forming a contact opening in a wafer, wherein the wafer comprises a substrate, a gate structure over the substrate and a dielectric layer over the substrate and surrounding the gate structure, and the contact opening passes through the dielectric layer and exposes the substrate;

forming a recess in the substrate such that the recess is connected to the contact opening;

performing an oxidation process to convert a portion of the substrate exposed in the recess to form a protection layer lining a sidewall and a bottom surface of the recess of the substrate;

etching back the protection layer to remove a first portion of the protection layer in contact with the bottom surface of the recess of the substrate such that a second portion of the protection layer is in contact with the sidewall of the recess of the substrate; and

forming a metal alloy structure at the bottom surface of the recess of the substrate.