# Sputtered niobium pentoxide layers for optical applications

N. Massoni, L.E. Nistor, C. Licitra, E. Nolot, G. Rodriguez

► To cite this version:

N. Massoni, L.E. Nistor, C. Licitra, E. Nolot, G. Rodriguez. Sputtered niobium pentoxide layers for optical applications. Thin Solid Films, 2024, 800, pp.140424. 10.1016/j.tsf.2024.140424. cea-04770970

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Submitted on 7 Nov 2024

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# Sputtered niobium pentoxide layers for optical applications

N. Massoni<sup>a, *</sup>, L.E. Nistor<sup>b</sup>, C. Licitra<sup>a</sup>, E. Nolot<sup>a</sup>, G. Rodriguez<sup>a</sup>

## ARTICLE INFO

### Keywords:

Sputtering
Niobium oxide
Image sensors
Bragg mirror

## ABSTRACT

The use of pulsed DC magnetron sputtered niobium pentoxide as a high index material in optical devices as Anti-Reflecting Coating (ARC) and optical filters has been investigated. When deposited on a silicon substrate with native oxide, the few nanometers of niobium pentoxide were optically distinct from the rest of the niobium pentoxide layer, effect not observed on thermally oxidized silicon substrates. The dispersion curve of this interfacial layer was determined and used to simulate the reflectometry curves. Next, an ARC consisting of a bilayer of niobium pentoxide and silica was fabricated and its optical response was simulated with this interfacial layer. Finally, a Bragg mirror consisting of seventeen layers of niobium pentoxide and silicon dioxide was realized, underlining that this interfacial layer does not affect the optical response when several layers were at stake.

## 1. Introduction

For the electronics market, the conversion of light into an electrical signal is a fundamental process provided by image sensors based on Charge Coupled Device (CCD) or Complementary Metal Oxide Semiconductor (CMOS) technologies [1,2]. Over the last twenty years, continuous research has improved CMOS image sensors (CIS), making them the dominant technology, particularly for mobile phones. The current trend is to move from a simple optical imager to a smart imager, integrating functions such as depth information [3]. These new capabilities require some spectral bands of the light spectrum to be isolated by optical filters [4]. These optical functions can be an anti-reflection coating (ARC), where reflection is minimized for a specific wavelength to maximize light through the photodiode, or a Bragg mirror, commonly referred as a bandpass filter, where reflection is maximized over a continuous range of wavelengths. These optical functions are deposited directly on the CIS by a combination of high index (HI) and low index (LI) layers, the design of which is described in detail in the literature [5]. For a Bragg mirror, the thickness e of each layer is constant and is given by $\lambda_0$, where $\lambda_0$ is the center wavelength of the bandpass filter and n is the refractive index at the wavelength considered $\lambda_0$. As an example, a combination of five bilayers [HI/LI], denoted [LI/HI]x5, deposited on a silicon substrate produces a Bragg mirror centered at $\lambda_0$ with a bandpass $\Delta\lambda$ estimated by Eq. (1).

$$ \Delta\lambda \approx \frac{4}{\pi} \sin^{-1} \left[ \frac{n_{HI} - n_{LI}}{n_{HI} + n_{LI}} \right] \times \lambda_0 \quad (\text{Eq. 1}) $$

It can be seen that the larger the refractive index difference, the larger the bandpass filter. The most common LI material is silicon dioxide, which has a refractive index of about 1,46 in the visible range. HI materials include titanium oxides [4], tantalum oxides [6] or tantalum nitrides [7]. This study focused on sputtered niobium oxide as HI material which main advantages are (i) a higher index than tantalum pentoxide and (ii) a constant refraction index for wavelengths above 500 nm [8]. As a single layer obtained by sputtering, the most stable phase is niobium pentoxide Nb$_2$O$_5$ (Nb$^{+V}$) [9,10]. Many studies report the tuning of niobium pentoxide properties by changing the process parameters such as power, pressure, oxygen flow and DC pulse ratio for pulsed DC power [11–17]. The deposition of niobium suboxides such as niobium monoxide NbO (Nb$^{+II}$) or niobium dioxide NbO$_2$ (Nb$^{+IV}$) by sputtering is also possible in a low oxygen atmosphere from a metallic niobium target [18,19]. These optical layers can be realized by Atomic Layer Deposition (ALD) or Chemical Vapor Deposition (CVD). In this work, we investigate the Physical Vapor Deposition (PVD) operated at room temperature as an alternative to ALD or CVD. Moreover, films with higher purity are expected since there are no precursor by-products. Since fewer defects are present, a reduction of the dark current is then expected for the optical device. In this work, niobium pentoxide was obtained from an alloyed target in a non-reactive atmosphere. In combination with silica, niobium pentoxide/silica Bragg mirrors have also been reported in the

