# Sputtering yield and induced contamination of Kapton, PEEK and PTFE under xenon ions bombardment

Lucas Nicolas, Marc Villemant, Thierry Paulmier

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Lucas Nicolas, Marc Villemant, Thierry Paulmier. Sputtering yield and induced contamination of Kapton, PEEK and PTFE under xenon ions bombardment. Journal of Physics D: Applied Physics, 2025, 58 (13), pp.135210. 10.1088/1361-6463/adaf32. hal-04937700

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Sputtering yields and induced contamination of Kapton, PEEK and PTFE under xenon ion bombardment

To cite this article: L Nicolas et al 2025 J. Phys. D: Appl. Phys. 58 135210

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# Sputtering yields and induced contamination of Kapton, PEEK and PTFE under xenon ion bombardment

L Nicolas*, M Villemant and T Paulmier<fnref n="1" />

DPHY, ONERA, Université de Toulouse, 31000 Toulouse, France

E-mail: lucas.nicolas@onera.fr

Received 17 July 2024, revised 6 January 2025

Accepted for publication 28 January 2025

Published 14 February 2025

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## Abstract

In this article, we present the results of sputtering yield measurements of Kapton, polyether ether ketone (PEEK) and polytetrafluoroethylene (PTFE) due to xenon ion bombardment. We measured these yields using the weight loss technique, for energies ranging from 350 to 800 eV and incidence angles from 0 to 75°. Polymers sputtering yield dependence versus incidence angle appears to be much lower than what we usually observe in other materials (such as metals). Furthermore, PTFE exhibits a very high erosion rate, which is 2 orders of magnitude larger than Kapton and PEEK ones. We also performed collection measurements using a quartz crystal microbalance apparatus. Such a method provides differential sputtering that yields data which are key pieces of information regarding satellite contamination issues. We also computed the total sputtering yields obtained by this second method, and it appears that they are much smaller than the ones measured previously by weight loss, especially for PTFE. This suggests that erosion products may be of different types (atoms, molecules) which are not all able to stick on a facing surface.

Supplementary material for this article is available online

Keywords: erosion, contamination, polymers, plasma thrusters

## 1. Introduction

When an incident ion, with sufficient kinetic energy, hits the surface of a material, it generates some collision cascades in the matter, which potentially lead to the ejection of atoms out

of the surface [1]. This process is called ion sputtering. When a material is exposed to a plasma flux for a long period of time, numerous particles may be sputtered out of the matter, resulting in an important ablation of the surface. This is an erosion phenomenon.

Nowadays, electric propulsion devices equip a large number of spacecrafts, and can operate for more than 10000 hours [2]. The most common electric propulsion systems are grid-ded ion thrusters and Hall thrusters. Both these devices rely on the generation of a noble gas plasma (mostly xenon) and the ejection of swift ions. However, thruster plumes are usually quite divergent [3], and some ions may impact various parts of satellites. Moreover, due to imperfect ionization of the gas, some neutral atoms with almost no kinetic energy may coexist with plasma ions. Collisions between these two particles may occur, resulting in the creation of a fast neutral atom and a slow

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Original Content from this work may be used under the terms of the [Creative Commons Attribution 4.0 licence](https://creativecommons.org/licenses/by/4.0/). Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

<footnote n="1">¹ Present address: Department of Physics, Instrumentation, Environnement, Space, ONERA-The French Aerospace Lab, 2 Av. Edouard Belin, Toulouse 31055 CEDEX, France.</footnote>

<footnote>* Author to whom any correspondence should be addressed.</footnote>
ion via charge exchange reaction [4–6]. These slow ions may then be accelerated toward any negatively charged surface of the spacecraft. As a result, several parts of the satellite may get eroded during thruster operation [7].

In most recent spacecrafts, many surfaces are covered with polyimide foils (Kapton® from DuPont) for electrical and thermal insulation [8]. Moreover, it is employed as a substrate for flexible solar arrays in development [9]. Kapton tape is also widespread in vacuum chambers for ground testing. Another technical polymer, the polyether ether ketone (PEEK), is increasingly used for its good mechanical properties compared to its low weight [10]. We find that it may be employed in structural parts as well as in newer coatings. Finally, polytetrafluoroethylene (PTFE) serves as a patch antenna substrate in GEO orbit [11], as well as a coating in LEO, thanks to their good atomic oxygen resistance [12]. In order to estimate satellite's lifetime, it is of primary interest to understand how these materials will react under heavy ion bombardment and how fast they will get eroded. But material degradation is not the only concern when we talk about the sputtering process on spacecrafts. Erosion products, which may be either atoms, clusters or molecules, may indeed be ejected toward other surfaces and stick on them [13], leading to spacecraft contamination. This erosion induced contamination may be quite problematic when it occurs on critical devices such as solar panels, optics or thermal controllers [14–16].

Erosion and sputtering product emission have been extensively studied in ground facilities for a large number of metallic and ceramic materials [17–20]. However, very few data are available regarding polymers [21–23]. In this contribution, we provide experimental measurements of the Kapton, PEEK and PTFE sputtering yields under xenon ion bombardment. The measurements are performed using the weight loss technique, where samples are weighed before and after erosion. Energies between 350 and 800 eV are investigated as well as incidence angles between 0° (normal incidence) and 75°. We also provide the angular distribution of ejected particles, measured using quartz crystal microbalances (QCM), which is of primary interest when one is concerned with satellite contamination.

## 2. Materials and method

### 2.1. Sample description

Kapton is a polyimide foil manufactured by DuPont. Its structure is shown in figure 1 and the chemical formula of its repeat unit is C₂₂H₁₀N₂O₅. PEEK structure is shown in figure 2 and its chemical formula is C₁₉H₁₂O₃. PTFE is a linear polymer and its repetition unit is C₂F₄. PTFE samples are 1 mm thick while the Kapton and PEEK samples are 50 and 100 µm thick respectively. For sputtering yield measurements using weight loss technique, samples are rectangular pieces of 3.5 × 4 cm. For emitted particle measurements by QCMs, the whole 5 × 5 cm sample is exposed to the plasma to ensure a good signal on the microbalance. In both cases, the same sample may

<figure><img src="image_7.png" /><figcaption>Figure 1. Kapton structure.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Figure 2. PEEK structure.</figcaption></figure>

be used for several measurements. To ensure good repeatability, each sample, prior to the first measurement, is sputtered cleaned for 1 hour under 500 eV Xe⁺ ions.

