Method of forming suspended structure

A method of forming a suspended structure is disclosed. Initially, a substrate is provided. A patterned first sacrificial layer and a patterned second sacrificial layer are formed on a front surface of the substrate. The second sacrificial layer has an opening exposing a part of the substrate and a part of the first sacrificial layer. A structural layer is formed covering the abovementioned sacrificial layers. Thereafter, a lift-off process is performed to remove the second sacrificial layer and define the pattern of the structural layer. A first etching process is performed on a back surface of the substrate utilizing the first sacrificial layer as an etching barrier and a through hole is formed under the first sacrificial layer. A second etching layer is performed to remove the first sacrificial layer and a suspended structure is thereby formed.