Reliability of multi-layer aluminum capped copper interconnect structures

Driven for further silicon reduction, wireless applications utilize copper interconnection and increase metal layer count from three to five layers. More aggressive ESD structures placed under the bond pads offer a significant opportunity for additional die area and cost reduction. Capping copper bond pads with aluminum was selected as the primary approach for probing and wire bonding of copper devices. There is an integral relationship between probe damage on the bond pads and subsequent wire-bondability. As the pad geometry decreases, the ratio of the area of probe damage to the bond pad size becomes proportionally larger, thereby reducing the available aluminum necessary to form reliable gold-aluminum intermetallic coverage. This paper describes probe and assembly processes developed for a fine pitch three-metal layer copper interconnect device with ESD structures placed under the bond pads. The relationship between probe conditions and wire-bondability were examined. Ball shear, wire rip and corresponding failure modes were evaluated at various read points of thermal aging studies to evaluate the integrity of ball bonds to the metal stack. Reliability assessment was also performed. Based on the investigations studying the relationship between the pad structures, probe and wire bond quality, recommendations were derived to ensure high quality, stable and reliable bonds for fine pitch wire bonding on multi-layer copper interconnect devices.