Mechanical and electrical design of a novel RF MEMS switch for cryogenic applications

RF MEMS switches operating at cryogenic temperature have not been reported so far. We have designed, fabricated and successfully tested a novel RF MEMS switch operating at 77K. This MEMS switch is composed of a switch plate suspended by two prestressed cantilever beams. These cantilever beams are made of two layers: 0.5 micronthick gold and 1.5 micron-thick polysilicon layers. The thermal mismatch between the gold and polysilicon layers causes the cantilever beams to warp in the direction of the gold layer. At room and cryogenic temperatures, the deformed beams press the switch plate onto the CPW line. As a result, the switch plate is normally in contact with the CPW line in the absence of a bias voltage. This corresponds to the OFF-state in the shunt switch design. An actuation electrode is placed on the side of MEMS structure away from the CPW. The switch beams act as the other electrode. When a DC voltage is applied, the switch beam and plate moves away from the CPW structure. This corresponds to the ON-state for the shunt switch design. For low operating temperature design, mechanical and electrical models are constructed to predict the switch behavior. The pre-stressed beams, the switch plate and the coupling structures in between are designed with the help of the thermal-mechanical model in CoventorWare, in order to achieve large out-of-plane switch displacement and reliable surface contact at cryogenic temperatures. The optimal switch design shows the out-of-plane displacement of 30 microns for the 500 micron-long beam. For the electrical design, the effect of the bias electrode on the switch insertion loss is modeled and discussed. For design reported, the measured insertion loss is optimized to be less than 0.8 dB up to the 40 GHz. The difference of the insertion losses between the switch and the same length CPW line is less than 0.3 dB.