Non-stoichiometric silicon nitride for future gravitational wave detectors

Silicon nitride thin films were deposited at room temperature employing a custom ion beam deposition (IBD) system. The stoichiometry of these films was tuned by controlling the nitrogen gas flow through the ion source and a process gas ring. A correlation is established between the process parameters, such as ion beam voltage and ion current, and the optical and mechanical properties of the films based on post-deposition heat treatment. The results show that with increasing heat treatment temperature, the mechanical loss of these materials as well as their optical absorption decreases producing films with an extinction coefficient as low as k = 6.2(±0.5) × 10₋₇ at 1064 nm for samples annealed at 900°C. This presents the lowest value for IBD SiN_x within the context of gravitational wave detector applications. The mechanical loss of the films was measured to be Φ = 2.1(±0.6) × 10⁻⁴ once annealed post deposition to 900°C.