Self-Assembled Polystyrene Micro-Spheres Applied for

Self-Assembled Polystyrene Micro-Spheres Applied for

JOURNAL INTEGRATED CIRCUITS AND SYSTEMS, VOL 1, NO. 3, JULY 2006.
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Self-Assembled Polystyrene Micro-Spheres Applied for Photonic
Crystals and Templates Fabrication
Daniel S. Raimundo1, Francisco J. R. Fernandez2 and Walter J. Salcedo3
Laboratório de Microeletrônica, Departamento de Engenharia Elétrica,
Escola Politécnica, Universidade de São Paulo, SP, Brazil
Av. Prof. Luciano Gualberto, Trav. 3, no 158, São Paulo, SP, Brazil, 05508-900
Tel: +55(11)3091-5310, Fax: +55(11)3091-5585, email: [email protected]
Abstract— The present work reports the fabrication of polystyrene self-assembled micro-spheres structures on silicon and
glass substrates by vertical deposition process using polystyrene micro-spheres monodispersion solution. The temperature
of the process, surface conditions and solution concentrations
were carried on the self-assembling process. Applications of
self-assembled polystyrene micro-spheres structures for
photonic crystals and templates fabrication, in silicon technology, were shown. Multilayer polystyrene structures showed
that could be applied to the fabrication of photonic crystal
structures and inverse opal structures.
Index Terms— self-assembling, polystyrene, template,
photonic crystals, metallic mask.
INTRODUCTION
Self-assembly method has been currently utilized by the
scientific community in the nanofabrication of materials.
The self-assembly method consists of a spontaneous process
by which molecules and nanophase entities may materialize
into organized aggregates or networks. Through various interactive mechanisms of self-assembly, such as electrostatic,
chemistry, surface properties, and via other mediating
agents, the technique proves indispensable to recent nanomaterials fabrication and device realizations.
In recent years, the systems composed of self-assembled
nanometric beads become important for different applications, as patterned arrays in lithographic techniques, microlenses and templates for inverse opal structures for sensors and optical applications [1-2]. In optical applications,
special attention has been given to photonic crystals enginnering area [3-4].
The idea of the periodic photonic crystals was conceived
by Yablonovitch [5]. One-dimensional (1-D) photonic crystal structure consists of alternating layers of materials with
different refractive index. A two-dimensional (2-D)
photonic crystal structure is periodic along two of its axes
and homogeneous along the third. A three-dimensional (3D) photonic crystal is periodic along the three axes. The
possibilities of controlling light emission, absorption and
propagation in optoelectronic and photonic devices have
generated intense research in the field of photonic crystals
(PCs) engineering [6].
A simple and effective process to obtain organized
structures is the self-assembling technique of colloidal
spheres. The combination of the capillary forces, the convective transport of spheres towards the substrate surface,
the horizontal force induced by the evaporation process and
the self-assembling effect, lead to the formation of the organized array structures of spheres based on self-assembling
concept [7].
A special characteristic of structures obtained by microspheres self-assembling is that the porous materials acquired have spherical voids, which were occupied originally
by the colloids. The monodisperse micro or nanometer colloids (organic or inorganic) [8] usually adopt face-centered
cubic (fcc) packing with approximately 26% voids in volume, into which the nanoparticles are easily infiltrated. The
templates subsequently can be removed by sintering for organic or acid extracting for inorganic templates to acquire
the organized porous network, in which the spherical voids
are periodic and interconnected via small interstitial channels. A lot of nanoporous materials including inorganic, organic, metallic and ceramic have successfully been made
using this method. Both organic and inorganic porous materials are very interesting for theoretical research and practical applications such as making optical filters, switches, and
chemical sensors, and as carriers for catalysts and medicines
[9]. Through various interactive mechanisms of selfassembling, such as electrostatic, chemistry, surface properties, and via other mediating agents, the technique proves
indispensable to recent nanomaterials fabrication and devices realization.
