# In-situ monitoring of tungsten oxides reduction during deuterium plasma exposure by spectroscopic ellipsometry

Federica Pappalardo, L. Rayneau, Céline Martin, Martiane Cabié, Éric Salomon, Thierry Angot, Gilles Cartry, Régis Bisson, Marco Minissale

► To cite this version:

Federica Pappalardo, L. Rayneau, Céline Martin, Martiane Cabié, Éric Salomon, et al. In-situ moni-
toring of tungsten oxides reduction during deuterium plasma exposure by spectroscopic ellipsometry.
Nuclear Materials and Energy, 2024, 41, pp.101751. 10.1016/j.nme.2024.101751. hal-04751773

HAL Id: hal-04751773

https://hal.science/hal-04751773v1

Submitted on 24 Oct 2024

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

# In-situ monitoring of tungsten oxides reduction during deuterium plasma exposure by spectroscopic ellipsometry

F. Pappalardo<sup>a,*</sup>, L. Rayneau<sup>a</sup>, C. Martin<sup>a</sup>, M. Cabie<sup>b</sup>, E. Salomon<sup>a</sup>, T. Angot<sup>a</sup>, G. Cartry<sup>a</sup>, R. Bisson<sup>a</sup>, M. Minissale<sup>a</sup>

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

**Keywords:**

Tungsten oxides

Deuterium plasma

In-situ monitoring

Ellipsometry

XPS

FIB-SEM

## ABSTRACT

In this work, we investigate the optical properties variation of thermally grown W oxides during the exposure to low energy deuterium (D) plasma. *In-situ* ellipsometry in the 400–1000 nm range was coupled to *ex-situ* diagnostics, such as X-ray photoelectron spectroscopy (XPS) and focused ion beam coupled to scanning electron microscopy (FIB-SEM), to probe the evolution of chemical and morphological properties of the W oxides. First, a 70 nm thick *WO*₃ layer was exposed to D plasma at a surface temperature of 650 K. An important reduction of the oxide layer was observed by FIB-SEM, and *in-situ* ellipsometry showed the evolution of the optical constants n and k of the oxide towards the ones of pure W metal. Secondly, a 300 nm thick *WO*₃ layer was exposed to D plasma at a surface temperature below 373 K. In order to follow the oxide evolution step by step, we alternated *in-situ* ellipsometry and XPS surface characterization. A quite similar evolution of the optical constants was observed, in particular an increase of the extinction coefficient k in the near infrared, which was linked to a progressive reduction of the oxidation level at the surface, as seen by XPS. Interestingly, the reduction of the oxide at 373 K was below the resolution of the FIB-SEM. This observation indicates that ellipsometry in the 400–1000 nm range is able to follow *in-situ* the reduction of *WO*₃ oxides by D plasma with high surface sensitivity.

## 1. Introduction

Tungsten (W) is a refractory metal which, among other applications like electronics, aerospace and astrophysics, will be used for plasma-facing components (PFCs) in nuclear fusion reactors such as ITER, due to its peculiar properties such as high melting temperature (around 3687 K), high mechanical resistance, high energy sputtering threshold and low erosion under concomitant heat and particles fluxes [1]. It is expected to withstand ion fluxes of the order of 10²⁴ ions/m²s, coming from the fuel plasma (hydrogen isotopes such as deuterium, D, and tritium, T) and one of the reaction products (helium), together with high heat fluxes of around 20 MW/m² in the stationary regime [2]. In this framework, the oxidation of W PFCs should also be taken into account, as oxygen will be present inside the reactor as a natural (intrinsic) impurity, and could also be injected during an accidental scenario involving air or water leaks. Knowing that W temperature during discharges can increase up to 1300 K [3], it is likely to have W oxide formation on W walls [4]. For this reason, it is fundamental to gain insight on the interaction mechanisms between hydrogen isotopes

and W oxides, which depend on their structure, stoichiometry and thickness.

Tungsten trioxide (*WO*₃) is extensively studied in the context of electrochromic materials, whose optical properties can be modified by the insertion/extraction of ions or electrons, mainly used in the context of smart windows and low emissivity coatings [5,6]. Previous studies have already shown the change of *WO*₃ from transparent to absorbing state (in the visible range) upon hydrogen or lithium intercalation, evidenced by a color change [7]. Furthermore, in the context of fusion it is known that the interaction between *WO*₃ and D ions can lead, among other things, to a higher fuel retention [8], to oxide erosion [9] (which may then introduce impurities in the fuel plasma) and to W bronzes (*D*<sub>x</sub>*WO*₃) formation [10]. It is clear from such studies that the oxide reduction by D ions is a temperature-driven process. Recently, Kremer et al. [11] exposed thin (< 100 nm) thermally grown *WO*₃ samples to low energy (5 eV/D), low flux (≈ 6 × 10¹⁹ D/m²s) and high fluence (≈ 1 × 10²⁴ D/m²) D plasma at 370 K, and observed with Rutherford Backscattering Spectroscopy (RBS) the appearance of a W-enriched layer about 13 nm thick. In a subsequent study [12], they also

