Guillaume Collet

Antonio Hrvat

Svetlana V Eliseeva

Céline Besnard

Anton Kovalenko

Stephane Petoud

Stéphane Petoud

A near-infrared emitting MOF: controlled encapsulation of a fluorescein sensitizer at the time of crystal growth

The emission in the near-infrared (NIR) spectral range offers breakthrough perspectives in fields of applications such as material science and biology. As examples, NIR-emitting probes and materials have a strong potential for the creation of secret tags and night-vision devices 1 while the in vivo fluorescent optical imaging will benefit from an improved light penetration due to a reduced tissue absorption, a lower scattering and the minimization of the detrimental contribution of tissues autofluorescence. [1][2][3] NIR signals can be advantageously used for diagnostic imaging in which the ability to accurately detect small foci is a key advantage to act early in order to treat a pathology. 4 The applications for intra-operative optical imaging and guided surgery can also be considered with NIR wavelengths as they allow to perform surgical acts in ambient light. 5,6 Despite the large amount of attractive theoretical evidences related to the use of NIR signals for in vivo imaging and promising experimental demonstrations 3,[START_REF] Zhang | Proc. Natl. Acad Sci[END_REF] , the number of NIR-emitting probes that can potentially be used in vivo remains scarce. The chemistry and the spectroscopy of lanthanide(III) cations (Ln 3+ ) is an active field of research as several of them have a unique potential to fulfil the needs described above by emitting light at wavelengths above 800 nm. [START_REF] Fan | [END_REF]9 In addition, Ln 3+ offer complementary advantages such as very narrow emission bands, the wavelengths of which are not affected by experimental conditions (pH, temperature, biological environment). 10 Moreover, most of Ln 3+ -based compounds are insensitive to photobleaching which allows (i) their excitation with powerful sources such as lasers, (ii) the extension of the experimental time or their repetition.

As most of f-f transitions are forbidden resulting in very low absorption coefficients of free Ln 3+ , the presence of highly absorbing sensitizers is required to generate their characteristic emission, an approach named "antenna effect". 9 In this process, the antenna absorbs the light from the excitation source and transfers the resulting energy to the accepting electronic levels of the Ln 3+ resulting in the enhancement of its emission intensity. 9,11 The antenna effect is mainly controlled by the electronic structure of the sensitizing chromophore, by the nature of the Ln 3+ , by the Ln 3+sensitizer distance and by their ratio. Various approaches have been tested to obtain such sensitization. 12 The vast majority of Ln 3+ compounds described in the literature have excitation wavelengths located in the UV range. Such high energy light can induce damages to biological systems. Another limitation of most of currently described Ln 3+ compounds emitting in the NIR are their quantum yield values that are often modest. To create Ln 3+ compounds able to emit a large number of photons despite low quantum yields, we took advantage here of the "polymetallic" approach that we have used previously in which a high density of luminescent Ln 3+ and sensitizers are located in a given volume. 10,[START_REF] Foucault-Collet | Proc. Natl. Acad. Sci[END_REF][START_REF] Douvali | [END_REF] To achieve this goal, the use of metal-organic frameworks (MOFs) is of particular interest due to the unique rigidity and the crystalline nature of this family of materials. 15 The specific advantages are (i) the precise location of all atoms determined by single crystal crystallography, including Ln 3+ and distances separating them allowing the rationalization of their photophysical properties, (ii) the possibility to synthesize large amounts of materials that are often essential for practical applications and (iii) the use of MOFs as carriers of molecules, drugs, etc. within their pores. [16][17][18] Several structures of MOFs have been used for the sensitization The cartoon in the square exemplifies the Yb 3+ sensitization mechanism: the excitation light (< 600 nm) is absorbed by the FL entrapped in the MOF pores followed by the energy transfer to Yb 3+ which emits a narrow NIR signal with a maximum at 980 nm.

of NIR-emitting Ln 3+ , however only a few of them demonstrated to be compatible with practical applications such as biological imaging. [19][20][21][22] As a major breakthrough, we have described in 2013 the first example of the use of a nano-MOF as a NIR-imaging agent in living cells. [START_REF] Foucault-Collet | Proc. Natl. Acad. Sci[END_REF] The limitation of this MOF is the excitation wavelength around 355 nm which is incompatible for in vivo biological imaging. Moreover, the excitation wavelength of this system is controlled by the nature of the rigid chromophoric ligand that is forming the network. To shift the excitation wavelength toward lower energy, we have tested different post-synthetic strategies, the entrapment of chromophores such as the LDS 750 into pores of the widely used Yb 3+ -BPDC MOF, 20 the covalent modification of MOF linkers, 23 or the in-situ coupling of acetylene units to form a high density of π-conjugated systems throughout MOF pores. 24 An alternative strategy for long-wavelength excitation of NIR-emitting compounds would be by two-photon absorption. However, such approach requires the use of specific equipment including femtosecond lasers. 25,26 One of the originalities of the present work lies in the design, synthesis and characterization of a new NIR-emitting Yb 3+ -based MOF (FL@CD-MOF-161, Figure 1, S1-2) that possesses a biocompatible excitation wavelength. Fluorescein (FL) has been described as sensitizing the NIR emission of Yb 3+ and exhibits a high absorption of light in the range of 400-550 nm (ε > 10 5 M -1 cm -1 ). 27 When exposed to excitation light, a NIR emission signal arises from Yb 3+ cations that are used as coordination metals ions in FL@CD-MOF-161. In this tridimensional system, FL molecules located inside the MOF pores act as antennae to sensitize Yb 3+ . As an additional novelty and advantage of our work, the FL has been introduced during MOF assembly at the time of crystal growth, leading to an easy, controlled and efficient loading up to the centres of crystals in contrast to a post-synthesis loading which is limited by diffusion. 16 The new FL@CD-MOF-161 was synthesized using a methodology inspired by a protocol that we had described recently combining 9anthracene carboxylic acid dimers (Di-9AC) as ligands, Yb 3+ as coordination metal ions and FL. 28 Crystallites with sizes around 50 µm were isolated (Figure 2A, 2B, S3), showing a high level of reproducibility from one synthesis to another. They emit in the NIR range upon excitation at 400-550 nm (Figure 2C,S3). The PXRD analysis confirmed the isostructural nature of FL@CD-MOF-161 with the previously described CD-MOF-161 (Figure S4). 28 Therefore, the presence of the FL does not affect the structure of the CD-MOF-161. Monitoring the FL emission by itself, its distribution inside the entire volume of crystals was evaluated by confocal laser scanning microscopy. Resulting pictures show that FL molecules are entrapped in the CD-MOF-161 structure up to the crystal centre and are indeed distributed evenly inside the entire volume (Figure S5-7). The photophysical properties of the FL@CD-MOF-161 crystals have been evaluated. The diffuse reflectance spectrum exhibits broad bands in the range 400-550 nm that correspond to typical FL absorption (Figure S8). Emission spectra were collected upon excitation at 480 nm. These data reveal the presence of a NIR emission band with a barycentre at 980 nm which can be attributed to the Yb 3+ 2 F5/2 → 2 F7/2 transition (Figure 3). As Yb 3+ does not possess electronic levels in the energy domain corresponding to the visible range [START_REF] Carnall | Handbook on the Physics and Chemistry of Rare Earths[END_REF] and as the Di-9AC ligands that form the architecture of the CD-MOF-161 exhibit absorption bands below 300 nm, the only way to sensitize Yb 3+ upon excitation at 480 nm is through the "antenna effect", i.e. energy transfer from the excited electronic states of the FL encapsulated in the pores of the MOFs to the accepting levels of Yb 3+ . 9 Moreover, the excitation spectrum (Figure 3, left) recorded upon monitoring Yb 3+ emission at 980 nm coincides well with the diffuse reflectance spectrum (Figure S8). This type of sensitization process results in a large difference in energy (~500 nm) between the maxima of the excitation and the emission wavelengths observed for FL@CD-MOF-161 crystals. Such large energy shift represents an important advantage for optical imaging experiments since the Please do not adjust margins Please do not adjust margins entire emission signal can be collected without taking into account (i) a compromise in filters choice as it is the case with most organic fluorophores due to small Stokes shift, (ii) the possible reabsorption by organic dyes of their own emitted photons, minimizing consequently the intensity of the detected signal. It is worth noting that the emission spectrum observed for the FL-free CD-MOF-161 crystals upon a 480 nm excitation does not contain any Yb 3+ -centred emission bands in the range of 940-1100 nm, confirming that the NIR signal arising from FL@ CD-MOF-161 is indeed obtained by the FLmediated sensitization (Figure S9). However, the sensitization process is not quantitative and the residual broad-band emission of FL in the visible range was observed upon 480 nm excitation (Figure S13). Yb 3+ -centred quantum yields (𝑄 𝑌𝑏 𝐿 ) were determined in order to quantify the efficiency of the FL@CD-MOF-161 to convert the excitation light absorbed by the FL to generate the NIR emission of Yb 3+ . A 𝑄 𝑌𝑏 𝐿 value of 0.308(3) % was obtained (Table S1). Remarkably, this value is 15-to 30-times larger than those observed for the postsynthetically modified MOF-1114(Yb) and MOF-1140(Yb), or for the LDS750@Yb 3+ -BPDC that we have described previously, demonstrating a much higher sensitization efficiency. 20,24 The experimental luminescence decay recorded for the FL@CD-MOF-161 crystals upon monitoring Yb 3+ emission is best fitted by a threeexponential function indicating the presence of emissive Yb 3+ in different coordination environments (Table S1). The calculated average lifetime value of the 2 F5/2 state is equal to 10.8 µs and is at least 2-times longer than those recorded for the other low-energy absorbing MOF-1114(Yb) and MOF-1140(Yb), or for the LDS750@Yb 3+ -BPDC that we have reported previously. 20,24 Such results probably reflect a better protection of Yb 3+ in FL@CD-MOF-161 from high-energy vibrations. Chemical stability of materials during extended periods of storage as well as the stability under continuous illumination referred to as photostability are both crucial properties to consider in view of potential applications. Both parameters were evaluated for FL@CD-MOF-161 crystals. The exact match of PXRD patterns recorded on freshly synthesized samples and on samples stored for one year in quartz capillaries at room temperature without protection from light (Figure S10) confirmed the extremely high stability of FL@CD-MOF-161 crystals. This stability was further supported by brightfield microscopy experiments that reveal a high level of similarity of crystals shapes for these two samples. Photophysical studies performed on the sample stored for one year showed that the characteristic Yb 3+ emission is still efficiently sensitized by the entrapped FL (Figure S11) while the 𝑄 𝑌𝑏 𝐿 value decreased only by a modest factor of 1.7. On the other hand, the average luminescence lifetime value of the 2 F5/2 state is 1.4-times longer for the FL@ CD-MOF-161 crystals after the one year storage period (Table S1). Such phenomenon may be due to a redistribution of water, FL and/or DMA molecules inside of the MOF network that impacts the Yb 3+ environment(s) and therefore the probabilities of non-radiative deactivations. These changes are also reflected by a slight variation of the relative intensities of the different Stark components of the 2 F5/2 → 2 F7/2 Yb 3+ transition observed in the emission spectrum (Figure S11). It should be noted, however, that the UV-vis absorption spectrum recorded on the supernatant solution of the FL@CD-MOF-161 crystals measured after the one year of storage does not indicate the presence of any traces of free FL (data not shown). Moreover, FL-centered quantum yield in the visible range remains the same after storage (2.2(1)%). All these results confirm (i) the good stability of FL@CD-MOF-161 crystals over an extended period of time with no requirement of protection of the sample from light and from the temperature, and (ii) the efficient entrapment of FL insides of MOF pores demonstrated by the absence of detectable release. Such absence can be also explained by the perfect docking of the FL molecule in cavities of the MOF, which was confirmed by the crystallography-determined structure Figure S12). In addition, the photostability of FL@CD-MOF-161 crystals upon continuous illumination at 480 nm was evaluated (Figure S13). Only minor decreases (less than 10 %) of the Yb 3+ signal at 980 nm or FL emission in the visible were observed after 2 hours of continuous irradiation, indicating an excellent photostability.

