MEMS-based Circuits and Systems for Wireless Communication

Miniature bulk acoustic wave (BAW) resonators are components that exhibit very interesting properties for communication systems, as confirmed by their extensive use nowadays in front-end filters for mobile phones. This chapter reviews the technology enabling the fabrication of these devices and the different models used to describe their electrical performances. Finally, a simple empirical model, mainly based on geometrical parameters, is proposed. It does not require massive computing power, but it can nevertheless predict very accurately the main characteristics of the thin-film BAW resonators.

This chapter focuses on NEM devices and their technologies and applications for information processing with special emphasis on NEM switches and resonators. Top-down and bottom-up fabrication of NEMS are presented. Then NEM switches for ultralow-standby-power digital applications and their fabrication and design challenges are described; silicon or metal nanowires and carbon nanotube switches are able to achieve interesting figures of merit for nanoscale electromechanical information processing (logic and memory functions). The chapter continues with NEM resonators; the extreme scaling requires active detection such as resonant gate/body transistors. Finally, the concept of NEMS-based radio is introduced and its promises and challenges are discussed.

Piezoelectricity and longitudinal elastic wave propagation constitute the basic physical mechanisms involved in the classical bulk acoustic wave resonator. Innovative acoustic MEMS will rely on other types of elastic waves (shear waves, guided waves in free plates or plates bonded on substrate, waves in periodic media) or transduction mechanisms (electrostriction) which are described in the first section of this chapter. Operation and characteristics of emerging acoustic RF MEMS devices, such as shear and guided wave resonators, tunable resonators and phononic crystal-based resonators and filters, are reviewed in the other sections.

For the last few years, one of the main challenges in circuit design has been the integration of frequency references in applications where phase noise requirements are very stringent. To overcome the usual limitation of (Bi)CMOS integrated circuits (ICs) phase noise, mainly due to the low Q-factor of standard integrated passive devices (R, L, C) inherent to low resistivity substrate, a solution has been to use BAW resonators. Indeed, thin film BAW resonators based on piezoelectric material, generally AlN or ZnO, sandwiched between two metallic electrodes, exhibit a high Q-factor, can handle high power, and can operate at high frequencies (above 10 GHz Hz), while keeping a small size and being compatible with (Bi)CMOS IC processes. The very first oscillators using FBAR and SMR resonators were designed with separately wire-bonded resonators connected to the IC circuit. This chapter deals with the world premiere realization of two 5-GHz FBAR-based oscillators, where the FBAR is directly integrated above the IC with some further process steps, compatible with (Bi)CMOS. A single-ended and a balanced version were designed. The circuits were implemented in a 0.35-μm SiGe BiCMOS process from AMI Semiconductor. From the obtained results, we show that post-processing the FBAR directly over the IC eliminates much of the parasitics and modelling issues associated with bondwires. Furthermore, it reduces the circuit area. The single-ended and balanced oscillators are based on the Colpitts configuration and achieve respectively a state-of-the-art phase noise performance (at the time of design) of − 117. 7 d b c Hz and − 121 d b c Hz at 100 K Hz offset from the 5.4-GHz carrier frequency. The balanced version allows direct driving of balanced dividers and mixers without the need of a single-ended to balanced converter. Some comparisons are also made with standard LC balanced oscillators.

Aggressive silicon technology scaling, combined with immense unexplored MEMS potential, leads to new advancements in the field of low-power, high-performance RF-MEMS codesigned architectures. This chapter introduces the approach, theory, and design of an important block in RF transceivers: a quadrature voltage-controlled oscillator (QVCO) stabilized with bulk acoustic wave (BAW) resonators. A new time-varying source degeneration coupling mechanism has been used to quadrature-couple the two oscillator cores for I/Q signal generation, reducing the required headroom of the series coupling transistors. The phase accuracy of the quadrature outputs determines the image rejection capability of integrated RF front ends. To improve phase error in the presence of process mismatch, we introduce a self-calibration loop for phase control of quadrature oscillators. Design concerns like temperature stability of BAW-tuned oscillators are addressed. Side-by-side comparisons of the BAW- and LC-based oscillators are made, and assembly challenges are discussed.

