# Etching parameter optimization of photonic integrated waveguides based on chalcogenide glasses for near- and mid-infrared applications

Abdelali Hammouti, Sofiane Meziani, Loïc Bodiou, Jonathan Lemaitre, Parastesh Pirasteh, Albane Benardais, Nathalie Lorrain, Radwan Chahal, Virginie Nazabal, Joel Harrier

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Abdelali Hammouti, Sofiane Meziani, Loïc Bodiou, Jonathan Lemaitre, Parastesh Pirasteh, et al. Etching parameter optimization of photonic integrated waveguides based on chalcogenide glasses for near- and mid-infrared applications. Optical Materials Express, 2025, 15 (4), pp.775-787. 10.1364/OME.553742. hal-05080133

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Submitted on 22 May 2025

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# Etching parameter optimization of photonic integrated waveguides based on chalcogenide glasses for near- and mid-infrared applications

ABDELALI HAMMOUTI,<fnref n="1" /> ID SOFIANE MEZIANI,<fnref n="1" /> LOÏC BODIOU,<fnref n="1" /> ID

JONATHAN LEMAITRE,<fnref n="1" /> PARASTESH PIRASTEH,<fnref n="1" /> ALBANE

BÉNARDAIS,<fnref n="2" /> NATHALIE LORRAIN,<fnref n="1" /> RADWAN CHAHAL,<fnref n="2" /> VIRGINIE

NAZABAL,<fnref n="2" /> AND JOËL CHARRIER<fnref n="1" />,*

**Abstract:** This study presents the etching parameters optimization of GeSbSe-based chalcogenide glass (ChG) for near and mid-infrared integrated photonic applications. Single-mode $Se_4$ ($Ge_{19.4}Sb_{16.7}Se_{63.9}$) on $Se_2$ ($Ge_{28.1}Sb_{6.3}Se_{65.6}$) waveguides were designed and fabricated. The $Se_2$ and $Se_4$ layers were first deposited via the RF magnetron sputtering technique. The waveguide structures were then patterned using combined reactive ion -inductively coupled plasma (RIE-ICP) etching and optimized by introducing argon gas to the fluorine ($CHF_3$)-based chemistry etching. Parametric investigation of etching conditions, particularly the $Ar/(CHF_3 + Ar)$ ratio and total gas flow rates, led to significant improvements in waveguide sidewall morphology and roughness. Consequently, propagation losses were reduced from 7.5 dB/cm to 2.6 dB/cm, at near-infrared wavelengths ($\lambda=1,55~\mu m$). In the mid-infrared region, the optimized process achieved a low propagation loss of ($1.45 \pm 0.81$) dB/cm at 4.11 $\mu m$, with an average loss of approximately 4 dB/cm across the 4.1–4.55 $\mu m$ wavelength range. This marks a substantial improvement over the initial process, which exhibited an average loss of 15 dB/cm. Advanced characterization techniques, including SEM-based roughness extraction and optical scattering loss modeling, were employed to correlate surface morphology with etching parameters. The Payne-Lacey model was used to predict propagation losses, showing good agreement with experimental results. This comprehensive approach provides valuable insights into the relationship between etching conditions and waveguide performance, contributing significantly to the development of low-loss chalcogenide-based photonic devices for near- and mid-infrared applications.

Published by Optica Publishing Group under the terms of the [Creative Commons Attribution 4.0 License](https://creativecommons.org/licenses/by/4.0/). Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

## 1. Introduction

Integrated photonics is a highly promising technology for various applications, as it enables miniaturization and thus a reduction of manufacturing costs of optical devices. Chalcogenide materials (ChG) have many advantages to implement integrated optical devices in the near- and mid-infrared wavelength ranges [1–3]. In particular, their high optical nonlinearities [4] can be leveraged to perform all-optical signal processing in the 1.55 µm optical communication window. Furthermore, these materials display a wide transparency band in the mid-infrared (MIR). Integrated spectroscopic sensors can thus benefit from the development of these materials to detect molecules with absorption bands in the mid-IR [5]. For these applications, the development of ChG-based optical platforms with low propagation losses would pave the way to the implementation of high-performance integrated devices. These propagation losses come from the intrinsic materials absorption, the presence of contaminants/impurities and scattering from

<footnote n="1">¹Institut FOTON, UMR 6082 CNRS, Université de Rennes, 22300 Lannion, France</footnote>

