# High surface quality Y2SiO5 silicate-crystal waveguides etched by chlorine-based inductive coupled plasma reactive ion etching

Anne Talneau, M. Chan, F. Laourine, F. Maillard, S. Welinski, P. Berger, A.

Ferrier

► To cite this version:

Anne Talneau, M. Chan, F. Laourine, F. Maillard, S. Welinski, et al. High surface quality Y2SiO5 silicate-crystal waveguides etched by chlorine-based inductive coupled plasma reactive ion etching. Journal of Vacuum Science & Technology A, 2025, Journal of Vacuum Science and Technology A, 43 (4), 10.1116/6.0004314. hal-05088359

HAL Id: hal-05088359

https://hal.science/hal-05088359v1

Submitted on 28 May 2025

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

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

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

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

# High surface quality Y₂SiO₅ silicate-crystal waveguides etched by Chlorine-based Inductive Coupled Plasma Reactive Ion Etching

A.Talneau¹, M. Chan², F. Laourine¹, F. Maillard¹, S. Welinski², P. Berger² and A. Ferrier³,⁴

¹ CNRS C2N, 10 Bd Thomas Gobert F-91120 Palaiseau

² Thales Group, Research & Technology, 1 Av. Augustin Fresnel, F-91120 Palaiseau

³ Chimie ParisTech, PSL University, CNRS, Institut de Recherche de Chimie Paris, F-75005 Paris, France

⁴ Sorbonne Université, Faculté des Sciences et Ingénierie, UFR 933, F-75005 Paris, France

¹) Electronic mail: anne.talneau@c2n.upsaclay.fr

## Abstract

Among Rare-earth ion doped crystals which are of high interest for quantum technologies, Yttrium Orthosilicate crystal Y₂SiO₅ has demonstrated the longest coherence times. Etching optical guiding structures enables the creation of complex structures offering the advantage of technological scalability. We report on dry etching of Y₂SiO₅ -YSO- crystal wafers performed by Inductive Coupled Plasma Reactive Ion Etching. The material etching rate, the etch rate selectivity versus the material mask and the etched side walls shape have been measured for various ICP process parameters and gas combinations including Ar-alone, Cl₂:Ar and Cl₂:N₂ plasma chemistry.

These gas-combinations choices have enabled identifying the etching mechanisms involved. Etching results have evidenced that Ar ions are the primary contributors for YSO etching, Cl ions being also efficient while Cl neutrals play a marginal role. ICP YSO etching is predominantly a physical process carried out by ions.
<figure><img src="image_3.png" /></figure>

Several ICP powers and RF powers have been investigated. The highest 80nm/min etch rate is obtained under Ar-alone plasma, but at the expense of trenching, while Chlorine-based etching provides no trenching and smooth sidewalls. The choice of the dielectric material mask is also discussed.

# I. INTRODUCTION

Rare-earth ion -REI- doped crystals are of high interest for quantum technologies, such as quantum repeaters ¹, single-photon emitters ², microwave-optical quantum transduction ³, and RF signal processing ⁴. Among the many matrices already explored for laser applications, some exhibit particularly long optical and hyperfine coherence times when cooled to cryogenic temperatures. In particular, Yttrium Orthosilicate crystal $Y_2SiO_5$ -YSO- has demonstrated the longest coherence times ⁵,⁶, primarily due to its relatively low internal magnetic noise.

Current storage experiments in bulk REI doped crystals display limited efficiency for long storage time due to low absorption ⁷ and poor light-matter interaction strength due to free space beam divergence, especially when optically long samples are needed.

A promising direction to address this issue involves using guided optics and integrated photonics to confine light within a small and stable modal volume. Waveguides in REI doped crystals appear as a good solution to enhance memory performances in particular the efficiency and multimode capacity: as the optical Rabi frequency remains constant in the material, the interaction length can be extended to match the desired absorption depth⁸ to reach maximum efficiency while enabling a much higher temporal multiplexing
<figure><img src="image_4.png" /></figure>

capability. In particular, it was calculated that for waveguides displaying a fundamental mode of 10µm radius at waist, the temporal capacity is doubled compared to bulk material⁹.

