MICROELECTRONICS T ake a long-exposure aerial photograph of a city at night, and you will see traffic patterns traced in the bright streams and dense clusters of car headlights. The image (below), obtained by a team of researchers at the IBM Thomas J. Watson Research Center in Yorktown Heights, New York, is the equivalent shot of a functioning microprocessor, the 1997 S/390 used in the current generation of IBM mainframes. The “traffic” consists of electrons emitting light as they pass through the transistors, or crossroads, of this silicon city. By directly viewing such traffic patterns, circuit designers can look for weak spots and bottlenecks in the millions of components on a chip. ![Figure][1] IBM “Researchers have known since the 1980s that electrons emit light as they pass through the field-effect transistors [FETs] at the heart of most modern microchips,” says Jeffrey Kash, of the IBM team that made the images. The light, which is in the near infrared and is extremely weak, can be detected only with cooled charge-coupled devices or special photomultiplier imaging tubes. Kash and his colleague James Tsang investigated this particular microprocessor because it consumed two orders of magnitude more current than it should have when it was not performing any operations. Kash and his colleagues obtained images of the chip in this “quiescent” state that showed a series of spots indicating that this excessive current was confined to a small portion of the chip. They couldn't tell which of the 7.8 million transistors were at fault, however, because they couldn't identify individual transistors in their images. “The issue was, how do you know where you are, how do you navigate,” says Kash. The team solved the navigation problem by spying on the individual FETs as they shuttled electrons around the chip when it was operating normally. “The only time a current is flowing is when you have a change of logic state,” says Kash. The FETs produce picosecond light pulses as they switch on and off, so by photographing the chip in normal operation, Kash and his colleagues obtained a “road map” of the positions of the FETs. When the researchers superimposed this image on the photo showing the excess leak currents, they pinpointed exactly where the leaks occur. The technique is useful for more than troubleshooting. “We can look at hundreds of thousands of FETs on a chip,” says Kash, “and that is very helpful” in improving the design of subsequent chips. Indeed, Ingrid De Wolf of IMEC, Belgium's Interuniversity Microelectronics Center in Leuven, says optical-emission diagnosis of chips is beginning to spread throughout the microelectronics industry. Researchers are going beyond simple imaging, she adds: “We are now also trying to get more information from the spectrum of the emitted light, so we can measure the energy of the electrons.” [1]: pending:yes
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Mar 15, 2002
Richard Layne + 1
Open Access