Fabrication of cone-shaped subwavelength structures by utilizing a confined convective self-assembly technique and inductively coupled-plasma reactive-ion etching
ABSTRACT Cone-shaped subwavelength structures (SWSs) were fabricated on a GaAs substrate by utilizing a confined convective self-assembly process followed by inductively coupled-plasma reactive-ion etching. A self-assembled polystyrene monolayer was used as an etch mask for pattern transfer onto the GaAs substrate. The fabricated SWS, having a cone profile with an aspect ratio of 1.5 and a 300 nm pitch, exhibited very low reflectance throughout the solar spectrum range and exhibited wide tolerance to different optical incidence angles. Reflectance of the cone-shaped SWS on the GaAs surface was less than 4% in a spectral range of 300–1000 nm under a normal incidence condition.
- Journal of Vacuum Science & Technology B Microelectronics and Nanometer Structures 01/2013; 31(3):1202-. · 1.36 Impact Factor
Fabrication of cone-shaped subwavelength structures by utilizing
a confined convective self-assembly technique and inductively
coupled-plasma reactive-ion etching
Dae-Seon Kim and Min-Su Park
School of Information and Communications, WCU Department of Nanobio Materials and Electronics,
Gwangju Institute of Science and Technology, 1 Oryongdong, Buk-gu, Gwangju 500-712, South Korea
School of Information and Communications, WCU Department of Nanobio Materials and Electronics, and
Research Institute of Solar and Sustainable Energies, Gwangju Institute of Science and Technology,
1 Oryongdong, Buk-gu, Gwangju 500-712, South Korea
?Received 9 September 2010; accepted 31 January 2011; published 7 March 2011?
Cone-shaped subwavelength structures ?SWSs? were fabricated on a GaAs substrate by utilizing a
confined convective self-assembly process followed by inductively coupled-plasma reactive-ion
etching. A self-assembled polystyrene monolayer was used as an etch mask for pattern transfer onto
the GaAs substrate. The fabricated SWS, having a cone profile with an aspect ratio of 1.5 and a 300
nm pitch, exhibited very low reflectance throughout the solar spectrum range and exhibited wide
tolerance to different optical incidence angles. Reflectance of the cone-shaped SWS on the GaAs
surface was less than 4% in a spectral range of 300–1000 nm under a normal incidence condition.
© 2011 American Vacuum Society. ?DOI: 10.1116/1.3556962?
Cone-shaped subwavelength structures ?SWSs? can be
used as an antireflection coating ?ARC? on various optical
devices, such as solar cells, photodetectors, and optical am-
plifiers, which require low reflectance over a wide spectral
range and wide incidence angles. Single-layer or multilayer
thin film coatings have conventionally been used as ARC on
these devices to minimize optical reflection loss at the sur-
face. Although thin film coatings are ideal as ARCs in nar-
rowband applications, SWS-based ARCs are preferred for
both broadband1–3and wide-incident-angle4applications. For
solar cells, the broadband antireflection properties of SWS
are beneficial because solar power is distributed throughout
the UV-vis-near-infrared ?NIR? wavelengths.5Low reflec-
tance over wide-incident angles is also desirable for ARCs in
concentrated solar cells because sunlight passing through the
concentrator is focused onto the solar cell with a wide inci-
dence angle. When the area of the concentrator is increased
and the height of the receiver is maintained, the solar inci-
dent angle onto the solar cell tends to increase. Considering
this wide range of solar incident angle in concentrated pho-
tovoltaic systems, SWSs with wide angle tolerance are good
candidates for ARCs.
Previously reported studies of SWSs have utilized nano-
lithographic fabrication techniques such as electron beam
lithography,6nanoimprint lithography,7,8and holographic
patterns. Although SWSs fabricated via these methods
have demonstrated excellent optical performance, colloidal
lithography techniques have the advantage of simple and
low-cost manufacturing capability without requiring sophis-
ticated semiconductor fabrication facilities. Colloidal lithog-
tion,14and evaporation-driven self-assembly15methods, have
been utilized to form periodic nanostructures on semiconduc-
tor surfaces. The advantage of colloidal lithography in the
fabrication of SWSs is the large-area formation of a closely
packed monolayer via a simple method without using special
equipment.16,17In particular, among colloidal lithographic
approaches, the confined convective self-assembly ?CCSA?
