DESIGN AND FABRICATION OF MEMS ELECTROSTATIC ACTUATORS

The research presented in this thesis is focused on the design and development of MEMS (Micro Electro Mechanical System) electrostatic actuators. Different kinds of electrostatic actuators are analyzed and presented . The study of torsional electrostatic actuator is carried out and the design of an micromirrors array for beam steering optical switching in a thick polysilicon technology is presented. The main advantage of these devices is that they are realized in a commercial surface micromachining technology (THELMA by STMicrolectronics) hence they are less expensive with respect to other solutions present in literature. For the torsional actuators, a novel semi-analytical model which allows prediction of the behaviour of the structures and takes into account the flexural deformation of the structure is presented. A very good agreement between the model, FEM simulation and experimental results has been obtained. The possibility of using another competitive surface micromachining technology (developed at IMEC) which uses poly silicon germanium as structural layer to realize the micromirrors is also investigated. Poly Silicon Germanium has the advantage that can be integrated on top of commercial CMOS. The development of optimized (low stressed) layers of poly silicon germanium has been addressed and some test micromirrors using this technology have been designed. Moreover an analysis of the levitation effect in electrostatic comb fingers atcuators is presented. The use of this actuation for vertical or torsional motion of micromachined structures is exploited. Two different levitational mechanical resonators have been designed and fabricated in THELMA. Because of the high thickness of the structural layer, classic springs shows several limitation, so specifically designed suspension springs (rotated serpentines), with better performance for high thickness, have been used. A full analysis of this kind of spring and a comparison with other springs are also presented. Finally a study of the dependence of the levitation force intensity on the geometric parameters of the actuators is performed using FEM simulations, and information about critical geometrical parameters in the design of operative levitational actuators is obtained. The devices are characterized and the obtained results are compared with FEM simulations.