<footnote><sup>a</sup> Univ. Grenoble Alpes, CEA, LETI, F-38000 Grenoble, France</footnote>

<footnote><sup>b</sup> Applied Materials, 864 chemin des fontaines, F-38190 Bernin, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: nicolas.massoni@cea.fr (N. Massoni).</footnote>
literature [20–22]. In the present study, the deposition of niobium pentoxide was reported and characterization methods were used to identify an interfacial layer between the silicon surface and the niobium pentoxide. This interfacial layer is a mixture of niobium, silicon and oxygen. The optical properties of the interfacial layer, such as its dispersion curve, have been reported. This interfacial layer was used to simulate the reflectometry of different niobium pentoxide films and can be used to better match the simulation of the optical properties of different stacks with experience.

## 2. Experimental procedure

### 2.1. Samples fabrication by magnetron sputtering

Deposition was performed by Physical Vapor Deposition (PVD) in a 300 mm Clover® PVD chamber on an Applied Materials ENDURA® system. Typical 300 mm silicon wafers were used for all tests, with no cleaning prior to deposition. Where specified, 100 nm-thermally oxidized Si wafers were used. The films were grown at room temperature, at a low pressure around 0,133 Pa, without heated substrate and no bias applied. Self-heating is limited due to large target-to-substrate distance, 3 times larger than classical planar chambers. The 15 cm-diameter targets were silicon dioxide (5 N purity) and niobium pentoxide (2N5 purity), both supplied by from JX Nippon Mining & Metals Europe, powered in RF mode at a power density of 5 W/cm² and pulsed DC mode (p-DC) at a power density of 11 W/cm² respectively. The use of pulsed DC mode allows to discharge the target surface reducing in film particles. The substrate-to-target distance is high resulting in low energy incident sputtered species on the wafer and good control of interfaces reducing possible mixing expected to high plasma energy. Prior to deposition, the targets were properly conditioned. After deposition, the wafers were exposed to ambient air. The backside of the wafers was chemically treated to ensure compatibility with the metrology tools without affecting the front layer. Characterization is described in Section 2.3.

### 2.2. Samples description

For clarity, all samples are summarized in Table 1. As a first test, 50 nm layers of niobium pentoxide and silicon dioxide were deposited (samples #1–3). Spectroscopic ellipsometry was carried out to extract the refractive index *n* and the extinction coefficient *k*. The experimental details are given in Section 2.3. A single layer model was sufficient for samples #1 and #3, indicating optical homogeneity throughout their thickness. However, this was not the case for sample #2, suggesting that a 2-layer model was required. In this 2-layer model the upper layer was formed by niobium pentoxide. The lower layer was the so-called

<table><caption>**Table 1**
Sum-up of the samples.</caption><thead><tr><th>Sample #</th><th>Type</th><th>Substrate</th><th>Materials</th><th>t (nm)</th><th>Characterization</th></tr></thead><tbody><tr><td>1</td><td>Single layer</td><td>Thermal Oxidized Si</td><td>Nb<sub>2</sub>O<sub>5</sub></td><td>50</td><td>XRR, SE, XPS</td></tr><tr><td>2</td><td></td><td>Si</td><td></td><td>50</td><td></td></tr><tr><td>3</td><td></td><td></td><td>SiO<sub>2</sub></td><td>50</td><td>SE</td></tr><tr><td>4</td><td></td><td></td><td>Nb<sub>2</sub>O<sub>5</sub></td><td>2,5</td><td>XRR, XRD, SE, XPS</td></tr><tr><td>5</td><td></td><td></td><td></td><td>5</td><td></td></tr><tr><td>6</td><td></td><td></td><td></td><td>10</td><td></td></tr><tr><td>7</td><td></td><td></td><td></td><td>15</td><td></td></tr><tr><td>8</td><td>400 nm-ARC</td><td>Multilayer Nb<sub>2</sub>O<sub>5</sub> / SiO<sub>2</sub></td><td>Fig. 1a</td><td></td><td>XRR</td></tr><tr><td>9</td><td>400 nm-mirror</td><td></td><td>Fig. 1b</td><td></td><td>TEM, AFM</td></tr></tbody></table>