method entails a simple process of controlling the lift-up rate
and substrate temperature.18A well-ordered monolayer can
be deposited by the CCSA method with very small quantities
of a polystyrene solution. Owing to the use of a small
amount of solution and the simple process, a low-cost pro-
cess can be realized using CCSA. Deposited polystyrene
monolayer has been employed as an etch mask to fabricate
antireflective surface of solar cells.19–21Polystyrene mono-
layer allowed transferring the pattern having a pillar or cone-
shaped SWSs onto the Si substrate and SiNxlayer. However,
precise pillar or cone-shaped SWSs cannot be achieved be-
cause of sphere-shaped etch mask.
In this study, the shape of polystyrene spheres deposited
by CCSA methods was transformed to cone shape by O2and
Ar plasma treatment before inductively coupled-plasma
reactive-ion etching ?ICP-RIE?. By using the cone-shaped
polystyrene monolayer as an etch mask, cone-shaped SWS
could be successfully fabricated on a GaAs substrate. The
surface reflectance of the fabricated SWS under various in-
cident angles was compared with those of a conventional
ARC fabricated with a single layer of SiNxthin film. The
reflection spectrum of the cone-shaped SWS and SiNx-based
ARCs at irradiation of the solar spectrum of AM1.5 and
AM0 were also compared.
a?Electronic mail: email@example.com
020602-1 020602-1J. Vac. Sci. Technol. B 29„2…, Mar/Apr 20111071-1023/2011/29„2…/020602/5/$30.00©2011 American Vacuum Society
Cone-shaped SWSs were fabricated according to the pro-
cess flow presented in Fig. 1. A self-assembled monolayer of
300 nm diameter polystyrene spheres was prepared on a
GaAs substrate using the CCSA method. The CCSA tech-
nique utilized in this study consists of the following steps.
First, a GaAs substrate backed with a glass carrier was
loaded onto a dipping machine in order to control the lift-up
rate. Another glass substrate was placed in front of the GaAs
substrate with a gap of about 100 ?m between them. The
gap was then filled with a solution containing polystyrene
spheres. Finally, the GaAs substrate backed with a glass car-
rier was lifted up at a rate of 43 ?m/s. To maintain a con-
stant temperature of 35 °C, the dipping apparatus was placed
in an oven. The resulting closely packed monolayer was sub-
sequently treated by O2RIE ?30 mTorr chamber pressure and
30 SCCM ?SCCM denotes cubic centimeter per minute at
STP? O2? to reduce the size of the polystyrene spheres while
maintaining the distance between the spheres. After exposure
to O2plasma for 60 s, the diameter of the polystyrene
spheres decreased from 300 to 250 nm. Subsequent Ar ion
bombardment in an ICP-RIE chamber ?at a chamber pressure
of 10 mTorr with 50 SCCM of Ar? for 4 min was utilized to
change the shape of the spheres from spherical to conical.
The resulting cone-shaped polystyrene, with a diameter of
250 nm and a pitch of 300 nm, was used as an etch mask for
the pattern transferring process. The use of the cone-shaped
etch mask ensured an accurate pattern transfer by preventing
the additional etching of the empty space between the poly-
styrene sphere and the GaAs substrate. The final cone-shaped
SWS was realized by etching the GaAs substrate with ICP-
RIE ?7.5 SCCM SiCl4and 30 SCCM Ar?.22The etching time
of the ICP-RIE process was determined from the etch selec-
tivity between the polystyrene and the GaAs material. The
ICP-RIE etching process was carried out until the polysty-
rene was completely removed. For a comparative study, a
single-layer SiNxARC was deposited on a GaAs substrate
using a plasma-enhanced chemical vapor deposition tech-
nique. The reflectance values of the GaAs substrate covered
with a cone-shaped SWS and a SiNxARC were measured at
wavelengths between 300 and 1000 nm and at various inci-
dence angles ?0°, 20°, 30°, 40°, 50°, 60°, and 70°? using a
UV-vis-NIR spectrophotometer ?Cary 5000, Varian?. Dif-
fused component was used to measure the reflectance at nor-
mal incidence condition, and specular component was used
to study angle-dependent reflection characteristics.
