Inexpensive Laser Technique Finds Circuit Hot Spots

Richard Gaughan

Each year, integrated circuits grow in on-chip density, complexity and cost. Increasing the density means that current must be carried by smaller structures that are susceptible to thermal damage, and the increasing complexity makes it difficult to identify those structures early in the design and fabrication process. Infrared cameras can provide the thermal maps to do so, but they are a relatively expensive solution.

In response, researchers at the National Institute of Physics at the University of the Philippines in Quezon City have developed an inexpensive method that can detect thermal defects and identify the suspect structures. Their approach has the potential to address thermal management issues not only in logic and memory chips, but also in LEDs, solar cells and solid-state detectors.

The technique uses a 793-nm diode laser from Sharp Corp., driven by a Melles Griot current-temperature controller. An Olympus objective lens focuses the output of the diode onto the active layer of an integrated circuit, creating currents within the semiconductor.

In general, current increases with device temperature, so with the diode laser operating at a constant power level, alterations in the optical beam-induced currents should be caused by changes in the semiconductor’s temperature. Anomalous regions where the current decreases with temperature can indicate a “hot spot” where the electrical resistance increases with temperature, creating a runaway heating problem.

The circuit to be tested is mounted on a thermoelectrically controlled platform that runs through a range from 26 to 86 °C. When the sample is scanned through the focused diode beam, two thermal maps are produced. The first shows the optical beam-induced current profile of the normal regions, and the second displays anomalous regions.

The more difficult task is correlating the thermal maps with the integrated circuit structures. The solution that has been provided by Caesar Saloma and his colleagues at the institute is to use the same laser source to measure active-layer architectural features.

An inexpensive imaging method uses confocal laser reflectance to identify regions where thermal problems are likely to develop in an integrated circuit. Overlaid images combine a current map generated by laser absorption with reflectance from the active layer to identify and localize potential hot spots.

The laser is sensitive to external optical pumping, so the focused diode beam reflects off structures within the active layer of the integrated circuit and modifies the output power. By tracking the diode laser power as a function of location, the researchers generate a confocal reflectance map of the active layer with diffraction-limited resolution. When the active layer map is combined with the thermal maps, the points of interest are clearly identified, with a resolution about three times better than other, similarly inexpensive techniques and at a cost much less than that of an IR imaging system.

Saloma said that distinguishing dielectric structures from metal or semiconductor ones is straightforward, but that confocal reflectance images display relatively low contrast between metal and semiconductor structures. The group is working on a spectral microscope that will detect the differences in reflectance spectra. It also is working on methods of predicting failure in the active layer of an integrated circuit and on multidimensional techniques that would offer spatial, spectral and temporal information in one measurement run.

Given the menagerie of semiconductor devices currently manufactured, he said, the availability of a less expensive approach to failure analysis in product development and production will be good for consumers.

Applied Physics Letters, Dec. 5, 2005, 231104.