Konstantina Christina Topka

Babacar Diallo

Maxime Puyo

Paris Papavasileiou

Charlotte Lebesgue

Cecile Genevois

Yann Tison

Cedric Charvillat

Diane Samelor

Raphael Laloo

Daniel Sadowski

François Senocq

Thierry Sauvage

Hugues Vergnes

Marie-Joelle Menu

Brigitte Caussat

Viviane Turq

Nadia Pellerin

Constantin Vahlas
email: constantin.vahlas@ensiacet.fr

Critical Level of Nitrogen Incorporation in Silicon Oxynitride Films: Transition of Structure and Properties, toward Enhanced Anticorrosion Performance

Keywords: Silicon oxynitride, tris(dimethylsilyl)amine, chemical vapor deposition, buffer oxide etchant, hydrolysis resistance, thin film, material network, wet etching corrosion, AFM: atomic force microscope ALD: atomic layer deposition ARC: anti-reflective coatings BEMA: Bruggeman effective medium approximation BOE: buffer oxide etchant CAE: constant analyzer energy CCD: charge-coupled device CFD: computational fluid dynamics CMOS: complementary metal-oxide-semiconductor CVD: chemical vapor deposition electron energy loss spectroscopy ERDA: elastic recoil detection analysis FIB: focused ion beam FT-IR: Fourier-transform infrared spectroscopy spectroscopic ellipsometry SEM: scanning electron microscope SPT: set-point temperature STEM-HAADF: scanning transmission electron microscopy-high angle annular dark field Si3N4: silicon nitride SiC: silicon carbide SiO2: silicon dioxide SiOxNy: silicon oxynitride

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Silicon oxynitride (SiOxNy) films have gathered increasing attention in international technology policy concepts such as the recent Key Enabling Technologies European Union one. [START_REF] Alford | Materials for Key Enabling Technologies[END_REF] With structure and properties transitioning from stoichiometric silica (SiO2) to silicon nitride (Si3N4), [START_REF] Shi | A review: preparation, performance, and applications of silicon oxynitride film[END_REF] amorphous SiOxNy films have been widely used in the fields of micro-and opto-electronics [START_REF] Šimurka | Effect of deposition conditions on physical properties of sputtered silicon oxynitride thin films on float glass[END_REF] . They have especially garnered attention since early 2000s as high-k replacements for the classical SiO2 gate dielectrics. [START_REF] Wong | On the scaling issues and high-kappa replacement of ultrathin gate dielectrics for nanoscale MOS transistors[END_REF] Their denser network in comparison to SiO2 lends them promising scratch-resisting, encapsulation, anti-corrosion and ion diffusion-limiting properties, finding them a spot in applications such as Na and B diffusion barriers in light emitting devices (LED) [START_REF] Palm | CIS module pilot processing applying concurrent rapid selenization and sulfurization of large area thin film precursors[END_REF] , passivation film on surface acoustic wave devices [START_REF] Nishimura | The Temperature Coefficients of Frequency of Surface Acoustic Wave Devices With SiOxNy Passivation Films on LiTaO3 Substrates[END_REF] or at the TiO2/SiON/n-Si gate-stacks for advanced CMOS devices [START_REF] Gupta | Structural and Electrical Characteristics of ALD-TiO2/SiON/n-Si Gate-Stack for Advanced CMOS Device Applications[END_REF] , and for hard encapsulation of carbon dots [START_REF] Barman | Transparent Hard Coatings with SiON-Encapsulated N-Doped Carbon Dots for Complete UV Blocking and White Light Emission[END_REF] .

Their oxygen permeability and low water vapor transmission rate is also in demand for food packaging and electronic devices, [START_REF] Shahpanah | Improving the oxygen barrier properties of PET polymer by radio frequency plasma-polymerized SiOxNy thin film[END_REF] given their ability to prevent degradation or failure of chemically reactive electrodes or conductive components after exposure to humidity or oxygen. Although this material has been widely studied due to the above outstanding properties, further investigation of the structural and chemical disorder is of essence for new upcoming applications of SiOxNy. [START_REF] Chen | Luminescence mechanism in amorphous silicon oxynitride films: band tail model or N-Si-O bond defects model[END_REF] Moreover, control of the chemical composition of such films is essential. Adjustable SiOxNy film composition, and by extension tunable properties such as refractive index or network density, are often achieved by controlling the starting reaction chemistry.

Throughout the past decades, several technologies have been studied for the production of SiOxNy films. Oxidation of SiNx or high-temperature nitridation and ion implantation on SiO2 have been proposed as one option, typically involving thermal or plasma nitridation processes. [START_REF] Yadav | Synthesis of buried silicon oxynitride layers by ion implantation for silicon-on-insulator (SOI) structures[END_REF] In addition, physical or chemical vapor deposition have also been investigated and used extensively. Conventionally, reagents such as NH3 and N2O have been utilized, in combination with silicon-containing precursors such as hexamethyldisilazane (HMDS), trisilylamine (TSA), silane (SiH4) and others. [START_REF] Tomar | Deposition and characterization of SiON films using HMDS for photonics applications[END_REF] Most thermal processes however involve high temperatures (800-1200°C), resulting in films with compressive internal stresses and degraded performance. [START_REF] Kaghouche | Investigation on optical and physico-chemical properties of LPCVD SiOxNy thin films[END_REF] The choice of precursors is of importance as it affects the operating thermal window, the chemical composition and properties of the resulting film. For instance, use of SiH4 and NH3 may produce films with relatively high hydrogen content, diffusing and generating traps in the gate dielectric, [START_REF] Parsons | Low hydrogen content stoichiometric silicon nitride films deposited by plasma-enhanced chemical vapor deposition[END_REF] causing decreased electrical performance.

Moreover, many of the previously described applications involve heat-sensitive and/or 3Dcomplex structures. Common literature that targets low deposition temperature reports SiOxNy films that are amorphous and produced by plasma enhanced chemical vapor deposition (PECVD) processes. [START_REF] Lin | Enhanced scratch resistance of polycarbonate by low temperature plasma-polymerized organosilica[END_REF][START_REF] Walkiewicz-Pietrzykowska | Deposition of Thin Films of SiOxCyH in a Surfatron Microwave Plasma Reactor with Hexamethyldisiloxane as Precursor[END_REF] However, increased hydrogen incorporation is often encountered in PECVD prepared films, [START_REF] Perani | Annealing effects on SiOxNy thin films: Optical and morphological properties[END_REF] which can be detrimental to barrier performance. As a cost-effective alternative, atmospheric pressure thermal CVD process is proposed, operated using reactive chemistries complying with deposition at moderate temperatures, compatible with heat sensitive substrates such as glass.

In the present work, the processing of SiOxNy films from the rarely-reported tris(dimethylsilyl)amine (N(SiHMe2)3, TDMSA) precursor is studied. Selected as a potential dual source of Si and N, TDMSA is supplied in presence of NH3 and/or O2. A parametric analysis based on the process conditions is carried out and the evolution of the network structure and properties is investigated as a function of the nitrogen incorporation. Combining information input from spectroscopic ellipsometry (SE), ion beam analysis (IBA), high-resolution transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared analysis (FT-IR), the structural order and chemical characteristics are correlated to mechanical and functional properties.

Finally, the link between distinct levels of nitrogen incorporation, Nx-Si-Oz tetrahedral configuration and their influence on the material properties is investigated. Particular attention is paid on the interdependence between film composition and wet etching rate, targeting applications such as protective, hydrolysis-resisting coatings for glass containers or etch-stop liners in field effect transistor devices. This work is the final part of a trilogy on CVD of SiOxNy films. Firstly, starting with the analysis of the gas phase during SiOxNy deposition from the same chemistry, information on by-products and active intermediates has been collected in order to elucidate the chemical mechanism leading to film production. [START_REF] Decosterd | An innovative GC-MS, NMR and ESR combined, gas-phase investigation during chemical vapor deposition of silicon oxynitrides films from tris(dimethylsilyl)amine[END_REF] Secondly, the acquired results were applied in the development of a chemical model dealing with the simulation of the SiOxNy deposition process for the first time, and offering grounds for process optimization studies using computational fluid dynamics (CFD) calculations. [START_REF] Topka | An innovative kinetic model allowing insight in the moderate temperature chemical vapor deposition of silicon oxynitride films from tris(dimethylsilyl)amine[END_REF] Being the last in this collection of new information on the SiOxNy deposition, the present work contributes to a holistic approach of the processing, characterization and functionality of such films.

