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Soft nanoimprint lithography on SiO 2 sol-gel to elaborate sensitive substrates for SERS detection

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This paper presents a new alternative fabrication of biochemical sensor based on surface enhanced Raman scattering (SERS) by soft nanoimprint lithography (S-NIL) on SiO2 sol-gel. Stabilization of the sol-gel film is obtained by annealing which simplifies the manufacturing of these biosensors and is compatible with mass production at low cost. This detector relies on a specific pattern of gold nanodisks on a thin gold film to obtain a better sensitivity of molecules’ detection. Characterizations of SERS devices were performed on a confocal Raman microspectrophotometer after a chemical functionalization. We report a lateral collapse effect on poly(diméthylsiloxane) (PDMS) stamp for specific nanostructure dimensions. This unintentional effect is used to evaluate S-NIL resolution in SiO2 sol-gel.
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Soft nanoimprint lithography on SiO2 sol-gel to elaborate sensitive substrates for
SERS detection
Frédéric Hamouda, Jean-François Bryche, Abdelhanin Aassime, Emmanuel Maillart, Valentin Gâté, Silvia
Zanettini, Jérémy Ruscica, Daniel Turover, and Bernard Bartenlian
Citation: AIP Advances 7, 125125 (2017);
View online: https://doi.org/10.1063/1.5004122
View Table of Contents: http://aip.scitation.org/toc/adv/7/12
Published by the American Institute of Physics
AIP ADVANCES 7, 125125 (2017)
Soft nanoimprint lithography on SiO2sol-gel to elaborate
sensitive substrates for SERS detection
Fr´
ed´
eric Hamouda,1,aJean-Franc¸ois Bryche,1,3 Abdelhanin Aassime,1
Emmanuel Maillart,4Valentin Gˆ
at´
e,2Silvia Zanettini,2J´
er´
emy Ruscica,2
Daniel Turover,2and Bernard Bartenlian1
1Centre de Nanosciences et de Nanotechnologies, CNRS, Univ. Paris-Sud,
Universit´
e Paris-Saclay, C2N – Orsay, 91405 Orsay cedex, France
2SILSEF, 382 rue Louis Rustin-Archamps Technopole, 74160 Archamps, France
3Laboratoire Charles Fabry - Institut d’Optique Graduate School, CNRS,
Universit´
e Paris-Saclay, 91127 Palaiseau, France
4HORIBA Europe Research Center, Avenue de la Vauve, Passage Jobin Yvon, 91120 Palaiseau,
France
(Received 11 September 2017; accepted 18 December 2017;
published online 29 December 2017)
This paper presents a new alternative fabrication of biochemical sensor based on sur-
face enhanced Raman scattering (SERS) by soft nanoimprint lithography (S-NIL) on
SiO2sol-gel. Stabilization of the sol-gel film is obtained by annealing which simpli-
fies the manufacturing of these biosensors and is compatible with mass production at
low cost. This detector relies on a specific pattern of gold nanodisks on a thin gold
film to obtain a better sensitivity of molecules’ detection. Characterizations of SERS
devices were performed on a confocal Raman microspectrophotometer after a chem-
ical functionalization. We report a lateral collapse effect on poly(dim´
ethylsiloxane)
(PDMS) stamp for specific nanostructure dimensions. This unintentional effect is
used to evaluate S-NIL resolution in SiO2sol-gel. © 2017 Author(s). All arti-
cle content, except where otherwise noted, is licensed under a Creative Com-
mons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
https://doi.org/10.1063/1.5004122
I. INTRODUCTION
Raman Spectroscopy is an efficient analysis technique to characterize chemical composition
of molecules, this optical characterization is relied on inelastic light scattering1,2though for low
concentration it gives a weak signal. To overcome this limitation, rough metallic surfaces or metal
nanostructures or both can be used.3,4This method refers to surface enhanced Raman scattering
(SERS) and thus allows to identify precisely the molecules with their specific peaks and high enhance-
ment factors. The interest to develop new SERS-based spectroscopic sensors combined with other
spectroscopic detections for chemical or biological applications were increased during these past few
years.5,6Sample design used in this work with thin gold film under nanostructures has been already
reported for his interest in SERS and Surface Plasmon Resonance Imaging (SPRI).79
To pattern such device two kinds of lithographic processes can be used. The first one, refer-
