High Frequency Switches
It is generally accepted that RF MEMS are a key technology for future telecommunication systems. The integration of MEMS into traditional radio frequency circuits result in systems with a new level of performance at decreased manufacturing costs. MEMS switches are basic elements of such RF systems. The switches can be of ohmic or capacitive type, see figure 1. With ohmic series switches the signal path can be closed or interrupted directly. Main advantage of such devices is an excellent performance over a very wide frequency range from dc to many tens of gigahertz. The upper limit depends mainly on the contact resistance. To minimize it, typically noble metals are used as contact materials. Capacitive switches are commonly arranged in a shunt configuration. Here the RF signal is blocked when the membrane is in the down-state while it can pass if the membrane is up. In contrast to ohmic ones the performance of capacitive switches is constricted in the low frequency range due to the limited ratio between up-state and down-state capacitancies. However, state-of-the-art RF design allows for their adaptation within narrow bands even at low gigahertz frequencies. A crucial advantage of capacitive switches is their particular suitability for an integration into standard IC processes. Another important point is the good power handling capability.
During the recent years at Fraunhofer ISIT a metal-micromachining process for capacitive RF MEMS switches has been developed. The device consist of a thin membrane suspended by thick anchoring structures. The special anchoring design ensure an effective compensation of thermally induced and intrinsic stresses. The drift of up-state capacitance as well as pull-in and release voltages can be kept very low over a wide temperature range. Moreover, the released MEMS structure can withstand prolongued heat treatments at up to 350°C without changes in the behaviour. The application of sputtered AlN as dielectric in the active area of the switch helps to suppress charge induced degradation effects. Robust phase shifters have been demonstrated showing reliable operation under hot switching conditions in harsh environments (up to 50 % humidity) as well as under high temperature variations even without hermetic packaging.