interfacial layer, which was located between the niobium pentoxide and the silicon substrate. In order to emphasize the thickness of the interfacial layer compared to the niobium pentoxide, thinner deposits were made (samples #4–7). These were then characterized with the multi-sample analysis procedure described in Section 2.4 to obtain the interfacial layer dispersion curve. Once the optical descriptions of samples #1–7 were complete, the individual layers were stacked with samples #8 and #9, whose designs were defined using the Optilayer software [23]. Sample #8 corresponds to an ARC bilayer whose objective is to minimize reflection at 400 nm (Fig. 1a). Sample #9 is a Bragg mirror centered at $λ_0 = 400$ nm with a width estimated by Eq. (1) (Fig. 1b). The reflection target is $R = 100\%$ along the bandpass width and the reflections, defined as the lateral zones outside the 100% reflection zone, have not been minimized.

Considering that all recipe parameters for niobium pentoxide films were identical, except deposition time, the substrate / niobium pentoxide interface were considered to be the same in sample #2 and #4–9.

### 2.3. Characterization

X-ray reflectivity (XRR) measurements were taken using a Jordan Valley JVX5200T instrument. X-rays were generated using a copper anode and monochromated before being focused onto the wafer. The spot size was 1 mm x 70 µm. Data was processed using JVXRR Diagnostic Modelling Software v2.6b0024. X-ray diffraction (XRD) was performed using an automated in-line Bruker Delta-XM instrument equipped with a Cu anode with parallel beam geometry. The source was positioned at a grazing angle of incidence $ω = 0,5°$, allowing good surface sensitivity. Inplane 2θ scans in the range 10° to 94° were then performed by moving the detector (12°/min) in the sample plane, with high angular acceptance (2,5° Soller slits) for high throughput. Reflectometry data acquisition was performed using an Onto Innovation Atlas XP+ in-line automated tool metrology system. The system included a spectroscopic reflectometer (SR) with normal incidence in the wavelength range from UV to VIS (190–780 nm, step size 0,56 nm). The SR module provided both microspot capability and an optimized reflecting aperture for high resolution purposes. Each point on the reflectance versus wavelength curve was the average reflectance of 49 measurements taken on the wafer. The error bar was the standard deviation of these 49 values for each wavelength. The theoretical reflectometry calculations were performed using IMD software [24]. The initial film thicknesses were coming from either XRR or Transmission Electron Microscopy (TEM) for the initial values. Where given, the thickness was refined using the Levenberg-Marquardt algorithm to fit the experimental reflectometry data. Ellipsometry measurements were done with a dual rotating compensator ellipsometer (Woollam RC2 instrument) at three angles of incidence (65°, 70° and 75°) and from 210 to 2500 nm. TEM analysis was performed using a Titan Themis microscope (Thermo Fisher Scientific) operating at 200 kV, equipped with a probe corrector and a Super-X system consisting of four silicon drift detectors for energy dispersive X-ray (EDX) spectroscopic analysis. High-angle annular dark-field scanning TEM (HAADF-STEM) images were acquired using TIA software, and STEM-EDX data sets were acquired and processed using Bruker Esprit software. XPS analyses were performed on a Thermo Scientific Theta 300 spectrometer using a monochromatic Al Kα source (1486,6 eV). The two-dimensional detector at the output plane has the photoelectron energy dispersed in one direction and the angular distribution dispersed in the other direction. All spectra include XPS data integrated over the full angular range. Pure gold and copper samples were used to calibrate the binding energy [25]. The base pressure of the instrument was $2,7 \times 10^{-7}$ Pa. High resolution spectra were collected using an analysis area of ~400 µm² and a pass energy of 40 eV. The energy resolution was 0,45 eV, determined from the Fermi level of a gold reference sample. Spectra were analysed using Thermo Avantage software (v.5.9902).
<figure><img src="image_4.png" /><figcaption>Fig. 1. Theoretical thicknesses for (a) 400 nm-ARC [Nb₂O₅/SiO₂]x1 (sample #8) and (b) 400 nm-Bragg mirror [Nb₂O₅/SiO₂]x8/Nb₂O₅ (sample #9). Thicknesses computed with Optilayer.</figcaption></figure>