Experimental Section

Films processing CVD of SiOxNy films was carried out in a horizontally aligned hot-wall reactor, illustrated in Figure SI1 of Supporting Information. The reaction chamber was defined by a 700 mm long quartz tube (Ø 46 mm internal). When heated, a thermal gradient develops, as presented in Figure 1 in the Results and Discussion section, featuring a 140 mm long isothermal section spanning from 360 to 500 mm. Deposition experiments were carried out at four different set-point temperatures (SPT), i.e. 550, 580, 625 and 650°C, corresponding to the temperature of the isothermal zone of the reactor. This temperature range is significantly lower than 760-820°C that is the state of the art for atmospheric pressure thermal CVD SiOxNy films. [START_REF] Tomar | Deposition and characterization of SiON films using HMDS for photonics applications[END_REF] Unless specified otherwise, the majority of the analyzed samples were taken from the isothermal region, an example of which is shown exemplified in Figure 1 in the Results and Discussion section.

Three lines supplied N2 (99.9999 %, Messer), O2 (99.9995 %, Messer) and NH3 (99.999 %, Linde) gases, as illustrated in Figure SI1. The flow rates were established by mass flow controllers (Brooks 5850TR). TDMSA (95 %, abcr) was conditioned in a bubbler and was transported in the deposition zone through N2 serving as carrier gas. Details on the bubbler and carrier gas line temperatures, pressure gauges and pumping equipment have been described elsewhere. [START_REF] Topka | An innovative kinetic model allowing insight in the moderate temperature chemical vapor deposition of silicon oxynitride films from tris(dimethylsilyl)amine[END_REF] The operating pressure was kept constant at 9.73*10 5 Pa (730 Torr) for all runs. Thirteen experiments were realized at deposition conditions that are summarized in Table 1. It is noted that runs TD9 and TD5 supplied 0 standard cubic centimeters per minute (sccm) of O2 at the inlet, however potential microleaks or H2O desorption from the reactor internal surfaces of the reaction chamber cannot be excluded. Monocrystalline silicon (100) n-type wafers (Neyco S.A., 32 x 24 mm 2 ) served as substrates.

Their preparation, degreasing and placement in the 57-500 mm region of the reactor have been described elsewhere. [START_REF] Topka | Large temperature range model for the atmospheric pressure chemical vapor deposition of silicon dioxide films on thermosensitive substrates[END_REF] Eighteen such samples were used in each experiment; their position is exemplified in Figure 1 in the Results and Discussion section. [START_REF] Topka | Large temperature range model for the atmospheric pressure chemical vapor deposition of silicon dioxide films on thermosensitive substrates[END_REF] The sample code for each position "p" contains a letter of the alphabet (e.g. pA) for samples located outside of the isothermal region, and a number (e.g. p1) for samples located in the isothermal region of the reaction chamber.

Measurement of film thickness and composition

The SiOxNy film deposition rates (DR) were calculated from thickness measurements performed by SE, using a Semilab SE-2000 instrument operating at an incidence angle of 70°. Each sample was probed at 5 points along the vertical center line as illustrated in the inset of Figure SI1 These techniques are shortly referred to as ion beam analyses. The absence of other elements polluting the samples was also verified. The middle of a sample positioned at 38.4 cm from the reactor inlet (i.e. located within the isothermal region), was probed for all runs (Figure SI1 of Supporting Information).

The composition of the films was obtained by simulation of the RBS, NRA and ERDA spectra using the SIMNRA software. [START_REF] Mayer | SIMNRA, a simulation program for the analysis of NRA, RBS and ERDA[END_REF] Details on the above characterizations have been described in previous publications. [START_REF] Decosterd | An innovative GC-MS, NMR and ESR combined, gas-phase investigation during chemical vapor deposition of silicon oxynitrides films from tris(dimethylsilyl)amine[END_REF][START_REF] Diallo | Network hydration, ordering and composition interplay of chemical vapor deposited amorphous silica films from tetraethyl orthosilicate[END_REF] Chemical structure

The chemical structure of the deposited SiOxNy films was assessed in transmission mode by FT-IR (Frontier FT-IR MIR/NIR spectrometer), equipped with a custom-made substrate support that facilitated rotation of the sample in relation to the incident beam. Two angles (0° and 60°) were used in this work. After removal of the background signal, sixty four accumulations in the 400-4000 cm -1 spectral range were collected for each sample with 4 cm -1 resolution. Post-acquisition processing of the data included removal of the substrate signature and baseline correction. The absorbance values were normalized by the corresponding thickness of each film, given that in transmission mode, the intensities of absorptions relating to various Si-O-Si vibrational modes are affected by the thickness of the film. [START_REF] Ponton | Investigation of the densification mechanisms and corrosion resistance of amorphous silica films[END_REF] Spectra-deconvolution using Gaussian function was carried out in OriginLab 2019 to distinguish overlapping absorptions. A deconvolution was considered successful for a R 2 goodness fit of the deconvolution product higher than 0.98. The fit was achieved by allowing a maximum tolerance of ±20 cm -1 for the wavenumber, ±30 for the full width half maximum (FWHM) and allowing only positive values for the peak area. A set of 9 peaks was utilized in the fingerprint region (700-1300 cm -1 ) and their fitting was executed within the specified tolerance margins. These tolerance values were necessary in order to account for the substantial chemical modification of the species caused by changes in the material composition (i.e. enrichment in nitrogen).

Complementary chemical analysis was performed by X-ray Photoelectron Spectroscopy (XPS), performed in a Thermofisher K-alpha system. An Al K-alpha source (1486.7 eV) was used, the analyzer was set in CAE (Constant Analyzer Energy) mode with 20 eV pass energy. Measurements were taken after a 30 s etching step using Ar + ions (1000 eV). Surface charges during data acquisition were neutralized with the aid of a low-energy flood gun providing electrons and Ar + ions (<5 eV). The spectra were deconvoluted using CasaXPS © to ascertain the environment of the elements. References of silicon carbide (SiC) and nitride (Si3N4) were used to verify Si-C and Si-N bond energies and valence.

Surface morphology, roughness, nanostructure and interface

The surface roughness of the films was measured in tapping mode with an Agilent Technologies 5500 atomic force microscope (AFM). Scan size and tip details are provided elsewhere. [START_REF] Topka | Tunable SiO2 to SiOxCyH films by ozone assisted chemical vapor deposition from tetraethylorthosilicate and hexamethyldisilazane mixtures[END_REF] The nanostructure of the film was studied with high-resolution transmission electron microscope (HR-TEM) operating at 80kV, as described in a previous work. [START_REF] Diallo | Network hydration, ordering and composition interplay of chemical vapor deposited amorphous silica films from tetraethyl orthosilicate[END_REF] Nanodiffraction and HR-TEM imaging were used to determine the morphology and the amorphous or crystalline state of the film. Additional chemical information was obtained by scanning transmission electron microscopy-high angle annular dark field (STEM-HAADF) imaging and electron energy loss spectroscopy (EELS). STEM-HAADF images were acquired with a 68-174.5 mrad inner-outer collection angle, using a 0.1 nm probe size. A cross-section of the SiOxNy film was prepared by focused ion beam (FIB) method described elsewhere, [START_REF] Diallo | Network hydration, ordering and composition interplay of chemical vapor deposited amorphous silica films from tetraethyl orthosilicate[END_REF] stored under inert atmosphere and analyzed with minimal delay to reduce contamination.

Functional and mechanical properties

Nanoindentation tests were carried out on thick films (ca. 500 nm) to limit substrate contribution, using an UltraNanoIndenter apparatus (CSM Instruments, Anton Paar) equipped with a Berkovich diamond indenter. A gradually increasing load up to 0.5 mN was applied, then maintained for 30 s, and lastly reduced to zero, with 1 mN.min -1 load/unload rates. Load and displacement resolutions were 0.25 µN and 0.1 nm, respectively. Elastic modulus and hardness data were calculated from the load vs. depth curves, utilizing the method proposed by Oliver and Pharr. [START_REF] Oliver | An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments[END_REF] Nanoscratch tests were performed using a Nanoscratch Tester (CSM Instrument, Anton Paar) equipped with a spherical diamond indenter (2 µm radius). Scratch load, displacement speed and further details on the nanoscratch tests have been reported in a previous work. [START_REF] Diallo | Network hydration, ordering and composition interplay of chemical vapor deposited amorphous silica films from tetraethyl orthosilicate[END_REF] Track analysis and determination of the critical normal loads describing the coating adhesive failures were facilitated by the use of an optical microscope (Nanoscratch CSM) and scanning electron microscope (SEM) apparatus (Vega 3 TESCAN). Special attention was given to film delamination from the substrate.

Wettability was measured with a GBX apparatus using 0.35±0.01 µl distilled-water droplets that were carefully placed at five different locations of the sample surface at ambient. The equilibrium static contact angle was measured using a CCD camera.

