Multi-step process for forming high-aspect-ratio holes for MEMS devices

A method of forming an integrated circuit structure, the method comprising:

• providing a substrate;
• forming an opening in the substrate, wherein the opening extends from a top surface of the substrate into the substrate;
• filling a first layer of a photo resist into the opening;
• removing a portion of the first layer of the photo resist on the surface of the substrate;
• baking the first layer of the photo resist;
• filling a second layer of the photo resist into the opening and over the first layer of the photo resist, wherein at a time the second layer of the photo resist is filled into the opening, the second layer of the photo resist comprises more solvent therein than the first layer of the photo resist at a time the first layer of the photo resist is filled into the opening;
• removing an additional portion of the second layer of the photo resist on the surface of the substrate;
• baking the second layer of the photo resist;
• forming a micro-electro-mechanical-system (MEMS) device over the top surface of the substrate, wherein the MEMS device comprises an opening adjoining the first layer of the photo resist;
• grinding a backside of the substrate until the photo resist is exposed;
• and
• removing the photo resist from the opening.