<footnote><sup>a</sup> Aix-Marseille University, CNRS, PIIM, Marseille, France</footnote>

<footnote><sup>b</sup> Aix-Marseille University, CNRS, Centrale Marseille, CP2M, Marseille, France</footnote>

<footnote>* Corresponding author.
E-mail address: federica.pappalardo@univ-amu.fr (F. Pappalardo).</footnote>
observed that the oxygen removal rate could be increased 3 fold when increasing the ions kinetic energy above 35 eV or when increasing the surface temperature to 500 K. However, in the latter case the plasma exposure temperature is relatively close to the growth temperature used for their WO₃ synthesis (600 K), raising questions about thermal stability.

In this study, we aim to contribute to the understanding of WO₃ layers evolution under low energy D plasma. To this end, we prepared thermally grown WO₃ samples synthesized at 1073 K and thermally stable up to 800 K (following a similar procedure as the one described in [13]). Then, we exposed these samples to 15 eV/D plasma at 373 K or 650 K. The novelty of our study is that the surface evolution of WO₃ is followed continuously using in-situ spectroscopic ellipsometry (SE). Such technique is extensively used to study thin films structures on substrates, for example to determine their optical constants (refractive index n and extinction coefficient k), as well as their thickness and roughness [14,15], together with the possibility of making real-time measurements in a wide spectroscopic range. By coupling the results of in-situ ellipsometry with ex-situ surface characterization methods, our goal is to test the use of in-situ ellipsometry as a possible non-contact and non-intrusive diagnostics for the oxidation/reduction evolution of PFCs materials in fusion applications.

In Section 2, we describe the synthesis of W oxides via thermal oxidation, the plasma exposure experimental set-up equipped with ellipsometry, and the ex-situ characterization methods that we have used before and after the exposure. We also explain the theoretical models used to fit the ellipsometric data. In Section 3.1, we present and discuss our results of WO₃ samples exposed to D plasma. Finally, in Section 4, we draw the main conclusions and offer some perspectives.

## 2. Methods

### 2.1. Materials and methods

We used two 10 × 10 × 0.5 mm³ polycrystalline W samples with a 99.95% purity, provided by GoodFellow. The samples were mechanically polished in PIIM laboratory (Aix-Marseille University), in order to have a mirror-like surface, and have a root mean square (RMS) roughness of about 10 nm (on an area of 130 × 175 µm²). Afterwards, the samples were electro-polished using a 1.5% NaOH solution and applying a tension of 7 V for 15 s and annealed in vacuum (10⁻⁷ mbar) up to 1073 K with a high-power laser for 30 min, in order to reduce surface contamination [16]. Finally, the samples were washed in ultrasonic baths successively, in acetone, ethanol and distilled water, and dried with optical paper. The experimental parameters used for W oxidation, resumed in Table 1, reproduce the realistic conditions in which oxide layers could be formed on W PFCs inside an ITER-like reactor considering water and/or air leak at high temperatures. The samples were oxidized in vacuum (base pressure: 10⁻⁷ mbar), reaching the oxidation temperature of 1073 K through an oven. Different values of gas pressure and oxidation times were used, in order to grow oxides with different thickness. The first sample (Wox1) had a thickness of about 70 nm while the second one (Wox2) was about 300 nm thick. The purpose of realizing two different thicknesses was to observe a fast reduction by D plasma in the sample Wox1 (exposed at high temperature), in order to extract by ellipsometry the optical indexes of a fully reduced layer. These optical indexes will then be used to estimate with ellipsometry the thickness of the W-enriched layer in the sample Wox2 (exposed at low temperature). As we were not certain of the reduction rate at low temperature, we decided to grow a thicker oxide (300 nm), in order to avoid full reduction upon D plasma exposure. The thickness of the oxide layers was measured on cross-section images using a FEI Helios 600 NanoLab dual beam, which combines a focused ion beam and a scanning electron microscope (FIB-SEM). The value taken as the oxide layer thickness, using the software ImageJ, is the mean value from several images and the error corresponds to its standard deviation. The

surface morphology was characterized using Confocal microscopy (Sneox SENSOFAR) and the surface chemical composition was evaluated with a Scienta Omicron X-rays Photoelectron Spectrometer (XPS) that uses a Mg Kα source (further details are given in Salomon et al. [17]).

