Effects of sidewall scallops on the performance and reliability of filled copper and open tungsten TSVs

Abstract

The effects of the presence of scallops along the sidewalls of filled (copper) and open (tungsten) TSVs are studied. The Bosch process is used in order to generate highly vertical deep trenches; however, the process results in scallops along the etched sidewalls. A model for the Bosch process is implemented in an in-house level set simulator in order to generate various TSV structures with small and large sidewall scallops. The resulting geometries are imported into a finite element tool in order to analyze the performance and reliability of the devices. The electrical parameters of the TSVs are shown to vary when scallops are present for both types of TSVs. In addition, the maximum thermo-mechanical stress increases in the presence of scallops, while the average stress along the interfaces remains relatively unchanged. Electromigration analyses were also performed on the structures in order to determine stress development during the early stages of operation. It was found that the filled TSV with scalloped sidewalls experiences a higher current density and suffers from increased stress, while the sidewall scallops do not cause variation in the stress of open tungsten TSVs. The open tungsten TSVs experience most Electromigration-induced stress in the connecting metal layers and not along the sidewall.