Thin film acoustic resonator (FBAR)

Thin Film Bulk Acoustic Resonator (FBAR) or FBARs is a device consisting of a piezoelectric material sandwiched between two electrodes and acoustically de-coupled from the surrounding medium. FBAR devices using aluminium nitride piezoelectric with thicknesses ranging from tenths of micrometres to several micrometres resonate in the frequency bands of cell phones and other wireless applications. Filters made from a series of connected resonators (either in half-ladder, full-ladder, lattice or stacked FBAR topologies) are designed to remove unwanted frequencies from being transmitted in such devices, while allowing other specific frequencies to be received and transmitted.

FBARs were developed to provide smaller, more efficient radio frequency (RF) signal components for RF devices. They consist of a thin film of piezoelectric material spanning a cavity etched in a substrate (usually silicon). These devices can be made to be strongly resonant with narrow bandwidth at high frequencies (several gigahertz), consistent with wireless communication frequencies. This discriminates against unwanted frequencies to provide a cleaner signal.

Among several applications, such as radio frequency (RF) filters, duplexers, etc., these devices are widely used in cell phones for rejecting noise and sidebands in the received signal, prior to amplification. They have partially replaced an earlier technology based on surface acoustic wave (SAW) devices, due to smaller size and increased fabrication and operating efficiencies.