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Mitigating Material Damage in Lithium Niobate After Plasma Etching for Nonlinear Photonics Applications

Introduction

Lithium niobate (LN) has established itself as a versatile material platform for photonic integrated circuits due to its exceptional electro-optic and nonlinear optical properties. The ability of lithium niobate to span the entire spectral range from radio to optical wavelengths demonstrates its versatility as a platform material in integrated photonics. [1] Despite its significant development over past decades and its large electro-optic coefficient, lithium niobate (LN) faces challenges in device miniaturization compared to advances in integrated silicon photonics. [2]

However, one of the major challenges in fabricating LN-based photonic devices is the difficulty in etching and processing this material. Unlike silicon, which is easy to etch, lithium niobate is not amenable to chemical etching, a technique commonly used in the silicon-based industry to produce nanometer-sized structures like waveguides. [3] Lithium niobate is difficult to process via dry etching, which has led researchers to explore alternative materials and methods for manufacturing photonic integrated circuits, such as using diamond-like carbon (DLC) as a hard mask to achieve deeply etched, tightly confining, low-loss waveguides. [4]

This report explores the various approaches to mitigate material damage in lithium niobate after plasma etching, which is crucial for developing high-performance LN-based nonlinear photonic devices.

Types of Damage from Plasma Etching

Plasma etching of lithium niobate leads to several types of damage that affect device performance:

1. Micromasking and Redeposition

Optical losses in lithium niobate circuits are limited by scattering from sidewall roughness, which is introduced by micro-masking effects partly due to material redepositing on the etched walls. [5] While argon-based processes have been shown to provide the smoothest sidewalls and facilitate the manufacturing of photonic integrated circuits of the best optical quality, the physical nature of argon sputtering introduces complications, most prominently the formation of non-vertical sidewalls and the redeposition of non-volatile etched material accumulating on the sidewalls. [6]

2. Sidewall Roughness and Angle

The byproducts from fluorine-based dry-etching evaporate only at 800°C, thus remaining on the surfaces and sidewalls of the structures. This not only reduces the etch rate of the target material but also prevents obtaining vertical etching profiles. Additionally, typical mask materials can delaminate or be consumed during the etching process, resulting in roughness on the etched structure, which can degrade performance (e.g., increased optical losses due to surface roughness). [7]

3. Lattice Damage and Material Modification

The high-energy electron beam utilized in the Electron Beam Lithography (EBL) process introduces material damage and significantly increases optical loss in thin-film lithium niobate (TFLN). [8] Smart-cut method TFLN wafers exhibit lattice damage caused by He+ implantation, and this damage needs to be repaired by annealing. [8]

Mitigation Strategies

Several approaches can be employed to mitigate the material damage in lithium niobate after plasma etching:

1. Thermal Annealing

Post-fabrication annealing can effectively repair the damage caused during the fabrication process. [8] A slow heating post-process annealing process has been designed to repair the damage caused during the fabrication process. This method has demonstrated the ability to reduce waveguide loss by approximately 50% and increase the intrinsic Q-factor of micro-ring resonators by around 100%. [8]

Systematic analysis comparing different lithographic techniques (EBL and DUV) has shown that TFLN is affected by high-energy electron beams generated by EBL, introducing additional material absorption loss. However, annealing can enhance the crystallinity of TFLN and effectively repair the damage caused during the fabrication process, reducing waveguide loss by approximately 50% and increasing the intrinsic Q-factor of micro-ring resonators by around 100%. [8]

Pump–probe techniques have determined that the material-limited loss in ion-sliced LN is approximately 1.5 dB/m, and an annealing process can reduce this to around 0.2 dB/m, approaching the limit of bulk LN. [9]

For waveguides made in lithium niobate by femtosecond laser writing, thermal annealing at temperatures up to 450°C (stepped by 25°C from room temperature) significantly improves guiding performance. This improvement is attributed to anisotropy expansion of the waveguiding region, reduction of color centers, and waveguide border smoothing after thermal treatment, as well as better mode matching for coupling. Waveguides treated at around 250°C show higher output guided power by up to 10 dB compared to those without thermal treatment. [10]

High temperature (1000°C) annealing of Ti-doped X-cut LN can be utilized for optical waveguide applications. Additionally, nanofaceting of LiNbO3 X-cut surfaces can be achieved through high-temperature annealing and titanium diffusion. [7]

