Investigations on free-standing polysilicon beams in view of their application as transducers

Abstract

Abstract Free-standing polysilicon structures can be fabricated by lateral etching of a sacrificial silicon dioxide layer that has been sandwiched between the substrate and the polysilicon film. If the fabrication process of these three-dimensional structures is IC compatible, it is possible to realize integrated transducers such as resonant microbeam sensors, microswitch actuators, microvalves or electrostatic micromotors. Undoped and phosphorus-doped (P-doped) sacrificial silicon dioxide/polysilicon beam material combinations are investigated. The P-doped polysilicon beams show larger free-standing beam lengths than the undoped beams. Slight buckling is observed only for P-doped beams longer than 500 μm. The maximum length of all the other beams is limited by downward deflection to the substrate, which might be due to electrostatic forces induced by surface charges. A test chip containing IC-compatible capacitive microbeam transducers has been realized. Applications are switch actuators and resonant sensors. For a 300 μm long and 1 μm thick doubly supported polysilicon beam, the first resonance frequency is calculated to be about 100 kHz and an estimation of the capacitance change due to the beam deflection yields 8.6 fF. For the same structure, evaluation of the voltage necessary to switch the beam gives 13.5 V and measurement (in an SEM) about 15 V.