Lithium niobate (LiNbO₃ or LN) is a versatile nonlinear optical crystal, valued for its strong electro-optic coefficients, piezoelectric properties, broad transparency range, and wide intrinsic bandwidth. When implemented as lithium niobate on insulator (LNOI), it enables the fabrication of high-index-contrast optical waveguides and densely integrated photonic circuits (PICs), opening the door to the next generation of miniature radio-frequency modulators and other advanced photonic devices.
Challenges in LiNbO₃ Ridge Waveguide Fabrication
Creating smooth ridge waveguides is a critical step in producing high-performance PICs. LiNbO₃’s hardness makes it challenging to etch: conventional dry etching can produce rough surfaces and sidewall redeposition, which increase optical losses and degrade device performance.
Advanced Plasma Etching Solutions
Sulfur hexafluoride (SF₆), used alone or mixed with argon (Ar), is a common plasma etching gas for LiNbO₃. SF₆/Ar mixtures have proven effective for producing smooth ridges. However, due to the low volatility of reaction by-products such as LiF and NbFₓ, relatively high bias power is required to physically remove these residues. Without sufficient energy, micro-masking can occur, resulting in surface roughness.
Optimized Process Using Chromium Masks
This application note demonstrates an optimized etching process for LiNbO₃ ridge waveguides with smooth, high-quality sidewalls performed using a SENTECH Instruments SI500 ICP plasma etcher. A chromium (Cr) mask was used for its resistance to high bias power, which preserves the etch profile and ensures high selectivity. Using an SF₆/Ar plasma, this method achieves clean, smooth sidewalls without redeposition, producing ridge waveguides suitable for high-performance photonic circuits.
Key Benefits
- Smooth LiNbO₃ ridge waveguides with minimal optical loss
- No sidewall redeposition
- High etch selectivity and pattern fidelity using Cr masks
- Compatible with next-generation PICs and compact RF modulators
This approach allows photonics engineers to reliably fabricate high-quality LiNbO₃ waveguides, supporting the development of innovative, miniaturized photonic devices.
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