Abstract
Large Michelson interferometer set up by Laser Interferometer of Gravitational Wave Observatory (LIGO) is conducted to detect gravitational wave. Owing that the signal of gravitational wave is extremely weak, it’s necessary to reduce the noise caused by the interferometer to approach accurate result. According to the noise spectrum, the sensitivity of interferometer is mostly limited by coating Brownian noise and Quantum noise at approximately 100Hz. The coating Brownian noise, which is proportional to mechanical loss stated by fluctuation-dissipation theorem, comes from high reflective coating on the mirror while Quantum noise is laser related. In research of reducing noise, deducting the mechanical loss of optical thin films becomes critical topic as well. Based on our recent study, the mechanical loss of SiN0.40/SiO2 stacks is lower than “600℃ annealed 14.5% TiO2 - doped Ta2O5 / SiO2 stacks” which is the material used in current gravitational wave detectors. In order to eliminate the stress effect on silicon cantilever, a double-side coating process is designed. Due to the wet-etching process, conventional silicon cantilever is one-side roughened. Thus, the thin films are coated under the different surface condition with conventional silicon cantilever which the roughened side contributed extra mechanical loss. Our team fabricated a double-side smooth silicon cantilever from Silicon-on-Insulator (SOI), hence the thin films could be coated under both smooth surface condition. Discovered in our study, applying different gas flow rate ratio leads to unique IR absorption spectra of silicon nitride films by using Fourier Transform Infrared Spectroscopy (FTIR). From literatures, the peak at 2150 cm-1 wavenumber is given by Si-H bonds when the other one is at 3350 cm-1 wavenumber given by N-H bonds. These two peaks are observed in our IR spectra and we can evaluate the bond density of each bond by related equation. As SiH4 /NH3 gas flow rate ratio increasing, the Si-H bond density increases while the N-H bond density decreases. The SiN0.40H0.79 film is annealed in air for 30 min at 450℃. As a result of thermal anneal would induce damages, we investigate a three-layer structure which the SiN0.40H0.79 film is sandwiched between SiO2 films to improve the surface without appearance of damages and the mechanical loss of three-layer structure is reduced from 1.23×〖10〗^(-4) to 4.13×〖10〗^(-5).