<footnote n="2">²ISCR, UMR 6226 CNRS, Université de Rennes 1, 35042 Rennes, France</footnote>

<footnote>*joel.charrier@univ-rennes.fr</footnote>
sidewalls roughness generated during the etching processing of the photonic integrated structures. Reactive Ion Etching (RIE) with Inductively Coupled Plasma (ICP) has been widely employed for fabricating chalcogenide waveguides for various applications across different wavelengths. Numerous studies have explored different chalcogenide glass systems including GeSbSe, GeSbS, GeSbTe, GeAsSe, GaGeSbSe, GaGeSbS, and AsS [6–18], each requiring specific optimization of etching conditions. Fluorinated gases, particularly CF₄, CHF₃, and SF₆, are commonly used due to their ability to form fluorine radicals that react with layer elements to produce volatile compounds [17]. The use of chlorine-based gases (Cl₂, BCl₃) for etching chalcogenide materials has shown etching quality equivalent to conventional fluorinated gases in terms of propagation losses. While both types of gases achieve similar performances in the near-infrared domain. In the mid-infrared range, using BCl₃ instead of CHF₃ led to enhanced quality factor around λ = 3.4 µm and eliminated an absorption peak centered at this wavelength. [15,19]. Xiong et al. [10] observed that while CF₄ demonstrated high chemical reactivity with various chalcogenide glass systems, CHF₃ proved to be a superior alternative for deep etching of GeSeSb films, offering lower roughness and improved control over the etching process. Furthermore, gas mixtures combining CHF₃ and CF₄ have shown effectiveness in achieving notable etching quality, minimizing optical propagation losses and contributing to enhanced performance of optical devices [7–9,13]. Mixing these fluorinated gases with O₂ or Ar has proven particularly effective in controlling roughness and etch profile [15,20–22]. The addition of argon, specifically, allows modulation of ion bombardment and influences etching kinetics. Varying the percentage of argon in CF₄/Ar or SF₆/Ar mixtures significantly affects the roughness and profile of etched patterns in layers, potentially leading to lower optical propagation losses and thus more efficient devices. This study aims to improve the etching process of integrated waveguides based on ChG through the optimization of dry etching parameters (RIE-ICP), with a focus on reducing propagation losses and achieving a selectivity ratio above 4 for thick layers dedicated to mid-infrared applications. The optimization of the CHF₃/Ar mixture could enhance several critical aspects of GeSbSe waveguide etching, such as sidewall roughness and residue removal, while maintaining an etching rate around 300 nm/min for adequate process control. A comprehensive investigation is presented, correlating dry etching conditions with waveguide sidewall roughness and optical propagation losses. This approach provides insights into the relationship between fabrication parameters and device performances, contributing to the development of high-quality chalcogenide-based photonic devices for near and mid-infrared (NIR and mid-IR) applications.

## 2. Design and fabrication

### 2.1. Design

Simulations to design optical waveguides consisting of a Se₂ (Ge₂₈.₁Sb₆.₃Se₆₅.₆) cladding layer and Se₄ (Ge₁₉.₄Sb₁₆.₇Se₆₃.₉) guiding layer were performed, at the operating wavelength of 1.55 µm. FIMMWAVE software was used to calculate the propagating modes based on geometric and optical parameters of the guiding structures, in particular the refractive indices, measured by ellipsometry and respectively equal to 2.54 and 2.74 at 1.55 µm, for Se₂ and Se₄ chalcogenide layers. Effective index maps for both transverse electric (TE) and transverse magnetic (TM) polarizations were generated by varying waveguide height (H) from 0.05 µm to 3 µm and width (W) from 0.05 µm to 3 µm, with 50 nm increments. A 5 µm-thick Se₂ confinement layer was found sufficient to avoid radiation losses through the Si substrate. Thickness of the Se₄ guiding layer of 1.2 µm allows obtaining single-mode propagation at a wavelength of 1.55 µm for a waveguide width of 1.5 µm for both TE and TM polarizations. These dimensions yield effective refractive indices (nₐeff) of 2.64 both TE and TM modes. For mid-IR applications around wavelength of 4 µm, Se₆ (Ge₁₂.₅Sb₂₅.₀Se₆₂.₅) was used as the guiding layer on a Se₂ cladding layer. Refractive indices at this wavelength were determined to be 2.49 for Se₂ and 2.77 for Se₆. Using the same simulation approach, optimal dimensions for single-mode propagation at 4 µm were found to
be $H = 1.2 \ \mu\text{m}$ for the $Se_6$ guiding layer height and $W = 5 \ \mu\text{m}$ for the waveguide width. The $5 \ \mu\text{m}$-thick $Se_2$ cladding layer remained sufficient for preventing substrate radiation losses at this longer wavelength.