Plasma exposure of W oxides samples was performed in the CAMITER set-up, described in more details in Yang et al. [18]. The plasma reactor is a stainless steel chamber (base pressure 10⁻⁸ mbar) where inductively-coupled D plasma (the dominant ion species being D⁺) is created via a planar coil antenna, placed on a quartz window on top of the chamber and connected to an Advanced Energy Cesar 1310 RF generator (200–1000 W) through a L-type matching box. The sample holder was not biased, resulting in an ion energy of about 15 eV. D plasma parameters such as electron density and temperature were measured with a Langmuir probe, resulting in the following ranges: nₑ ~ 10¹⁰ ~ 10¹¹ cm⁻³ and Tₑ ~ 1.2–3 eV. The ion fluxes and fluences ranges were respectively 10¹⁹ ~ 10²⁰ D⁺ m⁻² s⁻¹ and 10²² ~ 10²⁴ D⁺ m⁻². The sample temperature was measured via a pyrometer (METIS M323, SensorTherm) working in the 2–2.6 µm spectral range and allowing measurements of temperatures in the range 323–1073 K. The accuracy of the pyrometer was tested by changing the emissivity value from 0.3 (clean W surface) to 0.4 (oxidized W surface), with a change in temperature <5 K. It was installed in front of one of the quartz viewport and focused on the center of the sample. An in-situ Spectroscopic Ellipsometer (iSE, J. A. Woollam) was used to monitor the optical properties of samples during plasma exposure. The polychromatic light source is made of a quartz halogen lamp emitting in the wavelength range 400–1000 nm and a rotating compensator. The incident angle of light was fixed at 61°. The area of the beam spot onto the samples was of about 4 × 2 mm². The reflected light passes through a rotating analyzer and is then dispersed onto a CCD detector connected to an optical fiber, with a spectral resolution of ≈3 nm and a temporal resolution of 0.3 s.

### 2.2. Models

Ellipsometry measures the quantities Ψ and Δ, which are linked to the two Fresnel reflection coefficients of s and p polarized light reflected from the sample, r_s and r_p through the following relation:

$$ \rho = \frac{r_p}{r_s} = \tan \Psi \exp(i\Delta) \quad (1) $$

referring, respectively, to their amplitude ratio (Ψ) and to their phase difference (Δ). The experimental data of this study are fitted using the software CompleteEase [19]. The quality of the fit is determined by the mean square error (MSE), whose formula can be found in Ref. [20].

As we have a system composed by a W substrate and an oxide layer on top, we model the optical properties of each material separately. For the W substrate, we determine the optical constants n and k (or, equivalently, the complex dielectric function ε, with its two components ε₁ and ε₂) directly from the experimental data, through the following:

$$ (n + ik)^2 = \epsilon = \epsilon_1 + i\epsilon_2 = \sin^2(\theta) + \left[ 1 + \tan^2(\theta) \left( \frac{1-\rho}{1+\rho} \right)^2 \right] \quad (2) $$

where θ is the incident angle of light and ρ is defined in Eq. (1). Those are often called “pseudo” optical constants, since they are derived analytically from the measured Ψ and Δ (no surface roughness or overlayers are taken into considerations), and are valid for bulk materials.

In case of a thin film on a substrate, we observe the typical oscillations in the ellipsometric data Ψ and Δ due to the optical interference of light reflected from the top and from the bottom of the film. In this case, it is possible to retrieve the layer optical constants analytically only if its thickness is known (and viceversa).

To model a layer in which two different material phases coexist, we have used the so called effective medium approximation (EMA) layer, which has a dielectric function that depends on the single dielectric functions of the constituents materials and on their volume
<table><caption>**Table 1**
Experimental parameters of the samples oxidation process (temperature, oxygen pressure and time), the thickness of the oxide layer before plasma exposure and the plasma conditions (ion flux, fluence and temperature).</caption><thead><tr><th rowspan="2"></th><th colspan="4">Oxide growth conditions</th><th colspan="3">Plasma conditions</th></tr><tr><th>Temp. K</th><th>Pressure mbar</th><th>Time s</th><th>Thickness nm</th><th>Flux D<sup>+</sup> m<sup>-2</sup> s<sup>-1</sup></th><th>Fluence D<sup>+</sup> m<sup>-2</sup></th><th>Temp K</th></tr></thead><tbody><tr><td>Wox1</td><td>1073</td><td>0.73</td><td>20</td><td>70 ± 18 nm</td><td>1 × 10<sup>20</sup></td><td>6 × 10<sup>22</sup></td><td>650</td></tr><tr><td>Wox2</td><td>1073</td><td>6.73</td><td>5</td><td>328 ± 84 nm</td><td>2 × 10<sup>19</sup></td><td>1 × 10<sup>24 a</sup></td><td>373</td></tr></tbody></table>

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** FIB-SEM images of Wox1 (a) and Wox2 (c) samples, before plasma exposure, and their correspondent confocal microscopy images on the right (b,d).</figcaption></figure>

fraction [21]. From a general point of view, if the two material phases are called A and B and their dielectric functions $\epsilon_A$ and $\epsilon_B$, respectively, the EMA layer will have the following dielectric function:

$$ \epsilon_{EMA} = \epsilon_A \frac{2\delta(\epsilon_B - \epsilon_A) + \epsilon_B + 2\epsilon_A}{\epsilon_B + 2\epsilon_A - \delta(\epsilon_B - \epsilon_A)} \quad (3) $$

where $\delta$ is the percentage volume fraction of the material phase B in the layer. Further details on how we use this model (see Figs. 3, 4) will be given in Section 3.1.

