Vibrating RF MEMS for Next Generation Wireless Applications
Introduction
Today's wireless transceivers are generally designed under a near mandate to minimize or eliminate, in as much as possible, the use of high-Q passives. The reasons for this are quite simple: cost and size. Specifically, the ceramic filters, SAW filters, quartz crystals, and now FBAR filters, capable of achieving the Q's from 500-10,000 needed for RF and IF bandpass filtering, and frequency generation functions, are all off-chip components that must interface with transistor functions at the boardlevel, taking up a sizable amount of the total board volume, and comprising a sizable fraction of the parts and assembly cost.
Pursuant to reducing the off-chip parts count in modern cellular handsets, direct-conversion receiver architectures have removed the IF filter, and integrated inductor technologies are removing some of the off-chip L's used for bias and matching networks. Although these methods can lower cost, they often do so at the expense of increased transistor circuit complexity and more stringent requirements on circuit performance (e.g., dynamic range), both of which degrade somewhat the robustness and power efficiency of the overall system. In addition, the removal of the IF filter does little to appease the impending needs of future multi-band reconfigurable handsets that will likely require high-Q RF filters in even larger quantities— perhaps one set for each wireless standard to be addressed.
Fig. 1 compares the simplified system block diagram for a present-day handset receiver with one targeted for multiband applications, clearly showing that it is the high-Q RF filters, not the IF filter, that must be addressed. In the face of this need, an option to reinsert high Q components without the size and cost penalties of the past would be most welcome.
Recent advances in vibrating RF microelectromechanical systems ("MEMS") technology that have yielded on-chip resonators operating past GHz frequencies with Q's in excess of 10,000, may now not only provide an attractive solution to the above, but might also enable a paradigm-shift in transceiver design where the advantages of high-Q (e.g., in filters and oscillators) are emphasized, rather than suppressed. In particular, like transistors, micromechanical elements can be used in large quantities without adding significant cost. This not only brings more robust superheterodyne architectures back into contention, but also encourages modifications to take advantage of a new abundance in low loss ultra-high-Q frequency shaping at GHz frequencies. For example, an RF channelselect filter bank may now be possible, capable of eliminating not only out-of-band interferers, but also out-of-channel interferers, and in so doing, relaxing the dynamic range requirements of the LNA and mixer, and the phase noise requirements
Fig. 1: Expected progression of transceiver front-end architectures when vibrating RF MEMS (shaded) are employed. (a) Present-day superheterodyne. (b) Multi-band architecture, where the number of RF filters could reach >10. (c) Highly reconfigurable, low-power, RF channelselect architecture, where the number of RF filters could reach >100.
Fig. 2: Cross-sections (a) immediately before and (b) after release of a surface-micromachining process done directly over CMOS.
Fig. 3: SEM of a fully integrated watch oscillator that combines CMOS and MEMS in a single fully planar process.
of the local oscillator, to the point of perhaps allowing complete transceiver implementations using very low cost transistor circuits (e.g., perhaps eventually even organic circuits).
MEMS Technology
There are now a wide array of MEMS technologies capable of attaining on-chip micro-scale mechanical structures, each distinguishable by not only the type of starting or structural material used (e.g., silicon, silicon carbide, glass, plastic, etc.), but also by the method of micromachining (e.g., surface, bulk, 3D growth, etc.), and by the application space (e.g., optical MEMS, bio MEMS, etc.). For the present focus on portable communications, MEMS technologies amenable to low capacitance merging of micromechanical structures together with integrated transistor circuits are of most interest. In this regard, surface micromachining technologies, where structural materials are obtained exclusively via deposition processes, are perhaps most applicable to the present discussion.
Fig. 2 presents key cross-sections describing a polysilicon surface micromachining process done directly over silicon CMOS circuits. As shown, this process entails depositing and patterning films above the CMOS circuits using the same equipments already found in CMOS foundries until a cross section as in
Fig. 2(a) is achieved. Here, the structural polysilicon layer has been temporarily supported by a sacrificial oxide film during its own deposition and patterning. After achieving the crosssection of Fig. 2(a), the whole wafer is dipped into an isotropic etchant, in this case hydrofluoric acid, which attacks only the oxide sacrificial layer, removing it and leaving the structural polysilicon layer intact, free to move in multiple dimensions.
Fig. 3 presents the SEM of a watch oscillator that combines a 16 kHz folded-beam micromechanical resonator with sustaining CMOS transistor circuits using this very process flow, but with tungsten as the metal interconnects in order to accommodate 625o structural polysilicon deposition temperatures.
Cesar Augusto Suarez
CI 17394384
CAF
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