The wet etching corrosion resistance of the films was assessed using a buffered oxide etchant (BOE) wet etchant, used in microfabrication. [START_REF] Gao | Buffered Oxide Etchant Post-Treatment of a Silicon Nanofilm for Low-Cost and Performance-Enhanced Chemical Sensors[END_REF] It is a 6:1 solution consisting of 6 parts NH4F (40 wt%) and 1 part HF (49 wt%). The samples were immersed for up to 60 s under continuous stirring at 25°C.

The etching rate (Å.s -1 ) of each SiOxNy film was determined by measuring the thickness on ten different locations using SE before and after immersion.

Results and Discussion

SiOxNy deposition

It must be noted that the experimental runs TD1 and TD2, which featured a maximum temperature of 550°C and 580°C in their respective isothermal region, did not result in any visible SiOxNy deposition, well coherent with recent results detailing the decomposition of the TDMSA precursor above 580°C. [START_REF] Decosterd | An innovative GC-MS, NMR and ESR combined, gas-phase investigation during chemical vapor deposition of silicon oxynitrides films from tris(dimethylsilyl)amine[END_REF] Figure 1 presents results of runs TD3, TD6, TD7 and TD8 performed at 650°C, utilizing the TDMSA-O2-NH3 chemistry. As illustrated in the photograph of Figure 1(a), increasing the temperature to 650°C through run TD3 proved successful as noted by the iridescence. In these conditions, film formation occurs with a maximum deposition rate (DR) of 0. 92 nm.min -1 on sample p2 and a maximum nitrogen content of 9.5 at.% on sample p6, as measured by SE. Visual inspection of Figure 1(a) shows that minimum temperature of 610°C is necessary for film formation, with possible, albeit minimal, deposition between 585-610°C. No deposition is noted below 585°C, an observation coherent with the lack of deposition in runs TD1 and TD2. Run TD4, although utilizing a lower precursor flow rate, still resulted in film formation. A maximum DR of 0.96 nm.min -1 was noted in the isothermal region, higher than the one noted for TD3, attributed to the higher O2:TDMSA flow ratio (1 in TD4 compared to 0.33 in TD3, Table 1). However, this higher O2:TDMSA ratio decreased the nitrogen content of the SiOxNy films to a maximum of 0.7 at.% N, as a result of increased oxidation of the native Si-N bond in the precursor molecule. Run TD5 was performed in absence of O2, increasing the maximum nitrogen content to 16.5 at.%. The lack of O2 supply however resulted in a drastic decrease of the DR, which dropped to values below 0.2 nm.min -1 . It is obvious that a trade-off exists between film formation (promoted through higher O2 supply) and enrichment of the composition in nitrogen (promoted by lower O2 supply). The competitive incorporation between O and N has been encountered in other literature detailing the CVD of silicon oxynitrides. [START_REF] Cova | Effect of unintentionally introduced oxygen on the electron-cyclotron resonance chemical-vapor deposition of SiNx films[END_REF][START_REF] Lee | SiOxNy thin film deposited by plasma enhanced chemical vapor deposition at low temperature using HMDS-O2-NH3-Ar gas mixtures[END_REF][START_REF] Fainer | Study of the structure and phase composition of nanocrystalline silicon oxynitride films synthesized by ICP-CVD[END_REF] By fixing the TDMSA precursor flow rate at 2 sccm, the O2 flow rate was reduced to 0.6 sccm for runs TD6, TD7 and TD8 to revert the O2:TDMSA flow ratio back to ca. 0.3 (Table 1). The NH3 flow rate was varied between 0-40 sccm across these runs. The utilization of the BEMA SE model provides additional information on the TD6, TD7 and TD8 films composition. Nitrogen is first encountered at 0.22 m from the inlet and presents a continuous increase. Interestingly, although film formation decelerates beyond the maximum DR, the nitrogen content of the material increases, reaching ca. 7.5 at.% on the final sample of the array (p6) for all three runs. Given that the temperature is constant in the isothermal region, the decrease of the DR in the 0.36-0.50 m range is accredited to the O2 consumption in the gas phase. Consumption of the TDMSA precursor was excluded given its detection in the effluent gases of all three runs, in similar quantities. [START_REF] Decosterd | An innovative GC-MS, NMR and ESR combined, gas-phase investigation during chemical vapor deposition of silicon oxynitrides films from tris(dimethylsilyl)amine[END_REF] Considering the aforementioned competition between oxygen and nitrogen, the continuous increase of the N content in this region (0.36-0.50 m) is logical.

For the tested conditions however, variation of the NH3 supply did not aid in increasing the N composition. The nitrogen profiles remained almost identical across the three runs (Figure 1(b)).

Moreover, nitrogen was probed even in absence of NH3 (TD7). These results demonstrate the role of the TDMSA precursor as a dual Si and N source. The most significant contribution of NH3 is a decrease in carbon incorporation. [START_REF] Weiller | Chemical Vapor Deposition of TiN from Tetrakis(dimethylamido)titanium and Ammonia: Kinetics and Mechanistic Studies of the Gas-Phase Chemistry[END_REF][START_REF] Ray | Low-Temperature deposition of dielectric films by microwave plasma enhanced decomposition of hexamethyldisilazane[END_REF] A maximum carbon concentration of 11.6 at.% is measured for TD7, dropping to values below 2.3 at.% for the higher NH3 supply used in run TD8.

Globally, the refractive index of the films ranges from 1.465 to 1.621. The extinction coefficient of the SiOxNy films is zero independent of the deposition conditions, allowing their application in multilayer anti-reflective coatings (ARC). [START_REF] Farhaoui | Reactive gas pulsing sputtering process, a promising technique to elaborate silicon oxynitride multilayer nanometric antireflective coatings[END_REF] A similar parametric analysis based on the variation of the O2 flow rate in absence of NH3 (runs TD7, TD11, TD12 and TD14) has been reported elsewhere. [START_REF] Topka | An innovative kinetic model allowing insight in the moderate temperature chemical vapor deposition of silicon oxynitride films from tris(dimethylsilyl)amine[END_REF] In that report, the extracted compositional and thickness characteristics of all 18 samples from these runs have been utilized for the development of a kinetic model for the computational fluid dynamics simulation of SiOxNy deposition. factor that influences the chemical bonding configuration and physical properties of the film. [START_REF] Chen | Luminescence mechanism in amorphous silicon oxynitride films: band tail model or N-Si-O bond defects model[END_REF] Chen et al. proposed that when the O/(O+N) ratio is above 0.4, nitride-like structures in the film are transformed and lean towards oxide-like structures. In consequence, the electronic properties change when the SiOxNy layer is intended for gate dielectric applications. [START_REF] Habraken | Silicon nitride and oxynitride films[END_REF] In the present work, all films exhibit O/(O+N) ratios between 0.75-0.9, clearly showing a more oxide-like structure.

Composition trends

Carbon incorporation also decreases for higher O2 supply, being reduced from 19.7 at.% at 0. Lastly, changes in film composition due to the modification of the deposition temperature (TD7, TD14) are shown in Figure SI2. Although a trend is difficult to conclude with only two datasets, N incorporation is noted to increase from 4.2 to 5.2 at.% N when the temperature is raised from 625°C to 650°C. A co-deposition of C and N through a common intermediate species could be supposed, given the parallel increase in carbon content. The more prominent increase of hydrogen content could also support the assumption that most H in the films is inserted through N-H and/or -CHx moieties. noted, along with the TO3-LO3 pair observed after application of a 60° oblique incidence angle. [START_REF] Taraskin | Nature of vibrational excitations in vitreous silica[END_REF][START_REF] Nagasawa | Microporous organosilica membranes for gas separation prepared via PECVD using different O/Si ratio precursors[END_REF] Absorptions relating to organic carbon incorporation, e.g. the bending vibration of CH3 in Si-(CH3)x groups at ca. 1270 cm -1 , are also observed, their origin being the incompletely decomposed -CH3 groups in the TDMSA precursor molecule. In addition, an absorption at ca. 936 cm -1 is identified, corresponding to the stretching vibration of Si-N. [START_REF] Ay | Comparative investigation of hydrogen bonding in silicon based PECVD grown dielectrics for optical waveguides[END_REF][START_REF] Hallam | Effect of PECVD silicon oxynitride film composition on the surface passivation of silicon wafers[END_REF] Furthermore, additional nitrogen-related absorptions are noted at approximately 470 and 840 cm -1 , corresponding to the symmetric and asymmetric stretching modes of the Si-N bond, respectively. [START_REF] Giorgis | Optical, structural and electrical properties of device-quality hydrogenated amorphous silicon-nitrogen films deposited by plasma-enhanced chemical vapour deposition[END_REF] At higher frequencies, the N-H stretching vibration is noted at around 3300 cm -1 ), attributed to hydrogen bonds between hydrogen and the lone electron pairs of nearby nitrogen atoms. [START_REF] Yin | Optical dielectric function and infrared absorption of hydrogenated amorphous silicon nitride films: Experimental results and effective-medium-approximation analysis[END_REF] A summary of all encountered absorptions is listed in Table SI1 of Supporting Information. A Si-OH absorption at ca. 3650 cm -1 and organic -CH3 asymmetric stretching absorptions appearing as small shoulders at ca. 2900 cm -1 are noted in the band as well. [START_REF] Nakamura | Photochemical Reaction of Ozone and 1,1,1,3,3,3-Hexamethyldisilazane: Analysis of the Gas Reaction between Precursors in a Photochemical Vapor Deposition Process[END_REF] In light of the FT-IR results

