Experimental Study on Electromigration by Using Blech Structure

Abstract

In the advanced semiconductor industry, modern electronic devices are expected to realize complex functions with minimized size, which requires an increase in the density of on-chip interconnects. To meet this demand, the dimension of interconnects is reduced and it requires the narrowing of metal interconnects to carry the increasing current density. With such a developing trend, electromigration is one of the significant reliability challenges for electronic devices. Although lots of works focus on the formulation and simulation for electromigration, they are not complete and consistent with experimental results. Recently, A fully-coupled and self-consistent electromigration theory was developed by Cui et al, but new and extensive experimental data and analysis are needed to further validate their theoretical results.
This research work focuses on the experimental study on electromigration (EM), and the aim is to investigate the different effects on EM. The Blech structure was proposed as our experimental structure. The fabrication of Blech structure is conducted at the EKL cleanroom, and main structures with various dimensions are fabricated by employing sputtering technique for deposition and lithography for defining patterns. The measurement is carried out by accelerated tests with high current stress and at elevated temperature. Experimental results are characterized and analyzed by different tools, such as Keyence 3D laser profilometer and scanning electron microscope
With the current density of 1×1010퐴/푚2 and temperature of 250 ℃, the result shows that the critical length under certain conditions is 10 μm, and longer stripes have larger drift lengths and a shorter time to form voids in electromigration. Furthermore, it is found that with elevated temperature, the drift length increases and the electromigration lifetime decreases. In addition, the covered SiN passivation layer only for the annealed Al stripes suppresses electromigration and this is because the annealing process improves the coalescence of grain in Al film, reducing the defects at the grain boundary and finally forming a denser microstructure. The influence of the atmosphere on electromigration indicates that the additional oxide on Al interconnects increases that actual current stress and results in a short electromigration lifetime. In general, present experimental results were consistent with existing results in the literature, but several problems are still unsolved, which will be part of our future work.