### 2.2. Material preparation

Sputtering yields are measured using the weight loss technique. A Sartorius balance (ME36S) with 1 µg resolution is used to weight the samples. In many materials, polymers are subject to water adsorption when exposed to the atmosphere. When they are in vacuum, this water evaporates, in a process called desorption, resulting in a net mass loss of the samples.

When they are immersed in high vacuum, polymers are also subjected to outgassing [24]. This phenomenon is characterized by the removal of small organic molecules from the material's surface. This may also result in weight loss.

Several tests have been carried out in order to assess the magnitude of these phenomena. Samples were initially weighed, then stored under high vacuum (5 × 10⁻⁶ mbar) for 7 hours, before finally being taken out of the chamber and weighed again. The mass lost right after storage could be as high as 1 mg. However, after 2 h of air exposure, every sample has recovered its initial mass. During the plasma exposure, the temperature of the sample may reach 90 °C due to the external neutralizer (a heated tungsten wire). As outgassing is temperature dependent, we also performed a 3 h test in high vacuum, during which the sample is heated up to 90 °C. As before, the samples were around 1 mg lighter right after they were taken out, but recovered their initial mass 24 h later

Without any erosion performed, the maximum mass variation of the sample between the two weightings is around 20 µg, which may be related to temperature or humidity changes in the weighing room. We thus conclude that only water desorption is responsible for this mass loss and that outgassing is negligible in our experimental conditions.

### 2.3. Sputtering yields measurements

Before each experiment, the sample is stored under vacuum, and is moved outside right before the initial scale. The erosion
<figure><img src="image_9.png" /><figcaption>**Figure 3.** Ion energy distribution function at 500 eV.</figcaption></figure>

<table><caption>**Table 1.** Parameters of the Gaussian fit of the ion energy distribution function at the 4 studied energies.</caption><thead><tr><th>Set energy (eV)</th><th>350</th><th>500</th><th>650</th><th>800</th></tr></thead><tbody><tr><th>E<sub>0</sub> (eV)</th><td>346</td><td>498</td><td>648</td><td>796</td></tr><tr><th>σ (eV)</th><td>6.8</td><td>8.5</td><td>10.4</td><td>12.1</td></tr></tbody></table>

was performed at the IDEFIXe facility at ONERA's laboratory. It is a cylindrical vacuum chamber of 0.5 m in diameter and height, connected to a 700 l/s turbomolecular pump. Xenon ions are generated using a 3 cm Kaufman source from Ion Tech Inc. It operates under a neutral gas background pressure of $2 \times 10^{-4}$ mbar and a discharge voltage of 30 V, delivering approximately 10 mA of ionic current. An external tungsten wire delivers low energy electrons in order to neutralize the plasma. The ion energy is selected from the source controller. We performed ion energy distribution function measurements using a 3 grid retarding potential analyzer to ensure that the source was delivering the proper energy. In this device, the first grid is grounded so that it does not disturb the plasma, the second one is polarized at -20 V to prevent electrons from reaching the collector, and the third grid is the energy filter. A typical ion energy distribution function curve is presented in figure 3. Experimental measurements were fitted with a Gaussian function, centered at $E_0$ (mean energy) with a standard deviation $\sigma$. The values of these parameters at the four studied energies are reported in table 1. As we can see, the mean energy is really close to the set value and the standard deviation remains below 2% of $E_0$.

The sample is mounted 15 cm downstream of the ion source. As the plasma beam is divergent, a few ions hit the chamber's steel wall which leads to iron atom ejection. To prevent sample contamination, they are placed in a sample holder, which is designed so that the central area of 12 cm² is exposed to the plasma. The rest of the sample is covered by aluminum plates so it is protected from the plasma flux as well as from any erosion product. This holder can be tilted so that we can adjust the incidence angle of the ions in the sample ($\theta$ angle).

<figure><img src="image_10.png" /><figcaption>**Figure 4.** Schematic diagram of experimental setup for sputtering yield measurement by weight loss.</figcaption></figure>

The schematic diagram of the experimental setup is shown in figure 4.

During erosion, the ion current delivered by the source is monitored using a planar Langmuir probe. The probe is an aluminum rectangle of the same dimension as the sample exposed area. The measurement consists in biasing the probe from -20 V to +10 V and recording the collected current. The Keithley 2410 Sourceometer allows one to both polarize the probe and measure the current. Under negative bias, electrons are repelled and only ions are collected, leading to a negative current going through the probe, known as ion saturation current. We take this value as the ion current received by the sample. As the polarization exceeds 0 V, electrons are also collected and the current quickly increases. A typical I-V characterization is shown figure 5. Depending on the ion energy, the ion current density received by the sample ranges approximately from $8 \times 10^{-2}$ mA cm$^{-2}$ at 50 eV to $2 \times 10^{-1}$ mA cm$^{-2}$ at 800 eV. We have also performed current density measurements with a Faraday cup, biased at -15 V to prevent electrons from entering. Variations between the values obtained with this instrument and the Langmuir probe were around 8% at maximum. As the ion current density is not homogeneous along the sample, a precise ion current measurement would require a large Faraday cup (aperture of 3 × 4 cm) which would be quite intrusive. Hence, in order to assess the ion current in the sample, we believe that it is more representative to use a Langmuir probe with the same dimensions as the sample than a Faraday cup. However, we will add the 8% variation in the ion current uncertainties (see later).

