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Chemical Alkaline Etching of Silicon Mie Particles
Julien Proust,* Frédéric Bedu, Stéphane Chenot, Ibrahima Soumahoro, Igor Ozerov,
Bruno Gallas, Redha Abdeddaim, and Nicolas Bonod*
magnetism or photodiodes.[21–23] Spontaneous or assisted dewetting of thin silicon
Chemical etching via alkaline solutions is associated with electronic litholayers under ultrahigh vacuum is an altergraphy to structure at a nanometer scale silicon Mie resonators. Two difnative and original technique that leads in
ferent alkaline solutions are employed and their influences on the shape of
a single annealing step to the formation of
the resonators are discussed. The method is applied on both amorphous
monocrystalline Si-based resonators over
and crystalline silicon coatings and silicon resonators are characterized by
the whole sample.[24] The nanoimprint is
electron microscopy and optical spectroscopy, and then the different modes
also a very interesting method for large
scale nanostructures and it was applied
are identified thanks to numerical simulations. This method avoids the use
as mask for reactive ion etching (RIE) to
of reactive ion etching and appears to be very well adapted to design silicon
design antireflective coatings on silicon
particles at a nanometer scale for applications in nanophotonics.
wafer.[25] The electronic lithography with
RIE is well adapted to fabricate monomers or oligomers of particles.[10,11,16]
These
methods
benefi
ted from recent progress in plasmonics
1. Introduction
to design at a nanometer scale metallic nanostructures. However, all these techniques include expensive machines as rapid
Silicon has attracted intense interest and development over
thermal annealing ovens or RIE systems.
the last 60 years in due to its semiconducting properties and
Here we develop an alternative and cost effective method
this material plays a key role in a wide variety of optoelecdedicated to semiconductor materials exclusively, and in partronic components. Recently, silicon particles triggered a keen
ticular to silicon. This method is based on the possibility to etch
interest for their ability to feature electric and magnetic Mie
silicon with an alkaline solution and it was first introduced in
resonances in the visible and near infrared spectral regions.[1–5]
microelectronics for metal-oxide-semiconductor transistor.[26]
These particles are very promising to design subwavelength
sized photonic resonators because they exhibit weak losses and
The etching rate of silicon by the alkaline solution depending
can be used to enhance either the electric or magnetic field
on the atomic density, i.e., to the crystalline facets, it leads to
intensities.[6–9] The interplay between the electric and magnetic
silicon structures with very sharp edges. In this paper, we show
that this method can be extended to the fabrication of submiresonances provides to Mie resonators unique light scattering
crometric sized silicon particles of high quality. The design of
properties.[10–19] Different techniques have been developed to
the mask is carried out by electron beam lithography and the
fabricate single or coupled Si-based Mie resonators. The laser
wet etching is performed with an alkaline solution. The method
lithography technique proved its ability to fabricate high-quality
can be used on both amorphous and crystalline silicon and
amorphous or crystalline silicon particles on transparent suboffers a valuable alternative to the classical RIE that can raise
strate.[4,5,17,20] Chemical vapor deposition leads to high-quality
locally the temperature of the sample. We present in the next
crystalline silicon particles with a very low size dispersion that
sections the principle of this soft etching technique and show
have been used to fabricate metamaterials with strong optical
how the shape of the resonators can be controlled by the shape
of the mask and the choice of the alkaline solution. The samDr. J. Proust, R. Abdeddaim, Dr. N. Bonod
ples are characterized by electron microscopy and confocal scatAix-Marseille Université, CNRS
tering spectroscopy.