enced as conventional like electron beam lithography (EBL) requires long writing times for large
areas and a significant cost for large sample productions. The second one, considered as a next
generation lithography is nanoimprint lithography (NIL) which is rapid, with low cost fabrication,
and allows large area structuring. The principle is to pattern a polymer with a mold which may be
rigid or flexible. Two main ways in NIL have been developed, thermal process (T-NIL), commonly
known as hot embossing lithography (HEL), demonstrated for the first time by S. Y. Chou in 199510
and UV-NIL developed by J. Haisma from Philips research laboratories in 1996.11 Thermal NIL
aElectronic mail: frederic.hamouda@c2n.upsaclay.fr
2158-3226/2017/7(12)/125125/6 7, 125125-1 ©Author(s) 2017
125125-2 Hamouda et al. AIP Advances 7, 125125 (2017)
allows high resolutions but needs high pressures (between 20 and 100 bar) and high temperatures
to reach glass transition temperature (Tg) of the imprinted polymer.12 While UV-NIL resists are
liquid and UV-curable, advantages are patterning at low pressures and carried out at ambient tem-
perature. It requires a transparent mold or substrate. With this method, mainly two techniques can
be distinguished: Hard UV-NIL and Soft UV-NIL. The first uses a rigid transparent stamp, generally
in quartz,13,14 which permits to achieve high resolutions close to those obtained by EBL. How-
ever, with a non-planar surfaces, critical resolutions cannot be reached. In this case, Soft UV-NIL
is useful thanks to the use of a flexible stamp which can be conforming to the shape of the sur-
face. A common polymer material used as flexible stamp is poly(dim´
ethylsiloxane) (PDMS) in early
works1519 for its interesting physical properties such as flexibility, elasticity, low surface energy, and
UV-transparency.
To improve performances of Soft UV-NIL, technological developments have mainly focused
on new processes for mold’s manufacture with new polymers,2024 new resists and anti-sticking
treatment.2527
Among recent developments for NIL resists, sol-gel resist appears as an alternative.2830 Sol-gel
is based on the polymerization of inorganic monomers to obtain partially or totally mineral films.
The polymerization uses a soft chemistry method which relies on controlled reaction mechanisms,
taking place in liquid solution and usually at ambient temperature. These mechanisms are based
on the transformation of a liquid solution (sol) into a solid material (a dry gel called xerogel) via
an inorganic polymerization process. Then sol-gel thin film can be stabilized and cured either by
annealing31,32 to permit the evaporation of residual solvent still present inside the xerogel thin film or
by photocuring.33 The interest to use such inorganic polymers as material for nanoimprint lithography
has been growing over the last decade.2830 In another way, studies have shown the benefit of using
sol-gel films directly to obtain rough surfaces which promote the production of SERS signals.34
In our study, we have developed a new method for the manufacturing of SERS sensors with
different sizes and pitches of gold nanodisks on gold film. This alternative technique is based on a
soft nanoImprint lithography (S-NIL) in SiO2thermo-curable sol-gel deposited on a sacrificial resist
layer and a lift-off process to obtain gold nanodisks. The added value of this fabrication method
with respect to classical UV-NIL process is the replacement of an UV-curable resist by a thermo-
curable one. This offers the possibility for other applications requiring a stamp and substrate together
opaque to UV light. Thereby this method makes it possible to produce easily sensor with large
quantity comparable to previous soft method.35 Hence, we have patterned various holes’ diameters
(150-400 nm) with a periodicity of 400 nm and 600 nm on the bilayer resists (SiO2sol-gel/PolyMethyl
Methacrylate A2) with a flexible stamp and without external pressure. PMMA resist is used for the
lift-off process after gold deposition. Then we report on optical characterizations of the device with
thiophenol molecules (C6H6S) by SERS measurements. For specific dimensions, we obtained a lateral
collapse of pillars on PDMS stamp. This unintentional effect has been used to evaluate print resolution
in SiO2sol-gel.
