A method includes etching a dielectric layer of a substrate to form an opening in the dielectric layer, forming a metal layer extending into the opening, performing an anneal process, so that a bottom portion of the metal layer reacts with a semiconductor region underlying the metal layer to form a source/drain region, performing a plasma treatment process on the substrate using a process gas including hydrogen gas and a nitrogen-containing gas to form a silicon-and-nitrogen-containing layer, and depositing a metallic material on the silicon-and-nitrogen-containing layer.