Abstract
Laser Interferometer Gravitational-Wave Observatory (LIGO) detected the gravitational wave came from two black holes merged in 2015. Weiss, Thorne and Barish received Nobel Prize because of their contribution. From now on we can observe astronomical phenomena using not only electromagnetic wave but also gravitational wave. Gravitational wave signal caused about 10-21 change to the arms of the interferometer. It was so small that LIGO couldn’t detect weaker signals came from other astronomical phenomena. This research is dedicated to enhancing the signal to noise ratio of LIGO. As the frequency of signal is about 100Hz, the noise is dominated by quantum noise and coating Brownian noise. Coating Brownian noise comes from the material coated on the mirrors of the interferometer. It is a kind of thermal noise which is proportional to mechanical loss and temperature according to Fluctuation-Dissipation Theorem. We can investigate this noise by measuring mechanical loss of materials. In the first part of this thesis, we fabricated SiNxHy thin films by Plasma Enhanced Chemical Vapor Deposition (PECVD). We coated it on cantilevers and measured its cryogenic mechanical loss. The loss of SiN0.40H0.79 was lowest and SiN0.87H0.93 was largest. We deduced that loss increased as silicon to nitrogen ratio increased. There was a loss peak in SiN0.87H0.93 at around 40K. We used two level system to explain its loss mechanism and calculated its activation energy. In the second part of this thesis, we fabricated 4-pair and 8-pair SiN0.40H0.79/SiO2 stacks by PECVD. Their cryogenic mechanical losses were similar. It was believed that SiN0.40H0.79/SiO2 interface would not cause too much extra cryogenic loss. There was a cryogenic peak in the stacks at around 40 K. We believed the loss peak was caused by SiO2. The mechanical loss of 4-pair and 8-pair stacks were 2.07×10-4 and 2.10×10-4 respectively at 675 Hz, 120K. SiN0.40H0.79/SiO2 stack was a potential material to be used for LIGO in the future.