II. EXPERIMENTAL
A. PDMS stamp fabrication
First step to obtain soft NIL stamp is the silicon master mold fabrication using an electron beam
lithography system (NB4 from NanoBeam Limited, UK). Exposure was performed on PMMA A4
resist (200 nm) using an accelerating voltage of 80 kV and a current of 2 nA. After development
in methyl isobutyl ketone/ isopropanol (MIBK/IPA) (1/3, 1min 30sec) and a rinse in IPA (30 sec)
a reactive ion etching (RIE) process has been optimized to transfer nanoholes into a silicon sub-
strate. To compare SERS intensities versus structures’ density several holes’ patterns were designed
(300 µm300 µm of lateral dimensions) with diameters (D) ranging from 150 nm to 350 nm for two
periods (P) 400 nm and 600 nm on the same master mold. Before cast molding, an anti-adhesion layer
treatment based on trimethylchlorosilane (TMCS) was used to reduce surface energy of the silicon
and make easier the demolding step. Then to reduce PDMS’s viscosity and improve the penetration
of the polymer in nanoholes, we used 5% hexane diluted standard PDMS (RTV615). The mixing
125125-3 Hamouda et al. AIP Advances 7, 125125 (2017)
solution with his curing agent (1 curing agent : 5 PDMS) ration by weight, is deposited on Si master
mold and cured at 60C during 4 h.
B. Gold nanodisk fabrication and thiophenol molecules deposition
Gold nanodisks on continuous film were obtained by a nanoimprint lithography in SiO2sol-gel
and lift-off process with the bilayer resist method. As sol-gel is an inorganic thermo-curable polymer
we have used a sacrificial PMMA A2 layer spinned on a glass substrate previously covered with 2 nm
of Ti and 30 nm Au layers. These metallic layers were obtained using a Plassys evaporator equipment.
The sol-gel used in these researches provides from SILSEF Company. 90 nm thick layer was deposited
by spin coating on 120 nm PMMA layer before manual imprint with the flexible stamp. An annealing
at 110C during 10 sec was done before demolding. Etching process of the sol-gel and sacrificial layer
have been performed by a RIE equipment (STS). RIE process for sol-gel was based on 50 sccm CHF3,
at low pressure (15 mTorr) and 325 W power. For PMMA resist, etching was adjusted with 10 sccm O2,
a chamber pressure of 4.7 mtorr, and 10 W power and stopped when the level of gold film is achieved.
Then, an Au layer (30 nm) is evaporated and sol-gel was removed by a lift-off process in acetone
thanks to the PMMA underlayer. To test SERS sensitivity of this substrate, functionalization was
performed with thiophenol (C6H6S) molecules before characterizations. Samples were immersed in
0.1 mM solution of thiophenol for 2.5 h and rinsed in ethanol for 5 min before dried up with nitrogen.
III. RESULTS AND DISCUSSION
A. Flexible stamp
As describe above the flexible stamp was obtained by a molding process on a silicon master mold
on which several holes networks with different diameters have been patterned by E-beam lithography.
Figure 1(a) shows an AFM view of PDMS stamp with dots. They have a conical shape with a diameter
(D) of 200 nm at half of their height (h= 210 nm) and a periodicity of 600 nm. This shape is the result
of an optimized etching process in Si master mold.
In Figure 1(b) AFM view for diameter 200 nm and periodicity of 400 nm shows a lateral collapse
effect where PDMS pillars stick to each other randomly. Dilution has permitted to obtain aspect ratio
(h/D) 1.4 but greater than critical aspect ratio.23 This unintentional collapse effect was particularly
interesting because it allowed us to evaluate ultimate imprint resolution in sol-gel resist.
B. Nanoimprint in sol-gel and gold nanodisks
Printing tests were performed by placing gently the flexible stamp on top of SiO2sol-gel layer,
without any additional pressure.17 An annealing at 110C during 10 sec was used to cure the resist.