Chemical bonding and nanostructure

FT-IR

of Figure 3 x complex vibrating at higher frequencies. Similarly, the LO3 mode for runs at higher O2 flow rates also shifts towards higher frequencies, an effect that hints again at more oxygen-rich films, as confirmed by IBA results (Figure 2(b)). When the O2 supply is low (0.3 sccm) wowever, the LO3 position shifts substantially from the anticipated ~1250 cm -1 down to 1170 cm -1 . It is proposed that a different vibration absorbs at the 1170 cm -1 frequency, and that the LO3 is absent. Referring to Table SI1, this absorption could be associated to the TO4-LO4 pair. [START_REF] Ay | Comparative investigation of hydrogen bonding in silicon based PECVD grown dielectrics for optical waveguides[END_REF][START_REF] Bensch | An FT-IR study of silicon dioxides for VLSI microelectronics[END_REF] TO3 behaves similarly, shifting from ca. 1050 cm -1 at 1.2 sccm O2 to lower wavenumbers as the oxygen supply is decreased (Figure 3(d)). The lowest position noted for TO3 is 1022 cm -1 . Given that the Si-CH2-Si stretching vibration absorbs at 1000-1020 cm -1 , [START_REF] Lee | SiOxNy thin film deposited by plasma enhanced chemical vapor deposition at low temperature using HMDS-O2-NH3-Ar gas mixtures[END_REF][START_REF] Trunec | Deposition of thin organosilicon polymer films in atmospheric pressure glow discharge[END_REF] it is likely that the absorption noted at this wavenumber might be assigned to the Si-CH2-Si bond instead of the typical Si-O-Si TO3 mode when low O2 flow rates are supplied. Similar discussion can be made for the 800-820 cm -1 peak (Figure 3(d)). Both Si-C and Si-O (TO2) absorb in this range. [START_REF] Bulou | The influence of CH4 addition on composition, structure and optical characteristics of SiCN thin films deposited in a CH4/N2/Ar/hexamethyldisilazane microwave plasma[END_REF][START_REF] Pecora | A comparative analysis of silicon dioxide films deposited by ECR-PECVD, TEOS-PECVD and Vapox-APCVD[END_REF] A high absorbance is noted at 0 sccm O2, decreasing once O2 is added to the inlet gas composition, then increasing again for higher O2 flow rates. This is likely linked to decreasing absorption of Si-C bonds with parallel increase of the Si-O The evolution of the intensity of the absorption found at ca. 850 cm -1 is shown in Figure 3(e), plotted as a function of the nitrogen content. The graph contains results of deconvolution analyses carried out on FT-IR spectra from samples produced under various conditions, at different positions in the reactor, and from both TDMSA-O2-NH3 and TDMSA-O2 chemistries. The bonds attributed to this absorption are the rocking of CH3 vibrations in Si-(CH3)3 groups [START_REF] Ngamou | Plasma-deposited hybrid silica membranes with a controlled retention of organic bridges[END_REF] or the Si-N asymmetric stretching vibrational mode. [START_REF] Giorgis | Optical, structural and electrical properties of device-quality hydrogenated amorphous silicon-nitrogen films deposited by plasma-enhanced chemical vapour deposition[END_REF] Despite the variety of the samples, three regions with almost linear evolutions can be distinguished, seemingly being dependent entirely on the nitrogen content. The first region (Level 1) is found between 0.0 and 4.0 at.% N, followed by a short region (Level 2) at 4.0-4.5 at.% N. Lastly, a region beyond 5.0 at.% N (Level 3) is encountered. The evolution of the 840-870 cm -1 absorption at each level, although monotonous, follows different slopes, decreasing at Level 1, then increasing for Levels 2 and 3. Given this behavior and the overlap of the Si-(CH3)3 and Si-N asymmetric stretching absorptions at this wavenumber, we suppose that for Level 1, namely for samples with <4.0 at.% N, the probed signal corresponds to a Si-(CH3)3 absorption. As the nitrogen content increases, these vibrations decrease as a result of more Si atoms being connected to N. For Levels 2 and 3 (4.0-4.5 and >5.0 at.% N), the absorption is most likely connected to the appearance of the Si-N asymmetric stretching vibrational mode, in good coherence with the increase in nitrogen content in the films.

The evolution of the position of this particular absorption is presented in Figure 3(f) as a function of the nitrogen content of each sample. For the previously defined Level 1 (<4.0 at.% N), the absorption is probed at ca. 840 cm -1 , corresponding well to the position of the rocking vibration of CH3 in Si-(CH3)3 groups. [START_REF] Ngamou | Plasma-deposited hybrid silica membranes with a controlled retention of organic bridges[END_REF] The wavenumber decreases down to 834 cm -1 for Level 2 (4.0-4.5 at.% N), with the vibration at lower frequencies indicating a lengthening of the bond between the atoms in question. The lengthening may be assigned to the replacement of electronegative atoms (O) in the surrounding network by atoms with lower electronegativity (N). It is likely that the bond absorbing at Level 2 is still the rocking vibration of CH3 in a Si-(CH3)3 complex, that is most likely transitioning towards a Si-(CH3)x complex. At Level 3 (>5 at.% N), the position shifts substantially to 870 cm -1 , in good correlation with the appearance of the asymmetric stretching vibration of the Si-N bond, noted between ca 880 and 840 cm -1 in the literature. [START_REF] Behrens | Geometric structure of thin SiOxNy films on Si(100)[END_REF] The shift of this absorption towards lower wavenumbers, similarly to the one observed between 7 and 13 at.% N (Figure 3(f)), has been associated with a decreasing N-H concentration. [START_REF] Bustarret | Configurational statistics in a-SixNyHz alloys: A quantitative bonding analysis[END_REF] The higher wavenumber (855 cm -1 ) noted for 14.6 at.% is potentially linked to deposition conditions.

It is noted that the Si-N asymmetric stretching absorption has been described as an indicator of

XPS

For samples presenting high carbon content (e.g. runs TD9 and TD11), the distinction between the Si-C and TO2 absorptions at 800 cm -1 by FT-IR analysis is not easily discernible. The two absorptions are oftentimes found to overlap, as is the case for the p1 sample of run TD11with 20.1 at.% C, the FT-IR spectrum and deconvolution of which are shown in Figure SI4 of Supporting Information.

To help better ascertain the presence of carbon incorporation as carbidic Si-C and the overall chemical environment of the elements, high resolution XPS spectra were considered.