The present work reports the influence of the monodisperse concentration, temperature setup and initial surface
condition of the substrate on the self-assembly of polystyrene microspheres formation, and also the possibility of the
structures to be applied as photonic crystals and templates
in porous silicon microelectronics technology.
EXPERIMENTAL PROCEDURE
In this experiment, polystyrene spheres were assembled
onto glass and silicon substrates by vertical deposition to
fabricate 2-D and 3-D self-assembled structures. The solutions that were used consisted of polystyrene micro-spheres
monodispersions in aqueous solution. The diameter of polystyrene spheres was 660 nm. Before the self-assembling
process, the glass substrates were cleaned in extran solution
at 60oC and sonicated during 10 minutes. After it, the glass
substrates were rinsed using deionized water during 10 minutes. Finally, these substrates were immersed into an
ethanol solution and heated until completely dry to finish
the cleanness. Silicon substrates, before the self-assembling
process, were cleaned using conventional chemical microelectronics cleanness and were oxidized at 1000 oC to form a
300 nm SiO2 thickness, approximately. For the deposition
40 RAIMUNDO et al.: SELF-ASSEMBLED POLYSTYRENE MICRO-SPHERES AP. FOR PHOTONIC CRYSTALS AND T. FABRICATION
of polystyrene spheres, the substrates were immersed into
the solutions in vertical configuration and all the system was
heated at temperatures of 35oC, 50oC and 65oC, using two
different concentrations (1.72%wt and 0.86%wt) of polystyrene dispersion, to each temperature. In addition, to
evaluate the surface conditions for the deposition, glass
substrate surfaces were treated with HF (fluoridric acid)
after the cleanness process and before the deposition. To the
formation of metallic organized structures, a gold metallic
thin film (12 nm) was deposited on the structure by the
Sputtering technique. These metallized structures were sintered at 200 oC in air for 1 hour to promote good adherence
of the metallic film (gold), at interstitial regions, on the substrates; and to sinter the polymer.
Finally, the samples were immersed in sonicatingchloroform solution in order to remove the polystyrene
spheres (the template) by chemical dissolution of the polymer.
A 515 Philips Scanning Electron Microscope (SEM)
was used for direct morphological characterization of lattice
of the samples using the secondary electrons technique with
a 30 KeV energy. A Varian Cary 500 UV-VIS-NIR
Spectrophotometer was used for optical characterization to
verify the band gap existance.
RESULTS AND DISCUSSION
The self-assembled polystyrene structures for all the
samples showed hexagonal compact periodic array with lattice constant of c.a 660 nm (Figures 1 and 2). The hexagonal compact array showed thermodynamic preference because of the existence of the lowest minimum Gibbs free
energy for these structures (3). As follows, it will be discussed the self-assembled polystyrene structures in function
of the monodisperse solution concentration and the temperature level used for the solvent evaporation during the
PCs formation. In addition, it is worth discussing the surface substrate condition.
The self-assembled structures obtained on glass substrate by setting up temperature levels at 35 oC and 65 oC
and using two different concentrations of polystyrene
spheres aqueous solution c.a 1.72%(wt) and 0.86%(wt) are
showed in figure 1. The mechanisms of the self-assembled
structure with polystyrene spheres can be explained as follows: in initial state, a positive meniscus region is formed
on the substrate due to wetting from the solution and hydrophilic surface condition. Water evaporation out of this thin
meniscus, which leads to the flux and accumulation of polystyrene spheres and lateral capillary force, plays important
roles in the colloidal crystals formation (7). The samples
obtained at 35 oC (figures 1(a) and 1(b)) showed two-layer
structure with highly local organized aggregation. The
structures obtained at 65 oC showed four-sphere layers
(a)
(b)
(c)
(d)
Fig. 1 SEM images of photonic crystals structures obtained on glass substrates at (a) 35 oC and 1.72%wt (b) 35oC and 0.86%wt (c) 65 oC and
1.72%wt, and (d) 65 oC and 0.86%wt. The images (a), (b) and (c) were
10,000 times magnified and the image (d) was 5,000 times magnified.
