An integrated circuit structure includes a substrate having a top surface; a first conductive layer over and contacting the top surface of the substrate; a dielectric layer over and contacting the first conductive layer, wherein the dielectric layer includes an opening exposing a portion of the first conductive layer; and a proof-mass in the opening and including a second conductive layer at a bottom of the proof-mass. The second conductive layer is spaced apart from the portion of the first conductive layer by an air space. Springs anchor the proof-mass to portions of the dielectric layer encircling the opening. The springs are configured to allow the proof-mass to make three-dimensional movements.