Confining Bulk Acoustic Modes by Planar Thin-Film Fabrication

Bulk acoustic wave (BAW) devices have attracted great attention recently because of their high operation frequencies, large number of modes, high Q factors, and simple fabrication process. Particularly in the field of quantum information science, BAW resonators have been used for preparing and controlling quantum states, coupling with superconducting circuits, and interfacing microwave and optical photons. However, confining BAW is not a simple task because of the design and fabrication challenges in large bulk device volumes. Although the fabrication technique for creating a 3D plano-convex geometry has been previously developed to transversely confine BAW modes, the process is complicated. Here, we present a very simple planar fabrication process in a thin film to realize strong lateral confinement of the BAW modes. By fabricating a disk with engineered edge geometry in a ~500-nm-thick piezoelectric film, we achieve effective transverse confinement of the BAW modes across ~mm scale substrate with high Q factors at cryogenic temperatures. Multiple resonant phonon modes are read out through the piezoelectric coupling with a superconducting resonator. The effectiveness of mode confinement is studied by comparing acoustic Q factors of BAW resonators with different disk geometries. Our device design and fabrication offer a simple and effective scheme for confining BAW, hence creating new opportunities for BAW devices in both classical and quantum applications.