Micromachining for rf Communications
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Micromachining for
rf Communications Darrin J.Young
Introduction The increasing demand for wireless communication applications, such as cellular and cordless telephones, wireless data networks, and global positioning systems, motivates a growing interest in building miniature radio transceivers that can support multistandard capabilities. Such transceivers will greatly enhance the convenience and accessibility of various wireless services. At present, most wireless transceivers rely on a large number of discrete frequency-section components, such as voltage-controlled oscillators, bandpass filters, and solid-state switches, to perform the necessary signal processing. These off-chip devices occupy a majority of the transceiver area, severely restricting miniaturization. Increasing the level of integration to include these components is important for reducing the overall size and ultimately achieving multistandard capabilities. Micromachining offers a potential solution for integrating these frequency-selection devices on silicon substrates. Off-chip macroscopic components can be replaced by on-chip microelectromechanical versions. This article provides a brief overview of research progress made in this area.
tional VCO designs use discrete highperformance tuning capacitors, which are fabricated using materials and processing steps that differ substantially from standard microelectronics fabrication, to achieve low-noise performance. The off-chip components are ultimately responsible for the bulky dimension of the oscillator. Micromachining technology enables high-performance tunable capacitors integrated on silicon substrates together with CMOS (complementary metal oxide semiconductor) transistors, resulting in a significant size reduction. Figure 1 shows a scanning electron micrograph of such a capacitor.1 The device consists of a 200 m 200 m aluminum plate 1 m thick suspended in air 1.5 m above the bottom aluminum layer and anchored with four mechanical suspensions acting as springs. A dc bias voltage applied across the capacitors introduces an electrostatic pull-down force and consequent reduction of the air gap,
thereby increasing the capacitance value. Aluminum is chosen as the structural material for its low resistance, critical for minimizing the oscillator noise characteristics. The low processing temperature is attractive for integration with CMOS electronics. Four such capacitors connected in parallel have achieved a nominal capacitance value of 2 pF with a 16% tuning range under 3 V and a high quality factor above 60 at 1 GHz, which matches that of discrete counterparts. The micromachined tunable capacitors have been employed to realize a miniature rf VCO that achieves the low noise requirements for cellular telephone applications.2 This type of capacitor also has certain drawbacks. Special packaging methods are required to provide mechanical protection for the devices. Capacitor susceptibility to mechanical vibration or microphonics needs to be considered for oscillator performance optimization
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