## 2.4. Multi-sample analysis procedure for samples #4–7

The samples #4–7 were analyzed taking into account an interfacial layer between the silicon substrate and the niobium pentoxide film. Although ellipsometry is a very sensitive technique, under these conditions it is difficult to simultaneously determine the optical indices and the thickness of the interfacial layer due to model parameter correlations. In order to minimize the parameter correlations, the ellipsometry data of samples #4–7 were treated with a multi-sample analysis (MSA) procedure [26]. In this approach, the data of samples #4–7 were fitted simultaneously and the following parameters were kept identical for all samples: (i) the optical constants of niobium pentoxide, (ii) the optical constants and the thickness of the interfacial layer. The latter was fixed at 2 nm as a starting value, in agreement with the TEM measurement shown in Fig. 8. The outputs were the dispersion curve and the thicknesses of the niobium pentoxide and the interfacial layer. A schematic view of the MSA procedure is proposed in Fig. 2.

This was coherent with the deposition procedure of samples #4–7 because films were deposited in one shot (no loops). The optical constants of the niobium pentoxide and the interfacial layer were modelled with Tauc-Lorentz oscillators. The stack used in the model is shown in Fig. 2.

In this paper, the results of the samples #1–3 were presented in Section 3.1, then the results of the interfacial layer characterization were presented in Section 3.2 with the results of the samples #4–7. Then the multilayer results were presented in Section 3.3, followed by a discussion of the interfacial layer in Section 3.4, and finally a conclusion was proposed.

# 3. Results and discussion

## 3.1. Single layers of 50 nm niobium pentoxide and silicon dioxide (sample #1–3)

For niobium pentoxide, the typical dep rate achieved was ~12 nm/min. The average thickness as measured by ellipsometry was ca. 51±1 nm with 1,2% 49 pts non-uniformity (t-NU%49pts). The dispersion curves are shown and compared to literature in Fig. 3.

Both dispersion curves are quite in agreement with literature values. These values were achieved with magnetron sputtering films, some with niobium oxide targets or with niobium targets in a reactive atmosphere [10,14,17,27,28]. In Fig. 4a, the experimental and calculated reflectometry data are shown.

The reflectance response of sample #1 is correctly modelled with a 1-layer niobium pentoxide model over thermal silica. Using the same 1-layer model for sample #2 resulted in a slight discrepancy with the measured data (green line). The addition of a second layer, called the interfacial layer, improves the fit (red line). The properties of the interfacial layer are reported in Section 3.2.

The average thickness as measured by spectroscopic ellipsometry was ~55±2 nm, including 3,5 ± 0,5 nm of roughness and a 49 pts non-uniformity (NU%) of 3,9%. The optical index curve is shown in Fig. 5a and compared to literature data [27,29,30]. The data is in agreement with typical thin layer values. The coefficient extinction value is a k = 0 flat curve along the measured range (not shown).

The reflectance was calculated with the n/k data in a one single layer of silicon dioxide on a silicon substrate. It is compared to the measured reflectance in Fig. 5b. The good fit indicates reliable index values for sputtered silicon dioxide.

## 3.2. Characterization of the interfacial layer (samples #4–7)

Following the procedure described in Section 2.4, the MSA has converged to the expected outputs. The dispersion curves of the interfacial layer, modelled from the MSA algorithm are shown in Fig. 6 and compared to some literature values.

