Micro-nanostructured plasmonic TiN layer produced using rapid thermal nitridation of a nanoimprinted TiO2 sol-gel

Abstract

Titanium nitride (TiN) is a very promising new plasmonic material to replace traditional plasmonic materials like gold and silver, especially thanks to its thermal and chemical stability. However, its chemical resistance and its hardness make TiN difficult to microstructure. An alternative approach is to micro-nanostructure a titanium dioxide (TiO2) coating and then to use a nitridation reaction to obtain a micro-nanostructured TiN coating. This is an easy, rapid and cost-effective structuring process. In this paper, we demonstrate that rapid thermal nitridation (RTN) can be combined with nanoimprint lithography (NIL) to rapidly micro-nanostructure a TiN layer. This innovative approach is applied to a micro-nanostructured TiN layer for plasmonic response in the near infrared range. Experimental and theoretical approaches are compared.

Full text

Research Article Vol. 12, No. 10 / 1 Oct 2022 / Optical Materials Express 3846
Micro-nanostructured plasmonic TiN layer
produced using rapid thermal nitridation of a
nanoimprinted TiO2sol-gel
NICOLAS CRESPO-MONTEIRO,1,* ARNAUD VALOUR,1EMILIE
GAMET,1MARIA A. USUGA HIGUITA,1VALENTIN GÂTÉ,2COLETTE
TURBIL,2DANIEL TUROVER,2STÉPHANIE REYNAUD,1JULIEN
JONEAU,1AND YVES JOURLIN1
1Université de Lyon, UJM-Saint Etienne, CNRS, Laboratoire Hubert Curien, UMR 5516, 42000
Saint-Etienne, France
2SILSEF, 382 rue Louis Rustin, ARCHPARC, 74160 ARCHAMPS, France
*nicolas.crespo.monteiro@univ-st-etienne.fr
Abstract:
Titanium nitride (TiN) is a very promising new plasmonic material to replace
traditional plasmonic materials like gold and silver, especially thanks to its thermal and chemical
stability. However, its chemical resistance and its hardness make TiN difficult to microstructure.
An alternative approach is to micro-nanostructure a titanium dioxide (TiO
2
) coating and then to
use a nitridation reaction to obtain a micro-nanostructured TiN coating. This is an easy, rapid and
cost-effective structuring process. In this paper, we demonstrate that rapid thermal nitridation
(RTN) can be combined with nanoimprint lithography (NIL) to rapidly micro-nanostructure a
TiN layer. This innovative approach is applied to a micro-nanostructured TiN layer for plasmonic
response in the near infrared range. Experimental and theoretical approaches are compared.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Titanium nitride (TiN) is an attractive alternative material to replace gold and silver in plasmonic
applications, particularly in the visible to near infrared spectral range [1–3]. Indeed, TiN
layers have intrinsic physico-chemical and optical properties that make them very promising:
low resistivity, high reflectance in the infrared spectral range, good corrosion resistance, good
chemical inertness, good thermal stability (melting temperature 2930
°
C), and high hardness
(
∼
30 GPa) [4–8]. Manufacturing TiN films generally requires a vacuum technology, such as
reactive magneton sputtering [1,9–12], molecular-beam epitaxy [13,3], chemical vapor deposition
(CVD) [14–16], atomic layer deposition (ALD) [17–20] or pulsed laser deposition (PLD) [21–23],
under a nitrogen or ammonia atmosphere. Unfortunately, due to its good hardness and chemical
resistance, TiN is difficult to micro-structure or etch directly. An alternative solution, proposed
in many studies, is to use titanium dioxide (TiO
2
) as starting material, as it is easier to structure,
and then to use a nitridation reaction to obtain TiN [24,25]. During the nitridation reaction,
ammonia breaks down into hydrogen and nitrogen; hydrogen facilitates the creation of oxygen
vacancies, and the nitrogen reaction fills the vacancies, thereby transforming TiO
2
material into
TiN material [26,27]. The advantages of this process are that the structuring process is easy
[25,28–30] and is compatible with sol-gel approaches [27,31,32].
However, the nitridation reaction between the ammonia gas (NH
3
) and TiO
2
occurs at high
temperatures (
∼
1000
°
C) and the process is usually carried out in traditional ovens with a long
exposure time (several hours) [27,33,34] which limits its use to substrates that remain stable at
high temperatures and also limits its industrial use. Recently, we demonstrated the possibility of
using a rapid thermal nitridation (RTN) process that produces TiN thin films in a very short time
(a few minutes) and under much less restrictive conditions [32,35].
#468682 https://doi.org/10.1364/OME.468682
Journal © 2022 Received 23 Jun 2022; revised 13 Jul 2022; accepted 26 Jul 2022; published 6 Sep 2022

