In the last two years, the development and deployment of devices based on microelectromechanical systems for radio-frequency applications, or RF MEMS, have gained rapid ground. These new technologies have made possible discrete micro-scale mechanical circuits, which are capable of low-loss filtering, mixing, switching, and frequency generation. While the successful demonstration and implementation of RF MEMS devices is a cause for celebration, the real accomplishment is the beginning integration of RF MEMS into the module and IC. Devices such as RF switches and on-chip inductors are moving into production at several major semiconductor manufacturers. In this way, the industry is making measurable strides toward a major intermediate objective: the single-chip transceiver.

This goal is not as far away as one might think. Consider the existence of validated technologies that merge micromechanics with transistor circuits onto single silicon chips. Following the line of development, it seems that single-chip transceivers will be feasible in the next few years. The solution will not be found solely in the development of silicon-compatible MEMS devices, however. It also will require the use of alternative architectures that deploy large numbers of passive, high-Q MEMS circuits to reduce power consumption for portable applications.

This article looks closely at these micromechanical circuits, along with the associated technologies that will most likely play key roles in reducing size and power consumption for future communications transceivers. It also examines ways of integrating RF MEMS components for the creation of a complete RF front end.

DESIGN CHALLENGES
The architecture of today's transceivers has enabled many advances. Yet the creation of communications devices continues to rely on evolutionary techniques and a hodgepodge of technologies. Engineers must combine a variety of building materials, such as silicon, gallium arsenide, quartz, and ceramic. They mix on-chip and off-chip circuits for high-Q inductors, filters, capacitors, and oscillators. Plus, they insist on hanging on to challenging packaging methods like LTCC, CSP, and modules. This lack of an integrated approach has created inherent design challenges.

Standard mechanical circuits, such as quartz-crystal resonators and surface-acoustic-wave (SAW) filters, provide essential functions in the majority of transceiver designs. But manufacturers tend to avoid using too many of them. For this trend, the circuits can fault their larger size and cost. The downside of this practice is that when designers minimize the use of high-Q components, they often trade power for selectivity (in other words, Q) and thus sacrifice transceiver performance.

Additionally, the use of current high-Q passive solutions prevents the tailoring of termination impedances, which is required by RF and IF filters. This factor can be an added advantage when designing power-hungry, low-noise amplifiers (LNAs) and mixers in CMOS technology. At the output of an LNA, higher impedance can enable a major power savings.

RF MEMS ACTIVITY
Currently, the industry is seeing a broad number of RF MEMS devices in development for strategic applications. In the high-Q micromechanical domain, for example, Agilent has already launched a successful MEMS-based, film bulk-acoustic wave resonator (FBAR). It is now shipping in cell phones worldwide. Similarly, companies such as Discera have conducted extensive work in micro-resonator technology to enable the replacement or integration of front-end components. The first of these components is a micro-oscillator that can replace crystal oscillators.

As far as research goes, one of the most popular areas has been the MEMS-based radio-frequency switch. Now, companies such as Teravicta and Microlab are commencing production of switches based on differently actuated devices.

Another hot area is on-chip inductors. These inductors have long been a focus for several companies, including PHS MEMS. This company helped to develop and industrialize a thick copper process that enables very high Q. Micromachined variable capacitors may soon reach the market in volume, thanks to intensive development by STMicroelectronics and other companies. On-chip micro-antennae are also the subject of intense ongoing research and development.

POSSIBLE SOLUTIONS
The simplicity of micromechanical devices, coupled with their small size, translates into a range of possible solutions. Such solutions can help address the disparate nature of transceiver architecture today, as well as the need for lower power consumption. In addition, MEMS can be integrated on-chip using silicon-style batch-fabrication techniques. This capability offers the industry a cost-effective way to move toward additional functionality and high-performance circuitries.

The following micromechanical-solution scenarios use vibrating micro-resonator beams. They offer examples of the kind of gains that can be achieved using the partial and full-scale deployment of micromachined technologies. The current generation of micro-resonators, freed at both ends of the beam, offers Q factors of more than 10,000 at frequencies relevant to CDMA cell-phone applications (FIG. 1).

Perhaps the most direct way to harness micromechanical circuits is via the direct replacement of the off-chip ceramic, SAW, and crystal resonators. Such devices are used in RF pre-select and image-reject filters, IF channel-select filters, and crystal-oscillator references in conventional superheterodyne architectures (FIG. 2). Micromechanical switches also can be used to replace FET T/R switches. The micromechanical switches greatly reduce wasted power in transmit mode. For example, they can cut wasted power by as much as 280 mW if the desired output power is 500 mW. For further miniaturization, medium-Q micromachined inductors and tunable capacitors can be used in VCOs and matching networks.

Although they are beneficial, the performance gains afforded by the mere direct replacement by MEMS are quite limited—especially when compared to the more aggressive uses of the technology. To fully harness the advantages of micromechanical circuits, one must first recognize that these circuits offer the same system complexity advantages over off-chip discrete components that planar IC circuits provide over discrete transistor circuits. This is due to the MEMS circuits' microscale size and zero-dc power consumption. Because of this fact, micromechanical circuits should be utilized in large numbers in order to maximize performance gains. Even with banks of micromechanical circuits (100 high-Q resonators on a 2-mm² die), the area and height consumed by the devices is a fraction of the area consumed by their off-chip counterparts.