A comparison of via overetch variations between conventional Al-W and dual-inlaid copper integrations

A comparison of via overetch is made between a conventional integration using aluminum interconnects plus tungsten via plugs and a dual-inlaid integration using copper. Excessive overetch for Al interconnects can cause reliability problems because of veil formation, as well as create very high aspect ratio recesses that are difficult to fill. The reasons for variation in interlevel dielectric (ILD) thickness and via etch rate are discussed. For the Al-W interconnect system, oxide CMP controls the ILD thickness. In addition, the via etch rate has been observed to drop over time. Both contribute significant variations to via overetch. For a via-first dual-inlaid integration, the ILD deposition determines the via depth uniformity. In metal-first dual-inlaid, the metal trench etch controls the via depth. In both cases, the via etch rate has been optimized to be more stable over time. Overall, the Al-W integration has a very wide range of via depths and etch rates that must be tolerated, whereas the dual-inlaid integrations have only a few percent of variation