Centrale Marseille
Institut Fresnel
UMR 7249, 13013 Marseille, France
E-mail: [email protected]; [email protected]
F. Bedu, I. Ozerov
Aix-Marseille Université
CNRS, CINAM
UMR 7325, 13288 Marseille, France
S. Chenot, I. Soumahoro, B. Gallas
Sorbonne Universités
UPMC Univ Paris 06, CNRS UMR 7588
Institut des Nanosciences de Paris
75005 Paris, France
DOI: 10.1002/adom.201500146
Adv. Optical Mater. 2015,
DOI: 10.1002/adom.201500146
2. Chemical Etching of Silicon Mie Resonators
2.1. Principle
The principle of this method is to combine the versatility of the
electron beam lithography in term of design with a chemical
etching that offers an additional degree for the control of the
nanostructure, particularly with crystalline silicon. It can be
applied via minor adjustments to crystalline, polycrystalline or
amorphous silicon substrates.
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Si + 2OH− + 2H2 O− > Si(OH)4 + H2 (g)
Figure 1. Schematic of the fabrication method of Mie resonators by alkaline chemical etching. a) A crystalline silicon wafer or an amorphous silicon layer on a quartz substrate is cleaned. b) Spin coating of the e-beam
resist (PMMA). c) E-beam lithography. d) Development of the resist.
e) Evaporation of a gold layer. f) Lift-off of the resist. g) Alkaline etching.
h) Etching is stopped in water. i) The gold mask is removed revealing the
silicon Mie resonators.
The main steps of the fabrication method are summarized
and displayed in Figure 1 (see the Experimental Section for more
details). An e-beam resist layer of poly(methyl methacrylate)
(PMMA) is first used as positive resist. After the development
of the resist, the mask is obtained by evaporation of chromium
and gold. After the lift-off of the resist layer, an alkaline solution is used to etch the unprotected zones of silicon. The influence of the Si crystallinity and alkaline solution on the shape
of the Si particles will be discussed later. The alkaline etching
process is stopped in water. An acid solution is used to remove
the metallic mask that reveals the silicon Mie resonators.
2.2. Influence of the Alkaline Solution on the Shape of the
Crystalline Si Resonators
The rate of the chemical etching with respect to the atomic density, i.e., on the crystallographic planes, has been thoroughly
studied in microelectronics and applied to etch deep and sharp
grooves in crystalline silicon.[27] However, it remains to know
whether this property can be used in nanooptics (i) to design
silicon particles with sharp edges and (ii) to offer an additional
degree of control of the shape of the resonator.
In order to address the second point, we define a mask with a
square pattern oriented following the 〈110〉 crystallographic directions on silicon-on-insulator (SOI) substrate (see Figure 2a,d).
Two alkaline solutions are investigated to etch silicon: an inorganic solution of potassium hydroxide (KOH) and an organic
solution of Tetramethylammonium hydroxide (TMAH), with
different etching rates of the different crystallographic planes.
The alkaline solutions being not very efficient to etch native
silicon oxides, the substrate is immerged in a Fluorhydric acid
(HF) aqueous solution at 25% during 30 s in order to remove
the native silicon oxide, and then rinsed in water. Immerging
the substrate in a hot alkaline solution performs the alkaline
etching. The etching rate depends on the temperature, on the
concentration of the solution (usually 40% for KOH and 25%
for TMAH used between 50 and 70 °C) and on the crystallinity
of the silicon. The chemical reaction between hydroxide ions
and silicon is given in first approximation by[28]
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(1)
The dihydrogen (H2) emanation is a good indicator of the
reaction kinetics that also depends on the orientation of the
substrate. When orienting the silicon substrate towards a vertical position, it can be observed that the reaction rate increases
and that the surface roughness decreases.
