Laser interferometric method for ns-time scale thermal mapping of Smart Power ESD protection devices during ESD stress

A backside heterodyne interferometric technique is presented to study thermal effects in smart-power electrostatic discharge (ESD) protection devices during the ESD stress. The temperature increase in the device active area causes an increase in the silicon refractive index (thermo-optical effect) which is monitored by the time-resolved measurements of optical phase changes. Thermal dynamics and spatial temperature distribution in different types of npn transistor structures biased in the avalanche multiplication or snapback regime are studied with nanosecond time and micrometer spatial resolution. The activity and inactivity of the bipolar transistor action is indicated by the dominant signal arising from the emitter or base region, respectively. Hot spots have been found at the edges of the structures and attributed to the current crowding effect in the emitter. © 1999 Elsevier Science Ltd. All rights reserved.