Novel methods of synthesis of a-Si(H)/Mo multilayers for Extreme UV applications

This report is focused on the control of the plasma sputtering process used for the synthesis of multilayer mirrors for extreme UV and soft X-ray optical devices. The case of a- Si/Mo and of a-Si(H)/Mo multilayers will be discussed, with emphasis on the study of the basic parameters that control the growth of the single Mo and a-Si(H) layers. The deposition apparatus will be described. The effects of the plasma composition and of the bombardment of the growing layers by Ar+ ions and electrons on the properties of the layers are reported. The hydrogen concentration can be varied up to about 30 at% in the a-Si(H) layers: the hydrogen incorporation leads to a decrease of the material density. The accurate control of the plasma density and of the electron temperature and plasma potential is accomplished through the Langmuir probe method, that permits the accurate measurement of the energy of the Ar+ ions and of their fluence. The possibility of using the present apparatus for the synthesis of a class of ML designed for smaller wavelength applications (down to the water window spectral region) is discussed.