# Optimizing chemical resistance of PECVD amorphous silicon carbide thin films

Scott Greenhorn, Valérie Stambouli, Edwige Bano, Andrei Uvarov, Sonia Ortega-Murillo, Konstantinos Zekentes

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Scott Greenhorn, Valérie Stambouli, Edwige Bano, Andrei Uvarov, Sonia Ortega-Murillo, et al. Optimizing chemical resistance of PECVD amorphous silicon carbide thin films. Journal of Non-Crystalline Solids, 2025, 666, pp.123723. 10.1016/j.jnoncrysol.2025.123723. hal-05379544

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# Optimizing chemical resistance of PECVD amorphous silicon carbide thin films

Scott Greenhorn<sup>a,b,c,*</sup> <sup>ID</sup>, Valérie Stambouli<sup>a</sup>, Edwige Bano<sup>b</sup>, Andrei Uvarov<sup>d</sup>,
Sonia Ortega-Murillo<sup>a</sup>, Konstantinos Zekentes<sup>b,c</sup>

## ARTICLE INFO

### Keywords:

Silicon carbide

a-SiC

PECVD

Chemical resistance

Accelerated aging

## ABSTRACT

Amorphous silicon carbide thin films have been shown to be an appealing passivation strategy for biomedical implants due to their great chemical resistance and biocompatibility. This study investigates the chemical etching resistance of amorphous silicon carbide thin films grown by plasma-enhanced chemical vapor deposition (PECVD) with silane and ethylene gas precursors using varying deposition parameters. The chemical resistance is tested with an accelerated aging treatment in phosphate buffered saline. The films' etching rate variation is shown to be highly dependent on the deposition conditions. Indeed, the results emphasize the main role of the ratio of silicon and carbon-containing gas precursors, the deposition temperature and the formation of Si-C bonds. High-quality, long-lifetime films are achieved using high-temperature, silane-rich deposition conditions, resulting in accelerated etch rates as low as 0.03 nm/day. The amorphous SiC structure is confirmed using Raman spectroscopy, FTIR, and XPS, and outliers are explained in terms of their composition and chemical bonding. High chemical resistance is associated with high Si-C bonding and eventually nanocrystalline silicon regions in the films.

## 1. Introduction

Amorphous silicon carbide (a-SiC) thin films deposited by plasma-enhanced chemical vapor deposition (PECVD) show great promise as highly chemically-resistant and biocompatible coatings for various biomedical applications. It has been demonstrated as a protective layer for stents [1,2], dental and orthopedic implants [3–5], and as a passivation material for implantable neural interfaces [6–10]. a-SiC’s chemical inertness is expected to result in significant increases in biodevice lifetime. As a consequence, accelerated aging involving immersion in Phosphate Buffered Saline (PBS) at temperatures near 90 °C [6,10–13] is often necessary in order to test the films’ performance within a reasonable amount of time.

It is expected that the a-SiC films’ chemical resistance depends on intrinsic structural and chemical properties of the film determined from the PECVD deposition process. Films deposited with a typical silane-methane (SiH₄/CH₄) precursor gas mixture have been reported to show significantly delayed failure time when the deposition temperature

is increased above 300 °C [13]. Indeed, a previous comparative study of PECVD a-SiC and SiNₓ as well as thermal SiO₂ films found that a-SiC deposited at 325 °C significantly outperforms other materials, with no appreciable etching for a-SiC compared to 0.1 nm/day for thermal SiO₂ and 18 nm/day for SiNₓ over 112 days [12]. Cogan et al. [6] report an etch rate of 2.4 nm/day for a-SiC deposited at 400 °C and other parameters comparable to the previously-mentioned study. In another study [10], the addition of H₂ gas flow also improves the chemical resistance, in particular reducing the appearance of defects during aging but the etch rate has not being studied. A last study also uses accelerated aging, but applies the technique to entire neural interface devices passivated with a-SiC, and measures only changes in the neural interface performance, showing superior encapsulation for a-SiC coated devices [11]. All of these studies use silane-methane PECVD precursors for a-SiC film deposition as well as PBS at 87–90 °C for accelerated aging.

This study presents the chemical resistance, through PBS-based accelerated aging testing, of PECVD a-SiC films deposited by using a less-common chemistry involving a silane-ethylene (SiH₄/C₂H₄) gas

<footnote><sup>a</sup> Laboratoire des Matériaux et du Génie Physique, Grenoble, France</footnote>

<footnote><sup>b</sup> Centre de radiofréquences, optique et micro-nanoélectronique des Alpes, Grenoble, France</footnote>

<footnote><sup>c</sup> Foundation for Research and Technology - Hellas, Heraklion, Greece</footnote>

