Innovative (and courageous) microwave engineers can visualize the possibilities of newer technologies and translate these into practical high-frequency designs.
Emerging technologies often have a gradual but long-term effect on how high-frequency design engineers work. For example, when gallium-arsenide (GaAs) transistors and integrated circuits became commercially viable in the mid-1980s, amplifier designers shifted their focus from bipolar devices to the new semiconductors. In the current decade, several emerging technologies threaten to disturb the status quo for high-frequency designers, including microelectromechanical systems (MEMS) and nanotechnology, ultrawideband (UWB) communications, multilayer circuits, and wide-bandgap semiconductors.
MEMS technology applies silicon semiconductor fabrication processes to the creation of mechanical devices, such as variable capacitors, electromechanical switches, optical lenses, and miniature motors. Early RF MEMS designs have focused on simple structures, including variable capacitors, microwave switches, and relays. Because packaging is critical to isolating MEMS devices from the operating environment, critics of initial MEMS devices cautioned that reliability would be a problem with these miniature components. But several companies have delivered reliable commercial products that dispel these complaints.
RadantMEMS (www.radantmems.com), for example, has developed the model RMSW100 single-pole, single-throw (SPST) switch for use from DC to 12 GHz, as well as the model RMSW200 SPST switch for use from DC to 40 GHz (the highest-frequency commercial MEMS switch currently available). The lower-frequency device has been performance tested at 10 GHz for high reliability at more than 100 billion switching cycles (see Microwaves & RF, July 2004, p. 102). It features less than 0.27 dB insertion loss and more than 25 dB isolation at 2 GHz.
Similarly, the model MICO6-CDK2 single-pole, double-throw (SPDT) switch from Dow-Key Microwave Corp. (www.dowkey.com) has been rated for 100 million switching cycles at frequencies from DC to 6 GHz. The high-isolation (45 dB isolation to 3 GHz and 40 dB isolation to 6 GHz) component minimizes insertion loss to 0.2 dB at 3 GHz and 0.5 dB at 6 GHz.
Of course, not all MEMS devices are switches. Discera (www.discera.com) has focused on the MEMS fabrication of microminiature oscillators. The company's first product, the model MRO 100, is the world's smallest multifrequency oscillator at one millimeter on a side (see Microwaves & RF, August 2003, p. 84). Supplied in wafer-level vacuum packaging, the 19.2-MHz oscillator is a miniature replacement for quartz-crystal oscillators in low-power circuits, such as Bluetooth wireless devices and cellular telephones. The source draws just 2.7 mA current from a +3-VDC supply.
For companies interested in exploring the boundaries of MEMS technology, MEMSCAP (www.memscap.com) features a wide range of standard MEMS devices, including high-Q inductors, variable capacitors, and RF switches. The firm also offers its customizable "Above-IC Technology," which allows the placement of RF MEMS devices directly on top of a silicon IC.
MEMS technology is also well suited for optical applications, such as movable mirrors for tunable lasers. MEMS Optical, Inc. (www.memsoptical.com), for example, is a leading supplier of refractive and diffractive microminiature optics and MEMS devices for optical applications. The company offers lines of standard devices such as scanning two-axis tilt mirrors and moving mirrors for tunable lasers as well as a complete array of optical MEMS foundry services.
With the aid of many well-known corporate sponsors, the Carnegie Mellon University Microelectromechanical Systems Laboratory (www.ece.cmu.edu) is pursing the design and development of MEMS devices using batch-fabrication processes, particularly IC fabrication processes. Jointly associated with the university's Department of Electrical and Computer Engineering and the School of Computer Science's Robotics Institute, as well as the school's Institute for Complex Engineering Systems the MEMS Lab is investigating nanometer-scale data storage, microsensors and microactuators, embedded microinstruments, microrobots, and modeling and design tools for simulating these devices. Industrial Affiliates include ADtranz, Benchmark Photonics, Coventor, DARPA, Intel Corp., the National Science Foundation, STMicroelectronics, and XACTIX. The MEMS Lab includes a 4000-sq.-ft. Class 100 clean room for fabrication, an advanced wafer-probe system for testing, and a long list of computer-aided-engineering (CAE) tools for modeling, including software from Ansoft, Cadence Design Systems, Coventor, and The MathWorks. Also, Sandia Laboratories (www.sandia.com) offers a comprehensive lineup of MEMS fabrication, modeling, and testing services for those interested in creating their own MEMS devices.
Modeling a technology like MEMS poses a challenge for software developers since both electrical and mechanical characteristics must be represented. Coventor (www.coventor.com) offers one of the most widely used design tools with their CoventorWare software suite. The integrated set of tools offers a comprehensive methodology for the design, optimization, and analysis of microminiature devices, including MEMS and fluidic components and subsystems. Individual software engines handle schematic entry, two-dimensional layout, three-dimensional model generation, and model synthesis. The tools are available separately or bundled in any combination. The firm, which recently launched an updated version of its website, offers a free (30-day) software evaluation of the 3D analyzer EM3DS on its website.
