Semiconductor device and manufacturing method thereof

A semiconductor device, comprising:

• a substrate comprises a semiconductor fin;
• a plurality of spacers disposed over the semiconductor fin;
• a dielectric layer disposed over the semiconductor fin and between the spacers;
• and
• a plurality of metallic layers disposed over the dielectric layer, wherein the metallic layers respectively have compositions Xa1Y1-a1, Xa2Y1-a2 XanY1-an, and the compositions XanY1-an substantially satisfy Xa(n-1)Y1-a(n-1)≠XanY1-an.

A method, comprising:

• forming a gate stack over a fin structure of a substrate, wherein forming the gate stack comprises:
• forming a dielectric layer over the fin structure;
• forming a first metallic layer having a first element;
• forming a second metallic layer having the first element over the first metallic layer, wherein a percentage of the first element in the first metallic layer is greater than that in the second metallic layer, and forming the first and second metallic layers are in-situ performed in a same chamber;
• and
• forming a gate electrode over the second metallic layer.