Various methods for guiding light in YSO are currently investigated, including focused ion beam (FIB) techniques¹⁰, femtosecond laser micromachining (FLM) ¹¹, and evanescent coupling with silicon cavities ¹². However, many of these techniques face challenges, either showing inferior spectroscopic performance compared to bulk materials or struggling to accommodate complex functionalities such as couplers, curvatures, or cavities.

In this paper, we report, to our knowledge, the first study of lithographic and dry-etching processes of Y₂SiO₅ in order to create optical guiding structures. This research paves the way for a low-cost and highly versatile fabrication method that should maintain the spectroscopic performance of YSO while enabling the creation of complex structures.

## II. Dry Etching Parameters

In order to realize high-quality active devices and low-cost structuring, we consider dry-etching of shallow ridge waveguides within the YSO crystal. The Yttrium Orthosilicate Y₂SiO₅ -YSO- crystal is a very stable crystal elaborated from the melt by the Czochralski method ¹³, at temperature above 1200°C. Within the unit cell, both Si-O and Y-O bonds exist. Four types of oxygen sites are involved forming SiO₄ tetrahedron, and one type of interstitial non-silicon-bonded oxygen site is surrounded by four Y atoms. Y-O bonds are reported to be weaker than Si-O bonds ¹⁴.
A. ICP-Chorine-based plasma etching of oxides and glass

A comprehensive review of dry-etching technology of glasses is provided in ¹⁵, presenting Reactive Ion Etching -RIE- processing parameters and related etching mechanisms. During RIE etching, both chemical and physical contributions interplay, determining etched surfaces quality and shapes, etch rate and mask selectivity.

Dry-etching of oxides and glasses is extensively reported. Oxide materials are well known to be difficult to etch related to the high binding energy of oxide links. C. Weigel et al. ¹⁶ propose dry-etching mechanisms and processing for alumino-silicate glasses and glass ceramics using Fluorine-based chemistry. J.B. Bradley et al. investigated a large span of halide chemistry, Fluorine, Chlorine and Bromide-based, for both Al₂O₃ and Y₂O₃ oxides ¹⁷. As a conclusion, the Chlorine-based process has shown the highest etch rate for Y₂O₃ ¹⁸.

Although these results report on oxides, we consider them as relevant starting points for investigating dry-etching of silicate Y₂SiO₅ here under concern.

Chlorine-based Reactive Ion Etching is a widely used chemistry for a large range of materials. It has been thoroughly developed for silicon technology and also for III-V materials. In particular, for InP-based photonic crystals structures, we previously investigated ICP -Inductive Coupled Plasma- Reactive Ion Etching -RIE- Cl₂ chemistry in order to produce smooth and vertical etched holes surfaces with a high SiO₂-mask selectivity when operated at high temperature in the 170-190°C range. Several Cl₂-based processes have been developed and fully characterized, measuring physical plasma parameters, thus evidencing their role related to the etching mechanisms involved ¹⁹.
<figure><img src="image_5.png" /></figure>

## B. Plasma parameters choice

The choice of several dry-etching parameters investigated here is driven by our previous expertise in ICP etching of Silicon and III-V materials¹⁹, and rely upon the already published Y₂O₃ oxide etching results¹⁸.

These parameters are the following:

*Temperature: A high operating temperature has been chosen, 190°C, which is well known for enhancing the removal of etched products. The sample is stuck on the Silicon carrier with a conductive grease. Nonetheless, due to the poor thermal conductivity of the crystal, the sample temperature could exceed the set temperature for long etching processes, which is not detrimental for these very stable materials.

*Pressure: The operating pressure has been set to 8mTorr. This low pressure leads to a reduced angular distribution of positive ions flux in the plasma, related to lower collision rates. Additionally, a low pressure increases the mean free-path, thus reducing collisions that produce recombination ¹⁵,¹⁹, and as a consequence keeping a high ion flow. Ions are beneficial for desorption of reaction products at the surface. Such a low pressure is also implemented in ¹⁷,¹⁸, all reported plasma compositions being investigated at pressure below 10 mTorr.

