The invention discloses a formation method of an interfacial layer and a formation method of a
metal gate
transistor. The formation method of the interfacial layer is different from a conventional formation method of an interfacial layer. The formation method of the interfacial layer involves forming a high-K
dielectric layer first and then forming the interfacial layer and specifically comprises performing annealing
processing in a gas
atmosphere containing an oxidation gas, wherein during an annealing process, the oxidation gas with quite
high energy in a high temperature environment can penetrate the high-K
dielectric layer and is diffused to an interface between the high-K
dielectric layer and a substrate so as to be contacted with the substrate, such that the surface, which is contacted with the high-K
dielectric layer, of the substrate can be oxidized and the interfacial layer is grown. Since the interfacial layer is formed after the high-K
dielectric layer, some defects of the high-K
dielectric layer can be restored during the process of forming the interfacial layer, for instance, in the annealing process when the interfacial layer is formed, the oxidation gas can supplement
oxygen atoms to the high-K dielectric layer to enable the actual components of the high-K dielectric layer to be more similar to corresponding components in an ideal chemical molecular formula.