Reticle blank inspection and its role in zero-defect manufacturing

Stringent specifications require that reticle makers carefully examine the role blanks play in reticle quality. Photronics and KLA-Tencor are jointly examining several aspects of this issue. As part of this investigation, PBS blank quality was tested in a production environment using the KLA-Tencor STARlight inspection system. PBS blanks were inspected using a 500-nm pixel with the highest sensitivity settings. We completed a comprehensive study using an effective blank defect test pattern. The test pattern was chosen to maximize the probability that a blank defect will fall on a chrome-to-quartz transition. Several test reticles were inspected and reviewed before writing, and reviewed a second time after processing. 452 defects were classified using three variables: blank defect size, blank defect type and reticle defect type. Some blank defect sizes and types transferred to the test reticles with probabilities exceeding 80%. False defect rates were less than 0.5%. Defect statistics for two blank suppliers are presented. We outline the phases of the research, present the results and discuss the implications for production reticles. We demonstrate techniques that can be used before writing and processing to assess the probability of defect transfer. Plans for a trial protocol for blank inspection are presented.