*Flux: The total gas flux is kept in the 20-40 sccm (standard cm cube per minute) range, in order to reduce the collision rates, thus enhancing the ion flux directivity. Keeping a high ion flux directivity should reduce the formation of trenching at the bottom of the etched waveguide.
*ICP Power: A high ICP power has been chosen in order to create a large density of species, a high ICP Power generally improving the density of neutrals versus the density of ions. The maximum ICP power available on the Sentech SI 500S ICP-RIE equipment is 800W.

*Rf Power-Bias: The role of Rf power and its related bias is investigated. A large bias increases ions energy, thus enhancing their contribution to the physical part of the etching and their ability to remove the products of reactions at the surface. The maximum Rf Power available on the Sentech SI 500S ICP-RIE equipment is 300W.

*Gaz mixture: The investigated chemistries cover both Cl₂:Ar and Cl₂:N₂ based processes with different composition proportions. The role of BCl₃ and an Ar-alone plasma are also considered.

Electrons excited by ICP power are reactive and generate ions and neutrals within the plasma. Ions are produced when molecules are ionized, neutrals are produced when molecules are dissociated. Within Cl₂:Ar or Cl₂:N₂ gas mixtures, Ar or N₂ are involved in the generation of Cl₂⁺ ions and Cl neutrals, but not on the same way. Ar and N₂ have similar ionisation energies. Ar is an atom, N₂ is a molecule. When excited in the plasma, Ar is ionized generating electrons, whereas N₂ molecules are first excited before being ionized. In a Cl₂:Ar plasma, Ar ionization produces electrons that contribute to the dissociation of Cl₂ molecules generating Cl neutrals, while in a Cl₂:N₂ plasma the N₂ molecule being first excited does not generate as much electrons, so the Cl₂:N₂ plasma is not as rich in neutrals as the Cl₂:Ar plasma ¹⁹. We processed both gas mixtures in order to investigate the role of neutrals versus ions in YSO etching mechanism.
<figure><img src="image_6.png" /></figure>

We have also considered the role of BCl₃. When ionised, BCl₃ produces heavier
ions that Cl₂, thus contributing in a larger part to the physical etching mechanism. We
have developed Cl₂:Ar:BCl₃ mixture processes in order to investigate this possible
contribution.

Aside ICP RIE etching investigation, an Ion Beam Etching -IBE- process has
been investigated, in order to compare ICP etching results involving both physical and
chemical contributions with large densities of species with IBE etching result involving
only a physical etching mechanism with a low density of Ar ions.

**III. Etching Experiments**

Etching processes investigation is undertaken on bulk YSO material, which is a
very hard material that cannot be cleaved. As a consequence, YSO etched waveguides
Scanning Electron Micrograph -SEM- pictures shown are obtained on broken facets.

**A. Mask materials**

For etching processes investigation, we have chosen dielectric material masks,
SiN or SiO2. SiO2 is reported to being more selective in Cl2-based plasma environment
than SiN ¹⁹, while SiN is more resistant under pulverization than SiO2. We will consider
one or the other mask material according to the plasma composition investigated for
selectivity calculation. Fig.1 shows SEM pictures of both dielectric masks etched with a
Fluorine-based CCP process, evidencing the almost vertical walls. Fig.1-(a) shows the
SiO₂ side wall of a 3µm-thick SiO₂ layer with the remaining resist on top, while Fig. 1-(b) shows the SiN side wall of a 3.5µm-thick SiN layer etched, with the remaining resist on top. Evidencing the vertical shape of the dielectric mask will be of concern when we will discuss the role of plasma composition versus the mask faceting and the related YSO waveguide sidewalls shape. According to the resist thickness used, a relatively thick dielectric mask can be produced.

