Semiconductor device structure and method for forming the same

A method for forming a semiconductor device structure, comprising:

• forming a spacer layer and a dielectric layer over a fin structure, wherein the fin structure is over a substrate, the spacer layer has an opening exposing a portion of the fin structure, and the dielectric layer surrounds the spacer layer;
• forming a metal silicon nitride layer over an inner wall and a bottom surface of the opening;
• forming a first work function metal layer over the metal silicon nitride layer;
• removing a first top portion of the first work function metal layer;
• after the removal of the first top portion, removing a second top portion of the metal silicon nitride layer;
• forming a conductive layer in the opening;
• and
• removing a third top portion of the conductive layer and a fourth top portion of the metal silicon nitride layer.

A method for forming a semiconductor device structure, comprising:

• forming a spacer layer and a dielectric layer over a substrate, wherein the spacer layer has an opening exposing a portion of the substrate, and the dielectric layer surrounds the spacer layer;
• forming a gate dielectric layer over an inner wall and a bottom surface of the opening;
• forming a metal silicon nitride layer over the gate dielectric layer;
• forming a first work function metal layer over the metal silicon nitride layer;
• removing a first top portion of the first work function metal layer;
• after the removal of the first top portion, removing a second top portion of the metal silicon nitride layer;
• forming a conductive layer in the opening;
• and
• removing a third top portion of the conductive layer, a fourth top portion of the metal silicon nitride layer, and a fifth top portion of the gate dielectric layer.