An automatic null ellipsometer with picometer sensitivity has been developed for in-situ sputtering rate monitoring of extreme ultraviolet multilayer fabrication. For realtime analysis of the in-situ ellipsometric data, we developed a layer-by-layer analysis method to obtain the optical constants and the thickness of each layer of the multilayer. The method was applied for fabrication of 40 periods Mo/Si multilayer. The results of analysis well demonstrate the picometer thickness sensitivity as accurate as necessary enough for multilayer period control. (C) 2004 Elsevier B.V. All rights reserved.