A pragmatic approach to high sensitivity defect inspection in the presence of mask process variability

As design rules continue to shrink towards 4x nm, there are increase usage of aggressive Optical Proximity Correction (OPC) in reticle manufacturing. One of the most challenging aggressive OPCs is Sub Resolution Assist Feature (SRAF) such as scattering and anti-scattering bars typically used to overlap isolated and dense feature process windows. These SRAF features are sub-resolution in that these features intentionally do not resolve on the printed wafer. Many reticle manufacturers struggle to write these SRAFs with consistent edge quality even the most advanced E-Beam writers and processes due to resolution limitations. Consequently, this inconsistent writing gives reticle inspection challenges. Large numbers of such nuisance defects can dominate the inspection and impose an extraordinarily high burden on the operator reviewing these defects. One method to work around inconsistent assist feature edge quality or line-end shortening is to adjust the mask inspection system so that there is a substantial sensitivity decrease in order to achieve good inspectability, which then compromises the sensitivity for the defects on main geometries. Modern defect inspection tools offer multiple modes of operation that can be effectively applied to optimize defect sensitivity in the presence of SRAF feature variability. This paper presents the results of an evaluation of advance inspection methods and modes such as die to database selective thinline desense, transmitted & reflected light inspections, review system and die to die selective desense to increase inspectability and usable sensitivity using challenging production and R&D masks. Key learnings are discussed.