Most current radio receivers use SAW bandpass filters for band selection. Their center frequency depends on the dimensions of the interdigital structure. Therefore, in the next few years, SAW technology will face some fabrication process limits because of the need of higher operating frequencies. BAW resonators are emerging as an alternative technology. One of the potential advantages is the compatibility of the BAW process with standard silicon processing technology. BAW resonators exhibit high Q factors around 1,000 which make them very attractive for low insertion loss RF filters. Nevertheless, process dispersions on the thickness of AlN lead to a shift of the BAW resonator characteristic frequencies and thus a shift of the filter’s center frequency. Moreover, BAW resonators suffer from thermal drift of around 20ppm/°C. Therefore, the need of designing a tunable filter and an automatic tuning circuitry appears for SoC in order to correct process and/or temperature deviations. After presenting the way of synthesizing BAW filters for a given mask, the feasibility of tuning a 2-GHz bandpass BAW filter is demonstrated in this chapter by a very first flip-chip assembly of a BAW solidly mounted resonator (SMR) die on the top of a BiCMOS 0.25-μ m chip.

Although technology scaling of CMOS devices has allowed great advances in device speed and digital computational efficiency, the quality and size of on-chip passive structures have not enjoyed similar scaling. As a result, the power consumption, physical footprint, and performance of modern integrated transceivers are typically limited by their passive components. In this chapter, we explore the benefits of augmenting CMOS ICs with high-quality RF MEMS resonant structures. In addition to improving the performance of standard RF building blocks, these RF MEMS components enable fundamentally new radio architectures. Various techniques for utilizing RF MEMS resonators will be discussed. We introduce two new receiver topologies based on these techniques: one is an always-on wake-up receiver with an uncertain intermediate frequency (IF); the other is a high-sensitivity super-regenerative receiver.

This chapter presents an innovative wireless transceiver architecture that rely on MEMS components to achieve further miniaturization and significant power dissipation reduction compared to low-power radios targeting LDR to MDR applications. It is shown in particular how the limitations of MEMS devices can be waived at the architectural level and how their combination can lead to innovative concepts preserving or even surpassing the performances of current mainstream optimized solutions. Besides the architectural aspects, the chapter also focuses on the design of some ultra-low-power and MEMS-specific circuits and reports measurement results of the complete system. The synthesizer, which is based on a low-phase-noise fixed-frequency BAW DCO and a variable IF LO obtained by fractional division from the RF carrier, achieves a phase noise of − 113 dBc/Hz at 3 MHz. To correct for its ageing and thermal drift, the BAW DCO can intermittently be phase locked to a 3-μ A, ± 5-ppm, 32- K Hz reference, which is obtained after temperature-dependent fractional division of the signal of a 1- M Hz silicon resonator so as to compensate the non-idealities of the latter (frequency tolerance, large thermal drift). An all-digital PLL implementation guaranties a nearly immediate synthesizer settling when returning from an idle period, owing to the memorization of the previous lock conditions eliminating a multi-MHz XTAL and its associated start-up time. A sensitivity of 87 dBm was obtained in receive mode at 100 kb/s for a global consumption of 6 m A. The transmitter demonstrates a high-data-rate quasi-direct 1-point modulation capability with the generation of a 4-dBm, 1-Mbps, GFSK signal with an overall current of 20 m A. Both the receiver and transmitter further take advantage of BAW filters to implement interferers, image, and spurious rejection.

Crystal oscillators have been the primary frequency reference sources used in communication circuits. However, their size, frequency of operation, and cost are hard to scale. Push toward miniaturization and low cost demands smaller form factor and crystal-oscillator alternatives, and MEMS frequency references have been manufactured recently. In this chapter, a digitally controlled FBAR (freestanding bulk acoustic resonator)-based oscillator is presented as an alternative frequency reference.

In this chapter, we describe the architecture of an in-tire-pressure sensing system for automotive applications. Challenges are the supply of sufficient power, the proper functioning of the sensors (pressure, acceleration, and temperature), and a transceiver architecture with extremely low power consumption. Power consumption is reduced by use of a high-frequency resonator based on the bulk acoustic wave (BAW) technology. Such devices are used both in the transmit and in the receive paths. Further power-saving measures are taken introducing a specially adapted on/off cycling operation. The power supplied to the system is provided either by two small batteries or an energy scavenger, both capacitor buffered. The electronic system is integrated into 3D stacks using either through-silicon vias or the ultrathin chip stacking method to provide short interconnects for further reduced power consumption. The overall system is mounted in a compact molded interconnect device to save space and weight.