The advent of CD-SEM technologies for metrology on advanced photomasks

This paper chronicles the implementation of the low voltage CD-SEM (Critical Dimension-Scanning Electron Microscope) as a metrology tool for mask production. Accuracy and precision were investigated with the emphasis on improving both dynamic and static precision. Algorithm evaluation was focused at improving the confidence in the measurement, and its correlation with the on-wafer CD. Linearity was compared to known standards as well as absolute pitch values written by an advanced e-beam lithography tool. Another concern that was addressed was contamination due to the beam/sample interaction. Most notably comprised of elemental carbon, the dose/contamination relationship was investigated. Utilizing a KLA-Tencor starlight inspection tool, the dose required to register a surface defect was determined. Subsequent printing of the mask to a wafer, and examination of the same regions on the wafer by the CD-SEM, looked for variations in measured CD to determine whether the contamination affected the printed wafer.