A semiconductor device and manufacturing method thereof having a diffusion barrier layer formed on a semiconductor wafer, whose surface region is provided with a silylation layer, wherein the silylation layer is formed on the diffusion barrier layer which is formed on the semiconductor wafer, by a plasma process using silicon hydride or by a reactive sputtering method using SiH.sub.4. When the metal layer is formed on the silylation layer, the wettability between the diffusion barrier layer and the metal is enhanced and large grains are formed, thereby increasing the step coverage for the contact hole of the metal layer or for the via hole. Additionally, when heat treatment is performed after the metal layer is formed on the silylation layer, the reflow characteristic of the metal layer becomes good, to thereby facilitate the filling of the contact hole or the via hole easy. When the wiring layer is thus formed, the metal wiring having good reliability can be obtained and the subsequent process is rendered unnecessary.