The vast majority of these voltage alteration tools use a laser as the outside stimulus. The tightly focused beam of the laser allows the tool to precisely direct energy onto a given spot on the electronic circuit, facilitating failure analysis by allowing the user to pinpoint even the smallest of defects. To find a defect, the laser is scanned across the surface of the die, while the electrical bias conditions (the voltage and current applied to the device) are closely monitored. As the laser crosses a defect, a change can be observed in the bias applied to the part, corresponding to the type of defect that is present. This change can then be mapped onto an image of the device, providing an analyst with a detailed picture showing the precise location of a defect.
Depending on the type of laser used, this method of electronic circuit failure analysis will provide several different data sets. In many cases, an infrared laser with photon energy below the silicon bandgap of 1.12 electron volts will be used, which causes thermal changes on the die; the heating and subsequent relaxation cooling of the semiconductor and metal constituents of the circuit creates resistance changes which, though insignificant on a properly functioning device, are easily mapped on a defective unit. The counterpart to the thermal laser is a laser with energy at or above the silicon bandgap, used to generate free carriers in the semiconductor materials of the device causing an increase in the circuit’s current which can be measured and correlated to a normally-functioning unit to locate a defect.
The benefit of these types of voltage alteration techniques is that they can be applied to perform failure analysis of electronic circuits that are somewhat more unconventional than the typical IC. Indeed, in a pinch one can create a field-expedient way to test photovoltaic devices in this manner with nothing more than a laser pointer and a steady hand!
Derek Snider is a failure analyst at Insight Analytical Labs, where he has worked since 2004. He is currently an undergraduate student at the University of Colorado, Colorado Springs, where he is pursuing a Bachelors of Science degree in Electrical Engineering.