## 2.2. Fabrication

Chalcogenide bulk glasses are synthesized using the melt and quenching technique to ensure the high purity of the material [23]. Thin films of $Se_2$ and then $Se_4$ are then deposited by RF magnetron sputtering on 2-inch silicon substrates as displayed in Fig. 1. Approximately 600 nm of photoresist S1805 was spin coated on ChG films, then a classical i-line photolithography process Suss Microtech MJB4 aligner using a with a 365 nm UV lamp was performed to transfer the waveguide patterns. The exposure is carried out through a quartz mask with chromium patterns, whose design is based on numerical simulations. After development using an appropriate developer, the $Se_4$ (or $Se_6$) guiding layer is then etched by reactive ion etching with inductively coupled plasma (RIE-ICP) using a Corial 200IL equipment. Trifluoromethane ($CHF_3$) and Argon (Ar) gases can be used for the etching process.

<figure><img src="image_2.png" /><figcaption>Fig. 1. Chalcogenide waveguides fabrication process steps.</figcaption></figure>

The etching process allows control of the following dry etching parameters: the power of the plasma, the chamber pressure, and the flow rate of the etching gases. All along the manufacturing process, etching rates and selectivity were regularly checked using a mechanical profilometer. Roughness of waveguides sidewalls were extracted from top view scanning electron microscopy (SEM) images.

## 3. Results and discussion

Starting from the etching parameters displayed in Table 1, initial investigations were focused on etching gases and their flow rates as these parameters were identified as the main variables for process optimization. Further assessment of plasma power effects confirmed that the initial RF and ICP values were already optimal.

<table><caption>Table 1. Initial RIE-ICP etching parameters for GeSbSe waveguides.</caption><thead><tr><td>Pressure (mTorr)</td><td>CHF<sub>3</sub> (sccm)</td><td>Ar (sccm)</td><td>RF Power (W)</td><td>ICP Power (W)</td></tr></thead><tbody><tr><td>5</td><td>5</td><td>0</td><td>25</td><td>75</td></tr></tbody></table>

The optimization of the etching process was initially conducted in the NIR range with $Se_4$ waveguides, where sidewall roughness has a stronger impact on propagation losses and characterization is technically more convenient. Then the results have been transposed to the $Se_6$ waveguides for mid-IR applications.
## 3.1. Etching rate and selectivity

The effect of argon introduction in a CHF₃-based gas mixture on the etching characteristics of GeSbSe layers was investigated. The total gas flow was varied from 5 sccm to 30 sccm. The Ar/(Ar + CHF₃) ratio was progressively increased for each total flow rate. All experiments were conducted at 5 mTorr chamber pressure, 25 W RF power, and 75 W ICP power. Etching rate and selectivity were measured after each experiment using a DekTak 150 mechanical profilometer. Selectivity was calculated as the ratio of material etching rate to photoresist etching rate.

As shown in Fig. 2(a), the Se₄ etching rate decreases quasi-linearly with increasing argon ratio for all studied flow rates. This trend is consistent with previously reported observations for chalcogenide materials etched with CF₄/Ar mixtures [20]. The decrease in etching rate can be attributed to the reduction of CHF₃ concentration in the gas mixture, leading to decreased formation of reactive fluorine radicals. At constant Ar/(Ar + CHF₃) ratio, the etching rate increases with total gas flow, which can be explained by the increased quantity of etchant gas in the chamber, intensifying physico-chemical reactions at the chalcogenide surface. Figure 2(b) presents the selectivity evolution as a function of argon ratio for different total flows. Selectivity generally decreases with increasing argon ratio, following a trend similar to that of the etching rate. This correlation indicates that the mechanisms responsible for the Se₄ etching rate reduction also affects the photoresist mask etching in different proportions. At constant argon ratio, an increase in selectivity is observed with rising total flow rate, likely attributed to the enhanced density of reactive species in the plasma. The simultaneous increase in selectivity and etching rate with total gas flow offers an opportunity for process optimization, allowing a favorable compromise while maintaining the same photoresist thickness and thus warranting constant mask resolution during the i-line photolithographic step, even for thicker films required for mid-IR applications. Similar trends for etching rate and selectivity were observed for Se₆ attributed to its close compositions. The etching mechanism can be explained by two competing phenomena, analogous to those observed in SiO₂ etching with CF₄/Ar mixtures [24]. Initially, argon incorporation may promote the formation of reactive CFₓ radicals, intensifying ion-assisted chemical etching of Se₄. However, increasing argon concentration progressively reduces the collision rate of CFₓ radicals with the surface relative to that of Ar⁺ ions. As highlighted by Schmitt et al. [24] for SiO₂, this reduction in chemical reaction rate at the surface leads to a decrease in overall etching rate.