<figure><img src="image_7.png" /><figcaption>FIG. 1. SEM pictures of (a) SiO₂ mask and (b) SiN mask with remaining resist evidencing the vertical side walls obtained while etching with a Fluorine-based CCP dry-etching process.</figcaption></figure>

Nickel mask, which is the only relevant metallic mask in a Chlorine environment, is shortly addressed. Metallic masks are of interest because of their stronger resistance
<figure><img src="image_8.png" /></figure>

versus dielectric mask during physical etching, but the very large strain in the Ni deposited material prevents the elaboration of lift-off Ni waveguides thicker than 250nm. This limited thickness of the mask will translate in a limited etched depth in YSO. We do not report here on the use of Nickel mask for ICP YSO etching.

## B. IBE etching experiment and results

Experiment is performed on a Roth&Rau IonSys 500 equipment. A SiN mask has been chosen due to its large selectivity under Ar+ Ion Beam Etching -IBE-. 6µm wide ridges have been patterned in an 850nm-thick SiN layer. YSO etching is performed with 350eV ion energy, 250µA/cm² current density, cooling 0°C with back side Helium flow and 20°tilt angle, providing the highest etching rate. This SiN mask thickness allowed a 2h duration etching, divided in 4 running steps in order to reduce sample heating. A 510nm etched depth in YSO was measured, corresponding to an etch rate of 4.25nm/min. The selectivity, calculated as the ratio of material etch rate divided by mask etch rate is 0.6. This IBE process is only based on the physical contribution of Ar⁺ ions. Figure 2-(a) shows a SEM picture of a 5µm-wide 510nm-thick YSO etched waveguide. The edges' shape reflects the shape of the SiN mask, the edges' slope is produced by IBE etching conditions. The very low etching rate is a consequence of the low ions density in IBE process, preventing any reasonable implementation of this technique.
<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /><figcaption>FIG. 2. SEM pictures of etched YSO according to dry-etching conditions.</figcaption></figure>

(a) IBE Ar-based etching.

(b) ICP Cl₂:Ar etching, ICP=800W, Bias=-250V, Cl₂:Ar=16:24 sccm.

(c) ICP Cl₂:N₂ etching, ICP=800W, Bias=-350V, Cl₂:N₂=25:15 sccm.

C. ICP etching experiments and results

Experiments are performed on a Sentech SI 500S ICP-RIE equipment. Several operating conditions for ICP plasma parameters have already been set following the discussions above: the temperature is fixed to 190°C, the pressure is fixed to 8mTorr, the total gas flux is kept in the 20-50 sccm range. We investigate gas plasma composition and proportion, Rf Power and related Bias and ICP Power. We report on YSO material etch rate and selectivity according to the mask material used.

1. Cl₂-Ar plasma

For the choice of gas proportion in this gas mixture, we have taken advantage of the theoretical analysis of the species found in a Chlorine discharge diluted in Argon provided in ²¹, for a Capacity-Coupled Plasma -CCP-. The total reaction rate for Cl⁺ creation is simulated versus the percentage of Ar gas flow. An Ar proportion of 60% is
<figure><img src="image_13.png" /></figure>

found to produce the highest generation of Chlorine ions in the plasma. Although the plasma investigated here is an ICP plasma with Rf Power and ICP Power, so not being the same as the one simulated in ²¹, we have considered this Argon ratio as a starting point for the investigation of plasma conditions.

We have fixed the gas flux to Cl₂=16sccm, Ar=24 sccm, the ratio being 0.6, the total flux being 40sccm. We have varied ICP power from 500W to 800W, and we have varied Rf power in order to generate a Rf bias from -200 to -350V.