Erosion lasts between 1.5 and 4 h, depending on the incidence energy and angle (we always want to remove at least 1 mg of material to limit the uncertainties). Both the sample and the Langmuir probe are mounted on the same translation manipulator, which can move vertically, in front of the source. During regular erosion, the sample faces the source, and the probe is 10 cm above. Every 15 to 30 min (depending on the total duration of the test), the manipulator is taken down, and the probe gets to the position where the sample previously was. An ion current measurement is made, and the sample is moved back to its initial position. Finally, we compute the average current
<figure><img src="image_11.png" /><figcaption>**Figure 5.** Typical I-V curve obtained with a Langmuir probe, with a focus on the ionic branch (negative bias).</figcaption></figure>

value. The final sample was taken out of the facility and immediately weighed. Under these conditions, the sputtering yield is given by:

$$Y_m = \frac{\Delta m}{It}, \qquad (1)$$

where $\Delta m$ is the sample mass loss in g, I is the average ionic current collected by the sample (in A), and t is the total erosion time (in s). The mass sputtering yield $Y_m$ is given here in g/C and corresponds to the mass of sputtered material per incident coulomb. A volumetric sputtering yield (in mm³ C⁻¹) may also be expressed:

$$Y_{\text{vol}} = \frac{Y_m}{\rho * 10^{-3}}, \qquad (2)$$

where $\rho$ is the sample's volumetric mass in g cm⁻³. In the case of multi-component materials, these mass or volumetric units are preferred to the more standard atoms/ions since we do not know exactly what kind of particles are sputtered. Indeed, they may be either atoms or clusters (groups of atoms or small molecules).

## 2.4. Ejected particle measurement setup

During the sputtering process, sputtered particles are ejected anisotropically, in every direction facing the sample. We call the emission profile a diagram that shows the angular distribution of sputtered particles, i.e. the number of particles emitted in each direction. In order to collect and measure these particles, we use a QCM. This device relies on the piezoelectric effect. The quartz crystal is polarized and, hence, oscillates at a given eigenfrequency (typically, around 6 MHz) [25]. This frequency depends on the mass of the crystal, and the potential deposit on it.

When such a crystal is positioned in front of the sample that is being eroded, some sputtered particles may be ejected

<figure><img src="image_12.png" /><figcaption>**Figure 6.** Schematic diagram of experimental setup QCM collection measurements.</figcaption></figure>

toward the QCM, and stick on its surface. This results in a frequency drift, which is measured by the QCM controller. Given a particular value of the acoustic impedance, which depends on the deposited material, the controller is able to compute the evolution of the mass of the crystal, and thus the number of collected particles.

The experimental setup is shown in figure 6. The sample stands in the middle of the chamber, 25 cm downstream of the ion source. The QCM is located at the end of a horizontal arm, 15 cm away from the sample, mounted on a rotary feedthrough. This allows us to move the QCM all around the sample. The incidence angle, $\theta$, refers to the angle between the ion beam and the sample surface normal ($\theta = 0$ for normal incidence). The polar collection angle, $\alpha$, defines the position of the QCM with respect to the sample surface normal ($\alpha = 0$ when the QCM is on the axis of the sample surface normal). The variation of $\alpha$ corresponds to a QCM rotation around the $x_1$ axis on figure 6. Finally, the azimuthal collection angle is noted $\phi$ and represents the QCM rotation around the $x_2$ axis.

In order to plot the emission profiles, the QCM is moved all around the sample, from $\alpha = -90^{\circ}$ to $\alpha = 90^{\circ}$, with 10° steps. If one wants to map the entire sample-facing hemisphere, the QCM should also perform a scan from $\phi = 0^{\circ}$ to $90^{\circ}$. However, this contribution only presents results obtained in the $\phi = 0^{\circ}$ plane. At each location, a collection acquisition is performed, and the evolution of the mass of the QCM is recorded. We call $R(\alpha)$, in mm/s, the deposit growth velocity for a given collection angle. Based on this signal, it is possible to assess the differential sputtering yield, which refers to the number of particles emitted toward a given solid angle per ion impacting the sample surface:

$$y(\alpha) = \frac{R(\alpha) A_{\text{QCM}}}{I \Omega}, \qquad (3)$$

with $\Omega$ the collection solid angle in sr:

$$\Omega = \frac{A_{\text{QCM}}}{D^2}. \qquad (4)$$

$y$ is the differential sputtering yield in mm³ C⁻¹ sr⁻¹, I the average current in the sample (in mA), D the distance between
the sample and the crystal (13 cm) and A$_{QCM}$ the QCM exposed area (0.48 cm$^2$). Using (3) and (4) these may be simplified as:

$$y(\alpha) = \frac{R(\alpha) D^2}{I}. \qquad (5)$$

Once again, the ion current is measured using a Langmuir probe of the same area as the sample. For position value of α, a current measurement is performed at the beginning and at the end of the acquisition by placing the probe at the sample’s location. We consider the average value of the ion current in the sample.

In fact, the crystal oscillation frequency is quite sensitive to temperature shifts. For this experiment, we use ‘AT-cut’ crystals, which are very stable around 25 °C [25]. However, as we move the QCM toward the source, it gets closer to the neutralizer and its temperature starts rising. To prevent heating and to ensure a good thermal stability, the crystal is put onto a water-cooled holder. Water temperature may be adjusted and set as low as 0 °C. Thanks to a thermocouple located inside the holder, we monitor its temperature in order to keep it between 20 °C and 30 °C, with a drift below 0.02 °C min$^{-1}$ during an acquisition.