As a first observation, the general trend of the curves is common for oxide materials. However, the refractive indexes are quite high compared to niobium monoxide or niobium dioxide monolayers, which do not exceed 3,0 in the same wavelength range [19]. Cheng et al. have investigated the optical properties of a NbₓSiᵧO composite prepared by co-sputtering [31]. It was shown that the refractive indexes at 600 nm of the composites varied almost linearly from 1,5 (pure SiO₂) to 2,3 (pure Nb₂O₅) as a maximum value. Similar values are also reported by Leitel et al. [22] using the co-evaporation technique. In both studies, the refractive indexes do not exceed 3,0 in the 400–900 nm range, irrespective of the composition. A similarity with the dispersion curve of amorphous silicon [32] was remarked.

The reflectometry of the samples #4–7 is measured and fitted with the 1-layer and the 2-layers models and are reported in Fig. 7.

Globally, the fit is better with the interfacial layer included in the model, especially for the thinnest films. The refined niobium pentoxide thickness is quite close to the theoretical value. More interestingly, the interfacial layer thicknesses converged between 2,9 to 3,5 nm, in agreement with the HAADF-STEM image in Fig. 9b.

To evaluate the density of the interfacial layer, the XRR data of

<figure><img src="image_5.png" /><figcaption>Fig. 2. Schematic view of the MSA procedure applied for niobium pentoxide films (samples #4–7).</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Fig. 3. Refraction index n and extinction coefficient k of 50 nm niobium pentoxide (sample #1) compared with literature data.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 4. Measured (circles) and calculated reflectances (lines) of 50 nm niobium pentoxide film deposited on (a) on a thermal oxidized silicon substrate (sample #1) and (b) on a silicon substrate (sample #2).</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 5. (a) Refraction index of sample #3 (plain line) compared with literature data (dotted lines) and (b) measured and calculated reflectance of sample #3.</figcaption></figure>

samples #4–7 are fitted with a 2-layers model (not shown). The fit converges with a niobium pentoxide density in the range 4,5–5,3 g.cm⁻³ and 2,5–3,1 g.cm⁻³ for the interfacial layer with an average value of 2,8 g.cm⁻³. The density values for niobium pentoxide are slightly above the theoretical value for an amorphous phase, about 4,6 g.cm⁻³ [33]. The fits with the 2-layers model give a better fit factor than the 1-layer model. However, the visual difference in the fit is not significant because the

density shift between the substrate (d~2,33 g.cm⁻³) and the interfacial layer (d~2,8 g.cm⁻³) is not as high as for niobium pentoxide (d~4,6 g.cm⁻³). Therefore, the presence of the interfacial layer was initially missed by the XRR results alone. Later it was proven by TEM and solved by ellipsometry and reflectometry results.

The XPS results for samples #4–5 are shown in Fig. 8. For samples #6–7 (films thicker than 10 nm), the interfacial layer contribution is too
<figure><img src="image_9.png" /><figcaption>Fig. 6. Refraction index n and extinction coefficient k of the interfacial layer as modelled by the MSA algorithm.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Fig. 7. Measured (dots) and calculated reflectances (lines) of a niobium pentoxide film of (a) 2,5 nm (sample #4), (b) 5 nm (sample #5), (c) 10 nm (sample #6) and (d) 15 nm (sample #7), all deposited on a silicon substrate. The tables indicate the refined values, except the one with the star.</figcaption></figure>

low compared to the probed depth (~5 nm).

The Nb 3d$_{5/2}$ binding energies are typical of Nb$^{5+}$ into the niobium pentoxide compound, i.e. between 207,5 eV (sample #5) and 207,8 eV (sample #4) for a 206,6–207,6 eV theoretical Nb 3d$_{5/2}$ range [34]. A second contribution Nb 3d$_{3/2}$ measured at 206,2 eV (sample #5) and 206,5 eV (sample #4) appears, whose content represents 3,6% and 6,7% of the niobium binding state. In the Esfahani et al. study [39], Nb$^{2+}$ and Nb$^{5+}$ states were separated by 3 eV, suggesting 1 eV for one oxidation state. Considering an energy difference of 1,3 eV between the main and second contribution, these peaks are assigned to an Nb$^{4+}$ oxidation state. However it is not sure whether this contribution lies in the niobium pentoxide layer or in the interfacial layer. For Si 2p binding energies, the theoretical binding energies of Si$^{0}$ (99,2 ± 0,1 eV for both samples) and Si$^{4+}$ (102,6 ± 0,1 eV for both samples) are assigned to the substrate and

native silicon oxide, respectively [35]. An intermediate contribution located at 100,6 eV for both samples is assigned to Si$^{2+}$ oxidation state. This contribution could be related to silicon atoms located in the interfacial layer, eventually bound to niobium atoms.

