A method of forming an integrated circuit structure includes forming an opening in a substrate, with the opening extending from a top surface of the substrate into the substrate. The opening is filled with a filling material until a top surface of the filling material is substantially level with the top surface of the substrate. A device is formed over the top surface of the substrate, wherein the device includes a storage opening adjoining the filling material. A backside of the substrate is grinded until the filling material is exposed. The filling material is removed from the channel until the storage opening of the device is exposed.