Mechanical characterization of polysilicon at the micro-scale through on-chip tests

The issue of mechanical characterization of polysilicon used in Micro Electro Mechanical Systems (MEMS) is discussed in this chapter. An innovative approach based on a fully on-chip testing procedure is described; three ad hoc designed electrostatically actuated microsystems are here used in order to determine experimentally the Young’s modulus and the rupture strength of thin and thick polysilicon. The first device is based on a rotational test structure actuated by a system of comb finger capacitors which load up to rupture a couple of tapered beams under bending in the plane parallel to the substrate. The second microsystem is based on a large plate with holes. It constitutes, with the substrate, a parallel plate capacitor moving in the direction orthogonal to the substrate itself. A couple of tapered beams placed at the center of the plate is loaded up to rupture in bending in the plane orthogonal to the substrate. The third device is again based on a system of comb finger capacitors which load up to rupture a notched specimen of a thick polysilicon layer. Experimental data are obtained which allow for the determination of Young’s modulus and rupture strength on the basis of an accurate data-reduction procedure relying on electromechanical numerical simulations. The rupture values are interpreted by means of the Weibull approach; statistical size effects and stress gradient effects are taken into account thus allowing for a direct comparison between the data obtained from the test structures.