Acoustic resonators above 100 GHz

Piezoelectric resonators are a common building block for signal processing because of their miniature size, low insertion loss, and high quality factor. As consumer electronics push to millimeter wave frequencies, designers must increase the operating frequency of the resonator. The current state-of-the-art approach to increase the operating frequency is to decrease the thickness of the piezoelectric film to shorten the acoustic wavelength or to use higher order modes. Unfortunately, maintaining high quality factors typically requires thicker piezoelectric layers. Thinner individual layers suffer from higher defect densities and increased relative losses from surface related damping, which degrade the electromechanical coupling and quality factor. While acoustic high order modes can also increase operating frequency, the electromechanical coupling rapidly decreases with increasing mode number. Here, we overcome these limitations by utilizing a piezoelectric stack of three layers of lithium niobate with alternating crystallographic orientations to preferentially support higher order modes and thereby enhance the electromechanical coupling without degrading the quality factor. Our approach improves the figure of merit of millimeter wave acoustic resonators by roughly an order of magnitude greater compared to state-of-the-art piezoelectric resonators above 60 GHz. This concept of alternating crystallographic orientations facilitates a path to develop millimeter wave resonators with high figures of merit, low insertion loss, and miniature footprints, enabling applications in millimeter wave signal processing and computing.