7 August 1997, Electronic Engineering Times: MEMS could hold key to cell-phone-on-a-chip -- Darpa sows seeds of a telecom revolution Arlington, Va. - Researchers at the Defense Advanced Research Projects Agency (Darpa) here, working in tandem with a number of defense contractors around the country, are making significant progress in crafting micro-electromechanical systems (MEMS) in silicon that can replace radio-frequency components such as ceramic filters and wirewound inductor coils. RF MEMS techniques have already been used to fit a handheld reconnaissance radio into a standard PCMCIA form factor and create a smart or "tunable" antenna for military aircraft. While the technology is not yet ready for use in creating the single-chip cellular phone or other long-anticipated consumer-electronics devices, the advances thus far are beginning to raise eyebrows. Elliott Brown, a program manager for Darpa's Electronics Technology Office, believes the impact of RF MEMS on portable communications will rival the stir caused by the application of FETs in RF power amps in the 1970s. Rick Ridgely, deputy program manager of Darpa's Airborne Communication Node, is even more optimistic. "This will do for communications what the microprocessor did for computing," he predicted. Darpa believes RF MEMS will supplant discrete inductors, capacitors, ceramic filters and even transistor switches in navigation radios, airborne communications equipment and, by extension of the technology into the commercial sector, a host of portable personal communications devices. Because the mechanical structures are etched in silicon, they can be integrated with amplifiers, voltage-controlled oscillators (VCOs), phase-locked loops (PLLs) and other ICs, dramatically cutting the size, weight and power consumption of RF communications systems. By micro-machining movable capacitor plates into silicon, researchers have built antenna interface circuits with quality factors-known as Qs-as high as 20,000. (Qs are a measure of a resonant circuit's ability to oscillate at a particular frequency.) Darpa is overseeing research on three types of electrically tunable mechanical structures that could replace discretes in RF devices operating over a range of frequencies. "Mechanically resonant" structures, such as surface acoustic wave (SAW) filters, will support "low frequency" applications, from several MHz up to 1 GHz. A number of projects toward that end are under way, including those at the California Institute of Technology and at the University of Michigan. A 300-kHz three-resonator micromechanical filter constructed at Michigan, for example, is reported to have obtained a Q of 590 and a stopband rejection of better than 38 dB. For mid-band frequencies (1 GHz to 10 GHz), meanwhile, MEMS techniques will build lumped-element filters. A third type, distributed filters, will serve applications above 10 GHz. Darpa's Brown believes the biggest impact will be on the switches used as antenna-interface units for mobile and airborne transceivers-that is, the switches and filters that optimize an antenna to transmit at one frequency but to receive at another. A single MEMS switch, according to Brown, can replace the 13- or 15-component PIN-diode switch typically employed. That kind of component reduction, Darpa predicts, will reduce the RF section component count a hundredfold, reduce the size of the antenna interface units by 10,000 times, cut power consumption by a factor of 1,000 and yield a thousandfold increase in off-state isolation. "There is nothing that can beat a mechanical switch for effecting an infinite impedance," said Brown. The key metrics in which the devices excel, he said, are insertion loss and dc power dissipation. Whereas an electrical or electronic component presents its own resistances, capacitances and reactances to high frequencies, the insertion loss for a MEMS switch is negligible. When PIN diodes are employed for RF routing, for example, each diode can consume as much as 10 to 20 mA. The current drive needed to open or close a MEMS switch, Brown said, is on the order of nA or even pA. Darpa's Web site characterizes its MEMS program as an "interdisciplinary research and development" effort on both "advanced MEMS devices and advanced MEMS fabrication processes, all with the goal of demonstrating innovative systems concepts." Responsibility for roughly $10 million in RF MEMS project awards is shared between Brown and colleague Albert Pisano, an industry recruit and an expert in such traditional MEMS applications as inertial instruments and fluid sensing, control and transport. Brown oversees the Phase III MAFET (microwave and analog front-end technology) of which RF MEMS is a part. The lion's share of RF MEMS funding now comes out of the MAFET program. Ridgely of Darpa's Airborne Communication Node said the agency's overall RF MEMS expenditures could approach $20 million in the