SAW, BAW and the future of wireless
A high-end smartphone must filter the transmit and receive paths for 2G, 3G, and 4G wireless access methods in up to 15 bands, as well as Wi-Fi, Bluetooth and the receive path of GPS receivers. Signals in the receive paths must be isolated from one another. They also must reject other extraneous signals whose causes are too diverse to list. To do so, a multi-band smartphone will require eight or nine filters and eight duplexers. Without acoustic filter technology, it would be impossible.
SAW: Mature but Still Growing
Surface acoustic wave (SAW) filters are used widely in 2G receiver front ends and in duplexers and receive filters. SAW filters combine low insertion loss with good rejection, can achieve broad bandwidths and are a tiny fraction of the size of traditional cavity and even ceramic filters. Because SAW filters are fabricated on wafers, they can be created in large volumes at low cost. SAW technology also allows filters and duplexers for different bands to be integrated on a single chip with little or no additional fabrication steps.
The piezoelectric effect that exists in crystals with a certain symmetry is the ‘motor’ as well as the ‘generator’ in acoustic filters. When applying a voltage to such a crystal, it will deform mechanically, converting electrical energy into mechanical energy. The opposite occurs when such a crystal is mechanically compressed or expanded. Charges form on opposite faces of the crystalline structure, causing a current to flow in the terminals and/or voltage between the terminals. This conversion between electrical and mechanical domains happens with extremely low energy loss, achieving exceptional efficiency of 99.99% in both directions.
In solid materials, alternating mechanical deformation creates acoustic waves that travel at velocities of 3,000 to 12,000 m/s. In acoustic filters, the waves are confined to create standing waves with extremely high quality factors of several thousand. These high-Q resonances are the basis of the frequency selectivity and low loss that acoustic filters achieve.
In a basic SAW filter (see Figure 1), an electrical input signal is converted to an acoustic wave by interleaved metal interdigital transducers (IDTs) created on a piezoelectric substrate, such as quartz, lithium tantalite (LiTaO3) or lithium niobate (LiNbO3). Its slow velocity makes it possible to fit many wavelengths across the IDTs in a very small device.
SAW filters, however, have limitations. Above about 1 GHz, their selectivity declines, and at about 2.5 GHz their use is limited to applications that have modest performance requirements. SAW devices also are notoriously sensitive to temperature changes: the stiffness of the substrate material tends to decrease at higher temperatures and acoustic velocity diminishes.
An alternative approach is to use temperature-compensated (TC-SAW) filters, which include overcoating of the IDT structures with layers that increase stiffness at higher temperatures. While an uncompensated SAW device typically has a temperature coefficient of frequency (TCF) of about -45 ppm/oC, TC-SAW filters reduce this to -15 to -25 ppm/oC . However, because the process doubles the number of required mask layers, TC-SAW filters are more complex and thus more expensive to manufacture, but still less expensive than bulk acoustic wave (BAW) filters.
While SAW and TC-SAW filters are well suited for up to about 1.5 GHz, BAW filters deliver compelling performance advantages above this frequency (see Figure 2). BAW filter size also decreases with higher frequencies, which makes them ideal for the most demanding 3G and 4G applications. In addition, BAW design is far less sensitive to temperature variation even at broad bandwidths, while delivering very low loss and very steep filter skirts.
Unlike SAW filters, the acoustic wave in a BAW filter propagates vertically (see Figure 3). In a BAW resonator using a quartz crystal as the substrate, metal patches on the top and bottom side of the quartz excite the acoustic waves, which bounce from the top to bottom surface to form a standing acoustic wave. The frequency at which resonance occurs is determined by the thickness of the slab and the mass of the electrodes. At the high frequencies in which BAW filters are effective, the piezo layer must be only micrometers thick, requiring the resonator structure to be made using thin-film deposition and micro-machining on a carrier substrate.
To keep the waves from escaping into the substrate, an acoustic Bragg reflector is created by stacking thin layers of alternating stiffness and density. The result of this approach is called a solidly-mounted resonator BAW or BAW-SMR device (see Figure 4). An alternative approach, called a film bulk acoustic resonator (FBAR), etches a cavity underneath the active area, creating suspended membranes.
Both types of BAW filters can achieve very low loss because the density of their acoustic energy is very high and the structures trap acoustic waves very well. Their achievable Q is higher than any other type of filter of reasonable size employed at microwave frequencies: 2,500 at 2 GHz. This results in superb rejection and insertion loss performance even at the critical passband edges.