After annealing, stamp was released leaving the sol-gel patterned. Figure 2shows SEM views of the
imprint results.
Figure 2(a) is a SEM tilted view of the imprint in SiO2sol-gel. The residual layer thickness
obtained was lower than 10 nm. When diameter increases to 200 nm with the same period, we obtain
figure 2(b) result which corresponds to the imprint with the PDMS stamp having lateral collapse
FIG. 1. AFM images of stamp PDMS: with diameter 200 nm pitch 600 nm (a), diameter 200 nm pitch 400 nm (with collapse
effect of simultaneously 2 or 4 dots) (b).
125125-4 Hamouda et al. AIP Advances 7, 125125 (2017)
FIG. 2. SEM images of the imprint in SiO2sol-gel: diameter 350 nm and periodicity 600 nm (a), diameter 200 nm and
periodicity 400 nm with a collapse effect (b), diameter 150 nm and periodicity 400 nm (c).
effect. Figure 2(c) shows an imprint with diameter 150 nm and pitch 400 nm. Diameter 150 nm is
the lower limit that we can obtain in a reproducible manner with a PDMS. We can notice a consistent
transfer with the stamp. Etching processes of sol-gel and PMMA was performed as described in
“EXPERIMENTAL” section and stop at gold film. A gold deposit (30 nm) and lift-off process in
solvent with ultra sonic were performed to obtain gold nanodisks as shown in Figure 3. Figure 3(a)
shows regular dot arrays with diameter 230 nm and pitch 600 nm. A certain roughness around gold
nanodisks is attributed to the RIE process of SiO2sol-gel, this was confirmed by a SEM observation
after etching step. This process step also induces also the widening of the nanostructures.
Figure 3(b) shows gold nanodisks for diameter 200 nm and pitch 400 nm. We can notice a gold
nanostructure randomization due to the lateral collapse effect in PDMS which has been transferred
until this last step. Some discs can touch each other with a random distribution. As shown in black
square, others are very close with a gap smaller than 50 nm. Hence, this result shown in Figure 3(b)
makes possible to evaluate a print resolution in SiO2sol-gel around this value.
C. Optical characterizations
To characterize the performance of substrate as a sensor, SERS measurements were performed
on several nanostructured areas functionalized by thiophenol molecules. Some Raman characteristic
peaks of thiophenol molecules are 419, 1000, 1075, 1575 cm-1. These Raman spectra were recorded
using a XploRA spectrophotometer from Horiba Scientific. The acquisition time was fixed to 30 s.
A 638 nm laser was used for these measurements with a power of 2.6 mW. The laser excitation was
focused on the substrate using a microscope objective (x20, N.A.= 0.7). The same objective was
used to collect the Raman signal from the SERS substrates in a backscattering configuration. SERS
spectra were recorded with a spectral resolution under 4 cm-1.
Figure 4shows an increasing SERS signal versus nanostructures density as expected.
We calculated enhancement factor (EF) defined by the following equation:
EF =
Isers
IRaman
×NRaman
Nsers
(1)
FIG. 3. SEM image of gold nanodisks on gold film after lift-off process: (a) diameter 230 nm and periodicity 600, (b) diameter
200 nm and periodicity 400 nm.
125125-5 Hamouda et al. AIP Advances 7, 125125 (2017)
FIG. 4. SERS measurements with pitch 600 nm and diameters 230-360 nm (a).
Where Isers and IRaman are respectively the SERS and Raman intensities. NRaman and Nsers are
defined as the number of excited molecules in Raman and SERS experiments, respectively NRaman is
obtained from Ref. 35 and estimated to 4.22 ×1015.Nsers is evaluated from the following equation:
Nsers =NA×Scollected ×σSurf ×Sstructures
P2with NAthe Avogrado’s number (mol-1), Scollected the illumi-
nated area around 7 µm2,σSurf is the surface coverage of thiophenol molecules and is approximately
around 0.544 nmol/cm2,36,37 Sstructures is the sum of the lateral and top surface of nanodisks. P
corresponds to the periodicity of the nanostructures array.
