Semiconductor device and manufacturing method thereof

1. A method for manufacturing a semiconductor device comprising:

forming a semiconductor strip over a substrate, wherein the semiconductor strip comprises a first semiconductor stack and a second semiconductor stack over the first semiconductor stack;

forming a dummy gate stack crossing the semiconductor strip, wherein the dummy gate stack comprises a first dummy gate layer, an etch stop layer, and a second dummy gate layer, the etch stop layer is between the first dummy gate layer and the second dummy gate layer;

replacing the dummy gate stack with a first metal gate stack and a second metal gate stack, wherein the first metal gate stack is in contact with a first semiconductor layer of the first semiconductor stack and the second metal gate stack is in contact with a first semiconductor layer of the second semiconductor stack, wherein a bottom surface of the first metal gate stack is lower than a bottom surface of the second metal gate stack;

wherein replacing the dummy gate stack with the first metal gate stack and the second metal gate stack comprises:

replacing a first portion of the second dummy gate layer, a first portion of the etch stop layer, and the first dummy gate layer with the first metal gate stack; and

replacing a second portion of the second dummy gate layer with the second metal gate stack.

2. The method of claim 1 , wherein replacing the first portion of the second dummy gate layer, the first portion of the etch stop layer, and the first dummy gate layer with the first metal gate stack comprises:

removing the first portion of the second dummy gate layer and the first portion of the etch stop layer to form a trench;

forming an inner gate spacer on sidewalls of the second portion of the second dummy gate layer facing the trench;

removing the first dummy gate layer and a second semiconductor layer of the first semiconductor stack to extend the trench; and

forming the first metal gate stack in the trench.

3. The method of claim 2 , wherein the second semiconductor layer of the first semiconductor stack and the first semiconductor layer of the second semiconductor stack are made of a same material.

4. The method of claim 1 , further comprising forming an outer gate spacer on a sidewall of the dummy gate stack, and the first metal gate stack and the second metal gate stack are formed to be in contact with the outer gate spacer.

5. The method of claim 1 , wherein the first metal gate stack is one of an N-type metal gate stack and a P-type metal gate stack, and the second metal gate stack is another one of the N-type metal gate stack and the P-type metal gate stack.

6. The method of claim 1 , wherein the first semiconductor layer of the first semiconductor stack and the first semiconductor layer of the second semiconductor stack are made of different materials.

7. A method for manufacturing a semiconductor device comprising:

forming a semiconductor strip over a substrate, wherein the semiconductor strip comprises a first semiconductor stack and a second semiconductor stack over the first semiconductor stack;

forming a gate stack crossing the semiconductor strip;

removing a portion of the semiconductor strip uncovered by the gate stack to expose a remaining portion of the semiconductor strip;

forming a first epitaxy structure in contact with a first semiconductor layer of the first semiconductor stack; and

forming a sacrificial epitaxy structure in contact with a first semiconductor layer of the second semiconductor stack;

removing the sacrificial epitaxy structure; and

after removing the sacrificial epitaxy structure, forming a second epitaxy structure in contact with the first semiconductor layer of the second semiconductor stack and separated apart from the first epitaxy structure, wherein the second epitaxy structure is over the first epitaxy structure.

8. The method of claim 7 , wherein the first semiconductor stack further comprises a second semiconductor layer between the second semiconductor stack and the first semiconductor layer of the first semiconductor stack, and the method further comprises:

removing a portion of the second semiconductor layer of the first semiconductor stack to form a recess in the first semiconductor stack; and

forming a sidewall spacer in the recess before forming the first epitaxy structure.

9. The method of claim 7 , further comprising forming an insulation layer in the substrate after the portion of the semiconductor strip uncovered by the gate stack is removed.

10. The method of claim 7 , further comprises:

forming a bottom interlayer dielectric (ILD) covering the first epitaxy structure.

11. The method of claim 10 , further comprising forming a top ILD covering the second epitaxy structure and over the bottom ILD after forming the second epitaxy structure.

12. The method of claim 11 , further comprising:

forming a first contact in contact with the first epitaxy structure; and

forming a second contact in contact with the second epitaxy structure.

13. The method of claim 12 , wherein forming the first contact comprises:

removing a first portion of the top ILD to form a trench;

forming a contact spacer on a sidewall of the first portion of the top ILD facing the trench;

removing the bottom ILD to extend the trench and expose the first epitaxy structure; and

forming the first contact in the trench.

14. The method of claim 13 , wherein forming the second contact comprises:

removing a second portion of the top ILD to form a recess exposing the second epitaxy structure after forming the contact spacer; and

forming the second contact in the recess.

15. A method for manufacturing a semiconductor device comprising:

forming a semiconductor strip over a substrate;

forming a first sacrificial layer over the substrate;

forming a first semiconductor stack over the first sacrificial layer;

forming a second sacrificial layer over the first semiconductor stack;

forming a second semiconductor stack over the second sacrificial layer;

patterning the first sacrificial layer, the first semiconductor stack, the second sacrificial layer, and the second semiconductor stack to form the semiconductor strip;

forming a dummy gate stack crossing the semiconductor strip;

removing a portion of the semiconductor strip uncovered by the dummy gate stack to expose a remaining portion of the semiconductor strip;

after removing the portion of the semiconductor strip, replacing the patterned first sacrificial layer with a first inner gate spacer;

forming epitaxy structures respectively in contact with a first semiconductor layer of the first semiconductor stack and a first semiconductor layer of the second semiconductor stack;

removing a portion of the dummy gate stack to form a recess adjacent to a remaining dummy gate stack;

forming an inner gate spacer on a sidewall of the remaining dummy gate stack; and

forming a first gate stack in the recess, wherein the first gate stack extends from a sidewall of the inner gate spacer to a bottom surface of the inner gate spacer.

16. The method of claim 15 , further comprising replacing the patterned second sacrificial layer with a second inner gate spacer after removing the portion of the semiconductor strip.

17. The method of claim 15 , wherein the first sacrificial layer and the first semiconductor layer of the first semiconductor stack comprise SiGe but have different germanium percentages.

18. The method of claim 15 , wherein the first semiconductor stack further comprises a second semiconductor layer, and the first and second semiconductor layers of the first semiconductor stack and the first sacrificial layer have different etching selectivity.

19. The method of claim 15 , further comprising:

removing the remaining dummy gate stack to form a trench; and

forming a second gate stack in the trench, wherein the second gate stack is formed above a portion of the first gate stack.

20. The method of claim 19 , wherein the second gate stack has a top surface flush with a top surface of the first gate stack.