# 3. Results and discussion

## 3.1. Pre-exposure characterization

Fig. 1 (a,c) shows SEM cross-sectional images of the oxides after a FIB cut. The platinum layer, deposited on a few µm² area where the cut is performed, has a protective purpose and it is not removed before plasma exposure. The as-grown oxide layer thickness of the samples, measured from FIB-SEM images, was 70 ± 18 nm for Wox1 and 328 ± 84 nm for Wox2. It is possible to see that both oxides presented an irregular structure and were rather inhomogeneous in thickness. Furthermore, they were differently colored (dark gold for Wox1, dark blue for Wox2) which, as it was already observed in previous studies [10], depends both on the layer thickness (causing different light interference phenomena) and on the crystalline structure. Raman spectra (not shown here) displayed the typical peaks of $WO_3$ monoclinic crystalline phase, which resulted to be quite broad, indicating a disordered structure with the presence of defects (mostly oxygen vacancies), making us refer to the oxides as $WO_{3-x}$ or, more simply, $WO_x$.

## 3.2. D plasma exposure at high temperature

The sample Wox1 was exposed to a D plasma, with an ion flux of $1 \times 10^{20} D^+ m^{-2} s^{-1}$ for 10 min, reaching a fluence of $6 \times 10^{22} D^+ m^{-2}$

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Refractive index n and extinction coefficient k of the sample Wox1 before and after plasma exposure, and comparison with the typical indexes values of ideal $WO_3$ and W.</figcaption></figure>

Source: Retrieved from Kulikova et al. [22] and Rakic et al. [23], respectively.

and a maximum temperature of 650 K. Fitting the ellipsometric data before and after the exposure, we observe a substantial change of the layer's optical constants. As seen in Fig. 2, before D plasma exposure the indexes resemble the ones of a dielectric layer (with the extinction coefficient k close to 0), while after the exposure they resemble to the ones typical of a metal (k increasing at higher wavelengths). The spectral behavior of the refractive index n is not trivial, hence, even if we show both, we prefer to use k as a clear-cut marker of the electronic properties of the material. For comparison, in Fig. 2 we also show the optical constants n,k of pure W (from Rakic et al. [23]) and of an ideal $WO_3$ (from Kulikova et al. [22]), to better highlight the change in the optical constants of Wox1 that we observe after plasma exposure. It

<footnote>a This is a value of cumulated fluence that was reached with an average ion fluence of 5.5 × 10²² D⁺ m⁻² for each exposure.</footnote>
<figure><img src="image_6.png" /><figcaption>Fig. 3. Plots of Wox1 experimental (solid lines) and EMA model (dashed lines) Ψ (a) and Δ (b), as a function of time during D plasma exposure (starting at* t* = 0 min. and ending at* t* = 10 min.), for 5 different wavelengths.</figcaption></figure>

is important to specify that in our case even before the exposure we
do not have perfectly zero absorption, as our oxides are not perfectly
stoichiometric WO₃ and probably have some lattice defects which
induce light absorption from color centers. We then fitted the in-situ
ellipsometric data obtained during D plasma exposure (Fig. 3) using
an effective-medium approximation (EMA) layer on a W substrate. As
mentioned in Section 2.2, such layer is composed by two coexisting
material phases, in which the material phase A would have optical
constants n,k of Wox1 before plasma exposure and the material phase B
would have the n,k of Wox1 after plasma exposure. The bigger variation
of Ψ and Δ at 400 nm compared to the other wavelengths is probably
due to the fact that the penetration depth of 400 nm light in the oxide
is closer to its thickness (≈70 nm), causing light interference effects
which thus impact more on this wavelength. In the fitting procedure,
the layer thickness was constrained to the thickness range measured
with FIB-SEM, resulting in the value 54 ± 3 nm. The EMA percentage
(corresponding to the percentage of the second material, Wox1 after
plasma exposure) was left as a free parameter evolving with time. In
Fig. 4, we show that the EMA percentage goes from 0% before D plasma
exposure (i.e., the EMA layer entirely corresponds to a dielectric layer)
and increases during plasma exposure, until it reaches 100% (i.e., the
EMA layer resembles to a metallic material) after about 5 min, and
maintaining this value until the end of D plasma exposure (at *t* = 10
min) and even after plasma exposure.