Figure 4 presents XPS Si 2p, N 1s and C 1s spectra for p6 SiOxNy samples produced under varying O2 flow rates (runs TD7, TD9, TD11 and TD12) and their deconvolution based on literature data. [START_REF] Lo | Preparation of superhydrophobic polyimide films modified with organosilicasol as effective anticorrosion coatings[END_REF][START_REF] Nakaya | Impact of hot wire and material gas species on the Cat-CVD coating of gas barrier SiOC thin films onto PET bottles[END_REF][START_REF] Oulachgar | Chemical and Structural Characterization of SiONC Dielectric Thin Film Deposited by PSCVD[END_REF] A summary of the atomic concentrations obtained by integration of each binding energy peak is found in Table SI2 of Supporting Information. It is noted that the electron configuration gives information on the composition and the bonding state of atoms, which can effectively evolve and change based on the applied process conditions. For high O2 flow rate, as is the case for the TD12 film eV). [START_REF] Zhang | Higher than 60% internal quantum efficiency of photoluminescence from amorphous silicon oxynitride thin films at wavelength of 470 nm[END_REF] The XPS results further underline the transition from a RMM state (Ox-Si-Ny) to RBM (distinction of Si-Nx rich configuration), as revealed by FT-IR. eV) [START_REF] Chainani | Electronic structure of carbon-free silicon oxynitride films grown using an organic precursor hexamethyl-disilazane[END_REF] and Si-Si2-O-N (101.2 eV). [START_REF] Zhang | Higher than 60% internal quantum efficiency of photoluminescence from amorphous silicon oxynitride thin films at wavelength of 470 nm[END_REF] The final component with a binding energy of 100.3 eV is attributed to Si-Si-C or Si-O1-C3. [START_REF] Nakaya | Impact of hot wire and material gas species on the Cat-CVD coating of gas barrier SiOC thin films onto PET bottles[END_REF] No low energy component near 99.0-99.6 eV (Si-(Si)4) was detected, testifying the absence of Si clusters in the samples and confirming the amorphous state. [START_REF] Zhang | Higher than 60% internal quantum efficiency of photoluminescence from amorphous silicon oxynitride thin films at wavelength of 470 nm[END_REF] The N 1s spectrum obtained for TD12 conditions (Figure 4(h)) is deconvoluted into a primary peak (398.0 eV), intermediate to N-Si3 (397.4-398.4 eV) and Si2-N-H (398.0 eV). [START_REF] Shallenberger | Characterization of silicon oxynitride thin films by x-ray photoelectron spectroscopy[END_REF] The observed binding energy hints at a predominant surrounding of N by Si (N-Six) evidencing the deposition of a nitride-type material, non-excluding the presence of N-H bonds. This binding energy shifts towards 397.7 eV as the O2 flow rate is decreases (Figure 4(f) and (e)), evidencing a loss in N-H bonds and increasing N-Si3 bonding peak. [START_REF] Shallenberger | Characterization of silicon oxynitride thin films by x-ray photoelectron spectroscopy[END_REF] This is coherent with the transition towards a RMM state revealed by the FT-IR results. A second component attributed to Si-N-H2 groups is probed with a binding energy of 398.9 eV. [START_REF] Shallenberger | Characterization of silicon oxynitride thin films by x-ray photoelectron spectroscopy[END_REF] This binding state is unique to TD12 and is not probed for the other runs. Instead, the emergence of a component at 399.8 eV is noted for TD7 (Figure 4(g)), corresponding to a mixed nitrideoxide environment with a O-N-Si2 bonding scheme. [START_REF] Shallenberger | Characterization of silicon oxynitride thin films by x-ray photoelectron spectroscopy[END_REF] As the O2 flow rate decreases for TD11 and TD9

(Figure 4(f) and (e)), its position shifts to 399.2 eV, likely linked to an aliphatic C-NH2 amine.

The C 1s XPS spectrum obtained for TD12 conditions (Figure 4 [START_REF] Lo | Preparation of superhydrophobic polyimide films modified with organosilicasol as effective anticorrosion coatings[END_REF][START_REF] Nakaya | Impact of hot wire and material gas species on the Cat-CVD coating of gas barrier SiOC thin films onto PET bottles[END_REF] If present, a C(sp2)-N environment could contribute to the peak at 286.3 eV, [START_REF] Fainer | Thin silicon carbonitride films are perspective low-k materials[END_REF] A small percentage of Si-C is also probed. However, only up to5 % of the incorporated carbon is found in a Si-C-Si environment.

Correlating this back to the FT-IR results (Figure 3(d)), it could be proposed that the peak probed at ca. 1020 cm -1 under low O2 flow rates (TD9, TD11) should be attributed to mixed Si-O-Si TO3 and Si-CH2-Si absorptions. Similarly, the peak at ca. 800 cm -1 is very likely a mixed TO2/Si-C absorption.

Globally, the increased O2 supply is reflected in the higher oxygen content of films as seen in the atomic concentrations of Table SI2 ( The zoom at the interface in Figure 5(a) reveals a porosity-free, intimate and regular contact between substrate and film. The interface is also easily observable through Z-contrast imaging in STEM-HAADF mode at the atomic scale in Figure 5(c). The Si-substrate being heavier compared to the SiOxNy film, it appears brighter. It is noted that the Si/film interface region constitutes a critical factor for the photoelectronic properties of luminescent silicon and consequently the performance in electronic and optical devices. According to Daldosso et al., a sharp and stable Si/SiO2 interface provides passivation of non-radiative defect states on the surface, aiding in stable light emission. [START_REF] Daldosso | Role of the interface region on the optoelectronic properties of silicon nanocrystals embedded in SiO2[END_REF] In order to ascertain the particular binding configuration, i.e. RMM or RBM, a Principal Components Analysis (PCA) decomposition is carried out on the EELS Spectrum-Imaging data after denoising, using the Multivariate Statistical Analysis (MSA) program in the GMS 3.4 Gatan software. This process allows highlighting different components represented by maps (Figure SI5 of Supporting Information). Coupling this purely mathematical treatment with the elemental compositions (Figure 5(d)) reveals the complexity of the interface with the presence of 5 different domains. First, the Si-substrate is noted, followed by a carbonrich layer (C layer), a nitrogen-rich layer (N layer) and a SiOxNy layer with carbon-enriched domains (C-rich area). These variations of compositions occur over a very small thickness, around 2 nm, representing the first 3 min of the deposition process during which equilibrium may not have been reached yet. Globally through the film, the matrix seems to contain all the elements, indicating a varying SiOxNyCz composition, without the presence of separate SiO2 and Si3N4 phases. The film is generally amorphous and adheres to the RBM model, with simultaneous presence of Si-O and Si-N towards a homogeneously distributed Nx-Si-O4-x fourfold coordination. Both H and E are noted to evolve in a similar manner, exhibiting a slightly increasing trend for higher nitrogen incorporation. Based on the previously discussed levels of nitrogen incorporation (Figure 3(e)), this evolution can be divided in three regions. Samples with up to 5.5 at.% N exhibit an average hardness and elastic modulus of 4.8 and 53 GPa, respectively. The second region is noted between 5.5 and 7 at.% N, in which the H and E values are observed to increase with the enrichment of the films in nitrogen. A third region could be defined for samples with a composition above 7 at.% N, for which the H and E values appear to stabilize at ca. 7.0 and 70 GPa, respectively. The results illustrate how the partial and progressive replacement of O atoms by N prompts a change in the material network and density despite the constant deposition temperature, which as a result enhances the mechanical properties. The values of H and E have been noted to increase with increasing nitrogen contents in the literature as well. [START_REF] Tang | Optical, structural, and mechanical properties of silicon oxynitride films prepared by pulsed magnetron sputtering[END_REF] It is also recalled that a potential decrease of N-H concentration in the films, based on the discussion of Figure 3(e) and (f), replaced by the creation of additional N-Si bonds in the material network, might also be contributing to the gradual increase of the mechanical properties in the films. In general, both E and H measure comparably to values obtained from PECVD SiOxNy films containing ca. 10 at.% N, reported at 78.9 and 7.8 GPa, respectively. [START_REF] Gan | Material Structure and Mechanical Properties of Silicon Nitride and Silicon Oxynitride Thin Films Deposited by Plasma Enhanced Chemical Vapor Deposition[END_REF] The critical load, namely the load required for achieving complete delamination of a film from its substrate, is also shown in Figure 6. Critical load values for films produced from various runs are presented as a function of their nitrogen content. The films exhibit good adhesion to the Si substrate, with critical load values ranging between 12.2 and 22.5 mN, for compositions up to 7 at.% N. Beyond 7 at.% N, the resistance to delamination increases drastically to 89.7 mN. This increased adhesion can be attributed to the enrichment of the films in nitrogen and subsequent increase of N-Si bonds between deposited material and substrate and to the increased film mechanical properties. Recalling Figure 5(d),

Mechanical and functional properties

Mechanical properties

the EELS elemental composition profile of the sample (ca. 6 at.% N) exhibited indeed a prominent SiOxNy presence at the interface, alongside N-rich areas. Localized and/or partial delamination may be observed for low values of load, as shown in Figure SI6 of Supporting Information. These local phenomena were however observed rarely, and did not appear systematically.

Wettability

Water wettability measurements on SiOxNy films from various runs are presented in Figure 7.