Research Article Vol. 12, No. 10 / 1 Oct 2022 / Optical Materials Express 3847
In this paper, we show that, used with a TiO
2
SILSEF formulation sol-gel and combined with
the nanoimprint lithography (NIL), the RTN process makes it possible to rapidly manufacture
micro-nanostructures of TiN. This approach, which combines NIL and RTN, produces micro-
nanostructured TiN coatings in a few minutes, and to our knowledge, is completely new and
represents a break with the state of the art. Going further with this new process, we also
demonstrate the production of a micro-nanostructured plasmonic (SPR) device with plasmon
resonance in the near infrared range using a surface plasmon resonance from a TiN based grating.
We compare our experimental results with modeling using an electromagnetic model based on
coupled wave analysis.
2. Experimental methods
2.1. TiO2sol-gel coating
TiO
2
sol-gel films were prepared by spin-coating the SILSEF formulation on a TiN coated layer
on a silica substrate for 10 seconds at 5 000rpm with an acceleration of 3 000 rpm using a
laurel WS-650 spin coater. This sol-gel developed by SILSEF is composed of Titanium (IV)
isopropoxide (TIPT) which react in the presence of complexing agents allowing the stabilization
of the sol and alcohols as solvent.
2.2. Nanoimprint lithography process
SILSEF is a proprietary technology to pattern its sol-gel formulation using a soft NIL process.
The stamp was made of a soft polymer using a micro-nanostructured master mold with a sinusoidal
structure of period 1
µ
m with a grating depth of 450 nm. The stamp was applied on the sol-gel
film after spin coating and baked at 110 °C under 1 bar pressure for 5 minutes.
2.3. Rapid thermal nitridation of TiO2coatings
TiO
2
thin films and planar gratings were nitrided as detailed in previous works [32,35] in an
As-One 100 (Annealsys) RTA high-temperature furnace under an ammonia gas (NH
3
) atmosphere.
However, this configuration does not enable accurate measurement of the temperature of the
sample during the heat treatment, which is why only the power of the lamps used is indicated
hereafter. Before the nitridation process began, the chamber was successively purged and
evacuated with nitrogen (N
2
) to minimize the oxygen content in the furnace atmosphere. The
samples are then irradiated for 10 min at 30% of the power of the halogen lamps, followed by a
second IR irradiation at 1% for 30 seconds to protect the lamps and maximize their lifetime. The
samples were then cooled to room temperature. During the nitridation process, a 1 000 sccm
pure NH3flow was introduced in the chamber at a pressure of 10 mbar.
2.4. Modeling method
The structures were simulated by MC GRATINGS commercial software using the Chandezon
method (C-method). This model is based on solving Maxwell’s equations in curvilinear
coordinates, and enables the simulation of continuous grating profiles. Considering continuous
profiles of different shapes is appropriate for our fabricated structures since no perfect sinusoidal
profiles were obtained. In practice, the direct space is transformed as a function of the grating
profile to simplify the boundary conditions at the interfaces. The method performs the Rayleigh
expansion of the field in the new curvilinear coordinates system and thus resolves the wave
equation.
The goal of modeling was to determine the surface plasmon resonance which occurs for the TM
polarized incident beam that can be expected with the achievable opto-geometrical parameters
in the near infrared (NIR) region. A retro-simulation was run to observe the impact of the real

