Characterization of extreme ultraviolet imaging systems

The optical performance of extreme ultraviolet (EUV) imaging systems is investigated. Wavefront-measuring point diffraction interferometry is implemented at extreme ultraviolet wavelengths near 13 nm to evaluate aberrations in near diffraction-limited, all-reflective, multilayer-coated optical systems intended for use in projection lithography at critical dimensions of 0.1 {dollar}mu{dollar}m and below. Measurements at the operating wavelength yield the overall EUV wavefront quality, which is influenced both by mirror surface profiles and by multilayer coatings. The interferometer design, based on the properties of light diffracted from small pinhole apertures, is suited for highly accurate measurements of wavefront aberrations over a wide range of wavelengths.; A phase-shifting point diffraction interferometer is used to characterize the aberrations of a 10{dollar}times{dollar} Schwarzschild multilayer-coated reflective optical system at an operating wavelength of 13.4 nm. A sub-aperture of the optic with a numerical aperture of 0.07 is measured to have a wavefront error of 0.090 wave (1.21 nm) rms at 13.4-nm wavelength, due mainly to astigmatism. Chromatic vignetting effects due to the limited transmission passbands of the multilayer coatings are observed via measurements at different wavelengths. The multilayer coating properties that match the measured wavelength-dependent coating effects are found and compared to the coating characteristics from previously reported measurements on individual mirrors.; The EUV interferometry experiments indicate measurement repeatability of {dollar}pm{dollar}0.008 wave ({dollar}pm{dollar}0.11 nm) rms at 13.4-nm wavelength in a numerical aperture of 0.07. The wavefront measurement accuracy is assessed by defocusing the wavefront, by detecting known systematic effects, and by investigating the alignment sensitivity. The measurement quality is probably limited by reference wavefront errors caused by somewhat oversize reference pinhole apertures. The errors in the reference wave are estimated to be roughly {dollar}pm{dollar}0.015 wave ({dollar}pm{dollar}0.20 nm) rms in a numerical aperture of 0.07. An independent qualitative verification of the interferometric measurements is also obtained from photoresist exposure experiments performed on the extreme ultraviolet lithography system for which the Schwarzschild optic was designed. The image quality observed experimentally is consistent with calculations that include the effects of the measured aberrations.; The performance of lithographic optical systems is also investigated analytically by considering the image degradation caused by aberrations. The relationships between the spatial frequencies of the aberrations, the object feature dimensions, and the degree of partial coherence are explored using the theory of imaging with partially coherent light. The effects of aberrations are also evaluated by using aerial image calculations for aberrations having spatial frequencies up to ten cycles over the radius of the imaging system pupil. The aberrations considered correspond to the spatial-frequency regime represented by the first several hundred Zernike polynomials. Furthermore, two figures of merit for quantifying permissible aberrations in imaging systems are proposed.