The etching of crystalline silicon is first simulated thanks to
the Anisotropic Crystalline Etch Simulation software (ACES).[29]
The etching rates along the different directions 〈100〉:〈110〉:〈111〉
used for the simulations have the ratio 3:5:1 for KOH and 10:2:1
for TMAH according to Sato et al. works.[30,31] The etching rate
along the 〈111〉 direction is the lowest in all cases.[32] Different
facts can explain this property: (i) the {111} plane is extremely
reactive to the oxidation and a thin layer of SiO2 protects this
surface[33] and (ii) the Si atoms have two dangling bonds in the
{100} and {110} planes against only one for the atoms from
{111} plane.[34–36]
The numerical simulations displayed in Figure 2 with the
two alkaline solutions on the same square patterned mask
highlight the strong sensibility of the final pattern to the alkaline solution. In the case of KOH etching, the final structure is
an octagonal cylinder resulting from the ranking of the etching
rates: {110} > {100} > {111}. In the case of TMAH, the etching
leads to the creation of an orthogonal parallelepiped resulting
from the ranking of etching rates: {100} > {110} > {111}. A
nonionic surfactant has been added to the TMAH solution (2%
of polyoxyethylene-alkyl-phenyl-ether) in order to reverse the
etching rate rank. Indeed, the surfactant sleeks the surfaces and
sharpens the corners by protecting the {110} facets and privileges the etching of the {100} plane.[37]
Consequently, the final shape of the silicon resonators
depends on both the shape of the mask and on the alkaline
solution. The structure obtained with TMAH exhibits a shape
similar to that of the original mask while KOH leads to a
very different particle shape with an octagonal pattern. Very
importantly, it can be observed that in both cases, the alkaline
etching leads to very sharp grooves and edges. The TMAH
solution has shown its ability to perform trimers with very thin
gaps (Figure 2g,h) that are estimated to be ranged between 7
and 32 nm. Importantly, the gap size can be well controlled
by the mask and the high aspect ratio of the silicon particles
provided by the alkaline etching is well adapted to fabricate
nanogap silicon antennas (Figure 2g,h). This technique could
be applied to design complex oligomers such as heptamers
that were recently proved to exhibit well pronounced Fano
anomalies.[38]
2.3. Influence of the Concentration and Temperature
An increase in hydroxide ions concentration accelerates the
etching of silicon without modifying the ratios of etching rates
between the different planes. It has been demonstrated that an
aqueous solution of KOH at 20% offers the fastest etching but
also that a concentration lower than 30% allows for the creation
of pyramidal shape holes in {100} surface.[39] Consequently, a
concentration of 40% was chosen in accordance to these two
assessments.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Optical Mater. 2015,
DOI: 10.1002/adom.201500146
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Figure 2. Influence of the alkaline solution composition on the shape of the crystalline silicon Mie resonators. Both samples (a) and (d) are obtained
with the same mask featuring a square pattern oriented following the 〈110〉 direction. First line for KOH etching a,b,c) and second line for TMAH
etching d,e,f). b,e) 3D numerical simulations of wet chemical etching of crystalline silicon performed with ACES software (Anisotropic Crystalline
Etching Simulation). c,f) SEM images of the crystalline Si resonators. g) Thin gap trimers are obtained by TMAH etching. h) Dark field scattering
images of the trimers for different gap values.
The dependence of the etching rate on the temperature is
governed by the Arrhenius law that predicts an increase of the
kinetics with the temperature. The diffusion being increased
with the temperature, the replenishment of the hydroxide
groups near the surface will be facilitated with the temperature:
a stronger diffusion does not affect the etching of large and
opened areas but it will strongly increase the reaction kinetics
in deep grooves and nanometric areas.[33] Consequently, the
temperature has to be high enough to ensure a good diffusion in the grooves but not too high to avoid the creation of gas
bubbles in the solution. The temperature range is then chosen
between 50 and 70 °C.
2.4. Alkaline Etching of Amorphous and Polycrystalline Silicon
Contrarily to the case of crystalline silicon, the etching rate of
amorphous silicon is isotropic since the atomic density is isotropic. In that case, the final structure shape depends only on
the shape of the mask (see Figure 3a).