Fig.3-(a) shows YSO etch rate and selectivity versus SiO₂ mask, for different ICP powers, the Rf Bias fixed at -300V. YSO etch rate is in the 40-60nm/min range, with a selectivity versus SiO₂ mask in the 0.2-0.3 range. Performances are similar for several bias investigated. Fig.3-(b) shows YSO etch rate and selectivity versus SiO₂ mask, for different Rf bias, the ICP power fixed at 800W. It can be noticed on Fig. 3-(b) that selectivity versus SiO₂ slightly increases when the bias increases. This could be related to the formation of an SiClx layer at the surface of the SiO₂ layer, as reported in ²², which reduces the SiO₂ etching rate. Such a reduced etching rate of SiO₂ in Cl₂-rich plasma is also measured in ²³. Increasing the total gas flux to 60sccm, still keeping the same ratio (Cl₂=24sccm, Ar=36 sccm) does not change significantly neither the YSO etching rate nor the selectivity versus SiO₂. These results suggest that there is a limiting factor on YSO etched surfaces in the Chlorine environment. J.D.B. Bradley et al. proposed in ¹⁷ that the limitation of Y₂O₃ etching in a Chlorine-based plasma is related to the formation of a YCl₃ non-volatile etch product. YSO material containing Y-O bonds as Y₂O₃ material, we can reasonably suggest that the formation of YCl₃ could also occur here during Cl₂:Ar etching, thus limiting the etching rate when the gas flow is increased. A
<figure><img src="image_14.png" /></figure>

SEM picture of an Cl₂:Ar plasma etched wall is shown in Fig.2-(b), evidencing reasonably smooth etched surface.

<figure><img src="image_15.png" /><figcaption>FIG. 3. YSO etch rate and selectivity versus SiO₂ in the Cl₂:Ar plasma, changing (a) ICP power or (b) Rf Bias. Gas composition is Cl₂:Ar=16:24 sccm.</figcaption></figure>

## 2. Cl₂:Ar:BCl₃ plasma

We investigated the introduction of BCl₃ in the Cl₂:Ar mixture, considering that BCl₃ ions being heavier than Cl₂ ions could improve the removal of etched products from the surface. We operated at 8mTorr, keeping the total gas flux to 40 sccm. We operated at an ICP power = 500W, a Rf Bias= -300V. We varied the proportion of BCl₃. YSO etching rate and selectivity versus SiO₂ are plotted on Fig. 4. We notice that YSO etch rate does not depend on the proportion of BCl₃ in the mixture, the etch rate is reduced by a factor of 2 versus a gas mixture without BCl₃, and the selectivity versus SiO₂ is reduced below 0.2. These results indicate that the ions density is reduced when BCl₃ is introduced, leading to a lower YSO etching rate and BCl₃ ions do not improve etch products removal.
<figure><img src="image_16.png" /><figcaption>FIG.4. YSO etch rate and selectivity versus SiO2 in Cl2:Ar:BCl3 plasma, changing BCl3 proportion.</figcaption></figure>

3. Cl₂-N₂ plasma

Cl₂:N₂ plasma is then considered related to its lower Chlorine neutrals density than the one in the Cl₂:Ar plasma as discussed above. Keeping a total gas flux of 40sccm, we investigated two gas proportions: Cl₂=10: N₂=30sccm, and Cl₂=25: N₂=15sccm, for an 800W ICP power and a -350V Rf Bias. Fig.5 shows that YSO etch rate is increased to 36nm/min for the large Cl₂ proportion, and selectivity versus SiO₂ mask is also increased to 0.23. This result indicates that Chlorine ions density is increased when Chlorine gas proportion is increased. In the case of a total gas flow=40sccm, we can notice than YSO etch rates are here lower than the ones reached in the case of Cl₂:Ar plasma. A SEM picture of a Cl₂:N₂ plasma etched wall is shown in Fig.2-(c). The side-walls smoothness is comparable to the one obtained in Cl₂:Ar plasma.
<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /><figcaption>FIG.5. YSO etch rate and selectivity versus SiO2 in Cl2:N2 plasma, changing Cl2 proportion.</figcaption></figure>

Comparing the results obtained between Cl2:Ar plasma and Cl2:N2 plasma, we
suggest the following discussion for understanding YSO etching mechanism. Both
plasmas contain chlorine ions and chlorine neutrals, only Cl2:Ar plasma contains Ar ions.
In Cl2:Ar plasma, the role of Chlorine ions, Ar ions and a large amount of Chlorine
neutrals have to be considered, while in Cl2:N2 plasma, only a large density of Chlorine
ions are to be considered. We have measured YSO etching rates larger in Cl2:Ar plasma.
This would indicate that ions have the most significant contribution to etching
mechanism, the chemical part of the etching provided by neutral being minor. In Cl2:Ar
plasma, both Chlorine ions and Ar ions are involved, while in Cl2:N2 plasma , only
Chlorine ions are active. Both etching mechanisms are ions-driven.