## 2.5. QCM data post-processing

For each experimental configuration (i.e. ion energy and incidence angle) we measure the collection rate at 16 to 18 positions. This gives us a differential sputtering yield profile as a function of the polar collection angle. As mentioned before, only the evolution of y along α is studied here, hence no dependence over φ is assumed (we consider an axisymmetrical distribution around the φ axis). In order to compute the total sputtering yield (Y) in a given configuration, we fit a dataset to a sixth order polynomial:

$$y_{\text{poly}}(\alpha) = \sum_{i=0}^{6} k_i \alpha^i. \qquad (6)$$

The seven fitting parameters are extracted, and the resulting expression must be integrated over the whole 2π solid angle hemisphere to get the total sputtering yield value [26]. Considering no dependence of y over φ, the total sputtering yield may actually be expressed as a single integral:

$$Y = \pi \int_{-\frac{\pi}{2}}^{\frac{\pi}{2}} y(\alpha) \sin(\alpha) d\alpha. \qquad (7)$$

Such a post-treatment method is quite robust to assess the total sputtering yield from a series QCM measurements; however, (6) does not have a real physical meaning. A research group from the Colorado State University (CSU) [13, 23] have extensively used and discussed the QCM collection method, and they proposed another fitting formula, where the sputtering yield is a free parameter. It is a semi empirical relation, based on the work of Zhang and Zhang [27] who developed

<figure><img src="image_13.png" /><figcaption>Figure 7. QCM measurement of differential sputtering yield of aluminum under normal incidence bombardment and comparison with MZ and polynomial fitted profiles.</figcaption></figure>

a semi-empirical expression to predict the angular distribution of sputtered particles. The so called Modified Zhang (MZ) expression resulting in CSU work is:

$$y(\alpha) = \frac{Y}{1 - \sqrt{\frac{E^*}{E}} \cdot \cos(\theta)} \cdot \frac{\cos(\alpha)}{\pi} \cdot \left[ 1 - \frac{1}{4} \sqrt{\frac{E^*}{E}} \left( \cos(\theta) \gamma(\alpha) + \frac{3}{2} \pi \sin(\theta) \sin(\alpha) \cos(\phi) \right) \right], \qquad (8)$$

with:

$$\gamma(\alpha) = \frac{3 \sin^2 \alpha - 1}{\sin^2 \alpha} + \frac{\cos^2 \alpha (3 \sin^2 \alpha + 1)}{2 \sin^2 \alpha} \cdot \ln \left( \frac{1 + \sin \alpha}{1 - \sin \alpha} \right). \qquad (9)$$

E refers to the ions’ energy (in eV) and the two free parameters are Y (the sputtering yield), and E* (called characteristic energy). Figure 7 shows the collection measurements made on an aluminum sample under normal incidence, and the curves corresponding to polynomial and MZ fits. One can notice a fairly good agreement between experimental points and fitted profiles. However, as we will discuss in section 3.3, the MZ formula fits really poorly with ejected particle profiles obtained in polymers. Although this expression is based on more physical considerations, a polynomial fit is preferred for total sputtering yield determination from QCM measurements.

QCM only measures the flux of particles that stick on their surface. For metals and carbon, we assume that the sticking coefficient equals the unity [28–30]. However, in the case of multicomponent samples, we do not know exactly which species are sputtered, and we are not sure how easily each of them is able to condense. Sputtered species resulting from collisions cascades inside a material are supposed to be mainly single atoms in our energy range [26]. During boron nitride erosion, Rubin et al [31] demonstrated that only boron atoms were collected onto the QCM. The authors assumed that sputtered nitrogen atoms quickly reacted to form N$_{2}$. Regarding Kapton
<table><caption>**Table 2.** Elemental concentrations measured by XPS after PTFE erosion.</caption><thead><tr><th></th><th>C at%</th><th>F at%</th></tr></thead><tbody><tr><td>Sample</td><td>43.56</td><td>56.44</td></tr><tr><td>QCM</td><td>41.97</td><td>58.03</td></tr></tbody></table>

and PEEK, we assume that only carbon atoms are able to stick on the QCM, while H, O and N are not, which was also the assumption made by Yalin *et al* [23]. Hence, the measured sputtering yield for these polymers is actually the carbon sputtering yield. Considering a stoichiometric sputtering, it is, however, possible to compute the global sputtering yield, $Y_{glob}$ using the following relation:

$$Y_{glob} = Y_{QCM} \frac{M_{tot}}{M_{cond}}, \qquad (10)$$

with $M_{tot}$ the molar mass of the polymer repeat unit, and $M_{cond}$ the condensable fraction molar mass, i.e. the carbon molar mass is multiplied by the number of carbon atoms in the repeat unit. $Y_{QCM}$ refers to the uncorrected sputtering yield. Regarding PTFE, XPS measurements of the QCM at the end of a collection test exhibited a high fraction of fluorine on the surface, which was not far from the one in the PTFE sample (table 2). We do not yet know whether this fluorine comes from atomic or molecular ($C_xF_y$) deposition, but for this polymer, we will consider that the condensed yield ($Y_{QCM}$) is equal to the real sputtering yield. Unfortunately, performing XPS analysis requires transferring the samples toward the dedicated facility under the atmosphere. This leads to surface contamination by hydrocarbons, which are molecules made of carbon, hydrogen and oxygen. Such atoms are the ones that we are looking for when studying Kapton and PEEK erosion products, hence it is not possible to perform the same analysis for these two polymers.

### 3. Results and discussion

#### 3.1. Weight loss measurements

Kapton, PEEK and PTFE sputtering yields at normal incidence, below energies from 350 to 800 eV is reported in table 3. Results are provided in $mm^3 C^{-1}$, given a Kapton density of $1,42 g cm^{-3}$, a PEEK density of $1.32 g cm^{-3}$ and a PTFE density of $2.20 g cm^{-3}$. For the first two polymers, the evolution of their sputtering yield versus ion incidence energy is shown in figure 8. Furthermore, we compare these results in the Kapton sputtering yields obtained by Yalin *et al* [23], using a QCM collection technique, and show that our values are slightly higher than theirs. We will discuss these discrepancies in section 3.3. We can also tell that PEEK erodes 20%–40% slower than Kapton. As these materials are mainly made of carbon atoms, we also plotted the sputtering yield of chemical vapor deposited (CVD) carbon measured by Kolasinski *et al* [32]. It appears that these two polymers exhibit a higher sputtering yield than CVD carbon. PTFE, on the other hand,

