Thin-film bulk acoustic resonator
Industrial application areas of thin film bulk acoustic resonators include high-frequency signal filtering (e.g. for mobile telecommunication devices), crystal replacements, energy harvesting, sensing, sound emission (e.g. in hearing aids) and as part of mechanical qubits.
For high frequency purposes like filtering of signals the energy conversion efficiency is the most important item and therefore longitudinal (L) waves are favored and targeted to be used.
For sensing and actuation purposes the structural deformation might be more important than energy conversion efficiency and shear-mode wave excitation will be the target of the manufacturing of the piezoelectric film.
Despite the lower electromechanical coupling coefficient compared to zinc oxide, aluminum nitride, with a wider band gap has become the most used material in industrial applications, which require a wide bandwidth in signal processing.
With the development of thin film technologies it was possible to keep the Q factor high enough, leave out the crystal and increase resonance frequency.
Most smartphones in 2020 include at least one FBAR-based duplexer or filter and some 4/5G products may even include 20–30 functionalities based on FBAR technology mainly due to the increased complexity of radio frequency front end (RFFE, RF front end) electronics – both receiver and transmitter paths – and the antenna/antenna system.
Sensing and actuation is a developing area for FBAR resonators and structures based on them like in micro-mirror displays (DMD)s,[17] as well as energy harvesting by utilizing nanogenerators.
At an early phase of the development, basic finite element method (FEM) based modelling techniques that are used for crystals can also be applied and modified for FBARs.
Future requirements of new applications like filtering bandwidth with steep stopband attenuation and lowest possible insertion loss have effects on resonator performance and show development steps needed.
[29] FBARs can also be used in oscillators and synchronizers to replace a crystal/crystals in applications where frequencies more than 100 MHz and/or very low jitter is one of the performance targets.
Although monolithic integrated of FBARs on the same substrate with the electronic circuitry like CMOS has been demonstrated it requires several additional process steps and mask layers on top of IC technology increasing the cost of the solution.
FBARs can be integrated in complex communication like SimpleLink modules for avoiding area/space requirements of an external, packaged crystal.
Therefore, FBAR technology has a key role in electronics miniaturisation specifically in applications where oscillators and precise high performance filters are needed.
TFR Technologies Inc., founded in 1989, was one of the pioneering companies in the field of FBAR resonators and filters mostly for space and military applications.
In early 2015, RF Micro Devices (RFMD), Inc. and TriQuint Semiconductor, Inc. announced a merger to form Qorvo active providing FBAR-based products.
Infineon Technologies AG started to work with SMR-FBARs in 1999, concentrating in telecommunication filters[35] for mobile applications.
Infineon's FBAR (BAW) filter group was acquired by Avago Technologies Ltd 2008 which later became part of Broadcom as described before.
After acquiring Panasonic's filtering business in 2016 Skyworks Solutions became one of the major players in BAW/FBAR devices additionally to Broadcom and Qorvo.
Additionally after acquiring rest of RF360 Holdings in 2019 Qualcomm and Kyocera are offering thin film resonator based products like RFFE modules and separate filters.
Still many companies like Akoustis Technologies, Inc. (founded in 2014),Newsonic,[37] Saiwei Electronics,[38][39] Texas Instruments (TI), several universities and research institutes are offering and studying to improve FBAR technology, its performance, manufacturing capacity, advancing design capabilities of FBARs and exploring new application areas jointly with system manufacturers and companies providing simulation tools (Ansys, Comsol Multiphysics, and Resonant Inc.
