Functional flatness impact targets (Key Performance Indicators) for the high-volume manufacturing of EUV photomask blanks

We investigate the current state of four functional specifications as they relate to EUV photomask blank flatness and the ways in which each functional specification limits, constrains, and controls aspects of image placement and focus error at the wafer following exposure. We explain the relationship between the functional specifications and the ultimate impact they have at wafer, by connecting the key performance indicators (KPIs) to their topographically related features at the mask. Sample mask data sets are then evaluated against the KPIs to demonstrate how such tolerances can open up the process window for certain photomask processing steps, while still achieving the overall lithography process specifications. This exploratory paper aims to offer an interpretation of what these topographic tolerances mean and ultimately help end users understand the potential implications of adopting such functional tolerances. A thorough understanding of such specifications will be necessary as the industry moves forward to future nodes, in order to successfully develop processes which adhere to the upstream tolerances necessary to create mask topographies which enable the mask to meet the high-performance requirements for successful wafer exposures.