### 3.3. Multilayers results (samples #8–9)

The interfacial layer is clearly observed in between silicon substrate and niobium pentoxide (Fig. 9a). It can be reasonably supposed that the same observation would have been made for the samples #4–7 since the deposition parameters were the same except time. Moreover reflectometry fits of Fig. 7 with the 2-layers model have confirmed the interfacial layer presence. The elemental profiles along the drawn EDX line in Fig. 9a are reported in Fig. 9b.
<figure><img src="image_11.png" /><figcaption>Fig. 8. XPS data of a niobium pentoxide film of 2,5 nm (sample #4) and 5 nm (sample #5).</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>Fig. 9. (a) HAADF-STEM image of the substrate / niobium pentoxide interface of the 400 nm-ARC (sample #8) (1 px=11.8 pm) and (b) elemental profiles along the EDX line (step size~0.17 nm).</figcaption></figure>

The dotted vertical lines are added to estimate the limits between the substrate (100 wt.% Si) and a niobium pentoxide compound (70 wt.% Nb and 30 wt.% O). The interfacial layer arises between both zones. The chemical profiles confirm that silicon, niobium and oxygen were simultaneously present in this zone. An averaged composition of 45 wt. % Nb, 35 wt.% Si and 20 wt.% O (%at. Nb₀,₁₆Si₀,₄₂O₀,₄₂) is defined in this zone. Following the vertical lines, the interfacial layer thickness could be estimated around 2 nm. The Fig. 10 shows the reflectometry data of sample #8.

The reflectometry data fit is also better with a 2-layers model with a 4 nm-interfacial layer. The silicon dioxide film thickness was measured at 26 nm instead of 24,5 nm targeted. So the experimental zero reflection wavelength has shifted from 400 nm targeted to a higher value. The cross-sectional TEM image of sample #9 is reported in Fig. 11a and reflectometry data in Fig. 11b. The thicknesses extracted from the HAADF-STEM image were used for the calculated reflectance, i.e. 65 nm for SiO₂ and 39 nm for Nb₂O₅.

The experimental width of the mirror is 67+69~136 nm, considered at its half maximum. This is close to the calculated value of 137 nm with Eq. (1) with n<sub>HI</sub>=2561 and n<sub>LI</sub>=1484, both considered at 400 nm from the dispersion curves. The calculated reflectance with the 1-layer model and the 2 layers model are superimposed, indicating that the interfacial layer influence is negligible when several layers were at stake.

<figure><img src="image_13.png" /><figcaption>Fig. 10. Measured and calculated reflectances of the 400 nm-ARC (sample #8). The tables indicate the refined values, except the one with the star.</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>Fig. 11. (a) HAADF-STEM and (b) measured (circles) and calculated (line) reflectance of the 400 nm-mirror (sample #9).</figcaption></figure>

### 3.4. Discussion on physics of formation of the interfacial layer on si substrate

It is now clear that there is an interfacial layer of an alloy of niobium, silicon and oxygen between the silicon substrate and the niobium pentoxide. As shown in the previous sections, reflectometry and TEM converge on a plausible thickness estimated to be around 2 nm. Within this interfacial layer, the concentrations of niobium, oxygen and silicon vary. Although it is known that a native silica is likely to contain defects [36,37] which could facilitate the penetration of species into the defected layer during the initial moments of deposition, the TEM shows that the initial native silica layer is still present. Lohner et al. studied silicon damage during niobium oxide sputtering [38]. By modeling spectroscopic ellipsometry data, they suggested that argon ions and neutral atoms penetrate into the substrate to a depth of several nm, inducing the displacement of silicon atoms and damaging regions. In our study, the typical incident energy of the sputtered species is expected to be very low, due to the Applied Endura Clover® PVD system the large target-to-sample distance (~30 cm), far from the high energy used by Lohner et al. Even if high energy were responsible for the formation of an interfacial layer, this layer should also be seen on SiO₂ substrates, which is not the case.