<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Influence of argon ratio on (a) etching rate and (b) selectivity for different total gas flows in RIE-ICP etching of Se₄ layer.</figcaption></figure>
## 3.2. SEM image and characterization

The evolution of etching profiles as a function of argon ratio for different total flow rates was examined using SEM imaging. Figure 3 presents slightly tilted cross-sectional SEM images showing the influence of argon ratio and total flow rate on the $Se_4$ on $Se_2$ etching profiles.

<figure><img src="image_4.png" /><figcaption>Fig. 3. Cross-sectional SEM images showing the influence of Argon ratio and total flow rate on the etching profiles of $Se_4$ on $Se_2$.</figcaption></figure>

At a total flow rate of 5 sccm, a gradual change in waveguide sidewall morphology was observed with increasing argon ratio. Smoothing of the sidewalls was noted up to an argon ratio of 40%, as quantitatively confirmed by roughness measurements presented in Table 2. Simultaneously, the concentration of non-volatile fluorinated residues redeposited on the etching bottoms decreased progressively. This phenomenon is consistent with previous observations demonstrating the efficiency of argon in reducing roughness and removing etching by-products [21]. However, increasing the argon ratio beyond 60% led to significant degradation of etching quality, with a marked increase in sidewall roughness. This transition marks a critical point where argon ion bombardment starts causing damage to the waveguide sidewalls, degrading the quality of the etched structures. The trend observed for the total flow rate of 5 sccm was also confirmed for higher flow rates of 20 sccm and 30 sccm with increasing argon ratio. However, for these higher flow rates, the optimum shifts to ~60% Ar, where an equilibrium between physical sputtering and chemical etching is reached.

Figure 4 illustrates the roughness parameter extraction process from SEM images using a custom-developed Python code. A top-view SEM image of the waveguide is shown in Fig. 4(a), with the extracted edges plotted in Fig. 4(b). The SEM image was captured with a pixel size of 3.30 nm, providing high-resolution details of the waveguide edges. For this representative etching process, the Root Mean Square (RMS) for both upside and downside edges were determined. Autocorrelation functions for both edges are presented in Fig. 4(c) and Fig. 4(d). These functions were utilized to determine the correlation length ($L_c$), which characterizes the distance over which surface irregularities remain correlated under the given etching conditions. The autocorrelation data was fitted with a model (red dashed line) to extract specific $L_c$ values specific to each edge.