Adv. Optical Mater. 2015,
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We studied how to include anisotropy during the etching in
this configuration. For that purpose, the Si amorphous layer
coated on a transparent substrate was annealed to create polycrystalline films (see the Experimental Section).[40] It has been
demonstrated that the annealing creates crystals of a few tens
of nanometers in the original amorphous layer that form polycrystalline films (p-Si).[41] But the alkaline etching of p-Si films
remains isotropic in a standard alkaline solution. However,
by adding a surfactant to the TMAH solution, we were able
to protect some crystalline facets and to improve the sharpness of the resonators by forcing an etching anisotropy (see
Figure 3b). We experimentally evidenced that the etching rate
in the deep direction can be four times higher than that in the
surface direction. This property allows for the creation of well
controlled nanogap structures such as dimers or trimers on
transparent substrates as showed in Figure 3d,e, respectively.
The highest aspect ratio (thickness over diameter) of a structure that can be achieved by this method is defined by the clearance angle of 8.5° of silicon which leads to an aspect ratio limit
of 3.4. Experimentally, we estimate the aspect ratio of silicon
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Figure 3. a,b) The influence of the surfactant added to TMAH solution on the etching anisotropy. The isotropic ratio is 1:1 in (a) and 1:4 in (b). c–g) SEM
images of monomers 200 nm c), 700 nm f), and 500 nm tilted g) in diameter, dimers d) and trimers e) made with TMAH/surfactant etching. The
crackles visible on (a–c) come from the metallic percolated film used as conductive layer for the observation and not from the structures. The clearance
angles of the particles are measured and reported on the histogram (h) and give a clearance angle of 8.5° ± 3.6°.
cylinders around 2.5. Figure 3g shows a tilted image of silicon
cylinders that allows us to estimate the clearance angle of the
structures. The angle was measured on 200 structures and the
statistics on the clearance angle reported in Figure 3h shows
a clearance angle of 8.5° ± 3.6°. The observation of an average
clearance angle lower than 10° is a very promising result for
wet alkaline etching. Let us emphasize that this result is mainly
due to the use of surfactants that provide additional anisotropy
in the etching direction.
3. Optical Characterization and Oxidation
Thickness Estimation
In this part, we present the experimental and numerical optical
characterization of monomers on transparent substrates created by the etching of 300 nm thick p-Si obtained by the protocol detailed in the previous section. Optical measurements
are made by a home-made confocal microscope. The excitation
is made in 60° normal-tilted transmission by a quartz–tungsten–halogen white lamp, injected in a 400 µm core fiber and
collimated by a parabolic metallic mirror. The light beam is
gently focalized on the substrate by a Cassegrain objective (×15,
NA = 0.3) in order to average the scattering which is dependent
on the excitation orientation.[42] The forward scattering signal is
collected by a Mitutoyo objective (×100, NA = 0.7) working with
a 200 mm focal lens tube and the spatial filtering is obtained
thanks to a 50 µm core fiber located at a confocal position to the
sample in order to collect a theoretical circular area of 500 nm
diameter.
Figure 4a presents the forward scattering spectra of six silicon cylinders with diameters increasing from 160 to 195 nm
(with 300 nm of thickness). We can observe the formation of
two well-defined modes for the diameters around 175 nm. Two
other resonances at shorter wavelengths appear for diameters
around 185 nm. The dark field images of the particles are displayed in Figure 4b. The scattering spectrum can be tuned by
modifying the diameter of the particles and the distance of the
gap in oligomers as shown in Figure 2h. This ability to control
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the structural colors of silicon particles offers a great opportunity to design spectral filters coupled with image sensors such
as CCD or CMOS sensors.[43] The samples being immerged
in hot aqua Regia solution to remove the gold mask, it can be
expected that the surface of silicon nanoparticles is oxidized
during the process.[44,45] Numerical simulations are performed
with CST Microwave software to estimate the thickness of the
silicon dioxide and to identify the different modes (Figure 4c).
An oxide layer of thickness 5 nm leads to the best fit between
the experimental and numerical spectra. This silica layer could
be a valuable platform for functionalizing the surface of silicon
resonators by the silane chemistry. This method can be easily
transferred to larger particles that will exhibit resonances in
the near infrared spectrum in which the absorption of silicon
becomes negligible.