<footnote><sup>d</sup> Plasma-Therm Europe, Bernin, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: scott.greenhorn@grenoble-inp.fr (S. Greenhorn).</footnote>
<table><caption>**Table 1**
PECVD deposition parameters for a-SiC films.</caption><thead><tr><th>Film</th><th>Temperature (°C)</th><th>RF Power (W)</th><th>Chamber Pressure (mTorr)</th><th>SiH<sub>4</sub> flow (sccm)</th><th>SiH<sub>4</sub>/C<sub>2</sub>H<sub>4</sub> ratio</th><th>H<sub>2</sub> (%)</th><th>Total Carrier flow (sccm)</th><th>Thickness (nm) ± 2 nm</th><th>Deposition rate (nm/min)</th></tr></thead><tbody><tr><td>1</td><td>320</td><td>220</td><td>1600</td><td>55</td><td>0.8</td><td>95</td><td>1000</td><td>382</td><td>109.1</td></tr><tr><td>2</td><td>120</td><td>220</td><td>800</td><td>55</td><td>0.3</td><td>95</td><td>700</td><td>592</td><td>131.6</td></tr><tr><td>3</td><td>320</td><td>140</td><td>800</td><td>55</td><td>0.8</td><td>0</td><td>700</td><td>378</td><td>75.6</td></tr><tr><td>4</td><td>120</td><td>140</td><td>1600</td><td>55</td><td>0.3</td><td>0</td><td>1000</td><td>748</td><td>187.0</td></tr><tr><td>5</td><td>320</td><td>220</td><td>1600</td><td>25</td><td>0.3</td><td>0</td><td>700</td><td>503</td><td>100.6</td></tr><tr><td>6</td><td>120</td><td>220</td><td>800</td><td>25</td><td>0.8</td><td>0</td><td>1000</td><td>393</td><td>78.6</td></tr><tr><td>7</td><td>320</td><td>140</td><td>800</td><td>25</td><td>0.3</td><td>95</td><td>1000</td><td>443</td><td>39.1</td></tr><tr><td>8</td><td>120</td><td>140</td><td>1600</td><td>25</td><td>0.8</td><td>95</td><td>700</td><td>498</td><td>78.6</td></tr><tr><td>EC1</td><td>220</td><td>180</td><td>1200</td><td>40</td><td>0.55</td><td>47.5</td><td>850</td><td>477</td><td>95.4</td></tr><tr><td>EC2</td><td>220</td><td>180</td><td>1200</td><td>40</td><td>0.55</td><td>47.5</td><td>850</td><td>464</td><td>92.8</td></tr><tr><td>EC3</td><td>220</td><td>180</td><td>1200</td><td>40</td><td>0.55</td><td>47.5</td><td>850</td><td>454</td><td>90.8</td></tr><tr><td>EC4</td><td>220</td><td>180</td><td>1200</td><td>40</td><td>0.55</td><td>47.5</td><td>850</td><td>417</td><td>83.4</td></tr><tr><td>EC5</td><td>220</td><td>180</td><td>1200</td><td>40</td><td>0.55</td><td>47.5</td><td>850</td><td>442</td><td>88.4</td></tr><tr><td>9</td><td>220</td><td>180</td><td>1200</td><td>40</td><td>0.55 (SiH<sub>4</sub>/CH<sub>4</sub>)</td><td>47.5</td><td>850</td><td>432</td><td>86.3</td></tr></tbody></table>

precursor mixture. While other chemical etchants, including KOH, HF, and aqua regia [14,15], have been used, high-temperature PBS was selected for this study as it is a more appropriate brain fluid analogue for long-term in-vivo implantation. The influence of several process parameters is investigated to further optimize the film performance in terms of chemical resistance, including the deposition temperature, radio frequency (RF) power, ratio of silane and ethylene, the magnitude of the precursor flow, the chamber pressure, the inclusion of hydrogen gas, and the dilution of the inert Ar carrier gas with H₂. The accelerated aging results are compared with structural (atomic bond) measurements obtained from Raman spectroscopy, Fourier Transform Infrared (FTIR) spectroscopy, and X-Ray Photoelectron Spectroscopy (XPS) to further investigate the reasons for the differences in chemical resistance. As this study varies multiple deposition parameters, the trends observed are expected to be more generally applicable than those found in previous studies.

## 2. Methods

### 2.1. Investigated a-SiC films

Eight a-SiC films (1–8) deposited by PECVD using a Corial D250L reactor. This apparatus is a parallel-plate isothermal plasma box-style PECVD reactor, mounted inside a turbomolecular-pumped vacuum chamber. The system is equipped with a vacuum load lock with a base vacuum of 50 mTorr. The standby reactor pressure without gas flow is 1–2·10⁻⁶ Torr. The reactor was seasoned before introducing the samples by the of deposition of approximately 100 nm of a-SiC film on the reactor sidewalls. After loading in the reactor, the samples were subjected to thermal soak step of 5 mins and exposed to Ar plasma pre-cleaning of 1 min before the deposition process started.

Periodic plasma cleaning of the reactor was done to remove a-SiC film from its walls, thus assuring that no a-SiC films or particles peeled off during deposition on the wafers. This plasma cleaning process based on SF₆/N₂O chemistry was prior qualified as well compatible with a-Si deposition for photovoltaics (no contaminations associated to residues from plasma cleaning).

Silane (SiH₄) and ethylene (C₂H₄) are used as precursor gases while inert Ar is included as a carrier gas, in some cases diluted with H₂. a-SiC is deposited on 2'' Si wafers using various deposition parameters described in Table 1. C₂H₄ was chosen as a carbon-containing precursor as high-quality depositions were obtained using that chemistry with the D250L reactor. Hydrogen gas is investigated as a carrier gas since some studies report increased chemical resistance for films deposited with H₂ flow at the expense of higher stress [10]. A 100 nm thick SiO₂ passivation layer was deposited by PECVD immediately prior to a-SiC

deposition. The a-SiC films deposition parameters were chosen following a two-level, seven-parameter Plackett-Burman experimental design [16] procedure to minimize bias between the parameters under study. For each parameter, there are two values chosen representing a high (H) and low (L) level of the parameter. For example, for temperature, H is 320 °C while L is 120 °C, and for the SiH₄/C₂H₄ ratio H is 0.8 and L is 0.3.