In order to establish a preliminary proof of principle of the FL@CD-MOF-161 to be used as in vivo NIR imaging agent, an epifluorescence macroscopy experiment was performed using a biomimicking phantom tissue. This model system contains haemoglobin and intralipids in order to simulate the absorption and the scattering of excitation and emitted light, respectively, and to reproduce the optical characteristics comparable to living tissues. [START_REF] Pleijhuis | [END_REF] A mask with a 500 µm diameter circular hole was used to spatially simulate a small size emitting foci that potentially could be an early pathological site (Figure 4A). This mask was positioned on the top of a capillary containing crystals of FL@CD-MOF-161 (Figure S14). The 1 mm thick bio-mimicking tissue was placed on the top of the mask and the specific Yb 3+ NIR signal was monitored with a 996 band pass 70 nm filter upon excitation into FL-centred bands (482 band pass 35 nm filter). One of the important novelty provided by the creation of this This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins MOF lies in its ability to efficiently sensitize Yb 3+ using an excitation wavelength which is significantly shifted to the visible spectral domain in comparison to the actual NIR-emitting Ln 3+ compounds described in the literature. 31,32 The reduced autofluorescence present in the NIR allowed to unambiguously record the specific Yb 3+ signal through the bio-mimicking phantom tissue (Figure 4B, S15). In addition, this macroscopy experiment was realized in challenging conditions by detecting the signal through a small size focus of 500 µm of diameter, reducing significantly the total amount of collected light. Its success shows that it is possible to monitor the Yb 3+ signal, non-invasively, with a good level of sensitivity highlighting the potential of FL@CD-MOF-161 for in vivo imaging.

In conclusion, taking advantage of the rigid cavity of a MOF to encapsulate an organic fluorophore in a controlled way and at the time of crystals growth, we have synthesized an Yb 3+ -based NIRemitting MOF that can be excited with a low energy wavelength. Quantum yield are higher than those reported by us for other MOF systems with low excitation wavelength. This new MOF system is highly robust, being insensitive to long term storage and to photobleaching. The FL sensitizer remains in the cavity of the MOF. Proof of principle NIR imaging experiments were performed with the excitation and emission light passing through 1 mm of bio-mimicking phantom tissue. High quality pictures were obtained from the NIR signal arising from Yb 3+ . These results highlight the promising potential of FL@CD-MOF-161 for NIR imaging and diagnostic applications and ultimately for in vivo imaging.

The result incorporated into this study have received funding from the Ligue Régionale Contre le Cancer, comité du Loiret et d'Eure-et-Loir, the network «Molécules marines, métabolisme et cancer» from the Cancéropôle Grand Ouest, la Région Centre, l'Agence Nationale de la Recherche (NIRA-ANR-13-BS08-0011), the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 665790. A.H. and S.P. acknowledge support from Erasmus+ Programm (HR ZAGREB01) and the Institut National de la Santé et de la Recherche Médicale (INSERM), respectively.

Figure 1 .

 1 Figure 1. Schematic representation of the NIR-emitting FL@CD-MOF-161. Ligands: black lines; Yb 3+ and its coordination sphere: red and purple; FL: green spheres. The cartoon in the square exemplifies the Yb 3+ sensitization mechanism: the excitation light (< 600 nm) is absorbed by the FL entrapped in the MOF pores followed by the energy transfer to Yb 3+ which emits a narrow NIR signal with a maximum at 980 nm.

Figure 2 .

 2 Figure 2. Optical microscopy images of FL@CD-MOF-161 crystals. (A) Brightfield, objective 20×, scale bar: 50 µm. (B) Brightfield, objective 60×, scale bar: 10 µm. (C) NIR luminescence. λex: 482 nm band pass 35 nm, λem: 996 nm band pass 70 nm, τexp: 100 ms, objective 60×, scale bar: 10 µm.

Figure 3 .

 3 Figure 3. Corrected and normalized (left) excitation (λex = 480 nm) and (right) emission (λem = 980 nm) spectra of FL@CD-MOF-161 crystals at room temperature.

Figure 4 .

 4 Figure 4. NIR fluorescence images of FL@CD-MOF-161 crystals obtained by epifluorescence macroscopy upon excitation with light selected with a 482 band pass 35 nm filter and collecting the Yb 3+ emission with a 996 band pass 70 nm filter. (A) A capillary filled with crystals (Figure S6) was observed through a mask (500 µm of diameter); τexp: 5 s. (B) The same sample was observed through a 1 mm thick bio-mimicking phantom tissue positioned on the top of the mask; τexp: 30 s. Macro-objective 5×; scale bar: 500 µm.

Conflicts of interest

There are no conflicts to declare.

Notes and references


Manon Letiche

Maxime Hallot

Marielle Huve

Thierry Brousse

Pascal Roussel
email: pascal.roussel@ensc-lille.fr.

Christophe Lethien
email: christophe.lethien@iemn.univ-lille1.fr.

Tuning the Cation Ordering with the Deposition Pressure in Sputtered LiMn 1.5 Ni 0.5 O 4 Thin Film Deposited on Functional Current Collectors for Li-Ion Microbattery Applications

The development of high voltage spinel LiMn 1.5 Ni 0.5 O 4 (LMNO) sputtered thin films on functional current collector was reported within the framework of this study. We have first solved the technological issue due to the PtSi phase which originates from the interdiffusion between silicon wafer and chromium/platinum current collector to form PtSi phase under annealing treatment. By substituting the Cr layer with a very dense and pinhole free Al 2 O 3 thin film deposited by ALD acting as a barrier diffusion between the silicon substrate and the LMNO layer, the synthesis process (sputtered thin films annealed under air at 700 °C) has been validated. In the second part of this study, the behavior of the sputtered LMNO thin films as a function of the deposition pressure and the film thickness has been investigated. The deposition pressure has been found to play a key role on the Mn-Ni cations ordering in spinel-like structures (P4 3 32 ordered vs Fd3̅ m disordered spinel) and consequently on the electrochemical performance. We have thus shown that LMNO thin films deposited at 10 -2 mbar and annealed at 700 °C deliver a normalized capacity of 65 μAh•cm -2 •μm -1 with good capacity retention upon cycling thanks to its disordered spinel structure. Moreover, the Coulombic efficiency has been shown to be highly dependent on the film thickness for high voltage LMNO thin film electrodes. To the best of the authors' knowledge, this work is one of the most complete study combining structural and electrochemical characterizations of the LMNO thin film obtained by sputtering deposition on Si/Al 2 O 3 /Pt functional current collectors.

■ INTRODUCTION

Nowadays, connected electronic devices such as smartphones or tablets are widespread within the society. The trends for the next ten years is to move toward the Internet of Thing (IoT) concept where physical devices embedding electronics and sensors are connected together to collect and exchange data. Unfortunately, the main bottleneck slowing the development of smart and miniaturized sensors lies in the fabrication of high performance power sources at the millimeter scale. All solid state Li-ion microbatteries seem to be interesting candidates, but the energy densities of such commercially available devices are limited (0.5 mWh•cm -2 ) in spite of a high technological readiness level (industrial level). Getting autonomous miniaturized and connected sensors will be unachievable if the energy density is not significantly increased.

