铜工艺的电迁移，UG上的话理解不了

魅影变奏曲

Copper Interconnect
State-of-the-art process technologies use copper as chip interconnect material because copper
offers not only lower resistivity but also higher resistance to electromigration wearout. However,
copper interconnect structures are not as deep as aluminium structures, so the cross-section area
is smaller and therefore the current densities are higher. As a result, electromigration-based
failures still occur in copper structures.
In contrast to aluminum-based processes, vias built with copper increase the likelihood of
electromigration wearout and require more detailed verification.

这是EPS ug  13.1 版本，第九章，EM效应中的话。 在129/400 页

他说铜的互联结构不如铝的深，还说交界面的面积小导致电流密度大，所以对比铝工艺，孔的位置更容易EM

请大家帮忙从工艺的角度分析一下是什么原理，造成了这个现象。

谢谢大家！！

我先前发在了工艺板块，没人理我，发在这里试试。