A method for increasing adhesion between polysilazane and silicon nitride is disclosed, comprising, providing a substrate comprising a trench, forming a silicon nitride liner layer on a bottom surface and a sidewall of the trench, performing a treating process to the silicon nitride liner layer for producing a hydrophilic surface with OH groups that can increase adhesion between the silicon nitride liner layer and a subsequently formed polysilazane coating layer, and forming a polysilazane coating layer into the trench and on the silicon nitride liner layer.