  Diamond-based microelectromechanical systems (MEMS) enable direct coupling
between the quantum states of nitrogen-vacancy (NV) centers and the phonon
modes of a mechanical resonator. One example, diamond high-overtone bulk
acoustic resonators (HBARs), feature an integrated piezoelectric transducer and
support high-quality factor resonance modes into the GHz frequency range. The
acoustic modes allow mechanical manipulation of deeply embedded NV centers with
long spin and orbital coherence times. Unfortunately, the spin-phonon coupling
rate is limited by the large resonator size, $>100~\mu$m, and thus
strongly-coupled NV electron-phonon interactions remain out of reach in current
diamond BAR devices. Here, we report the design and fabrication of a
semi-confocal HBAR (SCHBAR) device on diamond (silicon carbide) with $f\cdot
Q>10^{12}$($>10^{13}$). The semi-confocal geometry confines the phonon mode
laterally below 10~$\mu$m. This drastic reduction in modal volume enhances
defect center electron-phonon coupling. For the native NV centers inside the
diamond device, we demonstrate mechanically driven spin transitions and show a
high strain-driving efficiency with a Rabi frequency of
$(2\pi)2.19(14)$~MHz/V$_{p}$, which is comparable to a typical microwave
antenna at the same microwave power.
