As Advanced IC Substrates (AICS) add more RDL layers, requiring additional via connections between the RDL layers, the potential for cumulative overlay shift increases. This overlay shift can lead to longer RDL traces, which increases interconnect resistance, resulting in lower yield. Keith Best, director of product marketing, for lithography at Onto Innovation, talks with Semiconductor Engineering about total overlay - the sum of the whole stack - and the challenges of tracking these overlay errors over the entire stack.