We previously demonstrated that the introduction of silicon oxide (SiO2) layers into the interfaces of the Mo/Si multilayer was quite effective in improving the heat stability of the multilayer [M. Ishino etal, Surf. Coat Technol. (to be published)]. However, the soft x-ray reflectivity of the Mo/Si multilayer with SiO2 layers decreased with the thickness of the SiO2 layer because of the large absorption of x rays by oxygen. In this article, an optimization of the thicknesses of the SiO2 layers inserted in the Mo/Si multilayer has been performed so as to minimize the decrease in the soft x-ray reflectivity with the high heat stability maintained. An asymmetric SiO2 layer thicknesses of 0.5 and 1.5 nm are found to be most favorable at the Si-on-Mo interface and at the Mo-on-Si interface, respectively. © 2002 American Institute of Physics.