High Reflectivity, Compact, and Widely Tunable Distributed Bragg Reflector Based on Silicon-Rich SiNx-SiOy at 80 °C PECVD

This study investigates the mechanical and optical characteristics of silicon nitride thin films deposited with PECVD at 80 °C for tunable silicon-rich SiNx-SiOy-based MEMS optical cavities. Varying the deposition parameters using SiH4 and N2 as precursor gases for silicon-rich SiNx thin films allows us to tune the refractive index to a value as high as 2.40 ± 0.013 at an extinction coefficient of only 0.008, an extremely low surface roughness of only 0.26 nm, and a compressive stress of about 150 MPa. We deposited 6.5-layer pairs of silicon-rich SiNx/SiOy-distributed Bragg reflector (DBR) micro-electro-mechanical system (MEMS) mirror that covers the whole 1300 and 1550 nm range. Cavity architectures of 6.5 top and 6 bottom layer-pairs were fabricated in the clean room providing a variety of cavity lengths between 0.615 µm and 2.85 µm. These lengths were then simulated in order to estimate the Young’s Modulus of silicon-rich SiNx, obtaining values from 56 to 92 GPa. One of the designs was characterised electro-thermally providing a tuning range of at least 86.7 nm centred at 1585 nm. The tunable filters are well suitable for implementation as tuning element in lasers for optical coherence tomography.