Phase measurement of reflection of EUV multilayer mirror using EUV standing waves

Projection optics of an EUV lithography system consists of multilayer mirrors. Phase of the incident beam is shifted on reflection at the multilayer mirror [Y. Watanabe et al., Jpn. J. Appl. Phys. 30, 3053 (1991)]. If the phase shift at reflection of a multilayer is not well controlled, it becomes the cause of wavefront aberration. The phase shift depends on the incident angle and the wavelength of the beam. The phase shift is also dependent on the structure of the multilayer. Certain kinds of structural change cause non-negligible variation of the phase shift with very little change of wavelength dependency of reflectivity. Therefore, not only reflectivity measurement but also measurement of phase shift is essential to manufacture multilayer mirrors for projection optics. X-ray standing wave technique has been used to characterize multilayer structure [B. Lai et al., Nucl. Instrum. Methods Phys. Res. A 266, 684 (1988); T. Kawamura and H. Takenaka, J. Appl. Phys. 75, 3806 (1996)]. Intensity of electric field, which generates photoelectrons, near a multilayer surface depends not only on the intensity of incident beam but also on the phase shift and the reflectivity. EUV reflectivity and photo-yield of Mo∕Si multilayer samples were measured using a laser produced plasma source (LPP) based EUV reflectometer [A. Miyake, Proc. SPIE 5037, 647 (2003)]. Thickness ratio Γ is the ratio of the thickness of molybdenum layer to the multilayer period. Γ of each sample is changed by 2% step. According to a simulation, it corresponds to variation of phase shift by 0.02πstep, in other words, variation of wavefront by 10mλstep. Measured wavelength dependencies of photo-yield for each sample are clearly separated. From the results of this experiment, it is shown that phase shift on multilayer reflection can be detected at a resolution of 0.02πrad.