Effect of non-uniform current distribution in the via patterns on microelectronics reliability

A lifetest experiment performed on ECL (emitter coupled logic) ring oscillators, to establish the reliability of circuits fabricated with an oxide-isolated Si bipolar process, displayed an unusual failure mode. Even though the metal current density was well below the electromigration-limited current density specified by MIL-M-38510, electromigration type failures were observed in a standard current-distributing via pattern at a specific location of the circuit. The metal deposition and etch process were carefully monitored and metal quality did not indicate any abnormality. A very careful data analysis showed that, at higher currents, the current crowding through a single via explained the low median time to failure. A uniform current distribution pattern is necessary to maintain quality circuit reliability. If multiple vias are required, each via chain should be placed in a separate routing path, with properly adjusted metal line widths between the vias to account for the via resistance and the metal line resistance ratio