Water contact angle (WCA) values range from 76.9-89.4° and seem to be independent of the nitrogen content (Figure 7(a)). Trevino et al. [START_REF] Trevino | Isoelectric points of plasma-modified and aged silicon oxynitride surfaces measured using contact angle titrations[END_REF] reported similar values (73.6°) for a thermally grown SiOxNy surface with 14.5 at.% N and 14.4 at.% C. No clear correlation was noted when plotting WCA values as versus the carbon and hydrogen contents. Given this non-linear behavior, a multi-factorial dependency of the WCA can be supposed. Such factors can be the overall film composition and surface termination, the roughness and/or surface energy. In the case of silicon nitride materials, surface termination is affected by -OH, -NH2 and -CH3 groups, with respective contact angles of 42°, 53° and 75° being reported. [START_REF] Tsukruk | Adhesive and Friction Forces between Chemically Modified Silicon and Silicon Nitride Surfaces[END_REF] It is also worth recalling the reduction in wettability for Si-H terminated surface compared to Si-OH. [START_REF] Zhang | Growth of silicon oxynitride films by atmospheric pressure plasma jet[END_REF] Similarly, replacement of NH or NH2 surface bonds by the less polar Si-H increases the WCA of Si3N4 films. [START_REF] Arafat | Tailor-made functionalization of silicon nitride surfaces[END_REF] For oxynitride materials, WCAs spanning at 98-110° were noted by Zhang et al. for SiOxNy coatings with high Si-H content. [START_REF] Zhang | Hydrophobic, transparent and hard silicon oxynitride coating from perhydropolysilazane[END_REF] Based on the above, the measured WCA values are plotted as a function of the intensity of the Si-H wagging or rocking vibrational mode.

Although this particular bond absorbs very weakly at 630-650 cm -1 for the films produced in this work, plotting of the WCA values yields an almost linear trend (Figure 7(a)). This result should be consolidated with dedicated, on purpose experiments. It is also noted that the trend line does not pass through the origin of the coordinate axes, presenting an intercept value of 76.9°. The wettability behavior of these films is segregated and examined as a function of their respective process parameters. More specifically, Figure 7(b) shows the change in WCA for increasing NH3 supply under a fixed O2 supply (runs TD6, TD7, TD8), and for increasing O2 flow rate in absence of ammonia (runs TD7, TD11, TD12). In the first dataset, higher NH3 supply appears to decrease the WCA. This may be assigned to -CH3 cleaving of organic moieties by NH3 and reduction in carbon as was revealed by IBA (Figure 2(a)), the presence of which conventionally increases the hydrophobic character of the films. 65 67 An alternative interpretation is likely the insertion of further surface -N-Hx bonds in the films for higher NH3 supply. Si-NH-Si bonds are hydrophilic given their polarity and quick reaction with air humidity. [START_REF] Zhang | Hydrophobic, transparent and hard silicon oxynitride coating from perhydropolysilazane[END_REF] No correlation was noted however when plotting the WCA as a function of the N-H 3300-3400 cm -1 infrared absorption.

An increase of the O2 supply in ammonia-free experiments (runs TD7, TD11, TD12) slightly increases the WCA (Figure 7(b)). This behavior is unexpected, since higher O2 supply decreases N incorporation and enhances the oxide character of the material, reported to facilitate dipole interaction and hydrogen bonding between Si-OH and H2O, resulting in more negatively charged and hydrophilic surfaces. [START_REF] Hong | The effect of nitrogen incorporation on surface properties of silicon oxynitride films[END_REF] The slight increase in WCA is thus difficult to explain, but could be related to a replacement of hydrophilic NHx groups by slightly less hydrophilic Si-OH ones.

Although only two points were analyzed, the effect of the deposition temperature is also considered (Figure SI7 of Supplementary Information). A lower contact angle is noted, although this result cannot be attributed solely to the deposition temperature and may be also linked to the higher N concentration and by extension higher amount of hydrophilic NHx bonds. Overall, given the multifactorial dependency of the wettability behavior on the surface termination, composition and surface roughness, the WCA results cannot be explained solely as a function of isolated deposition conditions.

Wet etching corrosion resistance

The wet etching corrosion resistance of SiOxNy films is presented in Figure 8. The evolution of the BOE 6:1 etching rate of films produced from various runs is shown in Figure 8(a), plotted as a function of their nitrogen content. An overall decreasing trend is noted, with the etching rate being reduced as the nitrogen content of the films increases. A maximum etching rate of 74.6 Å.s -1 is measured for SiOxNy films with low nitrogen incorporation, decreasing to near-zero values of 0.1 Å.s -1 for films with higher composition in nitrogen. The power curve added for visual aid reveals a convergence to values below 4 Å.s -1 beyond ca. 7 at.% N. It can be proposed that, for the tested conditions, ca. 7 at.% N is sufficient in providing an excellent corrosion resistance and that further N incorporation beyond this value is not necessary. Such information is important for the definition of operating conditions that are aimed at to produce films with a targeted corrosion resistance behavior. temperatures, plotted as a function of their nitrogen content. The local temperature of each sample is also included. For a sample with 0.1 at.% N produced at a local temperature of 609°C outside the isothermal region, the BOE etching rate is 57.5 Å.s -1 . In contrast, a sample with 9.5 at.% N taken from the isothermal region (local temperature of 650°C) exhibit an etching rate of 0.9 Å.s -1 . Similar overall reductions are observed for a simultaneous increase in temperature and nitrogen content. Additionally, the evolution of Figure 8(b) presents a break in its slope: a steep decrease in the 0-2.5 at.% N range is noted, attributed to the temperature effect. Between 2.5 and 10 at.% N the slope appears milder, with the temperature remaining constant for these samples. Thus, although the etching rate seems to depend strongly on the process temperature, is must be recalled that this process parameter also influences the level of N incorporation. is added in the inlet gas composition, a BOE etching rate of 3.9 Å.s -1 is noted. An increase of the O2 flow rate to 0.6 sccm results in a moderate increase of the BOE etching rate to 7.4 Å.s -1 . This value increases sharply to 16.2 Å.s -1 for an O2 supply of 1.2 sccm, and the reduced corrosion resistance is correlated to the higher oxide and loss in nitride character. Similar results were reported for Si3N4-to-SiO2 films produced with an increasing addition of O2 in the inlet gas composition, where BOE etching rates increased dramatically up to 83 times with gradual increase in O incorporation. [START_REF] Volkov | Influence of oxygen on the composition and some properties of the films obtained by r.f. sputtering from a Si3N4 target[END_REF] Resistance to wet chemical etching is therefore strongly linked to the structural modifications brought about by the O/N substitution. Further N incorporation increases the overall compactness and rigidity of the network as it changes towards RMM.

Surface roughness

The surface roughness of the films was examined by atomic force microscopy before and after subjection to the BOE 6:1 solution. As presented in Figure 9, the produced SiOxNy films are smooth with uniform surfaces. Their root mean square (RMS) roughness decreases from 1.05 to 0.62 nm with increasing nitrogen, showing an asymptote beyond 6 at.% N. A similar decreasing trend was noted by Tang et al., [START_REF] Tang | Optical, structural, and mechanical properties of silicon oxynitride films prepared by pulsed magnetron sputtering[END_REF] who reported a RMS decrease from 1.33 to 0.97 nm with parallel increase of the nitrogen content. In the present work, the RMS roughness also follows an increasing trend with increase of the deposition rate of the individual samples, independently of the deposition conditions (results not shown). Solid symbols refer to samples that were produced at temperatures below 650°C. Trend lines added as visual aid.

After subjection to the BOE etching solution, the RMS roughness is noted to increase for films containing up to 5 at.% N, as noted by the ΔRMS roughness. No singular behavior is observed in this region however. Starting with a sharp increase from 0.2 to 4.8 nm, the ΔRMS roughness decreases back to 0.2 nm when the N content is increased from 4 to 5 at.%. Delta values below 0.2 nm are noted once the film composition surpasses 5 at.% N, in agreement with the limited sensitivity of these films to the corrosion test. Plotting of the post-BOE or delta RMS roughness as a function of the deposition rate or the particular deposition conditions, such as temperature or addition of NH3 in the starting inlet composition, did not reveal any correlation. The results indicate a difference in etching mechanism, strongly linked to the short-range order. Corrosion resistance is globally improved upon further addition of nitrogen as illustrated in Figure 8(a), yet results in a non-uniform etching profile. Interaction between the etching solution and the extreme sample surface appears to be dependent on the particular Ox-Si-Ny tetrahedral configuration of the atoms in the network, resulting in a quasi site-selective etching that is likely more pronounced in regions where the Ox-Si-Ny tetrahedra exhibit x>2>y or even y=0. Moreover, it is recalled that a significant change in the C and N incorporation types was observed at around 5 at.% N, as revealed by FT-IR (Figure 3(e) and (f)). The presence of highly covalent Si-C bonds was reported to reinforce the nitride network, resulting in improved etching resistance. [START_REF] Gumpher | Characterization of low-temperature silicon nitride LPCVD from bis (tertiary-butylamino) silane and ammonia[END_REF] In contrast, organic carbon (e.g. -CH3, -CHx, -C=O) as observed by FT-IR and XPS in the present work, could be localized, given the C-rich areas revealed by HR-TEM (Figure SI5 of Supporting Information). This may lead to more etching-susceptible sites, increasing the RMS roughness.

