  Here we present the first demonstration and in-depth study of unreleased
acoustic resonators in 14nm FinFET technology in the IEEE X band, which offer a
zero-barrier-to-entry solution for high Q, small footprint, resonant tanks
integrated seamlessly in advanced CMOS nodes. These devices leverage phononic
waveguides for acoustic confinement, and exploit MOS capacitors and transistors
inherent to the technology to electromechanically drive and sense acoustic
vibrations. Sixteen device variations are analyzed across thirty bias points to
discern the impact of phononic confinement, gate length, and termination scheme
on resonator properties. The limiting factor in FinFET resonator performance
among design variations tested is shown to be Back End of Line (BEOL)
confinement, with devices with acoustic waveguides incorporating Mx and Cx
metal layers exhibiting 2.2x higher average quality factor (Q) and peak
amplitude, with maximum Q increasing from 115 to 181 and maximum amplitude
scaling from 0.8 to 4.5 uS. A detailed analysis of biasing in the highest
performing device shows good fit with a derived model, which addresses the
velocity saturated piezoresistive effect for the first time in active resonant
transistors. Peak differential transconductance that is dominated by changes in
the silicon band-structure, as expected from an analysis that includes
contributions from the piezoresistive effect, electrostatic modulation, and
silicon bandgap modulation.