For exfoliated thin films of LN obtained through helium ion implantation, a thermal treatment followed by hydrofluoric acid etching can be used. After direct bonding to a silicon carrier, rapid thermal annealing (RTA) at 1000°C for 30s can repair the crystal lattice and restore the electro-optic properties. [11]

2. Optimized Etching Processes

Extensive progress has been made to improve the fabrication of integrated optical circuits using lithium niobate. Although argon etching of LN waveguides provides the best optical quality, the process remains challenging to optimize due to its physical nature, particularly due to micro-masking effects from material redeposition and close-to-one etch mask selectivity for deep etches. Researchers have developed workflows to identify parameter sets that offer the best etching results independent of the plasma system being used. [5]

To fully benefit from the polishing character of Ar sputtering, redeposition removal during the dry etching can enable smoother sidewalls and lower optical losses. Three methods to achieve redeposition-free lithium niobate dry etching have been proposed, with workflows to identify the parameter set independent of the ICP-system used. [5]

Methods to achieve redeposition-free lithium niobate dry etching include a careful balance of ICP parameters (especially DC bias and chamber pressure), proximity of additional features to the target structure to aid redeposition removal through ion deflection between neighboring walls, and tailoring the etching mask toward a trapezoidal shape to favor redeposition removal. [6]

3. H2-Plasma Surface Treatment

An optimized processing methodology using Ti/Al/Cr stack as a hard mask during long plasma dry etching, with periodic pausing of the etching and chemical cleaning between cycles, can avoid thermal effects and byproduct redeposition. Additionally, H2-plasma treatment can be implemented to relieve surface tension by modifying the top surface atoms, resulting in structures with etch depths as deep as 3.4 μm with smooth sidewalls and near-perfect verticality across various crystallographic facets. [12]

4. Wet Etching and Combined Approaches

Atomic Layer Etching (ALE) followed by a wet cleaning process (RCA clean) has been shown to reduce surface roughness. The roughness induced by the ALE process primarily comes from redeposition of lithium and magnesium fluorides, which can be removed by wet cleaning. This supercycle approach, consisting of ALE followed by a wet process, could facilitate even larger decreases in surface roughness. [13]

A directional ALE process combined with a wet post-process to remove redeposited compounds can yield a 50% decrease in surface roughness. With additional optimization to reduce redeposited compounds, these processes could effectively smoothen surfaces of TFLN waveguides etched by physical argon processes. [13]

For NF3/Ar plasma etching, the wavelength region (220–290 nm) can be particularly useful for LiNbO3 etching control, as it helps determine the "endpoint" in multistep processes involving plasma etching and wet cleaning for LiF barrier layer removal. [14]

5. Advanced Mask Materials and Techniques

Diamond-like carbon (DLC) has been demonstrated as a superior material for manufacturing photonic integrated circuits based on ferroelectrics, specifically LiNbO3. Using DLC as a hard mask enables the fabrication of deeply etched, tightly confining, low-loss waveguides with losses as low as 4 dB/m. [4]

Conclusion

Mitigating material damage in lithium niobate after plasma etching is a critical challenge for developing high-performance nonlinear photonic devices. The most promising strategies include:

1. Thermal Annealing: Post-fabrication annealing at temperatures ranging from 250°C to 1000°C can significantly repair lattice damage, enhance crystallinity, and reduce optical losses.

2. Optimized Etching Processes: Carefully controlling etching parameters, using interrupted etching with cleaning steps, and designing appropriate mask geometries can minimize redeposition and improve sidewall quality.

3. H2-Plasma Surface Treatment: Pre-treatment of LN surfaces with hydrogen plasma can modify surface atoms, improving etch quality and reducing damage.

4. Combined Dry and Wet Approaches: Sequential use of plasma etching followed by wet chemical cleaning can effectively remove redeposited compounds and reduce surface roughness.

5. Advanced Mask Materials: Materials like diamond-like carbon (DLC) can serve as superior hard masks for LN etching, facilitating better pattern transfer and reducing overall damage.

By implementing these strategies, researchers can achieve significant improvements in the quality of LN photonic devices, enabling the development of advanced integrated photonic circuits for nonlinear applications with low optical losses, high Q-factors, and improved overall performance.

