Effect of an Ag buffer layer on a Cu/Ag/Si system

It is well known that Cu reacts with Si to form copper silicide, even at room temperature. To prevent copper silicide from forming on an Si substrate when Cu is used for wiring or as electrodes, it is necessary to place a buffer layer between Cu and the Si substrate. Silver works as a buffer layer between Cu and the Si substrate to form a layered Cu/Ag/Si structure. In the present paper, we determine the minimum effective Ag buffer layer thickness to prevent Cu diffusing into the Si substrate. The results show that Cu can reach the Si substrate through a 5 nm thick Ag film, but that a 50 nm film prevents Cu from diffusing through to the Si substrate. Using scanning electron microscopy (SEM) to observe the interface layer, it was found that round Ag islands form on the Si substrate, whereas Cu appears as a few large square islands and a lot of small clusters. The small Cu clusters tended to wedge into the gaps between the Ag islands. Furthermore, the results indicate that Cu and Ag repel each other.