AlN-on-Diamond FBAR for Coherent Acoustic Control of NV Centers*

Spin systems in negatively charged Nitrogen-Vacancy (NV) centers in diamond have been shown to be promising platform for quantum information science and sensing at room temperature. Spin-phonon interactions have been demonstrated to drive magnetically forbidden transitions coherently using high overtone bulk acoustic resonators (HBARs) with planar and solid immersion lens (SIL) geometry for improved strain coupling to the NV spin manifolds. In this work we demonstrate surface micromachining of diamond integrated with AlN transducers to realize thin film bulk acoustic wave resonators (FBARs) that are free standing mechanical resonators for enhanced phonon mode confinement. The AlN/Diamond FBARs are fabricated thinner than HBARs while retaining uniform in plane stress profile for a given depth location. Our microfabrication technique with double side processing on diamond allows desired thickness and lateral apodization of diamond plates that require no ion implantation. We also confirm that the spin dephasing time (T₂*) is not affected by the microfabrication processing. The impedance of the packaged diamond resonator is designed to match the 50 Ω RF source for maximum power transfer strain generating and microwave driving. We provide efficient strain coupling to spin systems in diamond NV centers via acoustic impedance matching of transducer layers to the thin released diamond membrane while generating stress wave confined in the diamond layer by utilizing 2^nd order harmonic of the resonator.