Figure 5(a) illustrates the roughness evolution as a function of argon ratio for different total flow rates. The graph confirms the trend observed in SEM images of Fig. 3, with an initial
<table><caption>Table 2. Summary of etching parameters (total flow rate, CHF₃ and Ar flow rates), extracted RMS roughness (σ) and Lc, estimated propagation losses (α<sub>estimated</sub>) using Payne-Lacey model, and measured propagation losses (α<sub>measured</sub>) using cut-back method at 1.55 µm for Se₄ waveguide</caption><thead><tr><th>Flow rate (sccm)</th><th>CHF<sub>3</sub>(sccm)</th><th>Ar (sccm)</th><th>L<sub>c</sub> (nm)</th><th>σ (nm)</th><th>α<sub>estimated</sub> (dB/cm)</th><th>α<sub>measured</sub> (dB/cm)</th></tr></thead><tbody><tr><td>5</td><td>5</td><td>0</td><td>57,4 ± 16,2</td><td>7,1 ± 1,3</td><td>5,7 ± 1,5</td><td>7,6 ± 0,6</td></tr><tr><td>5</td><td>4</td><td>1</td><td>75,9 ± 16,2</td><td>4,9 ± 0,7</td><td>3,1 ± 1,0</td><td>4,2 ± 0,5</td></tr><tr><td>5</td><td>3</td><td>2</td><td>28,8 ± 8,8</td><td>6,2 ± 0,6</td><td>2,7 ± 1,2</td><td>3,0 ± 0,2</td></tr><tr><td>5</td><td>2</td><td>3</td><td>79,7 ± 10,0</td><td>11,1 ± 1,3</td><td>15,9 ± 3,6</td><td>15,6 ± 0,4</td></tr><tr><td>20</td><td>16</td><td>4</td><td>51,1 ± 8,9</td><td>6,9 ± 0,4</td><td>5,1 ± 1,0</td><td>5,9 ± 1,0</td></tr><tr><td>20</td><td>12</td><td>8</td><td>49,9 ± 8,4</td><td>6,1 ± 0,3</td><td>3,9 ± 0,8</td><td>2,6 ± 0,8</td></tr><tr><td>20</td><td>8</td><td>12</td><td>60,1 ± 21,3</td><td>4,9 ± 0,6</td><td>2,6 ± 0,2</td><td>2,6 ± 0,6</td></tr><tr><td>20</td><td>4</td><td>16</td><td>67,6 ± 45,6</td><td>5,8 ± 0,5</td><td>4,0 ± 0,6</td><td>6,9 ± 2,4</td></tr><tr><td>30</td><td>18</td><td>12</td><td>42,9 ± 10,9</td><td>7,3 ± 1,0</td><td>5,1 ± 2,3</td><td>6,7 ± 0,3</td></tr><tr><td>30</td><td>12</td><td>18</td><td>39,9 ± 3,8</td><td>5,8 ± 1,2</td><td>3,0 ± 1,1</td><td>3,6 ± 0,7</td></tr><tr><td>30</td><td>6</td><td>24</td><td>25,5 ± 21,6</td><td>7,1 ± 0,9</td><td>3,0 ± 2,0</td><td>4,5 ± 1,0</td></tr></tbody></table>

<figure><img src="image_5.png" /><figcaption>Fig. 4. Process of extracting roughness parameters from SEM images: (a) Top-view SEM image, (b) Edge extraction, (c) Upside autocorrelation, (d) Downside autocorrelation.</figcaption></figure>
decrease in roughness followed by an increase beyond a certain threshold. Error bars represent the variability in measurement repeatability at different points of waveguides, separated by 100 µm, from the same etching process. The Payne-Lacey model [25,26] was then employed using these extracted RMS data and correlation length $L_c$ to estimate optical propagation losses due to sidewalls surface scattering. The fundamental equation of this model is expressed as:

$$ \alpha(dB/cm) = \frac{10}{Ln(10)} \frac{\sigma^2}{k_0 H n_c \sqrt{2}} g(k_0, H, n_c, n_{sp}, n_{eff})f(x, \gamma) \quad (1) $$

where:

$$ g(k_0, n_c, n_{sp}, n_{eff}) = \frac{\left(k_0 H \sqrt{n_c^2 - n_{eff}^2}\right)^2 \left(k_0 H \sqrt{n_c^2 - n_{sp}^2}\right)^2}{1 + k_0 H \sqrt{n_{eff}^2 - n_{sp}^2}} \quad (2) $$

<figure><img src="image_6.png" /><figcaption>Fig. 5. (a) Roughness extractions from SEM images (top view of the waveguide), (b) Modeling of scattering optical losses as a function of Ar/(CHF₃ + Ar) ratio for different total gas flow rates.</figcaption></figure>

And:

$$ f(x, \gamma) = \frac{x\sqrt{1 - x^2 + \sqrt{(1 + x^2)^2 + 2x^2 \gamma^2}}}{\sqrt{(1 + x^2)^2 + 2x^2 \gamma^2}} \quad (3) $$

where:

$$ x = k_0 L_c \sqrt{n_{eff}^2 - n_{sp}^2} \quad (4) $$

And:

$$ \gamma = \frac{n_{sp} \sqrt{n_c^2 - n_{sp}^2}}{n_c \sqrt{n_{eff}^2 - n_{sp}^2} \sqrt{\frac{n_c^2 - n_{sp}^2}{2n_c^2}}} \quad (5) $$

where $\sigma$ represents the RMS roughness, $k_0$ the wave vector at the study wavelength ($\lambda$ = 1550 nm), $L_c$ the correlation length, $H$ the guiding layer thickness and $n_c, n_{sp}, n_{eff}$ are the indices of the guiding layer, superstrate, and effective index of the guided mode, respectively.

