Advances and future perspectives for super-high frequency, wide-band, and miniaturized acoustic wave filters

Radio frequency (RF) filters for communication have been developed rapidly, driven by new communication standards and the dramatic expansion of wide-range applications. Although they are currently playing crucial roles in applications such as mobile communication, space-to-ground communication, and the Internet of Thing, there are significantly stringent and challenging requirements demanded for their rapid and successful applications. Compared with conventionally adopted low-temperature co-fired ceramics, integrated passive device filters, and dielectric filters, acoustic wave filters have been regarded as the competitive choice, mainly attributed to their wide bandwidth, small size, and low insertion loss. This paper reviews the advances and outlines future perspectives of high frequency acoustic wave devices for RF communication, focusing on several critical issues including bandwidth, roll-off, frequency, power-handling, insertion loss, out-of-band rejection, tunability, and size/package. It is focused mainly on the extreme performance breakthroughs of RF acoustic wave filter, e.g., how to achieve acoustic devices with operating frequency above 8 GHz, bandwidth around 1 GHz, and quality factor exceeding 2000. Various principles, strategies, and technologies for achieving the superior performance of super-high frequency RF filters are discussed, e.g., applying advanced materials such as scandium-doped AlN or single crystals of AlN and LiNbO3, creating new topology structures such as hybrid filters, and generating new types of vibration modes of acoustic waves.