<table><caption>**Table 3.** Sputtering yield of Kapton, PEEK and PTFE under several incidence energies.</caption><thead><tr><th>Ion energy<br>(eV)</th><th><i>Y</i><sub>Kapton</sub><br>(mm<sup>3</sup> C<sup>-1</sup>)</th><th><i>Y</i><sub>PEEK</sub><br>(mm<sup>3</sup> C<sup>-1</sup>)</th><th><i>Y</i><sub>PTFE</sub><br>(mm<sup>3</sup> C<sup>-1</sup>)</th></tr></thead><tbody><tr><td>350</td><td>0.0313 ± 0.0049</td><td>0.0167 ± 0.0039</td><td>4.80 ± 1.98</td></tr><tr><td>500</td><td>0.0435 ± 0.0081</td><td>0.0320 ± 0.0082</td><td>7.04 ± 2.74</td></tr><tr><td>650</td><td>0.0482 ± 0.0097</td><td>0.0405 ± 0.0087</td><td>10.44 ± 2.01</td></tr><tr><td>800</td><td>0.0594 ± 0.0101</td><td>0.0513 ± 0.0127</td><td>16.48 ± 2.94</td></tr></tbody></table>

<figure><img src="image_14.png" /><figcaption>**Figure 8.** Sputtering yield of Kapton and PEEK versus incidence energy. Results are compared with Kapton [23] and Carbon [32] measurements from literature.</figcaption></figure>

<table><caption>**Table 4.** Sputtering yield of Kapton and PEEK under 500 eV ions for several incidence angles.</caption><thead><tr><th>Incidence angle<br>(deg)</th><th><i>Y</i><sub>Kapton</sub><br>(mm<sup>3</sup> C<sup>-1</sup>)</th><th><i>Y</i><sub>PEEK</sub><br>(mm<sup>3</sup> C<sup>-1</sup>)</th><th><i>Y</i><sub>PTFE</sub><br>(mm<sup>3</sup> C<sup>-1</sup>)</th></tr></thead><tbody><tr><td>0</td><td>0.0435 ± 0.0081</td><td>0.0320 ± 0.0082</td><td>7.04 ± 2.74</td></tr><tr><td>30</td><td>0.0493 ± 0.0077</td><td>0.0337 ± 0.0068</td><td>7.21 ± 4.69</td></tr><tr><td>45</td><td>0.0613 ± 0.0109</td><td>0.0378 ± 0.0097</td><td>6.86 ± 3.93</td></tr><tr><td>60</td><td>0.0725 ± 0.0115</td><td>0.0464 ± 0.0147</td><td>7.11 ± 3.51</td></tr><tr><td>75</td><td>0.0879 ± 0.0187</td><td>0.0561 ± 0.0181</td><td>7.89 ± 5.01</td></tr></tbody></table>

exhibits a much larger sputtering yield, more than 2 orders of magnitude higher than the other two polymers. Such values are close, although still bigger, to those we can get on high sputtering yield metals such as silver for instance [18].

The evolution of the sputtering yield with ion incidence angle is reported in table 4. $Y/Y_0$ ratio represents the sputtering yield normalized by the value at 0° incidence. Figure 9 shows the evolution of this ratio versus incidence angle, and compares our results with the same data from the literature as before. For most materials, we usually observe a strong sputtering yield that increases with incidence angle, up to a maximum for an angle called $\theta_{max}$ and then a sharp decrease [33]. This increase is due to the collision cascade getting closer to the surface at oblique incidence. The carbon curve clearly illustrates this behavior. For Kapton and PEEK, this increase
<figure><img src="image_15.png" /><figcaption>**Figure 9.** Y/Y(0°) ratio versus incidence angle at 500 eV for Kapton, PEEK and PTFE. Results are compared with Kapton [23] and CVD carbon [32] measurements from literature.</figcaption></figure>

is much weaker, which was also an observation by Yalin et al [23]. θmax value is 75° or more for Kapton and PEEK, while it is about 65° for carbon. At this energy, the maximum sputtering yield (Y(θm)) of carbon is 4.5 times larger than the one at 0°, while it is only 2 times larger for Kapton and 1.75 times for PEEK. For its part, PTFE does not show any increase with incidence angle.

We can mention as well that, as a matter of fact, light materials, or compounds of light (low Z) elements exhibit an important sputtering yield increase with incidence angle [33]. Thus, this particular behavior for polymers is quite surprising and could suggest that regular collision cascades and particle sputtering are not the only mechanisms inducing erosion of these materials.

We computed the uncertainties in our sputtering yield measurements using the following formula:

$$ \frac{\Delta Y_m}{Y_m} = 2 \frac{\Delta m}{m} + \frac{\Delta I}{I} + 2 \frac{\Delta S}{S}. \qquad (11) $$

Δm refers to the uncertainties on each weighing (20 µg). For ΔI, we take the standard deviation of the measured ion current during an erosion test, and we add a net 8% from comparison of the ion current density measured with a Faraday cup. ΔS is the uncertainty on the sample and Langmuir probe area and is equal to 0.35 cm². This gives us uncertainties of around 20% for Kapton and PEEK. However, the ion current is quite unstable during PTFE erosion, leading to uncertainties reaching 60% in some cases. They are represented by error bars on the graphs.

## 3.2. Differential sputtering yields

QCM acquisitions give us the deposit thickness evolution over time, visible on figure 10. We show that the monitoring resolution limit is 1 Å and since the collected particle flux is very low, the thickness evolution is not continuous but appears

<figure><img src="image_16.png" /><figcaption>**Figure 10.** Deposit thickness evolution over time for acquisitions on a PTFE sample. A 20 s shift has been added on orange data to distinguish the curves.</figcaption></figure>

as a series of plateaus. This metric accounts for a 'pseudo-thickness', as a 1 Å deposit is irrelevant. We apply a linear regression to these data to compute the deposit growth velocity R(α). For Kapton and PEEK, every plateau has about the same length, and the linear regression fits well. However, in some (not all) acquisitions on PTFE samples, plateaus get longer over time, indicating a non-constant growth, with a decreasing velocity. These two cases are shown to occur on figure 10, with a constant (blue curve) and no constant growth (orange curve). Even after several reiterations, this behavior persists. This is neither due to thermal nor ion current instabilities, thus we do not yet know what is responsible for it. In these cases, as plateaus' length does not stabilize, the linear regression is not a proper fit, though it is the only way to get an R(α). One should keep in mind that such values overestimate this growth velocity, as well as the resulting differential sputtering yield, therefore large uncertainties are related to these measurements. Nevertheless, this points out that some phenomena taking place during PTFE bombardment are not yet understood, and still have to be investigated.

