Separately, semiconductor makers used MTTS to argue the merits of gallium arsenide (GaAs) compared with alternative processes for transmitters in cellular basestations and handsets. While Anadigics (Warren, N.J.) and Toshiba America (Irvine, Calif.) introduced GaAs transmitters for cellular and satellite applications, Motorola SPS (Phoenix) promoted its LDMOS process for basestation transmitters, and its Mosaic V bipolar process for handsets. Fujitsu Compound Semiconductor Inc. (San Jose, Calif.) vied with Motorola for the "largest supplier" title in low-power GaAs FETs.

But announcements on multilayer RF circuit boards remain the biggest attention-getter on the MTTS show floor. Merrimac Industries touted its "Multi-Mix" technology as an enabler for 16 percent wireless market growth. The Multi-Mix technology is a 3-D packaging technology that allows RF transmitter/receiver modules to be "layered" on the same circuit board with digital and other signal-processing elements.

By condensing the packaging and metal-shielding requirements, Multi-Mix provides substantial reductions in the size and weight of RF equipment. For instance, a transponder module (used for aircraft "IFF" -- identification, friend or foe -- broadcasts) was once a 13-pound box occupying a cubic foot. Merrimac's Multi-Mix technology reduced it to a 1.68-ounce module about the size of a credit card. The manufacturing technology for doing this, said Merrimac's chairman and chief executive officer, Mason Carter, is now cheap enough to apply it to cellular basestations and handsets.

The problem with using any kind of multilayer structure for RF, said Arthur Oliner, Merrimac consultant, is that the printed-circuit-board materials need to be carefully selected for their dielectric properties. And their dimensions need to be carefully controlled to prevent the transmitted/received RF signals from being attenuated or propagated through the noise-sensitive parts of the circuit.

While high-temperature co-fired ceramics (HTCC) offer the dielectric properties associated with RF substrates, it is difficult to control the geometries of the interconnects. The interconnect dimensions must always reflect some multiple of the RF wavelength -- and this will be increasingly smaller, and more difficult, the higher the operating frequency of the RF module. The HTCC materials, added Oliner, need gold conductors to reduce line losses -- gold will melt at usual ceramic-firing temperatures.

Thus, microwave system packagers have been forced to use a high-loss tungsten alloy for interconnects and then nickel-gold plating in an effort to reduce the line loss.

The packaging technique developed by Merrimac and another aerospace supplier, the Labtech Ltd. (Presteigne, U.K.) subsidiary of Intelek plc, replaces the ceramics with ordinarily flexible PTFE materials (fluoropolymers like Teflon), which offer a high-dielectric constant. The PTFE materials are stiffened with ordinary circuit-board materials like class fiber. In some cases, the copper interconnects serve as the stiffener; in other cases, a metal plate, also providing RF shielding, is bonded to one side of the circuit board.

The first manufacturing hurdle, explains Brian Mazonas, Labtech's managing director, is getting the metal to stick to polyimide layers -- by definition a non-stick material. The second hurdle is controlling the dimensions of the metal interconnects, chemically etched (like semiconductors) to a tolerance of ý10 microns.

The third hurdle -- the biggest, according to David Brannock, managing director of Pascall Electronics Ltd. (Isle of Wight, U.K.) -- is controlling the alignment between successive layers. This alignment must be tighter than ý 5 microns, according to Pascall, a packager of electronic systems and Labtech's sister company under the Intelek plc umbrella.

These geometries are not a challenge for semiconductor makers, Brannock suggests, but still represent a challenge for pc-board makers -- especially those using ordinarily pliable PTFE and FR4 materials. Pascall and Labtech are pleased that their multilayer aerospace technology is finding its way into mobile-communications markets.

As cellular basestations continue to shrink, turning from entire Quonset huts into metal canisters hung from telephone poles, manufacturers are taking advantage of the multilayer packaging techniques. Pascall's modules, for example, are used as what Brannock calls "the spine" of a cellular system -- the station-to-station microwave links operating at 7, 8 and 15 GHz. But Brannock is not optimistic about cracking the cost-sensitive market for cellular handsets anytime soon.

Like that of Intelek, Merrimac's packaging technology is currently used for smart antennas, cellular basestations and satellite uplinks. Merrimac uses a 300ýC "fusion-bonding" process to make copper-to-copper interconnects and get the metal to stick to the PTFE dielectrics. Interconnect patterns are etched to within 0.003 inches with a 0.0005 tolerance, and plated with nickel and gold.

Merrimac's James Logothetis, vice president for advanced technology, is a bit more upbeat about commercial markets. He believes his company's Multi-Mix technology will accommodate chips as well as passive components on the intermediate layers (below the RF modules). And that will make Multi-Mix vital in applications like cellular handsets, which trade on microminiaturization.

To push the technology into the hands of commercial users, Merrimac is offering a service called "On-line Co-Design." This is essentially a CAD tool for RF circuit designers, which allows them to select and place not only RF components, but also specially shaped, dimensionally controlled multilayer circuit elements.

These circuit elements include dielectrics, passive networks and components, interconnects and transmission-line components. Once the components of the design are selected and interconnected, the RF propagation effects can be simulated and analyzed, and the design can be optimized for low-cost manufacturing.

In fact, both Merrimac and Intelek say their multilayer packaging technology would not be feasible without new-generation EDA tools like those of HP Eesof's (Westlake Village, Calif.). "This is an 'enabler,'" Brannock insists. It would not be possible to build a multilayer RF circuit without being able to predict the propagation effects of the dielectrics, the interconnects and the changes brought on by small changes in their geometries. The tools for modeling 40-GHz systems were once very crude, he said, "but we can now believe their output."

On the semiconductor side, MTTS was dominated by suppliers of GaAs semiconductors for cellular basestation and satellite applications -- and manufacturers trying to find alternatives to GaAs.

Toshiba America introduced 1-, 2- and 0.2-W Ku-band Mimic modules. Abdul Aslam, Microwave marketing manager, cited the wide frequency range for these parts (13.75-14.75 GHz). Ordinarily, multichip GaAs modules like these are specially matched for input-output impedances (50 ý) and frequencies of interest. The Mimic modules are matched, but their wide frequency range allows them to be used for a number of different VSAT and wireless applications.

Other Toshiba introductions included high-power (45 W) C-band GaAs FETs (for 5.1- to 7.2-GHz applications), and Mimic modules for local multipoint distribution systems (LMDS), video-broadcast systems operating from 21 to 40 GHz.

Basestation applications, explains Anadigics' strategic marketing manager, Jennifer Palella, generally do not have to run from batteries and don't need to be as efficient as the parts used in cellular handsets. However, they do need to be high-power and low-cost.

Anadigic's first GaAs parts for cellular-basestation transmitters, introduced at MTTS, include the AWT921, an integrated multistage power amp (with a +39 dBm output) for 900-MHz AMPS systems, the AWT1921 for 1.6-GHz satellite links and the AWT1922 for 1.9-GHz PCS and GSM basestations.

Elsewhere at MTTS, Motorola's Wireless Infrastructure Systems Division promoted its LDMOS process -- a high-frequency DMOS process -- as the alternative to GaAs for cellular-basestation transmitters. "The LDMOS is not as efficient as 3-5 materials [such as GaAs, which occupies columns 3 and 5 on the periodic table of elements]," said Don Sundby, product manager. Since cellular basestations do not require the same power efficiency as battery-powered cellular handsets, Sundby felt basestation manufacturers would be attracted to LDMOS' lower costs.

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