On-chip testing and characterization of polysilicon thin films fracture mechanisms

The characterization and understanding of the mechanical response of freestanding nano-objects are of prime interest for fundamental research and for supporting reliability analysis of various Micro- and NanoElectroMechanical Systems (MEMS/NEMS). A versatile on-chip mechanical testing platform has been developed using MEMS-based structures in order to explore the mechanical properties of freestanding nano-objects. These structures take advantage of the mismatch stress present in a long silicon nitride beam to apply a deformation to a specimen beam attached to it owing to the release of the underneath sacrificial layer. The connection between the both beams is ensured by an overlap. At the specimen beam ends, dogbone shapes are located to concentrate the loading where the width is uniform. The in-plane dimensions of both, the actuator and the specimen beams, are varied to induce different strains and stress levels. Then, a large range of deformation can be applied to the specimen allowing the extraction of the strain-stress curve as well as the creep/relaxation behavior. Thousands of tests can be performed using a single processed wafer making statistical data analysis possible. The specimen stress and strain are extracted from the measured total displacement of the structure after its release using an analytical model based on beam theory. Recently, in order to improve the accuracy of the stress and strain estimation, additional geometrical and material features contributing to the total displacement of the structure have been implemented into the analytical model. These geometrical features are the specimen dogbones, the overlap between the actuator and specimen, and the under etching of the specimen and actuator fixed parts after the structures release. The presence of a stress gradient in the actuator constitutes an additional feature that must be taken into account. 3D finite elements simulations have been performed to evaluate the improved model precision and to determine what extent these additional features have to be considered or neglected. The technique has been used to study the deformation and fracture of phosphorus-doped polysilicon thin films. Two different film thicknesses have been tested: 240 and 40 nm. The Young’s modulus and the fracture strain have been extracted. For the two film thicknesses, the fracture strain increases with decrease specimen surface area. The thinnest film exhibits the lowest fracture strains. More surprising, post mortem analyses of broken samples reveal a change of fracture mode between the two film thicknesses, from transgranular to intergranular. The fracture strain difference and the fracture mode change observed between the two films thicknesses will be discussed in the light of surface roughness and microstructural characterization using the ACOM-TEM method