(3-D structure) at two different polystyrene sphere concentrations (figures 1(c) and 1(d)).
However, the structure obtained with lowest concentration solution of polystyrene spheres (0.86%wt), showed
non-organized local aggregate sphere structure
JOURNAL INTEGRATED CIRCUITS AND SYSTEMS, VOL 1, NO. 3, JULY 2006.
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(a)
Fig. 3 Transmittance spectra of two-dimensional (2-D) and threedimensional (3-D) photonic crystals structures. The curves present
photonic band gaps at near infrared region.
(b)
(c)
(d)
Fig. 2 SEM images of photonic crystals structures obtained on silicon substrates at (a) 35 oC and 1.72%wt (b) 35oC and 0.86%wt (c) 65 oC and
1.72%wt, and (d) 65 oC and 0.86%wt. The images (a) and (b) were 10,000
times magnified and the images (c) and (d) were 5,000 times magnified.
(figure 1(d)). The high evaporation speed of the solvent
with low concentration solution, promote the rise of kinetic
energy of polystyrene spheres, producing non-organized
structure. It is worth mentioning that samples obtained at 50
o
C and with both solution concentrations showed three-
layer structures (figures not shown at this paper). Then, it
was verified that the increasing of the temperature promote
the increasing of the number of structure layers, perhaps because of the velocity of the evaporation rate. High evaporation rates and high flow velocities promote the aggregation
of spheres, forming multilayers. Then the evaporation temperature during vertical deposition is an important parameter during crystal growth. An evaporation temperature
near 50oC with 0.22wt% polystyrene monodispersion
concentration showed the point of optimum balance between the particle transfer process an its assembly into the
array in order to obtain a monolayer structure (Figure 5
(a)).The samples obtained on oxidized silicon substrate at
temperature of 35 oC and 65 oC using two polystyrene
spheres aqueous solution concentration c.a 1.72%(wt) and
0.86% (wt) are showed in Figure 2. The sphere aggregated
structures are similar to those obtained on glass substrate,
but at 65 oC temperature setup with 0.86%(wt) solution
concentration, the spheres aggregation was dramatically
non-organized at all regions of the substrate (figure 2(d))
that was completely different from the structure obtained on
glass substrate. The glass surface was treated with KOH
aqueous solution in order to generate the hydrophilic surface on glass substrate. The silicon oxide layer was obtained
by thermal oxidation process, so the SiO2 surface has had
slightly hydrophobic condition. This different surface condition obtained at 65 oC and 0.86%(wt) solution concentration, could have promoted the dramatic difference between
the structures that were obtained on SiO2 and glass substrates.
All the samples obtained on SiO2 and glass substrates
showed crack defects between PCs domains. The formation
of these cracks is associated with liquid evaporation during
the growing process. Cracks are distributed on many regions of the films, and the pattern of the hexagonal packing
is preserved between them. Finally, at low temperature
(35oC), the evaporation rate and evaporation velocity are
low, promoting the formation of structures with a reduced
number of defects.
42 RAIMUNDO et al.: SELF-ASSEMBLED POLYSTYRENE MICRO-SPHERES AP. FOR PHOTONIC CRYSTALS AND T. FABRICATION
Fig. 4 SEM image of an amorphous silicon inverse opal structure obtained
by three-dimensional self-assembled polystyrene structure. The image was
20,000 times magnified and presents an 1µm-dimension bar.
(a)
(b)
phenomena, and then a good defined absorption band can
be observed (Figure 3). The appearance of band gaps shows
that the obtained structures are photonic crystals. The
photonic band gap obtained with polystyrene spheres was so
narrow because of the low contrast of the refractive index
between polystyrene spheres (n=1.54) and air (n=1.00). To
increase the width of photonic band gaps is necessary to increase the contrast of refractive index of materials that
compound the structure.