Conclusions

Given the tunable character of SiOxNy films and their attractiveness for numerous applications, especially those challenged by deposition at lower temperatures, it is pertinent to investigate the influence of the process parameters on the material properties. The thermal chemical vapor deposition of SiOxNy films from a new precursor, tris(dimethylsilyl)amine (TDMSA), was executed at 580-650°C, a thermal range lower compared to the ranges utilized for conventional disilazane precursors. A parametric analysis of composition trends was performed as a function of operating conditions such as NH3 and O2 flow rates and deposition temperature. It was demonstrated that higher NH3 supply decreases the film C content. For the studied range of parameters, deposition rate and film N content remained insensitive to variations of the ammonia flow rate, yielding films with maximum 7.5 at.% N.

Inclusion of NH3 in the starting chemistry was not mandatory for the production of SiOxNy films, thanks to the role of the TDMSA precursor as a dual Si and N source. In the case of O2, higher supply promoted O-rich film formation, with a competition existing between O and N incorporation.

Compositional trends highlighted by spectroscopic ellipsometry were confirmed using ion beam analyses. The produced films were hydrated, with hydrogen being involved in Si-OH, CH3, CHx, and N-H moieties as revealed by FT-IR. The films were generally amorphous, globally described as a SiOxNy

For Table of Contents Only

Figure 1 .

 1 Figure 1. Photograph of Si-sample array after deposition of run TD3 (a) and evolution of the deposition rate (DR, squares), the N (triangles) and C (diamonds) contents, as measured by spectroscopic ellipsometry, for variable NH3 flow rate (runs TD6, TD7, TD8) at SPT 650°C, with the corresponding temperature profile (grey triangles) (b).

Figure 1 (

 1 b) presents the evolution of the average DR and film content in N and C along the reaction chamber for runs TD6, TD7 and TD8. Deposition is initiated at ~510°C, albeit with a notably low DR of ca. 0.05 nm.min -1 for all three runs and in absence of nitrogen in the films. Incorporation of nitrogen is encountered only on films for which the local temperature exceeds 610°C. A maximum DR of 0.64 nm.min -1 is achieved in the isothermal region (650°C) for TD7, decreasing slightly to 0.59 and 0.55 nm.min -1 with NH3 flow rate increase from 20 to 40 sccm in TD6 and TD8, respectively.

Figure 2

 2 Figure2presents the film composition in Si, O, N, C and H, as obtained from ion beam analyses (IBA), for runs TD6 to TD14. Sample p2 was analyzed for all runs, targeting a minimum film thickness of 80-100 nm required for IBA.

Figure 2 .

 2 Figure 2. Ion beam analysis composition results for films produced under varying NH3 (a) and O2 (b) flow rates.

Figure 3

 3 Figure 3 explores the FT-IR chemical bonding in SiOxNy films. A representative spectrum of sample p1, taken from the isothermal region of run TD7 is shown in Figure 3(a). Multiple absorptions are identified. The characteristic transverse optical TO1, TO2 and TO3 modes corresponding respectively to the rocking, bending and symmetric stretching vibrational modes of the Si-O-Si bond are

  Figure 3. FT-IR analysis. Survey spectrum of the p1 SiOxNy sample from run TD7, analyzed at 60° incidence angle (a). Evolution of the fingerprint region as a function of the O2 flow rate (b). Evolution of Si-(CH3)x and LO3 positions (c), TO3 position and TO2 intensity (d) as a function of the O2 flow rate. Evolution of the intensity (e) and position (f) of the absorption probed between 830-870 cm -1 for SiOxNy samples produced under various conditions, as a function of their nitrogen content.

  TO2 mode. Therefore, we propose that for low O2 flow rates, and by extension low film composition in O, a disappearance of the LO3-TO3 pair (Figure 3(b)) results in carbon-related counterparts absorbing at wavenumbers that are classically attributed to Si-O absorptions, i.e. Si-CH2-Si instead of TO3, and Si-C instead of TO2.

  the binding state in a SiOxNy network. Two types of local structures are suggested by Behrens et al.,[START_REF] Behrens | Geometric structure of thin SiOxNy films on Si(100)[END_REF] i.e. a random mixture model (RMM) and a random binding model (RBM). RMM is defined as a mixture of randomly distributed SiO2 and Si3N4 phases, resulting macroscopically in an amorphous SiOxNy network. In RBM on the other hand, "the bonds between Si, O and N constitute a fourfold coordination and represent a homogeneous O-Si-N network in five different tetrahedral configurations of SiOvN4-v, where v=0-4", as per Behrens et al. and Shahpanah et al.[START_REF] Shahpanah | Improving the oxygen barrier properties of PET polymer by radio frequency plasma-polymerized SiOxNy thin film[END_REF][START_REF] Behrens | Geometric structure of thin SiOxNy films on Si(100)[END_REF] Multiple authors have linked the presence of this asymmetric stretching Si-N absorption to Si3N4, and by extension to the RMM model.[START_REF] Scardera | Formation of a Si-Si3N4 nanocomposite from plasma enhanced chemical vapour deposition multilayer structures[END_REF][START_REF] Yonekura | Influence of film structure on gas barrier properties of SiOxNy films[END_REF]

(1. 2 sccm

 2 photoelectron peaks is encountered when the valence state of the bonding atoms changes, namely a multiphase configuration shift from N-Si-O3 (103.0 eV), to N2-Si-O2 (102.8 eV) and to N3-Si-O (102.2

Figure 4 .

 4 Figure 4. Si 2p (spectra a, b, c and d), N 1s (spectra e, f, g and h) and C 1s (spectra i, j, k and l) XPS spectra of SiOxNy p6 samples produced under variable O2 flow rate mentioned on the right (runs, TD7, TD9, TD11, TD12).

56 TEM

 56 13.54 at.% for TD9, 0 sccm O2, increasing to 49.69 at.% for TD12, 1.2 sccm O2). The evolution of all elements are coherent with IBA results. The environment of Si appears to be changing substantially as a function of the O2 supply. At the highest O2 flow rates, Si is involved primarily in Si-O4 and O-N-Si-O configurations. Overall, the XPS measurements verify the presence of a Nx-Si-Oy bonding complex in the amorphous SiOxNy films through formation of individual Si-O and Si-N bonds. It is noted that the cumulative Si 2p peak position shifts from ca. 103.1 to 101.4 eV with decreasing O2 flow rate, notably starting close to the SiO2 binding energy (103.4eV) and ending up near the Si3N4 binding energy (101.9 eV).To ascertain the structure of the deposited films, high-resolution TEM has been carried out, shown in Figure5, for a 500 nm thick film with ca. 6 at.% N, deposited in the isothermal region of run TD10. The high-resolution cross section of Figure5(a) illustrates the amorphous SiOxNy film in contact with the [011] oriented monocrystalline Si substrate. A profile of nanodiffraction patterns has been collected along the depth of the film, spaced of 60 nm apart and probing an area of approximately 110 nm². All the diffraction patterns (i.e. at the surface, interface and core of the film), are similar to the one of Figure 5(b), and reveal no diffraction spots, therefore confirming the amorphous nature of the film, independently of the probed depth.

Figure 5 .

 5 Figure 5. High resolution TEM image of the film-substrate interface (a). Nanodiffraction pattern taken from the core of the SiOxNy film (b). STEM-HAADF image of the interface (c). Elemental composition profile obtained from EELS Spectrum-Imaging (Si in green, C in yellow, N in blue and O in red) and where the 5 different domains determined by PCA decomposition are specified (d).

Figure 6

 6 Figure 6 presents the mechanical properties of SiOxNy films as a function of the nitrogen content. Samples from the isothermal region of run have been analyzed for assessment of the elastic modulus E and hardness H. Since the temperature in this region was constant and film thickness measured ca. 550 nm, any change in these values can be safely attributed to an evolution of the local film composition and, by extension, the network structure.

Figure 6 .

 6 Figure 6. Mechanical properties of films deposited from TDMSA as a function of the nitrogen content. Elastic modulus and Hardness of various samples taken from the isothermal region of run TD10. Nanoscratch critical load results were obtained from p1 samples of various runs. Trend lines added for visual aid.

Figure 7 .

 7 Figure 7. Water wettability results for SiOxNy films from various runs (TD6, TD8, TD7, TD11 and TD12). Evolution of the contact angle as a function of the nitrogen content and Si-H FT-IR absorption at ca. 630-650 cm -1 (a), and NH3 flow rate and O2 flow rate in absence of NH3 (b). Trend lines serving as guide to the eyes.