In order to discriminate the relative role of Chlorine ions versus Ar ions in Cl2:Ar
plasma etching mechanism, we investigate an Ar-only plasma.
<figure><img src="image_19.png" /></figure>

This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.
PLEASE CITE THIS ARTICLE AS DOI: 10.1167/6.0004314

4. Ar-only plasma

In this Ar-only plasma, we used a SiN mask, related to its reduced pulverization
versus SiO2 as discussed above. A SEM picture of this mask is shown on Fig.1-(b). The
gas flux is set to 40sccm, both ICP Power and Rf Bias are varied. Fig.7 plots some ICP/
Rf Bias conditions investigated. For an 800W ICP Power and a -100V Rf Bias, a YSO
etch rate of 40nm/min is obtained, with a very high selectivity versus SiN reaching 1.
Increasing the bias to -300V doubles the etch rate up to 80nm/min; this large etch rate is
obtained at the expense of a reduced selectivity and a trenching visible on Fig. 6-(b), as a
consequence of the only physical mechanism provided by Ar ions. Reducing the ICP
power down to 500W in order to reduce Ar ion density, for a -150V Rf Bias, the etch rate
is reduced to 53nm/min and trenching has been greatly reduced, see Fig. 6-(a). As a
consequence of the physical etching contribution only under Ar+ ions, YSO etched
sidewalls are not as smooth as in the case of Chlorine-based plasma where a chemical
contribution is also involved <sup>16</sup>. Mask faceting is clearly visible (Fig. 6-(a)), due to the
large Ar ions density.
<figure><img src="image_20.png" /><figcaption>FIG.6. SEM pictures of YSO etched in an ICP Ar-plasma, with remaining SiN mask.</figcaption></figure>

(a) 500W ICP, -150V Rf Bias.

(b) 800W ICP, -300V Rf Bias.

<figure><img src="image_21.png" /><figcaption>FIG.7. YSO etch rate and selectivity versus SiN in Ar plasma, changing Rf Bias, for (a) 500W ICP and (b) 800W ICP.</figcaption></figure>
<figure><img src="image_22.png" /></figure>

Comparing the results obtained between Cl₂:Ar plasma and Ar-only plasma, we can conclude that Ar ions are the ions driving the etching mechanism. This is consistent with the lower etching rate measured in the Cl₂:N₂ plasma where only Chlorine ions are present.

This conclusion is consistent with the one reached in ¹⁸ for Y₂O₃ ICP RIE etching. Such a consistency could be expected from the results reported by W.Y Ching et al. in 2003 ²⁴. They performed first principles local density calculations of Electronic and Optical Properties of Y₂SiO₅ and Y₂Si₂O₇ with comparisons to α-SiO₂ and Y₂O₃, and reported that Y-O bond in the ternary crystals tends to be stronger than the one in Y₂O₃. It is also consistent with the conclusion reached in the case of Cl₂:Ar etching of silicon ²⁵.

### 5. Mask material choice and mask removal

Related to previous publications as discussed above, silicon nitride mask has been chosen for Ar-only etching while silicone oxide mask has been chosen in the case of Chlorine-based plasmas. We performed etchings tests with both masks in order to assess up to what extend this choice was the correct one. For Cl₂:Ar plasma, all conditions included, SiN etch rate is twice SiO₂ one, as expected related to Chlorine etching of SiN, while in Ar-only plasma, all conditions included SiO₂ etch rate is twice SiN one, comforting the stronger resistance of SiN under pulverization. These considerations on mask material choice do not take into account any possible impact of the mask material choice on the material etched rates, according to the chemistries investigated.

