Abstract
In order to detect the gravitational wave (GW) directly to confirm the general theory of relativity from Albert Einstein and for future gravitational wave astronomy, the international Laser Interferometer Gravitational Wave Observatory (LIGO) utilize Michelson interferometer to detect GW signals. However, the signal of gravitational waves is extreme weak, so the system noise and the environment noise should be reduced as low as possible. In all of the background noise, the coating Brownian noise which caused by optical coating is a limiting source of thermal noise at 40-400 Hz. It depends on the properties of coating materials and surrounding temperature. Thus, investigating a low mechanical loss material to reduce the coating Brownian noise is necessary. In our previous work, the stress of the SiNx increased with value of x. Moreover, it was found that high stress SiNx film has lower mechanical loss. Even mechanical loss of the high stress coated cantilever was lower than that of the uncoated. This unusual phenomenon is referred to as "inversion" for high stress SiN0.87 film. The inversion appears in most of the bending modes, but none of the torsion modes. It may attribute to reduction of the thermoelastic loss of the silicon substrate, which were warped due to the tensile stress of the SiN0.87 film. Because the torsion modes of the silicon cantilever are not susceptible to thermoelastic loss. In this paper, we studied the fabrication, material properties and mechanical loss of SiN0.87 film which is deposited by PECVD in detail. Finally, we developed a formula by considering the energy dissipation, and then substitute the results of measurement into the formula to obtain the loss angle of high stress SiN0.87 film. Key word：silicon nitride film, stress, mechanical loss, double-side coating, Young’s modulus, refractive index, extinction coefficient.