Fiducial mark requirements from the viewpoints of actinic blank inspection tool for phase-defect mitigation on EUVL mask

For Extreme Ultra-Violet Lithography (EUVL), fabrication of defect free multi-layered (ML) mask blanks is one of the difficult challenges. ML defects come from substrate defects and adders during ML coating, cannot be removed, and are called as phase defect. If we can accept ML blanks with certain number of phase defects, the blank yield will be drastically up. In order to use such blanks, the phase defects need to be identified and located during ML blank defect inspection before absorber patterning. To locate phase defects on the blanks accurately and precisely, Fiducial Marks (FM) on ML blanks are needed for mask alignment and defect location information. The proposed requirement of defect location accuracy is less than 10 nm [1]. In addition to the previous study for which FMs were etched by Focused Ion Beam (FIB) [2], we fabricated FMs by resist exposure by E-Beam (EB) writer and etching process, and inspected FMs with EUV Actinic full-field mask Blank Inspection (ABI) prototype developed at MIRAI-Selete, EB writer and other mask inspection tools. Then we estimated FM registration accuracy for several line widths and depths. In this paper, we will present the result of feasibility study on the requirements of FM on EUVL mask by experiments to establish the phase defect mitigation method. And the optimum ranges of FM line width, depth, and fabrication method on EUVL mask based on above results are 3 - 5 m line width, not less than 100 nm depth FM etched into ML respectively.