To improve the energy density of the Li-ion microbattery, an attractive solution consists of increasing the nominal cell voltage using high potential positive electrode material deposited using thin film technology. The use of Ni substituted LiMn 2 O 4 spinel material and their derivatives is really promising. [START_REF] Manthiram | A Perspective on the High-Voltage LiMn1.5Ni0.5O4 Spinel Cathode for Lithium-Ion Batteries[END_REF] Indeed, the spinel LiMn 1.5 Ni 0.5 O 4 (LMNO) exhibits a mean operating potential of 4.75 V vs Li/Li + , a theoretical specific capacity of 147 mAh•g -1 , and a good cycling stability despite the decomposition of standard liquid electrolytes at such a high potential. [START_REF] Aurbach | Review of Selected Electrode-solution Interactions Which Determine the Performance of Li and Li Ion Batteries[END_REF] As the performance of power sources per mass unity is not really meaningful for miniaturized devices, it makes sense to report on the storage capacity using either the footprint area or the volume (footprint × thickness) of the electrode. Therefore, assuming a bulk density of 4.47 g• cm -3 , the corresponding volumetric capacity is found to be close to 65.7 μAh•cm -2 •μm -1 . This density is strongly dependent upon the material porosity. As an example, shadowing effect occurring during sputtering deposition, when adsorbed atoms mobility is low, is known to be responsible for porous thin film morphology. [START_REF] Mahieu | Biaxially Aligned Titanium Nitride Thin Films Deposited by Reactive Unbalanced Magnetron Sputtering[END_REF][START_REF] Freixas | Sputtered Titanium Nitride: A Bifunctional Material for Li-Ion Microbatteries[END_REF][START_REF] Letiche | Sputtered Titanium Carbide Thick Film for High Areal Energy on Chip Carbon-Based Micro-Supercapacitors[END_REF] LMNO thin films have already been deposited by several techniques such as electrostatic spray deposition, [START_REF] Mohamedi | Electrochemical Investigation of LiNi0.5Mn1.5O4 Thin Film Intercalation Electrodes[END_REF] sol-gel, [START_REF] Kunduraci | Synthesis and Characterization of Nanostructured 4.7 V LixMn1.5Ni0.5O4 Spinels for High-Power Lithium-Ion Batteries[END_REF][START_REF] Lee | Structural and Electrochemical Evolution with Post-Annealing Temperature of Solution-Based LiNi0.5Mn1.5O4 Thin-Film Cathodes for Microbatteries with Cyclic Stability[END_REF] pulsed laser deposition [START_REF] Xia | Electrochemical Properties of Nonstoichiometric LiNi0.5Mn1.5O4 -δ Thin-Film Electrodes Prepared by Pulsed Laser Deposition[END_REF][START_REF] Xia | The Influence of Preparation Conditions on Electrochemical Properties of LiNi0.5Mn1.5O4 Thin Film Electrodes by PLD[END_REF][START_REF] Striebel | Electrochemical Studies of Substituted Spinel Thin Films[END_REF] (PLD), and RF magnetron sputtering deposition. [START_REF] Baggetto | Fabrication and Characterization of Li-Mn-Ni-O Sputtered Thin Film High Voltage Cathodes for Li-Ion Batteries[END_REF][START_REF] Li | An Artificial Solid Electrolyte Interphase Enables the Use of a LiNi 0.5Mn1.5O4 5 v Cathode with Conventional Electrolytes[END_REF][START_REF] Su | Structural Evolution of Bias Sputtered LiNi0.5Mn1.5O4 Thin Film Cathodes for Lithium Ion Batteries[END_REF][START_REF] Konishi | Structure and Electrochemical Properties of LiNi0.5Mn1.5O4 Epitaxial Thin Film Electrodes[END_REF] Among these techniques, magnetron sputtering (MS) is widely used for the elaboration of thin film electrodes such as LiMn 2 O 4 , LiCoO 2 , or LiFePO 4 (positive electrodes [START_REF] Tintignac | High Rate Bias Sputtered LiCoO2 Thin Films as Positive Electrode for All-Solid-State Lithium Microbatteries[END_REF][START_REF] Baddour-Hadjean | Raman Microspectrometry Applied to the Study of Electrode Materials for Lithium Batteries[END_REF][START_REF] Xie | Li-Ion Diffusion Kinetics in LiFePO4 Thin Film Prepared by Radio Frequency Magnetron Sputtering[END_REF][START_REF] Zhu | Preparation and Characteristics of LiFePO 4 Thin Film by Radio Frequency Magnetron Sputtering for Lithium Microbatteries[END_REF] ), current collectors (W, Al, Cu), and lithium diffusion barriers [START_REF] Freixas | Sputtered Titanium Nitride: A Bifunctional Material for Li-Ion Microbatteries[END_REF][START_REF] Baggetto | On the Electrochemistry of an Anode Stack for All-Solid-State 3D-Integrated Batteries[END_REF] (TiN), solid electrolytes [START_REF] Hamon | Influence of Sputtering Conditions on Ionic Conductivity of LiPON Thin Films[END_REF][START_REF] Bates | Thin-Film Lithium and Lithium-Ion Batteries[END_REF][START_REF] Pichonat | Further Studies on the Lithium Phosphorus Oxynitride Solid Electrolyte[END_REF] (LIPON), and Au, Ag, Si, or CuO for the negative electrodes. [START_REF] Taillades | Metal-Based Very Thin Film Anodes for Lithium Ion Microbatteries[END_REF][START_REF] Phan | High-Performance All-Solid-State Cells Fabricated With Silicon Electrodes[END_REF][START_REF] Pecquenard | Thorough Characterization of Sputtered CuO Thin Films Used as Conversion Material Electrodes for Lithium Batteries[END_REF] An allsolid-state thin film microbattery can be described as a stack of successive layers including the adhesive/insulating layer, the current collectors, the active materials (negative and positive electrodes separated by a solid electrolyte), and protective layers (hence at least seven layers). While "classical" studies focus only on the deposition of active material on commercial current collectors (Pt, Ti, Cu, stainless steel, or Al foils [START_REF] Xia | The Influence of Preparation Conditions on Electrochemical Properties of LiNi0.5Mn1.5O4 Thin Film Electrodes by PLD[END_REF][START_REF] Striebel | Electrochemical Studies of Substituted Spinel Thin Films[END_REF][START_REF] Morcrette | Non-Stoichiometry in LiMn2O4 Thin Films by Laser Ablation[END_REF][START_REF] Reddy | Thin Films of Vanadium Oxide Grown on Vanadium Metal[END_REF][START_REF] Cotte | Lithium-Rich Manganese Oxide Spinel Thin Films as 3 V Electrode for Lithium Batteries[END_REF] ), we propose in this paper to investigate the stacking of the first layers (from the substrate to the positive electrode) to fully understand, and further to control, the influence of the underlying layer (interdiffusion process, preferred orientation) on the active material of the microbattery. The ultimate goal is actually to develop a synthesis process easy scalable using microelectronic manufacturing.

Subsequently, we mainly focused on the elaboration of LMNO thin film by radiofrequency (RF) MS deposition. Among all the synthesis parameters (RF power, deposition time, pressure, temperature, etc.), a preliminary study has shown that the deposition pressure within the chamber and the afterward annealing temperature were key parameters to meet the electrochemical performance requirements. The investigation of the deposition pressure on the microstructure, the surface morphology, the Mn-Ni cation ordering in the structure of LMNO thin films deposited by MS, and the electrochemical performance are reported in the present paper. The influence of the film thickness is also reported as a key parameter that can improve both the surface capacity and the Coulombic efficiency of Li-ion microbatteries.

■ EXPERIMENTAL SECTION Film Preparation. Prior to LMNO thin films deposition, layers of Cr and Pt were first evaporated on (100) silicon wafer as adhesive and current collector layers, respectively. Consequently to the formation of an unexpected phase (PtSi) responsible for poor electrochemical performances of the material when the LMNO thin film is annealed at high temperature, the Cr layer was replaced by an Al 2 O 3 layer deposited by atomic layer deposition (ALD) in a Picosun R200 reactor.

LMNO thin films were deposited by radiofrequency magnetron sputtering. The CT 200 cluster sputtering equipment has been used to achieve the thin film deposition after pumping to a base vacuum of 10 -7 mbar. Homogeneous deposits were obtained by keeping the distance between the target and the substrate at 60 mm. A pure LiMn 1.5 Ni 0.5 O 4 target (99.999%, 10 cm diameter, 6 mm thick) was sputtered under argon atmosphere in the CT 200 sputtering equipment (Alliance Concept). Four operating pressures have been studied: 6 × 10 -3 , 10 -2 , 5 × 10 -2 , and 9 × 10 -2 mbar, respectively. The argon flow rate is kept constant close to 50 sccm for all the deposition pressures as well as the RF power used to bias the target (2.7 W•cm -2 ). In a preliminary study, three annealing temperatures were investigated (650, 700, and 750 °C). The optimized electrochemical performances of the LMNO thin films were achieved using a 700 °C annealing temperature. Subsequently, LMNO thin films were annealed under air atmosphere at 700 °C for 2 h (dwell) with a heating/cooling ramp of 5 °C/min. Finally, the deposition time has been tuned to deposit LMNO thin films with different thicknesses.

Sample Morphological and Structural Characterization. The thickness and microstructure of the films were determined using the scanning electron microscope (SEM Zeiss supra 55-VP). The surface morphology was analyzed by AFM Dimension 3100 ICON (Bruker) measurements operated under ambient conditions. Images were acquired in the tapping/peak force tapping mode with a Mikromach tap 300-Al cantilever. The scanning rate was 0.5/1 Hz, and 5 μm × 5 μm images were obtained. The mechanical stress has been investigated by measuring the curvature radius of a 3 in. Si(100) wafer before and after deposition of LMNO thin film. A Rigaku SMARTLAB multipurpose six-axis diffractometer (9 kW rotating anode) using Cu Kα radiation (λ = 1.5418 Å) and a Horiba Jobin Yvon LabRam HR UV Raman spectrometer with a 471 nm UV laser source were used to identify the structure type and the polymorph (ordered/disordered) of the films, respectively. XRD in plane pole figures was recorded as a series of phi values (from 0 to 360°) at different chi values (from 0 to 90°) for a fixed 2 theta angle depending on the recorded planes (2 theta = 36.44°for the 311 peak and 44.31°for the 400 ones). Transmission electron microscopy (TEM) studies were performed on FEI Technai G220 and Philips CM30 microscopes equipped with an EDS spectrometer. Cross-sectional samples were prepared by focused ion beam (FIB) technique. Li, Ni, and Mn contents were measured Electrochemical Characterization. Electrochemical measurements were conducted in homemade flat cells operated in an Ar filled glovebox. The cells were assembled with LMNO thin film as positive electrode and pure Li as counter and reference electrode. A mixture of 1 M LiClO 4 dissolved in ethyl carbonate (EC)/dimethyl carbonate (DMC) (1:1) was used as the liquid electrolyte. The electrochemical properties of LMNO thin films deposited at different pressures and annealed at 700 °C were characterized. Galvanostatic cycling, cyclic voltamperometry, and electrochemical impedance spectroscopy (EIS) were performed with a Biologic VMP3 potentiostat/galvanostat. Electrochemical impedance spectroscopy spectra were recorded between 100 kHz and 5 mHz with an AC voltage of 10 mV. This frequency range is restricted between 500 Hz and 5 mHz when ionic conductivity and diffusion coefficient are extracted from the slope of the Warburg impedance as a function of the square root of the angular frequency (linear region).

■ RESULTS AND DISCUSSION

Microstructural and Morphological Characterization of As Deposited Thin Films. Figure 1A-D depicts the SEM cross section and the AFM top surface observations of as deposited LMNO thin films obtained at different pressure conditions (0.4 μm thick). A microstructural evolution is clearly evidenced.

The melting temperature of the LMNO is close to the one of the LiMn 2 O 4 spinel compound [START_REF] Ledwaba | Molecular Dynamics Simulations of Spinels. LiMn2O4 and Li4Mn5O12 at High Temperatures[END_REF] (1700 K). Consequently, the T/T m ratio is lower than 0.2 for LMNO deposited at room temperature, and as expected, the film morphology is found to be in zone T from the Thornton [START_REF] Thornton | The Microstructure of Sputter-Deposited Coatings[END_REF] structure zone model (SZM). As a matter of fact, at low pressure, the LMNO films exhibit highly dense and homogeneous morphologies, whereas at higher pressures, the microstructure is found to be more columnar and porous (Zone 1 of the Thornton SZM). The observed morphology at low pressure results from the film densification which is a direct consequence of the atomic peening effect where high energy atoms strike the film surface under growth and induce atomic resputtering in the column boundaries. [START_REF] Detor | Stress and Microstructure Evolution in Thick Sputtered Films[END_REF] The surface analysis as a function of the deposition pressure is depicted in the insets of Figure 1 A-D. At low pressures (6 × 10 -3 and 10 -2 mbar), few small grains randomly distributed can be similarly noticed on the surface. Hence the surface roughness is very low (6-8 nm). At higher pressures (5 × 10 -2 and 9 × 10 -2 mbar), the size and the number of the random grains is increased which significantly enhances the surface roughness (20 nm). The films were then annealed under air atmosphere for 2 h at 700 °C. The SEM cross section analysis of the corresponding films is depicted in Figure 1 E-H. At low pressure, the microstructure is kept dense but a columnar-like structure is highlighted. When the pressure is increased, the morphology of annealed thin films moves from a porous columnar-like structure to a porous granular-like structure.

