RF MEMS, which covers a very wide range of applications, can be confusing as far as performance and cost requirements are concerned. But all RF MEMS devices try to reach one common goal—integrated high-quality factor (Q) properties with good tuning capabilities. Early researchers took advantage of micromachining technologies to enhance the performance of traditional RF devices, such as on-chip inductors, capacitors, and quartz crystal and surface-acoustic-wave (SAW) devices. Besides high-Q design, good electrical isolation is key in such devices. Applications in this category include on-chip high-Q inductors, baluns, and micro-strip filters.
Another group of RF MEMS devices consists of mechanical structures switching or moving around the kilohertz frequency range, while the electronic signal they're conducting is at the gigahertz frequency level. This property is useful for switches, tunable filters, and variable capacitors. Tuning capability without degrading the Q and the reliability of the moving structure are two essential goals.
We've now reached the point where the device's mechanical vibration frequency is equal to the frequency of the electronic signal (see the figure). Such devices are often used for frequency selection and reference. Examples include film bulk acoustic resonators (FBARs), piezoelectric resonators, and MEMS resonators.