SERS EF are between 2.6x106and 3.1x106depending of the nanodisks’ sizes for the period
600 nm. The order of magnitude of 106agrees with characterization of gold nanostructures function-
alized on gold film.8,9SERS measurements have also been performed for 400 nm period nanodisks
and shown due to collapse effect lower peak intensities.
IV. CONCLUSIONS
In this work we have presented an original fabrication process for biochemical surface enhanced
Raman scattering sensor based on soft nanoimprint lithography (S-NIL) on thermo-curable SiO2
sol-gel at low temperature. This soft nanoimprint method, mainly manual, and the lift-off process
with sacrificial layer under SiO2sol-gel permit to product easily sensors at low cost, thereby this
method is compatible with mass production. Otherwise for a periodicity of 400 nm we have obtained
a collapse effects of the pillars on PDMS stamp. We used this effect to evaluate the resolution of the
imprint in sol-gel. SERS characterization with thiophenol were performed with spectrophotometer.
An evaluation of SERS intensities on various structures’ density has been shown with the periode
600 nm. This new alternative manufacturing provided devices with specific coupling properties due
to the periodicity and the size of gold nonodisks on a gold film underlayer. These results consistent
with previous studies validate this new sol-gel-based process.
ACKNOWLEDGMENTS
This work was done with the C2N facility and partly supported by the RENATECH network and
the General Council of Essonne. More over the authors acknowledge ANR-12-NANO-0016-03.
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... The procedure is simple, cost-efficient and could achieve high resolution on nanoscale feature down to 20 nm [8][9][10][11][12][13][14][15]. Especially, thermally assisted nanoimprint lithography (T-NIL) has several advantages as compared to light assisted nanoimprint lithography (UV-NIL) because it allows the nanostructure replication process in both thermoplastic polymer and sol-gel films [16,17]. In addition, T-NIL provides the flexibility of using both transparent and opaque master stamp in nanofabrication process. ...
... To produce nanoscaled 1D (arrays of pillars) on epitaxial α-quartz thin films by silicon micromachining, we have tested cost efficient lithographic techniques such as laser transfer lithography technique 12 , Soft nanoimprint lithography 13 and a novel plasma-assisted self-assembled SrCO3 nanoparticles reactive nanomask etching. Such procedures do not require any lithographic mask and allow obtaining a large scale and precise control of epitaxial quartz nanostructures (see Fig. 1). ...
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The monolithic integration of sub-micron quartz structures on silicon substrates is a key issue for the future development of telecommunication to the GHz frequencies. Here we report unprecedented large-scale fabrication of ordered arrays of piezoelectric epitaxial quartz nanostructures on silicon substrates by the combination of soft-chemistry and three cost effective lithographic techniques: (i) laser transfer lithography, (ii) soft nanoimprint lithography on Sr-doped SiO2 sol-gel thin films and (iii) self-assembled SrCO3 nanoparticles reactive nanomasks. Epitaxial {\alpha}-quartz nanopillars with different diameters (down to 50 nm) and heights (up to 2000 nm) were obtained for the first time. This work proves the control over the shape, micro- and nano-patterning of quartz thin films while preserving its crystallinity, texture and piezoelectricity. This work opens up the opportunity to fabricate new high frequency resonators and high sensitivity sensors relevant in different fields of application.
... SERS EF are around 4.2 × 10 6 , 2.6 × 10 6 and 1.5 × 10 6 respectively for e-beam sample, soft UV-NIL and hard UV-NIL samples for the characteristic peak at 1575 cm −1 (see Table 1). The order of magnitude of 10 6 agrees with previous characterization of samples done by e-beam lithography [33,34] or soft nanoimprint lithography on SiO2 sol-gel [48]. The difference and lower value for NIL samples can be explained by a modification of the shape of nanostructures and small variations of periods in comparison with ebeam sample. ...
