Research of wet etching in HF-based solution to release SOI-based gyroscope micro-structures

During SOI-based gyroscope fabrication, wet etching in hydrofluoric acid is utilized to etch buried oxide layer(BOX) and release micro-structures of the gyroscope. The BOX layer is beneath the micro-structures, and it's not easy to observe when the release process is finished, so a gradual release experiment is conducted to find the right etch time. Release experiment is carried out in two kinds of etchant, and the surroundings for the two types of etchant are the same: Etchant is contained in a plastic container which is located in water bath at a constant temperature of 20 °C. One etchant (Etchant A)consists of 5 volume 40% HF(HF concentration is 25%) and 3 volume DI water, and the other (Etchant B, buffered HF, also named BHF)) is composed of 60g ammonium fluoride, 30ml 40% HF and 90ml DI water. By experiments, it's found that etching rate of BOX layer in etchant A is from 2000nm/min to 2915nm/min; In etchant B, etching rate is between 830nm/min and 1250nm/min. By comparation, it's found that etching rate in etchant B(BHF)is slower than that in etchant A, and so etching in BHF is more easily controlled. It's also found that etching rate of BOX layer in release holes with 10μ side length is smaller than the etching rate in release holes with 30μ side length.