| Summary: | 碩士 === 國立清華大學 === 光電工程研究所 === 103 === According to Einstein’s general theory of relativity, it will produce gravitational wave when a object was accelerated. In order to meaure gravitaitonal wave directly, LIGO set up large-scale Michelson interferometers to observe whether gravitational wave exist or not. But the signal of gravitational wave is so weak to be detected that it must to reduce the background noise below to the gravitational wave signal. One of them, Coating Brownian noise is the urgent problem that is needed to be reduced. However, it’s very difficult to measure coating Brownian noise directly. Fortunately, it shows that coating Brownian noise is propotional to mechanical loss from fluctuation-dissipation theorem. so our goal is to search and investigate the films which behave low mechanical loss and to reduce mechanical loss as possible as we can then use for LIGO application
In this article, CVD process is used for amorphous silicon deposition. There are some advantages that why we choose CVD and this material below. In the aspect of fabrication: CVD process is well-established in semiconductor technologies and it behaves perfect large area (18” wafer) uniformity, this advantage is suitable for LIGO mirrors which size are 35cm diameter. In the aspect of material: Refractive index of amorphous silicon is 3.5 at 1550nm, this high index value makes it desirable for quarter-wave lens coating. On the other hand, mechanical loss of amorphous silicon deposited by ion‐beam sputter is 10-4 order at low temperature in literatures. We expect that mechanical loss of the amorphous silicon films deposited by CVD will be similar low to films deposited by ion‐beam sputter.
In the article, optical loss and mechanical loss of the amorphous silicon films which deposit by different temperatures were measured and analyzed. When utilizing PECVD to deposit amorphous silicon film on silicon wafer directly, it existed some hilllocks on the surface. In order to prevent this phenomenon, a buffer silicon dioxide film was deposited between siliocn wafer and amorphous siliocn film. Finally the surface quality improved.
The stress of amorphous silicon is compressive stress as deposition temperature from 200 oc to 400 oc. Total mechanical loss after coated amorphous silicon is lower than uncoated substrate. This result is similar to the result of high stress SiNx film coated on siliocn cantilever. We think that silicon cantilever is bent by high stress from film and the bending mechanism reduced some part of the mechanical loss of silicon cantilever first(probably thermo-elastic loss). So even after coated the mechanical loss of coated is still lower than unbending siliocn cantilever substrate.
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