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 NH3 plasma enhanced chemical vapor deposition (NH3-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 count for the cryogenic mechanical loss. In this report, we studied an NH3-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 NH3-free PECVD method was reduced to below the detection limit (<1020 cm-3). The composition of the optimized film is SiN0.33H0.58 which shows the lowest extinction coefficient (1.21×10-5 @1550 nm), a high refractive index (2.68 @1550 nm), and excessively low stress (20.8 MPa), respectively. From 10 K to 120 K, cryogenic mechanical loss of the as-deposited SiN0.33H0.58 varies from 5 × 10-5 to 8 × 10-5 which is two to three times lower than that of the best NH3-PECVD silicon nitride previously obtained. No distinctive cryogenic loss peaks were found as well.

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