Thermal characterization is useful for identifying the cause of circuit failures. Unusually high local temperatures can occur due to unexpected and localized power dissipation in a particular device, which was not anticipated nor designed in the device. Unintended high temperatures may be the source of circuit damage or it might be caused by the failure itself, such as a tiny short-circuit. However, it is often difficult to obtain enough information about the physical structure and characteristics of the circuit to clearly identify the cause of the failure. The method we present here is an imaging technique with both thermal distribution and emission intensity superimposed on a single image.
Thermoreflectance thermal imaging takes advantage of the differences in light reflectivity of the surface with a changing temperature. This relationship is quite linear in a typical temperature range of interest. Using 1300 nm wave length for the illuminating source, the metal circuitry of interest is directly observed through the almost transparent silicon substrate of a flip chip mounted device. With the same photo detecting imaging sensor, i.e. InGaAs junction array, the emission from the target device through the silicon substrate is also detected. If there are some local electron collisions, e.g. due to a current concentration at a defect or a tiny whisker, a localized excess energy by electron-electron collision yields the photon generation and emits an electromagnetic wave. The photons having a sufficient energy level at wavelength transparent to silicon, reach the photo detecting imaging sensor. With software processing and precise synchronization of the driving circuit, illumination, and imaging, we can separate the thermal signal from the emission and observe transient behavior on a nanosecond time scale. The time response is helpful in some cases to separate cause and effect. As an example, an unintended small short-circuit resulting from the fabrication process may be reproduced for a small number of samples. The emission would be observed first, followed by the thermal signal which propagates in time. Another example is for a circuit with low design margin, in this case only a single sample exhibits a difference between emission and/or thermal image while numbers of good samples are identical with respect to the emission and thermal image. This difference clearly indicates the location of a potential failure.