Figure 8 .

 8 Figure 8. Evolution of the BOE etching rate as a function of the nitrogen content, for samples deposited at various conditions (a), for samples of run TD3 (empty black squares) and for samples deposited for varying O2 flow rates (blue triangles) (b). Trend lines added as guide for the eyes.

Figure 8 (

 8 Figure 8(b) also examines the effect of the O2 flow rate on the BOE etching rate. When no O2

Figure 9 .

 9 Figure 9. Evolution of the RMS roughness of SiOxNy before subjection to etching by the BOE solution (blue squares) and ΔRMS roughness after BOE (red circles), produced at various conditions.

  network, occasionally enriched locally in C and N. The organic character of the incorporated carbon and the low percentage of carbidic Si-C configuration was confirmed using XPS. The electron configuration revealed a substantial change in the bonding state of the atoms depending on the process conditions. Primarily, a shift from Si-N-H2 and Si-O-N-Si2 towards N-Si3 with decreasing O2 flow rate was highlighted, indicating a shift toward a random mixture model (RMM) of homogenously mixed oxide and nitride configurations.Simultaneously, the evolution of various film properties reflected the transition in the network structure. Evolution trends could arbitrarily be defined at 2 or 3 distinct levels, depending on the studied property. Globally, an increase in the nitrogen content resulted in an easily observable transitional region detected in the 3-6 at.% N range, within which material properties follow non-linear trends. Beyond ca.6-7 at.% N, properties such as the film hardness, elastic modulus, surface roughness and etching rate adhered to linear behaviors again and appeared to converge to constant values. Other properties on the other hand, such as film adhesion established through nanoscratch critical load, present a dramatic increase beyond 7 at.% N. Incorporation of C and H atoms could play a significant role in the evolution of these transitional regions. Increased RMS roughness in the 3-5 at.% N range after subjection to hydrofluoric solution further supports the influence of the particular tetrahedral configuration and medium-range order on the material properties. Potential site-selective etching induced by regions rich in O, H and/or organic C is assumed.Despite the presence of C and H, the produced SiOxNy films exhibited excellent resistance to wet etching corrosion in BOE 6:1 solution, with very low etching rates (0.1 Å.s -1 ) noted for films with >7 at.% N. Definition of this minimum required value is helpful for framing appropriate deposition parameters for etch-resistant SiOxNy films applied as wet corrosion protective coatings or etch-stop liner layers. The illustrated methodology serves as a demonstration for linking film composition, structure and properties to the CVD process conditions, and engineering amorphous SiOxNy films tailored for the envisioned applications. Supporting Information: Schematic representation of experimental setup, FT-IR absorptions encountered in the produced SiOxNy films and their deconvolution, atomic concentrations of binding states for N 1s, C 1s, O 1s and Si 2p XPS spectra after deconvolution and EELS Spectrum-Imaging details.

Table 1 .

 1 Experimental conditions for the production of SiOxNy films from TDMSA.

	.

  of Supporting Information. The SE data were fitted using a Bruggeman Effective Medium Approximation (BEMA) model in the Semilab SEA software. A three-component model was applied, comprised of SiO2, Si3N4 and amorphous-C. Details on how this model was calibrated for extraction of SiOxNy film composition with non-negligible amounts of carbon are described elsewhere.[START_REF] Topka | An innovative kinetic model allowing insight in the moderate temperature chemical vapor deposition of silicon oxynitride films from tris(dimethylsilyl)amine[END_REF] Results were deemed appropriated for a linear regression fit R 2 ≥ 0.99. The average thickness (and by extension average deposition rate) and average film composition in Si, O, N and C were calculated from the aforementioned five points.

The stoichiometry of SiOxNy films was measured by coupling Rutherford Backscattering Spectroscopy (RBS) and Nuclear Reaction Analysis (NRA) techniques. Additional composition results considering the composition in H were also obtained by Elastic Recoil Detection Analysis (ERDA).

  3sccm O2 to 7.3 at.% at 1.2 sccm O2. In terms of hydrogen content, the IBA results reveal concentrations varying non-monotonously from 17.2-22.7 at.% H, with lower levels of H observed for 1.2 sccm O2. In view of the composition trends, it is concluded that the evolution of the hydrogen content is complex, due to the multiple forms it can be incorporated as: N-H, -CHx, adsorbed H2O, Si-OH terminal group and potential Si-H. Evidence for the presence of such bonding states relies on infrared analysis, as presented in the next section.

Acknowledgements

This work was supported by the Agence Nationale de la Recherche (ANR) under the contract HEALTHYGLASS ANR-17-CE08-0056. This project has benefited from the facilities of the platform MACLE-CVL. This platform was co-funded by the European Union, the Region -Centre Val de Loire and The French minister of research (MESRI -DRRT). Europe is committed to the Centre-Val de Loire region with the European regional development fund (ERDF). The authors are indebted to Olivier Debieu, Olivier Marsan and Claudie Josse for their advice and assistance with spectroscopic ellipsometry, FT-IR and FIB techniques, respectively.


Luis A Casillas

Maulik K Patel

Kurt E Sickafus

Gianguido Baldinozzi
email: gianguido.baldinozzi@centralesupelec.fr

Rationalization of the influence of the chemical bonds on the radiation tolerance of compounds related to the M 7 O 12 oxygen

Cerium, praseodymium, and terbium stand out among other rare-earth systems because they exhibit two possible oxidation states that produce a series of anion deficient phases that are fluorite-related and have a generic formula Ln n O 2n-2 . Since the seminal works of Eyring, and the valuable contributions of many other scientists, many of the occurring phases were determined using scattering techniques. It was then possible to propose a paradigm for the particular oxygen vacancy order that is observed in most of these phases We would like to discuss how the structural characteristics of these systems can affect their radiation tolerance. In this contribution, we analyze using Bader's analysis the charge density and the chemical bonds of the A-type, B-type, and the i phase, which corresponds to the generic compound formula with n=7. Then we try to correlate these results with irradiation experiments on simple lanthanide oxides and with the behavior of the broad family of d phases that are pseudo-binary compounds with generic formulas A 6 BO 12 or A 4 B 3 O 12 that share many structural features.

Polyhedra and structural blocks Polymorphic and morphotropic transformations Topologigal charges and bond strength [3] Conclusion

The existence of a cation ordering temperature T co tells us that below some energy per site E co the space bifurcates into regions corresponding to different "solutions," and that as we go lower and lower in energy (or temperature) the space breaks up even more. This gives us a clear reason why reorganizing is hard: If we are in the wrong region, we have to cross an ordering energy and entropy barrier that is very high to achieve a more stable state. This also motivates a thorough study of the radiation behavior of non-stoichiometric lanthanides as their properties are still largely unknown but the absence of cation selectivity simplifies the overall problem.

'Heating' up quickly with Swift Heavy Ion irradiation …

As d-electrons in many transition-metal oxides, the f-electrons play an important role for the transport properties in rare-earth oxides. We investigate how the local environments change the charge density in various reference structures. It is common belief (e.g. CeO 2 ) that in LnO 2 4 valence electrons nominally leave the lanthanide atoms and transfer into the p-bands of two oxygen atoms while in Ln 2 O 3 f-electron localization occurs. The O vacancy formation process involves the localization of 2 electrons into f-level traps onto two lanthanide atoms. This implies a strong correlation between formation, migration, and ordering of virtual Ln III -vacancy complexes [START_REF] Adachi | Binary Rare Earth Oxides[END_REF][START_REF] Zinkevich | [END_REF].

A-type B-type i-type

Results of the AIM Bader analysis and bond covalency character according to the Bader-Essen rules.

The pseudobinary compounds RE 3 B IV 4 O 12 with d phase structure (isostructural to the i phase Ln 7 O 12 ) exhibit excellent radiation resistance Fast amorphization was induced in a few A 2 O 3 compounds irradiated by Swift Heavy Ions and a dependence on cation ionic radius was claimed [4] but differences among their electronic configurations (Eu III can reduce, Nd III cannot, and Mn is a compound with only d-states) still leave this matter about the influence of cation radii open.

On the other hand, ordered d phase structures (RE 6 B VI O 12 ) can experience higher migration entalpies because of the cation selectivity and this justifies the easy amorphization of these compounds [6].

Lack of cation selectivity and the easy reorganization mechanism of the anion sublattice seem essential [5]. In this respect, the weakness of the 7th bond of the polyhedra of the i structures seems a critical ingredient for the easy rearrangement of the anion sublattice as the diffusion barriers are known to be low. Larger charges at the BCPs are representative of shared contributions among the atoms.

The predicted strength of the 7th bond (and of all bonds) is significantly weaker in Tb-based compounds.