In addition, five error calibration (EC) films (films EC1-EC5) were deposited with the central deposition parameters (C) (exactly halfway between the high and low values for each deposition parameter) in order to study the consistency of the etch rate for a given recipe.

Finally, two additional films were added to the study for enhanced comparison. First, a film of a-SiC (film 9) deposited with average conditions (same as EC films, and similar to those used in previous studies [6,10,17]), but using silane and methane as precursors, has been studied in order to ensure that the experiment was consistent with previous accelerated aging tests on a-SiC [6,12]. Second, a film with 100 nm of SiO₂ and no a-SiC was prepared to compare against a non-a-SiC material.

### 2.2. Accelerated aging process

The samples were diced into 1 cm x 1 cm squares, and partially covered with a thick layer of polyimide (PI) tape as a mask to protect part of the surface upon immersion in PBS and thus serving as a reference. The samples were placed individually into sealed 50 mL plastic bottles to prevent evaporation. Accelerated aging was performed in phosphate-buffered saline (PBS, 0.01 M) (Sigma-Aldrich), at pH 7.4 and held at 90 °C in an oven. The experiment lasted 168 days (over 5 months) or until a film was too damaged to continue. If the PBS became murky due to crystallization, the solution was replaced for all samples. Sample positions were periodically rotated to avoid systematic effects from possible temperature gradients in the oven.

After removing the samples from PBS at the end of the experiment, the PI tape was removed from the surface, and the films were cleaned in acetone and isopropyl alcohol using an ultrasonic bath. The etch rate was found using mechanical profilometry (Bruker Dektak XT), measuring the step height between the region exposed to PBS and the region previously protected by the PI tape. The measurement was repeated five times at different points on the film to obtain an average etch rate. The validity of the measurement was verified by optical profilometry (ZeGage Pro HR) and spectroscopic ellipsometry (Ellipso Semilab SE-2000) to ensure the thickness of the polyimide-covered region did not change over the course of the experiment and that the difference between exposed and protected regions matched that found using mechanical profilometry.

The acceleration factor, a multiplicative value showing the rate of
<figure><img src="image_4.png" /><figcaption>Fig. 1. Optical image of four representative a-SiC films at t = 0, diced into 1 cm x 1 cm squares prior to the etching test.</figcaption></figure>

accelerated aging compared to physiological temperature at 37 °C, can be defined using the 10° rule, where an increment of 10 K increases the reaction rate by a factor of 2 [11,18].

$$A_f = 2^{\frac{\Delta T}{\theta}} \qquad (1)$$

where $A_f$ is the multiplicative acceleration factor, $\Delta T$ is the difference in temperature between the experiment and physiological temperature (37 °C), and $\theta=10$ K is the increment for doubling. At 90 °C, the acceleration factor is 39.

## 2.3. Raman spectroscopy

Raman spectroscopy measurements were performed using a Horiba Evolution HR spectrometer connected to an Olympus BX41 microscope with a 488 nm blue laser allowing the Raman shift from 80 to 1800 cm⁻¹ to be studied. More details on the use of this technique as well as of the following two techniques for a-SiC film characterization are given in [19].

## 2.4. Fourier transform infrared spectroscopy

The films are also studied by FTIR from 600–3100 cm⁻¹ using a Bruker Vertex 70 V apparatus in attenuated total reflection (ATR) mode using a Germanium crystal. ATR is a surface-sensitive technique using evanescent waves to study vibrational modes in thin films.

## 2.5. X-ray photoelectron spectroscopy

XPS measurements were undertaken using a Thermo-Fisher scientific K-Alpha XPS system. Sputtering was performed at 2 keV for 60 s to

<table><caption>**Table 2**
Accelerated aging results for all films.</caption><thead><tr><th>Film</th><th>Temperature</th><th>SiH<sub>4</sub>/<br>C<sub>2</sub>H<sub>4</sub><br>ratio</th><th>Accelerated Etch<br>Rate (nm/day)<br>± 0.1 nm/day</th><th>Equivalent Etch Rate<br>@ 37 °C (nm/day)</th></tr></thead><tbody><tr><td>1</td><td>H</td><td>H</td><td>0.06</td><td>0.002</td></tr><tr><td>2</td><td>L</td><td>L</td><td>0.74</td><td>0.019</td></tr><tr><td>3</td><td>H</td><td>H</td><td>0.03</td><td>0.001</td></tr><tr><td>4</td><td>L</td><td>L</td><td>2.23</td><td>0.057</td></tr><tr><td>5</td><td>H</td><td>L</td><td>0.35</td><td>0.009</td></tr><tr><td>6</td><td>L</td><td>H</td><td>0.42</td><td>0.011</td></tr><tr><td>7</td><td>H</td><td>L</td><td>0.27</td><td>0.007</td></tr><tr><td>8</td><td>L</td><td>H</td><td>0.60</td><td>0.015</td></tr><tr><td>EC 1</td><td>C</td><td>C</td><td>0.48</td><td>0.012</td></tr><tr><td>EC 2</td><td>C</td><td>C</td><td>0.36</td><td>0.0091</td></tr><tr><td>EC 3</td><td>C</td><td>C</td><td>0.42</td><td>0.011</td></tr><tr><td>EC 4</td><td>C</td><td>C</td><td>0.50</td><td>0.013</td></tr><tr><td>EC5</td><td>C</td><td>C</td><td>0.54</td><td>0.014</td></tr><tr><td>9</td><td>C</td><td>C</td><td>2.42</td><td>0.061</td></tr><tr><td>PECVD<br>SiO<sub>2</sub></td><td>-</td><td>-</td><td>3.60</td><td>0.091</td></tr></tbody></table>

remove the majority of surface oxides. A low-resolution survey spectrum with a pass energy of 100 eV and step size 0.5 eV was measured for each sample, followed by high-resolution spectra for the C1s and Si2p peaks with a pass energy of 30 eV and step size 0.1 eV to study the chemical bonds present in the films. The resulting spectra take the average of 15 passes across the energy range.