Research Article Vol. 12, No. 10 / 1 Oct 2022 / Optical Materials Express 3848
profile on the plasmon resonance. For this purpose a mathematical function and decomposition
in Fourier series were used [36].
2.5. Optical and structural measurements
The films were characterized before and after nitriding. Spectrophotometric measurements (Cary
5000 UV-Vis-NIR from Agilent Technologies) were performed in the infrared and visible range.
The phase composition of the layer following the RTN process was analyzed using Raman micro
spectroscopy measurements (LabRam ARAMIS) with excitation at 633 nm (He-Ne laser).
2.6. Transmission electron microscopy characterization
Advanced structural investigations were also conducted with a JEOL Neo-ARM 200F Transmission
Electron Microscope. First, focused ion beam (FIB) lamella was extracted from the TiN micro-
nanostructured samples using a FEI (Thermofisher) Helios 600i dual beam FIB/SEM microscope.
The lamella was thinned at different Ga
+
ion voltages and currents, and finally thoroughly cleaned
down to 1 kV to optimally remove ion beam artifacts, such as re-deposition or amorphization.
Electron energy loss spectroscopy (EELS) and spectrum image (SI) data were collected in EELS
mode together with HAADF-STEM using a Gatan imaging filter (GIF Quantum ER) and a Gatan
ADF STEM detector. Spectra and mapping were aligned and extracted from the dual-EELS SI
raw data, the appropriate background was selected with DM software.
3. Results and discussion
3.1. Unstructured thin films
Optical and structural characterizations were carried out on the unstructured TiO
2
and TiN layers
before the thin films were micro-nanostructured to produce a TiN plasmonic device. Raman
spectroscopy was used to confirm the phase conversion from TiO
2
thin film into crystallized
TiN during the RTN process. The TiO
2
thin film presented no vibrational features in the range
100 to 1 000 cm
−1
(data not shown), which indicates an amorphous TiO
2
phase of the sample
before irradiation. After the nitridation step, the Raman spectrum of the resulting film, shown in
Fig. 1(a), was characterized by five broad bands, in agreement with the Raman-active modes
of titanium nitride reported in the literature [37–39]. The first two bands centered at 208 cm
−1
and 320 cm
−1
, were assigned to transverse acoustic (TA) and longitudinal acoustic (LA) modes,
respectively. The band centered at 464 cm
−1
corresponds to the second-order acoustic (2A) mode,
the one at 536 cm
−1
corresponding to the transverse optical (TO) mode, and the last broad band
around 820 cm−1was assigned to the LA +TO modes of TiN.
Optical measurements of the obtained films were also performed using optical spectroscopy in
the visible and NIR region. Figure 2(b) shows the reflectance spectra of the synthesized TiO
2
and TiN thin films at wavelengths ranging from 400 nm to 2 000 nm. The amorphous TiO
2
layer
shows good transparency in these regions with a reflectance of less than 15-20% over the whole
range of wavelengths analyzed. After the nitridation step, a typical reflectance spectrum of a
TiN film was obtained in the visible-NIR wavelength region as shown in Fig. 2(b). The TiN film
showed a minimum reflectance of 20% at 500nm and a maximum reflectance slightly higher than
70% in NIR wavelengths, highlighting the metallic character of TiN, which is in good agreement
with the values reported for TiN thin films in the literature [40].
3.2. Micro-nanostructured films
As the TiO
2
SILSEF formulation sol-gel can be converted into TiN by RTN, micro-nanostructures
were carried out by NIL (NanoImprint Lithography) on the samples of TiO
2
and subsequently
transformed into TiN by RTN. The TiO
2
SILSEF formulation sol-gel was deposited by spin-
coating on the buffer TiN thin films previously deposited the silica substrates (Fig. 2(a)). Before

Research Article Vol. 12, No. 10 / 1 Oct 2022 / Optical Materials Express 3849
6000
11000
16000
100 400 700 1000
Intensity (arb.units)
Raman shift (cm
-1
)
208
320 464
536
820
0
20
40
60
80
400 800 1200 1600 2000
Reflectanc e (%R)
Wavelength (nm)
TiO
2
TiN
TiN
(a) (b)
Fig. 1.
(a) Raman spectrum of TiN thin film and (b) UV-visible-NIR reflectance spectra for
an incidence angle of 10°of TiO2and TiN thin films deposited on silica substrate.
(a) (b) (c) (d) (e) (f)
Fig. 2.
Schematic of the fabrication procedure for a TiN grating structure using pattern
transfer printing. (a) deposition of a TiO
2
xerogel by spin-coating on a TiN (b). (c)
Micro-nanotructuring of the TiO
2
film by the nanoimprint process to obtain a sinusoidal
grating (d). Nitriding of the structured TiO2film to obtain our nano-structured TiN layer.
structuring the films by nanoimprinting, a layer of TiN is deposited on the silica substrate in
order to obtain a continuous metallic layer under the micro-nanostructured TiN patterns. The
thickness of this layer must be thick enough to consider an infinite metallic layer according to
the penetration depth of the plasmonic mode, which is close to 30 nm. To obtain this TiN layer
the process described in Ref. [32] was used. The TiO
2
sol-gel described in this reference was
deposited by spin-coating at 3000 rpm during 30s in order to obtain a uniform TiO
2
layer of
300 nm. This layer was then nitrided by RTN using the same process described in experimental
and method. After nitriding we obtain a uniform TiN layer of 50 nm thickness. Next, the soft
stamp was pressed on the TiO
2
films for 5 minutes to transfer the sinusoidal microstructure to the
TiO
2
film (Fig. 2(c)). The soft stamp was then removed leaving a sinusoidal micro-nanostructure
on the TiO
2
sol-gel film. Finally, the micro-nanostructure was subjected to RTN treatment for 10
minutes to obtain the micro-nanostructured TiN layer (Fig. 2(e & f)) on top of the TiN planar
layer.
The structure of the patterned TiO
2
and TiN layers can be seen from the AFM measurements
and SEM imaging. Figure 3(a) unequivocally shows a TiO
2
grating structure after the embossing
process, with a sinusoidal periodicity of approximately 1
µ
m and a nearly consistent depth of
450 nm. After the nitridation step, the AFM measurements (Fig. 3(b)) clearly showed a change
in the structure of the diffraction grating, with a cycloid profile and a densification effect due
to heating during the process inducing a decrease of the porosity and a crystallization of the

