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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.
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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 (TiO2) 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 TiO2 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 (NH3) and TiO2 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].
In this paper, we show that, used with a TiO2 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 TiO2 sol-gel coating
TiO2 sol-gel films were prepared by spin-coating the SILSEF formulation on a TiN coated layer on a silica substrate for 10 seconds at 5 000 rpm 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 µm 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 TiO2 coatings
TiO2 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 (NH3) 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 (N2) 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 NH3 flow 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 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 TiO2 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 TiO2 thin film into crystallized TiN during the RTN process. The TiO2 thin film presented no vibrational features in the range 100 to 1 000 cm−1 (data not shown), which indicates an amorphous TiO2 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−1 was assigned to the LA + TO modes of TiN.
Fig. 1. (a) Raman spectrum of TiN thin film and (b) UV-visible-NIR reflectance spectra for an incidence angle of 10° of TiO2 and TiN thin films deposited on silica substrate.
Fig. 2. Schematic of the fabrication procedure for a TiN grating structure using pattern transfer printing. (a) deposition of a TiO2 xerogel by spin-coating on a TiN (b). (c) Micro-nanotructuring of the TiO2 film by the nanoimprint process to obtain a sinusoidal grating (d). Nitriding of the structured TiO2 film to obtain our nano-structured TiN layer.
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 TiO2 and TiN thin films at wavelengths ranging from 400 nm to 2 000 nm. The amorphous TiO2 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 500 nm 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 TiO2 SILSEF formulation sol-gel can be converted into TiN by RTN, micro-nanostructures were carried out by NIL (NanoImprint Lithography) on the samples of TiO2 and subsequently transformed into TiN by RTN. The TiO2 SILSEF formulation sol-gel was deposited by spin-coating on the buffer TiN thin films previously deposited the silica substrates (Fig. 2(a)). Before 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 TiO2 sol-gel described in this reference was deposited by spin-coating at 3000 rpm during 30s in order to obtain a uniform TiO2 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 TiO2 films for 5 minutes to transfer the sinusoidal microstructure to the TiO2 film (Fig. 2(c)). The soft stamp was then removed leaving a sinusoidal micro-nanostructure on the TiO2 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 TiO2 and TiN layers can be seen from the AFM measurements and SEM imaging. Figure 3(a) unequivocally shows a TiO2 grating structure after the embossing process, with a sinusoidal periodicity of approximately 1 µm 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 film. Each pattern has a very thin, pointed tip with a much wider base, unlike the TiO2 patterns, which have a thicker, rounded tip. This densification effect leads to a TiN grating with the same period as the structured film of TiO2, i.e. 1 µm, 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 µm, 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 TiO2 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.
Fig. 3. (a) and (b) AFM profiles of TiO2 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 4 also shows the results of mapping the STEM/EELS elements on the patterned TiO2 sample after RTN treatment.
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.
Fig. 5. Experimental reflection (%) measurements for TM and TE incident light for an angle of incidence of 30°.
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 SiO2 substrate. Nevertheless, an oxygen K edge was detected everywhere on 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 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 nTiN = 3.61 + i6.03 and nSiO2 = 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. 6 shows 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.
Fig. 7. Reflection (%) of TM and TE incident light for an angle of incidence of 30°. The structure considered is a TiN 1 µm period grating. The profile was fitted by an inverse cycloid to approximate the measured AFM profiles.
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 µm 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%.
4. Conclusion
In this article, we have shown that the RTN process can be combined with nanoimprint lithography (NIL) using an embossing SILSEF formulation TiO2 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 µm 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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