Figure 5(b) shows calculated optical scattering losses from waveguide sides as a function of argon ratio for different total flow rates. The curves exhibit a similar trend for the three studied
flow rates, with minimum scattering losses observed at 60% argon and 40% CHF₃ for flow rate of 20 sccm. This evolution is consistent with the roughness measurements presented in Fig. 5(a) and SEM observations in Fig. 3. In Fig. 5(a), each point represents the average roughness measured from three SEM images taken on different waveguides of the same sample, whereas in Fig. 5(b), the loss values are calculated based on the averaged roughness measurements for each data point. The error bars indicate the variability in measurement reproducibility in Fig. 5(a) and the propagation of this variability in the loss calculations in Fig. 5(b). This approach establishes a direct relationship between fabrication parameters and expected scattering losses. The Payne-Lacey model calculations consider both roughness (σ) and correlation length (Lc). As shown in Table 2, Lc exhibits no clear dependence on etching parameters. The modeling of scattering losses at λ = 1.55 µm shows that small variations in roughness lead to significant loss amplification when Lc approaches 100 nm. The roughness has a dominant impact on loss magnitude. The calculations are valid for both polarizations due to the similar effective indices of the fundamental TE and TM modes.

3.3. *Optical characterization*

3.3.1. Near infrared measurements

To experimentally measure the etching-dependent propagation losses, the set-up displayed in
in Fig. 6(a) was used. It consists in a 1.55 µm source (IRE-POLUS EAD-60) coupled to a
single-mode fiber and whose power stability is monitored via a coupler connected to an optical
power meter (Thorlabs PM100D). A microlensed fiber was employed to optimize mode matching
between the fiber and the waveguide modes therefore reducing coupling losses at the waveguide
entrance facet. After propagation through the waveguide, the output power was collected by
an objective lens and focused onto an electrically cooled InGaAs high sensitivity detector
(DSS-IGA020 T) with a 4 mm² active surface. A mechanical chopper and a lock-in amplifier
were utilized to enhance measurement precision. A high-resolution NIR camera (SU320M-1.7RT
InGaAs) was also used to confirm proper propagation in the waveguide.

<figure><img src="image_7.png" /><figcaption>**Fig. 6.** (a) Optical waveguide propagation loss characterization bench, (b) Example of cut-back method measurements for Se₄ waveguides (H = 1.2 µm, W = 1.5 µm) showing reproducibility test for the 20 sccm (40%Ar) recipe with fixed parameters: Pressure = 5 mTorr, P<sub>RF</sub> = 25 W, P<sub>ICP</sub> = 75 W.</figcaption></figure>

Propagation losses measurements were conducted for all explored etching conditions using the
cut-back method. Figure 6(b) shows a representative example of these measurements, performed
on a Se₄ waveguide on Se₂ with dimensions H = 1.2 µm and W = 1.5 µm. The etching conditions
for this sample were: 5 mTorr pressure, 25 W RF power, 75 W ICP power, with an Ar flow rate of
8 sccm and a CHF₃ flow rate of 12 sccm. Three independent measurement series were performed to ensure reproducibility and quantify both process and measurement variability. Optical loss values (α) were fitted using Beer-Lambert law. The mean value of these three measurements, indicated in green at the bottom of the graph, is 2.8 ± 0.7 dB/cm, accounting for both fabrication and optical measurement variabilities.

Figure 7 presents a synthesis of propagation losses for the range of explored etching processes, comparing results obtained via the cut-back method with Payne-Lacey modeling approach. An optimum is observed at 60% Ar in a total flow rate of 20 sccm (Ar + CHF₃), yielding minimal propagation losses of 2.62 ± 0.59 dB/cm. Measurements performed on the photoresist pattern before etching revealed a sidewall roughness of σ = 10.0 ± 0.6 nm, which would lead to scattering losses of 13.0 ± 1.5 dB/cm if this roughness was completely transferred from the resist to the waveguide. Several optimized etching conditions achieved lower roughness values, demonstrating that the etching process parameters strongly impact the final waveguide quality. Additional tests with different P<sub>ICP</sub>/P<sub>RF</sub> ratios, ranging from 2 to 6 (data not shown), did not show any improvement in measured propagation losses. This optimal condition corresponds to an etching rate of 246 ± 46 nm/min and a selectivity of 4.3 ± 0.3. Table 2 summarizes all experimental conditions investigated and their corresponding roughness and optical loss measurements.

