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

Abstract: A fabrication process is developed to grow c-axis textured aluminum nitride (AlN) films on the sidewall of single crystal silicon (Si) micro-fins to realize fin bulk acoustic wave resonators (FinBAR). FinBARs enable ultra-dense 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 super-high-frequency regimes. Si micro-fins 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 micro-fin. 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 deg and arc-angle of ~12 deg around (002) diffraction spot over film thickness. 4.2 GHz FinBAR prototype is implemented showing a Q of 1,574 and effective electromechanical coupling (keff2) of 2.75%, when operating in 3rd width-extensional resonance mode. The lower measured Q and keff2 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 multi-frequency and multi-band FinBAR spectral processors for the emerging carrier aggregated wireless communication systems.