One attractive solution to increase the energy density of the planar thin film microbattery is to deposit a thicker layer of active material. However, the LMNO microstructure has to remain stable, i.e., without any cracks or failures, a problem often encountered when higher thickness is achieved. Indeed, it is well-known that the mechanical stress of dense thin films should be carefully tuned when the level of pressure is significantly low. Regarding the film morphology observed in Figure 1, a dense LMNO layer is obtained at low deposition pressure and the film densification is a consequence of the ion peening effect. Subsequently, the growth stress induced by the LMNO deposition on the silicon wafer has been investigated. For that purpose, the curvature radius before and after LMNO thin film deposition has been measured. The growth stress σ growth is calculated according to Stoney formula 1:

σ ν = - - ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ E t t R R 1 6 1 1 1 Si growth Si Si 2 LMNO LMNO Si (1)
where E Si is the Young modulus of the substrate (130 GPa for the silicon wafer), ν Si is the Poisson coefficient of the silicon substrate (0.27), and t Si and t LMNO are respectively the thickness of the substrate (380 ± 15 μm) and the deposited film (0.4 μm in our deposition conditions). The bending radius of the LMNO (R LMNO ) is measured just after the sputtering deposition and compared with the initial one (R Si ), i.e., before the deposition.

As it is shown in Figure 2A, the thin film deposited at 6 × 10 -3 mbar exhibits a highly compressive intrinsic stress around -1350 MPa. Then, the growth stress is relaxed starting from a critical pressure located between 10 -2 and 5 × 10 -2 mbar, which is a true asset to reach high thickness without any cracks or failures in the films. The deposition rate is also slightly affected, decreasing from 11 down to 8 nm•min -1 when the pressure is increased (Figure 2B). For similar deposition time, the LMNO films are thus thinner at high pressure. From this analysis, it is clear that deposition pressure is a key parameter allowing the microstructure and the morphology of the LMNO films to be tailored while keeping the thin film strongly adhesive on the substrate. The stress of the sputtered LMNO thin films has been reduced after the annealing process as already observed in refs 33 and 34 on other thin film materials (B 4 C, W), probably owing to the formation of voids within the From this study, an annealing temperature of 700 °C seems to maximize the normalized capacity.

thin films as depicted on the SEM cross sections shown in Figure 1 or due to the crystallization process. Subsequently, an in-depth study dealing with the influence of the pressure on LMNO structural and electrochemical properties is proposed in the following paragraphs. The LMNO morphology exhibits a porous shape after an annealing at 700 °C as depicted in the SEM cross sections reported in Figure 1. Such morphology is quite interesting when the LMNO thin film is cycled in liquid electrolyte because the liquid fills the porous morphology. Consequently, the surface capacity is increased as compared to the electrochemical performance reached with dense thin films. This remark is questionable for future solid state microbatteries based on sputtered thin films. The most investigated solid electrolyte is LiPON where 1 μm-thick layer [START_REF] Bates | Thin-Film Lithium and Lithium-Ion Batteries[END_REF][START_REF] Pichonat | Further Studies on the Lithium Phosphorus Oxynitride Solid Electrolyte[END_REF] has to be deposited to act as good electronic insulator and a good Li + ionic conductor between the two electrodes. Sputtering technique is known to be unadaptable to achieve the deposition of a uniform/homogeneous coating inside micro/mesoporous structure. As a matter of fact, the thickness of the sputtered LMNO will not be totally active if LiPON deposition is performed on such a porous thin film because the LiPON deposition will not enter through the entire film thickness. Nevertheless, the actual trend is to reduce the electrolyte thickness in order to decrease the resistivity of the full cell and to produce a pinhole free layer. Atomic layer deposition (ALD) of lithiated solid electrolyte [START_REF] Letiche | Atomic Layer Deposition of Functional Layers for on Chip 3D Li-Ion All Solid State Microbattery[END_REF][START_REF] Kozen | Atomic Layer Deposition of the Solid Electrolyte LiPON[END_REF][START_REF] Nisula | Atomic Layer Deposition of Lithium Phosphorus Oxynitride[END_REF] is now widely developed. As ALD is known to be a powerful technique to coat micro-/ mesoporous structure with a pinhole free thin layer (<100 nm), such LMNO morphology is quite interesting when combined with ALD deposited solid electrolyte. We have recently demonstrated the growth of 10 nm-and up to 60 nm-thick Li 3 PO 4 solid electrolyte on a 3D complex surface to act as a good ionic conductor and electronic insulator for a thin film microbattery. [START_REF] Letiche | Atomic Layer Deposition of Functional Layers for on Chip 3D Li-Ion All Solid State Microbattery[END_REF] Classical techniques such as BET measurement could not be used to evaluate the porosity of thin films due to the small quantity of active material as compared to bulk material. Hence, we evaluate the bulk density of thin film (g•cm -3 ) by measuring the mass of the substrate before and after the deposition using a XP6U Mettler Toledo microbalance (±0.1 μg accuracy). Taking into account the surface of the sample as well as the thickness of the layer, the volume of active material is deduced. The measured bulk density is estimated to be 80% of the theoretical value.

Influence of the Substrate: Formation of the PtSi Phase at High Temperature Annealing. First, LMNO thin films (0.4 μm-thick) were deposited at 10 -2 mbar on Cr (20 nm)/Pt (50 nm) coated silicon wafer, i.e., a typical electrical current collector used in microelectronic applications. The CVs of the as-deposited and 700 °C annealed LMNO samples are displayed in Figure 3A. If the electrochemical signature is the one expected for the nonannealed sample (absence of redox peaks within the studied electrochemical windows between 3.8 V (or 4.2 V) and 4.9 V vs Li/Li + ), the annealed sample has to be electrochemically active.

To answer this question, this issue has been investigated by X-ray diffraction analysis (XRD). From the corresponding patterns (Figure 3B) it is clear that the only visible diffraction peak is due to the (111) plane of the platinum layer as expected on the as-deposited sample, leading to the conclusion that this sample is amorphous (red curve). After a 2 h annealing at 700 °C under air atmosphere, reflection peaks related to the LMNO spinel structure (JCPDS PDF 01-080-2162) can be observed (as awaited), but two additional phases are also evidenced. Indeed, another lithiated phase Li 2 MnO 3 (JCPDS PDF 00-027-1252) can be identified as well as an unexpected PtSi (JCPDS PDF 01-083-0152) phase triggering a significant decrease of the platinum (111) Bragg peak at 2θ = 40°. However, from an electrochemical point of view, none of these phases should be responsible for the bad electrochemical behavior of the film annealed at 700 °C. Indeed, Li 2 MnO 3 exhibits an irreversible Li 2 O loss above 4.6 V, and PtSi has no electrochemical activity within this potential window. However, the PtSi phase is likely damaging the current collector. In situ HT-XRD (high temperature X-ray diffraction) analysis has thus been undertaken on the as-deposited sample to investigate this PtSi issue vs crystallization of the LMNO thin films. XRD patterns have been recorded every 25 °C from 50 to 800 °C during annealing treatment in order to determine the annealing temperature required to obtain the LMNO phase without PtSi formation. Three steps are clearly highlighted (Figure 3C). From 50 to 225 °C the thin film is amorphous, and only the platinum reflection can be seen. From RT to 575 °C the (111) reflection peak of LMNO is raising. At 575 °C, the LMNO spinel structure is identified as well as the Li 2 MnO 3 phase and PtSi additional phases while the intensity of the (111) Pt peak drastically declines. This HT-XRD experiment clearly provides evidence that it is impossible to obtain only the spinel-like structure avoiding the PtSi phase formation. According to these results, we replaced the Cr adhesive layer by an Al 2 O 3 (70 nm) thin film deposited by ALD, which is expected to be very dense and able to prevent the Pt-Si interdiffusion responsible for the formation of the PtSi alloy. The XRD analysis corresponding to this new Si/Al 2 O 3 /Pt/LMNO sample annealed at 700 °C is depicted in Figure 3D. Subsequently, only the reflection peaks of the platinum layer and those of the LMNO cathode (with a small contribution of the Li 2 MnO 3 phase) are visible. Thus, the PtSi issue has been successfully solved. To the author's knowledge, this issue has never been evidenced in this context before and demonstrates that a careful control of the stacked layers has to be carried out in order to design sustainable microbatteries. Figure 3E displays the typical electrochemical signature of LMNO during the first cycle showing a redox activity around 4.7 V vs Li/Li + . The double peaks in the CV appearing during the oxidation (4.7 and 4.8 V vs Li/Li + ) and reduction processes (4.65 and 4.70 V vs Li/Li + ) are related to the reversible redox activity of the nickel transition metal from Ni 2+ to Ni 4+ . An additional oxidation peak is depicted at 4.6 V vs Li/Li + owing to the presence of Li 2 MnO 3 phase and subsequent oxygen evolution. [START_REF] Yu | Electrochemical Activities in Li2MnO3[END_REF] One pair of minor peaks is found around 4 V vs Li/Li + and could be attributed to the redox reaction of Mn 3+/4+ which corroborates the existence of a small amount of Mn 3+ ions inside the film. Those peaks have already been observed on bulk and thin film LiMn 1.5 Ni 0.5 O 4 electrodes owing to nickel or oxygen deficiency. [START_REF] Xia | The Influence of Preparation Conditions on Electrochemical Properties of LiNi0.5Mn1.5O4 Thin Film Electrodes by PLD[END_REF][START_REF] Lee | Influence of Doping on the Cation Ordering and Charge -Discharge Behavior of LiMn1.5Ni0.5O4 Spinel between 5 and 2 V[END_REF] The electrochemical signature of the sputtered LMNO thin films also suggests that the PtSi phase was responsible for the inhibition of the LMNO electrochemical signal as previously observed. In Figure 3F, the evolution of the potential as a function of the normalized capacity between 4.4 and 4.8 V vs Li/Li + is reported at a 1C cycling rate regarding the annealing temperature (the second cycle is shown). These constant current charge/discharge galvanostatic measurements of a sample deposited at 10 -2 mbar confirm that 700 °C is the optimal annealing temperature to maximize the normalized capacity (65 μAh•cm -2 •μm -1 ).

Structural Characterization of LMNO Thin Films. A full structural characterization has been undertaken with three complementary techniques (transmission electron microscopy, X-ray diffraction, and Raman spectroscopy) enabling an imaging of the film at different scales, going from long-range (XRD) to a short one (TEM).

The TEM images reported in Figure 4 on a cross section obtained by the FIB technique (focused ion beam) shows the different stacked layers on silicon wafer where the thicknesses have been measured close to 85 nm for Al 2 O 3 , 30 nm for the Pt current collector, and 480 nm for the LMNO positive electrode (Figure 4A). The chemical mapping analysis (EDS-STEM) has been performed on the cross section (Si turquoise blue, Al yellow, Pt red, Mn blue, Ni green, Ti pink). It clearly confirms the absence of interdiffusion between the stacked layers: the PtSi alloy formation issue is no longer depicted and neither is any other diffusion from a layer to another, thus evidencing the pertinence of our layer stacking design. Another interesting point is observed in Figure 4B, where we can see that the LMNO layer exhibits a polycrystalline behavior with a different crystallized orientation (domains of about 20 nm), except for a thickness of 26.5 nm along the Pt layer where a (111) preferred orientation is observed. To discriminate between epitaxial or fiber texture behavior, X-ray diffraction pole figure analysis has been performed. This study will be detailed in a following section depicting the influence of the thickness on electrochemical performance, but by anticipation, this first layer is a (111) fiber-textured one, not epitaxial.