Both SiO₂ and SiN masks are usually removed using HF, the etched rate for SiN being lower than the one for SiO₂. In the case of YSO, HF hardly degrades the surface,
generating orthorhombic pyramids of Yttrium fluoride YF₃. Fig. 8-(a) shows SEM pictures of these quite large pyramids and Fig. 8-(b) reports on the atomic percentages of elements identified during an EDX analysis on one orthorhombic pyramid, comforting its YF₃ stoichiometry, and close to it on YSO surface. Specific mask removal procedures are under investigation in order not to degrade YSO surface.

<figure><img src="image_23.png" /></figure>

<table><caption>FIG. 8. (a) SEM pictures of YSO surface degraded after SiO₂ mask removal using HF. (b) Atomic percentages table of elements identified during EDX analysis on one pyramid and on YSO surface beside a pyramid.</caption><caption>(b) EDX analysis</caption><thead><tr><th>Element</th><th>Pyramid<br>Atomic %</th><th>YSO<br>Atomic %</th></tr></thead><tbody><tr><td>O</td><td>6.1</td><td>48.9</td></tr><tr><td>Si</td><td>2</td><td>8.3</td></tr><tr><td>Y</td><td>18.6</td><td>17.8</td></tr><tr><td>F</td><td>62</td><td>14.7</td></tr></tbody></table>

## IV. Conclusion

Several ICP Chorine-based etching processes have been investigated in order to etch shallow ridge waveguides in Y₂SiO₅ silicate-crystal material. Cl₂:Ar plasma, Cl₂:N₂ plasma and Ar-only plasma have been considered, this choice of plasma compositions allowing an investigation of etching mechanisms involved. SiO₂ or SiN dielectric material masks have been considered, the choice being driven by plasma composition and the related etch selectivity.

The experiments results demonstrate that the etching process is ions-driven. Ar-only plasma leads to the highest YSO etch rate of 80nm/min and a related selectivity
<figure><img src="image_24.png" /></figure>

versus SiN mask= 0.5 for a large ICP power =800W and a large Rf Bias =-350V but produces trenching. Reducing the Rf Bias to -100V greatly reduces the trenching, at the expense of a lower etching rate but a larger selectivity, both figures being a consequence of the lower Ar ions energy. Both Cl₂:Ar plasma and Cl₂:N₂ plasma operating with chlorine ions provide lower Y₂SiO₅ etching rates and lower selectivities versus SiO₂ mask, but produce smoother side walls and no trenching. The choice of etching conditions will be driven according to the targeted applications.

These results open a now road for the realization of Rare earth ion-doped Y₂SiO₅ integrated photonic devices, potentially leveraging limitations present in current bulk samples.

## ACKNOWLEDGMENTS

This work was partially supported by the French Renatech network.

## AUTHOR DECLARATIONS

### Conflicts of Interest

The authors have no conflicts to disclose.

## DATA AVAILABILITY

The data that support the findings of this study are available from the corresponding author upon reasonable request.
<figure><img src="image_25.png" /></figure>

REFERENCES

¹K. Azuma, S. E. Economou, D. Elkouss, P. Hilaire, L. Jiang, H-K Lo and I. Tzitrin, Rev Mod Phys, **95**, 045006, (2023) doi: 10.1103/RevModPhys.95.045006.

²A. M. Dibos, M. Raha, C. M. Phenicie, and J. D. Thompson, Phys Rev Lett, **120**, 243601 (2018) doi: 10.1103/PhysRevLett.120.243601.

³X. Gonzalvo, (2017) [Online]. https://api.semanticscholar.org/CorpusID:125459947

⁴P. Berger, Y. Attal, M. Schwarz, S. Molin, A. Louchet-Chauvet, T. Chanelière, J-L Le Gouët, D. Dolfi and L. Morvan, J. Light. Technol., **34**, 4658 (2016) doi: 10.1109/JLT.2016.2556008.

⁵M. Zhong, M.P. Hedges, R.L.Ahlefeldt, J.G. Bartholomew, S.E. Beavan, S.M. Wittig, J.J Longdell and M.J.Sellars, Nature, **517**, 177, (2015) doi: 10.1038/nature14025.

⁶M. Rančić, M. P. Hedges, R. L. Ahlefeldt, and M. J. Sellars, Nat. Phys., **14**, 50, (2018) doi: 10.1038/nphys4254.

