  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.