X-ray diffraction measurements were realized in Bragg-Brentano reflection configuration using Ni-filtered copper Kalpha radiation. In order to amplify the intensity of the LMNO layer, 2θ/ω scans were done with a ω shift of 5°to inhibit the signal issued from the silicon substrate. XRD patterns of LMNO thin films deposited at different pressures and annealed at 700 °C are displayed in Figure 5A,B. For the sake of clarity, two sets of diffractogramms (with intensity normalized to the (111) peak of LMNO at 18.8°) are reported with different sets of deposition pressure. For all the samples (Figure 5A,B), the main reflection peaks at 18.8°, 36.4°, 38.1°, 44.3°, 48.5°, 58.8°, and 64.5°are assigned to the LMNO spinel structure (JCPDS PDF 01-080-2162). The peak corresponding to the (111) diffraction plane of the platinum current collector (JCPDS PDF 00-004-0802) is observed at 40°, while the peak at 36°( highlighted on the four XRD patterns) is also due to the (111) platinum layer, but to the K-β X-ray emission line that is not completely filtered. The crystallite size has been evaluated between 16 and 20 nm according to the Scherrer formula, whatever the pressure is. Moreover, at low pressure (Figure 5A), an additional Li 2 MnO 3 phase (JCPDS PDF 00-027-1252) is depicted from the (020) diffraction peak located at 21°. A preferential orientation is also observed according to the pressure. Indeed, since the entire patterns are normalized on the peak at 18.8°((111) diffraction plane), Figure 5A,B clearly shows an increase of the peak intensity at 44.3°((400) diffraction plane) when the pressure increases from 6 × 10 -3 up to 9 × 10 -2 mbar. As a matter of fact, the I (400) /I (111) ratio moves from 0.04 at low pressure to 2.83 at high pressure (i.e., a factor of ∼70). Except for this preferred orientation, all the diffractogramms of the four samples are similar. At this stage of the study, Mn-Ni cation ordering/disordering on the LMNO spinel structure is suspected to have a significant influence on the electrochemical performance. However, with Cu based wavelength, it is impossible to distinguish using XRD the ordered spinel structures from the disordered one. Even if the space groups of the two polymorphs are different (P4 3 32 vs Fd3̅ m), Cu XRD patterns are the same because scattering factors at the Cu edge are too close to be discriminant. To overcome this problem, Raman spectroscopy was used to further investigate these samples (Figure 5C,D). At this stage of the structural analysis, a question is raised about knowing whether the deposition pressure directly influences the orientation of the thin film or if the change in deposition rate which is induced by the deposition pressure affects the orientation of the films. As shown in Figure 2, the deposition rate has been evaluated between 8 and 11 nm•min -1 depending on the deposition pressure. Consequently, the deposition time has been tuned for the four samples being tested to have the same thickness as depicted on the SEM cross sections of Figure 1. To the author's knowledge, deposition pressure is responsible for the densification of sputtered thin films owing to the high energy of the condensed species within the plasma [START_REF] Letiche | Sputtered Titanium Carbide Thick Film for High Areal Energy on Chip Carbon-Based Micro-Supercapacitors[END_REF][START_REF] Detor | Stress and Microstructure Evolution in Thick Sputtered Films[END_REF] (atomic peening effect). At low pressure, high From this analysis, the atomic plane seems to be well organized with a preferential orientation perpendicular to the substrate for the first 26.5 nm (out of plane analysis). Above this threshold, the LMNO exhibits a polycrystalline behavior.

F energy atoms strike the film surface under growth and induce atomic resputtering in the column boundaries leading to dense thin films. Competitive growth between several lattice planes occurs during the sputtering deposition of LMNO layers, and pressure directly changes the orientation of the thin films as shown on the diffractogramms reported in Figure 5.

Indeed, since Raman spectroscopy clearly enables distinguishing between Mn-O bonds and Ni-O ones, it has been used to identify which polymorph of the spinel phases has been formed. First, all the Raman peak intensities are higher in an ordered spinel (hereafter denoted by its space group, i.e., P4 3 32) than in a desordered one (space group Fd3̅ m). Moreover, it is known [START_REF] Kunduraci | Synthesis and Characterization of Nanostructured 4.7 V LixMn1.5Ni0.5O4 Spinels for High-Power Lithium-Ion Batteries[END_REF][START_REF] Amdouni | Structure and Insertion Properties of Disordered and Ordered LiNi 0.5Mn1.5O4 Spinels Prepared by Wet Chemistry[END_REF] that an increase in the Raman peak intensity corresponding to the Ni-O band (166 and 410 cm -1 ) occurs with ordering (P4 3 32), i.e., at high pressure (Figure 5C), while samples deposited at low pressure correspond to the disordered spinel structures (Fd3̅ m). The Raman spectra of the four samples under test clearly evidence the A 1g , F 2g [START_REF] Manthiram | A Perspective on the High-Voltage LiMn1.5Ni0.5O4 Spinel Cathode for Lithium-Ion Batteries[END_REF] , F 2g [START_REF] Aurbach | Review of Selected Electrode-solution Interactions Which Determine the Performance of Li and Li Ion Batteries[END_REF] , and F 2g [START_REF] Mahieu | Biaxially Aligned Titanium Nitride Thin Films Deposited by Reactive Unbalanced Magnetron Sputtering[END_REF] peaks assigned to the vibration modes of a cubic spinel type structure. [START_REF] Allen | Chemical Characterization of Transition Metal Spinel-Type Oxides by Infrared Spectroscopy[END_REF] Nevertheless, a slight shift of these modes is visible as a function of the pressure. Those shifts have already been observed when a comparison is done between disordered vs ordered spinel compounds. [START_REF] Amdouni | Structure and Insertion Properties of Disordered and Ordered LiNi 0.5Mn1.5O4 Spinels Prepared by Wet Chemistry[END_REF][START_REF] Wang | A Comparative Study of Fd-3m and P4332 "LiNi0.5Mn1.5O4[END_REF] From the Raman spectra of the low pressure samples (Figure 5D), the presence of an additional Li 2 MnO 3 phase is confirmed by a set of seven peaks assigned [START_REF] Julien | Lattice Vibrations of Materials for Lithium Rechargeable Batteries III. Lithium Manganese Oxides[END_REF] to the phonon vibrations (248, 308, 330, 372, 412, 433, and 565 cm -1 ).

The lattice parameters of LMNO (Figure 5E) issued from Rietveld refinement are found to decrease from 8.167 down to 8.105 Å when the deposition pressure increases. This last conclusion has a significant importance as it is known 7 that disordered spinel LMNO structures have a larger volume cell than the ordered spinel LMNO one. In such a way, the Fd3̅ m disordered spinel LMNO phase seems to be formed at low pressure while a P4 3 32 ordered one is obtained at higher pressure, which corroborates the conclusion issued from Raman analysis. However, according to ref 44, nanodomains of ordered or disordered spinel can occur in our LMNO thin films and we cannot exclude a fraction of the ordered phase in a disordered matrix and vice versa.

The atomic ratio Mn/Ni and Mn/Li are found to increase when the deposition pressure increases from 6 × 10 -3 to 9 × 10 -2 mbar (Figure 5E).

Electrochemical Characterizations of the LMNO Thin Films. In order to have a better understanding of the diffusion process of the lithium ion in thin film LMNO electrodes, an electrochemical impedance spectroscopy (EIS) study was undertaken on the four samples with different deposition pressures. For each sample, EIS has been performed between 100 kHz and 5 mHz at 4.6 V vs (Li/Li + ) after 10 CV cycles. Such a frequency range is restricted between 500 Hz and 5 mHz to plot the evolution of the Warburg impedance as a function of angular frequency in the linear region. Nyquist plots are represented in Figure 6A. Again, clear differences can be noticed between low and high pressures. Disordered spinel exhibits faster kinetics than the ordered one with an area specific resistance (ASR) contribution of 200, 600, 2500, and 2400 Ohm•cm 2 for the 6 × 10 -3 , 10 -2 , 5 × 10 -2 , and 9 × 10 -2 mbar samples, respectively. The ASR obtained for the disordered spinel is comparable to the value reported by the Dudney 12 et al. and Mohamedi 6 et al. groups. All plots have been fitted according to the equivalent circuit proposed in Figure 6B. This model inspired from previous works [START_REF] Aurbach | Common Electroanalytical Behavior of Li Intercalation Processes into Graphite and Transition Metal Oxides[END_REF] consists of (i) an electrolyte bulk resistance, (ii) a capacitance and resistance coming from the electrolyte, (iii) the Li/electrolyte interface, (iv) the LMNO films (charge transfer resistance coupled with a double layer capacitance of the electrolyte/ LMNO interface, (v) a Warburg impedance coming from the diffusion the electroactive species, and (vi) an accumulation capacitance describing lithium accumulation at very low frequencies.

Under semi-infinite conditions, the apparent chemical diffusion coefficient of the lithium ion (D app ) can be calculated from the Warburg impedance analysis [START_REF] Ho | Application of A-C Techniques to the Study of Lithium Diffusion in Tungsten Trioxide Thin Films[END_REF] which is expressed in the complex plane using

σ ω ω = - = - - - Z j V AFn D E x j (1 ) 2 d d (1 )
where V m is the molar volume of the LMNO, F the Faraday constant, n the charge transfer number, A the surface area of the film, E the potential, and x the lithium concentration. Finally, after a series of equation transformations, σ can be expressed [START_REF] Mohamedi | Electrochemical Investigation of LiNi0.5Mn1.5O4 Thin Film Intercalation Electrodes[END_REF] as

σ = Q Ml nFC D 2 i acc app (4)
where Q i is the capacity of the LMNO layer (147 mAh•g -1 ), M the molecular weight of LMNO (183 g•mol -1 ), l the diffusion pathway (corresponding to the film thickness), and C acc the accumulation capacitance obtained with the fitting procedure. σ (ionic conductivity) can be obtained from the slope of the Warburg impedance as a function of the square root of the angular frequency (Figure 6C). After calculation, the apparent diffusion coefficient D app is found close to 2 × 10 -10 cm 2 •s -1 for the disordered spinel and 3 × 10 -11 cm 2 •s -1 for the ordered one, almost 1 order of magnitude more, which reflects the resulting kinetics. Note that, at this potential, these values are in good agreement with those reported by Xia The electrochemical analysis of the sputtered LMNO as a function of the operating pressure is displayed in Figure 7. Cyclic voltammetry (CV) is first performed on the four samples at a scan rate of 0.2 mV•s -1 in 1 M LiClO 4 in EC/DMC (1/1) between 3.8 and 4.9 V vs Li/Li + . In Figure 7A,B, a small redox peak due to the Mn 3+ /Mn 4+ redox couple is detected, thus highlighting the presence of a small amount of Mn 3+ in the sample, whatever the pressure is. This redox activity is generally attributed to oxygen deficiency in the films owing to the high temperature of the post deposition annealing. Indeed, to compensate the oxygen deficiency, a small amount of Mn 4+ is reduced to Mn 3+ . Similar results have been observed previously on bulk or thin film LMNO electrodes. [START_REF] Manthiram | A Perspective on the High-Voltage LiMn1.5Ni0.5O4 Spinel Cathode for Lithium-Ion Batteries[END_REF][START_REF] Xia | Electrochemical Properties of Nonstoichiometric LiNi0.5Mn1.5O4 -δ Thin-Film Electrodes Prepared by Pulsed Laser Deposition[END_REF][START_REF] Xia | The Influence of Preparation Conditions on Electrochemical Properties of LiNi0.5Mn1.5O4 Thin Film Electrodes by PLD[END_REF] In Figure 7A, only two oxidation peaks at high potential (4.7 and 4.8 V) are depicted during the first cycle demonstrating the change of the oxidation state of the Nickel element (from Ni 2+ to Ni 3+ and Ni 3+ to Ni 4+ ). Only one large peak (4.6 V vs Li/Li + ) with a small shoulder is visible during the reduction process which is characteristic of ordered LMNO spinel [START_REF] Wang | A Comparative Study of Fd-3m and P4332 "LiNi0.5Mn1.5O4[END_REF] where the double peaks are closer than in the disordered spinel.