The 3-D polystyrene spheres can be used as an excellent
molding structure in order to obtain inverse opal structures.
In this sense polycrystalline silicon film was deposited onto
the polystyrene 3-D structure by PECVD technique after the
film annealing at 400 oC in N2 ambient. After, the polystyrene spheres were dissolved by chloroform solution and finally, the obtained final film showed an inverse opal as can
be seen in Figure 4.
The monolayer polystyrene spheres array was used to
obtain a gold metallic mask fabrication throughout the following procedure: The gold film (~12nm) was deposited
onto the polystyre monolayer structure by Sputtering technique. After, the polystyrene spheres were removed by chloroform solution, remaining the gold metal at the spheres interstitial regions. The remaining metal showed a wellorganized pattern with regular distributed region withouth
metal. (the previous site of polystyrene spheres) with separation distance between then within the resolution of polystyrene spheres size as can be seen in Figure 5 (b).
Finally we can conclude that the regular and organized
structures of polystyrene spheres not only can be used to
photonic crystal structure fabrication, but also can be used
to fabricate metallic masks using self-organized structures
as template. Furthermore, the self-asembling technique is an
easy manner to reduce the resolution of the masks in silicon
technology, especially because of the easy implementation
and low cost. At present this gold metallic mask is being
used for photonic crystal fabrication on silicon substrate using porous silicon technology which results will be reported
in the next paper.
CONCLUSIONS
Fig. 5 SEM images of (a) a monolayer structure of polystyrene microspheres obtained with a 0.22wt% monodispersion at 50 oC, and (b) a metallic mask obtained by the structure (a) after the total removal of spheres.
(a) indicates a angular view (60 degrees) and (b) indicates a top view.
Images (a) and (b) were 40,000 times magnified and present 1µmdimension bar.
The transmittance spectra of samples with one and two
layer structure are showed in figure 3. It can be observed
that absorption band (band gap) shifts down as number of
layers decreases. The absorption spectra were obtained by
normal incidence of light beam relative to surface of structure. The monolayer absorption band structure is probably
due to a single layer (660 nm) diffraction phenomena, i.e,
the hexagonal array structure doesn’t contribute to diffraction phenomena. For the samples with two or more layers,
the polystyrene spheres array is responsible for diffraction
The self-assembly method used to the fabrication of
two-dimensional (2-D) and three-dimensional (3-D) regular
periodic structures showed a very good and cheap alternative to the formation of 2-D and 3-D photonic crystal structures. The temperature and polystyrene monodispersion
concentration were showed as sensible parameters for selforganized structure formation. The number of selfassembled layer is correlated with temperature used at experimental processes. Multilayer structure (>2 layers) was
formed at high temperature set-up (65 oC). At this temperature, the lowest dispersion concentration showed to be the
worst condition to the formation of arranged films, especially when they are associated with a slightly hydrophobic
condition of the SiO2 surface. The transmittance spectra at
near infrared region showed the existence of more defined
photonic band gap structure feature with the increasing of
the number of self- organized layers. Multilayer polystyrene
structures showed that can be applied to the fabrication of
JOURNAL INTEGRATED CIRCUITS AND SYSTEMS, VOL 1, NO. 3, JULY 2006.
photonic crystal structures and inverse opal structures.
Monolayers polystyrene structures showed that could be
applied as template in the fabrication of photonic crystal
structures based on porous silicon technology. Furthermore,
the self-assembling technique is an easy manner to reduce
the resolution of the masks in silicon technology, specially
because of the easy implementation and low cost. Then, it is
possible to reduce the masks resolution to values by 660
nm.
ACKNOWLEDGEMENTS
The authors thank CNPq and FAPESP for financial support, and Laboratório de Microeletrônica and Laboratório
de Sistemas Integráveis at Escola Politécnica, Laboratório
de Cristais Iônicos, Filmes Finos e Datação at Instituto de
Física and Laboratório de Espectroscopia Molecular at Instituto de Química, from Universidade de São Paulo, for technical support.
43
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