
techniques, respectively.
The shape of
nanostructure can be also modified by using
different combinations of metal masks and
etching recipes.
Acknowledgements
This research was supported in part by
National Science Council under contract
91-2113-M-001-021.
Figure 1. Illustrations for the fabrication of
large-area periodic nanostructures by a
combination of double layer nanosphere
lithography and nanoimprint lithography.
Figure 2. (a) SEM image of a nanoimprint
stamp fabricated by a 350 nm polystyrene
template. (b) The imprinted patterns on
PMMA. The base of triangular hole is about
55 nm. (c) Large-area imaged of periodic
metal dots formed by nanoimprint
lithography. (d) SEM image of nanodots
formed by nanoimprint lithography. The
diameters of nanodots are around 50nm. (e)
SEM image of the imprinted patterns using
stamp treated with chromium etchant. The
lateral dimension of triangular holes is about
30 nm.
Figure Captions
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中央研究院 應用科學與工程研究所籌備處
摘要
大範圍週期性的奈米模具製造技術已
經結合了奈米球微影術與離子反應蝕刻製
作；而單、雙層之高分子球也被利用於週
期性的矽柱陣列。矽柱陣列的製造也成功
的在奈米壓印使用。經本實驗室之研究可
製做出低於 50nm 以下大範圍週期性之奈米
結構。