RF MEMS Switch Overview
mems switches provide the ultra-low losses, high isolation, and high linearity of relays, but with the significant size, power consumption and cost advantages of high volume wafer fabricated solid state switches.
MEMS switches are also unique in that they are broadband (meaning they can operate over a wide frequency range). The unique attributes of TeraVicta’s RF MEMS switch significantly increase the battery life and/or range of any radio, including cell phones, wireless LANs, and PDAs.
TeraVicta’s switch is an electrostatically actuated cantilever-beam which consists of a metal beam suspended above a control electrode (gate) and a signal electrode (source).
The base of the beam is anchored to the source and the free standing tip of the beam is suspended above a third electrode (drain). When a sufficiently large voltage is applied to the gate relative to the source, the resultant electrostatic force pulls the beam toward the drain until the signal tip and the drain contacts come together. At that point, the switch is closed and a signal path is formed through the beam from the source to the drain. Because the actuation is electrostatic, no quiescent current is required to maintain closure.
A typical device using this technology is TeraVicta’s TT712-68CSP, a 7 GHz single-pole double throw (SPDT) RF micro-electromechanical systems (MEMS) switch that combines the RF characteristics of mechanically switched electrical pathways with the switching speed and reliability benefits of solid-state switches.
With low insertion loss, low distortion and low power consumption, the device is suited for applications such as Tx/Rx switching, antenna bank switching and source/detector multiplexing for test equipment.
The TT712-68CSP is composed of two MEMS switches. One side of each switch is tied to a common terminal on the device package. The other side of each switch is connected to a separate terminal so that the three pads form the electrical terminals of an SPDT switch.
To achieve high switching reliability, the MEMS switching elements are driven by 70 Vdc.
The two switches are engaged in a complimentary fashion to perform the SPDT switching function. Because the MEMS devices are true dc switches and because they are independently controllable, make-before-break or break-before-make switching action can be emulated.
The TT712-68CSP features an insertion loss of 0.1 dB at 1 GHz, a maximum dc on-resistance of 1 O, and a switching speed of less than 100 microseconds. The peak input signal power is 30 W, while the power consumption is 3 mW when operated with the TT6820QFN charge pump at 3 V.
MEMS are ideally suited for applications where high performance electro-mechanical, reed relay and other single function switching technologies are currently employed.
Additionally, MEMS chip scale package significantly improves RF performance and eliminates wire-bonds and leads.
MEMS switches can be electrostatically activated. This approach is ultra low power because typically only a nano-joule of power is required for each switching event and no power is consumed when the switch is in the closed or open state. This approach is far better suited to power sensitive applications than the more power hungry magnetic switch activation approach that is traditionally used by mechanical relays in such applications as central office telecommunication switches, test equipment and many aerospace applications. The semiconductor industry has conclusively demonstrated over its 50-year history that the most effective methodology for ensuring device reliability is to first understand basic failure mechanisms and then to design processes and devices that eliminate the failure mechanisms from the final product.
Product qualification then becomes almost just a formality, since the devices are reliable by design. Unfortunately, there is almost no overlap between the well-understood failure mechanisms that occur in semiconductors and those that occur in MEMS devices.
The products that are most similar to RF MEMS switches, and whose reliability has been extensively studied, are High Density Interconnect (HDI) printed wiring boards and conventional electro-mechanical relays. Even in these cases, there are major differences between their failure mechanisms and those of MEMS switches and relays.
For example, whereas conventional relays operate with high mechanical forces (contact and return) for short lifetimes (typically around one million cycles), MEMS switches operate with much lower forces for much longer lifetimes.
There is a key benefit to low contact forces: They dramatically increase contact life! However, nothing is free. The lower contact forces qualitatively change contact behaviour, especially increasing sensitivity to surface morphology and contaminants and the corresponding low return forces make the switches susceptible to sticking. The longer lifetimes demand a more thorough understanding both of the electrical contact and mechanical structure evolution, including methods for accelerating the mechanisms responsible for any performance degradation, to ensure reliable devices. One of the greatest challenges facing the MEMS industry is the need to understand the novel failure mechanisms associated with the various types of MEMS devices.
Attempting to ensure device reliability without fully understanding the basic failure mechanisms will likely result in a customer experiencing “unexpected” field failures. Using industry-standard practices, such as design and process Failure Mode and Effects Analyses (FMEA), TeraVicta is working to identify all of the potential failure modes of MEMS switch and relay devices and eliminate the root causes.
6-Oct-2006