The effect of aspect ratio scaling on hydrostatic stress in passivated interconnects

In this study, numerical work using ANSYS and analytical work based on Eshelby models were performed to examine the effect of aspect ratio scaling on the hydrostatic stress in passivated metal interconnects. Aluminium and copper interconnects passivated with phosphosilicate glass (PSG) with aspect ratios ranging from 1 × 10^{- 4} to 100 were studied. Copper interconnects in damascene structure were also studied. The results from analytical models agreed well with numerical results and relevant experimental results. The results showed a decreasing trend of hydrostatic stress with aspect ratio for narrow interconnects, and increasing trend of hydrostatic stress for wide interconnects, with a maximum hydrostatic stress at an aspect ratio of 1. It was observed that there is a large scaling effect. For example, in the case of aluminium interconnect, stress values vary between 50 MPa and 463 MPa. It was also observed from the hydrostatic stress contours that the regions of highest stress do not correspond to the void locations seen experimentally. This implies that it is insufficient to look only at hydrostatic stress for determination of failure sites. Another factor that should be examined is the stress gradient.