We measured differential sputtering yields of Kapton, PEEK and PTFE for three different energies (500, 650 and 800 eV). We did not manage to perform the 350 eV collection due to the very low deposition rate of these materials. Figure 11 shows the results under normal incidence while those under 45° incidence are presented in figure 12. The very low sputtering yield of these polymers induces a small collection rate which is responsible for the large noise in these measurements. On the other hand, Kapton and PEEK exhibit a similar behavior, with emission profiles presenting the same shape. Under normal incidence, the sputtered particle flux is maximal for directions ranging between 80° and 50° (in absolute values) and is smaller closer to the surface normal. On the other hand, one can notice that for PTFE samples, the maximal flux of ejected particles is closer to the normal, at around 30° and 40°. Under 45° incidences, profiles are no longer symmetrical and most particles are ejected in the forward direction,
<figure><img src="image_17.png" /><figcaption>**Figure 11.** Differential sputtering yield of Kapton (a), PEEK (b) and PTFE (c) under normal incidence bombardment.</figcaption></figure>

with a maximum from 70° to 50° for the first two polymers and between 60° and 20° for third one. Once again, the sputtered particle flux is closer to the surface normal in the case of PTFE samples. The bulge at -90° is probably due to the collection of particles coming from the edge of the sample holder and, hence, this point should not be considered.

From (5), we can compute the uncertainties on differential sputtering yields using the following formula:

$$ \frac{\Delta y_{\text{vol}}}{y_{\text{vol}}} = \frac{\Delta R(\alpha)}{R(\alpha)} + \frac{\Delta I}{I} + 2\frac{\Delta D}{D}, \qquad (12) $$

with $\Delta D/D = 3.8\%$. In order to estimate the stability of the ion source over time, we computed the relative difference between the two current measurements. As $\Delta I$, we take the standard deviation of this shift along the entire experiment, which is about 2%–5% for Kapton and PEEK and 6%–12% for PTFE. As before, we also add an 8% error from comparison with the ion current density measured using the Faraday

<figure><img src="image_18.png" /><figcaption>**Figure 12.** Differential sputtering yield of Kapton (a), PEEK (b) and PTFE (c) under 45° incidence bombardment.</figcaption></figure>

cup. Finally, in order to assess the uncertainties on the deposit growth velocity, we computed the slope between each consecutive plateau of an acquisition. For $\Delta R(\alpha)$, we take the standard deviation of these slope datasets. For Kapton and PEEK, it gives us 3%–12% error, while it may reach 25% for PTFE. Error bars on figures 11 and 12 are obtained by summing these three contributions.

### 3.3. Total sputtering yield estimation from QCM measurements

Differential sputtering yield data were fitted using the MZ formula (8). However, the measured ejected particle profiles fit quite poorly with this expression, as one can notice on figure 13. Discrepancies could be caused by the noise in our measurements, but we mainly believe that this semi-empirical formula was developed to fit ejection profiles from metals and monoatomic samples [27, 29]; hence, it is not suitable for polymers sputtering related physics. Since we noticed quite
<figure><img src="image_19.png" /><figcaption>**Figure 13.** Comparison of QCM data with best fit MZ profiles under 650 eV ions bombardment for Kapton (a), PEEK (b) and PTFE (c).</figcaption></figure>

different behavior between Kapton and PEEK profiles, on the one hand and PTFE ones on the other, we cannot yet propose a better fitting formula. Hence, the total sputtering yields are computed with a polynomial fit (6), and integration with (7).

Extracted parameter values for the polynomials are reported in table A1. QCM sputtering yield ($Y_{QCM}$) as well as the corrected global sputtering yield ($Y_{glob}$) (computed using (10) for Kapton and PEEK) are presented in table 5. Absolute uncertainties in these results (6th column) are defined as the average uncertainty on every $y(\alpha)$ for a given configuration. The last column reports the average deviation between the QCM data points and the values of the fitted expression at the same $\alpha$ angle. This last metric indicates how good the fit is. Under oblique incidence, the differential sputtering yields depend on both $\alpha$ and $\phi$ angles. Since we only performed

<table><caption>**Table 5.** Total sputtering yield ($Y$) under normal incidence, uncertainties and standard error between data points and polynomial fit.</caption><thead><tr><th>Polymer</th><th>Energy (eV)</th><th><i>Y</i><sub>QCM</sub> (mm<sup>3</sup> C<sup>-1</sup>)</th><th><i>Y</i><sub>glob</sub> (mm<sup>3</sup> C<sup>-1</sup>)</th><th>Uncert. (%)</th><th>Error (%)</th></tr></thead><tbody><tr><td rowspan="3">Kapton</td><td>500</td><td>0.011</td><td>0.016</td><td>25.8</td><td>5.5</td></tr><tr><td>650</td><td>0.016</td><td>0.022</td><td>25.5</td><td>8.8</td></tr><tr><td>800</td><td>0.021</td><td>0.031</td><td>24.8</td><td>9.5</td></tr><tr><td rowspan="3">PEEK</td><td>500</td><td>0.011</td><td>0.014</td><td>24.8</td><td>9.8</td></tr><tr><td>650</td><td>0.018</td><td>0.023</td><td>22.0</td><td>4.5</td></tr><tr><td>800</td><td>0.022</td><td>0.028</td><td>25.0</td><td>8.2</td></tr><tr><td rowspan="3">PTFE</td><td>500</td><td>0.088</td><td>0.088</td><td>40.2</td><td>37.7</td></tr><tr><td>650</td><td>0.193</td><td>0.193</td><td>32.7</td><td>19.7</td></tr><tr><td>800</td><td>0.274</td><td>0.274</td><td>28.2</td><td>12.8</td></tr></tbody></table>

<figure><img src="image_20.png" /><figcaption>**Figure 14.** Comparison of sputtering yield values of Kapton and PEEK obtained by weight loss measurement and QCM method (in this case, data correspond to $Y_{glob}$ values). Black crosses are the results obtained by Yalin *et al* [23].</figcaption></figure>

measurements along $\alpha$, integration with (7) is not relevant in this case. Hence, we only computed total sputtering yields for normal incidence.