We find a good agreement between the model and the experimental
data, except during the first 3 min of the D plasma exposure, which
correspond to an exponential increase of the surface temperature up to
650 K (after 3 min, the temperature remains constant at this value until
the end of the plasma exposure). This could be due to the fact that the

<figure><img src="image_7.png" /><figcaption>Fig. 4. EMA percentage (of the metallic WOₓ layer) as a function of time, for the sample Wox1.</figcaption></figure>

EMA model described by Eq. (3) is only valid when the second material
fraction is low in comparison to the first material. Since between about
1 and 3 min the layer is evolving from the pre-exposure state to the
post-exposure state, the two fraction are comparable and the validity
of the model is not fulfilled. We presume that the synergistic effect of
plasma irradiation and temperature increase causes an abrupt change
in the surface state, for which we cannot find a satisfying agreement
with other models (i.e. homogeneous layer, graded layer) in this time
range. The drastic change in the oxide surface state and optical response
is visible by naked eye, since the Wox1, initially dark gold, turned
brighter and metallic once exposed to the D plasma (Fig. 5(b)). FIB-SEM
measurement (Fig. 5(a)) evidenced a substantial reduction of the oxide
layer, as seen by the brighter and more inhomogeneous oxide layer
as compared to the pristine one before plasma exposure. As the ion
energy is far below the physical sputtering threshold for D ions on W
oxide (around 65 eV [24]), we suppose that a chemical sputtering of the
oxide by D ions, enhanced by the simultaneous temperature increase,
is responsible for the oxide layer reduction. This would result in a
more disordered and oxygen-depleted oxide layer due to the creation
of defects such as oxygen vacancies, with physical properties that are
the ones typical of a metallic surface. We stress that the temperature
rise is directly linked to the presence of the plasma and its ion flux, as
we do not heat the system before the exposure, and that we could not
investigate the two effects separately as in the current version of our
set-up it is not possible to efficiently cool down the sample.

3.3. D plasma exposure at low temperature

In order to slow down the oxide reduction and to follow its evolu-
tion with increasing fluence, we exposed the second sample (Wox2) to
a “gentle” D plasma, i.e. with a lower ion flux of 2 × 10¹⁹ D⁺ m⁻² s⁻¹,
and thus a lower heat flux, as the sample temperature was not ex-
ceeding 373 K during this “gentle” D plasma exposure. We performed
ellipsometric measurements during 10 successive D plasma exposures
of various times, until reaching a cumulated fluence of 1 × 10²⁴ D⁺m⁻².
In addition, between each D plasma exposure, we performed XPS
measurements, in order to characterize any change in the W oxidation
state. We stress that we do not see any kinetic evolution of the oxide
because of the sample transfer between the plasma chamber and the
XPS apparatus, since the air transfer was systematically realized in less
than a few minutes.

For the Wox2 sample, we do not model the *in-situ* data during
each exposure, as we do not observe big variation of the optical signal
during the plasma, but rather focus on the values of Ψ and Δ after each
exposure. In this case, we do not use the EMA model as we do not have
<figure><img src="image_8.png" /><figcaption>Fig. 5. Wox1 after D plasma exposure: FIB-SEM image for thickness measurement (a) and confocal image (b).</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 6. Optical indexes n and k of sample Wox2 as a function of wavelength after different plasma exposures (e.g., with increasing cumulated fluence), assuming that the oxide layer is homogeneous. These indexes are compared with the ones of ideal $WO_3$ and W (from Kulikova et al. [22] and Rakic et al. [23], respectively) and of the exposed sample Wox1.</figcaption></figure>

two co-existing material phases, i.e. the FIB-SEM measurement did not show a full reduction of the oxide but instead a seemingly unmodified oxide. Thus, in a first step, we modeled the system as an homogeneous layer with unknown optical constants, in which we constrained the layer thickness to the range measured with FIB-SEM, resulting in a thickness value of 260 ± 40 nm, and set the optical constants of the layer, n and k, as free parameters of the fit. We show in Fig. 6 the indexes n and k as a function of wavelength, at the end of each exposure (so with increasing cumulated fluence). While for n the behavior is not continuous with increasing fluence, it seems here that a similar process as the one occurring in the thin sample Wox1 is taking place, namely a progressive increase of k with increasing cumulated fluence, going from values close to 0 (typical of a dielectric layer) before the exposure, to higher values of k at higher wavelengths (typical of a metallic surface). We also represent the indexes of an ideal $WO_3$ and a pure W material, to better show the transition from one kind of surface to the other one after each plasma exposure. Considering the similar evolution of n and k observed with Wox1 and Wox2, in the following we will base our discussion on the behavior of k, as the spectral behavior of n is not trivial and it is not a clear indicator of the electrical properties of the material, unlike k (which is directly proportional to e.g. the electrical conductivity).

