Taller Microcomponents Lead to New Applications
Just a sliver of extra metal may make new types of wireless com-munications systems possible. By precisely extending the height of microscopic metallic structures on electronic printed circuits, Electrical and Computer Engineering Assistant Professor Steven S. Gearhart is improving communications systems and inventing new antennas and filters that work at very high electromagnetic frequencies. That's important both because the lower frequencies are, by and large, already occupied and because effective high-frequency devices will make new applications possible.
Any system that uses electromagnetic waves to communicate requires its own space in the spectrum of electromagnetic frequencies. If two systems try to transmit at the same frequency, they'll interfere with each other. But as communications technology has blossomed, the world's airspaces have become jammed with signals from such wide-ranging sources as television transmitters, citizens band radios, navigation satellites and cellular telephones. "There's just not a lot of frequency spectrum left," says Gearhart. "That's one reason communications systems are being pushed into higher and higher frequencies, toward the millimeter wave frequencies, which are between 30 and 300 gigahertz and have corresponding wavelengths between 1 and 10 millimeters." These frequencies can also transmit more information than lower frequencies. And millimeter-wave frequencies hold promise for automotive and aeronautic radar systems. To frequencies from about 35, 94, and 140 gigahertz, fog and clouds appear transparent. Radar operating at these frequencies would be able to "see" where current systems can't.
Extremely high frequencies, however, pose special problems for designers of electronic circuits. As frequencies increase, wavelengths-the distance between two points on adjacent peaks in a wave-decrease. When wavelength decreases, the size of the circuit elements such as transistors, filters, and antennas must also decrease, making constructing circuits more difficult. Usually, circuit components are linked with wires and "interconnections," which are strips of metal on the surface of a printed circuit. But at millimeter-wave frequencies, the inductance of the these small bits of metal becomes too high. To minimize these effects, designers would like to bring different parts of circuits as close together as possible, eliminating connecting wires and reducing the length of interconnections.
That's where Gearhart's techniques come in handy. "One of the main research areas in millimeter waves is putting the transistors and diodes, the transmission lines, and the antennas all on the same semiconductor substrate," he explains. He has developed a way to build these and other critical circuit components on the same surface-the substrate-as the rest of an integrated circuit.
The type of integrated circuits that Gearhart studies are made from semiconducting substrates on which electronic components are formed by layering specific areas. Metal components can be added by plating these surfaces through a thin mask, like spraying paint through a stencil. But this process produces broad, flat- or "planar"-metal components that lie very close to the substrate. "Unfortunately, the best filter and coupler designs are not planar, but are three dimensional," Gearhart explains. One reason planar components don't work well is because "high dielectric" substrates alter the paths of high frequency signals, trapping the signals the way fiber-optic cable traps light. The trick, then, is to distance the metal components from the substrate to reduce the negative effects of the high dielectric while keeping them on the substrate to avoid the added inductance of interconnections. Gearhart's solution is to build tall structures, so that the bulk of the component is far from the substrate.
Using LIGA micromachining techniques, Gearhart's research group is developing miniature 3-D transmission lines and filters for a wide variety of millimeter-wave applications. Gearhart constructs these "tall" structures-which in the Lilliputian world of a microchip means between only about 100 microns to one millimeter high-by plating the substrate through a much thicker mask than usual. He uses synchrotron radiation-X-rays-from an atomic particle accelerator to cut patterns in the plastic to form a sort of mold. "Where the X-rays hit we get these very vertical sidewalls," Gearhart explains. Because the mold's sides are so straight, the heights and widths of the resulting metal structures can be controlled to within a few microns, which is vital to the performance of millimeter-wave circuits. And because the structures are so tall, they are stiff enough to extend off the edges of the substrate without additional support. That lets designers build antennas that avoid the electrical problems associated with substrates.
The precisely cut forms also let Gearhart construct devices with much smaller "foot-prints" than previous techniques allowed. "It's smaller, so it takes up less semiconductor space, which is very important for the electronics industry," he says. "Gallium-arsenide is very expensive and Indium-phosphide is even more expensive."