Advances in MEMS technology will aid both commercial and military systems. Both commercial and military interests are also pursuing UWB technology for its elegance in handling high date rates at low power levels and sort distances. In essence, a UWB transmitter sends billions of pulses occupying a fairly wide bandwidth. The pulses are arranged according to a temporal sequence known to the receiver, which can then extract the voice, data, or video content carried by the pulse train.
Two years ago, the Federal Communications Commission (FCC) mandated the use of the spectrum from 3.1 to 10.6 GHz for UWB transmissions in the United States at a limited transmit power of 41 dBm/MHz. The FCC had grappled with concerns over UWB interference with existing applications, such as the Global Positioning System (GPS), C-and satellite communications, and the Microwave Landing System (MLS) before finally agreeing to the 7.5-GHz slice of bandwidth for UWB use.
In order for UWB technology to earn widespread acceptance for applications such as short-range data and video transfer, a universal transmission standard must be adopted by product designers. So far, UWB supports have divided into two camps.
This past September, the MultiBand OFDM Alliance (MBOA) announced the formation of the MBOA Special Interest Group (MBOA-SIG) to support standard specifications for short-range UWB technology. MBOA-SIG promoter companies include Alereon, Hewlett Packard, Intel Corp., Nokia, Philips Electronics, Samsung Electronics (SAIT), Staccato Communications, Sony, Texas Instruments, and Wisair. According to UWB strategist at Intel and MBOA co-founder Stephen Wood, "Our membership of more than 170 companies includes the leading semiconductor, personal-computing, mobile-phone, and consumer-electronics companies." The organization (www.multibandofdm.org) has developed specifications based on orthogonal-frequency-division-multiplex (OFDM) UWB for a physical layer (PHY) and progress is being made on specifications for the UWB Media Access Control (MAC) protocol layer.
The MBOA MAC and PHY specifications, adopted by the WiMedia Alliance and the Wireless Universal Serial Bus (USB) Promoters Group, will serve as the common radio platform for those industry standards. The MBOA specifications are becoming the basis for Wireless USB applications, adding wireless connectivity to the large installed base of USB products. The MBOA and WiMedia Alliance are also working closely with the IEEE 1394 Trade Association.
The UWB Forum (www.uwbforum.org), which was formed earlier this year, promotes the use of common signaling mode (CSM) and direct-sequence UWB (DS-UWB) approaches for product interoperability rather than frequency-hopped orthogonal frequency-division multiplex (OFDM) UWB. The group claims that DS-UWB is well understood with clearly defined emission limits compared to OFDM UWB, and is available now through the XtremeSpectrum™ chip sets. The CSM technique allows different classes of devices (e.g., MB-OFDM and DS-UWB) to communicate with each other in order to coordinate their actions and interoperate within the same wireless network.
The XtremeSpectrum technology was acquired by Motorola Semiconductor, now known as Freescale Semiconductor (a wholly owned subsidiary of Motorola) and one of the major supports of the DS-UWB format. The company announced during this past June's Wireless Connectivity (WiCon) World Expo in Amsterdam that it planned to deliver three advanced UWB product families, including the industry's first 1 Gb/s UWB solution. The first-generation XtremeSpectrum achieves data rates exceeding 110 Mb/s with DS-UWB technology. The planned next-generation product families will be engineered to deliver data-transfer rates of 220, 480, and 1000 Mb/s in peer-to-peer and ad hoc networking applications. The new product families will also be designed to integrate sophisticated power-management tools to help extend battery life, a critical requirement for mobile applications. According to Martin Rofheart, director of UWB Operations at Freescale (www.freescale.com), "As we plan commercial shipments of our current UWB solution, it's clear that a variety of speeds--from 100 Mb/s up to 1 Gb/s--as well as a variety of power requirements and ranges are needed to serve the broad range of emerging handheld, mobile, and in-room video and audio applications."
The planned product families, which are to be designed to comply with the FCC's current Ultra-Wideband Report & Order, are scheduled to include driver support for multiple operating systems. The Freescale MAC chip is compliant with the IEEE 802.15.3 MAC protocol, while the Freescale PHY, which is based on the 802.15.3a DS-UWB proposal, provides data transfer rates ranging from 110 Mb/s to 1 Gb/s.
Although multilayer circuits have been commonplace in computer and digital industries, they have only recently been embraced by high-frequency engineers. The use of multilayer circuits allows a designer to think in terms of three-dimensional, rather than planar, layouts with a consequent savings in size and often power consumption. One of the more popular multilayer circuit approaches is based on low-temperature-cofired-ceramic (LTCC) substrates. Mini-Circuits (www.minicircuits.com), for example, offers triple-balanced mixers and quadrature splitters based on the technology to cut insertion loss and shrink these designs to a fraction of the size of printed-circuit components.
