Silicon Inclusions in Aluminum Interconnects

A significant reliability related problem was detected during the course of a failure analysis performed on a 16K dynamic RAM. The problem was due to the presence of large silicon nodules in the aluminum metalization which, in comparison to the cross-sectional area of the metal stripe, were large enough to severely restrict current flow. Although silicon nodule formation, has been previously analyzed as a processing variable, it has not been regarded as a significant reliability concern at normal die temperatures. With the advent of VLSI technology and the resultant shrinking line widths, nodule formations must be re- evaluated as a potential yield and reliability concern. The nodule problem becomes serious when the nodule size reduces the effective metal line cross- sectional area such that significant current flow restriction occurs. MIL-STD-883C, Method 2018, Scanning Electron Microscope (SEM) examination procedures also do not readily detect these silicon nodules; nor do most other normal industry screening procedures. This paper discusses techniques used to locate the nodules, comparison of several different vendors product, theory of silicon nodule formation, ramifications to the VLSI industry and the reliability risk to the end user.