Recent developments in EUV reflectometry at the advanced light source

In order to satisfy the metrology requirements of multilayer coatings for EUVL optics and masks, improvements have been made to the reflectometry beamline at the Advanced Light Source. The precision in determining multilayer peak reflectance and wavelength has been improved by reducing the measurement noise. The peak reflectance of a typical Mo/Si multilayer can now be measured with a precision of 0.08% rms,(relative) and the centroid wavelength with a precision of 0.007% rms. It has now been possible to determine the contribution of scattered light to the spectral purity. Under the typical measurement conditions the scattered light accounts for about 1.3% of the incident beam. With an appropriate slit it is possible to reduce the scattered light to 0.25%. By correcting for the remaining scattered light, it is estimated that a reflectance accuracy of 0.1% (absolute) is obtained for a typical Mo/Si multilayer.