Growth of C-Axis Textured AlN Films on Vertical Sidewalls of Silicon Microfins

A fabrication process is developed to grow c-axis textured aluminum nitride (AlN) films on the sidewall of single-crystal silicon (Si) microfins to realize fin bulk acoustic wave resonators (FinBARs). FinBARs enable ultradense integration of high-quality-factor (Q) resonators and low-loss filters on a small chip footprint and provide extreme lithographical frequency scalability over ultra-and superhigh-frequency regimes. Si microfins with large aspect ratio are patterned and their sidewall surfaces are atomically smoothened. The reactive magnetron sputtering AlN deposition is engineered to optimize the hexagonal crystallinity of the sidewall AlN film with c-axis perpendicular to the sidewall of Si microfins. The effect of bottom metal electrode and surface roughness on the texture and crystallinity of the sidewall AlN film is explored.The atomic-layer-deposited platinum film with (111) crystallinity is identified as a suitable bottom electrode for deposition of c-axis textured AlN on the sidewall with c-axis orientation of 88.5° ± 1.5° and arc-angle of ~12° around (002) diffraction spot over film thickness. A 4.2-GHz FinBAR prototype is implemented showing a Q of 1574 and effective electromechanical coupling (k
_eff
²
) of 2.75%, when operating in the 3rd width extensional resonance mode. The lower measured Q and k2eff compared to simulations highlights the effect of granular texture of sidewall AlN film on limiting the performance of FinBARs. The developed c-axis textured sidewall AlN film technology paves the way for realization and monolithic integration of multifrequency and multiband FinBAR spectral processors for the emerging carrier aggregated wireless communication systems.