Determining the mechanical properties of AlN films using micromechanical membranes

The mechanical properties of thin films can vary significantly due to variations in the deposition process. Monitoring these properties is essential when, e.g., engineering micro-mechanical devices. Established methods such as wafer bending and bulge testing provide only average values for the entire wafer or impose constraints on the sample fabrication process. Therefore, a method to efficiently characterize mechanical properties locally with simple sample fabrication is desirable. In this study, we determine the stress in direct-current magnetron-sputtered aluminum nitride films on prefabricated silicon nitride membranes by measuring the fundamental eigenfrequencies of the membranes and combining it with mechanical band structure simulations. We find that the stress, which is directly related to the strain, transitions from compressive to tensile as a function of the film thickness. Our simple and robust method allows in situ measurements that are compatible with existing nanofabrication techniques of, e.g., hybrid photonic circuits and can probe the stress locally over the chip.