Semiconductor device and fabrication method thereof

1. A method, comprising:

forming a gate dielectric layer over a semiconductor substrate;

forming a metal element-containing layer over the gate dielectric layer; and

thermal soaking the metal element-containing layer in an aluminum-containing gas, such that aluminum atoms from the aluminum-containing gas are diffused into an upper portion of the metal element-containing layer to form an aluminum-containing layer over the metal element-containing layer; and

thermal soaking the aluminum-containing layer in a silicon-containing gas, such that silicon atoms from the silicon-containing gas are diffused into an upper portion of the aluminum-containing layer to form a silicon-containing layer over the aluminum-containing layer.

2. The method of claim 1 , wherein forming the metal element-containing layer and thermal soaking the metal element-containing layer are performed in the same processing chamber.

3. The method of claim 1 , wherein thermal soaking the metal element-containing layer in the aluminum-containing gas is performed at a temperature from about 200° C. to about 500° C.

4. The method of claim 1 , wherein thermal soaking the metal element-containing layer in the aluminum-containing gas and thermal soaking the aluminum-containing layer in the silicon-containing gas are performed in the same processing chamber.

5. The method of claim 1 , wherein thermal soaking the first metal element-containing layer in the aluminum-containing gas is performed without using plasma.

6. The method of claim 1 , further comprising:

providing an inert gas to react with the aluminum-containing gas, such that a compound of the aluminum-containing gas is dissociated.

7. The method of claim 1 , further comprising:

after thermal soaking the aluminum-containing layer in the silicon-containing gas, forming an etch stop layer extending along an upper surface of the silicon-containing layer.

8. The method of claim 1 , further comprising:

after thermal soaking the aluminum-containing layer in the silicon-containing gas, forming a P-work function metal layer over the silicon-containing layer; and

forming an N-work function metal layer over the P-work function metal layer.

9. A method, comprising:

forming a gate dielectric layer over a channel region;

forming a metal element-containing layer over the gate dielectric layer;

performing a surface cleaning process to remove oxide materials from the metal element-containing layer; and

after removing oxide materials from the metal element-containing layer, treating the metal element-containing layer with an aluminum-containing plasma, such that aluminum atoms from the aluminum-containing plasma are doped into a surface layer of the metal element-containing layer to convert the surface layer of the metal element-containing layer to an aluminum-containing layer.

10. The method of claim 9 , wherein the aluminum-containing plasma comprises an aluminum hydride plasma.

11. The method of claim 9 , wherein the aluminum-containing plasma comprises a trimethylaluminum plasma or a triethylaluminum plasma.

12. The method of claim 9 , further comprising:

treating the aluminum-containing layer with a silicon-containing plasma, such that silicon atoms from the silicon-containing plasma are doped into a surface layer of the aluminum-containing layer to convert the surface layer of the aluminum-containing layer to a silicon-containing layer.

13. The method of claim 9 , wherein after treating the metal element-containing layer with the aluminum-containing plasma, the aluminum-containing layer has an aluminum concentration that ranges from about 5% to about 30%.

14. The method of claim 9 , wherein after treating the metal element-containing layer with the aluminum-containing plasma, a bottom portion of the metal element-containing layer below the aluminum-containing layer has an aluminum concentration less than about 1%.

15. The method of claim 9 , wherein the surface cleaning process is performed by a plasma process.

16. A method, comprising:

forming a gate dielectric layer over a semiconductor fin and in a gate trench between gate spacers;

forming a metal-containing layer conformally over the gate dielectric layer;

thermal soaking the metal containing layer in an aluminum-containing gas to convert an upper portion of the metal-containing layer into an aluminum-containing layer;

after the thermal soaking the metal containing layer in the aluminum-containing gas, further thermal soaking the aluminum-containing layer in a silicon-containing gas to convert an upper portion of the aluminum-containing layer into a silicon-containing layer; and

after forming the silicon-containing layer, depositing a tungsten layer to fill up a remainder of the gate trench by using a tungsten fluoride precursor, wherein fluorine atoms from the tungsten fluoride precursor bond with silicon atoms in the silicon-containing layer.

17. The method of claim 16 , wherein the thermal soaking the metal containing layer in the aluminum-containing gas and the thermal soaking the aluminum-containing layer in the silicon-containing gas are performed in the same processing chamber.

18. The method of claim 16 , wherein after thermal soaking the aluminum-containing layer in the silicon-containing gas, the silicon-containing layer has a silicon concentration that ranges from about 5% to about 30%.

19. The method of claim 16 , wherein after thermal soaking the aluminum-containing layer in the silicon-containing gas, a bottom portion of the metal-containing layer below the silicon-containing layer has a silicon concentration less than about 1%.

20. The method of claim 16 , further comprising:

after thermal soaking the aluminum-containing layer in the silicon-containing gas, forming a titanium nitride layer extending along an upper surface of the silicon-containing layer.