Ultrafast Optoacoustics Reveals Intricate 3D Anisotropic Elasticity in Nanocrystalline Membranes

The mechanical performance of nanocrystalline membranes plays a critical role in determining the reliability and stability of advanced integrated circuit devices and nano-electromechanical systems. Conventional characterization techniques such as nano-indentation and micro-scale mechanical testing, while widely used, are generally destructive and incapable of resolving three-dimensional (3D) anisotropic properties. Targeting nanocrystalline membranes, we introduce an ultrafast optoacoustics-based approach for characterizing their 3D anisotropic elastic constants and thickness simultaneously. Gigahertz Lamb waves are thermoelastically generated to propagate in nanocrystalline copper membranes. Both non-propagating zero-group-velocity resonances and propagating modes are captured in high spatial and temporal resolution. The resonance and dispersion characteristics are employed to determine the thickness and anisotropic elastic constants of the membrane via a nontrivial multi-parameter inversion algorithm. Accordingly, substantially different elastic properties are observed by changing the substrate, especially the shear stiffness, which could be a result of the underlying microstructural mechanisms. The developed ultrafast optoacoustic approach provides a fully non-destructive and in situ metrology tool for characterizing parameters such as 3D anisotropic elastic constants and thickness of freestanding membranes. This capability not only enables a deeper understanding of the mechanisms of nanocrystalline materials but also facilitates improvements in the design and fabrication of reliable, high-performance next-generation micro- and nano-devices.