Semiconductor device structure and method for forming the same

A semiconductor device structure, comprising:

• a substrate;
• a fin structure over the substrate;
• and
• a gate stack wrapping around a first upper portion of the fin structure, wherein the gate stack comprises a first work function layer, a second work function layer, a gate electrode and a hard mask layer over the gate electrode, the second work function layer is over the fin structure, the first work function layer is between the second work function layer and the gate electrode, a first portion of the gate electrode extends into the hard mask layer, the first portion of the gate electrode is made of a conductive material, the gate electrode comprises a second upper portion and a lower portion, the second upper portion is wider than the lower portion, the second upper portion has a lower surface facing the substrate, and a second portion of the first work function layer is between the lower surface and a topmost surface of the second work function layer.

A method for forming a semiconductor device structure, comprising:

• forming a first fin structure over a substrate;
• forming a dielectric layer over the substrate and the first fin structure, wherein the dielectric layer has a first trench exposing a first portion of the first fin structure;
• forming a first work function layer in the first trench;
• forming a first mask layer over the first work function layer;
• forming a second mask layer over the first mask layer, wherein the second mask layer has an opening over the first trench;
• removing a second portion of the first mask layer in the first trench through the opening, wherein after the removal of the second portion, a third portion of the first mask layer in the first trench is remained over a bottom surface of the first trench;
• removing the first work function layer exposed by the first mask layer;
• removing the first mask layer and the second mask layer;
• and
• forming a first gate electrode in the first trench.