Angle-optimized ion-beam etching for high-verticality and low-loss lithium niobate microresonators

Lithium niobate-on-insulator (LNOI) has emerged as a compelling platform for integrated nonlinear photonics and electro-optics. While notable advances have been made in developing low-loss LNOI waveguides and resonators, their sidewall verticality typically remains below 70°. Here, we present an angle-optimized ion-beam etching (IBE) process with a soft positive-tone resist as the mask. By investigating the correlation between IBE incidence angles (0^∘-30^∘) and critical device characteristics, we achieved trench-free LNOI waveguides with sidewall verticality approaching 80° while preserving low propagation loss. This approach enables the fabrication of compact spiral microresonators with intrinsic quality factors up to 5.1 × 10⁶, corresponding to a propagation loss of 0.08 dB/cm. We further validated the device performance by demonstrating pulse-driven Kerr soliton microcombs with repetition rates near 24 GHz. These results establish angled, resist-masked IBE as a simplified fabrication process for realizing high-verticality, low-loss LNOI photonic circuits.