Controlling short wavelength x-ray multilayer period variation on focussing optics

The variation of multilayer period, or d-spacing, across the reflecting surfaces of soft x-ray normal-incidence focussing optics is of primary importance in the optical performance of these systems. Focussing necessarily implies a specific optimal variation of period across the surfaces, and the multilayer bandpass sets the tolerance scale for acceptable deviations from this ideal variation.1,2 Mo/Si multilayers for use at wavelengths above 12.4 nm have relatively broad bandpasses, easing these tolerances. Multilayers for use at shorter wavelengths have significantly narrower bandpasses, thus placing significantly greater demands on the control of the period variation. Other design considerations, such as higher magnification systems and larger optics, also place more stringent demands on the control of multilayer period.