Research Article Vol. 12, No. 10 / 1 Oct 2022 / Optical Materials Express 3850
film. Each pattern has a very thin, pointed tip with a much wider base, unlike the TiO
2
patterns,
which have a thicker, rounded tip. This densification effect leads to a TiN grating with the same
period as the structured film of TiO
2
, i.e. 1
µ
m, with a consistent depth reduced to 110 nm. These
results were confirmed by SEM images of the top view of the samples, illustrated in Fig. 3(c)
and (d). Indeed, these images show diffraction gratings with no apparent defects with the same
periodicity, approximately 1
µ
m, as that measured by AFM. The decrease in the width of the
diffraction grating lines after the RTN process is also visible, related to the change in the structure
of the grating from sinusoidal TiO
2
to inverse cycloid TiN caused by the densification of the
material during the nitriding process. Despite densification and the change in structure, the
nanostructured TiN layer still diffracts light and has plasmonic optical effects.
0.00
0.20
0.40
0.60
0246
Y [μm]
X [μm]
450 nm
1 μm
0.00
0.05
0.10
0.15
0.20
0246
Y [μm]
X [μm]
110 nm
1μm
TiO
2
TiN
(a) (b)
3 μm
(d)
3 μm
(c)
TiO
2
TiN
Fig. 3.
(a) and (b) AFM profiles of TiO
2
and TiN planar gratings with their respective SEM
images of the top view of the samples (c) and (d).
The HAADF STEM images in Fig. 4(a, b) show a cross section of a line whose respective size
is in agreement with AFM measurements (around 100 nm). Figure 4also shows the results of
mapping the STEM/EELS elements on the patterned TiO2sample after RTN treatment.
The energy loss spectrum corresponding to the nitrided sample shows, respectively, an N K
edge at 401 eV, a Ti L2,3 edge at 456 and 462 eV, and a reduced O K edge at 532 eV. EELS
mapping of titanium, oxygen and nitrogen confirmed that the nitridation process was complete
throughout the layer. EELS oxygen mapping based on the oxygen K edge shows a predictable
strong signal in the SiO
2
substrate. Nevertheless, an oxygen K edge was detected everywhere on

Research Article Vol. 12, No. 10 / 1 Oct 2022 / Optical Materials Express 3851
(d)
(b)
(c)
(a) (e)
Fig. 4.
(a), (b) Cross-section of a line of grating imaged by HAADF-STEM (HAADF), and
(c), (d), (e) images of the mapping of the N, Ti, and O elements in the TiN layer, respectively.
0
20
40
60
80
500 750 1000 1250 1500 1750
Reflctance (%R)
Wavelength (nm)
30° P polarization 30° S polarization
Fig. 5.
Experimental reflection (%) measurements for TM and TE incident light for an
angle of incidence of 30°.
the FIB lamella, including on the TiN film and on the carbon-protective carbon layer. Concerning
the weaker intensity of the O K edges on the TiN layer and the protective FIB layers, one can
reasonably assume that both the top surface of the TiN layer and the entire surface of the FIB
lamella are contaminated by ambient oxygen following the nitridation process and the final FIB
lamella step, respectively. To conclude, EELS data indicate that the layer was entirely nitrided in
the form of titanium nitride following the nitridation process.
3.3. Plasmonic device
According to the previous characterizations (both optical and structural characterizations), TiN
can be considered as a metallic material and one of the applications of a microstructured layer
is demonstrating surface plasmon resonance (SPR) in the infrared range. SPR is the resonant
excitation of electromagnetic modes called surface plasmon polaritons (SPP), supported at
the metal–dielectric interface. SPR consists of electromagnetic waves coupled to conduction
electrons collective oscillations [41–44]. Such optical components are suitable for many
applications especially in the field of sensors (biosensors or gas sensors) due to confinement of
the electromagnetic field at the metal/dielectric interface. The main methods of optical excitation
of surface plasmons include attenuated total reflection (prism coupling) and grating coupling
46. In grating coupling-SPR (GC-SPR), the resonance conditions are provided by the
−
1st or
+
1st evanescent diffracted order of the TM polarized incident light in this case, there is a dip
in the reflectance curve of the reflected incident beam at the resonant wavelength, i.e. when

