MEMS Motion Sensors Based on the Variations of the Fringe Capacitances

In this paper, a MEMS motion sensor is described which uses fringe field capacitances as sensing elements. Each capacitance basic cell is formed using thin strips of standard electrical paths, running side by side, and close one another in a plane parallel to the substrate: in this way, the capacitance between the paths is mainly formed by the fringe field streamlines. A grounded seismic mass, suspended in the air volume where the electric fringe field streamlines are concentrated, determines changes in the capacitance value when subject to an external force that makes it move. With respect to parallel plates capacitive sensors, this implementation has the advantage to be more immune from the pull-in instability. A test device is built using an industrial surface micromachining process. The experimental results show a sensitivity close to 1 fF of capacitance variation per unit of earth acceleration for a device taking up an area of (430 mu m)(2). A detailed analysis performed through FEM simulations predicts possible consistent sensitivity improvements.