Using microfabricated devices to determine the fracture strength of materials

Abstract

AbstractMeasuring the fracture strength of brittle materials is often difficult, because alignment of the load with the test specimen is critical. Any off-axis misalignment of the load will produce fracture in an undesired mode. This report describes several micrometer-scale devices, in which the “load cell” is fabricated simultaneously with the fracture mechanics specimen, so that precise alignment is assured. The devices utilize the residual stresses contained within the materials, which are deposited onto Si substrates, to create the loading forces and determine the fracture strength. The devices are passive, meaning that the fracture results are obtained immediately upon fabrication, and no external loading is required. Stress concentrations are generated at notches, and the fracture strength is determined by the critical stress required for failure. A variety of devices have been fabricated from materials of interest for microelectromechanical systems devices, including polysilicon, silicon nitride, and aluminum, with both tensile and compressive residual stresses.