The Effect of Wafer Edge Cu Contamination on FinFET Devices

This paper presents the results of an investigation to gain a better understanding of the impact of wafer substrate copper (Cu) contamination on FinFET devices. A chip from a wafer free of Cu contamination and several chips near a Cu contaminated wafer edge were sampled for chemical, structural, and morphological analysis and electrical device performance testing. The contaminated wafer was also annealed at high temperature, trying to drive Cu diffusion further into the Si substrate. TEM analysis revealed that the Cu interacted with Si to form a stable η-Cu₃Si intermetallic compound. SIMS analysis from the backside of the wafer detected no Cu even after most of the backside material was removed. Likewise, electrical nanoprobing showed no parametric drift in the FinFETs near the edge of the wafer, comparable to device behavior in a Cu-free Si substrate. These results indicate that the formation of η-Cu₃Si with a well-defined crystalline structure and stable stoichiometry immobilizes Cu diffusion in the Si substrate. In other words, the impact of Cu diffusion in silicon has no effect on device performance as long as η-Cu₃Si does not form in the FinFET channel or short any structures within the chip.