Practical considerations for Wafer-Level Electromigration Monitoring in high volume production

Abstract

Reliability monitoring is an important part of process control in high volume production. For the back end of line (BEOL), a wafer-level electromigration (WL-EM) test is usually the method of choice to get a good indication of process variation (Schuster, 2001). In this work we present practical normalization procedures to ensure an appropriate wafer to wafer comparison which is independent of variation in cross-sectional area as well as of the initial resistance spread. The measurements have been performed on a commercially available 300mm multi-side probe station, using custom-made software to implement the current ramp and resistance measurement. The test conditions were achieved through Joule heating; the test structures used were 800mum long single lines (no vias) in metal 1 to metal 3, varying in width from 0.14mum to 10mum. After several normalization steps described in this paper we found a strong activation energy dependence on line width. This dependence was linked to issues in temperature investigation using a constant TCR value. Additionally we found a simple way to estimate the current density exponent by optimizing the Arrhenius relation. Overall a comprehensive guideline for constant current WL-EM is presented