Putting the pieces together
Scientists have known since the 1980s that electrons emit light when they speed through field effect transistors (FETs), the building blocks of CMOS (complementary metal-oxide-semiconductor) microchips. All microprocessors and memory chips are made from CMOS. The electrons move only when the CMOS circuits change from one state to another, switching on or off. Putting two and two together, Jeff Kash and Jim Tsang at IBM's Thomas J. Watson Research Center realized that they could use this very faint light emission to monitor the switching of individual components of advanced CMOS chips.
Proving the theory
To confirm the effect, the pair used high-speed optical detectors to monitor light emissions from simple high-speed circuits. Next, they adapted a more sophisticated detector that permitted them to image and time-resolve light emission from hundreds or thousands of devices on a chip simultaneously. Working in collaboration with IBM's Microelectronics Division, they showed that the technique produces movies of information flowing through complex chips, such as IBM microprocessors. Because the pulses of light last for only picoseconds (trillionths of a second), the technique has been dubbed "Picosecond Imaging Circuit Analysis", or PICA.
Easier design, better chips
That basic technology has led to a diagnostic tool with great significance for the semiconductor industry worldwide. So far, it has found its chief application in spotting and diagnosing faults in chips at the design and prototyping level. Late in 1996, for example, Kash and Tsang used the light emission to identify a problem in a high-speed chip, based on CMOS technology, that IBM was designing for its high-end S/390 servers.
Check out the demos