Design of via and stacked via test structures to evaluateelectromigration in thick metal AlCu metallizations

摘要

Thick metal modules are more and more important for mixed signal processes. For thick top metal lines it is possible to use conventional test structures. The mirror image via test structure with bottom metal as feeding line which is suggested in standard JESD 87 [2] doesn't work. It is caused by the geometrical relation of grain size, width and thickness and the required high operating current density. In an accelerated electromigration test the lower metal line is more sensitive for electromigration effect. The problem can't be solved by a change of metal width. The test structure is important to assess the quality of the interface of thick via, liner and metal and the via bottom region. To avoid electromigration effects in the bottom metal a very short lower metal line and an additional via is used. Optical inspections showed the expected migration effects in the thick metal line near force low site of the test structure.