## 3. Results

Before etching, the film morphology was characterized by optical

<figure><img src="image_5.png" /><figcaption>Fig. 2. a) Optical image of an a-SiC sample after the removal of the polyimide tape and cleaning. b) Optical profilometry image (film 1, 2.5x magnification, t = 168 days) showing the interface between the region covered by polyimide (bottom) and the region exposed to PBS (top) with no visible defects. c) optical profilometry image (film 4, 2.5x magnification, t = 123 days) showing interface between the region covered by polyimide (right) and the region exposed to PBS (left) with many defects. Red color indicates higher elevation, blue indicates lower elevation.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Fig. 3. a): Accelerated etch rates of a-SiC films grown under SiH₄ and C₂H₄ as a function of the ratio of SiH₄ to C₂H₄. b): Accelerated etch rates of these films as a function of the deposition temperature.</figcaption></figure>

microscopy. The film thickness and color were highly homogeneous for each film, indicating consistency for each film being studied (Fig. 1). The observed colors are dependent on a combination of the thickness and composition of each film. For example, the thin and near-stoichiometric Film 6 is a bright green color, while the Si-rich Film 1 with similar thickness is more red. The thicker Film 2 appears orange, and Film 9, deposited with CH₄ and highly Si-rich in composition, exhibits a brown color.

### 3.1. Etching results

After etching, polyimide removal, and cleaning, some films (in particular the SiO₂ film, Film 9 (grown with CH₄) and the Film 4) showed significant degradation, while others showed negligible change (Fig. 2). In some cases, small amounts of PBS residue were visible in microscopic images even after repeated cleaning in ultrasound; further

cleaning was halted to avoid damaging the surfaces. Point defects of up to several hundred µm in diameter were visible in optical microscope in the fastest-etching a-SiC film (film 4).

The resulting accelerated etch rates are presented in Table 2. For the films grown under silane/ethylene, the etch rate ranges from 0.03 nm/day up to 0.74 nm/day, except film 4 exhibiting a value of 2.23 nm/day. This film, deposited with low SiH₄/C₂H₄, low temperature, and no H₂ flow, showed very low Si-C bonding, explaining the significantly degraded performance. This effect will be examined in detail in Sections 3.2–3.4. It must also be noted that the etch rate of film 9 with silane/methane match closely with those reported by [6]. Furthermore, we note that all SiH₄/C₂H₄ recipes result in films with higher chemical resistance compared to the SiH₄/CH₄ recipe (film 9). Naturally, a single Si-rich film with silane/methane quite probably does not represent the best possible performance obtainable with this C-precursor and for the employed PECVD system. To compare the performance of a-SiC with

<figure><img src="image_7.png" /><figcaption>Fig. 4. Accelerated etch rates of a-SiC films as a function of the RF power for two values of SiH₄/C₂H₄ ratio. a) All films. b) Removing film 4.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Fig. 5. Accelerated etch rates of a-SiC films as a function of the film thickness. Left: all films. Right: Same graph by omitting Films 4 and 9 to observe the trend from other films.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 6. Raman spectra for a-SiC films (films 1–8 deposited with SiH₄ and C₂H₄ and film 9 deposited with SiH₄ and CH₄) before starting the accelerated aging, ascending from most chemically-resistant to least chemically-resistant.</figcaption></figure>

another typical passivation material, the PECVD SiO₂ film was etched at 3.6 nm/day.

These results demonstrate the possibility of great performance of the silane/ethylene gas precursor recipe under a variety of conditions.

The EC films showed a maximum difference in etch rate of 0.186 nm/day, or a standard deviation of 0.073 nm/day. This magnitude is 10 times smaller than the standard deviations of films 1–8 (0.7 nm/day).

This result indicates that the differences in etch rate are most likely due to the changes in the film properties due to their differing recipes rather than to random variations.

Given the complexity of the plasma physics during PECVD deposition, the influence of all deposition parameters has been considered. The accelerated etch rate is seen to depend strongly on the ratio of silane and ethylene precursors, but is confounded by the additional influence of
<figure><img src="image_10.png" /><figcaption>Fig. 7. Raman spectra for EC a-SiC films showing little variation in the bands for films deposited under the same conditions.</figcaption></figure>

temperature (Fig. 3). A high SiH₄/C₂H₄ ratio (0.8) and high temperature (320 °C) result in more chemically-resistant films compared to films deposited at low temperature (120 °C) or low SiH₄/C₂H₄ ratio (0.3). Varying the SiH₄ flow rate for a given SiH₄/C₂H₄ ratio has only a small and inconsistent effect on the etching rate (compare films 1–4 against films 5–8).

The effect of the RF power has also been studied. This effect is temperature-dependent (Fig. 4). For films deposited at low temperature, increasing the RF power slightly improves the chemical resistance. By contrast, films deposited at high temperature show a small decrease in chemical resistance when the RF power is increased.

A small correlation between the film thickness and accelerated etch rate was noted for the films deposited with C₂H₄ gas precursor, with the thinner films exhibiting lower etch rates (Fig. 5). Again, films 9 and 4 exhibit a substantially higher etch rate.

