What Are Bulk Acoustic Wave Devices?

Bulk acoustic wave (BAW) devices are electromechanical components that convert between electrical signals and mechanical vibrations propagating through the bulk of a piezoelectric material, rather than along its surface. The core structure of a BAW resonator is a thin piezoelectric film, most commonly aluminum nitride (AlN), sandwiched between two metal electrodes. When an alternating voltage is applied across the electrodes, the inverse piezoelectric effect excites a thickness-mode acoustic resonance at a frequency determined by the film thickness and the acoustic velocity of the material. These resonances produce very high quality factors (Q) at frequencies in the range of 1 GHz to 10 GHz, making BAW devices the dominant technology for RF bandpass filters and duplexers in mobile handsets and wireless infrastructure.

The field draws on thin-film deposition technology, acoustic physics, and microelectronics fabrication. BAW devices emerged as a commercial technology in the early 2000s, driven by the need for RF filters that could operate at the frequencies of 3G cellular bands above 2 GHz, where surface acoustic wave (SAW) filters face increasing fabrication challenges from the shrinking wavelengths required.

Film Bulk Acoustic Resonators

A film bulk acoustic resonator (FBAR) is constructed by suspending a piezoelectric thin film membrane over an air cavity. The air gap provides acoustic isolation from the substrate, allowing the resonance to be highly confined and minimizing energy loss. FBAR technology is manufactured using surface micromachining processes compatible with standard CMOS facilities, which enables integration with RF circuits and allows wafer-scale production. The coupling coefficients and Q factors achievable with AlN FBARs are sufficient to meet the insertion loss and rejection specifications of LTE and 5G NR filter bands, and aluminum scandium nitride (AlScN) alloys have been explored to increase the electromechanical coupling for wideband filter applications.

Solidly Mounted Resonators

Solidly mounted resonators (SMRs) replace the air cavity beneath the piezoelectric film with a Bragg acoustic mirror: a stack of alternating high- and low-acoustic-impedance layers that reflects acoustic energy back into the resonating film. The Bragg mirror eliminates the suspended membrane, producing a mechanically rugged structure that can survive dicing and assembly operations without the handling fragility of FBAR membranes. SMRs are favored in applications where long-term mechanical reliability is a primary concern, such as automotive and industrial environments. Both FBAR and SMR devices can be assembled into coupled resonator filter (CRF) configurations, in which two resonators are stacked with a controlled acoustic coupling layer between them, enabling the synthesis of balanced (differential) filter responses for direct connection to differential circuit topologies.

Piezoelectric Materials and Frequency Scaling

The frequency of a BAW resonator is set by the thickness of the piezoelectric film, which shrinks as operating frequency increases. For the sub-6 GHz bands of 5G New Radio, AlN films of a few hundred nanometers are required, and deposition uniformity across a wafer determines filter frequency tolerance. A review published in Microsystems and Nanoengineering describes how AlScN alloys with scandium concentrations of 10 to 40 atomic percent provide a higher piezoelectric coupling coefficient than pure AlN, allowing filter bandwidths to widen to accommodate the wider channel allocations of 5G without proportional increases in insertion loss.

Applications

Bulk acoustic wave devices have applications in a range of wireless and sensing systems, including:

• RF duplexers and bandpass filters in 4G LTE and 5G mobile handsets
• Base station and small cell RF front-end filtering
• Timing references and oscillators for precision frequency control
• Mass-sensitive chemical and biological sensors using resonant frequency shifts
• Radar front-end signal conditioning in automotive and defense systems