Seed-layer engineering of Mo/AlN piezoelectric stacked structures: Microstructure evolution and piezoelectric enhancement

This work systematically studies how AlN, Ti, TiN and Ti/TiN seed layers modify the microstructure and piezoelectric properties of Mo/AlN stacks, targeting high-quality, low-stress AlN films for high-frequency FBARs. Seed layers tune the texture and surface features of Mo bottom electrodes, further altering AlN’s orientation, defects and residual stress. The AlN seed layer delivers small lattice mismatch with Mo (110), inducing well-aligned Mo and perfect c-axis AlN growth with better crystallinity, milder stress and flatter surfaces. By contrast, Ti, TiN and Ti/TiN seeds cause severe interfacial mismatch and defect aggregation, damaging Mo texture, raising stress and weakening AlN piezoelectricity. PFM tests show the AlN seed lifts the effective d₃₃ from 0.42 pm/V to 1.1 pm/V. This work clarifies seed-layer interfacial modulation on Mo/AlN stacks and offers experimental support for optimizing high-performance AlN piezoelectric MEMS.