## 3.2. Raman spectroscopy results

The Raman spectra of the films prior to aging are shown in Fig. 6 as a function of increasing etch rate. Three main regions are of interest. First, a Si-Si bond-related band appears in a range from 250–550 cm⁻¹. Within this range, it can be observed a first peak at around 480 cm⁻¹. Its large width indicates randomly oriented Si bonds corresponding to an

amorphous state (a-Si as indicated on the Fig. 6). In some cases, the peak is observed to be sharper and narrower suggesting Si-related micro or nanocrystallinity in the films [20,21]. Second, the a-SiC band, comprised of Si-C bonds, again with random orientations, is observed from 575–900 cm⁻¹. The peak is located at the same energies as that of crystalline Si-C bonds but is broader due to the amorphous structure [22]. Third, a C—C bond related band appears from 1300–1650 cm⁻¹. This peak is large which means the C bonds correspond to an amorphous, randomly-oriented structure (a-C as indicated on the Fig. 6). Graphitic carbon bonds can appear in this band as two distinct peaks, at 1350 cm⁻¹ (D-band, “disordered”) and 1550 cm⁻¹ (G-band, “graphitic”) [20,23–25]. Note that, due to its low bond polarizability (40 times smaller than that of C—C bonds [26]), the Si-C bond peak is smaller and relatively weak compared to a-C and a-Si peaks, making it more difficult to compare across samples. Finally, in some cases there are two peaks (a sharp peak at 517 cm⁻¹ and a smaller peak near 950 cm⁻¹) which correspond to crystalline Si bonds, due to the substrate. The presence and magnitude of these peaks from the substrate depend on the density of the a-SiC layer as well as on the optical interference in the multilayer (a-SiC, SiO₂) structure. In summary, the reported Raman spectra confirm the amorphous structure of the SiC films and closely match those reported in the literature [23,24].

No significant trends in the Raman spectra as a function of the

<figure><img src="image_11.png" /><figcaption>Fig. 8. Raman spectra comparison for the bulk of Film 4 and a graphitic cluster on the same film. Inset: Optical image of graphitic cluster.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>Fig. 9. FTIR ATR spectra for a-SiC films (films 1–8 deposited with SiH₄ and C₂H₄ and film 9 deposited with SiH₄ and CH₄) before starting the accelerated aging, ascending from most chemically-resistant to least chemically-resistant.</figcaption></figure>

chemical resistance are observed across all films. Comparing only the films deposited with SiH₄ and C₂H₄ (films 1–8), it appears that films with a narrower Si-Si peak near 490 cm⁻¹ are more chemically resistant. Other films show a variety of peak sizes and structures, not clearly corresponding to the etch rate. Films 4 and 5 are outliers with a C—C double peak, corresponding to graphite, which is most prominent in film 4. Graphitic clusters are reported in C-rich a-SiC depositions [27,28], and appear here in the films deposited with low SiH₄/C₂H₄ ratio and no H₂ flow.

Film 9, deposited with silane and methane gas precursors, is an important outlier that does not follow the trend of films 1–8, displaying a spectrum similar to that of the highly chemically-resistant C₂H₄ films (Films 1 and 3) but exhibiting very poor chemical resistance. There is nearly no amorphous carbon peak visible in the Raman spectrum, consistent with a high Si composition (see XPS results later on).

The Raman spectra of the EC films are more consistent, with only small variations in the a-Si and a-C band heights between the five films (Fig. 7). No other variations in the bands are observed.

<figure><img src="image_13.png" /><figcaption>Fig. 10. Si-C stretching and Si-CH₃ wagging absorbance peak intensities from FTIR-ATR vs. accelerated etching rate. Labels are provided to identify peaks with similar trends.</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>Fig. 11. FTIR ATR spectra for EC a-SiC films showing small variation in the Si-C stretching and Si-CH₃ wagging bands.</figcaption></figure>

Before the aging test, it was noted that films deposited with C₂H₄-
rich plasma (low SiH₄/C₂H₄ ratio of 0.3, films 2, 4, 5, and 7) showed
defects visible under optical microscope. These defects were dark in
color and ranged from several tens to hundreds of µm in size. Raman
spectroscopy was performed directly on one of these dark defects (again
prior to aging) (Fig. 8). The defects are found to be graphitic, with no
visible Si or Si-C bonds.

3.3. *FTIR results*

FTIR-ATR measurements were performed for all films. The corre-
sponding spectra are reported as a function of the etching rate in Fig. 9.
A previous study has identified the main FTIR peaks for a-SiC [29],
including the Si-C stretching peak at 670 cm⁻¹, Si-CH₃ wagging bond
peak at 780 cm⁻¹, C—Hₙ wagging at 1000 cm⁻¹, Si-Hₙ stretching bands
between 2000–2100 cm⁻¹, and C—Hₙ stretching bands between

2800–3100 cm⁻¹. Note that the C—Hₙ stretching band is very small in ATR measurements.

As reported previously by [10], there is some correlation between
the Si-C stretching peak height at 670 cm⁻¹ for the films (1–8) deposited
with SiH₄/C₂H₄ and the chemical resistance. The particularly small Si-C
stretching peak of film 4 could explain the significantly faster etching
rate compared to other films.

The position of the Si-H<sub>n</sub> band varies as a function of the amount of
hydrogen: if Si-H bonds are plentiful, the band appears closer to 2000
cm<sup>-1</sup>, while films with more Si-H<sub>3</sub> bonds will have a band closer to 2090
cm<sup>-1</sup> [29,30]. The locations of the Si-H<sub>n</sub> stretching peaks are mostly
consistent between films, with a small shift to higher energy (more
Si-H<sub>3</sub>) for film 4.

