Reservoir effect and maximum allowed VIA misalignment for AICu interconnect with tungsten VIA plug

In this paper the maximum allowed VIA misalignment is determined from VIA electromogration (EM) performance. It is found that positive overlay of metal interconnect at VIAs serves as a reservoir, leading to increased VIA lifetimes. On the onther hand decreasing metal overlap with VIA does nog reduce the VIA EM lifetime. Besides the reservoir effect, the area or length of the metal lines plays a direct role too in determining the maximum allowed misalignment. Further investigations on mis-aligned VIAs confirm the hypothesis of an electro-chemical process of VIA W corrosion due to charging. This leads to the identification of a sensitive test structure to monitor the concerned VIA W corrosion problem and results in a few options for process improvement. A safe relaxation of the misalignment control, which reduces re-work in the fab, can therefore be allowed without increasing reliability risks.