<figure><img src="image_8.png" /><figcaption>**Fig. 7.** Comparison of propagation losses measured at λ=1.55 µm by cut-back method and modeled (Payne-Lacey) as a function of Ar/(CHF₃ + Ar) ratio for Se₄ waveguides (H = 1.2 µm, W = 1.5 µm) on Se2 cladding for total flow rates: (a) 5 sccm, (b) 20 sccm, (c) 30 sccm (fixed parameters: Pressure = 5 mTorr, P<sub>RF</sub> = 25 W, P<sub>ICP</sub> = 75 W)</figcaption></figure>

Regarding propagation losses, various contributions must be considered, notably volume and surface scattering losses. Given the transparency of chalcogenide glasses in the NIR, surface scattering constitutes the primary source of propagation losses [27,28]. This factor, strongly influenced by etching conditions, directly impacts the optical performance of the waveguides. Modelling results inherently contain errors due to employed approximations, but the tendencies are still conserved. Specifically, the model assumes a uniform roughness pattern across the entire lateral surface of the waveguide, whereas in reality, it is a complex and irregular rough surface. The representations in Fig. 7 confirms the consistency between experimental measurements and theoretical results, underscoring the influence of argon in the etching of Se₄ chalcogenide glass. Combined with Fig. 5, it also establishes a correlation between waveguide sidewall roughness and propagation losses in the integrated photonics platform. Recent works on thermal reflow have shown promising results for further reducing propagation losses and enhancing non-linear optical properties. Grayson et al. [29] have demonstrated that thermal annealing of GeSbSe waveguides can lead to a significant reduction in propagation losses, from 2.5 dB/cm to 1 dB/cm. This improvement is attributed to the reduction of surface scattering losses through thermal reflow, which smoothens the guiding structure's surface. Moreover, their research revealed a four-fold increase in optical non-linearity after optimal annealing, an effect attributed to structural reorganization of the material, reducing deposition-induced crystallinity and promoting an
amorphous structure. As highlighted by Dory et al. [30], post-deposition annealing significantly affects the optical properties of amorphous Ge-based chalcogenide thin films due to structural relaxation. This approach could potentially be applied to our chalcogenide waveguides to further optimize their performance in integrated photonics applications.

### 3.3.2. Mid-infrared measurements

For mid-IR characterization, $Se_6$ was selected as guiding layer due to its higher refractive index contrast with $Se_2$ confinement layer, allowing thinner layers for single-mode operation and thus requiring shorter etching time and lower selectivity. Due to their similar composition, the etching rates are nearly identical for both $Se_4$ and $Se_6$ glasses. This was confirmed by roughness measurements on $Se_6$ waveguides processed with the optimal recipe (20 sccm, 60%Ar), showing an extracted roughness of 4.95 ± 0.8 nm Fig. 8. and associated optical losses of 3 ± 0.8 dB/cm at $\lambda = 1.55$ µm. The optimized process was then evaluated in the mid-infrared for only the initial process and the optimal one determined from the near-infrared study.

<figure><img src="image_9.png" /><figcaption>**Fig. 8.** (a) Top-view SEM image of $Se_6$ waveguide etched with optimized recipe (20 sccm, 60%Ar), and (b) corresponding sidewall roughness extraction showing roughness of 4.95 ± 0.8 nm.</figcaption></figure>

Mid-infrared (MIR) optical characterizations around 4.5 µm were performed using a similar optical bench as the one displayed in Fig. 6(a). Light coupling from a tunable quantum cascade laser (QCL) emitting from $\lambda = 3.9$ to 4.6 µm (MIRCAT, Daylight Solutions) to the integrated waveguide was achieved through a mid-IR transparent chalcogenide fiber. The output signal was collimated to a mid-IR detector (DSS-PSE020TC, Horiba) through a ZnSe microscope objective. More details on the experimental setup and procedures can be found in Ref. [31].

