In this paper, silicon oxynitride layers deposited with different plasma-enhanced chemical vapor deposition (PECVD) conditions were fabricated and optimized, in order to make an interferometric sensor for detecting biochemical reactions. For the optimization of PECVD silicon oxynitride layers, the influence of the N₂O/SiH₄ gas flow ratio was investigated. RF power in the PEVCD process was also adjusted under the optimized N₂O/SiH₄ gas flow ratio. The optimized silicon oxynitride layer was deposited with 15 W in chamber under 25/150 sccm of N₂O/SiH₄ gas flow rates. The clad layer was deposited with 20 W in chamber under 400/150 sccm of N₂O/SiH₄ gas flow condition. An integrated Mach–Zehnder interferometric biosensor based on optical waveguide technology was fabricated under the optimized PECVD conditions. The adsorption reaction between bovine serum albumin (BSA) and the silicon oxynitride surface was performed and verified with this device.