III. RESULTS AND DISCUSSION
Figure 2 shows a scanning electron microscopy ?SEM?
image of the deposited monolayer with closely packed poly-
styrene spheres. At a temperature of 35 °C, the lift-up rate of
43 ?m/s was appropriate to achieve a polystyrene mono-
layer. Either monolayer or multilayer polystyrene could be
assembled by controlling the lifting rate and the substrate
temperature. Figure 3?a? shows a SEM image of a monolayer
of polystyrene spheres with reduced size. A subsequent O2
plasma treatment was used to reduce the diameter of poly-
styrene and to control the duty ratio of the fabricated SWS.
As shown in Fig. 3?a?, the O2plasma treatment allowed con-
trol over the sphere size without altering the pitch. After
exposure to the O2plasma for 60 s, the diameter of the
polystyrene spheres decreased from 300 to 250 nm. The de-
pendence of the polystyrene diameter on the O2plasma treat-
ment time is shown in Fig. 3?b?. As the O2plasma treatment
time is increased, the diameter of the polystyrene tends to
decrease. As-deposited sphere-shaped polystyrene monolayer
is not suitable for etch mask because unwanted undercut
etching can take place under the spheres as shown in Fig.
4?a?. Cone-shaped polystyrene monolayer is preferred to
avoid the excessive undercut etching. Subsequent Ar plasma
treatment was carried out in ICP-RIE chamber to transform
the shape of polystyrene from spherical to conical as shown
in Fig. 4?b?. Figure 5 shows a top and cross-sectional view of
the fabricated cone-shaped SWS. Transformation of the size
FIG. 1. ?Color online? Fabrication process sequence of the cone-shaped
SWS. A monolayer of polystyrene was deposited on a GaAs substrate using
the CCSA method. After depositing the monolayer, the profile of the etch
mask was controlled by an O2and Ar plasma treatment. Finally, the cone-
shaped SWS was realized on a GaAs substrate using an ICP-RIE etching
FIG. 2. SEM top view image of the closely packed monolayer formed by the
Kim, Park, and Jang: Fabrication of cone-shaped subwavelength structures020602-2020602-2
J. Vac. Sci. Technol. B, Vol. 29, No. 2, Mar/Apr 2011
and profile of the etch mask by the O2and Ar plasma treat-
ment allowed precise pattern transfer onto the GaAs using
the ICP-RIE etching method. The resulting cone-shaped
SWS exhibited an aspect ratio of 1.5 and a pitch of 300 nm.
The aspect ratio and profile of the SWS were controlled by
the ICP-RIE etching conditions. On the other hand, the pitch
and duty ratio were independently controlled by the size of
the polystyrene in the solution and by the O2plasma treat-
Figure 6?a? shows the reflectance as a function of the
incident wavelength ranging from 300 to 1000 nm which
was by using diffused component only. The reflectance of the
bare GaAs substrate is higher than 30% throughout the
wavelength range investigated in this study. The reflectance
of the GaAs substrate coated with the single-layer SiNxARC
is nearly 0% at 550 nm but is very high at a short wavelength
and a long wavelength range, as shown in Fig. 6?a?. The
GaAs substrate covered with a cone-shaped SWS exhibits
reflectance lower than 4% throughout the wavelength range
from 300 to 1000 nm. Low reflectance of the SWS is
achieved because the cone-shaped SWS continuously trans-
forms the effective refractive index from the top to the bot-
tom of the structure. The continuously transformed refractive
index from the top to the bottom of the structure is free from
any abrupt refractive index mismatch at the interface be-
tween the air and semiconductor surface, which can give rise
to optical reflection.23–26The GaAs substrate coated with
SiNxARC exhibited almost perfect transmission characteris-
tics at a wavelength of 550 nm but suffered from narrowband
characteristics. However, the cone-shaped SWS demon-
strated low reflectance over the entire solar spectrum range
compared with the SiNxARC. The cone-shaped SWS main-
tains lower reflection compared to the SiNxARC throughout
the entire wavelength range of 300–1000 nm. Figure 6?b?
shows the angular dependent mean transmittance, T???,
which is defined to be
?1 − R??,???d?,
where R??,?? is the angular dependent reflectance measured
by using specular component. The mean transmittance of the
FIG. 3. ?a? SEM top view image of polystyrene arrays after O2RIE treat-
ment. ?b? Dependence of the sphere diameter on the O2RIE treatment time.
