Fast joint in-device metrology of overlay and tilt of multi-tier 3DNAND devices

Multilayer stack height in 3DNAND has reached the limit of the aspect ratio that etching technologies can cost-effectively achieve. The solution to achieve further bit density scaling is to build the stack in multiple tiers, each etched separately. While lowering the requirements on etching aspect ratio, stacking two tiers introduces a critical overlay at the interface between the stacks. Due to the height of each stack, stress- or etch-induced effects, channel holes can be subject to strong tilt and overlay. Characterizing and controlling the resulting complex overlay and tilt fingerprints requires dense and frequent overlay metrology. The widely used electron beam metrology after etch-back is too slow and expensive for frequent measurements of overlay and tilt. This paper introduces a fast, non destructive optical data-driven method that is accurate & robust for In-Device Metrology (IDM) which allows for simultaneous monitoring of overlay and tilt on etched 3DNAND devices. A method for accurate, precise, robust and fast overlay measurements has been developed previously and successfully applied to DRAM [2] and 3DNAND [1]. The method relies on ASML Self-Reference (ASR) targets to create a data-driven model for in-device overlay measurements. Because ASR targets are special targets that are positioned on the scribe line, they allow model training and monitoring on any process wafer, without the need of manufacturing Design of Experiment (DoE) wafers or measuring wafers with external (destructive) reference metrology system. In this work we expand the already demonstrated capabilities of ASR overlay recipe creation to tilt metrology. We demonstrate that ASR targets can be successfully employed to create recipes that can simultaneously measure in-device overlay and tilt without throughput penalty. Using this unique capability, we characterize the relationship between overlay and tilt, allowing for a better breakdown and budgeting of overlay of 3DNAND structures. The potential of IDM to help process characterization and control is also demonstrated by illustrating how tilt monitoring can be used to characterize the behavior of etchers and the effect of their tuning parameters on after etch process.