In Figure 7B, two sets of redox peaks (4.72/4.82 V and 4.65/ 4.7 V vs Li/Li + ) have been found at low deposition pressure during the first cycle, thus indicating the redox activity of the nickel (Ni 2+ /Ni 3+ and Ni 3+ /Ni 4+ ) in the LMNO film. An oxidation peak is depicted at 4.6 V vs Li/Li + only at low deposition pressure and is attributed to the presence of Li 2 MnO 3 additional phase already observed by XRD and Raman analyses and occurring only for disordered spinel phase (low deposition pressure). As no reduction peak is observed in the similar region, this oxidation reaction is irreversible. When the pressure is increased, the oxidation peaks attributed to the nickel element are shifted toward higher potentials while reduction peaks shift toward lower potentials, thus indicating that the deposition pressure is a crucial parameter to stabilize the initial oxidation state of the nickel element. During the second cycle (Figure 7C), the oxidation peak at 4.6 V vs Li/Li + is no longer observed, indicating that the Li 2 MnO 3 additional phase only exhibits electrochemical activity during the first cycle.

Cyclic voltammetry at different sweep rates has been used to check the lithium transport within the LMNO thin film deposited at 10 -2 mbar (Figure 7D). Not surprisingly, the peak current is linearly proportional to the square root of the sweep rate indicating that the lithium transport is under diffusion control (Figure 7E). The b-coefficient issued from the power law (I = aν b , where a and b are arbitrary coefficients and ν is the sweep rate in mV•s -1 ) is equal to 0.524 ± 0.100 for anodic and cathodic currents meaning that the stored charge within the LMNO layer arises from a semi-infinite diffusion process.

Galvanostatic measurements were also performed on these samples. The first goal was to determine the suitable electrochemical window providing a stable cycling behavior of the LMNO thin films. When the samples are cycled between 3.8 and 4.8 V vs Li/Li + a small plateau is observed at 4.2 V vs Li/Li + typically assigned to the Mn 3+ /Mn 4+ redox couple. [START_REF] Kunduraci | Synthesis and Characterization of Nanostructured 4.7 V LixMn1.5Ni0.5O4 Spinels for High-Power Lithium-Ion Batteries[END_REF][START_REF] Xia | Electrochemical Properties of Nonstoichiometric LiNi0.5Mn1.5O4 -δ Thin-Film Electrodes Prepared by Pulsed Laser Deposition[END_REF] The small amount of Mn 3+ within the LMNO thin film is disproportionated when cycling in liquid electrolyte leading to the formation of Mn 2+ and Mn 4+ cations that is accompanied by the loss of soluble Li 2 O and MnO into the solution. [START_REF] Xia | The Influence of Preparation Conditions on Electrochemical Properties of LiNi0.5Mn1.5O4 Thin Film Electrodes by PLD[END_REF] This behavior was typically observed for thin films deposited by PLD or RF sputtering where oxygen loss could occur during the vacuum deposition process or during the high temperature annealing step. [START_REF] Xia | The Influence of Preparation Conditions on Electrochemical Properties of LiNi0.5Mn1.5O4 Thin Film Electrodes by PLD[END_REF] For the stoichiometric LiMn 1.5 Ni 0.5 O 4 layer, the manganese is at a +4 oxidation state while the nickel is at +2. The surface discharge capacity of the sample deposited at 10 -2 mbar (Figure 7F) is found to be close to 22 μAh•cm -2 at 1C, but this capacity quickly fades from the 20th cycle.

When the electrochemical window is reduced progressively from 3.8-4.8 to 4.4-4.8 V vs Li/Li + , the discharge capacity at 1C exhibits a good cycling behavior during the first 50 cycles and reaches 21 μAh•cm -2 . The evolution of the normalized capacity (in μAh•cm -2 •μm -1 ) within 4.4-4.8 V vs Li/Li + is reported in Figure 7G for the four samples under test at 1C. On this figure, two different behaviors regarding the operating pressure are observed again. The low-deposition samples exhibit the highest normalized capacity close to the theoretical one (66 μAh•cm -2 •μm -1 taking into account a bulk density of 4.5 g•cm -3 and a gravimetric capacity of 147 mAh•g -1 ). Nevertheless, the retention capacity seems to be better for the sample deposited at 10 -2 mbar as compared to the one deposited at 6 × 10 -3 mbar. The capacity reached by the two samples deposited at higher pressure is equal to 40 μAh• cm -2 •μm -1 demonstrating the superior performance of the disordered spinel as compared to the ordered one. Figure 7H depicts the evolution of the potential as a function of the normalized capacity at the 20th cycle (1C rate) which clearly illustrates the redox activities of the nickel (Ni 2+ /Ni 3+ and Ni 3+ / Ni 4+ ) within the LMNO thin films at different pressures. The charge capacity is higher than the discharge one for all the samples. The irreversible capacity is never whittled down to zero, which can be attributed to a continuous electrolyte decomposition over 4.3 V vs (Li/Li + ) during cycling. At such high voltage, unwanted electrolyte oxidation occurs with standard liquid electrolytes leading to the decomposition of inorganic compounds such as lithium salt and organic solvent. [START_REF] Aurbach | Review of Selected Electrode-solution Interactions Which Determine the Performance of Li and Li Ion Batteries[END_REF] These decomposed products form a layer at the electrode surface acting as a solid electrolyte interphase (SEI). Consequently, low Coulombic efficiency is generally obtained for such a high voltage electrode with conventional liquid electrolyte. In our case, for the four films under test, the Coulombic efficiency is measured close to 75% (with a 400 nm layer), in good agreement with results reported by the group of Dudney. [START_REF] Baggetto | Fabrication and Characterization of Li-Mn-Ni-O Sputtered Thin Film High Voltage Cathodes for Li-Ion Batteries[END_REF] To understand this relatively bad efficiency, one has to come back to the Coulombic efficiency definition, i.e., Q discharge /(Q charge + Q SEI ). On one hand, for bulk electrodes (powder or thick layer), the amount of active material is high (>10 mg•cm -2 ) and the thickness (20 nm) of the SEI (and hence its negative contribution) is negligible as compared to the thickness of the active layer (>100 μm). On the other hand, for the thin film (0.5 μm thick), the amount of active materials is low (0.2 mg•cm -2 ), and its thickness has a strong influence onto the Coulombic efficiency: low Coulombic efficiency (75%) has been reported by Dudney et al. for 500 nm-thick LMNOs while the 2-μm LMNO layer exhibits a Coulombic efficiency [START_REF] Li | An Artificial Solid Electrolyte Interphase Enables the Use of a LiNi 0.5Mn1.5O4 5 v Cathode with Conventional Electrolytes[END_REF] higher than 98%. In our study, for the sample deposited at 10 -2 mbar (the best compromise between capacity retention and performance) the Coulombic efficiency is 70% for 0.4-μm thick LMNO which is low for long-term application. A simple way to increase the Coulombic efficiency is thus to increase the thickness of the active material.

Figure 8 reports the behavior of the films deposited at 10 -2 mbar as a function of the film thickness. Six different thicknesses have been tested from 60 to 1450 nm. For the sake of clarity, the diffractogramm of the 0.4 μm thick LMNO (already given in Figure 5A,B, red curve) is not reported again in Figure 8A. Two important facts have to be noted on XRD analysis: (i) for the thinnest film, only (111) and (222) peaks are clearly visible, and (ii) with increasing thickness, all the intensities of other peaks are increasing without any index dependence (as a classical powder). These two observations are in good agreement with the previous TEM study where, at this pressure, a fully (111)-oriented deposition is observed on the first 25 nm of the LMNO layer.

At this stage, both XRD and TEM results converge to an orientation in the direction perpendicular to the silicon wafer, but there is no information about the in plane orientation. Indeed, either the film is randomly distributed in the horizontal direction (giving rise to a so-called fiber texture), or it is also oriented (giving rise to a so-called epitaxial growth). To answer this question, XRD pole figures were recorded on (311) and (400) peaks of the LNMO 60 nm-thin film (Figure 8B). In this pole figure, strong peaks that would represent single-crystal-like texture (i.e., epitaxy) are absent, whereas continuous rings are observed, indicating a random in-plane orientation of the grains in the film. The different observed rings have been unambiguously modeled (blue rings) with the software Stereopole, [START_REF] Salzmann | STEREOPOLE: Software for the Analysis of X-Ray Diffraction Pole Figures with IDL[END_REF] and we can conclude the presence of a (111) fiber-textured LMNO thin film on a (100) Si substrate.

The surface capacity, measured between 4.4 and 4.8 V vs Li/ Li + at 1C, is presented in Figure 8C as a function of cycle number. The 60 nm-thick LMNO film does not provide any capacity while the capacity of the 140 nm-thick quickly fades at the seventh cycle. This behavior can be attributed to the preferred orientation observed in the film, orientation that would be detrimental and very sensitive for low thickness where this effect is predominant. [START_REF] Liu | Morphological Evolution of High-Voltage Spinel LiNi0.5Mn1.5O4 Cathode Materials for Lithium-Ion Batteries: The Critical Effects of Surface Orientations and Particle Size[END_REF] As soon as the film thickness is higher than 260 nm, the capacity retention of the LMNO films is stabilized over at least the first 50 cycles. Not surprisingly, the surface capacity increases with the thickness. As shown in Figure 8D, the surface capacity, measured at 1C, does not vary linearly with the thickness and seems to saturate when the thickness is higher than 0.6 μm. The evolution of the Coulombic efficiency (CE) as a function of the cycle number is presented in Figure 8E. The surface capacity due to electrolyte decomposition and SEI contribution is evaluated close to 5 μAh•cm -2 based on the CV reported in the inset of the Figure 8E. Regarding the formula used to calculate the Coulombic efficiency [START_REF] Li | An Artificial Solid Electrolyte Interphase Enables the Use of a LiNi 0.5Mn1.5O4 5 v Cathode with Conventional Electrolytes[END_REF] for thin film electrodes (CE = Q discharge / (Q charge + Q cell + Q SEI )), the contribution of Q cell + Q SEI is 5 μAh•cm -2 . For 450 nm-thick LMNO thin films exhibiting 20 μAh•cm -2 surface capacity (Figure 8C) at the 30th cycle, such capacity is not negligible and CE = 20/(20 + 5) = 80% (Figure 8E). For the thinner film (260 nm thick), CE = (12/(12 + 5) = 70%, and for thicker film (1450 nm), CE = 48/(48 + 5) = 91%. Consequently, the Coulombic efficiency (CE) is degraded due to decomposition of the electrolyte and SEI contribution. CE is demonstrated to be thickness-dependent for thin film electrodes. Finally, as demonstrated in Figure 8F, we successfully pattern the LMNO thin films with a standard plasma etching process (Cl 2 /Ar reactive ion etching process) available in the microelectronic industry which paves the way toward the fabrication of full cell Li-ion microbattery using CMOS compatible equipment. The process is found to be used for mass production. No physical mask has been used to achieve the patterning (only a photoresist layer spin coated on the top the LiMn 1.5 Ni 0.5 O 4 layer is proposed as an etching mask).