For these two polymers, one can notice that the sputtering yield values ($Y_{glob}$) obtained by QCM measurements are 3 to 4 times smaller than those measured by weight loss, as one can see on figure 14. Although Yalin *et al* [23] used the collection technique, their results are closer to those we obtained by weight loss measurements. For PTFE, QCM sputtering yields are 2 orders of magnitude smaller (figure 15), although carbon and fluorine have been found in stoichiometric proportions on the QCM surface (table 2).

Such results indicate that the assumption leading to (10), i.e. stoichiometric sputtering of single atoms, must be wrong; otherwise, $Y_{glob}$ values ($Y_{QCM}$ for PTFE) would be equal to weight loss sputtering yields. Hence, we assume that not only single atoms are sputtered during erosion but also clusters or
<figure><img src="image_21.png" /><figcaption>**Figure 15.** Comparison of sputtering yield values of Kapton and PTFE obtained by weight loss measurement and QCM method.</figcaption></figure>

molecular fragments that would be generated by bond breaking due to moving particles. Rather than being physically sputtered, such species would easily outgas under vacuum. This could explain the small (even nonexistent for PTFE) sputtering yield increase with incidence angle observed on figure 9. Since collision cascades are responsible for this increase, we assume that they are not the only mechanism leading to matter loss. Molecular fragments can be created by incoming ions breaking bonds, but their generation rate must not depend on the projectile incidence, but only on deposited energy.

Moreover, the lower sputtering yields measured by the collection method tell us that some of these fragments may not be condensable, at least not at room temperature. The large sputtering yield of PTFE, as well as the large difference between the results with the two methods, suggest that in this case, erosion mostly occurs via molecular fragment emission, rather than atom sputtering. According to the Kapton and PEEK results, the molecular fragmentation contribution seems weaker. In order to quantify this, low temperature collection measurements should be performed, so the eroded compound would be able to stick to the QCM. Since our cooling system cannot cool down the QCM surface below 15 °C, we are not yet able to carry out such measurements. Liquid nitrogen cooling should be considered to do so.

## 4. Conclusion

We report sputtering yield values measured by weight loss for Kapton, PEEK and PTFE under xenon bombardment at energies ranging from 350 to 800 eV. PTFE exhibits a huge mass loss, while for the other two polymers the sputtering yield is of the order of magnitude of the carbon one. PTFE does not show any sputtering yield increase with incidence angle, and this dependence is also quite limited for Kapton and PEEK. This behavior, which is quite surprising, suggests that particular mechanisms take place during polymer bombardment.

We also performed collection measurements with a QCM to compute the differential sputtering yield of these materials, which is a key source of information regarding satellite contamination. Kapton and PEEK ejected particle profiles exhibit the same shape, while PTFE sputtered particles are emitted closer to the sample's surface normal. Uncertainties in differential sputtering yields are significant, especially for PTFEs (as high as 50%) but these results reveal specific and new erosion features in polymer bombardment by low energy Xe ions, that imply dedicated physicochemical processes.

We used a sixth order polynomial to fit the QCM data and integrated the resulting expression to assess the total sputtering yield values obtained by the collection method. These were much smaller (up to 2 orders of magnitude) than those obtained by weight loss measurements, which leads us to suggest that non-condensable molecular species are emitted during ion bombardment. In this way, polymers do not behave like other materials (metals, ceramic etc) as low energy ion bombardment usually leads to single atom sputtering.

Following these first observations, we have studied polymer decomposition under ion bombardment performing residual gas analysis measurements during erosion. These measurements brought out the emission of molecular fragments with heavy atomic mass. Results will be published in a further paper.

## Data availability statement

All data that support the findings of this study are included within the article (and any supplementary files).

## Acknowledgment

Mr Lucas Nicolas has benefited from an ONERA/CNES PhD fellowship.
Appendix. Extracted parameters values for polynomial fit

