Fast Times in Silicon Circuits

By Peterson, Ivars | Science News, July 11, 1987 | Go to article overview

Fast Times in Silicon Circuits


Peterson, Ivars, Science News


Fast Times in Silicon Circuits

In the ultrafast world of microelectronics,the blink of an eye seems to last a century. It is a world in which time is measured in picoseconds--trillionths of a second. Recently, a team of IBM researchers generated electrical pulses so short that each one lasts only half a picosecond. In that fraction of a second, light travels a distance of less than a millimeter.

The IBM technique is one of severalmethods now being developed for measuring the characteristics of high-speed integrated circuits. As researchers develop electronic devices that switch on and off faster and faster, the need for measurement techniques that can keep up with such devices grows.

"The problem,' says IBM's DanGrischkowsky, "is that standard electronic measurement capability is not as fast as the fastest devices.' Techniques for generating and detecting extremely short electrical pulses make it possible to time brief events and to track a pulse as it travels along microscopic transmission lines laid down on a silicon chip.

To generate ultrashort electricalpulses, scientists at IBM in Yorktown Heights, N.Y., refined a technique first developed more than a decade ago. They fabricated a transmission line consisting of a pair of parallel, micron-wide aluminum strips, 2 microns apart, on a thin piece of silicon. That transmission line, in normal operation, is maintained at a certain voltage.

When a laser pulse strikes the siliconbetween the two aluminum strips, the light frees electrons. The presence of that electrical charge in the gap momentarily shorts the circuit, abruptly changing the voltage that moves down the line. A similar "photoconductive' switch is used to detect an electrical pulse later in its travels.

To keep the electrical pulse sharplydefined so that it matches the shortness of the triggering laser pulse, IBM scientists bombard the silicon surface with atoms to create a large number of defects capable of quickly swallowing up the loose charge carriers. Thus, the short circuit is brief, and the resulting pulse starts and stops sharply.

By incorporating a set of transmissionlines within experimental, high-speed integrated circuits, researchers can use ultrashort electrical pulses to test how well the devices work. The pulses can also be used for scientific studies, says Grischkowsky. IBM researchers have, for instance, watched what happens to such electrical pulses as they travel down a superconducting transmission line. "We see an enormous amount of structure and ringing,' says Grischkowsky. For these and similar scientific studies, he says, "you need as short a pulse as you can get. …

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