Method of forming suspended structure

A method of forming a suspended structure, comprising the steps of:

• providing a substrate;
• forming a first patterned photoresist on a front surface of the substrate;
• performing a thermal process to solidify the first patterned photoresist;
• forming a second patterned photoresist on the substrate and the first patterned photoresist, the second patterned photoresist having an opening to expose a part of the first patterned photoresist and a part of the substrate;
• forming a structural layer covering the first patterned photoresist, the second patterned photoresist, and the substrate;
• performing a lift-off process to remove the second patterned photoresist and a part of the structural layer;
• performing a thinning process on a back surface of the substrate;
• performing a first etching process on the back surface of the substrate and forming a through hole positioned below the first patterned photoresist;
• and
• performing a second etching process to remove the first patterned photoresist and form a suspended structure.