Again, film 9 (deposited with SiH₄ and CH₄) is an outlier compared to
the other films (1–8) deposited with SiH₄ and C₂H₄, showing a spectrum
distinct from these films. The major differences is a relatively lower Si-

<figure><img src="image_15.png" /><figcaption>Fig. 12. XPS survey spectrum for film 3 showing Si2p, Si2s, C1s, and O1s bonds. This survey was taken after sputtering for 60 s, and is representative of the other films with corresponding variations in the relative peak heights.</figcaption></figure>
<table><caption>**Table 3**
XPS peak deconvolutions.</caption><thead><tr><th>Film</th><th>Accelerated etch rate (nm/day)</th><th>Si2p Si-Si energy (eV) [FWHM]</th><th>Si2p Si-C energy (eV) [FWHM]</th><th>Si2p Si-O energy (eV) [FWHM]</th><th>C1s C—C energy (eV) [FWHM]</th><th>C1s C-Si energy (eV) [FWHM]</th><th>C1s graphite energy (eV) [FWHM]</th></tr></thead><tbody><tr><td>1</td><td>0,06</td><td>99,7<br>[1.3]</td><td>100,5<br>[2.1]</td><td>-</td><td>283,5<br>[1.9]</td><td>282,9<br>[1.1]</td><td>-</td></tr><tr><td>2</td><td>0,74</td><td>99,8<br>[1.2]</td><td>100,4<br>[1.2]</td><td>101,2<br>[1.2]</td><td>283,8<br>[1.9]</td><td>283,2<br>[1.2]</td><td>-</td></tr><tr><td>3</td><td>0,03</td><td>99,6<br>[0.9]</td><td>100,1<br>[1.5]</td><td>-</td><td>283,5<br>[1.8]</td><td>283,0<br>[1.0]</td><td>-</td></tr><tr><td>4</td><td>2,2</td><td>-</td><td>-</td><td>102,2<br>[2.4]</td><td>-</td><td>-</td><td>284,3<br>[1.9]</td></tr><tr><td>5</td><td>0,4</td><td>-</td><td>100,3<br>[1.4]</td><td>-</td><td>283,8<br>[1.8]</td><td>283,2<br>[1.0]</td><td>-</td></tr><tr><td>6</td><td>0,42</td><td>99,7<br>[1.2]</td><td>100,5<br>[1.8]</td><td>-</td><td>283,7<br>[1.8]</td><td>283,1<br>[1.3]</td><td>-</td></tr><tr><td>7</td><td>0,27</td><td>-</td><td>100,3<br>[1.3]</td><td>-</td><td>283,8<br>[1.8]</td><td>283,2<br>[0.9]</td><td>-</td></tr><tr><td>8</td><td>0,60</td><td>99,8<br>[1.2]</td><td>100,2<br>[1.7]</td><td>102,1<br>[1.0]</td><td>283,6<br>[1.8]</td><td>283,0<br>[1.2]</td><td>-</td></tr><tr><td>EC1</td><td>0,47</td><td>99,6<br>[1.1]</td><td>100,3 [1.8]</td><td>-</td><td>283,5<br>[1.9]</td><td>283<br>[1.2]</td><td>-</td></tr><tr><td>EC2</td><td>0,35</td><td>99,6<br>[1.0]</td><td>100,1 [1.8]</td><td>-</td><td>283,2<br>[1.8]</td><td>282,9<br>[1.0]</td><td>-</td></tr><tr><td>EC4</td><td>0,5</td><td>99,9<br>[1.3]</td><td>100,3<br>[1.8]</td><td>-</td><td>283,6<br>[1.9]</td><td>283<br>[1.2]</td><td>-</td></tr><tr><td>9</td><td>2,42</td><td>99,6<br>[1.2]</td><td>100,3<br>[2.1]</td><td>-</td><td>283,4<br>[2.1]</td><td>283,0<br>[1.0]</td><td>-</td></tr></tbody></table>

CH₃ wagging bond peak relative to its Si-C stretching peak and a larger Si-Hₙ stretching peak, which is shifted to slightly lower energies (more Si-H relative to Si-H₃) compared to the C₂H₄ films. With only one sample we cannot suggest a causal relationship between these properties and the film's performance.

Quantitatively, the Si-C stretching and Si-CH₃ wagging peak heights for the a-SiC films without nanocrystalline Si are inversely proportional to the etching rate (Fig. 10). The trend continues for film 4, with the smallest Si-C and Si-CH₃ peaks and the fastest etching rate of the C₂H₄ films. The films with nanocrystalline Si visible in the Raman spectra do not follow the same trend, as films 1 and 3 have lower Si-C and Si-CH₃ peaks compared to top-performing C₂H₄ films despite their lower etch rate, while film 9 again shows peaks comparable to Si-rich films 1 & 3 while having the fastest etching rate of all films studied.

The FTIR spectra of the EC films show very little variation in the Si-Hₙ and C—Hₙ stretching bands as well as the C—Hₙ wagging band (Fig. 11) with two films (EC1 and EC5) exhibiting the extreme cases in terms of intensity of Si-C stretching and Si-CH₃ wagging bands. The variations are not as extreme as those seen in films 1–8.

### 3.4. XPS results

XPS measurements allow for a precise determination of the film compositions as well as an examination of the types of bonds present in the films. The peaks for all films show a similar structure to Fig. 12, with peaks corresponding to Si2p and Si2s, C1s, and O1s bonds.

