I. Introduction
The ever-increasing demand for the sophisticated signal processing functions on-chip has driven the developments of the radio frequency (RF) acoustic devices [1], [2], [3], [4], [5]. Particularly, the RF front-end architectures for the latest mobile terminal devices must support an increasing number of frequency bands in 5G era, due to the compact spectrums and the narrower frequency band gaps, as schematically shown in Fig. 1. Therefore, RF acoustic filters with low insertion loss, steep skirts, and enhanced temperature stability are highly desirable. To address these requirements, surface acoustic wave (SAW) resonators with ultrahigh quality factor (Q) and stable temperature characteristics are strongly required.
Schematic of a typical RF front-end module, and the diagram of the frequency bands that the mobile terminal devices need to support in 5G era.