Numerical spectra feature two well defined modes that can
be associated to electric and magnetic dipoles, with apparent
better quality factor than the experiment. The weaker quality
factor observed experimentally is assumed to be due to the
from the grain boundaries of the polycrystalline silicon that
provide additional scattering and absorption losses.
This last section is aimed at evidencing that a wet alkaline
etching is an efficient and original method to etch silicon
Mie resonators featuring electric and magnetic resonances.
For that purpose, the distributions of the electric and magnetic fields are reconstructed in Figure 5c–f at the four peaks
observed in Figure 5a when calculating the scattering spectrum of a cylinder with a diameter equal to 220 nm including
an oxide layer of 5 nm. The excitation conditions of this single
resonator are detailed in Figure 5b. The observation plane is
in the plane parallel to the two faces of the cylinder and positioned in the center of the height. The different field distributions are characteristic of the excitation. Figure 5c–f presents
two modes which can be attributed to magnetic and electric
modes, of dipolar (Figure 5c,e) and quadrupolar (Figure 5d,f)
orders.[2–5,20–24] These different modes were observed experimentally by near-field optical measurements.[46,47] This result
highlights the reliability of the alkaline etching for the fabrication of silicon Mie resonators.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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DOI: 10.1002/adom.201500146
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Figure 4. a) Normalized experimental forward scattering spectra of six nanoparticles with diameters ranging between 160 and 195 nm (thickness of
300 nm). The spectra are stacked one above the other. The diameter includes the oxide layer of thickness 5 nm. b) Dark field images in transmission
of each resonator. c) Numerical simulations of cylinders with the oxide layer. Each resonance is identified by MD for magnetic dipolar resonance, ED
for electric dipolar resonance, MQ for magnetic quadrupolar resonance, and EQ for electric quadrupolar resonance.
4. Conclusion
In conclusion, alkaline etching is proved to lead to the formation of high-quality silicon Mie resonators. Their optical properties are characterized in the optical domain and the formation of
dipolar electric and magnetic dipolar modes is observed when
the aspect ratio tends toward the unity. The etching of crystalline silicon being dependent on the crystallographic planes, this
method can lead to the formation of particles with very sharp
edges. Two alkaline solutions of KOH and TMAH are studied.
We proved that on crystalline silicon, they lead to the formation
of particles with very different shapes as confirmed by numerical simulations. The alkaline etching is also employed on a
silicon film evaporated on a transparent substrate. In this case,
it is showed that the annealing of the silicon film together with
the use of a surfactant accelerates the etching in the deep direction leading to the formation of sharp grooves. The influence
of the concentration of the alkaline solution is also discussed,
together with the influence of the temperature that must range
between 50 and 70 °C, making this approach particularly advantageous when low temperatures are required.
5. Experimental Section
Silicon Coating: The crystalline substrates feature either a (100) or a
(110) crystalline plane at the surface. We worked on silicon wafers and
SOI substrates from NEYCO. Polycrystalline or amorphous silicon is
composed by silicon layers on SiO2 substrates. Thin films of amorphous
silicon were obtained by e-beam evaporation of solid source in high
Adv. Optical Mater. 2015,
DOI: 10.1002/adom.201500146
vacuum. We used a 1 mm thick UV-grade fused silica substrates with
two polished sides. Prior to introduction in the chamber, the substrates
were degreased in acetone in an ultrasonic bath, rinsed using distilled
and deionized water, and dried using dust free nitrogen in a clean
room environment. The substrates were then introduced in the growth
chamber where they underwent a smooth plasma cleaning to remove
any surface contamination. The substrates were maintained at room
temperature during growth. The growth rate was monitored during
evaporation using a crystal quartz microbalance which, after a calibration
procedure, provided the amount of silicon deposited onto the substrate
as a function of time.
