Abstract
In 1916, Albert Einstein predicted the existence of gravitational waves based on general relativity. However, the existence of gravitational waves is not easy to be proved by experimental measurement. Until 1974, Russell Hulse and Joseph Taylor observed the Hulse-Taylor binary pulsar and found out the period shift of binary pulsar becomes larger every year. This observation is the first indirect evidence for the existence of gravitational waves [1]. Laser Interferometer Gravitational-Wave Observatory (LIGO) set up two large Michelson interferometers to measure the gravitational wave signals. On September 14, 2015, the first signal of gravitational waves was measured, which confirmed the existence of gravitational waves directly [2], It is a breakthrough in the history of science The University of Western Australia (UWA) designed an optical cavity [3], which is the external cavity of the detector using to increase the detector sensitivity of the laser interferometer. The Cat-flap structure is one of the components in the resonant cavity. The Cat-flap is composed by a silicon substrate coated with high reflective mirror on the both sides and this silicon is suspended by a silicon nitride film so that it is able to swing easily. This paper elaborate on the fabrication and problem discussion of Cat-flap structure. The first chapter describes the motivation of Cat-flap investigation. The second chapter describes the design and process steps of bare cat-flap in detail, and the key point of this section is how to take the Cat-flap out of the water. The author had tried the natural evaporation method, the critical point drying method and pinhead assisted evaporation method. Finally, the author used pinhead assisted evaporation method to take Cat-flap out of the water successfully. The third chapter shows the production process of double-side high reflective mirror coated cat-flap and discusses the problems in the process. Especially, when the water-jet guided laser cutting double-sided high reflection mirror, a cutting damage is generated on the backside mirror surface. Eventually, the damage is improved by dual laser cutting (UV laser and water-jet guided laser). Furthermore, this section also shows that high reflective mirrors are almost undamaged during the silicon nitride removed process in phosphoric acid. The corner undercutting phenomenon caused by KOH wet etching so that the high reflection mirror is dangled at corner. The fourth chapter focuses on the fabrication testing of structure optimization including how to improve the high reflection mirror corner undercutting in Chapter 3, and how to improve the cleanliness on the high reflective mirror surface and remove the excess silicon nitride film. The result shows that the cleanliness is improved and the excess silicon nitride film can be removed successfully. In order to avoid undercutting phenomenon, the area of the high reflection mirror coating is reduced on purpose. Nevertheless, the shadow-effect area of high reflective mirror is etched by KOH, because it cannot be protected by silicon nitride film, which induces the extra issues. In the future, we expect to improve the production process to avoid this phenomenon.