Low Loss Aluminum Nitride Waveguide Fabrication: Propagation Loss Reduction Through ALD and RTA

Aluminum nitride (AlN) has emerged as a leading platform for integrated photonics in the visible and ultraviolet spectral ranges, particularly for quantum information applications involving trapped atoms. However, achieving low propagation loss in tightly confining single-mode AlN waveguides remains a challenge, especially at sub-micron wavelengths where scattering losses scale unfavorably. In this work, we present a reproducible and detailed fabrication process for low-loss AlN waveguides on sapphire, achieving a record loss of 2~dB/cm at 852~nm. Our results are enabled by systematic process optimization including high-resolution electron beam lithography with shape-based proximity effect correction, atomic layer deposition (ALD) of \ce{Al2O3} for waveguide surface passivation, and post-fabrication rapid thermal annealing (RTA). We provide a study of the contributions of each technique to propagation loss reduction and support our findings with quantitative loss measurements and comparison with an exhaustive literature review. This work represents the first detailed report of ALD passivation and post-cladding RTA applied to AlN waveguides.