In order to understand the strong variation observed on the extinction coefficient k upon D plasma exposure despite a seemingly unmodified FIB-SEM measurement, we performed XPS measurement to characterize the near-surface region of Wox2. Top panel in Fig. 7 shows XPS spectra in the 28–41 eV binding energy region (W4f) of the sample Wox2, as introduced and after multiple exposures to D plasma (with increasing cumulated fluence). XPS spectra denote a progressive change in the surface chemical environment (W oxidation state) with increasing cumulated fluence. As evidenced in the histogram in the bottom panel of Fig. 7, which depicts the abundance of each W oxidation state

<figure><img src="image_10.png" /><figcaption>Fig. 7. Top: XPS spectra of Wox2 for different values of cumulated D fluence. Bottom: Histogram representing the peaks predominance in XPS spectra for each value of cumulated fluence, going from highest binding energy ($W^{6+}$) to the lowest ($W^{4+}$), to finally metallic W.</figcaption></figure>

(corresponding to a peak at a specific binding energy), we start with a predominance of $W^{6+}$ peak (relative to $WO_3$, at 35.8 eV) for the as introduced sample, as well as after the first D plasma. With increasing cumulated fluence, also other oxidation levels $W^{5+}$ and $W^{4+}$ (relative to the sub-stoichiometric oxides $WO_{2.5}$ and $WO_2$, at 33.4 and 32.9 eV, respectively) appear, until reaching, at the end of plasma exposures, a predominance of metallic W 4f peaks (at 31.3 eV), which already start to appear at a cumulated fluence of $2.8 \times 10^{23} D^+ m^{-2}$. After the multiple D plasma exposures, we measured the oxide layer thickness with FIB-SEM, which we do not show here as we do not observe any thickness variation or other change in the oxide layer morphology. Therefore, we conclude that the variation in the optical properties and in the chemical composition of the oxide that we detect with ellipsometry and XPS, respectively, are related to an oxygen-depleted (or W-enriched) surface layer of no more than 5 nm thick, consistent with previous studies with similar exposure conditions [11,13]. The
<figure><img src="image_11.png" /><figcaption>Fig. 8. Schematic representation of the stack layer model (a) used to fit the W-enriched layer thickness (b) and the optical indexes n,k of the underlying layer WOₓ (c,d).</figcaption></figure>

formation of such layer could be due to the same chemical sputtering process that occurs in sample Wox1, however with a less drastic change because of the lower sample temperature during D plasma exposure.

In order to confirm the appearance of the W-enriched layer at fluences >2.8×10²³ D⁺ m⁻² observed by XPS, we use ellipsometry to model the system as a stack of layers on a W substrate (Fig. 8(a)). We use for the W-enriched layer the same indexes of sample Wox1, i.e. an oxide fully reduced after the plasma exposure, while we use as fitting parameters its thickness and the indexes n,k of the underlying oxide. We find that the optical indexes of the underlying oxide do not evolve significantly with increasing fluence (Fig. 8 (c,d)), while the thickness of the W-enriched layer (reduced oxide) grows up to about 5 nm after the last plasma exposure (Fig. 8(b)).

To summarize, we are able to detect the presence of a W-enriched layer though ellipsometry and XPS, which are both surface sensitive techniques, while with FIB-SEM we are not able to observe it, as the W-enriched layer thickness should be close to the resolution limit of the technique (≈5 nm). We believe that the surface temperature increase during D plasma may be a decisive parameter in determining the surface modification of W oxides and its extension in depth, independently from the thickness of the oxide. This would agree with the studies of Alimov et al. [8] and Tu et al. [9], and complete the ones of Kremer et al. [12] which also explore the role of ion energy. Despite similar experimental conditions, we do not observe a ≈10 nm W-enriched layer as the one in Kremer et al. [11], possibly due to slight differences in the plasma exposure or more significant differences in the thermal growth of the samples.

## 4. Conclusions

In this study, we performed ellipsometric measurements on W thermally grown oxides exposed to D plasma at two different surface temperatures of 650 K and 373 K, respectively. In the case of the high temperature exposure, we observed a drastic change in the optical constants n and k, which go from the values typical of a dielectric layer before the exposure, to the ones typical of a metallic surface after plasma exposure. Since the ion energy (15 eV) is below the physical sputtering threshold, we attribute the surface modification to a chemical sputtering of the oxide layer by D ions, confirmed by ex-situ FIB-SEM measurements. In the case of the low temperature exposure, we observed a similar change of the optical constants and, in particular, an important increase of the extinction coefficient k in the near infra-red, suggesting again a transition towards a surface with

metallic physical properties. XPS measurements also showed a change in W oxidation state with increasing cumulated fluence, in particular, going from a predominance of WO₃ peaks to a mixture of WO₃ and WO₂, and finally to the predominance of metallic W4f peaks at a fluence of 2.8 × 10²³ D m⁻². However, for the low temperature exposure we did not observe any change in the layer morphology, i.e. the change occurs below the FIB-SEM resolution of ≈5 nm. These results show that the W-enriched layer is limited to the near-surface layer (1–5 nm), to which we are sensitive with both ellipsometry and XPS. As for the high temperature exposure sample, the similar optical properties evolution with D plasma exposure is also due to the chemical change in the near-surface layer, but the thermally accelerated oxide reduction allowed to observe these effects with FIB-SEM on the whole oxide thickness.

