Single-shot nanosecond thermal imaging of semiconductor devices using absorption measurements

A nonscanning optical method for single-shot thermal imaging of semiconductor devices is presented. The method detects changes in the band-to-band absorption due to local self-heating effects. The device is illuminated from the substrate side and the image reflected from the device topside is detected. The illumination wavelength is set near the semiconductor absorption edge. The time resolution is 5 ns, determined by the laser pulsewidth and the space resolution is about 2 μm. The method is applied to study the transient current distribution in electrostatic discharge (ESD) protection devices fabricated in smart power technology. The observed current spreading with time is explained in terms of a negative temperature dependence of the impact ionization coefficient. The method allows a fast analysis of the current-flow homogeneity in ESD protection and power devices.