Soichi Inoue

EUV actinic blank inspection tool with a high magnification review mode

Because the realization of defect-free Extreme Ultra-violet Lithography (EUVL) mask blanks is uncertain, the defect mitigation techniques are becoming quite important. One mitigation technique, Pattern shift, is a technique that places a device pattern to cover multilayer (ML) defects underneath the absorber pattern in such a way that the ML defects are not printed onto wafers. This mitigation method requires the defect coordinate accuracy of down to tens of nanometers. Consequently, there is a strong demand for a Blank Inspection tool that is capable of providing such defect coordinate accuracy. To meet such requirement, we have started to develop a high accuracy defect locating function as an optional feature to our EUV Actinic Blank Inspection (ABI) system which is currently being developed aiming at HVM hp16 nm-11 nm node. Since a 26x Schwarzschild optics is used in this inspection tool, it is quite difficult to pinpoint defect location with high accuracy. Therefore we have decided to realize a high magnification review optics of 600x or higher by adding two mirrors to the Schwarzschild optics. One of the additional two mirrors is retractable so that the magnification can be switched according to the purpose of inspections. The high magnification review mode locates defect coordinates accurately with respect to the fiducial position. We set the accuracy target at 20 nm so that the mitigation technique can be implemented successfully. The optical configuration proposed in this paper allows both a high speed inspection for HVM and a high accuracy defect locating function to be achieved on one inspection system.