Morphological investigation of aluminium nitride films on various substrates for MEMS applications

Piezoelectric films, such as aluminium nitride (AlN), are of great interest for the fabrication of thin film bulk/surface acoustic resonators, where the growth parameters and the substrate material influence the morphological properties. Herein, AlN films were deposited using RF reactive sputtering on Si, SiO₂, metal (Al, Cu, Cr and Au) coated silicon, GaAs and InP substrates. C-axis (002) oriented AlN films were observed on most of the substrates, where the surface morphologies of the films grown on the 1.5 mm thick SiO₂ layer and GaAs substrate were found to be non-uniform. Moreover, AlN films deposited on the Cr electrode exhibit well textured film with fairly uniform grains, while the films deposited on other metal electrodes exhibit a granular type of structure with mixed small and large grains. After optimisation of growth parameters, silicon micromachining was performed by the wet chemical etching method. Suspended Cr-AlN-Cr-SiO₂ cantilevers of 20 μm in width were fabricated for futuristic microelectromechanical systems (MEMS) applications.