Design and fabrication of high performance RF MEMS devices

High performance transmission lines have been fabricated with a bulk-micromachining process on silicon. These lines are composed of high-aspect-ratio single-crystal-silicon beams coated with metal and show an attenuation of <0.2 dB/mm from DC up to 50 GHz. They also show a phase constant, normalized to the free space phase constant, very close to 1 with little dispersion. This is due to the fact that the substrate floor is coated with metal and the lines are suspended in air.; We have integrated these transmission lines with electrostatic comb-drive actuators that allow for the impedance tuning of the lines by varying the spacing between the beams that constitute them. We have used this impedance-tuning concept to make a high performance continuous phase shifter that operates in the 20% bandwidth about the design frequency of 44 GHz. This frequency is chosen because the device is designed to be integrated with RF power amplifiers in a parallel-plate array for the purpose of steering and amplifying signals around this frequency. The device has a measured phase shift of 38° at 45 V applied bias at the center frequency. The insertion loss is less than 1.8 dB over the 20% bandwidth about 44 GHz and the reflection is less than –10 dB over this frequency range.; We have also developed new processes for integrating our RF process with more conventional MEMS (MicroElectroMechanical Systems) processes. This integration will give us the flexibility to make a wide variety of RIF and conventional MEMS components simultaneously. Specifically, we have developed two new processes for selectively depositing metal on our very deep and compliant structures. We have also developed a novel procedure for controlling the heights of the different components of our comb-drive actuators. These technological developments should enable us to make integrated RIF MEMS systems that operate at very low biases.