参考文献：

[1] https://www.science.org/doi/10.1126/science.abj4396 - Lithium niobate photonics: Unlocking the electromagnetic spectrum | Science [2] https://www.spiedigitallibrary.org/journals/Advances-in-lithium-niobate-photonics-development-status-and-perspectives/volume-4/issue-03/034003/Advances-in-lithium-niobate-photonics-development-status-and-perspectives/10.1117/1.AP.4.3.034003.full - Advances in lithium niobate photonics: development status and perspectives

[3] https://spectrum.ieee.org/lithium-niobate-a-new-and-viable-platform-for-optoelectronics - Plasma Etching Lithium Niobate on an Optoelectronic Chip - IEEE Spectrum

[4] https://www.nature.com/articles/s41467-023-40502-8 - High density lithium niobate photonic integrated circuits | Nature Communications [5] https://www.researchgate.net/publication/367129327_Redeposition-free_inductively-coupled_plasma_etching_of_lithium_niobate_for_integrated_photonics - (PDF) Redeposition-free inductively-coupled plasma etching of lithium niobate for integrated photonics

[6] https://www.degruyter.com/document/doi/10.1515/nanoph-2022-0676/html?lang=en - Redeposition-free inductively-coupled plasma etching of lithium niobate for integrated photonics

[7] https://www.mdpi.com/2079-4991/12/16/2836 - High-Quality Dry Etching of LiNbO3 Assisted by Proton Substitution through H2-Plasma Surface Treatment

[8] https://www.sciencedirect.com/science/article/abs/pii/S0925346724002325 - Reduced material loss caused by Electron Beam Lithography in thin-film lithium niobate through post-process annealing - ScienceDirect

[9] https://pubs.aip.org/aip/app/article/7/8/081301/2835188/Reduced-material-loss-in-thin-film-lithium-niobate - Reduced material loss in thin-film lithium niobate waveguides | APL Photonics | AIP Publishing [10] https://www.sciencedirect.com/science/article/abs/pii/S0030399222000287 - Significant improvement in optical propagation of lithium niobate written waveguides by a thermal annealing - ScienceDirect

[11] https://opg.optica.org/oe/fulltext.cfm?uri=oe-21-22-27003&id=274011 - 12.5 pm/V hybrid silicon and lithium niobate optical microring resonator with integrated electrodes

[12] https://pmc.ncbi.nlm.nih.gov/articles/PMC9415737/ - High-Quality Dry Etching of LiNbO3 Assisted by Proton Substitution through H2-Plasma Surface Treatment - PMC

[13] https://arxiv.org/html/2310.10592v2 - Directional atomic layer etching of MgO-doped lithium niobate using sequential exposures of H₂ and SF₆/Ar plasma

[14] https://www.sciencedirect.com/science/article/abs/pii/S0924424721006117 - Optical diagnostic of LiNbO3 etching in NF3/Ar ICP plasma: Part I - ScienceDirect

Generated Article 📖

Mitigating material damage in lithium niobate (LN) after plasma etching is crucial for enhancing the performance of LN-based nonlinear photonic devices. Several strategies have been explored to address this issue:

1. Atomic Layer Etching (ALE): An isotropic ALE process using sequential exposures of H₂ and SF₆/Ar plasmas has been developed for MgO-doped LN. This method achieves an etch rate of approximately 1.59 nm per cycle with high synergy, effectively reducing sidewall surface roughness by 30% without additional wet processing. (arxiv.org)

2. Photolithography Assisted Chemo-Mechanical Etching (PLACE): This technique involves patterning a chromium mask via femtosecond laser micromachining, followed by chemo-mechanical etching of the LN thin film. PLACE has been used to fabricate high-index-contrast single-mode waveguides with low propagation losses, indicating its effectiveness in minimizing etching-induced damage. (arxiv.org)

3. Focused Ion Beam (FIB) Milling: Direct FIB milling on LN substrates, as well as a combination of FIB milling on a deposited metallic layer followed by reactive ion etching (RIE), have been employed to create nanostructures. These methods produce homogeneous structures with well-reproduced periodicity, suggesting their potential in reducing material damage during the etching process. (arxiv.org)

Implementing these techniques can help mitigate plasma etching-induced damage in LN, thereby improving the performance and reliability of LN-based nonlinear photonic devices.

References

[1] https://arxiv.org/abs/2310.10592?utm_source=openai - Isotropic atomic layer etching of MgO-doped lithium niobate using sequential exposures of H$_2$ and SF$_6$ plasmas

[2] https://arxiv.org/abs/2006.11562?utm_source=openai - High-index-contrast single-mode optical waveguides fabricated on lithium niobate by photolithography assisted chemo-mechanical etching (PLACE)

[3] https://arxiv.org/abs/0801.4201?utm_source=openai - Nanostructuring lithium niobate substrates by focused ion beam milling