After the growth process, the substrates were transferred in a tubular
furnace where a thermal annealing at 600 °C for one hour in vacuum
was performed in order to increase the crystalline part of silicon. The
thickness and optical constants of the annealed films were determined
ex situ from analysis of spectroscopic ellipsometry measurements.
Electron Beam Lithography and Chemical Etching:
(a) The silicon surface of the different samples (Si wafer, SOI, or
Si-based coatings on transparent substrates) is first cleaned under
ultrasonic waves in a solution composed by acetone:propan-2-ol
(1:1) during 10 min to remove dusts. In a second step, the substrate
is cleaned under Ar:O2 plasma at 300 W of RF power (DSB-3 tool
from Nanoplas, France) during 10 min to remove residual organic
materials, to oxide and to activate the surface in order to improve its
wettability.
(b) A 200 nm thickness e-beam resist (AR-P 679.04 from ALL-RESIST,
Germany) is spin-coated at 6000 rpm. A soft backing is performed
on the sample, 10 min at 170 °C on a hot plate, following the
conventional e-beam lithography protocol. Then, in case of the use
of a dielectric substrate allowing an electric charge effect during
e-beam exposure, a 30 nm thick conductive resist is spin-coated
on the e-beam resist (SX AR-PC 5000/90.1 from ALL-RESIST) and
backed 2 min at 85 °C.
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Figure 5. a) Simulated scattering spectrum of a single silicon cylinder, 230 nm in diameter, surrounded by a 5 nm thick oxide layer. b) Schematic of
the cylinder and the orientation of the incident plane waves. We show in dotted line the observation plane used in (c)–(f). c,d) Distributions of the
magnetic field intensity at the respective wavelengths associated with the magnetic dipolar and quadrupolar resonances in (a). e,f) Same as (c) and
(d) but with the electric field intensity.
(c) The e-beam lithography is performed by a SEM-FEG system (PIONEER
from RAITH). The main advantage of the e-beam lithography is the
high versatility of the potential shapes such as oligomers or arrays of
particles, isolated particles, split-ring resonators, etc.
(d) The conductive resist is removed by water if necessary. Then the
substrate is dried under nitrogen. The development is performed
during 60 s in a commercial solution (AR 600-55 from ALL-RESIST).
The development is stopped in propan-2-ol (IPA) and dried under
nitrogen.
(e) A metallic mask is evaporated under vacuum (Auto 306 tool from
Edwards). Gold is chosen for its resistance to the chemical attacks.
A thin layer of 3 nm of chromium is first evaporated as adhesive
layer between silicon and gold. Then a 30 nm thick gold layer is
evaporated as mask.
(f) A lift-off process removes the e-beam resist and the excess of gold
during some hours in acetone.
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(g) The chemical etching is carried out with two alkaline solutions:
KOH or TMAH. The alkaline solutions are not very efficient to etch
native silicon oxides. The substrate is in that case immerged in a HF
aqueous solution at 25% during 30 s in order to remove the native
silicone oxide and then rinsed in water. Immerging the substrate in a
hot alkaline solution performs the alkaline etching. The etching rate
depends on the temperature of the solution (usually between 40 and
80 °C) and on the crystallinity of the silicon.
(h) A cold water bath is used to stop the etching when the desired
thickness or the SiO2 surface is achieved. The substrate is then
cleaned by water and dried under nitrogen.
(i) The mask is finally removed. The gold part is removed by immersion
in a fresh aqua regia solution (1 min). The chromium part is
removed by a commercial etchant (Chrome Etch18 from Micro
Resist technology) (1 min). The substrate is finally rinsed by water
and dried under nitrogen.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Optical Mater. 2015,
DOI: 10.1002/adom.201500146
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This work was carried out with the support of the A*MIDEX project (No.
ANR-11-IDEX-0001-02) funded by the Investissements d’Avenir French
Government program and managed by the French National Research
Agency (ANR). Nanofabrication processes were performed in PLANETE
cleanroom facility.
Received: March 12, 2015
Revised: April 3, 2015
Published online:
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