This study clearly shows that a careful control of the sputtering deposition parameters and annealing conditions is fundamental to understand the local ordering of the nickel and manganese atoms in the LiMn 1.5 Ni 0.5 O 4 spinel structure. Such local ordering has been shown to induce a significant contribution to the electrochemical properties of LMNO thin films. A similar conclusion dealing with the structuralelectrochemical properties relationship has already been reported [START_REF] Kim | Integrated Nano-Domains of Disordered and Ordered Spinel Phases in LiNi0.5Mn1.5O4 for Li-Ion Batteries[END_REF] on the powder based electrode. Moreover, we have shown that deposition pressure higher than 10 -2 mbar combined with a 2 h-annealing step at 700 °C allows to limit the presence of an additional phase such as Li 2 MnO 3 .

■ CONCLUSION

To summarize, we have first solved the technological issue of the PtSi phase which originates from the diffusion of Si through the Cr layer to form the PtSi phase under annealing treatment, damaging the current collector. The proposed solution is to substitute the Cr layer by a very dense and pinhole free Al 2 O 3 thin film deposited by ALD acting as a barrier diffusion. This Al 2 O 3 /Pt stacked layers operating as a functional current collector for high potential positive electrode material are compatible with the fabrication process of the 3D microbattery, [START_REF] Letiche | Atomic Layer Deposition of Functional Layers for on Chip 3D Li-Ion All Solid State Microbattery[END_REF][START_REF] Eustache | Silicon-Microtube Scaffold Decorated with Anatase TiO2 as a Negative Electrode for a 3D Litium-Ion Microbattery[END_REF] where a careful control of the 3D deposition process and annealing step has to be taken into account to promote the crystallization of the positive electrode thin film without inducing the formation of any additional phase and interdiffusion between each layer. In this manuscript, we report a full investigation of RF-sputtered LMNO thin films as a function of the deposition pressure and the film thickness. We have demonstrated that the pressure parameter plays a key role on the Mn-Ni cations ordering in spinel-like structures and consequently on the electrochemical performance. We have thus shown that LMNO thin films deposited at 10 -2 mbar and annealed at 700 °C deliver a normalized capacity of 65 μAh• cm -2 •μm -1 with good capacity retention upon cycling thanks to their disordered spinel structure. A kinetic study performed with CV and EIS measurements confirms these performances, since the samples deposited at low deposition pressure (disordered spinel) exhibit the highest chemical diffusion coefficient as compared to the ordered one. We have also demonstrated a significant dependence of the Coulombic efficiency with the thickness for high voltage LMNO thin film electrodes. This work is, to our knowledge, the most complete study combining structural and electrochemical characterizations of LMNO thin film obtained by the RF sputter deposition process.

Figure 1 .

 1 Figure 1. SEM cross sections (bare scale = 100 nm) and top surface (AFM scanning area = 5 μm × 5 μm) analyses of as-deposited LMNO thin films as a function of the deposition pressure, from 6 × 10 -3 up to 9 × 10 -2 mbar (A-D). SEM fracture and top surface (AFM scanning area = 5 μm × 5 μm) analyses of the four studied LMNO layers annealed at 700 °C (E-H).

Figure 2 .

 2 Figure 2. Evolution of the intrinsic stress (A) and the deposition rate (B) as a function of the operating pressure.

Figure 3 .

 3 Figure 3. Illustration of the PtSi phase formation. A. Cyclic voltammetry of the as-deposited and (700 °C) annealed LMNO/Pt/Cr/Si samples between 3.8 (or 4.2) and 4.9 V vs Li/Li + at 0.1 mV•s -1 . B. Diffractogramms of the as-deposited and annealed LMNO/Pt/Cr/Si sample at 700 °C. C. HT-XRD analysis of the as-deposited LMNO/Pt/Cr/Si sample from 50 to 800 °C. D. High temperature XRD patterns of the LMNO/Pt/Al 2 O 3 /Si sample, i.e., where the Cr adhesive layer has been replaced by an Al 2 O 3 insulating layer. E. Cyclic voltammetry of the LMNO/Pt/Al 2 O 3 /Si sample annealed at 700 °C demonstrating the redox activity of nickel cations at 4.7 V vs Li/Li + . F. constant current galvanostatic charge-discharge between 4.4 and 4.8 V vs Li/Li + at 1C rate as a function of the normalized capacity reported regarding the annealing temperature (650, 700, and 750 °C).From this study, an annealing temperature of 700 °C seems to maximize the normalized capacity.

Figure 4 .

 4 Figure 4. Transmission electron microscope analysis of the LMNO thin film deposited at 0.01 mbar and annealed at 700 °C. A. TEM analysis of the cross section Si/Al 2 O 3 /Pt/LiMn 1.5 Ni 0.5 O 4 /Ti/SiO 2 prepared by focus ion beam technique. This fracture analysis confirms the stacked layers, from Al 2 O 3 insulating layer up to the LMNO thin films. The titanium and SiO 2 coatings correspond to protective layers used for the film preparation by the focus ion beam technique. The chemical mapping analysis (EDS-STEM) of the cross section (Si turquoise blue, Al yellow, Pt red, Mn blue, Ni green, Ti pink) shown at the bottom of part A clearly confirms the absence of interdiffusion between the stacked layers. B. HRTEM image of the LMNO layer with a focus close to the Pt current collector in order to check the Pt/LMNO interface.From this analysis, the atomic plane seems to be well organized with a preferential orientation perpendicular to the substrate for the first 26.5 nm (out of plane analysis). Above this threshold, the LMNO exhibits a polycrystalline behavior.

Figure 5 .

 5 Figure 5. Structural analyses of the sputtered thin films deposited at 4 different pressures and annealed at 700 °C. A. XRD analysis of the low pressure Al 2 O 3 /Pt/LMNO deposited samples (6 × 10 -3 and 10 -2 mbar). An inset of the diffractogramm is proposed between 35 and 45°. B. XRD analysis of the Al 2 O 3 /Pt/LMNO samples deposited at higher pressure (5 × 10 -2 and 9 × 10 -2 mbar). An inset of the diffractogramm is proposed between 35 and 45°. C. Raman spectra of the thin films. Clearly, the Raman analysis confirms that the deposition pressure influences the atomic ordering within the studied positive electrode material: the disordered spinel Fd3̅ m is obtained at low pressure while the ordered spinel P4 3 32 is synthesized at high pressure. D. Raman spectra of the low pressure samples (0.006 and 0.01 mbar) demonstrates the presence of the Li 2 MnO 3 monoclinic phase. E. Evolution of the lattice parameter and atomic ratio regarding the deposition pressure.

Figure 6 .

 6 Figure 6. A. Nyquist spectra of the sputtered thin films recorded between 100 kHz and 5 mHz. B. Focus on the sample deposited at 10 -2 mbar is proposed as well as an equivalent circuit used to fit the corresponding Nyquist plot. C. Plots of the Warburg impedance as a function of the inverse square root of the angular frequency for the four thin films under test.

Figure 7 .

 7 Figure 7. Electrochemical analysis of the sputtered LiMn 1.5 Ni 0.5 O 4 thin films as a function of the deposition pressure. All the layers are annealed at 700 °C under air atmosphere during 2 h. A specific color, depending on the deposition pressure, is applied for each sample on all the reported graphs from A to H. The electrolyte is 1 M LiClO 4 in EC/DMC (1/1). A, B. Cyclic voltammogram (CV) of the four tested samples (first cycle) as a function of the deposition pressure between 3.8 to 4.9 V vs Li/Li + . The sweep rate is fixed at 0.2 mV•s -1 . C. CV of the sample deposited at 10 -2 mbar: comparison between the first and the second cycles. D. Cyclic voltammograms of the LMNO thin film deposited at 10 -2 mbar as a function of the sweep rate (0.1 up to 0.8 mV/s). E. Evolution of the anodic and cathodic peak current regarding the square root of the sweep rate illustrating that the lithium transport within the layer is diffusion controlled (b = 0.52). F. Evolution of the surface capacity as a function of the working electrochemical window. G. Galvanostatic plots of all the tested samples between 4.4 and 4.8 V vs Li/Li + at 1C. The specific capacity is normalized to the layer thickness to have a good comparison between the tested samples. H. Evolution of the potential as a function of the normalized charge/ discharge capacity for the four tested pressures (20th cycle). On those galvanostatic plots, each sample clearly exhibits two plateaus occurring during the oxidation (4.68 and 4.75 V vs Li/Li + ) process and two plateaus occurring during the reduction process (4.65 and 4.72 V vs Li/Li + ).

Figure 8 .

 8 Figure 8. Evolution of the LMNO performance as a function of the layer thickness. A. Bragg-Brentano X-ray diffraction analysis (all diffractogramms are normalized to the 111 peak of LMNO at ∼18°). B. X-ray in-plane pole figures on peaks (311) and (400) of the LMNO thin film. Rings (modeled by Stereopole in blue) are systematically observed, indicating a fiber texture rather than an epitaxy. C and D. Evolution of the surface capicity vs cycle number and as a function of the film thickness. E. Enhancement of the Coulombic efficiency during the first 50 cycles. The inset of the Figure 8E reports the cyclovoltammogram (second cycle) of the Si/Al 2 O 3 /Pt stacked layers at 0.1 mV•s -1 between 4.4 and 4.8 V vs Li/ Li + . F. Patterning of the LiMn 1.5 Ni 0.5 O 4 thin films (as clover shape) achieved by using a dry etching (plasma) process compatible with microelectronic fabrication facilities: an SEM fracture analysis and a tilted optical view are reported.

  9,10 et al. and Mohamedi 6 et al., using the same calculations.

X B

D D


Tristan Pichonat

Christophe Lethien

Djamila Hourlier

Nicolas Tiercelin

David Troadec

Paul-Alain Rolland

Towards the 3-D microfabrication and integration of a complete power unit used for energy autonomous wireless system

This paper deals with the fabrication of an all-solid lithium-ion microbattery for autonomous wireless sensors networks nodes, focusing especially on the solid electrolyte and the negative electrode made of silicon nanowires. Regarding the electrolyte, amorphous thin solid films of lithium phosphorus oxynitride (LiPON) were prepared by radio-frequency sputtering of a mixture target of P2O5-Li2O in ambient nitrogen atmosphere. The morphology, composition, and ionic conductivity were characterized by X-ray diffraction, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and A.C. impedance spectroscopy. A microstructuring process is proposed, including a double masking with silicon oxide and silicon nitride layers. Finally, the combination of silicon nanowires and LiPON is presented.

