Optical and structural properties of silicon nitride thin films deposited by plasma enhanced chemical vapor deposition for high reflectance optical mirrors

Silicon nitride has been extensively studied as high-refractive index material for distributed Bragg’s reflectors planned to be used in the 3rd generation of Gravitational Wave Detectors working at cryogenic conditions. The absence of mechanical loss of this material at cryogenic conditions and its high refractive index, make this material be considered one of the best options for the mirrors of the GWDs. The optimization of composition and structure of SiN_x thin films to refine optical (refractive index, and optical absorption), and morphology (surface roughness, defects) have been carried out mainly using ion beam sputtering (IBS), plasma enhanced chemical vapor deposition (PECVD) and low-pressure CVD (LPCVD). This work reports the characterization of both silicon nitride (SiN_x) and a new alternative silicon oxynitride (SiO_xN_y) thin film, deposited by ammonia free based PECVD. We measured and analyzed the composition of the films, as well as their stress, surface roughness, and optical constants, including refractive index and extinction coefficient at λ = 1550 nm. Under our deposition conditions, superior properties in terms of high thickness uniformity – free of cracks – at wafer scale, low compressive stress (range of kPa), low surface roughness (<1 nm), and high refractive index 2.2 were achieved in both materials, with pure composition lacking contaminants.