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This paper reviews the latest developments in microwave acoustic wave devices. After an introduction and brief history of bulk acoustic wave (BAW) and surface acoustic wave (SAW) devices, a review is given for guided SAWs and XBARs -two new technologies, which are promising for future 5G applications. Following this, we discuss recent simulation techniques, such as 3D finite element method (3D FEM) and simulation of nonlinearities, as well as filter synthesis. Next, a review on tunable and reconfigurable acoustics is given. Finally, we present the latest developments in microwave acoustics for millimeter-wave (mm-wave) operation as well as BAW oscillators.INDEX TERMS Bulk acoustic wave (BAW) devices, filter, filter-synthesis, front-end, high power, MTT 70th Anniversary Special Issue, multiplexer, nonlinearity, resonator, surface acoustic wave (SAW) devices.

Improving the quality factor (Q) of electromechanical resonators is of paramount importance for different applications, ranging from RF filtering to sensing. In this paper, we present a modified fabrication process flow for contour mode resonators to simultaneously obtain the (i) Q insensitive to the Si undercut geometry and (ii) in-phase reflectors for anchor loss control and Q optimization. To assess the potential of the reflector, we vary its distance from the resonator's anchor. This results in a periodic trend of Q when the distance between the anchor and the reflector changes. Further confirmation of the trend is obtained via a finite element (FE) model. Interestingly, when in the FE model, the step between consecutive reflectors is decreased by a factor of 6× with respect to the experimental step, we observe a fast modulation of Q, superimposed onto that seen experimentally. The origin of this fast modulation is likely the coexistence of waves with different wavelengths traveling through the released region. Our results show that the profile of the region undergoing Si undercutting (released area) can be easily set by design. Furthermore, engineering the introduced acoustic mismatch provides unprecedented control of anchor loss.