Thermal and free carrier concentration mapping during ESD event in Smart Power ESD protection devices using an improved laser interferometric technique

Spatial distribution of temperature and free-carrier concentration during h igh-current stress is studied in smart power electrostatic discharge (ESD) protection devices using a backside laser interferometric technique. The me thod is based on detecting changes in the refractive index of silicon due t o thermo-optical and plasma-optical effects. We use a modified version of a heterodyne interferometer, where the reference beam is reflected from an e xternal mirror outside the sample chip, which allows one to perform measure ments without any restriction to the size of the scanning area. We have fou nd two pronounced heat dissipating regions due to a vertical and a lateral current flow path in the device. In addition, two regions with increased cu rrent density due to carrier injection related to the two current paths hav e been found. These temperature and carrier concentration distributions fou nd by the experiment agree very well with the results of 2D device simulati on. (C) 2000 Elsevier Science Ltd. All rights reserved.