Amorphous silicon nitride deposited by an NH 3 -free plasma enhanced chemical vapor deposition method for the coatings of the next generation laser interferometer gravitational waves detector

Cryogenic mechanical loss of the mirror coatings will result in thermal noise and limit the sensitivity of the next generation laser interferometer gravitational wave detectors operated at cryogenics. Amorphous silicon nitride (aSiN) films deposited by NH 3 plasma enhanced chemical vapor deposition (NH 3 -PECVD), a coating method with potential in large area uniform coatings for the next generation detectors, were found previously to have a low cryogenic mechanical loss and without loss peaks that are common in current coatings for room temperature detectors. A positive correlation between N–H bond density and cryogenic mechanical loss in the aSiN films has been observed previously, and the existence of an N–H bond-related asymmetrical two-level system was postulated to account for the cryogenic mechanical loss. In this report, we studied an NH 3 -free PECVD process to reduce the N–H bond concentration and hence reducing the cryogenic mechanical loss. The N–H bond density of all films deposited by the NH 3 -free PECVD method was reduced to below the detection limit (