20nm MOL overlay case study

As the process nodes continue to shrink, overlay budgets are approaching theoretical performance of the tools. It becomes even more imperative to improve overlay performance in order to maintain the roadmap for advance integrated circuit manufacturing. One of the critical factors in 20nm manufacturing is the overlay performance between the Middle of Line (MOL) and the Poly layer. The margin between these two layers was a process limiter, it was essential that we maintain a very tight overlay control between these layers. Due to various process and metrology related effects, maintaining good overlay control became a challenge. In this paper we describe the various factors affecting overlay performance and the measures taken to mitigate or eliminate said factors to improve overlay performance.