Finally, in this study we have proved that we are able to investigate the surface modification of W oxides during D plasma irradiation using an easy, non-destructive and non-invasive technique as in-situ ellipsometry, whose results are confirmed by ex-situ techniques such as XPS and FIB-SEM. In future works, we aim to investigate on the single and separate effect of ion flux and surface temperature in the oxide reduction, as well as to perform Transmission Electron Microscopy (TEM) measurements on “gentle” plasma exposed oxides, in order to test the presence of <5 nm W-enriched layer.

We believe that our findings are in the interest of the fusion community and can lead to a better understanding and monitoring of the surface modification of the PFCs.

## CRediT authorship contribution statement

F. Pappalardo: Writing – original draft, Validation, Investigation, Formal analysis, Data curation, L. Rayneau: Investigation, Formal analysis, C. Martin: Visualization, Resources, Investigation, Funding acquisition. M. Cabie: Writing – review & editing, Visualization, Investigation, E. Salomon: Writing – review & editing, Validation, Resources, Data curation. T. Angot: Supervision, Funding acquisition. G. Cartry: Writing – review & editing, Validation, Supervision, Resources, Funding acquisition. R. Bisson: Writing – review & editing, Supervision, Resources, Project administration, Funding acquisition, Conceptualization. M. Minissale: Writing – review & editing, Visualization, Validation, Supervision, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation.
Declaration of competing interest

The authors declare that they have no known competing finan-
cial 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

This work has been carried out within the framework of the French Federation for Magnetic Fusion Studies (FR-FCM) and with the help of French government under France 2030, within the framework of the Initiative d'Excellence d'Aix-Marseille Université - A*MIDEX (AMX-19-IET-013 - ISFIN and AMX-20-IET-015 AMUTech). The project leading to this publication has received funding from the ANR under Grant No ANR-20-CE08- 0003. This work has been carried out within the frame-work of the EUROfusion Consortium, funded by the European Union via the Euratom Research and Training Programme (Grant Agreement No. 101052200 — EUROfusion). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Commission. Neither the European Union nor the European Commission can be held responsible for them.

References

[1] H. Bolt, V. Barabash, G. Federici, J. Linke, A. Loarte, J. Roth, K. Sato, Plasma facing and high heat flux materials-needs for ITER and beyond, J. Nucl. Mater. 307 (2002) 43-52.

[2] J. Gaspar, C. Pocheau, Y. Corre, N. Ehret, D. Guilhem, M. Houry, T. Loarer, T. Loewenhoff, C. Martin, C. Pardanaud, G. Pintsuk, M. Richou, F. Rigollet, H. Roche, G. Sepulcre, M. Wirtz, Emissivity measurement of tungsten plasma facing components of the WEST tokamak, Fusion Eng. Des. 149 (2019) 111328.

[3] R. Pitts, S. Carpentier, F. Escourbiac, T. Hirai, V. Komarov, S. Lisgo, A. Kukushkin, A. Loarte, M. Merola, A. Sashala Naik, R. Mitteau, M. Sugihara, B. Bazylev, P. Stangeby, A full tungsten divertor for ITER: Physics issues and design status, J. Nucl. Mater. 438 (2013) S48-S56, Proceedings of the 20th International Conference on Plasma-Surface Interactions in Controlled Fusion Devices.

[4] C. Martin, M. Diez, A. Campos, M. Cabié, G. Giacometti, M. Balden, A. Gallo, B. Pegourié, E. Bernard, E. Tsitrone, First post-mortem analysis of deposits collected on ITER-like components in WEST after the C3 and C4 campaigns, Phys. Scr. 96 (12) (2021) 124035.

[5] R. Bringans, H. Höchst, H. Shanks, Defect states in W O 3 studied with photoelectron spectroscopy, Phys. Rev. B 24 (6) (1981) 3481.

[6] H. Camirand, B. Baloukas, J.E. Klemberg-Sapieha, L. Martinu, In situ spec-
troscopic ellipsometry of electrochromic amorphous tungsten oxide films, Sol.
Energy Mater. Sol. Cells 140 (2015) 77-85.

