Abstract
Since gravitational waves (GW) had been predicted by Albert Einstein in 1915, scientists have tried to direct measure it, but nevertheless no one has succeeded until September 14th, 2015, Laser Interferometer Gravitational-Wave Observatory (LIGO) used large-scale Michelson interferometer to measure gravitational wave signals. Two observatories of LIGO located in Hanford, WA, and Livingston, LA had measured the gravitational waves from coalescence of two binary black holes. For further improving sensitivity of the detectors toward practical application for gravitational waves astronomy, acitivities have been taken place in every aspect of the detector related technologies. A “cat-flap” structure proposed by University of Western Australia (UWA) is one of such activities. The aim of this paper is to study the fabrication methods for the cat-flap structure. It is a double-sided high reflector coated silicon pendant that is suspended by a silicon nitride film. At first, the high reflective mirror coatings will be coated on the silicon wafer, then SiN thin film is deposited for suspension. After this, we identify the window area through photolithography process before removing SiN film by using dry etching to make silicon surface in the window area be exposed, then we etch the sliced sample pieces from wafer. During KOH etching process, we use SiN film and ProTEK B3 on other area to prevent etching. After KOH etching is done, we will preserve the SiN film for suspension and remove the other part to form a complete Cat-flap resonator. Considering that the most difficult part of this process: etching the Cat-flap structure with KOH, we proposed two etching methods: Direct-formation and Well-etching. The two etching methods are used to define the pattern on the double-side SiN coated silicon wafer through the photolithography, then etching the SiN film in the window by dry etching to expose the silicon and using KOH to etch the silicon in the window area. These two methods can provide quick verifications whether the Cat-flap structure can successfully support the pendant below the SiN film. With the direct-formation method, the Cat-flap structure can be successfully formed by using silicon wafers with a thickness of 100μm and window widths between 30μm and 400μm after etching silicon, and we could observe the SiN stress-release process that causes the substrate to bend and deform during the etching. After etching, sample is lifted off the water, yet the structure is fragile because it is supported by thin film. The first attempt was to replace the KOH solution in the beaker with dilution to DI Water and then slowly withdraw the water to make the sample be exposed. But due to strong surface tension of water, the sample was broken; To our delight, we found that the sample could be successfully removed from the liquid through natural evaporation. But silicon <111> crystal plane on both silicon pieces (the pendant and the main body) form an optical contact on curving during the stress release such that two pieces stuck together. Another etching method: Well-etching is to define a SiN membrane on the silicon, then gradually removing one pair of the opposite side walls to form the suspended SiN out of the membrane. However, when the <110> plane was etched, the <111> plane was exposed simultaneously and that stopped the etching process. Therefore, the well-etching method was not successful.