⁷Y. Ma, Y-Z Ma, Z-Q Zhou, C-F Li and G-C Guo, Nature Commun., **12**, 2381 (2021)

⁸M. Afzelius, C. Simon, H de Riedmatten and N. Gisin, Phys. Rev. A, **79**, 052329 (2009)

⁹M. Chan, A. Ferrier, P. Berger, L. Morvan, A. Talneau and S. Welinski, Rare-Earth for Quantum Information workshop, München (2024)

¹⁰I. Craiciu, M. Lei, J. Rochman, J.M. Kindem, J.G. Bartholomew, E. Miyazono, T. Zhong, N. Sinclair and F. Faraon, Phys. Rev. Appl., **12**, 024062, (2019), doi: 10.1103/PhysRevApplied.12.024062.

¹¹G. Corrielli, A. Seri, M. Mazzera, R. Osellame, and H. de Riedmatten, Phys. Rev. Appl., **5**, 054013, (2016) doi: 10.1103/PhysRevApplied.5.054013.
<figure><img src="image_26.png" /></figure>

¹²I. Craiciu, M. Lei, J. Rochman, J. G. Bartholomew, and A. Faraon, Optica, **8**, 114,
(2021) doi: 10.1364/OPTICA.412211.

¹³G. Wolfowicz, H. Maier-Flaig, R. Marino, A. Ferrier, H. Vezin, John J.L. Morton, and P. Goldner, Phys. Rev. Lett., **114**, 170503 (2015)

¹⁴B. Liu, J. Wang, F. Li, J. Wang and Y. Zhou, J. Am. Ceram. Soc., **95** 1093, (2012) doi:
10.1111/j.1551-2916.2011.05046.x

¹⁵A.F. Moore, J.A Frantz, L.E. Busse and J.S. Sanghera, Naval Research Laboratory, NRL/5620/MR--2021/2 (2021)

¹⁶C. Weigel, U. Brokmann, M. Hofmann, A. Behrens, E. Rädlein, M. Hoffmann, S. Strehle and S. Sinzinger, Journal of Optical Microsystems, **1**, 040901 (2021) doi: 10.1117/1.JOM.1.4.040901

¹⁷J.D.B. Bradley, F. Ay, K. Worhoff and M. Pollnau, Appl. Phys. B **89**, 311 (2007), doi: 10.1007/s00340-007-2815-3

¹⁸J. D. B. Bradley, F. Ay, K. Wörhoff, and M. Pollnau, ECS Trans. **3**, 117 (2006) doi: 10.1149/1.2392925

¹⁹K.H. Lee, S. Guilet, G. Patriarche, I. Sagnes and A. Talneau, J. Vac. Technol. B, **26**, 1326 (2008)

²⁰S. J. Pearton, Int. J. Mod. Phys. B **8**, 1781 (1994).

²¹G. Thorsteinsson, A. Th. Hjartarson and J.T. Gudmunsdsson [online] http://langmuir.raunvis.hi.is/~tumi>clar_CIO09

²²C. Petit-Etienne, M. Darnon, L. Vallier, E. Pargon, G. Cunge, M. Fouchier, P. Bodart ; M. Haass, M. Brihoum and O. Joubert J. Vac. Sci. Technol. B **29**, 051202 (2011), doi: 10.1116/1.3622311
<figure><img src="image_27.png" /></figure>

23 Seong Jae Yoo, Ji Eun Kang, You Jin Ji, Hyun Woo Tak, Byeong Ok Cho, Young Lae Kim, Ki Chan Lee, Jin Sung Chun, Yongil Kim, Dong Woo Kim, Nanotechnology 34, 465302 (2023) doi 10.1088/1361-6528/acec7a

24 W. Y. Ching, L. Ouyang, and Y. Xu, Phys. Rev. B, **67**, 245108 (2003)

25 S Tinck, W Boullart and A Bogaerts J. Phys. D: Appl. Phys. **41**, 065207, (2008)
doi:10.1088/0022-3727/41/6/065207