The major materials supplier for these LTCC circuits is DuPont Microcircuit Materials (www.dupont.com). The company's Green Tape™ LTCC materials are used in a wide variety of electronic and electrical applications in the wireless and wired telecommunications, automotive, military, medical, instrumentation, industrial, data processing, components and consumer industries. Green Tape is a combination of tape dielectric materials with screen-printed, thick-film conductors. The dielectric tape is cut or punched to the desired geometry; via holes are mechanically formed where needed, and conductors are printed on the tape sheets. For multilayer LTCC components, layers are stacked and laminated into a monolithic structure, which is dried and fired at normal thick-film firing temperatures to produce the desired circuit. LTCC circuits can form both passive and active components, since active devices and semiconductors can be added to the LTCC circuitry.
For those interested in developing LTCC, IMST GmbH (www.ltcc.de) offers a wide range of services including design consulting, circuit and module development, prototype manufacturing, RF/microwave characterization of circuits and antennas, and access to a host of software design tools. The company, which has cooperative arrangements with leading materials suppliers, such as Ferro, Heraeus, and DuPont, and LTCC foundries, is a partner in a variety of European Space Agency (ESA) research and development (R&D) projects.
Synergy Microwave (www.synergymwave.com) has developed its own multilayer technology called SYNSTRIP®. The approach achieves a smooth transition from stripline to microstrip at the component level, and helps designers to attain efficient RF signal transfer from the surface-mount component to 50 (omega) microstrip on the system board using single-layer microstrip via holes. The company has applied the technology to lines of couplers, hybrids, and power splitters for applications from 650 MHz to 2.8 GHz.
Merrimac Industries (www.merrimacinds.com) recently launched its Multi-Mix PICOT™ version of the company's Multi-Mix multilayer technology. This new version reduces the size of single-function microwave components from 84 to 87 percent compared to the previous Multi-Mix designs. The approach provides the basic performance advantages of the original multilayer circuit process, but with significant reductions in size and cost. The technology has been applied to a wide range of miniature hybrids and directional couplers, with power-handling capability of 100 W CW.
While much of the technological advances in semiconductors have focused on harnessing performance benefits from nanostructure processes for microprocessors and memory, a great deal of work continues in commercial and military sectors on gaining more RF power per device for radars, communications, and other applications. Major military systems houses such as BAE Systems (www.baesystems.com) and Northrop Grumman (www.northropgrumman.com) have invested in the future of silicon-carbide (SiC) and gallium-nitride (GaN) high-power RF transistors. The excellent thermal conductivities of these materials make them ideal candidates for high-power RF devices. Unfortunately, production wafers have been small in size and limited in yield due to high defect densities.
That may change with advances achieved by the Advanced Material Group of Sumitomo Electric Industries (www.sumitomo.com), which has just begun manufacturing relatively defect-free 50-mm-diameter GaN wafers. Additional materials research on GaN is being conducted at the General Electric Global Research Center (www.ge.com) and ATMI Inc. (www.atmi.com) with similar goals to reduce defect densities in the wafers and create more usable GaN semiconductor real estate. The number of companies and research institutions developing GaN substrates is about 20 and climbing, since the material is also well suited for optical light-emitting diodes (LEDs).
The Office of Naval Research (ONR, www.onr.mil) is a long-time sponsor of wide-bandgap semiconductor devices for low-noise and power applications, annually seeking research proposals from industry and academic institutions. The ONR's objective is to develop devices for power supplies, communications, fire control, surveillance, EW, and multifunctional RF systems. Goals include wide instantaneous bandwidths, high power-added efficiency (better than 60 percent), and output-power levels that are five times higher than existing GaAs devices.
Cornell University has long been one of the leading academic developers of advanced semiconductor technology and, at present, a team led by Cornell's Lester Eastman and including researchers from Rensselaer Polytechnic Institute (RPI) and Northrop Grumman is exploring new forms of amplifiers based on unipolar GaN-based transistors on SiC substrates. The team will apply two types of transistors: heterojunction field-effect transistor (HFET) and vertical ballistic transistor (VBT) devices. Northrop Grumman's SiC vertical devices have already achieved more than 450 W of pulsed UHF power, with modules reaching more than 1 kW output power. GaN epitaxial transistor layers will be grown by molecular beam epitaxy and organometallic vapor phase epitaxy at Cornell on bulk and epitaxial SiC from Northrop Grumman. Part of the mulidiciplinary university research initiative (MURI) at Cornell University, the research team includes Michael Shur at RPI and Kevin Webb at Purdue University. John Zolper of ONR is the MURI contract monitor.