Introduction

As wireless sensor applications and installations continue to grow just as the concept of smart environments, the way to autonomous power supply is more than ever one of the key factors [START_REF] Lewis | Smart environments: technologies, protocols, and applications[END_REF] to lean towards autonomous and self organizing wireless sensor networks (WSN) (fig. 1). To comply with autonomous and miniaturization concepts, the WSN node (fig. 2) has to include a small power unit mainly composed of three parts, namely the energy harvesting system, the energy storage device, and the energy management subsytem. The ultimate goal of our current research is to provide miniaturized and integrated autonomous power supply for WSN nodes, especially for the supply of both the management module and the low consumption wireless communication transceivers. According to the relative low power consumption of the wireless module designed in our group (target consumption: 100 µW), the short radiofrequency range (10 m) mainly due to the operating carrier frequency (millimetre wave: 60 GHz) and the low data rate transmission of such WSN (several bits/s), it is expected to develop a small energy device integrated in a volume close to a few cubic centimetres (~cm 3 ) in order to provide a low cost, small size, robust and packaged node.

To follow the specifications previously described, effort is notably put on the storage unit with the fabrication of an all-solid-state thin film lithium-ion (Li-ion) microbattery on a glass substrate.

This study especially focuses on the 3-D microstructuration of a solid electrolyte as well as its combination with silicon nanowires as the anode of the microbattery. Combining the advantages of silicon material such as its low cost, a high specific insertion capacity (up to 4.4 lithium per silicon), a high theoretical capacity (4200 mAh.g -1 ), and the advantages of nanowires for battery electrode applications (large surface to volume ratio, efficient 1-D electron conducting pathways, accomodation of the volumic expansion associated with the lithiation/delithiation during the charge/discharge of the battery), silicon nanowires are currently one of the emerging solutions that could potentially replace carbonaceous materials [START_REF] Laïk | [END_REF](3)(4). A complete structure of the battery is sketched in fig. 3. Miniaturization can be achieved through the use of micro-and nano-technology facilities, particularly used in the fields of microelectronics and MEMS fabrication. Solid-state vacuum-sputtered Lithium Phosphorous Oxynitride (LiPON) was chosen as the solid electrolyte. Even if LiPON material has been extensively investigated over the last 15 years (5-9), micropatterning is still a challenge [START_REF] Oukassi | Proc. of Design, Test, Integration, and Packaging of MEMS and MOEMS (DTIP)[END_REF][START_REF] Song | [END_REF]. The physicochemical characterization of the LiPON layer has been performed (XPS, XRD, EDX), as well as AC impedance spectroscopy to exhibit the ionic conductivity of the solid electrolyte. This study also gives a way for structuring LiPON films through dry and wet chemical etching processes. Moreover, as mentioned above, silicon nanowires were associated to the LiPON layer electrolyte in order to provide the negative electrode of the microbattery. One of the many challenges of this study is to provide a technological process allowing the electrical connection between all the nanowires on an insulating glass substrate, contrary to the common use of stainless steel substrates in battery fabrication [START_REF] Laïk | [END_REF]3).

Experimental details

Nanowires growth

A 500 nm-thick n-doped (10 18 cm -3 ) polycrystalline silicon layer is first deposited by low pressure chemical vapor deposition (LPCVD) at 700°C on a glass substrate. This material will be used as a contact layer to gather the electronic contributions of each nanowire. A 10 nm-thick gold layer is evaporated and structured through a standard lifoff process on the polycrystalline silicon layer. Gold is used as the metal catalyst for Si nanowires growth. The silicon nanowires (SiNW) are then traditionally grown under SiH4 atmosphere through a vapor liquid solid CVD process [START_REF] Laïk | [END_REF].

Glass substrate

Currentcollector

Solidelectrolyte

Cathode

Currentcollector

Nanostructured anode

Thin film deposition LiPON thin film electrolyte is deposited over the nanowires by a radiofrequency sputtering of a 4-inch mixture target of P2O5-Li2O (0.25:0.75) prepared by Super Conductor Materials, Inc. The target material was sputtered in a Leybold Z550 equipment after pumping down to a base vacuum of 10 -4 Pa. A diode RF-sputtering was used at 13.56 MHz with a constant forward power of 3.7 W.cm -2 in a pure nitrogen atmosphere (N2) and at a pressure of 0.6 Pa.

For the AC impedance measurements, a LiPON layer is deposited between two 500 nm-thick layers of Cr-Pt elaborated by evaporation within the same conditions than previously described.

Fig. 4 displays the structuring process of the LiPON layer. It includes the deposition of silicon nitride and silicon oxide masking layers by a PECVD low temperature process (100°C). Further details will be given in the next section.

Characterization

The thickness of the films was determined using surface profilometer (Dektak II a) and SEM (Zeiss Ultra 55) / FIB (FEI Strata DB 235) imaging. Bruker D8 Advance X-ray diffractometer (XRD) was used to define the nature of the films (crystalline or amorphous). The composition was measured with X-ray photoelectron spectroscopy (XPS, instrumentation based on a Physical Electronics 5600 model) and energy dispersive X-ray spectroscopy (EDX, Xflash 4010 system from Bruker AXS). XPS measurements were performed with a monochromatic Al Kα x-ray source (1486.6 eV) and an analyzer pass

Results and discussion

Silicon nanowires

In situ n-doped SiNW of 400-nm diameter were obtained on the polysilicon layer. A resistivity of 10 -2 Ω.cm was previously measured for the polysilicon layer thanks to fourprobe V(I) measurements. The aspect of the obtained SiNW is observed on fig. 5. Actually, at this stage, accent was not put on the size nor the density of the SiNW, but rather on how the LiPON electrolyte was going to deposit on them.

LiPON electrolyte

First step was to confirm the quality of the LiPON films, i.e. to show the conformity of the sputtered layers with the literature, as well as to bring further results. Morphological aspects. XRD was performed on a multilayer composed from bottom to top of Cr (100 nm), Pt (100 nm), and LiPON (1.4 µm). No sharp diffraction peaks appear in the XRD pattern (Fig. 6a) except those attributed to underlying Pt (40°, 46°, and 68°) and Cr (44°). A broad peak from 15 to 40° such as the ones observed in (5,13) suggests that the deposited LiPON electrolyte exhibits an expected amorphous structure, contrary to the P2O5-Li2O powder from the target which clearly exhibits the pattern of a crystallized material (Fig. 6b). Thin film composition. EDX and XPS analyses were performed on the LiPON films with Cr-Pt as the underlying layer. EDX technique can not detect the lightest elements such as Li but enables to quantify the other components (P, O, N) of the films and therefore to deduce the ratios between atoms (N/P and O/P). XPS analysis on the same samples has also demonstrated the appearance of the main peaks representative of the LiPON signature (Li, P, O, N peaks).

Results obtained from both EDX and XPS confirmed the presence and incorporation of nitrogen into the film in the proportions N/P and O/P respectively at 1.2 and 2.4, close to those measured by Hamon (6) and Hu (7) in similar conditions. XPS analysis also provides information about elemental bonding environments. Nitrogen 1s core level has been decomposed into 3 Gaussian-Lorentzian mixed components, at 399, 400.5, and 405.5 eV (Fig. 7), corresponding respectively to P -N = P, P -N < P P and O -N = O bondings. A good agreement with previous studies (5, 6, 9) is found. LiPON depositing on silicon nanowires A 1.4 µm thick LiPON layer was sputter-deposited onto the nanowires (step 1 of the process presented in fig. 4). Results are displayed in fig. 9. These SEM pictures clearly show the complete coating of the nanowires by the LiPON layer. This is really important that it covers completely all the nanowires in order to avoid any possible short circuit with the future additional cathode layer that will be deposited over.

Other views of the coated SiNW were provided after vertical (fig. 10a) and horizontal (fig. 10b) cross-sections performed with a focus ion beam (FIB) device. They allow to show how the LiPON surrounds the nanowires. Because of the dielectric nature of the the LiPON layer, a thin FIB-sputterred layer of Pt had to be deposited over in order to practically make the cross-section possible without dissolving the LiPON layer with the ion beam.

Conclusion

With the final aim of powering autonomous wireless sensors networks nodes, we have first put efforts on the energy storage part, constituted by a Li-ion microbattery.We have achieved the sputter deposition of thin LiPON electrolyte films with similar characteristics than previous literature. Time was especially devoted to the material characterization, as one of the keypoint for the development of efficient batteries. A process for microstructuring the LiPON layer was proposed, and further results will soon be published. Wishing to use silicon nanowires as negative electrode for the microbattery, the complete coating of the nanowires with LiPON was performed.

Figure 1 .

 1 Figure 1. Schematic approach of a wireless sensors network.

Figure 2 .

 2 Figure 2. Details of one node from a WSN.

Figure 3 .

 3 Figure 3. Sktech of the complete battery structure.

Glass substrate 1 .

 1 Sputtering of LiPON (1.4 µm) 9. Wet etching of LiPON (diluted solution of TMAH) 10. CF4/CHF3 reactive ion etching of SiO2 and Si3N4 remaining films 8. CF4/CHF3 reactive ion etching 4. Spin-coating of photoresist of Si3N4 film

Figure 4 .

 4 Figure 4. Diagram of the complete LiPON microstructuring process.

  Photoresist

Figure 5 .

 5 Figure 5. SEM front view of the silicon nanowires grown on polycrystalline silicon.

Figure 6 .

 6 Figure 6. XRD patterns of the deposited 1.4 µm-thick LiPON thin film on Cr-Pt layer (a) and of the P2O5-Li2O (0.25:0.75) mixture target powder (b). The four sharp peaks in (a) are identified as underlying Pt (40, 46 and 68°) and Cr (44°).

Figure 7 .

 7 Figure 7. XPS spectrum of the deposited 1.4 µm-thick LiPON thin film on Cr-Pt layer in the region of the N 1s peaks (406-395 eV range).

Figure 8 .

 8 Figure 8. Nyquist AC impedance plot of the Cr-Pt|LiPON|Cr-Pt stack at room temperature.

Figure 9 .

 9 Figure 9. SEM cross section views of LiPON coated silicon nanowires.

Figure 10 .

 10 Figure 10. SEM views of FIB cross section LiPON coated silicon nanowires. (a) vertical cross section and (b) horizontal cross section.

Acknowledgments

This work was supported by the ERDF (European Regional Development Fund) and by the Nord-Pas-de-Calais Region (France). The authors want to thank C. Boyaval (IEMN) for the EDX studies, S. Godey (IEMN) for the XPS studies, Pascal Roussel, Marie Colmont, and E. Capoen (UCCS, CNRS UMR 8181, Villeneuve d'Ascq, France) for both the XRD studies and their help on impedance spectrometry measurements.