Properties of a MEMS electric field sensor with optical readout

Abstract

The subject of this thesis is a MEMS electric field sensor. It makes use of the electrostatic induction of two conducting domains of the MEMS chip in the electric field. The induced charges effect an electric force between the domains, leading to a displacement of an illuminated grid built into one domain. The displacement leads to a modulation of the light flux through the grid, which is measured with an optical readout. In the scope of this thesis, various properties of the sensor are studied by means of analytical models, simulations and measurements. By combining the results of the different investigation methods, a deeper understanding of the operating principle of the sensor is obtained. Two analytical models are developed, yielding a closed-form expression for the electric force as a function of the electric field and the geometrical parameters. The models help to get a better understanding of the underlying processes associated with the electrostatic induction. The equation of motion for the displaced grid is set up and solved. The solution describes the response of the grid to the electric force caused by the induced charges. The calculated displacement is used to determine the output signal as a function of the electric field and the properties of the grid. Using FEM simulations, it has been calculated how the dimensions of the MEMS chip and the domains, affect the electric force responsible for the displacement of the grid. The simulation results are compared with the results of analytical models, assessing their reliability. The behaviour of the sensor for different orientations with respect to the gravitational force and the electric field is measured and investigated with FEM simulations. An existing setup can be used to determine the influence of the gravity on the output signal, however the direction between the MEMS chip and the electric field can not be changed. Therefore, a new measurement setup has been designed, to alter the orientation of the horizontally placed chip with respect to the electric field.