In terms of the chemical composition of the interfacial layer, Esfahani and others [39] described a close compound for fuel cell applications called "silicon-doped niobium suboxide" using powder metallurgy. The reported atomic composition was Nb<sub>0.24</sub>Si<sub>0.30</sub>O<sub>0.46</sub>, not far from the Nb<sub>0.16</sub>Si<sub>0.42</sub>O<sub>0.42</sub> described here. In addition, the niobium binding spectra of Esfahani et al. also reported niobium suboxide contributions along with the main Nb<sup>5+</sup> peak. This is quite similar to the XPS spectra of this study in Fig. 8. Esfahani et al. reported a fully crystallized suboxide consisting of NbO, NbO₂, Si and SiO₂ phases resulting from an elaboration process at 900 °C. In our study, the interfacial layer appears to be rather poorly ordered as shown by the XRD patterns of Fig. 12.

As seen in Fig. 12, there are no peaks emerging from the background. Hence, the 5 to 15 nm niobium pentoxide deposits can be considered as X-ray amorphous. The lack of long range order in the interfacial layer microstructure may be due to the limited temperature during its growth. Two papers relating to the reduction of niobium pentoxide by silicon monoxide vapor were found regarding the presence of Nb<sup>4+</sup> [40,41]. Assuming that there is no significant heating during deposition due to the chamber design, such reactions do not appear to be possible.

The atomic layer deposition of thin niobium pentoxide (<5 nm) was reported by Macco et al. on HF-dipped and wet-chemically oxidized silicon substrates [42]. The Si XPS spectra showed that the formed silicon oxides were rather substoichiometric (Si<sup>+</sup>, Si<sup>2+</sup>, Si<sup>3+</sup> and Si<sup>4+</sup>) for the HF-dipped samples, i.e. with a native oxide, and close to

<figure><img src="image_15.png" /><figcaption>Fig. 12. In plane XRD scans.</figcaption></figure>

stoichiometric (Si<sup>4+</sup>) for the wet-chemically oxidized samples. We could therefore assume that these near-stoichiometric silicon oxides strongly limit niobium bonding with silicon, explaining why no interfacial layer is detected for sample #1. We could also suppose that Macco et al.'s HF-dipped samples might have an interfacial layer, similar to the one described here, made of niobium bonded to silicon. However no niobium XPS data were reported in the Macco et al.'s paper to check this out. This could reveal whether an interfacial layer would also be present with the ALD technique.

### 4. Conclusions

This paper deals with the sputtering of niobium pentoxide films on silicon for optical applications. The existence of an interfacial layer between the native silica layer of the substrate and the niobium pentoxide layer, composed of niobium, silicon and oxygen, is reported. Although such an interfacial layer had previously been observed in powder metallurgy, it is not observed by sputter deposition at room temperature. A chemical similarity is found, but differences in the microstructural state are highlighted. The optical effect of the interfacial layer (ARC) is significant for thin niobium pentoxide films, but negligible for multilayer films (Mirror). However, its dispersion curves are reported in this paper and used to simulate reflectometry data.
CRediT authorship contribution statement

**N. Massoni:** Writing – original draft, Visualization, Validation, Supervision, Methodology, Data curation, Conceptualization. **L.E. Nistor:** Writing – review & editing, Supervision, Investigation, Data curation, Conceptualization. **C. Licitra:** Writing – review & editing, Methodology, Investigation. **E. Nolot:** Writing – review & editing, Methodology, Investigation. **G. Rodriguez:** Writing – review & editing, Project administration.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

Data will be made available on request.

Acknowledgments

Part of this work, carried out on the Platform for Nano-
characterisation (PFNC), was supported by the 'Recherches Tech-
nologiques de Base' program. This work was carried out in the
framework of the joint laboratory of CEA-LETI and Applied Materials.
The authors would like to thank Eugénie Martinez for XPS measurement,
Denis Mariolle for AFM, Zineb Saghi for TEM, Agathe André, Jérémy
Marchand and Yann Mazel for metrology support, Clément Cammarata
for fruitful discussions and Catherine Pelle for Optilayer support.

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