FIG. 4. ?Color online? ?a? Scheme of etching for cone-shaped SWS using
etch masks consisting of spherical and conical profiles. ?b? SEM top view
image of polystyrene arrays after an Ar plasma treatment. The Ar ion bom-
bardment changed the profile of the etch mask from spherical to conical.
FIG. 5. SEM images of the fabricated SWS: ?a? top view and ?b? cross-
Kim, Park, and Jang: Fabrication of cone-shaped subwavelength structures020602-3020602-3
JVST B - Microelectronics and Nanometer Structures
cone-shaped SWS was nearly 11% higher than that of SiNx
at all incident angles investigated in this study.
Figure 7 shows the calculated reflectance spectrum of a
GaAs substrate coated with cone-shaped SWS and SiNx
ARCs under solar power irradiation. The reflection spectrum
?R??? and reflected solar irradiance R??? throughout the en-
tire wavelength can be calculated from
?R??? = ?sun???R???,
where ?sun??? is the standard solar spectrum at AM1.5 and
AM0 and R??? is the reflectance under normal incidence
measured by using diffused component. The total solar irra-
diances integrated from 200 to 860 nm were 563.3 and
826.9 W m−2for solar spectrum at AM1.5 and AM0, respec-
tively. The reflected solar irradiances from the SWS and SiNx
ARCs were 19.0 and 46.4 W m−2at AM1.5 and those from
the SWS and SiNx samples at AM0 were 27.6 and
90.9 W m−2. Low reflection of the cone-shaped SWS over a
wide wavelength range contributed to a low reflected solar
irradiance. Additionally, the reflected solar irradiance under
various incident angles was presented in Fig. 8. The reflec-
tance used in this calculation was measured by using specu-
lar component. The values of the cone-shaped SWS were far
lower than those of the SiNxARC at AM1.5 and AM0 for all
incident angles investigated in this study. The reflected solar
irradiance values of the cone-shaped SWS were 30 and
64 W m−2lower than those of the SiNxARC for all incident
angles at AM1.5 and AM0, respectively.
The CCSA method was utilized to fabricate cone-shaped
SWSs. This method involves a much simpler and less expen-
sive process compared to other approaches and opens the
possibility of large-area SWS fabrication at a low cost.
Transformation of the polystyrene shape from spherical to
conical by utilizing a plasma treatment was exploited to form
an ideal cone-shaped SWS. The fabricated SWS exhibited
optical properties that are comparable to those of previously
reported SWSs fabricated via conventional nanolithography.
The cone-shaped SWS exhibited lower reflectance under a
broadband wavelength and a wider incident angle than a con-
FIG. 6. ?a? Measured optical reflectance of the GaAs substrate without ARC,
with SiNxARC, and cone-shaped SWS. ?b? Mean transmittance of cone-
shaped SWS and SiNxARC under wide-incident angles.
FIG. 7. ?Color online? Calculated reflection spectrum of cone-shaped SWS
and SiNxARC at irradiation of the solar spectrum of AM1.5 and AM0.
FIG. 8. Angular dependence of the reflected irradiance at the solar spectrum
of AM1.5 and AM0.
Kim, Park, and Jang: Fabrication of cone-shaped subwavelength structures020602-4 020602-4
J. Vac. Sci. Technol. B, Vol. 29, No. 2, Mar/Apr 2011
ventional SiNxARC. In particular, the cone-shaped SWSs
can find highly promising applications in concentrated pho-
tovoltaic devices and photovoltaic devices for terrestrial and
space applications due to their low reflection characteristics
over a wide wavelength range and wide tolerance to different
The work was supported by a NRF grant ?Grant No. R01-
2007-000-10843-0?, a grant ?Code No. 20090006? from the
Development of Marine Environmental Sensors Using Nano
and Photonics Technology Program funded by the Ministry
of Maritime Affairs and Fisheries of the Korean Govern-
ment, and the Center for Distributed Sensor Network at
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