The peaks were deconvolved using the Thermo-Fisher Avantage software following the same procedure as [19], finding consistent, for most films, peaks in terms of energy and full width at half maximum (FWHM) for Si-Si, Si-C, C—C, and C-Si (Table 3). Film 4 has significantly shifted peaks that do not correspond to typical a-SiC bonds seen in the other films.

Most of the films deposited with C₂H₄ display a mix of C—C, Si-C, and Si-Si bonds [19,31] (Fig. 12). The outlier film 4 is immediately visible with a distinct peak in the XPS measurements, with negligible non-graphitic C—C, Si-C, and Si-Si bonds for both C1s and Si2p peaks while showing instead a distinct peak at 284.3 eV attributed to graphite [32] and Si-O (Si2p peak) bonds, suggesting a lack of typical SiC formation.

<caption>**Fig. 13**</caption>

Film 9, deposited with CH₄, shows spectra similar with the most chemically-resistant C₂H₄ films, albeit with smaller C-Si (C1s peak) bonds and a much higher Si-Si (Si2p peak) bonds corresponding to its lower concentration of C (see Table 4 below).

By comparing the evolution of the peak areas as a function of the accelerated etching rate, we can infer the influence of each bond on the chemical resistance (Fig. 14). As with Fig. 10, there are clusters of similar behavior for the Si-rich films with nanocrystallinity (1 & 3), the remaining C₂H₄ films, the film 4 with significant oxygen presence, and the film 9. For the majority of C₂H₄ films, Si2p Si-C and C1s C-Si bonds generally correlate to etching rate value. The films with nanocrystalline Si show much larger Si2p Si-Si peaks as expected. Film 9 shows large Si2p Si-Si peak but quite low and C1s C-Si peak, further emphasizing its very high Si content. Film 4, by contrast, shows negligible Si-C bonding and instead has graphite and Si-O bonds in high quantities.

The composition for each film was extracted from the peak area corresponding to each bond [19] (Table 4). It is noted that the worst-performing films (4 and 9) are distinct in different ways. Film 4 shows significantly higher O content (approx. 3x higher than the other films), again supporting that this film shows a significant presence of oxides rather than Si-C bonding. On the other, Film 9 shows an Si composition of 73.4 % and a C composition of only 17.2 %, significantly more Si-rich than even the high-performing Si-rich films 1 & 3.

### 4. Discussion

Our results show that the a-SiC films grown with silane and ethylene precursors (films 1–8) all display etch rates lower than the comparative film deposited with silane and methane (film 9), as well as those reported by Cogan et al. [6], and significantly lower than the 3.6 nm/day of the PECVD SiO₂ film. For a 100 nm thin film of a-SiC, the lifetime is expected to be between 44 and 3300 days under accelerated aging conditions, depending on the deposition parameters, or between 5 and 300 years under physiological conditions using formula (1). This demonstrates that high-performing passivating a-SiC coatings can be achieved with the SiH₄/C₂H₄ chemistry.

The films deposited with high temperature (320 °C) and high SiH₄/C₂H₄ ratio (0.8) exhibit the best chemical resistance of those tested.
<figure><img src="image_16.png" /><figcaption>**Fig. 13.** (a) XPS spectra of the C1s band. Most films exhibit Si-C (100.3 eV) and C—C bonds (283.5 eV), while Film 4 shows a large peak corresponding to graphite (284.4 eV). (b) XPS spectra of the Si2p band. Most films exhibit Si-Si (99.6 eV) and Si-C (100.3 eV) bonds. Film 4 shows only a broad peak corresponding to silicon-oxygen bonds (102.3 eV).</figcaption></figure>

<table><caption>**Table 4**
Film compositions determined by XPS.</caption><thead><tr><th>Film</th><th>Accelerated<br>etch rate (nm/<br>day)</th><th>Composition Si<br>% ± 5 % (XPS)</th><th>Composition C<br>% ± 5 % (XPS)</th><th>Composition O<br>% ± 5 % (XPS)</th></tr></thead><tbody><tr><td>EC1</td><td>0,47</td><td>50.2</td><td>41.6</td><td>8.2</td></tr><tr><td>EC2</td><td>0,35</td><td>51.0</td><td>40.6</td><td>8.4</td></tr><tr><td>EC4</td><td>0,5</td><td>52.3</td><td>40.2</td><td>7.5</td></tr><tr><td>1</td><td>0,06</td><td>62.2</td><td>29.1</td><td>8.7</td></tr><tr><td>2</td><td>0,74</td><td>40.6</td><td>50.5</td><td>9.9</td></tr><tr><td>3</td><td>0,03</td><td>61.1</td><td>28.4</td><td>10.5</td></tr><tr><td>4</td><td>2,2</td><td>32.9</td><td>34.1</td><td>33.0</td></tr><tr><td>5</td><td>0,4</td><td>40.4</td><td>52.3</td><td>7.3</td></tr><tr><td>6</td><td>0,42</td><td>47.2</td><td>45.8</td><td>7.0</td></tr><tr><td>7</td><td>0,27</td><td>40.6</td><td>42.5</td><td>6.9</td></tr><tr><td>8</td><td>0,60</td><td>51.6</td><td>39.6</td><td>8.8</td></tr><tr><td>9</td><td>2,42</td><td>73.4</td><td>17.2</td><td>9.4</td></tr></tbody></table>

More particularly, the films deposited with simultaneously high SiH$_4$/ C$_2$H$_4$ ratio and high temperature, namely films 1 and 3, exhibit sharp, narrow Si peaks in their Raman spectra and large Si-C stretching bond peaks in FTIR, suggesting a relatively high degree of chemical organization compared to typical a-SiC films. High deposition temperature has previously been reported to result in high Si-C bonding [10,13], which contributes to the chemical resistance. Other studies have suggested that

high H content, usually incorporated as C—H$_n$ bonds from methane (or ethylene) precursors [33], lead to microvoids that are less resistant to chemical attack [10].

