# A Method for Fabricating Arrays of Nanopatterns with the Feature Size beyond Diffraction Limit

Authors: Shuhong Li; Lifang Shi; Xiaochun Dong; Chunlei Du; Yudong Zhang
Journal: Research Letters in Nanotechnology
 (2008)

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## Abstract

A convenient lithographic technique is proposed in this paper, which can be used to produce subdiffraction-limit arrays of nanopatterns over large areas (about several square centimeters). An array of polystyrene spheres (PS) is arranged on the surface of a layer of silver which has a thickness of about tens of nanometers. With the normal illumination light of wavelength 365 nm perpendicular to the substrate, PS can generate an array of optical patterns with high intensity at their contact points with silver. By designing the silver slab, the evanescent waves that carry subwavelength information about the optical patterns are substantially enhanced, while propagating components are restrained. In the photoresist which is on the other side of silver, the optical intensity is redistributed and subdiffraction-limit patterns are obtained after exposure and development. Simulation by finite-difference time-domain (FDTD) and experiments were carried out to verify the technique. The results show that by using PS with diameter of 600 nm, nanopatterns with dimension of less than 80 nm can be obtained.


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## Body

## 1. Introduction

Arrays of nanopatterns are useful for
a wide range of applications such as chemical sensors [1],
optical filters [2–4], photonic crystals [5], digital optical systems [6, 7], and displays [8].
Conventional photolithography [9] cannot be
used to fabricate arrays of nanopatterns because of the diffraction limit. Holographic
lithography [10], E-beam lithography [11], and laser pattern writing [12] are currently used to fabricate them. Although
high-quality patterns can be produced with those techniques, they require the expensive
facilities, and the multistep processing is required normally. In 2002, the group of
Whitesides proposed a lithographic technique by which the simple, repetitive
micropatterns over large area can be produced [13].
However, the technique can only be used to fabricate patterns with dimensions
larger than 100 nm. In 2005, Zhang Xiang's group proposed a method to produce
subdiffraction-limited optical imaging with a silver superlens [14]. However, masks fabricated by focused-ion-beam
lithography or E-beam lithography are needed in this method.To obtain nanopatterns for the
potential applications, we demonstrate a convenient process to produce
subdiffraction-limit arrays of nanopatterns over large areas in this paper. An array of PS is placed on the surface of a layer of silver
and forms contact points of medium and metal. When there is illumination
light, PS can generate an array of optical patterns
with high intensity at these contact points. Evanescent waves that carry
subwavelength information of the optical patterns can be enhanced when they are
transmitted through the silver layer to form intensity distribution in
photoresist on the other side of silver. Meanwhile, propagating
components are restrained. Therefore, subdiffraction-limit nanopatterns
can be obtained in
photoresist after development. In this technique, the nanopattern masks produced
by E-beam lithography can be omitted. The contact
points of PS and silver layer can be considered as the mask, where PS diameter
is about hundreds nanometers which can be easily produced. Nanopatterns with
dimensions below 80 nm can be obtained by this technique, which are about one-fifth of
the illumination wavelength.


## 2. Principle and Simulation

The schematic view of
this technique is shown in Figure1(a). Figure 1(b) shows the location of PS in the PDMS membrane, where PS cannot be enwrapped completely by PDMS because of
the capillary effect. Figure 1(c) shows that the PS is arranged in hexagonal
arrays.(a) The scheme of the technique; (b) PS embedded in PDMS membrane; (c) the arrangement of PS.
(a)(b)(c)We begin the process by making a mask
constituted by PS embedded in PDMS membrane. Then, the mask is put onto the
surface of a silver layer, where a photoresist (PR) layer is placed on the
other side. Here, PS converges the incidence light to the limited area, where optical patterns with relatively high
intensity are generated on the surface of the silver. At the nanoscale contact
points of PS and silver layer, some of the light in optical patterns can
be coupled into evanescent waves with wave vectorkx, where kx≫k0 (k0 is the wave vector in the vacuum). Here,
evanescent waves and propagating components are included in the optical
patterns, where evanescent waves carry the subwavelength information. When the real part of the
metal permittivity equals the dielectric of adjacent medium, most of the
evanescent waves that carry subwavelength information can be transmitted by extremely
amplifying in the silver layer. Meanwhile, the propagating components are restrained
by reflecting and
absorbing of the silver slab. Therefore, in the photoresist on the other
side of silver, the optical intensity is redistributed and subdiffraction-limit
patterns are obtained after exposure and development.To verify the feasibility
of this technique, simulation with FDTD method was carried out to obtain the
intensity distribution in the photoresist. The permittivities of PS, PR, and silver are 2.5281 [13], 2.886+i0.059 [15],
and −2.4012+i0.2488 [15], respectively. The
results are shown in Figure 2. The wavelength
of the illumination light is 365 nm, and the diameter of PS is 600 nm.The simulation results: (a) the intensity distribution in photoresist with a layer
of silver on its interface; (b) the normalized intensity distribution on the
surface of photoresist according to the scheme in (a);
(c) the intensity distribution in photoresist without silver on its interface;
(d) the normalized intensity distribution on the surface of photoresist
according to the scheme in (c).
(a)(b)(c)(d)The left part of Figures2(a) and 2(c) is
the simulated schemes with and without a layer of silver on the surface of
photoresist, respectively, where the thicknesses of optimum silver slab and
photoresist are 30 nm and 100 nm. The right parts in these two figures are the
intensity distributions in photoresist within 40 nm away from the surface.
Figures 2(b) and 2(d) are the normalized intensity distributions on
the surface of photoresist according to Figures 2(a)
and 2(c), from which we can see that the full
width half maximums (FWHMs) are 57 nm and 247 nm. Here,
we consider the FWHM as the size of photoresist which is exposed and can be dissolved
in the development process. In the above two situations, the dimensions
of the patterns to be obtained in photoresist are 57 nm and 247 nm,
respectively.From above simulation, we can know
that with silver slab in the exposure procedure, the dimension of the obtained
patterns is extremely reduced. The reason is the amplifying ability of the
silver to the evanescent waves and restrained to the propagating components.