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Shortly after its inception, nanoimprint lithography (NIL) was primarily used as tool for the thermal embossing and flash curing of thermoplastic resists and polymer precursors, respectively. Driven by a need for high density material architectures, the semi-conductor industry served as the primary inspiration for NIL development for decades. However, as new resist materials have been explored, the variety of fields investing resources into the technology has grown, and NIL now finds applications actuating numerous organic and inorganic materials, imparting unique manifestations of elastic and plastic deformations. From photovoltaics to water filtration, the role of NIL is growing steadily, and this review illustrates the breadth of its impacts.
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The monolithic integration of sub-micron quartz structures on silicon substrates is a key issue for the future development of piezoelectric devices as prospective sensors with applications based on the operation in the high frequency range. However, to date it has not been possible to make existing quartz manufacturing methods compatible with integration on silicon and structuration by top down lithographic techniques. Here we report unprecedented large-scale fabrication of ordered arrays of piezoelectric epitaxial quartz nanostructures on silicon substrates by the combination of soft-chemistry and three lithographic techniques: (i) laser transfer lithography, (ii) soft nanoimprint lithography on Sr-doped SiO2 sol-gel thin films and (iii) self-assembled SrCO3 nanoparticles reactive nanomasks. Epitaxial α-quartz nanopillars with different diameters (from 1 µm down to 50 nm) and heights (up to 2000 nm) were obtained. This work demonstrates the complementarity of soft-chemistry and top-down lithographic techniques for the patterning of epitaxial quartz thin films on silicon while preserving its epitaxial crystallinity and piezoelectric properties. These results open up the opportunity to develop a cost-effective on-chip integration of nanostructured piezoelectric α-quartz MEMS with enhanced sensing properties of relevance in different fields of application.
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The calcium ferrite nano-particles (CaFe2O4 NPs) were synthesized using a sol-gel method for targeted drug delivery application. The proposed nano-particles were initially prepared by mixing calcium and iron nitrates that were added with citric acid in order to prevent agglomeration and subsequently calcined at a temperature of 550°C to obtain small particle size. The prepared nanoparticles were characterized by using an XRD (X-ray diffraction), which revealed the configuration of orthorhombic structures of the CaFe2O4 nano-particles. A crystallite size of ~13.59 nm was obtained using a Scherer's formula. Magnetic analysis using a VSM (Vibrating Sample Magnetometer analysis), revealed that the synthesized particles exhibited super-paramagnetic behavior having magnetization saturation of approximately 88.3emu/g. Detailed observation via the scanning electron microscopy (SEM) showed the calcium ferrite nano-particles were spherical in shape.
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Metal nanoparticle arrays have proved useful for different applications due to their ability to enhance electromagnetic fields within a few tens of nanometers. This field enhancement results from the excitation of various plasmonic modes at certain resonance frequencies. In this article, we have studied an array of metallic nanocylinders placed on a thin metallic film. A simple analytical model is proposed to explain the existence of the different types of modes that can be excited in such a structure. Owing to the cylinder array, the structure can support localized surface plasmon (LSP) modes. The LSP mode couples to the propagating surface plasmon (PSP) mode of the thin film to give rise to the hybrid lattice plasmon (HLP) mode and anti-crossing phenomenon. Due to the periodicity of the array, the Bragg modes (BM) are also excited in the structure. We have calculated analytically the resonance frequencies of the BM, LSP and the corresponding HLP, and have verified the calculations by rigorous numerical methods. Experimental results obtained in the Kretschmann configuration also validate the proposed analytical model. The dependency of the resonance frequencies of these modes on the structural parameters such as cylinder diameter, height and the periodicity of the array is shown. Such a detailed study can offer insights on the physical phenomenon that governs the excitation of various plasmonic modes in the system. It is also useful to optimize the structure as per required for the different applications, where such types of structures are used.
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In this paper, we report on an improved enhancement of the surface-enhanced Raman scattering (SERS) effect. Such improvement is obtained by using a continuous gold film (underlayer), which is added below an array of gold nanostructures. Two types of nanostructures were studied to validate our results: regular disk arrays with two diameters (110 and 210 nm) and lines with a width of 110 nm, all on a gold film of 30 nm thick. A supplementary gain of one order of magnitude on the SERS enhancement factor (EF) was experimentally demonstrated for several excitation wavelengths: 633, 660, and 785 nm. With such SERS substrates, EFs of 10^7 are observed for thiophenol detection. This opens the way towards routine and reliable detection of molecules at low concentration.