According to our results, in cases where high temperature or silane-rich flow is not possible, for example when trying to achieve films with simultaneously high electrical resistivity using high C content or low deposition temperature [34,35], the negative effects can be mitigated by increasing the RF power. This mitigating technique does not achieve the same degree of chemical resistance as high-temperature, silane-rich depositions, but can still achieve state-of-the-art performance.

The worst-performing C$_2$H$_4$ film (film 4) showed very low Si-C bonding, and instead showed significant graphitic and Si-O bonds (or equivalently a very high O content). This film was deposited at low temperature and low RF power, along with a low SiH$_4$/C$_2$H$_4$ ratio and no H$_2$ flow, all of which are expected to lead to lower Si-C bonding [10,13, 35]. This failure mode seems to be avoided by using higher temperature depositions, higher Si content, or H$_2$ gas flow.

Film 9, deposited using a CH$_4$-based recipe, resulted in a much more Si-rich film than those deposited with C$_2$H$_4$. While it appears structurally similar to the highly chemically-resistant films 1 and 3, we suggest that film 9, being 73 % Si compared to <62 % for films 1 and 3, may have insufficient C content to obtain the high chemical resistance of a-SiC, instead behaving more like a-Si. Further study of this Si-rich region of a-SiC composition is necessary, with both C$_2$H$_4$ and CH$_4$ precursors, in order to verify this result and identify more precisely the transition
<figure><img src="image_17.png" /><figcaption>**Fig. 14.** XPS bond peak intensities as a function of the accelerated etch rate of the films for major bonds. The films are identified based on similarity in their Raman spectra with the same groups used for Fig. 10.</figcaption></figure>

between high-performing and low-performing Si-rich a-SiC films.

Note that the fastest-etching a-SiC films deposited with C₂H₄ also showed the appearance of point defects which accelerate the failure time locally.

The range of deposition parameter values, in particular the temperature and RF power, used for the experiments in this study was bounded by the deposition system to a maximum of 325 °C and 220 W, respectively. However, studies of PECVD a-SiC films [6,7,36–38] have shown high-quality films deposited at higher temperatures (up to 400 °C) and RF powers (up to 300 W), which could further extend the trends reported here. Another approach, using very CH₄-rich precursor flows and low RF power to achieve a plasma in the silane-starving regime, has also been employed for methane deposition chemistries [10,39–41]. This regime has been shown to increase Si-C bond density in the deposited films which is therefore expected to result in high chemical resistance. This regime has not been studied for ethylene-based depositions, but has potentially been observed for very C-rich depositions and requires further investigation [19].

## 5. Conclusion

This study demonstrates that a-SiC thin films deposited by PECVD using silane and ethylene gas precursors can exhibit accelerated chemical etch rate as low as 0.03 nm/day. Our most chemically-resistant films are deposited at high temperature (320 °C) and high SiH₄/C₂H₄ ratio (0.8). Acceptable results can also be obtained by using a high RF power (220 W) for films deposited at low deposition temperature (120 °C). Regarding the film structure, the most resistant films exhibit high Si-C bonding and a possible presence of nanocrystalline Si regions, while films displaying graphitic bonding, high O content, and very low Si-C bonding etch more quickly. A film with extremely high Si concentration (73.4 %) and deposited with different C-precursor gas was also found to etch quickly.

These results clearly show that a-SiC films deposited with silane-ethylene gas precursors are competitive, in terms of chemical resistance, with other a-SiC films which are more commonly deposited with silane-methane. Furthermore, the high variances observed between the different films demonstrate that the deposition recipe used for a-SiC passivating films is an important factor that must be considered when selecting a-SiC films for a given application. A more exhaustive study including multiple depositions with both CH₄ and C₂H₄ precursors as well as more samples in the Si-rich range is needed to determine more

precisely the influence of chemical structure on the etching rate.

## CRediT authorship contribution statement

**Scott Greenhorn:** Writing – review & editing, Writing – original draft, Visualization, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Valérie Stambouli:** Writing – review & editing, Supervision, Methodology, Funding acquisition, Conceptualization. **Edwige Bano:** Writing – review & editing, Supervision, Project administration, Funding acquisition, Conceptualization. **Andrei Uvarov:** Writing – review & editing, Resources. **Sonia Ortega-Murillo:** Data curation, Investigation, Writing – review & editing. **Konstantinos Zekentes:** Writing – review & editing, Supervision, Project administration, Funding acquisition, Conceptualization.

## 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.

## Acknowledgments

This study has received funding from the French National Research Agency under the project SiCNeural (ANR Grant number ID ANR-21-CE19-0054-01) and by the French National Research Agency in the framework of the "Investissement d'avenir" program (ANR-15-IDEX-02). The authors thank Dr. Michel Mermoux (LEPMI, Grenoble, France) for beneficial discussion on Raman spectra, Dr. Carmen Jimenez (LMGP, Grenoble, France) for fruitful help on FTIR-ATR spectra, and Matthieu Weber (LMGP, Grenoble, France) for help with the XPS data acquisition as well as Grégory Berthomé (SIMaP, Grenoble, France) for help with XPS interpretation.

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