The increasing usage of soft x-ray radiations due to the development of synchrotron radiation source and others demands varied optical manipulation in the soft x-ray spectral region. The phase manipulation is important because this leads to the control of the polarization state of a soft x-ray radiation. A new criterion for selecting material pairs for the maximum phase difference between the s and p polarization through a multilayer (ML) structure was proposed and tested theoretically. Transmission characteristics of ideal ML. structures in the soft x-ray spectral region have been studied by simulation. The results have been applied to optimize ML structures for quarter-wave plates at 4.4 and 13 nm. It was found that Rh/Si and Co/K MLs are good candidates for quarter-wave plates at 13 and 4.4 nm, respectively.