For pure Mo/Si-multilayers with period thicknesses between 6.5nm and 7.0nm the deposition parameters of the magnetron sputtering process have been optimized so that normal incidence reflectivities of REUV=68.7% (l=13.39nm, a=1.5°) can be realized with a 60-period Mo/Si multilayer. Although this is one of the best experimental results achieved so far, there is still a gap between this experimental value and its theoretical limit (REUV=75.5%). One of the main reasons for this discrepancy is the formation of intermixing zones at the interfaces. From HRTEM micrographs it can be concluded that the thicknesses of the intermixing zones are 1.2nm and 0.7nm for Mo-on-Si and Si-on-Mo, resp. Therefore a further improvement of the EUV reflectivity requires the complete suppression or at least reduction of these intermixing zones. This can be achieved by the introduction of tiny interdiffusion barrier layers on the individual interfaces having thicknesses between 0.2 and 1.0nm. The resulting EUV reflectivity of this new type of EUV mirrors is 69.8% (l=13.42nm, a=1.5°) and 71.4% (l=12.52nm, a=22.59°) (measured at PTB/BESSY2).
For pure Mo/Si-multilayers we have developed a multilayer structure model consisting of 4 layers per period. Calculations of the EUV reflectivity using this model and replacing the MoSi2 intermixing layers by B4C and C, resp., show that the resulting reflectivity is strongly depending on the thicknesses and the material of the barrier layers on the different interfaces, Mo-on-Si and Si-on-Mo. Whereas the thicknesses of the barrier layers have to be <0.5nm on the Si-on-Mo interfaces, the thicknesses of the barrier layers on the Mo-on-Si interfaces can be of the same size as the MoSi2 layers without losing EUV reflectivity.
The analytical characterization of the two different barrier layers within the Mo/Si-multilayer stack shows that very smooth interfaces are achieved. The number of observable bragg reflections is similar to that of pure Mo/Si multilayers. From HRTEM investigations it can be concluded that the typical intermixing zones are no longer observable. Furthermore, barrier layers on the Mo-on-Si interfaces lead to a changed growth mechanism of the Mo-layers. In difference to pure Mo/Si multilayers, where lattice fringes within the Mo layers are detected in the HRTEM micrographs, no indication of crystalline matter can be found. Investigations of the multilayers by X-ray diffraction confirm this effect: the intensity of Mo diffraction peaks is considerably decreased. Additionally a peak shift is found, which corresponds to an increase of the lattice parameter of Mo by 2%.