Thin-film bulk acoustic resonator


A thin-film bulk acoustic resonator is a device consisting of a piezoelectric material manufactured by thin film methods sandwiched between two electrodes and acoustically isolated from the surrounding medium. FBAR devices using piezoelectric films with thicknesses ranging from several micrometres down to tenth of micrometres resonate in the frequency range of roughly 100 MHz to 20 GHz,. Any material like lead zirconate titanate or barium strontium titanate from the list of piezoelectric materials could act as an active material in an FBAR resonator. However aluminium nitride and zinc oxide are two most studied piezoelectric materials for FBAR realisations. As two compound material and compatibility with the silicon integrated circuit technology AlN has become the most widely used in commercial volume manufacturing of FBAR resonator based products like radio frequency filters, duplexers and power amplifier modules.
Doping or adding new materials like scandium are new directions to improve material properties of AlN for FBARs. Research of electrode materials like by replacing one of the metal electrodes with very light materials like graphene for minimising the loading of the resonator has been shown to lead better control of resonance frequency.
FBAR resonators can be manufactured on ceramic, sapphire, glass or silicon substrates. However silicon is the most common substrate due to its scalability towards mass manufacturing and compatibility with various manufacturing steps needed.
During early studies and experimentation phase of thin film resonators in 1967 cadmium sulfide was evaporated on a resonant piece of bulk quartz crystal which served as a transducer providing a quality factor of 5000 at the resonance frequency . This was an enabler for tighter frequency control, for needs to use higher frequencies and utilising FBAR resonators.
Most of the smartphones include year 2020 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 electronics and antenna system.

Structures

Currently there are two known structures of thin-film bulk acoustic wave resonators: free-standing and solidly mounted resonators . In a free-standing resonator structure air is used to free the resonator from the substrate/surrounding. Manufacturing of a free-standing resonator structure uses some typical manufacturing steps used in micro-electromechanical systems MEMS. In an SMR structure acoustic mirror providing an acoustic isolation is constructed between the resonator and the surrounding like the substrate. The acoustic mirror typically consists of odd total number of materials with alternating a high and low acoustic impedance. The thickness of mirror materials must also be optimized to the quarter wavelength for maximum acoustic reflectivity.
If needed resonator structures can be stacked e.g. built on top of each other like in some filter applications. However this approach increases the complexity of manufacturing.
Some performance requirements like tuning of the resonance frequency may also require additional process steps like ion milling, which complicates the manufacturing.
Because realising FBAR structures needs many precise steps simulation is actively used during the design phase to predict purity of resonance frequency and other performance. At early phase of the development basic FEM modeling techniques used for crystals were also applied and modified for FBARs,. Several new methods like a scanning laser interferometry were needed to visualise the functionality of the resonators and helping to improve the design to achieve purity of the resonance and wanted resonance modes.

Applications

In many applications temperature behavior, stability vs. time, strength and purity of the wanted resonance frequency are forming the base for the performance of the applications based of FBAR resonators. Material choices, layout and design of resonator structures are contributing to the resonator performance and the final performance of the application. Mechanical performance and reliability are determined by the packaging and structure of the resonators in the applications.
A common application of FBARs is radio frequency filters for use in cell phones and other wireless applications like positioning systems and modules. Such filters are made from a network of resonators and are designed to remove unwanted frequencies from being transmitted in such devices, while allowing other specific frequencies to be received and transmitted. FBAR filters can also be found in duplexers. FBAR filters are complementing surface acoustic wave filters in areas where increased power handling capability, and electrostatic discharge tolerance is needed. Frequencies more than 1.5 GHz are well suited for FBAR devices. Because FBARs can be manufactured on a silicon substrate they are well suited in high volume manufacturing supported by all development of semiconductor device fabrication methods. Future requirements of new applications like band width have effect on resonator performance and show development steps needed.
FBARs can be used by oscillators and synchronizers to replace a crystal in applications where frequencies more than 100 MHz and/or very low jitter is one of the performance targets.
FBARs can be used in sensor applications. For instance, when a FBAR device is put under mechanical pressure its resonance frequency will shift. Sensing of humidity and volatile organic compounds are demonstrated by using FBARs. A tactile sensor array may also consist of FBAR devices, and gravimetric sensing can be based on FBAR resonators.
FBARs can also be integrated with power amplifiers or low noise amplifiers to form either a module solution or a monolithic integrated solution on the same substrate with the related electronic circuitry. Typical module solutions are a power amplifier-duplexer module, or a low noise amplifier-filter module where FBAR and the related circuitry are packaged in the same package possibly on a separate module substrate.
FBARs can also 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.

Historical and Industrial Landscape

The use of piezoelectric materials for different applications begins in the early 1960’s at Bell Telephone Laboratories/Bell Labs, where piezoelectric crystals were developed and used as resonators in applications like oscillators with frequencies up to 100 MHz. Thinning was applied for increasing the resonance frequency of the crystals. However there were limitations of the thinning of crystals and new methods of thin film manufacturing were applied in the early 1970's for increasing accuracy of resonance frequency and targeting increasing manufacturing volumes.
TFR Technologies Inc founded in 1989 was one of the pioneering company in the field of FBAR resonators and filters mostly for space and military applications. The first product were delivered to customers in 1997. TFR Technologies Inc was in 2005 acquired by TriQuint Semiconductor Inc. Beginning of 2015 RF Micro Devices, Inc. and TriQuint Semiconductor, Inc. announced to merge to form Qorvo active providing FBAR based products.
HP Laboratories started a project on FBARs in 1993 concentrating in free-standing resonators and filters. In 1999 FBAR activity became part of Agilent Technologies Inc, which delivered year 2001 25000 FBAR duplexers for N-CDMA phones. Later in 2005 FBAR activity at Agilent was one of the technologies of Avago Technologies Ltd, which acquired Broadcom Corporation year 2015. In 2016 Avago Technologies Ltd changed its name to Broadcom Inc currently active to provide FBAR based products.
Infineon Technologies AG started to work with SMR-FBARs in 1998 concentrating in telecommunication filters for mobile applications. The first product was delivered to Nokia Mobile Phones Ltd, which launched the first SMR-FBAR based GSM three band mobile phone product 2001. Infineon's FBAR filter group was acquired by Avago Technologies Ltd 2008which later became part of Broadcom Inc.
After acquiring Panasonic's filtering business year 2016 Skyworks Solutions became one of the major players in BAW/FBAR devices additionally to Broadcom and Qorvo.
Additionally some other companies like Kyocera and Qualcomm are offering few thin film resonator based products.
Still many companies like Akoustis Technologies, Inc., Texas Instruments, several universities and research institutes are offering and studying to improve FBAR technology, its performance, manufacturing, advancing design capabilities of FBARs and exploring new application areas jointly with system manufacturers and companies providing simulation tools.