## 3. Experiments and Results

To clean the Si substrate completely,
it was dipped in a 3 : 1 (v:v) mixture ofH2SO4 (98%) and
H2O2 (30%) for about an hour in the temperature of 80°C. Then, it was processed to be
hydrophilic by dipping in a 1 : 1 (v:v) mixture of H2O2 (30%)
and NH4OH (20%) for an hour. Then, a monolayer of PS (from Duke Scientific Corporation, Calif,
USA) with diameter 600 nm was spin-coated onto the surface of the substrate Si. It
was worth mentioned that the PS was arranged in hexagon which could be seen in Figure 3.Figure 3
Monolayer of PS on the surface of substrate Si.The substrate was placed in a plastic petri dish. A 10 : 1 (v:v) mixture of
PDMS-Sylgard Silicone Elastomer 184 and Sylgard Curing Agent 184 (Dow Corning
Corp. Midland, Mich, USA)
was poured into the petri dish. The PDMS cured at room temperature in the
laboratory ambient for 30–60 minutes. This cure
was followed by additional curing at 65°C for approximately 1 hour or until the
polymer was rigid. After cooling to room temperature, the PDMS was carefully
peeled from the dish and substrate. The membrane of PDMS embedded with PS was
obtained.Silicon was taken as the substrate material and photoresist AZ3100 as the exposure
material. The resist was spin-coated onto the substrate with rotation speed of 4000 rad/s. Process parameters of coating time, prebake temperature, prebake time,
and resist thickness were 30 seconds, 100°C, 5 minutes, and 100 nm,
respectively. A layer of silver with optimum thickness 30 nm was then deposited
to the surface of the photoresist. The PDMS membrane was put onto the surface
of silver with the PS contacted with silver. Then, the above structure was
exposed from the top of the PS by illumination light source with central
wavelength of 365 nm. The exposure time was 7 seconds.After exposure, the PDMS membrane embedded with PS was peeled off from the surface of
silver. The substrate was dipped in the solution of HNO3 (30%) to
wipe off the layer of silver. After developing for about 20 seconds, the
nanopatterns with dimension of about 75 nm were obtained in photoresist as
shown in Figure 4(a),
which is a little larger than the theory value 57 nm we got in the part
principle and simulation. The reason may be the infection of the PS not
contacting with silver tightly and the miss control in the development process.Experiment results: (a) patterns in photoresist
obtained with silver in the exposure procedure; (b) patterns
in photoresist obtained without silver in the exposure procedure.
(a)(b)Control experiment was also carried out. There was no silver layer on the surface of
photoresist. The optimum process parameters of exposure time and development
time were 1 second and 5 seconds, respectively. The obtained patterns in
photoresist are shown in Figure4(b) from which
we can see that the dimension of holes is about 260 nm.From the above
results, we can know that the pattern's dimension we got with silver layer in
the exposure procedure is much smaller than that without silver layer. The
effect of silver can be proved.


## 4. Conclusions

This paper demonstrates a technique which can be used to fabricate
nanopatterns. The technique does not require the use of masks made by expensive
optical facilities. It provides a convenient method for fabricating
subdiffraction-limit nanopatterns. By reasonably choosing the size of PS, the
arrangement of PS array, the kind of the photoresist, and different kinds of nanopatterns
can be obtained. Nanostructures generated by this technique can be useful for
the fabrication of optical elements. The research group in our laboratory is
carrying out deep research about the technique. Nanopatterns with other periods
and arrangement arrays are in fabricating.



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*Source: 492478-2008-12-14.xml*