<table><caption>Table A1. Extracted parameters values for polynomial fits with (6).</caption><thead><tr><th>Sample</th><th>θ (°)</th><th>E (eV)</th><th>k<sub>0</sub></th><th>k<sub>1</sub></th><th>k<sub>2</sub></th><th>k<sub>3</sub></th><th>k<sub>4</sub></th><th>k<sub>5</sub></th><th>k<sub>6</sub></th></tr></thead><tbody><tr><td rowspan="6">Kapton</td><td rowspan="3">0</td><td>500</td><td>1.61 × 10<sup>-3</sup></td><td>3.05 × 10<sup>-4</sup></td><td>-1.55 × 10<sup>-3</sup></td><td>2.43 × 10<sup>-4</sup></td><td>3.14 × 10<sup>-3</sup></td><td>-1.51 × 10<sup>-4</sup></td><td>-1.15 × 10<sup>-3</sup></td></tr><tr><td>650</td><td>1.89 × 10<sup>-3</sup></td><td>4.90 × 10<sup>-43</sup></td><td>8.39 × 10<sup>-4</sup></td><td>5.44 × 10<sup>-5</sup></td><td>1.23 × 10<sup>-3</sup></td><td>-1.20 × 10<sup>-4</sup></td><td>-7.91 × 10<sup>-4</sup></td></tr><tr><td>800</td><td>3.21 × 10<sup>-3</sup></td><td>1.28 × 10<sup>-3</sup></td><td>-6.25 × 10<sup>-4</sup></td><td>-7.28 × 10<sup>-4</sup></td><td>2.93 × 10<sup>-3</sup></td><td>5.04 × 10<sup>-5</sup></td><td>-1.34 × 10<sup>-3</sup></td></tr><tr><td rowspan="3">45</td><td>800</td><td>1.35 × 10<sup>-3</sup></td><td>1.74 × 10<sup>-3</sup></td><td>4.88 × 10<sup>-3</sup></td><td>1.32 × 10<sup>-3</sup></td><td>-2.54 × 10<sup>-3</sup></td><td>-8.95 × 10<sup>-4</sup></td><td>2.92 × 10<sup>-4</sup></td></tr><tr><td>650</td><td>3.28 × 10<sup>-3</sup></td><td>2.86 × 10<sup>-3</sup></td><td>3.37 × 10<sup>-3</sup></td><td>1.14 × 10<sup>-4</sup></td><td>-1.75 × 10<sup>-3</sup></td><td>-5.75 × 10<sup>-4</sup></td><td>2.53 × 10<sup>-4</sup></td></tr><tr><td>800</td><td>4.52 × 10<sup>-3</sup></td><td>3.92 × 10<sup>-3</sup></td><td>1.04 × 10<sup>-3</sup></td><td>6.79 × 10<sup>-4</sup></td><td>1.60 × 10<sup>-3</sup></td><td>-1.01 × 10<sup>-3</sup></td><td>8.00 × 10<sup>-4</sup></td></tr><tr><td rowspan="6">PEEK</td><td rowspan="3">0</td><td>500</td><td>1.14 × 10<sup>-3</sup></td><td>3.26 × 10<sup>-4</sup></td><td>7.66 × 10<sup>-4</sup></td><td>6.36 × 10<sup>-4</sup></td><td>7.32 × 10<sup>-4</sup></td><td>-4.03 × 10<sup>-4</sup></td><td>3.63 × 10<sup>-4</sup></td></tr><tr><td>650</td><td>1.45 × 10<sup>-3</sup></td><td>1.17 × 10<sup>-3</sup></td><td>3.09 × 10<sup>-3</sup></td><td>-8.29 × 10<sup>-4</sup></td><td>-7.53 × 10<sup>-4</sup></td><td>5.61 × 10<sup>-5</sup></td><td>2.04 × 10<sup>-4</sup></td></tr><tr><td>800</td><td>2.21 × 10<sup>-3</sup></td><td>3.05 × 10<sup>-6</sup></td><td>2.20 × 10<sup>-3</sup></td><td>1.23 × 10<sup>-3</sup></td><td>4.44 × 10<sup>-4</sup></td><td>-5.98 × 10<sup>-4</sup></td><td>-5.93 × 10<sup>-4</sup></td></tr><tr><td rowspan="3">45</td><td>500</td><td>1.66 × 10<sup>-3</sup></td><td>2.62 × 10<sup>-4</sup></td><td>3.54 × 10<sup>-3</sup></td><td>2.75 × 10<sup>-3</sup></td><td>-2.50 × 10<sup>-3</sup></td><td>-1.29 × 10<sup>-3</sup></td><td>4.50 × 10<sup>-4</sup></td></tr><tr><td>650</td><td>2.44 × 10<sup>-3</sup></td><td>-4.91 × 10<sup>-4</sup></td><td>5.81 × 10<sup>-3</sup></td><td>4.39 × 10<sup>-3</sup></td><td>-4.44 × 10<sup>-3</sup></td><td>-1.78 × 10<sup>-3</sup></td><td>9.17 × 10<sup>-4</sup></td></tr><tr><td>800</td><td>2.84 × 10<sup>-3</sup></td><td>2.39 × 10<sup>-4</sup></td><td>9.34 × 10<sup>-3</sup></td><td>3.41 × 10<sup>-3</sup></td><td>-6.79 × 10<sup>-3</sup></td><td>-1.46 × 10<sup>-3</sup></td><td>1.23 × 10<sup>-3</sup></td></tr><tr><td rowspan="6">PTFE</td><td rowspan="3">0</td><td>500</td><td>2.84 × 10<sup>-3</sup></td><td>2.84 × 10<sup>-3</sup></td><td>2.84 × 10<sup>-3</sup></td><td>2.84 × 10<sup>-3</sup></td><td>2.84 × 10<sup>-3</sup></td><td>2.84 × 10<sup>-3</sup></td><td>2.84 × 10<sup>-3</sup></td></tr><tr><td>650</td><td>2.77 × 10<sup>-2</sup></td><td>5.96 × 10<sup>-3</sup></td><td>7.95 × 10<sup>-2</sup></td><td>-2.15 × 10<sup>-2</sup></td><td>-8.58 × 10<sup>-2</sup></td><td>7.66 × 10<sup>-3</sup></td><td>2.06 × 10<sup>-2</sup></td></tr><tr><td>800</td><td>5.70 × 10<sup>-2</sup></td><td>-1.45 × 10<sup>-2</sup></td><td>3.92 × 10<sup>-2</sup></td><td>9.38 × 10<sup>-3</sup></td><td>-5.87 × 10<sup>-2</sup></td><td>-1.80 × 10<sup>-3</sup></td><td>1.47 × 10<sup>-2</sup></td></tr><tr><td rowspan="3">45</td><td>500</td><td>1.77 × 10<sup>-2</sup></td><td>4.26 × 10<sup>-2</sup></td><td>1.25 × 10<sup>-2</sup></td><td>3.39 × 10<sup>-2</sup></td><td>7.86 × 10<sup>-3</sup></td><td>7.32 × 10<sup>-3</sup></td><td>-1.69 × 10<sup>-3</sup></td></tr><tr><td>650</td><td>3.16 × 10<sup>-2</sup></td><td>5.59 × 10<sup>-2</sup></td><td>-2.20 × 10<sup>-2</sup></td><td>-4.84 × 10<sup>-2</sup></td><td>1.41 × 10<sup>-2</sup></td><td>1.14 × 10<sup>-2</sup></td><td>-3.58 × 10<sup>-3</sup></td></tr><tr><td>800</td><td>4.16 × 10<sup>-2</sup></td><td>2.70 × 10<sup>-2</sup></td><td>-3.75 × 10<sup>-2</sup></td><td>-1.67 × 10<sup>-2</sup></td><td>3.08 × 10<sup>-2</sup></td><td>3.49 × 10<sup>-3</sup></td><td>8.30 × 10<sup>-3</sup></td></tr></tbody></table>
ORCID iDs

L Nicolas https://orcid.org/0009-0002-2097-1552

M Villemant https://orcid.org/0000-0003-3026-0420

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