The market transition from 2D to 3D-NAND in recent years requires strict focus control and monitoring solutions. ASML’s μDBF targets (micro Diffraction Based Focus) enable on-product focus measurement which can be used to optimize scanner correction. Additionally, dense computational focus maps can be generated by combining μDBF measurements with scanner metrology such as non-correctable leveling error. This paper discusses the focus variability observed on memory layers through on product focus monitoring. This work will show how exposure at best focus can be performed for immersion lithography in the case of strong focus fingerprints. Focus monitoring data from μDBF and computational focus metrology will be used to generate and apply corrections on two 3D-NAND layers.
With each technology node, overall focus budgets have become increasingly tighter in order to meet the necessary product requirements. The 7nm node has required us to define new opportunities for addressing top contributors to the focus budget. Field curvature in particular has been identified as a key contributor to the intrafield focus budget, contributing around 50%. This paper will introduce two new methodologies for improving field curvature; one a hardware solution and one a software solution.