Photomask CD correlation of an optical linewidth measurement tool and a scanning electron microscope with reference to a Stylus NanoProfilometer

摘要

Abstract: As photomask critical dimensions extend well into the submicron range, optical measurement techniques are approaching the end of their useful life. While offering advantages of being nondestructive, relatively fast, and very precise, optical measurement tools may have accuracy problems due to diffraction. Linearity between measured and actual values is typically lost due to interference, resonance, and shadowing effects even before the diffraction limit is reached. In addition, standards for submicron features do not exist for optical tools. These limitations have given rise to using a Scanning Electron Microscope (SEM) for reticle dimension measurement. Another complication is that no NIST standard exists for chromium photomask dimensions. A CD SEM must therefore be calibrated to some other trusted standard or measurement method. A Stylus NanoProfilometer (SNP) has better inherent resolution than a SEM and was chosen as the standard measurement instrument for both CD SEM and optical tool metrology. This paper describes the cross-calibration of the Leica LWM-250 (white light) optical metrology tool and the KLA-Tencor 8100XP-R CD SEM, with reference to a Surface Interface SNP9000. The primary motivation was to define the usable ranges of both SEM and optical CD measurement systems, allowing for flexible implementation into a photomask manufacturing environment. !5