MEMS for frequency control

Abstract

Due to their need for high frequency selectivity and low noise frequency manipulation, portable wireless communication transceivers continue to rely on high-Q, off-chip resonator technologies that must interface with transistor electronics at the board-level, thus contributing to the substantial percentage (often more than 50%) of portable transceiver area taken up by board-level, passive components. Recent advances in IC-compatible micro-electroechanical system (MEMS) technologies that make possible micro-scale, mechanical circuits capable of low-loss switching, filtering, mixing, and frequency generation, now suggest methods for board-less integration of wireless transceiver components. In fact, given the existence already of technologies that merge micromechanics with transistor circuits onto single silicon chips, single-chip transceivers may eventually become possible, perhaps using alternative architectures that maximize (rather than minimize) the use of passive, low-loss, micromechanical circuits to enhance robustness and reduce power consumption for portable applications. In particular, given that vibrating micromechanical resonator technologies have already achieved on-chip Q's in excess of 10,000 at 1 GHz at room temperature—something previously not possible—even RF channel-selection becomes plausible, where channels are selected right at RF and inteferers removed before they reach any demodulation circuits. This course presents a detailed overview of the micromechanical circuits and associated technologies expected to play key roles in reducing the size and power consumption of future communication transceivers. It begins with a review of transceiver operation, emphasizing the need for low-loss and high-Q in both the transmit and receive paths, and identifying the functional blocks that stand to benefit most from MEMS implementation. Detailed coverage of the operation, design, and fabrication of the micromechanical devices most useful for communication applications then follows, including expositions on high-Q micromechanical resonators, filters, and mixer-filters; low-loss micromechanical switches; and medium-Q micro-machined inductors and tunable capacitors. Receiver architectures are then proposed that best harness the tiny size, zero dc power consumption, and ultra-high-Q (or low-loss) of micromechanical resonator and switch circuits. The course concludes with discussions pondering micro-scale physical phenomena that may eventually limit the scalability, and hence application range, of RF MEMS.