At-wavelength characterization of an extreme ultraviolet camera from low to mid-spatial frequencies with a compact laser plasma source

The final alignment and characterization of an extreme ultraviolet (EUV) lithographic projection optics system must be performed at the operating wavelength in order to account for errors associated with the combined effect of multilayer coating and substrate fabrication error. Two complementary metrology techniques have been developed which can operate on a compact laser plasma source: EUV lateral shear interferometry (LSI) and aerial image monitoring (AIM), LSI quantifies residual low spatial frequency wave front errors associated with the "figure" of each optical element. To characterize the effect of mid-spatial frequency errors that lead to scatter in the image plane, an EUV aerial image monitor was employed. EUV interferometry was compared to visible light interferometry measurements of the optical system and was found to be in reasonable agreement for low spatial frequency errors. The contrast as measured by AIM and photoresist exposures indicates an additional reduction of contrast associated with scatter due to large mid-spatial frequency errors. (C) 1997 American Vacuum Society.