Mechanical properties of microstructures: experiments and theory

Micromaterial's parameters, such as stiffness constants, critical stress values and fatigue limits are determined with mechanical tests on microfabricated samples. Single crystal silicon, nickel or nickel-iron alloys are investigated using newly developed testing devices. In the microsample tensile test, force and elongations are measured independently with high accuracy. 10(-5) N is the resolution of the force measurement with a precise balance. Elongations down to 10 nm are resolved by analysing light optical microscope images of the testing region with the Least Square Template Matching (LSM) algorithm. The stiffness constants obtained from the tensile test are in excellent agreement with the results of the vibration tests. In addition, Q-factor measurements are performed, using a phase-locked loop for precise frequency stabilization. The same technique is applied for fatigue experiments, in which crack growth is measured with a resolution of 15 nm. Using a new apparatus, torsion tests are performed: a microfabricated sensor is used to quantify the applied torque and the twist angle is determined from the deflection of a laser beam, measured with a photodiode. The theoretical analysis presented in this paper concerns two typical problems of microstructure design: the stress concentration at sharp notches in single crystal silicon microstructures and the process induced anisotropy in LIGA structures.