A method for fabricating semiconductor device is disclosed. The method includes the steps of: providing a substrate having an interlayer dielectric (ILD) layer thereon; forming a first recess, a second recess, and a third recess in the ILD layer; forming a material layer on the ILD layer and in the first recess, the second recess, and the third recess; performing a first treatment on the material layer in the first recess; and performing a second treatment on the material layer in the first recess and second recess.