[7] M. Lagier, A. Bertinotti, O. Bouvard, L. Burnier, A. Schüler, Optical proper-
ties of in vacuo lithiated nanoporous WO3: Mo thin films as determined by
spectroscopic ellipsometry, Opt. Mater. 117 (2021) 111091.

[8] V. Alimov, B. Tyburska, M. Balden, S. Lindig, J. Roth, K. Isobe, T. Yamanishi, Surface morphology and deuterium retention in tungsten oxide layers exposed to low-energy, high flux D plasma, J. Nucl. Mater. 409 (1) (2011) 27-32.

[9] H. Tu, C. Li, W. Ding, L. Shi, Y. Zhang, The erosion and retention properties of tungsten trioxide films exposed to low energy deuterium ions: Temperature dependence, J. Nucl. Mater. 562 (2022) 153590.

[10] Y. Addab, C. Martin, C. Pardanaud, J. Khayadjian, K. Achkasov, D. Kogut, G. Cartry, G. Giacometti, M. Cabié, J.L. Gardarein, P. Roubin, Formation of thin tungsten oxide layers: characterization and exposure to deuterium, Phys. Scr. T167 (2016) 014036.

[11] K. Kremer, T. Schwarz-Selinger, W. Jacob, Influence of thin tungsten oxide films on hydrogen isotope uptake and retention in tungsten – evidence for permeation barrier effect, Nucl. Mater. Energy 27 (2021) 100991.

[12] K. Kremer, T. Schwarz-Selinger, W. Jacob, Deuterium plasma exposure of thin oxide films on tungsten-oxygen removal and deuterium uptake, Nucl. Mater. Energy 35 (2023) 101406.

[13] M. Ialovega, Surface Conditions of Tungsten Components: Impact on Hydrogen Inventory (Ph.D. thesis), CEA Cadarache, 2021.

[14] D. Lehmann, F. Seidel, D.R. Zahn, Thin films with high surface roughness: thickness and dielectric function analysis using spectroscopic ellipsometry, SpringerPlus 3 (1) (2014) 82.

[15] I. Valyukh, S. Green, H. Arwin, G. Niklasson, E. Wäckelgård, C. Granqvist, Spectroscopic ellipsometry characterization of electrochromic tungsten oxide and nickel oxide thin films made by sputter deposition, Sol. Energy Mater. Sol. Cells 94 (5) (2010) 724-732.

[16] F. Pappalardo, F. Romero Lairado, C. Louis de Canonville, C. Martin, G. Giacometti, G. Serin, E. Salomon, T. Angot, L. Gallais, R. Bisson, M. Minissale, Optical properties of tungsten: A parametric study to characterize the role of roughness, surface composition and temperature, Optics 3 (3) (2022) 216-224.

[17] E. Salomon, M. Minissale, F.R. Lairado, S. Coussan, P. Rousselot-Pailley, F. Dulieu, T. Angot, Pyrene adsorption on a Ag (111) surface, J. Phys. Chem. C 125 (20) (2021) 11166-11174.

[18] X. Yang, D. Kogut, L. Couëdel, T. Angot, P. Roubin, J.-B. Faure, G. Cartry, TALIF measurements of hydrogen and deuterium surface loss probabilities on quartz in low pressure high density plasmas, Plasma Sources Sci. Technol. 30 (1) (2021) 015013.

[19] J. Woollam, Complete EASE 6, 2017.

[20] P. Janicek, M. Putri, K.H. Kim, H.J. Lee, M. Bouska, S. Šlang, H.Y. Lee, Spectroscopic ellipsometry characterization of As-deposited and annealed non-stoichiometric indium zinc tin oxide thin film, Materials 14 (3) (2021) 578.

[21] Principles of spectroscopic ellipsometry, in: Spectroscopic Ellipsometry, John Wiley and Sons, Ltd, 2007, pp. 177-184.

[22] D.P. Kulikova, A.A. Dobronosova, V.V. Kornienko, I.A. Nechepurenko, A.S. Baburin, E.V. Sergeev, E.S. Lotkov, I.A. Rodionov, A.V. Baryshev, A.V. Doro- feenko, Optical properties of tungsten trioxide, palladium, and platinum thin films for functional nanostructures engineering, Optics Exp. 28 (21) (2020) 32049-32060.

[23] A.D. Rakić, A.B. Djurišić, J.M. Elazar, M.L. Majewski, Optical properties of metallic films for vertical-cavity optoelectronic devices, Appl. Optics 37 (22) (1998) 5271-5283.

[24] M. Guseva, A. Suvorov, S. Korshunov, N. Lazarev, Sputtering of beryllium, tungsten, tungsten oxide and mixed W-C layers by deuterium ions in the near-threshold energy range, J. Nucl. Mater. 266 (1999) 222-227.