not-too- distant future. The Darpa program managers are firm in their belief that micromechanical switches and resonator filters will be smaller and offer superior performance to their electrical component equivalents, but they're hesitant to project when the devices will be accessible to manufacturers of cell phones and other cost- sensitive appliances. Pisano believes polysilicon structures can be honed to make cost-effective micromachines. "You need very tight mechanical dimensioning," he said, "but it is essentially single-mask photo etching." The low-frequency mechanical resonator filters present conceptual as well as cost issues. You can electrically tune a MEMS frequency filter by altering its dimensions-for example, the gap between one mechanically vibrating section and another. The resonant frequencies will be amplified; the non-resonant frequencies will be dampened. But engineers will need to rethink voltage-to-frequency conversion, phrasing it in terms of the electrical energy necessary to change the mechanical spacings that affect resonant frequency. Mechanical signal processing, as Brown called it, is especially critical when one frequency is used to modulate another. A mechanical filter may be more economical than an electrical filter for particular frequencies, Brown said, but it will never be cheap. He believes the cost of the filters will follow the same learning curve as other silicon semiconductors and will be easily competitive with those made in GaAs. Controlled dimensions can also make transmission lines with variable (i.e., "tunable") impedances. Examples include what Hughes Aircraft calls smart antennas, in which the radiating characteristics of the antenna itself-the length, width and gap impedance-can be changed on the fly. The technology is most useful for phased-array antennas operating in the S and K bands and for point-to-multipoint broadcasts at 38 GHz. At very high frequencies, Brown explained, all radiation assumes a point-to-point pattern: "The signal is 'teledesic'-it doesn't wrap around the horizon; it doesn't penetrate buildings." Special radiation patterns are required. But tunability results in a tenfold performance increase in the phased-array antennas used for such applications. MEMS may be the best bet for miniaturizing RF systems. The typical aircraft navigation receiver, for example, may include dozens of hand-wound coils and hybrid amps. "The antenna-interface unit is a 6-pound brick," Ridgely said. MEMS approaches can take the weight out by slashing the parts count. A technology-demonstration project at the Avionics and Communications Division of Rockwell Collins (Thousand Oaks, Calif.), for example, rebuilt the UHF/VHF antenna-interface unit (AIU) for the F-22 jet fighter with a MEMS tunable- capacitor circuit. The original switch matrix included UHF/VHF filters for four separate receivers. It used 576 varactors, 216 inductors, 144 resistors and 108 capacitors, for a total parts count of 1,044. The replacement devised by Rockwell was a tuning circuit with 36 MEMS capacitors. Raytheon's E-Systems unit (Falls Church, Va.), which builds remote airborne reconnaissance systems whose core electronics include low-noise amplifiers and interference-cancellation systems, turned to RF MEMS for a handheld communication radio for battlefield applications. Called Ultra-Comm, the receiver is interoperable over a wide range of frequencies and modulation schemes-and is small enough to fit on a PCMCIA card. The company is developing the device as a "software radio" to ensure compatibility with a wide range of changing communication standards, said William Rinard, E-Systems' director of technology development. Heavy legacy The new device will conform to standards mapped by the government-sponsored Programmable Modular Communication System (PMCS) and the Modular Multifunction Information Transfer System (MMITS). While government and industry are collaborating on standards for multiband, multimode radios, neither sector can ignore the substantial investments already made in legacy receivers and transmitters. The software approach, E-Systems noted, allows for modular transmitters and receivers (packaged on VMEbus boards, PCI-bus cards or PCMCIA modules) that can be programmed to retain compatibility with existing communications equipment. The device will be completely tunable from 800 MHz to 2.8 GHz. E-Systems engineers say the tight integration provided by RF MEMS filters accounts for the small form factor. They say they remain uncertain about the suitability of a MEMS device for consumer applications, but they note that $10 or $15 is still a lot cheaper than the $150 to $300 currently charged for the front-end filters used in military aircraft multiband radios. "If these guys want a true Dick Tracy radio," said Michael Clingempeel, E- Systems manager for RF hardware development, "they're not gonna do it with ceramic filters." ----------