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We report on the use of soft UV nanoimprint lithography (UV-NIL) for the development of reproducible, millimeter-sized, and sensitive substrates for SERS detection. The used geometry for plasmonic nanostructures is the cylinder. Gold nanocylinders (GNCs) showed to be very sensitive and specific sensing surfaces. Indeed, we demonstrated that less than 4 ×10^6 avidin molecules were detected and contributed to the surface-enhanced Raman scattering (SERS) signal. Thus, the soft UV-NIL technique allows to obtain quickly very sensitive substrates for SERS biosensing on surfaces of 1 mm^2.
Article
Surface-enhanced Raman scattering (SERS) is a sensitive and widely used as spectroscopic technique for chemical and biological structure analysis. One of the keys to increase the sensitivity of SERS sensors is to use nanoparticles/nanostructures. Here, we report on the density effect of gold nanodisks on SERS intensity for a highly sensitive detection of chemical molecules. Various densities of gold nanodisks with a height of 30 nm on gold/glass substrate were fabricated by electron beam lithography in order to have a good uniformity and reproducibility. The evolution of the enhancement factor (EF) with nanodisk density was quantified and compared to numerical calculations. An EF as high as \(2.6 \times 10^{7}\) was measured for the nanodisk with a diameter of 110 nm and a periodicity of 150 nm which corresponds to the highest density (42.2 %).
Article
Polymeric molds replicated from a master structure can provide intrinsic anti-sticking behavior and UV-transparency. They can be replicated from various substrates and offer cost efficient replication of multiple working stamps from only one master. They also allow the use of various imprint methods including UV- or thermal-assisted ones. Usually, the polymer material exhibits mechanical and surface-chemical properties which differ from hard mold materials like silicon, silicon dioxide or metals. Due to this, the molds might be deformed or even destroyed during imprint or cleaning. This is pronounced for high aspect ratio patterns, as they occur, if imprint is used as direct pattering method. The affinity to pattern damage of polymeric molds during cleaning is investigated in this paper. Different possible polymeric mold materials are considered. Experimental data is compared to simulation results and shows good agreement. Different exemplary patterns are investigated and a best suitable material is found. It is stable for feature aspect ratios up to 10 for half pitch gratings in the considered range of dimensions.
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In this study, an extension of the soft UV nanoimprint process is presented with improved control of the residual layer thickness and significant reduction of the nanoimprint proximity effect. The process is based on the consecutive halving of the liquid resist layer by a liquid transfer process. In the initial stage, this liquid transfer process uses a thick initial resist layer to ensure complete filling of the stamp cavities. The thick residual layer is then thinned down to about half by peeling off the applied imprint stamp, a process that can be repeated until the desired residual layer thickness is achieved. The information carrying layer remaining on the stamp can be transferred conformally to any substrate even with nonplanar surfaces. The fabrication of silicon photonic waveguides and photovoltaic antireflection textures are two applications where the advantage of this process becomes particularly clear.
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The resolution, dimension stability, and reproducibility of the Soft UV-Nanoimprint is investigated. The potential for imprinting nanostructures with flexible molds in UV-curable resists in the 100 nm regime are explored and the limitations analyzed. The dimensional stability of imprinted patterns is determined by the deformation of the mold that in term depends on the geometry of the structures and the imprint pressure applied.
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This paper describes the fabrication steps developed to pattern nano scale features on thin silica wafers. The optimization of e-beam exposure dose is presented. The use of a chrome layer on top of the silica wafer implies higher doses to crosslink the negative NEB22 resist, but the results show a very large window process. Specific etching processes have been developed. It is demonstrated how the micro-trenching and CD bias are reduced. Thanks to the optimization of both the exposure dose during e-beam lithography and the plasma dry etch steps, features with a resolution as low as 30nm have been achieved.