nce benefits. A MEMS resonator is a few tenths of a millimetre across. A quartz crystal is a few millimetres across, one hundred times the surface area. MEMS resonator performance is a function of device geometry. As CMOS geometries reduce, the electrode gap reduces and the sense signal and signal-to-noise ratio will improve, giving the oscillators a better phase noise and jitter specification. As MEMS resonators get smaller they get less expensive. As quartz crystals get smaller they get more expensive. MEMS resonators therefore become increasingly attractive over time. 22 SAW filters use semiconductor processes to produce combed electrodes that are a metallic deposit on a piezoelectric substrate. SAW devices are used as filters, resonators and oscillators and appear both in the RF and IF (intermediate frequency) stages of present cellular handset designs. SAW devices are now being joined by a newer generation of devices known as BAW (bulk acoustic wave) devices. In a SAW device, the surface acoustic wave propagates, as the name suggests, over the surface of the device. In a BAW device, a thin film of piezoelectric material is sandwiched between two metal electrodes. When an electric field is created between these electrodes, 22 A white paper on MEMS resonators is available via this URL http://www.sand9.com/white-papers/ Making Telecoms Work an acoustic wave is launched into the structure. The vibrating part is either suspended over a substrate and manufactured on top of a sacrificial layer or supported around its perimeter as a stretched membrane, with the substrate etched away. The devices are often referred to as film bulk acoustic resonators (FBAR). The piezoelectric film is made of aluminium nitride deposited to a thickness of a few tens of micrometres. The thinner the film, the higher the resonant frequency. BAW devices are useful in that they can be used to replace SAW or microwave ceramic filters and duplexers in a single component. BAW filters are smaller than microwave ceramic filters and have a lower 