What is a BAW Filter?

Bulk Acoustic Wave (BAW) Filters are compact, low-cost RF filters that can be used in a wide range of applications up to 6 GHz. Like SAW Filters, BAW filters also operate by converting electrical energy into acoustic or mechanical energy on a piezoelectric material. Since they can operate at higher frequencies, BAW filters are used for many of the new LTE bands above 1.9 GHz. They are also highly effective for LTE/Wi-Fi coexistence filters. Compared to SAW filters, BAW filters can operate at higher frequencies, are less sensitive to temperature changes, however, are more expensive.

A BAW filter has interdigital transducers (IDTs) to convert electrical energy to mechanical acoustic waves that travel through the surface of the piezoelectric material and are stored in it. But the acoustic waves propagate vertically as shown in figure below. 

BAW filters have a quartz crystal substrate with metal patches on the top and bottom side of the quartz. These metal patches excite the acoustic waves, which propagates vertically between them to form a standing acoustic wave. The resonant frequency is determined by the thickness of the slab and the mass of the electrodes.

To keep the waves from escaping into the substrate, there are two main configurations of BAW filters:

An acoustic Bragg reflector is created by stacking thin multiple layers of alternating materials with varying refractive index to prevent the waves from escaping into the substrate. This type of BAW filter is called as solidly-mounted resonator BAW or BAW-SMR device.

An alternative approach, called a film bulk acoustic resonator (FBAR), etches a cavity underneath the active area, creating suspended membranes.