Research Article Vol. 12, No. 10 / 1 Oct 2022 / Optical Materials Express 3852
synchronism occurs between the incident wave vector and the surface plasmon wave vector. Even
though this method is known to be less sensitive than the prism-based coupling method, it has
higher miniaturization and integration capabilities than the prism-based coupling method 47.
3.4. Simulations
Simulations were performed to predict the effect of SPR on the 0th reflected order. The refractive
indices considered for our proposed structures were obtained from ellipsometric results on a TiN
layer from previous published works [32,35]. The values at
λ=
1550 nm are n
TiN =
3.61
+
i6.03
and n
SiO2 =
1.44. Dispersion in the range of the wavelength considered was taken into account and
the values of the TiN layer refraction index were based on experimental data. Both polarizations,
(TE and TM) were plotted to show that there was no resonance effect in TE, plasmon resonance
occurs only in TM polarization (Fig. 5). The first simulation took into account a 160 nm layer
with a sinusoidal corrugation with a depth of 110 nm; Fig. 6shows that in such a structure,
plasmon resonance occurs at 1.5 µm for an angle of incidence of 30°.
Fig. 6.
Reflection spectrum (%) for TM and TE incident light for an angle of incidence of
30°. The structure considered is a sinusoidal TiN 1 µm period grating.
As shown in Fig. 7, the profile of the TiN structure is quite far from a sinusoidal grating. The
profile obtained was fitted using an inverse cycloid of 1
µ
m period and a depth of 110 nm. The
approached profile was then taken into account in the second model, or retro-simulation, based
on a more realistic profile of the fabricated structure, and the results are in good agreement with
the experimental results. Indeed, the reflectance dip is lower, rising to 30% whereas when the
structure is purely sinusoidal, the dip is 10%.

Research Article Vol. 12, No. 10 / 1 Oct 2022 / Optical Materials Express 3853
Fig. 7.
Reflection (%) of TM and TE incident light for an angle of incidence of 30
°
. The
structure considered is a TiN 1
µ
m period grating. The profile was fitted by an inverse
cycloid to approximate the measured AFM profiles.
4. Conclusion
In this article, we have shown that the RTN process can be combined with nanoimprint lithography
(NIL) using an embossing SILSEF formulation TiO
2
sol-gel to micro-nanostructure TiN coating
in a production time of less than 15 minutes. The time required for the process can be further
optimized, but is already compatible with industrial use. The TiN coatings obtained have
an optical reflectance of more than 70% in the near-IR region, meaning they can be used as
plasmonic material in the visible and near infrared regions. A TiN plasmonic device with a
surface structure with a period of 1
µ
m and a depth of 110 nm was produced using with this
combined process. The results were highly consistent with an electromagnetic model based on
coupled wave analysis. This shows the strong application potential of this approach, in particular
for plasmonic sensing and optical components, specifically compared to conventional well-known
plasmonic materials such as Au and Ag. TiN based plasmonic devices open the way for cost
effective plasmonic devices and sensors (bio-sensors, gas sensors) for use in severe and high
temperature environments, even though the quality (e.g. SPR efficiency) may be lower than the
one of materials based on pure metals.
Funding. Agence Nationale de la Recherche (ANR-21-CE08-0042-01).
Acknowledgments.
The Authors would like to thank the French Region Auvergne Rhône-Alpes for financial
support in the framework of Pack Ambition Recherche 2018, MICROSOLEN project and the French National Research
Agency (ANR) for financial support in the framework of project NITRURATION (ANR-21-CE08-0042-01). This work
was partly supported by the French RENATECH+network led by the CNRS, on the NanoSaintEtienne platform.
Disclosures. The authors declare no conflicts of interest.
Data availability.
Data underlying the results presented in this paper are not publicly available at this time but may
be obtained from the authors upon reasonable request.
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