Propagation losses measured between 4 and 4.6 µm wavelength are presented in Fig. 9 for two etching processes performed on the same wafer. A significant reduction in propagation losses is observed when using the recipe optimized for near-infrared propagation losses (20 sccm with 60% of Ar + 40% of $CHF_3$) compared to the initial process (5 sccm with 100% of $CHF_3$) in Table 1. The optimized process significantly reduces the propagation losses from an average of 15 dB/cm with the initial process to approximately 4 dB/cm across the studied spectral range (4.1-4.55 µm), reaching a substantially low minimum of (1.45 ± 0.81) dB/cm achieved at 4.11 µm. This lower loss value compared to near-infrared is reasonable as surface roughness has less impact at longer wavelengths. The relatively large measurement uncertainties are inherent to the increased complexity of mid-infrared characterization setups and alignment procedures compared to near-infrared measurements. This improvement can be attributed to the addition of Argon in the gas mixture. Previous studies by Meyer et al. [21]. using XPS technique have shown
that Argon can reduce fluorocarbon formation during etching processes. Additionally, Ma et al. [32] demonstrated that fluoropolymers produced from fluorine plasma can introduce absorption between 3 and 5 µm. This suggests that our optimization using Argon significantly reduced redeposition products. However, the remaining propagation losses could be attributed to either residual redeposition or intrinsic material absorption (volume or surface scattering unintentionally introduced during deposition). Bulk chalcogenide materials have shown negligible absorption (close to 0) for wavelengths above 1 µm [33], but such characterization has not been performed on thin films. Despite these residual losses, the achieved results with the optimized process are promising for sensing applications in the mid-infrared molecular fingerprint region.

<figure><img src="image_10.png" /><figcaption>**Fig. 9.** Propagation losses in dB/cm as a function of wavelength for ChG monomode waveguides with Se~6~ on Se~2~ compositions. Two sets of etching parameters were applied on samples from the same wafer: initial parameters (5 sccm with 100% of CHF~3~) and optimized parameters (20 sccm with 60% of Ar + 40% of CHF~3~).</figcaption></figure>

**4. Conclusion**

In this study, we present a successful optimization of Se~4~ chalcogenide glass waveguides for near and mid infrared integrated photonic applications. A single-mode waveguide design on the Se~4~ on Se~2~ platform was initially developed. The etching conditions were investigated, with a focus on the influence of gas flow rates. A significant improvement in etching performance was realized through the introduction of argon and CHF~3~ gases mixture. This optimization led to an important reduction in propagation losses from 7.5 dB/cm to 2.6 dB/cm, representing a 65.3% decrease, while maintaining well-defined etching profiles. This improvement was achieved while preserving a sufficiently slow etching rate for precise manual monitoring and acceptable selectivity. The optimized process parameters, specifically at 60% Ar in a total flow rate of 20 sccm (Ar + CHF~3~), resulted in minimal losses of 2.62 ± 0.59 dB/cm, corresponding to an etching rate of 246 ± 46 nm/min and a selectivity of 4.3 ± 0.3. In the MIR range, propagation losses were reduced from an average of 15 dB/cm with the initial process to approximately 4 dB/cm across the studied spectral range (4.1-4.55 µm), reaching a substantially low minimum of (1.45 ± 0.81) dB/cm at 4.11 µm. This reduction in losses is attributed to decreased fluorocarbon formation during the etching process, which is known to absorb in this spectral region. The integration of theoretical modeling with experimental validation provides a robust framework for further
optimization of etching processes in chalcogenide-based photonic devices. Future work may
explore the influence of additional parameters, such as pressure or temperature, to further enhance
the performance of these integrated photonics platforms for both NIR and mid-IR applications.

**Funding.** Agence Nationale de la Recherche (ANR-21-CE04-0011-02); HORIZON EUROPE Framework Programme (N° 101092723).

**Acknowledgments.** This work was supported by a CDU (Contrat Doctoral Unique – University of Rennes) and benefited from funding through the ANR AQUAE project (contract N°ANR-21-CE04-0011-02) and the European project IBAIA (grant agreement N°101092723). This work was also supported by the French Renatech + Network (national facilities for micro-nanotechnology), and the authors acknowledge the CCLO platform for technological support (SEM imaging + sample processing).

**Disclosures.** The authors declare no conflicts of interest.

**Data availability.** Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.

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