Real case study for isothermal EM test as a process control methodology

The isothermal electromigration (EM) test is a very quick method to evaluate metal quality (Jones and Smith, 1987). However, most people do not have adequate confidence in its accuracy due to the fact that its failure mechanism is not easily controlled. By carefully controlling the failure criteria, we have shown that the failure mechanism in isothermal is similar to that of conventional EM tests. The isothermal test has been shown to be a good method to evaluate the metallization process in a production line. In this work, we first determined the failure criterion of isothermal EM from resistance degradation with stress time. We found that the failure mode of isothermal test under control was the same as that of package level electromigration. Furthermore, we used the failure criterion (delta R/R₀=2%) to extract activation energy and found the result to be extremely encouraging. The activation energies from conventional EM tests and isothermal tests are almost the same. From failure mode and activation energy analysis, the isothermal failure mechanism could be clearly identified. Furthermore, skew lots and maverick lots were differentiated from normal production lots, thereby successfully demonstrating the value of this new test methodology in volume manufacturing